By Jan H. Schut
Screws turning in barrels melt nearly100% of all thermoplastic, whether for extrusion, injection, blow molding, thermoforming, compounding or fiber—pretty much everything except compression and rotomolding. Over time new variations of screws and barrels have been invented, but the basic melting mechanism is the same–shear pressure and friction on pellets dragged along a barrel wall. So when a radically new extruder comes along, it’s exciting, whether the new idea catches on or not.
Such a radical new extruder appeared at the SPE’s recent ANTEC conference, co-located with NPE in Orlando, Fla. (www.antec.ws) in April. The newcomer is called a vane extruder, and it bears little resemblance to a screw extruder. It was invented by Jin-ping Qu, a professor of material processing at South China University of Technology in Guangdong(www.scut.edu.cn), who caught attention in the 1990s for patents on vibration-assisted extrusion using electromagnetics (U.S. Pat. # 5217302 in 1993).
Qu’s ANTEC paper presents drawings on the vane extruder for the first time anywhere. He had presented the concept at the previous SPE ANTEC/NPE 2009 in Chicago, but without schematic drawings because the patent application hadn’t published yet (EP 2113355 A1 published Nov. 4, 2009). Papers from ANTEC 2012 are available on a CD from the SPE (www.4spe.org) for $50 for members or $100 for non-members.
The vane extruder was also exhibited for the first time in the U.S. at NPE by Wuhan Handern Machinery Co., Wuhan China (www.handern.com) at Booth 47013. Handern ran the vane extruder in a sheet line with a 1200 mm wide die making corrugated HDPE sheet at 280 kg/hour with a 55 Kw motor. Handern had shown the vane extruder previously at China Plast beginning in 2008 and also in operation at the K 2010 show in Germany. The developmental extruder, which was built by Handern and the university, isn’t available commercially yet.
ADVANTAGES OF THE VANE EXTRUDER
Its advantages, Qu says, are short thermo-mechanical history, low volume, and a different melting mechanism that aligns molecular chains, rather than breaking them. So it reportedly processes plastics with better mechanical properties than a screw extruder. “The vane extruder is more effective in orienting molecular chains than the shear flow in the screw extruder, and the orientations are very important in the formation of some special crystal structures,” Qu’s ANTEC paper claims.
The vane extruder has been tested successfully with LDPE, HDPE and PS with nano CaCO3 at 8%-10% fill levels at rotor speeds of 30, 60, 90, 120 and 150 rpm and with various die openings. The tensile strength of LDPE and PP made in a vane extruder is significantly higher than that of the same LDPE made in a screw extruder, Qu reports. LDPE made in the vane extruder showed tensile strength of 13.4 MPa vs. 11.0 MPa for the same LDPE made in a screw extruder. PP processed in the vane extruder showed tensile strength of 36.5 MPa vs. 32 MPa for PP processed in a screw extruder.
The vane extruder also reportedly causes less degradation. Thermo-mechanical history is 64% shorter than for a screw extruder, Qu claims, with low energy consumption. PS loses less molecular weight in a vane extruder. PS with an initial Mw of 212506 g/mol. drops only 3.18% to 205741 g/mol. after extrusion at 150 rpm in a vane extruder vs. a drop of 18.3% to 173447 g/mol. after extrusion at 150 rpm in a screw extruder. Another potential benefit is that since the vane extruder doesn’t rely on shear and viscosity to move plastic, it could process lower viscosity resins than a screw extruder.
HOW THE VANE EXTRUDER WORKS
The vane extruder has a short rotor with vanes turning in an off-center cylinder with baffles, which is believed to be unique. Rotors have been placed off-center in round cylinders before, but not vice versa. HPM Corp., formerly in Gilead, Ohio, which closed in 2009, built a Wave screw for Prodex extruders for about 10 years with an off-center screw in a round barrel for better mixing. Varying depths between the Wave screw and barrel wall were offset along the length of the screw like the handles of a crank engine to balance wear on the screw.
Instead the vane extruder consists of a series of Vane Plasticating and Conveying Units, placed off-center around a straight rotor. Each plasticating unit consists of an eccentric static cylinder, a rotor, and four vanes installed in pairs in rectangular radial slots through the rotor. Each pair of vanes slides reciprocally back and forth in its through-rotor slot as the rotor turns. “The inner bottoms of the two vanes are in contact with each other, and the outer top surfaces are in contact with the inner surface of the cylinder,” the patent application says. The eccentric gap is adjustable.
“Embodiment 2” of Qu’s patent application describes offsetting each plasticating cylinder slightly to the left or right of the preceding cylinder, so viewed from the top, the cylinders zigzag. Thus the cylinder sections are off center on the rotor. The illustration from the ANTEC paper shows a rotor with a feed section and three such plasticating units, but the number of units can be varied, depending on the type of plastic and other factors.
The ANTEC paper gives test results for a vane extruder with a 400 mm long rotor, 15 plasticating units, five temperature zones, and an L/D of 12. The vane extruder operating at NPE had a 750 mm long rotor, 12 plasticating units, four temperature zones, and an L/D of 10. (In a vane extruder L/D means the length to diameter ratio of the rotor, not of the cylinder, which aren’t as similar as they would be in a screw extruder.)
Eccentricity in the vane extruder creates the melting mechanism. Plastic pellets are crushed, compressed, and melted simultaneously between the rotor and cylinder wall as the gap between them narrows. “Solid materials are stretched in the rotating and discharging direction and are encapsulated with the melt,” Qu’s paper says. Melting is therefore based on elongational flow with compression and barrel heat, not on friction and drag, according to the patent application.
Baffles on the cylinder separate one plasticating unit from the next, the discharge baffle of one unit becoming the feed baffle of the next, and so on. Melted and compressed plastic passes into the large volume area of the next plasticating unit. To keep the discharge of plastic from pulsing, the vanes in each subsequent unit are set at a 45 degree angle to the vanes in the preceding unit. This seems to work because the extruder was demonstrated at NPE without a melt pump. The rotor can either be built in one piece or assembled in sections.
WHERE OTHER RADICAL EXTRUDERS ARE TODAY
The vane isn’t the only radical extruder ever built. Over the past 15 years, at least two other unusual alternative extruders were introduced–the Conex conical extruder from Conenor Ltd. in Tampere, Finland (www.conenor.com), and the stubby Fimex rotor with tightly grooved barrel from the University of Paderborn in Germany (www.uni-paderborn.de). Both are also much shorter than a screw extruder and claim greater energy efficiency. Both were commercialized in a small way and then more or less forgotten, even though they worked. Both are still available, though neither is actively being built or sold today.
The patented Conex extruder (U.S. Pat. # 6450429 and 6722778) made a show-stealing debut at the K ’98 show in Germany with its tiny footprint. It nests two or more hollow low rpm counter-spinning cones between heated cone-shaped stators, allowing each rotor to melt and extrude materials on its inside and outside surfaces. Each rotor is powered by two motors from behind. Feed augurs are located on opposite sides of the stator.
Conex was developed by a consortium of Nextrom (today Maillefer), a maker of optical fiber machinery in Vantaa, Finland (www.mailleferextrusion.com); Uponor Corp., a pipe maker also in Ventaa (www.uponor.com); and NK Cables, formerly Nokia Cables and now Draka NK Cables in the Netherlands (www.draka.fi). Nokia-Maillefer built 23 Conex extruders in all, mostly for R&D. Eight were sold commercially: four to Uponor, which has worldwide exclusivity for pipe; three to NK Cables, which had an exclusive for wire and cable coating until 2001; and one to Nexans, a cable maker in Nurnberg, Germany (www.nexans.de).
The Conex business was spun off in 2001 as Conenor in Finland, which developed a Conex variation for wood plastic composites with Maillefer and built a demo line in Finland. One was sold in 2007 to a WPC start-up in Canada, using the largest cone size with 500 mm rotor root and 200 mm tip diameter. The nested rotors are also a different design for WPC with holes allowing material flow through the rotor for better mixing.
The University of Paderborn’s short Fimex rotor was not initially intended for plastic. It was designed to process ground corn using only the heat of friction and the moisture in the corn to make loose fill packaging. The Fimex rotor, which has no heater bands, starts with a conventional looking conveying section for roughly half its length. Then the root diameter expands to almost the full width of the barrel, creating a barrier flight. The remainder of the rotor has narrow, tightly spiraled flights. The barrel starts with a grooved feed throat, then becomes smooth over the conveying flights and spirally grooved over the grooved part of the rotor.
Researchers tested the Fimex with plastic pellets and found that it could also melt plastic. It was shown at K 2001 in Germany, called the “intensive plastifier.” But without heating or cooling, the temperature was too hard to control for plastics. The technology was invented by Helmut Potente, a professor at the university, and spun off in 2003 to Emendo-Tec GmbH in Horn-Bad Meinberg, Germany (www.emendo-tec.de), which offers the Fimex in three sizes: 80 mm 1.5:1 L/D, 100 mm 1.5:1 L/D, and 100 mm 4:1 L/D. Emendo-tec has sold three to G&G Naturpack GmbH, a maker of wooden pallets in Borgentreich, Germany (www.naturpack.de) to make corn into loosefill for packaging.
Both Conex and Fimex have advantages in specialized applications. But machine costs are high, and neither caught on. The new vane extruder, however, will be built in China, so it could be more cost competitive. Big operating questions remain. How hard will it be to clean out all the unusual baffles, vanes and through-holes in rotors? Does polymer hang up in the slots the vanes slide in. Will output rates be economical? Can a vane extruder process plastics with conventional sized fillers? The tests were done with nano fillers. The answer, the patent says, is that it has “wide adaptability and small volume.” It’s inventor also says a lot of work is needed before it will be commercialized. We can’t wait to see the next iteration!