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.

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