Thin-Film Sensors in Injection Molds
Researchers at the Plastics Institute (Kunststoff-Institut), Lüdenscheid (KIMW) have developed temperature sensors that can be applied to part-forming mold components by means of the PVD process. The entire layered structure has a thickness of less than 10 μm. The sensor layer itself is only 1 μm thick. The thermocouple pairing NiCr/Ni has been selected for thin-film sensors. This corresponds to a Type K thermocouple.
The thin-film thermocouple is first electrically isolated from the mold insert by means of an Al2O3 layer over its entire surface. To ensure a long sensor service life, the thin-film thermocouples are covered with a wear-resistant passivation layer, also of Al2O3. Because of their low thermal mass, the thin films have a short response time compared to conventional wire thermocouples. The temperature sensors can also be placed in poorly accessible areas of the mold.
A major benefit of this technology is that several sensors can be manufactured simultaneously in a single coating process. For instance, five measuring points were placed at different locations on the surface of a mold insert. A flexible sealing element separates the contact points from the incoming plastics melt. To permit rapid replacement of the sensor in the event of damage, the KIMW has developed a version where the temperature sensing location is on the top surface.
Temperature-sensitive mold coatings permit complete online monitoring of the injection molding process. With the aid of a graphical representation of the temperature curve, sampling, startup and optimization of injection molds can be accelerated. Process fluctuations are recognized faster, which allows reproducibility of molded part quality to be increased. Development of sensitive mold coatings that also permit measurement of the cavity pressure in addition to temperature is the object of further investigations.