[CitizenBrian]

I have a part made from Polyethylene that is 1/2" in diameter, 1/8 thick, and with a 1/8 hole thru the center. I am having a problem with the part expanding in diameter out of my +/-.0015 tolerance after they leave my department. The parts are being made in a Citizen L20 from ground material with all uncoated carbide tools. I think the problem is being caused during the cleaning process, may be the water is too hot or too cold, maybe they are being left near a hot or cold window(winter here so it's cold window) or may be even the cleaner being used (LPS Precision clean) could be the problem. I should also point out I ran a test were I left the parts in my department over night then had our qc person check the diameter in question and they were fine. Any help will be great. Thanks Brian

[txcncman]

Almost any type of material can stress relieve after machining. I had some polycarbonate that would grow 0.010" or more in diameter after sitting on a work bench for a couple of days. You might can use moderate heat to pretreat the material before machining. The other option is to find the amount of change and account for it during machining. I would contact your material vendor for additional suggestions.

[CitizenBrian]

Thanks txcncman, for pretreating the material do you have any suggestions on temp and time?

[txcncman]

One half the melting point for 2-4 hours and allow to cool as slowly as possible. Basically annealing the plastic. You may find it softens and changes the cutting properties.

And, I still recommend you contact your material vendor. They may be able to supply you better material. Sometimes the difference between cast and extruded makes a difference also.