Metal injection molding has emerged as a rapidly developing and highly promising near-net-shape forming technology in the field of powder metallurgy. It is recognized as one of the “hottest metal component forming technologies” globally.
This article introduces the fundamental knowledge of the metal injection molding process, including basics, process, advantages, comparisons with other processes, suitable part types, and applications.
For engineers, learning and understanding the metal injection molding process is essential for effective product design, as it often provides significant opportunities for cost reduction.
What is Metal Injection Molding？
Metal injection molding(MIM) is a method that blends metal powder with a binder system for injection molding. The process begins by mixing selected powders with binders, followed by pelletizing the mixture to inject it into the required shape. Through debinding and sintering, the binder is removed, resulting in the desired metal product. Subsequent processes such as sizing, surface treatment, heat treatment, or machining can be applied to further refine the product.
Metal Injection Molding = Powder Metallurgy + Injection Molding. Metal injection molding is a quintessential interdisciplinary technology that integrates two completely different processing methods: powder metallurgy and plastic injection molding. This integration allows engineers to break free from traditional design constraints, using plastic injection molding techniques to obtain low-cost, complex-shaped parts made of stainless steel, nickel, iron, copper, titanium, and other metals, thereby offering greater design freedom than many other production processes.
Metal Injection Molding Process
The metal injection molding process is divided into four stages: granulation, injection molding, solvent debinding, and thermal sintering. Secondary processes such as machining, brushing, or electroplating can be performed if required.
1. Granulation
Fine metal powders are mixed with binders and thermoplastics in precise proportions. This mixing occurs in specialized equipment heated to a specific temperature to melt the binders. In most cases, mechanical mixing is used until the metal powder particles are uniformly coated. Once cooled, the mixture forms granules (feedstock) that can be injected into mold cavities.
2. Injection molding
The granulated feedstock is fed into an injection molding machine, heated, and injected into mold cavities under high pressure to produce a “green part.” This process is very similar to plastic injection molding. Molds can be designed with multiple cavities to improve productivity, and cavity dimensions must account for the shrinkage that occurs during the metal component’s sintering process.
3. Solvent Debinding
Debinding is the process of removing the binder from the green part to obtain a “brown part.” This process is usually completed in several steps. The vast majority of the binder is removed before sintering, while the remaining portion supports the part as it enters the sintering furnace.
Debinding can be achieved through various methods, with solvent extraction being the most common. After debinding, the parts become semi-permeable, allowing residual binders to volatilize easily during the sintering stage.
4. Thermal Sintering
The debinded brown parts are placed in a furnace with controlled high temperature and pressure. The brown part is slowly heated under a protective atmosphere to remove residual binders. Once the binders are eliminated, the part is heated to a very high temperature, and the gaps between particles disappear as they fuse. The brown part undergoes directional shrinkage to its design dimensions and transforms into a dense solid, resulting in the final product.
During the sintering process, the brown part undergoes an overall dimensional shrinkage of approximately 20%.
Advantages of Metal Injection Molding
Metal injection molding combines the advantages of powder metallurgy and plastic injection molding, breaking through the geometric limitations of traditional metal powder forming. It uses the ability of plastic injection molding to produce complex-shaped parts in large batches with high efficiency. As a near-net-shape technology for high-quality precision parts, it offers advantages incomparable to conventional powder metallurgy, CNC machining, and investment casting.
Capability to Form Highly Complex Parts
Compared to other metal forming processes such as sheet metal stamping, powder metallurgy, forging, and CNC machining, Metal injection molding can form parts with highly complex 3D geometries. Generally, structures achievable through plastic injection molding can also be realized via metal injection molding. Using this feature, metal injection molding offers the opportunity to merge multiple parts originally made by other processes into a single component, simplifying product design and reducing assembly costs.
High Material Utilization
Metal injection molding is a near-net-shape process where the part shape is close to the final form. Material utilization is high, which is particularly significant for reducing losses when processing precious metals.
Uniform Microstructure
Metal injection molding is a fluid forming process. The presence of binders ensures uniform powder distribution, eliminating microstructural non-uniformity and allowing sintered products to reach 95% to 99% of their theoretical density. This high density improves strength, toughness, ductility, conductivity, and magnetic properties.
Conversely, traditional powder metallurgy only reaches about 85% density due to friction between the mold wall and the powder, leading to uneven pressure distribution and non-uniform shrinkage. This results in high porosity and poor mechanical performance, often requiring lower sintering temperatures to mitigate these effects.
High Efficiency and Scalability
Metal injection molding uses injection machines to form green parts, which significantly improves production efficiency and is suitable for high-volume production. The consistency and repeatability of the molded products provide a guarantee for large-scale industrial production.
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Wide Range of Materials
The range of metal materials suitable for metal injection molding is very broad. In principle, any powder material that can be sintered at high temperatures can be processed via metal injection molding, including difficult-to-machine and high-melting-point materials.
Materials include low-alloy steel, stainless steel, tool steel, nickel-based alloys, tungsten alloys, carbides, titanium alloys, magnetic materials, Kovar alloys, and fine ceramics. What’s more, metal injection molding allows for custom alloy formulations to create composite materials. While non-ferrous alloys like aluminum and copper are technically feasible, they are usually processed more economically via die casting or machining.
| Material | Grades | Application |
| Low-alloy Steel | 4605, 4140 | Various structural components for automotive, machinery, and other industries |
| Stainless Steel | 316L, 17-4PH, 420, 440C | Medical devices, clock and watch parts |
| Cemented Carbide | B771 | Various cutting tools, clocks, and watches |
| Ceramics | ASTM C1161 | IT electronics, daily necessities, and clocks |
| Titanium Alloy | Ti-6Al-4V | Medical and military structural components |
| Magnetic Materials | ASTM A753 | Various magnetic components |
High Precision
The dimensional accuracy of metal injection molding parts is typically ±0.5%, with precision levels reaching ±0.3% or higher. For small parts, metal injection molding’s precision is higher than other casting processes, often eliminating the need for secondary machining. Like other processes, higher precision requirements increase costs, so moderate tolerances are encouraged where performance allows. Tolerances unreachable by initial molding can be achieved through surface treatments.
Comparison with Other Processes
| Attribute | Metal Injection Molding | Powder Metallurgy | Investment Casting | CNC Machining |
| Weight (g) | 0.01 – 1000 | 5g – 1kg | > 1 | > 1 |
| Tolerance (%) | < 0.3 | 0.1 | 0.5 – 1.0 | < 0.1 |
| Density (%) | 98 – 99 | 85 – 92 | 95 – 99 | 100 |
| Strength (%) | > 97 | 75 | > 95 | 100 |
| Surface Roughness (μm) | 1 | 1 – 5 | 5 | 0.2 – 4 |
| Wall Thickness (mm) | 0.2 – 10 | > 2 | > 2 | > 1 |
| Complexity | High | Low | Medium | High |
| Design Flexibility | High | Medium | Medium | Low |
| Production Capacity | High | High | Low | Low |
| Material Range | High | Medium | Medium | Medium |
| Cost | Medium | Low | Medium | High |
Which Parts are Suitable for Metal Injection Molding?
Although metal injection molding is known as the fifth generation of metal forming technology, not all metal parts are suitable or economically viable for this process. Only high-volume, small, precision metal parts with complex 3D geometries and special requirements are suitable for metal injection molding.
| Characteristic | Minimum | Typical | Maximum |
| Weight (g) | 0.03 | 10–15 | 300 |
| Dimensions (mm) | 2.0 | 25 | 150 |
| Wall Thickness (mm) | 0.025 | 1–3 | 15 |
| Batch Size (Annual) | 1,000 | 100,000 | 100,000,000 |
Small Weight
Metal injection molding is best suited for small metal parts. Typical parts weigh between 10g and 15g. Those under 50g offer the most economic value, with a maximum limit of 300g.
Small dimensions
The process is ideal for small dimensions. Typical sizes are around 25mm, with a maximum of 150mm. Metal injection molding is less suitable for large parts because tolerances are generally 0.3% to 0.5% of the size. As the size increases, absolute tolerances grow, which may fail design requirements or necessitate costly secondary machining.
The typical wall thickness for metal injection molding parts is 1.0mm to 3.0mm.
Complex Geometry
Metal injection molding is suitable for parts with external undercuts, external threads, tapers, cross-holes, blind holes, bosses, keyways, ribs, or surface knurling. Simple shapes may be more economical via stamping, forging, or powder metallurgy.
Batch Volume
Since metal injection molding requires molds, high production volumes are necessary to offset tooling costs. Generally, an annual volume of over 100,000 units is required for the process to be economically viable.
Applications of Metal Injection Molding
Metal injection molding is widely used in consumer electronics, automotive parts, medical devices, power tools, industrial equipment, and daily necessities.
Consumer Electronics
This includes smartphones, tablets, laptops, digital cameras, wearables, and drones. In 2010, the logo components of Blackberry phones utilized the metal injection molding, initiating its mass use in mobile phones. Apple followed in 2010, expanding and leading the use of metal injection molding for power connectors, SIM trays, hinges, camera rings, and buttons. As devices become thinner and components more precise, the prospects for Metal Injection Molding in this field grow.
Automotive Components
As a no-cut forming process, metal injection molding saves material and costs. The automotive industry has highly valued it since the 1990s. Currently, the industry uses metal injection molding for complex parts, bimetallic parts, and micro-component sets, such as turbocharger parts, adjustment rings, fuel injector nozzles, vanes, gearbox components, and power steering parts.
Medical Devices
Metal injection molding produces components with high precision and excellent mechanical properties, meeting the requirements for small, complex surgical tools. It is increasingly used for surgical tool handles, scissors, forceps, dental parts, and orthopedic joint parts.
Power Tools
Power tool accessories involve complex machining and low material utilization, making them dependent on metal injection molding. Typical products include specially shaped milling cutters, cutting tools, fasteners, micro-gears, and components for textile or crimping machines.
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