Once you have a finished silicone mold, casting resin copies is relatively straightforward — but there are enough variables to turn a good mold into a bad casting if you don’t understand the process. This guide covers the full casting workflow from mold preparation through demolding.
For the mold-making steps that precede this, see silicon rubber mold making.
Choosing a Casting Resin
Two-part urethane resins are the standard for prop casting. They cure hard, are paintable, can be tinted with colorants or filled with metal powders, and are available in a range from flexible to rigid.
Smooth-On Smooth-Cast Series
The Smooth-Cast series covers the range from very hard rigid to semi-flexible:
Smooth-Cast 300 — White, Shore D 70 (rigid plastic), 7-minute pot life, 30-minute demold. The most-used resin for small prop casting. Good detail reproduction, sands and paints well.
Smooth-Cast 305 — Off-white, same hardness, 7-minute pot life. Slightly slower cure than 300 — useful in warm weather when 300 may be too fast.
Smooth-Cast 320 — Brilliant white, Shore D 70+, 6-minute pot life. Slower to demold than 300 but excellent surface finish.
Smooth-Cast 65D — Semi-rigid (Shore D 65). Good for parts that need some flex — thin-walled castings that would crack if fully rigid.
For most B9 robot parts and solid prop pieces, Smooth-Cast 300 is the default.
Task Series (Filled Resins)
The Task series resins are formulated for specific performance properties:
Task 3 — Rigid, good impact resistance. Used for parts that will be handled or subjected to stress.
Task 4 — Higher impact resistance than Task 3. Good for prop weapons, handles, anything that gets dropped.
Alumilite and Polytek make comparable products — brand choice often comes down to what your local supplier stocks.
Mold Preparation
Even a good silicone mold needs preparation before casting:
Inspect the mold face — Check for tears, defects, or debris. Pick any particles from the surface with a cotton swab.
Apply mold release — Silicone molds release from most resins without a dedicated release agent, but applying Ease Release 200 or a light coat of paste wax extends mold life and ensures clean pulls. This is especially important for older molds that have been used many times.
Warm the mold — Cold silicone slows the cure of urethane resin and can cause problems with thin-walled castings. If your shop is below 70°F, warm the mold to room temperature before casting.
Measuring and Mixing
Urethane resins are typically mixed 1:1 by volume (measure A and B in equal volumes) or by weight following the manufacturer’s specification. Check the technical data sheet — some resins have off-ratio mixes that look like 1:1 but aren’t.
Use a fresh mixing cup and stick for every pour. Resin left in a cup from a previous pour may contain partially cured material that accelerates your new mix.
Mix procedure:
- Measure Part A into one cup, Part B into another
- Pour Part B into Part A (not the reverse — minimizes unmixed material clinging to the cup bottom)
- Stir slowly and thoroughly for 30–45 seconds, scraping sides and bottom
- Pour before pot life expires — for Smooth-Cast 300, that’s 7 minutes from start of mixing
Do not shake or stir fast — you’ll whip air into the mix.
Pour Technique
One-part molds — Pour into the mold cavity in a thin stream from a height of 8–12 inches. Let the resin self-level. Fill to the lip of the mold for a flat-backed casting, or leave a slight underfill if you’re planning to mate two halves.
Two-part molds — Close the mold and secure the halves (rubber bands, clamps, or dedicated mold clamps). Pour through the pour gate — the channel cut into the mold parting line or a separate sprue. Fill until resin appears at the vent holes.
Rotational casting — For hollow castings, pour a small amount of resin into the closed mold, close all openings, and rotate the mold in all directions while the resin gels. The thin layer that coats the mold cavity cures into a hollow shell — ideal for large pieces that would be too heavy if solid.
Cure Time and Demolding
Smooth-Cast 300 can be demolded in 30 minutes at room temperature. However, full cure (full hardness) takes 24 hours. Freshly demolded pieces are softer and more flexible than they’ll be when fully cured — this is normal.
Flex the mold slightly to release. For two-part molds, separate the halves first, then flex each half away from the casting.
Finishing the Cast
Fresh resin castings have a parting line (seam at the mold’s parting line), possibly small air bubble pits, and sometimes a rough pour gate that needs trimming.
Parting line — Sand with 150-grit sandpaper along the seam, followed by 220 and 320. Fill any remaining gaps with two-part epoxy putty or CA glue + baking soda filler.
Bubble pits — Fill with CA glue (thin viscosity), let cure, sand flush. For multiple pits, a surfacer primer will fill small defects.
Pour gate — Cut flush with flush-cut nippers and sand smooth.
After finishing, prime with automotive sandable primer (Rust-Oleum Automotive Filler Primer works well). The primer reveals any remaining surface defects and provides tooth for paint.
Next Steps
For more complex casting techniques including urethane foam fills and pressure casting for bubble-free results, see advanced casting techniques. For applying these skills to a complete prop project, see the Mystic Seer replica guide or the B9 robot construction overview.