By Jan H. Schut
As commercial interest in micro injection molding picks up for high-precision, speck-sized medical and electronic parts, so is R&D. Probably the most unusual development in decades is a novel one-piece melting and injection unit for micro molding, which melts with flights in a stationary barrel, not the rotating shaft. The idea behind the new technology is to reduce melt volume in the barrel, residence time, and risk of thermal degradation.
The technology was invented at the Institute for Plastic Processing (IKV) at RWTH Aachen University in Germany (www.ikv.rwth-aachen.de) and presented for the first time outside of Germany at the SPE’s ANTEC conference in Las Vegas last April (www.4spe.org) by Torben Fischer, a research assistant and PhD candidate at the IKV, and Christian Hopmann, head of the IKV. The technology was first presented at the 29th Polymer Processing Society conference in Nuremberg, Germany (www.pps29.com) in 2013.
Their ANTEC paper describes an “Innovative Plasticizing Method for Micro Injection Molding,” which is a colossal understatement. Plasticizing is done by an “inverse screw” with flights in a stationary barrel, not on the rotor. It’s believed to be the first plastic processing of any kind that melts with stationary flights, not turning ones. The fixed barrel flights correspond to feeding, compression, and metering zones in a conventional screw. Barrel flights are 4 mm deep in the feed zone tapering to 2 mm in the metering zone. So they are nothing like the shallow regular grooves used in the feed sections of some extruders to increase throughput. The 160-mm-long “inverse screw” barrel has an 80-mm feed zone, 48-mm compression zone, and 32-mm metering zone with three temperature zones for compression, metering, and the nozzle.
The flighted barrel allows a much smaller diameter rotor to be used. The IKV has tested shafts as small as 8 mm diameter, whereas 14-15 mm diameters are typically the smallest used in conventional micro injection molding machines (or 12 mm with special micro granules). With a 15-mm diameter screw, a typical injection stroke of three diameters in length makes a shot of roughly 18,000 cubic mm of material—far more than needed for micro injection molded parts. An 8-mm shaft reduces shot volume to 1,200 cubic mm, the IKV presentation says—15 times less.
The “inverse screw” technology is also unusual because it’s a single machine unit with one rotating, reciprocating shaft that melts plastic and feeds molds. Micro injection molding is conventionally done in two units-one melting, one feeding. The Babyplast micro injection molder, built by Chronoplast S.L. in Barcelona since the mid-1980s and distributed by Rambaldi Co. I.T. Srl, Molteno, Lecco, Italy (www.babyplast.com), for example, uses two smooth-bored plungers. The first melts plastic, the second meters it into molds. Other commercial micro-injection molding machines typically use either screw-to-plunger or screw-to-screw processes.
BUILDING THE FIRST ‘INVERTED SCREW’
The IKV’s patent-applied-for plasticizing method (German Pat. Applic. # DE102009057729) was invented by Thomas Kamps, a previous doctoral candidate at the IKV. Kamps’s dissertation was actually on vibration in micro molding, presented as “Heating and Plasticizing Thermoplastics with Ultrasound for Micro Injection Molding” (see blog May 30, 2010). In the course of his thesis work, Kamps hypothesized that plastic melting could occur whether flights rotate against a smooth barrel or vice versa.
To prove the concept Kamps built a simple hand-cranked aluminum model and tested it with PP, which showed that it didn’t matter where the movement occurred. Research on the concept was subsequently funded for three years starting in 2011 by the Deutsche Forschungsgemeinschaft, Bonn, Germany (www.dfg.de), together with Arburg GmbH, Lossburg, Germany (www.arburg.com). Fischer did the practical work and built the first motorized test rig as part of his dissertation.
Because they were putting “screw flights” into a barrel for the first time, they built the barrel in two halves out of hardened stainless steel (X5CrNiCuNb16-4), using 3-D laser sintering from BKL Lasertechnik GmbH, Roedental, Germany (www.bkl-lasertechnik.de). The “inverse screw” test unit was driven by a small synchronous torque motor from Beckhoff Automation GmbH, Verl, Germany (www.beckhoff.com), and set up to run continuously like an extruder. Fischer’s ANTEC presentation describes tests done with this initial motorized test rig using PP, PMMA, and POM. The test rig was designed so they could try different shapes and surfaces for the flights and shaft.
They polished the barrel flights, but left the shaft unpolished, just as a conventional screw has polished flights and an unpolished barrel to prevent melt slippage. But there were feeding and pressure issues at first. So the IKV tested also knurled, and lengthwise grooved shafts. Initially no plunger geometries could feed PP and PMMA, which had larger pellets. Pellets bridged in the feed zone of the barrel and wedged into the longitudinal grooves in the shaft. The grooved shaft, however, could process POM, which came in smaller pellet size.
The unit also didn’t build up enough pressure before the nozzle for injection. POM melted properly in the feed zone, but the melt still contained air bubbles at the end of the metering zone. So the IKV did more nozzle development and was able to build enough head pressure for injection molding.
The grooved shaft also showed excessive wear on the groove over the feed section. So the feed opening was optimized to prevent jamming by “applying different feed geometries to the upper half barrel shell and analyzing the feed behavior of different pellets,” IKV’s Fischer explains. The feed opening described in the ANTEC paper is the length of one pitch of the barrel flights (11.2 mm). Ultimately the feed opening became three pitches long (33.6 mm). The feed opening is also parallel to the shaft, whereas in conventional injection molding the feed opening is cross-wise to the screw.
Arburg has since built two industrial “inverse screw” prototype molding machines based on its electric Allrounder 270A injection molding machine, the first reciprocating versions of the “inverse screw.” One was given to the IKV in February 2014, officially presented during the IKV’s technology colloquium in March. The other is at Arburg. These prototypes can process PP, PMMA, and POM, provided pellets are small enough (roughly 2.5 mm). The next step now is for the IKV to test Arburg’s industrial prototype against conventional two-step micro injection molding machines to benchmark its performance. The “inverse screw” technology could also potentially be used to melt pellets for 3D printing and is so compact that it would be small enough for desktop 3D printers (see blog Oct. 27, 2014).