This page covers silicon rubber mold-making methods previously documented at this URL. Content is being restored from archive sources.

Topics covered originally:

  • Two-part silicon rubber (MoldMax-style products)
  • Mold box construction
  • Pour technique and cure times
  • Releasing the master pattern

Related: plastic casting (part 1, part 2), prop fabrication reference.

Frequently Asked Questions

What are the two main types of silicon rubber for mold making? The two primary chemistry types are platinum-cure (addition-cure) and tin-cure (condensation-cure) silicones. Platinum-cure is preferred for detailed prop molds because it has no shrinkage, better tear resistance, longer shelf life, and excellent detail capture — but is inhibition-sensitive to sulfur and latex. Tin-cure is less expensive and less sensitive to inhibition but shrinks slightly during cure.

What is the MoldMax series used for in prop mold making? Smooth-On’s MoldMax series (MoldMax 10, 20, and 30) are platinum-cure silicones used for most prop and B9 robot mold work. MoldMax 20 (Shore A 20) is the most-used option — it has enough body to hold shape while remaining flexible enough to release complex shapes, with a mix ratio of 100A:10B by weight and a 30-minute pot life.

What is the standard mold box material for silicon rubber mold making? LEGO bricks became the standard mold box material in the prop community because they interlock precisely, create relatively leak-resistant walls without glue, can be built to any footprint and height in 8mm increments, are completely reusable, and allow for perfectly flat parting surfaces critical for two-part mold alignment.

How is the master pattern prepared before pouring silicon rubber? Porous masters must be sealed with spray lacquer (2–3 light coats), shellac, or Smooth-On Smooth-Prime urethane primer and allowed to cure overnight. After sealing, a thin coat of petroleum jelly (Vaseline) or Ease Release 200 is applied with a soft brush and excess wiped away to leave only a thin release film over all surfaces.

Further Reading from Authoritative Sources

  • silicon rubber — Wikipedia’s silicone rubber article provides authoritative coverage of both cure chemistry systems including their properties, inhibition factors, and comparative characteristics.
  • mold making — Wikipedia’s Molding (process) article provides authoritative context on mold design and production principles that the silicon rubber series is built on.