Urethane Casting (vacuum casting) is ideal for producing small quantities of parts, especially if aestheticsvacuum casting are important. The master part, typically produced by stereolithography or CNC machining is dressed and textured as required before establishing parting planes, feeds etc. The master is then mounted within a frame and liquid silicone cast around it. After curing the master is cut out to leave a cavity into which a variety of polyurethanes can be cast under vacuum.
The silicone has sufficient flexibility that undercuts can be released by flexing the tool, although deep draws may require additional tooling splits.
Tool life varies with resin cast, but is typically around 20 parts by which time the tool surface has started to harden and the tool flexibility deteriorates. Using multiple masters and producing multi cavity tools can produce greater quantities, although cavity life will remain around 20 parts.
The process is relatively labour intensive and typically requires between 2 andurethane casting 5 days to produce first off castings. This is on top of the time to manufacture and dress the master part. These times are dependent upon part size as this affects material curing times.
In evaluating vacuum casting, especially if quantities require multiple cavities, it may be worth considering RIM or plastic injection molding. Alternatively if quantities are low it may prove faster and more cost effective to make multiple laser sintered parts. These alternatives are dependent upon part size, complexity and time available.
Tolerances vary depending on material and supplier, however as a general rule using ± 0.25mm to 100mm and ± 0.25% thereafter will provide a 'safe' estimate. Note these are process tolerances and should be applied on top of any master part tolerance.
Little post processing is usually required, the finish is achieved by dressing and texturing the master part before casting the tool. After casting the part there is only the feed and riser removal, although in some very colour critical conditions the parts are sometimes painted. (Usually this is a colour pigment added to the resin)
The process steps can be summarised as
1. Manufacture a master part and dress to achieve the required aesthetics.
2. Ensure the master part is clean and sealed (some materials will react with the silicone).
3. Establish a parting line and apply coloured tape around the it.
4. Construct a casting frame and after attaching a feed and risers, suspend the master part in the casting frame.
5. Degass the silicone and pour into the casting frame, degassing the cast tool again to remove any entrained air and then leaving to cure.
6. When fully cured, remove the casting frame and cut open the silicone to the coloured tape / parting line.
7. The master model can now be removed, the tool reassembled, taped together and sent for casting.