Large Solid Parts with Thick Cross-Sections
Reason: Excessive material shrinkage and internal voids. Thick sections cool unevenly, leading to warping or sink marks.
Alternative: Use processes like injection molding or machining for bulky parts.
High-Precision Mechanical Components
Reason: Vacuum casting uses silicone molds, which wear after ~20-30 casts, causing gradual dimensional inaccuracies.
Alternative: CNC machining or injection molding for tighter tolerances.
High-Temperature Applications
Reason: Most castable resins (e.g., polyurethanes) have low heat resistance (typically <100°C).
Alternative: Use high-temperature resins (e.g., epoxy) or metal casting.
Parts with Extreme Undercuts or Fine Internal Channels
Reason: Silicone molds lack rigidity, making demolding difficult without damage.
Alternative: Use dissolvable cores or 3D printing for complex internals.
Transparent Parts with Optical Clarity Requirements
Reason: Air bubbles or flow lines may form, reducing optical quality.
Alternative: Injection molding with polished molds or specialized casting resins.
Shrinkage Control and Dimensional Tolerance Management
Shrinkage Compensation:
Mold designers scale up the master pattern (typically +0.15–0.25%) based on the resin’s shrinkage rate.
Example: For 2% shrinkage, the pattern is enlarged by 2% to offset shrinkage.
Process Controls:
Degassing: Resin is vacuum-degassed to eliminate bubbles causing uneven shrinkage.
Curing: Controlled slow curing reduces thermal stress and warping.
Mold Design: Use of mold releases and strategic venting to ensure complete filling.
Tolerance Standards:
Typical tolerances: ±0.1–0.3% per 100 mm (e.g., ±0.15 mm for 100 mm).
Critical dimensions: Machined post-casting or adjusted via iterative mold corrections.
Summary
Avoid vacuum casting for large solid parts, high-precision/high-temperature
components, and complex internal features. Control shrinkage through
design compensation and process optimization, maintaining tolerances
within ±0.2% for most applications.
