Sundar V. Atre

Optimisation of process conditions in powder injection moulding of microsystem components using a robust design method. part I. primary design parameters

Abstract Powder injection moulding (PIM) is a net fabrication technique that combines the complex shape forming ability of plastic injection moulding, the precision of die casting, and the material selection flexibility of powder metallurgy. For this study, the design issues related to PIM for the fabrication of thin walled, high aspect ratio geometries were investigated. These types of geometries are typical to the field of microtechnology based electro chemical, mechanical and biological systems, which are multiscale (sizes in at least two or more different length scale regimes) fluidic devices working on the principle of heat and mass transfer through embedded micro- and nanoscale features. Stainless steel was the material chosen for the investigations because of its high temperature resistivity and chemical inertness necessary for typical microfluidic applications. The investigations for the study were performed using the state of the art computer aided engineering design tool, PIMSolver. The effect of reducing part thickness on the process parameters, including melt temperature, mould temperature, fill time and switchover position, during the mould filling stage of the injection moulding cycle was investigated. The design of experiments was conducted using the Taguchi method. It was found that the process variability generally increased with reduction in thickness. Mould temperature played the most significant role in controlling the mould filling behaviour as the part thickness reduced.

Integrated filling, packing and cooling CAE analysis of powder injection moulding parts

Abstract A coupled numerical analysis of the filling, packing and cooling stages of powder injection moulding (PIM) has been implemented. Finite element method/finite difference method methodologies were used in the filling and packing stages while Boundary Element Method (BEM) was used for the cooling stage. Using these methodologies, a numerical simulation program for the injection moulding process of PIM parts, PIMSolver was developed by taking into account the peculiar rheological behaviour of powder–binder mixtures. Specifically, the apparent slip phenomena at the mould wall and the yield stress were incorporated into the above analysis. The coupled analysis among the filling, packing and cooling stages was performed because the viscosity and slip phenomena of powder–binder mixture highly depend on temperature. In order to evaluate the significance of the coupled analysis and slip phenomena, several PIM experiments were performed using 316L stainless steel powders dispersed in a paraffin wax–polypropylene binder system. Using the examples of a U-shaped test specimen and an electronic package part, the importance of coupled numerical analysis for PIM parts and the significance of slip dependency of temperature during the coupled analysis were demonstrated.

Master decomposition curve analysis of ethylene vinyl acetate pyrolysis: influence of metal powders

Abstract Polymer burnout (pyrolysis or delubrication) is a crucial step in sintering die compacted powders. To systematically analyse and design the thermal delubrication step, the master decomposition curve (MDC) has been formulated based on the intrinsic kinetics of polymer pyrolysis. The Kissinger method was used to estimate the activation energy from thermogravimetric analysis (TGA) experiments. The activation energy of poly(ethylene-co-vinyl acetate) (EVA) was determined and an MDC analysis was performed to map the weight loss of the polymer as a function of time and temperature. The developed MDC was used to investigate the effects of powder chemistry, powder shape, and particle size of 316L stainless steel on the decomposition behaviour of EVA. The activation energies for decomposition of EVA decreased in the presence of gas and water atomised 316L stainless steel powders, indicative of a catalytic effect. This effect was more pronounced for the first decomposition step suggesting the possible role of a carboxylate ion – metal transition state complex that promoted decomposition. In addition, the gas atomised 316L stainless steel had a greater effect on lowering the activation energy for decomposition compared to water atomised 316L stainless steel, emphasising the influence of powder surface chemistries. Based on the MDC analysis, the required hold time can be predicted for a given temperature and target binder weight loss. This reduces the experimentation required to optimise the delubrication cycle. Furthermore, when extrapolating to very small particle sizes, this approach is of particular interest for predicting the behaviour of nano-particulate materials.

Effects of lubricant and part geometry on the ejection characteristics during die compaction

ABSTRACT The ejection of a part following die compaction is a critical step in manufacturing powder metal and ceramic parts as well as pharmaceutical tablets. In this paper, the ejection of die-compacted hollow cylinders of heights 0.5 and 1.2 cm from Fe–2%Cu–0.5%C powders mixed with various amounts of (0.2–0.8 wt-%) of ethylene-bis-stearamide (EBS) was studied to understand the effect of lubricant amount and part geometry on the ejection process. Additionally, the ejection data of gears of 0.5 and 1.2 cm die compacted from Fe–2%Cu–0.5%C powders with 0.8 wt-% EBS was analysed to understand the effect of geometry on the ejection process. Several ejection parameters were found to be sensitive to the amount of EBS as well as the size and shape of the parts. The results from the present study indicated that the major portion of the ejection cycle involved the movement of the part within the die.