Metal injection molding, or MIM as it’s commonly referred to in the firearms community has something of a bad name. Go to any forum, especially forums that specialize in S&W revolvers or 1911 pistols and you’ll no doubt see a post that reads “that gun has MIM parts in it, and MIM sucks” or something along those lines. But the question is does MIM really suck? What’s all the hubbub…bub?
First, let’s take a look at how MIM works – this is going to be a very simple 10,000 foot look at the process, because I don’t want to put you guys to sleep reading about binders and powders. A MIM part starts its life as metal powder, which is mixed with what’s called the binder to create “feedstock”. The feedstock is then injection molded into the part mold, creating the shape of the part to come. Following that step is the debinding process – the raw part is placed into a low temperature debinding oven which vaporizes the binding agent. The final step in the manufacturing process is sintering; the sintering oven raises temperatures to just below the melting point of the raw material, causing science to happen.
Post sintering operations can include heat treating, additional surface hardening, polishing, welding, etc. MIM parts are “real” metal just like a forged or cast part in that any metalworking operation you can do on one of those parts you can also perform on a MIM part. However, the greatest benefit of using MIM to a firearms manufacturer is that the process produces “finished” parts – if your formula was good, you shouldn’t need to use a post manufacturing process to prepare the part for use.
MIM parts can be good – Colt’s MIM sears in their 1911 pistols are generally regarded as excellent. Your Glock/M&P is full of MIM part. The problem with MIM isn’t the process itself, but rather the shop that’s making the MIM part. For an example, we’ll look at 1911 sears, a part which is commonly MIM in factory guns. During the sintering portion of the MIM process, the sear acquires its final, finished hardness. While the part can be additionally heat treated for great surface hardness, doing that negates one of the greatest advantages of using MIM parts, which is the general lack of need for post manufacturing processes. In the MIM process, the debinding steps and sintering steps are critical to making sure the part has the correct hardness. If either of those steps aren’t done correctly, you can get a part that’s too soft, such many of the Kimber MIM sears, or a part that’s too hard and thus brittle. Those steps can also be performed correctly and still result in a part that’s too soft if your initial formula wasn’t quite right.
The other problem with MIM is that it’s hideously expensive to tool up to use MIM – molds and raw material costs a ton of money. That means if you get a run of parts that aren’t quite right, such as slightly too-soft sears your choice is either toss them out and watch a giant pile of money go into the garbage or to put them in guns because they’re “good enough.” The latter decision is obviously a quality control decision, and we could have a massive discussion on quality control as an entire other post.
So here’s the truth about MIM – there are plenty of excellent MIM parts in current use in pistols right now. MIM parts can be polished, filed, ground, welded, heat treated – any operation you could perform on a forged part you can perform on a MIM part post-manufacturing. The big weakness in the MIM process is quality control. That’s really what it comes down to. Bear in mind that many gun companies don’t manufacturer their own MIM in house, because of the noted cost of tooling up MIM production, but instead shop the work out to metalworking and dedicated MIM manufacturing houses. That ads another step of quality control, which some companies aren’t necessarily willing to perform.
MIM is just another manufacturing process. The problem with the soft sear in your 1911 isn’t because it’s a MIM part, the problem is that it’s a part that was manufactured with indifferent attention to detail and a casual attitude towards quality control.