Powder injection molding (PIM) is a cost effective technique for producing complex and precise metal or ceramic components in mass production. The used raw material, referred as feedstock, consists of metal or ceramic powder and a polymeric binder mainly composed of thermoplastics. The thermoplastic binder composition gives plasticity to the feedstock during the molding process and holds together the powder grains before sintering.
Most binder systems are made of multi-component systems with a range of modifiers which fulfill the above mentioned requirements. The flow behavior of the feedstock is the result of complex interactions between its constituents. The viscosity of the feedstock and its reproducibility batch by batch is the base for a production of high quality green parts with low scrap rates. Thus rheology is a key factor for the production of high quality PIM parts, the characterization of feedstocks themselves and for reliable results of numerical simulation of the PIM process.
From the rheological point of view the PIM-feedstocks are highly-filled polymeric suspensions. The flow behavior is further complicated by particle–particle interactions, which cause their redistribution and reorientation in the binder, and thereby influence the bulk rheological behavior. Due to these interactions PIM feedstocks, compared to thermoplastics, have their own specific rheological behavior. Furthermore effects such as yield stress, wall slip, phase separation, and pre-shearing can have a significant effect on the flow behavior, the accuracy of the measurement and thus on related results of injection molding simulation.
In general, the flow properties of feedstocks depend on the temperature, binder composition, powder content and powder characteristics (particle size distribution and shape of particles).
The viscosity curve describes the dependence of the viscosity on the shear rate. At very low shear rates, for thermoplastics the viscosity curve usually changes to a horizontal viscosity line. The viscosity value at very low shear rates is more or less constant or independent from the shear rate (Newtonian behavior). This constant viscosity is called zero-viscosity η0 (Fig. 1). After a certain shear rate (View the MathML source), viscosity starts to decrease rapidly as a function of shear rate, this is known as shear thinning or pseudoplastic behavior. For highly filled compounds like PIM feedstocks a yield stress can be observed. Thus the viscosity increases dramatically when decreasing the shear stress and the zero shear viscosity is hard to measure and thus shear thinning is observed even at very low shear rates. Around a certain higher shear rate View the MathML source a second Newtonian plateau can be observed and at very high shear rates (View the MathML source) the plateau can change to an increasing viscosity curve due to formation of particle agglomerates that can restrict the flow of the binder system. In various industrial processes the shear rate usually ranges between 10−3 and 107 s−1. The very high shear rates occur, e.g. in the molding of thin walled parts.
This article appeared in the Jan/Feb issue of Metal Powder Report.
