Heard about “MIM” parts? MIM is an injection molding process for metal parts, and it has been revolutionizing many industries. In the revolver business, both Smith & Wesson and Taurus have made use of MIM parts. Like any new process, however, there are those who decry the new technology; some gunsmiths spread the misinformation that MIM parts can’t be worked on, and refuse to take in guns using MIM parts. Adding fuel to the fire are a few well-publicized parts breakages, most notably with 1911 autopistol sears.

Is there something inherently wrong with MIM parts? No, but the story is a bit more complex than that.

I have some experience with MIM parts in revolvers; I’m not at all averse to the use of MIM parts, where appropriate. Note those last two words!

MIM is just another metalworking method, like forging and casting. Like those well-established metalworking methods, it has strengths and weaknesses. Far too few engineers apparently understand them.

First off, a steel MIM part can be treated like any other steel part; it can be welded, soldered, blued, hardened, and tempered. This is important to understand, as there is a perception out there that the parts are not “real” steel. They are!

The advantages of an MIM part do not generally include raw cost; the material is expensive, and the molds are horrendously expensive. The benefits come in the area of post-fabrication. The MIM part, as noted, can be heat treated – the benefit is that they don’t need to be, as the hardness of the part can be engineered in when the part is made. The parts come out ready to use; no additional surface finishing is generally needed. Finally, the parts can be made in shapes that would be extremely expensive or nearly impossible to economically machine.

The downsides? Cost, as already noted. Additionally, the tolerances for an MIM part generally need to be larger; it’s hard to hold them to .001″ in all dimensions (though they’re getting better all the time.) Another problem is that the technology doesn’t work all that well for parts that are more than about 3/8″ thick (again, this gets better on an almost monthly basis), nor on stressed parts that are very thin.

There are other, less obvious pros and cons of MIM parts, but you get the idea – MIM, like anything else, is a balancing act.

Now here’s the part that those of you who aren’t fond of MIM should understand: the problem isn’t with the technology, but with the engineering behind the part itself.

As noted, MIM on a per-part basis is pretty expensive, but since they can be engineered with specific traits they can eliminate some expensive secondary operations – hardening, for example. Here’s the problem: let’s say that you are building 1911 sears, and MIM seems a good method for producing them. You decide that the sear has to have a certain hardness (so that it doesn’t wear), and since the surface finish is good “as produced” you think you’re home free.

The trouble is that the MIM part is the same hardness all the way through, since that’s how it was engineered. This is great for reducing sear face wear, but with hardness comes brittleness – and that thin edge is quite brittle. What you need is a surface hardening of some sort for wear resistance, with the underlying material left softer for strength. You COULD do that with an MIM part, but if you did you’d negate one of the primary benefits of the method: the elimination of secondary operations. So the company chooses to continue to use the MIM part as designed, and which is a poor choice for the application. No wonder some people don’t like them!

The bottom line: if you have trouble with MIM parts, it’s not the part’s fault – it’s the fault of the engineers in the company that designed the part. (Frankly, I wouldn’t want to buy an entire gun from a company that botched the engineering that badly, regardless of whether or not I replaced the parts in question. I’m funny that way!)

-=[ Grant ]=-