Posted: 1/2/2010 10:17:05 PM EDT
| What gives away a component as being MIM material, vs. tool steel? Curious because I just bought an SA GI model, have heard these have a MIM slide stop, but don't see any visible difference between it, and my Sig GSR or Remington Rand slide stops. What gives away the MIM construction? |
| Usually, there will be a square, circular, or rectangular marking where a rod pushes the part out of the mold. The seam, if there is one, will usually be alot cleaner than a cast part, too. I have two slide stops for SA Mil-specs. One is MIM and the other is cast. |
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Usually, there will be a square, circular, or rectangular marking where a rod pushes the part out of the mold. The seam, if there is one, will usually be alot cleaner than a cast part, too. I have two slide stops for SA Mil-specs. One is MIM and the other is cast. MIM uses the same tooling as plastic injection molding. Ejector pin marks, gates and parting lines will be present. |
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The ejection marks are fairly obvious. Cast parts are worse about showing a seam. One sure fire give-away is the presence of letters or numbers raised from the surface. This doesn't happen on 1911 parts, but the US made triggers for AKs have Made in USA markings molded on them, to properly identify them for 922r requirements. Think how expensive it would be to do that without MIM!
Just looking at a new S&W revolver it is easy to tell that the hammer and trigger are MIM. The trigger has a hollow on the back side, and pulling the hammer back on my 625 reveals a pin mark on the right hand side. I suppose one could also go to the trouble of weighing an individual part, and comparing it to a known machined part. MIM is less dense. Sounds like a lot of trouble, though. I usually change the firiing pin stop for an EGW, because I prefer a small radius fps. After that, as long as the extractor is not MIM, I just shoot it instead of worrying about it. No breakages yet. Eventually, anything can break. |
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Reading up on it it seems MIM parts are 96% metal and 6% bonding agent. When heated the bonding agent melts away leaving a metal piece that is 96% metal.
Someone a lot smarter than me will be along shortly to correct spelling, definition, and sentence structure. Probably post in Latin or something... Thanks for the picture - that helped a lot. |
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The bonding agent burns away, leaving a more porous structure. In load bearing or high wear parts, this translates as more areas where stress risers can form, leading to shear mode failures (in extractors especially) and crack propagation b/c of the voids. Generally, the fabrication of barstock from the mill tends to align the grain of the steel, and proper heat treat/temper cycles will provide the requisite hardness/flexibility for the part to withstand many thousands of rounds. There is a reason the original specs called for specific alloys of medium and high carbon steel (1050 and 1075 IIRC). With MIM, it seems the rallying cry is: "If it ain't broke, fix it till it is."
D-R |
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The bonding agent burns away, leaving a more porous structure. In load bearing or high wear parts, this translates as more areas where stress risers can form, leading to shear mode failures (in extractors especially) and crack propagation b/c of the voids. Generally, the fabrication of barstock from the mill tends to align the grain of the steel, and proper heat treat/temper cycles will provide the requisite hardness/flexibility for the part to withstand many thousands of rounds. There is a reason the original specs called for specific alloys of medium and high carbon steel (1050 and 1075 IIRC). With MIM, it seems the rallying cry is: "If it ain't broke, fix it till it is." D-R <Rimshot> |
Ed Brown 'Hardcore' slidestops. Partially machined investment castings. My choice for my full-house builds. Excellent quality.
Colt. Machined from a forging. Dimensionally excellent. I have seen more than a few of these break off the internal lobe. I do not use these on serious builds. 
EGW bar-stock. Dimensionally excellent. As of yet, my experience with these is minimal. 
Major manufacturer MIM. These two were from consecutive serial numbered pistols. I am sure the manufacturer would have gladly replaced them. Note the identical voids on the shafts and the MIM ejection pin marks. 
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Reading up on it it seems MIM parts are 96% metal and 6% bonding agent. When heated the bonding agent melts away leaving a metal piece that is 96% metal. Someone a lot smarter than me will be along shortly to correct spelling, definition, and sentence structure. Probably post in Latin or something... Thanks for the picture - that helped a lot. <rimshot> DOH! 
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Ok, why is MIM so bad? Because the technology is newer than 1911. It is therefore bad and evil. Seriously, only 1911 guys worry about MIM parts. The Glock guys don't worry about MIM parts in their guns, the Remington guys don't worry about MIM parts in the 870. The Ford guys don't worry about MIM crankshafts and other engine parts... The only issue with MIM is when it is used for parts that should have differential hardness and need to flex, like extractors, or take an impact like ejectors. |
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Quoted:
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Usually, there will be a square, circular, or rectangular marking where a rod pushes the part out of the mold. The seam, if there is one, will usually be alot cleaner than a cast part, too. I have two slide stops for SA Mil-specs. One is MIM and the other is cast. MIM uses the same tooling as plastic injection molding. Ejector pin marks, gates and parting lines will be present. Yep, I have a Auto Ordnance GI style (MIM) thumb safety and a genuine USGI thumb safety on my bench. If you look carefully, you can see parting lines on the Auto Ordnance piece. You can see machine marks on the USGI piece. |