While silicone is great for making medical devices and their parts, the material is also a great choice for areas of those parts or devices which need the properties that only a silicone can provide, whether it’s excellent compression set, softness, or chemical inertness. Overmolding provides medical device designers the flexibility of incorporating silicone’s advantages where they’d have the most benefit. From handles and grips to gaskets and catheters, silicone overmolding is used for many medical device applications.

A strong bond between the overmolded silicone and the base material (called the substrate), is crucial to the finished device’s performance and safety. When the silicone and substrate fail to adequately jointogether, the two materials can become detached, resulting in premature failure of the device. Frequently for medical devices, this bond not only must withstand the stresses of normal use, but also the temperatures, chemical agents, and radiation used for sterilization.

Tips for Good Overmolding Bonds

Because silicone overmolding has been used extensively for medical devices, the industry has learned several tips to help ensure a good bond between the silicone and substrate.

Keep the surface of the substrate clean and dry. Contaminants such as oils and mold release agents can thwart both the chemical and mechanical bonding of the silicone to the base metal or plastic. Some contaminants can even hinder the curing of the silicone.

Design features into the substrate for mechanical bonding. Loops and other such features in the substrate allow the silicone to flow into the substrate and mechanically lock into it. This is especially true if Liquid Silicone Rubber (LSR) is used to overmold.

When necessary, treat the substrate’s surface with primers or plasma. Sometimes the silicone and substrate are incompatible when it comes to chemical bonding. Care should be taken with using primers for medical devices, since the primer can impact the biocompatibility of the finished device. That’s why plasma surface treatment is preferred, as that process doesn’t introduce new substances.

How to Confirm Bond Strength

The above tips and other industry best practices provide great guidance, but testing is what ultimately reveals the true bond strength of an overmolded part. The regulatory requirements for medical devices require such testing to provide the proof required for FDA approval, not to mention the continued testing of production samples for ongoing Quality Assurance (QA).

Medical device OEMs should follow these steps in order to verify that their chosen combination of substrate, silicone material, surface treatment (if any), and molding process can produce adequate bond strengths:

  1) Have the molder create sample plaques with your substrate material, surface treatment (or primer), and silicone material. This should be done early in the development process, before a final material selection has been made and molds have been machined. An OEM may need to experiment with several different silicone formulations or substrate surface treatments before one is found which not only delivers the required bond strength, but also meets all the other design requirements (e.g. durometer and biocompatibility).

 2) Perform thorough destructive testing on the sample plaques. The best way to measure the strength of a bond between two different materials is to measure the amount and type of stress needed to make that bond fail. Thus, destructive testing covering tensile strength, lap shear, cyclic fatigue, compression, and torsion is a must. Many OEMs may find it helpful to consult the ASTM D429 standard, which covers methods for testing the adhesion of rubber to rigid substrates.

The resulting test data will definitely guide an OEM toward making the best material and surface treatment decisions for the medical device.

3) Produce device prototypes and subject them to the same destructive testing as well. This can be done both on early stage prototypes and on production equivalent parts.

4) Once production has started, incorporate bond strength testing. This can be done in a number of ways, from quick and simple hand stripping or peeling tests to much more quantified testing using machines. Either way, bond strength testing should only be performed on a subset of parts, due to the destructive nature of the testing.

At ProMed, our passion and expertise is silicone molding for medical devices—and that includes overmolding. From selecting the best silicone formulation for a substrate to optimizing molding parameters for a strong, durable bond, we produce exceptional overmolding results for medical device OEMs. Contact us and let us know how we can help you.

4 Steps for Confirming Bond the Strength in Silicone Overmolded Medical Devices
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4 Steps for Confirming Bond the Strength in Silicone Overmolded Medical Devices
While silicone is great for making medical devices and their parts, the material is also a great choice for areas of those parts.........
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ProMed Molding
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