PP-403 L-02 Printing Processes

8/11/2019 PP-403 L-02 Printing Processes

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Intaglio: The image consists of tiny indentations or

wells in a plate or cylinder that hold the ink; the non-

image areas are scraped clean with a wiper or metal

doctor blade that contacts the smooth outer surface of

the plate or cylinder. This is the principle of gravure,

copper etching, and steel die engraving.

Planographic:

The printing and nonprinting areas are

essentially on the same plane but differ in their

receptivity to ink; the printing areas are ink receptive

and the nonprinting areas are ink repellent. This is

the principle of lithography, collotype, and driography.

Lithography is based on two principles:1. The greasy ink used will transfer to any dry

surface.2. It will not readily mix with water.

The transfers easily to a dry metal surface but not toa wet surface. As seen the right hand side picture, the water flows out, forming a thin film thatadheres strongly to the metal, preventing the ink from contacting the metal surface.

Lithographic image carrierThe image areas - the ship - have been made water-repellent; the non-image areas water-accepting.

A damping roller passes across the plate fromleft to right, leaving a water film adheringstrongly to the non-image areas, but only water

droplets on the image areas.

When an inking roller passes over, it inkins-upthe image areas, leaving clear the non-imageareas that are protected by the water film.

On a lithographic press, the plate must be dampedduring each revolution in order to maintain a water film over the non-image areas so that theyare kept free of ink.

Stencil or Porous

It is a simple method of separating image and non-image areas, The stencils shown on the left

are familiar to most of us. The image areas have been cut out of the background, giving open

areas through which ink can be applied to produce a print. A rather stylized reproduction is

made due to the connecting links needed to hold the design together in the stencil.


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Screen process solves the problem of theconnecting links by supporting all parts of thestencil on a fine mesh material stretched tightlyacross a frame, as shown on the right side. Ink ispoured into the frame, then spread across it andpushed through the open mesh areas with asqueegee. Screen process is the only type ofprinting that prints through the image carrier,rather than from it.

A metal, silk, or nylon screen or a fibrous material

is used on which the nonprinting areas are

blocked so that the ink goes through only the porous areas, which represent the image.

There are five major commercial printing processes based on the above principles: letterpress,

flexography, gravure, lithography, and screen-printing.

DIRECT PRINTING AND OFFSET PRINTING

In printing, ink may be transferred directly to

the paper, as in direct printing, or it may be

transferred from the inked plate to an

intermediate cylinder covered with a rubber

blanket, which then transfers it to the paper

or other substrate. This latter method of

indirect ink transfer is called offset printing.

Letterpress, gravure, and screen-printing are

essentially methods of direct printing, butthey can also use the offset principle.

Lithography, on the other hand, is almost

exclusively offset printing, so much so that

the term offset has become synonymous with

lithography. The offset principle provides a number of important advantages when it is used in

lithography and to some extent in letterpress and gravure. These advantages include the

following:

The printing surface of the blanket, made of rubber, conforms to the irregularities in the

paper surface. Less printing pressure is needed. Print quality is improved making

possible halftones of good quality on rough-surfaced papers.

Paper does not contact the metal printing plate, thereby reducing the possibility of

abrasive wear from direct contact of the paper with the plate and increasing the life of

the plate.

Speed of printing is increased. Speeds over 10,000 impressions per hour are possible

on sheet-fed presses and over 40,000 per hour on web presses.

The image on an offset plate is "straight" reading instead of "reverse" reading. This

facilitates both preparation of the plates and correction of errors.

Less ink is required for equal coverage. This speeds up ink drying, reduces the tendency

for the ink to smudge or set-off (from the front of one sheet to the back of the next sheet)

in the delivery pile, and reduces problems with ink trapping.


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In offset printing, both sides of paper can be printed simultaneously in a single nip. The

process is called blanket-to-blanket printing or perfecting. Simultaneous two-side printing

is not possible in such direct printing processes as letterpress, flexographic, and

gravure, because of the uneven pressure generated by pressing two profiled plates

together. In offset printing, there is little height difference between the printing and non-

printing areas. The even surface of offset cylinders aids two side printing.

PRINTING PRESSES

Presses may be flat-bedor rotarytype. In the flat-

bed press, the image carrier (type or plate) is

mounted on a flat bed that may either be horizontal

or vertical. Flat-bed presses are used chiefly for

letterpress, screen-printing, and for proofing. In a

rotary press the plate is mounted on a cylinder that

rotates during the printing cycle.

Presses may also be classified as sheet fedor web

or roll fed. On a sheet-fed press sheets of paper are

fed into the press one at a time, the impression is

made, and the sheet is removed or delivered into pile

or a supplementary inline operation. On a web press

paper is fed from a roll, and printing is done

continuously during each revolution of the rotating

cylinder as the paper passes under the inked plate or

blanket. The printed paper eventually feeds into a

sheeter, which cuts it into sheets, or into a folderwhich folds it into signatures. All of the major printing

processes can be used on sheet-fed and web-fed

presses.


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Printing unit of an offset lithographic press

The unit has three main cylinders: the platecylinder; blanket cylinder; and impressioncylinder. The inked image is transferred from theplate to the blanket, and then on to the paper.

The substrate passes between theblanket and impression cylinders, but nevercontacts the plate.

The plate must be damped before it is inked, and

kept damp throughout the run, to desensitize thenon-image areas. As shown, the plate passesunder a set of dampers beforeit passes underthe plate inkers.

The damping unit applies an aqueous solution called a fountain solution - to the plate at eachrevolution. Fountain solutions are mainly water,with small quantities of additives to enhance thewater-receptivity of non-image areas, preventthe growth of bacteria or fungi, and keep the

solution at the correct acidity.

High surface tension of the water causes the fountain solution to form beads when in smallquantities or thick films of solution when in larger quantities. To combat these effectsoccasionally alcohol is added to the fountain solution to lower its surface tension. In doing this,the fountain solution is able to spread out over the non-image areas of the plate in a muchthinner film. As a result, the minimum amount of fountain solution is needed to desensitize thenon-image areas of the plate so that any adverse effects of water on the ink or paper areminimized.

Inks for lithography have to be attracted to and remain on the image areas and mix with the

fountain solution to form a water-in-ink emulsion. Inks used for lithography therefore tend to betacky.


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Lithographic inks differ from other paste inks in that they have to split twice: from plate toblanket and from blanket to paper. Therefore a thin ink film is deposited using the lithographicprocess. To maintain the required color strength or saturation, litho inks are more highlypigmented.

Inks used for lithography dry by a number of means:1 By oxidation, which on some presses can be accelerated by warming the sheets under a

bank of infrared lamps.2 By curing special inks under a UV rich light source.3 By heatset, drying the inks by passing the printed web through a hot, turbulent air, oven

which dries the inks, which are then set by cooling the web.4 By using an absorbent paper and relying on the ink being absorbed into it. This form of

drying is known as coldset drying.

Gravure Printing Process

The picture on the right shows a gravure printing imagecarrier, a copper plated, steel cylinder that is etched orengraved in the image areas and chrome plated, somaking it durable enough to withstand very long runs. Itis positioned in the ink duct of a gravure press.

Gravure can be classified by the two types of cell that are etched or engraved into the cylinder.They are: conventional, where the cells remain constant in area but vary in depth, or, halftone,where the cells vary in area as well as depth.


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The most commonly used method to produce the recessed image is to engrave each individualcell using a diamond tipped stylus. The recessed image area is produced by scanning reflectioncopy to create a digitized signal that drives the stylus into and out of the copper plated cylinder.Inverted pyramid shaped cells are produced, varying in depth and area depending on the signalsent to the stylus. The cylinder is rotated at great speed while the stylus traverses along itslength, to slowly build up the image areas. The cylinder is then chromium plated to make it

durable.

Traditionally there were many stages in producing a gravure cylinder. Following copper platingof the steel or iron cylinder, a light sensitive, acid resist was applied to it. The design was thentransferred to it photographically and broken up into tiny cells using a gravure screen. Theresist was then washed away in the image areas and then the cylinder surface was etchedusing a series of varying strength acids. The resist prevented the cell walls from being etchedaway so that a screened, recessed image area remained. Again the cylinder was chromiumplated to increase its useful life.

Copper plate cylinders are chromium plated for extended life and will produce runs in the order

of several million.

The ink used for gravure printing is very fluid, flowing likewater. Because the cylinder rotates in the ink duct, and along chain of inking rollers is not required, a solvent-basedink can be used that dries in seconds by evaporation of thesolvent, aided by passage through a heated drying tunnel.

Gravure inks are therefore suitable for printing at high speedon non-absorbent materials such as plastic films and metalfoils.

The intensity and cost of the cylinder preparation makesgravure more economic for medium to long run work - and itis particularly suited to long run multi-color printing.

Printing unit o f a flexographic press

The ink is contained in the ink duct orfountain. There are only two inking rollers:a rubber covered fountain roller whichrotates in the fountain, carrying ink up to atransfer or forme roller, which in turn inks theplates or stereos mounted on the platecylinder. The transfer roller is called an'Anilox', and is engraved or etched with anoverall pattern of small cells, which act asink-carrying reservoirs. The amount of inkcarried to the plate by the Anilox roller iscontrolled by the number and size of these

cells. Ceramic Anilox rollers and doctor blades have been introduced for greater control of inksystems. Water-based and UV cured inks have been introduced to enhance image quality.


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Because the inking system has been reduced to two rollers only, it is possible to use fast dryingsolvent-based inks, similar to those used in gravure. Thus the process is eminently suitable forpackaging work on non-absorbent plastics, films and foils.

The function of the impression cylinder is to support the web of printing stock, and bring it intocontact with the photopolymer plate with just enough pressure to transfer ink from the raisedareas to the web.

Since the ink is very fluid, and the plate has a raisedimage area, there is a pronounced tendency for theink to be squeezed out round the edges of the image.This gives a distinctive rim around the printed image,called 'squash'. It is essential to print with the lowestpossible pressure to minimize the effects of squashand produce good quality prints. Cushion backadhesives are used to reduce the effect of plateimperfections, and enable minimum impression to be

achieved more easily, so relieving the inherent characteristic of squash.

An image carr ier of screen pr inting

The open areas of a stencil form the imageareas. The stencil is supported on a mesh.This enables images such as the centers of theletters 'b' and 'a' to be printed without the needfor linking bridges to hold the non-image areaof the stencil.

During printing, ink is forced through the openareas of the mesh using a flexible blade called

a squeegee.

Meshes are made from dimensionally stablematerials including nylon, polyester andstainless steel and are stretched tightly acrossa frame. The thickness of ink film that can beprinted is largely controlled by the thickness of the mesh material. Various types of mesh areavailable for different kinds of work.

Stencils can be produced manually or photographically; the simplest method being hand cutstencils. These are produced by cutting the non-image areas out of a special stencil film whichis then attached (normally by ironing) to the mesh. Halftone and fine detail work is reproducedusing either direct or indirect stencils. A direct stencil is produced by applying a light sensitive

coating onto the mesh which is then exposed to UV light through a film positive. The exposedcoating then hardens to produce the stencil and the unexposed areas are developed awayleaving open, image areas. Indirect stencils work in a similar way except the photopolymer isexposed and developed out on a carrier sheet before being applied to the mesh.

Screen printing is one of the most adaptable printing processes as it is able to print a widevariety of inks onto a range of substrates, including preformed objects such as bottles andcontainers. Most screen printing is associated with textiles, posters, showcards, generalprinting, and fabricated materials such as glass, ceramics and plastics.


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A f lat bed screen pr inting unit

In this arrangement, the screen frame remains

stationary, while the squeegee moves backwards and

forwards. The substrate moves on carrier belts under

the screen plate.

A type of screen pr inting press - the cy linder

press

In a cylinder press, the squeegee remainsstationary during the printing cycle, while thescreen moves backwards and forwards.

The printing unit consists of an impressioncylinder, a carriage for the screen, and asqueegee. The stock to be printed is clamped bygrippers at the leading edge of the cylinder andheld firmly to its surface by vacuum. As the

cylinder and screen move in unison, the fixed squeegee pushes the ink through the mesh on tothe substrate being carried between the cylinder and the screen to produce the print.

Fully automatic flat-bed and cylinder presses are used for long run work. Speeds vary

according to the type of work and machine, with cylinders generally being faster. The printing ofrigid materials is not possible on most cylinder presses.

Screen printing on prefabricated articles

Screen process is used extensively for industrialand packaging printing on a range of prefabricatedarticles - containers, bottles, knobs and dials,equipment and machinery, sport and leisure holdalls, toys, and a range of clothing including T-shirtsand designer jackets.

Many articles are printed on semi-automaticpresses, and there are few limitations on size orshape. Objects ranging from oil drums down topens and lipstick cases can be dealt with.

The picture shows polythene containers being

printed on a semi-automatic press. The principle is

that of the cylinder press, with the container taking the place of the cylinder. The squeegee is

fixed, while the screen moves to and fro. A gear mechanism turns the container in a

reciprocating action in time with the screen. Thin walled bottles are inflated to give a stable

printing surface.


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Special, screens and jigs can be designed to print on peculiar shaped objects, such as cupswith handles, ovals, tapers or recessed panels. Print heads are available that can be bolted onto and over any automated process line. In some cases printing may form part of a completelyintegrated production line from polythene bottle manufacture to filled bottles. A complete linewould comprise a blow-molding machine, pre-treatment unit, printing units and a filling, capping

and packing station.Screen process can compete in many areas with other printing processes. Its ability to printvery thick films and to print on any shape and size of article also gives it areas of applicationoutside the range of other processes.

Screen process can print on almost any shape and size of prefabricated article.

Comparison of thickness of ink filmsprinted by different printing processes

Screen process can deposits ink-films 30times thicker than that by the lithographicprocess and six times thicker than that bythe gravure.

Screen inks may dry by oxidation forconventional surfaces, but with somesubstrates the process of drying will beextended and will require the material tobe racked.

These 'buttery', opaque inks are ideal forprinting bold and solid colors well suited

for posters and advertising displays.

Screen printing can deposit an ink on most surfaces, including those that are rigid and flexible.Screen also prints on a range of textiles.

Recent developments include the introduction of water-based inks, which in common with otherprinting processes, such as flexography, reduce toxicity. Improved stencil production has alsoenhanced the quality of image for screen printing and it is now possible to print fine detail, four-color process printing and complex images.

The dot structure for halftone printing still remains less defined than for offset or gravure, butimprovements in mesh types have increased the resolution considerably.

Basic characteristics of image carriers

In flexography and letterpress, both relief processes, the printing surface is raised above thenon-printing area. Flexography uses flexible, deformable relief plates made from eitherphotopolymer or rubber. Letterpress uses hard, non-deformable photopolymer plates.

In lithography, a planographic process, the image and non-image areas are carried on the samesurface of the image carrier. Each area has a different tolerance to water, the non-image areasaccept water to act as a barrier to reject ink and the water repellent image areas reject water toleave them dry enough to accept the ink. An area that can be dampened with water will bemade repellent to ink and is said to be desensitized, while an area that remains undampenedand accepts ink is said to be sensitized.


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In gravure, the image areas are recessed belowthe surface of the image carrier. The imageareas are screened (divided into tiny cells) toprovide support for the doctor blade whichwipes the non-image areas clear of ink.

In screen printing, the image areas are the openareas of a stencil that is supported on a meshmaterial which is stretched tightly across aframe.

The profile of the printing surface gives a

characteristic appearance to prints from each

process, and imposes specific demands on the

nature of the ink, the requirements of the

printing stock and the construction of the

printing press. Both directly and indirectly, it will

determine which process is most suitable for a

particular job of work.

OTHER PRINTING PROCESSES

Collotype(Also known as photo-gelatin):

The process is similar to lithography in some respects but differs in the nature of plate used. It is

the oldest planographic printing method capable of reproducing continuous tone images without

halftone photography. The process uses photomechanically produced image carrier. Collotype

plates are metal or glass sheets coated with a gelatin that is made light sensitive with

ammonium bichromate. The gelatin receives the ink for printing.

The variation of tone in the printed image is achieved by varying the ink receiving properties of

the gelatin by its exposure to light. The plate is exposed through a positive image. The areas

exposed to light become hard and tend to resist the ink. The darker areas are less hard andabsorb more ink. Thus a variation in the darkness or lightness of the ink results. The plate is

without a grain or halftone pattern. The softness of the gelatin gives the plate poor wear

qualities so the process is practical only for short run work where detail is important. Direct

collotype allows short plate life (about 5000 impressions), while the offset collotype is not

generally used. The process is commercially insignificant today.

Screenless lithography

The printing process, in limited use today, is based on a photo-responsive resin coated on a

base plate. The resin is initially strongly water absorbing but at the same time insoluble andwater resistant. Exposure to light causes the resin to become oleophilic in proportion to the


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degree of exposure. The method provides a high quality reproduction medium ability to

deposit exceptionally strong layers of ink reproduction of fine art subjects/ prints of paintings. It

allows easy color correction (masking). The image reproduction is through reticulation, i.e.,

random pattern of image elements that are much smaller than the halftone dots. Varying ink

receptivity allows variation in the thickness in the deposited ink film

Letterset

Hybrid of letterpress and offset lithography. The process was developed to provide

letterpress with the advantages of an offset configuration while eliminating the problems

caused by the dampening system in the lithography.

In package printing applications, it typically handles jobs that need an offset process but

cannot be done by lithography. Letterset provides low-odor printing for food cartons and

confectionery wraps that require that require the use of water miscible inks.

It is also used to print plastic bottles, caps, and tubes and and some metal packages such

as toothpaste tubes.

Hot stamping

Hot-stamping involves the use of a heated relief plate to transfer ink and release waxes to

labels, paper, film, bottles, boxes, and lids. Hot-stamping highlights certain areas in the print,

giving it sizzle or sparkle. Metallic colors are typically used in this process. Hot-stamping has

limited use because of the low speeds and high costs involved in printing large areas.

Heat transfer

Heat transfer printing resembles heat stamping. In this method an image is transferred from a

preprinted image on a carrier web to a container or package, usually a bottle, tube, vial, or lid.

The image on the carrier web can be printed by gravure, flexography, or silk screen. The quality

of the art work is controlled by the process used to print the image on the carrier web. Heattransfer requires special machinery, and its expense limits its use in packaging applications.

Decalcomania

Decalcomania is another method of transferring an image printed on a carrier substrate to a

package. The printed design is released from the backing with water, solvent, or heat. Typically

it is used on bottles or china. As in heat transfer, the system is slow. The artwork is controlled by

the printing process used to create image on the carrier web.

QUESTIONS

1. Describe mechanism of image formation in the following printing processes:

Letterpress, Flexography, Lithography, Gravure, Screen Printing, Mimeography, Daizo

Process, Electrostate, Ink-Jet Printing, Thermographic Imaging, No Carbon Required

(NCR) Copying.

2. Describe the techniques of plate making for: Letterpress, Flexography, Lithography,

Gravure, and Screen-printing.

3. Differentiate between conventional gravure and variable-area variable-depth gravure

printing.

PP-403 L-02 Printing Processes

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