Some aspects concerning the powder injection moulding of hardmetal components
The application of a powder injection moulding process to the production of fully sintered hardmetal components has been studied. Salient, highly interdependent process variables investigated include; powder and binder characteristics, mixing techniques, feedstock rheological characteristics, mould design features, moulding parameters, debinding and sintering parameters. Fundamental studies were conducted to determine the effect of powder and binder characteristics on the powder loading capacity of feedstocks. Various methods of mixing were investigated. The most favourable methods were identified from the rheological response of their respective feedstocks as determined by capillary rheometry. Thermogravimetric analyses were used to; (a) identify binders and feedstocks essig beneficial debinding kinetics, (b) in the study of suitable debinding atmospheres and (c) to develop thermal debinding profiles for selected feedstocks. A spiral mould was used to assess the mouldability and optimum moulding parameters of selected feedstocks. Feedstock properties and mould design features which promoted moulding defects were identified and solutions developed. It was found that the maximum hardmetal powder loading achievable in a given feedstock was dependent on the powder size, size distribution and level of agglomeration. Low viscosity binders with high dielectric permittivities were found to promote highly loaded feedstocks. Feedstock viscosity increased with powder concentration. This relationship was modelled by a simple exponential power function over a narrow range of shear and powder concentration. Compounding methods utilising high shear melt mixing principles were found most effective in producing low viscosity feedstocks of consistent rheological response. Feedstock compositions of high powder concentrations and based on single, crystalline, wax binder systems were found to exhibit a high thermal dependence of viscosity, high activation energies of viscous flow, a high shear sensitivity and tended to segregate when subjected to shear. Such propensities were found detrimental to moulding behaviour. Spiral mould analysis revealed feedstock compositions were sensitive to changes in thermal parameters. Compositions based on multi-component binder systems were found most preferential in producing defect free mouldings of sound integrity and offered favourable debinding characteristics. Thermal debinding of mouldings was only completely effective by careful control of heating rates and when performed in hydrogen rich atmospheres. The reaction order and activation energy of the binder volatilisation was found to be dependent on the level of binder decomposition. Melt wicking was most effective using a hydrated magnesium aluminium silicate substrate. Sintered engineering components were produced by an injection moulding process with near theoretical densities and acceptable microstructures.