What wall thickness is best for vacuum casting? Most vacuum cast polyurethane parts work best with wall thickness in the 1.5–3.0 mm range. Thinner walls increase the risk of short shots and fragile silicone mold edges, while very thick sections cure slowly and may cause sink or voids. Use ribs and coring to balance stiffness, weight, and cosmetic quality.

Keep walls as uniform as possible. Large, abrupt thickness changes trap heat in the silicone mold, slowing cure and causing uneven shrinkage. This shows up as warpage, sink, or print-through of internal structures on cosmetic faces.

Good practices:

• Transition between wall sections with gradual tapers instead of sharp steps.
• Where stiffness is needed, use ribs at roughly 40–60% of the wall thickness instead of solid mass.
• For very thick areas you cannot avoid, add internal pockets or lightweighting features in the CAD model.

If you are unsure whether a thin wall is manufacturable, share the 3D model with our team through the vacuum casting RFQ form. We can simulate fill paths and suggest small geometry changes that avoid defects.

Silicone molds are flexible, which helps them release parts more easily than hard steel tools, but they still benefit from consistent draft angles.

• Aim for ≥ 1° draft on all vertical faces.
• Use 2–3° draft around deep pockets, tall ribs, and textured surfaces.

Draft reduces friction during demolding and lowers the stress on silicone around sharp corners. Less stress means longer mold life and more consistent parts.

Handling undercuts:

• Small undercuts (for example, light snap features) can often be handled by flexing the mold, especially if radii are generous.
• Large or sharp undercuts rapidly damage silicone and reduce mold life. For these, we may split the part into two components for assembly, use removable silicone inserts, or redesign the feature into a through-slot or accessible latch.

Mention critical undercuts in your RFQ notes. We can recommend whether vacuum casting is still suitable or if CNC machining or injection molding is a better route.

Ribs and bosses are common in housings, covers and brackets. For vacuum casting they largely follow injection molding rules, with a few additional considerations for silicone molds.

Ribs

• Design rib thickness at 40–60% of the adjoining wall.
• Use generous fillets where ribs meet walls to avoid stress concentrations in the mold.
• Avoid very tall, thin ribs that are hard to fill; add draft and radii to make resin flow more predictable.

Bosses

• Keep boss outer wall thickness similar to the surrounding wall to minimise sink.
• Add support ribs instead of thick bosses for strength.
• For threaded inserts, leave enough clearance for press-fit or heat-set inserts after casting, or discuss overmolding metal inserts directly into the silicone mold.

Fine details

• Text and logos can be reproduced accurately in silicone molds, but avoid extremely sharp or tiny features that may not fill or may tear the mold.
• As a rule of thumb, keep relief depth ≥ 0.3–0.4 mm and radius ≥ 0.2 mm on raised features.

Vacuum casting delivers excellent dimensional accuracy for low-volume parts, but it does not match the tightest CNC or steel-mold tolerances.

• For most geometries, plan for ±0.2 mm general tolerance.
• For simple, small dimensions and critical fits, ±0.1 mm is often achievable with good process control.
• Larger parts and long dimensions may need looser limits or specific GD&T callouts.

Polyurethane resins shrink during curing, and silicone molds also expand slightly with temperature. Typical overall shrinkage is 0.2–0.4%, depending on the chosen resin and cure schedule.

How to handle critical dimensions:

• Flag critical fits (bearing seats, sliding interfaces, gasket grooves) in your drawing or RFQ.
• We can bias the master pattern dimensions to compensate for expected shrinkage.
• For very tight fits, consider finishing critical faces by CNC machining after casting.