By Jan H. Schut
It’s hard for unusual extruders to graduate from universities into production. So a commercial trial for Germany’s “S-Truder,” developed at the University of Duisburg-Essen (www.uni-due.de),would be exciting after over ten years and four different prototypes. S stands for Schmelzetrennung, or melt separation in German, but think of it as “sieve.” The extruder uses a fairly conventional screw with a very unconventional solid/melt separation device – a sieve – which functions like the barrier section of a screw, but reportedly doesn’t cause shear. The sieve is a slightly conical sleeve with bore holes between the screw and barrel that draws melted plastic away from solid material in the melt channel, so melt doesn’t overheat.
Perforated melt separation devices have been tried before. In the 1970s, Imrich Klein and Zehev Tadmor (who wrote the early bible on extrusion, “Engineering Principles of Plasticating Extrusion”) patented an SDS Solids Draining Screw (U.S. Pat. # 3924842) for a screw-within-a-screw with holes drilled into undercuts in the channel of the outer screw, which drained melt to the inner screw. New Castle Industries, New Castle, PA, now part of NordsonXaloy (www.xaloy.com), built a few SDS screws in two sizes with up to 8-inch outer/3-inch inner screw diameters, but it remained an oddity.
In the early 1990s screw designer Robert Barr Inc. in Virginia Beach, VA (www.robertbarr.com), developed and patented a “melt bleeder” extruder (WO 2002004184) for a hollow screw with slots that bled melt off through the center of the screw. Barr called it the Barr 2000 and built two of them for testing with blow molding. The patent included another embodiment for “a cylindrical melt sleeve” with radial slots between the screw and barrel, but this was never built.
PROTOTYPE #1: THE FIRST SIEVE EXTRUDER
The University of Duisberg’s S-Truder is believed to use the first perforated device to drain melt off the outside of a screw, not the inside, which has advantages in cleaning and temperature control. The S-Truder is the creation of three successive doctoral students at the university under Professor Johannes Wortberg. Two students, who have since graduated, designed and built the first three prototypes, shown at K Shows in Dusseldorf, Germany, in 2004, 2007, and 2010 respectively. A third student is building the fourth prototype now. Each iteration solves some problem of the one before. The fourth and latest S-Truder, being built now, will be at the K Show in October. If it passes its tests at university, it also could be the first S-Truder to make production trials.
The target is high speed extrusion of blown film – a process where high-speed has been tried before, but with little success. High-speed extruders are typically 75-85 mm diameter with long 30-40 L/D screws that go 800 to 1500 rpm, and extrude over 2000 lb/hour of plastic – the output of an extruder twice that size. But high-speed extruder screws are optimized for one or two polymers—PP, PS, or ABS—and can’t do anything else. A high speed screw for PP, for example, would burn PE since PE’s viscosity doesn’t decrease as much with increasing shear.
The idea behind the S-Truder was to develop a high output extruder, which could process different polymers. “We wanted high-speed and flexibility,” explains Marcel Grossmann, who built the second and third prototypes during his PhD work at the university in 2007-2011. “We didn’t want a high-speed extruder that could only process one or two materials.” Grossmann now works as a project manager in development for the South African packaging company Mondi Group in Gronau, Germany (www.mondigroup.com).
The first S-Truder was built as a research project by Dirk Kaczmarek, a PhD student at the university from 2000 to 2003, who also works for Mondi as a technical account manager. The first S-Truder was shown at K 2004 by ETA Kunststofftechnologie GmbH in Troisdorf, Germany (www.eta-kunststofftechnologie.de), a builder of screws and dies. ETA is co-owned by Professor Wortberg and built the components for all the S-Truders. Kaczmarek and Wortberg published the first description of the S-Truder in a 2003 ANTEC paper on “A New High Speed Extruder with Melt Separation.”
“The sieve was a strange idea,” Kaczmarek recalls, “but we did a lot of calculations to see if it would work, and it worked.” Barrel diameter nearly doubles from 35 mm to 60 over the sleeve, which is fixed to the barrel and slightly conical, wider upstream, tapering downstream. Plastic melts underneath the sleeve and is forced out through the bore holes, which are designed so molten material leaves the screw channel without losing significant pressure.
The first S-Truder was short (16 L/D) with a 35-mm screw with five zones: a grooved feeding zone (2 L/D), a shallow-flighted metering zone (4 L/D), a tapered transition zone (3 L/D),then a long plastifying zone with the sieve sleeve (5 L/D), followed by a static mixer (2L/D). L/Ds are approximate based on a schematic drawing in a university publication.
PROTOTYPE #2: KOAX S-TRUDER
The coaxial or Koax-S-Truder was a joint development. The “Koax” part was designed and built by Kaczmarek. It has two co-rotating screws, one inside the other, a more or less standard 35-mm plasticating screw, inside a short 70-mm coaxial feed screw. The screws are separately driven by one four-motor Compact Motor Gear (CMG) drive from K&A Knoedler GmbH in Ostfildern, Germany (www.knoedler-getriebe.de). One motor drives the feed screw up to 330 rpm. The other three motors drive the main screw as slowly as possible.
The feed screw has elongated holes in the flight channels, which pass some pellets through to the main screw. Most plastic is fed via the outer feed screw, but a second feeding zone flows between the feed screw and the main screw. Both feed zones are grooved. “Using the separately driven feed screw allows higher specific throughput, no longer influenced by the bulk density or physical properties of the raw material,” explains Grossmann. “It also allows low speed plasticating for lower melt temperature.”
Grossmann combined the coaxial dual-screw feeding with the sieve sleeve into the Koax-S-Truder, which was shown by ETA at K 2007. Grossmann and Wortberg published the first description of it in a 2009 ANTEC paper called “Koax-S-Truder—A New Flexible and High Efficient Plasticating Concept.”
The coaxial S-Truder is 20 L/D with five zones; first the screw-over-screw feeding zone (4 L/D), next the transfer zone (3 L/D) on the main screw, followed by the plasticating zone with a slightly longer sieve sleeve (7 L/D), where barrel diameter again doubles to accommodate the sleeve. Last comes a low pressure homogenizing zone with a static mixer (6 L/D).
PROTOTYPE #3: HIGH-SPEED S-TRUDER
The High-Speed S-Truder, also built by Grossmann, is shorter, simpler, and a lot faster. It was shown by ETA at K 2010 and described in an article in Kunststofftechnik magazine in Germany the same year. Instead of the 4-motor CMG drive, it uses a 75 kW high-speed synchronous motor from Oswald Motoren GmbH in Miltenberg, Germany (www.oswald.de), mounted directly onto the 35-mm screw without gear reduction.
The screw is again20 L/D, but with six zones: first a grooved, water-cooled feeding zone (4 L/D), followed by a transfer zone with deeper channels (4 L/D), then a plasticating zone with a much longer sleeve, which is partly sieve (6 L/D) and partly static mixer (3 L/D), followed by a conveying zone (3 L/D).
The High-Speed S-Truders goes six times faster and the screw-within-a-screw is gone compared to the Koax S-Truder. The sleeve over the plasticating zone has also changed dramatically. The sleeve is much longer – 9 L/D in all – with a sieve part upstream and a new static mixer with 500 smaller bore holes downstream. A blocking element on the screw builds melt pressure up under the sieve part and forces melt through the bore holes into the gap between the sleeve and barrel. This gap widens downstream as in earlier S-Truders. Melt reaches the static mixer and is redirected via the 500 holes back onto the screw again.
PROTOTYPE#4: HIGH-SPEED, ROTATING SIEVE
The fourth S-Truder is being redesigned and built now by a third doctoral student at the university, Gregor Karrenberg. It uses the same high-speed synchronous Oswald motor and feed section as Prototype #3, but with improved melt homogenization. Karrenberg developed a new melt flow simulation in order to redesign the sieve. He presented the new simulation in a recent ANTEC paper in Cincinnati in April:“3D-CFD Simulation of Polymer Plastification in a Single Screw Extruder under High-Speed Conditions.”
His new math reportedly has more nuances of viscosity than previous simulations of fractional melt. Using ANSYS Fluent 14.0 software, he simulates the viscosity of material that is partly solidly packed pellets and partly melted plastic filling the melt channel of a 35-mm conventional high-speed extruder (not an S-Truder). He assumes melting of the solids bed by shear heat and heat dissipation within the bed and allows for expansion of semi-crystalline LDPE, when melting causes the crystalline structure to come apart, and for thermal expansion.
He simulates a virtual high speed extruder at 500 to 2000 rpm for LDPE with throughputs ranging from 110 kg/hr at 500 rpm to 465 kg/hr at 2000 rpm. Then he simulates the High-Speed S-Truder. The simulation showed polymer backing up outside the boreholes that didn’t get through the 500-hole static mixer. So Karrenberg redesigned a floating sieve sleeve with a flight on the outside that rotates around the screw, turned by melt flow. Karrenberg will present his S-Truder simulation in July at a conference of the Polymer Processing Society in Nuremburg. He also intends to use a newly designed dynamic mixer this time.
A high output extruder for PE for blown film or blow molding is a great idea. In fact after over a decade, Barr plans to continue work on the “melt bleeder” screw again. The practical issue both the S-Truder and Barr 2000 screws face isthat they increase melting so dramatically, that it’s hard to feed plastic and homogenize melt fast enough to keep up. It’s not easy to convert radical research ideas into production machines, but it could help that the first two engineers on the S-Truder, who built the first three prototypes, work for the same packaging firm and must itch for a test drive.