MIM Feedstock

Figure 7 shows a plot of viscosity versus shear rate for a MIM feedstock (58 vol. % carbonyl iron powder with EVA/beeswax binder) for 130, 150 and 170 °C. It can be observed that within the range of shear rate investigated, the feedstock exhibited shear thinning behavior. 

Yield stress has been observed in MIM feedstock suspensions at low shear rates and low shear stress conditions [1,11,13], A yield stress of a fluid can be defined as the limiting shear stress that has to be exceeded to initiate a shear flow. A fluid that exhibits such a yield stress is termed viscoplastic [29]. When an applied stress is less than a particular stress (yield stress), the... 

The two basic deformation mechanisms in a polymer are by shear yielding and crazing. In a composite system, such as the MIM feedstock, an additional mechanism, dewetting, is observed [75]. The yield point is actually due to crazing effect or a dewetting effect in which the particle-matrix interface is destroyed and resulting in a dramatic drop in the modulus of the material [59]. 

This chapter focuses on the apphcation of polymers in the formulation of binder blends for MIM. A brief summary of the different classifications of binder blends is presented. The rheological, thermal and mechanical properties of a feedstock with EVA/beeswax binder is discussed to highlight the influence of binder-powder interactions on these properties. 

Figure 6. Range of shear rates experienced by a feedstock during a typical MIM process.

However, while the Cox-Merz rule is valid for general unfilled and low concentration filled systems, it was discovered that the rule fails to hold for concentrated suspensions. It is important at this point to recognize that MIM or PIM feedstock is essentially a concentrated suspension at elevated temperature. A modified Cox-Merz rule was developed for cmicentrated suspensions by Doraiswamy et a [20]. The modified rule stated that the complex viscosity versus shear rate amplitude plot is equal to the corresponding viscosity versus shear rate plot. That is... 

A low molecular weight component is usually added to the binder system as a plasticizer to reduce the viscosity of the feedstock. Usually this component is in the form of a wax. Beeswax, paraffin wax, microcrystalline wax and camauba wax are the common types of wax used in MIM. 

Throughout the entire MIM process, the feedstock is repeatedly subjected to elevated temperatures. Hence,... 

The application of polymers to binder formulation for metal injection molding (MIM) has been discussed. The different classifications of binder formulations were listed and described. Rheological, thermal and mechanical properties of the feedstock was described with respect to the effects from powder loading, temperature, binder composition and binder-powder interaction. Morphology inspection by SEM was also discussed. It could reveal the state of adhesion between the powders and binders and is an important technique in the study. 

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