Extrusion International 5-2026

28 Extrusion International 5/2026 PIPE EXTRUSION, EXTRUSION TOOLING Picture 5.1 shows the normal ow with shear rate and melt veloc - ity pro le over die gap. Picture 5.2 shows what happens if there is no melt ow, but the die and mandrel are rotating. Picture 5.3 shows the combination of the both situations. This creates a 3D shear that is gener- ated by the ow and rotation. Inuence of rotation to properties It was reported that die an man- drel rotation leads to reduced pres- sure drop, eliminates melt +fracture, homogenised and oriented ller distribution, moderate increase in strength and elongation at break, enables the control of orientation of short glass bres, up to 40% increase on circumferential tensile strength for PP with short glass bres Negative effects due to rotation The die/mandrel rotation has some negative effects, in addition to the positive effects. These are the limits for the rotation speed. At higher speeds, the die swells chang- es shape. This means that after reaching a certain rotation speed, you have to redo the die centering. Agians, the die rotation doesn't cor- rect any eccentricity. Technical solution CONEXTRU uses either helical or radial spiral technology for poly- ole n pipes, or a combination for multilayer pipes, for the pipe head concept itself. The rotation is achieved by inde- pendently turning the die and man- drel at different speeds using servo motors. The rotation speed depends on the diameter, line speed, and the expected effect, such as the pitch of the ribs or the orientation. Making rotation possible through mechanical is quite challenging. On the one hand, the rotation should not require a high level of torque from the drive. Torque is inuenced by backpressure due to the man- drel's shape and the closing force required for a seal between the xed and rotating parts of the man - drel. The viscosity of the melt within the die and mandrel gap must also be considered. A seal is needed between the rotating part and the non-rotating part. Experience has shown that Teon O-rings are the best option for seals because Teon has good gliding properties and is temperature resistant. The picture 6 shows a 400 mm die with a rotating mandrel – a short front part with grooves that cre- ate ribs inside the pipe later on. The front part of the mandrel turns and is connected to a shaft that extends backwards to the end of the head. There is a unit that generates the closing force, which is adjusted by springs. This force can be adjusted by tightening the springs using screws. Outlook Tubes with internal ribs repre- sent the state of the art, and there is nothing more to add – as demon- strated by numerous practical ex- amples. However, the inuence of orientation due to rotation still re- quires further research. It is evident that an increasing number of scien- ti c institutions are investigating the inuence of rotation on tube properties. So far, however, this has no practical relevance. It would be worthwhile to pursue further re- search in this area, as there is poten- tial to improve product properties. By J. Dobrowsky CONEXTRU GmbH Klosterstarsse 19, 3011 Irenental, Austria Dobrowsky.j@conextru.eu Picture 5.3 Picture 5.1 Picture 5.2 Drive Spring package to built up sealing force Rotating mandrel Teon O-Ring sealing Pipe head PO 400 with rotatable mandrel tip