Metal Injection Molding (MIM) is a new powder metallurgy forming technology that combines modern plastic injection molding technology with powder metallurgy. The basic process involves mixing solid powder and organic bonding agents to create a feedstock, which is then injection molded and cured in a mold at a high temperature. The resulting part is then subjected to chemical or thermal decomposition to remove the bonding agent, and then sintered to produce a dense and compact final product. Compared to traditional methods, MIM offers high precision, uniform microstructures, excellent properties, and low production costs. MIM products are widely used in various industrial sectors, such as electronics, biomedical devices, automotive, machinery, sports equipment, watchmaking, knives and defense, and aerospace. The development of MIM technology is considered a revolutionary manufacturing technology, and has been regarded as the hottest part-forming technology and the molding technology of the 21st century.
Metal Injection Molding is a cross-disciplinary technology that combines plastic molding, polymer chemistry, powder metallurgy, and metal material science. By using molds, MIM can rapidly produce high-density, high-precision, three-dimensional complex shape parts through injection molding and sintering. It can effectively materialize design concepts into products with specific structural and functional characteristics, and can also produce parts in large quantities. The process has many advantages over traditional powder metallurgy, including fewer steps, less or no cutting, and high economic benefits. Furthermore, it overcomes the shortcomings of uneven material, low mechanical performance, difficult to manufacture thin-walled and complex structures, etc. It is especially suitable for mass production of small, complex and special-shaped metal parts.
Powder Preparation for Metal Injection Molding (MIM) Process
MIM requires high-quality raw material powders, including their shapes, particle sizes, composition, specific surface area, loose packing density, etc. Due to its high powder requirements, MIM powders are more expensive, costing approximately 1-10 times higher than traditional PM powders. This has been a key factor limiting the widespread use and promotion of MIM technology. Currently, the two major methods of producing MIM raw material powders are hydroxide method and atomization method. To meet the demand for extremely fine spherical powders in injection molding, some companies have developed new atomization techniques, such as high-pressure gas atomization and supersonic laminar gas atomization, resulting in aluminum alloys and stainless steel powders with an average diameter of ≤10μm.
Binder for Metal Injection Molding (MIM) Process
Binders play a crucial role in metal injection molding technology. Only by adding a certain amount of binder can the powders flow and be suitable for injection molding. After molding, the binders also help maintain the shape of the product. The binder content is generally 40%~60vol%.
The general requirements for binders are: small contact angle with powders and strong bonding ability; pure binders should have a viscosity below 0.1Pa.s at injection temperature and should not undergo phase separation with powders; after cooling, binders should have a certain strength and brittleness. To ensure smooth injection and debinding, binders, which have dual functions of conveying flow and maintaining shape, generally use multi-component systems. This includes good flowability, low-melting-point components (such as paraffin and vegetable oil) and high-melting-point polymer components (such as polyacetal resins). A small amount of surfactant also needs to be added.
Mixing for Metal Injection Molding (MIM) Process
Mixing involves mixing and effectively stirring raw material powders and binders under certain conditions and temperatures to achieve uniformity and meet injection requirements.
Since the properties of the feed determine the performance of the final injection-molded product, the mixing process is critical. This involves various factors such as the way and order of adding binders and powders, mixing temperature, mixing apparatus characteristics, etc. An important indicator of good mixing process is the uniformity and consistency of the resulting feed.
Common mixing devices for MIM include twin-screw extruders, B-type turbine mixers, single-screw extruders, plunger extruders, double planetary extruders, and double cam mixers. These devices are suitable for preparing mixed feed of viscosity between 1~1000Pa.s.
Injection molding is a critical process in the entire manufacturing process. During the injection molding process, various defects such as cracks, pores, welds, delamination, and powder and binder separation can easily form. These defects often can only be discovered after degreasing and sintering are completed and injection stress is released. The formation of defects is mainly due to factors such as unqualified raw material powder, improper selection of binders, and unqualified feeding mixing, and mainly depends on the process conditions during injection molding.
When injection molding, control of potential defects should be considered from two aspects: 1) setting of molding process parameters (injection temperature, injection time, and mold opening time); 2) flow behavior of the feeding material in the mold cavity. Because most MIM products are complex in shape and have high precision requirements, the flow behavior of the feeding material in the mold cavity involves mold design issues, including the position of the feed port, the shape and length of the runner, and the setting and distribution of exhaust holes. Therefore, in mold design and manufacturing, detailed analysis of the rheological properties of the feeding material, the temperature distribution in the mold cavity, and the residual stress distribution is necessary.
Degreasing is the process of removing all binders from the formed blanks by using a suitable method. There are two basic methods for degreasing: solvent extraction and thermal decomposition.
Sintering is an important link in powder metallurgy (PM) and also the final process in the MIM process. Through sintering, MIM products can achieve full density or near full density. Modern powder metallurgy sintering technology is developing towards "three highs and one low" (high strength, high performance, high precision, and low cost) and "three resistances" (wear resistance, high temperature resistance, and corrosion resistance). With injection molding combined with modern sintering technology, the molded parts or products can achieve 96% of the theoretical density.
Compared with traditional processing methods, MIM parts have the characteristics of high precision, high strength, high shape complexity, material diversity, and low cost, so MIM technology has been widely used in automobiles. Currently, MIM parts used in automobiles are generally iron-based materials, mainly including Fe-Ni alloy steel, Fe-0.4C-1Cr-0.75Mn-0.2Mo alloy steel, pre-alloyed Cr-Mo-C steel, Ni-Cr-Mo-C steel, 316L, 17-4PH, 400 series, HK series stainless steel, and Inconel713C nickel-based heat-resistant alloy steel.
The turbocharger is mainly composed of a turbine, impeller, rotor, and blades. It uses the inertia force generated by the high-pressure exhaust gas discharged from the engine to drive the impeller to rotate, which in turn drives the turbine to increase the engine's intake pressure.
In recent years, the research and development of turbocharger MIM components have become a focus of scientific research, and the turbocharger is also one of the iconic components manufactured by MIM. Its structure is extremely complex, the working environment is harsh, and the precision requirements are high. However, other machining methods have higher costs and are difficult to control accuracy. Turbocharger components are mainly made of nickel-based high-temperature alloys, titanium alloys, etc., and traditional powder metallurgy technology was used for manufacturing them.
Many small precision composite components on automobiles can be manufactured using MIM technology. The manufacturing of composite parts generally uses forging, precision casting, and other methods, resulting in high costs and low precision, and cannot achieve better economic benefits. When using MIM technology for manufacturing, production efficiency can be improved, precision can be increased, materials can be saved, processes can be reduced, and costs can be lowered.
The fuel injectors on automobile engines consist of more than 20 parts, among which iron cores, retaining iron, magnetic sheets, and guide bodies are the components that make up the injector's magnetic circuit structure, and these parts are all made of soft magnetic alloy materials.
Compared with traditional fuel injectors, MIM-manufactured fuel injectors made using iron-based nano-crystalline soft magnetic powder have improved comprehensive performance.
With the progress of technology, the types and functions of sensors applied to automobiles tend towards diversified, intelligent, and miniaturized development. According to the different application areas, sensor shells are used for engine, chassis, body, navigation and other systems.
Many sensors used in engine, chassis, body, navigation, and other systems have been manufactured using MIM technology, such as pressure sensor components, embedded components for airbag sensors, oxygen sensors, steering sensors, cruise control sensor seats, sensor shells, etc.
With the progress of technology, the main direction of automotive development is towards energy-saving, environmental protection, comfort, intelligence, and lightweight. The application prospects of MIM automotive products are huge. Using MIM technology, tiny automotive parts can be designed as a whole or multiple components, and through one-time injection moulding using MIM technology, components can achieve micro, integrated, low-cost, and other characteristics while meeting performance requirements. Meanwhile, components with external grooves, external threads, cross-holes, blind holes, reinforcing rib plates, grooves, and key pins can be manufactured with complex shapes.
Die casting is a method of casting in which liquid or semi-liquid metal or alloy is poured into the pressure chamber of a die casting machine, filled in a mold cavity under high pressure and high speed, and molded and crystallized under high pressure to obtain a casting. The high pressure and high speed are the two main characteristics of the liquid metal filling and molding process in die casting, and the fundamental difference between die casting and other casting methods. The filling process in die casting is affected by many factors, such as pressure, speed, temperature, properties of the molten metal, and filling characteristics. Throughout the entire die casting process, the molten metal is always pushed by pressure and remains solidified under pressure when the filling is complete. The presence of pressure is the main feature that distinguishes this casting process from other casting methods, and it has an impact on speed, temperature, gas in the mold cavity, and a series of filling characteristics. Therefore, in the die casting filling process, there should be an overall concept of the variation in pressure.
The time required from when the liquid metal begins to enter the mold cavity until the mold cavity is filled is called the filling time. The length of the filling time depends on the size and complexity of the casting. For large and simple castings, the filling time is relatively longer, while for complex and thin-walled castings, the filling time is shorter. The filling time is closely related to the cross-sectional area, width, and thickness of the sprue, and must be accurately determined.
Holding pressure and mold opening time
The duration from when the liquid metal fills the mold cavity to when the sprue is completely solidified under the action of the injection plunger is called the holding pressure time. The length of the holding pressure time depends on the material and wall thickness of the casting.
After holding pressure, the mold should be opened to remove the casting. The time from the end of injection to the opening of the mold is called the mold opening time, which should be accurately controlled. If the mold opening time is too short, the low strength of the alloy may cause deformation when the casting is ejected and the mold is dropped. However, if the mold opening time is too long, the casting temperature will be too low, the shrinkage will be large, and the resistance to core pulling and ejecting the casting will also be significant. Generally, the mold opening time is calculated based on a casting wall thickness of 1 millimeter requiring 3 seconds, and then adjusted through trials.
Die casting coating
To avoid the welding of the casting to the die casting mold, reduce the friction resistance when the casting is ejected and prevent the die casting mold from overheating, coating is used during the die casting process.
Casting cleaning is a heavy workload, often 10 to 15 times that of die casting. Therefore, as the productivity of the die casting machine increases and the production volume increases, it is essential to realize the mechanization and automation of casting cleaning work.
Die casting machines are generally divided into two categories: hot chamber die casting machines and cold chamber die casting machines. Cold chamber die casting machines are further divided into horizontal and vertical (including full vertical) based on their chamber structure and layout.
The hot chamber die casting machine (also known as the hot air die casting machine) has the chamber immersed in the liquid metal of the insulating melting crucible, and the injection components are not directly connected to the machine base, but mounted on top of the crucible. The advantages of this type of die casting machine are simple production process, high efficiency, low metal consumption, and stable process. However, the long-term immersion of the chamber and injection nozzle in liquid metal affects their lifespan and increases the iron content of the alloys. Hot chamber die casting machines are currently mainly used for casting low-melting-point alloys such as zinc, but they can also be used for small aluminum and magnesium alloy castings.
The cold chamber die casting machine has its chamber separated from the insulating furnace. During casting, liquid metal is taken from the insulating furnace and poured into the chamber for die casting.
Die casting is a highly-efficient, non-cutting, and precise metal forming casting method that has developed rapidly in modern metal processing technology. Compared with other casting methods, die casting has advantages such as short production process, simple and concentrated process, good quality and high precision of castings, smooth surface finish, and reduction or elimination of mechanical processes, equipment, and working hours. It has a high production rate, saves energy and raw materials, and is a highly cost-effective casting method. This casting method has been widely used in various industries in the national economy, including military industry, automobile and motorcycle, aerospace products, electronic appliances, instruments, radio communication, television, computers, agricultural machinery, medical equipment, washing machines, refrigerators, clocks, cameras, and daily hardware parts. Currently, die castings range from a few grams to a maximum weight of 50kg for aluminum alloy castings and a maximum diameter of 2 meters.
The size and weight of die castings depend on the power of the die casting machine. As the power of the machine increases, the size of the casting can range from a few millimeters to 1-2 meters, and the weight can range from a few grams to tens of kilograms. Die casting is no longer limited to the automotive industry and instrument industry but has expanded to various industrial sectors such as construction machinery, machine tool industry, electronics industry, defense industry, computers, medical equipment, clocks, cameras, and daily hardware parts. New technologies, such as vacuum die casting, oxygen injection die casting, rapid die casting, and the application of soluble cores, have emerged in the die casting industry.
What are the differences between metal injection molding and die casting? The main differences lie in the material, temperature, and control methods.
Compared with powder metallurgy and metal injection molding parts, die castings may be the same size or much larger. When high strength, high operating temperature, high wear resistance and high friction reduction performance, and corrosion resistance are the primary requirements, powder metallurgy is more suitable. For example:
High strength: The tensile strength of some iron-based sintered alloys is more than three times higher than that of die-cast alloys.
High operating temperature: Iron-based and copper-based sintered alloys can be used to meet this requirement.
High wear resistance and high friction reduction performance: These can be solved by using iron-based and copper-based sintered alloys soaked in lubricating oil.
Corrosion resistance: Copper-based sintered alloys and sintered stainless steel can meet the requirements. Zinc die-castings may be a substitute for iron-based powder metallurgy products under conditions requiring medium strength and using temperatures not exceeding 65°C.