All About Metal Injection Molding (MIM)

Like our plastic injection molding process, metal injection molding involves injecting material into a mold to create parts. The difference with metal injection molding is that it requires a few extra steps, which we'll outline in this article. Metal injection molding is used to create small and intricate metal parts with a mass of 1–100 grams.

This article explores the process of metal injection molding, as well as its uses, advantages, and disadvantages.

What Is Metal Injection Molding?

Our metal injection molding process, also known by the acronym MIM, begins with a polymerized metal feedstock containing tiny metal particles with a plastic binder. This feedstock is then injected into a mold cavity under great pressure. After the molding stage, the mold opens, and the form is released. After runners and gates are trimmed away, the resulting parts are known as "green parts." The parts go through de-binding, which removes the polymer binders in the green parts, resulting in porous metal "brown parts." The final step involves heating the parts to high temperatures in a furnace to sinter the metal particles, creating non-porous, virtually fully dense metal parts. Some shrinkage occurs during the sintering process as the metal particles fuse together; however, we can plan for this during manufacturing by increasing the size of the mold to offset shrinkage.

Metal injection molding produces high-quality products. The process can create detailed parts with intricate geometry and accurate dimensions. The high pressure and heat used in the process reduce inclusions and porosity, resulting in a dense, high-quality part. 

When using the MIM process, parts will typically be around 96%-99% dense after sintering. This is much higher than some other processes because metal injection molding uses more refined powders, which results in less porosity. 

Materials That Can Be Molded using MIM

The materials used in metal injection molding are all metals and are divided into four categories. The first category is ferrous alloys including steel, stainless steel, tool steel, iron-nickel alloys, invar, and kovar. The second category is tungsten alloys, the third is hard metals which include: cobalt-chromium, cemented carbides, and cermets. The last category is special metals including aluminum, titanium, nickel, and molybdenum.

Uses of Metal Injection Molding

Metal injection molding is almost exclusively used for high-volume production of small metal parts with complex geometry. This is because the initial setup and tooling costs are high, making the process less economically viable for low—or medium-batch production. We recommend our CNC machining service to help bridge the gap between low volume and production quantities. MIM can also be used to produce geometry, which is challenging in other processes, such as very thin walls.

Applications of Metal Injection Molding

Metal injection molded parts have applications in a wide of range industries. Below are some examples that can take advantage of the MIM process:

1. Automotive: To produce sensor housings, gear synchronizers, fuel injectors, rivets, and fasteners.
2. Weapons: One of the biggest uses of metal injection molding in the USA. Metal injection molding is used to create sight mounts, safety levers, and firing pins.
3. Medical: Metal injection molding can process medical-grade metals and, therefore, can be used to produce endoscopes, laparoscopic devices, implants, and dental crowns. 
4. Consumer Goods: Used to create small and intricate parts such as heat sinks, watch cases, buttons and switches, and eyeglasses frames. 

Advantages of Metal Injection Molding

Metal injection molding has many advantages, including:

1. Large volumes of metal parts produced at one time. Commonly, these have complex geometries and detailing. The result is the production of small, precision parts that have tight tolerances.
2. Compared to some other metal processes, there are fewer design restrictions, which allows designers more freedom to produce a variety of shapes.
3. High-quality surface finish, with the potential for further post-processing if desired.
4. The process can be used to consolidate multiple components or assemblies into a single piece.
5. Components are produced with high-quality mechanical properties in terms of hardness and strength. 
6. The production process produces less material waste and scrap when compared to machining processes. Up to 95–98% of the material can be transformed into usable metal parts, making it cheaper when using expensive materials such as superalloys or specialty metals are involved. 
7. At higher quantities, the process can be cheaper than investment casing, machining, and even stamping in some instances.
8. Used across a wide variety of metals, such as copper alloys, nickel alloys, and iron. 

Disadvantages of Metal Injection Molding

There are several disadvantages to metal injection molding, including the following:

1. The operation of metal injection molding can be costly, requiring a high initial investment for tooling production and other process costs.
2. Piece price can be higher at smaller volumes.
3. Suitability is limited to small/medium-sized parts as larger parts decrease the capacity of furnaces and of the mold, therefore increasing production costs. 
4. Shrinkage will occur in the parts between their green stage and sintered stage, and thus must be offset during the manufacturing process. Designers don't need to worry about this, as the manufacturer will address it, however it is still a factor to consider.

MIM Metals and Powders Available Through Xometry

Table 1 shows the different metal injection molding metals and powders available through Xometry:

Metal Injection Molding Accuracy

Metal injection molding is highly accurate and can achieve dimensional tolerances of between +/- 0.3–0.5%. Xometrys MIM manufactures can also post-machine critical features to further ensure accuracy. Metal injection molding is considered to be superior in accuracy when compared to other similar methods of manufacturing, such as die casting, which often requires post-process machining to hold tight dimensions and a finer finish. The accuracy of metal injection molding can vary more for larger parts, as larger parts shrink more during sintering and, therefore, are more susceptible to variation.

Metal Injection vs. Plastic Injection Molding and Die Casting

The injection stage of metal injection molding and plastic injection molding are very comparable. Both involve taking a plasticized raw material or feedstock and injecting it into a mold cavity at high pressure. The primary difference between the processes is the post-processing required. With plastic injection molding the parts are nearly complete when being ejected from the mold. With metal injection molding, the parts require de-binding and sintering. Additionally, while both processes are used for high-volume parts, metal injection molding is typically only used for small parts with complex geometry. In contrast, plastic injection molding is used for both small and large parts that may or may not be complex in geometry. 

To learn more, see our full guide on What is Plastic Molding.

The biggest difference between metal injection molding and die casting is that die casting requires limited, if any, post-processing to create a final part, whereas metal injection molding requires multiple stages, such as the debinding and sintering steps we've described earlier in this article. These extra steps and the increased cost of the raw material used in metal injection molding mean that die casting is generally more cost-effective. That said, the MIM process is better suited to the production of smaller and more intricate parts and can create pieces with accurate dimensions without necessitating post-machining operations. Die casting, on the other hand, is useful for producing larger metal parts. In terms of speed, while the initial injection stage of metal injection molding is faster than die casting, the overall process, including post-processing, for die casting is quicker.

Summary

This article provided an overview of metal injection molding, explained it, and discussed its various advantages and applications. We hope you found the information helpful!

Disclaimer

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