Silicone / Polyurethane Molding
Cast custom rollers, treads, and compliant grippers from silicone or polyurethane. Mold-making and pour-cast fundamentals you'll reach for whenever a COTS part is the wrong shape.
Before you start
Pick the right resin. Shore hardnessShore A is the scale for soft rubbery materials. ~20A is gummy-bear soft; ~60A is car-tire firm; ~80A is shoe-sole firm. Pick by feel and what the part has to grip. determines how much your part will deform under load.
1Design the part you want to cast
Model the final part shape in Onshape. Account for ~0.5% shrinkage on cure for most polyurethanes; silicones shrink less but still some.
Avoid undercuts unless you're willing to build a multi-part mold.
2Design the mold around the part
Use Onshape's Boolean Subtract to subtract the part body from a block — what's left is the mold cavity. Add pour spout and ventResin enters through a spout at the lowest point and air escapes through a vent at the highest. Without a vent you'll trap an air pocket exactly where you don't want one. channels.
Add alignment pins or tabs if your mold is multi-part.
3Print the mold halves
Print in PLA or PETG with 0.2mm layers. Smooth the inner cavity surface with a quick wipe of acetone (for PETG) or by post-printing primer for a glass-smooth finish — surface texture transfers 1:1 to the casting.
Apply mold releaseEven if the resin claims no release is needed, use one anyway. Ease Release 200 or a simple wax coat means you'll actually be able to demold without tearing the part. before pouring.
4Mix and pour the resin
Mix Part A and Part B by weight (not volume) — most kits are 1:1A but read the data sheet. Stir slowly for 90 seconds, scraping the cup sides. Pour in a thin steady stream from one corner to let air escape ahead of the resin.
If you have a vacuum chamber, degas the mixed resin for 60 seconds before pouring; without one, pour from height to let bubbles burst on impact.
5Let it cure, then demold
Most polyurethanes need 16–24 hours at room temperature; silicones are usually 4–6 hours. Resist the urge to demold early — green parts tear and never recover their full strength.
Trim the flash with a sharp knife. The cast part is now the actual rubber roller, tread, or finger pad you designed.
You can now cast any shape you can model. Rollers with embedded hex bores, finger pads with built-in V-grooves, treaded wheels — anything COTS doesn't sell.