Injection molding machine manufacturer ENGEL (York, Pa., USA) reported on Jan. 23 that it is working with the Fraunhofer Institute for Chemical Technology (ICT, Pfinztal, Germany) to develop a process whereby the low-viscosity monomer ε-caprolactam could be injected under low pressure into and around a continous fiber form, and then polymerized to polyamide 6 in-situ in the mold.
ENGEL says the only limitations of this process are due to the drapability requirements of the reinforcing fabric or scrim for high degrees of shaping. By using this method, says ENGEL, thermoplastic fiber composite components can be produced directly from dry continuous filament structures – other production methods still require semi- finished products such as organic sheet or tapes embedded in thermoplastic matrix.
Performing polymerization directly in the mold makes use of the monomer's qualities for infiltrating and wetting the fibers. The viscosity of ε-caprolactam during the infiltration is about 4 mPa, which is only slightly above the value of water, yet lower than the viscosity of epoxy and polyurethane systems, and significantly lower than the viscosity of polyamide. This allows high injection speeds for low injection pressures; forces that affect the preform are low, leaving the fibers undisturbed during the impregnation. Converting the ε-caprolactam into highly crystalline, long-chain polyamide produces thermoplastic fiber composites that offer good mechanics and high impact resistance.
At the Fraunhofer Institute for Chemical Technology (ICT), ε-caprolactam was prepared and injected for the first time using a reciprocating screw and a modified mixing nozzle. Based on this development, ENGEL Austria (Schwertberg, Austria) and the ICT have jointly built a fully functional prototype injection molding machine for the in-situ polymerization of ε-caprolactam. The tie-bar-less ENGEL e-victory injection molding machine, with a clamping force of 130 US tons and energy-saving ecodrive servohydraulics, has two electric injection units mounted side-by-side at a 30° horizontal incline. Solid ε-caprolactam with an activator is supplied in the material hopper of one injection unit and ε-caprolactam with a catalyst in the barrel of the second unit.
The prototype machine is part of a near-series manufacturing cell for the highly automated production of brake pedal inserts. Starting with glass fabric layers, first preforms are produced. Once dried, the preforms are inserted into the oil-heated mold (manufactured by ZF Friedrichshafen, Friedrichshafen, Germany). Once the mold closes, the cavity is filled within two seconds. The subsequent polymerization takes only a few minutes. Once polymerization is completed the mold is reopened, the component part is removed, sent to a CO2 laser cell for cutting and put on a conveyor belt. At the current stage of development, the brake pedal insert is then placed offline in a second mold to injection-mold ribs and other functional elements made of glass fiber-reinforced polyamide. For large-scale production, both the subsequent injection molding and the cutting of the fabric can be integrated inline.
ENGEL says the process of in situ polymerization using continuous fibers in an injection molding machine opens new opportunities for the mass production of thermoplastic fiber composites. Costs in the range of PA 6 and the direct use of fabric or scrim provide a relatively low-cost base of raw materials. The fact that these starting materials are readily available is also essential for secure large-scale production. ENGEL adds that it will take several years before production with such a system could start, noting that further joint development work by raw material suppliers, processors, research institutes and machinery manufacturers is required.