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Old 01-28-2013, 18:18   #1
flw
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MIM parts vs cast?

Its my turn (again) to be the dummy in the room.

What is the difference between MIM (Metal injection molding) and traditional cast metal parts ? Not in how its made terms but in performance terms like hardness, tensile strength etc...
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Old 01-28-2013, 19:03   #2
collim1
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PROPERLY made, properly being the key term, it won't make a bit of difference.
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Old 01-28-2013, 19:14   #3
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What he said.

Quote:
Originally Posted by collim1 View Post
PROPERLY made, properly being the key term, it won't make a bit of difference.
The ones who say different are usually the ones selling the replacement parts.

Last edited by Narkcop; 01-28-2013 at 19:15..
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Old 01-28-2013, 19:14   #4
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^^^This.^^^
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Old 01-28-2013, 19:54   #5
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If both done properly MIM will always be stronger than cast. Problem people have when comparing the two is some parts, do to their geometry, can't be made by MIM.
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Old 01-28-2013, 20:28   #6
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MIM is about making parts for less money not better parts.

Late 3rd generation & 4th generation Glocks had ejection issues (brass to face) about the same time as when the MIM extractors replaced cast.

Most people would much rather have paid an extra $15 to keep Glocks the way they were (perfection) prior to 2009 or whenever the ejection issues started.
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Old 01-28-2013, 22:09   #7
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Quote:
Originally Posted by flw View Post
Its my turn (again) to be the dummy in the room.

What is the difference between MIM (Metal injection molding) and traditional cast metal parts ? Not in how its made terms but in performance terms like hardness, tensile strength etc...
Fatigue strength, shear strength, torsional shear strength are more accurate measures of strength as concerned with firearms parts.

Hardness is independent of grain size, porosity, or any other physical attribute of the part. A MIM 4140 steel part will be the same hardness as a forged or cast 4140 steel part. Hardness is more of a factor of heat treatment.

If you have identical parts, MIM will be superior to casting. However, even the slightest changes in part geometry will require a more thorough examination to determine which is "stronger". For instance, a forged part that includes sharp 90 degree angles in its geometry will be weaker than cast or MIM parts that have more oblique radii.
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Old 01-29-2013, 00:17   #8
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Given only the choice between the two and nothing else, I'd take a good quality and properly made MIM part over a cast part any day.
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Old 01-29-2013, 00:48   #9
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A lot of generalization in the short replies in this thread, so I'll add some long-winded specifics...

First lets talk castings:

Castings are melted metal poured/injected into a mold and them allowed to solidify, which of course, everyone knows. Large differences in part quality are typical depending on the casting method - Sand casting, Die Casting, or investment (lost wax) casting, the last being used with super high quality metals because the ceramic molds are heated to above the melting point of the metal, so no cooling whatsoever happens while the metal is filling the mold. Only after the mold is filled does cooling start to happen, and this makes the best castings you can get with high quality steel. As cooling happens, the metal part and the ceramic mold contract together.

Die casting is a much faster method for high volume production, but the mold is made of steel and must be kept VERY cool compared to the temperature of the melted metal being injected into it. This affects the part quality because some of the melt starts to cool and solidify as soon as it touches the walls of the mold. You absolutely cannot get the same quality, but production speed happens at about warp 9 compared to investment casting. Die casting is done almost exactly like plastic injection, and the molding machine runs automatic, kicking out parts every 30 seconds or so. Very nice aluminum parts can be made this way for a much lower price than you could ever reach with the lost wax casting method in ceramic molds, but casting in hot ceramic is truly the king of part quality - just a lot more expensive to do. It gives you parts easily about 99 percent as good as if they were milled out of bar stock if you are using good metal for your melt.

Sand casting is how they make engine blocks, and like investment casting, has the advantage of making shapes that would be hard to do in a metal die-cast mold, since the sand & ceramic molds are destroyed in order to get the cast parts out. Sand castings (mold is made of sand glued together) are always inaccurate compared to the other methods, and will require precision machining to finish the parts.

Now MIM: more properly called Sintered Metal parts.

It's done in metal molds that are designed almost exactly like plastic injection & die cast molds are, but nothing very hot or melted is injected into them. What's injected into a MIM mold is finely ground up particles of solid metal in a powder-like form, mixed with various types of binders that could be anything from plastics to vegetable oil. When it's injected (and under MUCH higher pressure than even plastic - many thousands of PSI) it gets compacted so tightly that when the mold kicks the part out, the particles & binder stick together sort of like a cookie, or like the way drug companies make pills out of compressed powder. The parts are fragile and weak at this point, and you can usually break them like cookies. The parts are then usually put through a binder removing chemical process, depending on what binder was used. Next, comes the sintering process. The parts are more or less placed on a cookie sheet and baked in an over with the temperature very precisely controlled so that they are heated up just enough to make the metal particles melt to the point where they fuse together, but not heated enough to make the part melt down into a useless blob. Because microscopic voids are left in the metal from the previous removal of the binder, the parts always shrink during sintering, and they shrink a lot. 10 to 15 percent is typical, and this is a lot more than plastic shrinks when cooled. It's also a mile more shrinkage than happens when cast metal parts cool off. The voids in the MIM part get filled in because of the shrinkage during sintering, and you wind up with a part than can be almost as good as a part cast in hot ceramic. Accuracy of the MIM parts can be excellent and they will often require no additional machining, but that is not always the case. Depending on the part geometry and the thickness of the walls, the extremely large amount of shrinkage can distort the part shape a little in some cases during sintering, so certain surfaces might need machining to make them perfect. But the real idea of MIM is to try to make accurate parts that won't need this - or much of it.

This is one reason why lost wax casting in hot ceramic is truly the king of casting quality since the ceramic mold holds the part perfectly in shape while it cools off, so no heat distortion is possible.

But no matter what casting/molding method is used, it's the type & quality of the metal you are using that really sets the quality of the finished part, and it's not even slightly realistic to generalize about the part quality you can expect from each method when you are not even talking about what metal you parts are made of. Good powered metals are expensive, but cheaper stuff is also available, and who knows what a particular manufacturer used?

Bottom line is that MIM may or may not be a great part, same as it is with all other methods of casting. To say all MIM is junk or all MIM is great is as silly as judging people's honesty by their hair color. Even lost wax casting - the king of cast part accuracy & quality - can just as easily be done with crappy metals and you'd get crappy parts that break too easily. What the metal is, is usually a lot more important than how it was put into it's final shape.
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