Screen Printing Hb

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Handbook forscreen printers

Published by:Sefar Inc.Printing Division

Handbookfor screen printers

Published by:

Sefar Inc.Printing DivisionCH-9425 Thal/SGSwitzerland

February 1999

©Copyright by SEFAR

i©Copyright by SEFAR, 2/1999 Introduction

Foreword

Welcome to this, the first edition of the Sefar Screen PrintingHandbook.

It is intended as a comprehensive, up-to-date reference work forscreen printers. Written by and for screen printers, it aims to coverall aspects of the screen printing applications.

WE SHARE OUR KNOWLEDGE: This is a cornerstone of the SefarGroup’s business philosophy. In this book, we offer a comprehensivebody of reference material to screen printers. We have always consi-dered customer feedback essential to our success as mesh manufac-turers. This handbook is an ideal reference to our screen printingcourses and customer-specific training programmes.

We have gathered the knowledge and experience of Sefar’s applica-tion technologists. We would like to thank everyone who contribu-ted to this book and provided text and illustrations.

Sefar Inc.

Printing Division

Applications Technology Thal

ii Contents ©Copyright by SEFAR, 2/1999

Contents

1. Monofilament screen printing fabrics 1.1

1.1 Raw materials 1.1

1.2 Physical fabric properties 1.4

1.3 Elongation characteristics of polyester fabrics 1.6

1.4 Geometry of screen printing fabrics 1.7

1.5 Coloured screen printing fabrics 1.16

1.6 Calendered screen printing fabrics 1.18

1.7 History and development of screen printing fabrics 1.20

1.8 Conveyor belts for textile printing dryers 1.22

2. Screen printing frames 2.1

2.1 Materials used for screen printing frames 2.1

2.2 Sections 2.3

2.3 Frame size 2.5

2.4 Pre-treatment of frames 2.7

3. Stretching 3.1

3.1 Hand stretching 3.1

3.2 Mechanical stretching 3.2

3.3 Pneumatic stretching 3.6

3.4 Stretching at an angle 3.12

3.5 Multiple stretching 3.13

3.6 Correct stretching 3.15

3.7 Recommended tensions 3.17

3.8 Sefar tension control instruments 3.21

4. Gluing 4.1

4.1 Preparation 4.1

4.2 Marking stretched frames 4.2

4.3 Adhesives 4.3

4.4 Gluing the fabric to the frame 4.5

4.5 Screen storage 4.6

4.6 Stretching services 4.6

iii©Copyright by SEFAR, 2/1999 Contents

5. Diapositive manufacture 5.1

5.1 Manually made diapositives 5.1

5.2 Photographically made diapositives (films) 5.1

5.3 CTS (Computer to Screen) 5.2

5.4 Tips for external production of diapositive films 5.4

6. Stencils 6.1

6.1 Pre-treatment of screen printing fabrics 6.1

6.2 Mechanical stencils 6.1

6.3 Photomechanical stencils 6.4

6.4 Direct stencils with emulsion 6.5

6.5 Troubleshooting direct stencils with emulsion 6.6

6.6 Stencils for water-based inks 6.7

6.7 Direct stencils with film and emulsion 6.14

6.8 Direct stencils with film and water 6.15

6.9 Indirect stencils 6.17

6.10 Exposure 6.19

6.11 Step exposures 6.22

6.12 Rinsing 6.25

6.13 Influence of coating thickness on print sharpness 6.25

6.14 Influence of stencil thickness on ink volume 6.26

6.15 Hardening stencils for printing water-based colours 6.28

6.16 Reclaiming 6.29

7. Register 7.1

7.1 Summary of key recommendations 7.1

7.2 Problems of accurate register 7.2

7.3 The diapositive 7.3

7.4 The stencil 7.4

7.5 The printing substrate 7.14

8. Raster printing 8.1

8.1 AM raster (amplitude modulated rastering) 8.1

8.2 FM raster (frequency-modulated rastering) 8.2

8.3 Types of raster rulings 8.4

8.4 Raster ruling 8.7

8.5 Tone values of half-tone dots 8.10

8.6 Raster printing process line 8.11

8.7 Printing control strip 8.14

iv Contents ©Copyright by SEFAR, 2/1999

8.8 Types of stencils 8.16

8.9 Avoidance of moiré effects 8.16

8.10 General recommendations 8.20

8.11 Improved printing stability through achromatic reproduc-tion 8.23

8.12 Heuristic rastering for textile printing 8.26

8.13 Objectives of heuristic half-tone printing 8.31

8.14 Technical considerations 8.32

9. Printing 9.1

9.1 Setting up of a flat bed machine 9.2

9.2 The squeegee 9.4

9.3 Flood bar (Doctor blade) 9.10

9.4 Printing speed 9.11

9.5 Object printing 9.12

9.6 Single operation multiple colour printing 9.13

9.7 Ink deposit 9.13

9.8 UV inks 9.14

9.9 Printing systems 9.15

10. Measuring instruments 10.1

10.1 Coating thickness 10.1

10.2 Roughness 10.2

10.3 Radiometer (incident radiant energy measurement) 10.3

10.4 Hardness meter (Shore measuring instrument) 10.3

10.5 Viscosimeter 10.4

10.6 Wet coat thickness 10.4

10.7 Grindometer for measuring particle size 10.5

10.8 Recording thermometer/hygrometer 10.5

11. Recommended choice of fabrics 11.1

1.1Monofilament screen printing fabrics

1. Monofilament screen printing fabrics

Standard polyester fabrics make ideal stencil material for screenprinting. Precision weaving techniques, using state-of-the-artequipment optimised for screen printing applications, results inexcellent fabric quality.

Polyester fabrics woven from high-viscosity polyester are a furtherdevelopment of standard polyester fabrics. The material’s reducedelasticity enhances the already good properties of standard fabrics.These stencil fabrics offer increased process reliability, andsignificantly higher tensioning that is retained over large print runsand long periods of time.

Polyamide fabrics (nylon) have exceptional mechanical durability. Thismakes them highly suitable for printing abrasive media (ceramiccolours, reflective inks). The stencil fabric’s high elasticity makes iteasier to print uneven surfaces (e.g. shaped objects).

Stencil films and emulsions adhere better to polyamide fabrics thannormal polyester fabrics.

Note: This book uses the abbreviations PET for polyester, PA forpolyamide.

1.1 Raw materials

Physical properties

The raw materials generally used for screen printing fabrics – ”stencilcarriers” according to DIN 16610 – are monofilament chemicalfibres made of synthetic polymers.

The fibres most frequently used are:

– Polyamide, abbreviated to PA 6.6

– Polyester, abbreviated to PET

They both belong to the group of polycondensation orpolymerisation fibres.

The group to which they belong governs the fibre’s physicalproperties.

Polyester PET

The main properties of polyester fibres are:

– high resistance to stretching

– good mechanical durability

– good abrasion resistance

©Copyright by SEFAR, 2/1999

1.2 Monofilament screen printing fabrics

– high resistance to light

– insensitivity to climatic factors

Other properties are listed in the table "Fibre properties” below.

Polyamide PA (Nylon)

Polyamide fibres are excellent in the following respects:

– very good mechanical durability

– high abrasion resistance

– good surface-tension characteristics

– high elasticity

– good dimensional recovery characteristics (100 % following 2 %elongation)

Other properties are listed in the table ”Fibre properties” below.

Fibre properties

6.6APedimayloP)nolyn(

tnemalifonom

TEPretseyloPtnemalifonom

ytivargcificepS 41.1 83.1nihtgnertselisneT

mm/Nad 2 )yrd( 76-14 57-54%)tew(yticanet.leR 59-09 001

%kaerbtanoitagnolEyrd-tew-

53-0204-52

03-5103-51

ta%noitprosbaerutsioM02 ° .ler%56dnaC

ytidimuh

4-5.3 4.0

tniopgnitleM °C 352-742 062-042tniopgninetfoS °C 532-522 042-022

ecnatsisererutarepmeT° gnitimiletamixorppa(C

yrdrednuerutarepmet)snoitidnoc

egnahcruoloCotetihwmorf

eergedehT.wolleydnaegnahcfo

noitcuderhtgnertsehtnosdnepeddnaerutarepmet

.emitnoitcaer

otputaehyrD051 °C

suounitnoCerusopxe

dnathgilotecnatsiseRrehtaew

egarevaotwoL doogyrevotdooG

ecnatsisernoisarbA doogyreV dooG



Chemical resistance

Remarks:

Polyamide (nylon) Sensitive to acidsFibres are either weakened or destroyed,depending on concentration, temperature andreaction time.

Good resistance to alkalis.

Polyester Sensitive to alkalisFibres can be weakened to the point ofdestruction, depending on concentration,temperature and reaction time.

Highly resistant to inorganic acids.

6.6APedimayloP)nolyn(

tnemalifonom

TEPretseyloPtnemalifonom

yllarenegsdicA)a roop–detimiL doog

dicaciruhpluSdicacirolhcordyH

dicacirtiN

ecnatsiserdecudeR,noitulossidronotnedneped,noitartnecnoc

dnaerutarepmetemitnoitcaer

woltadetceffanU,snoitartnecnoc

dnaserutarepmetsemitnoitcaer

dicacimroF elbuloS detceffanU

dicacionahtE -erutarepmet,dooGtnedneped

detceffanU

yllarenegsilaklA)b -erutarepmet,dooGtnedneped

roop,detimiL

edixordyhmuidoSedixordyhmuissatoP

tadetceffanUmoorlamron,erutarepmettagninekaew

rehgihserutarepmet

rehgihtaelbuloS,serutarepmet

dnasnoitartnecnocsemitnoitcaer

yllarenegstnevloS)c otecnatsiserdooGneercsnommocstnevlosgnitnirp

otecnatsiserdooGneercsnommocstnevlosgnitnirp



1.2 Physical fabric properties

Screen printing fabrics with differing properties can be producedfrom the same fibre group by modifying the fibre and/or fabricmanufacturing processes.

In any screen printing application, the elongation characteristics ofthe screen printing fabric are crucial.

Elongation characteristics govern:

– tensioning procedure

– mesh strength

– mesh stability

Elongation characteristics have a direct influence on usagecharacteristics such as:

– register and dimensional precision of the printed image

– snap-off behaviour

– conformance with the object to be printed, important when thesubstrate has an uneven or irregular surface

– selecting a fabric type appropriate to the requirements of theprinting parameters

Having decided which fibre type to use, printers must then choosebetween polyamide (nylon) or polyester fabric.

Polyamide fabric PA 1000

Nylon fabrics were the first and most enduring monofilamentchemical fibres to be used in screen printing. Although a relativelyold technology, nylon fabrics still remain viable in certain areas oftoday’s screen printing industry, thanks to their desirable properties:

– good mechanical durability

– good abrasion resistance

– good surface-tension properties

– relatively high elasticity



Modified polyamide fabric PA 2000

This type of fabric combines the properties of regular polyamide(nylon) fabrics – good mechanical durability, abrasion resistance, andsurface tension properties – with lower elongation.

The benefits:Improved snap-off and ink release, with sufficient elasticity toconform with uneven substrates.

Polyester fabrics

The classic stencil substrate for screen and textile printing. Correctlyhandled, it can be used in a wide variety of applications.

ytreporP stifeneblacitcarP

noitagnolewoL ruoivahebgninoisnetdooG•ruoivahebffo-pansdooG•

ycaruccanoitartsigerdooG•

dnaytilibarudlacinahcemhgiHecnatsiserlacimehc

snurtnirpgnolrofelbatiuS•esu-erdnagnimialcerdooG•

scitsiretcarahc

ecafruserbifhtoomS noitartenepknidoogyreV•esaelerknitsaF• → hgih

gnitnirpdeepsliatedfonoitcudorperdooG•

etamilcnisegnahcottnatsiseR)erutarepmet/ytidimuh(

ytilibatslanoisnemiddooG•,gninaelcretfagniyrddipaR•

gnipoleveddnagnitaoc



Modified polyester fabric PET 1000

Monofilament polyester fabric with reduced elongation, also knownas ”high-modulus fabric” is distinguished from regular polyesterfabric by its low elongation and mechanical resistance.

1.3 Elongation characteristics of polyester fabrics

Force/elongation characteristics of screen printing fabricsmanufactured by Sefar AG from monofilament synthetic fibres.

The elongation characteristics are vital to the performance of thestencil system. This is illustrated in the force/elongation diagramabove.

The graph shows the relationship between the tensioning force andthe resulting fabric elongation, including the force and elongationjust before the sample tears. Tensioning tests are an integral part ofSEFAR’s quality control, and are conducted using tension testingmachines under constant controlled conditions.

High-modulus polyester

Standard polyester

Fabric elongation (%)

Ten

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N/c

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ytreporP stifeneblacitcarP

noitagnolewolyreV hgihdnatshtiwotelbA•snoisnet

knidnaffo-tfildooG•htiwscitsiretcarahcesaeler

tcatnocffolaminimnoitartsigerdesaercnI•

ycaruccarevoycaruccatnetsisnoC•

snurtnirpgnolnoisnetninoitcudertsedoM•

esuhtiwefilnoitcudorpregnoL•



1.4 Geometry of screen printing fabrics

Fabric geometry describes all two- and three-dimensional aspects ofthe fabric’s structure.

The basic factors in fabric geometry are mesh count and threaddiameter.

Mesh count is specified as the number of threads per cm.

The thread diameter is specified as a nominal value, referring to thediameter of the unwoven thread.

When selecting fabric for a particular application, fabric geometry isof greater importance than the elasticity character.

Fabric geometry directly affects:

– printability of fine line and half-tone images

– edge definition in the print

– ink release characteristics

– maximum printing speed (in conjunction with ink viscosity)

– thickness of the ink volume

– ink consumption

– ink drying

The following values, listed in technical datasheets, such as

– mesh opening in µm, abbreviated to (w)

– mesh opening in %, abbreviated to (ao)

– mesh thickness (fabric thickness) in µm, abbreviated to (D)

– theoretical ink volume in cm3/m2, abbreviated to (Vth)

are all derived from the mesh count (Fn) and the thread diameter (d).

The fundamental geometrical unit is the fabric pitch (t).

Pitch (t) is the sum of one mesh opening and thread diameter (t = w + d). This value is calculated as follows: t = 10'000/Fn.

The flat surface of a fabric is the result of weaving perpendicularwarp and weft threads. A high quality fabric is characterised byextremely close tolerances for both the overall mesh count Fn, aswell as mesh counts in the warp and weft directions.

Sefar AG products guarantee the closest fabric geometry tolerances;these are published in the technical datasheets.



Mesh count and thread diameter

The terms ”mesh type” or ”fabric number” are similar descriptionsof the mesh count per centimetre / inch, together with the threaddiameter.

Example: 120-34 indicates 120 threads per cm, each with anominal thread diameter of 34 µm.

The nominal thread diameter refers to the diameter of the unwoventhread.

”Mesh type” is a relatively new term that replaces the widespreadSL/S/M/T/HD nomenclature.

New nomenclature Old nomenclature

120-31 120 S120-34 120 T120-40 120 HD

150-27 150 SL

Mesh type description

W = White

Y = Yellow

CY = spun dyed, yellow

PW = Plain Weave

TW = Twill Weave

OSC = One Side Calendered

Example:

PET 1000 140-34Y PW OSC


CalenderedPlain WeaveYellowThread diameterMesh-countQualityMaterial


From the «Technical data sheet for SEFAR© PET 1000 10.99»

mc hcni mc/n-/+ µm µm % µm -/+ µm mc 3 m/ 2 m/g 2

WPW13-021 WPW13-503 1:1 0.3 94 13 0.53 94 3 2.71 62

WPY13-021 WPY13-503 1:1 0.3 94 13 0.53 94 3 2.71 62

WPW43-021 WPW43-503 1:1 0.3 54 43 6.92 55 3 3.61 43

WPY43-021 WPY43-503 1:1 0.3 54 43 6.92 55 3 3.61 43

WPW04-021 WPW04-503 1:1 0.3 73 04 1.02 56 3 0.31 44

WPY04-021 WPY04-503 1:1 0.3 73 04 1.02 56 3 0.31 44

WPY72-051 WPY72-083 1:1 0.4 63 72 6.82 14 2 7.11 62

WPW13-051 WPW13-083 1:1 0.4 23 13 3.32 74 3 9.01 23

WPY13-051 WPY13-083 1:1 0.4 23 13 3.32 74 3 9.01 23

WPW43-051 WPW43-083 1:1 0.4 32 43 1.21 55 3 6.6 24

WPY43-051 WPY43-083 1:1 0.4 32 43 1.21 55 3 6.6 24

WTW43-051 WTW43-083 1:2 0.4 62 43 4.51 26 3 6.9 24

WTY43-051 WTY43-083 1:2 0.4 62 43 4.51 26 3 6.9 24


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Weave type

The mesh type is specified along with the weave type. This describesthe pattern in which the weft and warp fibres cross over each other,and is expressed as a weave number.

Screen printing fabrics are either plain or twill weave. Plain weave isa 1:1 weave.

Various types of twill weave have differing weave numbers e.g. 1:2 or 2:2.

Plain 1:1 = PW Twill 1:2 = TW

Mesh opening

Mesh opening w is the spacing between adjacent warp or weftthreads, and is measured perpendicular to the plane of the fabric.

Mesh opening governs:

– the maximum particle size to be used in a screen printing ink

Mesh opening affects:

– the level of printed detail in line and half-tone artwork

– ink release characteristics

– the thickness of the ink volume

Note: For adequate ink penetration, the average particle size p ofthe screen printing ink must be smaller than 1/3 of the mesh opening.



Resolution characteristics

”Resolution” refers to the level of printed detail in line and half-toneartwork that a given fabric is capable of reproducing. It is governedprimarily by the mesh count, and the relationship of thread diameterto mesh opening.

Examining the relationship of thread diameter to mesh opening wmore closely, screen printing fabrics fall into the following categories:

a) Mesh opening greater than thread diameter (ω > d)PET 1000 150–27PW ω =36µm

b)Mesh opening comparable to thread diameter (ω ≅ d)PET 1000 150–31PW ω =32µm

c) Mesh opening less than thread diameter (ω < d)PET 1000 150–34PW ω =23µm

In general, fabrics where the mesh opening is greater than thethread diameter are capable of higher resolution than fabrics wherethe opposite is true.

Next to the relationship of thread diameter to mesh opening, thethread diameter itself also affects the printable dot / line size.

Secondary factors in the printability of fine line and half-toneartwork are the flow, viscosity and rheology of the screen printingink.

PET 1000 150-27PW PET 1000 150-31PW PET 1000 150-34PW

The theoretical resolution capability Ath for a given screen printing

fabric can be estimated from the following formula:

Ath = √2 * t * d/ω(t = ω + d)



Table: Theoretical resolution Ath for a variety of screen printingfabrics

Key:

Ath = theoretical resolution

Fn = Mesh count

d = Nominal thread diameter

ω = Mesh opening

The theoretical resolution value of a fabric should be taken as arelative guideline for better understanding of geometrical factors onthe relationship between mesh number, thread diameter and meshopening.

rebmuncirbaF mµhtiwhseM =hctiP+d(t )ω mµ

A htmµ

WP04-0010001TEP 75 79 69WP43-0110001TEP 45 88 87WP04-0110001TEP 74 78 501WP13-0210001TEP 94 08 27WP43-0210001TEP 54 97 48WP04-0210001TEP 73 77 811WP13-0410001TEP 63 76 28WP43-0410001TEP 13 56 101WP72-0510001TEP 63 36 76WP13-0510001TEP 23 36 68WP43-0510001TEP 32 75 911WP72-5610001TEP 92 65 47WP13-5610001TEP 32 45 301WP72-0810001TEP 22 94 58



Open area, αo (open mesh area in %)

The sum of all the mesh openings over the total area. A fabric withan αo of 30.5% has an open mesh area of 30.5%, and a closed,impervious area of 69.5%.

αo % is one of the variables used to calculate the theoretical inkvolume.

Theoretical ink volume Vth cm3/m2

This value is calculated from the open area and mesh thickness. Thevolume of the open mesh governs the effective amount of ink that ascreen printing fabric can accept. The calculated effective ink volumeis higher than, but proportional to, the theoretical ink volume.

Under realistic conditions, the degree to which a fabric is filled withink depends on the squeegee speed, squeegee blade characteristics– hardness, angle, and finish – and the consistency of the ink itself.Given the difficulty of calculating a value based on so manyvariables, theoretical ink volume provides a more practical alternativefor determining ink consumption and the thickness of the printedvolume.

Given optimally filled fabric and clean ink release, the thickness ofthe wet printing ink deposit can be estimated from the theoreticalink volume:



Theoretical ink volume cm3/m2 = αo * D

100

A fabric with a theoretical ink volume of 18 cm3/m2 produces aprinted layer with a thickness (when wet) of 18 µm.

Basic ink consumption Mf is then estimated by:

m2/Lt = 1000

Vth

The relative accuracy of the estimated value may be improved byincorporating factors to compensate for the absorbtion of theprinting stock and the percentage thinning of the screen printingink.

Compensation factor S for porosity of the printing stock:

S for highly porous printing stock = 0.5

S for slightly porous printing stock = 0.8

S for non-porous printing stock = 1.0

Compensation factors for ink thinning percentage (V)

V 0% = 1

V 5% = 1.05

V 10% = 1.10

V 15% = 1.15

V 20% = 1.20

etc.

Taking all these factors into account, the approximate inkconsumption Mf can be derived from the screen printing fabric’stheoretical ink volume using the following formula:

m2/Lt = (theoretical coverage) * S * V

= 1000

Vth * S * V



Fabric geometry summary

Key:

D = Fabric thickness (mesh thickness)

d = Thread diameter

Fk = Warp threads

Fs = Weft threads

n = Mesh count

t = Pitch = ω + d

ω = Mesh opening


d / µm

d / µm

d / µm

D / µm

n / cm

ω / µm

t / µm

d / µm Fs / µm

Fk /

µm


1.5 Coloured screen printing fabrics

By exposing a direct stencil to light, the illuminated areas becomehardened. Light rays striking the white fibres of the fabric arereflected, and scatter under the black edges of the film.

Light is also conducted through the fibres themselves, leading to yetmore under cutting.

The results are unsharp printed edges, causing colour shifts inmulticolour half-tone artwork. There is a reduction of the openprinting areas, particularly in fine detail work. To keep thesephenomena under control, it is necessary to calculate exposure timesleading to proper exposure.

White fabric

Emulsions and films are sensitive in the UV range, from approx. 350to 420 nanometres. To be effective, light scatter protection mustabsorb UV light over this wavelength range. The obvious choice inachieving this is to use the complementary colour, which bydefinition absorbs the desired wavelengths. Absorption tests showthat the most effective absorber in the 350 – 420 nanometre rangeis a warm yellow colour.

When UV light strikes a yellow fibre, only yellow light is reflected –and this has no effect on the emulsion. This is why it is advisable towork with emulsion in yellow light. Emulsions are sensitive only toblue UV light. The results are pin-sharp edges, and open details.Also, because light scatter is no longer an issue, it is now possible togive exposure times that thoroughly harden the emulsion. In general,exposure times on dyed SEFAR fabrics are 75% – 125% longer thanon their plain white counterparts due to less light scatter of blue UVlight; this results in tougher, more durable stencils. Generousoverexposure latitude reduces the risk of underexposure.



Dyed fabric

Dyed fabrics should always be chosen for printing the finest lines,text and half tones.



1.6 Calendered screen printing fabrics

Screen printing inks contain solvents that evaporate during thedrying process, thus reducing the thickness of the ink volume.

UV-hardening inks , by contrast, contain very little or no solvents.This means that hardening does not significantly reduce thethickness of the ink volume.

The high ink film left by UV-hardening inks often produces problems:

- UV light does not adequately penetrate a thick ink layer, especiallywhen it is heavily laden with pigment. The ink is incompletelyhardened as a result.

- Multicolour half-tone printing:If the first two colours are too thickly layered, there is barely spacefor the third and fourth colours between or on top of the pointsleft by the first two. This results in colour shifts, smeared print,and moiré effects.

Recent years have seen major progress in weaving technology. Thismakes it feasible to produce even the finest fabrics in a 1:1 weave.The resulting trend is towards increased use of finer, uncalenderedfabrics that offer increased printing resolution and better ink controlcharacteristics. For this reason, the range of calendered fabric typeshas been reduced.

The current range still includes the following fabric numbers,available with a maximum width of 206 cm:

SEFAR PET 1000 140-34Y PW OSC




SEFAR PET 1000 165-34Y TW OSC

SEFAR PET 1000 180-31Y TW OSC

These fabrics are suitable for printing with UV inks and lacquers.PET 1000 OSC are one side calendered, yellow-dyed fabrics.The calendered side is shiny, the other side is dull.



There are two techniques for reducing the ink deposit:

1.By stretching the fabric with the shiny surface facing thesqueegee, OSC fabrics reduce the ink deposit by around 10 – 15% compared to uncalendered fabrics.

2. If the shiny surface is stretched facing the substrate, the inkdeposit is reduced by around 15 – 25%.

The degree of ink volume reduction depends on a variety ofadditional factors in the printing process, particularly the ink’srheological characteristics, which vary depending on the colour. It istherefore impossible to give exact figures.

Fabric cross-sections: Comparative ink deposit:

Regular fabric 100%

Calendered side = squeegee side (RK) approx. 10–15% reduction

Calendered side = substrate side (DK) approx. 15–25% reduction



1.7 History and development of screen printing fabrics

Several hundred years separate the first stencils made from humanhair, and contemporary high-precision screen printing materialsbased on modified synthetic fibres. Despite this, screen printing is acomparatively new printing technique, the first documentedappearance being a patent submission from 1907 in which SamuelSimons recommended silk gauze (used to sieve flour) as a stencilmaterial. Shortly thereafter, silk weavers began manufacturing plainweave fabrics especially for screen printers, allowing finer printingand improved ink control.

The rise of synthetic fibres not only improved screen printing quality,it also increased the range of potential applications. Serigraphy,originally used by artists, thus became an industrial printingtechnique. Research and development in the weaving industryproduced continual advances: fabrics with up to 200 threads per cm,widths up to 365 cm, and extraordinarily high tensioning capabilityopened radically new vistas for screen printing in all branches ofindustrial manufacturing.

The silk gauze recommended by Samuel Simons was woven fromselected multifilament silk yarn. To prevent threads from sliding andblocking the mesh as the flour was sieved, a special technique calledtwill weaving was used:

Silk gauze, multifilament

The first fabrics specially developed for screen printing were alsomade from multifilament silk yarn, but plain woven. The mesh countcould thus be increased to 90 threads/cm.

The first synthetic yarns were likewise multifilament, plain woven,but considerably easier to stretch than silk. Furthermore, they wereinsensitive to water and resistant to chemical attack. Theseproperties represented a breakthrough, because they allowed screenprinting technology to be used with every imaginable ink system andprinting stock.



The spinning industry’s success in manufacturing monofilament yarnsopened the way for a further step in screen printing development.Monofilament yarns can be manufactured in considerably thinnerand more consistent diameters than their multifilament counterparts.Thus, fabrics with up to 200 threads/cm could be manufactured,with no loss of mesh opening compared to multifilament fabrics.This opened up entire new markets to screen printing, withapplications in electronics, ceramics, packaging, CD labels, etc.

Despite satisfying growth in the screen printing industry, researchand development continues apace. New materials are being tested,along with varied fabric treatments and novel weaving techniques.The screen printing industry can continue to look forward to newgenerations of screen printing fabrics, able to keep pace withincreasing demands and expectations.



1.8 Conveyor belts for textile printing dryers

Conveyor belt:a = Printer tableb = Printed stockc = Conveyord = Dryer

PET 1000 HD PET Mono-Multi

PET Mono-Multi PLUS PET Soft

ab

cd



Conveyor belts made of strong polyester monofilament, for examplePET 1000–HD, have the following advantages:

Air-current pervious – fresh air reaches all sides of the conveyedgoods. The benefits:

– lower drying temperatures are possible

– higher running speeds are possible

– energy savings

– permissible temperatures up to 150 ºC

– no squeezing of the conveyed goods

– anti-static treatment possible

– good running characteristics

– minimal soiling, easy to clean.

Belts are available with a variety of closures (mainly of the snapvariety), or stitched together as an endless belt.

Belts are supplied with either a woven or welded edge trim. Bothvariants can be hemmed with reinforcing tape to protect againstfraying.

For temperatures above 150 ºC we recommend conveyor belts madeof PTFE coated glass fibre.

Textile endless belts are subject to a certain degree of stretching andelasticity. They should therefore be tensioned over compensationrollers.

Please address questions and queries to:

Sefar Inc.Filtration DivisionMoosstrasse 2CH-8803 Ruschlikon

Switzerland

Phone ++41-1-724 65 11Fax ++41-1-724 15 25

Internet: http://www.fd.sefar.ch


2.1Screen printing frames

2. Screen printing frames

A screen printing frame is constructed from hollow sections, anddesigned to hold a tightly stretched piece of fabric. Screen printingframes should be as resistant as possible to mechanical deformation,both during stencil-making and during the printing process. Thesurface should also be resistant to chemicals used in stencil-making,printing inks, solvents and cleaning materials.

Frame sections must be welded to lie perfectly flat, and straightenedwhere necessary. Warped frame sections are a considerable nuisancewhen printing, and lead to register errors.

2.1 Materials used for screen printing frames

Wooden frames

Wooden frames are easily handled, especially the small woodenframes for container printing. They should not be used, however, forprecision register prints. Wood swells and shrinks, often within a fewhours, due to fluctuating humidity and temperature. Wooden frameshave a shorter life than metal frames; they warp in the course oftime and no longer lie flat.

Painting the wood with a two-component lacquer protects it fromwater and solvents.

Metal frames

Aluminium and steel are the metals most commonly used to makescreen printing frames.

Aluminium is 2.9x more flexible than steel of similar cross-section. Togive aluminium comparable strength, the cross-sectional area mustbe enlarged, the section walls thickened, or an alternative sectionprofile chosen.


2.2 Screen printing frames

Aluminium frames

Aluminium’s specific gravity (approx. 2.7) means that even very largeframes are easily handled. However, larger frames must be designedwith increased cross-sectional area and reinforced walls. Aluminiumframes do not rust, but are less resistant to alkalis and acids.

Advantages:

– can be used to stretch all kinds of fabric

– light weight

– wide choice of cross sections

– low-cost

– good corrosion resistance

– easily cleaned

Disadvantages:

– less robust than steel

Steel frames

Given the good rigidity of steel frames, the cross-section can besmaller than a comparable aluminium frame. A major disadvantage,especially with large frames, is their associated weight (specificgravity of steel approx. 7.8).

Regular steel frames tend to rust, and require appropriate treatment(galvanising or painting).

Advantage:

– low-cost

Disadvantages:

– susceptible to corrosion

– heavy

– require repainting before re-use (unnecessary if two-componentglues are used).



2.2 Sections

Next to the material used, the type of section and the wall thicknessare decisive factors influencing the dimensional stability of screenprinting frames.

We distinguish between rectangular and special sections.

Rectangular sections

Special sections

Sections with reinforced verticalwalls

40

60

2

3

30

50

2

4.5

40

40

2

4.5

Sections with 4 identical wallthicknesses

40

40

340

60

3



Flat steel strip for CD framesSpecial-purpose L-section (e.g. for object printing)

25

4

25

4

25

Section with concave edge,tapered to the outside (for textile printing)

Section with concave edge

38.6

236.74

2

50

80

Section with inner supportSection with sloping inner edge

50

100

340

65

2.5

55

4.5



Recommended sections, frame dimensions and tension values

2.3 Frame size

The choice of frame size depends on the desired printable area andthe type of printing. There should always be an adequate zonereserved outside the printable area, for use as an ink rest.

In machine printing the squeegee motion is usually in the direction ofthe frame width, in other words, contrary to the usual practice inhand printing. The horizontal and in particular the vertical distancesbetween the frame and printable area (the ink rest) has to bedetermined by test for every type of machine. Too small an ink restcan lead to register difficulties and poor print quality. The sizes amachine can use have to be determined through individual trials.

In textile printing, the size of the printable area and the frame mustbe adapted to the squeegee system, and set up in accordance withthe machine manufacturer’s instructions.

In contrast to graphic screen printing, textile printing is generally donein ”contact”, i.e. there is no physical separation between the stenciland the printing stock. (See the chapter on printing.)

noitcesmuinimulAhtgneLta)mc(mc/N21

htgneLta)mc(mc/N51

htgneLta)mc(mc/N02

htgneLta)mc(mc/N52

8.1/0.3x03x035.2x5.42x1.83

5.2x53x53

570808

075757

568607

062636

8,1/0.3x03x03/04EPOLS5.2x03x04

0.2/8.2x04x04

5809021

5708001

075709

367658

8.1/1.3x04x53/54EPOLS0.2/5.4x04x040.2/5.4x03x05

031051081

021041051

011521031

001011021

0.2/2.3x04x050.2/2.3x04x54/55EPOLS

0.2/5.4x04x05

081012032

051081002

031051071

021021031

0.2/0.3x04x065.2/5.4x04x55/56EPOLS

0.3/0.6x04x06

052062072

022032042

091012022

051071081

8.2/0.5x05x56/57EPOLS0.3/0.6x04x08

8.2/0.5x05x57/58EPOLS

082003013

062072082

032052062

002032042

0.3/0.6x04x0015.2/0.5x05x521/531EPOLS

0.4x05x0510.6/0.01x06x071/081EPOLS

004066007008

083065006057

053------

033------



Recommended frame sizes

Warning: In textile printing applications, follow only the machinemanufacturer’s instructions. In the case of extremerectangular dimensions, consult the frame manufacturerfor information.

A A 1B/B 1C/CmuinimulA

dnasnoitcesllaw

sessenkcihtmmni

muinimulAsnoitces

suoiravhtiwllaw

sessenkcihtmmni

leetSdnasnoitces

llawsessenkciht

mmnitamroF

NIDaeraelbatnirP

mmni

stserknIpot/edis

mmni

edisniemarFsnoisnemid

mmni

4A 003x012 051/051 006x01504/040.3-5.2

04/040.2/5.2

04/045.1

3A 024x003 051/051 027x006

2A 095x024 051/051 098x027

1A 048x095 061/061 0611x019 05/040.306/04

0.3

05/040.2/0.3

05/040.2/5.4

05/040.2

0A 0811x048 081/081 0451x0921

0061x0021 002/002 0002x006104/060.3/0.6

0081x0041 022/022 0422x048104/080.3/0.6

0012x0061 052/052 0062x001204/0010.3/4.6

B1

B1

B B

C1

D

AD C



2.4 Pre-treatment of frames

Screen printing frames should not have any sharp edges or pointed corners, since these candamage the fabric which might tear when tightly stretched.

Sandblasted frames

Screen printing frames that have been sandblasted must bethoroughly degreased with a solvent (acetone) immediately prior touse. Greasy cleaning agents must not be used. After degreasing,frames that will be used with fine fabrics (OSC fabrics and otherswith a mesh count of 100 or more) should be primed using thesame adhesive that will later be used for gluing.

Metal frames that have not been sandblasted

Metal frames with a smooth surface must be roughened prior touse.

Roughening

The recommended way to roughen adhesive surfaces, as well asremove stray glue from used frames, is to use a rotary grinder withan abrasive paper or fibre disc fitted to a rubber backing. The disksthemselves should be no. 24 or 36 grit.

When working on the frame, it is essential to keep the frame surfacetruly flat and level, otherwise there could be contact and gluingproblems later.

Frame surface after grindingRotary grinder with integrateddust extractor



Ensure that all edges and corners have been de-burred.

Shortly before gluing, frames must be thoroughly degreased with asolvent (acetone). Greasy cleaning agents must not be used. Afterdegreasing, frames that will be used with fine fabrics (OSC fabricsand others with a mesh count of 100 or more) should be primedusing the same adhesive that will later be used for gluing.

Frame surface after beltsanding. Because the groovesrun parallel to the frame,solvents cannot penetratebetween the frame and thefabric.

Belt sander, no. 24–36 grit. Thistechnique gives a flatter frame



Cleaning used frames

Frames that have been used previously must be stripped of leftoverfabric, ink and glue. The edges must be rounded, to avoid the risk oftearing the fabric. Old glue may be left on the frame surfaces,providing it is even (no holes or bumps) and the layer is not toothick.

Poor frame edge must be rounded-off

Frame after rounding-off the glue edges

Frames prepared in this way are now ready for gluing.


3.1Stretching

3. Stretching

Stretching systems

There are three basic approaches to stretching fabric, offeringvarious degrees of precision.

– Hand stretching

– Mechanical stretching

– Pneumatic stretching

3.1 Hand stretching

The traditional method of stretching fabric on wooden frames byhand (aided by stretching grippers and staples) is still used by somescreen printers, especially for printing on solid objects.

It is essential that the fabric is also glued to the frame.

This technique does not give uniformly, tautly stretched fabric.Warning: staples tend to tear the fabric.

Hand stretching


3.2 Stretching

3.2 Mechanical stretching

Mechanical stretching apparatus produces tensioning forces in thewarp and weft directions. Depending on the equipment dimensions,several frames can be stretched at once. Also angled positioning offrames is possible. The ability to stretch several frames at onceincreases productivity. However, mechanical apparatus is unable topre-stress the frames. However, this can be achieved using separateequipment.

Mechanical stretching apparatus may be classified into two groups:

– Self-tensioning frames

– Spindle tensioning machines

Self-tensioning frames

Roller frame

Fabric is clamped in the frame. Stretching is done by rotating theframe rails, for example.

Self-tensioning frames have the advantage that the fabric does notneed to be glued to the frame.

Caution: Excessive fabric tension and risk of tearing at thecorners.


3.3Stretching

Spindle tensioning machines

Spindle tensioning is another mechanical approach. The machineconsists of a sub-frame supporting four guide-rails, which in turncarry pin rails or carriage clamps that grip the fabric. The grippers aremoved by means of a threaded shaft, which is turned either by ahand-wheel, a ratchet, a torque wrench, or an electric motor.Tensioning is achieved by increasing the distance from the guiderails.

Spindle tensioning machine

During the stretching process, the screen printing frame rests on aheight-adjustable holder, to avoid contact with the fabric. The frameis pressed onto the fabric during gluing.

When gluing at an angle, the printing frame is laid on the holder atthe desired angle, while the fabric is stretched at right angles in thenormal way.


3.4 Stretching

Stretching machines with pin rails

In this type of machine, the fabric is hung over rigid pin railsattached to the stretching battens.

Pin rails are dangerous, particularly with fine-count fabrics. Theserequire particularly careful handling to reduce the risk of tearing.

In stretching machines with rigid, laterally fixed stretching battens,the corners of the fabric must be handled with special care to avoidover-stretching. Over-strained corners often cause torn fabric duringand after stretching.

To reduce the possibility of this happening, the corners must be laidfree and then gradually pinned, so they have just the right tensionafter stretching is complete.


3.5Stretching

Tensioning machines with single clamps

This type of machine replaces pin rails with movable clamps. Theyrun on ball bearings and can follow the fabric as it stretches. Thisaccommodates length changes during stretching.

Clamp

Stretching machine


3.6 Stretching

Semi-automatic mechanical stretching machines

Semi-automatic mechanical stretching machines providecontinuously variable tensioning by mechanical means alone. Force isapplied by means of an electromechanical drive mechanism thatpulls the stock clamps outwards on both guide-rails at once, or eachone in turn.

These machines are suited to large-size frames, or for coveringseveral smaller frames at once.

Mechanical stretching machine

3.3 Pneumatic stretching

Pneumatic stretching machines consist of many individual stretchingclamps, linked together and acting in concert. The clamps areoperated by compressed air, and the number used depends on theframe size.

Tabletop pneumatic stretching machine


3.7Stretching

The clamps are constructed in such a way that they prop themselvesagainst the printing frame during stretching. The tension applied tothe fabric also comes to bear on the frame edges. The frame is thusautomatically given the necessary pre-stress to avoid loss of fabrictension after gluing.

Since the clamps are held under steady pre-set air pressure, fabrictension remains constant right up to the time of gluing.

SEFAR 3

The SEFAR 3 stretching clamp is pneumatically operated, with amanual closure.

The fabric is clamped between two dissimilar rubber profiles(round/flat) to prevent slippage. The jaw clamping force may be setusing a torque wrench.

The clamp is designed to be equally suitable for fine or coarsefabrics, and low to extremely high tensions.

SEFAR 3 stretching clamps are available in jaw widths of 150 mmand 250 mm. Both types can be used simultaneously, one besidethe other, since the pulling strength of both clamps isproportionately the same. Therefore, the clamps can be arranged tosuit any frame size.


3.8 Stretching

Sefar 3 clamps with 150 mm and 250 mm jaw widths

Pneumatic unit with distributor mounted on table

SEFAR 4

SEFAR 4 stretching clamps feature a progressive jaw action thatapplies a steadily increasing clamping force. The force grows inproportion to the fabric tension, preventing slippage. Thecombination of constantly adjusted clamping force and plastic jawinserts allows higher tensions than can be achieved withconventional clamping systems.


150mm 250mm

3.9Stretching

SEFAR 4 clamps are elevated by a short-throw cylinder during thestretching process. This avoids undesired rubbing of the fabricagainst the frame.

The clamps are propped against the frame, flexing it in proportion tothe applied tension. These ensures consistent tension even after thefabric is glued to the frame.

SEFAR 4 stretching clamps feature automatic closing and locking, aswell as retracting and opening automatically after stretching iscomplete. The progressive clamping force means that the clampsmay easily be opened and closed by hand.

SEFAR 4 stretching clamps are available in jaw widths of 150 mmand 250 mm. Both types can be used simultaneously, one besidethe other. Therefore, the clamps can be arranged to suit any framesize.

Pneumatic circuits

There are two ways to arrange the air supply for pneumaticstretching machines: the one-circuit system, and the two-circuitsystem. They provide complementary alternatives for optimum, eventensioning of the fabric over any frame size.


3.10 Stretching

One-circuit system

The one-circuit system is used for frames with edges up to approx.150 cm long.

With the one-circuit system, clamp positioning must correspond withthe inside edge of the frame.

The one-circuit system uses a single control box, with two air outlets.One is connected directly to the first clamp, the other to the clampdiagonally opposite, in order to supply compressed air to both setsof clamps. They are also linked by a pneumatic coupling in the otherdiagonal corner.


3.11Stretching

Two-circuit system

The two-circuit system is used when the side length exceeds 150 cm.

In the two-circuit system, the clamps on both short sides of theframe must be positioned so that the clamp ends overhang by oneframe section width. The clamps on each of the long sides must bespaced 4–6 cm away from the outside edge of the frame.

The two-circuit system uses two independent control boxes. Onesupplies the short side (warp), the other supplies the long side(weft). This allows pre-tensioning the warp to half its final value,before clamping the weft and likewise pre-tensioning to half thefinal value. Both circuits can now be adjusted in tandem to attainthe final desired tension. This technique improves the evenness ofthe fabric tension.


3.12 Stretching

3.4 Stretching at an angle

Lines running parallel to the screen frame can be accurately printedwhen the mesh is stretched at an angle. It is important that themesh and the lines to be printed are not parallel to each other.

Mesh stretched parallel to the frame.

Mesh stretched at a 15° angle.

Stretching methods

With mechanical stretching, the printing frame is laid in thestretching machine at the desired angle.

With pneumatic stretching, two techniques may be used.

The mesh is cut at the desired angle and placed straight into thestretching clamps. Stretching difficulties arise when the mesh angleis greater than 15°, because the mesh is not stretched in thedirection of the threads.

There are fewer problems when using a wooden support board. Thisis placed into the stretching apparatus, and the screen frame cannow be positioned at the desired angle. The mesh is stretchedstraight in the direction of the threads. There is a loss of tension ifthe frames are too weak, because the clamps are propped againstthe support or master frame, and not against the stencil frame.


3.13Stretching

Aluminium profiles should be at least 80/40/6 mm for lengths up toapprox. 2 m. In order to quickly adjust the profile to variousformats, bore holes at 3 cm intervals along the profile.

4 adjustable prop profiles transfer the tensioning force to thetensioning frame. This reduces the loss in fabric tension.

3.5 Multiple stretching

Several frames may be covered at once, using a master frame andone stretching machine.

Master frames are especially useful for covering small stencil frames.The master frame is tensioned, the small stencil frames are placed ona foam rubber underlay, and the master frame is laid over them.Small weights can be positioned on top, to improve contactbetween the mesh and the frame.


3.14 Stretching

Covering several small frames with a master frame

A wooden or plastic board can be placed in the stretchingapparatus. Several identical or different sized frames can bepositioned straight, or at an angle.

It is essential to position weights on the fabric between theindividual frames, to ensure optimal contact with all the frameedges.


3.15Stretching

3.6 Correct stretching

After stretching, the screen printing fabric is mounted onto theframe. The permissible tension depends on the tearing strength ofthe particular fabric. The resistance to stretching of a particular fabricis an important factor in ensuring correct register, and indetermining the proper distance between the screen and thesubstrate.

Tension is measured in Newton per cm (1 N = 0.102 kp) withmechanical or electronic equipment placed on the fabric.

See also the section ”Checking fabric tension”.

Optimum tensioning force for different fabrics.

The optimum stretching force to be applied in kg per cm of selvedgedepends, as previously mentioned, on the tearing strength andstretching resistance of the particular fabric.

The tearing strength and stretching resistance of modern syntheticyarns depends on the material and manufacturing process used.

Polyester and polyamide (nylon) have very similar tearing strengths,but considerably different stretching characteristics. Polyester is morestretch-resistant than polyamide, and high-viscosity polyester is morestretch-resistant than standard polyester.

Apart from this difference between the tearing strength and thestretching resistance of different yarn materials, for one and thesame material it may be stated in principle that both these valueswill be roughly proportional to the cross-sectional area of the yarn.The cross-sectional area of a round yarn thread is obtained by thefamiliar expression r2 x p, i.e. 3.14 x square of the radius or .785 xdiameter squared. This means that a round thread A that has doublethe diameter of another thread B of the same yarn material will beabout four times as strong in tearing and stretching. With increasingthread diameter, therefore, the strength values increase according totheir squares.

Stencil fabrics are made in different degrees of fineness (counts).The count represents the number of threads per linear cm.

In general, the higher the number, the finer the threads. Coarsefabrics with relatively thick threads can be tensioned more tightlythan fine fabrics, even though they stretch less.


3.16 Stretching

77-48 120-40

Moreover, in the same weave counts (same number of threads perlinear cm), fabrics can be woven from combinations of thinner andthicker yarns.

120-31 120-34 120-40

Considering the different fabric grades with the same number, i.e.the same mesh count, it is obvious that the grade with the thickeryarn is stronger than the grades with the thinner yarns. This shouldbe taken into account when stretching on printing frames.


3.17Stretching

Fabric numbers with threads of differing thickness used to bespecified as:

SL = thinnest threadS = thinner threadM = medium threadT = thicker threadHD = thickest thread

Now, these symbolic terms have been replaced by the nominalthread diameter. Nominal thread diameter refers to the diameter ofthe unwoven fibre.

Fabric number Thread diameter former identification

120 31 S120 34 T120 40 HD

The complete fabric identification is comprised of:

Fabric type + fabric number + thread diameter + special treatment.

As yet, there are no standardised symbols for special treatments;fabric manufacturers use their own abbreviations.

Example: PET 1000 120-34Y PW

3.7 Recommended tensions

The recommended tensions refer to target values for the fabric inthe tensioning apparatus, before it is attached to the printing frame.These are reliably attainable using correct tensioning methods andwell-maintained tension measuring equipment. Higher thanrecommended tensions increase the risk of tearing during handlingand printing. Lower tensions may be necessary for specificapplications (hand printing, printing solid objects).


3.18 Stretching

Maximum tensions for PET 1000 fabric:

The tensions listed in the table refer to controls made using theSEFAR Newtontester or TETKOMAT.

Preconditions:

– tensioning system with pre-stressed frame

– SEFAR-3/4 stretching clamps or other devices capable of providinguniform tension

– slip-proof fabric clamping system

– stable frame

Applicability

The specified tensions are valid for frame edge lengths up to approx.1m.For larger sizes, the specified tensions should be reduced by15–20% for edge lengths up to approx. 2m, and 20–25% for edgelengths up to approx. 3m.

32 43 45 48 51 54 61 64 68 73 77 81 90 95 100 110 120 130 140 150 165 180 190

55

50

45

40

35

30

25

20

15

70

80

70

8080

70

55

80

70

64

64

70

6470

55

55

64

70

55

48

48

4848

4040

40

34

31

3434

3434

34

31

31

2731

27

31

27

31

48

55

64

40

40

N/c

m

Nominal thread diameter

Mesh count /cm


3.19Stretching

Standard tensioning procedures

The fabric can be brought to the desired tension within 1 to 3minutes. Before fastening the fabric to the frame, wait 10 minutesand again increase the tension to its final value. Repeating thisprocedure several times will reduce future loss of tension.

With modern pneumatic equipment or SEFAR clamps, tensioningtime may be reduced to an absolute minimum. (1 minute)

Rapid tensioning procedure

Within 1 to 3 minutes, the fabric can be brought to a tension some15% higher than desired, then attached to the printing framewithout delay (no relaxation phase).

Loss of tension

When tensioning procedures are correctly followed, there is a loss oftension of 15–20% with standard fabrics, and 10–12.5% with PET1000 fabrics. These values do not take the printing framecharacteristics into account. Loss of tension can be reduced bylonger relaxation phases.


N/cm

Time →

N/cm

Time →

3.20 Stretching

Possible causes for loss of tension

The following points should be investigated if loss of tension isproblematic:

– weak frame sections

– fabric incorrectly inserted in the stretching clamps

– stretching clamps pull unevenly: one side of the frame is laying toohigh

– large temperature changes

– insufficient waiting time before gluing

Extreme climatic or mechanical influences can also affect the fabrictension.

Because the mesh must have a certain degree of elasticity during theprinting process, undue demands should not be made on thetension. Differences of 1–2 N/cm are permissible. Multi-colourgraphics printing experience has shown that good register isachieved at mesh tensions above 12 N/cm. It is important that allthe meshes used are tensioned similarly.

Tension control

There is a relationship between the applied stretching force and theresulting elongation of the fabric. The relationship (cause and effect)is, however, not constant for different types of stretching equipmentand fabric.

We recommend using a commercial measuring instrument, e.g. theSEFAR Newtontester or TETKOMAT, for determining fabric tension.

If specialised measuring equipment is unavailable, fabric tension canbe roughly checked by monitoring elongation during tensioning.

Elongation in percent at 15–20 N/cm:

Fabric number Polyester fabric Polyamide fabric

10 - 20 1 - 1.5% 2 - 3%20 - 50 1.5 - 2% 3 - 4%50 - 100 2 - 2.5% 4 - 5%100-200 2.5 - 3% 5 - 6%


3.21Stretching

3.8 Sefar tension control instruments

Newtontester

Screen printing quality largely depends on perfectly controlled screentension. The Sefar Newtontester instantly controls the screen tensionin Newton/cm and displays it on a large, highly legible dial. Sturdyand precise construction guarantees consistent, reliable and exactscreen tension reading. Tension values from 5–60 Newton/cm can bechecked.

Instructions for use

Calibration

The accuracy of the indicator dial should be checked periodically. TheSefar Newtontester is placed on the glass plate delivered with theinstrument for this purpose:

– The needle of the dial must be precisely in the 12 o’clock position. Any deviation can be corrected by turning the hexagonal set screwat the bottom of the tester. The appropriate Allen wrench ispacked with the instrument.

– The calibration point on the dial should be made to exactlycoincide with the indicator needle by turning the outside ring ofthe dial.

The Sefar-Newtontester is now ready for use.

Controlling

Place the Sefar-Newtontester onto the fabric and read the tension.


3.22 Stretching

Tetkomat

TETKOMAT characteristics

The TETKOMAT is suitable for all screen printing fabrics. It is a purelymechanical instrument, needing no battery or other electrical supply.

The Tetkomat allows reliable and simple checking of fabric tension inwarp and weft directions at any time. This makes it possible toproduce uniformly tensioned, reproducible stencil sets with aminimum of fuss.

Fabric tension control

Before starting to use the Tetkomat, check that the display is correct,i.e. the indicator needle must rest exactly over the scale calibrationpoint (see Checks and Adjustments below).

To control fabric tension, place the instrument on the tensionedfabric so that the long edge of measuring head (1) is parallel to thethread direction. If the measuring head is aligned with the warpthreads (along the fabric), the instrument displays the warp tension.If it is aligned with the weft threads (across the fabric), it displays theweft tension. This allows the tension to be balanced in bothdirections.


3

4

2

1

5

3.23Stretching

Checks and adjustments

To avoid control errors, the instrument should be regularly checked,and adjusted if necessary.

– Clean the measuring bar and the supplied glass plate of any dirt.

– Place the instrument on the glass plate.

– The needle should rest exactly over the scale calibration mark (2).

– If the needle shows a deviation, loosen the scale set screw (3).

– Turn the outside ring of the dial (4) so that the needle aligns withthe calibration mark (2).

– Remove and replace the instrument on the glass plate severaltimes, to verify reproducibility. (The instrument should be cleanedif reproducibility is poor.)

– Tighten the scale set screw (3); the instrument is now ready foruse.

Important: Minimum distance from frame: 10 cm. To avoid erroneous readings, the instrument’s contactrollers (5) must lie parallel to the direction of thethreads.Follow the mesh tension recommendations in thetechnical datasheets published by Sefar AG.

Loss of tension

A newly stretched screen loses approx. 10–20% of its tension withinthe first 24 hours, depending on the type of tensioning device used,the original mesh tension, the frame stability and the waiting timebefore gluing. For printing jobs with accurate register, therefore, it isrecommended to let the screens rest for 24 hours before stencilling.When stretching frames, please take into account this loss oftension.

We recommend working with Newton controlling instruments at alltimes.

For multi-colour work, all screens should have the same tension.Checking with a Newton controlling instrument is thereforeespecially important.

Practical experience has shown that variations in screen tension of1–2 N on the same or different screens do not have any noticeableeffect on screen printing precision.


3.24 Stretching

During long print runs, or after several screen reclaimings, the loss oftension can amount to several Newtons.

Caution: Unequal warp and weft tension can result in thefollowing:

– uncontrollable register– deteriorating surface roughness of the ink volume– higher ink volume (depending on the squeegee

direction)– increased mechanical abrasion of the fabric and

squeegee


4.1Gluing

4. Gluing

Applying two-component glue through the mesh is the techniquemost used at present for securing fabric to printing frames.

One-component glue, UV or reserve adhesives are other alternatives.

The choice of glue depends largely on the solvents used by theprinting process.

4.1 Preparation

Screen printing frames must be thoroughly cleaned and degreasedprior to gluing. There must be no traces of dust, grease or oxidation.

The tools used are:

– a brush with hard bristles, optional brush-holder for storage

– degreasing agent

– adhesive tape

– felt or fibre-tip marker

– a knife

Cleaning and degreasing the printing frame

First, the side of the frame to be glued must be cleaned, and ink andadhesive residues removed. If the old glue coating is non-porous andflat, it may be left on the frame.

Sharp edges and corners must be rounded off.

It is always advisable to roughen the adhesive surface of metal, inparticular aluminium, using a coarse emery wheel or emery disk.Sand-blasting the surface to be glued is also a good method.

Printing frames should only be roughened or sand-blasted on theside to be glued, otherwise it is more difficult to remove inkresidues.

Metal frames should be thoroughly degreased shortly before gluing,using a suitable solvent (cellulose thinner, acetone, refined petrol, oralcohol). Prepared frames should be glued right away, to avoid therisk of recontamination.

When gluing fabrics with weave counts of 100 and up, it isadvisable to pre-coat the frame with the same glue that will be usedlater. This improves adhesion.


4.2 Gluing

4.2 Marking stretched frames

Before gluing, it is advisable to mark the tensioned fabric along theframe edge, using a felt or fibre-tipped pen. The followinginformation should be recorded:

– fabric brand

– weave count, including thread diameter

– roll/batch number

– tension in N/cm

– date

– operator’s initials

Example:PET 1000 120-34Y PW, 2189203101, 20N.,18.08.98/gh

Next, adhesive tape is applied inside of the marked fabric, approx. 1-2cm from the frame. This helps to give a cleaner glue boundary,and protects the transitional area between the frame and themarked fabric.

To make the frame easier to find in a storage rack, a sticker with thesame information is applied to the outer edge of the frame. Thesticker can be made of self-adhesive plastic film or paper, andwritten with permanent marker. A piece of polyester film glued overthe label protects against solvents.

Glued and marked fabric

PET-1000 120-34, 2189203101, 20N./cm, 18.08.98/gh

Gewebefeinheit: .................. Datum: ...................Rollennummer: .................... Winkel: ...................Neuspannung: ..................... Unterschrift: ............


4.3Gluing

4.3 Adhesives

There are various adhesive systems, falling into the followingcategories:

– two-component adhesives

– reserve adhesives

– UV adhesives

– contact adhesives

Two-component adhesives

Two-component glue is a catalytic mixture consisting of the adhesiveitself, plus hardener. This type of adhesive has generally goodresistance to solvents, although the adhesive should be tested withthe solvent used for ink removal.

Adhesive and hardener must be mixed before use, in the proportionsspecified by the manufacturer. It is important to observe the correctratio, to avoid impaired adhesion and hardening.

Two-component adhesives harden in two phases. The solventevaporates first, then the chemical hardening process begins.

The initial drying (evaporation) time depends on the fineness of thefabric, tension, thickness of the glue coat, room temperature andrelative air humidity. With so many variables, it is difficult torecommend precise drying times. It is therefore advisable to followthe manufacturer’s instructions before removing the frame from thestretcher.

As a general rule, the higher the tension and the lower the meshcount, the longer the drying time required.

It should also be noted that two-component adhesives only remainworkable over a limited period, since the reaction between theadhesive and the hardener begins in the pot. The delay betweenmixing and the onset of the chemical reaction is known as the potlife.


4.4 Gluing

Reserve adhesives

Reserve adhesives are applied to frame in advance. The coated framecan then be stored indefinitely.

Painting acetone or some other activator through the tensionedfabric in contact with the frame revives the adhesive.

If frames glued in this way are to be used with solvents, the adhesivemust be protected by a coat of alcohol-based lacquer.

UV adhesives

UV adhesives are one-component glues that cure (harden) throughexposure to ultra-violet light from a special lamp. The hardeningprocess is faster than with two-component adhesives.

UV adhesives are resistant to solvents.

Contact adhesives

Contact adhesives cause a frame to adhere to a stretched fabric sofirmly that no further pressing is required. The process takes about30 seconds. After several more minutes of drying, the frame can betaken out of the stretcher.

The adhesive is applied to the frame as well as onto the stretchedfabric. When the adhesive is dry, both glued surfaces are pressedtogether and the gauze is then smoothed down with a plasticspatula for better contact.

Although a hardener is added, this type of adhesive has insufficientresistance to certain powerful solvents. Therefore, the glued surfacemust be protected with a coat of lacquer.


4.5Gluing

4.4 Gluing the fabric to the frame

It is important to ensure positive contact between the fabric and theframe during gluing. If there are problems, weights may be placedon the fabric to force it onto the frame surface.

It is important to ensure that the frame edges are thoroughly gluedto the fabric, so there is no possibility of solvent penetrating andweakening the glue.

If the frame is not flat, it is impossible to establish good contact, andthe fabric bond is correspondingly weaker. There is a risk of thefabric becoming detached later.

Gluing


4.6 Gluing

4.5 Screen storage

Screen storage and transport

4.6 Stretching services

Efficient networks of SEFAR trade supply houses guarantee promptand reliable delivery of perfectly stretched screen printing frames.You can be certain that the mesh has been stretched using state-of-the-art equipment, and the tension checked with appropriatemeasuring instruments.

It should be evident that prefabricated, ready-to-use frames are thesafest and best prerequisite for making perfect stencils. Successfulprinting largely depends on this.

A stretching service saves you warehousing costs for various weavecounts and widths, as well as the investment in a stretchingmachine. Put your premises and expensive labour to better use!


5.1Diapositive manufacture

5. Diapositive manufacture

In screen printing, stencil preparation requires a positive transparencyof the artwork to be reproduced.

It is important that the diapositives are reading right, with theemulsion side up. Diapositives can be made manually,photographically, or digitally.

5.1 Manually made diapositives

– Draw with opaque ink on transparent polyester foil. For artisticprints, a wax pencil can also be used. It is preferable to use one-sided matt polyester foil.

– Cut-film process. The diapositive is prepared on a masking film,consisting of a polyester backing and an emulsion coating. Cuttingmay be done using a special knife, or a computer-controlledplotter.

Masking films are available in orange and red. Both types aresuitable for screen printing, but for photographic reproduction onlyred masking films should be used.

5.2 Photographically made diapositives (films)

At present, the conventional technique generally used to makescreen printing diapositives requires a positive, reading right film. Inthis context, reading right means that the artwork must bereproduced the right way round on the emulsion side of the film.This is the opposite to films intended for offset printing.

If diapositives are made externally, it is essential to instruct the reprobureau to expose the artwork reading right on the emulsion side ofthe film.

This is important, because it allows during exposure the emulsionside of the film to rest directly on the stencil emulsion. If offset-stylereversed films (reading wrong) are used, the film polyester basebecomes interposed between the emulsion and the film layer. Thiscauses undercutting, giving stencils that lack sharpness and detail.


5.2 Diapositive manufacture

Films (diapositives) are currently produced on laser imagesetters.Computer data is converted to screens and lines by a PostScript RIP(Raster Image Processor), and translated into the imagesetter’smachine language. The result is then output to film.

5.3 CTS (Computer to Screen)

CTS is a newer process for making diapositives. Just like filmproduction, computer data is converted by a RIP and output on aninkjet plotter. However, no actual film is involved: instead, the plottersprays UV-opaque ink or wax directly onto the coated mesh. Themesh is then exposed and rinsed in the same way as stencils madefrom films. This technique does not require vacuum retention duringexposure, because the ink or wax is deposited directly on theemulsion surface.


Conventional

Conventional:

5.3Diapositive manufacture

This technique has the advantage of eliminating expensive films. Forproducing screen printing stencils, inkjet technology has proved aclear winner over laser techniques (same principle as CTS). Only afew laser machines are in use world-wide; they are much slower andrequire special emulsions and fabric types.


Develop stencil

Colour separation

Film production

Film development

Full-sheet production

Mount filmon stencil

Exposure

Archive film

CTS:

Develop stencil

Colour separation

Spray and exposestencil

Conventional film-based production vs. CTS

5.4 Diapositive manufacture

5.4 Tips for external production of diapositive films

If pre-press facilities are not available in-house, it is necessary towork closely with an expert partner. The best choice is a repro housewith an established track record in making screen printingdiapositives.

Most repro houses are oriented towards offset printing. It isabsolutely crucial to discuss the specific job requirementsbeforehand.

1.Positive films must be made reading right with high density (d ≥ 3.5) to UV-A illumination.

2.Raster films must not be made using PostScript round dots. Whileeminently suitable for offset printing, these are a big problem inscreen printing. This is because dots for 50% tones are square,and frequently cause screen printing moirés.

3.Most repro houses offer a wide variety of raster techniques(round, linked, etc.). Trials with various dot shapes are essential.

4. It is a good idea to make a test film with a variety of raster dotshapes with different screen rulings (L/cm or lpi). Proof prints fromtest stencils made from various film materials on different meshcounts provide a basis for establishing subsequent standardisedpractice.

5.Multi-colour raster prints present a further dimension. Havingestablished the optimum raster point shape, the best raster anglemust then be found. A proof print with various four-colour rasterangles is made; the print results then determine which standardraster angle to use.

Once all these parameters have been established, there ought to befewer moiré problems in production runs. Informing customers ofthe optimum parameters beforehand can also save much troubleand expense.

To reiterate, here is a summary of the parameters:

- correct side (i.e. reading right), with high density

- optimum raster dot shape

- optimum raster angle

- optimum raster ruling


©Copyright by SEFAR, 2/1999 6.1Stencils

6. Stencils

6.1 Pre-treatment of screen printing fabrics

Degreasing

Special finishing methods mean that SEFAR screen printing fabricsare fundamentally very clean. Irrespective of this, all fabrics, whethernew or old, must be degreased shortly before use. Fabrics can becontaminated by handling, or airborne dust.

Degreasing is done with the normal screen printing degreasingproducts available from dealers. Household detergents must not beused. These often contain other chemical additives, e.g. lanolin forskin protection, which can seriously affect the adhesion ofphotofilms or photographic emulsions.

After degreasing, the fabric should not be touched again. Photofilmor photographic emulsions must be applied immediately afterdegreasing the mesh. If screens are allowed to lie around, the fabricmay again attract grease or dust.

Degrease by spreading a modest quantity of degreasing agent overthe wet fabric, using a soft brush. Leave to stand for a few minutes,then rinse thoroughly using a high-pressure water jet.

6.2 Mechanical stencils

The hand-cut stencil

This type of stencil has the advantage of producing perfectly sharpedges. It is mostly used for lettering and large solid objects. Thestencils for the individual colours are easily cut using a special cuttingknife, e.g. a swivel knife.

Suitable cutting tools and masking films can be obtained throughyour screen printing dealer.

Plotter techniques have created a resurgence in hand-cut stencils.

©Copyright by SEFAR, 2/19996.2 Stencils

Water soluble hand-cut film

This kind of hand-cut film has the following advantages:

– film adheres to the fabric by water surface-tension

– suitable for all solvent-based inks

– easy removal with hot water

Cellulose hand-cut film

The following points should be observed with this film:

– The fabric must be prepared and degreased as with a photostencil.

– For perfect adhesion, the solvents recommended by the filmmanufacturer must be used.

– Hand-cut films bonded with solvents resist only water-based inks.

– Can be removed using solvents.

Problem causes

– Hand perspiration, hand cream, or dirt on the film side can createbonding difficulties.

– When cutting, the hand should rest on a protective sheet of paperto avoid grease-stains.

– Insufficient pre-treatment and degreasing of the fabric (see thesection on pre-treatment).

– Creased film.

– Poor contact while bonding.

– Excess liquid while bonding results in swollen edges.

– Drying at excessive temperature.

– Using an unsuitable instrument to lift off the film.

6.3Stencils

Cutting

Lifting


6.4Stencils

©Copyright by SEFA

R, 2/1999

6.3Ph

oto

mech

anical sten

cils

A B C DDirect stencil Direct stencil Direct stencil Indirect stencilwith emulsion with film and with film and water

emulsion (capillary film)

– Mesh cross-section– Emulsion– Film

Mechanical resistance very good very good good poor

Solvent resistance good good good good

Contour sharpness good – very good very good very good very good

Average press run 50 000-75 000 20 000-50 000 10 000-30 000 2000-5000

Time and labour high high low medium

Applications Printing of flat and Printing of flat and Printing of flat and Flat printshaped objects shaped objects certain shaped objects

Reclaiming difficult difficult easy easy

6.5Stencils©Copyright by SEFAR, 2/1999

6.4 Direct stencils with emulsion

General procedure

Degreasing Before making any stencil, the fabric should bedegreased with a suitable degreasing agent. Do not use household detergents.

Drying Remove all water by suction. Dry thoroughly atroom temperature.

Coating Uniform coating with photo emulsion (diazo,photopolymer or dual cure), wet-on-wet. Use a suitable coating trough.

Drying Dry the stencil in a horizontal position, print sidedown. Maximum temperature 40ºC.

Additional coatings Apply additional coatings on the print side, tosmooth out unevenness.

Drying Same drying procedure as after the first coating.

Exposure Use a suitable light source, e.g. a metal halogenlamp. Determine the correct exposure timeusing step exposures and a test diapositive.

Developing Develop with a moderate water-jet. Observemanufacturer’s temperature instructions. Afterdevelopment, rinse out the printing sidethoroughly with a powerful water-jet.

Drying Remaining moisture can be removed by gentleapplication of unprinted newspaper or windowcleaning suede, or using specialised watersuction equipment. Final drying in a dryingchamber.

Retouching Pinholes and film edges can be covered withscreen filler.

6.6 Stencils ©Copyright by SEFAR, 2/1999

6.5 Troubleshooting direct stencils with emulsion

Formation of fish-eyes after coating

– Insufficient degreasing of the fabric.

– Dust particles on screen fabric.

– Poorly mixed photo-initiator (diazo) and emulsion. (inhomogeneous coating)

Air inclusions during coating

– Coating too fast can trap air in the mesh openings. (Formation ofbubbles leads to premature printing failure.)

Poorly bonded photo emulsion after exposure

– Emulsion insufficiently dried before exposure.

– Exposure time too short. Failure to compensate for a highly light-absorbent diapositive.

– Intensity fall-off in the exposure lamp. (Measure the output usinga light integrator.)

– Insufficiently sensitised emulsion. Incomplete dissolution of thediazo component in water. Some of the diazo sensitiser remainsundissolved in the bottle.

– Caution at very high humidity! The coated screen may feelcompletely dry, but is only superficially dry due to the high airhumidity. Allow for longer exposure times under thesecircumstances.

– After coating, leave coarse meshes to dry overnight at roomtemperature.

Undercutting when copying (loss of detail)

– This can occur with white mesh. Use a dyed fabric. Compared towhite fabrics, dyed screen fabrics require a 75–125% exposureincrease.

– Only use non-reversed positives.


Saw-tooth effect

– Poor coating.The coating on the print side is too thin. The coating bridges themesh openings, but has sunk into the depressions in the fabricstructure. Printing sharp outlines requires multiple wet-on-wet coatings. The greater part of the photo emulsion must be, in any case, onthe print side. After drying, additional coatings can be applied to smooth outunevenness.

Half-tone printing

– Half-tone screen printing requires thinly coated mesh. Apply anadditional coat to the print side.

Reclaiming difficulties

– Underexposed emulsion.

– Ink was not immediately washed off after printing.

– Insufficient cleaning after printing. Ink deposits cling to the mesh.After a certain time, the dried ink particles can no longer becompletely removed.

– The stencil is still greasy from solvent. The reclaiming solutioncannot dissolve the photo emulsion. Additional degreasing isneeded before applying the reclaiming agent.

– Unsuitable reclaiming agent.

6.6 Stencils for water-based inks

Water-proof emulsions must be used with water-based inks, e.g. fordirect printing on textiles or ceramic.

Carefully observe the manufacturer’s instructions.

Sensitizers

DIAZO and/or PHOTOPOLYMERS are used as sensitizers in screenprinting.

Important: For environmental reasons, DICHROMATE should nolonger be used.


Diazo and photopolymer photo emulsions

Both types are characterised by prolonged storage capability forcoated screens. Neither material represents an environmental hazard.

1. Coating on 2. Coating onprint side, 1–2x squeegee side, 1–4x

3. Dry, 20–30 °C 4. Additional coatings onprint side down! print side, 1–2x

5. Dry, 20–30 °C 6. Exposure

7. Develop with cold water 8. Dry at 20–30 °C

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A uniform coating is crucial to a perfect direct stencil. The fabricshould be completely covered by the emulsion, with a slightly thickercoating on the print side of the stencil.

The screen is coated 1–2 times on the print side, followedimmediately by 1–4 times on the squeegee side, wet-on-wet, thendried. The drying temperature must not exceed 40º C.

After drying, surface quality can be significantly improved by 1–2additional coatings that are allowed to dry between coating. The depth of the surface roughness should be less than 10% of thefabric thickness.

With very coarse meshes of 5 – 40 threads/cm, the screen can, afterintermediate drying, be given 1–2 additional coatings as well on thesqueegee side. This improves the stencil life.

The number of coatings depends on various factors, partlyinfluenced by the particle content and viscosity of the emulsion, andalso by the fineness of the fabric and the demands of the printingjob.

Coating thickness vs. printing job

Lines: Sharply defined prints are attained with acoating thickness of 10 –18 µm on fabrics with90 threads/cm and finer.

Rule-of-thumb: Coating thickness on the printing side approx.10 - 20% of the mesh thickness. The Rz value isdetermined using roughness depth measuringequipment. This value represents the average ofthe highest and lowest points on the surface(see chapter 10).

Half-tones: The thinnest possible coat of 4 – 8 µm results inthe thin ink volume required for half-tone prints.

Rule-of-thumb: Coating thickness on the printing side: approx.10% of the mesh thickness. Roughness depthless than the coating thickness.

UV inks: When printing with UV inks, the ink volumeshould generally be as low as possible. As arule, the coating thickness on the print side ofthe screen should not exceed 5 µm.


The fineness and quality of the fabric are the determining factors forthe number of stencil coatings.

Fabric fineness

Fabric number Mesh-opening Open area Fabric thickness

120-34 (T) 45 µm 29.6% 55 µm

150-34 (T) 23 µm 12.1% 55 µm

These examples clearly depict the differing open areas in % infabrics of similar thickness. The larger mesh-opening causes moreemulsion to be pressed through the fabric per coating. Achieving thesame coating thickness on both fabrics requires a different numberof coatings.

Fabric quality

Fabric number Mesh-opening Open area Fabric thickness

120-31 (S) 51 µm 37.9% 49 µm

120-34 (T) 45 µm 29.6% 55 µm

120-40 (HD) 37 µm 20.1% 65 µm

Various fabric qualities in the same number also influence thecoating thickness, because of differences not only in the meshopening but also in the fabric thickness.

Example: number of coats:

Fabric number Open area Fabric Coating thickness wet-on-wet+ coating

D / R

120-31 (S) 37% 53 µm 2 + 1

120-34 (T) 30% 61 µm 2 + 2

120-40 (HD) 22% 72 µm 2 + 3

The screen should be coated immediately after degreasing, to avoidrecontaminating the fabric with dust, etc. The fabric must beperfectly dry before coating.


Coating example

Correct coating

Coating too thin

The easiest way to apply the photo emulsion onto the fabric is witha coating trough. The coating edge must be rounded, and should becambered over its entire length. This guarantees an overall uniformcoating, even in the centre part of the screen.


Hand coating + trough

Most DIAZO sensitisers and, consequently, the sensitised emulsions,are strongly acidic. Therefore, photo emulsions should be appliedonly with a V2A-stainless steel coating trough.

Warning: Aluminium is very easily damaged.

Galvanised steel troughs oxidise after a short time, which destroysthe photo emulsion. This is accompanied by the formation of finebubbles or scum, which also happens after the photo emulsion hasbeen kept for several hours in an aluminium trough. It indicates thatthe emulsion can no longer be used.

Care should be taken that the photo emulsion never stays in thecoating trough for longer than necessary. Covering the troughmerely protects against dust and drying out.

Technical data for coating troughs:

Trough Trough (stainless steel)

length/mm profile/mm

< 50 30/30/1.5

50-150 40/40/1.5

150-1000 50/50/1.5

> 1000 50/50/2.0 - 60/60/2.0

For screen sizes over 1000 mm and a fabric of less than 20threads/cm, we recommend a profile depth of 60–80 mm.

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r=0.75 mm

r=0.25 mm

1.5 mm

20 °

0.1-0.2%


Storage rack for coating troughs

Automatic coating machine


6.7 Direct stencils with film and emulsion

General procedure

Degreasing: Before making any stencil, the fabric should bedegreased with a suitable degreasing agent.Do not use household detergents.

Drying: The fabric must be completely dry beforetransferring the film. To prevent later difficulties,avoid dust when transferring.

Transfer: Place the film on a glass plate, emulsion side up.Bring the stencil, in the printing position, incontact with the film. Avoid trapping dust. Pour the sensitised photo emulsion into thestencil and sweep the emulsion over the filmusing a soft squeegee.

Important: Wait approx. 3 minutes beforeputting the stencil into the dryer.

Drying: For accurate register, dry at room temperature(max. 40 ºC). Remove the plastic backing sheetafter drying, and allow to dry for a few minuteslonger.

Exposure: Determine the correct exposure time throughstep exposures. Underexposure causes poor film adhesion, andreduced coating durability.

Developing: Rinse with cold water.

Drying: Dry at room temperature. Excess water can beremoved with unprinted newspaper or watersuction equipment.

Retouching: Pinholes and film edges can be covered with awater-based screen filler.


Troubleshooting direct stencils with film and emulsion

Poor film adhesion to the fabric

– Fabric used is too fine Fabric with low ink penetration prevents sufficient photo emulsioncoming into contact with the film. The results are inadequateadhesion of the film on the fabric.

– Squeegee is too hard or too soft Incorrect squeegee hardness results in insufficient emulsion beingpressed onto the film, leading to inadequate film sensitisation. Theideal squeegee hardness is 60 º - 70 º shore.

– Trapped dust This results from failure to clean the film with an antistatic clothbefore transfer. Dust can also be a problem when there has been adelay between degreasing the mesh and applying the film.

– Exposure time too short This results in poor adherence of the film on the fabric.

– Insufficient drying before exposure Also results in poor adherence of the film on the fabric.Unexposed, and hence unhardened particles on the emulsion sideare washed out during development.

– Copying error The film backing sheet was not removed prior to exposure.

6.8 Direct stencils with film and water

General procedure


Wetting agent: Wetting agent encourages the formation of auniform water film on all fabrics, to facilitatesafe transfer of the capillary film.

6.16 Stencils ©Copyright by SEFAR, 2/1999

Transfer: The capillary film is transferrred onto the wetstencil mesh.This procedure has two advantages: 1. No additional drying time.2. Dust problems are practically eliminated.

The film is placed on a flat surface, emulsionside up. The wet mesh fabric is carefullybrought into contact with the film. The film issucked onto the fabric through capillary action.Sweep off excess water with a squeegee. With large sizes in particular, we recommendthat the film be tightly rolled, so that it can beeasily unrolled on the wet upright screen.

Drying: For accurate register, dry at room temperature(max. 40 ºC). Remove the plastic backing sheetafter drying, and allow to dry for a few minuteslonger.

Reinforcement: Attention: For large print runs, the capillary filmcan be further reinforced after drying byapplying emulsion to the squeegee side.

Exposure: Determine the correct exposure time throughstep exposures.

Developing: Wash out with tepid water, concentrating onthe print side until the image is open.Thoroughly rinse the squeegee side. Excesswater can be removed with unprintednewspaper or water suction equipment. Dryafterwards.

Retouching: Pinholes and film edges can be covered with awater-soluble screen filler

Troubleshooting direct stencils with film and water

Poor film adhesion to the fabric

– Inadequate water film on the print side of the mesh whentransferring the film. For this reason, it is advantageous to rinsethe wetting agent out of the fabric from the squeegee side, sothat a homogenous film of water is formed on the print side.


– Insufficient exposure time results in poor adhesion of the film onthe fabric.

– Insufficient drying before exposure also results in poor adhesion ofthe film on the fabric. Unexposed, and hence unhardened particleson the emulsion side are washed out during development.

– Copying errorThe film backing sheet was not removed prior to exposure.

6.9 Indirect stencils

Indirect stencils are made independently, then attached to the fabricmesh. Indirect stencils are thin and have little influence on inkvolume.

Since indirect film is exposed separately from the stencil, fabriccolour has no effect on exposure time or stencil edge sharpness.However, there is a uniform amount of light scatter because theexposure is made through the film substrate.

The disadvantage of indirect stencils is their loose attachment to themesh, which makes them insufficiently durable for long print runs.

General procedure for making indirect stencils

Roughening: New fabric should be roughened on the printside with silicon carbide 500.


Exposure: The pre-sensitised indirect film is exposedthrough the polyester substrate.

Important: Determine correct exposure timethrough step exposures.

Fixing: The exposed film is fixed in a bath of hydrogenperoxide, or the manufacturer’s proprietarypowder.Follow the film manufacturer’s instructions.


Developing: Rinse the film in warm water, emulsion side up.Follow the film manufacturer’s instructionsabout water temperature. Rinse cold; thoroughrinsing is absolutely essential.

Transfer: Place the film emulsion side up on a sand-blasted glass plate. Bring a corner of the wetstencil in contact with the film, and let the filmdraw itself up onto the mesh. Remove excesswater from the squeegee side, using unprintednewspaper.

Drying: Drying must be at room temperature. Whenthoroughly dry, remove the polyester backingsheet.

Retouching: Pinholes and film edges can be covered with awater-soluble screen filler.

Troubleshooting indirect stencils

Poor film adhesion to the fabric:

– Insufficient pre-treatment of the fabricFabric must always be roughened on the print side with siliconcarbide 500. Household scouring powder is an unsuitable abrasive,because the particles are of uneven size and can clog the mesh.

– Insufficient fabric degreasing Having mechanically abraded the fabric, it must be degreased aswell. Roughening and degreasing are not the same thing!

– Excessive exposure timeThis is the main cause of poor adhesion of indirect film with thefabric. Longer exposure tends to make the film harder and morebrittle, so it cannot attach itself to the fabric during transfer.

– Inactive developerIt is best to use the developer recommended by the filmmanufacturer. Hydrogen peroxide deteriorates with prolongedstorage.

6.19Stencils

– Drying the stencil with warm airDrying an indirect stencil in warm air tends to make the edges curlup. Therefore, indirect stencils should be dried only at roomtemperature.

– The polyester backing sheet should be removed only afterthorough drying.

6.10 Exposure

Exposing the dried photosensitive layer to UV light causes uncoveredareas to harden (polymerisation) and cease to be water-soluble.Unexposed areas remain water-soluble and can later be washed outusing cold or lukewarm water.

Many UV light sources are suitable for exposing the photo-sensitivelayer. The emission spectrum should peak in the range from approx.350–420 nm, to coincide with the maximum sensitivity of stencilfilms and photo-emulsions.

Invisible and visible light spectrumA = wavelengthB = spectral emission W/5 nm

Suitable UV light sources are:

– metal halogen lamps from 2000 to 6000 watts

– mercury vapour lamps

– high pressure mercury lamps

– mercury halogen lamps

– super-actinic fluorescent lamps

Although xenon lamps are used in offset printing, their spectralrange is not sufficient for screen printing.

We recommend a point source (spotlight) for exposurereproducibility.

250 300 350 400 450 500 550 600

A

B

650 7000

10

20

30

40


6.20 Stencils

Light tubes can also be used if fine line or half-tone reproduction isnot required. If several tubes are stacked in parallel, their separationmust be no more than the distance to the stencil.

The larger the area to be exposed, the stronger the light sourceneeded.

The distance between the exposure lamp and the copy frame shouldbe at least as great as the diagonal width of the area to be exposed,and also at least 1.5 times the diagonal of the image area to beexposed. Under no circumstances should the angle of the light coneexceed 60°.

Increasing the distance between the lamp and the copy reduces theradiant intensity in proportion to the square of the distance increase.Therefore, to maintain constant exposure, the exposure time mustalso be increased in proportion to the square of the distanceincrease.


1 m

2 m1 Lux

1 m

1 m

Lamp

Lamp

1/4 Lux

6.21Stencils

Formula:

Example:

new distance = 150 cmold distance = 100 cmold exposure time = 1 minute (60 seconds)

The new exposure time is thus 2 minutes 15 seconds.

Bear in mind that coloured fabrics require longer exposure timesthan white fabrics.

Tests with step exposures are therefore vital to determining thecorrect exposure time.

We recommend using a light meter for the following reasons:

– to compensate for light intensity at various distances.

– to compensate for reduced light intensity caused by lamp ageing.

Exposure too short Correct exposure(squeegee side) (squeegee side)

150

100x 60sec.=1.52 x 60sec.= 2.25 x 60sec.=135sec.=2min.15sec.

2

New exposure time = new distance

old distancex old exposure time

2



6.11 Step exposures

Step exposures are a means of determining the optimum exposuretime. Correct exposure time depends on the characteristics of thephoto emulsion or film, the fabric, overall thickness, the light source,and the distance between the lamp and the material to be exposed.

Underexposed stencils do not harden all the way through; photoemulsion on the squeegee side is washed away during development.A smudged photosensitive layer is a sure sign of underexposure.With inadequate rinsing, some of the dissolved photo emulsionsticks in the open parts of the stencil. A barely visible scum is leftbehind after drying, which impedes ink flow during printing.

Underexposed stencils also have poor resistance to solvents, printinginks and mechanical wear. The stencil is also difficult to reclaimafterwards.

Overexposed stencils may suffer from reduced resolution; this isespecially noticeable with white fabric. The un-dyed threads of thewhite fabric reflect light during exposure, which rapidly leads toundercutting problems.

Underexposed Correctly exposed

Making a step exposure

A step exposure is best made with a test positive containing at least5 identical images featuring positive and negative fine lines and half-tones.

5 times are chosen, progressing through 50% – 75% – 100% –125% – 150% of the nominal exposure. If the nominal exposuretime is unknown, this must be calculated from the emulsionmanufacturer’s data.


The test positive is laid on the stencil, emulsion side down, andplaced in the vacuum frame. Once the vacuum has established firmcontact between film and stencil, all five images are exposed for thefirst step (50%). One image is covered up, and the remaining fourgiven a further 25% exposure. The second image (75%) is thencovered as well, and the remaining three given a further 25%exposure. The third image (100%) is then covered as well, and theremaining two given a further 25% exposure. The fourth image(125%) is now covered, and the last image given a final 25%exposure. This provides the fifth step (150%).

During development, it becomes apparent that different exposuretimes produce a varying degree of stencil discoloration. Thedifference between steps is especially visible if the first two steps(50% and 75%) are underexposed. There should be no colourdifference between the remaining steps (100%, 125% and 150%).We can therefore assume that step three (100%) is the minimumexposure time. At step three, the stencil should no longer besmudged on the squeegee side. This indicates a correct exposure.

Should there be a colour difference between the fourth and fifthsteps (not hardened through), make another step exposure based ona longer exposure time. On the other hand, when there is no colourdifference between the first and second steps (already hardenedthrough), make another step exposure based on a shorter exposuretime.

With diazo coatings, the colour difference between individual stepsis highly recognisable. The effect is more subtle with purephotopolymer coatings, although it is possible to judge exposurebased on the presence of smudging: no smudging = hardenedthrough.


ESMA test film

The ESMA test film is designed for checking the optimum exposuretime for stencil-making.

Discoloration through a step exposure (no half-tone factors) is usedto find the best compromise between hardening and optimumresolution (sharp details).

The test film incorporates the following features:

– five identical images for step exposures

– positive and negative details

– radial lines from 0.5mm to the limiting resolution of the silver film

– circular lines, and straight lines at varying angles

– various line widths (0.025 – 1.00 mm)

– text at various sizes

– raster ruling 24 L/cm / 45° / tone values 0% – 100%

6.25Stencils

6.12 Rinsing

For rinsing an exposed stencil, we recommend using a nozzle wherethe water pressure can be regulated.

A water suction unit withdraws excess water from the stencil. Thisprevents scum formation (clouding) and considerably shortens thedrying time.

Rinsing trough with water suction equipment

Small screens can be drawn over a stationary water suction nozzle.

6.13 Influence of coating thickness on print sharpness

A) stencil too thin → sawtooth effect

B) correct stencil → sharp print

C) stencil too thick → unclear print

A B C


Direction of squeegee stroke

6.26 Stencils

Stencils with direct or indirect film system

Stencils using these film systems print the thinnest ink layer (3–5 µmstencil profile on the print side of the mesh) without a saw-tootheffect. The film coating bridges the screen mesh evenly. Theproblems shown under A, B and C are therefore less important(assuming the correct choice of mesh and film thickness).

6.14 Influence of stencil thickness on ink volume

Printing open areas

When printing open areas, and line widths exceeding approx. 1.5mm, the squeegee can press quite hard onto the substrate. A thickstencil will then give an elevated ink volume at the edges of the areato be printed.

A) correct stencil → uniform ink volume

B) stencil too thick → higher ink volume at the edges

A

B


6.27Stencils

Half-tones and fine lines

Stencil thickness heavily influences ink volume in half-tone printing.The half-tone dots support the fabric over the entire area to beprinted. The greater the stencil thickness, the higher the ink volume.

A stencil that is too thick causes:

– loss of print in the light areas, and smudging of the dense half-tone areas (change of tone values)

– incorrect colour reproduction due to the high ink volume.

A) correct stencil → good ink volume

B) stencil too thick → ink volume too high

A

B


6.28 Stencils

6.15 Hardening stencils for printing water-based colours

General procedure

– Exposure and development as with graphic stencils

– Retouching with the same emulsion, or a special lacquer

– Dry and re-expose

– Brush hardener onto both sides, allow it to react for 15–20minutes

– Blow or suck the mesh free

Hardening procedure

Textile and ceramic printing generally use water-based inks. Screensare made using photo lacquer, i.e. emulsions that can be processedin the normal way and given a final chemical treatment to makethem resistant to water and chemicals.

Hardener may be applied using a wide brush (not a polyamidebrush), a felt squeegee, or a sponge. Hardener is evenly applied toboth sides of the horizontal stencil.

Attention: as little excess as possible!

It is important for the hardener to penetrate the coating before thestencil is finally fixed. It should therefore rest for approx. 15–20minutes at room temperature. Thereafter, it can be finally hardenedby heating at 50 °C for 1 hour, or leaving at room temperature for24 hours.

After final hardening, the stencil is practically insoluble and cannotbe removed from the mesh by chemicals generally used in screenprinting.

Take care to follow the lacquer manufacturer’s instructions.

Attention: Most hardeners are acidic, which makes them detrimental to nylon fabric. Polyamides are sensitive even to weak acids.

Note: This kind of stencil hardening process is generally usedfor printing water-based inks used in graphic and ceramic screen printing.


6.29Stencils

6.16 Reclaiming

After printing, ink is washed off the screen with the recommendedcleaning fluid.

Reclaiming is best done immediately after printing, before thecleaning fluid has a chance to dry with any ink particles left on thescreen.

Reclaiming process:

– Wash the screen until the screen filler is removed.

– Apply reclaiming agent to both sides of the screen, until theemulsion dissolves.

– Wash clean with a high-pressure water jet (50–100 bar, 3 – 5 cm distance)

– Remove any remaining ink deposits with special solvents.

Again, we recommend following the film and emulsionmanufacturer’s instructions.


7.1Register

7. Register

7.1 Summary of key recommendations

Fabric

– High-modulus POLYESTER

– Dimensionally stable

– Unaffected by heat and humidity

– Optimum handling and treatment using modern equipment

Stencil frames (see also the chapter on frames)

– Do not use wooden frames

– Use steel or aluminium frames

– Use a profile of adequate strength

– Consider side-reinforced profiles

– Check frame flatness

– Select an ideal relationship between the printing area and the sizeof the frame

Stretching machine (mechanical)

– Use a stretching machine with movable clamps

– Clamps must hold the fabric without letting it slip

– Clamps must be free of old glue

– Ease the strain on the fabric by pulling it out at the corner clamps

– If possible, pre-stress the frame using suitable equipment

– Continually check the tension using a measuring device (seeTETKOMAT, NEWTONTESTER)

– Observe the fabric manufacturer’s recommended tension

Stretching machine (pneumatic)

– Use a stretching machine with movable clamps

– Clamps must hold the fabric without letting it slip

– Clamps must be free of old glue

– Continually check the tension using a measuring device (seeTETKOMAT, NEWTONTESTER)

– Observe the fabric manufacturer’s recommended tension


7.2 Register

Gluing

– Use a two-component adhesive to prevent subsequent fabricslippage caused by temperature and solvents

– Observe the manufacturer’s recommended ratio of adhesive tohardener

– Observe correct drying times

– Observe the glue’s pot life

Printing

– A perfectly flat printing table is crucial

– Minimum lift-off

– Minimum squeegee pressure

– Optimal lift-off conditions (consistent snap-off angle over theentire printing area)

– Squeegee speed

– Print stencil laterally (shorter squeegee travel)

– In multi-colour printing, always use the same length of squeegee

– Ink viscosity

Conditioning of the working area and printing stock

– 55 – 65% RH (Relative Humidity) is considered ideal

– Room temperature 18 – 21°C

– High drying tunnel temperatures can affect the dimensions of theprint stock. It is common practice to send unprinted stock throughthe tunnel before the first printing pass.

7.2 Problems of accurate register

We can define accurate register as:

– Exact congruence between an original (e.g. a diapositive) and theimpression on the printing stock; in multi-colour printing, exactcongruence between the printed impressions of the variouscolours (colour register); further, exact agreement of theimpressions at the beginning and at the end of a printing run, orbetween any individual, intermediate impressions.

Accurate register further includes constant location of the printedimpression on the successive, individual printed units, i.e. constantdistance and angle between the printed impression and the marginedges or locating holes in the printing stock.


7.3Register

Absolute accuracy is unattainable in practice, and we mustconsequently define what we mean by ”accuracy” in screenprinting. This depends firstly on the purpose and intention of theindividual screen print, and secondly on the results attainable inscreen printing generally. Textile screen printers, poster printers andcircuit printers will each have their own ideas on the subject.

Despite this individuality of aims and means, we can and mustconsider all the possibilities of faulty register and for each causedetermine the possible order of magnitude. This indicates whichpoints should receive attention if register is to be improved, and inwhat cases it would be pointless to seek higher accuracy, since theattainable improvement would be insignificant.

7.3 The diapositive

The diapositive consists of a polyester base and a photographicemulsion. As temperature and humidity rise, the diapositive spreads,principally by swelling of the emulsion. The polyester base itself canbe considered unconditionally stable for screen printing purposes.Cellulose-based layout or mounting foils are not recommended.

The polyester film most commonly used for diapositives is 0.1 mmthick. A temperature increase of, say, 5°C causes it to shrink byabout 0.135 mm per metre. However, it expands by about 0.21mm per linear metre in response to a 10% increase in relativeatmospheric humidity (RH). A similar drop in temperature andhumidity causes the polyester film to respond in the opposite sense.Any hysteresis effect can be disregarded. (Temperature and RH areconsidered separately; it should be remembered RH falls with risingtemperature and constant absolute humidity.)

The 0.18 mm polyester diapositive base frequently used in circuitprinting responds to temperature changes in a similar way to thethinner film. However, the RH change mentioned above causes thefilm to change by only 0.16 mm.

The comparatively insignificant dimensional changes in diapositivematerial are of little importance in screen printing, considering thefar greater dimensional changes in the printing stock, especiallypaper and cardboard (see the section on printing substrates, below).


7.4 Register

7.4 The stencil

Steel and light alloy frames

Stencil frames are of considerable importance in securing accurateregister.

a) The linear coefficient of thermal expansion

Given our standard temperature increase of 5°C, which we haveadopted for practical reasons, a steel frame expands by about0.06–0.07 mm per linear metre. Aluminium frames expand byabout double this amount.

b) Frame distortion by fabric pull

Screen printing gauze stretched to about 20 Newton/cm exerts a pullof around 2 kg on every centimetre of the frame (i.e. everycentimetre of the fabric edge). This works out at around 200 kg permetre.

For instance, consider the straight longitudinal section of a DIN A0steel frame, with a mid-side sag of about 3 mm due to the tensionin the stretched gauze. The polyester gauze, initially strained toabout 2% elongation, will lose 1/4 of its stretch. The sag increasesmore than linearly with increasing size, even with the usualreinforced section (cf. recommended frame sizes and profiles).

Concave frame

A permanently sagging frame has less influence on register than afluctuating sag. The fluctuations depend on the stability or rigidity ofthe frame, the stability of the fabric, and the distance between thescreen and the printing stock. Fluctuations can be minimised inpractice by applying countermeasures to the frames:


7.5Register

Mechanical pre-stressing device

Pre-stressed frame with SEFAR clamps

Before gluing the fabric, the frame is given sufficient concavecurvature either by a strap, or the force of the stretching clamps asthey prop themselves against the outside edge of the printing frame.Fabric pull and frame tension thus balance each other.

It is possible to give the long sides a convex bend of approx. 5–10mm per metre during frame fabrication, then welding them at thisangle which is slightly in excess of 90°.

Convex frame


7.6 Register

Frame warping under various mechanical stresses

The fabric exerts a powerful pulling force. Depending on the meansof securing the frame in the printing machine, the danger of framesbecoming warped in use should not be underestimated. Roughhandling of printing frames is another common cause of warping.Warped or twisted frames always give difficulties with printing, andin obtaining correct register. The distance between the stencil andthe print substrate becomes non-uniform, which interferes withsqueegee pressure and uniform ink release.

This defect cannot be corrected merely by selecting a frame with astable cross section. Levelling frames during manufacture or repairrequires a very expensive levelling plate or slab, which should be partof every good frame manufacturer’s equipment.

Steel versus aluminium

Steel of the type used for screen printing frames has a specificgravity of about 7.8, light alloys about 2.7, or only about 1/3 theweight. On the other hand, sections and sides of aluminium frameshave to be somewhat thicker than steel frames.

The high weight of large steel frames is a disadvantage, both forlabour and equipment. There can be especial difficulties when ahinge of a hand printer is too weak to hold the frame, on one sideonly.

To achieve satisfactory bonding of adhesive to the frame, aluminiumrequires heavier roughening than steel.

Virtually all screen printing frames are now blasted with sand orsteel. Steel frames are protected by galvanising or two-componentlacquer. New aluminium frames must be cleaned with solvent (e.g.alcohol-based cleaner) before glue is applied for the first time.

Recommendations for frame size and profile

In machine printing, squeegee motion is usually in the direction ofthe frame width, in other words, contrary to the usual practice inhand printing. The size of the ink rests, along the sides and inparticular at the top and bottom, has to be determined byexperiment for every printing machine. Ink reservoirs that are toosmall give rise to a range of problems, including uncertain registerand smeared print. The sizes a machine can use have to bedetermined through individual trials.


7.7Register

Insufficiently strong frame profiles inevitably lead to problems suchas:

– loss of tension at the centre of the printing area

– poor register

– reduced stencil life, etc.

The following table shows the amount of frame flexing (mm) for agiven profile and frame length (cm) under a given tension (N/cm).

Example: A frame 100 cm long fabricated from 40 x 30 x 2.5 mm profileunder a load of 18 N/cm will flex by 0.94 mm.

Flexures in mm

htgnelegdEmc001

htgnelegdEmc002

htgnelegdEmc003

eliforP N81 N82 N81 N82 N81 N82

5.2x03x04 49.0 64.1

7.1/8.2x04x04 67.0 81.1

8.1/5.3x04x05 14.0 46.0 35.6 71.01

0.2/0.3x04x06 72.0 24.0 82.4 66.6

0.4x04x08 94.1 23.2 45.7 47.11

0.4x04x001 48.0 13.1 72.4 46.6


7.8 Register ©Copyright by SEFAR, 2/1999

Frame size and profile sections:

A A 1B/B 1C/CmuinimulA

dnasnoitcesllaw

sessenkcihtmmni

muinimulAsnoitces

suoiravhtiwllaw

sessenkcihtmmni

leetSdnasnoitces

llawsessenkciht

mmnitamroF

NIDaeraelbatnirP

mmni

stserknIpot/edis

mmni

edisniemarFsnoisnemid

mmni

4A 003x012 051/051 006x01504/040.3-5.2

04/040.2/5.2

04/045.1

3A 024x003 051/051 027x006

2A 095x024 051/051 098x027

1A 048x095 061/061 0611x019 05/040.306/04

0.3

05/040.2/0.3

05/040.2/5.4

05/040.2

0A 0811x048 081/081 0451x0921

0061x0021 002/002 0002x006104/060.3/0.6

0081x0041 022/022 0422x048104/080.3/0.6

0012x0061 052/052 0062x001204/0010.3/4.6

B1

B1

B B

C1

D

AD C

7.9Register

Screen printing fabric

Non-contact printing with lift-off necessarily distorts the stencilcarrier, even before consideration of the fabric shift caused by thesqueegee action. The change or distortion of the impressiondepends primarily on the distance between the stencil and theprinting substrate (snap-off).

Mesh distortion as a function of the lift-off distance:

Internal screen dimension: 1000 mm

Mesh distortion „V“, front view:A = 1 mm V = 0.008 mmA = 2 mm V = 0.032 mmA = 3 mm V = 0.072 mm

Mesh distortion „V“, side view:A = 1 mm V = 0.002 mmA = 2 mm V = 0.008 mmA = 3 mm V = 0.018 mm

The friction of the squeegee on the stencil leads to a shifting ordistortion of the imprint in the direction of the squeegee motion.Register inaccuracies are influenced by:

– ink viscosity

– squeegee pressure

– squeegee shape and position

– squeegee hardness

– printing speed

– surface condition of the printing substrate

– screen stability

��

��

500

A

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125 750

A

125



This requires investigation of the qualities of the stencil carrier(fabric) in regard to elongation resistance.

a) Polyamide

Even with maximum stabilisation, nylon fabrics (polyamide),cannot achieve the stretch resistance of polyester fabrics. They areprimarily used for printing curved surfaces, where highstretchability is desirable.

b)Polyester / steel

For precision-register prints, especially in larger sizes, the choice isbetween polyester and steel (V2A) screen fabrics. Steel has evenhigher stretch resistance than polyester. Nevertheless, polyesterstencils are often preferred because, correctly tensioned, theysatisfy register requirements and are less sensitive to blows andshock.

It is impossible to specify reliable coefficients for registerdifferences and the differences between polyester and steel mesh,since other factors previously mentioned always interfere.Numerous comparative tests have been made, particularly bycircuit printers.

Steel’s higher susceptibility to fatigue is well known in practice,but cannot be quantified.

c) Fabric strength

The strength and stretch resistance of a monofilament threadincrease in proportion to the square of the diameter, while thestrength of the fabric only increases in linear proportion to themesh count.

A fabric woven from relatively thick threads is thus more stretchresistant and prints with better register. The choice of mesh count,and thread diameter depend on the fineness of the print, thedesired ink volume, and fabric permeability.

d)High-modulus polyester fabrics

High-modulus polyester fabrics are characterised by increasedstrength and dimensional stability. They find use in virtually allscreen printing applications.

e) OSC fabrics for printing UV inks

Calendered (i.e. heat-flattened) fabrics offer less resistance tosqueegee motion, hardly move, and promote good register.Calendering inevitably reduces the mesh opening, which can bedesirable for very low-viscosity lacquers and UV inks.

7.11Register

f) Static electricity

Electrostatic charging of polyamide and polyester fabrics isprevented by pre-treating the gauze during manufacture on theone hand, as well as by appropriate conditioning of the workingarea (RH > 55%). Ionisers, and anti-static additives in printing ink,can limit the build-up of electrostatic charge during printing.

Optimal fabric tension

1)Degree of stretch:

See also the chapter on fabric stretching.

The tensioning force is limited by the nature and capacity of thestretching equipment, but even more so by the strength of thefabric and the rigidity of the printing frame. Fabrics woven fromhigh-viscose polyester can be stretched to extraordinary levels.Very high fabric tension allows a small distance between thescreen and the printing stock, and precise adjustment of thesqueegee pressure.

The degree of tension can be measured in different ways:a) pressure gauge on pneumatic stretching equipment

b) measuring elongation of a predetermined length of fabric before and after stretching (2–4% elongation for polyester fabrics, 4–6% for polyamide)

c) using instruments that measure the sag of the fabric under applied weight, and display the result scaled in mm or N/cm (NEWTONTESTER, TETKOMAT)

Good quality fabrics have the same stretch resistance in warp andweft, and do not require different degrees of stretching. Differingtensions can be useful in very narrow or curved frames, as usedfor printing skis, as well as in rotary printing.

2)Choice of stretching machine:

Pneumatic stretching equipment is recommended for best registerprecision, and for stretching frames of generally similar size.

Mechanical stretching machines are suited to stretching frames ofmany differing sizes.


7.12 Register

3)Pneumatic tensioning:

The fabric must be clamped parallel to the warp and weft threads.Take care that the sum total of the clamp lengths does not exceedthe length of the printing frame. The clamps should line up flushbeside each other, and pull uniformly and simultaneously at thefabric. Unsynchronised clamp movements produce shear forcesthat can tear the fabric. The stretching equipment must beextremely well treated and maintained.

Stretching with pneumatic clamps

4)Stretching with mechanical machines:

The fabric must be clamped parallel to the warp and weft threads.Fabric is pulled out a little from the corner clamps, to avoid localover-stretching (see table). The clamps must hold the fabric gently enough not to causedamage, but tight enough to avoid slippage. The warp and weftmust be stretched equally.


7.13Register©Copyright by SEFAR, 2/1999

a = pocket

Gluing fabric to the printing frame

1)Frame pre-treatment:

The adhesion surface of new frames must be roughened anddegreased. Old glue should be removed from used frames wherepossible. Sharp edges and corners must be smoothed down.

2)Solvent-proof adhesives (two-component glue):

Modern glues are required to be:

– easily spread, approx. 1 hour pot life

– fast drying time (depends on fabric number)

– mechanical tensile strength 80 – 90 kg per 10 cm of selvedge

– solvent-proof within less than 24 hours

– hot water proof (max. 70ºC)

RAFES 0001TEProf2Xdna1XsnoitisoptakcalS

:foecrofgninoisnetmumixam

htgnelemarF2Yro1Y

mc/N01 mc/N02 mc/N03

m1 mm5 mm01 mm51

m2 mm01 mm51 mm02

m3 mm51 mm02 mm52

m4 mm02 mm52 mm03

X1X2

X1

X1X1

X2

X2X2

Y1

Y2

a

a a

a


7.5 The printing substrate

Importance of air and material conditioning to the dimensionalstability of the material to be printed.

Composition

Composite substrates that have been woven, coated or laminatedfrom several materials should undergo an ink test to determinedimensional changes brought about by the ink and solvents(curling/buckling).

Conditioning

Optimum climatic conditions exist for achieving high-quality resultson paper and cardboard.

It is extremely important to allow the substrate to becomeacclimatised to the environment in the print room. If possible,atmospheric conditions should be the same in the print room and inthe warehouse.

Paper and cardboard

1)Effect of temperature:

Normal changes in room temperature have surprisingly little effecton paper and cardboard characteristics by themselves. (Printersrequire a certain temperature level and stability more because ofink viscosity and drying characteristics.) However, temperatureaffects the relative moisture content of the printing substrate,which is extremely important.

2)Effect of relative humidity:

All vegetable fibres, of which paper and cardboard are made, arehygroscopic. Water absorption strongly depends on the quality ofthe paper.

Rag papers (made from textile waste) absorb the least water,cellulose (wood pulp) is somewhere in the middle, and mechanicalpulp absorbs the most moisture of all.

Water absorption is increased by heavy milling, and reduced byhigh filler content. Paper’s hygroscopic behaviour further dependson its preconditioning. If the paper has been previously dried, itwill absorb less water than if it has been left moist (hysteresis).


The wet elongation curves for the papers mentioned above are allS-shaped, i.e. they are flattest between 40 – 60% relativehumidity. Moreover, elongation is less along the paper’smanufacturing length than across its width, because the fibrestend to orient themselves in the machine direction and fibres swellmany times more diametrically than longitudinally. Papers withclosely compacted fibres spread more.

During manufacture, a paper is exposed to stresses that mayremain largely latent. As soon as the paper becomes softened bymoisture, it tends to contract in the machine direction whileexpanding transversely.

Degree of elongation

As a rule of thumb, it can be assumed that under workingconditions of around 50% relative humidity, a change of 10% willcause the following dimensional changes in the printing stock:

– transverse to the machine direction: 0.8 – 1 mm per linear metre

– longitudinally to the machine direction: 0.3 mm or less per linearmetre.

The average dimensional changes between 20 and 80% relativehumidity (e.g. for SK* 95 g/m2 offset paper) can be estimated at:

– transverse: 1.25 mm per linear metre

– longitudinal: 0.48 mm per linear metre

for a 10% change in relative humidity.

Plastics

Here again, it is first necessary to establish whether the printing baseconsists of a single material or a composite: e.g. whether a foil hasbeen rolled on or affixed to a backing. The foil to be printed may, insome cases, be so elastic that it will follow all the dimensionalchanges of the base or backing to which it is affixed.

The same applies, of course, to self-adhesive foils before beingattached to a base, if the adhesive is covered with a protectivepaper.

7.16 Register

Relatively rigid, e.g. polyester-based, foils are an exception, wherefluctuating humidity can lead to bulging as the protective paperexpands and contracts more than the foil itself.

Dimensional changes in plastics, brought about by temperature andhumidity fluctuations, vary widely. Conductivity is generally very low,and moisture absorption proceeds so slowly that the plastic can takehours or days to respond to a change in relative humidity. Changescaused by temperature fluctuations are of more importance thanthose due to varying humidity, which are practically insignificant. Theinfluence of solvents on PVC, however, has to be considered.

Dimensional change in mm/linear metre for a temperature variationof 5°C:

– polyester 0.135

– PVC 0.35 - 0.5

– transparent acrylic 0.35

Glassine foils

When printing on glassine foils (transparent viscose foils, which arenot counted among the synthetic plastics), considerable dimensionalchanges must be expected as a result of their strongly hygroscopicnature. It depends on whether these foils are unlacquered (PT foils),or lacquered with cellulose (MSAT foils) or PVDC (MXXT or K foils).

Acetate

Where accurate printing register is important, acetate foils (viscosetreated with ethanoic acid) are preferred, provided no difficultiesarise in regard to inks and printing procedures. The dimensionalchange in response to a 5°C temperature change is 0.7 mm permetre, not considering dimensional changes due to moisture.

Summary

The factors discussed affecting accurate register show that theprimary stencil requisites for best printing results are:

– frame stability

– correct fabric stretching

– the stencil making method



Factors such as dimensional changes of

– polyester-based diapositives

– the frame material itself

– use of polyester or steel as the stencil carrier

are less important.

Register is further influenced by:

– squeegee pressure

– squeegee hardness

– ink viscosity

– lift-off height

– type of facility

– printing machine quality

Proper conditioning of the print room and printing stock are crucialwhen printing on paper and cardboard.

Below, we again compare the various possible dimensional changesto compare their importance. This is intended to help pinpointsources of error whose elimination would be advantageous.

For dimensional changes caused by temperature and humidity, weuse as standards:

– 5°C temperature variation, or

– 10% change in relative atmospheric humidity

We disregard the inverse relationship between temperature andrelative humidity.

It should further be noted that air conditioning within tolerances of±2°C and ±5% humidity represents an advanced standard for amodern screen printing facility, and requires highly accuratesupervision. In general, the following numerical values will have tobe multiplied according to the actual atmospheric fluctuations ineach individual case.

Data are given in terms of mm per linear metre.


1)Dimensional changes induced by climatic conditions in mm:

5°C 10%temperature rel. humidity

change change

Diapositive, polyester0.1 mm thick 0.135 0.21

Diapositive, polyester0.18 mm thick 0.135 0.16

Steel frame 0.065 0

Aluminium frame 0.13 0

Printing stock:

Paper, transverse elongation insignificant approx. 0.8 - 1

Longitudinal elongation insignificant approx. 0.3

2)Mechanically induced dimensional changes:

Fluctuations in frame warping up to approx. 2 mm

Distance between stencil and printing stock:

3 mm lift-off DIN A1 impression length 0.025impression width 0.065

5 mm lift-off DIN A1 impression length 0.075impression width 0.180

Distortion by squeegee friction:

Well-stretched steel stencil approx. 0.02 and over

Well-stretched polyester stencil approx. 0.03 and over

Indirect stencil:

Film shrinkage through washing and drying up to approx. 0.3

8.1Raster printing

8. Raster printing

The original artwork to be half-tone printed is frequently aphotographic image. Images of this kind cannot generally be rasterprinted in their original form: photographs are always continuous-tone images, where the gradient from light to dark has nodiscernible point structure.

The artwork must first be converted into a printable form. This isdone by translating the continuous-tone image into half-tone dots,using an AM or FM raster.

8.1 AM raster (amplitude modulated rastering)

An area of continuous tone is resolved into a grid, with rows oflarger and smaller dots. Viewed under a microscope, it becomesclear how the dot size is related to colour intensity, while thedistance between the dots is fixed. In other words, we have a fixeddot spacing with variable dot area. If we draw a plot of dot area onthe vertical axis against dot spacing on the horizontal axis, the resultis a curve whose amplitude changes according to the dot coverage,while the dot spacing remains fixed. The general shape of the curveis not unlike an ”amplitude modulated” waveform, with fixedfrequency and changing intensity or amplitude.

Analogue raster with variable area (amplitude)


8.2 Raster printing

Although the printing process is inherently digital (1/0, ink/no ink),the end result is an apparently analogue (smooth tone)representation of the original image. Applied to colour printing, thevery nature of the technique encourages moiré patterns.Countermeasures, such as angling the individual colour separations,are required to minimise the moirés inevitably produced by multi-colour printing.

8.2 FM raster (frequency-modulated rastering)

The ability to produce laser-generated points smaller than thesmallest practicable analogue raster dot opens the possibilities ofemulating analogue rasters, or pursuing a new, digital half-tonemodel. The first laser and film recorders were designed merely toreproduce the well-established analogue raster angles and pitches.However, the technique’s growing popularity has given rise to newrendering models that go under the general name of ”frequency-modulated rasters”.

The recording technology allows representing a continuous-toneimage by varying the distribution of uniformly sized, extremely smalldots. Varying coverage is achieved by varying the number ofrecorded dots per unit area.


Conventional (AM)

Agfa CristalRaster

HeidelbergPrepress Diamond Raster

UGRA/FOGRA Velvet Raster

Scitex Fulltone

Crosfield

20 % tonevalue

80 % tonevalue

8.3Raster printing

Examining the dots, it is clear how they are much finer distributedthan with an AM raster, as well as being all the same size. If wedraw a plot of dot size as amplitude on the vertical axis against dotspacing on the horizontal axis, we obtain curves of constantamplitude, but variable distance between adjacent peaks. The curve’sgeneral character corresponds to a frequency-modulated waveform– which is why we speak of a frequency-modulated raster.

Coverage and frequency modulation

Various methods have been devised for positioning the dots in anoptimal manner.

On a practical level, it is clear that an FM raster results inconsiderably finer dots on the film, and hence uniform colourcoverage. It should also be apparent that this type of half-tonetranslation virtually eliminates moiré effects. In their place comeother disturbances such as clustering or heaping.

It is important for the raster printer that the FM dots are of aminimum printable and reproducible size. We recommend a dotdiameter corresponding to at least 2 threads plus 1 mesh opening ofthe raster printing fabric.


8.4 Raster printing

8.3 Types of raster rulings

For a half-tone image to be effective at a given size and viewingdistance, one must determine a number of factors, including thetype and fineness of the raster ruling.

Monochrome half-tone images can be achieved using so-called”effect rasters”. These are:

– Corn raster

– Worm raster

– Line ruling

– Circular ruling

Corn raster example

The uneven structure of corn and worm rasters is less prone tomoiré effects than line, point or bead-string rulings.

For textile printing, there is a long-established type of grainingknown as the ”DIRACOP method”. Even now, diapositives are oftenprepared by hand , using transparent foils with a grained surface toachieve the half-tone structure.


8.5Raster printing

Four-colour printing uses the following dot shapes:

– round dots

– ellipsoidal dots (bead ruling)

– square dots (chequerboard ruling)

1 = Dot raster, dots amalgamate at approx. 70%

2 = Bead ruling, dots amalgamate at a) approx. 40%b) approx. 60%

3 = Chequerboard ruling, dots amalgamate at 50%

Dot amalgamation

Formation of discontinuities makes the way in which the raster dotsamalgamate especially important in raster printing. Here, weexamine each dot shape in turn at 46% and 52% coveragerespectively.


8.6 Raster printing

Round dots at 46% and52% coverage, and at amalgamation

Round dots only amalgamate at 65–70%. When this happens,though, it affects four neighbouring dots simultaneously, and thisresults in a steep tone value transition.

Ellipsoidal dots at 46% and52% coverage

With a beaded raster, the dots amalgamate at two distinct tonevalues. This results in a virtually unnoticeable transition. Dotamalgamation is direction-dependent: two neighbouring dots firstchain together, then the two parallel chains combine.

Square dots at 46% and 52% coverage

Square dots amalgamate with four neighbouring dotssimultaneously, resulting in a steep tone value transition. In rasterprinting, the effect is further emphasised by the high ink deposit.


8.7Raster printing

8.4 Raster ruling

The fineness of the raster ruling (L/cm) is always linked with thefineness of the mesh and the type of stencil.

Fabric and half-tone fineness

The finest details should properly adhere to the gauze. Areas withthe highest ink coverage, i.e. where the smallest dots of emulsionmust cling to the fabric, are particularly critical. The smallest pointsshould not be allowed to rest on just one thread, or even fallthrough the mesh opening.

Critical dot sizes

Strictly speaking, the diameter of the smallest half-tone dot on thediapositive should be microscopically measured, in order to select amesh of the correct fineness.

Table of dot sizes

From the examples above, it is clear that the diameter of the smallestdot must correspond to two threads plus one mesh opening, if theraster dot is to be adequately supported by the mesh.

egarevocmc/L

%5 %01 %51 %02 %03 %07 %08 %58 %09 %59

02 621 871 812 252 903 903 252 812 871 62122 411 261 891 922 082 082 922 891 261 41152 101 241 571 202 742 742 202 571 241 10182 09 721 651 081 022 022 081 651 721 0903 48 911 541 861 602 602 861 541 911 4823 97 111 631 751 391 391 751 631 111 9743 47 501 821 841 281 281 841 821 501 4704 36 98 901 621 451 451 621 901 98 3684 25 47 09 501 821 821 501 09 47 2545 64 66 18 39 411 411 39 18 66 6406 24 95 27 48 301 301 48 27 95 24


8.8 Raster printing

Ink flow in shadow areas

Raster printing is a print-through process and not – as in offset – atransfer process. The raster printing form (mesh + stencil) produces athicker ink deposit than a litho plate. This is the characteristic andadvantage of raster printing, namely, the intense and effective depthof colour. In half-tone printing, however, the ink deposit should berelatively thin, because the finer the raster ruling and the higher thedegree of coverage, the more difficult it is to avoid ink running inthe high-coverage, dark print areas (smudging).

The thinner and finer the fabric, the less ink is deposited. Thesefabrics are therefore better suited for fine half-tone printing.

When printing areas with low percentage coverage, the ink shouldflow freely through the smallest mesh openings without anyobstruction from fabric threads or stencil thickness. In this respect, arelatively light fabric is more advantageous than one with thickthreads.

5% coverage 10% coverage 95% coverage 90% coverage

Fineness and viewing distance

The half-tone simulates continuous tone in as much as the eyecannot distinguish the individual dots anymore.

Under normal conditions, the human eye discerns two adjacentpoints or lines as separate when their images do not impinge on twoneighbouring retinal cells (rods or cones). There has to be at leastone intervening retinal cell.


8.9Raster printing

The dots therefore become indistinguishable when their images fallon the same or two adjacent retinal cells.

The minimum angular resolution of the human eye is approx. 0.02°.

Raster rulings discernable to the eye

Guidelines:

The following factors should be considered when determiningwhether a given print job is feasible:

tamroF ecnatsidgniweiV stodenot-flaHmcrep

4ANID< m5.0< 84-63

4ANID m5.0.xorppa 63-42

3ANID m1-5.0 42-81

2ANID m3-1 02-51

1ANID m5-2 81-21

0ANID m01-3 51-21

0ANID> m02-3 21-

16 L/cm30 L/cm

80 L/cm

0,6 mm0,3 mm0,1 mm

30 cm

1 m

2 m


8.10 Raster printing

– Adapt the raster ruling to the surface structure of the printingmaterial.

– Glaring or highly intensive colours require a relatively coarser rasterruling than pastel colours. The coarser the half-tone ruling, thehigher the contrast of the print.

– For finer or softer images, choose a finer raster ruling.

From the commercial viewpoint, a raster printer is advised to limithimself to about three different raster rulings in order to gather thenecessary initial experience.

For artistic raster printing, one can choose finer raster rulings than forpurely commercial raster prints, such as for advertising.

8.5 Tone values of half-tone dots

Tone value expresses the relative area coverage of the half-tone dotscompared to maximum (100%) coverage.

In offset printing, the percentage coverage of the dots can rangefrom 95% to 5%, whereas the raster printer must be satisfied with atone value range from approximately 85% to 10%. This applies toraster rulings of approx. 30 dots/cm and finer.

For a perfectly printed dot of, say, 15% lightness, the ink must berelatively fluid in order to keep the dots open in the mesh. This leads,however, to difficulty in obtaining the tonal range, because the dotsfor 85% coverage tend to smudge if the ink is too thin. On the otherhand, if the ink is made more viscous, the smallest dots tend to drytoo quickly and block the mesh.

Tone value:Therefore, ink viscosity is a compromise in order to prevent smudgingin dark areas, while permitting fine dots in the light areas of theimage.

Tone value of the colour separation:For half-tone raster printing, the repro photographer should calculatethe maximum coverage for all four colours together, to reach anoptimum density of 300%. For reproductions with predominantlydark areas, the colour black should not attain more than 75%coverage. Yellow, however, can show higher coverage in order toproduce the correct tone values for green and red.


8.11Raster printing

The finer the raster ruling, the greater the difficulties. This illustratespresent-day limits to commercial half-tone raster printing.

Examples for perfect reproduction of half-tone values in rasterprinting:

up to 24 dots/cm 5-90%



These examples are based on the following general rules:

The finest printable dot should have a diameter of 80–100 µm (Thiscorresponds to the sum of 2 thread diameters + 1 mesh opening ina PET 150-31 fabric).

8.6 Raster printing process line

The process line is the characteristic curve describing therelationship between tone values on the diapositive and those of thecorresponding printed image.

The raster printing process line serves as a correction guideline,which helps the repro specialist avoid the problems that wouldotherwise arise in half-tone printing.

The raster printer therefore needs no measuring instrument. A half-tone step wedge with at least 10 tone values should be printedalongside the artwork. The repro specialist uses a transmissiondensitometer to measure the half-tone values on the positive film,and a reflection densitometer on the print. The results can becompared on a chart.



Densitometer

Example of a printing process line

enotmliFeulav

%

enottnirPeulav

%

toDssol/niag

99 001 1+ 00119 89 7+ 0938 49 11+ 0867 98 31+ 5796 48 51+ 0736 67 31+ 5635 95 8+ 0674 35 6+ 0504 44 4+ 0423 53 3+ 0382 82 0 5222 22 0 0271 51 2- 5111 7 4- 01

6 2 4- 51 0 1- 0


8.13Raster printing

The variances in tone values, plotted as a curve, result in the so-called process line:

Schematic representation of the printing colour scale

Every process line must also specify the following operationalparameters:

– raster lines/cm, type

– fabric type, threads/cm, tension N/cm

– type of stencil emulsion, capillary film, indirect film

– stencil thickness specified in µm

– surface roughness Rz value in µm

– ink type, manufacturer, composition, viscosity

–machine type, manufacturer

– squeegee hardness, thickness, clearance, angle,pressure

–printing stock precise description, e.g. type of paper, etc.

The colour scale can be significantly influenced by a change in anysingle item in the list above.

1614121086420

-2-4

%

%10 15 20 30 35 40 50 60 65 70 75 80 90 1000 5



8.7 Printing control strip

The FOGRA DKL-S1 control strip has been specially developed forhalf-tone raster printing, and may be used for visual anddensitometric monitoring of the following:

– Stencil production

– Changes in tone values

– Colour caste

– Colour balance

– Smearing effect

Half-tone field

This area features half-tone dots with coverage levels from 5% to95%. The raster count is 24 dots/cm. This facilitates visual and(preferably) densitometric checks on tone value transfer duringprinting.

Full tone field

A further check, which is of utmost importance in raster printing, isto measure the intensity of the printed ink.

A reflection densitometer is used to measure the ink intensity of thefour colours in the full tone fields. To achieve a good grey balance,all three process colours must lie within a close tolerance range.

Raster and full tone

Example full tone measurement:

Colour Required density ToleranceCYAN 1.45 ± 0.10YELLOW 1.00 ± 0.05YELLOW 47B 1.40 ± 0.10MAGENTA 1.40 ± 0.10BLACK 1.85 ± 0.15

Measuring instrument: Densitometer

Printing stock: Artistic printing papers


8.15Raster printing

Overlaid printing fields M/Y, C/M, C/Y, and C/M/Y

These areas allow colour acceptance to be assessed both visually andby measurement. It is important to use the same print-coloursequence for proof prints as well as for the production run.

M/Y C/M C/Y C/M/Y

Overlaid print

Ring field

This allows monitoring of transfer errors during printing, which canbe caused by smearing effects.

Ring field

Balance field

The combined three-colour print in the balance area should be aneutral grey, with a tone value approximately equal to the 40% half-tone field. This is a very sensitive indicator of shifting colour balanceduring a print run.

Balance field



8.8 Types of stencils

Generally speaking, one can use any kind of stencil for half-toneprinting. However, one should take into consideration a few pointsthat are typical for this kind of printing.

The difficulty with printing single or multi-coloured half-tones is thatboth light and dark areas must be neatly printed. There should alsobe no shifts in tone values. To maintain a perfect tonal range, thestencil emulsion should be as thin as possible.

The preferred stencils for half-tone printing are therefore indirectstencils, or direct stencils with film and water (capillary film), andminimal film thickness.

Direct stencils with photo emulsion are also used for long press runs.However, it is essential that they have a thin coating (5–10% offabric thickness) and low Rz value (less than the coating above thefabric in µm).

Important: Direct stencils should use yellow dyed fabrics to avoidlight scatter during exposure.

Only impeccable diapositives are suited for half-tone work. Forcorrect reproduction of the complete range of tone values, it isessential that all the dots are completely opaque right up to theiredges. (See chapter 5.4)

8.9 Avoidance of moiré effects

In repro techniques, suitable angling controls the moiré effectproduced between the raster lines of the individual componentcolours.

Half-tone angling is often given in two different ways:

– within 90° for rulings with two axes of symmetry (e.g.chequerboard and dot rulings)

– within 180° for rulings with one axis of symmetry (e.g. beadruling)


8.17Raster printing

Example:

Strong colours like CYAN, MAGENTA and BLACK must always be atan angle of 30° from each other. This minimises visible moirépatterns due to the interplay of the raster rulings. YELLOW, being aweaker colour, can be set at a 15° angle from a darker colour. Inraster printing, yellow should be on the vertical axis of the image,since a moiré caused by the stencil mesh is barely perceptible.

Angling of 4-colour half-tone rulings

Images with a high black content (deep tones)

Within 90° Within 180°

YELLOW 0 ° 0 °MAGENTA 15 ° 15 °CYAN 75 ° 75 °BLACK 45 ° 135 °

Images where YELLOW + MAGENTA predominate, e.g. skin tones,orange tones



75° Cyan 75° Cyan

135° Black

45° Black

15° Magenta

0° Yellow

15° Magenta

0° Yellow




Images where YELLOW + CYAN predominate, e.g. green andturquoise tones



General suggestions:

The strongest, most dominant colours should be at 45° in the 90°disposition or at 135° in the 180° disposition (45° left).

For five, six or more colours, the angling should be chosen in such away that the light colours coincide with their complementarycolours, e.g. dark red and light blue, dark blue and light red. Anadditional grey plate should be angled in such a manner that it doesnot coincide with colours strongly related to grey.



Dark colour 45 ° a) 45 ° b) 135 °

Light colour 15 ° 105 ° 75 °

75 ° 165 ° 15 °



Dark colour 45 ° a) 45 ° b) 135 °

Light colour 75 ° 105 ° 75 °

Angling of single colour half-tone rulings


45 ° a) 45 °


8.19Raster printing

Moiré between the film and the fabric

In raster printing, an additional moiré effect can result fromunsuitable angling of the raster lines of a particular componentcolour in relation to the raster fabric. This effect is most plainlyvisible in monochrome prints, whereas multi-colour printing tends toconceal it. The moiré effect is most apparent in the 40–60% range.

The moiré can be wholly or partially eliminated in the followingways:

1.By the type of stencil:

The moiré effect is less apparent with an indirect stencil, since theinfluence of the fabric is less pronounced than in the case of directstencils.

2.By the fineness of the fabric:

The finer the fabric in relation to the raster ruling, the less visible isthe moiré effect.

Recommended ratio between mesh count and the fineness of theraster ruling:

Mesh count/cm : Raster L/cm

2.50 : 1.003.75 : 1.005.00 : 1.00

Examples:

Fabric Ratio Raster L/cm

PET 140-31 yellow: 2.50 : 1 56 dots/cm3.75 : 1 37 dots/cm5.00 : 1 28 dots/cm

i.e. the fabric number is divided by the ratio.

If these recommendations are followed, there will be hardly anyvisible moiré effect at half-tone angles of 15°, 45° and 75°. In therare event of a moiré still appearing, it is advisable to increase ordecrease the number of half-tone dots/cm by one or two:

either 0.5-2 half-tone dots moreor 0.5-2 half-tone dots less.

3.By the fabric angling:

a) An ideal, universal angle for the fabric on the raster printingframe lies between 4° and 9°, provided the raster lines of thecomponent colours, as in the previous examples, are align withthe vertical or horizontal axis.



Angling the fabric, e.g. to 7.5°, has the additional advantagethat the cause of striped print appearance can be traced withcertainty to the fabric, or a poorly ground squeegee blade.

Stretching at a certain angle can be ordered from yourstretching service. A well-equipped stretching service canprovide an evenly straight stretched frame, which is essentialfor avoiding moiré.

b) It is impossible to feed an automatic printing machine at otherthan right angles. Therefore, it is impractical to angle the imageon the stencil, and feed the printing stock at a correspondingdiagonal.

c) There remains the angling of the whole set of diapositives (e.g.+7.5°) with respect to the image axis.

4.By the type of half-tone raster

a) According to current theory, grained raster, line raster andcircular are only suitable for monochrome printing. There islittle danger of moiré with these types of half-tones. For a lineraster, the fabric is angled.

b) Half-tones with elliptical dots (bead ruling) can, under certaincircumstances, lessen moiré development. It is not for thisreason that they are selected, but rather to soften abruptcolour-tone transitions.

8.10 General recommendations

– A basic requirement for perfect image reproduction is the qualityof the diapositive, with opaque half-tone dots right up to theedges.

– The colour separations and the printing inks should follow thesame colour scale, e.g. EUROSCALA.

– Place the half-tone positive on a glass plate, illuminated frombelow. Place the stretched raster on the positive, parallel to theimage axis. If a moiré appears, turn the raster left or right until themoiré effect disappears (approx. 7° is sufficient in most cases).

– The critical zones for the formation of moiré lie in the direction ofthe threads and their cross-overs.

– Dominant or darker colours tend to cause more moiré problems.

– For 4-colour prints, stable metal frames of the same dimensionsshould be used.


8.21Raster printing

– All frames are stretched with the same mesh.

– Use a dyed fabric for direct stencils.

– Stretch the fabric tightly along the directions of the threads.

– Stretch all 4 frames with the same tension.

– A perfectly ground squeegee is crucial to a high quality print.

– The squeegee blade should be of about 70° shore hardness.

– The squeegee should be set at an angle of 75°. If the squeegee isset at too flat an angle, it tends to smudge. Set it too steep, andthere is a greater risk of fabric distortion.

– The doctor blade should not be set too low. The stroke shouldleave a thin film of ink on the stencil. If the doctor blade is set toolow, the stencil becomes overfilled with ink and the print issmudged.

– Half-tone images are printed with high-viscosity inks.

– Initial half-tone printing trials should be carried out with coarseraster rulings.

– The finest fabrics require relatively high colour pigmentation.

– UV inks tend to smudge, but do not dry in the mesh. For thisreason, the half-tone diapositive should have a tone value rangeof 5–80%.

– In 4-colour printing using UV inks, ensure that the additionalstencil thickness and the Rz value are no higher than 5 µm.

– To keep UV ink smearing under control when overprinting colours,we recommend the following colour printing sequence:

CYAN – MAGENTA – YELLOW – BLACK

– For UV inks, the squeegee should have 75° shore hardness, i.e.generally somewhat harder than with conventional inks.

– The squeegee angle should be approx. 75°.

Recommended raster ruling (L/cm) in relation to fabric number (threads/cm)

The following basic parameters must be established before makinguse of the recommendation:

a) Type of printing job

b)Type of ink

c) Fineness of half-tone ruling



Graphical work, CDs:

Ink type Half-tone Fabric Threadruling threads/cm diameter

Solvent-based inks up to 48 L/cm 120 - 165 27, 31 and 34 µm

UV inks up to 60 L/cm 140 - 180 27, 31 and 34 µmup to 15 L/cm 90/2 34 µm

Water-based inks up to 48 L/cm 140 - 180 27 and 31 µm

Direct printing onto ceramics:


Water-based inks Floor tiles up to 24 L/cm 40 - 77 80, 70, 64, 55

and 48 µm

Solvent/waterbased inks Wall tiles up to 36 L/cm 120 - 140 34 and (31) µm

Ceramic transfers:


Solvent-based inks up to 48 L/cm 120 - 165 31 and 27 µm

T-Shirt printing:


Pigment ink up to 24 L/cm 61 - 77 64, 55 and 48µm90 40 µm

Plastisol ink up to 36 L/cm 90 - 140 48, 40, 34 and (31) µm


8.23Raster printing

8.11 Improved printing stability through achromaticreproduction

Helmut Acker, Production Manager in a repro house

Helmut Acker writes about achromatic reproduction from therepro house’s point of view.

Regardless of the printing process used, multi-colour reproductions,where the various hues are produced by rastering, now tend tooriginate on scanning equipment. Specialised raster types, andrulings, dot shapes and raster angles are either built-in, or areavailable from dealers as plug-ins.

Modern, customisable colour scanners facilitate colour separationsbased either on traditional chromatic techniques, or newerachromatic methods. Given the right software, achromatic films canbe produced which are just as reliable and accurate as traditionalthree-colour separations.

There are wide differences of professional opinion about theproduction of achromatic separations. Some authorities maintainthat greyscales should be achieved using black alone, and allcomposite colours obtained from two process colours plus black.This offers the prospect of saving on expensive process colours,while improving or even matching the brilliance of deep colours – a result much sought-after by offset printers.

A technical discussion of achromatic films must distinguish betweenachromatic in a narrower sense, and increased UCR (under-colourremoval). Practical experience shows that repro specialists mustadopt a very different approach to UCR. For technical illustrations ofobjects like radios, televisions, cameras, binoculars and the like, UCRtogether with a corresponding combination of full black is verysuccessful in reducing coverage from at least 280% to approx.200%. This gives a more brilliant result, while avoiding colourinterference effects in areas of dark red or green tones.



On the other hand, illustrations of predominantly full-coloured, darksubjects should be given only minimal UCR, to counter the risk ofbeing unable to use black to compensate for strong colour removal.This is mainly a problem in gravure and offset printing.

Process colour buildup with under-colour reduction

In our experience of making software-generated achromaticseparations, we generally assemble the separation from cyan, yellow,magenta, and a skeletal black mainly for enhancing the dark tones.We also remove any trace of impure composite colour and replace itwith black. This results in colour separations with much reducedcolour saturation and an unusually rich black. As a result, browntones no longer contain cyan, while green to olive contains no red,for example. The missing colour is always replaced by black. Thisbrings great advantages during the print run. The image generallygains in brilliance, and we notice time and time again: achromatic ismore colourful!

100%

50%

0%

40% C 50% M 60% Y 30% K 180%

C M Y K


8.25Raster printing

Achromatic buildup

Experience shows that, while technically possible, it is inadvisable tocarry removal of the third process colour to extremes. We know thata grey made up of process colours is more pleasant visually than ahalf-tone black. We therefore recommend not completely removingthe third process colour in impure colours, in order to retain aharmonious effect. This technique is known as achromatic buildupwith process addition.

Achromatic buildup with process addition

100%

50%

0%

25% C 35% M 45% Y 45% K 150%

C M Y K

0% C 10% M 20% Y 70% K 100%

C M Y K

100%

50%

0%



It is fair to ask the reason for discussing achromatic buildup in araster printing context. It is an established fact that achromatic filmsoffer particular economic and qualitative advantages to offsetprinters working with four-colour and web offset machines. Thesame can be said of gravure, where offset films are used. Initialresults with raster printing indicate that this process stands to gainmost of all from achromatic techniques. The missing process colourin the final result accelerates drying considerably. The reduction inthe amount of ink overprinting virtually eliminates velvet effects.Reduced ink usage brings its own set of advantages. Registerproblems are much less visible, since black is the only detail-printingcolour.

Given that on-raster assessment of the colour separation is not yetperfect, proof prints remain essential. This inevitably leads to higherfilm production costs. Extreme use of achromatic technique exacts aprice in side-effects and artefacts which have to be taken intoconsideration. Complete removal the third process colour can makecertain tones appear harsh, the absence of certain dots in the colourrosettes may even lead to moirés, and the smallest differences ofregister can cause white flashes on some print run specimens. Allthese aspects, in particular the degree of colour removal, must begiven due attention by the repro specialist making the colourseparations.

8.12 Heuristic rastering for textile printing

Heuristic rastering is a digital colouration and design system in whichthe desired colour nuances are produced by mixing textile printinginks on the fibres themselves. A precision-calculated raster doses theindividual colour elements. A combination of specialised litho work,precision stencil production, and a fine-tuned printing processdeposits successive individual ink doses on the fabric fibres, wherethey subsequently blend together. Four stencils suffice to print adesign with a virtually unlimited range of colour nuances. Thistechnique is suitable to producing attractive, multi-coloured designsat low cost, with minimal environmental impact.


8.27Raster printing

History of half-tone textile printing

There was a time when the printer, as well as the colourist, wereartisans if not indeed artists. The requirements were primarilyaesthetic, and technical resources were limited.

Times change. It was no longer considered acceptable for theprinting machine to limit the maximum number of colours on aprinted fabric. Various half-tone techniques were developed,including half-tone rasters. One well-known example is the”DIRACOP” process, which uses a kind of corn raster. Even now,suitable colour separations are prepared by hand, using transparentfoils with a grained surface to achieve the half-tone structure.

The corn raster is produced photographically using a magentacontact raster.

There have been repeated attempts at introducing standardised 4-colour half-tone techniques to the textile printing industry.

Incentives for developing heuristic rastering

The driving forces behind the development of heuristic techniquesare:

– ability to realise attractive designs

– improved economic factors

– reduced environmental impact

Heuristic half-tone printing can emulate the appearance of thefollowing techniques:

– half-tone

– gradation

– overlay

– multi-colour half-tone print

– combinations of the above techniques



The basic principles of heuristic rastering can also be found in:

– offset printing

– ink-jet printing

– raster printing

Principles of various printing techniques

The basic principles of the following printing techniques aresummarised here, to aid understanding:

– traditional raster printing

– standardised 4-colour half-tone printing

– heuristic multi-colour half-tone printing

Traditional raster printing

In traditional textile printing, the design is separated into itsindividual colours. A separation film is used to make a stencil foreach colour. Areas of the stencil are either permeable orimpermeable to ink. These areas may also be occupied in whole orpart by various half-tone rasters, with due attention to colouroverlays and underlays. The choice of stencil fabric depends on thetextile material to be printed, and is a factor in determining theamount of ink-paste to apply. Ink-paste is pre-mixed to the rightcolour and pressed through the stencil onto the appropriate placeson the textile. The maximum number of colours allowed in a designis governed by the size of the printing machine.

Standardised 4-colour half-tone printing

This technique is similar to graphic raster printing in that the designis separated into primary colours: cyan, magenta, yellow and black.The colour parameters used in separation originate from a standardcolour scale (e.g. EUROSCALA). The colour separations are used asfor graphic raster printing.

Recreating an original design using textile dyes is very difficult, needsmuch experience, and involves a large number of tests and proofprints. One must be prepared to compromise in matters of colourrendition. A restricted colour space and colour elements producesdull, over-saturated designs. Amalgamation of the basic colours isusually unavoidable, and the results are unsatisfactory.


8.29Raster printing

Heuristic multi-colour half-tone printing (Ciba Specialty Chemicals)

In this technique, the first step is to digitise the design. The chosenraster ruling defines the number of lines/cm, as well as the resolutionof the final print. This should take account of the characteristics ofthe textile stock to be printed, the expected viewing distance, andthe desired visual effects.

The dot size controls the brightness (ink quantity) for every singlecolour component. The dots are calculated from tone value scalesspecific to a given substrate and set of printing conditions. Theheuristically calculated dots are recorded on film (half-toneseparation)

The films are used to make printing forms, which reintegrate theindividual component dots to reproduce the original design.Correctly sized dots, correctly located, at a calculated angle, on asuitable textile substrate and with the correct inks andconcentrations, combine to reproduce the form and colour of theoriginal design.

Let us examine the build-up characteristics of inks used in traditionaltextile printing. Ink build-up is responsible for generating a visuallyperceptible printed colour depth that is related to the ink-pasteconcentration. It is also a characteristic of the ink. The way in whichthe inks build up is affected by the following factors:

– the substrate

– pre-treatment

– ink-paste composition

– ink-paste volume

– fixing

With heuristic half-tone printing, the tone value behaviour of variousraster values is similarly analysed under given productioncircumstances. The half-tone value describes the proportional size ofa dot relative to 100% (maximum) coverage. The tone valuebehaviour on the textile substrate describes the colour combination(visual colour depth) as a function of percentage coverage (half-tonedot size).

The tone value characteristic is fundamental to heuristic half-toneprinting, and deserves a corresponding amount of attention.



Unlike traditional printing, the heuristic method varies the ink-pasteconcentration. This gives the fully-saturated colour at 100% solidarea, while a 1% tone value gives the smallest possible addition tothe overall colour mix.

The tone value characteristic, coupled with a suitable choice of inks,determines the achievable colour space.

In heuristic multi-colour half-tone printing, the colour space is notbound to a fixed number of inks. The original design may containcolours that lie outside the space that is printable using 4 colours. Toreproduce a brilliant blue, for example, requires an extra blue ink. Itis the same in the orange area, where an orange ink is necessary.Taken to the other extreme, there are designs with a colour spacethat requires just 3 inks.

Example of multi-colour half-tone printing

Colour values are first defined, then described in terms ofcorresponding raster ruling. Separations are then made, based onthe textile inks specifically chosen for the job. The basis is thecalculated tone value / raster ruling, taking all contributory factorsinto account (type, raster, angle, stencil fabric, printing sequence,etc.). Production factors are recorded during process analysis, andrequired degree of reproducibility is achieved through optimisationand standardisation of the individual steps in the process.

The raster films are adjusted to the entire gravure technique(specially selected raster printing fabric), technical printing data,colour influences and characteristics of the lithographic material. Theresulting stencils, used in correct sequence with the defined inkcolours and concentrations, give accurate colour rendition of theoriginal design on a given textile substrate.

Cyan

Black

Magenta

Yellow

Cyan

Violet

Black

Magenta

Orange

Yellow


8.31Raster printing

8.13 Objectives of heuristic half-tone printing

– more attactive designs

– cost/economic factors

– environmental considerations

More attractive designs

The Ciba multi-colour technique with 100 brightness levels allows atheoretical palette of 4 million reproducible colours.

These can be printed using 4 stencils. Textile printers can achieve softcolour gradations, defined half-tone values, calculated overlays and3-D effects.

Cost/economic factors

Using this technique, even multi-coloured designs require only aminimum number of stencils. This has a strong influence on theoverall printing cost.

Given that production printing never requires more than a fewstencils, the number of personnel operating the printing machinecan also be reduced. Retooling times for printing a new design areshorter, since there are fewer stencils to change and the printing inksare the same for all designs, meaning that they do not have to bechanged.

Heuristic half-tone printing also reduces the workload in the inkmixing shop. It is no longer necessary to calculate ink quantities andmix their individual components. Checks and controls are neededonly for the basic colours, which are produced in large quantitieswith associated economies of scale.

The number of expensive proof prints is minimised. Experience hasshown that heuristic half-tone technique, correctly applied,eliminates the need for corrections and modifications.

Environmental considerations

The problem of processing and recyling old ink does not arise withthis technique. Where the same ink-pastes are used for all designs,left-over ink is no longer an issue. Whatever is not consumed oneday can be used the next.



Installations like ink scoops, delivery pipework, squeegee systemsand ink cisterns no longer need to be rinsed down during everydesign change; instead they can be used straight away for the nextone. With traditional methods, rinsing-down wastes approximately10 kg of ink per stencil. Eliminating this step reduces the demandson the wastewater system.

In half-tone printing, the ink-paste deposit depends not on coverage,but the average colour density of the design. Lighter shades areachieved through a lower ink-paste deposit. From the heuristicdesigns produced to date (approx. 400), the estimated averagecolour intensity is around 40%. This in turn represents anapproximate 60% reduction in liquid effluent from chemicals in theink-paste (urea, alginate, additives, etc.).

8.14 Technical considerations

This technique requires cooperation between the end user and Ciba.Successful results depend on the quality of cooperation. A heuristicraster based on incorrect data is unusable. It is absolutely essentialthat the inks specified in the half-tone calculation are also deployedin production.

The half-toning mechanism (mixing ink-pastes on the fibresthemselves) necessitates transparent pigments. If non-transparentpigments are used, one can expect reproducibility problems within aproduction batch. Where half-tone dots overlap, a non-transparentpigment will obscure the previous ink layer.


9.1Printing

9. Printing

The following are recommendations for screen printing by hand andon flat bed machines.

Various factors influence the results of the printing process. Themost important are:

– design and construction of the printing machine: heavy, light, orprecision construction

– stability of the printing bed, type of register adjustment, squeegeeaction, etc.

– type of screen fabric, especially the quality of printing form /screen tension

– squeegee set-up, i.e. blade hardness, accuracy of the groundedge, pressure, speed and angle

– snap-off distance (distance between the stencil and the substrateto be printed)

– lift-off adjustment

– positional accuracy of the print substrate (register stops, print bedstability)

In view of the interplay between all the above factors, it is advisable,where practical, to limit the selection of screen frames to a fewstandard sizes.

To gain practical printing experience in a systematic manner, certainbasic rules should be observed, the two most important being:

– Limit the initial job diversity, i.e. restrict the variety of designsattempted.

– When proofing, alter only one factor at a time – i.e., neverattempt to change two or more settings simultaneously.


9.2 Printing

9.1 Setting up of a flat bed machine

For clean prints with accurate register, correct setting of the snapand lift-off distances is important.

a) screenb) substratec) printing tabled) snap-off

”Snap-off” is the distance between the screen and the substrateimmediately before printing, when the frame is in the loweredposition.

The ”snap-off” distance is necessary to prevent the screen fromtouching and possibly smearing the substrate before printing, andalso to allow the tightly stretched screen to rise clear of the printimmediately after the squeegee has passed.

The snap should be as small as possible, e.g.

– for a DIN A3 screen 1 - 3 mm;

– for a DIN A0 screen 3 - 5 mm;

The snap is usually slightly greater for hand printing than onmachines with mechanical squeegee action.

SEFAR measuring wedge

With flat bed printing machines, an even snap-off distance is veryimportant for exact register and a perfect print image. If a stencil isnot placed perfectly flat in the machine, squeegee pressure isunbalanced because the blade presses down harder on the raisedpart of the stencil. This results in an uneven, distorted image.

d

abc


9.3Printing

The SEFAR measuring wedge provides a simple means to verifyuniform snap distance. Simply slide the wedge under all four edgesof the stencil frame and the machine bed in turn, and read off thesnap distance in mm. The optimum snap distance depends on thedimensions of the frame and the printed image, the tension of thefabric, on the ink composition, and the desired printing job.

Snap measuring wedge

In order to improve clearance between the screen and the substrate,many printing machines incorporate an automatic lift-offmechanism, which raises the screen progressively higher as thesqueegee moves forward. The snap-off angle behind the squeegeeremains constant from start to finish of each print stroke.

a) screenb) substratec) printing tabled) lift-offe) printing stroke

The quality of stretch, the snap-off and the lift-off all help to raisethe screen fabric from the printing immediately after the squeegeepassage. If the screen were to remain in contact with the freshlyprinted substrate, the slightest shift of substrate or screen wouldsmudge the wet ink.

d

abc

e


9.4 Printing

All three factors are adjustable:

– fabric tension (see the section on stretching)

– snap-off distance

– lift-off motion

If any of these is increased, the squeegee pressure must also beincreased a little.

Excessive snap and lift-off are detrimental to accurate register.

Reducing the printing speed also helps to reduce the adverse ”drag”effect of the screen on the freshly printed substrate.

On cylinder printing machines, no lift-off is necessary as rotation ofthe cylinder bed gives the same effect.

9.2 The squeegee

Material

Squeegees for screen printing are made of natural or syntheticrubber (Neoprene), or of polyurethane (Vulkollan, Ulon, etc.).

Those of natural or synthetic rubber wear out faster, but, on theother hand, they are much less prone to accumulating electrostaticcharge.

Polyurethane squeegees have better abrasion resistance, but agreater tendency to accumulate electrostatic charge during printing.

Both materials harden with age, and overlong exposure to solventscauses the squeegee to swell, leaving the blade wavy and unusable.Squeegees should therefore be cleaned and dried immediately afterprinting. The edges and sides of the squeegee should be free ofblemishes like pock-marks and scratches, as these tend to givestreaky prints. Squeegees should be frequently re-ground: sharp,clean edges are essential to good quality printing.

Hardness

Squeegee hardness is expressed in degrees shore, the generallyrecommended region being 60° – 75° shore hardness.

Harder squeegees (70° – 75° shore) are suitable for large formatsand half-tone printing.


9.5Printing

Softer squeegees (60° – 65° shore) are preferable for solid overallpatterns and substrates with an uneven surface.

Excessive squeegee hardness can cause difficulty in maintainingregister, due to the high frictional drag with subsequent distortion ofthe screen fabric. This also tends to reduce the stencil life.

Excessive softness, combined with high pressure, can cause thesqueegee to flex backwards. The squeegee angle becomes tooshallow, so that ink, instead of being swept over the mesh, ispressed onto an elongated section of the printing substrate. Ink thenruns under the stencil.

Dimensions

Squeegee profile

Squeegee front view

The dimensions of the printing frame should be such as to leave aclearance of at least 12 cm between the inside edge of the frameand each end of the squeegee. Insufficient clearance can result invisible distortion.

15cm3cm 3cmimage area

15cm

a) Squeegee hardness

b) Squeegee thickness

c) Squeegee (free) height

d) Squeegee angle

e) Squeegee pressure

f) Squeegee speed

g) Squeegee grinding (profile / surface)

a

b

c

d

e

f

g

10-2

0mm


9.6 Printing

Squeegee grinding

The condition of the edge of squeegee blade is important to theproduction of sharp, clear prints.

A sharp-edged squeegee will precisely meter the amount of inkpassing through the screen. This is an important factor in theproduction of fine detailed work and half-tones.

A blunted or intentionally rounded squeegee edge fails to skim theink off the screen in a precisely metered fashion. Instead, it forces anexcessive amount through to the substrate, smudging details.However, ink application at such high levels may be desirable to givegood coverage, especially in solid areas.

A badly ground squeegee blade will produce streaky prints; however,this effect can sometimes also be caused by irregularly woven screenfabric. It is often extremely difficult to determine whether thesqueegee or the fabric is causing streaky prints. The only certain wayis to stretch the screen fabric on the bias (i.e. at an angle to theframe) so that the threads run at an angle to the frame and theprinting stroke.

Squeegee streaks are cured by carefully rubbing the cleanly-groundsqueegee edge with a polishing cloth.

The squeegee grinding machine must have a firm clamping devicefor the squeegee. The blade should be ground parallel to the length,so as to allow accurate re-grinding.

Avoid overheating the squeegee during the grinding process.

Round-off the squeegee ends.

Emery belts are used for squeegee grinding. Belts may be fitted oversingle or multiple rollers, as shown in the diagrams below.


9.7Printing

Types of squeegee grinding machine

The emery belts should be of. 80 – 180 grit, depending on thesqueegee rubber material and the printing application.

Squeegee sections

Squeegee sections

Squeegee grinding profiles


9.8 Printing

Special squeegee grinding for screen printing on circuit-board tracks of 70 µm ormore

5° – 30° angle-grind at the squeegee edge.

Proof prints are essential. The viscosity of the ink paste has aconsiderable effect on the result.

Special angle-grinding at the squeegee edge

Squeegee angle

Squeegee angle: a) printing strokeb) steep c) normal d) low

The optimum squeegee printing angle is 75° from the screen. Largedeviations from this angle can adversely affect ink metering andaccuracy of register.

a

b c d

75°

25-3

0

5-30°

min. 1.5mm

10

75°


9.9Printing

Steep angle: In this position, the flexibility of the squeegee blade isimpaired, increasing friction with the screen fabric. The blade’sincreased cutting action reduces the ink deposit, while dragging andstretching the fabric, which causes loss of register.

Low angle: The squeegee blade flexes too far backwards, pressingmore ink through the stencil mesh. Ink tends to run under thestencil.

Notes for textile printing:

Rounded squeegee profiles are most commonly used. Depending onthe ink absorbency of the printing substrate, profiles with a suitableradius and corresponding hardness are used.

Squeegee pressure

As already stated, excessive pressure adversely affects the accuracyof register, since the squeegee drags the screen fabric in thedirection of the stroke. Therefore, pressure should be kept to theminimum necessary for good print results. Pressure adjustment canbe made as follows:

– Adjust the squeegee so that it is clear of the screen while in thenormal printing position.

– Place the squeegee in the centre of the image area, i.e. the centreof the print stroke.

– Lower the squeegee until it is just clear of the screen, and parallelto it.

– Adjust both set screws to bring the squeegee into contact withthe printing substrate.

– Make final adjustments to the squeegee setting during the firsttrial pulls on waste material.

Squeegee pressure must not be altered during the print run, as thiscan cause colour and register changes. In multi-colour printing,squeegee pressure should be identical for all screens. Raising thepressure lengthens the printed image, leading to register problems.

The squeegee should be cleaned immediately after printing.Prolonged exposure to solvents impregnates the blade, making itsoft and subsequently useless.


9.10 Printing

RKS squeegee system

Squeegee profiles

Advantages of the RKS system are:

– quick fitting and removal

– constant blade angle

– easy control of squeegee pressure

– low wear

RKS squeegees can also be fitted to conventional squeegee holders.

9.3 Flood bar (Doctor blade)

The flood bar or scraper squeegee is attached parallel to the printsqueegee, but with a lighter pressure. It spreads a thin film of ink onthe screen, which prevents the ink from drying.

It is important that the flood bar or scraper edge is not damaged inany way, with no sharp corners or edges, splinters, etc.

Camber grinding of the flood bar gives a uniform ink lay on thescreen, especially with large formats.


9.11Printing

9.4 Printing speed

The ink flow or metering effect through the screen depends onseveral factors, such as printing speed, ink viscosity, type ofsqueegee, etc.

When the printing speed is too high in comparison to the otherconditions, the mesh openings are not completely loaded with ink.This results in poor prints. Printing speed should always be set inconjunction with other determining factors, as per example:

– high ink viscosity

– thick stencil coatings

– stencil fabric with fine mesh openings

– steep squeegee angle to prevent smearing of fine detail

– large areas requiring good ink coverage.

The foregoing are not the only relevant factors affecting printquality, but any of the above will necessitate a reduction in printspeed.

Changing speed during the print run will result in a correspondingchange in the print quality. If printing results are unsatisfactory(smearing, poor register, etc.), one or more of the preceding factorsare generally responsible. Bear in mind that in endeavouring torectify any misprint, only one factor at a time should be checkedand, if necessary, altered. The prime condition for success is a wellmade screen equal to the requirements of the printing task. (See alsoother chapters of this handbook.)


9.12 Printing

9.5 Object printing

This section deals with the printing of shaped objects such asbottles, glassware, crockery, jars, tools, instruments, boxes, sportinggoods, machine parts, etc.

a) stationary squeegeeb) movement of screenc) direction of rotation

Schematic concept for printing round objects:

Printing shaped objects needs a more flexible screen to make fullcontact with the curved surfaces. Polyamide fabrics (PA) have theideal elasticity to allow a perfect fit to various shaped objects andsurfaces.

Direct stencils are most frequently used for printing shaped objects,since indirect stencil films are not elastic enough to follow the fabricas it adapts to the shape of the substrate.

It is usually uneconomic to reclaim screens used on long print runswith difficult or abrasive printing substrates. Mounting new fabric isoften faster, cheaper and more reliable.

Squeegees for printing round objects are normally cut with an equalbevel, often referred to as ”V” cut or ”double cut” squeegees.

Rectangular squeegee profiles are generally chosen for UV inks, andangled at approx. 75° to the stencil surface.

Recommendations laid down by the manufacturer of the printingmachine should be duly noted.

c

b

a


9.13Printing

9.6 Single operation multiple colour printing

Partitioning the squeegee and the stencil allows two colours to beprinted in the same pass. This is only possible when the respectivecolours are separated by 10 mm or more.

Split stencil for two-colour printing

The stencil is partitioned by inserting a divider (cardboard, plastic, ora thin wooden batten), secured and sealed with two-componentglue.

9.7 Ink deposit

The theoretical ink volume of the fabric is an indication of the wetink deposit, as well as being used to calculate ink consumption.

Example:

An ink or paste with 60% solid content, printed with PET 1000 77-48 results in a wet ink deposit of 28 cm3/m2, which correspondsto a thickness of 28 µm.

During the drying process, the 40% solvent content evaporates. Theremaining dry ink deposit is only 17 µm.


9.14 Printing

Fabric number

Refer to the technical information in the fabric datasheets.

9.8 UV inks

UV inks have a very high solid content, approaching 100%. Extrathin and fine fabrics (120-31 – 180-27) are necessary to reduce inkconsumption and ink deposit. For extreme ink deposit reduction (UVvarnish) we recommend a one side calendered fabric such asSEFAR® PET 1000 OSC.

In 4-colour half-tone printing, care should be taken that the stencilthickness on the mesh does not exceed 3–5 µm.

84.4

57.3

40.230.5 28.0

21.1 16.3 10.9

6.5

0.030

-120

40-8

0

51-7

0

61-6

4

77-4

8

100-

40

120-

34

150-

31

180-

27

20.0

40.0

60.0

80.0

100.0Theoretical ink volume in cm3 per m2


9.15Printing

9.9 Printing systems

Flat bed printing

Flat bed screen printing is used for printing both flexible and rigidstock, e.g. paper, cardboard, plastic foils, wooden, plastic or ceramictiles, textiles, and flat objects.

a = printing directionb = squeegeec = printing framed = substrate

Special technique for industrial textile printing

The squeegee is a circular steel rod. Ink quantity is controlled by thesqueegee diameter and the magnetic force exerted from below theprinting machine bed. The stencil is in contact with the substrate.

a = printing directionb = squeegeec = printing framed = magnete = magnet carriagef = rubber matg = substrate

abc

d

ef g

��

��

abc

d


9.16 Printing

Cylinder bed printing

Cylinder bed machines are suitable only for flexible print stock likepaper, plastic foils, etc.

a = printing directionb = squeegeec = printing framed = back pressure cylindere = substrate

Printing on cylindrical objects

These machines use the substrate itself as the back pressure cylinder,e.g. bottles, tubes, jars, etc.

a = substrate

��

��

a

��

��

a

bc

d

e


9.17Printing

Rotary printing (single substrates)

Example: Ceramic tile printing

Rotary printing (reel to reel)

Printing occurs continuously, either from a feed to a take-up reel, oron a flat substrate fed by conveyor belt under the rotating cylinder(paper, foils, textiles, ceramic tiles, etc.).

a = print directionb = squeegeec = cylindrical stencild = substrate (on reels or conveyor belt)

c

a

b

d


10.1Measuring instruments

10. Measuring instruments

Consistently high printing quality can only be achieved andmaintained through the use of internal standards. These in turndemand reproducible data and tolerances, which can only begathered using suitable measuring instruments.

10.1 Coating thickness

This instrument measures the stencil coating thickness on the mesh.The thickness on the mesh determines edge sharpness, resolution,and the wet thickness of the ink deposit on the printing substrate.


10.2 Measuring instruments

10.2 Roughness

This instrument is used to check the stencil surface. The measuringprobe is simply laid on the surface to be measured at an angle of22.5° to the fabric threads. During the measuring sequence, theprobe moves a few millimetres in order to take a predeterminednumber of measurement samples at the highest and lowest pointson the surface. The measured average in µm appears on theinstrument’s digital display. A perfectly flat surface would show avalue of 0.

The Rz value (DIN nomenclature for average roughness) for screenprinting stencils should always be less than the measured coatingthickness. A relatively smooth stencil surface is essential to printingsharp edges and avoiding saw-tooth effects.

Poor: Rz > coating thickness

Good: Rz < coating thickness

12µ 8µ

coating thickness roughness (Rz)

12µ 20µ

coating thickness roughness (Rz)



10.3 Radiometer (incident radiant energy measurement)

Stencils cure and harden best when exposed to a good light source.The source should radiate predominantly in the UV range between350 – 420 nm. Lamps have a limited life, and the radiant energyfalls off as the lamp ages, requiring exposure compensation. Aradiometer provides the simplest means for measuring lampefficiency.

10.4 Hardness meter (Shore measuring instrument)

This instrument is used to check the hardness of the squeegeerubber. The rubber is affected by various solvents and is also subjectto a natural ageing process that causes the hardness to change overtime. Every colour pass in a printing batch should use the samesqueegee size and hardness. Different squeegees lead to registerproblems and colour shifts.


10.4 Measuring instruments

10.5 Viscosimeter

The viscosity of the printing medium (ink, lacquer, paste, etc.) has astrong influence on the attainable printing speed, amount of inkpassing through the stencil, wet film thickness, edge sharpness, etc.For consistently reproducible printing results, the viscosity of theprinting medium must be as constant as possible.

A viscosimeter is used either to measure printing medium viscosity,or adjust the viscosity to a predetermined value. Printing mediumviscosity is usually measured in Pascal or Poises.

10.6 Wet coat thickness

Certain screen printing applications require carefully controlled andreproducible coating thickness. The solution is this simple device formeasuring the thickness of the wet ink deposit.

The instrument is rolled carefully over the freshly printed surface. Thecoat thickness in µm is then read off the scale at the point wherethe ink deposit ceases.



10.7 Grindometer for measuring particle size

The particle size of the printing medium must be at least 3x less thanthe mesh opening of the screen printing fabric. A test sample issimply smeared over the grindometer. The particle size in µm is readoff the scale at the point where the smear ends.

10.8 Recording thermometer/hygrometer

Printing media, printing stock and printing frames are all stronglyaffected by temperature and atmospheric humidity. Monitoring airquality in the printing room can avoid many problems, or helppinpoint their cause.


11.1Recommended choice of fabrics

11. Recommended choice of fabrics


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gnitnirprevoreuqcaL 04-09 43-021 43-041 13-081mc/L82otpusneercsenot-flaH *43-021

*13-021*43-051*13-051

*43-041 *72-081

morfsneercsenot-flaHmc/L45otmc/L82

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*13-561*72-561

*43-051*13-561

*13-051*72-081

sdraobtiucricdetnirPksamredlosyalrevO 041-21 052-81

ksamredlosevitisnesotohP 021-42 55-86etsapredlosTMS 07-23

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tsiserhctE *84-09 *43-021 *43-021 *43-041tsisergnitalP *84-09 *43-021 *43-021tnirpgnikraM *43-021 *13-041 *43-041 *13-051

sdraobyekenarbmeMreuqcalnoitalusnI 001-63 55-86

etsapevitcudnocrevliS *07-84 *55-86evisehdA 07-84 84-77

reuqcallarutcurtsVU 84-77 13-561liofroceD *84-09 *43-021

swodniwtnerapsnarT 43-021

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Continued

11.2 Recommended choice of fabrics ©Copyright by SEFAR, 2/1999


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refsnartlositsalP 08-84 *43-021cirbaflasrevinU 55-84

tceridlositsalP *46-45 *43-021enot-flah,sknitnemgiP *46-16 *04-001

refsnartnoitamilbuS *84-77 *43-021smliftalf,selitxeTscirbafrocédyvaeH)mined,htolcyrret(

081-81 07-84

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*03-051*03-561*03-081

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Continued

Continued

11.3Recommended choice of fabrics©Copyright by SEFAR, 2/1999


sknilanoitnevnoC


***skniVU

cirbafnolyN**0001AP


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08-34*84-77

46-45

55-86*04-001

*04-001:senihcamtnemesumA

slenapedisdnatnorF *84-77 *43-021:srinevuoS

serutcipdlareH 46-45 *04-001:gnisitrevdA

dnatnaruatser,letoHsngispohs 08-34 *84-77

).cte,scitsalp(stcejbOsaeraeuqapO *04-001 *43-021 *43-041 *13-051 *83-001 •*53-051

senilenifdnaenot-flaH *43-021 *72-561 *13-051 *72-081 *53-021 •*03-081ylno1:1=WP:skniVU)•

Continued

Sefar Inc.Printing DivisionP.O. BoxCH-9425 Thal SGSwitzerland

Tel. ++41 71 886 32 32Fax ++41 71 886 35 91E-mail [email protected]

http: //www.sp.sefar.ch

Order n° 901CHF 30.–

Screen Printing Hb

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