M A T E R I A L S

Moulds for vacuum forming are mostcommonly made from a timber product,aluminium or epoxy resin. Othermaterials such as steel, copper, brass,paper, card, glass, thermosets like‘Bakelite’ (Urea formaldehyde), plaster ofparis, dental plaster, clay or plasticine canbe used on their own or in combination.

Details can be added by mesh, perforatedsheet, wire and even sandpaper (which isused for providing surface texture onvacuum formed braille diagrams).

T IMBER BASED MOULDS

One of the best materials to work with isMedium Density Fibre Board (MDF) as it has nograin, is easy to shape, and is relatively stablewhen heated. It is easy to work into reasonablycomplex shapes using cutting and abrasive toolssuch as bandsaws or sanders, and other materialssuch as card or metal can be fixed to its surfaceusing adhesives. Other fibre boards or regularclose grained timber can be used.

Tools needed to produce a prototype mould canbe found in most carpentry workshops including:

1 Saws2 Chisels3 Gauge4 Drill bits5 Sanding block6 Tilt table disc sander7 Drill press8 Small high speed PCB drill such as a

‘Dremel’9 Band saw10 Sash clamps

The tilt table disc sander is particularly useful forquickly developing blocks with draft angles and radii.

_ _ _ _ _ _ _ _ _ _1. Prepare the base blocks

If necessary, assemble a composite block or blocksof MDF or some other timber or timber basedboard. Glue boards together with PVA woodglue, clamp together with sash clamps and cureovernight.

If you have an oven or a hot box (see 4: VacuumForming), you can improve the curing time.

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _2. Marking out

Get an accurate set of scale drawings to workfrom and if it’s a complex shape, break it downinto simple components that can be assembledlater.

Mark out or paste photocopies of the drawings ordrawing sections onto the composite block orblocks (use the same face for each part - usuallythe base - so that angles are generated from thesame datum).

_ _ _ _ _ _ _ _ _ _ _ _ _3. Shaping & assembly

Cut roughly to shape using saws and then finishthe shapes more accurately with the disk sander.Assemble the mould frequently to check onprogress.

If the mould endsup too long, cut itin half, remove aslice from one ofthe ends and gluethe two halvesback together.

If the mould endsup too short, cut itin half, add asection to one of the ends and glue the two halvesback together.

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _4. Adding detail

When you are happy with the mould’s basicshape, add detail using wire, mesh, sandpaper or

4 -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 -

Mould Making 4

PHOTO COPYPROFILE

WOOD BLOCKS

CARD STRIPS

EXPANDED MESH

First stages of mould manufacture

Tilt table disc sander

4 .2

any other heat resistant material that achieves thedesired result (metals can be glued to MDF usingLocktite 499).

Dimples and hollowscan be drilled into themould using a customshaped flat drill bit.

Off the shelf mouldingscan be used to makedecorative bases andsurrounds - mitre at thecorners and pin inposition.

_ _ _ _ _ _ _ _ _ _5. Mounting on a baseboard

Once the mould is complete it should be mountedon a baseboard. This is necessary for threereasons:

1. To hold down the mould during the blowcycle.

2. To locate the mould (or moulds) on the platenin the best vacuum forming position.

3. To prevent material stretching all the waydown to the platen tray causing unnecessarythinning.

The baseboard can be made from a timber basedproduct or metal and should be cut so that it fitsneatly inside the platen tray.

The mould is placed on the baseboard on shimms(double sided sticky pads work well) and screwedinto position from underneath. The shimmsprovide an air track around the bottom edge ofthe mould.

At least one hole should be drilled in the centre ofthe base board but, if evacuation is slow, you candrill more.

_ _ _ _ _ _ _ _ _ _ _ _ _6. Forming & modifying

Place your mould in a vacuum former andattempt a forming. The first forming will usuallyshow up areas of the mould which needmodifying. Keep on modifying the mould andtesting it until you are satisfied with the result.

RESIN CAST MOULDS

A timber based mould is good for mosteducational or experimental applications andshould last for a hundred formings or so -depending on how well it is made. For a moredurable, resin cast mould, first make a timberbased mould as described previously, then followthese steps:

_ _ _ _ _ _ _ _ _1. Vacuum forming a pattern

Take a forming of the timber based mould in 1.5 -3.0 mm PS, forming at maximum temperature toattain the best definition. The PS sheet has a glossside and a matt side - remove the film from thegloss side and form with it on the inside.

_ _ _ _ _ _ _ _ _ _ _ _2. Making a casting box

Construct a timber frame and fix the forming in itusing small, large headed nails or tacks. Turn theassembly upside down and fill with dry sand(moisture may permeate and distort the PS).Strike off the sand level with a straight edge andfix a timber board over the box to prevent thesand tipping out.

Turn the assembly back over, and you will have asturdy hollow mould, that will not distort evenunder several kilos of resin or with the heatcreated by curing.

CLEARANCEANGLE

STANDARDFLAT BIT

Modified drill bit for cavities

Solitaire mould

Mould preparation prior to casting

WOOD FILLER BLOCK WITH SUPPORTBATTENS

POLYSTYRENEFORMING

SUPPORT FRAME

DRY SAND

BASE BOARD

LOCATION NAILSAND HOLES INBATTENS

_ _ _ _ _ _ _ _ _ _ _3. Packing out the cavity

Suspend blocks of expanded PS or timber overthe main cavity, making sure that there is alwaysa gap of approximately 20 - 25mm (3/4 - 1”)between the forming wall and the packing. Thiswill reduce the amount of resin required to makethe mould and make drilling evacuation holeseasier (see later).

The blocks can be suspended on wooden battenswhich will need to be weighted down or fixed toprevent them floating when the resin is poured.

If you are making more than one mould, it mightbe worth making a vacuum forming of the innershell which can be suspended over the blocks inthe same way.

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _4. Mixing the resin

Calculate how much resin the mould is likely toneed and mix that amount.

The best resin for vacuum forming moulds isaluminium epoxy but, there are others on themarket including fast curing types, which can beused. Whichever resin you use it will come as atwo part pack and need to be mixed thoroughlybefore pouring.

Most resins are too viscous to be mixed by handso you will need to make some form of paddlemixer, fit it to an electric drill and mix for 3 - 4minutes, making sure all the resin from thecorners of the container is mixed in, to ensure youhave a good resin that doesn’t have any soft spotswhen it’s cured.

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _5. Pouring

Brush the forming with resin to prevent any airbubbles spoiling the mould surface (air bubbleswill have been formed during mixing and will notbe able to escape due to the resin’s viscosity - thisdoesn’t cause a problem on the inside).

Once the entire forming surface has been coated,pour resin in until the forming is about half full.

Place the packing blocks in place and pin orweight them down and then pour the rest of theresin. Slight shrinkage may occur during curingso overfill to just above the top of the forming.

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _6. Curing

Aluminium epoxy resin needs 24 hours at aminimum 20˚C (68˚F) to cure after which time itcan be taken out of the forming box. The packingblocks will probably remain inside the mould andthe original forming may be destroyed in theprocess of removing it. Once it is out of the box,it should be post cure heated at 80˚C (176˚F) forabout one hour per kg. Other resins will havedifferent curing requirements and you shouldrefer to the manufacturers’ recommendations ineach case.

_ _ _ _ _ _ _ _ _ _7. Drilling evacuation holes

An epoxy mould differs from a wooden mould inthat it is not porous and will require evacuationholes for the vacuum to be able to draw materialdown into all the details.

You can easily tellwhere evacuationholes will be neededinitially, by holdingthe original formingup to the light andseeing where it hasthinned the most.These points areexactly the pointswhich need evacuation holes.

4 -3

Mixing paddlesOPTION 1 -PROPELLER STYLE

OPTION 2 - WIREHOOK STYLE

3mm (1/8") DIAWIRE HOOK

BOLT WITH SHEETMETAL PADDLESECURED WITHLOCK NUTS

NB: The twists in the paddle blades will act like apropeller, pushing the drill out of the resin. If they havethe opposite effect ie. they suck the resin pot onto thepaddle, then you should reverse the direction of the drillor turn the paddles over.

PILLAR DRILL

SMALLDRILLFROMTOPOF CAVITY

LARGE DRILLFROMBASE WITHLOCATION PIN

WOOD BASE FORLOCATION PIN

Aligning small and large evacuation holes

FLATTEN THE END OF1.0-1.5mm (.040-1/16")DIAWIRE AND GRINDTOA BASIC DRILLPOINT

CLEARANCEANGLE

Piano wire drill

4 .4

Drill with a 1.0 - 1.5mm (0.04 - 1/16”) diameterdrill bit, or a section of flattened and sharpenedpiano wire or similar, in a high speed PCB drillsuch as a Dremmel.

As well as the thin spots, hemisperical cativitiesand large flat or gently domed areas such asmodel car roofs, may need evacuation holes too.

Sand down or grind the bottom of the moulduntil it is flat and mount onto a baseboard. Tryforming it before drilling too many evacuationholes as you may need less than you think.

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _8. Finishing

Final finishing, patching up with resin or car bodyfiller, sanding with fine grade wet & dry paperand buffing should leave you with a high qualitymould that will last for many thousands offormings.

Note: Large aluminium epoxy resin moulds for highvolume production may need to be water cooled.

ALUMINIUM MOULDS

Commercial vacuum formers tend to usealuminium moulds which is a technologybecoming increasingly prevalent in schools andcolleges as CNC milling equipment becomes morewidely available.

CNC milling has the potential to make mouldproduction very simple - all you have to do isdraw or 3D scan your model into the computerand then wait while the computer and the milldo all the hard work; mount the finished pieceon a baseboard, drill the evacuation holes andyou are ready to vacuum form.

Aluminium moulds are very stable whenheated and the larger ones have such a highthermal mass that they don’t need watercooling.

Some complex moulds have moving partswhich are easier to engineer in aluminium.

PROBLEMS WITH MOULD DESIGN

T H I N N I N G

One of the most common problems in the designof vacuum formed products is thinning. Differentmould shapes present different thinning problemstoo numerous and diverse to discuss here.

The circular cup mould though, is helpful to bearin mind. If a cup mould is straight sided withsharp corners, the maximum depth of cavity willbe about half the diameter at the top of the cup.Radiusing the corners at the bottom will improvethe depth and rounding the bottom completelywill treble it.

The thinning effect in cavities

30mm

Material thickness9% 20% 1.5mm (.060”) 43%13% 26% 3.0mm (1/8”) 40%

MOULD

LOWER PLATENSEAL

UPPER AIR SEAL

CLAMPFRAME

AIR EVACUATION

AIR DISTRIBUTION MESH

ADHESIVE PADSPROVIDE AIR PASSAGEBETWEEN MOULD ANDBASE BOARD

MAIN MACHINECASING

Base board details and evacuation considerations

BASE BOARD

Escalation of thinning effect in a sharp corner

Original material thickness

Actual material thickness

MINOR WEB FORMEDON A CORNER

STRETCH BLOCKS TOPREVENTWEBBING ATCORNERS

The causes of Webbing and possible solutions

MAJOR WEB BETWEENTWO HIGH POINTS

4 -5

If you go beyond these parameters the material atthe bottom of the cup becomes paper thin, like acarrier bag, and the cup won’t stand on it’s base.

W E B B I N G

Webs will form between points that are too highand/or too close together. As the material isdrawn down it meets itself and forms a foldbefore it comes into contact with the mould.

Commercial vacuum formers use a plug assist topush the material down before the vacuum takeseffect.

This can be done by hand on a manual machine,using a piece of rounded timber, stiff card or TPoffcut.

CHILL MARKS

A mark is often left around the top of a forming asa result of the mould cooling or ‘chilling’ theheated TP sheet as the platen rises (see 3. VacuumForming).

These marks are difficult to eliminate completelybut there are two things you can try:

1 Increase the heater output over the high spots.

2 Increase the heat of the mould - this reducesthe chill effect and helps material to ‘flow’ over

the mould togive a moreeven forming.

On some machinesit is possibletoblow the materialbefore forming,which can help thechill marks butmay aggravatewebbing.

S O L U T I O N S

D E S I G N

Some products can’t be vacuum formed in onepiece. But vacuum forming doesn’t have to be aone stage solution to any design problem and canproduce components for larger products.

For example, if we tried to vacuum form the CDrack pictured below, we would have problemswith thinning, webbing and trapping asillustrated.

THERMOPLASTICSHEET

CLAMP FRAME

MOULD

WOODENGRID

OVERHEADRAM

An overhead grid on the plug assist ram as used on a commercial machineto eliminate webbing

Chill marks on a forming

Excessive thinningWebbing

Undercut

FINN’S MADE FROM SHEETMETALUSING A STANDARD SHEETMETALCORNERNOTCHING MACHINE AND INSERTED INTO SAW CUTS IN BASEBOARD

FOLD AFTER VACUUM FORMING. 0.5MM (.020”) WILLFOLDCOLD. FOR THICKER MATERIALUSE A STRIPHEATER.

Design solution to thinning in a deep mould

4 .6

But if the rack dividers were vacuum formed flatand folded up afterwards, either cold or on a stripheater, they could be an insert in a casing madefrom another material.

I N V E S T I G A T I O N

By drawing a grid on a TP sheet before vacuumforming as commercial vacuum formers do whensetting up large machines (see 3. VacuumForming) you can show where the areas ofthinning are originally heated and reduce the heatover those parts.

OTHER THINGS TO TRY

NEGATIVE DRAFT ANGLES ORU N D E R C U T S

Normally, a negative draft angle will prevent aforming from being released from a mould.

But if a negative draft at one end of a mould, iscomplemented by a generous positive draft atthe other end, it can be possible to get aforming off by releasing the front end firstand pulling the forming up and back.

E N C A P S U L A T I O N

Skin packaging is carried out using very thin PVCfilm - less than 0.5mm - and vacuum forming itonto a cardboard base, specially prepared withperforations (for the air track) and heat sensitiveadhesive. Products are placed on the board andvacuum formed, with the film.

Using thicker material creates a permanent skinfor a product that gives it strength, colour anddurability.

If the mould is going to remain inside theforming, there is no need to worry about draftangles and you can use simple materials, likerectangular sections of balsa wood.

Coins or other artefacts can be simply mountedby encapsulating them in clear PVC, PETG or PSon a decorative base.

PART ENCAPSULATION

You may find yourself faced with the prospect ofa complicated mould who’s formings are gettingstuck on just one part. If you make that partdetachable, then you can leave it in the formingand you only have to make another one of thoseparts and attach it to the main mould with blu-tack or dowel pegs to make another forming.

VACUUM FORMING EXPANDEDP O L Y S T Y R E N E

Expanded polystyrene, which is easily cut on aHot Wire Cutter, is an interesting material to useas a mould. The heat of the sheet as it forms overthe expanded polystyrene, heats it up to the pointof collapse, creating an interesting surface texture.Different density foams collapse to differentdegrees, creating different surface textures.

Body of car mould remains intact

Rear aerofoil is detatchable and replaced for each new forming

Formula One car mould concept with encapsulated aerofoil

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

A “one-off” tool tray using encapsulated Balsa or Ply wood as the core

CORE OF MOULDIN BALSAOR PLY

WOOD

VACUUM FORMED“SKIN”

MOULD

DIRECTION OFREMOVAL

FORMING

Forming undercuts