Designing for metal injection molding (MIM)

doi:10.1533/9780857096234.1.29

Handbook of Metal Injection Molding

Woodhead Publishing Series in Metals and Surface Engineering

2012, Pages 29-49

https://doi.org/10.1533/9780857096234.1.29 Get rights and content

Abstract:

In this chapter, a basic design guide for MIM components is presented. Although many of these design guidelines are similar to those used for plastic injection molding, there are subtle differences in how MIM components are designed to account for the subsequent thermal processing. For example, wall sections should not exceed a certain thickness to prevent defect formation during thermal debinding and a flat section is often designed into the component to provide uniform support for sintering. These design concepts and many others are described here as suggestions to provide guidance, since many potential components will not perfectly fall into the ideal design. As such, the opportunity exists to find clever solutions outside the advice provided here to take advantage of the MIM technology for particular applications.

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Citation Excerpt :
The green part densities in MBJ (generally 50% to 65% of a fully dense body) are remarkably lower than that of PM components processed by a cold isostatic pressing before sintering (almost 80% of a fully dense body [19,20]), leading to higher shrinkage/distortion during sintering of MBJ parts. Besides, in comparison to MIM parts with a typical mass of 15g [21], MBJ parts not only have bigger dimensions and wall thicknesses but also more complex geometries (with channels, lattice structures and in general overhang areas). These attributes of MBJ components escalate the effect of gravity and friction, leading to considerable distortion during sintering, which is detrimental to dimensional accuracy [9,20,22-24].
Sintering, as a post-processing step in metal binder jetting (MBJ), often results in distortion. Numerical simulations can predict sintering distortion and minimize costly trial-and-error experiments. The present paper implements a numerical approach based on a phenomenological model of sintering to capture the creep deformation during free sintering. To qualify and calibrate the material model for MBJ, metallographic studies, dilatometry experiments, and viscosity measurements are carried out instead of empirical models for viscosity and sinter stress. Using the calibrated material model, final sintering distortions are predicted in two industry-relevant parts. The reproducibility of MBJ products is illustrated by manufacturing seven specimens for each geometry, and a statistical method to evaluate the simulations’ accuracy is suggested. The predictions have a good agreement with experimental results, particularly for specimens with a lower build height. As the build height increases in MBJ specimens, the number of interlayer gaps in the build direction grows, resulting in anisotropic densification. This causes a lower prediction accuracy under isotropic shrinkage assumption, especially in overhang areas. Consideration of anisotropic shrinkage and heterogeneous density distribution of green parts in sintering simulation is essential to improve the accuracy of sintering predictions for MBJ components.
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