Injection molding is the first step in the production chain of MIM applications. It determines part quality, as faults induced here cannot be mended out later on. The green part quality and the scrap are therefore permanently targets for optimization. Additionally MIM producing companies start to care about cycle times of each sub-process. It becomes especially important when continuous sintering ovens are used. Consequently, the performance of injection molds for MIM becomes critical. Appropriate analysis tools are required to identify optimization potentials, in order to reduce cycle times and increase quality. This paper describes the experience of a MIM producer about how process-integrated injection molding simulation supported efficiently a new mold development. By understanding the interaction between feedstock, mold components and mold tempering system, drawbacks in the mold design were identified early. Innovative solutions were found to avoid local hotspots; cycle time was reduced and article quality improved in parallel.
Paper EuroPM 2012
In the past, when discussing possible optimization potentials with MIM producers, the answers often lead towards topics like scrap, segregation, sinter warpage etc. Cycle time, the key topic in thermoplastics production, didn’t play a significant role for feedstock injection, due to the bottleneck of batch-wise sintering. Nowadays, based on the increasing success of continuous sinter ovens, the minimum achievable cycle time during the molding phase becomes economically interesting and thus the thermal performance of the injection mold.
In the past years, MIM simulation and corresponding pu- blications focused mainly on rheology [1, 2]. This topic was important as feedstock rheology is different compared to for e.g. thermoplastics simulation. Accurate rheology is the key to simulate flow patterns right, which is essential to reliably design gates and to decide for gate location. The feedstock solidifica- tion however was not discussed in detail and neither were the consequences regarding induced stresses or required cycle time. Even the performance of the injection mold was interpreted regarding the significant influence of rheology  but not regar- ding economical savings due to possible cycle time reductions. It can be concluded that injection simulation has been available in the industry for some years now, as an efficient tool to reduce time to market and to identify flow-related issues, such as jetting, pressure or flow front propagation. It has been proved that simulation can help identifying part quality issues related with the filling behaviour of the part. Effects such as particle segregation can nowadays be predicted . It has also been proved that conventional thermoplastic simulation is not accurate enough to reproduce the complex rheological pheno- mena occurring in MIM applications and that expanded rheological models have to be used, in order to capture the increase in the viscosity which occurs at low shearing rates . However, the state of the art in MIM simulation goes far beyond. Not only the part itself can be simulated, but the whole mold, with all its components, can be considered easily [3, 4]. The thermal behaviour of the mold, the way it heats-up due to heat exchange with the tempering channels, can be today predicted accurately. Furthermore, the coupled heat exchange between melt and cavity can be calculated .
Additionally, this can be done over several production cycles, so that the effect of residual heat from one cycle to another can be considered in the simulation, until the mold-melt system has achieved a steady production state - in the same way as it occurs in reality. And the case study presented in this paper nicely shows that MIM producers start to care about these details of their molding processes. They better understand how the green part quality develops. They can reduce mold trials and iteration in steel significantly. And they appreciate that reducing cycle time saves money.