Vacuum casting

Vacuum casting (also called urethane casting, polyurethane casting or silicone mold casting) is a process for making small runs of plastic and rubber-like parts by casting a two-part polyurethane resin into a silicone mold inside a vacuum chamber. The mold is made by embedding a master pattern, usually an SLA print or a machined part, in liquid silicone rubber. The castings faithfully replicate the shape and surface of the pattern, and their properties imitate common injection-molded plastics.

The master pattern is first finished to the required surface quality (sanded, polished or textured), fitted with gates and vents and embedded in silicone inside a frame. After curing, the mold is cut open along the chosen parting line and the pattern is removed. The resin is mixed and degassed under vacuum and poured into the mold under vacuum, which limits bubbles and fills even thin walls and fine details. The parts are then cured at elevated temperature, typically around 60 to 70 °C, and finally trimmed of gates.

Polyurethane resins come in many grades that imitate ABS, polypropylene, polyamide, polycarbonate or clear PMMA, plus rubber-like materials of roughly 30 to 90 Shore A, flame-retardant (self-extinguishing) grades and resins for higher temperatures. Parts can be colored in the resin or painted, metal threaded inserts can be placed in the mold, and overcasting produces parts that combine rigid and soft materials.

A silicone mold typically lasts about 15 to 25 castings, depending on part complexity and the resin used; after that, a new mold is made from the same master pattern. Dimensional accuracy is lower than with machining, roughly ±0.15 to 0.3 mm per 100 mm of length depending on material and geometry. Part size is limited mainly by the vacuum chamber, commonly from small components to housings about one meter long. First parts are usually available within one to three weeks.

Vacuum casting suits runs of tens, at most hundreds, of parts, where 3D printing each part would be expensive or fall short on surface quality and an injection mold does not pay off yet. Typical applications are instrument and electronics housings for testing and early customers, visual prototypes and trade show samples, rubber-like parts (seals, grips, soft covers) and parts for marketing photos before series production.

A single master pattern gives you repeatable parts with a surface close to that of a molded part, in different colors and materials, so you can compare several material options before ordering a mold.

Polyurethane casting resins are thermosets that only imitate the properties of thermoplastics. Stiffness and hardness may be similar, but heat and chemical resistance, UV aging and fatigue behavior differ. Do not carry test results from castings over to the injection-molded part without careful thought, and for certification testing check whether the lab will accept vacuum-cast parts.

Casting quality depends directly on the master pattern: every layer line or scratch is copied to every part. In your request for quote, state the required surface (gloss, matte, texture), color, Shore hardness for rubber-like materials, quantity and tolerances of critical dimensions. Long thin walls can warp, and thick sections tend to show sink marks. Also check whether the price covers the master pattern, the mold and painting, and how many parts one mold yields, because a larger run means additional molds.