Metal powder injection molding, combines the advantages of powder metallurgy and injection molding technologies, breaking through the limitations of the traditional metal powder molding process in product shape. At the same time, it uses plastic injection molding technology to form parts with complex shapes in large quantities and with high efficiency. Characteristics, it has become a near-net shape technology for modern manufacturing of high-quality precision parts. It has advantages that conventional powder metallurgy, machining, precision casting, and other processing methods cannot match.
MIM, the metal injection molding process, has become a rapidly developing and promising new near-net forming technology in the field of powder metallurgy, and is known as one of the “most popular metal parts forming technologies in the world”.
This article will introduce the basic concepts, process flow, advantages, comparison with other processes, suitable part types, and MIM applications of the MIM process.
II. What is Metal Injection Molding?
Metal injection molding, referred to as MIM (Metal Injection Molding), is a method of mixing metal powder and binder for injection molding.
It first mixes the selected powder with a binder, then granulates the mixture, and then injects it into the required shape. After degreasing and sintering, the binder is removed to obtain the metal product we want, or it is then processed Subsequent shaping, surface treatment, heat treatment, machining, and other methods make the product more perfect.
MIM = powder metallurgy + injection molding
MIM is a typical cross-disciplinary product. It integrates two completely different processing technologies (powder metallurgy and injection molding), allowing engineers to get rid of traditional constraints and obtain low-cost, special-shaped stainless steel, nickel, iron, copper, titanium, and other metal parts, allowing greater design freedom than many other production processes.
III.Detailed Explanation of the MIM Process
The MIM process is mainly divided into four stages, including granulation, injection, degreasing, and sintering. If necessary, secondary processing such as machining or wire drawing and electroplating can be performed later.
Fine metal powders are mixed with paraffin binders and thermoplastics in precise proportions. The mixing process is carried out in a special mixing equipment, which is heated to a certain temperature to melt the binder.
In most cases, machinery is used to mix until the metal powder particles are evenly coated with a binder and cooled to form granules (called raw materials), which can be injected into the mold cavity.
The granular raw materials are fed into the machine to be heated and injected into the mold cavity under high pressure, and the green part is obtained through injection molding. This process is very similar to plastic injection molding. Molds can be designed with multiple cavities to increase productivity, and the size of the mold cavity should be designed to take into account the shrinkage generated during the sintering process of metal parts.
Degreasing is the process of removing the binder from the green embryo, and the brown part is obtained after degreasing. This process is usually completed in several steps. Most of the binder is removed before sintering, and the remaining part can support the part into the sintering furnace.
Degreasing can be accomplished by a variety of methods, the most commonly used being solvent extraction. Degreased parts are semi-permeable and residual binder is easily evaporated during sintering.
The degreased brown billet is put into a furnace controlled by high temperature and high pressure. The brown blank is slowly heated under the protection of gas to remove the remaining adhesive. After the binder is completely removed, the brown blank is heated to a very high temperature and the gaps between the particles disappear due to the fusion of the particles. The brown blank shrinks directionally to its designed size and transforms into a dense solid, resulting in the final product.
IV.Advantages of MIM process
MIM combines the advantages of powder metallurgy and plastic injection molding technologies, breaking through the limitations of the traditional metal powder molding process in product shape. At the same time, it uses plastic injection molding technology to form parts with complex shapes in large quantities and with high efficiency. It has become a near-net shape technology for modern manufacturing of high-quality precision parts. It has advantages that conventional powder metallurgy, machining, precision casting, and other processing methods cannot match.
Can form highly complex parts
Compared with other metal forming processes, such as sheet metal stamping, powder forming, forging, and machining, MIM can form parts with highly complex geometric shapes.
The complex part structures that can be achieved by injection molding can generally be achieved by MIM.
Taking advantage of this feature, MIM has the opportunity to combine multiple parts that were originally formed by other metals into one part, simplifying product design, reducing the number of parts, and thereby reducing product assembly costs.
Parts have uniform microstructure, high density, and good performance
Metal injection molding is a fluid molding process. The presence of the binder ensures the uniform distribution of the powder, thereby eliminating the uneven microstructure of the blank so that the density of the sintered product can reach the theoretical density of the material.
Generally speaking, Metal injection molding can reach 95% to 99% of the theoretical density. High density can increase the strength, toughness, ductility, electrical and thermal conductivity of MIM parts, and improve magnetic properties.
The density of parts pressed by traditional powder molding can only reach up to 85% of the theoretical density. This is mainly due to the friction between the mold wall and the powder and between the powder and the powder, which makes the distribution of the pressing pressure uneven, which leads to the microstructure of the blank being uneven, which will cause the pressed powder metallurgy parts to shrink unevenly during the sintering process. Therefore, the sintering temperature has to be lowered to reduce this effect, resulting in large porosity, poor material density, and low density. Seriously affects the mechanical properties of parts.
High efficiency, easy-to-achieve mass, and large-scale production
MIM uses an injection machine to mold green products, which greatly improves production efficiency and is suitable for mass production. At the same time, injection-molded products have good consistency and repeatability, thus providing a guarantee for mass and large-scale industrial production.
Wide range of applicable materials and broad application fields
The metal materials suitable for MIM are very wide. In principle, any powder material that can be cast at high temperatures can be manufactured into parts by the MIM process, including materials that are difficult to process and high melting point materials in traditional manufacturing processes.
The metal materials that Metal injection molding can process include low alloy steel, stainless steel, tool steel, nickel-based alloy, tungsten alloy, cemented carbide, titanium alloy, magnetic materials, Kovar alloy, fine ceramics, etc.
High precision of parts
The dimensional accuracy of MIM parts is usually ±0.5% of the size, and the precision level can reach more than ±0.3%.
For smaller part sizes, MIM has higher accuracy than other die-casting processes and generally does not require secondary processing or requires only a small amount of finishing, thereby reducing the cost of secondary processing.
As with other processes, the higher the dimensional accuracy requirements, the higher the cost. Therefore, moderate relaxation of tolerance requirements is encouraged when quality permits.
Tolerances that MIM cannot achieve in one molding can be achieved with the help of surface treatment.
V. Comparison: MIM vs. Traditional Plastic Injection Molding
The core distinction in the world of injection molding pivots around the materials used. Traditional plastic injection molding primarily uses various polymers and plastics. Conversely, Metal Injection Molding (MIM) employs a blend of fine metal powders and a polymer binder. This fundamental difference in raw material and composition not only dictates the process but also influences the final product’s properties and applications.
Molding Mechanics and Apparatus:
Albeit both methodologies deploy akin molding contrivances, the operative parameters diverge markedly, attributable to disparate material characteristics. Metal injection molding necessitates elevated temperatures to meld the metallic powder with the binder, and it typically encompasses more intricate debinding and sintering phases after molding.
Properties of the Culminating Product:
MIM yields metallic components boasting traits paralleling those crafted through orthodox metalworking techniques, such as augmented strength and thermal endurance. This contrasts with the predominantly plastic outputs of traditional injection molding.
Focus on Material Heterogeneity and the Feasibility of Complex Shapes
Variability in Materials: MIM endows a broader gamut of material options, spanning various steels, titanium, and bespoke alloys. These can be customized for specific requisites in robustness, longevity, and other mechanical attributes, unlike traditional injection molding, which is confined to the intrinsic characteristics of plastics.
Sophistication in Shapes:
MIM is adept at fabricating parts with elaborate geometries and refined details, a feat often unattainable with conventional plastic molding. The minuscule metal powder utilized in MIM facilitates heightened detail and stringent tolerances, rendering it apt for diminutive, complex components in sectors such as aerospace, medical, and electronics.
Wall Thickness and Internal Configurations:
MIM is capable of producing components with slender walls and intricate internal configurations without sacrificing strength, a challenge with plastic materials due to their divergent mechanical properties.
Surface Finish and Subsequent Treatments:
Components fashioned through MIM generally possess a superior surface quality compared to those from traditional plastic molding. They can undergo various enhancements like polishing, plating, or thermal treatments to augment their functional properties or visual appeal.
In summation, while Metal Injection Molding and traditional plastic injection molding share procedural parallels, their disparities in material usage, product characteristics, and shape complexity are significant. These distinctions render MIM more apt for scenarios demanding the resilience and precision of metallic components coupled with the design versatility characteristic of plastic injection molding machine parts.
In the article, we delve into the nuances of metal injection molding (MIM), a process that stands out in the world of manufacturing. Compared to conventional plastic injection molding, a significant diversity of materials used in MIM becomes apparent, enabling the production of more complex structures. MIM’s ability to manufacture metal injection molded parts with complex designs, exceptional precision, and strong mechanical properties has cemented its unique position in manufacturing, particularly in industries such as aerospace, healthcare, automotive components, and consumer electronics.
Overall, metal injection molding represents a significant advancement in manufacturing technology, blending the best aspects of metal and plastic production methods. Its continued development and adaptability will play a key role in the future of manufacturing, driving innovation and progress across multiple industries.