Imbalance filling of multi-cavity tooling during powder injection molding

• Symmetry in geometry of multi-cavity mold do not guarantee balanced filling.
• Imbalance filling is commonly observed in PIM multi-cavity injection molding.
• Competitive roles between conduction/convection and viscous heating cause imbalance.
• CAE study is essential to avoid imbalance filling in PIM.

A shortened cycle time with improved productivity is a key justification for multi-cavity molds. However, filling imbalance is an offsetting barrier for multi-cavity molds. The sources for an imbalance are often subtle, in both conventional plastic injection molding and powder injection molding (PIM) industries. The greater thermal conductivity of the PIM feedstock and the subsequent large sintering shrinkage amplify the issue for PIM. Due to its characteristic, the size of PIM parts is relatively smaller than the conventional plastic injection molding and so multi-cavity molds are widely used. Despite the geometrically systematic delivery system and cavities, asymmetric branching of flow paths, uneven viscous heating, and cooling variations exist for the molten feedstock flow to the cavities. These variations significantly limit the benefit of a multi-cavity mold. We investigated the imbalance filling between the near- and far-side cavities in a geometrically symmetric multi-cavity mold for PIM. Via molding simulations and experiments we found that imbalanced filling arises from subtle factors such as non-uniform mold cooling, uneven conduction of the molten feedstock into mold, and uneven viscous heating and their competitive roles between conduction/convection and viscous heating in the multi-cavity molds mainly contribute to imbalance filling the same way in the conventional plastic injection molding.

The difference in thermal properties of metal feedstock compared to plastic will show different behaviors in filling pattern so, CAE analysis will play a very important role in PIM imbalance filling analysis. We could observe that metal feedstock is less prone to imbalance filling compared to plastics but proper optimization of injection speed will enable balanced filling in both cases. We found out that the competitive roles between conduction/convection and viscous heating contribute to the imbalance filling in multi-cavity PIM.Figure optionsDownload as PowerPoint slide