By Jan H. Schut
Physical foam molding has been dominated for over 15 years by MuCell microcellular foam technology from Trexel Inc., Wilmington, MA (www.trexel.com), commercialized in 1999 and backed by a master part patent on “Injection Molding of Microcellular Material,” published in Europe in 1998 (EP # 952908) and the U.S. in 1999 (U.S. Pat. # 6884823). Trexel had 200 MuCell licenses by 2004 and 300 MuCell machines in operation by 2006, when it stopped requiring licenses and sold MuCell rights along with OEM machine installations. Since then Trexel hasn’t said how many MuCell machines are running, but it has licensed six OEM machine builders to sell MuCell-equipped injection molding machines directly and an additional six OEMs to install MuCell equipment purchased through Trexel.
Trexel, however, wasn’t first. Union Carbide Corp. developed physical foam molding with a continuously turning screw and melt accumulator, licensed to several machine builders in the 1960s and ‘70s, including Uniloy Springfield, now part of Milacron (www.milacron.com), which still builds them. Battenfeld GmbH in Meinerzhagen, Germany, now part of Wittmann-Battenfeld GmbH (www.wittmann-group.com), invented direct gas foam injection molding for an intermittent screw (U.S. Pat. # 4381272) in 1979 and built the machines until the late 1980s. In the 1990s Battenfeld improved control of gas injection and screw design with new patents (U.S. Pat. # 6451230) filed in 1999 in the U.S. and 1998 in Europe, just as Trexel was starting out. So Battenfeld and Trexel agreed not to sue each other’s customers.
Around 2000, two other physical micro foam technologies were developed and patented in Europe—Ergocell and Optifoam—both different from Trexel and Battenfeld, which add physical foaming into the barrel. Ergocell and Optifoam added blowing agent into the melt between the barrel and plunger, which meant dosing against much higher pressures of the injection mold vs. dosing against pressure in the barrel. A fifth microfoam technology, Profoam, developed around 2006, adds blowing agent to plastic pellets before melting with very low pressure. All five technologies—Battenfeld, MuCell, Ergocell, Optifoam and Profoam—describe using carbon dioxide and nitrogen blowing agents, but all known applications of any of them use nitrogen. All five could reportedly make microcellular foam parts that were similar or identical, which brought Ergocell and Optifoam into the shadow of Trexel’s part patent when they were commercialized.
Ergocell from Demag Ergotech GmbH in Schwaig, Germany, now part of Sumitomo (SHI) Demag Plastics Machinery GmbH (www.sumitomo-shi-demag.eu), injected physical blowing agent into the melt with a mixer and reservoir device. Ergocell was introduced at the K Show in Germany in 2001. Trexel cited its patent and insisted that molders needed a MuCell license to produce any micro foamed parts, regardless of method. By December 2001, Trexel had an agreement with Demag Ergotech to sell Ergocell machines only with a MuCell license, so Ergocell was effectively finished.
Optifoam, invented at the IKV Institute for Plastics Processing at RWTH University in Aachen, Germany (www.ikv-aachen.de), adds blowing agent to the melt using a porous spider-like torpedo in a porous sleeve (DE Pat. # 1983021). In 2003 the IKV licensed Optifoam exclusively to Sulzer Chemtech Ltd., Winterthur, Switzerland (www.sulzer.com), which commercialized it in 2005 with two automotive molders, Master Industries Inc. in Ansonia, OH, and a European molder. Sulzer, the IKV, and Huntsman Polyurethanes in Belgium also partnered to develop Optifoam thermoplastic PU shoe soles with 63% density reduction. Trexel filed a complaint in a U.S. district court against Master and in 2006 announced that Sulzer’s U.S. customer had stopped using Optifoam, while its European customer had taken a MuCell license.
A LONG PATENT STRUGGLE
Starting in 2003 Battenfeld, Sulzer, Peguform GmbH (now part of SMP Samvardhana Motherson Peguform GmbH), a molder of car cockpit systems in Boetzingen, Germany, and several other European companies with patents for micro foam molding began a long struggle to get Trexel’s master part patent revoked in the European patent office in Munich. In 2005 Trexel’s “Microcellular Molded Article” patent was revoked in Europe. Trexel appealed, and it was reinstated in 2007, revoked again in 2008, appealed again, and finally rejected by the European patent office in September 2011.
By 2008, Battenfeld had been acquired by Wittmann in Austria. Battenfeld’s second generation physical foam molding, now named Cellmould, was announced at a Wittmann Battenfeld open house in Austria that year, but the machine wasn’t yet commercial. Sulzer spun its static mixer business and Optifoam off as a management buyout in 2012 to Promix Solutions AG, also in Winterthur (www.promix-solutions.ch), which offered medium-to-high density Optifoam only for extrusion, not for injection molding (see this blog, Nov. 9, 2010). In 2014 Sulzer acquired low density extrusion foam specialist Aixfotech GmbH in Aachen, Germany, and took the Optifoam name back, but not the technology, which Promix continues to offer for extrusion.
The IKV’s patent-applied-for Profoam (WO Pat. Applic. # 200613660) combining blowing agent with pellets hadn’t been commercialized. The IKV’s first technology (DE Pat. Applic. # 102005061053) for adding physical blowing agent before the barrel used pelletized dry ice, or frozen CO2, blended with plastic pellets. Profoam, however, is like an autoclave, impregnating plastic pellets with nitrogen under low pressure before they are fed into the barrel.
Profoam was developed after 2007 in partnership with Arburg + Co KG in Lossburg, Germany (www.arburg.com); Volkswagen AG in Wolfsburg, Germany (www.vw.com); Ticona GmbH (www.celanese.com); Bayer MaterialScience (www.covestra.com); and LyondellBasell (www.lyondellbasell.com). The IKV first presented Profoam in an ANTEC paper in 2008 by Walter Michaeli, Thorsten Krumpholz, and Domenik Obeloer, who wrote his doctoral thesis on the process. Arburg and the IKV tested the prototype successfully with PP, PC, PC/ABS, PBT, PPS, PS, PA6, and even TPE. The IKV gave a second ANTEC paper on Profoam in 2011, but no partners are named on either ANTEC paper.
Arburg built a commercial version of the Profoam device with a control interface to an injection molding machine. The IKV had applied for a patent on Profoam, but Arburg, not the IKV, patented it (DE 2009100124810 published in 2010), including a critical valve for the autoclave chambers. The patent is under the name of Arburg’s inventor and senior partner, Karl Hehl, not under Arburg’s name. The device had two pressurized chambers, one on top of the other, in which pellets are impregnated with nitrogen before being fed into a standard injection molding machine.
Pellets are loaded into the top chamber under ambient pressure. The chamber is locked; blowing agent is added under low pressure (up to 50 bar). The airlock under the chamber opens, so pellets fall into the lower chamber, which locks. The upper chamber opens to ambient air and refills, as the lower chamber opens and releases pellets still under pressure into the feed throat. When the pellets melt, blowing agent dissolves evenly into the melt.
The amount and distribution of gas is far better controlled than by any of the higher-pressure injection approaches into melt, IKV’s Obeloer says. An important modification needed for the injection molding machine is to fit the back of the screw with a seal, so gas doesn’t escape. Profoam uses more nitrogen than other processes because of venting the upper chamber, but presumably this could be recaptured. Profoam molding is lower viscosity than solid molding, so it can make glass-filled PP with longer fibers than equivalent solid parts, Arburg says. Another plus is that the Profoam device can easily be moved from one injection molding machine to another or disconnected.
MICROFOAM COMPETITION REAPPEARS
In 2009 Battenfeld, which always had the least to fear from Trexel’s part patent, opened its new plant in Meinerzhagen, Germany, and showed a Cellmould machine for the first time, offering it commercially in 2010 with improved controls, an improved screw design capable of finer bubbles, and pneumatically controlled shut-off nozzles. Cellmould can be used with Battenfeld’s BFMold (ball-filled mold) technology for rapid mold temperature cycling and with other mold technologies like gas counter pressure and expanding molds.
Cellmould, which doesn’t need a Trexel license, was commercialized in 2011 for washing machine parts, using the technology more for part strength and rigidity than for light weighting per se. Battenfeld says it has about a dozen Cellmould customers now, including Tier 1 automotive suppliers.
By 2013, Arburg had commercialized the first Profoam part in-house, molding glass-filled PP instrument covers for its own injection molding machines. Arburg didn’t show Profoam publicly until 2015, first in-house at its Technology Days conference in March, then at Fakuma 2015 in October, but not at NPE 2015 in the U.S., where Arburg showed light weight composite sheet technology instead.
At Technology Days 2015 Arburg molded glass-filled PP automotive airbag housings in a two cavity mold on an Allrounder 820A in 70 second cycles. The 280-mm-long part weighed only 272 grams, 18% less than a solid part. At Fakuma 2015 Arburg molded an automotive sliding tray housing on a hydraulic Allrounder 630 S in 65-second cycles. The 20% glass fiber PC part weighed 361 grams, 13% less than a solid part.
At Technology Days this year, Arburg showed Profoam molding an ambitious automotive interior glove compartment door with a high gloss surface, using “variotherm” rapid mold surface temperature cycling. The PC/ABS part was molded in 60-second cycles on a hybrid Allrounder 630H with 2500 kN of clamp force. The part weighed 200 grams with wall thickness of 1.8 mm, 30% less than the previous solid part with 2.5 mm walls.
Arburg, which is a licensed seller of Trexel’s MuCell, ran Profoam and MuCell side by side at its Techology Days both years. In 2016, MuCell foamed a structural bracket out of nylon 6 using a mold designed for a solid part and achieving a 28 second cycle—20% faster than solid molding with a weight reduction of 7%, while meeting all strength and dimensional requirements.
Arburg won’t say how many Profoam devices it has sold, but Arburg builds only one Profoam size with one-liter chambers for small injection molding machines up to 200 tons, so it’s safe to assume they are for small parts and development. VW, which partnered with the IKV on Profoam in 2010-2012, has a Profoam device in its technology center, but doesn’t use it commercially, and the IKV has one. Automotive applications will presumably have to wait for the 2019 model year, when Trexel’s U.S. part patent will have expired.
With competition on the horizon Trexel’s business is also becoming more mature and competitive. In April Trexel introduced its T Series of new user friendly “smart” dosing control, which requires an operator to input only two parameter—shot weight and percent of supercritical nitrogen—setting the rest automatically. Previously MuCell dosing was much more difficult, requiring a highly trained technician to enter six parameters.
Trexel is also becoming a broader based foam company, offering engineering design for foam parts with up to 30% weight reduction instead of the usual 10%. In September 2015, Trexel even introduced a chemical foaming agent, called TecoCell, licensed for injection and automotive blow molding from Polyfil Corp., Rockaway, NJ. Polyfil continues to offer its EcoCell foaming agent for extrusion. So Trexel can now offer foaming for smaller production runs without capital investment.