for a thermoplastic polymer, if you are above melting temperature, your polymer become like a liquid and its viscosity decreases. however, for the stability it depends on the polymer, there are polymers that are stable at relatively high temperatures. so what i suggest is to test your polymer's thermal stability before using it by means of TGA for example.

In the case of high temperature advanced thermoplastics, which exhibit high glass transition and/or melting temperatures one should be very careful with overheating. Probably the processing window is very narrow. You should consult the brochures of the manufacturers of a given material. In addition, to perform blow molding or an analog processing technology you need a ceratin amount of melt stability and melt elasticity otherwise the bubble blows up. In addtion to the TGA test suggested above you should perform mechanical tests on samples processed at various temperatures to see the degree of degradation Change in color may also be indicative of degradation.

Answer involves a blend of chemistry, thermal analysis, rheology and tooling. As you can wonder by now, this is not a unique answer, only testing will tell. Nevertheless, the more you know about your polymer and the more combinations you have at hardware, the better your efforts.