The most advanced and expensive medical silicone grade available is the unrestricted grade for implantation. These materials can be implanted into the body for beyond 29 days. Essentially, this form of silicone used is for long-term implants that may remain in the body until end of life. Materials in this group, will undergo all USP tests as they are refined the most and inside the body for long periods of time. As with restricted grades of silicone material, these materials can be loaded with a low percentage of barium or tungsten for future medical detection inside the body.

There are many medical grades of silicone available from the premier silicone suppliers today. The three grades of medical silicone materials can be accessed by designers, inventors, and engineers with relative ease. If a certain project requires a material with special additives and or characteristics, then many of the silicone material suppliers will work with your company to create custom batches. The availability of medical grade silicone is better than ever and datasheets can be accessed via the internet.

Mold Processing

Silicone materials can be formed into parts utilizing multiple molding processes. In order to determine which process is used to mold the parts, many manufacturing variables must be reviewed. Four variables that immediately come to mind are: phase of the product lifecycle, production volume, part geometry, and material type.

Depending on where the part design is in the product lifecycle will largely determine which molding process is used. Compression molding offers a flexible option when the part design is in the prototyping, testing, and product launch phases. Compression mold tools can be altered in design and modified easily as design changes are made. Once the part design is finalized, then testing can be completed. As the medical device outfit prepares its silicone part for product launch, compression molding still offers a viable option. Compression molding allows for an easy switch between different material grades and durometers for testing purposes, while providing a dimensionally correct product in the right material. (See Figure 1)

Fig. 2 – Injection molding requires the two parts of liquid silicone rubber to be mixed in a static mixer before being sent to the screw. The screw then feeds the LSR material under pressure into the mold in a specified amount (shot size). This allows for continuous and often automated molding of quality parts.
Once the part is launched into production and beyond the product launch phase, volume becomes an important factor. Medical device companies must determine what their anticipated annual production forecasts are, in order to select which molding process to use for their silicone parts. If the volume is only a few thousand to tens of thousands then compression molding may be suitable. However, if hundreds of thousands to millions of parts are needed, then injection molding offers a faster rate of production and lower cost per unit. Some parts may require higher volumes that cannot be injection molded due to the part’s design geometry. (See Figure 2)

The part geometry will become a major factor as to which molding process can be used. If the part is simple in design and does not require undercuts or complex cores, then it can be injection molded or compression molded. However, if the part is highly complex in geometry and requires multiple cores then compression molding may be a better fit. It is important to note that complex tooling can require very extensive investment in injection molding tools as compared to compression.

The final variable that will determine what form of molding is selected is the material type. LSR is suitable for compression and injection molding. This is the most common and advanced silicone material widely available today. LSR is inserted into compression molds via dispensing guns, while it is fed to injection molds via meter and mixing systems. HCR utilizes a process known as transfer molding to mold silicone products. Transfer molding rams HCR into a mold tool and then transfers it into the cavities. Since HCR is a legacy material, its use is declining, while LSR’s usage continues to increase. The reason for this is that the properties that once made HCR desirable can now be achieved using LSR.

Applications

Fig. 3 – Electrical seals for surgical and medical diagnostic equipment.

Silicone has seen an increased utilization rate since the 1990s in the medical industry. As medical devices become more complex and demanding, silicone offers designers an excellent material to work with in terms of flexibility and stability. The material is capable of handling a diverse temperature range, with great tear strength and elongation properties. Silicone molding offers medical device companies and start-ups a viable solution for their most demanding products.

Silicone is commonly used in surgical tools for many reasons. Metal tools are commonly overmolded with silicone to form handles. In addition, seals are typically made of silicone within these surgical devices to reduce potential leak paths. Guards on surgical tools are made of silicone to protect surgeons and patients from sharp edges, insulate electrical currents, and dissipate heat. Many contact seals for instrumentation used to insert medical device implants in the body are also made of silicone. (See Figure 3)

Medical Design Briefs Magazine

This article first appeared in the May, 2015 issue of Medical Design Briefs Magazine.

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