Metal Injection Molding (MIM) is a variation on traditional plastic injection molding that enables the fabrication of solid metal parts utilizing injection molding technology.
In this process, the raw material, referred to as the feedstock, is a powder mixture of metal and polymer. For this reason, MIM is sometimes referred to as Powder Injection Molding (PIM). Using a standard injection molding machine, the powder is melted and injected into a mold, where it cools and solidifies into the shape of the desired part. Subsequent heating processes remove the unwanted polymer and produce a high-density metal part.
Metal injection molding is best suited for the high-volume production of small metal parts. As with injection molding, these parts may be geometrically complex and have thin walls and fine details. The use of metal powders enables a wide variety of ferrous and non ferrous alloys to be used and for the material properties (strength, hardness, wear resistance, corrosion resistance, etc.) to be close to those of wrought metals.
Also, because the metal is not melted in the MIM process (unlike metal casting processes), high temperature alloys can be used without any negative affect on tool life. Metals commonly used for MIM parts include the following:
Low alloy steels
Stainless steels
High-speed steels
Irons
Cobalt alloys
Copper alloys
Nickel alloys
Tungsten alloys
Titanium alloys
Metal parts manufactured from the MIM process are found in numerous industries, including aerospace, automotive, consumer products, medical/dental, and telecommunications. MIM components can be found in cell phones, sporting goods, power tools, surgical instruments, and various electronic and optical devices.
MIM is a process that merges two established technologies: plastic injection molding and powdered metallurgy. It offers a manufacturing capability of producing precise, complex parts in large quantities.
MIM Materials
There are a wide variety of metal alloys available for the MIM process including different types of steel, titanium, and copper—to name a few. OptiMIM specializes in:
Stainless steel – ideal for applications requiring strength, ductility, and corrosion resistance
Low alloy steel – generally used for structural applications especially when high strength and hardness are necessary
Copper– most commonly used for thermal management applications that require miniaturization, improved heat transfer, and electrical conductivity
Metal Injection Molding Process
Once the proper material is selected, the key steps for the MIM process are as follows:
Step 1: Feedstock – Very fine metal powders are combined with thermoplastic and wax binders in a precise recipe. A proprietary compounding process creates a homogenous pelletized feedstock that can be injection molded just like plastic. This achieves ultra-high density and close tolerances over high-production runs.
Step 2: Molding – The feedstock is heated and injected into a mold cavity under high pressure, allowing for extremely complex shapes. Once the component is removed it is known as a “green part.”
Step 3: Debinding – the “green part” is then put through a controlled process called debinding that removes the binder and prepares the part for the final step. Once the debinding is complete, the component is referred to as “brown.”
Step 4: Sintering – the “brown” part is held together by a small amount of binder and is still fragile. During sintering temperatures reach near the melting point of the material. Sintering eliminates the remaining binder and gives the part its final density and strength.
MIM Tooling
When choosing a MIM manufacturer you want to make sure that whatever tooling process you choose delivers consistent parts efficiently and repeatedly. Our conventional tooling process is designed to offer you efficiency in production and lower costs.
Wondering if your part is a good size for MIM? Read more here.
Heat Treating for MIM Components
MIM materials may be heat treated to increase strength, hardness, and wear resistance. The degree of hardening is determined by the percentage of carbon, alloying elements, and residual porosity of the component. Heat treating allows you to achieve ultimate properties for the alloy. To ensure optimum strength and durability, tempering or stress relief is required after quenching.
Other Secondary Operations after Sintering
After your components are completely rid of all binding material, OptiMIM offers many secondary operations to improve dimensional control, including:
Coining / Sizing / Straightening
Machining (miling, turning, grinding, etc.)
During the sintering process, parts can be distorted, and start to sag or drag. The processes above correct these issues and return the part to its original design.
Benefits of MIM
MIM offers greater design freedom than many other production processes by freeing designers from the traditional constraints associated with trying to shape stainless steel, copper, titanium, and other metals. Other benefits of MIM, include:
MIM makes it possible to integrate and consolidate several components into a single molded piece—reducing the need to work with several manufacturers and decreasing processing and assembly costs.
Texture, knurling, threads, lettering, and company logos can all be incorporated into the mold.
Design for MIM
Design for MIM manufacturability is one of the most important steps that can be overlooked. When you design for MIM there are a number of factors that you can plan for that reduce or eliminate the need for secondary operations after sintering—which can increase overall component cost. Ask your design engineer about sintering supports, draft, fillets and radii, as well as sink and knitlines and what they plan to do to prevent part distortion.
Metal Injection Molding is today a crucial technology for a wide range of manufacturing sectors, from consumer electronics to medical devices to automotive and aerospace applications.
In the following introduction we review the main process steps and present a representative applications.
We also look at the range of materials that can be processed along with the basic properties that can be expected when using Metal Injection Molding technology. Whilst Metal Injection Molding is the focus of this review, much of the content is also application to Ceramic Injection Molding.
Introducing a dynamic high growth global industry
An overview of the Metal Injection Molding process
Powders for Metal Injection Molding
Binders and binder removal techniques
Feedstock for Metal Injection Molding
Injection Moulding MIM parts
Sintering in the Metal Injection Moulding process
Applications: Medical and Orthodontic
Applications: The Automotive Industry
Applications: IT, Electronics and Telecomms
Applications: The Aerospace Industry
Applications: Consumer Products
Applications: Firearms and Defence
Conclusions
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