PROCESSING | SCREWS AND BARRELS Right:
Assymetric T-Profile kneading elements from CPM Extrusion Group
Degassing gains Another area being explored for HP elements is in the degassing section of the twin-screw extruder, where the novel geometries provide increased surface renewal and so improved devolatilisation. Current state-of-the-art for screw design in degassing zones uses traditional Erdmenger conveying elements, which do not produce any significant surface renewal outside of the intermesh region. By introducing mixing while under vacuum, this increases the diffusion of volatiles in the melt. CPM Extrusion Group is focusing this area of development towards solutions for VOC reduction for ABS and polyolefins, which includes use of stripping agents in combination with HP elements for degassing. The general trend in development of screws and
barrel sectors has focused more on materials than on new shapes for conveying or mixing sectors, according to Italian extruder manufacturer Comac. “Although the geometries now available on the market are numerous, most continue to use the traditional two-lobe shapes both for conveying and mixing. While these generally provide good mixing and maintain a perfect self-cleaning profile, good self-cleaning cannot be guaranteed in different gear tooth shapes or in the IGEL type, which — even if they are used only in particular circumstances — have the disadvantage of risking polymer block that can degrade thermally,” says Dr Riccardo Pellegrini, Research and Development Director. “On the construction steel side, our research has
instead shifted towards specific materials to address problems most common in a customer’s processes. Therefore, we do not use steels suitable for corrosion problems that have less resistance to mechanical wear for processes in which corrosive by-products are developed. Instead, we concentrate on steels that have a high hardness and wear resistance for customers producing highly abrasive compounds with glass, carbon or
Right: Comac says steel election is critical for screws and barrels
IMAGE: CPM EXTRUSION
other fibres. These special steels meet standards which ensure average values of resistance to both corrosion and wear. In the same screw composition, sectors of different steels can often be placed side-by-side, according to the area and functionality of the screw sector.” The same applies to steels used in the barrel segments, for which Comac has a standard length of L/D of 4. “This is even the case if the HRC hardness of the barrels has always to be kept higher than the screws to guarantee a longer life,” adds Pellegrini. “The novelty of barrels manufactured by Comac for a number of years now is the three piece ‘sandwich’ model, which has replaced the model with internal sleeves.”
Faster maintenance Comac adds that the main advantages provided by its concept barrels compared with a traditional design include that they offer faster and more convenient maintenance. In addition, barrel modules and sleeves can be disassembled and reassembled without special tools and with reduced handling of heavy parts. Correct alignment of modules at reassembling is easy, which makes spare parts perfectly and directly interchangeable. More convenient changing of the barrel configuration is also possible, which in some cases is required when changing the screw profile, and a full set of spare parts for the whole barrel is also said to be cheaper. The company also claims that more accurate thermoregulation can be achieved by temperature probes positioned very close to the inner surface in contact with the material being processed. And the design provides optimised tight coupling of modules to each other. JSW Japan is set to introduce a new screw element called V-Kneading, which is a special V-shaped kneading element designed to achieve higher dispersion in the compounding process. Currently under development, the company plans
58 COMPOUNDING WORLD | August 2020
www.compoundingworld.com
IMAGE: COMAC
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