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In-situ polymerization opens up exciting opportunities in the production of fiber-reinforced plastic components with a thermoplastic polyamide (PA) matrix. Based on dry reinforcement textiles, polymerization and injection molding are combined, enabling particularly efficient and automated processes in series production, for example, in lightweight automotive engineering. The decisive factor for processing efficiency and product quality is the reactive unit developed by Austrian machine builder Engel specifically for the preparation and injection of the monomer ε caprolactam. Engel has further optimized its solution with a new, smaller size reactive injection unit.
Now offering two sizes, Engel can now cater to an even wider range of applications and supports its customers from product and process development, through scale-up to series production. The new smaller reactive unit can handle matrix volumes between 10 and 600 cm³, offering great flexibility, especially for testing facilities, in the production of test parts, specimens, and parts up to a weight of 1.5 kg. The second, larger unit can process matrix volumes of up to 1500 cm³. Both units are very compact. They integrate the complete media supply and reduce the footprint required for the entire system.
In in-situ polymerization, a type of thermoplastic resin transfer molding (RTM) process, pre-shaped dry fiber preforms are impregnated directly in the mold cavity with the reactive matrix. Thanks to ε caprolactam's low viscosity in its molten state, the dry fibers can be wetted particularly well. Compared to conventional RTM employing a thermoplastic matrix, longer flow paths and higher fiber content are attainable. When the ε caprolactam is polymerized to create PA 6, a composite with particularly high load-bearing capacity is formed that can be functionalized by injection molding immediately after manufacture in the same process.
Servo-electric injection pistons are a proven solution for injecting the reactive components. They support particularly precise adjustment of the injection volume and absolutely synchronous injection of the two components. The recirculation common in reactive systems is deliberately avoided. The volume of monomer melted is limited to what can be processed directly. The reactive components have a particularly short residence time in the system and are not prone to residence time scatter. This, in turn, prevents thermal damage to the material.
A further benefit of Engel's system comes into play in testing and technology center operations with frequent recipe and batch changes: the residual material can be quickly removed from the system without the system needing to be flushed.
The magnetically coupled screw conveyors for feeding the solid reactive components are a new feature. They ensure reliable and process-assured feeding of the solids. The magnetic couplings are contact-free and provide a wear-free sealing to ensure that the entire material feed is evacuated.
Within the user-defined limits, the solids are continuously dosed and plasticized using an approach that is largely independent of the injection process. Up to the moment when the material is fed in, storage and conveying of the solids remain strictly separated thermally and spatially from the melting zone underneath. The vacuum above the molten material is maintained even when topping up the storage hopper's material, and this further boosts both process stability and product quality.
Both Engel reactive units can be combined with Engel injection molding machines from all series. A retrofitting option is available for injection molding machines with the CC300 control unit. Complete control integration ensures that the entire process can be managed centrally on the machine display. Optionally, the reactive unit can be operated as a stand-alone system with its own CC300 control unit.
The range of applications for in-situ polymerization extends from small parts with thin wall thicknesses through to large, highly stressed structural elements in lightweight automotive engineering, automotive electronics, technical molding, and sports equipment manufacturing. When overmolding metal inserts or cables in very small structures, in-situ polymerization can offer advantages over other processes – even without fiber reinforcement.
The new reactive unit is available for customer trials at Engel's Center for Lightweight Composite Technologies in Austria. At the Center, Engel is collaborating with the Johannes Kepler University in Linz, Austria, and mold maker Schöfer, on the further development of the in-situ polymerization process.
On account of the trend towards thermoplastic composites, this technology is increasingly shifting into the focus of lightweight engineering developers. The continuous thermoplastic material base enhances processing efficiency while at the same time paving the way for recycling composite parts. In the form of in-situ polymerization and the Engel organomelt technology, system supplier Engel has two production-ready processes for the manufacture of thermoplastic composite parts in its product range.
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