Casting Urethane for Rapid Prototyping Projects

Posted by Ainsley Aiken on December 28th, 2018

For companies focusing on rapid prototyping, producing small quantity detailed parts, cast urethane is ideal. The master part of the design of the product is dressed and textured as required before establishing parting planes, feeds etc. Then it is mounted within a frame and a liquid silicon cast is put around it. Later, the master is cut out to leave a cavity into which a variety of polyurethanes can be cast. The flexibility of the silicon that undercuts can be released by flexing the tool while deep draws may require additional tooling splits.

The tool life varies but is typically up to 20 parts after which the tool surface starts to harden and the flexibility of the tool deteriorates considerably. For producing the products in greater quantity, using multiple masters to produce multi cavity tools can help, though the cavity life for each one remains the same.

The cast urethane process is time consuming and labour intensive, hence, requires about 2 to 5 days to produce first off casting. Added to this is the time required for manufacturing and dressing the master part. The process depends upon the part size as it definitely affects material curing time.

Plastic injection mouldings are considered for the multiple cavities vacuum castings. As an alternative option, for low quantities it may prove to be more effective to make multiple lasers sintered parts. Though there is a safe estimate provided, tolerance may vary depending upon material and supplier. Before casing the tool, dressing and texturing the master part comes as the post processing finishing touch. After casting the part, the feed and the riser are removed. For colour critical conditions, the parts are sometimes painted.

To summarise, the whole process from manufacturing a master part and to dress it to achieve the required aesthetics, cleaning and sealing it in, establishing a parting line and applying a coloured tape on it, constructing a casting frame and attaching feed and riser to suspend the master part in the casting frame, degassing the silicone and pouring it into the frame degassing the cast tool again to remove any entrained air, removing the casting frame when fully cured by cutting open the silicone to the coloured tape to removing the master model to reassemble and tape together to send for casting, it is a work that requires skilled and focused employees and the companies that have acquired excellence in the field can boast of both the qualities. 

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