Machinable plastic for a composite mold

[Formula1]

Hello, I'm wondering if anyone knows of a cheap machinable plastic that could be used for a composite mold. I need to make intake runners out of carbon fiber and have made an aluminum mold, but I was looking for a cheap plastic that I can throw into a CNC.

[Geof]

Cheap machinable plastic is a bit of a contradiction. You need a plastic that the binder of your composite will not stick to so likely the best would be UHMW polyethylene. Maybe an alternate choice would be Delrin but it does react with more chemicals than the UHMW. Getting well away from cheap you could consider fiber filled Teflon.

One thing I think you should consider though is thermal expansion. I believe in same cases the carbon fiber composites are cured at a highish temperature? Any plastic will expand considerably. UHMW can handle up to about 80 degree C without losing much strength but Delrin might be starting to soften a bit.

[Formula1]

Thank you for your help, I forget to mention that this mold is only a female mold for a male mold that will be made from Aquapour. What we're doing is CNCing a female mold out of something cheap (hopefully plastic), then filling it with this Aquapour material, which will then be baked. Then it will harden and the carbon can be applied. Then the Aquapour will be dissolved out and leave the complete, hollow, intake runner. The Aquapour sets up in the mold, is removed, and then baked. Thermal expansion and withstanding heat is not a problem. What do you recommend for the cheapest option?

[Geof]

You make the female mold, cast the male mold in the Aquapour and bake the Aquapour.

Does the Aquapour solidify so you remove it from the plastic before baking? It sounds like it does if you do not have to worry about heating the plastic.

Your least expensive approach may be to use machinable wax. This way you can reuse it for other molds; if you have the ability to bake something you should be able to remelt the wax and recast a fresh block.