Polyurethane vacuum casts are molds made out of silicone rubber, usually based upon a master mold created from CNC machining or one of our rapid prototyping services (SLA, SLS or DMLM). This option is ideal for making duplicates of the master up to 50 pcs., with highly detailed resolution of the original. Molding cost is low, but it requires careful preparation of the master.
A master mold can be made using our own prototyping services or from a customer-supplied model. The model is placed inside of a chamber or casting box, which is then filled with liquid silicone rubber. Once the silicone is cured, it can be split open and the master removed. There is now an empty cavity exactly conforming to a reverse image of the master. This cavity is then filled with the casting material of your choice. A vacuum is applied to the silicone mold, which effectively pulls the material into any voids inside the mold.
A wide variety of PU resins are available for casting parts to suit your needs – heat resistant, flexible, clear, rigid, etc. Many polyurethanes will accept colorants in the mold or can later be painted or printed as required. For more information on available materials, see here.
Advantages of Vacuum Casting
Polyurethane vacuum casting makes high-quality finished parts that are nearly the equal of injection molded pieces, without incuring the related cost. The silicone mold lends itself well to replicating complex features and overhangs that would be difficult to 3D print, and the surface can be refined to produce a high-gloss clear acrylic finish. It is even possible to overmold dissimilar materials or threaded inserts. The disadvantage is that the silicone mold is not durable and cannot be relied upon to make more than approximately fifty pieces, depending on part geometry.
The finished dimensions of vacuum cast parts is dependent on the accuracy of the master model, part geometry and casting material. In general, a shrinkage rate of + 0.15% is to be expected. Oversized parts can be post-machined as required.