Influence of the pre-heating process of thermoplastic mono-material sandwich-structures for thermoforming

Continuously fiber-reinforced sandwich-structures are highly used for lightweight design applications. However, with an increasing number of recycling cycles, the fiber length decreases with each cycle and downcycles the material. One solution to this problem is the usage of mono-material sandwich-structures. The reinforcement fibers, the matrix material and the shear-resistant foam core consist of the same thermoplastic base-polymer and enable therefore mechanical recycling without the need to separate the different material components. Within a fusion bonding process the semi-finished products are joint to 2D sandwich-structures. The thermoforming of these materials requires precise temperature control before, during and after forming. The different process steps of preheating, thermoforming and cooling of sandwich-structures causes a time dependent temperature gradient in the direction of the structures thickness [1] while meeting three requirements: the material must be heated above glass transition temperature to enable the forming, while also preventing the foam cells from collapsing and the reinforcing polymer fibers from thermally induced relaxation. To investigate the pre-heating of mono-material sandwich-structures based on Polyethylene terephthalate (PET), four different process routes where compared: conductive, convective, microwave- and infrared- (IR) based. To enable the monitoring of the temperature during the conductive, convective and IR-based processes, thermocouples were introduced into the sandwich-structures bevor the fusion bonding process. They were placed into the foam core and the boundary layer between the foam core and the face sheets at different positions. The temperature control during the microwave-process was implemented by a thermocamera. The formation of the temperature gradient was observed by choosing 130 °C and 170 °C as temperature levels. The cooling behavior was observed by positioning the sandwich-structures between two flat steel plates which were kept at 40 °C. Subsequently, the experimental results were compared within the four process routes: a microwave-based process is not suitable for the preheating of PET-mono-material sandwich-structures, preheating by using conductive, convective or IR-based processes is possible, while conductive and the IR-based process route had comparable, fast cycle times.