Plastic globules on 316SS tubing after polishing

PET deposits on stainless steel tubing. We are using an orbital welding system to make fusion joints on 316 SS thin wall tubing and fittings. We are finding dark spots roughly 1/4" away from the weld. We had a lab analyze the tubing- the spots are PET plastic globules that burn due to the heat from the welding process. Farther away from the weld, our laboratory has found PET plastic globules. This tubing has been polished to 20 Ra, using mechanical methods. Where would PET globules be introduced to this tubing? Please call me if you have any experience with this issue, and how to clean the tube to remove the PET globules.

Thanks,

I've been told -- in typical polisher vs. plater disputes -- that it is possible to burn plastic onto the metal from a Scotchbrite polishing wheel. I don't actually know whether it's true or not :-)

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

Ted I will concur with you 100% "Scotchbrite" or nonwoven abrasives will smear on the surface. The trick is to limit pressure hence heat, and to allow the part to cool. I have made a tidy sum removing it from titanium and stainless steel medical devices.