VERFAHREN UND ROHRZIEHFORM ZUR EXTRUSION VON ROHREN AUS THERMOPLASTISCHEM MATERIAL

摘要

The thermoplastic material is extruded through a die comprising a diverging annular channel (4) connected directly to a straight annular forming channel (5) having a constricted annular flow zone (10) upstream of its outlet orifice such that the speed of deformation of the thermoplastic material in the constricted annular flow zone (10) is at least equal to 0.1 sec<-1>. For this purpose, use is made of a die in which the ratio between the flow section area upstream of the constricted annular flow zone and the section area of the constricted annular flow zone is comprised between 1.5 and 10. The constriction induces longitudinal stress into the material, which neutralises the effects of radial stresses produced in the diverging annular section and which would otherwise cause contraction of the pipe downstream of the outlet orifice, particularly in the production of pipes with large diameters greater than 250 mm. <IMAGE>