By Jan H. Schut
In situ molding could be the answer to mass producing engineering thermoplastic composites, especially metal-replacing structural parts for future cars. In situ molding mixes melted monomers of a polymer in two parts, one with catalyst, the other with activator, and puts them into a heated mold where they polymerize. For PA6 they polymerize at a temperature over the melt temperature of the monomer, but below the melt temperature of the polymer. The reaction is anionic “ring-opening” addition for PA 6 and PC or conventional chain addition for PA 6.6. Other condensation polymers that could potentially be molded in situ include PMMA, PBT, TPU, and PEK and copolymers are possible.
In situ thermoplastic reaction injection molding has been known for 50 years, primarily for nylon 6, but it’s done by a slow batch casting process with conventional RIM equipment. NYRIM, for example, is a PA6 copolymer system for thermoplastic RIM, commercialized in 1981 by DSM NV in the Netherlands (www.dsm.com), and acquired by Brueggemann Chemicals, Heilbronn, Germany (www.brueggemann.com) over 10 years ago. NYRIM combines caprolactam with different ratios of elastomeric prepolymer (7-40%) and polymerizes them into copolymers in situ using conventional RIM equipment. Caprolactam with activator is heated in one pot, elastomeric prepolymer with catalyst in another. They’re combined in a RIM mixing head and polymerized at low pressure in aluminum molds. It’s a niche process for specialty parts with high strength requirements and short production runs like treads for earth movers.
What’s new is in situ thermoplastic RIM based on conventional injection molding, which can be fully automated and fast enough for mass production for the first time. Three years ago Engel Austria GmbH in Schwertberg, Austria (www.engelglobal.com), built a prototype for the first reactive thermoplastic RIM machine for caprolactam using reciprocating screw injection units instead of heated tanks. Engel worked with the Fraunhofer Institute for Chemical Technology in Pfinztal, Germany (www.ict.fraunhofer.de), to develop the new approach.
The advantage of molding monomers is that they have much lower viscosity than polymers, so monomers can thoroughly impregnate dry continuous or woven fiber structures without disturbing fiber position. In situ molding can thus achieve directional woven fiber contents of up to 65 volume % and mold more complex shapes than viscous polymers. Compared to traditional thermoset RIM molding of epoxy and PU, continuous thermoplastic RIM has other advantages: short cycle times, greater toughness and impact strength, weldability, and recyclability. Below is the latest in thermoplastic RIM developments. The next blog will look at a similar surge of R&D in thermoplastic in situ RTM.
AUTOMATING IN SITU THERMOPLASTIC RIM
Engel’s in situ process for PA6 started as a PhD thesis by Lars Fredrik Berg at the Fraunhofer ICT, supervised by Peter Elsner at the Karlsruhe Institute of Technology in Germany (www.kit.edu), and Georg Steinbichler, head of R&D at Engel. Berg’s thesis, finished in 2011, yielded enabling machine developments, which Engel and the Fraunhofer continued to work on.
In 2011 Engel built a prototype for a commercial thermoplastic RIM machine using a modified tiebarless Engel e-victory reciprocating screw press with two all-electric injection units inclined at a 45 degree angle. One melts epsilon-caprolactam monomer (flake or pellets) with catalyst, the other melts caprolactam with activator. A non-return valve patented by Berg and the Fraunhofer (U.S. Pat. # 8684726) goes onto the end of the reciprocating screws and seals against the inside of the injection barrel, allowing precise feeding and injection of very low viscosity monomers. The two melts are combined in a high pressure mixing head and fed with very low pressure into a mold to cure.
Engel has a patent application on a way to make fiber composites or hybrid components (U.S. Pat. Applic. # 20130181373) using a modified high-pressure mixing head. Engel partnered with Hennecke GmbH, Sankt Augustin, Germany (www.hennecke.com) to optimize Hennecke’s high pressure RTM mixing head for PU for lower viscosity caprolactam instead. Caprolactam has a viscosity of 4mPas, so leakage was an issue. Hennecke developed new dynamic mixing technology for the high pressure, high temperature head for caprolactam. Throughout the molding process, caprolactam also has to be protected against moisture, since moisture absorption impedes or stops polymerization.
Engel’s injection molding-based RIM process was shown first at an Engel open house in June 2012, molding a continuous fiber PA6 insert for a passenger car brake pedal, developed with ZF Friedrichshafen AG in Friedrichshaven, Germany (www.zf.com). Then the IKV Institute of Plastics Processing RWTH in Aachen, Germany (www.ikv-aachen.de) and 14 partner companies worked with Engel to develop an automated in situ injection molding process combined with TPU overmolding, which was shown for the first time by the IKV at the K 2013 show in Germany. The automated process prototyped an athletic shin guard, starting with a woven fiber glass preform made of multiple layers, consolidated by a binder, robotically trimmed with ultrasonics, dried, and inserted robotically with needle grippers into heated injection molds. Temperature control is critical. Caprolactam melts in the injection barrels at about 70 C, passes through heated runners and the electrically heated mixing head at about 100 C, and into molds heated to about 150-160 C.
The two-cavity in situ molds for the shin guards were built by Schoefer GmbH in Schwertberg, Austria (www.schoefer.at), with a compression edge with silicone seals to contain the low viscosity monomers and to compress the fiber preform at the edges so that no flash forms. The e-caprolactam was specially developed for the application by LanXess AG in Koeln, Germany (www.lanxess.com). Cure time was below 3 minutes. In a separate injection molding machine the shin guards were over-molded on the back with soft-touch TPE.
Auto company interest in in situ molding is also visible. Toyota Motor Corp., Tokyo, Japan (www.toyota-global.com) got a patent years ago for in situ RIM molding of PC (U.S. Patent # 5514322), describing carbonate compounds with catalyst and activators, mixed and fed into a mold to polymerize. The patent says that the anionic addition reaction is controlled by the a third ingredient (BPh3 Lewis acid) so that the reaction starts inside the mold, not before. The patent cites 23 formulations of which the 23rd cures in situ in 4 minutes with thorough impregnating of continuous fibers and makes PC with molecular weight of 17,000. Another Toyota patent (JP Pat. # 194-157769) describes catalysts for in situ molding. It doesn’t appear that Toyota has applied in situ PC technology commercially, though light weight parts for the energy saving Prius would be a good candidate. The Prius-Alpha hybrid minivan already claims to have the largest PC panoramic roof in the world.