Which vacuum casting material should your parts be cast in?

Most vacuum cast parts are a rigid ABS-like resin; choose PP-like, POM-like or PA-like for clips and moving parts, clear PMMA-like or PC for see-through parts, and rubber-like (Shore A 40 to 90) or silicone for anything that bends. Every vacuum casting material behaves like a production plastic without being it, so casts suit prototypes, pilot runs and short runs of a handful to a few hundred parts.

We cast all ten in our studio in Singapore. New to the process? Start with what vacuum casting is and how it works.

Vacuum casting materials at a glance

“-like” means a cast resin made to behave like that plastic. It is not the plastic itself, so it does not carry the real plastic’s data sheet.

Shore A is the hardness scale for rubber: 40 is soft and squashy, 90 is firm but still bends.

UL94 V-0 is the top vertical-burn rating in UL 94, the common flammability test for plastics: the flame goes out on its own and nothing burning drips.

Tell us what your part does and we suggest a material

ABS-like casts: the everyday choice for rigid parts

Most rigid casts are ABS-like. It looks and feels like a moulded ABS part, it is rigid and tough, and it sands and paints well (paint always goes on over a primer). Colour is mixed into the resin, so a scratch shows the same colour underneath.

Two versions cover the jobs plain ABS-like cannot:

• Flame-retardant ABS-like carries a UL94 V-0 rating, for enclosures around power supplies, chargers and circuit boards.
• High-temperature ABS-like stays firm to about 120°C, for parts near motors, lamps, heaters or hot exhaust air.

Their honest limits:

• The rating is the resin’s, not your product’s. A flame-retardant cast is a strong start, but a product that needs a safety approval is still tested as a product.
• Heat: the high-temperature grade is the one made for it. If the part gets warm, tell us how warm.

Need a flame rating or heat resistance? Ask us

PP-like, POM-like and PA-like casts: for clips and moving parts

These three imitate the plastics used for parts that move, flex or slide. They let you test a snap fit or a mechanism in a material that behaves like the production one, before paying for a moulding tool.

• PP-like behaves like polypropylene, the plastic of bottle caps and food tubs. It bends without snapping, so it suits clips, lids and snap fits.
• POM-like behaves like acetal (POM), a stiff, hard plastic with a smooth, slippery surface. It suits catches, slides and look-alike gears.
• PA-like behaves like nylon. It is tough and takes a knock, so it suits brackets, clips and housings that get handled roughly.

Their honest limit: they imitate the feel, not every figure on the real plastic’s data sheet. Do not assume a cast matches the real plastic on wear, chemicals or a hinge flexed every day for years. Test a cast first, or make the part in the real plastic by CNC machining or injection moulding.

Need the real plastic? See injection moulding

Clear casts: PMMA-like for clarity, PC for toughness

We cast two clear materials. Clear PMMA-like behaves like acrylic and is the one to pick when the part must be as see-through as possible: lenses, light covers, display parts. Clear PC behaves like polycarbonate, the plastic of safety glasses, so it suits clear covers and guards that may take a knock.

• Clarity starts with the original. Silicone copies every mark, so the original is polished before the mould is made.
• The vacuum matters most here. In a clear cast every air bubble would show, and casting under vacuum is what keeps them out.

Need clear parts? Tell us about them

Rubber-like casts: Shore A 40 to 90, from soft seals to firm covers

Rubber-like casts bend, squash and spring back. You choose the hardness, from Shore A 40 (soft, like a pencil eraser) to Shore A 90 (firmer than a shoe heel), so one material family covers soft seals, grippy feet and covers that keep their shape.

• Soft end: seals, gaskets, grips and bumpers.
• Firmer grades: grommets, covers and parts that must hold their shape but not crack, like the screen-protector plates at the top of this page, cast at Shore A 80.

Its honest limit: rubber-like resin feels like rubber, but it is not a moulded rubber compound. For oils, fuel or outdoor sun all year, tell us and we check the grade.

See the run of 10 rubber-like screen protectors

Silicone casts: for soft-touch and food-safe parts

Silicone is soft, flexible and grippy. We cast it for soft-touch parts and, in a food-safe grade, for kitchen products. It is also what every vacuum casting mould is made of, which is why a mould like the one below flexes enough to peel off a detailed part.

Its honest limit: silicone is soft by nature, so it is the wrong choice for a part that must stay rigid or hold a screw.

See our silicone moulding and vacuum casting service

Timing and mould life: about 3 to 4 weeks, 20 to 50 casts a mould

As a rough guide for a normal-size part, a vacuum casting job takes about 1 week to design the original, 2 to 3 working days to make the silicone mould, then 7 to 10 working days of casting. Size and design change it, so check with us and tell us your deadline.

• One mould casts 20 to 50 parts before fine detail starts to soften.
• For a few hundred, we pour more moulds from the same original, so every batch matches.
• Past about 1,000 identical parts, injection moulding usually costs less per part. Our comparison of 3D printing, silicone moulding and injection moulding shows where each one wins.

Tell us how many you need and by when

Other routes: print one or two, or make the real plastic

Vacuum casting sits between printing each part and building an injection moulding tool. Choose something else when:

• You need one or two. Print them: see which 3D printing material suits your part, or which SLA resin to choose for clear and rubber-like resins.
• The part must be the real plastic. For genuine ABS, PP, POM, nylon or PC, use injection moulding for thousands or CNC machining for a few.

Not sure which route suits your part? Ask us