Characteristics of the mold filling simulation for a variothermal injection molding process with highly filled thermoplastics

Bipolar plates, repetition parts in a fuel cell, have to be preferably produced due to cost efficiency reasons by a mass production technique like injection molding. In order to meet thermal and electrical conductivity requirements as necessary product features, state of the art research focuses on highly filled thermoplastic compounds with carbon filler loadings of up to 65 percent by volume. Operating temperatures of the fuel cell (100-200 degrees C), corrosion stability in the chemical setting (hydrogen, water) and mechanical stability lead to the selection of the high performance thermoplastic polymer polyphenylene sulfide (PPS) as a matrix material. However, due to high filler loadings injection molding machines have to operate close to their stress limits. Only significant machine operating stress reduction allows the production of bipolar plates with injection molding machines for standard applications (injection pressures <= 2500 bar). Hence, the potential of operating stress reduction by means of a dynamic mold temperature control via oil is focused. Therefore, the mold temperature was set above the crystallization point during the filling phase of the injection molding process of a test specimen, based on the flow situation of bipolar plates for high temperature fuel cell operation. For modelling the rheological and thermodynamic material behavior, the realistic cavity pressure is prognosticated. For this reason measured cavity pressure profiles during the filling and holding phase of the injection molding process are compared with those of the injection molding simulation. For this purpose a quality criterion of forecasting the cavity pressure was defined.