Extrusion International 1-2026

27 Extrusion International 1/2026 mers that have a viscosity ranging from 5,000 Pas (adhesive for co-ex - trusion) to 300,000 Pas (PE 100 RC low-sagging). This is why different distributor geometries are needed. One version of the helical spiral is the radial spiral. Conextru GmbH was the first company to apply this technology, which is known from blown film dies, to pipe applica - tions. The geometry was naturally adapted for use with polymers in pipes. This is essentially a flat heli - cal spiral distributor. Similar to the helical spiral, the flow in the radial spiral goes from the outside to the inside of the distributor plate. This geometry can easily be derived from the helical spiral. The me- chanical design differs in that the distributor disc is much shorter in length, but larger in diameter, than the helical version. This distributor design is used for pipes of up to 110 mm and 2 to 7 layers. It can also be used as a single-layer addition of up to 10% of the main layer, with a maximum diameter of 400 mm. Conextru GmbH uses two types of radial distributor geometry, Pic. 4. Pre distribution of melt When designing a new head, which is mostly for a multilayer cus- tomer design, the number of chan- nels is selected depending on the di- ameter range, polymer and output. Basically, nothing has to be calculat- ed on the distributors anymore; the basic design is finished. However, we have extruder position adapters and a section that distributes a sin- gle flow into a number of channels, and it is obvious that each channel must receive the same quantity of melt flow, a process known as pre- melt distribution. For this part, CONEXTRU carries out new calculations for each new design. For example, a PO 630 CR crosshead will be designed for a PP coating on a PE pipe with an output of 200 kg/h. In compari- son, the same size of head for a steel pipe coating requires a 1200 kg/h output with PE 100. Clearly, the adapter design and pre-melt distribution will be different, but the spiral will be the same. For this rheological design work CONEXT- RU uses formulas to calculate pres- sure as following: - Shear rate based on output and geometry - Viscosity according to the curve of the polymer (viscosity over shear rate - Pressure based on geometry and throuput. These formulas used to calculate the pressure. In principle, there are 3 basic geometries – see table below. This is very useful when it comes to long adapters or nozzles with circu- lar cross-sections and narrow gaps. The melt is distributed from a cir- cular cross-section into the number of channels. This is simple with an even number of channels because it is symmetrical. It is slightly more complicated with an odd number of channels, e.g. 3, 5, 7, 11, etc. In this case, the pressure calculation is adapted so that each channel builds up the same pressure at the same throughput. This is only possible if the geometry of the pre-distribu- tion is changed accordingly. If this is well designed and finished, – in case of multilayer heads the next area which needs to pay attention is the area in which the melts flowing together. Melt merging area Particular attention should be giv- en to the dimensioning of the flow Pic. 3: scale up method Pic. 4: radial disc depending on the polymer and output Pic. 5: PO 630 CR melt pre distribution Pic. 6: PO 630 CR melt distribution Geometry Shear rate (1/s) Pressure drop bar round cross section GP = 4. VP*e/(3,142*R^3) DP = 8*ETA*VP*L/ Phi*R^4 Rectangular slot GP = 6*VP*e/(B*H^2) DP = 12*ETA.VP/ (B*H^3) B>>W Annular slit GP = 6*VP*e/ (3,142*(Ra+Ri)*((Ra-Ri)^2)) DP = 12*ETA.VP/ (B*H^3) H<<D helical rodia