Casting resin parts in a silicone mould

This page has safety advice that hasn't yet been checked by our safety reviewer. Treat it as general guidance, and follow your game's, venue's or product's own safety rules first.

How to mix two-part polyurethane resin and pour it into a silicone mould to make hard, identical costume parts, for makers ready to work carefully with chemicals.

Casting resin in a silicone mould is one of the most reliable ways to duplicate a part you have sculpted or found, whether that is a row of armour rivets, a decorative medallion, or a prop gem. You start with a finished silicone mould — made separately from mould rubber poured around a master — and your job at the casting stage is simply to mix the resin correctly and get it into the cavity before it begins to set. Because silicone is naturally non-stick, the cured resin part releases cleanly without a release agent in most cases, though a dedicated mould release spray can extend the life of a mould used heavily. Two-part polyurethane casting resin comes in two bottles, usually labelled Part A and Part B. The mix ratio and the working time before the resin becomes unworkable both vary between brands and product lines, so read the specific instructions for the product you have rather than relying on any general figure. Measure the two parts carefully — by volume or by weight as the product specifies — pour them into a clean mixing cup, and stir slowly and thoroughly, scraping the sides and bottom of the cup. Rushing the mix or being sloppy with the ratio leads to parts that remain tacky, warp, or simply do not cure. Mixed resin also gives off heat as it cures, and large or thick pours can get very hot, so stay within the sizes your product recommends. Pour the mixed resin into the mould in a thin, steady stream aimed at the lowest point of the cavity. This helps push air ahead of the liquid rather than trapping it. Gentle tapping or flexing the mould after filling encourages bubbles to rise. Some makers cure the filled mould in a pressure pot, which shrinks bubbles until they are hard to see; a pressure pot is a pressurised vessel, so use one made for casting and follow its instructions exactly. Once the resin has fully cured — again, check your product's stated time — flex the silicone mould to release the part, then inspect it and trim any mould lines with a hobby knife. Safety is not optional at this stage. Uncured polyurethane resin and its fumes can irritate skin, eyes, and lungs, and repeated skin contact can leave some people permanently sensitised to it. Work in a well-ventilated space — outdoors or with active airflow — and wear nitrile gloves, safety glasses or goggles, and a respirator fitted with organic-vapour cartridges, or whatever the product's safety data sheet specifies. Younger makers should have a responsible adult present throughout. Do not eat, drink, or touch your face while handling unmixed or mixed resin, and dispose of used mixing cups and stir sticks according to local regulations once the material has fully cured and hardened.