Making Dissimilar Molded Materials Work Together—A Medtech Makers Q&A

By Sean Fenske, Editor-in-Chief

Molding is a well-understood and commonly used component fabrication process in medical device manufacturing. That said, there are still complex and challenging aspects that are needed less often, which can cause delays or increase costs. One example is the joining of dissimilar materials through a molding process.

There are numerous examples where a device requires two dissimilar materials to be molded together, such as the overmolding of one material atop another. These may include a surgical handle to enhance the grip, a soft covering to provide comfort, or a plastic layer on metal to provide a specific tactile feel. Whatever the purpose, the bond between the two surfaces must be secure.

To further explain the challenges, advantages, and applications associated with the molding of dissimilar materials, two representatives from MRPC participated in this interview. In the following Q&A, Denise Sawdey, Process Engineer, and Brunson Parish, Director of Technology Development, address several questions on the topic, including important considerations and why working with an expert partner is critical.

Sean Fenske: Why can combining dissimilar materials be challenging in medical device manufacturing?

Brunson Parish: From the conception of a multi-material medical device design, the OEM is designing a device that provides a combination of enhanced physical and performance properties. Device manufacturers may want to design a device that is both soft to the touch and flexible externally yet structurally rigid, holding a very tight tolerance internally. In this case, a high-performance, strong thermoplastic backbone to the device may be chosen. In combination with this, a very flexible silicone outer shell may be desired for patient comfort.

The challenge to the manufacturer in the case of a design that combines thermoplastic and silicone is the fact that the manufacturing steps to produce either of these materials are processed in environments completely opposite of one another. Thus, the challenge becomes how to design a robust manufacturing process to provide the right combination of both manufacturing environments without compromising either material’s physical properties. The second, and no less important, challenge is that these materials naturally do not bond to one another. So, the manufacturer must develop a process that provides a permanent bond between two chemically incompatible materials and do so with confidence that this bond will remain throughout the product’s lifecycle.

Fenske: What needs to be considered to create a reliable multi-material component?

Parish: Manufacturers of these multi-material products must intimately understand the design intent of the device being developed. Also, the manufacturer must be capable of understanding the steps in developing reliable manufacturing processes to produce this product. These considerations can include the following:

• What is this product’s environment?
• What stresses or extremes will this product be exposed to?
• Does the device require sterilization or repeated sterilizations, and if so, what types?
• Is this a short-term or long-term implantable device?
• What are the regulatory requirements or classifications of the materials to produce this component?
• Is there an opportunity to explore alternative materials?
• Can this product be produced in one manufacturing step or multiple steps?
• How will this multi-material component be performance-tested?
• How will the manufacturing process of this multi-material component be validated?

Fenske: What are the challenges with making dissimilar materials bond together?

Denise Sawdey: The two main challenges while trying to bond silicone to thermoplastic are the materials’ poor adhesion to each other and the processing temperature mismatch. To circumvent the poor adhesion, manufacturers can select self-bonding silicones, create a molecular attraction between the two materials with a primer, or use a plasma treater to clean the insert at a micro level and increase the surface energy.

Fenske: How does MRPC approach material selection and determine the best way to bond dissimilar materials?

Parish: MRPC approaches material selection as a constantly evolving research and development activity. Reliance on our extensive experience in materials across a wide spectrum of the medtech field is not our only asset. MRPC has developed a strong, committed cooperation with our material supply chain to remain abreast of newly developed and redeveloped materials, additives, and performance enhancers to continually broaden our knowledge portfolio of materials that can enhance product designs or provide a technical solution to a challenging product’s manufacturability

Fenske: How can multi-material molding expand what is possible in medical device design and manufacturing?

Sawdey: Adding silicone to metal or thermoplastic allows the design to have geometry and properties that otherwise would be impossible. Due to silicone’s ability to stretch and flex, a design can now incorporate internal seals and undercuts. Adding a silicone component would now also allow your part to have biocompatibility, chemical resistance, and extreme temperature resistance in the area where that silicone is.

Fenske: What are some examples of medical devices and/or components that require multi-material molding?

Sawdey: Devices where doctors need a tactile way to feel that their surgical instrument is in the correct location may require silicone to be overmolded incrementally onto a steel or thermoplastic insert. For the comfort of patients, silicone may be overmolded onto rigid thermoplastic, and where that thermoplastic may have irritated the patient’s skin, the soft silicone will act as a comfort barrier. Along that same line, molding silicone over a device a doctor will be holding for long periods of time while in surgery could benefit from an ergonomic silicone overmolded grip.

Fenske: Why is it important to involve an experienced manufacturing partner early when developing a multi-material component?

Parish: MRPC prides itself on being involved early as the key to successful product development and launch. Our experience becomes most useful on the front end of design conception. Our history in prototyping, product and mold design, and state-of-the-art manufacturing equipment, as well as extensive knowledge based on a multitude of performance materials, both in thermoplastics and silicone elastomers, are key advantages. Further, success often involves failures along the way, which is a valuable experience in and of itself. We understand pitfalls or shortcomings in product development and can guide medical device designers toward a successful path.

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