Twin-screw extruders | machinery feature
Peter Mapleston looks at how manufacturers are lifting the productivity of their twin-screw extruders by raising torque levels, increasing free-volumes, reducing machine downtime, and rethinking screw element designs
The twin-screw compounding extruder continues to evolve. Some equipment producers continue to push the limits on torque, while others are concentrating on increasing output in other ways – most notably by reducing downtime. There is a lot of room to play with here, since it is far from rare for there to be several formulation changes each day on a small- or medium- sized extruder, each one requiring a purge or even an important reconfi guration of the line. Torque has been a major focus for twin-screw
extruder manufacturers over recent years with the maximum rating increasing from 15 to 16 to 18 Nm/cm3
.
Leistritz claims to have set a new record of 20 Nm/cm³ with its ZSE 50 XX concept extruder, which it unveiled at K 2013. Michael Thummert, head of marketing at the company, says that the screws are able to deliver such high torque because they are made out of a single piece of metal using special Leistritz technology. The company uses a whirling process to cut the
full-length one-piece high-precision screws, rather than grinding or milling individual screw elements. Using solid screws that are machined in one piece means that the screw confi guration cannot be changed for different compound formulations. However, the screw design can be optimised for a particular application prior to machining. In addition, the company says that the cost of a one-piece screw is lower than a segmented design, so it is possible to have different screws for different compounds, plus they are quicker to change. The drive unit, with a water-cooled AC drive and pneumatic safety coupling to a high-torque gear box, has a magnetic sleeve coupling behind the screws that
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Take twin-screw extrusion to new extremes
enables the exact torque on the screws to be measured. The screws also incorporate RFID (Radio Frequency Identifi cation) chips that enable the extruder, which has a sensor in the coupling box, to identify them as high-torque screws. This information is passed to the machine controller, “unlocking” full power. The RFID chip can also save data such as hours in operation and process parameters. Other features on the ZSE 50 XX include side feeders with modular screws that can be adapted to specifi c additives, making it easier, for example, to prevent air getting into the extruder. The degassing unit has a heated barrel to facilitate removal of volatiles. Both the side feeder and the side degassing unit are easy to attach to the extruder using four easily accessed tie rods. Thummert said at press time that it was still too
early to say when the commercial version of the ZSE 50 XX would go into production. ICMA San Giorgio has added to its MCM family of
co-rotating extruders, with the HP (High Performance) class, also introduced at K 2013. These offer torque density levels of up to 16 Nm/cm3
, and in addition have March 2014 | COMPOUNDING WORLD 29
Leistritz raised the bar for
torque density with the K 2013 launch of its ZSE 50 XX concept extruder
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