Urethane Rubber for Mold Making

A Durable Option

I've used silicone for most of my mold-making, but when it comes to casting concrete, urethane rubber is a more durable alternative. The downside? It sticks to just about everything, unlike silicone, which naturally releases from most surfaces. That meant a few extra steps to make sure everything separated cleanly.

Getting the Parting Line Right

A basic part of making the mold is ensuring the two halves of the mold register properly when assembled and separate without issue when building up the rubber layers when creating the mold.I tested different materials for creating the registration keys and parting line. HDPE - the white cutting board material you have probably seen a nd aluminum worked best as urethan won't bond to them. PETG, on the other hand, didn't release at all with no release agent.

I used a dental vacuum former to shape HDPE over a 1"steel ball bearing welded halfway into a 3/8" steel plate. This gave me a good set of registration a male and female interlocking piece that transferred well to each half of the rubber mold. To create this parting line, I painted on three thin layers of urethane ver the entire statue, then built up a thickened bead right where the mold would split. Once it set, I sliced into it and slid in my aluminum shims and HDPE registration pieces. Aluminum HVAC tape held everything together. Simple, effective.