What is vacuum casting, and when should you use it?

Vacuum casting copies one part many times: we 3D print and finish an original, pour liquid silicone around it to make a soft mould, then draw liquid polyurethane into that mould under vacuum so no air bubbles are trapped. It suits short runs of matching parts in hard, rubbery or clear material, and it is also called silicone moulding.

Vacuum casting at a glance

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

Lead times depend on the part’s size and design. Smaller parts can be faster, so check with us and tell us your deadline.

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How vacuum casting works: five steps from one original

1. Print the original. We 3D print the part, then sand and finish it until it is exactly what every cast should look like. Resin printing (stereolithography, SLA) starts smoothest, so it suits fine detail.
2. Make the mould. The original sits in a mould box and liquid silicone is poured around it. Making the mould usually takes 2 to 3 working days. The mould opens in two halves, the original comes out, and a hollow the exact shape of the part is left behind.
3. Fill it under vacuum. Polyurethane is mixed with its colour, and the closed mould goes into a vacuum chamber. The air is pumped out and the liquid is drawn into every corner of the hollow.
4. Open and finish. Once the cast has set, the soft mould bends open and the part lifts out. We trim off the feed and the faint parting line where the halves met, then sand it.
5. Repeat. One mould gives 20 to 50 casts before fine detail starts to soften. For a few hundred parts we pour more moulds from the same original, so every batch still matches.

How many parts make vacuum casting worth it, against printing or a steel tool, is a question of its own: see 3D printing vs silicone moulding vs injection moulding.

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Why the vacuum matters: no trapped air bubbles

Stirring two liquids together traps air. Poured in open air, those bubbles stay in the resin and set as pinholes on the surface or hollows inside the part. In the vacuum chamber the air is pumped out, so the bubbles rise and burst, and the resin flows into thin walls and fine detail that it would otherwise skip. Formlabs’ casting guide describes the vacuum the same way: it stops air getting into the part.

• Clear parts show it most. In a clear cast every bubble is visible, so the vacuum is what makes a clean, see-through part possible.
• Small detail needs it. Fine teeth, ribs and lettering fill completely instead of ending in a rounded air pocket.

Need clear or finely detailed casts? Tell us about the part

Finish: every cast copies the original

Silicone copies everything on the original: its shape, its texture, and every scratch or layer line. That is why the original is sanded and finished before the mould is made. Fix a flaw once on the original and it is gone from every cast; leave it, and it is on every cast.

• Smooth or textured. A smooth original gives smooth casts. A texture, like the vein detail below, comes through on every part.
• Parting line. A faint line shows where the mould halves meet. We trim and sand it on every part.
• Colour. It is mixed into the resin, so a scratch shows the same colour underneath. Paint, when you need it, goes on after a primer: see painting and surface finishes.
• Clear casts. A clear cast is only as clear as the original is smooth, so a clear original is polished first.

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Designing for vacuum casting: taper the walls, round the corners

A part designed for printing usually casts well too. These four rules make the mould last longer and the casts come out cleaner.

• Give the walls a slight taper. Straight walls with no taper (draft) grip the silicone as the part comes out, which wears or tears the mould. A small taper lets it slide free.
• Undercuts are fine, within reason. An undercut is a lip or groove that would lock a part into a hard mould. Silicone bends, so it peels over most of them. Formlabs notes a silicone mould allows significant undercuts. A very deep one may need the mould made in more pieces.
• Keep walls 1.5 to 4 mm thick. That is the range Formlabs’ guide recommends, with 1 mm as the absolute minimum. Thinner walls may not fill; thicker ones use more resin and take longer to set.
• Round the inside corners. A sharp inside corner becomes a sharp edge of silicone, which is where a mould tears first.

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Other routes: print one or two, injection mould thousands

Vacuum casting sits between printing each part and building a steel tool. Pick something else when:

• You need one or two parts. Print them. A resin print can be polished clear, and a flexible print in TPU (a rubbery filament) can stand in for a cast rubber part. See 3D printing.
• You need thousands of a final design. Past about 1,000 identical parts, injection moulding costs less per part. We build and test the design first, then arrange the tool.
• The part must be the real production plastic. Casts are look-alikes, such as ABS-like polyurethane. For genuine ABS, PP or PC, use injection moulding, or CNC machining for a few.
• You need a thin shell, like a tray or a cover. That is vacuum forming: a heated sheet pulled over a shape, not a mould filled with resin.
• The design is still changing. Keep printing until it settles. A new silicone mould is quick to make, but each change means a new original and a new mould.

Many products go from a printed prototype to a vacuum cast pilot run, then to injection moulding once demand is proven. Our product development team in Singapore covers the whole path.

Not sure which route suits your part? Ask us