If you ask the average gun enthusiast who frequents social media, you’ll likely hear that metal injection molding results in low-quality parts and led to the demise of one or more firearms manufacturers. Neither is true. 

The reality is that MIM wasn’t created for firearms manufacturing at all but began in the 1970s for use in aerospace applications. It was brought to the gun industry years later due to the low-cost, high-quality parts that it produces. 

Oddly enough, the 3D metal printing we recently went over is somewhat similar to MIM. Both use metal powders. The difference is that while 3D printing uses a laser to superheat the powder like a welder to bind it together, MIM uses a process more akin to polymer injection molding followed by a heating or sintering process. 

To make a part, they initially mix the desired metal powder with a polymer binder, like glue. In some instances, the binder is simply wax. This mixture of power and binder, referred to as feedstock, is then injected into a mold and the part is formed. Currently, MIM has a limit of one hundred grams of material per mold shot, but the mold can have multiple cavities to form parts. At this point, it is referred to as a green part and is the consistency of a crayon. Once the part is removed from the mold, it is placed in a special tray called furniture and is sintered. 

Sintering is done under high heat in a vacuum furnace at a temperature slightly below the melting point of the metal, which is often over 2,000 degrees Fahrenheit. This can take up to 48 hours. At this point, all of that heat vaporizes the binder and fuses the metal like happens with 3D printing. When wax was used, parts were simply dipped in solvent to remove the binder, and then sintered. 

Here’s an interesting point about this process. The sintering in the furnace results in the part shrinking up to 20%, so they have to be cast about 20% oversized to begin with. As you can imagine, this is a major challenge when creating the mold, with tooling costs being the most expensive part of the process. Engineers design oversized molds, but with so many variables in the process, if the final parts are still the wrong size, other processes are tweaked before a new mold is created. For example, they might change the ratio of binder to powder in the feedstock or the time and temperature spent in the vacuum furnace. 

Other challenges in the process include the introduction of bubbles as the feedstock fills the mold. This can be caused by an improper binder to powder ratio or a bad mold design. The latter is usually the result of the location of the gate, or position where the material is injected into the mold. For intricate parts, engineers must ensure that the feedstock can flow into all of the portions of the mold without creating bubbles. They also have to ensure that the gate isn’t located on a part that will affect its function. 

There’s another component of a part which is called the pin, or pins, if there is more than one. These are used to eject the unfinished part from the mold and are removed during later operations.

Speaking of gates, one of the ways to tell if a part was manufactured using MIM is the gate mark, which is a slightly raised bump on the part, unless it was made smooth during processing.

As mentioned earlier, some parts will need to be machined and/or heat-treated before they are ready for use. This can be followed with some form of surface treatment like applying Cerakote. Once the final process for making the parts is set, they have to ensure it is consistently followed. 

As for MIM parts being low quality, the opposite is true. MIM parts make a great alternative to die or investment cast metal if the parts are more intricate. Generally, cast parts can’t be as intricate and require extensive machining before they are suitable for use.

So, what types of parts are manufactured using the MIM process?  Parts like triggers, firing pins and even sights are made using MIM. In fact, the list is long. What’s more, MIM parts can be made from many materials, including titanium and various steel mixtures. This is one of the areas where MIM helps reduce costs. MIM only uses as much metal powder as is needed with a little extra for the gate and pins, leaving less waste. 

To be sure, there are made-in-America parts available, but like so many other forms of manufacturing, many MIM parts are made overseas, particularly in Turkey and India. This is where a lot of the idea that MIM parts are low quality comes from. Interestingly, one Indian MIM specialist has set up shop near Dallas, Texas, to better support the U.S. market. However, just because a part is made overseas doesn’t necessarily mean that it is good or bad. Once again, it all comes back to the design and specification of the part, along with quality control to maintain consistency. 

The reality is that Metal Injection Molding provides relatively intricate, low-cost parts at scale. It may also produce parts faster than 3D printing because it combines several processes. Finally, it is considered more environmentally friendly than other forms of manufacturing due to its reduced waste.