Regarding injection molding, residence time of the 
material being molded and proper plasticization are key factors to consider regardless of the resin being used and the end product being molded. The injection process starts when heat is introduced to the polymer after it enters the feed zone of the screw.
How long the material takes to travel from the feed throat to exiting the nozzle is the residence time of the material. Too short of a residence time and the material could not be uniformly plasticized. Too long of a residence time and the material could be degraded which may not always be visually apparent in the melt.
It is important to set the process up at the beginning with this in mind. You want to create the proper melt temp with the right amount of residence time to ensure a consistent viscosity of the material. This will make life much easier when the set up process moves to the mold.
Plasticization occurs when the screw rotates to move material through the 3 zones; feed zone, compression zone, and metering zone. After moving through the metering zone the material passes through the check ring into the shot where it will be ready to inject into the mold. Barrel heats, screw rotation speed, and back pressure all play a part into the temperature and consistency of the material in the shot. Proper process parameters for the plasticization of the material are crucial to achieving the correct viscosity for the material being processed.
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