On MIM gun parts
Three little letters that often get the most derision from the gun community is MIM.
There are two types of mass production techniques that are similar and are often confused. Powder Metallurgy (PM) and Metal Injection Molding (MIM). Both methods use a metal powder mixed with a binder to fill a mold cavity with material. The powder metal binder part that comes out is called a “green.” The binder is removed from the green and the green is then sintered into a solid piece.
The differences in the process is what separates PM from MIM.
Powder size is one of the biggest differences. PM powders are in the 50-100 micron range (0.002 – 0.004 in), while MIM powders are in the 2- 15 micron range (0.00008 – 0.0007 in).
The compaction of MIM is much higher than PM, using 40-50% binder in the green molding process, the material flows into the mold using processes borrowed from the plastic injection molding industry.
The debinding process in MIM is more involved, due to the higher volume percentage of binder. It can be done by washing in water if is a water-soluble binder is used; solvent debinding using an organic solvent; or catalytic debinding, which uses an acidic vapor to dissolve the binding agent. This converts the green to a brown part, which then goes to sinter.
In both PM and MIM, sintering is done at a minimum 70% of the melting temperature. The higher starting porosity means that the MIM part will experience significantly more shrinkage during sinter.
PM parts will generally achieve a 92% max density. MIM parts can achieve 95-98% max density, with much finer final porosity and higher dimensional tolerances during shrinkage. MIM parts can then be subsequently heat treated in a process called hot isostatic pressing (HIP), in which the part is subject to extremely high temperature and pressure (30,000 psi) in an inert atmosphere furnace the further compacts the part to near 100% dense.
When complete, the general rule of thumb is that PM and MIM parts have the fractional percent of strength of full density parts, i.e., a steel MIM part this is 98% dense will have 98% the strength of the same part if it were machined from the same steel. For static loads, this is accurate. The microporosity in MIM does reduce the fracture toughness by about 20%.
All sorts of materials can be produced by MIM. Common alloys include 4140, 4340, S7 tool steel, 9310, 8620, 17-4 PH (precipitation hardenable stainless steel), as well as custom blends. It’s possible to alloy in situ using carbon, carbonyl iron, and other alloying elements to make modified alloys that respond well to MIM processing.
MIM parts then can be subsequently heat treated and finished using the same heat treats and finishes as machined parts, such as quench and temper, carbonizing, nitriding, ferritic nitrocarburizing (FNC), aging, etc., that apply to each alloy.
MIM has the advantage of being a relatively low-cost process to manufacture small, complex geometry parts, with high degrees of tolerancing.
This is perfect for the gun industry.
Decades ago, when MIM first started to be used in the firearms industry, it gained a poor reputation for quality. This is common when a new technology enters the market. However, in the last forty years, MIM technology and quality has increased substantially.
All of your favorite guns contain MIM parts, a lot of MIM parts, muzzle devices, sights, firing pins, disconnectors, extractors, ejectors, frames, and there are at least two pistols out there that have MIM barrels.
These are guns that have been tested by the design engineers for the companies that manufacture them to tens of thousands of rounds.
With a property designed and manufactured MIM part, made from the appropriate material and given the correct heat treatment, a gun owner or shooter will never have a part fail because it was MIM and not machined.
The reflexive hatred of MIM parts is a vestigial belief that is almost half a century out of date from some early bad parts that were not made with modern MIM materials. But that belief is used to get gun owners to shell out lots of money in machined aftermarket parts that they don’t need to buy.
Your factory MIM parts are fine.
