From several perspectives the integration of renewable raw materials in plastics is a very important development objective for our time. From the ecological perspective, an ever-increasing proportion of plastic can be countered with recyclable natural fiber reinforced plastics. From an economic perspective, new material composites are being developed which are of great interest for the automotive industry and the lightweight sector. However, for the production of such natural fiber reinforced plastic parts in injection molding processes, increased wear occurs in the extruder area and in the mold, and this in turn results in shorter tool life, which causes process costs to increase significantly. Consequently, within the Cornet project “Smart NFR“ innovative, multi-functional film systems are being developed that reduce wear on the mold and extruder area of the plastic injection molding system. In addition, the process should be optimized through integration of thermo-resistive and piezo-resistive thin-film sensor structures in the coating system developed at the Fraunhofer IST.
Manufacturing of the sensory thin-film system
On steel inserts that can easily be installed in the injection mold, as base layer the piezo-resistive and wear-resistant DiaForce® film is deposited homogeneously in the thickness of 6 µm. On this layer individual chromium electrode structures are placed in such a manner that they are in the contact area of the plastic melt. This is followed by two insulating coatings of alumina or SiCON®, between which the conductive traces from the electrodes to the contacting areas and a temperature sensor in meander geometry are manufactured out of chromium. The entire film system has a thickness of approximately 10 µm. A mold insert with a complete film system, in which alumina as well as an insulating intermediate coating and a top coating have been deposited, is presented in the figure above. The two circular areas represent the piezo-resistive sensor areas that are half-enclosed by the meander-shaped structure of the temperature sensor.
Testing of the sensory film systems in the injection molding processing of different natural fiber reinforced plastics
At Tomas Bata University in Zlín, three inserts with sensory thin-film systems were installed in the mold of the injection molding machine of the company Arburg (Allrounder 470 H) and were tested with different natural fiber reinforced plastics. The individual systems have different colorations. This is caused by the different insulation coatings (see adjacent figure above): In the case of the left insert SiCON® coatings were deposited as intermediate coating and as top coating. On the other hand, for the two inserts arranged on the right, the transparent alumina was used as a wear-protection insulating coating. The figure in the middle shows an injection molded part after opening the mold, still in contact with the sensor systems. Using the voltage curves of the individual thin-film sensor structures, even during the injection molding process it is possible to detect whether or not a good part has been manufactured (see adjacent figure below). So far the thin-film systems for detection have been used in more than 500 injection molding processes with different fiber-reinforced plastics, without showing signs of wear.
The results described were achieved within the project “Smart coating systems for process control and increased wear resistance in processing of natural fiber reinforced polymers“, or in short “Smart NFR“, in which the Fraunhofer IST is working jointly with the Fraunhofer Institute for Machine Tools and Forming Technology IWU and Tomas Bata University in Zlín in the Czech Republic.