Well, I'm no "internet expert" but I do have several years of experience in the metals manufacturing field as everything from press operator (cold isostatic press and hydraulic), cnc and conventional lathe programmer/operator, INJECTION MOULDING OPERATOR, and even head of the Engineering Dept. - so I guess you could say I know a fair bit about the topic at hand. This is no armchair quarterbacking, no Chicken Little "The Sky Is Falling" hysteria, just the facts as I've experienced them:
The metal injection moulding process is
inherently flawed by its very nature and I'll explain why (I've done this once before on here):
Freshly manufactured powdered metal comes out of the "powder room" and is packed and pressed via cold isostatic press into a small sample ingot. This sample is turned on a lathe to a uniform diameter (we used 1.25"), cut to a predetermined length (we used 1") and the sizes logged. Then it's off to the furnace for sintering.
After sintering, the sample is inspected for size to determine length and diameter shrinkage factors, diameter factor is generally lower as "tray drag" has it's effect but we generally noted shrinkages in the 1.30 to 1.45 percentage range on most samples. Whether they were nickel or cobalt based binders or non magnetic samples was immaterial to shrinkage rates.
From this stage, the sample is polished and sent to the metallurgist for various testing. He (or she) will test for material particle density, Rockwell "C" hardness, porosity, lamination and a host of other "contamination based" material flaws.
Provided everything tests out satisfactorily, the material is released for production use, the shrinkage factors are applied to the production print and the resized print goes to the shop floor for production. At this point everything's fine as the majority of machined parts undergo the same cold isostatic press (C.I.P.) process, at the same pressure, for the same dwell time and will be sintered in the same manner as the original sample piece.
Now here's where injection moulding starts to go awry:
That same metal powder that was tested via C.I.P. is now used raw in something akin to a large kitchen mixer and blended with various waxes to attain a semi-liquid, flowing mixture that can be shot under pressure into a water heated mould. A small sample of this material is taken back to the metallurgist for density testing and adjustments to the wax percentages are made. This sample is tested unsintered, or "green." Since the mould voids are fixed sizes, shrinkage of the final part is adjusted by means of pressure and material density modification rather than machining at a larger size based on known sintered effects. It's more like an "educated guess" than scientific method.
Now consider that, on a microscopic level, metal particles are generally flat, jagged edged "plates" (for lack of a better word). In a high pressure C.I.P. or hydraulic press process these particles are more or less forced into uniform, interlocking alignment that gives the final product strength and rigidity. In a free flowing wax matrix, these particles are a jumbled haystack with far less interlocking structural integrity. See a problem yet?
So now the mixture is transfered into a small, hot water jacket heated, vertical mixer that resembles a concrete truck mixer. At the base of the mixer is a large nozzle that aligns with the opening in the part mould - the mould is also heated by means of a hot water jacket to keep the mixture from cooling and reverting to a solid mass. Now that mixture is shot into the preheated mould (that's sprayed with a silicon based release agent)
UNDER AIR PRESSURE. The duration of the "shot", mixer air pressure, temperature of the mixer and mould are all operator controllable variables.
The parts are removed from the mould, run through the C.I.P. to check for air voids (bubbles), cooled, rough cut to a percentage oversize and taken to "Dewax."
The Dewax process is designed to remove excess wax media now that the parts are in solid form. Here the parts are embedded in Koalin clay and run through a three day cycle is what is, essentially, a large industrial bread proofer/oven. When they're removed, they're warped, brittle and contaminated with multiple waxes and clay media. Remember also that the lost wax is what formerly filled the spaces between the microscopic metal particles - you've just lost structural integrity.
After cleaning off the clay (with an air compressor hose), the parts are deburred to have the cut-off flashings removed by hand, with steel wool. There goes your generally accepted .005 +/- size tolerance. Then it's off to the furnace department for sintering. This is usually a 12 to 15 hour cycle regardless of whether the part was injection moulding or manufactured by traditional machining.
After sintering the parts are cooled, removed from the trays and returned to the Injection guys. At this stage, the warpage of the parts is something you'd have to see to believe: Between the wax burn out, the residual clay media (which is also what knifemakers use to produce differential heat treating or "hammond lines"), tray drag (shrinkage deformation caused by the part's weight on the tray slowing its contraction), and particle alignment issues, I've seen three inch parts that look like bananas.
Now warpage is checked
! The majority of parts are returned to the furnace department for a second (and sometimes third) sintering run. This is done under weighted graphite plates or in graphite forms in an effort to reverse parts warpage into something resembling a usable part. From here the parts, when applicable, undergo surface grinding to achieve the final length, O.D. grinding to correct oversized outer diameters, or centerless grinding to correct for warped cylindrical parts of smaller sizes (imagine a warped Tootsie Roll being ground into a straight form).
From here the parts go to the Q.C. department - again,
! If you've got a 10,000 piece order (very common), standard practice is to inspect 10% of the order. If that 10% passes by an "acceptable margin" then the entire order will shipped to the customer. Any returns are remanufactured. It's only if that 10% fails inspection that the entire order will be inspected.
If the guys in inspection decide sizes are good but voids are evident, the parts are sent to the Hot Isostatic Press (H.I.P) - high pressure under high heat in an inert gas environment, such as Argon vs. C.I.P. which is pressurized under water. The purpose of the H.I.P. run is that it will "sometimes" close up smaller, internal voids (air bubbles) and larger voids can collapse under pressure where Q.C. can verify them.
At this point the parts are either sand blasted, packaged and shipped to the customer or the order is remade and the whole process is repeated.
Now in no particular order, the potential problems are:
- Improper wax to metal ratio leading to lamination (layers of metal prone to separation under use).
- Low structural integrity due to the process itself.
- Improper air pressure or low material level in the injection mixer leading to internal air voids.
- Out of spec parts due to human error at any stage during the process.
- Foreign contaminants causing structural weaknesses under use.
- A host of other variables I'm probably forgetting here.
Hopefully this will give everyone an idea of where the weaknesses of MIM parts are and then you can decide for yourself if it's worth buying the product or replacing those parts with forged steel when available. IMNSHO, I'll avoid them.
Jack