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In-situ polymerisation on the road to series production

In-situ polymerisation on the road to series production

Articles
02

Peter Egger (left) and Dr. Lorenz Reith (right) with a lightweight shovel produced at ENGEL’s stand during K 2016.

In the injection moulding industry it is standard practice to produce ready-to-use parts that drop out of the mould in a single work step. The production of fabric or fibre-reinforced lightweight components is still quite some way from this ideal model of efficiency, but the gap is getting smaller. In October, at K in Düsseldorf, ENGEL presented an example based on in-situ technology. Peter Egger, Director of the ENGEL Center for Lightweight Composite Technologies, and Dr. Lorenz Reith, a developer in the reaction technology area, speak about the opportunities presented by the new developments.

Mr. Egger, what was special about the manufacturing cell used to produce composite parts live at K?
PETER EGGER: For the first time, we realised an integrated and highly automated process for the in-situ polymerisation of Caprolactam to form fibre composite carrier structures which are functionalised through injection moulding. The multi-component process uses fibre fabric, ε-Caprolactam and thermoplastic granules to create lightweight, ready-to-fit parts. This is a major milestone on the road to series production. When we introduced the first prototype machine for in-situ polymerisation, we had to transfer the FRC preform to a second mould or machine for the purpose of functionalisation in injection moulding.

How does the new multi-component process work, Mr. Reith?
LORENZ REITH: At the trade fair, we demonstrated the new possibilities by producing lightweight shovels. To do this we used an ENGEL v-duo 700 vertical machine, with two differently temperature-controlled moulds supplied by our system partner Schöfer. One cavity is set up for the reactive process and the other handles the injection moulding process. After moulding of the continuous fibre reinforced carrier structure, the integrated viper robot transfers this to the second cavity. Here, detailed features such as edge surrounds, rib structures, screw domes and other functional structures such as a logo, are injection moulded in parallel with the manufacture of the next reactive part.

Why is the Center for Lightweight Composite Technologies devoting so much energy to in-situ polymerisation?
REITH: In-situ polymerisation offers major opportunities for us when it comes to producing complex composite parts as efficiently and cost-effectively as possible. Generally speaking, various procedures are available. The multi-stage approach, employing pre-consolidated semi-finished products which are trimmed, heated, shaped and overmoulded is useful where waste can be kept to a minimum and no local reinforcements are needed to improve stiffness and strength. Reactive processes, which include HP-RTM and in-situ polymerisation, offer advantages in connection with complex lay-up conditions, hollow profiles and hybrid layer structures. However, further functionalisation is relatively complex in the HP-RTM process. The components produced are made of epoxy resin or polyurethane so that joining elements and reinforcing structures typically need to be manufactured separately and attached to the structural component. On the other hand, in-situ polymerisation produces polyamide 6, onto which functional elements can be moulded directly. In-situ technology therefore enables us to improve production efficiency while following the trend towards an increased use of thermoplastic matrix materials.

What challenges did you need to overcome to integrate in-situ polymerisation and the injection moulding process?
EGGER: We have made significant design adjustments and developed a completely new reactive unit, in which machine parts for the preparation and injection of reactive components are separated. In this development, we have benefited greatly from our injection moulding experience. Many established injection moulding solutions such as high precision injection with the use of servo-electric drives can be utilized for in-situ polymerisation. The synchronous injection of the two reactive components is the most challenging sequence in the processing workflow.

Why is the injection process so critical?
REITH: For in-situ processing, a catalyst and an activating agent are added to the ε-Caprolactam in separate material containers. These additives ensure the polymerisation reaction starts immediately after mixing the two components. First the two injection plungers, which are not mechanically coupled, independently draw the required shot volumes, then inject these into the cavity in an electronically synchronised parallel movement. The nozzles on the mixing chamber also need to be opened and closed in sync with the stroke movements. ENGEL ensures this precision by means of servo-electric drives and high performance process control software specially developed for the purpose.

Melting units were one focus of development work.
EGGER: Exactly. As in the processing of thermoplastics, the aim is to prevent unnecessary thermal exposure of the raw materials. In the melting process, monomer mixtures are heated to 120°C, close to the polymerisation temperature of 140-160°C. For this reason, the material mixtures in the new reactive unit are only melted in the required quantities, and only immediately prior to processing. For the first time, we now have a technological system that supports the precise and robust processing of the necessary prepared quantity of ε-Caprolactam. This will open up further interesting application possibilities. The new reactive unit can also be combined with ENGEL clamping units of different design types, which means we can attain a generally high level of flexibility in made-to-measure production solutions.

What will be the next development  steps?
REITH: As far as efficiency is concerned, we have achieved a great deal, but we are not there yet. One aspect we are currently addressing is the synchronisation of the dissimilar processes. Since polymerisation has a longer cycle time than injection moulding, as things stand, it is not carried out in the injection moulding cycle. Then again, the cavity pressures required for in-situ production are much lower than with injection moulding production. With a suitable system layout, we can take advantage of this factor. For example, multiple reactive components can be manufactured simultaneously in a multiple cavity in-situ mould for downstream functionalisation in a single-cavity injection mould. Ideally, the cycle time for the in-situ process would precisely match the time needed for consecutive injection moulding processes.

The Center for Lightweight Composite Technologies is being expanded. How will customers benefit from this?
EGGER: We have even greater capacity for joint development work and customer trials. Alongside the v-duo 700, we now have a second vertical machine with a clamping force of 17,000 kN. It is equipped for traditional injection moulding as well as reactive process technology, equipped with an ENGEL easix multi-axis robot, and can be combined with the IR oven developed by ENGEL. This enables us to cover a very wide spectrum of applications and technologies. Our colleagues at the technology centre for large-scale machines in St. Valentin are providing excellent support for mould trials on horizontal systems, which are available for use in their centre.

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