The use of additives in thermoplastic polymer films allows for the film properties to be tailored to a specific application; such as photovoltaic backsheets or flame retardant films. The properties of the film are highly dependent on the extent of mixing achieved as well as the degree of polymer degradation during twin screw extrusion processing. The work aims to establish the effect of operating conditions such as screw speed, throughput and torque on the extent of dispersive mixing observed. More importantly, it attempts to relate the ‘calculated’ parameters which are directly dependent on the operating conditions such as specific mechanical energy input, specific output and residence time to the degree of mixing achieved. The impact of polymer viscosity is assessed through the use of higher molecular weight polymers as well as the changes associated with temperature differences induced as a result of changes to the barrel temperatures. The extent of polymer degradation is monitored though the analysis of intrinsic viscosity in order to understand the nature of the compromise between achieving adequate mixing and maintaining the polymer matrix properties. The work identifies key parameters required for effective scale up of twin screw extrusion mixing processes.