MIM Metal Injection Molding

[Homer]

I'm trying to get a better handle on MIM gun parts. More of the newer guns are using metal parts from this process and I simply want to get it straight. I view the MIM process similar to forge welding, where separate parts of metal are heated almost to the point of melting, then hammered together to achieve a bond.

The MIM process of course uses a mold where a metal powder slurry (including a wax/flux bonding material) is injected into a mold, making a part (hammer for instance) which is then, and then heated (under pressure) to the an almost molten state at which time the bonding material (wax with flux) melts out of the part while the metal powder bond together, producing a single part that is then used in a rifle or handgun. The process is used because it can make a part repeatedly to exact dimensions, which maintains consistency in the production of the part and firearm.

Anyway the process reminds me of forge welding (which I used to do a long, long time ago). Do I have this correct?

[7.62 Precision]

There is nothing wrong with MIM parts. MIM parts are also junk.

It all depends on how the part is manufactured and how it is used (which part it is).

Some people are too willing to discount the quality of a firearm because it contains MIMed parts.

Some manufacturers are too willing to MIM parts that shouldn't be, or use poor quality MIMed parts to save money.

Metal injection molding—MIM—offers a manufacturing capability for producing complex shapes in large quantities. The process utilizes fine metal powders (typically less than 20 micrometers) which are custom formulated with a binder (various thermoplastics, waxes, and other materials) into a feedstock which is granulated and then fed into a cavity (or multiple cavities) of a conventional injection molding machine. After the “green” component is removed, most of the binder is extracted by thermal or solvent processing and the rest is removed as the component is sintered (solid-state diffused) in a controlled-atmosphere furnace. The MIM process is very similar to plastic injection molding and high-pressure die casting, and it can produce much the same shapes and configuration features. However, it is limited to relatively small, highly complex parts that otherwise would require extensive finish machining or assembly operations if made by any other metal-forming process.

The advantages of the metal injection molding process lie in its capability to produce mechanical properties nearly equivalent to wrought materials, while being a net-shape process technology with good dimensional tolerance control. Metal injection molded parts offer a nearly unlimited shape and geometric-feature capability, with high production rates possible through the use of multi-cavity tooling.

Remember that the Winchester 94 built in the '60s and '70s had a sintered metal receiver (made a bit differently than MIM, but similar in concept) and was considered by the guys at Winchester who tested them to be the strongest 94 receivers made.

[7.62 Precision]

Anyway the process reminds me of forge welding (which I used to do a long, long time ago).

There are people who still use axe heads that were made this way 150 years ago.

[Homer]

Please don't misunderstand my post - I am neither for or against the MIM process or the product it produces. I'm just trying to understand it by means of association to another process.

Neither am I against forge welding, in fact in some applications it can be used to support gas or other forms of welding resulting in a stronger bond. I made parts for old (really old) ranch equipment; parts that were discontinued decades before. Some of these parts, one in particular, was used in a high stress part of an old hay grinder; and after gas and forge welding, it held up quite well. So, again, I'm not being critical, just curious.

[Grizz]

It's how Ruger makes their guns... seems pretty good to me. They have elevated the metalurgy to high art. There are poor specimens as well.

I think almost all major gun parts are made from cast metals. The difference being that the metal molded part is cast to shape, as you noted, while the other parts are made by machining away the parts that don't look right from a cast steel billet. I mean, all steel starts out molten before we get it.

I believe that the impeller blades in the jet engines that transport us are made by lost wax casting, and they mostly hold up pretty well most of the time.

But no, it's not like forging in the sense you mean it, it's more like sand casting with a furnace rather than a forge. In forging you strive to prevent the metal from melting.

Grizz

[Homer]

Grizz, Ruger uses investment casting for their guns. The difference between it and MIM is that the steel is in a fully melted liquid state when injected into a mold. MIM, on the other hand, uses metal powder that never achieves the liquid state when injected into the mold, nor does has it reached a melted state when heated to bond the metal powder and remove the bonding agents.

And yes, I agree with you Ruger has perfected the invested casting process which has resulted in some very high quality firearms.

[Grizz]

Interesting. I missed that post-it note about the process. Read a little and it sounds like the sintering process gets pretty close to liquid state temperatures. Perhaps analogous to forge welding heat, which makes it more like forging after all. Maybe. The parts can be hardened or annealed, so they are similar to metal.

I suppose I will have to get some and file them and heat them and forge them and quench them to see how they behave.

[Lastmohecken]

The main thing I know about MIN parts is that to many of them get used in guns these days. Kimber is about the worst and best. They make a lot of fantastic 1911's but at the same time, I personally have seen several of Kimber's MIN parts break after moderate or sometimes even fairly light use, but still yet sometimes they hold up quite well.

Even Colt uses a few MIN parts in their 1911's.

They have the ability to make a very precise and accurate part, but I don't like them much myself.

Even Smith and Wesson has messed up there revolvers with MIN Parts and I have been told by gunsmiths that it's hard to do an action job on the newer Smiths. I know personally I do not like the way the triggers are made on the newer Smith revolver even though they probably still work pretty good out of the box normally.

As far as I am concerned MIM parts are just a way to keep prices affordable on the new guns and other items, but they are not better, and lots of them will break sooner then forged steel parts of yesterday if it is a part subject to certain stresses.

[DPris]

Ruger's been introducing MIM parts into their guns for somewhere around 14 years now, but the steel frames on revolvers & other parts like the rifle bolts are still cast.
Denis

[7.62 Precision]

Please don't misunderstand my post - I am neither for or against the MIM process or the product it produces. I'm just trying to understand it by means of association to another process. So, again, I'm not being critical, just curious.

I know that, I was just trying to provide some info. I am for and against it, depending whether it is used right.

What bugs me more than MIMed parts is the plastic mainspring housings everyone puts on 1911s these days. They may work just fine, but I just never get used to them, and sure enough, I occasionally dump one in my solvent tanks and dissolve it.

[Rusty]

I remember when S&W made the switch to the MIM parts. Prices on older guns seemed to go up because they DIDN'T have the parts. Some people were taking the newer guns and replacing the MIM parts with older parts to get the best of both worlds.