Heating of Thermoplastic Sheet by Means of Resistance Heaters
Because of its good mechanical properties and low weight, reinforced thermoplastic sheet is finding increasing use in the automobile industry. The thermoplastic sheet is reinforced with carbon fibers and can be formed into three-dimensional parts by first heating the thermoplastic matrix and then subsequently allowing it to cool. Until now, heating was accomplished generally through use of contact heaters, recirculating air ovens or IR heaters, but these all involved high losses from either convection or radiation.
At the Institute for Plastics Technology (IKT) in Stuttgart, a research project has been investigating whether the electrical conductivity of carbon fibers can be used to heat the matrix. With this in mind, voltages in the range of 15 V were applied to test specimens of reinforced thermoplastic sheet with the aid of two electrodes and the temperature changes recorded by a pyrometer at a distance of 1200 mm from the part. The experiments demonstrated that this method makes fast and uniform heating of the reinforced thermoplastic sheet possible. In addition, greater efficiency is achieved with this method because of the low heat losses. The researchers see additional benefits in the good controllability and small footprint of the equipment needed for the process. According to the information provided the risk of thermal oxidative degradation or delamination of the thermoplastic sheets is very low for the newly developed process.
On the basis of these results, high-volume use of the process has received positive assessments. In further investigations, the reinforced thermoplastic sheet will be softened and formed following in-situ heating in order to analyze in depth the relationship between the structure of the carbon fibers and the heating method. In addition, plans call for investigating how the efficiency of heating can be increased with a specific orientation of the fiber structures.