A Complete Guide to the Metal Injection
Moulding (MIM) Process

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.

How metal injection moulding works (step-by-step guide)

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:

1. Creating a mould

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.

2. Feedstock preparation

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.

3. 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.’

4. Debinding (binder removal)

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.

5. Sintering

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.

Considerations of the MIM process

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:

• MIM part weight recommendation: Although MIM has been used to produce parts weighing up to 453g, for optimal results, we recommend producing parts under 100g. Weight reduction enhances precision in dimensional tolerances and reduces raw material consumption, lowering overall cost.
• Edge considerations: The maximum allowable radius is recommended for all edges, especially internal edges since sharp edges can be sources of potential cracks.
• Gradual section changes: Section changes should be gradual, to facilitate material flow during injection and prevent issues like isolated masses, voids or incomplete filling.
• Flat face for palletizing: A flat face will facilitate the palletizing of the part inside the MIM furnace. The more stable this support is, the smaller the deformations will be and the more precise the dimensional tolerance will be.

What types of materials can be used in metal injection moulding?

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.

Difference between MIM and plastic injection moulding

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.

Advantages of metal injection moulding

While the wide selection of materials for MIM is an advantage, the metal injection moulding process has four additional advantages.

• Creating complex parts – MIM excels at producing small, intricate and complex shapes with high precision, allowing for the manufacture of detailed and sophisticated metal components.
• Reduced Secondary Operations – MIM often requires fewer secondary operations compared to metal fabrication processes like CNC machining, as it can achieve complex shapes and features directly in the moulding process.
• Higher production volumes – MIM is pefect for higher-volume production, as it can produce a large number of parts in a single cycle, making it cost-effective for mass production.

Disadvantages of metal injection moulding

Though the benefits of metal injection moulding are great, it does have disadvantages worth considering:

• Initial costs – MIM may involve higher initial costs, including the expenses associated with equipment, materials, and specialised tooling.
• Tooling lead times – The process may require longer lead times for tooling development, impacting the overall production lead time.
• Design change costs – Implementing design changes in metal injection moulding can be costly, and the process is often less flexible in accommodating alterations compared to some other manufacturing methods.

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