The exact knowledge of the flow behaviour of polymer melts is essential for the
successful use of simulation programs for plastics processing processes.
New applications and high demands on material properties very often lead to
application of customer specific mixtures or functional compounds instead of using standardized materials from the manufacturers.
Thermoplastics are provided with fillers or additives in many ways. The flow processes of highly filled polymers are complex. Occuring effects are very hard to separate, like shear flow with yield stress, wall slip, elastic effects, etc.. Furthermore, the occurrence of phase separation due to multi-phase composition of compounds is quite probable.
In this work, the flow behaviour of extremely highly filled thermoplastics such as
powder injection moulding feedstocks (PIM-feedstocks) and wood plastic composites (WPC) was investigated.
The core objective of the present study was to investigate to what extent known
rheological models for the description of flow behaviour of thermoplastics can be
transferred to and applied on extremely highly filled polymers.
The approach consisted of rheological measurements with the slit die system with flush mounted pressure sensors on a high pressure capillary rheometer. Furthermore, the rheological material data obtained with slit dies were compared with values measured using the round die.
In contrast to studies on PIM-feedstocks and wood plastic composites, the influence of temperature and the practical melt processing conditions have been examined more closely. For this purpose, the appropriate correction algorithms have been used in the rheological evaluation.
The influence of preshearing on the flow behavior of PIM-feedstocks under practical conditions was examined and evaluated by a special PIM injection machine rheometer.
Furthermore, the influence of moisture on the flow behavior of wood plastic
composites was studied. The measurement results show that predried ceramic and metallic feedstocks exhibit shear flow behavior (wall adhesion) as a flow mechanism in the whole investigated shear rate ranges. Additional shear in the plasticizing unit during the rheological measurements with metallic feedstock on the injection molding machine led to a significant reduction in viscosity. The measured viscosity function of the injection molding machine was about 6 – 10 % lower than that of the high pressure slit rheometer. The measurements on the PIM-feedstock with plate-plate rheometer showed that at shear rates lower than 1 s-1 no zero shear viscosity was observed, but instead the viscosity further increased strongly. This indicates the possible presence of a yield point. This flow behavior could be described with the Cross-WLF approach with Herschel Bulkley extension very well. An increased content of humidity in WPC results in wall slip. The calculated total pressure drop in a profile extrusion tool was approximately 1.5 to 2 times lower than measured when applying the rheological model for shear flow. Using the wall slip model the pressure drop was predicted with good accuracy. The measured viscosity data were validated by the simulation of the filling stage of injection moulding and by the calculation of the pressure drop in a profile extrusion tool. The results showed that the developed methods for the description of the flow behavior of metal and ceramic feedstocks for PIM and wood plastic composites are well suited for simulation.