According to Regulation (EC) No 1907/2006 HL-6K

During the processing of polymer materials, internal friction arises from two sources:

Friction between polymer and polymer (ie between molecules together), this friction is called internal friction.
Friction between polymer and device wall: friction between polymer and screw surface, forming mold surface, guide system surface, etc. These frictions are called external frictions.
The role of lubricants is to reduce the internal and external friction mentioned above, to help minimize harmful effects arising during the machining process:

Large energy loss, increased processing costs
Generating uncontrolled energy, causing loss of quality control.
Damage to the molecular chain of the polymer (ie molecular chain breakage), reducing the properties of the plastic