Extrusion International 1-2025

32 Extrusion International 1/2025 PIPE EXTRUSION the material and utilizes the energy stored in the material. This energy can then be used to reduce the mo- tor power. Furthermore, the enthal- py of ABS (shear energy required to place the material) is signi cantly lower compared to PE. The preheating and the lower shear energy result in a signi cantly lower installed motor power com- pared to PE or PP. For this reason, it would make sense to design a spe- cial extruder for ABS instead of using a standard PE/PP extruder for ABS extrusion. The screw geometries are similar to the PE design, i.e. there is grooved feed, transmission, barrier, maddock and mixing. The shear en- ergy output of an ABS screw is ap- prox. 30 % lower than that of a PE screw and two specially designed extruders are used for the 3-layer ex- trusion of ABS: C 45 37 to 250 kg/h for the inner and outer layers and C 60 37 to 450 kg/h for the middle lay- er. Both extruders are equipped with a temperature-controlled feed zone and gravimetric dosing for output control or weight per meter control. The blowing agent is metered vol- umetrically via a metering unit at- tached directly to the extruder, and ABS foams well if the right blowing agent is selected, i.e. a short decom- position time at low temperatures of around 170 °C is required be- cause the dwell time in the extruder or pipe head is short due to the low volume. The blowing agent can be added volumetrically or gravimetri- cally as micro-granules. The foaming of ABS is only suc- cessful if the blowing agent is cor- rectly selected. A short decomposi- tion time at low temperatures of around 170 °C is required, as the residence time in the extruder or pipe head is short due to the low volume. Mixing is effective and is further enhanced by a more inten- sive mixing section at the end of the screw. The diagram illustrates the output and energy consumption at a back pressure of 200 bar. Pipe head In the past, mandrel holder tools, as used in PVC extrusion, were gen- erally used. However, these have the disadvantage of a high volume and a long dwell time. In addition, the web marks are usually visible and impair the quality of the cross- section. Furthermore, ABS is ther- mally sensitive, which can easily lead to oxidation of the butadiene content and a signi cant reduction in spiral mandrel manifolds are pre- ferred, which have a low volume, good melt distribution properties and the possibility of extracting the internal air in the pipe through the pipe head. Effective internal pipe cooling is not required, only the reduction of the internal tem- peratures in the pipe is necessary to reduce the possibility of oxidation, which would be visible through brown discoloration. ABS has an- other special property: the ow property, i.e. the ow of the melt, reacts strongly to the surface tem- perature of the ow channel. Air currents and temperature in- homogeneity lead to a change in ow around the circumference. For this reason, it is necessary to use insulated heating tapes to prevent cooling from the outside and to en- sure uniform temperature distribu- tion. The pressure build-up of ABS in