Does anyone have recommendations on type of plastic for CNC machining a plug for an airbox?
We have the airbox modelled (along with the inlet system) in Solidworks so can easily CNC machine a negative (plug) - it will be a split mould and the block of plastic will be about 120x175x350mm allowing for flanges and most of it will be machined out.

Searching around Heat Stabilised Blue Nylon seems a balance of friction coeff, machinability, rigidity and price but thats just from specs with no real life experience.

Thanks in advance
Stuart

I've always thought polyurethane was the best tooling block to use.

Yes polyurethane is what you need, it is called Ureol and it is not hard to get hold of.
Don´t you live in Coventry?
found this when looking around:
http://www.itpgroup.co.uk/about_itp/ITPGroupUK-CoventryUniversityClayMilling.html

http://motoform.blogspot.se