Problems with an articulation gear stripping in an instrument for minimally invasive surgical operations prompted a major medical instrument manufacturer to approach us to see about transitioning the gear from machining to metal injection molding (MIM). The gear was originally designed to be polymer injection molded, but during trials and development, the gear teeth would strip due to the forces involved. At the distal end of the instrument, the part has to articulate and lock in at a specific angle. Instead, it was snapping back and not holding.

The OEM had determined that machined aluminum was sufficiently strong to prevent the gear teeth from stripping, but the cost to machine the parts was too high. Machining the gear would have required cutting the gear teeth and the hole in the middle of the gear. In contrast, MIM can build the complete part using no secondary operations.

The challenge was to meet the plastic molder’s requirements. Historically, plastic molders have been hesitant to use MIM parts for insert molding, because machined metal tolerances are so tight (typically, within 0.0005 in) that if a MIM part is out of spec, the molder’s tools could be destroyed. But in this case, the insert molder agreed to try using MIM, with a little persuasion from the OEM. This gave us the chance to use our proprietary MIM technology with insert molding for the first time.

Because the OEM had already determined that machined aluminum met the part’s mechanical requirement for gear teeth strength, we had a large selection of alloys from which to choose. We selected one of the most commonly used alloys, MIM stainless-steel 17-4. It is corrosion resistant, has high strength, and can be hardened. The material also is low cost and has robust operating parameters and an extensive operational history in terms of as-sintered tolerance capability.

The insert molder let us know precisely how much room we had and the tolerances required. It was determined that the critical portion of our sintering stage is to ensure that the part feet have a certain pocket or envelope that must fit exactly. It was absolutely imperative to keep the feet within this envelope – one out-ofspec part could destroy the insert tooling.

It was also important to hold tolerances on the part diameter on which the insert molding closed. Tolerances must be tight enough, but with no metal-to-metal hard contact to wear out the surface.

Because the critical diameter being shut off was only acting as a seal, it needed just enough width to hold the plastic flow. We were able to hold diameter roundness tightly (< 0.003-in). We also repositioned the gate from the original location in the plastic part to an area more symmetric to the diameter. Although the design added complexity to the mold, it helped ensure flow uniformity.

To address the MIM process variation that occurs downstream, after molding, a custom setter was designed to minimize the effect of frictional forces resisting the part’s 20% shrinkage on the contact surface during sinter. We prepared a sizing operation in case the as-sintered MIM tolerance was not capable, but it was ultimately deemed unnecessary.

Two years into production, the spec is 0.0030-in maximum out-of-round. However, we have maintained 0.0012- in. out-of-roundness for the life of the product to date. Once joined together, the metal and plastic pieces become one unit, which is sent to the OEM for assembly into the instrument. MIM in this case cut costs by 50% as compared to machining.

Key to the project’s success was that the OEM, the insert molder, and Parmatech worked closely together to design for manufacturability and take advantage of all technologies, yet retain the complexity of the design. Every feature has a critical component mating or passing through it. The control linkages from the handle to the working end of the instrument have to pass through the subassembly containing the articulation gear.

The project shows that MIM can be used to increase part strength without the high cost of machining, even when insert molding operations are involved. Cost savings can be substantial compared to machining, when considering MIM’s higher production rate and low material waste.