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Pipe dies | machinery feature


From rotating dies to co-extrusion crossheads, the latest extrusion die technology ensures that pipes of all shapes and sizes are produced accurately and effi ciently. Lou Reade reports


In the round: pipe die technology


The sheer variety of plastic pipes available – which range from enormous polyolefi n water pipes to small medical tubes – means that the die technology to produce them must be adaptable and appropriately designed. Researchers from the Technical University of


Munich in Germany, for instance, have devised a method to make silicone rubber medical tubing using a heated die.


The method is an alternative to curing the tubing outside the die – which insists that the viscosity of the uncured silicone is high enough to be dimensionally stable. “Even with high viscosities, achievable precision and


tolerances are limited due to this fact,” said the researchers, in a paper presented at the recent Antec conference in the US. The new process relies on a heated die to vulcanize


the extrudate inside the die. The work has derived a formula for the fl ow rate for an in-die-curing silicone extrusion, in order to describe this process mathematically – and be able to predict the capabilities of this technology. For the extrusion process, an extruder was modifi ed


for silicone extrusion. It was used with a standard three-zone screw, with screw speed controlled by a


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variable-frequency drive (VFD). Experiments were conducted at die temperature at (or above) 200°C, and VFD frequencies up to 2Hz – as these conditions caused fast enough curing.


“In-die curing is unusual for silicone extrusion, but


would enlarge the possibilities for extruded geometries and the use of special silicones,” said the researchers. Comparison of the measured and calculated values


showed good correlation – though the researchers admitted that the model had several limitations. “The model gives a small hint on the effects of


several parameters – and can be used for future estimations in LSR extrusion – but it will not replace experimental testing,” they concluded.


Die rotation Conextru, for instance, has begun to supply rotating pipe dies for specialist applications. A pipe die does not normally have any rotating


elements, because the pipe – whether single- or multi-layer – does not include any structures that require rotation. However, there are now a few applica- tions of pipes where an inner structure caused by rotation will improve performance. An example is PVC/PP pipe with internal spiral ribs, for in-house sewer pipes. This increases fl ow down on


June 2016 | PIPE & PROFILE EXTRUSION 29 German researchers


have devised a method to


make silicone rubber medical tubing using a heated die


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