Viscosity

Most injection moulding processes use a reciprocating screw injection moulding machine.
The reciprocating screw injection moulding machine uses a large screw inside a heated barrel to melt the plastics pellets, and convey the polymer melt. The screw typically turns counterclockwise, and the friction created by the screw pushing the material down the barrel causes shear heating.
Shear heating is responsible for most of the heat required to melt the plastics pellets. The rest of the heat is provided by the barrel heaters which enclose the barrel.
During injection, the reciprocating screw forces the polymer melt into the injection mould. As the melt is injected into the mould, the flow characteristics are dependent on the material’s viscosity.

Viscosity

The viscosity of the polymer is a measure of the material’s resistance to flow. A material which flows easily has a low viscosity, while a material with a higher viscosity does not flow as easily.
Most polymers are available in different grades, each grade has its own flow characteristics. Typically, materials with lower viscosity’s have lower molecular weight. These materials are easier to process, but typically have lower mechanical strength than the same polymer with a higher viscosity.
The viscosity of the polymer can be used to compare the flow characteristics of different polymers, or different grades of the same polymer.
Viscosity data can also be used to qualify a new material. You can compare a newer lot of material to a previously used material to determine whether or not the new material is the same.
Two of the most common methods of determining viscosity are capillary rheometry and melt flow indexing.