Extrusion International 1-2025

33 Extrusion International 1/2025 spiral mandrel manifolds is low and the melt distribution is good. A triple spiral mandrel manifold is used for processing foam core pipes. The foamed middle layer usu- ally covers 60 percent of the entire cross-section. A smaller extruder is used for the two layers “compact inside” and “compact outside”, and the distribution from the inside to the outside takes place in the so- called Y-block. As the pressures of the inner manifold differ from the pressure of the outer manifold, this must be taken into account in the Y-block and is corrected via an ori- ce plate. In this way, the pressure build-up and layer thickness of both layers are equalized. Die design When designing the nozzle, it must be taken into account that a weight change of approx. 20 to 40 % occurs during foaming. This per- centage must also be taken into ac- count when designing the nozzle in order to ensure that the melt foams by this percentage after leaving the nozzle and that the diameter is cali- brated without upsetting or under- cutting. To ensure free foaming, a certain distance between the nozzle and the calibration is required, as the foaming process does not take place in the nozzle, but only when the melt leaves the nozzle and the pressure is removed. As a result, the nozzle diameter must be reduced by the percentage of foaming to be ex- pected, and this also applies to the wall thickness. For a foam density of 20 %, a reduction of the nozzle and the gap by 20 % each is therefore required. Weight and cost saving It should be noted that the calcu- lation for weight and cost savings is well known from PVC foam core pipe extrusion and is merely ad- opted here. In this example, a den- sity reduction of 40 % of the middle layer is assumed. This results in an overall density of 0,8 kg/dm^3 if the foamed layer accounts for 60 % and the wall thickness remains the same. This re- sults in a weight reduction of app. 25 %. (Tabelle) This also reduces the amount of material used, which is re ected in the lower weight per meter costs and an increase in the length of pipe produced. By J. Dobrowsky CONEXTRU Klosterstarsse 19, 3011 Irenental, Austria Dobrowsky.j@conextru.eu Full recyclability, excellent trans- parency and impressing mechanical properties: Ultra-thin MDO-PE and MDO-PP lm produced on SML’s latest generation of cast lm lines open new opportunities to manufacturers and form the technical basis for the mono-material packaging solutions of tomorrow. www.sml.at Engineered to perform. Maximum film performance at full recyclability.