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What is Metal Injection Molding? How does It work?

Metal Injection Molding (MIM) is a new type of metal injection molding near-net forming technology that is derived from the plastic injection molding industry. MIM combines the advantages of plastic injection molding, such as low cost and the ability to produce complex shapes, with the advantages of metal injection molding, such as high strength and good wear resistance.

What is Metal Injection Molding (MIM) Process?

The MIM process begins with the mixing of metal powder and a binder material. The mixture is then injected into a mold using a plastic injection molding machine. The molded part is then subjected to a debinding process, in which the binder is removed. The part is then sintered, which is a process that causes the metal powder to bond together and form a solid part.

MIM is a versatile process that can be used to produce a wide variety of metal parts. MIM parts are used in a variety of industries, including the automotive, aerospace, medical, and electronics industries. Although MIM is suitable for small metal parts with complex structures and hard metal parts （Such as Tungsten parts）, it has natural advantages, but not all metal materials are available for metal injection molding materials, which is also one of the limitations of the MIM process.

Metal Injection Molding Features

The MIM process is a manufacturing process that injects powdered metals to a die to create components with unique chemical compositions, and mechanical, and physical properties. There are multiple MIM advantages and properties cannot be achieved through traditional casting methods. Metal injection molding technology can directly create porous, semi-dense, or fully dense materials and products, such as oil bearings, gears, cams, guide rods, knives, and more. This process requires little to no cutting or machining.

Minimizes segregation of alloy components

metal injection molding technology minimizes the segregation of alloy components, which eliminates coarse and uneven casting structures. This is important for the production of high-performance materials, such as rare earth permanent magnet materials, rare earth hydrogen storage materials, rare earth luminescent materials, rare earth catalysts, high-temperature superconducting materials, new metal materials (such as Al-Li alloys, heat-resistant Al alloys, superalloys, powder corrosion-resistant stainless steel, powder high-speed steel, and high-temperature structural materials of intermetallic compounds).

High-performance non-equilibrium materials

Metal injection molding can produce high-performance non-equilibrium materials, such as amorphous, microcrystalline, quasicrystalline, nanocrystalline, and supersaturated solid solutions. These materials have excellent electrical, magnetic, optical, and mechanical properties.

Higher tolerance

When metal injection molding vs. casting, you will find that the MIM process is able to reach tighter tolerance than casting. MIM parts without post-processing can reach an accuracy of 0.02mm. This is not possible with the casting process.

Easy composite production

Metal injection molding makes it easy to produce composites, which allows for the full utilization of the characteristics of each component material. This is a low-cost production technology for high-performance metal matrix and ceramic composite materials.

Can produce materials with special structures and properties

Metal injection molding can produce materials and products with special structures and properties that cannot be produced by ordinary smelting methods. These include new porous biological materials, porous separation membrane materials, high-performance structural ceramic abrasives, and functional ceramic materials.

Near-net formation and automated mass production

Metal injection molding can be used to achieve near-net formation and automated mass production, which effectively reduces resource and energy consumption for production. The material utilization rate of metal and ceramic powder injection molding is about 98%

Scrap metal reuse

Metal injection molding is a new technology that can effectively regenerate and comprehensively utilize materials. It can use ore, tailings, steelmaking sludge, rolling steel scale, and recycled scrap metal as raw materials.

Hard metal alloy forming

Returning to the metal injection molding history, you will find that this process was originally used for military products. Such as tungsten alloy armor piercing warheads. Later, with the advancement of technology, it was gradually used in hard metals or high-temperature metals that are difficult to process. Many of our common machining tools and hardware abrasives are manufactured by metal injection molding technology. This technology is becoming increasingly important as we look for ways to reduce reliance on traditional manufacturing methods and conserve resources.

How Does Metal Injection Molding Work?

Metal powder injection molding combines the advantages of the plastic injection molding process and powder metallurgy process and has the manufacturing difficulty of both processes.

Powder preparation

Powder preparation is the first step in the metal injection molding (MIM) process. It is a critical step that determines the quality of the final part. The metal powder must be of high quality and prepared and controlled to produce high-quality parts. The preparation method of metal powder is usually divided into two categories: the mechanical method and the physical-chemical method.

The first step in powder preparation is to select the appropriate metal powder. The type of metal powder will depend on the desired properties of the final part. For example, if the part is to be used in a high-temperature environment, a powder made from a high-temperature alloy will be required. Once the metal powder has been selected, it must be processed to remove any contaminants. This can be done by sieving, ultrasonic cleaning, or combining methods.

The next step is to size the powder particles. The particle size will affect the flowability of the powder and the final properties of the part. The particle size is typically controlled by passing the powder through a series of sieves.

The final step in powder preparation is to blend the powder. This is done to ensure that the powder is uniform and that there are no clumps of powder. The powder is typically blended in a tumbling machine. Once the powder has been prepared, it is ready to be used in the MIM process.

Binder material mixing

The metal powder is first mixed with a binder material in a large mixing chamber. The binder is typically a thermoplastic material, such as polyethylene or polypropylene. It makes the powder flowable, which allows it to be injected into a mold and helps to bind the metal powder together, which creates a strong and durable part. It also can be used to create parts with a variety of properties, such as strength, lightness, and porosity. The amount of binder added to the powder depends on the desired properties of the final part. For example, a part that needs to be strong and durable will require more binder than a part that needs to be lightweight and porous.

Feedstock preparation and injection molding

The first step in MIM is to prepare the feedstock. This involves mixing the metal powder and binder together. The metal powder is typically made of a fine, high-purity metal, such as stainless steel, titanium, or copper. The binder is typically a thermoplastic polymer, such as wax or nylon. The metal powder and binder are mixed together in a special machine called a mixer. The mixer ensures that the metal powder and binder are evenly distributed throughout the feedstock. Once the metal powder and binder are mixed together, the feedstock is granulated. Granulating makes the feedstock easier to handle and inject into the mold. The feedstock is granulated by passing it through a series of rotating drums. The drums have blades that break the feedstock into small pieces.

MIM-feedstock

The next step is to inject the feedstock into the mold. This is done using a plastic injection molding machine. The plastic injection molding machine is a large machine that heats the feedstock and then forces it into the mold. The mold is typically made of steel or aluminum. The mold is designed to create the desired shape of the part. The plastic injection molding machine heats the feedstock to its melting point. The molten feedstock is then forced into the mold by a piston. The piston forces the feedstock into all of the small details of the mold. Once the feedstock is in the mold, it cools and hardens. Once the feedstock has cooled and hardened, the part is ejected from the mold. The part is then ready for debinding and sintering.

Debinding

The molded part is first heated to a temperature of 100-200 degrees Celsius. This temperature is low enough to prevent the metal powder from melting, but high enough to start to vaporize the binder. The binder vaporizes and is carried away by a flow of inert gas. The debinding process is typically carried out in two stages. In the first stage, the majority of the binder is removed. In the second stage, the remaining binder is removed to a high degree of accuracy.

The debinding process is an important step in metal injection molding. It is necessary to remove the binder in order to allow the metal powder to sinter together and form a strong, solid part. The debinding process can be carried out using a variety of methods, but the most common methods are thermal debinding, solvent debinding, and catalytic debinding.

Thermal debinding is the most common method of debinding. It is a simple and efficient process that can be carried out in a variety of furnaces. The temperature of the furnace is gradually increased to a point where the binder begins to vaporize. The binder vaporizes and is carried away by a flow of inert gas. The debinding process is typically carried out in two stages. In the first stage, the majority of the binder is removed. In the second stage, the remaining binder is removed to a high degree of accuracy.

Solvent debinding is a less common method of debinding. It is a more expensive process than thermal debinding, but it can be used to remove more of the binder. In solvent debinding, the molded part is immersed in a solvent that dissolves the binder. The solvent is then removed from the part, carrying the binder with it. Solvent debinding is a more time-consuming process than thermal debinding, but it can produce parts with a higher degree of accuracy.

Catalytic debinding is a newer method of debinding. It is a more efficient process than thermal debinding, and it can be used to remove more of the binder. In catalytic debinding, a catalyst is added to the furnace. The catalyst reacts with the binder, causing it to decompose and vaporize. The vaporized binder is then carried away by a flow of inert gas. Catalytic debinding is a more expensive process than thermal debinding, but it can produce parts with a higher degree of accuracy.

Sintering

The debinded part is then sintered. Sintering is a process that causes the metal powder to bond together and form a solid part. Sintering is typically done in a furnace at a temperature of 1,200-1,600 degrees Fahrenheit.

For example, let's say you are making a metal injection molded part for a car. The part is made of a metal powder that is mixed with a binder. The part is then injected into a mold and heated. The binder melts and flows out of the mold, leaving behind a porous metal part. This part is then sintered. During sintering, the metal particles heat up and bond together. This removes the pores and creates a solid metal part. The part is then cooled and removed from the mold. Sintering is an important step in the metal injection molding process. It is what creates the solid metal part from the porous metal powder. Sintering also improves the strength, toughness, and density of the part.

Sintering is a solid-state process, which means that the metal powder does not melt during sintering. Since the pores in the part are removed during sintering, the part will be smaller than before sintering. We call it the MIM parts shrinkage ratio. The sintering temperature is typically below the melting point of the metal powder. The sintering time depends on the size and shape of the part, the type of metal powder, and the desired properties of the part. Sintering can be done in a variety of atmospheres, including air, vacuum, and inert gas. The atmosphere used for sintering can affect the properties of the part.

Post-processing

The sintered part may require post-processing, such as heat treatment, machining, or plating to achieve more excellent properties. Typically, the sintered blank shrinks and deforms slightly. We need to correct the shape through post-processing. We will achieve our desired product requirements through the following post-processing steps:

Shaping

These tiny deformations are corrected by pressing the blank into the correcting mold.

Heat treatment

Heat treatment is a process that alters the properties of a metal by heating it to a specific temperature and then cooling it at a controlled rate. This can be done to improve the strength, toughness, or machinability of the part. For example, the hardness of 17-4 PH stainless steel before heat treatment is HRC 20, and it can reach HRC 40 after heat treatment

Machining

Machining is a process that uses cutting tools to remove material from a part to achieve the desired shape or dimensions. This can be done to improve the fit and finish of the part, or to add features that cannot be created by MIM.

Plating

Plating is a process that coats a metal part with a thin layer of another metal. This can be done to improve the appearance of the part, to protect it from corrosion, or to improve its electrical conductivity.

The specific post-processing steps that are required will depend on the design of the part and the desired properties. For example, a part that is required to be strong and durable may need to be heat treated, while a part that is required to have a smooth surface finish may need to be machined.

For example, A gear made of stainless steel is MIMed. The gear is then heat treated to improve its strength and toughness. The gear is then machined to improve its fit and finish. The gear is then plated with a thin layer of gold to improve its appearance and corrosion resistance.

The post-processing steps that are used to create a MIM part can have a significant impact on the cost of the part. The more complex the post-processing steps, the more expensive the part will be. However, the post-processing steps can also improve the performance and durability of the part, which can make it worth the extra cost.

Pros of the metal injection molding process

Complex geometries: MIM can be used to create parts with complex geometries that would be difficult or impossible to create using other manufacturing processes. For example, MIM can be used to create parts with internal features, such as gears, bearings, and connectors.

You can understand that the complexity and shape that the plastic injection molding process can achieve, metal injection molding can do it too.

High precision

MIM parts can be produced to very tight tolerances, making them ideal for applications where precision is critical. Min tolerances down to +/-0.02 mm, and min thin wall down to 0.4 mm can be achieved, which is not possible with hard metal forming processes such as lost wax casting. For example, MIM parts are often used in medical devices, aerospace applications, and the telecommunication industry.

Strong and durable

MIM parts are typically strong and durable, making them ideal for a variety of applications. For example, MIM parts are often used in automotive and industrial applications.

Cost-effective

Metal injection molding cost mainly includes mold fee, injection molding fee, degreasing fee, sintering fee, shaping fee, post-processing fee, etc. Even producing a single part has to go through all the processes. MIM can be a cost-effective manufacturing process for high-volume production runs. For example, MIM is often used to produce parts for the automotive and electronics industries. But not suit for low volume production.

Environmentally friendly

MIM is a relatively environmentally friendly manufacturing process, as it produces very little waste. For example, MIM does not require the use of solvents or hazardous chemicals.

Cons of the metal injection molding process

High initial investment

The cost of setting up a MIM production line can be high. This is because MIM requires specialized equipment, such as an injection molding machine, debing furnace, and a sintering furnace.

Longer lead times

The lead time for MIM parts can be longer than for other manufacturing processes. This is because MIM is a multi-step process that requires time for the metal powder to be compacted, sintered, and machined.

Limited material selection

The range of materials that can be used in MIM is limited. This is because not all metals are available in powder form. Currently, Neway provides copper alloys, iron-based and tungsten alloys, and titanium alloys in metal injection molding services. However, the range of materials that can be used in MIM is expanding as new technologies are developed.

Conclusion: Metal injection molding is a production process for small metal parts with high complexity, high precision and relatively large output. If you are looking for MIM supplier, please contact us.