中国国际医疗器械设计与制造技术展览会(Medtec China)2018

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1-3 September 2021 | SWEECC H2&H4

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How to improve ultrasonic weld quality

2020-04-17

As medical devices miniaturize, there’s an increased need to weld plastic parts that are smaller, lighter, thinner-walled and often more complex in shape or contour than in the past. The growing presence of parts that contain embedded electronics and sensors that require special care in the ultrasonic welding process only heightens the challenge.

Meeting the medical device industry’s demand for repeatable, strong and consistent welds of these delicate components has required the development of ultrasonic welding technology that provides more precise and responsive force control.

The challenge

The ultrasonic welding weld process requires downforce exerted by the welder’s actuator to maintain horn-to-part contact, ensure the smooth and efficient transmission of ultrasonic energy into the mating parts, and bond the parts together after melt has occurred at the part interface. For decades, actuators have relied on pneumatics to deliver and regulate downforce and pneumatic actuators continue to be an industry standard.

However, to meet the unique challenges posed by increasingly small and delicate parts, the developers of Branson ultrasonic welding technology had to reconsider the capabilities of pneumatic actuators in light of the rapidly advancing capabilities of servo control technology. They ultimately found an optimal solution in a new electromechanical actuation system that delivers downforce with greater precision and responsiveness throughout the ultrasonic welding process.

Extensive testing of the new actuation systems revealed that rapidly and precisely adjusting downforce during the weld process could significantly improve weld quality. For users, more responsive actuation and force control mean a higher likelihood of achieving 100% good parts or zero scrap. Or, it can mean the ability to complete good welds on plastic parts that could not be reliably ultrasonically welded before.

The role of weld force control

For a given set of weld parameters, the precision and responsiveness of the actuator can make a major difference. Variations in force control that apply too little force reduce compression of the mating surfaces, reduce the heat generation needed for plastic melt, and result in “cold,” relatively weak welds.

Similarly, force variations that apply too much force can cause part joints or energy directors to deform, deflect or break, or can suppress the delivery of enough vibratory energy for proper part friction, melt flow and polymer entanglement to occur. This also results in weak welds.

Applying just the right amount of force at just the right time results in higher quality welds with highly consistent characteristics and strength. But achieving ideal force levels throughout the welding process is difficult. Ideal force control requires rapid, dynamic changes in the clamp force/downspeed applied by the actuator following the melt of the plastic. This automated 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.

Ideal force control adjusts downforce milliseconds after the melt (right). In weaker welds (left), polymer chains reassemble without entangling. The center weld shows the impact of inadequate force control.(Image from Emerson)

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.

More consistent and complete polymer chain entanglement and stronger welds are a direct result of technical improvements in actuation and force control. By evening out even small force variations very quickly, advanced process controls and actuation systems 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, advanced process control and actuation systems like this can provide superior weld quality and improved yields, characterized by uniform and consistent weld collapse depths and minimal flash or part marking.

Benefits of improved force control

In a series of laboratory tests and customer trials, the advanced process controls and electromechanical actuator developed for the Branson GSX-E1 welder:

  • Produced parts with an even higher average pull strength and more consistent and repeatable levels of break force (e.g. lower standard deviation in results) than an industry-leading pneumatically actuated ultrasonic welder.
  • Demonstrated the ability to maintain extremely tight torque test values across multiple GSX-E1 welders performing the same task. In this trial, multiple welders held torque values to within a nominal range of approximately 0.2 Nm over a run of 50 production-quality welds. Such control proved impossible with the customer’s pneumatically actuated ultrasonic welders.
  • Consistently produced welds with higher-than-expected break-push strength on parts with a very thin (~0.5 mm) plastic shear joint. One customer’s production trial delivered 3,000 parts with an average break force of 152 lb — nearly double the customer requirement — and did so with zero scrap parts.

The improved force control capabilities in an ultrasonic welding platform from can enable it to produce these and similar results while reducing weld cycle time, peak power input, and total weld energy consumption compared to welders equipped with less-responsive and precise actuation systems.

From:medical design and outsourcing