High-temperature contact heating table expands potential of high-performance fiber composites

Fiber-reinforced composites are increasingly being used for lightweight construction applications in vehicles, aircraft and mechanical engineering. In particular continuous fiber-reinforced plastics based on thermoplastics are growing in popularity. Some resins, however, require high processing temperatures that may be beyond the capabilities of conventional processing equipment. Further, the heating of reinforcing structures such as fabrics made from thermoplastic pre-impregnated uni-directional fiber-reinforced UD (unidirectional) tape requires a high level of process control and reproducibility.

Press manufacturing company Wickert Maschinenbau GmbH (Landau/Pfalz, Germany) says it has come up with flexible system for tackling such high temperature challenges. The equipment was designed based on process engineering technology developed by the Fraunhofer Institute For Chemical Technology (ICT, Pfinztal, Germany) to consolidate UD tape supports.

The system has already entered trial operation. At the end of 2014, Wickert delivered a high-temperature contact heating table that can achieve processing temperatures of up to 425^oC. This is being used at Fraunhofer ICT by the thermoplastics processing (TP) working group, part of the Polymer Engineering Department. One of the research focuses of the TP working group is local continuous fiber reinforcement, which also covers the processing of continuous fiber-reinforced semi-finished products such as UD tapes.

"In the past, the most you could process was fabrics made from thermoplastics such as PA6 or PA66, but the market now requires increasingly high-performance matrices" says Raphael Jauch, project manager at Fraunhofer ICT. His aim is to be able to process tape materials with matrices from high-performance thermoplastics such as PPS or PEEK, thereby entering a new dimension in applications.

Fraunhofer ICT houses the entire process chain under one roof. The fabrics are manufactured at the Augsburg site by the Functional Lightweight Design project group (FIL). Here, the UD tapes are processed into the desired reinforcement structures in the so-called RELAY process (Rapid Efficient Layup) with any given layer set-up. The UD tapes are placed precisely on a moving table in layers and "spot-welded" using ultrasound.

The orientation of the tape, and therefore the fibers, can be set in an infinitely variable way by rotating the table and can therefore be adjusted optimally to any load. These fabrics are then processed further and consolidated in a two-step HTP process (heat/transfer/press). It is here that the newly developed high-temperature contact heating table from Wickert comes into play, heating up the fabrics before they are pressed. In order to manufacture laminates with the best possible quality and reproducible properties, the individual layers must be bonded together in this process step without any air pockets. The decisive factors when it comes to heating the fabrics include homogeneous temperature distribution and high plane parallelism for the heating plates.

The new system allows the manufacture of laminates based on high-performance thermoplastics (such as PPS, PEI or PEEK) that are of great interest to users in the automotive and aerospace industries, both in terms of material characterization and further processing into complex high-temperature components.

The maximum product dimensions that can be processed are approximately 1100 mm x 1100 mm. With a heat output of 2 x 50 kW, the maximum temperature of 425°C can be reached very quickly from room temperature. Both heating plates are equipped with six-zone temperature control designed specifically for this purpose, making it quick and easy to make material-specific adjustments. The control accuracy of the heating element zone monitoring system is +1.0°C.

Further, the wide variety of operating modes offers maximum flexibility: the contact heating table can be time- or temperature-controlled, which results in extremely high product reproducibility. All process-related data, such as the target/actual temperatures of the heating plates, product temperatures, heating and stop times and the entire cycle time, are recorded, monitored and displayed.