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

Vacuum forming involves pushing a mould into aheated TP sheet and evacuating the air frombetween mould and sheet, so that atmosphericpressure pushes the sheet onto the mould, makingthe forming.

There are many different kinds of vacuumforming machine available from small, manually

operated units to fully automatic, in-lineproduction machines, but no matter what thedifferences between units might be, they are allvariations on the same theme, ie:

1 The sheet is clamped in place on a heat proofair-tight seal.

2 The heater system moves under or over thesheet, or vice versa, and begins heating.

3 Once the sheet has reached it’sthermoforming temperature the vacuumpump is energised.

4 The heater moves back to it’s restingposition (or the sheet moves from theheating position to the mouldingposition).

5 The mould, mounted on a movingplaten, moves up into the sheet whichdrapes over it.

6 Once the platen reaches the top of it’sstroke, the space between the undersideof the sheet and the upper surface of themould forms an air-tight pocketconnected to the vacuum pump, whichthen pumps air from between the two.This removes air which is preventingatmospheric pressure from pushing thesheet down over the mould.

7 As the sheet cools it contracts, grippingthe mould. Hence the next step is toreverse the airflow, using air pressure toforce the forming off the mould andprevent it sticking, this step has becomeknown as the ‘blow cycle’. Blow cyclesare short - just long enough for theforming to release from the mould - andimmediately followed by anothervacuum cycle.

8 Vacuum/blow cycling continues untilthe sheet is rigid once more. At thistime, the vacuum is switched off or the

mould lowered and the forming isreleased from the clamp.

3 .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 -

Vacuum Forming 3

13

4

5

6

7

8

2

1 Heaters2 Sheet material3 Clamp frame4 Reflector

5 Material seal6 Mould

7 Platen seal8 Platen

REMOVE HEAT4

3

5

SWITCH ON VAC PUMP

ELEVATE PLATEN & MOULD

VACUUM PUMPFLOW REVERSEDBYCHANGEOVER VALVE FORMOULD RELEASE

1-1.5mm (.040-.060") EVACUATION HOLESIN THE CORNERS OF CAVITIES

Expanded mesh to allow airdistribution under base board

Base board

Mould

The components of a 725FLB Vacuum Former

Steps 3-5 on a 725FLB

Steps 7 & 8 on a 725FLB

3 .2

THE HEATER SYSTEM

Ceramic heaters are possibly the most commonamongst vacuum forming machines. They consistof coiled resistance wire elements set in mouldedchina clay. Available in round, square orrectangular shapes, they can be flat (for maximumproximity) or curved (to provide a parabolicreflector which radiates more effectively).

The mainadvantage ofceramics is thatthey radiate longwavelength heatwhich is readilyabsorbed byTP’s.

They can run atvery high power outputs but the normal level forhigh performance vacuum forming is around22.25 kw/sqm (2 kw/sqft).

Their only drawback is their high thermal mass,which means they take some time (10 - 15minutes) to warm up and are slow to respond toenergy regulation adjustments.

Quartz emitters are also used in vacuum formingand like ceramics, they have a coiled resistancewire element but housed in a quartz glass tube,rather like a bathroom heater. With much lessthermal mass there is hardly any warming uptime and the medium wavelength heat is moreresponsive to reflectors so that a greaterpercentage of heat can be projected downward.

The drawback of quartz emitters is that mediumwavelength heat is not so easily absorbed by TP’sas the long wavelength heat of ceramics.

HEAT ZONES

Heaters are arranged in a reflective hood ingroups or clusters which can be controlledindependently. This allows the operator tocontrol the distribution of heat over the sheetwhich is useful, particularly for complex moulds(eg. fridge linings).

To determine what each individual heat settingshould be, a grid is drawn on a sheetcorresponding to the pattern of heat zones. Aforming is taken and where there is excessthinning, heat output is reduced. Equally, heatoutput is increased in zones where definition ispoor.

DOUBLE S IDED HEATING

For material thicknesses over 5 or 6 mm, single

sided heating is usually too slow, and a doublesided system is used. In such a system the sheetwill normally move to the heating station, whichis pressurised, so that the sheet can be supportedby air as it sags to prevent it from sticking to thelower heaters.

THE PLATEN SYSTEM

Lifting the platen is an important part of theprocess as it needs to be done promptly and asquickly as possible so that forming takes placebefore the sheet begins to cool and go rigid.

On smaller machines it is done manually with alever. Bigger machines need pneumatic,hydraulic or other mechanical systems becausethey have bigger, heavier moulds and bigger,thicker sheets which offer more resistance.

THE VACUUM SYSTEM

Vacuum pumps, appropriate to the size of sheetbeing formed and the volume of air beingevacuated (approximately -0.83 bar [25 inHg])should be an integral part of the machine.

The type of pump is important because most TP’sgive off vapours when they are heated which cancorrode the delicate components found in somepump units, leading to reduced performance andultimately failure.

THE BLOW CYCLE

When vacuum forming manually, it is best tomake the first blow cycle as soon as the materialhas been drawn completely over the mould - itwill still be soft over much of it’s area but, if ashort blow cycle, to break the material’s grip, isfollowed by another vacuum cycle, the materialwill draw back down onto the mould andsubsequent blow cycles can be shorter. It’simportant to keep the blow cycles as short aspossible because they will put stress into thecorners of a mould if they are done for too longonce the material is rigid.

COOL ING AND TH INN ING

As soon as the heated sheet makes contact withthe mould, it will start giving up it’s heat to themould and becoming rigid again. This is why it iscommon to find vacuum formings with deepermaterial thickness at the top than the base; thematerial at the top cools first and stops stretching

REFRACTORYMATERIAL

RESISTANCEHEATING SPIRAL

FIXING CLIP

POWER LEADS

Section through a ceramic element

because it makes contact with the mould beforeany other part of the sheet. The material at thebottom of the mould is the last to cool so it’sstretching for longer than other parts (see HeatZones earlier).

THERMOFORMING MACHINE OPTIONS

A U T O M A T I O N

Any or all of the vacuum forming functions canbe automated and indeed they are for mostcommercial production purposes along with someothers, such as automatic cutting, stacking orcounting.

For example, vacuum formed pvc sandwichpackets are made 24 or so at a time on a largethermoformer. The material is fed from one roller,over the machine and onto a scrap collectingroller. The fixed heater heats an area of sheet atthe same time as one batch of packets are beingvacuum formed and the previous batch are beingpunched out ready for stacking and collection.Each cycle takes only seconds to complete andone machine produces thousands of packets eachday. The heaters may not be set to work at theirfull capacity - to allow enough time for thethermoforming to be completed.

VACUUM/PRESSURE FORMING

The pressure exerted by ordinary vacuumforming is limited to atmospheric pressure which,for some applications does not produce enoughdefinition. In such cases vacuum/pressureforming may be used. This technique involves amachine with a sealed pressure chamber over thetop of the forming into which compressed air ispumped during forming thus increasing thepressure drawing the material down onto themould.

PLUG ASSIST VACUUM FORMING

The depth of a forming is limited by the ability ofthe material to stretch into a long deep shape,

such as a cup. Ordinary vacuum formers canonly really vacuum form a depth of about half thediameter of a cup - any deeper and the material atthe bottom becomes so thin as to be unusable. Socups and yogurt pots are made with machinesthat have a male mould which pushes the heatedmaterial into a female mould before vacuumingcommences. This distributes the material moreevenly over the mould thus achieving deeperdraws.

P R E - D R Y I N G

As we’ve already mentioned TP’s absorb moisturewhich can lead to problems when vacuumforming -particularly Extruded Acrylic, ABS andPolycarbonate; these materials must be pre-driedbefore heating.

O V E N

Materials can be dried by placing them in a fanassisted oven at 40˚C (70˚F) below the start oftheir thermoforming temperature range for about1-2 hours per mm of thickness. Polycarbonategenerally takes longer than ABS or ExtrudedAcrylic and times will vary from batch to batch -depending on how much moisture has been

absorbed due to the conditions that thematerial has been stored in.

HOT BOX

Alternatively, you can construct asimple hot box using plywood and alow power heater such as agreenhouse heater or even light bulbs.If sheets are stored in the hot box then(as long as they’ve been in there formore than a day or so) they willalways be ready to use.

3 .3

1. Heating

Material roll feed V

Scrap collection

2. Vacuum forming 3. Punching

Productcollection

Schematic of a sandwich packet thermoformer

! SAFETY NOTE !For safe operation ensure that heaterpower will not exceed 60°C (140°F)

HEATERSVENT HOLESMESH TO PROTECT

HEATERS

Lid with vent

Simple Hot Box construction

3 .4

MATERIAL REFERENCE CHART

T R I M M I N G

LOW VOLUMES

Special machines are available that are designedspecifically for cutting out vacuum formings.

Our Profile Router 145, suitable for material up to3mm thick, comprises a slot drill mounted in awork table with a guard/fence that allows you tofollow the profile of a forming accurately whilstprotecting against injury from the cutting tool.

The whole table pivots sothat you can cut into thecorners of a forming orcut leaving a flangewhich is useful for gluing.

Thinner materials, up to1mm can be cut with anart knife and this processitself can be madesomewhat easier if someform of cutting guide orplug is used.

HIGH VOLUMES

Many mass production thermoformers aredesigned with an integral trimming or cuttingdevice.

Small, lightweight items such as packaging aresimply punched out by a die mountedimmediately after the vacuum press.

Yogurt cups or tubs designed to take lids aretreated specially with a rolling device as they arecut which turns over the top to form the lip.

Heavy gauge products can be cut out using handheld routers, hole saws, band saws or guillotine.

A five axis CNC router is probably the fastest andmost efficient form of trimming and finishing.These routers not only cut out the basic shape butcan also cut out windows from cars or boats andengrave features, lettering or details.

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.Clarke & Company (US) Inc.P.O. Box 470936, Celebration, FL.34747Tel: 1 800 676 7133 Tel2: 1 407 566 0755 Fax: 1 407 566 0756Email: m.m.roberts@usa.net Internet: www.crclarke.co.uk

FORMING

FORMING

KNIFE

METALPLATE

Using a plug to help cutting out

Material Pre-drying Definition Forming temp.˚C(˚F)

Polystyrene NO Good 120 (250)Extruded acrylic YES Good 160 (329)PVC NO Fair 130 (265)ABS YES Good 159 (300)Polycarbonate YES Good 200 (400)Polypropylene NO Very good 180 (350)Polyethylene NO Fair 120 (250)PETG NO Good 120 (250)

Using a trimming guide