In order to form internal cells in PVC extrusions, the die requires an insert or internal member that the material flows past. These inserts are supported to the outer die structure by so-called spiders that pass through to the outer wall of the extrusion die. The extruded material must separate and then recombine as it passes over the spider. The material must then form a bond at this interface shortly before exiting the die. These interfaces are referred to as knit-lines. In a recent project involving a large and complex PVC extrusion, difficulty was encountered in developing these knit lines within the webs of the extrusions. Upon visual inspection, these interfaces appeared to be without bond over portions of the cross section. However, mechanical testing in the worst knits revealed that bonding had occurred with the knit providing 85% of the bulk material strength although without supporting any significant ductility. At the same time, knits at different parts of the extrusions showed ductility comparable with the base material. Altering the process variables showed a means for improvement in the webs but this was limited by other constraints. In this work we describe the character of the knitlines and the resulting mechanical properties along with the testing methodology.

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