Metal injection molding (MIM) is a new powder metallurgy near net forming technology that has emerged from the plastic injection molding industry. It is well known that plastic injection molding technology produces various complex shaped products at a low price, but plastic products have low strength. To improve their performance, metal or ceramic powder can be added to plastics to obtain products with high strength and good wear resistance.
In recent years, this idea has evolved to maximize the content of solid particles and completely remove the binder and densify the formed billet during subsequent sintering processes. This new powder metallurgy forming method is called metal injection molding.
What is metal injection molding?
Metal injection molding, also known as composite forming technology, is a processing technique that involves injecting metal materials into specific molds under liquid or gas pressure. The process includes a series of steps such as heating, cooling, and forming, ultimately producing various specialized parts and components according to the design drawings of the molds.
Furthermore, metal injection molding can also refer to the mixing of fine metal powder with a binder material to create a “feedstock,” which is then solidified using injection molding. This molding process can create large-volume, complex parts. After molding, the components undergo adjustments to remove the binder and densify the powder. The finished small parts can be applied in various industries.
Purpose
Metal injection molding (MIM) aims to produce small, intricate metal components in large quantities. This technique is favored because it eliminates the need for machining, making it more cost-efficient than traditional methods such as forging, casting, or machining, especially for small, complex parts produced in high volumes.
Why use metal injection molding?
Metal Injection Molding (MIM), a cutting-edge metal fabrication process, has witnessed a significant surge in popularity over the years. From its humble beginnings in 1986 to the current market size of approximately USD 380 million, MIM has established itself as a preferred choice for many manufacturers. The reasons behind this rise in popularity are numerous, but two stand out: its unparalleled efficiency and cost-effectiveness.
The efficiency of MIM lies in its ability to produce numerous small parts simultaneously. Unlike traditional metalworking processes, MIM allows manufacturers to mold only quantities of 100 grams or less per “shot” into multiple cavities of the mold. This means that a single molding cycle can yield multiple products, significantly reducing the overall production time and cost per item.
Moreover, the process allows for greater flexibility and customization. The feedstock used in MIM can consist of various metals, the most common being stainless steel. This versatility enables manufacturers to create products with specific properties and characteristics, tailored to their specific requirements.
Once the products are molded, a refinement process follows. The binder material used to bind the metal particles together is carefully removed, and the metal particles undergo a sintering process to fuse them together. This sintering step not only enhances the strength and durability of the material but also shrinks the size of the item by approximately 15%. This shrinkage is a controlled process, ensuring that the final product meets the desired specifications.
The cost-effectiveness of MIM is another significant factor contributing to its popularity. By producing multiple small parts simultaneously, manufacturers can achieve economies of scale, reducing the overall production cost per item. Additionally, the refinement process eliminates the need for post-processing steps such as machining, grinding, or polishing, further reducing the overall cost.
Metal injection molding process
The MIM manufacturing process generally includes: mixing granulation, injection molding, degreasing, sintering, and secondary treatment. The equipment involved includes a mixing and granulation integrated machine, MIM specialized injection molding machine, degreasing furnace, sintering furnace, and various detection and secondary processing equipment.
1.Preparation of metal powder
Metal powder injection molding has high requirements for raw material powder, including powder morphology, particle size, particle size composition, specific surface area, loose density, etc.
There are two main methods for raw material powder used in metal powder injection molding: hydroxyl method and atomization method. Metal powder injection molding requires very fine raw material powder, so the requirements for metal powder injection molding are very high.
2.Binder
Adhesives are the core of metal injection molding (MIM) technology. In MIM, adhesives have two basic functions: enhancing flowability to fit injection molding and maintaining the shape of the billet. In addition, they should also have characteristics such as easy removal, no pollution, non toxicity, and reasonable cost. As a result, various types of adhesives have emerged, and in recent years, they are gradually shifting from selecting based solely on experience to meeting the requirements of degreasing methods and binder functions, The development direction of designing adhesive systems with targeted approaches.
The requirements for binder in metal powder injection molding include: small contact angle with the powder and strong adhesion; No two-phase separation occurs with the powder; After cooling, it should have a certain strength; When degreasing, it will not cause serious injection molding defects such as cracking and bubbling of the green body; The viscosity of pure binder at injection temperature should be less than 0.1Pa · s.
3.Mixing
Mixing is the process of mixing metal powder with a binder to obtain a uniform feed. The process of mixing and effectively stirring the raw material powder and binder to homogenize and meet the injection requirements. The nature of the feed determines the performance of the final product. This process step has currently remained at the level of experience exploration, and an important indicator for evaluating the quality of the mixing process is the uniformity and consistency of the obtained feed.
So, the mixing process has become very important, which will involve various factors such as the way and sequence of adding binders and powders, the mixing temperature, the characteristics of the mixing device, etc. Ultimately, an important indicator for evaluating the quality of the mixing process is the uniformity and consistency of the feed obtained.
The commonly used mixing devices for metal injection molding (MIM) include double screw extruders, Z-shaped impeller mixers, single screw extruders, plunger extruders, double planetary mixers, double cam mixers, etc. These mixing devices are all suitable for preparing mixtures with viscosity within the range of 1-1000Pa · s.
4.Injection molding
In the powder injection molding process, injection molding is an important process that determines the production of qualified green billets. The mixture is heated by screw stirring in the injection molding machine, and the plasticized mixture is injected into the mold cavity through the injection molding machine feeding system, and maintained under pressure to compensate for cooling shrinkage.
After cooling and solidification, when the part has sufficient strength, open the mold and use a ejector pin to push the part out to obtain a green body.
5.Defatting
The green body is manually and regularly placed on the ceramic plate, and then waiting to enter the degreasing furnace for degreasing.
From the degreasing steps, all degreasing methods can be roughly divided into two categories: one is the two-step degreasing method. The two-step degreasing method includes solvent degreasing+thermal degreasing, siphon degreasing – thermal degreasing, etc. The one-step degreasing method is mainly the one-step thermal degreasing method, and currently the most advanced is the amaetamold method.
6.Sintering
Sintering is an important process in powder metallurgy and also the final step in the metal powder injection molding process.
By sintering, metal powder injection molded products can achieve full density or near full densification. In the sintering stage, it is necessary to control the change in sintering temperature to obtain high-density metal parts, avoid any surface cracks, and maintain the original shape of the parts with the same size shrinkage rate.
7.Secondary treatment
After sintering, sintered parts usually have certain burrs. The main part of the burrs can be removed by laser first, and then the remaining part can be removed by sandblasting, etc.
Sandblasting is a technique that uses the impact of high-speed sand flow to clean burrs and roughen surfaces. Sandblasting generally includes copper ore, quartz sand, diamond sand, iron sand, Hainan sand, etc. Sandblasting can not only achieve a matte effect on the surface of the product, but also deburring.
Duration of the Metal Injection Molding Process
The entire metal injection molding (MIM) process, from metal powder to finished parts, typically spans between 24 to 36 hours. Most of this duration is dedicated to thermal debinding, a step that alone can take up to 24 hours. Compared to direct laser metal sintering, which might take up to seven days to produce a single part, MIM is significantly faster. This speed advantage makes MIM ideal for high-volume production.
Popular metal injection molding materials
Metal Injection Molding (MIM) is a highly versatile process that can utilize a wide range of materials to create intricate metal parts. Here are some of the most popular materials used in MIM:
- Stainless Steel: Stainless steel is a common choice for MIM due to its corrosion resistance, strength, and durability. It is widely used in medical devices, automotive components, and other applications where precise metal parts are required.
- Low-Alloy Steels: These materials offer good strength-to-weight ratios and are often used in applications where weight reduction is critical, such as aerospace components.
- Nickel-Based Alloys: Nickel alloys are known for their high-temperature resistance and corrosion resistance, making them suitable for applications in harsh environments like oil and gas exploration.
- Titanium: Titanium is a lightweight but strong material that is often used in aerospace and medical applications. It is also biocompatible, making it a popular choice for implantable medical devices.
- Tool Steels: Tool steels are designed to retain their hardness and wear resistance at high temperatures, which makes them suitable for cutting tools and other applications where durability is essential.
- Copper-Based Alloys: Copper alloys are excellent conductors of heat and electricity, making them suitable for electrical components and heat exchangers.
- Tungsten Carbide: Tungsten carbide is a hard and wear-resistant material that is often used in cutting tools and wear parts.
The choice of material for MIM depends on the specific application’s requirements, such as strength, corrosion resistance, wear resistance, or electrical conductivity. Manufacturers often select the most suitable material based on these requirements and the material’s compatibility with the MIM process.
Common metal injection molding machines
- Injection Molding Machine – Used for injecting the metal powder-binder mixture into the mold.
- Debinding Furnace – Utilized for removing the binder from the molded part.
- Sintering Furnace – Employed to heat the debound part, causing the metal particles to fuse together.
- Mixing Equipment – Used to blend the metal powders with the binder material.
- Mold Clamping Unit – Ensures the mold is held tightly during the injection process.
- Temperature Control Units – Maintains precise temperature control throughout the process.
- Conveyor Systems – Facilitates the transfer of parts between different stages of the process.
- Cooling Systems – Used to cool the parts after injection and sintering processes.
- Quality Inspection Machines – Equipment like coordinate measuring machines (CMM) and X-ray inspection systems to ensure part accuracy and integrity.
These machines work together in the metal injection molding process to produce high-precision metal parts.
Cost of Metal Injection Molding
Metal injection molding (MIM) involves several cost factors that can vary based on specific project requirements. The feedstock, which consists of metal powder mixed with a binder, is notably more expensive in MIM, costing around $24 per kilogram. This contrasts with other powder metallurgy methods, where the feedstock costs about $2–$4 per kilogram.
Investing in MIM machinery represents a significant expense. The cost of metal injection molding machines ranges from $50,000 to $200,000, depending on the machine’s capabilities and specifications. Additionally, tooling costs are another considerable factor, typically falling between $30,000 and $70,000.
Several other factors influence the overall cost of a MIM part, including the size, material, and complexity of the product. Given these variables, the price per unit can vary widely, but it is often in the range of $1 to $5. The specific project requirements, such as the volume of parts needed and the intricacy of their designs, play a crucial role in determining the final cost.
Thus, while MIM offers cost advantages for producing small, complex parts in large volumes, the initial investment in feedstock, machinery, and tooling must be carefully considered.
What are the advantages of injection molding?
Metal Injection Molding (MIM) has multiple advantages, and the following are some of the main benefits:
1.Complexity
Metal injection molding (MIM) has almost no limitations in shape design, making it an ideal choice for combining various independent parts into highly complex and multifunctional products. The design rules are similar to injection molding, which means that almost all types of products can benefit from this advanced process.
2.Precision
Metal injection molding can achieve high precision. The variation in size is usually within the range of ± 0.1, and some characteristics can even reach ± 0.005. This high level of precision makes metal injection molding the preferred process for pursuing high-quality and precision components.
3.Weight and dimensions
Metal injection molding performs particularly well in handling small components, especially for components weighing less than 80 grams, with the lowest range being below 40 grams. Metal injection molding reduces the weight of components by adopting new technologies, thereby minimizing raw material costs to the greatest extent possible.
4.Thinning
For components with a wall thickness less than 5 millimeters, metal injection molding is the most suitable choice. Of course, thicker outer walls can also be treated, although it may lead to increased processing time and additional material costs.
5.Production capacity
Metal injection molding is suitable for products with annual demand ranging from thousands to millions, achieving economical and efficient production. However, for small batch products, customers may need to invest in molds and tools, which may have a certain impact on costs.
6.Multiple material selection
Metal injection molding, as a multifunctional process, can handle various metal materials, including ferroalloys, superalloys, titanium alloys, copper alloys, etc. Although non-ferrous alloys such as aluminum and copper are also technically feasible, other more economical processing methods such as die casting or CNC machining are usually chosen.
What are the disadvantages of injection molding?
The disadvantages of metal injection molding mainly include the following points:
- 1.High cost: The metal injection molding cost is high because it requires higher equipment and mold costs, as well as higher raw material costs, resulting in relatively high production costs.
- 2.High process difficulty: Metal injection molding requires precise control of process parameters such as temperature, pressure, and time, otherwise it is prone to problems such as insufficient filling, overflow, and warping, requiring high process requirements.
- 3.Limited application scope: Metal injection molding is suitable for the production of small and complex shaped metal parts. For large or simple metal parts, traditional metal processing methods may be more suitable.
Application of metal injection molding
Metal injection molding is suitable for applications that require high precision, shape, and material properties, such as:
- Automotive manufacturing industry: Metal injection molding can be used to produce automotive engine parts, transmission parts, intake manifolds, etc. These parts need to have high precision, high strength, and high temperature resistance.
- Medical device industry: Metal injection molding can be used to produce precision parts in medical devices, such as dental implants, surgical instruments, etc. These parts need to have high precision, non-toxic and sterile characteristics.
- Aerospace field: Metal injection molding can be used to produce aircraft components, rocket engine parts, etc. These parts need to have high strength, high precision, and lightweight characteristics.
- Electronic products industry: Metal injection molding can be used to produce precision parts in electronic products such as mobile phones, tablets, etc., such as camera brackets, antennas, etc. These parts need to have high precision and lightweight characteristics.
What is the difference between metal injection molding and plastic injection molding?
Metal injection molding and plastic injection molding are two different processes, and the main difference between them lies in the materials and process used.
- Material selection: Metal injection molding mainly uses metal powder or metal alloys, such as aluminum alloy, magnesium alloy, zinc alloy, copper alloy, etc., and is processed through injection molding at high temperature and high pressure. Plastic injection molding mainly uses various plastic materials, such as ABS, PC, POM, PVC, etc.
- Injection molding process: Metal injection molding usually uses die-casting technology, which requires higher injection pressure and longer injection time. Plastic injection molding, on the other hand, adopts a compression molding process, which requires shorter process time and lower process pressure.
- Product application: Parts manufactured by metal injection molding process have the characteristics of high density, high precision, and high strength, which are suitable for producing complex shaped parts, especially some parts that are difficult to process through mechanical processing and other process methods. Plastic injection molding has a wider range of applications and can be used to produce parts of various shapes and sizes.
Is metal injection molding suitable for prototyping?
Metal injection molding (MIM) is generally not suitable for prototyping due to the high tooling costs involved, making it economically impractical. For prototyping, methods like metal binder jetting or direct metal laser sintering (DMLS) are more appropriate. These techniques are ideal for creating prototypes because they utilize production-grade materials, ensuring that the prototype closely mimics the properties and performance of the final product.
Accuracy of metal injection molding
Metal injection molding (MIM) is known for its high precision, achieving dimensional tolerances between +/- 0.3% and 0.5%, or 0.01–0.001 mm. This level of accuracy surpasses other manufacturing methods like die casting, which often necessitates post-process machining to meet dimensional and surface finish requirements. However, the accuracy of MIM can be influenced by the size of the parts and the metal powder used. Larger parts tend to shrink more during sintering, leading to greater variability. Additionally, larger metal particles can result in increased shrinkage, affecting the overall precision of the final product.
Difference between metal injection molding vs die casting
| Feature | Metal Injection Molding (MIM) | Die Casting |
|---|---|---|
| Material | Metal powders mixed with binder | Molten metal |
| Process | Injection molding, debinding, sintering | High-pressure injection into molds |
| Accuracy | +/- 0.3–0.5% (0.01–0.001 mm) | Requires post-process machining for accuracy |
| Tooling Cost | High | Moderate to high |
| Production Volume | Suitable for high volumes | Suitable for medium to high volumes |
| Part Complexity | Excellent for complex geometries | Good, but with limitations |
| Size Limitations | Best for small to medium-sized parts | Can handle larger parts |
| Surface Finish | Generally smooth, requires little finishing | Varies, often needs additional finishing |
| Material Range | Wide range of metal alloys | Commonly non-ferrous metals like aluminum, zinc |
| Production Speed | Moderate | High |
| Prototype Suitability | Not cost-effective for prototyping | More feasible for prototyping |
| Shrinkage | Significant, managed during sintering | Minimal, managed during cooling |
| Applications | Automotive, aerospace, medical, electronics | Automotive, consumer goods, electronics |
Conclusion
Overall, metal injection molding technology has become one of the highly favored advanced manufacturing technologies in today’s manufacturing industry due to its flexibility, high precision, economy, and suitability for various metal materials. By fully utilizing this process, manufacturers can achieve more complex, precise, and economical production of metal components, driving continuous innovation and development in product design and manufacturing.
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FAQ
Generally, metal injection molding machines are a significant investment, and prices can range from hundreds of thousands to millions of dollars. The cost may also be influenced by additional factors such as the complexity of the machine’s design, automation features, and the level of precision and control it offers.
This article was written by engineers from the BOYI team. Fuquan Chen is a professional engineer and technical expert with 20 years of experience in rapid prototyping, mold manufacturing, and plastic injection molding.
