With careful planning, complex injection molding can reduce costs, optimize functionality and improve aesthetics in medical devices.

Ken Glassen, Kaysun Corp. 

As medical device design evolves to provide ever-improving healthcare outcomes, manufacturers are partnering with full-service, experienced complex injection molders to gain the increased design freedom and process efficiencies necessary to keep pace with medical device advances. This partnership is a consistent value-add, from part and tooling design through material selection and process control, empowering many medical OEMs to address issues before they hit production or the bottom line. 

Where complex injection molding makes sense

Injection molding gives medical OEMs the latitude to integrate disparate materials — from dissimilar polymers to lenses, fastener threading, metal components and other non-plastics — to facilitate assembly, optimize functionality and increase durability.

The decision to use complex injection molding is commonly driven by several factors, including:

• Aesthetics: Incorporating a uniquely colored polymer into a device can make it more patient-friendly (notably in pediatric applications) or help differentiate products within a manufacturing line.
• Expedited manufacturing: Integrating an intricate part into the product leverages machine-paced production and reduces or eliminates worker involvement in assembly.
• Compliance: Addressing design and production challenges posed by strict regulatory controls efficiently validates all physical, chemical and compositional aspects of medical devices, especially implants.

Looking into part design

Many of the crucial decisions involved in complex injection molding should be made as early as possible in the design phase and with molder input. At this stage, adjustments have a reduced effect on the total cost and product timeline. When engineers are mindful of medical component design, tooling functionality and materials at the project outset, they can generally prevent missteps in three key areas:

• Aesthetics: Visible knit lines may appear on product surfaces if proper tooling design is overlooked.
• Bonding: If the product requires multiple materials, compatible polymers must be identified and chosen carefully to ensure a strong, permanent bond.
• Durability: Plastics selection is integral to producing robust medical parts that can withstand the wear and tear — or even abuse —of daily use, even when devices are routinely moved from room to room or from ambulances into hospitals.

Getting tooling right

Tooling development is at the heart of the injection molding process, the step from which everything else flows. The ultimate success of the part begins at the point when the engineering team selects the metal from which a tool is made, based on certain process criteria, including:

• Durability: Medical device production varies in volume and complexity. If the yearly usage is under 10,000 parts, aluminum is the preferred tooling material. Yearly usage exceeding 10,000 demands harder steels.
• Cost: Softer metals, such as P20 steel and aluminum, are easily machined and therefore less costly builds. The tradeoff for the lower cost, however, is that softer metals wear faster.
• Corrosion: Given the priority placed on cleanliness in most medical applications, stainless steel is the appropriate tool steel to avoid corrosion.

Material matters

Ideally, choosing materials for the injection-molded part is a collaboration between the plastics engineer and the OEM to clearly define the project’s requirements and reach a mutual understanding of five defining factors:

• Physical load: Can the part stand up to the conditions of everyday use without fatigue?
• Mechanical function: Can the characteristics of the polymer meet the demands of the medical application?
• Thermal conditions: Will the device be exposed to fluctuating and extreme temperatures, particularly if it functions outdoors?
• Environment: Will the device be implanted or used in direct contact with bodily fluids? If so, the polymers must be biocompatible in accordance with FDA regulations.
• Chemical compatibility: What exposure will the device have to chemicals, such as hospital-grade disinfectants?

Due to the unique nature of medical environments, it’s not uncommon for medical applications to have additional considerations that factor into materials selection, such as:

• Sterilization: Many medical devices must withstand regular sterilization treatment by radiation, chemicals or the high heat and steam of autoclaving.
• Molding dissimilar plastics: Multi-shot technology, or overmolding, is one of the most common complex injection molding techniques used in medical device manufacturing. This process involves adding softer polymers for ergonomic and waterproofing features (such as keypads, grips, protective bumpers and seals) over a hard-plastic substrate, typically an impact-resistant device body.

The high-heat resins required to withstand autoclaving, such as polysulfone (PSU), have their own set of process considerations. These materials are more difficult — and therefore costlier — to work with, mainly due to their higher melting points, which complicates everything from safety compliance to the molding process.

For instance, PSU has a melting point of 700°F, versus 500°F to 550°F for typical resins. These higher demands mean higher risks for both safety and deviation. Since the tooling itself can reach 325°F (as compared to 180°F for a water-heated tool) it is subject to higher levels of thermal expansion, which adds complexity to the overall tooling design process.

Conclusion

Complex injection molding can provide a medical device and equipment manufacturer with competitive differentiation in the progressive medical industry. Moreover, choosing to work with an experienced, full-service molder gives OEMs increased design freedom and process efficiencies that result in high-quality parts and devices that perform with optimum efficiency and at lower total production costs.

Ken Glassen is VP of engineering at Kaysun Corp. (Manitowoc, Wis.) — a provider of custom injection molding and engineering services. Glassen shares and applies his technical plastics engineering expertise to help OEMs in a variety of industries including medical, automotive and defense develop complex injection molding solutions.

The opinions expressed in this blog post are the author’s only and do not necessarily reflect those of MedicalDesignandOutsourcing.com or its employees.