MACHINERY | TWIN-SCREW EXTRUDERS
Mixer or pump?
The twin screw compounding extruder is an excellent mixing device but a poor efficiency pump, says Dean Elliott, Technical Processing Manager at US-based machinery maker Entek. “This is manifested as undesirable temperature rise of the molten compound occurring mostly at the screw tips,” he says. Elliott and his team have been running tests at the company’s pilot plant in Lebanon, Oregon, that aim to minimise this risk by making some simple processing changes without incurring capital expenditure. In a trial running a 50% talc filled MFI 12 polypropyl- ene masterbatch on a 43mm co-rotating twin screw extruder they found that 40-pitch convey screw elements (CoC-2-40) performed best, while 60-pitch convey (CoC-2-60), single flight convey 60 pitch (CoC-1-60) and undercut feed 60 pitch elements (CoF-2-60) all performed the same. Running the last two barrel sections at significantly lower barrel temperature set points significantly reduced compound melt temperatures. The trial was carried out by installing three different restrictions
at the outlet of the extruder to simulate low, medium and high back pressure and the final 240mm (5.5 L/D) of convey elements on the extruder screw configuration were altered with different styles and pitches. “Tribal knowledge amongst TSE [twin screw extruder] users is that the tighter the pitch of the screw element, the more efficient the pumping. We learned that this is true, however, once the pitch becomes tighter than 1D the pumping becomes less efficient,” says Elliott.
While pumping performance can be optimised it does not
match a melt pump. “What this study showed was that a convey screw element with a pitch equal to the diameter of the screw is best for pumping,” says Elliott. “The co-rotating TSE remains an inefficient pump at roughly 5 to 15%
efficient...the melt pump is roughly 25 to 35% efficient.” �
www.entek.com
Above: Comac was promoting its capabilities in compounding of biopolymers at K2019
combined with a low screw speed, because PLA does not tolerate stress,” says a company spokesperson. “The degassing system has been increased (in different points) to extract moisture from materials like starch. Liquid injection systems are required because liquids are necessary to allow starch plastification.” The company says a typical extruder size supplied for bioplastic compounding is 92mm, which will provide an output of around 800kg/h.
Graph showing the relationship between pressure and exit melt temperature according to convey elements used (from top: 80 pitch, 60, 30, 40, 40 with last two barrel sections around 110°C colder than upstream sections. 43-mm extruder, 200rpm, 115kg/h) Source: Entek
24 COMPOUNDING WORLD | March 2020
Custom configuration Italian company Maris says its prime advantage in the market is its ability to customise its co-rotating twin screw machines to the customer’s specific needs. “This is true also when it comes to highly filled compounds,” the company says. “In order to increase extruder performances over the years, Maris has made numerous technical process improvements maximising, therefore, both the filler percentage and the output capacity.” The most important of these developments are said to include: the ability to offer several Do/Di ratios to provide different free volumes; to offer a wide range of extruders with different specific torques; the possibility to add the filler in four different feeding points (one in the main feed and three in side feeders); incorporation of special equipment — such as a deaeration system — to facilitate the introduction of fillers that incorporate air; and optimisation of the screw profile and screw elements for a particular process. “The synergy of these technical solutions has allowed us to reach filler percentages up to 80-85%, preserving the output capacity and keeping excellent dispersion standards,” says a company spokesperson. Optimisation of screw profiles is also a priority at Japanese compounding extruder maker Toshiba
www.compoundingworld.com
IMAGE: COMAC
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