Identifying the best thermoplastic process method to produce thermoplastic medical device components is just as important as choosing the appropriate material. With many different processes available, factors such as design parameters, material properties, and equipment capabilities come into play. The careful selection of thermoplastic process technologies can help device makers create innovative designs, reduce manufacturing costs, boost quality, and accelerate time to market.
Major resin suppliers can be a valuable resource. They typically have extensive knowledge of processing methods and their suitability for different thermoplastics and applications. These companies can draw on cross-industry experience to benefit medical device manufacturers.
Based on the perspective and experience of a global thermoplastics supplier, this article examines criteria for choosing among processing methods used in medical device manufacturing. They range from established molding techniques to new and emerging technologies.
Supplier guidance on processing selection, optimization
Each processing technology has advantages and disadvantages. The key is determining the best method for a particular application according to parameters such as performance and regulatory requirements, design specifications, and physical and mechanical properties of the material, aesthetics, production volume, secondary operations, and capabilities of the molder.
All these considerations can make it complicated to find the best processing method. Suppliers can provide guidance and resources to help customers determine which method to use and optimize it for the specific application. They may be able to offer flow and structural simulation, application testing capabilities, laboratory facilities, and material-specific performance data.
Further, material suppliers are helping develop new processing methods that can overcome the limitations of current technologies and open the door to novel applications.
Standard injection molding
Injection molding is a long-standing processing method that remains very popular for medical device manufacturing because of its versatility, dimensional accuracy, and high productivity. Injection molding can be used for applications ranging from surgical tools such as staplers and trocars to diagnostic equipment housings.
It can also help device makers meet healthcare challenges such as:
- Improving patient outcomes: For drug-delivery devices with complex moving parts, injection molding provides high precision for accurate dosing.
- Reducing costs: Through part consolidation and elimination of secondary operations, injection- molded designs can help lower the cost of components such as chassis and frames in capital equipment.
- Increasing usability: Thin-wall injection molding reduces weight in portable medical devices so clinicians and patients can transport and operate them more easily.
One consideration for injection molding is system cost. Although mold tools require an upfront investment, this expense is typically offset by advantages such as high-speed, high-volume production. Further, advances in mold building and new material options for molds such as aluminum offer less-expensive alternatives. Finally, even at lower production volumes it’s possible to benefit from part consolidation, dimensional accuracy, and repeatability.
Another factor is the choice of many different resins and compounds. Options include polyetherimide (PEI), polycarbonate (PC) and PC/acrylonitrile-butadiene-styrene (PC/ABS) resins, and modified polyphenylene ether (PPE) materials for foam molding. Specialty compounds expand material choice by offering properties such as inherent lubricity or high strength and stiffness.
If injection molding is the chosen method, the next step is optimizing processing parameters for the part. Variables such as temperature, pressure, and resin moisture content, along with design specifications and equipment capabilities, affect throughput and quality. 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 complex geometries.
Process optimization using a material supplier’s resources can potentially save time and help avoid costly mistakes. Major suppliers understand the capabilities and limitations of existing injection molding technology and can use their engineering resources to overcome these hurdles.
Specialized injection molding
Advanced injection molding techniques can meet the specialized performance, aesthetic, and processing requirements of medical devices.
Gas-assist injection molding/structural foam: This involves injecting pressurized nitrogen into the mold’s interior. 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 molding may also reduce sink marks, improving surface quality. Complex geometries that can’t be created with a single-part, conventional molding process can benefit from this technology. It also offers economic advantages by requiring less material and accelerating throughput.
Gas-assist injection molding can improve medical device usability through weight reduction and ergonomic design. Typical applications include surgical tools used for retraction and impaction currently made from heavier stainless steel. The gas-assist process can eliminate the need for external ribbing, offering a smoother surface that’s easier to clean.
Materials well suited for gas-assist injection molding include PC copolymers and PPE resins. Processing considerations include locating gas channels correctly and adjusting to faster cooling resulting from hollow sections.
Structural foam processing can be confused with gas-assist molding because both reduce part weight. However, it’s a different approach. During the injection phase, gas under pressure is introduced and allowed to expand in the final part geometry. Expansion is often accomplished through chemical foaming agents, resulting in low-pressure molding conditions that can save on tooling costs and permit larger parts and better structural performance compared with traditional injection molding. However, without special attention, most structural foam parts require secondary operations, such as painting, to achieve an acceptable appearance.
Heat-cool molding: The temperature of the mold tool plays a key role in the surface quality of injection molded parts. Heat-cool molding technology thermally cycles the tool’s surface temperature within the injection molding process. 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 allows glass-reinforced materials to be used for parts that require a high-gloss finish by creating a resin-rich surface that can help avoid secondary painting. In addition to providing a competitive advantage, an attractive surface can increase the appeal of a home-use device, potentially improving a patient’s usage.
This method also reduces stress within the part, helping it resist cracking when exposed to aggressive healthcare disinfectants. Resins suitable for heat-cool molding include glass-filled PC and PEI and specialty compounds. A material supplier with established global application development centers can help customers use heat-cool molding effectively.
Overmolding (insert molding or two-shot injection molding): When a part’s requirements can’t be met by a single thermoplastic, two or more parts can be combined using mechanical fasteners, solvent, or adhesive bonding; thermal or laser welding; and press/snap fit assembly. However, these secondary operations can add costs and affect productivity.
Alternatively, overmolding a thermoplastic elastomer (TPE) 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. Overmolding using insert or two-shot molding techniques can improve a medical device’s ergonomics, safety, or aesthetics. Different sensory effects from the elastomer (grip, tactile feel, texture) give device designers a wider array of options.
Applications can include the handles of surgical tools or portable devices for comfort, and non-slip areas of durable medical equipment for patient safety and stability.
Successful overmolding must accommodate the potentially different shrinkage characteristics of the two materials. Choosing a higher-modulus substrate and providing stiffening ribs can help mitigate shrinkage of the elastomer.
Additive manufacturing
In medical device applications, additive manufacturing (AM) can provide patient-specific customization, exceptional design freedom, and cost benefits by avoiding the need for a mold tool.
Currently, AM is used to develop device prototypes and individual, customized parts. Large-scale production is a goal for the future. One challenge is the need for resins and compounds specifically tailored for this process. Polyetherimide and PC copolymers are among the materials currently being used for AM of medical devices.
Device designers need assurance that their chosen materials meet stringent criteria regarding origin, chain of custody, testing, and regulatory compliance. They must also meet basic requirements for healthcare applications, such as biocompatibility. As new technologies emerge, some suppliers are developing or modifying resins and compounds to improve properties and printing speed.
Suppliers contribute to successful commercialization
Medical device original equipment manufacturers (OEMs), particularly smaller companies and startups, may lack experience in plastic processing and may not fully appreciate how it affects part design and material choice. Device makers can benefit from collaborating with a material supplier early in the design process to select the best processing method.
When choosing a supplier, device companies should look for:
- Expertise in different processing methods
- Range of molding equipment and tooling
- Broad portfolio of healthcare materials
- Collaborative approach
- State-of-the-art material, processing, and application testing capabilities
- Analytical tools such as mold flow, stress, and cost analysis
- Understanding of regulations and environmental requirements
Which manufacturing method?
Device companies can choose from many thermoplastic processing methods, from classic injection molding to AM. These technologies vary widely in their demands, parameters, advantages, and challenges. Collaboration early in the design phase involving the OEM, a knowledgeable and experienced material supplier, and the molder or 3D printer can be invaluable in successfully optimizing devices that can withstand a variety of challenging medical environments.
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