I have a forming machine that measures 24" x 36". We use 1/8" and 3/16" sheets of HDPE for forming. We are getting lines in the hdpe from where the plastic kinks up when you heat it. Is there any good way to cure this problem? We have tried unclamping the plastic until it sags, cutting the sheets down shorter, preheating the sheets, controlling temps and wind, etc. The lines are never in the same spot. They are very shallow. I never have any problems when we use ABS or polypropylene. However, HDPE is our material of choice.