Robotic ultrasonic welding can be used to join metal or plastic parts. The technology is ideal for some applications, such as welding battery packs, but may not be difficult to justify for other projects.
The Edison Welding Institute (EWI) is the leading organization in North America dedicated to materials joining technology and engineering research. ASSEMBLY magazine recently asked several EWI specialists, including Matt Bloss, Sean Flowers and Karl Graff, to comment on the pros and cons of using robots for ultrasonic welding applications.
Boss specializes in ultrasonic metal welding, while Flowers is an expert in ultrasonic plastic welding. Graff is one of the world’s leading authorities on high-power ultrasonic welding technology, with extensive experience on both the metal and plastic side. Here’s what they had to say:
ASSEMBLY: How popular is ultrasonic robotic welding? Is it growing in demand?
Bloss: Ultrasonic robotic welding currently is limited in use. However, applications are emerging with much higher speed and quality requirements.
Flowers: For some plastic welding applications, ultrasonic robotic welding is growing in demand. The most demand is for flexible robotic systems that can be used to weld multiple components for low-volume applications that require several welding operations on a large component, or for high volume applications. Some robotic systems can incorporate multiple welding horns and don’t require manual tool changing. Via programming, weld parameters and welding horns can be changed as needed.
Graff: This allows one robot system to be very flexible and maintain the ability to be reprogrammed to weld future applications.
ASSEMBLY: What type of ultrasonic welding applications are typically done with robots?
Bloss: Manufacturing battery packs is one application where robotic welding is required. First, it is necessary to minimize operator contact with the battery pack terminals, because the packs are welded live and contain dangerous amounts of stored energy. Second, battery packs often have hundreds of welds in series, with no redundancy; if one weld fails the entire unit loses power.
Flowers: Common applications include bumper attachment, door panel components, dashboard assemblies, manifolds, filters, internal trim, and sensors for the automotive industry. The consumer electronics, sporting goods and cosmetic industries also utilize robotic ultrasonic welding for several applications.
Graff: Robotic ultrasonic welding is most popular for high-volume applications.
ASSEMBLY: What are the advantages of combining ultrasonic welding and robotics?
Bloss: The advantages are safety, speed, reliability and consistency. A manually-driven weld station can actually have more flexibility than a programmed robotic cell. It is desirable to have a manual weld station for prototype assembly and for a backup weld station to use during possible robotic downtime.
Graff: One robotic system can be programmed to weld multiple components on several applications, making them very flexible and appealing for many different industries. Ultrasonic welding requires the precise control of positioning that robot controls offer. Robots provide repeatable manipulation of the ultrasonic horn for consistent weld results.
ASSEMBLY: What are the challenges of using robots for ultrasonic welding applications?
Bloss: Ultrasonics, in general, is challenging because of a lack of experience and unfamiliarity with the process. To automate an unfamiliar process can be challenging. The automotive industry is well experienced with the automation of resistance spot welding , but rarely an ultrasonic metal welding system. It may also take some extra time to integrate an ultrasonic weld controller with a robotic cell. It is desirable to have the ultrasonic system held very rigidly while firing a weld cycle. This requires a powerful robotic system. In resistance spot welding, the heads can be much lighter, requiring smaller robots.
Graff: The use of robots requires the knowledge base of both robotic programming and ultrasonic processing. Because ultrasonic welding can be very sensitive, optimization of the welding process in addition to programming a robotic system can be challenging.
ASSEMBLY: Is any special end-of-arm tooling required for robotic ultrasonic welding?
Bloss: In ultrasonic metal welding, the end-of -arm tooling is usually something that is strong enough to reliably hold the heavy weld head, which can weigh more than 70 pounds. In some flexible welding stations, it may be desirable to have a quick-change end-effecter to change from one weld head to another, but this is more common in ultrasonic plastic welding.
Graff: Application-specific welding horns are required.
ASSEMBLY: Are special welding horns required?
Bloss: Horn and anvil design typically depends on the application. For robotics or automation, it may be desirable to machine chamfers and other features that will help guide the tooling in the event of a minor interference.
Graff: Fixturing and horn design depend on application.
ASSEMBLY: What type of software or controls is necessary?
Bloss: While a robotic cell or CNC station could be programmed without logic, it is not desirable to do so. It is also easy to get too involved with integration and sensors. Some level of feedback is desirable to ensure that the system doesn’t crash into anything, can recognize the presence of parts to weld, and can adapt the weld location if the weld components are slightly off from the designed coordinates. With a higher-speed system or a cell that makes many welds without pausing, the welding equipment needs to be able to recognize an “out of the norm” situation and generate a fault or error. Most commonly-used ultrasonic metal welding systems allow program limits to be set for each welding parameter, such as the pre-weld height, the post-weld height, the energy, the power and the time. For example, in battery manufacturing, if a cell tab (electrode) is out of place, the pre-weld height limit would be out of the limits and the system would not fire the weld cycle.
ASSEMBLY: Is any unique type of fixturing required?
Graff: The type of fixturing depends on the application. Most robotic ultrasonic welders require precise positioning. Depending on how close the tolerances are for each weld, the fixturing may have to involve high quality THK rails or be machined for closer tolerances. In the case of a battery pack, the fixturing has to be designed to reach down between sets of exposed battery tabs. There is always a potential for a short if done incorrectly, potentially damaging the welding system and the battery pack.
ASSEMBLY: Is robotic ultrasonic welding more expensive than traditional ultrasonic welding? If so, how can the extra expense be justified?
Bloss: I’m sure we’ve all heard this before, but it depends. Most applications can be addressed without robotics. For the applications that do make sense, either for safety, volume, or consistency, a robot has to be integrated well and usually has to be used to produce a fairly substantial lot size in order for it to be truly financially justified. If the integration has not been troubleshot, the programmer and technician may spend more time fighting with the robotic cell than it would take to simply manually weld the same product.
Graff: Robotic ultrasonic welding is typically more expensive than a standard ultrasonic welding press, but benefits can be realized when multiple, complex weld sequences are required for one or more applications.
ASSEMBLY: What types of robots typically work the best with ultrasonic welding applications?
Bloss: I would recommend an off-the-shelf six-axis integrated robot, just because of the flexibility, cost and ease of use. However, most ultrasonic metal welding applications involve some sort of CNC gantry cell. While the startup cost may seem lower with a CNC system, by the time it is fully integrated, it often costs more and takes substantially more time. In either case, most ultrasonic weld heads weigh at least 70 pounds and need to be moved quickly, precisely and held rigidly during the weld. This usually implies that the robot or CNC is a fairly substantial setup.
The Pros and Cons of Robotic Ultrasonic Welding
February 5, 2010