For those who have persevered this long, let's finally get to discussing MIM parts. Metal Injection Molding (MIM) is a relatively new process for the fabrication of metal parts. First developed in the 1970s, MIM was patented by Dr. Raymond E. Wiech Jr. It came into widespread industrial use in the 1980s.
In some ways, MIM is not a whole lot different than the standard plastic injection molding process, where heated plastic is forced under pressure into a mold, solidifying as the plastic cools. As a matter of fact, modified plastic injection molding equipment is usually used to produce MIM parts. The difference is how the MIM material is created, and how the parts are processed once they come out of the mold.
The MIM process starts by mixing finely powdered metal with a binder material, usually some sort of resin. The ratio is usually about 80% powdered metal to 20% binder. The resulting material is then heated so it will flow and injected under pressure into a mold inside the injection molding equipment. When the part cools it is ejected from the mold. At this stage the part is called 'green' meaning it has just popped out of the mold, and still consists of a matrix of metal powder and binder. The next step in the process is to drive out most of the binder material through a combination of solvents, heat, and/or catalytic processes. The result is called a 'brown' part. At this stage the part is porous because most of the binder was driven out, and it is very fragile. Finally, the brown parts are sintered, meaning they are placed in a furnace and heated to just below the melting temperature of the specific metal in the mix. This causes the surfaces of the individual particles of powdered metal to bind together. The part does not melt as one might picture happening in a die casting process, instead the individual particles of powdered metal bind to each other. When the part is sintered, it shrinks about 14% - 18% as the metal particles close up the gaps between them left behind when the binder was driven out. At this point, the finished part is 96% - 99% solid.
It sounds like a very complicated process, but MIM vendors have gotten it down to a science and produce huge quantities of MIM parts. The trick is they can produce small, detailed parts very quickly, and very inexpensively. Much cheaper than more traditional processes. MIM parts can then be plated, passivated, annealed, or carburized (case hardened). MIM parts can also be machined further if needed.
MIM is best suited to relatively small parts, because of current limitations in the size of molds in injection molding equipment, and because of the flow characteristics of the material in the molding process. But many parts can be molded in one 'shot'.
As for strength, it is claimed that MIM parts can be just as strong as parts made of similar materials by traditional methods. I do not know about that, but I had a conversation a few years ago with an engineer at S&W who was instrumental in introducing MIM parts. He said that their tests had proven that the MIM parts they were using were plenty strong enough for their specific applications.
MIM parts are usually designed on 3D CAD (Computer Aided Design). The part can have as much detail as any other molded part that will still pop out of a mold. Because the parts are designed on CAD, the shrinkage factor can be factored in so the finished part will meet dimensional specifications.
I hope I don't sound like too much of a cheerleader for MIM parts, because nobody admires a beautifully machined part more than I. But the cost savings of using MIM parts is what has driven S&W to using them.
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In the next photo, the traditional hammer assembly from the Model 17 is on the left, its MIM counterpart from the Model 617 is on the right. In accordance with my normal procedure, I have not further disassembled the hammer assembly on the Model 17. The Model 17 Hammer assembly consists of three parts; the hammer, the double action sear (the hinged piece protruding from the front of the hammer), and the stirrup which engages the main spring. The stirrup in turn consists of two parts, the flat, oval plate and a pin through it that engages the hooked end of the mainspring. The double action sear and the stirrup are pinned to the hammer, so that they are free to rotate. There is also a spring hidden inside the hammer which pushes the double action sear to the forward position as it is seen here. Obviously, from an assembly standpoint the hammer assembly requires some skill to assemble, and the time to drive in the two pins.
The MIM hammer assembly is also made of three parts. The stirrup is a single piece. It slips into the claw shaped receptacle at the rear of the hammer. It only fits in one way, it took me a little while to figure that out, but an experienced assembler could probably pop it into place in seconds. The double action sear is also a separate piece, but it is more easily seen from the other side. However in this view we can see the spring for the double action sear. While we are still looking at this side, notice the two odd shaped recesses coming off the top of the curved slot. Those are the recesses that capture the stud on the lock flag to lock up the gun.
On the other side of the assembly we see how the double action sear nestles in a pocket in the hammer. There is no pin it rotates around, the geometry is very cleverly designed, the double action sear simply rocks back in its pocket when the trigger is released. The 'open pocket' allows the assembler to compress the spring and simply drop the double action sear into its position, no pinning is necessary.
Regarding the hollows present on both sides of the hammer, the best I can figure is material was 'removed' where it is not necessary without affecting the strength of the part. Kind of like a girder with a thick section at the top and bottom and a thin web in between. Why make the hammer this way? Because less metal is required, driving down cost slightly.
From this angle we can see the parting line (the line indicating where the two halves of the mold separated) on the back side of the hammer.
There is one more detail visible in this view. There is a slight countersink around the hammer pivot hole. Remember when I mentioned the studs in the frame of the 617 are flat topped, rather than rounded? The rounded ends of the older studs makes it simpler to guide the hammer onto its stud. Assembling the hammer one is pulling the trigger back with one hand, holding the thumbpiece back with the other hand, and attempting to position the hammer onto its stud with still another hand. The rounded studs make this a bit easier. Here, the opposite strategy has been employed. The countersink at the pivot hole helps line up the hammer on the flat topped stud and guide it into place. It is still just as fussy to mount the MIM hammer as it was to mount the older one, but both parts have a different solution to help guide it on.
I really have no idea of the comparative costs to make up the two different hammer assemblies, other than to know the MIM parts are less expensive. But it is also interesting to see that the MIM part is simpler (cheaper) to assemble.
