2. Polymer extrusion processes

2.1 Extrusion in general

Polymer extrusion is one of the key processes used to form thermoplastic materials available in pellet or powder form . Extrusion involves conveying the solid polymer, then melting it and subjecting it to pressure by means of a screw rotating inside a heated barrel (cf. articles ). The polymer is then "forced" through an extrusion die and cooled, which defines the type of extrusion.

The question of extrusion defects becomes relevant when there are free surfaces. When the flow is confined (e.g. injection molding) or when secondary molding is used (e.g. thermoforming), part appearance defects cannot be associated with extrudate deformations (even though their origin may be similar in nature)....