THERMAL DEBINDING OF POWDER INJECTION MOLDED PARTS - OBSERVATIONS AND MECHANISMS

The mechanisms of thermal debinding for powder injection molded parts were investigated in this study. Mercury porosimetry analysis on specimens made from carbonyl iron powders shows that fine interconnected pores, smaller than 0.8 mum, are developed at the initial stage of debinding. These pores are formed due to the decomposition of low-temperature binders. In the intermediate stage, most pores consist of 0.8-mum interparticle voids. The fine pores which were developed at the initial stage are now filled by the binder and give a flat region on mercury intrusion curves. Scanning electron microscopy (SEM) pictures show that the binder-vapor interface moves slightly inward at the initial stage. As debinding temperature increases, the binder has a low viscosity and is pushed to the compact surface by the internal decomposed gas, leaving a nearly binder-free region at the center of the compact. Concurrently, liquid binder redistributes within the compact due to capillarity and causes nonuniform binder distribution within the part. At the final stage, residual binders are all trapped at contact points between particles. These binders, though accounting for less than 20 pct of the original amount, are holding particles together with the capillary force.