Vacuum casting pours resin into a silicone tool inside a vacuum chamber so that air is drawn out of both the resin and the cavity before it cures. The result is a dense, bubble free part that reproduces the master's surface exactly, which is why it is the standard route for launch quantities, appearance models and clear parts.
Why the vacuum matters
Resin poured in air traps bubbles, and bubbles at a surface are a cosmetic defect while bubbles in section are a structural one. Degassing the resin and casting under vacuum removes both. It also helps the resin reach thin sections and fine detail that surface tension would otherwise keep it out of, which is what makes fine texture reproduce faithfully.
Making the silicone tool
A master is printed or machined and finished to exactly the surface you want, because the silicone copies it. Silicone is poured around it, cured, then cut open along a planned split line and the master removed. Where the split falls is a decision: it leaves a witness line on the parts, so it is placed where the line will not matter.
Materials and quantities
Polyurethane systems approximate common thermoplastics for stiffness and appearance, elastomeric grades cover soft parts across a wide hardness range, and optically clear systems exist for lenses and light pipes. A tool typically gives a few dozen casts before the surface degrades and dimensions drift, so plan tools per hundred parts.
Where accuracy drifts
Silicone is flexible, so parts are slightly larger or smaller than the master depending on the resin's shrinkage, and the tool relaxes as it is used. Early casts and late casts from one tool differ measurably. Where a dimension is critical, measure it on each tool's first article and expect to compensate on the master rather than on the parts.
Questions people ask about vacuum casting
What is vacuum casting?
Casting resin into a silicone tool inside a vacuum chamber, so air is removed from both the resin and the cavity and the part cures free of bubbles.
How many parts per tool?
Commonly a few dozen before the cavity degrades. Abrasive or aggressive resins reduce that, so budget for several tools on a run of a hundred or more.
Are cast parts as accurate as moulded ones?
Close but not equal. Silicone flexes and degrades with use, so dimensions drift across a tool's life. Treat them as representative rather than certified.
Can clear parts be cast?
Yes. Optically clear polyurethane systems cast well under vacuum and are one of the strongest reasons to choose the process, since clear injection tooling is expensive.