Extrusion defects

Extrusion defects are extrudate deformations often observed during the high-speed extrusion of thermoplastic materials. Various forms of extrudate deformation can be identified. Typically, a distinction is made between localized deformations on the surface and deformations affecting the entire volume of the extrudate. Defects of the former type are commonly referred to as "shark skin" and those of the latter type as "extrudate rupture". Combinations of surface and volume deformations also occur and are known as "spurt" or "stick-slip melt fracture".

However, in industrial practice, surface deformations are perceived as the most important problem, because they appear first and limit extrusion throughput. The extruded object (film, sheet, tube, cable or hollow body) no longer conforms to quality standards (it shows a loss of optical transparency, gloss or mechanical integrity).

Corrective measures can be taken to delay the onset of extrudate deformation at higher extrusion rates: various solutions are available, but they are only of interest if they remain economically acceptable. Typically, the extrusion temperature of the molten polymer is increased, or the thermoplastic is blended with a similar polymer of lower viscosity, or additives are added to facilitate flow.

In this article, the various types of extrusion defects will be defined and described for a number of thermoplastic extrusion processes. Possible causes of extrusion defects will be analyzed from a scientific point of view. Some typical solutions for delaying the onset of extrusion defects will be presented.

For further details, readers may wish to consult the articles on extrusion in this treatise:

• Extrusion. Single-screw extrusion processes (Part 1) ;

• Extrusion. Single-screw extrusion processes (Part 2) ;

• Extrusion. Twin-screw extrusion processes ;

• Flow modeling in extrusion dies .