Drew Rogers, Trelleborg Sealing Solutions.

Silicone’s popularity for use in medical devices has been growing.  This is due to its compatibility with bodily fluids and strong chemical structure. Advancements in medical polymers are also part of this push toward next-generation single-use medical devices, implants and packaging technology.

In order to get the right material, configuration and manufacturing process for a device’s proper end use, it’s key to choose the proper technology to customize the silicone components for the right application. We’ll review the advantages and disadvantages of several types of processing techniques for silicone plus post-fabrication services like coating, sheeting, tubing and packaging.

Processing techniques

1. Injection Molding

Injection molding has its benefits: It is highly efficient in manufacturing and can create components that are very complex and intricate. Injection molding typically uses a single mold with multiple cavitations to manufacture a large number of parts.

The part and application may lend itself to liquid injection molding (LIM) or liquid silicone rubber (LSR) production.  LSR can also possibly be used in combination with an engineered plastic using a 2-shot (or more) fully automated injection molding set-up.

In line with the complexity of the finished product, developing a tool-grade steel mold, hot- or cold-runner blocks, and process automation equipment can be expensive upfront and this cost must be considered in any fabrication program. However, as injection molding, and even more so liquid injection molding, can produce high integrity parts over very high volumes, the tooling can be a relatively low priced considered on a piece-by-piece basis over the life of a program.

The efficiency of the mold depends on choices such as cavitation, parting-line geometry, gating, venting, surface finish and supporting automation. It’s important to ensure the mold is robust enough to mitigate the inherent lot-to-lot variability of raw silicone.  It will also need to integrate seamlessly with equipment that pumps, mixes, injects, compresses, heats, and ejects. Molds must be carefully designed and precisely manufactured, adding a layer of complexity to the process.

Creating a mold for a seal to be used in a medical device typically requires early, close collaboration between the device maker’s engineering team and the injection molding engineering experts at the seal fabricator. The idea is to ensure the correct material selection and adherence to regulations, while minimizing variability, maximizing yield, and reducing costs by optimizing seal geometry, tooling and process engineering.

Advantages of injection molding

• Liquid Injection Molding
  • Produces complex designs; ideal for parts with a large amount of detail such as undercuts or thin wall sections
  • Capability to mold micro- and nano-sized parts
  • Accommodates hard-soft combinations via a 2-shot LSR process
  • Highest efficiency of any molding method with short cycle times and possibility of full automation ideal for very high volumes
• Injection Molding
  • Enhanced strength; fillers can be used to reduce the density of the silicone while it’s being molded, further strengthening the molded part
  • Multiple silicone types can be utilized and tailored to the application conditions and molding process requirements
  • Metal or plastic elements Can be integrated into the part
  • Efficient process for technical parts in medium to high volumes in semi-automation

Disadvantages of injection molding

• Liquid Injection Molding: highest initial tooling cost that must be considered as an investment over the life of a tool; which is, however, the longest of any type of alternative molding tool, i.e. 1 million shots
• Injection Molding
  • Design restrictions, including the fact that all parts must be solid and must have drafting if they are perpendicular to the tool opening
  • There may be restrictions on part thickness to avoid shrinkage problems
  • Requires part de-flashing operation with additional cost

2. Compression Molding

The compression molding process is ideal for parts beyond the size capacity of extrusion or injection molding and for moderately complex parts in low quantities. Compression molding is used in medical applications such as diaphragms for respiratory equipment, lip seals for cylinder applications, and isolation bumpers used to inhibit vibrations.

Compression molding is also used to manufacture thermoset plastic parts. The raw materials for compression molding are either granules, putty-like masses or preforms. The raw material is placed in an open, heated mold cavity to which pressure is applied, forcing the material to fill the cavity.

Advantages of compression molding

• Cost effective for smaller volumes; low tool costs
• Parts can be made to customer specification from specified materials
• Flexible mold design
• Tools with multiple cavities can be created
• Quick turnaround of tools and parts
• Good surface finish

Disadvantages of compression molding

• Slower part production rates
• Can be difficult to control flash; requires post operations
• Precision is good but limited to a normal level for rubber parts
• Largely used to produce only flat or moderately curved parts with no undercuts

A further option for production of moderate quantities of complex rubber part geometries is transfer Molding.  here, the elastomer is first heated in a pod to then be injected into the hot cavity.

3. Extrusion

Thanks to advances in thermoplastic polymers, such as polyether ether ketone (PEEK), polyurethanes, and polyolefins, plastic tubing is replacing metal tubing in many medical devices. For example, PEEK is a strong alternative to stainless steel and other metals often used in medical-grade tubing, because it’s very strong and has a low friction coefficient. Similarly, both PEEK and polyphenylsulfone (PPSU) are being used to produce parts for long-term implantable components because of their biocompatibility.

Although thermoplastic polymers are more flexible, lighter, and generally less expensive than metal, there are some important factors to consider. First, the type of material used can dramatically affect the physical properties of the final product. Second, the degree to which the extrusion process is controlled can greatly impact the final result.

Advantages of extrusion

• Accommodates high production volumes
• Provides efficient melting
• With plastics, allows for post-extrusion manipulations
• Allows considerable flexibility in manufacturing products with a consistent cross-section

Disadvantages of extrusion

• Difficult to predict the exact degree of expansion
• There can be size variances
• There can be product limitations

4. Multiple-profile extrusion (MPE)

Originally developed to improve closed-wound drainage products, MPE eliminates secondary bonding operations through its ability to mate with a variety of tube profiles. The process produces a single, continuous tube, eliminating the need for leak testing. It also provides “on the fly” manipulations, allowing the cross-sectional profile of a silicone tube to change during extrusion, reducing costs. Just as importantly, the absence of a seam greatly enhances product performance, eliminating areas where bacteria can accumulate.

MPE facilitates the extrusion of balloons of any length via a moving mandrel that maintains a constant outside diameter while thinning the wall to vary the inside diameter. This eliminates the need for secondary bonding requirements, reducing costs and increasing production speed. Double extruder configurations allow for a wide range of stiffness and flexibility in tubes. The amount of flexibility can be controlled by thinning out the extrusion wall or switching to a softer or stiffer material anywhere along the extruded profile.

Within the MPE process, 2 or more lumens can easily be split off a center lumen or merge two lumens into a single lumen—all in a single continuous extruded tube. The multi-lumen process involves moving dies and mandrels in sync, reducing cross contamination of fluids in the separate lumens.

Advantages of multiple profile extrusion

• Facilitates the extrusion of balloons of any length
• Eliminates secondary bonding operations
• Allows seams to be eliminated
• Various types of tubing (single lumen, multi-lumen, transitional GeoTrans, etc.) can be produced, as well as rod, ribbon, and other non-standard profiles
• Suitable for extruding both elastomers and foams

Disadvantages of multiple profile extrusion

• Material choices limited to HCRs (high consistency rubber)
• Issues can arise from having to move dies and mandrels in sync
• Cross contamination of fluids in the separate lumens can occur

Finishes and post-manufacturing

1. Silicone dip molding and coating

Metal, plastic, fabric and glass medical components can all be coated with thin silicone films that are then vulcanized to produce a smooth, durable, biocompatible finish. For example, dip molding can be used to create silicone coatings on devices such as needles, cannulas, and syringes to enhance patient comfort.

Dip molding can also be a cost-effective alternative to silicone molding processes in instances involving costly metal molds. It is ideal for rapid prototyping of complex, thin-walled shapes that can later be scaled up for large-volume commercial production.

The dip molding process involves producing a mandrel in the shape of the final part. Mandrels are usually machined from metal but can be fabricated from engineered plastics and ceramics. The mandrel is immersed in a vessel containing silicone dispersion and then withdrawn. The mandrel, now coated with a thin liquid silicone film, is set and placed in an oven where the silicone is vulcanized. Following vulcanization, the silicone rubber is stripped from the mandrel, creating the finished product.

Wall thickness can be adjusted by varying the number of dips and by adjusting the percent of solid concentration of the silicone dispersion. Several variables contribute to the quality and reproducibility of each part, including evaluation of mandrel surface finish, immersion and withdrawal angle and speed, dispersion viscosity and temperature, ambient manufacturing conditions, and vulcanization parameters.

Advantages of dipping/coating

• Processes include mandrel-type dipping that produces discrete components (mammary shells, balloons) as well as durable coated surfaces (coated surgical blades, non-slip handles on surgical tools)
• Costly metal molds not required
• Ideal for prototyping and short runs

Disadvantages of dipping/coating

• Choice of silicone raw materials limited to solvent-based dispersions or low viscosity materials with marginal physical properties
• Possible variation over long runs dependent on multiple variables

2. Calendaring and sheeting

In high consistency rubber (HCR) calendaring, silicone is fed through multiple rollers to produce a film of uniform thickness. The film is then transferred onto a carrier sheet and may be cured or left uncured. Calendared sheeting can be pigmented, produced with a number of surface finishes and used to create vulcanized, un-vulcanized, reinforced, and non-reinforced products. The process works particularly well to create mandrel-wrapped hoses and ducts, as well as die-cut seals.

Custom sheeting is used in a variety of medical devices. Discs punched from vulcanized elastomeric sheets are key components in various valve assemblies, mesh-reinforced sheeting is integrated as sewing rings on artificial heart valves, and laminated sheeting containing both vulcanized and un-vulcanized layers is used to produce seal tissue expanders and mammary devices.

Advantages of calendaring

 

• Used to manufacture silicone sheeting in a continuous process
• Reinforcing mesh can be added to increase strength and tear resistance
• Sheets can be die-cut or punched into components

Disadvantages of calendaring

• Raw material choice limited to HCRs

3. Assembly, packaging and sterilization

Post-manufacturing – including complete device assembly, packaging and sterilization – is a final consideration for silicone processing. Not all suppliers have a full range of capabilities, and many silicone components are part of a larger, more complex device. Others, such as punctual plugs and small joint implants, function as complete medical devices but must be properly sterilized and packaged before shipping.

Conclusion

The ability of silicones to be formulated to attain specific performance, aesthetic, or therapeutic properties makes them ideally suited for many medical devices. However, suppliers must demonstrate a strong understanding of the details involved with both manufacturing silicone rubber parts and complex medical devices. With time-to-market such a critical component in the creation and sale of medical devices, the ability to produce rapid prototypes, quickly reach a final design, and consistently produce and deliver high-quality products, are the keys to success.

Drew Rogers is global director of healthcare and medical at Trelleborg Sealing Solutions.