application for PIM, the dimensional precision and variability of the manufactured components need to be further In this study, a correlation between green part dimenunderstood and improved. Recently, substantial part to sional variation and feedstock viscosity variation is part variability has been observed in the green (i.e. injection presented for the powder injection moulding (PIM) moulded) state before the components undergo substantial manufacturing process. A correlation of an increase shrinkage during sintering.4-6 in green part dimensional variation as feedstock vis-The importance of feedstock homogeneity has been cosity variation increases has been found and the pointed out by both part manufacturers and researchers correlation was independent of powder type (316L alike, all agreeing that variations induced in the early gas atomised and water atomised) and mixing process stages cannot be eliminated in the subsequent PIM technique (batch and continuous). The variation of process steps of moulding, debinding and sintering. feedstock viscosity was lowest over the greatest Therefore, the homogeneity of the feedstock can become a temperature range for high shear continuous comcritical characteristic for these later process steps in the pounding with a broad distribution of irregularly PIM process. Inhomogeneities can lead to powder/binder shaped powder. Thus, this feedstock material would separation, defects (e.g. cracks and voids) or increased have the greatest process window for injection moulddistortion and eventually to failure of the PIM manufacturing with the least variation.PM/1049 ing route. Supati et al.7 studied the effects of mixing time, mixing Dr Zauner
ranging from ±0•15% to ±0•5% (Vonderohe et al.5), compared with conventional PM processes, which are In this study, the effect of powder characteristics usually accurate to ±0•1%. and their variability on the dimensional variability Dimensional variation can be caused by factors stemming of green and sintered PIM components has been from any step of the PIM process. Li et al.6 looked at examined for 316L stainless steel. Three lots of gas rheological properties of binder systems in feedstocks and atomised and three lots of water atomised powders how each affected shrinkage homogeneity, which has a were characterised and used to make six batches of direct effect on controlling dimensional variation. Weaver PIM compound. These compound lots were injection and German7 found that reducing the dimensional variation moulded using a cavity pressure transducer and screw in green parts caused by injection moulding improved position regulation controls. The moulded geometry dimensional control in the final sintered part. Hu and was measured in the green state and sintered state for Hwang8 studied swelling and deformation of PIM parts dimensional variability. The general findings are that during solvent debinding and how it affects dimensional gas atomised powder produce less dimensional varichange. Cardamone et al.9 found that the dimensional ability than the water atomised powder from lot to variability increased from the green to the sintered parts, lot, however, the water atomised powders produce as did Ebenho ¨ch and Krueger10 who state that both less in lot dimensional variability and are generally sintering temperature and gravity have a significant effect less susceptible to distortion of cantilevered members on dimensional control and anisotropy of shrinkage, since during sintering. Also, the lot to lot variation in the the part is weaker during sintering, as compared with other powder characteristics, such as particle size and process steps, and stresses readily result in a permanent pycnometer density, have an effect on dimensional deformation. stability whereas variations in powder characteristics Although variability of the overall process has been such as surface area, tap and apparent density, and examined and cited in the literature, only limited attention chemistry have little effect on dimensional stability.has been given to the actual raw materials that are used PM/1064 for generating the feedstock, the 'feeding material' for the shaping process (injection moulding
Liquid-phase sintering (LPS) is a technique widely used to sinter hard and heavy metals such as tungsten carbide and tungsten heavy alloys. LPS involves formation of a liquid phase during sintering that promotes fast densification. However, the ratio of liquid to solid, microstructure and external forces (gravity, component/substrate friction) act to promote distortion as a function of sintering time and temperature. To understand and control distortion during LPS, a numerical model is being developed to solve continuity and momentum equations using a finite-element technique. In this article, transient distortion under gravity is calculated as a function of surface tension, density, and viscosity of the material. The effect of the friction force due to the component support during isothermal sintering is also evaluated and compared with experimental data acquired by in-situ recording of distortion during sintering.
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