MIM - what does it mean?

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UncleWalther

UncleWalther

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I've been doing lots of research on 1911 pistols, and keep coming across the term MIM to describe the ####ty ones. What is MIM anyway? Which manufacturers DON'T do it?
 
Yep, Metal Injection Molding.

Essentially, they mix metal dust with a polymer bonding agent, and inject it in to a mold that is slightly oversized. Once out of the mold, it is run through an oven, sintering the metal together and driving off the binding agent.

It is a very good method of making parts that have compound angles, curves, etc. The problem is the part can not handle the stress loads machined or even some cast parts can. That is fine if you are just using MIM to make sights, triggers, safetys, etc.
 
I'd say that depends on the type of metal used.

"Pot metal" is cheap metal that tends to be brittle. MIM doesn't necessarily mean cheap metal, it refers to the process used to form it. A part is formed using MIM instead of being machined, cast, or forged.
 
Not being an engineer, the 'street cred' goes to non MIM parts.
I can't comment on how durable they are, compared to forged or cast parts that are then machined, but there seems to be a prejudice against MIM being used in devices that are meant to be 'stone axe' reliable...
Any of the engineers/materials experts care to pipe in......?
 
MIM is a process whereby a metal powder is mixed with a thermoplastic... a plastic which softens before it burns. This mix is formed into pellets, and these pellets are used to injection mould in the usual way. The resulting parts are a mixture of metal powder and the thermoplastic.

The thermoplastic was chosen for a certain property, though: It sublimates almost completely when ehated above a certain point.

Thus, the moulded parts are sintered (stuck in a really hot oven) which causes two things: The metal particle boundaries form bonds due to a weak charge attraction as the grains deform very slightly and re-crystallize somewhat in the heat. So the metal parts become a porous solid, and the plastic goes away.

They are more brittle than non-MIM parts, in general, but, when used correctly, they will be fine. How strong is strong enough, sort of thing. For a thumb safety on a 1911? Game on! Walther uses an MIM *locking block* for pete's sake!
 
Pot metal is an impure alloy of magnesium. Melts at low temperature, great flow characteristics when heated, and very lightweight. Impurities have a huge impact on magnesium alloys, it is important to note.

Burns cool, too.
 
foxbat said:
Not being an engineer, the 'street cred' goes to non MIM parts.
I can't comment on how durable they are, compared to forged or cast parts that are then machined, but there seems to be a prejudice against MIM being used in devices that are meant to be 'stone axe' reliable...
Any of the engineers/materials experts care to pipe in......?
Click to expand...

There are more MIM parts in guns than you realise.

My S&W 625 has a MIM trigger, hammer and rebound slide and I'm very confident with them.

1911 safety? Time for a change because they do, not will, break.

1911 hammers and sears? It's up to you, but if you need to stone the parts, you have to be very careful that you don't break through the hardened surface which is very small. If you do, the it's new hammer and sear time.
 
Bartledan said:
Pot metal is an impure alloy of magnesium. Melts at low temperature, great flow characteristics when heated, and very lightweight. Impurities have a huge impact on magnesium alloys, it is important to note.

Burns cool, too.
Click to expand...

Magnesium alloys are generally not considered pot metal.

Pot metal is a negative generic term for low melting point metals. Typically zinc based alloys but some also consider aluminum alloys to be pot metal. While magnesium is often a component of many alloys (aluminum or zinc based) the percentage is generally very low.

Zinc alloys are most often associated with the term pot metal. The advantage of zinc alloys is the lower melting temperature. This allows it to be used in injection metal molds. Your trigger locks are most likely zinc (other than the cheapy plastic ones). Zinc alloys are a little lighter than steel. Aluminum is much lighter and magnesium a little lighter yet.

Injection castings of zinc alloys can be made quite thin. This made the parts cheaper to produce. Unfortunately many products that were made this way were also easy to break. This created the negative coronation associated with "white metals" and the term pot metal.

Magnesium melts at about the same temperatures as aluminum but there are serious issues with handling it. It was not typically used in low cost items as zinc based alloys are.

Many items that were made of pot metal are now made of plastics.
 
As per Robert K Campbell Gun Digests Shooters Guide To The 1911 on MIM parts Quote-"When properly executed the parts are nearly as strong as forged parts." Further "MIM is here to stay, and seems suitable for peripheral parts with low stress."
 
Many of your car engine parts are MIM, including connecting rods, cams and cranks, So don't count out the technology as being inferior.

I'd say a connecting rod travelling at 2000 - 3500 rpm for 250K miles subject to all that heat and impact may be more abusive.
 
I have guns with MIM parts, never had a problem with them. I'm no expert but from what I understand, if the MIM process is done correctly the parts are extremely close to forged as far as strength and reliability. Also they can me made to much tighter tolerances than forged so that saves manufacturers time and money by not having to spend as much time fitting them.

It must be a reliable parts making process because many parts in modern jet engines are manufactured with the MIM process.
 
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