Metal Injection Molding: Techniques, Benefits, and Applications

Metal injection molding is a relatively new manufacturing method, first developed in the late 1970s. It offers a way to make parts that very few other methods can achieve, producing intricate components and complex geometries.

What Is Metal Injection Molding (MIM)? Process Overview & Key Benefits

Metal injection molding is a manufacturing process most similar to casting, whereby the raw material is fluidized and able to flow into complex cavities (molds) to produce parts that would not otherwise be possible to make.

Other benefits of MIM include low wastage and high performance, in some cases parts can be stronger than machined counterparts, with less waste and time to produce. Materials can also be “alloyed” with one another to increase strength.

Step-by-Step MIM Process: From Feedstock to Finished Part

The MIM is relatively complex, with each step needing careful control and coordination to achieve acceptable results:

Feedstock Preparation

Once the material powders are selected and mixed, they are mixed with a thermoplastic binder to create a “fluid” with a homogeneous density and flow rate.

Once the solution is combined, it is cooled and split into fine pellets, a process called granulation.

Injection Molding

The feedstock granules are heated until the thermopolymers or waxes melt, this allows the product to be injected into the mold. This is done on specialized equipment, at extremely high pressures.

Debinding

The molding process produces a “green part” that is solid but is not homogeneous. Debinding allows the binding agents (thermopolymers or waxes) to melt away, leaving a brittle and porous part.

Sintering and finishing

The final stage of the process involves bringing the part up to near the metal’s melting point. This causes the powdered metals to fuse together, creating a strong, solid part.

In some cases the finished part will need to be finished, such as sprues and mold lines removed and cleaned up.

MIM Materials: Stainless Steel, Alloy Steel & Specialty Metals

The metal injection molding process works with many types of metals, the most common ones being

• Stainless steels, great for things like medical devices where corrosion resistance is key
• Low alloy steels such as nickel iron based alloys; these are commonly used for automotive components
• Titanium and titanium alloys: lots of complex, intricate aerospace parts are made using titanium alloys, molding in this way
• Specialized alloys such as those with tungsten, cobalt, and even carbides for extreme hardness.

Cost Breakdown of Metal Injection Molding: Tooling vs Production

Like any molding or casting process, the tooling presents a significant upfront cost, and the per-part cost reduces dramatically with volume.

In the majority of cases, bespoke molds will run anywhere from $5,000 to $50,000+ to have designed and made. Thankfully, they can last for hundreds of thousands of parts, with minimal maintenance.

Producing parts using MIM can be one of the most cost-effective ways to do so, provided it is even possible to make them any other way. The process is highly mechanized and automated, meaning per-part labor costs are generally low.

MIM vs CNC Machining & Die Casting: When to Choose Each

In some cases it may be possible to make the same components with MIM, die casting, or CNC machining. It’s important to select the correct method, as each has their own strengths and weaknesses:

Metal Injection Molding:

• Useful when parts are very intricate, and have details on more than one plane – something that adds extra steps when machining
• Great for exotic materials that are hard to machine

CNC Machining:

• Part revisions can be implemented easily; a simple program change is all that’s needed
• Many shops have CNC milling machines, meaning costs can be more competitive

Die Casting:

• If your part is needed in high quantities, die casting is a great option as it’s super fast and efficient
• It also allows for thin-walled components, something that is difficult for the other methods

Best Applications for MIM: Small, Complex Precision Parts

There are thousands of applications for MIM, it is very likely that you have encountered parts made using this process already, without realizing it.

Metal injection molding is perfect for parts that need to be small, intricately designed, strong and produced in high quantities.

This makes the process perfect for parts in industries such as:

• Aerospace
• Dental/surgical implants
• Firearms accessories and components
• Automotive
• Specialized tools and hardware