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Metal Injection Molding (MIM) VS. CNC Machining

Metal Injection Molding (MIM) VS. CNC Machining

Mar 17th 2023

MIM (metal injection molding) and CNC (computer numerical control) are two common manufacturing methods used to produce parts and components for various industries.

MIM is a process that combines the benefits of plastic injection molding and powdered metallurgy to produce complex, precision parts made of metal. Metal powders are mixed with a binder material to create a feedstock that can be injection molded into a desired shape. After molding, the part is heated to remove the binder and sintered to create a solid metal part with high density and strength. MIM is often used in the medical, aerospace, and automotive industries for producing small, complex parts with tight tolerances.

On the other hand, CNC is a manufacturing process that uses computer-controlled machines to produce parts and components from various materials, including metal, plastic, and wood. A computer program controls the movement of a cutting tool or other machining tool to shape and cut the material into the desired shape. CNC machining is commonly used to produce high-precision parts and components with complex shapes. It is ideal for applications where precision is critical, such as in the aerospace, automotive, and medical industries. CNC machines can produce parts quickly, efficiently, and with high accuracy and repeatability.

Now that we’ve outlined what both MIM and CNC manufacturing are, let’s dive into some additional differences between the two, starting with additive and subtractive manufacturing.

Differences between Metal Injection Molding (MIM) and CNC Machining

Additive Manufacturing VS. Subtractive Manufacturing

One of the first differences between the two is that MIM is additive manufacturing, which means that material is built/added until the desired product is reached. CNC is considered subtractive manufacturing due to the material being subtracted from a larger piece of stock until the desired product is achieved. MIM utilizes a powdered metal bonded together using heat and high pressure inside a mold.

Cost

The cost differences between MIM (metal injection molding) and CNC (computer numerical control) machining can vary depending on several factors, including the complexity of the part, the production volume, and the material used.

MIM can be more cost-effective than CNC machining for producing small, complex parts in high volumes. The initial tooling costs for MIM can be relatively high, but the cost per part decreases as production volume increases. MIM also allows for the production of parts with intricate geometries, which can be difficult or impossible to produce with CNC machining.

CNC machining, on the other hand, can be more cost-effective for producing larger parts or parts with simpler geometries in low volumes. CNC machines are highly versatile and can work with a wide range of materials, allowing for greater flexibility in production. However, the initial setup costs for CNC machining can be high, and the cost per part may not decrease significantly with increasing production volume.

Overall, the choice between MIM and CNC machining will depend on the specific requirements of the part or component, as well as the production volume and budget constraints. In general, MIM may be more cost-effective for producing small, complex parts in high volumes, while CNC machining may be more suitable for larger or simpler parts in low volumes.

Repeatability vs. Tolerancing

MIM tolerance is based on a percentage of the actual part, usually around +/- 2% of the total part. MIM has higher repeatability than CNC machining. However, CNC machining can hold much tighter tolerances +/- .0005, with some machines holding even tighter tolerances.

Design Flexibility and Modifications

MIM is not as flexible with design changes as CNC. The mold controls the part, and sometimes the supplier cannot make modifications, so a new mold must be created.

MIM is generally a less expensive option for extremely high production numbers that are not subject to revision changes.

MIM Vs CNC: 1911 Production

Beginning in the late 1990's, use of MIM parts in production 1911's became more common. MIM parts allowed manufacturers to build a 1911 that was still economically viable due to the lower per-part cost, especially for parts with complex geometries such as beavertail grip safeties and extended thumb safeties.

Regarding 1911's, MIM parts have a few obstacles and quirks that can affect their performance. Obviously, for traditionalists, MIM parts have a negative connotation. The 1911 was designed during a time when everything was forged, finish machined and then hand fit. Once that obstacle is overcome, there are still some potential issues with MIM for 1911 parts. Those issues include:

  1. Hardness: MIM parts will typically go through a case hardening process once sintered to increase the surface hardness of the finished part. Sometimes, the part finishes up too soft for the intended application and longevity suffers. Other times, it ends up being too hard and makes it difficult to properly fit and tune the part. For some parts, they need to be through treated (the steel is hardened all the way through the part rather than just the surface), which cannot be accomplished with case hardening.
  2. Density: MIM parts start out as a powder. If the powder size is not correct for the application or if the sintering process isn't performed correctly, the part in question isn't dense enough and longevity suffers.
  3. Design issues: As mentioned previously, the 1911 was designed over a century ago. At that time, metal injection molding had not been invented. The 1911 platform was never intended to use MIM parts, and some MIM parts are wholly inadequate for the design even if the MIM was done correctly.

For some parts, with proper design and construction, MIM is a viable construction method. Grip safeties, firing pin stops, single side thumb safeties and magazine catches should be okay, provided the quality control is good and that you, as the owner of the gun, like the look of the parts. These parts aren’t subject to a huge amount of stress and should hold up okay.

For other parts, a CNC machined part is preferrable. High stress parts such as the slide stop, ejector, plunger tube and ambidextrous thumb safeties will typically last longer if they're machined. The hammer and sear should ideally be CNC machined, especially if you intend to have a trigger job done on the gun. These items are subjected to considerably more stress and tend to hold up better when they are CNC machined.

XS Sights Tritium Night Sights

XS Sights produces high-quality gun sights, with one of the benefits of our sights being that they are CNC machined.

CNC machining allows us to produce gun sights with extremely tight tolerances and a high degree of consistency from one part to the next. This means that XS Sights' gun sights provide greater accuracy and reliability, which is essential for any shooter. The overall benefits of our sights being CNC machined include greater precision, consistency, customization, and efficiency.