Metal Injection Molding (MIM): Process, Uses, Advantages, and Disadvantages
Metal injection molding is a process very similar to plastic injection molding in which molten metal is injected into a cast and solidified to create a metal part. The difference with metal injection molding is that it requires post-processing including debinding and sintering. Metal injection molding is used to create small and intricate metal parts of around 1–100 grams. Advantages of metal injection molding include: high production volumes, low cost per part, high dimensional accuracy, and surface finish. The disadvantages of metal injection molding are that it is limited to large production batches or small parts and there is a large initial investment required for tooling.
This article explores the process of metal injection molding, as well as its uses, advantages, and disadvantages.
Metal injection molding, also known as powder injection molding, is a process whereby a powdered metal feedstock is used in an injection molding process to create intricate metal parts. The powder metal feedstock is bound using an organic binder which creates powdered-metal pellets. The metal pellets are then injection molded into the part shape before debinding and sintering.
The purpose of metal injection molding is to create small metal parts with complex geometry in high volumes. Metal injection molding is used because it can create parts that do not require machining and is a more cost-effective process than forging, casting, or machining for small, complex, high-volume parts. This process is widely used in the weapons, automotive, aerospace, electronic, telecommunications, medical, and dental industries.
Metal injection molding works by injecting a feedstock of powdered metal formed into pellets. The use of an organic binder at high pressure and temperature turns the feedstock into molten metal. The metal then cools and solidifies in the mold and can then be removed. The organic binder is then removed through debinding before the part can be compacted using sintering into its final form.
The injection stage of metal injection molding and plastic injection molding are very comparable. The difference between the processes is the post-processing required. Plastic injection molding usually requires the part to be trimmed. With metal injection molding the part is not trimmed but it does require debinding and sintering. Additionally, while both processes are used for high-volume parts, metal injection molding is only used for small parts with complex geometry, whereas 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 metal injection molding process can vary slightly — however, an overview of the process is as follows. Firstly, a metal powder is bound using an organic material such as wax or polypropylene to produce the feedstock. The feedstock can then be injected into the mold in a liquid state and then cooled. Once cooled, the part is removed and resembles the final shape of the part, this part is referred to as a “green part”. To create the final part, the binder used to turn the metal powder into feedstock must be removed. Some of the binder can initially be removed using a furnace, but to remove all of the binder one of three processes is used. The binder can be removed by applying heat which evaporates the binder, using a gas to chemically vaporize the binder, or using a solvent to dissolve the binder. When the binder is removed, the part is left in a porous state. To reduce this porosity, the part is sintered. Sintering is a process in which the temperature of the part is raised close to the liquid phase, which causes the part to shrink and compact.
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.
No, pure tin metal cannot be used for metal injection molding. Tin can be used as an alloying element for a copper alloy in injection molding but pure tin is not suitable. To make small parts with complex geometry out of tin, die casting is required.
Yes, tungsten metal can be used in metal injection molding, and so can tungsten alloys. Metal injection molding is a more favorable process since machining tungsten is more challenging than other metals.
From powder to metal parts—including the injection, debinding, and sintering—takes between 24 and 36 hours. The majority of this time is spent using a thermal debinding method which can take up to 24 hours to complete. This is much faster than direct laser metal sintering which can take 7 days to create one part. The difference in speed is one reason why metal injection molding is used for high-volume production. Especially when taking into account that one mold, debinder, and sintering oven can process multiple parts at a time. This drastically increases the production output.
Metal injection molding produces high-quality products. The process can create detailed parts with intricate geometry and accurate tolerances. The high pressure and heat used in the process result in a low number of inclusions and a low final porosity, resulting in a high-quality part.
When using the metal injection molding technique, parts will reach approximately 95% of their theoretical density after sintering. This is much higher than other processes because metal injection molding uses more refined powders which results in less porosity.
Metal injection molding is used to produce small metal parts which often have complex features on their surface. This is because creating such parts using other methods is often complex, not cost-effective, or impossible. Metal injection molding is also used exclusively for high-volume production, this is because the initial setup and tooling costs are too high for low- or medium-batch production. Metal injection molding is used to produce walls smaller than 100 micrometers, this thickness of material is hard to produce using other methods.
Metal injection molding has applications in a range of industries for different parts. Examples of the uses of metal injection molding are listed below:
- Automotive: To produce sensor housings, gear synchronizers, fuel injectors, rivets, and fasteners.
- 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.
- Medical: Metal injection molding can process medical-grade metals and therefore can be used to produce endoscopes, laparoscopic devices, implants, and dental crowns.
- Consumer Goods: Used to create small and intricate parts including: heat sinks, watch cases, buttons and switches, and eyeglasses frames.
Metal injection molding has many advantages including:
- There can be a large volume 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.
- Few restrictions are placed on the design of the end piece which allows manufacturers the freedom to produce a variety of shapes.
- High-quality surface finish, with the potential for further enhancements after the initial process.
- Multi-component parts can be manufactured as a single piece.
- Components are produced with high-quality mechanical properties in terms of hardness and strength.
- The production process produces less material waste and scrap when compared to a machining process. 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.
- Cheaper in comparison to investment casing, machining, and stamping over a long period.
- Used across a wide variety of metals, such as copper alloys, nickel alloys, and iron.
No, metal injection molding is not suitable for prototyping as the tooling cost required for producing a prototype is not cost-efficient. Instead, processes should include metal binder jetting or direct metal laser sintering (DMLS). These processes are well suited for prototyping as they can make use of production-grade materials that can exactly replicate the properties and performance of a product.
There are several disadvantages to metal injection molding, including the following:
- The operation of metal injection molding can be costly, requiring high initial capital injection and processing costs as a result of acquiring and maintaining machinery.
- Expensive for smaller production demands.
- Suitability is limited to small/medium-sized parts as larger parts decrease the capacity of furnaces and the mold, therefore increasing production costs.
- Complicated fabrication process.
Xometry can support your metal injection molding needs by providing metal injection molding services. To receive a quote from Xometry, complete a request here. Xometry will return a reply from a skilled team of engineers within 24 hours of submission. Metal injection molding services provided by Xometry are ISO 9001:2015, 13485, and AS9100D certified and ITAR registered.
Table 1 shows the different metal injection molding metals and powders available through Xometry:
316, 316L, 17-4 PH, 303, 304, 440C, and 420P
Steel alloys have increased resistance to corrosion and heat due to their high nickel and chromium content. This makes them ideal for surgical tools used in the medical industry.
Fe-Ni (Iron-Nickel), FN02, FN08, 4140, 8620, and 100Cr6
Low-alloy steels have a low carbon content and are low in other alloying elements. This improves their machinability and cost-effectiveness.
Tool steel has a high carbon content which gives it a high hardness and abrasion resistance. These properties along with stringent quality checks make them perfect for making cutting tools and mold tools.
Soft Magnetic Steel
Fe-Ni50, Fe3Si, FeCo50, and FeCoV
Soft magnetic steels have high magnetic permeability. This means they react well to magnetism. This leads to their use in solenoids, electric motors, and relays.
Tungsten Heavy Alloy
W-Ni-Fe (Tungsten Nickel Iron) and W-Ni-Cu (Tungsten Nickel Copper)
Tungsten heavy alloys are composed of tungsten with other alloying elements. These alloying elements vary, but heavy tungsten alloys are mostly used for electromagnetic shielding due to the materials’ high density.
Tungsten Carbide-Cobalt (WC-Co)
Cobalt particles within the tungsten carbide matrix increase the hardness and wear resistance of the material. This allows the material to be used for cutting tools, machine parts, and mining tools.
The cost of powder feedstock is more expensive for metal injection molding than for other powder feedstock methods at approximately $24 per kilo compared to $2–$4 per kilo for feedstock for alternative powder methods. Metal injection molding machines represent a large investment for production which is in-house; machines can cost anywhere between $50,000–200,000. Finally, tooling can also cost between $30,000–70,000. This, along with other factors including product size, material, and complexity, can all vary the cost of a metal injection molding part. The price per unit could likely be somewhere between $1–5, however, this will vary significantly depending on the specifics of the project requirement.
Metal injection molding is highly accurate and can achieve dimensional tolerances of between +/- 0.3–0.5%. This can result in tolerances of between 0.01–0.001 mm. 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 achieve the right surface dimensions and 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. Additionally, the size of the powdered metal can affect the accuracy as larger metal particles produce a higher level of shrinkage.
Metal injection molding is one of the most prevalent metal processing methods in use today due to its high productivity output and its low cost per part. Between 1986 and 2004 the metal injection molding market in America grew from $9 million to $380 million. Furthermore, the global market share in 2022 was $4,120 million and is predicted to grow to $8,020 million by 2030.
There is a big future for metal injection molding, with most sources citing a predicted 8–11% compound annual growth rate (CAGR) of the market value. The main reason for this growth is that metal injection molding is bridging the gap in the technology of metal production and providing manufacturers with the ability to injection mold metal in a similar manner to plastic injection molding. Ongoing experimentation to find new metals suitable for metal injection molding is increasing the usability and applications of this process and enhancing existing materials' properties and behaviors. Metal injection molding is also able to provide production capabilities for very small and intricate parts which are finding more uses in electronic and medical industry applications as devices get smaller and smaller. Metal injection molding also uses less power and creates less waste than other metal production techniques.
The biggest difference between metal injection molding and die casting is in the process of metal injection molding and die casting is that die casting requires limited, if any, post-processing, whereas metal injection molding requires debinding and sintering. These extra steps and the increased cost of the raw material used in metal injection molding mean that die casting is more cost-effective. The metal injection molding process is also better suited to the production of smaller and more intricate parts. Die casting, on the other hand, is used to produce larger metal parts. While the injection process of metal injection molding is quicker than die casting, the overall process, including post-processing, for die casting is quicker.
This article presented metal injection molding, explained it, and discussed its various advantages and applications. To learn more about metal injection molding, contact a Xometry representative.
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