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Sabic's guide to processing medical plastic

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Ashir Thakore, global segment leader - healthcare, Sabic explains how to choose the optimal plastic processing method for medical device applications.

Credit: Sabic

Finding the right thermoplastic materials for medical device applications is crucial to the entire product lifecycle, from design through use, reuse and disposal. However, it’s just as important to identify the best processing technology for each part or component, based on factors such as design parameters, material attributes and equipment capabilities. Optimised processing can help contribute to the overall success of a medical device by accelerating time to market, offering companies a more competitive advantage, helping reduce costs, promoting more consistent quality and helping to enable novel designs.

Major suppliers of thermoplastic resins can help in this area. Not only are they experts in their own materials but they also have in-depth processing knowledge and experience from across multiple industries that can benefit medical device companies. Tapping into this processing expertise, which is typically derived from a combination of in-house research and testing and application development work with customers, can help original equipment manufacturers (OEMs) optimise manufacturing.

Leveraging the perspective and experience of a global thermoplastic materials supplier, this article presents considerations for evaluating different processing methods for medical devices, from established moulding techniques to new and emerging technologies.

Processing pros and cons

There are advantages and disadvantages to each type of plastic processing. The challenge is determining the best method for a particular application according to parameters such as desired part and device performance, design specifications, physical and mechanical properties of the material, aesthetics, production quantity, secondary operations and capabilities of the moulder.

This long list of variables can make the selection process complicated. Guidance is available from major thermoplastics suppliers, such as Sabic, which offer resources to help customers pinpoint the appropriate processing method and optimise it for the application. These resources may include processing equipment, application testing capabilities, laboratory facilities and material-specific performance data.

Besides assisting with existing processing technologies, materials suppliers are working on enhancements and refinements. They are also helping to develop new methods that can surmount current economic or technical limitations and help enable production of novel parts.

Standard injection moulding

Injection moulding, a longstanding processing method, remains very popular for medical device manufacturing because it offers versatility, dimensional accuracy and high productivity. It can be used for a variety of applications ranging from surgical devices, such as staplers and trocars, to housings for major diagnostic equipment.

Along with these advantages, injection moulding can help device makers address some of the top healthcare challenges:

Considerations for injection moulding start with system cost. Although injection moulding tools may cost more up front, this expense is typically offset by the economic advantages of high-speed, long tool life and high-volume production.

Another factor is design freedom. Device designers can choose from the many different resins and compounds that are appropriate for injection moulding. These range from polycarbonate (PC) and PC/acrylonitrile-butadiene-styrene (PC/ABS) to modified polyphenylene ether (PPE) for foam moulding. Specialty compounds, such as those that offer inherent lubricity or stiffness under demanding conditions such as repeated autoclaving, provide even more choices.

Once an OEM decides on injection moulding, it is important to optimise processing parameters for the given part. Variables such as temperature, pressure and resin moisture content, along with design specifications and equipment capabilities, affect throughput and quality. For instance, careful analysis and testing are needed to ensure part-to-part consistency in multicavity tools, control warpage in thin-wall and extended-flow areas, and completely fill intricate or complex features.

This is where a material supplier’s investment in process development can pay major dividends to the device maker — potentially saving time and avoiding expensive mistakes. Major suppliers understand the capabilities and limitations of existing injection moulding technology and can use their engineering resources to overcome these barriers, helping customers achieve their goals.

Specialised injection moulding

Advanced injection moulding techniques are available to meet the specialised performance, aesthetic and processing requirements of medical devices.

This process enhancement to conventional injection moulding involves injecting pressurised nitrogen gas into the interior of the mould. The gas flows through strategically placed channels to displace resin in thick areas of the part by forming hollow sections. The resulting parts are lighter, with less moulded-in stress, more-uniform wall thicknesses and better dimensional stability. Gas-assist injection moulding may also reduce sink marks, producing a high-quality surface finish. Complex part geometries that cannot be created in a single-part conventional moulding process can benefit from this technology. This type of injection moulding can offer economic advantages by requiring less material and shortening cycle times to help increase productivity.

Gas-assist injection moulding can be an excellent technique for improving medical device usability through weight reduction and ergonomic design. Typical applications include surgical tools used for retraction and impaction that are currently made from stainless steel. In addition to better ergonomics, gas-assist injection moulded parts can eliminate the need for external ribbing, offering a smoother surface that is easier to clean and less likely to promote build-up of human tissue that can increase the risk of infections.

Processing considerations for gas-assist injection moulding include locating gas channels correctly and adjusting to faster cooling of the part due to hollowed-out sections (cooling takes place from the outside and inside of the part).

Finding the right materials for this niche process involves obtaining advice from the materials supplier, while manufacturing optimisation calls for a collaborative effort by the device company, the moulder and the resin supplier.

The temperature of the mould tool plays an important role in raising the surface quality of injection moulded parts. Heat-cool moulding technology thermally cycles the tool’s surface temperature within the injection moulding cycle. This requires heating the tool surface above the material’s glass transition temperature (Tg) prior to injection using specialised equipment such as superheated water systems or induction coils. After the resin is injected into the cavity, the tool is quickly cooled to solidify the moulded part prior to ejection.

This process enables glass-reinforced materials to be used for parts that require a high-gloss finish by creating a resin-rich surface. Achieving a high-quality surface finish in parts moulded with glass-reinforced materials can help eliminate the need for painting. An attractive surface can increase the appeal of a home-use device for patients and can help it stand out from the competition.

Another important benefit of heat-cool moulding is stress reduction within the part. With less moulded-in stress, the part has better resistance to cracking, especially when it is exposed to chemical cleaning agents used to combat hospital-acquired infections (HAIs). Resins suitable for heat-cool moulding include glass-filled PC and polyetherimide (PEI), as well as specialty compounds. A materials supplier with established global application development centres, such as Sabic, can provide customers with knowledgeable resources to help them use heat-cool moulding more effectively.

If a part’s requirements cannot be met by a single thermoplastic, two or more can be combined using techniques such as mechanical fastening, solvent bonding, welding, adhesive bonding and press/snap fit assembly. However, these time-consuming secondary operations can add costs and affect productivity.

Overmoulding a thermoplastic elastomer or a liquid silicone rubber (LSR) onto a rigid substrate avoids secondary operations and yields a tight bond – as long as the two materials are compatible. It is ideal for enhancing a medical device with better ergonomic performance, greater safety or improved aesthetics. Different sensory effects from the elastomer (grip, tactile feel, texture, etc.) give device designers a wider array of options. Specialised materials for noise and vibration damping are also available, and many of these materials come in a wide range of colours.

Some potential application examples include the handles of surgical tools or portable devices (such as defibrillators) for comfort and non-slip areas of durable medical equipment (walkers, canes, etc.) for patient safety and stability.

Unlike conventional design and processing, successful overmoulding must accommodate the potentially different shrinkage characteristics from the two materials. Significant shrinkage of the elastomer can be partially mitigated by using higher-modulus substrate materials and providing stiffening ribs in the substrate.

Additive manufacturing

Additive manufacturing is a disruptive technology that produces three-dimensional solid objects from a digital file by depositing successive layers of plastic. Its potential advantages for medical devices include patient-specific customisation, lower costs by avoiding the need for a mould tool and exceptional design freedom. The healthcare industry is benefiting from breakthrough parts produced by additive manufacturing – such as models of bones and organs for diagnosis or guidance in complex surgery, custom-designed casts and prosthetics, and medical and dental implants.

The use of additive manufacturing for medical devices is currently focused on prototyping and individual, customised parts. Large-scale production is a possibility for the future. One challenge is the need for resins and compounds that are specifically tailored for this process. Medical device designers need assurances that their chosen materials meet stringent criteria regarding origin, chain of custody, testing, regulatory compliance and the supply chain. Of course, these materials must also meet basic requirements for healthcare applications, such as  biocompatibility and sterilisation compatibility and must be suitable for additive manufacturing methods.

To help address these requirements, some material suppliers are developing or modifying resins and compounds for additive manufacturing. They are working to deliver improvements in speed and material properties – for instance, to improve surface finish and aesthetics.

Using its healthcare-grade resins, Sabic has developed Ultem Pei and Lexan PC healthcare filaments for fused deposition modelling to produce printed parts with excellent mechanical performance, sterilisation compatibility and biocompatibility. When they are used for prototyping, application development can become more efficient since the same base resin materials are available in injection moulding grades for production.

Suppliers contribute to processing optimisation

Medical device OEMs, particularly smaller companies and startups, may lack experience in plastic processing. Often, they rely on external partners and may not fully appreciate how the processing method affects design and material choice – and vice versa. Regardless of their level of expertise, device makers can benefit from collaborating with a materials supplier early in the design process to optimise manufacturing and help avoid costly errors and rework. When choosing a supplier, device companies should look for capabilities such as:

Which manufacturing method?

Today, device companies can choose from many different thermoplastic processing methods to mould their products or components, from classic injection moulding to newer additive manufacturing. These technologies vary widely in their demands, parameters, advantages and challenges. A strong collaboration involving the OEM, a materials supplier with a broad portfolio and deep processing knowledge, and the moulder can be invaluable in finding the optimal part design and process for a given component.

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