[WebHobbit]

So what does "MIM" mean in relation to S&W revolvers?

I just know it has to be something simple I'm missing!

:scrutiny: :o

[Coronach]

IIRC, it means "metal injection molding." This is a cost-saving technique that, supposedly, creates a product of similar durability to forging or milling of parts. I'm no metallurgist, so I'll not go much further than saying "supposedly." With guns, its never quite clear what is a righteous objection to a new and stupid idea, and what is a stupid objection to an idea that is just plain new, absent an in-depth knowledge of the field in question. We as a group tend to be a wee bit reactionary. ;)

Mike

[3 gun]

As I understand it, it's a way to make smaller, non stressed parts by injecting powdered metal into a mold under high temps and pressure. Some would/do believe that it results in an inferior part. Truth is if property done the part is just as good as a machined one, at lower cost and needs no/little finishing machine work to be fitted in final assembly.

[VictorLouis]

flomet (http://www.flomet.com)

[Sisco]

From S&W, on MIM-parts...
Mr. Herb Belin of S&W posted this on another board
-----------------------------------------------------------------------------------

"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 there "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 fo Metal Injection Molding. It
holds some similaritys 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 and 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 snaped 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. Dimensionaly 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.

Lets 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 dollars. 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."

[WebHobbit]

Thanks for that!

Explains a lot. People didn't used to trust Ruger's castings either! They were dead wrong.

[bountyhunter]

Don't be fooled into thinking it's a simple answer. As far as I know, SW has not had any specific increase in failures due to the use of MIM for hammers, triggers, rebound slides, and firing pins.

In the case of 1911's, there have been documented cases of MIM parts performing poorly when used for slide stops and extractors. Colt went to MIM extractors for a period of time and changed back (because of failures).

I have installed two MIM extractors in 1911's and found that they do not hold tension when first set, and have to be re-set at least once. Over time, they also weaken and have to be reset. IMO, this is due to the fact that the MIM part can not be made comparable in "spring" characteristics to 4340 steel and suffers from the phenomenon we engineers call "strain hardening". This just mens you bend a piece of metal and it doesn't come all the way back to it's original position. In the case of extractors, that means you lose tension.

[MrAcheson]

One issue with this sort of thing is that as the parts being fabricated with MIM get smaller, the process becomes less defect tolerant. What would be a minor defect in a large part become a significant defect in smaller ones. So because of porosity MIM doesn't tend to do as well with small structural parts.

The other thing is that forging, for instance, both shapes and works and heat treats a part. MIM does not do all these things so potentially the part is weaker as well.

[VictorLouis]

One issue with this sort of thing is that as the parts being fabricated with MIM get smaller, the process becomes less defect tolerant. What would be a minor defect in a large part become a significant defect in smaller ones. This makes absolutely no sense at all. A defect is a defect is a defect. It will either cause a problem, or not. You are aware that the shrinkage is accounted for in the whole design process of the part?

So because of porosity MIM doesn't tend to do as well with small structural parts. Porosity has nothing to do with it. Small parts are where the process excels. Didn't you read the site for some backround? It's imcompatible with large parts, not small.

The other thing is that forging, for instance, both shapes and works and heat treats a part. The technique cannot begin to approach the shape of many small parts. That's where the CNC machine comes into play. Have you ever seen a raw forging, for instance, like that of a 1911 frame?:confused: If you mean 'work-hardening', yes, it does do that to an extent. Because of stresses involved, though, a dedicated heat-treatment is needed. The latter is completely separate from the former.

MIM does not do all these things so potentially the part is weaker as well. Neither does forging, as I explained. You know, this is the same sort of armchair metallurgical reasoning that was levied against Ruger way back in the day with his introduction of casting. So much drivel...:fire:

[QuarterBoreGunner]

We as a group tend to be a wee bit reactionary

'yer dern tootin'! Kids these days with their fancy lah de daa injection molded nonsense! We had machined parts and we liked 'em!'

/cranky old man


Actually, I dislike the MIM hammers and triggers on the newer Smiths because they're not color cased anymore. I always liked that little touch on the older revolvers.

And IIRC I once heard a gunsmith (who shall remain nameless) that said that getting a good trigger job out of the MIM parts was much more difficult; something about the surface treatment.

[bountyhunter]

Before you throw it all out, this humble engineer would say there is truth in the statement that MIM is not well suited for very small parts or thin parts (such as extractors or slide stops). The stats I saw said that MIM was something like 97% as dense as steel machined from forged stock, so basic tensile strength can be comparable for similar size and hardness. However, defects can be a problem. A good MIM part requires the grain size of the metal powder be very precisiely controlled or a defect will occur. on a long, thin piece like a slide stop or extractor, that would lead to a shear fracture. A thick piece like a hammer or trigger might survive it.

Purely from my own experience: I got a 1999 SW model 66 and the MIM pieces looked really good, with a surface like a "bead blast" finish and quite even. The newer ones look horrible and have lumps and bumps. A machinist buddy says that is what happens if the grain size is not constant. They look like crap, but so far, none have failed. As for 1911 extractors and slide stops: plenty have failed.