Metal Injection Molding (MIM) or Powder Injection Molding (PIM) is a powder metallurgy process in which fine powdered metal (or ceramic) is mixed with a binder often comprising wax and polymers to form a feedstock. The molten feedstock is then injected in to a mold and cooled to give a ‘green part’. This undergoes a thermal or chemical debinding step to remove the binder and form a ‘brown part’ that is sintered to form the final dense component.
The rheological properties of the feedstock are of major importance for MIM applications since they influence the homogeneity of the molten feedstock, how well it flows through the die into the mold cavity, and the mechanical properties of the green part on cooling. Rheological properties are influenced by several factors including binder and solids composition, temperature and flow rate. Of particular importance is the so-called critical particle loading when the viscosity increases greatly and which is influenced by particle size and shape.
Both capillary rheometers can be used to measure the rheological characteristics of the feedstocks. The capillary rheometer is better able to mimic process conditions associated with injection molding.
Almost any metal can be used in MIM so long as it has the correct powder properties, including particle size and shape. High packing densities are required so spherical particles with the correct size distribution are preferable, as are smaller particles (< 38μm) that sinter more readily. Slight particle irregularity can be beneficial as it increases interparticle friction giving mechanical strength to green and brown parts prior to sintering. For particle size measurements laser diffraction is the preferred technique, while for particle shape analysis morphological imaging is an important tool. X-ray techniques such as X-ray diffraction (XRD) and X-ray fluorescence (XRF) can also be used to determine the crystal structure and elemental composition of metal and alloy powders which can affect sintering behaviour and the chemical and physical properties of fabricated components.
Binder properties are critical for the MIM process as they facilitate melt flow, provide structure to the green body on cooling and must to be readily removed in the debinding stage. The main binder components are natural waxes or synthetic polymers but other additives can be used. While the choice and ratio of components in the mixture defines its physical and rheological properties it is ultimately the molecular weight and molecular structure of the main components which is important. Such properties can be measured using Gel Permeation Chromatography (GPC).