Urethane casting design guidelines

About 0.040 in (1 mm) is the practical minimum for rigid urethanes, with 0.060–0.120 in (1.5–3 mm) preferred. Thinner walls fill poorly at the low pressure of a vacuum pour and tear during demold. Small features can go thinner over short distances, and elastomeric grades tolerate thin lips better than rigid ones.

No draft is required in most cases, because the silicone mold flexes to release straight walls. About 0.5° helps on walls deeper than about 2 in (50 mm) and extends mold life. If the part will move to injection molding, add 1–2° now so the same model works for the steel tool.

Yes. Moderate undercuts such as snap hooks and shallow grooves release by flexing the silicone mold, with no slide or tooling change. Deep or fully enclosed undercuts need an extra cut in the mold or a loose core piece, which adds labor per shot. Soft elastomeric parts can also be stretched off undercuts.

Use threaded inserts, either placed in the mold before the pour or pressed or heat-staked in after cure. Cast urethane threads wear quickly under repeated assembly. Size the boss wall at about 60% of the nominal wall and the hole to the insert maker's recommendation. Drill and tap only for parts assembled once.

Yes, if it is drawn to injection molding rules from the start: uniform walls of 0.060–0.120 in, 1–2° of draft, ribs at 50–60% of the wall and undercuts that a slide or pass-through core can form. Urethane casting tolerates less draft and more undercut, so a molding-ready design casts easily, but not the reverse.

Cast parts typically hold about ±0.010 in (±0.25 mm) on features under a few inches, after about 0.15% shrink from the master. A master built on a high-resolution SLA printer and hand finished carefully gives the best results. Accuracy drifts slightly as each silicone mold wears over its 20–25 shots.