Ultrasonic Assembly: Controlling Influence
A common attribute of successful grand prix race car drivers is their un-canny ability to withstand intense vibrations and g-forces while guiding sleek, fast-moving vehicles through a series of undulating twists and turns. Too much or not enough control over intense operating conditions can ruin their day. Precise control over process variables also influences the effectiveness of ultrasonic welders.
More and more ultrasonic welding machines are now equipped with digital power supplies and microprocessors that monitor and improve the joining process. In addition, more welding frequencies are available, ranging from 15- to 70-kilohertz systems, to handle a wider range of applications. As a result, assemblers have more opportunities for joining metal, plastic and nonwoven materials with much higher quality than ever.
Ultrasonic welding is a form of friction welding that joins parts together by vibrating them against each other. The parts are clamped together while a vibrating tool called a horn transfers ultrasonic energy to the joint interface. Heat is generated through a combination of friction and hysteresis. Amplitudes of 20 to 80 microns are usually produced at the end of the horn. When the ultrasonic vibrations stop, the molten material solidifies and a weld is achieved.
In plastic welding applications, ultrasonic vibrations are applied vertically. In metal welding, they are applied horizontally. In plastic welding, the parts are heated to their melting point. In metal welding applications, the parts get hot, but they don’t fully melt. Instead, the ultrasonic vibrations break up oxides and films on the part surfaces, permitting metal-to-metal contact.
Cycle Arial are short. In fact, the entire welding process takes 0.5 second or less. “There is no faster way to bond two pieces of the same or compatible plastic than ultrasonic welding,” claims Robert Bishop, president of Sonitek Corp. (Milford, CT).
Ultrasonic welding offers many advantages to assemblers. It is a fast, efficient, flexible process that offers quick tooling changeover. No warm up or cool down time is necessary, unlike other welding processes, such as hot-plate welding. Ultrasonic welding is an energy efficient cold-working process that does not require elaborate ventilation.
Unlike other joining processes, such as resistance welding, ultrasonic welding does not generate excessive heat that could damage components. The energy efficiency of ultrasonic welding technology appeals to manufacturers that are searching for ways to cut production costs. “Ultrasonic welding only uses energy on demand,” says Jeff Frantz, director of applications and acoustic tooling at Branson Ultrasonics Corp. (Danbury, CT). “The user is not paying for energy except when in use.”
Despite those advantages, ultrasonic welding is facing fierce competition from other thermal joining processes. “This is due to the increasing competitiveness of the vibration and spin welding processes,” claims Dr. Karl Graff, executive director of the Edison Welding Institute (EWI, Columbus, OH). “For example, vibration welder costs have come down significantly, and it has fewer material and geometry limitations.” Graff says lasers are another emerging competitor.
Other observers agree that there has been a decline in the ultrasonic assembly market. “More forgiving processes, such as spin welding and thermal staking systems, are replacing ultrasonic systems in many areas,” notes Dave Kralovetz, north central regional sales manager at Forward Technology Industries Inc. (Minneapolis). “Spin welders and thermal stakers are very price competitive with ultrasonics. Vibration welding and hot-plate welding are much more robust and less difficult to maintain. They are also much better suited for large part welding. But, sonic weld Arial are frequently much faster than thermal staking, vibration welding and hot-plate welding.”
“Ultrasonic welding is generally cost-effective,” adds Graff. “It is cheaper than laser welding and faster than hot-plate welding. Still, when all factors are considered with, say, spin or vibration welding, there may not be much to differentiate. Where pure processing speed is most important, such as consumer applications, ultrasonic welding may have advantages. However, there still may not be a great deal to differentiate ultrasonics over other processes.”
“Ultrasonic welders are more ex-pensive than resistance welders for metal and plastic,” admits Frantz. “However, the operation cost is lower and the process is more ‘in control.’”
In large volume manufacturing, ultrasonic welding is a more cost-effective process.
Because ultrasonics is the fastest thermal process for joining plastic parts, the growing use of that material is expected to continue to drive demand for ultrasonic technology. Many observers expect ultrasonic welding to remain a popular alternative to adhesives and mechanical fasteners. “In the automotive industry, ultrasonics is used frequently in replacement of fasteners wherever possible,” says Kralovetz.
New ApplicationsAutomotive and medical device assemblers continue to be the two largest users of ultrasonic welding technology. But, the joining process is gaining wider acceptance in other industries, such as packaging.
Ultrasonic welding is used to assemble a wide variety of medical devices, such as hearing aids, filters, monitors and diagnostic components. New applications include microfluidic devices, implantable devices, such as heart valves and pumps, blood analysis devices, and knee and hip joints.
“Advances in microfluidics and diagnostic devices offer substantial growth opportunities for ultrasonic plastic welding,” says Saeed Mogadam, president of Stapla Ultrasonics Corp. (Wilmington, MA). “The proliferation of handheld instruments manufactured with engineering-grade thermoplastics can generate additional applications suitable for the use of ultrasonic welding technology.”
According to Dr. Dominic Friederich, executive vice president and general manager of Herrmann Ultrasonics Inc. (Schaumburg, IL), the medical device industry had a calibration problem, due to FDA requirements, with microprocessor-controlled welders that utilized manual gauges, internal springs and other components. “The current high-end CNC welders provide full calibration capabilities assuring 100 percent repeatable performance over time and from welder to welder,” notes Friederich.
The automotive industry is one of the largest users of ultrasonic welding equipment. Many different plastic car parts are assembled with ultrasonics, including tail lamps, instrument panels, bumpers, fuel tanks, manifolds, fuel filters, valves and sensors.
Ultrasonic welders can also bond nonferrous metals, such as aluminum, brass, copper, nickel and silver. “The demand for ultrasonic metal welding is increasing at an annual rate of 15 percent,” claims Mogadam. “This is due to an increased awareness of ultrasonic metal welding.” He says the technology has become more common and understandable than in the past.
Rechargeable battery manufacturers, medical device assemblers and Tier One automotive part suppliers are increasing demand for ultrasonic metal welding. Typical applications include welding wires to terminals and flexible flat cables.
As long as the auto industry continues its weight loss program, ultrasonic welding should benefit. Indeed, automakers are using more aluminum to improve fuel economy, reduce emissions and enhance vehicle performance. The Aluminum Association Inc. (Washington, DC) claims that aluminum content in 2002 model year vehicles is at an all-time high—268 pounds per vehicle, compared with 255 pounds the previous year.
Ford Motor Co. (Dearborn, MI) is using ultrasonic technology for spot-welding aluminum body parts, such as trunk lids and door panels. “The new Lincoln LS is assembled using an ultrasonic welding gun attached to a robot arm,” says Janet Devine, president of Sonobond Ultrasonics (West Chester, PA). “For instance, plastic license-plate holders are attached to the trunk lid using ultrasonic welding. As a result, the company is using fewer rivets.”
Ultrasonic welding is also used to assemble nonwoven textiles for items such as medical gowns and face masks. Automotive air filters, oil absorbers and other filter media are a growing application for ultrasonics. “Filter manufacturers want to simplify the assembly process by eliminating stitching, gluing and heat sealing,” says Devine.
According to Branson’s Jeff Frantz, consumer products packaging is one of today’s hottest markets for ultrasonic welding. “The biggest change is occurring in packaging,” he points out. “Trends include larger parts, more blister packs and a change from polyvinyl chloride (PVC) to recycled polyethylene terephthalate (RPET).” Frantz claims ultrasonics is being used to weld or seal a variety of plastic packaging, such as clam shells and liquid container seals.
Process VariablesProcess variables are very important to ultrasonic welding. Controlling time, force, depth, amplitude and power is critical to making good welds. Key parameters include how much the material is compacted before the ultrasonic energy is turned on, how much the material is being compacted while the energy is being applied, total energy input to the joint and welding time.
While there are many different viewpoints on which variable is most important, most observers believe amplitude is the most critical. “Welding time and amplitude are the most critical factors in most applications,” claims Nitin Phadnis, president of Ultrasonics For Less (Sparks, NV). “There are a small group of applications that require the entire gamut of controls, but around 70 percent of applications weld on time and amplitude. Most manufacturers have improved on their consistency in controlling these variables so that you get repeatedly good results on the parts being assembled.”
“Welding amplitude is still and will always be the most critical process to control or deliver to the application,” adds Sonitek’s Bishop. “Many Arial I have seen applications fail because of insufficient or too much welding amplitudes delivered to the part through the horn and booster combination.”
Other experts claim that the depth of the weld is the most critical variable. The strength of an ultrasonically welded joint is a function of the depth of the weld, which can be adjusted to meet the requirements of the application.
“The actual traveled weld depth needs to be measured very precisely, including the weld travel during the hold time—the time where the weld horn remains on the part until the melt is solidified,” says Herrmann Ultrasonics’ Friederich. “Measuring the actual weld depth very precisely provides only a single value—the measured end point. It does not provide information about how this end point was reached. Therefore, it is very important to monitor the joining velocity or weld travel over time.” In most cases, Friederich says there is a direct relation between linear joining velocity and tensile strength.
According to Tony DiFinizio, engineering manager at Stapla Ultrasonics, the dominant process control variable is highly dependent on the application. “For example, time may be used for an automated process line where timing is critical and the parts are very consistent,” he explains. “Welding to a specific energy is helpful when parts may be contaminated or for assuring the best quality. In a few cases, the best correlation between quality and process control is found by welding to a specific compaction height.”
Important factors such as joint design, mold tolerances and material variability can be even more important than process variables. “One can end up chasing process variables when it is actually design and materials that are the issue,” warns EWI’s Graff.
The biggest challenge is gaining control over each of the process variables simultaneously to ensure consistency of results. Microprocessor- and software-based control schemes allow ultrasonic welders to automatically monitor a wide range of feature sets, ranging from controlling just welding time to controlling every variable and parameter of the weld.
Today, there is expanded control over all the variables that contribute to the ultrasonic welding process. High-low limits and tolerances are available for each of the main process variables. “By controlling all the process variables, we feel we can produce parts with consistency that don’t require total quality testing,” says Branson’s Frantz.
Control TrendsThe current crop of ultrasonic welders provide better monitoring of critical information than was available a few years ago. That’s because vendors are adding more bells and whistles to their products. For instance, PC-based control systems can measure the compaction as the tool compresses the parts against the anvil before the ultrasonic energy is turned on. If the measurement deviates from the correct value stored in the control system, the process is interrupted.
New technology allows end users to program weld parameters, profile process variables and maintain consistent quality. Indeed, the ability to view, collect and manipulate weld data is better than ever.
State-of-the-art systems allow precise control of the key variables in ultrasonic welding to verify that every part conforms with established requirements. A microprocessor-based welder also has the ability to communicate to outside devices. This allows concise reporting of weld data that shows the process is under control and verifies that good parts are being made.
High-speed microprocessors can provide closed-loop control of these variables, store welding programs in memory, and send welding data to a printer or another computer. “New technologies are constantly being introduced to get more accurate information and to utilize this information to assure the highest quality joints,” says Stapla Ultrasonics’ Mogadam. “Data collection software allows the ability to monitor a process and keep it in control.”
Several control trends are affecting ultrasonic welding:
- Digital generator technology is providing much more flexibility than traditional analog technology. All output signals can be adjusted via software and changed to operate critical weld horns that couldn’t be operated with analog generators.
- Computer numeric controlled (CNC) welders are allowing all pro-cess parameters to be numerically controlled.
- Higher power levels—up to 5,000 watts—are available, because more and more modified high-tech resins require more ultrasonic power to be welded.
- Industrial PCs with Windows-based operating systems are available, instead of custom-tailored microprocessor controllers.
- There is a growing trend toward touch-screen data access and analysis, in addition to data storage of performance parameters, such as curves for past welds.
“Today’s power supplies are far more sophisticated and really do take control of the process variables,” says Mogadam. Digital technology provides true closed-loop control with faster sampling rates. Digital technology also simplifies the connectivity of this equipment.
According to Sonobond’s Devine, digital power supplies take some of the variability out of the system and allow end users to get better control of key process variables. “They provide better overall control than analog pow-er supplies,” she points out. “The energy calculation is more accurate and reproducible.”
Branson Ultrasonics recently introduced a new generation of power supplies that provide real-time process control for amplitude stepping and built-in digital amplitude control by allowing the user to program amplitudes in steps from 10 percent to 100 percent of full amplitude output in 1 percent increments. Previously, ultrasonic welders could only change amplitude with mechanical devices known as booster horns and the changes were too extreme in reducing or increasing amplitude. These changes were fixed steps determined by the actual preset gain of a booster horn.
The increasing power and speed of microprocessors is expected to generate more applications for controlling the ultrasonic welding process. However, some observers are not sold on the new controls. “While ultrasonic welding systems continue to be developed that increasingly measure ‘more things,’ it is not certain that users are clear on taking advantages of this data,” warns EWI’s Graff. “In many cases, seat-of-the-pants systems adjustments may still be the primary method of setting up welders, with little full process review.”
“Many people make the mistake of believing that more controls mean better results,” adds Nitin Phadnis. “Actually, more controls mean more capital investment and higher maintenance costs. Most of the time, applications are simple. Assemblers just need time and amplitude control, with no real need for feedback or closed-loop systems.”
The biggest mistake many end users make is in the design of their parts for ultrasonic welding. Just because an ultrasonic welder has many process control features, it will not produce perfect welds if part quality is poor. In fact, Forward Technology’s Kralovetz claims that molded part imperfections prior to ultrasonic plastic welding are the cause of 95 percent of production problems.
“Regardless of how significant or advanced the welder features are, if you don’t design the part correctly for ultrasonics then even the most sophisticated welders won’t weld the part correctly or to the customer’s expectations,” concludes Mogadam.
Low FrequencyTraditionally, ultrasonic welding has been limited to relatively small-sized parts. When joining large parts or larger surface areas in the past, assemblers were forced to use other joining methods, such as vibration welding or hot-plate welding.
“Efforts continue on being able to weld larger parts through lower frequencies or multiple transducer drive of large horns,” says Graff. For instance, large part welding with ultrasonics is now being accomplished with lower frequency, high power systems.
According to Sonitek’s Bishop, 15-kilohertz welders are filling this void. “The 15-kilohertz welder is a significant new product,” says Bishop. “It allows every ultrasonic aspect to increase by 33 percent, including amplitude delivered to a horn face with deeper cuts.”
Dukane Corp. (St. Charles, IL) recently joined the low frequency trend and unveiled a 15-kilohertz model. “It is ideal for applications where ultra-sonics technology is typically passed over in favor of larger capacity, alternative assembly processes, such as vibration welding,” says Mike Johnston, Dukane’s national sales and marketing manager. He claims the welder can penetrate materials easier and faster than higher frequency devices. “The low frequency sound waves give the unit the ability to handle larger parts and softer materials thought by many to be difficult or impossible to assemble ultrasonically,” notes Johnston.
“Many companies are offering lower frequency units, such as 15 kilohertz,” says Kralovetz. “Unfortunately, applications for it are still quite limited in size and geometry. Vibration welding and hot-plate welding are much better suited for large part welding.” Kralovetz says these processes allow for hermetic seals on large, geometrically challenging parts.
Lower frequency systems can end up costing end users more money, notes Sonobond’s Janet Devine. “Noise is an issue,” she points out. “The noise level is much greater, which means you have to buy an enclosure. That can add several thousand dollars to the price tag.” But, Devine admits that lower frequency welders facilitate the welding of larger assemblies. Her company offers a 15-kilohertz metal welder.
“Lower frequency equipment, such as a 15-kilohertz welder, and higher power, such as 5 kilowatts, are two ways to weld larger parts or greater surface areas,” adds Branson’s Frantz. He says lower frequency enables larger horns to be designed.
Many new horn designs have been developed. “Composition horns, which combine several horns driven off of one common ‘mother horn,’ allow us to deliver several horn faces of varying amplitude purposely at various planes of welding,” says Sonitek’s Bishop. “Composition horns allow for deep reliefsas much as 10 inches or more when required. Combined with solid horn elements, they even allow for deep cuts into horn facesas much as 2.5 inches. That’s something unheard of a few years ago.”
“For irregular pieces and contours, multiple weld heads appear to be the only real way,” says EWI’s Graff. “Fundamental physics, in terms of horn breadth and vibration uniformity, is still a limitation.” He says there are two alternatives: using lower frequencies, such as 15 kilohertz, and using larger horns driven by multiple converters.
But, not all observers are bullish on low frequency systems. “For many plastic resins, low frequency systems do not provide enough mechanical motion per second to generate a true molecular bond,” warns Herrmann Ultrasonics’ Friederich. “The technically correct solution is to create larger joints with a single horn using twin converter technology. Two converters are mounted to a single 20-kilohertz horn, thus providing the possibility to achieve true molecular bonds nearly double the size.”
Consistent, repeatable ultrasonic welding requires that a uniform amount of energy is imparted to the joint, whether it is flat, curved or multilevel. Ongoing acoustic tooling research is one area in which advancements will be made to ensure that uniform energy is achieved.