Utilizing Dynamic Hold Capability of Servo-Driven Ultrasonic Welders

by Alex Savitski, Ph.D.; Hardik Pathak; Leo Klinstein; Kenneth Holt; and David Cermak; Dukane IAS, LLC
Figure 1
Figure 1. Ultrasonic weld phases
Ultrasonic welding of thermoplastics is widely used by many industries to fuse together two parts in a short time without introducing additional consumables, such as fasteners, adhesives or solvents. The recent development of servo-driven ultrasonic welders introduces unique levels of control throughout the welding cycle. Unlike pneumatically driven welding machines, servo welders provide the user with the ability to control both hold distance and the velocity during the final phase of the welding cycle.
Figure 2
Figure 2. Welder and tooling

The hold phase of the ultrasonic welding cycle is critically important for the joint quality. During this phase, the weld is formed and a final microstructure responsible for the joint strength is established. The ultrasonic welding cycle can arguably be divided into four major phases [Figure 1]. For the purpose of this article, we will define the phases as “Pre-Weld,” “Initial Heating,” “Melting” and “Cooling Under Pressure” (hold phase). During the first three phases, the weld distance, energy, weld time or any other primary control factor is met. The hold phase begins after the first three welding phases are complete and ultrasonic vibrations have ceased.

 

 

Figure 3. ISTeP test part

During the hold phase, a force is applied to the joined parts to allow material to cool under pressure, which results in an additional collapse – upon which the assembly attains its final geometry. A reason for the lack of focused research in this area is that pneumatically driven welders, which were until recently the most common welders utilized in the industry, do not facilitate an active control of this phase. Pneumatic welders do not offer the control for additional displacement of the material during the hold phase or to set the rate with which material is displaced, as does the servo welder. The only process parameter being controlled for a pneumatic-driven welder is the duration of time in which the weld force continues to be applied to the molten material after ultrasonic vibrations have ceased.

Figure 4
Figure 4. ISTeP joint design with a 90° energy director

For servo-driven welders, there are currently no technically based recommendations available for setting the hold distance or the displacement rate to achieve joint strength and consistency. For many instances, users set the hold distance too high, allowing the melt layer sufficient time to solidify prior to reaching the programmed displacement. There are common cases in which the hold depth values are as high as 50 percent or more of the weld distance (shear joint or energy director size). Such cases generally result in underperforming welds, which likely are due to higher stress levels. It can be speculated that the increased levels of stress are due to the additional pressure being applied to an already solidified material.

Table 1. Average failure loads for varying dynamic hold distance

At the other end of the spectrum, instances when a hold distance was too small or nonexistent are known as well. As often happens, if users are not sure why to use a certain feature, it is ignored altogether. This also has been seen to be counterproductive, leading to reduced strength and dimensional variation in assembled parts.

This study delves into investigating effects of the hold phase settings on weld strength, with the aim of alleviating the lack of scientifically based recommendations in setting the parameters of both hold distance and velocity. Experiments were conducted using Dukane’s 30 kHz 1800 W iQ Servo Ultrasonic Welder with Melt-Match® technology and custom-made tooling. iQ Explorer II software was used for data collection and analysis.

Figure 5. Average failure load vs. dynamic hold for variable displacement velocity

Parts used for this experiment were Dukane ISTeP parts with a 90° (sharp) ED (Figures 1-3), molded of a common Sabic grade Lexan 121R polycarbonate. This part was developed by Dukane to provide a test specimen for ultrasonic welding with changeable joint designs.

The approach taken in this study was based on varying the programmed hold distance from 12.5 microns to 175 microns, while monitoring the force applied to the weld and measuring the change in height of the assembly. The experiments demonstrated that displacement resulting from moderate force application to the molten material is beneficial to weld strength and consistency, as shown in tensile test results, with both reaching their maximum at 37.5 microns (0.0015″) of the dynamic hold displacement (Table 1, Figure 5).

Figure 6
Figure 6. Change in assembly height for varying dynamic hold settings

It was found that as material solidifies, a further increase in force required to gain additional weld collapse does not produce any added material displacement and results in increased residual stress in the weld. This was shown when the height of the samples was measured with the drop gauge before and after the welding (Figure 6).

Table 2. Force applied from equipment and melt layer thickness for varying dynamic hold settings Table 2. Force applied from equipment and melt layer thickness for varying dynamic hold settings

It was further evidenced by representative microscopic characterization samples of the weld zone with different hold distance settings, in which the melt layer thickness remains practically unchanged after reaching its minimum at 37.5 micron hold distance (Table 2). Further increase in collapse beyond this value did not produce any additional material displacement and reduction of the melt layer thickness.

Examination of the microscopic images also shows a marked increase in the number of fringe lines, which indicate increased residual stress, in the welds in the samples generated with higher programmed hold distance. This phenomenon is most pronounced in the images at and above 75 microns of hold distance (Figures 7, 8, 9), when the welder was programmed to achieve higher displacement values.

Figure 7
Figure 7. Cross-section illustrating stress lines for 37.5 µm dynamic hold displacement – 1 mm/s
Figure 8
Figure 8. Cross-section illustrating stress lines for 75 µm dynamic hold displacement – 1 mm/s
Figure 9
Figure 9. Cross-section illustrating stress lines for 175 µm dynamic hold displacement – 1 mm/s

Summary

The cooling phase of the ultrasonic welding cycle is critically important for the joint quality. It is during this phase that the weld is formed and the intermolecular bonds and final microstructure responsible for the joint strength are established. Servo-driven welding equipment allows the user to actively manipulate the hold phase by programming the amount of material displacement while controlling its squeeze flow rate; this feature is essential in maintaining dimensional stability of the assembly.

The measurements taken in this study have demonstrated that, as material solidifies, a further increase in force to gain additional weld collapse does not produce any added material displacement and results in increased residual stress in the weld. This event is confirmed by the examination of both post-weld height measurements and microscopic characterization of the weld zone.

Although the data established in this experiment correspond with the particular material and joint geometry, the investigation has provided scientifically based insights on the effect of using hold phase settings in creating a strong, consistent weld. It also has provided an informed practical approach for establishing these process settings in manufacturing. The study results provide value to process engineers, enabling a better understanding of the joining process and factors driving final joint geometry.

Alex Savitski, Ph.D., is chief engineer, advanced technologies, for Dukane. He is involved in development and implementation of advanced plastic assembly processes for critical applications in medical device, electronics, automotive and other industries. Savitski is a nationally recognized expert in joining methods and equipment for assembling plastic components and a published author. Dukane is a global leader in offering precision ultrasonic and plastic joining technologies to manufacturing operations. Over the years, through intense research and studies, the company has developed multiple patented technologies that create seamless welding processes. In the complex environment of plastic welding, Dukane takes the guesswork out of product selection by focusing on a customer-centered approach to develop solutions that meet unique requirements of each customer’s application. For more information, call 630.797.4900 or visit www.dukane.com/us

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