Metal Injection Moulding (MIM) is an advanced manufacturing technique that uses fine metal powder combined with binder material to create a feedstock that is then shaped and solidified using injection moulding. Like plastic injection moulding, MIM is a cost-effective and incredibly efficient process that can be used for a wide range of applications.
This metalworking process is ideal for making higher volumes of small, precise, and complicated metal components, especially when the parts need to meet specific performance standards. In this article, we will cover how the metal injection moulding process works, considerations of the process, materials used in the process, and advantages and disadvantages of MIM.
Metal injection moulding is a manufacturing process that combines the principles of plastic injection moulding with powdered metal sintering to produce complex-shaped metal parts. Here’s a step-by-step overview of the MIM process:
The first step is to design and fabricate a high precision mould that will be used in the process to mould the desired metal product. The mould is designed with features such as runners, gates, and cavities to facilitate the flow of the molten feedstock.
Fine metal powders are then mixed with a thermoplastic or wax binder material to achieve the desired material properties then heated into a viscous mass. The mass gradually cools down and is processed into granular pellets to create what’s known as a ‘feedstock.’ This binder helps the metal powder maintain its shape during the injection moulding process.
In the injection moulding process, the metal feedstock is heated and injected into the mould cavity under higher pressure to create complex shapes and parts efficiently and precisely. The material then solidifies to form the final MIM part, known as the ‘green part.’
The green part is removed then put through a controlled binder removal process called ‘debinding.’ The binder is removed either by heating the green part in a controlled atmosphere or by using solvents to dissolve the binder. Once debinding is complete, the component is referred to as ‘brown.’ The part will be semi-porous, which allows the remaining binder to escape during the sintering process.
Sintering involves using heat to make materials stronger and more durable. During sintering, the metal particles fuse together in a controlled atmosphere and the remaining binder is entirely removed, contributing to a high-density final component. At this stage, shrinkage will occur. It can vary significantly depending on factors such as the material composition, sintering temperature, and time.
Generally speaking, any structure or shape that can be achieved using plastic injection moulding can be replicated in steel by metal injection moulding, but there are some limitations or design rules and considerations to keep in mind:
A wide range of metals can be used in metal injection moulding including ferrous alloys, tungsten alloys, special materials like precious metals, and hard materials – all of which emulates specific properties and characteristics.
|Ferrous metals refer to any metal that contains iron. They are known for their tensile strength and durability.
|Tungsten alloys are known for their high density and exceptional hardness. They are often used in applications where weight is a critical factor, such as in the aerospace and aviation industry.
|Special materials refer to alloys or materials that have unique or specialised properties that meet particular performance criteria, such as high-temperature resistance, corrosion resistance, or biocompatibility.
|In the context of MIM, “hard metals” typically refer to materials that are known for their exceptional hardness and wear resistance. They are often used in applications where resistance to abrasion and deformation is critical.
The metal injection moulding and plastic injection moulding processes share many similarities but also have distinct differences. MIM involves the use of metal powders mixed with a thermoplastic binder, whereas plastic injection moulding involves melting and injecting thermoplastic or thermosetting polymers into a mould.
In the plastic injection moulding process, molten plastic cools within the mould to solidify and form the final plastic part. In contrast, the MIM process involves sintering at high temperatures to fuse metal particles and remove the binder.
While the wide selection of materials for MIM is an advantage, the metal injection moulding process has four additional advantages.
Though the benefits of metal injection moulding are great, it does have disadvantages worth considering:
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