Weighing in: The pros and cons of processing methods for medical polymers

Drew Rogers, Trelleborg Sealing Solutions, looks at extrusion and moulding, and the advantages and disadvantages of processing methods for medical silicone polymers.

With biofluid compatibility, favourable haptic as well as physical, chemical and processing attributes, silicone’s popularity for use in medical devices continues to grow.  Advancements in medical polymer architecture further facilitate many novel next-generation medical devices and implants. 

Bringing together the ideal combination of material, component design and manufacturing process within the right framework of regulatory compliance is the key to fulfilling a device’s intended fit, form and functions reliably. Expertise from a silicone and polymer processing specialist, and due diligence at the early stage of concept and development, pay tribute later for timely and smooth market launch and industrialisation. We’ll review the advantages and disadvantages of several types of processing for silicone and medical polymers.

1. Injection Moulding

Injection moulding allows highly efficient high volume manufacturing of components in – depending on sophistication of tooling – very complex and intricate geometries.  Cavitation per mould is tailored from one to several hundred depending on complexity of part and capacity needs.

A part and application may lend itself to be produced from liquid silicone rubber (LSR) in a liquid injection moulding (LIM) process. LSR has the potential to be used in combination with an engineered plastic using a 2-shot (or more) fully automated injection moulding set-up.

In line with the complexity of the finished product, developing a tool-grade steel mould, hot- or cold-runner blocks, and process automation equipment can be expensive upfront.  However, at high volume and over the life of a program, the tooling costs per part can actually be quite low.  Injection moulding, and even more so LIM, can produce high integrity parts over very high volumes.

The moulding efficiency depends on decisions and choices made around mould and process design as to details such as cavitation, basic tool construction, gating, venting, surface finish, and supporting automation. It will also need to integrate seamlessly with equipment that pumps, mixes, injects, compresses, heats and ejects.

Creating a mould for a seal to be used in a medical device typically requires early, close collaboration between engineering teams at the device maker and the seal supplier. This will ensure the correct material selection and adherence to regulations, while minimising variability, maximising yield, and reducing costs by optimising seal geometry, tooling and process engineering.

Advantages of Injection Moulding

• Liquid Injection Moulding

• Facilitates complex designs; ideal for parts with a large amount of detail such as undercuts or thin wall sections
• Ideal for micro- and nano-sized parts
• Accommodates hard-soft combinations via a 2-shot LSR process
• Highest efficiency f any moulding method with short cycle times and possibility of full automation
• Ideal for very high volumes in flashless quality

• Injection Moulding

• Enhanced strength; fillers can be used to reduce the density of the silicone while it’s being moulded, further strengthening the moulded part
• Multiple silicone types can be utilised and tailored to the application conditions and moulding 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 Moulding

• Liquid Injection Moulding

• 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 injection moulding tool, i.e. typically one million shots

• Injection Moulding

• 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 moulding

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

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

Advantages of compression moulding:

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

Disadvantages of compression moulding:

• Slower part production rates
• Involves largely manual process steps
• Requires post-moulding operations to remove flash
• Precision is good but limited to a normal level for rubber parts
• Largely used for simple to moderately complex shapes with no undercuts

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

3. Extrusion

Silicone is well established as a completely inert, biocompatible and very versatile material in medical extrusion.  In medical devices, both peroxide and increasingly platinum-cured silicone grades enjoy increasing popularity, the latter due to its increased purity and faster production cycle.  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. PEEK is an excellent alternative to stainless steel because it is very strong and has a low friction coefficient. Similarly, both PEEK and polyphenylsulfone (PPSU) are used for long-term implantable components because of their biocompatibility.

Advantages of extrusion

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

Disadvantages of extrusion

• Difficult to predict the exact degree of expansion
• Subject to size variances
• Some product limitations

4. Multiple-Profile Extrusion (MPE)

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. The absence of a seam also greatly enhances product performance, mitigating areas where bacteria can accumulate.

With MPE, there is no need for secondary bonding, thereby 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, two or more lumens can easily be split off a centre 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

•       Removes secondary bonding operations

•       Allows for 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

Conclusion

The ability of silicones and thermoplastic polymers to be formulated and processed to attain specific performance, aesthetic, or therapeutic outcomes makes them ideally suited for many medical devices. Device designers and makers – either at OEM or CMO basis – need to have a basic understanding of the diversity of processing options available, or, better yet, bring on board from the early concept stage of a new device a processing expert for silicone and other polymer components. With time-to-market being such a critical element 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.