By Sean Fenske, Editor-in-Chief

When it comes to medical devices, reliability is paramount. The devices must work each time, without fail. As such, the components within these technologies must perform equally as well. As a result, many components are made with metal to leverage its strength and durability. However, this can result in increased costs, weight, or other attributes not desired for the final product.

Fortunately, a plastic alternative may exist that retains the same level of strength, durability, and other favorable characteristics as the metal part. Resins can be filled with additives that alter the properties of the plastic to achieve features not typical for the raw material on its own. The result can lead to a part that’s lighter weight, smaller, less costly, or a combination of benefits.

To help further explain the advantages of working with filled resins for medical device components, Ray Scherer, global engineering manager at Aptyx responded to the following questions in this Q&A. He provides insights on why filled resins can benefit medical device manufacturers, critical considerations to keep in mind, and common challenges that may be encountered.

Sean Fenske: What is a filled resin? What are the commonly selected fillers for medical device applications?

Ray Scherer: Fillers in resin are materials added to improve or adjust the properties of a base polymer. Common fillers for medical device applications include glass fibers, mineral fibers, barium sulfate, and antimicrobial agents.

Fenske: Why use a filled resin? What are their benefits?

Scherer: Fillers can change performance characteristics like strength, rigidity, impact resistance, and heat resistance, as well as control shrinkage, add radiopacity or conductivity, prevent bacterial growth, reduce friction, and even optimize cost. We primarily use fillers to enhance performance, fine-tune material characteristics, and manage costs. Most commonly, fillers help boost mechanical strength, stiffness, and heat resistance, which are key for many medical applications.

Fenske: For what types of medtech applications are these filled resins being used? What benefits do each of the different fillers provide?

Scherer: In medical device design, we use fillers in thermoplastics to enhance specific properties. For decades, metal was the go-to material for a range of medical devices because conventional polymers couldn’t deliver the high modulus, strength, and chemical resistance of metal. However, high-performance specialty polymers have been a game-changer in many applications. For example, glass fibers add strength and rigidity that are ideal for converting components such as gears from metal to plastic.

With the trend from reusable to single-use devices, as well as miniaturization, the advantages of fiber components are driving metal-to-plastic conversions for robotic surgery components and many other medical applications. It’s about achieving cost reduction while maintaining or even improving overall performance.

Some long-established fillers include barium sulfate, which is added for radiopacity so catheters are visible under X-rays. For infection control, antimicrobial fillers like silver nanoparticles are used in wound dressings and catheters to prevent bacterial growth. Each filler plays a key role in ensuring devices meet high standards for safety and durability.

Fenske: What should be considered in terms of designing-in filled resins? Are there best practices for design for manufacturability?

Scherer: One of the most common issues that can impact design is shape control. Fillers with higher aspect ratios—such as glass and carbon fiber—create disproportionate shrink rates within the same part. The fiber aligns in the polymer-flow direction, creating lower shrink rates along the flow axis and higher shrink rates along the crossflow axis. These differential rates can cause shape distortion in the form of excessive warp.

When shape control is critical, low-aspect-ratio fillers such as minerals or glass beads should be considered, as they produce much lower disproportional shrink rates. Regardless of the choice of filler, the critical first step is to use simulation software to identify gate locations that optimize fiber alignment and shape control. We’ve had great success using Moldflow simulation software for predicting outcomes and have seen some drastic consequences when others neglect this step.

Along with optimizing for the as-manufactured shape, we also need to consider the as-manufactured properties. One of the challenges with glass-filled material is managing weld line locations and properties. With fiber-filled resins, as-manufactured properties can be half the strength of the published value, and this is especially true at weld line locations. So, in addition to part design, we also need to consider gate location(s), number of gates, gate types, sequential valve gating, gate-runner sizes, overflow features, and venting. Our goal is to move weld lines to less critical areas in the design to enable the flow fronts to meet and move toward ideal venting locations. Moldflow simulation is critical for these types of analyses.

Fenske: Following design, what are the manufacturing challenges involved with filled resins?

Scherer: Even after simulation and precise gate placement, the flow-front fiber alignment can still drive warp. The best trick we’ve found to manage differential shrink is to use Moldflow simulation to guide us in manipulating fiber alignment by modifying part design. We recently experienced great success applying this method to a family of heavily glass-loaded nylon components for a robotic-assisted surgical device. The customer, who had made many similar parts in the past, was excited to see such straight parts!

Sometimes the design can’t be modified, and we need to design adjustability into the mold from the outset. Using Moldflow simulation, we can predict part features that may need adjustment and ensure our mold design can economically accommodate future changes. During the process development phase, we often determine shape through CT scanning and use these models to create the necessary mold compensation. Old-timers (like myself) call this “Kentucky windage”—a throwback to the 1800s when a skilled marksman would make sight adjustments for factors outside their control, like wind direction and distance. In this case, you could say we make the mold “wrong” so the part will be right.

Fenske: What other challenges do manufacturers need to keep in mind? Or what is commonly overlooked with regard to filled resins?

Scherer: One aspect to keep in mind is fillers can often negatively affect the cosmetic appearance by showing up on the surface of a molded part. An experienced molder can improve this condition by adjusting the process. Combining proper fill speed with optimal steel temperature and pack control, the surface of a part can be made resin-rich, effectively burying the filler below the surface. I recently completed a program that used carbon black powder in a polycarbonate for conductivity purposes. We were able to create a surface so glossy it looked like a black mirror.

Fenske: Do you have any additional comments you’d like to share based on any of the topics we discussed or something you’d like to tell medical device manufacturers?

Scherer: We’ve all heard the expression, “The best offense is a good defense.” That certainly applies to successfully using fillers in plastics. Getting involved with a molder early in your project while you’re working through your design is a recipe for success. Partner with a molder with a strong engineering staff who can guide you through these steps. In the hands of an experienced engineer, the great predictive tools out there can be leveraged to ensure a successful program.

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