The European Commission defines personalized medicine as, “a medical approach tailored to the patient or a group of patients for prevention, prediction and treatment . . . that moves away from the ‘one size fits all’ medical model.”
Continued advancements in personalized medicine are making it easier for patients to be diagnosed and treated in the comfort of their own homes using highly complex, tailored treatments and devices. Increased availability and home delivery of healthcare devices and services, along with increased demand from aging populations, are pushing the market and expanding opportunities for medical device manufacturers. These advancements put pressure on device manufacturers to accelerate the design, production, and testing of high-reliability products and to get to market quickly, efficiently, and cost-effectively in a heavily regulated environment.
As the medical device industry continues to drive the development of more complex and revolutionary devices, it is also driving continued innovation in the plastics joining processes that are integral to production of everything from micro-fluidic drug-delivery systems to in vitro diagnostic tests, infusion pumps, and monitors for home patient care. Choosing the best method of assembly for a plastics application is crucial when reliability, performance, quality, and regulatory compliance are all mandatory for market success.
When selecting a technique or supplier for device assembly, it's imperative to examine and compare all the options to determine which may be best for a given application. It's necessary to understand the advantages and limitations of each process available and to work closely with equipment or system providers that have the technical expertise required to develop solutions that work for the application and production requirements.
While it can be daunting, the best bet in selecting a process for a given application is to go into the decision-making process with an open mind and be “process neutral.” Working with a supplier who does not favor one technology over the other at the outset also can provide the advantages of reduced time to market, lower costs, and improved product reliability.
A Growing Range of Plastics Joining Processes
Many joining technologies exist for medical device applications. While ultrasonic plastic welding is currently the leading process used for medical device production, there are a growing range of other plastic joining technologies whose capabilities are opening the doors to new advances in device design. And, for many applications, utilizing a combination of these plastic joining techniques often provides the best solution.
Plastics joining technologies include ultrasonic welding, vibration welding, clean vibration technology (CVT), clean laser technology (CLT), and clean infrared technology (CIT). All of the “clean” joining technologies are specially designed to minimize the generation of flash and particulates during the joining process (see Table 1).
Joining processes also can support ecological standards and requirements for recyclability, eliminate the need for chemical solvents and adhesives, and deliver high energy efficiency.
Ultrasonic Plastic Welding
Ultrasonic plastic welding is an extremely cost-effective and popular assembly method whose benefits include speed (most welds take less than a second), no need for added-cost consumables, minimal or no setup time, low cost of capital equipment, and easy integration with automated processes.
It utilizes a series of components — power supply, converter, booster, horn (or Sonotrode), and actuator — to deliver mechanical vibration and force to the mating parts that are held in weld tooling.
How Ultrasonic Plastic Welding Works
As seen in Figure 1, the power supply takes a standard electrical line voltage and changes it to an operating frequency (i.e., 20 kHz), which is sent through an RF cable to the converter. The converter, in turn, converts this electrical energy to mechanical vibrations at the operating frequency of the power supply. The amplitude of these vibrations is based upon the thermoplastic materials being welded.
In operation, mechanical vibrations are delivered to the parts to be welded, which are put under a mechanical load using an actuator that holds the booster and horn (or Sonotrode) [see Figure 2]. Under this load, the mechanical vibrations are transmitted to the interface between the material surfaces, which focuses the vibration to create intermolecular and surface friction. This friction creates heat and a subsequent melt, which solidifies into a welded bond.
The controlled mechanical vibration generates intermolecular and surface frictional heat at the interface of the mating surfaces, melting the plastic and creating a strong bond.
In medical devices, high precision and perfectly clean joints are typical requirements. Other joining methods can have drawbacks. Adhesives, for example, have much longer setup and processing times and can cause contamination, especially at the micro level where many of these devices are operating.
Ultrasonic welding can offer significant advantages for assembly of surgical instruments that are typically joined by screws and solvents. It uses the device material itself to form an immediate bond and does not introduce glues or adhesives into the device, eliminating consumables. It also offers a time savings in production compared to gluing, after which parts typically require curing time.
Clean Vibration Welding
Clean vibration combines infrared and vibration processes, offering more options and applications for smart molding joint design. The major advantage of vibration lies in its application to large (up to 1500 mm long and 700 mm wide) irregularly shaped parts. Even cross-ribs that create separate compartments can be sealed. The process also works with multiplane and curved surfaces. Vibration welding offers the capability of joining more than one part at a time and also readily lends itself to automation.
The advantage of “clean” vibration welding begins with the preheating of the part surfaces with an infrared heat source. This preheat can minimize particles generated during the vibration weld phase and produce clean, high-strength joints while reducing residual stress material-specific friction, and welding time for each part.
Vibration or clean vibration technology is typically used in manufacturing large two-part systems such as patient monitors, infusion pumps, or fluid collection vessels.
Clean Laser Welding
Clean laser welding of plastics is an innovative welding technique that passes laser energy through one plastic component (the transmissive part) that is absorbed by a mating component (the absorptive part). This absorption results in heating and melting of the material interface, enabling the parts to be permanently joined by the application of controlled clamping 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 can ensure attractive, reliable hermetic sealing in a single step that only takes a few seconds.
Laser is the technology of choice for joining plastics for in vitro diagnostics (IVD) devices and microfluidic devices. These include devices used in diabetic care management or peritoneal dialysis, where internal fluid pathways are very small, necessitating a joining process that generates zero particulate or flash and protects delicate structures.
Clean Infrared Welding
Clean infrared technology (CIT) can be used for clean joining of small, medium, and large parts. Precise plasticization occurs using noncontact heat input by medium-wave, metal-foil emitters that emit the same wavelength spectrum as the absorption range of most common thermoplastics. During the CIT process, the two-part halves to be joined are held in position a few millimeters from the metal-foil-emitter platen that follows the contoured profile of the weld seam. The platen uniformly pre-heats the weld area only, without risk of damage to pre-assembled inner parts. Once plasticization has occurred, the platen is removed, and the halves are brought together under pressure and allowed to resolidify, producing a clean, clear, weld that is virtually particle free.
How to Choose the Right Process
Although the many options available can make it difficult to determine which process is best for a particular application, the following thought process should work for a majority of applications.
The first consideration is material. Some materials are more readily compatible with a given process. Polyolefins, for instance, are somewhat limited when used with ultrasonic welding, but are recommended for all of the other processes. Ultrasonic welding is not recommended for use with thermoplastic rubber or thermoplastic elastomers (TPRs/TPEs), yet has limited capabilities in some applications and is recommended for others.
The second consideration is part geometry, which starts with the size of the part. One of the limitations of ultrasonic welding is the size of tooling. As a rule, the lower the frequency (20 kHz), the larger the tool (approximate maximum, 20 × 20 cm) while higher frequency processes (40 kHz) are limited to smaller tooling sizes (approximate maximum 6 × 6 cm) If the parts are larger than these ranges, it is necessary to consider either multiple units with ultrasonic welding or another joining process.
A second factor in part geometry is the complexity of the part and weld profile. Some assembly processes can accommodate part features easily while others cannot. Wall thickness and internal walls must also be considered. Clean vibration welding, due to its reciprocating motion, has difficulty welding long unsupported walls, while other technologies do not have an issue with these part features.
Production volumes cannot be overlooked. Some processes, like ultrasonic, can bond assemblies in fractions of a second, while other may take considerably longer. In some instances, it is possible to weld multiple parts in a single cycle to improve 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 the process selected is based on initial price, the decision may not have considered the long-term product or application development: 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 can be a valuable asset within the overall product development strategy. The “total cost of ownership” for the assembly process, including direct and indirect costs, should be considered.
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 that the appropriate joining technique is chosen. Again, start with all of the options by taking a “process neutral” approach.
This article was written by Tom Hoover, Sr. Market Segment Manager, Medical, Branson at Emerson, Danbury, CT. For more information, visit here .