Utilizing Dynamic Hold Capability of Servo-Driven Ultrasonic Welders in Studying Cooling Phase of the Ultrasonic Welding Process.

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1 Utilizing Dynamic Hold Capability of Servo-Driven Ultrasonic Welders in Studying Cooling Phase of the Ultrasonic Welding Process. Alex Savitski, Ph.D., Leo Klinstein, Kenneth Holt, David Cermak, Hardik Pathak, Keith Pedziewiatr, Dukane Corporation Abstract 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, as opposed to pneumatically driven welding machines, introduces unique levels of control throughout the welding cycle. This study focuses on the final phase of the welding process, i.e., the hold cycle, and the benefits that the servo-driven ultrasonic welders can provide to this final phase by controlling both hold distance and the velocity at which this final phase is accomplished. Introduction The ultrasonic welding cycle can be divided into four separate phases the last of which is the hold phase [1]. The hold phase of the ultrasonic welding cycle is critically important for the joint quality. During this phase, the weld is actually formed and intermolecular bonds and final microstructure responsible for the joint strength are established [2]. Currently there is very little research [ 3 ] and virtually no practical recommendations on how to control this phase in order to achieve joint strength and consistency. After the weld cycle progresses through the first three welding phases of contact, heating, and melting of the energy director, and the value of weld distance, energy, weld time or any other parameter set as a primary control factor for the process has been met, the ultrasonic vibrations are ceased, and the hold cycle begins. During this phase, a force is applied to the joined parts to allow melting material to cool under pressure, which results in an additional parts collapse and the assembly attains its final geometry. One of the reasons for lack of the focused research in this area is a fact 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. They do not allow the setting of a specific desirable value for the additional displacement of the material during the hold phase or setting of the rate with which material is displaced. The only process parameter being controlled in this case is the duration of time in which the weld force continues to apply on the molten material after ultrasonic vibrations stopped. Servo-driven welding equipment enables a user to actively manipulate the hold phase by specifically programing the amount of desirable displacement during the hold phase and applying variable pressure on the molten material to control the squeeze flow rate during the final stage of the process after the ultrasonic vibrations ceased. These new process control features have provided a unique tool for investigating the effect of the main factors of the hold phase critical to the weld formation, which was the primary focus of this paper. As there are currently no technically based recommendations available for setting the hold distance or the displacement rate, in many instances users set the hold distance too high, so the melt layer would solidify before programmed displacement value is reached. There are cases in which the hold depth values as high as 50% or more of the weld distance (shear joint or energy director size) and they are not rare. The resulting welds were seen to be quite high in stress levels, as it is likely that in these cases the pressure was likely applied to material which was already solidified. and very vulnerable to stress cracking during aging. Even with hold distances of 25% of the weld distance, a negative effect on the joint s mechanical characteristics is observed. At the other end of the spectrum, there are instances when a hold distance was too small or non-existent.. 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 and leads to reduced strength and a variation in assembled parts. This paper addresses this lack of study and delves into investigating effects of the hold phase settings on weld s strength with the aim to alleviate the lack of scientifically based recommendations in setting the parameters of both hold distance and velocity during this phase. It presents and compares test results of not only several hold distances but also the velocities used to achieve them. Materials The parts used for this studyare Dukane ISTeP parts with a 90 (sharp) ED (Figures 1-3), molded of a common Sabic grade Lexan 121R polycarbonate. This SPE ANTEC Anaheim 2017 / 1725

2 part was developed by Dukane to provide a test specimen for ultrasonic welding with changeable joint designs. [10] continues. This drop in force indicates the presence of an initial molten layer [4]. The Melt-Match feature controls weld velocity and allows direct control of the squeeze flow rate of the molten material. Figure 1: Innovative ISTeP Test Part The Dynamic Hold feature, which allows for control of the squeeze flow rate of the molten material and the collapse distance after ultrasonic vibration has ceased, is the most relevant to this study, and it was utilized to investigate the effect of Dynamic Hold settings on the weld formation and strength. These process control features of the servodriven ultrasonic welders are significantly different from those traditionally used in pneumatic welders, which were the basis of the previous research, they allow precise control of the welding process [3-11]. Figure 2: ISTeP 90 deg. E.D. design. Units: mm. Figure 3: A cross-section of a 90 ED on an ISTeP part prior to welding Equipment Experiments were conducted using Dukane s 30 khz 1800 W iq Servo Ultrasonic Welder, model # 30HS180-2Q-P7, with Melt-Match technology and a HMI running iq Explorer II software for data collection and analysis. The tooling was a flat-face, high-gain horn (with a gain factor of 2) and a custom-made, drop-in style fixture as pictured in Figure 4. Advanced control features of the servo-driven system were utilized in this experiment including Melt- Detect, which allows the press to hold its position on the assembly at the initiation of welding until a decrease 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 Figure 4: Welder and tooling used in this study. For tensile testing, a Com-Ten Industries ComTouch Total Control System with Variable Speed Test Stand and TSB3A load cell with 22,250 N capacity (accuracy of +/- 0.5%) was used with a custom designed fixture, as in Figure 5. Failure load was identified by peak tensile force at break. SPE ANTEC Anaheim 2017 / 1726

3 Weld strength and consistency were evaluated based on the tensile test results. The samples were also measured using a drop gage before and after the welding, and the total change in the assembly height resulting from the welding process was calculated. Ten samples were welded and tested in each trial. Representative welds were cross-sectioned for microscopic characterization, which included resulting melt layer measurement. Results and Observations Figure 5: Custom pull test fixture The samples height before and after the welding was measured using the Mitutoyo HD-18 AX drop gauge. Olympus SZX 10 microscope with polarized light capabilities an SC50 digital camera were used for microscopic characterization of the weld area. Experimental Details The purpose of this investigation was to evaluate the effect of the hold phase parameters on the weld formation and strength and to provide scientifically based insights for setting them. The approach taken was based on varying the programmed hold distance from 12.5 microns to 175 microns, while monitoring the actual force applied to the weld and measuring the actual change in the height of the assembly. The experimental matrix also included a trial in which there was no programmed displacement at all, and the parts were cooled off without applying any additional force once the ultrasonic vibrations were stopped. Two different Dynamic Hold velocities were tested, i.e. two different squeeze flow rates were applied to the material -0.4 mm/s and 1.0 mm/s to evaluate the effect of this parameter. The rest of the welding parameters were left unchanged throughout the trials and utilized a linear velocity profile, in which weld velocity was changing from 0.25 to 0.4 mm/sec. These and other process settings kept constant through the trials and were set based on the process parameters investigations reported in [6 ], which utilized ISTeP parts made of PC. These parameters are presented in the Table 1 below. Small displacement resulting from moderate force application to the molten material is beneficial for weld strength and consistency, as shown in tensile test results, with both reaching their maximum at a Dynamic Hold setting of 37.5 microns ( in) (Figures 6 and 7). As material solidifies, 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 by the direct samples measurements, when the height of the samples was measured with the drop gauge before and after the welding (Table 2). It was further evidenced by microscopic characterization of the weld zone of representative samples welded 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 4). Further increase in collapse beyond this value did not produce any additional material displacement and reduction of the melt layer thickness. This is shown in the following microscopic cross-sections. Examination of the microscopic images also show 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 (Fig ), when the welder was programmed to achieve higher displacement values. Displacement velocity effect. While the tested range was limited to two velocities, it appears that the higher displacement rate of the material during the hold phase is beneficial to both strength and consistency (Figures 6 and 7). Table 1: Fixed Process Parameter Settings Amplitude Trigger Force Melt- Detect (%) Distance (mm) Weld Velocity (mm/sec) Static Hold (sec) SPE ANTEC Anaheim 2017 / 1727

4 Average Deviation Average Failure Load Table 2: Height measurements of welded, un-welded, and change in height Dynamic Hold Settings Un-welded Assembly Height (mm) Welded Assembly Height (mm) Change in Height (mm) Table 3: Average Failure Loads & Standard Deviation for varying Dynamic Hold Distance and Velocity D. Hold 0.4 mm/sec Dynamic Hold Velocity Average Failure Standard Deviation 1.0 mm/sec Dynamic Hold Velocity Average Failure Standard Deviation (%) (%) Table 4: Force Applied reading from equipment and Melt Layer Thickness for varying Dynamic Hold Settings Dynamic Hold Settings Force Applied Melt Layer Thickness Figure 6: Average Failure Load vs. Dynamic Hold Distance for Variable Displacement Velocity Standard Deviaton vs. Dynamic Hold Distance Average Failure Load vs. Dynamic Hold Distance Dynamic Hold Distance 1 mm/sec - Average Failure Load 0.4 mm/sec - Average Failure Load Dynamic Hold Distance 1 mm/sec - Standard Deviation 0.4 mm/sec - Standard Deviation Figure 7: Average Failure Load vs. Dynamic Hold Distance for Variable Displacement Velocity SPE ANTEC Anaheim 2017 / 1728

5 Change in Height (mm) Dynamic Hold Settings vs. Postweld Change in Height Figure 10: Cross-section illustrating stress lines for 37.5 µm Dynamic Hold Settings Change in Height (mm) Figure 8: Change in Assembly height for varying Dynamic Hold Settings Figure 11: Cross-section illustrating stress lines for 50 µm Figure 9: Cross-section illustrating stress lines for 25 µm Dynamic Hold Displacement - 1 mm/s Figure 12: Cross-section illustrating stress lines for 75 µm SPE ANTEC Anaheim 2017 / 1729

6 for the joint strength are established. At the same time this is the phase subject to very limited if any research, and there are virtually no practical recommendations on how to control this phase in order to achieve joint strength and consistency. 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. These control features made it possible to investigate how changing hold phase conditions affect weld formation. Figure 13: Cross-section illustrating stress lines for 125 µm In this study, the material displacement i.e. Dynamic Hold Displacement was the primary controlled variable. It was found that some material displacement resulting from moderate force application to the molten material is beneficial to weld strength and consistency. 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. While the tested range was limited to two velocities, it appears that the higher displacement rate of the material during the hold phase is beneficial to both strength and consistency, as with the higher rate more material can be displaced before it solidifies. Figure 14: Cross-section illustrating stress lines for 175 µm Cross sections were next analyzed under a microscope using a polarized light filter. This technique is commonly used to evaluate residual stress levels within transparent plastic parts. There is always some level of residual stress within welded parts due to melting, reforming and cooling during the weld. Per J. Feingold s article [12], when viewed with polarized light, stressed areas of polymers are visible to the eye as a series of multicolored bands or fringes. This fringe pattern, sometimes referred to as birefringence, can be interpreted as varying levels of stress at a specific point and in a particular direction through the material. Although the data established in this experiment is limited to 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 has also provided an informed practical approach for establishing these process settings in manufacturing. The study results provide value to process engineers, providing them with a better understanding of the joining process and factors driving final joint geometry. Further research incorporating round energy director joint design, with an expended range of dynamic hold velocities, are recommended. These studies may provide additional insights into conditions for a stronger and more consistent weld formation for a wide variety of applications Conclusions The cooling phase of the ultrasonic welding cycle is critically important for the joint quality. It is during this phase that the weld is actually formed and intermolecular bonds and final microstructure responsible SPE ANTEC Anaheim 2017 / 1730

7 References 1. M.J. Troughton, Handbook of Plastic Joining: a practical guide, 2nd edition, Plastics Design Library, Norwich, NY, pg. 37 (1997), ISBN D. Grewell, A. Benatar, J.B. Park, Plastics and Composites Welding Handbook, Hanser Gardner Publishers, Cincinnati, pg. 150 (2003), ISBN Miranda Marcus, Satish Anantharaman, Bob Aldaz, Advantages of Servo-Driven Ultrasonic Welder ANTEC M. Marcus, P. Golko, S. Lester, L. Klinstein. Comparison of Servo-Driven Ultrasonic Welder to Standard Pneumatic Ultrasonic Welder ANTEC T. Kirkland. Ultrasonic Welding: The Need for Speed Control Plastics Decorating. July/August, A Savitski, H. Pathak, L. Klinstein, P. Golko, K. Holt, A Case for Round Energy Director: 7. Utilizing Advanced Control Capabilities of Servo-Driven Ultrasonic Welders in Evaluating Round energy Director Performance, ANTEC H. Turunen. Ultrasonic Welding for Plastics Bachelor s Thesis, Turku University of Applied Sciences, Finland A. Benatar. Servo-Driven Ultrasonic welding of Semi-crystalline Thermoplastics 39th Annual Symposium of the Ultrasonic Industry Association. Cambridge, MA Miranda Marcus, Bob Aldaz, Loc Nguyen, Ken Holt, Benefits of Servo-Driven Ultrasonic Welding for Critical Assemblies, ANTEC A. Mokhtarzadeh and A. Benatar. Comparison of Servo and Pneumatic Ultrasonic Welding of HDPE Shear Joints ANTEC US Patent 8,720,516. May 13, Klinstein et al. Ultrasonic Press Using Servo Motor with Delayed Motion 13. J. Feingold, Stress: Diagnose It Before It Ruins Your Parts, Plastics Technology, Dec SPE ANTEC Anaheim 2017 / 1731