Extrusion International 1-2026

28 Extrusion International 1/2026 PIPE EXTRUSION, EXTRUSION TOOLING channel in the area where the melt streams meet – the melt merging area. For polymer composites made from the same material e.g. only PE or only PP or only ABS, the size of the gaps at the melt merging point can be adjusted according to the layer thicknesses. The configuration of the system is designed in such a manner that it is capable of accom- modating a wide range of layer dis- tributions. For instance, it should be capable of accommodating distri- butions such as 10/90 to 90/10 for a two-layer pipe or 5/90/5 to 45/5/45 for a three-layer structure. For polymer structures of totally different materials and viscosities during processing like PE/ Adhe- sive/ PPS, PE/Adhesive/PA or PP/Ad- hesive PVDF the dimensioning of the gaps at melt merging must be optimized by calculation. Depend- ing on the shear rate in the gap before melt merging, viscosity ac- cording the shear rate, layer thick- ness and throughput, the gaps in this area are calculated with the aim of achieving minimal differences in shear stress and flow speed at the moment of merging. This ensures that there are no flow anomalies such as ripples in the boundary layer or long term waves or penetration of melt of one layer into the other layer (low viscosity penetrates higher viscosity. The result is the highest possible accuracy of the layer thickness dis- tribution and eccentricity, as well as flexibility in the event of large changes in throughput ratios. The analysis revealed that a wid- er gap better meets this require- ment than a narrow gap. It was also found that the process of pulling the melt out of the the gap, like a draw down, leads to a better thick- ness distribution than squeezing the melt to generate the right thick- ness. The use of outdated tools is an indication of this. After the melting point, the tool should have the shortest possible melt path to the final die sets. Long dies create pressure and shear stress, as well as a long residence time, so the new design aims to shorten the dies as much as possible. The new head design incorporates a centring unit with a master die after melt merging. This die covers a cer- tain range. Further die and mandrel rings are mounted on the master die. These die set rings can be larger or smaller in diameter than the melt merging area. This system is called add-on dies. By Josef Dobrowsky CONEXTRU Klosterstarsse 19, 3011 Irenental, Austria Dobrowsky.j@conextru.eu Pic. 7: layer ration 45/10/45 to 25/50//25 Pic. 8: Melt merging of PE/Ad/PA/Ad/PE Pic. 9: 3 layer die 400 with CSR and die set to 180 and mandrel mounting tool