MIM parts ?

2

224

New member
We've heard a lot of opinions about MIM parts around here. Quite a few people have told us how bad they are but I haven't read that many stories of firsthand failures. These are exactly what I want to hear about.

I would ask that everyone keep the discussion topic specific. Let's save the tirades on how bad MIM parts are. These include statements like "I'm an engineer and MIM parts are bad because...". On the other side of the coin let's not hear "my gun's fired a gazillion rounds without a problem..." By now we all know who's on which side of the MIM fence.

Stories about your slide stop not locking the slide back etc. don't count either. That's a problem with the parts dimensions, not the manufacturing proccess itself. Also, I'd like all of the shade tree gunsmiths to be honest with themselves and not comment. If you've decided that you can improve your pistols trigger and have filed away at the sear engagement surface, causing your pistol to go full auto that doesn't count. If you've decided to adjust the extractor with a pair of pliers and it's snapped in two, same thing. In short, don't blame the parts for your mistake.

Like I said, only firsthand accounts please. No "my brother talked to his friend Joe who was at the range and saw this guy..." stories. What I'm interested in are accounts of "I own pistol A and part B broke after X amount of rounds under these circumstances".

Thanks
 
There have been reports of specific failures of MIM parts. When a piece of machinery is designed to use milled steel, the use of other manufacturing techniques (casting, MIM, sintered metal, stamping, plastic molding) needs to be done carefully and with a lot of testing. All the older guns were over designed, and in many cases can be made lighter and cheaper with new techniques. But changes should be considered by design engineers and metallurgists, not by people seeking to make big bucks by making goods cheaper. Just because a part "looks about right" doesn't mean it is right.

Jim
 
That's great Jim, thanks.

Could everyone else please follow the instructions and remain on topic.
 
Now that the forum is back online... 224, I did indeed have a failure of a Kimber barrel bushing. The gun had around 4500 rounds through it at the time of faiulure. I was nearing the end of a shooting session when the recoil plug and spring took flight down range. Seems that the ears that hold the plug in place snapped off releasing the spring and plung on their flight. I had recently replaced the stock recoil spring with a Wolff 18.5 lb spring but don't feel that the spring had any effect on the failure of the bushing. Kimber's customer service did send out replacement parts right away, but I think I will be replacing all the MIM parts on the gun with quality machined parts. Okay who else? Mark / FL
 
Mark King - I just double-checked at the Kimber site on the page for Custom and Custom Target...their bushings are machined from stainless steel blanks. John
 
So Mark, aside from the fact that the bushing isn't an MIM part, why do you feel the replacement of the stock spring with an aftermarket couldn't have had any contributing factor to the bushing breaking?
I had the bushing break on a Springfield, but I was fairly certain it happened because I replaced the spring with an aftermarket spring and full length guide rod.
 
Well, I have heard experiences of others I know, but since you don't wanna hear those, I won't bother.

[This message has been edited by shamster (edited March 07, 2000).]
 
Regardless of whether or not the barrel bushing is or is not a MIM part, it wasn't stainless, nor was it replaced with a stainless part. The failure of the bushing IMHO a was due to an inferior part. Upon inspection of the broken part the steel was much more porous than what I expected . Regarding the 18.5 recoil spring, that's only 2.5 pounds more than the stock spring, I would think that a quality bushing should be able to stand up to a nominal increase in spring tension. Mark /FL
 
Johnbt, The web site does indeed say the parts are machined from stainless steel blanks. What it didn't say was whether or not the blanks were made from bar stock, forgings or the MIM process..Mark / FL
 
I've put several hundred rounds through a Kimber Ultra Carry with no problems. I intend to put several hundred more through it and I don't anticipate any problems. With the number of guns Kimber has produced in the last few years, I don't take the few reported cases of the MIM parts failing with much more than a grain of salt.

Alex Johnson
NRA Life Member
 
Shamster,

No insult meant. I have no reason to doubt your honesty. The problem with second and third hand information is that it is seldom truly accurate. that's why I'd like to stick with first hand accounts.
 
Sorry to be so dumb, but what does MIM mean? I even did a search, and looked it up in my online metallurical dictionary without success.

Walt
 
Hey Walt,

MIM means Metal Injection Molding. This is a casting proccess in which powdered metal is mixed with a polymer and then injected into a mold. The mold is then heated, therby removing the polymer and causing the metal to expand into the exact shape of the given part.
 
224; Thanks for the info. I take it that this is more precise than the lost wax process, which I also thought was called injection molding if it was done under pressure, but was with metal only; no polymer.

Why on earth would they make a piece of bar stock with this process and then machine it as mentioned above?

This sounds suspiciously like an old process called sintering. Could you elucidate as to the differences between MIM and sintering?

Thanks, Walt
 
Well, that's pretty close, but the metal part will actually contract (shrink) rather than expand, as the metal particles sinter together (the metal atoms move around to fill in the holes left by the missing polymer, which causes the outside dimensions of the part to shrink). Metal injection molding is basically similar to conventional powder metallurgy techniques where metal powders are pressed into shape and sintered with or without a polymer binder. The difference is that the metal/polymer mixture is squirted into a mold with the shape of the part (although oversize to account for shrinkage) rather than pressed into a shaped die under very high pressure, and so has a higher polymer binder content and more shrinkage. Advantages to these techniques are low cost, because machining of the resulting part can be reduced or eliminated, and the ability to make compositions that are not thermodynamically stable at room temperature (some powder mixtures would form brittle intermetallic compounds if melted, but the low temperature sintering prevents full reaction). May be too technical, but parts formed in this way have been accepted in many demanding applications, and in some cases offer properties much better than those of the parts they replace. The big problem is retained porosity; these parts will almost always have some pores present in the material that can act as initiation sites for failures. As Jim Keenan said, most parts in a handgun are a lot stronger than they really have to be, so there is a lot of potential for MIM.

This argument is similar to the steel vs. alloy frame argument; sure steel is stronger, but alloy frames do the job if the designer does his. A responsible manufacturer isn't going to use a MIM part for a job it isn't suited for, and if he does you probably don't want to buy his other products either.
 
Nick19, you did a BRILLIANT job explaining in reasonable terms what is meant by MIM!

What I'd like to know, from some Kimber owners, is WHICH parts are MIM?

My experience is that you have to look at the part, and know something about how the shape of the part affects the stresses it's subjected to, to know what manufacturing process is appropriate for it's manufacture.

I think the engineers out there would second this--'there are no bad processes, only bad engineers..' Perhaps someone at Kimber is falling down on the job.
 
Or perhaps they aren't, seeing as how NO ONE has come up with a first hand account of an MIM part failing yet in this thread.
 
I stand corrected, the Kimber barrel bushings are machined from bar stock, not the MIM process as I had been told by a well known pistolsmith, who shall remain nameless. I spoke with Joe at the Kimber factory this morning to get the information on the manufacturing process. So in the words of Emily Latella, "Nevermind!" Mark / FL
 
Nick19,

Thanks for elaborating on my inadequate explanation of MIM manufacturing. That was very well done.

Weegee,

The only parts that I'm certain are MIM on the Kimbers are the slide stop, hammer and sear (Wilson uses these parts as well). It's been claimed that the extractor is also MIM although I can't confirm that.
 
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