Engineering thermoplastics are widely used to manufacture a broad range of medical devices, from single-use syringes and applicators to diagnostic and monitoring equipment. One critical aspect of using thermoplastics effectively in devices is optimized manufacturing, which involves identifying the best molding process for the application; understanding and accommodating the specific attributes of the chosen material and equipment; and finding ways to meet the design intent while controlling costs and maintaining high quality.
Improved manufacturability of a device can help OEMs achieve strategic goals such as speed to market, cost reduction, extended useful life under demanding conditions (e.g., exposure to harsh cleaning agents), and competitive differentiation.
While thermoplastic resin suppliers are expected to provide innovative materials, their expertise in process technology — gained from working on application development with customers and from in-house R&D — is a valuable additional asset. Knowledgeable suppliers can make a significant contribution to the success of the final device. This article, based on that perspective and experience, offers guidance on different processing options when using engineering thermoplastics to manufacture medical devices and components.
Traditional and Emerging Processes
Each type of plastic processing has advantages and disadvantages. The challenge is finding the best method for the application, based on requirements such as desired part and device performance; design parameters and complexity; material properties; weight and aesthetics; high- or low-volume production; and secondary operations.
Major thermoplastics suppliers, such as SABIC, offer resources to help customers identify the appropriate processing method and to optimize part manufacturing. These tools may include processing equipment, application testing capabilities, laboratory facilities, and material-specific performance data. Further, suppliers continue to spearhead improvements to existing manufacturing methods and develop new ones to overcome economic or technical limitations of current manufacturing, and support production of novel parts.
Conventional Injection Molding
While injection molding has been used for decades, it remains extremely popular for device manufacturing, thanks to its exceptional versatility, dimensional accuracy, and productivity advantages. Injection molding can be used for parts as diverse as single-use devices (staplers, trocars, clip applicators, etc.) and large panels for diagnostic equipment. The capabilities of injection molding help device makers address top healthcare trends, including:
Improved patient outcomes: For example, drug-delivery devices such as injection pens and auto injectors have complex moving parts. Injection molding delivers the high precision needed for accurate dosing, which promotes positive outcomes.
Cost reduction: By consolidating parts and avoiding secondary operations, molded designs help to lower the cost of components such as chassis and frames used for capital equipment.
Usability: Thin-wall injection molding and metal replacement reduce weight in portable medical devices, making them easier for clinicians and patients to transport and operate.
Considerations for injection molding parts begin with system cost. Injection molding tools may require higher up-front investments to produce, but their cost is typically offset by the economic advantages of high-speed, high-volume production.
Design freedom is another consideration. Many different resins and compounds are appropriate for injection molding, giving device designers greater flexibility. These range from polycarbonate (PC) and PC/acrylonitrile-butadiene-styrene (PC/ABS) to modified poly-phenylene ether (PPE) for foam molding. Specialty compounds, such as those that offer lubricity or stiffness under demanding conditions, including repeated autoclaving, give designers even more choices.
Optimizing processing parameters such as temperature, pressure, and resin moisture content for high throughput and quality depends upon material properties, part design, and equipment. For instance, careful analysis and testing are needed to ensure part-to-part consistency in multicavity tools, control warp-age 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. Based on broad and deep experience, major suppliers understand the limitations of existing injection molding capabilities and can use their engineering resources to push beyond these barriers, helping customers achieve their goals.
Specialized Injection Molding
Beyond conventional injection molding, advanced injection molding techniques are available to meet the specialized performance, aesthetic, and processing requirements of medical devices.
Heat-Cool Molding. Tool temperature plays an important role in raising the surface quality of injection molded parts. Heat-cool molding technology thermally cycles the surface temperature of the tool within the injection molding cycle. This requires heating the tool surface above the material's glass transition temperature (Tg) prior to injection using specialized 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 molded part prior to ejection.
Heat-cool molding 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 molded with glass-reinforced materials can help eliminate the need for painting. An attractive surface can increase a home-use device's appeal to patients and can help it stand out from the competition.
Another important benefit of this process is stress reduction within the part. With less molded-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 molding include glass-filled PC and polyetherimide (PEI), as well as specialty compounds. A materials supplier such as SABIC, for example, with established global application development centers, can provide customers with a knowledgeable resource to help them use heat-cool molding effectively.
Gas-Assist Injection Molding. Gas-assist injection molding is a process enhancement to conventional injection molding. It involves injecting pressurized nitrogen gas into the interior of the mold. 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 molded-in stress, more-uniform wall thicknesses, and better dimensional stability. Gas-assist injection molding may also reduce sink marks, producing a high-quality surface finish. Complex part geometries that cannot be created in a single-part conventional molding process can benefit from this technology. This type of injection molding can offer economic advantages: less material is needed and, because it is a high-speed process, cycle times are shorter, helping to boost productivity.
Gas-assist injection molding can be an excellent technique for improving medical device usability through weight reduction and ergonomic design. Typical applications include the myriad of surgical tools used for retraction and impaction that are currently made from stainless steel. In addition to better ergonomics, gas-assist injection molded 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 molding 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 requires advice from the material supplier, while manufacturing optimization calls for a collaborative effort by the device company, the molder, and the resin supplier.
Overmolding (Two-Shot Injection Molding). When a single thermoplastic cannot meet a part's requirements alone, 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 add costs and affect productivity.
Overmolding a thermoplastic elastomer or a liquid silicone rubber 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 wide array of options. Specialized materials for noise and vibration damping are also available.
Application examples include the handles of surgical tools for portable devices (such as defibrillators) for comfort, a firm grip for vibration damping, and non-slip areas of durable medical equipment (walkers, canes, etc.) for patient safety and stability.
Unlike conventional design and processing, successful overmolding 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.
Injection molding and its variations provide many manufacturing possibilities to match the specific requirements of devices and components. However, new methods are also rapidly gaining ground. One of the most intriguing is additive manufacturing.
This disruptive technology produces three-dimensional solid objects from a digital file by depositing successive layers of plastic. Its advantages for medical devices include patient-specific customization, lower costs by avoiding the need for a mold tool, and exceptional design freedom. The news is filled with examples of breakthrough parts produced by additive manufacturing — models of bones and organs for diagnosis or guidance in complex surgery, custom-designed casts and prosthetics, and medical and dental implants, to name a few.
Additive manufacturing for medical devices is used primarily for prototyping and for individual, customized parts — not yet for large-scale production. One key hurdle 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, including bio-compatibility and sterilization compatibility, and must be suitable for additive manufacturing methods.
To meet these needs, 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 medical-grade materials, SABIC, for example, is developing PC and ULTEM™ PEI healthcare filaments for fused deposition modeling to produce printed parts with excellent mechanical performance, sterilization compatibility, and biocompatibility.
Suppliers Support Optimized Manufacturing
Medical device OEMs may lack experience in plastic processing because in many cases they rely on external partners and may not fully understand exactly 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 material supplier early in the design process to optimize manufacturing and avoid costly errors and rework. When choosing a supplier, device companies should look for capabilities such as:
Broad portfolio of healthcare materials for greater design freedom.
Material testing capabilities.
Range of molding equipment and tooling.
Staff expertise in different molding methods.
Collaborative approach to helping designers with their specific applications
State-of-the-art processing and application testing capabilities to support prototype development.
Analytical tools such as mold flow, stress, and cost analysis.
Regulatory compliance expertise.
Navigating the Manufacturing Maze
Today, device companies can choose from a growing array of thermoplastic processing methods to mold their products or components. From classic injection molding to newer additive manufacturing, these technologies vary widely in their demands, advantages, and disadvantages. To identify the optimal part design and process for a given component, a strong collaboration involving the OEM, a material supplier with a broad portfolio and deep process knowledge, and the molder can be invaluable.
This article was written by Ashir Thakore, Global Segment Leader – Healthcare, SABIC, Wixom, MI. For more information, Click Here .