Anyone tried MIM Metal Injection moulding or sintering?

[bry1975]

Any of you chaps,


Had the pleasure of trying MIM  Metal Injection Moulding process?


Bry

[Lew_Merrick_PE]

Bry,  I have designed and made (at other company's sites) both MIM and PM parts.  Most recently (which is more than a decade back), the locking bolt for the US Navy SEAL folding knife (designed by me and produced by McCann Industries) was a MIM part.  We got a 42 ksi shear strength out of a 4130 MIM part which gave us a "lock failure" at 850 lb-ft of torque applied to the blade pivot.  (The civilian version can be seen at http://www.mccannindustries.com/foldair.htm -- though the website is wrong as to the material for the locking bolt.)

While working on a project for Microsoft (which never saw the public light of day) I designed and had made (in China -- according to MS's policies) several dozen MIM and PM parts.  From a personal viewpoint, I avoid such things for one-off or small prototype runs as I find it easier and less expensive to either cast or forge a part that might be a candidate for MIM or PM in a production version.  It is no small feat to qualify MIM or PM parts if there is any critical loading or cycling involved.

It depends on what you are doing and the requirements that need to be met.

[bry1975]

Thanks for the info Lew certainly sounds like an interesting project with the navy seal folding knife.
I take it 4130 means 4130 Chromoly Steel?

42,000psi for shear strength sounds quite impressive.


I mainly asked about the MIM process due to what I heard about Swatch watch case manufacturing it certainly sounds like a useful manufacturing process for certain applications.

[Lew_Merrick_PE]

Thanks for the info Lew certainly sounds like an interesting project with the navy seal folding knife.
I take it 4130 means 4130 Chromoly Steel?

42,000psi for shear strength sounds quite impressive.

Yes, chrome-moly steel (4130).  It took us four years from the first Request for Proposal (RFP) to an actual contract in hand.  The first MIM'ed bolts were running about 28 ksi shear strength.  One of my "magician caliber" heat treaters solved that problem for us.  Everything had to pass a 5000 hour salt-fog corrosion test.  Yeah, it was "fun"...

I mainly asked about the MIM process due to what I heard about Swatch watch case manufacturing it certainly sounds like a useful manufacturing process for certain applications.

Quantity would justify it.  It would depend on your numbers.  Remember that your dieset has to withstand molten metal.

[bry1975]

That explains why they were probably aluminium swatch cases a nice low melting point.

So did you have to use a Tungsten die set?

[bambuko]

... Remember that your dieset has to withstand molten metal...

My experience of MIM/sintering involved injection moulding of metal powder mixed with plastic binders, using bog standard injection moulding machine and die.
This moulded part (called at this stage "green part") would be cured, binders would be removed and eventually sintered, resulting in finished part without the need for secondary processes.
No molten metal involved...? ?
Typical example of application of MIM would be making parts in tungsten - because of the extremely high melting point of tungsten it is not possible to produce finished parts by the usual methods (casting, extrusion, stamping etc.).
The project (as is often the case...) came to nothing, but experience was most interesting.
Most difficult thing, was getting shrinkages right.
See also:
http://www.epma.com/New_non_members/MIM.htm

[bry1975]

Sounds very interesting Bam I can imagine shrinkage being a right problem!

Bry

[Lew_Merrick_PE]

... Remember that your dieset has to withstand molten metal...

My experience of MIM/sintering involved injection moulding of metal powder mixed with plastic binders, using bog standard injection moulding machine and die.
This moulded part (called at this stage "green part") would be cured, binders would be removed and eventually sintered, resulting in finished part without the need for secondary processes.

MIM (Metal Injection Molding) is to PM/sinter (Powder Metallurgy) as HIP (Hot Isostatic Processing) is to casting.  That's the explanation I was given back in the 1970's.  A better analogy might be to state that PM is like unto soldering whereas MIM is like unto welding.  Both use metal powder material at the base of the operation.  A PM-part is held together by the binders (solder analogy) whereas MIM is metallurgically bonded to itself (welding analogy).  That is the difference within the (American) military standard and aerospace process definitions.

I can tell you that the little "blade bolt" for the knife was 4130 powder injected into a die that was then heated into the 2200°F range, held for a few minutes at that temperature, and then compressed to 55 ksi before it was cooled.  The resulting part is virtually indistinguishable (metallurgically speaking) from a part machined from solid bar.  Our supplier spent no small amount of time qualifying the process.

Back in the late-1980's and early 1990's I was the "chief mechanical engineer" on the program that developed the modern airbag restraint systems for automobiles.  (This sounds impressive until you realize that (A) airbags are chemical, not mechanical; and (B) I spent nearly three years developing a product that was designed to blow up in my face.)  The "problem" was (is) that the propellants are highly sensitive to changes in humidity and must be hermetically sealed in their containers.  PM parts do not qualify for hermetic seals, MIM parts do qualify.  Helium gas (the test of hermeticity) will pass right through a PM part, but not through a MIM part.

People are using these terms as if they are interchangeable today.  That may be fairly common, but it is incorrect from a technical standpoint.

[bry1975]

Thanks Lew,

Good to know what the differences are.

Certainly sounds a great responsibility developing parts for life savers/air bags!

Bry