A new possibility of melt cooling in extrusion dies to prevent sagging-effects in thick-walled pipes

Abstract

One challenge in the extrusion process of thick-walled pipes is the cooling of the product. Besides the output of the extruder, the line speed is also limited by the efficiency of the cooling line. The cooling time increases according to the wall thickness of the pipe under otherwise equal process conditions. State of the art is the cooling of the outer surface in water tanks or spray-cool-tanks. In addition to that, it is possible to cool the inner surface by air that is sucked through the pipe. Despite these technologies it is problematic to cool down thick walled-products with the right speed. Especially thick-walled pipes show problems by cooling the layers in the middle of the wall. On the one hand an intensive cooling of the outer and inner surface of the pipe entail the formation of shrink holes in the middle of the pipe wall. On the other hand without a quick cooling the melt flow in circumferential direction because of the gravity takes place (sagging-effect). Because of this reason in the presented paper new possibilities of melt cooling in extrusion dies to prevent sagging-effects are given. An aimed cooling of the inbound melt layers inside the extrusion die could prevent the effect of melt flow in circumferential direction after the extrusion die, allows the specification of a specific temperature profile over the radius of the pipe wall and helps to reduce the melt temperature for rising mass throughputs and screw driving speeds of the extruder. It is also thinkable to influence the crystallization process and thereby the mechanical properties of the end-product by an aimed cooling of the inner pipe layers.