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I N T R O D U C T I O N

Local line bending thermoplastic sheets has threestages:

1 Heating the sheet along the bend line to itselastic window using a strip heater.

Note: The best bends are produced whenmaterial is in its elastic state; the materialretains enough tensile strength to maintain itsform, producing uniform, aesthetically pleasingbends.

2 Folding the sheet, normally by hand, taking

care to avoid contact with the heated area.3 Securing the sheet in its folded position to

cool, normally using some form of coolingjig.

H E A T I N G A T H E R M O P LA S T I C S H E E T F OR

LI N E B E N D I N G

As already mentioned in part 1, there are 3 typesof strip heater commonly used for line bending:

1 Hot wire heaters

2 Radiant element or inconel sheathedelement heaters

3 Contact heaters

In this section, we will look at each heater type ingreater detail.

H O T W I R E H E A T E R S

Suitable for all thermoplastics, including PC, castand extruded PMMA, PVC, PETG, ABS, PS,HDPE and PP.

When current passes through a resistance wire,every point along the length of the wire emits thesame amount of heat.

If the wire is tensioned between two points then itwill be straight between those points. The resultis an extremely straight and uniform heat source.Add to this the ability to arrange wires alongsideeach other at distances as close as 1mm and youhave the most ideal and versatile heatingtechnique for bending thermoplastic sheets thathas been developed to date. Lines as long as 3M

can be heated with a negligable temperaturedifferential along their length.

The amount of heat emitted can be controlled (by

the voltage) so thatmaterial can be

placed very close tothe wire withoutburning. This hastwo benefits.

Firstly, a verylocalised heating isachieved - similar tothat produced bycontact heating butwithout the contact.

Secondly, because most of the heat beinggenerated is being used to heat the plastic, the restof the machine remains relatively cool, removingthe need for reflectors or water cooling. (Note:water cooling can be used on a hot wire heater toreduce bowing along the bend line)

Another advantage for clamping heaters is thatthe material can be clamped where it is cool,eliminating marks and blemishes.

Heat band widths can easily be controlled by

arranging different numbers of wires along a bendline. In material over 3mm for example, bendingtimes can be reduced and aesthetic propertiesenhanced if a wider heat band is applied to theoutside of the bend. This can also help whenmaking two opposing bends simultaneously in thesame sheet.

R A D I A N T EL EM E N T O R I N C O N E L

S H E A T H E D E L EM E N T H E A T ER S

Suitable for thermoplastics with a large elasticwindow, such as cast PMMA, PVC, PETG or ABS.

The heating element can have hot and cold spotsalong its length because of the way it ismanufactured and the way it is fixed (brackets actas heat sinks).

When a sheet is heated parts of the bend line willbe hotter than others. With a large elastic windowthis doesnt matter because the material will beelastic at say, 120C as well as 150C and fold justas well when parts of the bend line are at bothtemperatures.

If, however, the heating characteristics are mainlyplastic, then some areas of the sheet will be readyto fold (ie. elastic) when others are too cool. By

2 . 1

I N T R O D U C T I O N T O T H E R M O F O R M I N G -

Local Line Bending 2

A typical hot wire heatingconfiguration

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the time the cool areas are hot enough to bend, theareas that were previously hot enough, will havebecome too hot (ie. plastic - or even worse,degraded) and deform when folded, spoiling thelook of the bend.

The elements generally consume a lot of powerand produce a lot of heat which has to be directed

and controlled either by reflectors or water-cooledwork supports (or both), to prevent too much ofthe sheet being heated.

Radiant elements can be fragile and expensive toreplace.

CONTACT HEATERS

Suitable for thermoplastics with a large elasticwindow, such as cast PMMA, PVC, PETG or ABS.

The material is heated by a blade or blades withintegral heating elements, either mounted ingroups on a machine or built as an individual andportable heating sword.

Contact heating is particularly useful for very thinmaterial because it is very accurate - the heat goesexactly where you put it. Unlike thicker material,where a wide heat band is required, on thinmaterial you only want to heat an area one or twomillimetres wide and this can be governed veryaccurately by the section of the contact blade.

Generally we would say that only elastic materials

are suitable because the blades will have atendency to leave slight grooves in the sheet.Elastic material will recover its original shapeusing its plastic memory before it cools, whereasmaterial in its plastic state will not recover andwill be permanently blemished.

Sometimes thiscan be anadvantage, as inthe case of PP

and PE. Toimprove thespeed of

bending thickPP and PEsheets, a grooveis sometimesrouted on theinner facebefore bending.A V shaped

contact bladeapplied withenough forcewill form agroove during

heating - removing the need for routing andspeeding up the process even more.

When the sheetis folded, thematerial thathas beendisplaced either

side of the Vshaped contactblade will fusetogether,forming whatlooks like aweld and increasing the strength of the bend (thisweld can be trimmed off with a bead trimmingknife immediately after forming if it is notwanted).

Note: Some form of PTFE coating is essential when

contact heating PP to prevent the material sticking tothe blades.

B E N D C H A R A C T E R I S T I C S

The style of a bend, depends mainly on the heatband width, which should be adjustable.

The heat band width determines the radius of thebend, and can be roughly calculated using theformulas below:

90 bend: Heat band = 2 x inner radius [inner

radius must be greater than sheet thickness]180 bend: Heat band = 4 x inner radius [inner

radius must be greater than sheet thickness]

There is no limit to the radius you can give a bendbut, large radii only work well with materials thatcan be folded in their elastic state.

D O U B L E S I D E D H E A T I N G

Heating a sheet from both sides can cut theheating time almost in half. Double sided heatingwith hot wire heaters also enables control of the

2 . 2

Top V blade or sword

Lower broadheating blade

C O N T A C T H E A T I N G OF P E & P P

Homogeneous weldand internal bead

Bend section produced

Sharpcutting

edge

Bead trimming knife

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bend characteristics and produce opposing bendsin single sheets simultaneously.

B E N D I N G T H E S H E E T

Folding is normally done by hand. There are stripheaters on the market that are equipped withfolding apparatus but these are expensive. Thefolding work of most fabricators involves complexdesigns and large volumes, so the pieces arefolded and loaded onto cooling jigs by hand.

The bend must never be touched when it is hot,even if you are wearing gloves. When it is hot,even if it is only in an elastic state, a sheet isvulnerable to pressure marks from hands andfingers. A hot sheet can be slightly tacky and dirt

and fibres can stick to the heated line.C O O LI N G T H E S H E E T

Most fabricators use wooden or composite boardcooling jigs, some of them designed to be loadedwith 50 pieces or more, held in place with soft

jawed clamps if necessary.

When designing acooling jig, it isimportant toremember that the

heated bend shouldnot come into contactwith any othersurface to avoid

blemishes.The beauty ofTP line

bending is thatonly the

bending line isflexible so infact, a jig can

be nothing

more than four nailsin a board that stopthe four corners ofan angled sheetfrom splaying apart.But most fabricatorshave more complexrequirements andinvest a good deal

of timedesigningjigs that willdo the jobcleanly andefficiently.

Small

returnswhichwould beimpossibleto fabricate

by hand canbe made byplacing theheated edgeof the sheetinto a slot

and thenfolding therest of the sheet back against the jig.

D I S T O R T I O N

Distortion can be a problem for fabricators.Multiple bends in close proximity, bends innarrow sections or bends near the edges of sheetsall suffer from buckling during heating and

bowing along the bend line when cool.

The problem arises because the line of heated

material expands whilst the cool material eitherside of it does not.

During heating this can cause distortion whichcan be restricted by clamping the sheet.

When the sheet is folded, the bend line cools andcontracts after it has become rigid again. Thisproduces a bow as the bend line pulls the twoends of the sheet together.

C O U N T E R B OW I N G

The effects of bowing can be reduced in a numberof ways:1 Keep bends well spaced and away from edges -

if there is plenty of material either side of thebend it helps to restrict its movement.

2 . 3

U bendheating wire

configuration

Jiggle bendheating wire

configuration

fin ished productfin ished product

upper heating wires upper heating wires

lower heating wires lower heating wires

material

heated area

coolingjig

A d edi cat ed U bend j ig accuratel ycontrols the outside dimensions

Adjustable protractor jig to set angles

Jiggle bend jig used for the manufactureof slat wall display products

Adjustable right angle jig for U bends,

L bends and tent style products

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2 Use minimum width heat lines.3 Use water cooled heater beams to reduce the

heated area.4 Clamp in a straight cooling jig and anneal.5 Cool in a counter bowed cooling jig.

A counter bowed cooling jig incorporates a bow inthe opposite direction to the one expected in the

material, the stresses induced in the bend line due

to heating should pull the bend straight whenremoved from the jig.

It is best to establish counter-stress measurements

by experiment - a good place to start might be tomeasure the bow on a piece of material bent on anormal cooling jig and take if from there.

When producing multiple bends in closeproximity there is a tendency for the stressproduced by each bend to accumulate andcompound the distortion which becomesprogressively worse (if the bends are done one ata time).

It is better to use a multiple bending heater to

make all the bends simultaneously so that thepiece only picks up the distorting stresses from asingle heating cycle.

P S & P P

As already mentioned, bending TPs in theirplastic state can lead to unsightly bends and bellmouth ends - particular problems for materialslike PS and PP - so following are a few things thatcan help.

Avoid large radius bends; keep the heat bandwidth to a minimum.

Heat from one side and keep the heating time asshort as possible so that a very small area of thetop face becomes flexible and hopefully, remainselastic.

Bend with the heated side outwards and a sharp,well defined bend with a minimum radius should

be produced. (PP can be bent like this but,bending it the opposite direction works just aswell, as a bead of material builds up on the inside

of the bend in the same way as if it had beencontact heated).

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A long shallow item with little depth of sectioneither side of the bend line to oppose stress

Ji g t o correct bow i ng

near sheet edge

apply pressure at ends ofsheet to counter bow

anticipated bow withoutcorrection

saw slot to suit material

pins or dowels

bell mouth effectat bend ends

Charact eri sti c spreading of bend ends w hen

bending a materi al i n it s plasti c stat e

Heat from one side only for t hin pol ystyr ene

bend withheated face on

the outside

C.R.Clarke& Company (UK) Limited

Betws Industrial Park, Ammanford, Carms. SA18 2LS.Tel: +44 (0)1269 593860 Fax: +44 (0)1269 591890Email: sales@crclarke.co.uk Internet: www.crclarke.co.uk

C.R.Clark e & Company (US) I nc.

4407 Vineland Road, Suite #D5, Orlando, FL.32811Tel: 1 800 676 7133 Fax: 1 407 648 1905