An In-Depth Examination of Placement (Tray Layout – Support) and Gravity Effects
Metal Injection Molding (MIM) is a powerful manufacturing method capable of producing complex geometries with high precision. However, the most fragile point of this strength is the dimensional deviations and distortion problems that arise during sintering. In practice, these problems are often labeled as a "furnace error" or a "material problem". Yet when the root causes are examined, it is seen that a very large portion of distortion is directly related to part placement and gravity effects.
In this article, I will address distortion formation in the MIM sintering process under two critical but often neglected headings:
(1) Placement – tray layout, contact points, and support,
(2) Gravity effect – especially in thin and long parts.
1. Placement: Tray Layout, Contact Points, and Support
During sintering, MIM parts are in a porous and mechanically weak state at high temperature, completely stripped of their binder. At this stage, the part no longer behaves like a "hard metal" but like a viscoplastic body. Therefore, the contact geometry between the part and the tray (setter) directly determines the sintering result.
Effect of Tray Layout on Distortion
Small errors made while arranging parts on the furnace tray can turn into major dimensional deviations at the end of sintering. In the literature, this condition is mostly defined as friction-induced distortion.
- Friction between the part and the tray limits the isotropic shrinkage that must naturally occur during sintering.
- Regions whose lower surface contacts the tray more shrink less, while free regions shrink more.
- This condition manifests itself especially as warpage and trapezoidal deformation.
Studies show that as the surface area of the part in contact with the tray increases, the risk of distortion also increases. Therefore, "full surface contact" is not a safe choice in most cases.
Contact Points and Support Strategies
In most production facilities, support is considered only so that "the part does not tip over". However, from a sintering perspective, support is a design parameter that governs shrinkage behavior.
A well-designed support system:
- Balances the part's own weight.
- Minimizes the effect of gravity.
- Allows sinter shrinkage to occur as freely as possible.
A poorly designed support, on the other hand:
- Creates local stresses.
- Leaves contact marks at contact points.
- Leads to asymmetric distortion.
In particular, point supports, when correctly positioned, can yield much more successful results than planar contacts. However, if the number and position of the support points are not compatible with the part's center of gravity, distortion may increase further.
2. Gravity Effect: The Silent Danger in Thin and Long Parts
Gravity is often accepted as a "constant and unavoidable" effect in sintering processes. However, especially for thin-walled and long-geometry MIM parts, gravity is one of the primary causes of distortion.
Viscoplastic Behavior During Sintering
At sintering temperatures, MIM parts:
- Low elastic modulus,
- High diffusion rate,
- A structure that can change shape over time
exhibit. Under these conditions, the part tends to sag under its own weight.
For thin and long parts, this condition progresses as follows:
- The middle region of the part displaces downward more than the ends
- Compressive stress forms in the lower fibers and tensile stress in the upper fibers
- After cooling, this shape "locks in" and becomes permanent
Geometry–Gravity Interaction
Gravity-induced distortion is directly related not only to part weight but also to geometry:
- As the length / thickness ratio increases, the risk increases
- Asymmetric cross-sections show more bending
- Thin-walled regions deform earlier than thick regions
Therefore, two different parts made of the same material and sintered in the same furnace can exhibit completely different distortion behavior.
Approaches That Stand Out in Practice to Reduce Distortion
- Sintering thin and long parts in an orientation perpendicular to gravity rather than parallel to it, as much as possible,
- Using balancing supports that take the part's center of gravity into account,
- Avoiding extensive surface contact and creating controlled contact points,
- Selecting tray and setter materials whose thermal behavior is compatible with the part material,
- Questioning the assumption that more support = less distortion.
Conclusion
Distortion in the MIM sintering process often begins much earlier than furnace parameters. Part placement and gravity effects are the quietest but most decisive actors in this process. Especially in thin and long geometries, even the most ideal sintering recipe may be insufficient if proper placement and support strategies are not applied.
Therefore, for a MIM manufacturer that truly wants to bring distortion under control, the solution begins not only in furnace settings but in the first placement decisions on the tray.
Sources / References
- German, R.M., Sintering Theory and Practice, Wiley, 1996.
- German, R.M., Bose, A., Injection Molding of Metals and Ceramics, MPIF, 1997.
- Li et al., “Shape distortion due to friction and gravity during sintering”, Journal of Materials Processing Technology.
- Heaney, D.F., Handbook of Metal Injection Molding, Woodhead Publishing, 2012.
- Kukla et al., “Influence of support conditions on sintering distortion”, Powder Metallurgy.