Analysis of Deformation Causes after MIM Metal Powder Injection Molding and Sintering

 

Date：[2023/11/9]
 

The deformation of the product after sintering is a manifestation of poor dimensional accuracy of the product.
The deformation after sintering is a complex problem involving various links such as raw materials, mixing, injection molding, degreasing, and sintering.
The causes of deformation after sintering can be divided into the following types:
1、 Reason for material:
1. Because in the early development of MIM, there were no injection material suppliers, and these manufacturers could only obtain patent licenses or conduct their own testing and development of adhesive systems.
However, one of the secrets of MIM's successful production is that each batch of injection materials used must be absolutely consistent. If this is not achieved, defects may occur during molding and deformation may occur during sintering.
2. Deformation caused by segregation of powder and binder.
When the sintering temperature increases, the deformation of fine powder is relatively small; On the contrary, the deformation increases
4. The deformation of the parts decreases with the increase of powder loading; On the contrary, the deformation increases
5. The surface of the feeding material is not smooth, which may cause deformation later on. Residual stress can cause surface wrinkling or even deformation.
II Reason for mold design:
1. The mold design should strive for uniform wall thickness and remove the core and holes. To avoid deformation, it is best to have uniform wall thickness.
The change in wall thickness may also cause shrinkage changes during sintering. This makes it difficult to control the size. In some cases, if the thickness transition section cannot meet the requirement of uniform wall thickness, it should be designed as a gradual transition between different thicknesses.
Another method to improve the robustness and strength of thin walled parts is to use reinforcing ribs.
The thickness or width of the reinforcing rib shall not exceed the thickness of its connecting wall. Whenever possible, in principle, try to maintain consistency with the wall thickness.
However, although strengthening ribs can increase the strength of parts, improve material flow, and prevent deformation during processing, they may also cause warping, sinking, and stress concentration. Therefore, it is important to be cautious when adding reinforcing ribs to component design.
3. The design of the gate is unreasonable
The design of the gate must consider issues such as the shape of the formed billet, the flow direction of the molten injection material in the mold cavity, the occurrence of fusion marks, and the removal of the gate.
The cross-sectional area of the gate is large and the filling ability is good, but the sealing time of the gate is long, making it difficult to remove.
The most basic requirement for gate design is to choose a gate form that minimizes stress concentration and deformation caused by orientation.
4. Unreasonable design of mold waterway
On the premise of meeting the required heat transfer area for cooling and the allowable mold structure, the number of cooling circuits should be as large as possible, and the cooling channel holes should be as large as possible.
To ensure uniform heat dissipation in the mold cavity, low internal stress and deformation in the product, and high accuracy
5. Unreasonable mold ejection design
During demolding, products that require a large amount of ejection force locally are concentrated at the rod end. Therefore, a reasonable choice of ejection method should be made to ensure uniform and stable ejection and reduce deformation as much as possible.
3、 Unreasonable sintering support:
During the process of debonding and high-temperature sintering, the raw material of MIM parts shrinks by about 20%. To minimize the possible distortion and deformation, it is necessary to properly support the MIM parts.
Usually, when placing MIM parts on flat ceramic plates or pallets, it is best to design the sintering flat plates or pallets with a large flat surface to use standard brackets. MIM parts with long spans, cantilevers, or vulnerable areas may need to be supported by dedicated brackets or fixing devices for the parts.
When the upper part of the product is not restricted and the bottom is difficult to shrink due to friction, the sintered support material causes deformation. If smooth materials are used as sintering supports, the deformation caused by friction can be reduced.
4、 Unstable sintering process:
1. Uneven carbon concentration and uneven temperature during sintering are the reasons for deformation after sintering.
2. The air entering the sintering furnace cannot guarantee the required temperature, pressure, and atmosphere, resulting in non dense sintering and product deformation.
3. Inaccurate sintering temperature and insensitive control result in sintering deformation.