In recent decades, plastics have made healthcare simpler, less difficult, and make new techniques and prostheses possible. Therefore, it is more important than ever for medical device and component manufacturers to understand some of the latest advancements in plastic joining or welding technologies, which can assist them in developing new products.

Fig. 1 – Typical Distance vs. Time Graph for servo welder.
Two new technology advancements in plastic welding equipment to support medical device manufacturers are servo-driven ultrasonic welders and the recent introduction of two-micron lasers for welding optically clear plastic assemblies. Ultrasonic welders that utilize servo-controlled motion versus pneumatically driven welders offer unprecedented control of material flow during the welding cycle and result in significantly improved process repeatability. The recent introduction of two-micron lasers in plastic welding allow for a highly controlled melt through the thickness of unfilled and optically clear plastic parts without the need of expensive laser sensitive additives. This has resulted in a vastly improved and simplified process for laser welding of clear polymers, which make this most advanced assembly method available for medical device manufacturing.

Servo-Driven Ultrasonic Welding Systems

Ultrasonic welding has been a widely used technique to join plastic components in medical devices for decades. Older methods of welding plastic components using pneumatically driven ultrasonic welders may not provide the consistency and quality for most of today’s complex medical devices and components. The medical assembly industry requires a need for strong, dimensionally consistent parts that show good cosmetics. The process used to meet these increasing demands must be consistent and repeatable over time. Servo-driven ultrasonic welding technology is designed to meet this demand. This technology allows unprecedented control of material flow during the welding cycle and results in significantly improved process repeatability.

In the ultrasonic welding process, there are three fundamental process variables that have a direct effect on weld quality: amplitude, force, and duration. The first of these parameters, amplitude, has long been controlled through frequency selection, horn-booster design and modulation of the electrical input to the transducer.

The second of these parameters, duration, could only be controlled only by setting a specific weld time and not providing a closed loop controlled process. In 1988, ultrasonic welding equipment was revolutionized by the development of welding by distance, thus allowing greatly improved troubleshooting and process control.

A new precise method of additional process control was introduced with servo-driven ultrasonic welder technology in 2009. This new technology allows complete control of the material flow within the melting zone, directly controlling the squeeze flow rate of the molten material. In recent years, there has been a plethora of research conducted with the new servo-driven ultrasonic welders. Each of these experimental studies has demonstrated unique benefits of using servo-driven ultrasonic welders.

Advanced control capabilities offered by the servo-driven system, specifically, the ability to insure the presence of molten material in the contact area before applying the weld force, allow the user to control melt propagation into mating surfaces to a desired depth. This is achieved by programming the electric servo-drive to hold the press position on the assembly at the initiation of welding cycle until a drop in force is detected. When the magnitude of the force drop reaches a user programmable value, expressed as a percentage, the downward movement of the stack continues. This drop in force indicates the presence of an initial molten layer.

The other important feature of servo-driven systems allows one to control the weld velocity, as explained in Figure 1, directly controlling the squeeze flow of molten material and displacing the material in a highly controlled manner, matching the squeeze flow rate to the speed of melt propagation into the bulk of the material, thus creating the optimum conditions for weld formation and reducing residual stresses in the weld associated with high molecular orientation.

These unique process control features are significantly different than those utilized in pneumatic welders, which can only apply a constant pressure on the weld, regardless of the actual material condition within the weld zone. Based on previous research presented at ANTEC in 2015, this precise control of the welding process results in increased strength and consistency of welded assemblies built using servo-driven welders.

Benefits of Servo-Driven Ultrasonic Welding for the Medical Industry

Servo-driven ultrasonic welding systems have a proven track-record of delivering quality results in welding medical parts such as valves, ports, filters, surgical instruments, and implant devices. The servo process control features confirm when the material in the joint area becomes molten before prompting the press to initiate the weld collapse portion of the weld cycle. This is especially important in welding small parts and assemblies which require a hermetic seal, as it eliminates the risk of deforming the joint due to starting to collapse before the melting of the material has been initiated, which could result in a leak path and “cold-formed” welds.

The ability of the system to detect the presence of molten material enables a user to accurately establish the necessary amplitude required by specific material properties and part configuration, using the welder’s graphical output. This ground-breaking approach for amplitude setting allows the user to apply just the right amount of ultrasonic energy that is needed to initiate the melt, thus avoiding excessive heating and material degradation in the weld. This is especially important for building filter assemblies, where excessive amplitude is associated with tiny pin-holes occurring in the filter media.

In addition, servo technology eliminates the variability associated with pneumatic press components resulting in improved process repeatability and accuracy. This then produces fewer rejects for manufacturers. The elimination of compressed air helps to reduce clean room air filtration and significantly lowers overall manufacturing costs.

Two-Micron Laser Welding Systems to Weld Clear to Clear Plastic

In general, the laser welding technique is dependent on the upper component being transparent to the laser and having the presence of an absorbing agent in the lower component. This configuration limits the process applicability for manufacturing medical devices when a “clear-to-clear” or a “clear-to-colored” assembly is required. Recently, laser welders have successfully overcome this obstacle by integrating a newly developed two-micron laser. This laser type is characterized by the greatly increased absorption by clear polymers and enables a highly controlled melting process through the thickness of optically clear parts. This has resulted in a greatly improved and simplified technique for laser welding of clear polymers for the medical device industry, which now can fully capitalize on the benefits of this advanced Laser assembly process.

Fig. 2 – Laser welded clear-to-clear tube-to-tube and tube-to-port assembly without any laser absorbing additives (photographed in polarized light under microscope).
Laser beam delivery in this system integrates both programmable multi-axes servo gantry and a scan head, supported by advanced software which, harmonizes the action of both components moving the beam. This assures a precise and controllable laser beam delivery when welding mid-size and large components.

Benefits of Two-Micron Laser Welding for Medical Industry

Currently a laser welding based assembly process is the most advanced, flexible, and precise assembly technique for plastic components. This process is very repeatable and allows manufacturers to produce assemblies of better quality in a very efficient way and reduces costs associated with the reject rate. Because of these advantages laser plastic welding is broadly adopted by automotive, consumer goods, and many other industries. However, the adoption of this process by medical device manufacturing to date was very limited, as the process was based on utilizing either carbon black or expensive specially designed laser radiation absorbing agents in order to make laser plastic welding possible. This new method allows the joining of plastics without any additional absorbing agents, which facilitates a laser welding based assembly process for medical device manufacturing, and brings all the advantages associated with it, to medical device assembly.

Fig. 3 – Ultrasonic Welding System with patented Melt-Match® technology and Laser Welding Work Cell.
Typical applications for medical device manufacturers will include welding of medical device components made from unfilled polymers, including the most common assemblies like tube-to-tube, tube-to-port (See Figure 2) and tube-to-cap parts, which until recently, was not possible to join with laser welding. Blood filters, infant care devices, medical appliance components, laboratory ware, housings for surgical devices, and electrical components are also possible to weld using this efficient joining method. To summarize, any medical device or component that is molded in optically clear thermoplastic can be welded without laser absorbing additives.

Whether medical device manufacturers have high-volume automation lines to produce simple subassemblies for disposable IV sets, or the most complex and sophisticated medical devices custom built in single units on pilot plants, these manufacturers need equipment which provides them repeatability, reliability, and strong process control. Providers of plastic assembly solutions to the medical device industry are focusing their efforts to address these requirements by developing the most advanced plastic welding technologies. (See Figure 3)

This article was written by Alex Savitski, PhD, Chief Engineer, Advanced Technologies and Mike Luehr, Applications Technology Manager, Dukane Corporation, I.A.S. Division, St. Charles, IL. For more information, Click Here .


Medical Design Briefs Magazine

This article first appeared in the April, 2016 issue of Medical Design Briefs Magazine.

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