I have read with much interest the many comments in this [Smith and Wesson] 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 successfully this would truly be an "Acid Test". There is nothing more important to a revolvers feel than the all-important Single Action that is established between the hammer and the trigger. Mechanically 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 lose 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 customer’s 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 consistency. 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".
Let's 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 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 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 metallurgy 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 were 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 achieve 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 didn’t 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 $30,000.00 and $50,000.00; 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 misplaced punctuation. I have no spell checker on this and the phone continues to ring!
Have a Great Weekend,
Herb [Belin,
Project Manager, Smith & Wesson]
Originally Posted by Wildcat
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