Metal Injection Molding

The metal injection molding cycle starts with the creation of the feedstock (the raw materials used to produce another material or item) by combining a binder made up of lubricants, polymers, waxes, and surfactants with a smooth metallic powder. The final feedstock is then ground into granules. Next, the feedstock is heated using an injection molding machine before being injected under pressure into a mold cavity. Time is then allowed for the molten feedstock to cool and solidify. A portion of the feedstock becomes a very porous brown substance once the binder components are eliminated. This brown component is subsequently sintered into more solid material and normally achieves over 95% of the density of the rest of the material, with no pores.

Metal Injection Molding Considerations

Metal injection molding can process almost any metal manufactured in an appropriate powder form. The adhering oxide film constantly presents on the surface of aluminum metal makes it an exception, as it prevents the sintering stage of the metal injection molding process. Numerous popular and less-common metals and their alloys are included in the list of metals that have been utilized in the metal injection molding process. The various metals processed through MIM include high-speed steels, superalloys, magnetic alloys, stainless steels, cemented carbides (hard metals), magnetic alloys, and intermetallic alloys.

Several factors make the choice of the binding particle used significant, including a powder’s density, the amount of metal contained in the feedstock, etc. Another surprising factor in selecting a binder is the shape of its particles. Typically, a spherical or nearly-spherical shape is preferable for MIM processes, although this also increases the chance of the skeleton deforming during the debonding stage. Coating each particle’s surface with a binder is another major factor to consider during the mixing process.

The feedstock mix is converted into solid pellets through a process known as granulation during the molding stage. To fabricate the best products, the temperatures of various metal injection machine components, including the nozzle used to inject the feedstock mix into the die and the screw which extrudes the mix through the die cavity, must be carefully maintained. The temperature of the die itself must also be managed, and it needs to be kept low enough to guarantee that removed components are set and solid.

Debinding, which involves removing the binding agent from the mold, is another crucial factor in metal injection molding, requiring extremely precise management. Two different processes are typically involved during debonding. In the first process, heat is applied to the mold and other equipment components to disintegrate any remaining feedstock material found on them. The second debinding procedure, which is only applicable to specific binder systems, involves dissolving the binder agent using suitable solvents like trichloroethane; again, heating is typically needed as the last step to complete the elimination of the binder material and solvent by evaporation.

Sintering creates a solid mass of material by applying pressure and heat to it without melting it to the point of liquefaction. Sintering is what provides a final product its strength. The procedure is done at temperatures usually below the metal’s melting point in controlled-environment furnaces in a vacuum and is crucial to prevent oxidation of the metal. The specific atmosphere of the sintering furnace depends on the metal type. For many metals, a simple atmosphere of hydrogen is sufficient. Still, for steel, where carbon serves as a crucial alloy ingredient, the atmosphere must contain one or more carbon compounds to maintain equilibrium with the steel, not to change this material’s properties.

Advantages of Metal Injection Molding

• Large production runs of metal components with intricate geometries and features are supported by metal injection molding. It works well for the high-volume production of small, precise parts with exact tolerances.
• Metal injection molding can precisely manufacture features, including internal and external threads, undercuts, holes, markings, and slots, without the need for additional machining or fabrication steps.
• Few limitations are placed on an item’s design through MIM. This process allows the opportunity to produce a wide range of shapes.
• Metal injection molding provides a smooth surface that can be improved through polishing.
• Metal injection molding can produce multiple parts from a single material piece.
• A variety of materials can be used in metal injection molding.

Disadvantages of Metal Injection Molding

• Metal injection molding can be pricey for modest production requirements.
• MIM may not be the best method for creating challenging metal fabrications.
• Metal injection molding is appropriate for small and medium-sized parts. Large part shaping, however, can reduce the mold’s ability to function properly and the furnace’s capacity, increasing process costs.