A loss in part properties and service life is caused by thermal, mechanical, and thermo-oxidative stress during processing of thermoplastic materials. Process related thermal and mechanical stresses are inevitable. However, processing under exclusion of oxygen can reduce the thermo-oxidative stress. A lower thermo-oxidative stress can yield a higher transparency of optical parts, less reduction of average molecular weight for example in microinjection molding, and a longer service life due to less consumption of stabilizers during processing. For a cost-effective and gentle process it is important to know at which step of processing the polymer is exposed to thermo-oxidative degradation and where the oxygen comes from causing degradation. To study the local thermo-oxidative stress, an injection molding machine was encapsulated and run under local exclusion of oxygen. As reported in this article, thermo-oxidative degradation occurs in the plasticating unit due to oxygen from the surrounding air and oxygen dissolved in the polymer resin.