POWDER SELECTION FOR SHAPE RETENTION IN POWDER INJECTION-MOLDING

Flow by viscous creep during thermal debinding is a major cause of compact distortion in the powder injection molding (PIM) process. The ability to retain compact shape and attain uniform shrinkage after sintering depends on the inherent interparticle friction of the powder as well as other processing factors. In this study, the friction is measured using the angle of repose at the tap density (compacted angle of repose). Since the tap density closely corresponds to the critical solids loading in PIM feedstock, it is important to assess the interparticle friction when the particles are at this packing density. For these experiments, four iron powders and four binary mixtures of these powders were examined for compacted angle of repose and distortion. Statistical tests determined that the majority of distortion (approximately 75%) occurred in debinding. Small powders with large compacted angles of repose exhibit the greatest degree of shape retention. A compacted angle over 55-degrees is recommended for use in PIM with thermal debinding; although, it may be possible to use lower values with solvent or sublimation based debinding approaches. This provides a basis for selecting powders for the PIM process.