MIM (Metal Injection Molding) Mold

MIM molds are different from injection molds in several ways. The thermal conductivity of the feedstock is several times higher than that of plastic, which results in much worse fluidity. The shrinkage rate of MIM molds is usually 15%~22%, which is much larger than that of injection molds. This makes it more difficult to control the density and sintered size of the green embryo. Many companies make mistakes on this issue. In addition, metal powder is much more destructive to the barrel and mold than plastic. There are many differences, so we need to make some changes when designing MIM molds:

 a，Mold steel

----Highly wear-resistant heat-treated steel should be selected, but it does not mean that steel used for tungsten feedstock should be used to make stainless steel feedstock molds. This will result in a certain cost waste, so according to Feeding to select steel

b, Hot runner

----Liquid hot runner is more stable than electric hot runner, and will obtain green body with more uniform density and smaller deformation during sintering.

c, Runner and gate

----Due to the large shrinkage rate and high thermal conductivity of the feed, the layout of the runner and gate must be balanced, otherwise the shaping work after sintering will be more troublesome. Some serious ones need to be machined to correct the size, which will increase the manufacturing cost. d, The length of the head in the mold filling nozzle should be as short as possible to shorten the flow path of the feed and reduce heat loss. e, Surface treatment of mold cavity

----Wax pattern is a common problem in MIM products. The treatment of the mold surface will improve the wax pattern. Generally, VDI18 discharge treatment is performed during mold processing.

Molds and molding processes are different for each set of molds. Like injection molding, mold design evaluation, mold trial and measurement are required for different problem points of each product. Until the new product is qualified. This is also the most headache for most MIM factories at present. The structure of the mold is relatively simpler than that of injection molding, and even if the overall size of the mold core is enlarged or reduced, there is no need to repair the mold. Just adjust the percentage of metal powder in the feed. Generally, we choose another percentage of feed to solve the problem of unqualified mold size.

Key points of MIM mold design

1.1Mold material or wear-resistant steel with the same hardness

1.2. Runner and gate design

The runner should be as short as possible

A cold well needs to be designed

Avoid sharp corners (otherwise iron and plastic will separate)

The cross-sectional area of the runner should be as round as possible

In order to shorten the length of the head, the nozzle can be lengthened

3. Mold water channel design

The water channel design should be balanced, otherwise it will affect the fluidity, filling, appearance, deformation after sintering, dimensional instability and many other problems of the product

4. Mold exhaust

If the mold exhaust is poor, the product appearance will be combined with the line, black lines will be produced, and holes will be produced for thick parts. Poor bubbles are basically caused by poor mold exhaust. For molds with difficult exhaust, breathable steel can be considered.

Some other key points of mold design:

• Parting line should not be in the functional surface area
• The gate should not be directly facing the feed flow direction to avoid spraying
• When arranging the products, consider the convenience of robot picking and placing and whether the direction is correct when placing them on the sintering plate
• Do some discharge patterns or micro-processing on the surface of the mold cavity to prevent ferrite separation. Whether the size, shape, density and appearance of the sintered product are ok, these factors are inevitably related to the mold.
• Design the joint line and ejector pin in an unimportant position. Unless it is necessary, try to avoid using latent gates. According to our experience with existing products, latent gates are generally difficult to demold automatically due to the brittleness and lack of toughness of the raw materials. They are often broken back in the mold, and some can only be picked out with a steel needle. It is difficult to achieve fully automatic production when the next mold is made. Although MIM is basically isotropic, the size of the molded part will change due to external forces. The external forces include the viscosity of the mold during injection molding filling, the gravity when removing the binder, and the friction of the chassis during sintering shrinkage. The impact of these forces on the product is more complex and greater than that of injection molding. Therefore, these factors must be considered when designing the mold.