Silicone’s well-known biocompatibility has made it the material of choice for medical device components and complete products. From catheters to heart valves, silicone’s inherent chemical resistance, toughness, and thermal stability is conveniently augmented by the vast range of available durometers on the market.

As versatile as silicone is, fabricating high-quality parts from it is not foolproof. In this article, we’ll highlight some best practices recommended for medical silicone injection molding.

Make Prototypes

Producing prototypes is a standard part of product development and design improvement, but the variable and relatively high shrinkage rate of silicone (relative to thermoplastics) underscores the need to build and test more than one iteration of tooling in order to determine and accommodate for the exact shrink rate of the part. The earlier in the tooling design process a change is made, the lower the impact on total project cost and schedule.

Multiple prototype iterations are also useful for determining the best durometer (see below) for a device’s ergonomic and performance needs. While CAD simulations deliver accurate results for the latter, the former is inherently more subjective, requiring people to physically hold and attempt to use the device.

Dial in the Correct Durometer for the Application

When selecting a specific silicone material for a new product, the criterion given the most focus is often biocompatibility (especially true for long term implantable devices) and mechanical properties like compression set and tear strength. While this focus is appropriate, durometer should not be overlooked. Durometer is a measure of the hardness of the cured silicone, and it must match the application. If an implantable device will be located alongside or nearby soft tissues or fragile organs, a softer durometer would be preferable to a harder one. Conversely, if the part needs to provide mechanical support, then a stiffer durometer would be the better choice. Of course, the durometer is intertwined with other mechanical properties such as elongation and modulus of the finished part as well as the viscosity of the uncured silicone.

Adopt a Scientific Approach

Scientific Injection Molding (SIM) isn’t simply a best practice, it’s a requirement for molded silicone medical devices from a competitive standpoint. SIM centers on planning and conducting experiments in order to collect data about a particular injection molding process. Not only does this foundation of data show whether or not the process is under control, but it also quantifies how variables such as processing temperatures, injection speed, injection pressure, and cure time impact the performance of the molding process.

With that quantification, process engineers can develop a model of the process that can guide them to the optimal values of injection temperature, mold temperature, injection speed, etc. . . which produce consistent, high-quality parts, lot after lot (by reducing process variation). As an added bonus, scientifically determining those optimal parameters leads to less wasted material, time, and money.

Temperature and pressure sensors can be built into the injection press and even the mold as well, where they serve two purposes: gathering the data needed for the process experiments SIM requires, and providing the feedback for closed-loop control during production. Both efforts complement each other because that sensor data can be used to make the injection molding process reliable and repeatable. Thus, regulatory compliance is a natural outcome of efficient, traceable and reproducible production, not an obstacle to it.

Silicone molding of medical devices and components opens up new doors to innovation and breakthroughs in patient care, but it also presents technical challenges which the above three best practices can help resolve. These three best practices are only a sample of those implemented at ProMed, where our focus and passion is medical silicone molding and supporting the innovation of our OEM customers.

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Best Practices: Medical Silicone Molding of Components & Devices
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Best Practices: Medical Silicone Molding of Components & Devices
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Scientific Injection Molding (SIM) isn’t simply a best practice, it’s a requirement for molded silicone medical devices from a competitive standpoint
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ProMed Molding
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