machinery | Pipe dies In pipe extrusion we have polymers with very high


viscosity up to 250,000Pas (PE 100 RC low sagging) or very low viscosity materials up to 20,000Pas (adhesive for coextrusion). Practice has shown that it is sufficient to have a


range of distributors – for high and low output, and high and low viscosities. (Conextru has five different geom- etries which are used depending on the circumstances. With this range nearly all cases can be covered.) A version of the helical spiral is the radial spiral. At


Above: A radial spiral distributor for a 400mm die


is an optimisation process by slight changes to the geometry. The result is a curve – showing how much material remains within the spiral channel and how much is leaving the spiral channel into axial flow. The curve – volume over channel length – should look like a Gaussian distribution, which is well-known from statistical analysis. Precondition for the calculation is the viscosity curve of the polymer – which means viscosity over shear rate. This because a loop within the calculation is a pressure calculation based on volume, shear rate and viscosity. Because the calculation is based on one channel


Right: This poly- olefin die head, designed by Conextru and manufactured by Rollepaal India, has a


throughput of 1200kg/h


along its length, a new pipehead is generated at first by the number of channels. Therefore we know exactly how the distributor will behave in terms of melt distribution when it has X number of spiral channels. At the same time, based on the diameter, we know the output because the throughput is proportional to number of channels.


Conextru, we have applied this technology – which is known from blown film dies – to pipe application. Of course, the geometry was adapted for use with pipe-grade polymers. Within the radial spiral we have similar flow conditions to those in the helical spiral, and the flow goes from the outside to the inside of the distributor plate. In the radial version, the distributor disc is much shorter in length but larger in diameter. The main application for such distributors is for a thin layer and low output but there is no limitation in the diameter. (Conextru uses two types of radial distributor geometries depending on polymer and output.) Regarding melt distribution, there is one more


design called the pinhole or heart curve distributor. It works well for low viscosity materials. The main advantage is that this type does not need melt pre-distribution and the volume is very low. Conextru uses this type for large diameter pipe, or for temperature-sensitive, low- viscosity polymers. With these three systems


and, in total, six different distribution systems – which are all standard and proven – it is possible to design a new tailored tool that is suited to fulfil all


Pipe heads for PVC require larger volumes


For PVC, there are single-layer pipe heads for UPVC, MPVC and PVC-O. The tools are single spider or double spider pipe heads. There have been no signifi- cant improvements to these tools in the last 20 years. The technology is still fundamentally the same, and has seen no change in the design criteria. Compared to non-PVC tools, PVC pipe heads need much larger volumes in order to guarantee residence times of three to 10 minutes.


14 PIPE & PROFILE EXTRUSION | May 2017


PVC need a kind of compression with minimum ration of compression (torpedo square to die set sqare) for best welding after torpedo and also much longer die sets.


Multilayer tools are designed for PVC


foam core pipes or for three-layer compact pipes. Tools for this application were initially developed by pipe produc- ers, and later by machinery suppliers. A wide range of different types are on the market. The manifold for each system


will be completely different, and most are optimised by trial and error. However, traditional PVC tools have some disadvantages. It is time to review the spider technology for PVC pipes. One solution could be to use a


two-channel manifold die, as is used for PVC multilayer dies. Conextru has already built such a tool for PVC-O heavy wall pipes, but further optimisation is in progress. This tool will be tested in the near future.


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