SCREWS & BARRELS | MACHINERY
Turning point: latest in screws & barrels
Machine life and performance, as well as final product quality, can all be improved by choosing the best screw configuration – or by using a custom-designed replacement
Deteriorating performance in an extruder can often be traced to machine wear – so replacing original extruder screws and cylinders with third-party versions is an effective way to extend machine life and performance. “Replacing plasticising units in existing extruders
is a rapidly growing segment for us,” said Josef Dobrowsky, managing director of Austria-based Conextru. “The applications are diverse – such as special screws for highly filled materials with wear protection.”
Others include: replacing standard plasticising
screws with new wear-protected versions; and adding new screws to improve quality and output capacity. “Old extruders usually have oversized drives,” he said. Conextru is increasingly replacing or retrofitting
of plasticising units in single-screw extruders. This has an impact on several assemblies, including the flange connection to the gearbox, cooled feed section – and, of course, the screw. “The new plasticising units are not a copy of the
existing design, but a transition to our technology,” he said. This applies to the groove bushing with length, volume and the number and shape of grooves, as well as the screw – which is matched to the feed zone and the cylinder. It is important to know details of the mechanical design – such as motor power, torque and required output – as the screw may need to be modified slightly. To carry out such a replacement, some dimen-
sions of the existing extruder are needed. To obtain these, it may be necessary to remove the cylinder from the gearbox. This can be done in advance by the machine’s owner. Conextru can then visit the site, take measurements, and use them to create new production drawings. These are then sent to the manufacturer of the screw and cylinder with instructions on screw characteristics. A variety of new qualities can be manufactured.
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Main image: ReDeTec’s MixFlow separates the drive and melt sections with a thermal isolator
Popular ones include: bimetallic cylinders with carbide inserts; grooved feed zones made of H13 material; and screws fully coated with tungsten carbide. This usually extends lifetime compared to the original design.
Extruders can be supplied with special steel and surface finishes. Each classification will tend to use more and more sophisticated materials on different screw and barrel surfaces. Standard execution is for virgin material, or
virgin mixed with regrind and colour masterbatch. Wear-resistant execution is for large amounts of regrind and polymers with high content of filler or any other abrasive material. Corrosion-resistant execution is for corrosive molten polymers, such as fluoropolymers or polyamide. “The question of whether to go with a standard design – or invest more in a wear-resistant one – is down to the customer,” said Dobrowsky.
Screw configuration In twin-screw extrusion, it can be hard to decide whether a conical or parallel screw configuration is the best option. The answer depends on many factors – such as
the material to be processed, its rheological characteristics, the pressure required during
Summer 2025 | PIPE & PROFILE EXTRUSION 27
IMAGE: REDETEC
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