What are MIM internals?
Does it matter?
Are they better or worse than non MIM?
Please first see the FAQ, and the accompanying post by S&W on the issue of MIM:
http://smith-wessonforum.com/s-w-smithing/94072-faqs.html#post1029055
Wish I had seen this b4 writing all the mess below.
Metal Injection Molding. Oversimplified, MIM is a powdered metal mixed with a synthetic binding agent injected into a mold and then heated to a high temperture to cook off the synthetic binding agent leaving only the metal part after the firing process. Your Glock and or M&P is full of MIM parts. Heck, jet engines have moving parts that are MIM (but their parts are x-rayed before use).
Quality of the finnished metal product is dependent on the quality of the mixture put into the molds and the quality of the process. If the process is done correctly, the finished products arguably begin to rival that of other methods. If things are done wrong, parts can be too soft or too brittle.
Some gun manufacturers such as Wilson Combat's 1911's tout the fact that they do not use MIM parts instead selling their customers parts from machined tool grade forged steel. They market these parts as "bullet proof". Many will say that's overkill. And, Wilson typically charges North of 4K for their 1911's. Wilson's clever marketing may just mask their avoidance of the breathtaking expense of materials, machinery, and quality control for MIM production.
Most S&W revolvers have MIM. Maybe all, I do not know. Once in a while you'll see a S&W revolver that foregos the MIM hammer and trigger for forged steel such as the PC 586 L-Comp.
High quality MIM is 95 - 96% the density of forged steel. When MIM breaks people will on occasion point fingers at the fact that it is MIM; but figure if the failed part was machined from forged stock it had to be an anomaly. In other words, you can get the same forum hysteria surrounding MIM you get with the Internal Lock. There are caveats to both, but nothing approaching the ridiculous statements and purchase avoidance sometimes seen on this forum.
There are two equally extreme and equally unfounded positions abounding about MIM: 1) All MIM parts are horrible and should never be used (MIM stands for Maybe It's Metal). 2) There is absolutely no difference so why worry? MIM does introduce defect modes not found in machined from forged parts and the latter parts make more sense (to many) for key parts subject to high stress. Also, it is arguable that gun makers who use MIM for all internal parts are not using common sense.
Here is a hysteria position I recently read by a WC owner: "I have tons of MIM parts in my guns and so far, haven't had any fail. I have ZERO MIM parts in the gun that protects my life and my family. My choice." Koolaide consumed.
So when should MIM be avoided? Some will point out that MIM (like cast) is subject to casting voids (air bubbles) defects as well as metal powder size variation defects, to name the two biggest. As a result, the argument will be made that MIM is not best suited for use in long, thin parts which are subject to stress like a 1911 extractor or slide stop or safety lever where a slight defect would cause the piece to break; but MIM can be safely used for hammers and triggers and similar parts when properly done. While I'm aware those arguments are made, I don't have the expertise to judge their prudence. In revolvers it does appear S&W widely and successfully uses MIM for at least hammers hammers and triggers.
Here's the conclusion over at gunnuts.net, "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." That article gives a much more detailed rendering of the MIM process as well.
So no, when implemented with all diligence and prudence, it does not matter.
Peace
