Metal Injection Molding � A Primer

By David Tong

I do not claim to be a metallurgist. The purpose of this article is to discuss the basics of this process, which is now prevalent in the firearms industry, to produce geometrically complex, multi-faceted shapes with a maximum of automation and a minimum of materials wastage.

Metal injection molding is about a 25-year-old process. Essentially, what it does is combine the technology involved in the plastic injection molding process with a metal powder component at the micron level. Please note that this is also sometimes called �sintered metal,� which also promises a high level of dimensional accuracy, part density, freedom from air pockets and requires little or no additional processing, such as finish machining.

The process itself involves metal powders, which can be carbonized iron or inert gas atomized powder mixed with thermoplastic binders, creating a mixture that is approximately 60% of the total mixture volume. The parts are part of an expensive mold with �spruing,� which would be familiar to anyone who has ever built a plastic model kit, and which permits the flow of material into the mold, filling the voids and crevices of the mold with extreme accuracy.

This mixture is then fed into automated molding furnace machines at between 100 to 250 deg. C. Typically, there can be a shrinkage rate of 20-30 percent on all dimensions of the part, which strikes the layperson as excessive, yet the finished articles are quite dimensionally consistent as they are removed robotically from the cooled mold. The plasticizer used to fill the mold evenly is removed by high temperatures or chemically. After this, the parts are hardened and finished for the application.

The reason for the use of the process, also used in automotive, dental and other small part production, is simple: the economies of scale possible by removing labor and machine time in large batch parts production. In addition, the parts have a uniform density, more difficult to obtain with older processes such as sand or investment casting, and far less expensive than machining from bar stock, from a machine time, tool replacement and materials waste perspective. The big minus of the process in terms of firearms parts production is controlling the carbon content of the steel, especially in the 4000 series steels commonly found in these parts that may be structural, or load bearing, in nature.

Common parts in firearms made with MIM include rifle trigger guards, floorplates, bolt sleeves, shell lifters and telescopic sight bases. On handguns, you will find MIM hammers, sears, slide stops, magazine catches, sights and safety levers. Nearly all modern firearms manufacturers use MIM in their completed firearms.

Typically the parts produced with MIM are small, usually less than 1.5� in length and less than 0.5� in thickness, and the reason is that these formerly machined from stock parts can be highly automated and saving labor and machine time. While the process has its adherents, others disagree about its use, claiming that the process is simply a cost cutting measure and can, more importantly, become a quality control issue.

There have been a number of anecdotal reports of parts breakages on several popular brands of semi-automatic handguns, of manual safety levers, sears, hammers and firing pin blocks fracturing under recoil or pressure. The issue is that while MIM does allow for the economic production of geometrically complex components, it is the QC protocol of the particular arms manufacturer, the size or quantity of the random sampling, that causes heartburn, as no manufacturer I know of can possibly test every single part for density, freedom from air bubbles or voids, or even heat treatment. The problem doesn�t appear to be any great respecter of the retail cost of the arm involved, as even the high end products of current manufacturers have had issues with parts failure.

My own observation about this is that Ruger�s use of the precision investment casting technique might provide a higher level of reliability, although most of their arms seem �overbuilt,� in that the parts produced through investment casting typically have larger dimensions and thicknesses, creating bulkier feeling guns than would be the case with MIM or parts machined from stock. Note that Ruger also uses some MIM parts in their firearms, perhaps for this reason.

On my own Kimber .45 ACP pistol, MIM parts include the thumb and grip safeties, magazine catch, firing pin stop, sear, hammer, disconnector, firing pin block, slide stop and the sights. Again anecdotally, I have heard of the fire control parts failing and preventing intentional discharge of the weapon, which in a sticky situation would be troubling. I may budget in the future purchase of machined from stock critical components to allay my perhaps unfounded fears.

While we would all love to see lovingly crafted, machined from pre-heat-treated bar stock parts in our arms, the need to produce these parts at prices that people can reasonably afford means that it is likely that the adoption of the MIM process by the firearms industry will continue to grow in coming years; some authorities estimate this at 20-30% over the next five years. Cost containment in a highly regulated industry, which is also constantly suffering from both legal and legislative onslaughts, is sadly here to stay. This is why most modern production arms makers have adopted the MIM process.

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Copyright 2009 by David Tong and/or All rights reserved.