The theoretical maximum will to heat without complete melting will be the
glass transition temperature. This will be unique for the polymer you are using, and can vary with additives, molecular weight distribution, copolymers, etc. I suggest looking up the Tg of your specific polymer and staying below that to avoid deformation. If you go above Tm you are completely screwed, so don't do that. If you bought virgin polymer from a reliable source, there will probably be Tg and Tm listed on the packaging or with the properties sheet.