I keep seeing references to MIM parts in The Fourm.
Would somebody please explain to me exactly what MIM parts are.
When did they first start using them?
Are they only in revolvers, or in semi-autos too?
How can you tell if your gun has MIM parts in it?
What is the problem with them?

Thank you in advance for any information you can provide.

Stu

__________________
NRA Benefactor/Cert Instructor

Stu,

Here is a post from several years ago. I saved it, and it pretty well describes MIM and S&W.
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Here is an explantion by Herb Belin of Smith and Wesson on MIM parts. I first saw this posted by Victor Louis, and saved it.
Troy

By popular request, here's the post from Mr. Herb Belin of S&W -----------------------
"I have read with much interest the many comments in this forum pertaining to MIM, MIM Parts and the use of same in a S&W product. So far I have come away with several impressions and they are "people in general don't like/trust MIM parts" and no one has said why. I will take a stab at this issue and see where it goes.

As background to our decision to use MIM in some areas of our Mfg Process, we took a long hard look at our "Life Time Service Policy". It was clear to us that any change in any of our products such as the use of MIM components had to show equivalent or better performance and durability to those components that were being replaced or the "Lifetime Service" would haunt us forever. The second consideration was to determine if the change was too radical a departure from S&W mainstream design.

For the performance and durability issues, we decided that if MIM could be used for the fabrication of revolver hammers and triggers succesfully this would truly be an "Acid Test". There is nothing more important to a revolvers feel than the all important Single Action Sear that is established between the hammer and the trigger. Mechanicaly few places in a revolver work harder than at the point where the hammer and trigger bear against each other. If these surfaces wear or loose their "edge", the "feel" is lost. Initial testing was on these two critical parts. Over time we arrived at a point where our best shooters could not tell the difference between a revolver with the old style hammer and trigger and the new MIM components. Special attention was given to their endurance when used in our very light Magnum J frames such as the early prototype 340 & 360 Sc's. None of our revolvers work their components harder than these small magnum revolvers. Throughout this testing MIM held strong and finally we determined that this change, judged on the basis of durability and feel, was a good one.

The second area of concern to S&W was our customers reaction to this departure from the traditional. Many heated, intense discussions resulted, but in the end the decision was made to move ahead with MIM.

The issue of cost was only one of the considerations in making this decision. Equally as important was the issue of part to part uniformity and the result of this of course is Revolver to Revolver consistancy. We found that revolvers that used MIM hammers and triggers required almost no fitter intervention in those areas during final assembly and final inspection and Trigger Pull Monitor rejection rates dropped markedly on finished guns. From an internal process point of view, it appeared a "Winner".

Lets shift gears for a moment and talk about the MIM process. It is unclear to me as to the reason for many of the negative feelings on the forum concerning MIM. Typically when people complain and aren't specific in the reason why, the problem is often created by a departure from the "Traditional". Perhaps that is indeed what is bothering some people when they view MIM.

The term MIM stands for Metal Injection Molding. It holds some similarities to Plastic Injection Molding and many differences as well. To start we would take a finally divided metal powder. This could be stainless or carbon steel. Today even Titanium is being used in some MIM fabrications. We would mix the metal powder and a thermoplastic binder (generally a Wax) forming a slurry of sorts when heated, inject this mix into a precision mold, and finally form what is known as a"Green Part". This part is roughly 30% larger than the finished part it will become at the end of the process. Interestingly enough, the Green Part at this stage can be snapped in two with simple finger pressure. The Green Parts are then placed in a Sintering furnace filled with dry Hydrogen gas and the temperature is brought almost to the melting point of the metal being used. Over time the "Wax" in the Green Part is evaporated, the metal fuses and the part shrinks 30% to it's final correct dimensions. At this stage of the process the MIM part has developed 98 to 99% of the density of the older wrought materials and a metalurgy that is almost identical. Dimensionally, it is finished and no machining is required. However the job is not yet done and the MIM parts are brought to our Heat Treat facility for hardening, and in the case of Hammers and Triggers, Case Hardening. Depending on the particular metal alloy that was used at the start of the process, we apply a heat treat process that is the same as would be used if the material was the older wrought style. Final hardness, case thickness and core hardness are for the most part identical to parts manufactured the older way.

Let's look for a moment at how we acheive dimensional precision when comparing these 2 processes. The old parts were each machined from either bar stock or a forging. Each cut and every resulting dimension was subject to machine variations, cutter wear, operator variations, etc. If every operation was done exactly right, each and every time and the cutter didn't let you down you would have produced a good part, but sometimes this didnt happen resulting in a rejected gun and rework, or in the worst case an unhappy customer. With MIM parts you must still machine to very high tolerances and your cutters have to be perfect and your machinist has to be highly qualified, but all of this only has to come together one time. That time is when the injection mold is made. Typically a mold for this process costs S&W between $30,000 and $50,000. Once it is perfect, every part it makes mirrors this perfection and you have in my view a wonderful manufacturing process.

Hopefully this description will help us all better understand the MIM process.
Please forgive the spelling errors and missplaced puncuation. I have no spell checker on this and the phone continues to ring!

Have a Great Weekend,
Herb

Additional Point:
Currently S&W is paying about $1.20/Lb for stainless steel barstock. Raw MIM stainless steel injectable material costs $10.00/Lb."


Here is a link to more info on MIM parts

MIM Technologies: MIM Process

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Lorin

Thank you for your post Skeezix. That clears some of it up.
I am still unsure about the following though:
When did they first start using them?
Are they only in revolvers, or in semi-autos too?
How can you tell if your gun has MIM parts in it?
What is the problem with them?

Thanks in advance to anybody who has the answers to these questions.

Stu

__________________
NRA Benefactor/Cert Instructor

My understanding is the the MIM parts were first implemented in the mid 90's. However, because S&W has a very long history to batch building guns using parts stored in inventory it took until about 2009 or 2010 for the forged internals to start disappearing from some models. At this point I believe that all new revolvers now feature MIM internals.

The simplest way to spot an MIM gun is to look at the rear surface of the trigger. If it's hollowed out, it's MIM. If it's solid, the gun will have forged internals. The MIM parts also come in two colors, "blued" which has a resemblance to color case hardened forged parts, and Stainless which looks like the older flash chromed parts used on some of the older stainless steel models.

I have a distinct preference for the MIM parts. As for why, it's because they are so consistent in size and profile that tuning the action is almost a matter of simply exchanging springs. Because of this it takes about 1/2 the time to tune the DA and SA trigger pulls to a specific weight versus a gun with forged internals. It's also far easier to get a really smooth stage free trigger pull in double action.

For example I picked up a new 625JM last Friday. Saturday morning I woke up at 6AM, had a couple granola bars for breakfast then started tuning the trigger on that new 625. My target weights for this gun are a DA pull of 8 lbs. and a SA pull of 3 lbs. Shortened the strain screw by 0.030 inch, smoothed the frame recess where the rebound slide runs with 1200 grit wet sandpaper, took a Dremel with a sanding wheel and took the "bite" out of the serrations on the trigger then polished it up, stoned the bearing surfaces on the rebound slide, and installed a 14 lbs. rebound spring and a C&S extended firing pin. At 7 AM I had the sideplate buttoned down and checked the trigger with my Lyman Digital trigger gage. The DA weight measured at 7 lbs. 14 ounces and the SA trigger pull was 3 lbs. 2 ounces. Close enough for me to call it good, I'm not going to worry about a measly 2 ounces. At 9 AM I left for a local range and tested it with 50 rounds of American Eagle FMJ in moon clps and 50 rounds of Speer Lawman TMJ without moon clips. Function in both double and single action was 100% with no hint of a light strike on any of the fired casings. As for the quality of the trigger pull, smooth as butter with no hint of staging anywhere in the trigger stroke. Now you know why I prefer the MIM internals, I didn't have to do any fitting at all. I also knew what needed to be done to hit my target for trigger weights because of past experience tuning the MIM lockwork.

As for the durability of the MIM parts, I've seen posts about failures on just 2 occasions since 2008. In one case the revolver was dropped on concrete and the hammer spur snapped off. In the second case the trigger spur snapped of on a 686 that the owner stated had been used intensively for dry firing without the use of snap caps. As for the wear resistance of the MIM parts, I use diamond laps to do any stoning because these parts are noticeably harder than the forged parts. While I can't prove it at this point IMO the MIM parts will last much longer in use, especially the critical SA sear surfaces.

Bottomline, I believe the MIM internals are good to go and nothing to be concerned over.

Scooter123. Thank you too for your information.

__________________
NRA Benefactor/Cert Instructor

Bottom line is that the tensile strength of a MIM trigger is greater than the tensile strength of your finger!

Thanks for all the info on MIM.

I'm not sure if I understand the "Additional Point" at the end of the explanation Lorin posted. Does that mean MIM parts are actually more expensive (material wise) for S&W to make than traditionally machined parts?

That would appear to be the case but the total cost is offset by savings in machining/finishing, along with a supposedly superior end product.

Terrible locks next.

1997 was the year most revolvers were switched over to MIM parts, although some were earlier and some later.
I don't know about Smith & Wesson semi-autos, but most semi-autos these days do use them. There are notable exceptions, for instance Dan Wesson or Ed Brown 1911s.
The Standard Catalogue of Smith and Wesson lists engineering changes for particular models of revolvers. For instance, the model 686-5 (introduced in 1997) begins the use of MIM parts for the 686 series. Do you have a particular gun you'd like information on?
In many people's minds they are associated with being more breakage prone and are not to be trusted. For other people, they represent cost cutting measures and step away from old-world craftsmanship. Some just think the MIM parts look hideous when compared to the old flash chromed stuff.

While there is probably way more truth in Mr. Belin's write up on MIM than the typical internet lore, we should also consider that the source was a S&W employee - not someone without a dog in the fight.

Would I be worried about buying a gun with quality MIM parts? Not at all - I own several and will buy more. Do I usually seek out S&Ws that are 15 or more years old and lock and MIM free? Yep. They just represent exactly what I think a good revolver "should" be.

Just to add a little more to what other posters have said:

I only have one S&W semi-auto -- a 1911 shipped in 2005. I think some of the small parts are MIM -- IIRC, the slide stop on mine is MIM.

The use of MIM parts in revolvers coincided (for the most part) with the switch to frame mounted firing pins and the elimination of the square-butt frame. Most models underwent these changes in the 1997-1998 time frame, but the use of MIM could have been a little earlier for some models. The "case coloring" on MIM parts is a dark bluish-grey without the rainbow coloring of the old case-hardened forged parts.

Here are a couple of pics showing how to tell MIM hammers and triggers.

You can see what I call a "mould injection point" on the side of this hammer (this hammer was polished).
mim hammer.jpg

The rear of the trigger is hollow. You can also see this "mould injection point" on the side of the trigger.
mim trigger.jpg

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Lorin

It amuses me that people who fret and worry over cast, plastic and MIM parts in a firearm will jump in an automobile constructed almost entirely of cast, plastic and MIM parts,
and go roaring down the highway at 70 mph without a care in the world.

I don't like or dislike MIM parts---they work and i know of no one who has had a problem.
Blessings

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TEXAS, by GOD

There was a big reluctance to accept investment cast parts at one time too. Anything with CAST in the name called to mind something brittle. Many top gunmakers used it , but chose to hide the fact. Ruger went the other way and proudly boasted of the benefits and strength of investment cast parts. And Rugers are well known for their strength and durbility. Early investment cast parts often took a plum tone after bluing. According to Bill Ruger , this was due to the high content of silicon in the steels used. It made the steel extremely tough , but also made it flow into the molds better. He notes in the book , Ruger and his guns , about seeing the highly touted German-made Weatherby Mark V action with the same plum color , and he knew that those actions , made by J.P.Sauer , were investment cast.
High silicon content steels like AISI S-5 and S-7 are some of the toughest steels out there. Used to make 'battering' tools like jack-hammer chisels. I made a scope base out of S-5 tool steel bar stock and tried to slow-rust blue it. Yep , it would only get a purplish-plum color.

Early Colt Mk.III guns like the Trooper and Lawman used sintered internal parts. Powdered alloy steel is put in a mold , and heated till the steel melts or fuses together. The parts didn't hold up and were replaced.

Word is that S&W now MIMs transmission gears for Harley Davidson motorcycles. They still forge their shifter forks.

What does the 'MIM' stand for?

Metal Injection Method

From the 2nd post: The term MIM stands for Metal Injection Molding.

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Lorin

On one of the MIM threads I've seen, one poster commented (in response to a "they aren't as tough" or "they won't last as long" comment) that most automobile engines today use MIM connecting rods, among many other MIM parts.

I know of automobile connecting rods that are made in a powdered metal (PM) process, which uses similar materials and processes as MIM with some differences.

Some differences between MIM and PM are:

1. In the molding, there is no wax in the conventional powder metal (PM) process. That means the metal powder is dropped into a die cavity, a top die comes down and compresses the powder into a "green" part - fragile just like MIM, but the powder is held in its shape by being mechanically compressed, not injected with a binder like wax, like MIM.

2. In the sintering, PM parts don't shrink 30%, again due to the lack of that binder in the MIM process. Depending upon the PM formulation, the parts either shrink a tiny bit, expand a tiny bit, or stay the same size in the sintering.

In the sintering, each part's tiny metal particles touch other tiny metal particles, and all these contact points 'weld' together, giving the MIM and PM parts their strength. Both could be compressed in a secondary operation called 'sizing' to further densify the parts, and / or close up some of the tolerances as needed. A heat treat process can be added to remove stresses. Polishing, plating, and machining can also be added.

The biggest complaint of MIM and PM is usually due to the porosity. Each part has millions of tiny particles that contact other particles in a weld, and those parts also have gaps between them also. Connect enough gaps and you get what looks like a crack, and can be a weak point.

Advances in the past couple of decades allow producers to make stronger parts - above the strength required for storm window cranks, for example - so that PM parts can replace drive gears in power tools and engine cam gears, for example, ABS rings on everything, and connecting rods. In the case of the connecting rods, the one PM company that I know makes them with a proprietary process that makes them strong enough to do the job.

I am familiar with the MIM process but not the extent of the parts' usages - except of course gun parts. I do know the PM process and where those types of parts are used. I think MIM triggers and hammers will do the job just like machined or stamped metal parts, under normal use conditions.


google "Powdered Metal parts" and "Metal Injection Molding" and you will see videos and get more info than you will ever need.

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Marty 4513TSW 13-1 642 60-10

MIM = Made In Mexico - Really, that's what I thought it stood for, fender guitar acronym.

I like my connecting rods forged and shot-peened.