Advanced Thermoforming, 2nd Edition

Chapter 3Production of Semifinished Products, Extrusion, and Coextrusion

Written in cooperation with Rudi Salmang, Terneuzen, The Netherlands, and Horst R. Dänzer, Büchlberg, Germany

3.1 EXTRUSION

3.1.1 No Thermoforming without Extrusion

Experienced thermoformers have a proverb: It is possible to produce bad formed parts from good semifinished products, but it is impossible to produce good formed parts from bad semifinished products. Experience is also defined as the summation of the mistakes one has made in the past! Over time, thermoforming in packaging evolved from processing PVC to PS to PP and PET. Whereas PVC showed hugh and PS comfortable operating windows, PP and PET are known for their narrow operation windows. Knowledge about the extrusion process is essential to mastering the thermoforming process. The success of a thermoforming application depends on correct extrusion of the semifinished product. If the extrusion is not carried out according to the rules, this can have serious effects on the thermoforming. Film thickness variations can diminish the quality of a thermoformed part. For a multi‐cavity tool, different wall thicknesses of products will not meet the quality standards. Longitudinal and diagonal shrinkages of semifinished products are caused by different orientations during the extrusion process, and these too are recurring issues that lead to variations in the batch quality. Variations in the batch quality are therefore due to variations in the thermoforming ...