| Powder injection molding (PIM) is an advanced near net shape process with well recognized capability for efficiently producing complex parts. However, the typical PIM products' tolerance is only ?.3% compared to that of the Powder Metallurgy(P/M) products, which is +0.1 %, and the machining can even achieve the higher tolerance. So, the lower dimensional tolerance has become an obstacle in the process of the PIM's industrialization. Systematic research on the PIM products' tolerance has been made: in this dissertation, such as:1) Control of the feedstock. Powder loading and homogeneity of feedstock are the most factors influencing on the dimensional tolerance, and they are determined by the forces between the powder and binder. Addition of the proper quantities of surfactant can better the interaction between the powder and binder. The influence of surfactant on the wetting angle, rheological property, powder loading and homogeneity are investigated, and the minimum amount of surfactant which just form an adsorption layer on the surface of powder are calculated by creating the mathematic model. Moreover, some methods evaluating homogeneity of feedstock are offered.2) Control of injection process. In the process of injection, the overall appraisal of rheological property and shape retention of feedstock is the key factor in the control of tolerance. There is often contradiction between the requirements of rheological property and the compact's shape retention, and the better rheological property doesn't mean the compact's shape retention is also better. The tolerance of compacts made of PW-EVA-HDPE-SA binder system is the highest.The distortion of compacts in the debinding process can be traced to the residual stresses in the compacts. In this dissertation, the residual stresses due to viscoelastic behavior of the powder injection molding feedstock have been approximated by an equation developed from modificaitions of exiting equations as follows:τ =τ0exp[-(G/η)tc] where (G/η) =1/tr, tc is the cooling time and tr is the relaxation time. The results indicates that, if the relaxation time is greater than the coolingtime the compact will contain stresses which are related with the thermo-mechanical properties. In addition, lengthening the cooling time or lowering the cooling rate can reduce the residual stresses.The effect of molding conditions represented by molding temperature and pressure on the compact shape retention has been studied. The higher the molding pressure and temperature, the lower the distortion that occurs during the debinding up to a certain level followed by increase in distortion with increasing molding pressure and temperature due to the appearance of residual stresses. Influence of molding pressure and temperature on the products' tolerance can be explained by the empirical equation 8=23.218exp(-0.0341P) and 8=3460.1exp(-0.0642T).3) Control of the debinding process. Study of the compact's shape retention is essential to the control of tolerance in the debinding process. Influence of different debinding atmosphere, powder's characterizes, powder loading and heat rate on the compact's shape retention has been investigated in this thesis. The essence of distortion during the debinding has been analyzed from theory, and changing graph of the compact's strength during the debinding has been formulated.4) Control of the sintering process. The distortion during the sintering is due to the inhomogeneous density distribution in the green, temperature gradients in the sintering furnace, different sintering atmospheres, friction with the sintering support, gravity induced creep, and so on. In this thesis, the influence of temperature gradients in the sintering furnace and sintering atmospheres on the tolerance has been studied.5) A mathematic model to estimate the products' tolerance has been provided. In the condition of uniform shrinkage, by considering the feedstock's homogeneity, molding parameter's instability, the influence of debinding and sintering process on the... |
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