Tips for no-melt machining

It's relatively easy to machine advanced plastic stock shapes, but you have to know what you're doing. EDITED BY JEAN M. HOFFMAN Semiconductor-equipment manufacturers machine advanced plastic stock shapes such as this fully imidized thermoset polyimide (PI) into wafer clamp rings. With a continuous use temperature of 500°F along with high strength, stiffness, and chemical inertness the machinable PI helps ensure that the rings stand up to the ravages of wafer processing. Called Duratron XP, the material is a ductile PI and, as such, resists chipping, breakage, and tool wear during machining. Surgical probes machined from Ultem PEI rod are autoclavable. PEI offers high strength, rigidity, and dimensional stability. However, because it is an amorphous material, nonaromatic coolants should be used when machined. Aircraft engine electrical connectors machined from PBI plate are strong, dimensionally stable and can withstand temperatures over 400°F. Polycrystalline diamond tools are recommended when machining the PBI called Celazole, because it is an extremely hard material Process and test equipment manifolds machined from a PET bar stock called Ertalyte PET-P offer good dimensional stability, combined with excellent stain and chemical resistance. Machined PET-P shapes have low residual stress and are porosity-free. Coolants are not required, but generally improve surface finishes and tolerances. Makers of semiconductormanufacturing equipment have long been known as major users of plastics that work at high temperatures and that resist corrosion. What is interesting is that these materials serve in applications where conventional plastic-forming techniques either cost too much or won't allow quick and frequent design changes. Take the example of a clamp ring for polishing silicon wafers. Frequent dimensional changes cropped up during design that made large-scale injection molding impractical. The cost of repeated tool rework would have been quite high. In contrast, the only cost associated with shaving a few thousandths off one of the clamp ring's features was in material and reprogramming. Machinable plastic stock shapes span