When designing an assembly method for the plastic components in a medical device, stay process-neutral and weigh the advantages and disadvantages of the available methods.
As the medical device industry continues the rapid development of complex, innovative products, the need for joining plastic components is keeping pace. These requirements range from micro-scale tubes to macro monitors and pumps. Choosing a best method of assembly for plastic applications is crucial when reliability, performance, quality, and regulatory compliance are essential.
When selecting a technique or supplier for assembly requirements, it’s imperative to examine and compare all options to determine which may be best for a given application. It’s necessary to understand the advantages and limitations of each process, and work closely with equipment or system providers with the technical expertise to develop an application-specific solution.
Selecting a joining method can be overwhelming, so the best application choice is to enter the decision-making process with an open mind and be process-neutral. Working with a supplier who doesn’t favor one technology over the other at the outset can provide the advantages of reduced time-to-market, lower costs and improved product reliability.
Weighing the welding methods
There are many joining technologies for medical devices. Although ultrasonics is the dominant technology, others, such as laser systems, open the door to new opportunities. For many applications, a combination of techniques provide the best solution.
Technologies for joining plastics include ultrasonic welding, vibration welding, hot-plate welding, spin welding, and thermal processing. In addition, clean-joining techniques, which include laser welding, infrared (IR) welding, and vibration-plus-IR welding, are specially designed to minimize flash and particulates.
Joining methods also can support ecological standards and requirements for waste management, eliminate the need for chemical solvents and adhesives, and be highly-energy efficient.
Ultrasonic assembly is a cost-effective and widely used technique with the benefit of speed – most welds take less than a second, need no consumables, require minimal or no setup time, and offer low cost of capital equipment and easy integration into automation.
Ultrasound assembly uses a series of components, including power supply, converter, booster, horn (or stack), and actuator, to deliver mechanical vibration and force to the parts. This generates heat at the interface of the mating surfaces, melting the plastic and creating a strong bond.
Medical devices typically require high-precision, perfectly clean joints. Other joining methods have drawbacks: Adhesives, for example, have much longer set-up and processing times and can cause contamination during operation, especially at the micro level.
Ultrasonics can offer significant advantages for assembling surgical instruments that are typically joined by screws and solvents. The method uses the device material itself to form a bond and does not introduce glues or adhesives into the process, eliminating consumables. It also cuts time in production, compared to gluing.
Minimally invasive surgical instruments, as well as catheters, cannulas, luers, and trocars, use ultrasonic joining.
Clean laser technology. Laser welding of plastics is an innovative technique based on the principle of passing laser energy through one plastic component, the transmissive part, that is absorbed by the second component, the absorptive part. This absorption results in heating and melting of the interface. The parts are joined with the application of a controlled clamp force.
Laser welding is a gentle and clean joining process that enables welding of complex geometries and materials that are difficult to bond with other techniques. Laser welding ensures attractive, reliable hermetic sealing in a single step that only takes a few seconds.
What’s more, laser is the technology of choice for joining plastics for in vitro diagnostic devices, as well for as microfluidic devices. This includes those used in diabetes care, where fluidic pathways are small and a primary consideration is to have zero particulate or flash, and protect delicate structures.
Vibration welding is a proven friction technique capable of producing strong, pressure-tight joints in the thermoplastic parts.
The major advantage of vibration welding is in its application to large (up to 60-in.-long or 24-in.-wide), irregularly-shaped parts. Cross-ribs, which create separate compartments, can even be sealed. The process also works with multi-plane and curved surfaces.
Vibration welding can be used to weld more than one part at a time and also readily lends itself to automation. Two motion path options are employed in vibration welding:
- Linear vibration welding uses transverse, reciprocating motion: The vibration occurs in only one axis. The vibration motion occurs equally in both the x and y axes and all axes in between.
- Orbital vibration welding uses constant velocity motion: A non-rotating, offset, circular motion in all directions.
The clean vibration method combines infrared and vibration processes, offering more options and applications for smart molding joint design. Pre-heating the surface with infrared minimizes particles generated during the vibration weld phases and produces clean, high-strength joints, which reduces residual stresses and material-specific friction and welding time.
This technology is typically used in manufacturing large, two-part systems such as patient monitors, infusion pumps, or fluid collection vessels.
Spin welding is a friction process that joins circular thermoplastic parts by bringing the part interfaces together under pressure with a circular, spinning motion. One part is held stationary in a fixture, while the other rotates against it under pressure. The frictional heat generated causes the part interfaces to melt and fuse together, creating a strong, hermetic seal. The lower-mass half is usually located in the upper spinning portion, but exceptions are made based on part geometry.
Surgical trocars – which function as portals for the placement of instruments such as graspers, scissors, or staplers – lend themselves to spin welding because of their ports’ round geometry.
Choosing the right process
Even though the many options can make it difficult to determine which process is best for a joining application, here is a thought process that should work for a majority of applications:
- The first consideration is material. Some materials are more compatible with a given process. Polyolefins, for instance, are somewhat limited in ultrasonic welding, but are recommended for all of the other processes. Ultrasonics is not recommended for TPRs/TPEs and has limited capabilities in some applications, but is recommended for others.
- The second consideration is part geometry. Start with the size of the part. One of the limitations of ultrasonics is the tooling size. The lower the frequency (15 kHz) used, the larger the tool (approximate maximum: 10 x 10 in. (25 x 25 cm). The higher the frequency (40 kHz), the smaller the tool (approximately 2.5 x 2.5 in. or 6 x 6 cm). If the parts are larger than these ranges, you’ll have to consider using either multiple hits with ultrasonics or another assembly process.
• Delving further into part geometry, we come to the complexity of the part and weld profile. Some assembly processes accommodate certain features easily and others cannot.
- Other considerations are wall thickness and internal walls. Vibration welding, due to its linear reciprocating motion, has difficulty welding long, unsupported walls, while other technologies do not have an issue with these features.
- Another important aspect is production volume. Some processes, such as ultrasonic, spin, and laser welding, will process assemblies in seconds or faster, while hot-plate welding may take 40 to 50 seconds. In some instances, welding multiple parts in a single cycle improves throughput.
- Capital equipment cost should be the last consideration, although that may be easier to recommend than to put in practice. Keep in mind that if you select a process based on initial price, you may not have considered long-term product or application development. This includes time to market and processing costs such as scrap, downtime and mold changes.
Choosing a material joining equipment supplier with a broad portfolio of technologies, application engineers and experience is a valuable asset within an overall product development strategy. Also consider the total cost of ownership for the assembly process, including direct and indirect costs.
For new or modified medical devices, all parameters, including design, materials, prototypes, and product performance, as well as processing time and costs, should be thoroughly evaluated to ensure choosing an appropriate joining technique. The best way is to start by taking a process-neutral approach that considers all options.