How to improve ultrasonic plastic welding

Tom Hoover, senior medical business development manager – Americas, assembly technologies, at global technology and manufacturing company, Emerson, highlights the latest advances in ultrasonic welding and explores how to meet the demands of the medical device sector.

The latest advances in ultrasonic welding for medical devices are driven by the need to assemble plastic parts that tend to be smaller and lighter in weight, thinner-walled, and often more contoured than in the past. A growing number of these parts also contain embedded electronics and sensors that require special care in the ultrasonic welding process.

Meeting the medical device industry’s demand for repeatable, strong and consistent welds in these smaller and more delicate components has required the development of improved ultrasonic welding technology. Perhaps the most important recent improvement has been the development of new, more precise methods of force control. This has required a series of changes to the ultrasonic welding actuator and its microprocessor controls.

To achieve the greater and more precise levels of force control required, the developers of Branson ultrasonic welding technology considered not only the capabilities of pneumatic actuators—which remain an industry standard—but also the rapidly advancing capabilities of servo control and technology.

Their solution was a new electromechanical actuation system, recently introduced in Branson GSX welders from Emerson. A key attribute of this new welding platform and its advanced actuation system is substantially more precise and responsive force control through the entire weld process. Downforce is necessary to maintain horn/part contact and ensure the smooth, efficient transmission of ultrasonic energy into the mating parts. Managing downforce more rapidly and precisely has important implications for weld quality.

The role of weld force control

For a given set of weld parameters, variations in force control that result in applying too little force reduce compression of the mating surfaces, reduce the heat generation needed for plastic melt, and result in cold or weaker welds. Similarly, force variations that result in applying too much force can cause part joints or energy directors to deform, deflect or break and may not provide enough time for proper melt flow and polymer entanglement to occur.

Applying just the right amount of force at just the right time results in quality welds with highly consistent characteristics and strength. Ideal force control requires rapid, dynamic changes in the clamp force/downspeed applied by the actuator following the melt of the plastic. This adjustment, called ‘dynamic follow through,’ enables each weld cycle to adapt to part-to-part variances and other factors such as the type of plastic, joint style, and part geometry.

As the speed and precision of force control and dynamic follow-through increase, the strength, quality and consistency of plastic welds follow. For example, the strongest ‘pull force’ for a part weld results from a controlled force profile that allows for complete and random polymer chain entanglement that makes the weld as strong as the parent material (As shown in figure 1).

As seen in the right-most illustration in figure 1, ideal force control adjusts downforce milliseconds after the melt, allowing polymer chains to extend vertically across the part interface and entangle with each other across the bond line as melt and compression occur before cooling. By contrast, weaker welds, characterised by partial or no polymer chain entanglement, show polymer chains that reassemble parallel to the bond line without entangling across the part interface. The centre weld shows the impact of inadequate force control, while the ‘cold’ weld at left could be caused by too little or too much downforce in too short a weld time.

More consistent and complete polymer chain entanglement and stronger welds are a direct result of technical improvements in force control. By evening out even small force variations very quickly, the process control and actuator in the GSX welder maintain more consistent horn-to-part contact and enable weld parameters to be executed far more accurately and gently. So, for even hard-to-weld shapes and small or delicate parts, it can provide superior weld quality and improved yields, characterised by uniform and consistent weld collapse depths, with minimal flash or part marking.

Results of improved force control

In a series of laboratory tests and and customer trials, the advanced process controls and electromechanical actuator in the GSX-E1 welder reportedly consistently outperformed pneumatically actuated welders, enabling the latest ultrasonic welding platform to:

• Produce welded parts with more consistent and repeatable levels of strength. In a head-to-head comparison of welding performance on identical parts, the GSX-E1 was matched against an excellent pneumatically actuated legacy welder. Results showed that while both welders produced strong welds, the GSX-E1 was able to produce parts with an even higher average pull strength and more consistent and repeatable levels of break force (e.g., lower standard deviation in results), as well as physical characteristics that more closely matched those of parent material.

• Change downforce on a weld within milliseconds as part-to-part melt occurred, resulting in more precise closed loop control over heat generation and dissipation, weld-collapse depth, and weld quality. In another example, a customer evaluated the performance of the GSX-E1 welder when its legacy welder, a pneumatically actuated welder, was unable to change weld downforce quickly enough to avoid bubble formation in the weld zone between two polycarbonate parts. The result was an unacceptable level of scrap parts. The customer found that the more rapid, precise action of the electromechanical actuator and control on the Branson GSX-E1 eliminated the bubbles, producing consistently high-quality welds while reducing weld cycle time, peak power input, and total weld energy consumption.

• GSX-E1 repeatability across multiple welders. A customer with a low weld clamp force (~40N) application required a torque test specification on their welded parts maintaining a tight torque value test range of between 0.2Nm and 0.65Nm across multiple welders. Their trial with GSX-E1 welders exceeded the requirement, holding torque values to within a nominal range of approximately 0.2 Nm over a run of 50 production-quality welds—a feat impossible with the customer’s pneumatically actuated ultrasonic welder.
• Complete challenging part welds with exceptional consistency. The force control capabilities of the GSX-E1 were also proven in applications involving long, delicate, and thin-walled parts. In one application, a customer using a competitive, servo-actuated ultrasonic welder struggled to consistently produce welds with high break-push strength (>80 lbs.) on parts with a very thin (~0.5 mm) plastic shear joint. An extended production trial with the GSX-E1, working at 500 parts per hour, delivered 3,000 parts with an average break force of 152 lbs (nearly double the customer requirement). In addition, the GSX-E1 produced 100% good parts—zero scrap—a yield that the customer could not achieve with either the competitive servo-actuated ultrasonic welder or their own pneumatically actuated ultrasonic welder.

In another example, this time involving far-field welds of a long, thin (0.070” wall thickness) tube with a shear joint into a moulded base, a customer was able to produce strong welds with its legacy welder, but sometimes had quality problems including part marking and inconsistent weld depth, resulting in flashing. A weld trial set up in less than fifteen minutes with the new intuitive Branson GSX quickly resolved the problems. Its responsive process control and electromechanical actuator delivered weld amplitude consistently and smoothly, resulting in parts free of markings and flash with higher average pull strength.

The improved force control capabilities built into the new GSX ultrasonic welding platform from Emerson typically enable GSX welders to produce these and similar results while reducing weld cycle time, peak power input, and total weld energy consumption compared to welders with less-responsive and precise actuators.