BENEFITS OF SERVO-DRIVEN ULTRASONIC WELDING FOR CRITICAL ASSEMBLIES

Miranda Marcus, Bob Aldaz, Loc Nguyen, Ken Holt (Dukane Corporation)Mike DeVries (Parker Hannifin Corporation)

Abstract

Ultrasonic welding is one of the most widely used processes for bonding polymers, valued for its speed, flexibility, and low cost. Recently, there has been a call for more controlled and consistent ultrasonic welding processes, as part designs become more complex and requirements more stringent, especially in the medical field and for complex packaging applications. Additionally, the processes used to meet these increasing demands must be consistent and repeatable over time. Dukane has worked to meet this demand through the development of a new iQ series Servo-Driven Ultrasonic Welder with Melt-Match® technology, which is protected by US and international patents with other US and international patents pending [1, 2, 3, 4].

This study explores the potential benefits of the features solely available with Dukane’s servo-driven welders, such as Melt-Match® (matching welding speed with the melt flow rate of the plastic). A full scale Design of Experiments has been undertaken to identify the relational effects of these speed and distance control features and how they can be used to optimize the welding process. An effort has been made to detail and quantify the improvement to the weld joint based both on previous research, including a joint study with Value Plastics, a large medical device manufacturer, and new experimentation in collaboration with Parker Hannifin.

Introduction

Ultrasonic welding has long been the most common welding process used with thermoplastics [5]. In the ultrasonic welding process, there are three fundamental process variables that have a direct effect on weld quality: amplitude, duration, and force [6, 7, 8]. The first of these parameters, amplitude, has long been controlled through frequency selection, horn-booster design and regulation of the electrical input to the transducer. The second of these parameters, duration, could only be controlled by setting a specific weld time for 50 years [8]. In 1988 this was revolutionized by Dukane’s development of welding by distance [9], thus allowing greatly improved troubleshooting and process control [8].

Recently, a new precise method of motion and force trigger control has been introduced with Dukane’s servo-

driven ultrasonic welder in 2009 [9]. This new development allows complete control of the third parameter that defines ultrasonic welding, force.

It has long been known that better process control leads to improved part quality and consistency. As Robert Leaversuch noted in 2002, “in automotive, medical, and other demanding sectors, use of advanced controls is critical to meeting strict quality requirements” [8]. In this, servo welders are at the “top of the process-capability food chain” [6]. Dukane’s servo-driven ultrasonic welder further eases process optimization by allowing all the weld data to be exported in formats that can easily be imported to Excel or Minitab for analysis [5]. Additionally, the data produced by the generator, such as graphs or cycle data, is very useful for process optimization and troubleshooting [7]. Simply said, servo welders “take control of the process away from the plastic” [6].

As with any new technology, the servo-driven ultrasonic welder offers several advanced control features which are fundamentally different than those which have been traditionally available. We have therefore completed an in depth Design of Experiments to identify and measure how these newly available settings affect weld quality. These results can be used to expedite process optimization for critical assemblies.

In order to complete this fundamental exploration of the ultrasonic weld process, Dukane has also created a new standard test part. Previous experimentation has shown that the AWS I-beam test samples are prone to warp and sink which makes consistent measurements difficult to achieve [10]. With the advent of very accurate machinery and new highly complex critical applications, a more precise test part is needed.

Literature Review

In recent years, there has been a plethora of research conducted with the new servo-driven ultrasonic welder. Multiple studies have shown that servo-driven ultrasonic welders produce parts with increased weld strength over parts produced with pneumatic welders [5, 6, 10, 11]. Each of these experimental studies has demonstrated unique benefits of using servo-driven ultrasonic welders with regard to process consistency and performance.

However, none have investigated the full range of features available, providing only small windows into the capability of this exciting new equipment.

Distance Control

The servo-driven ultrasonic welder offers “excellent repeatability of collapse distance from part to part” [12]. In a pneumatic system there is a limit to the speed at which air can escape the cylinder, preventing abrupt changes in velocity and limiting distance control. Dukane’s servo ultrasonic welder can accelerate up to a rate of 1270 mm/s², allowing almost instantaneous velocity shifts during the weld and hold phases [9]. An initial study of the servo ultrasonic welder showed that the servo welder was able to achieve a standard deviation of 1.1% in measured collapse distance compared to 3.9% achieved with the pneumatic welder [10]. Value Plastics, a manufacturer of precision molded couplers and components for the medical industry, was able to achieve a standard deviation of collapse of 0.9% in production using Dukane’s servo-driven ultrasonic welder [9].

Clearly, servo-driven ultrasonic technology offers improved repeatability, measurement accuracy and more precision [5, 9]. This improved consistency will be needed as products become increasingly complex [5].

Velocity Control

Even before the introduction of Dukane’s servo-driven welder, the need for velocity control was recognized. As Mikell Knights wrote in 2005, “Research has proven that consistency of melting rate has a direct influence on bond strength.” A linear velocity means a steady melt rate which, in turn, creates a homogenous molecular structure and a stronger weld [7]. Precise speed control has been shown to be a great aide in welding under non-ideal circumstances. Servo welders were able to create stronger bonds than pneumatic welders when grease was present in the joint or the energy director was damaged [6].

In past years there has been much effort put into attempting to get consistent velocity control with pneumatic systems [6, 7]. These efforts have been in vain, as it is simply not possible to get precise velocity control with a pneumatic welder [13]. As one writer put it, trying to achieve precise speed control with a pneumatic press was “the equivalent of sending a ship on an ocean voyage with a map and a compass from a box of cereal” [6]. Dukane’s servo-driven ultrasonic welder offers clear improvement in this area by directly controlling velocity.

Studies using servo ultrasonic welders have shown that the programmed weld velocity can be directly correlated

with weld strength [10, 12, 13, 14, 15]. In a study at The Ohio State University it was shown that by using a defined velocity profile with a slower speed during melt initiation and a faster speed in the middle and end of the weld, strength could be increased with less weld time and reduced surface marking [14]. Dukane has provided a unique new weld control to achieve this initiation of melt before collapsing the weld joint through the use of the patented Melt-Detect™ feature. This features allows the press to contact the parts and turn on ultrasonic vibration with no vertical movement until a drop in force is detected which indicates that welding has initiated [13].

Experimentation

ISTeP Development

The ISTeP (Industrial Standard Test Part) design was developed to allow precise height measurement and consistent pull testing results. The round design prevents the effect of zippering where one side of a beam style part tends to fracture first, leading to a peel effect on the remaining weld, as seen with the ASTM I-Beam parts. Since peel strength is less than pull strength on ultrasonic welds, this leads to a less accurate and artificially reduced pull test value. Additionally, the mold was designed for even fill to prevent any part warp and wall thicknesses are consistent to prevent sink marks.

Figure 1: Innovative round ISTeP design

The mold design includes an insert to allow molding of an unlimited number of joint designs. Already, a shear joint, sixty degree energy director and ninety degree energy director have been designed. However, any joint design can be easily and quickly implemented [Figure 2]. For all the experiments in this study, the ninety degree energy director was used [Figure 3].

Figure 2: Drawing of ISTeP part used for testing

Figure 3: Cross-section of ISTeP at joint

The ISTeP part allows for testing via pull tests, pressure testing using the included tube opening, and by measuring part height before and after welding to determine actual collapse distance. These parts were compared to traditional I-Beam parts, per AWS G1.2 standardized ultrasonic welding test specimen [16]. A variety of four weld setups were compared [Table 1].

ISTeP I-Beam ISTeP I-Beam ISTeP I-Beam ISTeP I-BeamBooster o-ring o-ring o-ring o-ring resonant o-ring resonant resonantTrigger Force

N 250 250 250 250 250 250 250 250

Weld Distance

mm 0.3 0.3 0.3 0.3 0.25 0.25 0.305 0.25

Weld Speed

mm/s

10 10 10 10 2 2 2 2

Weld Amplitude

% 100 100 100 100 90 90 90 90

Melt Detect

% 10 10 10 10 5 5 5 5

Hold Distance

mm 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05

Hold Speedmm/s

12.7 12.7 12.7 12.7 5 5 5 5

Static Hold s 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

Setup 1 Setup 2 Setup 3 Setup 4

Table 1: ISTeP v. I-Beam Settings

Part height was measured before and after welding. Because the I-Beam parts did not self-locate, a fixture had to be used to perform these height measurements before welding [Figure 3]

Figure 3: Tooling used to align I-Beam parts for measurementFor all experiments, weld strength was measured using a Com-Ten Industries pull tester with an accuracy of +/- 2.5 lbs. Tooling was designed specifically for each part [Figure 4].

Figure 4: Pull testing tooling

Gage Repeatability and Reproducibility

Prior to completing an in depth Design of Experiments, it was necessary to first ensure that our means of part measurement could produce sufficiently consistent results to be usable. To this end, ten parts were welded at our low, nominal, and high weld settings, which were also used for the complete Design of Experiments [Table 2]. Results were interpreted using Minitab.

Nominal High LowTrigger Force (N) 250 400 100Weld Distance (mm) 0.3 0.35 0.25Weld Speed (mm/s) 2 3 1Weld Amplitude (%) 90 100 80Melt Detect (%) 5 10 0Hold Distance (mm) 0.05 0.1 0Hold Speed (mm/s) 3 5 1Static Hold (s) 0.5 1 0Table 2: Gage R&R and DOE Settings

Design of Experiments

In order to better use the new servo-driven ultrasonic welder technology, it is important to have complete understanding of how each parameter affects weld quality. Minitab was used to develop the DOE and to perform the analysis.

Seventy two runs were completed with eighteen samples for each parameter set for a total of one thousand two hundred and ninety-six samples [Table 3]. The entire set was welded by one operator before any measurements were done. After completing the welding, the same operator carried out all of the pull tests.

Run A B C D E F G H Run A B C D E F G H1 + - - + - - + - 37 + - + - + + + -2 0 0 0 0 0 0 0 0 38 + - - + + - + +3 + - - + - + + + 39 + + + - + - - -4 - - - + + + - + 40 - + - + + - + +5 - - - - + + + + 41 - + - - - + - +6 - - + + - + + - 42 - + - - - - - -7 - - - - - - + + 43 + + + + + - + -8 - - + + + - + - 44 + + + - - + - -9 - + - - + + - - 45 + - + - + - + +10 - + - - + - - + 46 + + + + - + + -11 + - + + + + - - 47 - - + - + + - +12 + + - + + - - - 48 - + + + + - - +13 - - - + - - - + 49 - + + - - + + +14 - + - + - + + + 50 + - + + - + - +15 + - + - - - + - 51 - - - - - + + -16 + + - - + + + + 52 - - - + - + - -17 - - - - + - + - 53 - - + - - - - +18 - - + - - + - - 54 + + + + - - + +19 0 0 0 0 0 0 0 0 55 + - - - + + - -20 + + + - + + - + 56 - - + - + - - -21 0 0 0 0 0 0 0 0 57 - + + + + + - -22 - - - + + - - - 58 - + + + - + - +23 + + - + + + - + 59 + + + - - - - +24 + + - - - - + + 60 + - + + - - - -25 - - + + + + + + 61 - + + - + - + +26 0 0 0 0 0 0 0 0 62 + - - - - + - +27 + + - + - + - - 63 0 0 0 0 0 0 0 028 + + - - - + + - 64 - + + - + + + -29 + + - - + - + - 65 + + - + - - - +30 - - + + - - + + 66 0 0 0 0 0 0 0 031 + - - - - - - - 67 - + + - - - + -32 + - - - + - - + 68 0 0 0 0 0 0 0 033 + - + - - + + + 69 - + + + - - - -34 + + + + + + + + 70 - + - + + + + -35 + - + + + - - + 71 - + - + - - + -36 + - - + + + + - 72 0 0 0 0 0 0 0 0

Table 3: DOE run parameters

A Dukane servo-driven ultrasonic welder with attached HMI, running iQ Explorer II, was used to complete these experiments. A flat face high gain horn and a simple drop-in style fixture were used in conjunction with it [Figure 5].

Figure 5: Welder and tooling used for experiments

Speed Trials

After completing the full DOE, a follow up experiment was conducted on the effect of welding speed on weld quality. Weld speeds were chosen to provide a closer look at the effect of this parameter within the range chosen for the DOE and at slower speeds. The other parameters were maintained at the nominal settings [Table 4].

Trigger Force

Weld Distance

Weld Speed

Weld Amplitude

Melt Detect

Hold Distance

Hold Speed

Static Hold

N mm mm/s % % mm mm/s s

250 0.25 0.5 90 5 0.05 3 0.5250 0.25 1.0 90 5 0.05 3 0.5250 0.25 1.5 90 5 0.05 3 0.5250 0.25 2.0 90 5 0.05 3 0.5250 0.25 2.5 90 5 0.05 3 0.5250 0.25 3.0 90 5 0.05 3 0.5

Table 4: Speed trial settingsResults

ISTeP Development

Across all weld settings, the ISTeP parts showed more consistent results for both the measured collapse distance and the pull strength. Additionally, the standard deviation of the collapse distance measurements of the I-Beam parts become more consistent after practice, showing that it is more dependent on operator skill than measurements of the ISTeP parts. These results are shown in Charts 1 and 2 below.

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Chart 1: Measured Collapse standard deviation comparison between ISTeP and I-Beam parts

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ISTeP I-Beam

Chart 2: Pull Strength standard deviation comparison between ISTeP and I-Beam parts

Gage Repeatability and Reproducibility

Two runs were completed for the Gage R&R study. In the first run, ten welds were tested at each weld parameter set by three separate operators. After this, some changes were made to the pull test fixture to improve consistency. The second run used only one operator, who later completed the entire DOE run independently. The average and standard deviation values from these runs are shown in Table 5.

Run 2

A B C A

Ave 1486 1497 1494 985

511 407 428 545

34% 27% 29% 55%

Ave 3342 3027 3369 2925

618 410 559 598

18% 14% 17% 20%

Ave 4230 4879 4343 5496

855 746 663 344

20% 15% 15% 6%

High

StDev

StDev

StDev

Operator (A, B, C)

Run 1

Pull Force (N)

Low

Nominal

Table 5: Gage R&R results

The results of the first run were analyzed with Minitab, which showed good distinction between the weld sets, verifying the repeatability of the test equipment. Also, all three operators produced virtually the same results, verifying reproducibility.

Part-to-PartReprodRepeatGage R&R

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UCL=991.2

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Gage name: Date of study:

Reported by: Tolerance: Misc:

Components of Variation

S Chart by Operators

Xbar Chart by Operators

Pull value By Parts ( Operators )

Pull value by Operators

Gage R&R (Nested) for Pull value

Figure 6: Minitab Gage R&R results

Design of Experiments

The complete DOE results were analyzed by Minitab which demonstrated some exciting results. As shown in Figure 7 below, the three weld parameters with the greatest effect on weld strength were Weld Speed, Weld Distance, and Melt-Detect™ setting. This is particularly interesting as precise weld speed and weld distance control are the hallmarks of servo-driven welder technology. Additionally, the Melt-Detect™ feature is exclusive to Dukane’s ultrasonic servo welders.

Figure 7: Pareto chart shows most significant weld parameters

Specifically, the analysis shows that a reduced Weld Speed, increased Weld Distance, and increased Melt-Detect™ percent all improved weld quality [Figure 8]. The locations of the red dots off of the lines indicate that there may be some curvature in each of these effect plots. A closer look at some of these factors would be warranted based on these results.

Figure 8: Main effects plot shows how each parameter effects weld strength

Surprisingly, there was little interaction between each of these weld parameters. If there was significant interaction in the settings, it would be indicated by the lines on the charts in Figure 9 crossing. However, there could be some interaction in these parameters that was outside this selected value set.

0.350.300.25 321 1009080 1050 0.100.050.00 531 1.00.50.06000

4000

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A - Force (N)

B - Dist (mm)

C - Speed (mm/ s)

D - Amplitude (%)

E - Melt Detect (%)

F - Hold Dist (mm)

G - Hold Spd (mm/ s)

H - Static Hold (s)

100 Corner250 Center400 Corner

Force (N)A Trigger

Point Type

Force (N)A - Trigger

Point Type0.25 Corner0.30 Center0.35 Corner

Dist (mm)B Weld

Point Type

DistanceB - Weld

1 Corner2 Center3 Corner

Spd(mm/s)C Weld

Point Type

SpeedC - Weld

80 Corner90 Center

100 Corner

D Amplitude Point Type

AmplitudeD - Weld

0 Corner5 Center

10 Corner

Detect (%)E - Melt

Point Type

Detect (%)E - Melt

Point Type

0.00 Corner0.05 Center0.10 Corner

Dist (mm)F Hold

Point Type

DistanceF - Hold

1 Corner3 Center5 Corner

(mm/s)G Hold

Point Type

Interaction Plot for AverageData Means

Figure 9: Interaction plot shows that the selected weld parameters work independently within this range.

Speed Trials

A follow up experiment was conducted to take a closer look at the effect of weld speed on weld strength. As can be seen in Chart 3 below, significantly slower weld speeds resulted in vastly improved weld strength results even at the nominal weld distance and Melt-Detect™ levels selected for this test.

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Average Std Dev

Chart 3: Speed trial average & standard deviation results

Conclusions

These experiments have resulted in a variety of significant findings. First, the advantages of using Dukane’s new ISTeP part design over the traditional I-Beam design have been shown. The ISTeP parts allow more accurate measurement which can help develop processes for critical and precise applications.

Second, it has been shown that the pull test equipment developed for this part can provide consistent results for accurate analysis of weld strength.But beyond this, perhaps the most exciting discovery is the importance of Weld Speed, Weld Distance, and Melt-

Detect™ to weld strength. Weld Speed and Weld Distance can each be precisely controlled by servo-driven Ultrasonic welders, and this research has demonstrated why this ability is so important to critical assemblies.

Melt-Detect™ is a component of Dukane’s Melt-Match® technology which allows the welder to turn on ultrasonic energy with no movement once part contact has been detected. The welder then waits at this position until a drop in force has been detected that indicates melt has initiated. Melt-Detect™ is a feature which can be enabled to define a percent of drop in force that must be observed, below the trigger point, before downward motion is initiated. This study has shown that this feature, which is only available with Dukane’s servo-driven ultrasonic welders, provides a key means of adjusting the weld for increased strength.

Finally, detailed investigation shows that the ability to weld at very slow weld speeds allows drastically greater weld strengths to be achieved.

Altogether, the use of Dukane’s ISTeP part design along with the advanced settings available with Dukane’s servo-driven ultrasonic welder allow greater experimental insight into the ultrasonic welding process than ever before possible. These abilities are necessary to the continued improvement of weld quality and consistency required for the increasingly complex and critical welded assemblies being created today.

Future Work

Similar Design of Experiments can be conducted in a variety of materials to determine how material properties affect weld quality and consistency. Additionally, in depth studies of weld speed control, including speed profiling should be investigated. Dukane’s Melt-Detect™ feature is also an area where further research can be beneficial.

Acknowledgments

Special thanks to Paul Cathcart for designing the ISTeP parts and pull testing apparatus. We also want to thank John O’Toole for making design revisions to the ISTeP pull test tooling. Finally, gratitude to Joe Stevens for stepping in as an operator for the Gage R&R experiments.

References

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2. US Patent 7,819,158. Oct. 26, 2010. Klinstein et al. Ultrasonic Press Using Servo Motor with Integrated Linear Actuator.

3. US Patent 8,052,816. Nov. 8, 2011. Klinstein et al. Ultrasonic Press Using Servo Motor with Delayed Motion.

4. European Patent EP 2 106 899 B1. May 22, 2013. Klinstein et al. Ultrasonic Press Using Servo Motor and Delayed Motion Technique.

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8. R. Leaversuch. “How to Use those Fancy Ultrasonic Welding Controls” Plastics Technology. Oct 2002.

9. P. Golko. “Boost Performance, Speed, Economy with Servo-Controlled Welding” Plastics Technology. Aug 2011

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11. M. Marcus, K. Holt, A. Mendes. “Benefit of Servo-Ultrasonic Welder to Medical Industry – A Case History” ANTEC 2012.

12. A. Benatar. “Servo-Driven Ultrasonic welding of Semi-crystalline Thermoplastics” 39th Annual Symposium of the Ultrasonic Industry Association. Cambridge, MA. 2010.

13. S. T. Flowers. “Servo-Driven Ultrasonic Welding of Biocomposites” ANTEC 2012.

14. A. Mokhtarzadeh and A. Benatar. “Comparison of Servo and Pneumatic Ultrasonic Welding of HDPE Shear Joints” ANTEC 2011.

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16. “Specification for Standardized Ultrasonic Welding Test Specimen for Thermoplastics” American Welding Society G1.2M/G1.2:1999.