Case Study: U Bracket & Stop

Process: Metal injection molding
Secondary Processes: Coining to the bracket only
Hardness: 32–38 HRC
Materials: MIM-17-4 PH stainless steel
Density: >7.5 g/cm³
Tensile Strength: 900 MPa (130,000 psi)
Yield Strength: 730 MPa (106,000 psi)

U Bracket & Stop
2016 Grand Prize: U Bracket & Stop

End Use and Function

The two metal injection molded (MIM) parts shown are a U bracket and stop, and are used in a Feedbox Support Improvement Kit (FSIK) for an M249 squad automatic weapon (SAW) used by the U.S. Military. The device is designed to securely hold various-size high-volume magazine ammunition packs to the gun.

Fabrication

The innovative I-beam and webbing design allowed the parts to meet the 32–38 HRC hardness range requirement and still maintain the total weight of the kit below 100 g (3.5 oz.). Molded from MIM-17-4 PH stainless steel, the parts have >7.5 g/cm³ density, 900 MPa (130,000 psi) ultimate tensile strength, 730 MPa (106,000 psi) yield strength, and 6% elongation. The only secondary processing of these near-net shaped parts are a coining operation to the bracket in order to achieve the tolerance required for the distance between the notch and the through-hole, as well as tapping the hole to provide necessary threading. Both parts are black oxided to remove their reflective properties, a critical consideration for the safety of the soldier.

Results

  • Extended the service life of the weapon by enabling the soldier to repair it in the field, thus avoiding the cost to taxpayers of a new weapon, which can be as high as $3,200.
  • MIM provided 100% conformity to these component elements and functional features in a high-production setting, whereas other processes such as casting and machining would not have been practical.
  • Drop testing of the firearm with the device attached proved the integrity of the FSIK even while other components were damaged.
  • Maintained a low overall weight.