Medical instruments used in surgery today often require more elaborate features and added functionality than they did in the past, largely due to the growth in minimally invasive surgery. For this reason, greater attention to detail is critical in product design and development. To meet the demand for small, complex, lightweight surgical products, medical device original equipment manufacturers (OEMs) are increasingly opting for disposable surgical tools and product offerings, which provide flexible, off-the-shelf single-use sterile solutions to surgeons and hospitals.

Much of the demand is also meant to address an increase in hospital acquired infections (HAIs), as single-use instruments offer a safer approach to preventing transmission of infections versus reusable instruments that undergo repeated use and sterilization.

OEMs also want their contract manufacturing organization (CMO) partners to be able to meet these ever-changing needs to support surgeons and their patients. It is why innovative technology continues to help achieve more advanced features that are cost effective and efficient to produce.

Producing small, high-strength disposable surgical components and products can be accomplished effectively with both a metal injection molding (MIM) process and traditional machining. However, efficiency is a key factor in determining which approach is best to use.

Both traditional machining and MIM have their advantages and disadvantages in relation to the production of components for medical devices, especially where parts may be smaller and require more maneuverability. To determine what is best to use, the following considerations needs to be factored into the decision making:

  • Material selection

  • Part size

  • Volume

  • Tolerance

MIM is ideal for parts that require assembly or multiple steps to assemble. (Credit: Micro)

The use of MIM in medical device manufacturing — in particular to make minimally invasive surgical tools and micro surgical assist devices — has been growing exponentially over the last several decades, and with good reason. Minimally invasive surgery offers many advantages for patients, including reduced pain, less injury to tissue, shorter hospital stays, and faster recovery. And, given the growth in minimally invasive surgery, MIM is a core capability for any CMO looking to stay competitive in today’s medtech marketplace.

A MIM process is ideal for producing very small, intricate parts and products with complex geometries at high volumes, such as handheld tools used for cutting and articulating during surgery. Devices produced through MIM have excellent strength and properties and often can be produced at a lower cost than traditional metal machining and die casting. Overall, MIM advantages include:

  • Greater design freedom. With MIM, parts can be designed and manufactured with minimal design restrictions. In addition, almost all design changes are possible within the shortest development cycle and turnaround time.

  • Complex and intricately shaped parts. MIM is ideal for producing complex-shaped components as well as parts that require assembly or multiple steps to put together.

  • High production requirements. MIM is most beneficial in high-volume production of small precision parts with complicated design geometry. The process lends itself to automation where high volumes and consistent quality are required.

  • Miniaturization. MIM technology is the best viable process for producing miniature parts economically.

The MIM process lends itself to automation where high volumes and consistent quality are required. (Credit: Micro)

The MIM Process

The MIM process is similar to plastic injection molding but uses metal feedstock instead of plastic. It is a hybrid technology that combines plastic injection molding with powdered metallurgy. The initial step to MIM is material selection and preparation. Once the appropriate combinations of metal powder and organic binders are blended and compounded, an injection moldable feedstock is produced. Using an injection molding machine, the parts produced are then subjected to a binder removal process. Depending upon the type of binder used, different methods of debinding are applied. After debinding, the parts then go through a sintering process to ensure that they achieve the desired material composition, physical properties, and correct geometry. Processes include:

  • Mixing — homogenous mix.

  • Molding — standard injection molding machine.

  • Debinding — solvent and/or thermal removal of organics.

  • Sintering — controlled shrinkage to full density.

Using a MIM process, 95– 98 percent of wrought materials properties can be achieved at significantly lower cost. When injection molding is being used, it is essential to keep in mind that not all feedstocks and methods are the same. For ideal mechanical properties, high density, and small dimensional variation, it is important to achieve constant shrinkage and diffusion of organic binders and surfactants from the metal matrix. Poor diffusion increases the empty spaces in the metal matrix by confining gas, and the lower density considerably decreases the strength of the part.

MIM versus Traditional Machining

MIM technology is the best process for producing miniature parts economically. (Credit: Micro)

A MIM process works best for small, complex precision components that traditional machining cannot produce cost-effectively. Adding complexity to traditional machining adds cost as well as setup and overall production time, and machining often requires multiple and secondary operations. Conventional machining has design limitations, is more labor intensive, and can lead to significant material waste. Machining requires assemblies, whereas MIM can produce complex geometries in a single highly automated process. MIM requires less labor and less production time, and it offers greater flexibility to scale up when volumes increase.

With MIM, the biggest cost is up front in producing the custom mold, but the initial investment is offset by high production runs, which is why using a MIM process is most suitable for high-volume development programs. However, keep in mind that if designs are likely to change downstream, machining might provide more design flexibility over MIM. Once a mold is created, it is difficult to accommodate design changes.

Both MIM and machining produce high-quality outputs. MIM provides a high degree of consistency at high production volumes because automation can be achieved very effectively with injection molding. Machining’s longer cycle times may be better suited for lower production volumes of complex shapes with tighter tolerances. Secondary operations, including finishing, with MIM are minimal in comparison to machining in cases where the application has less critical tolerances. A MIM process can incorporate challenging material and eliminate a secondary sharpening operation, which can be add significant costs to a project.

In terms of materials, MIM allows for customized material compositions according to the specific attributes required by the customers. Material compositions that can be used with a MIM process include stainless steels, low alloy steels, carbon steels, Ni-alloys, tool steels, and tungsten alloys. Another advantage of MIM versus machining is that it uses less raw material and reduces material waste, which then reduces cost of a project. MIM is better suited to thin wall sections, interior and exterior threads, and when words or numbers need to be molded into parts. Other design features such as undercuts, cross holes, and grooves can all be achieved with MIM.

For CMOs looking to produce precision components and unique geometries with exceptional mechanical properties at high volumes, MIM is an effective and time-tested manufacturing process for the right application that can save costs for customers without sacrificing quality.

CMOs can be invaluable partners to their OEM customers in providing specific manufacturing proficiencies when taking a design from an initial concept through production. CMOs can offer significant value through design for manufacturability (DFM) services that can help address any potential problems early during the design stage. Choosing a CMO partner with proven technical capabilities and expertise is crucial, and doing so helps save time and money and helps prevent wasting resources in the long run. It’s also important that CMOs are sensitive to pressures that design engineers face with cost targets, time to market, and other critical success factors. CMOs can play a vital role in helping mitigate issues when design engineers dismiss manufacturing considerations, such as an over-designed or underdesigned product. These can prove detrimental to ultimately producing the parts successfully.

This article was written by Steve Santoro, EVP of medical device contract manufacturer Micro, Somerset, NJ. He was recently appointed a charter board member of the School of Applied and Engineering Technology at the New Jersey Institute of Technology. For more information, visit here .


Medical Design Briefs Magazine

This article first appeared in the June, 2020 issue of Medical Design Briefs Magazine.

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