Traditionally, dedicated ultrasonic welding equipment has been used to assemble only one type of product at a time. But, in today's high-mix, low-volume environment, manufacturers are demanding more flexibility.
Several challenges confront engineers when using robots for ultrasonic welding applications. One concern is cycle time. If an application requires many weld points, the cycle time can become long.
“On traditional sonic welding equipment with dedicated welders for each point, the cycle time can be very fast,” explains Brunssen. “However, on parts that have many welds required, the cycle time has to be considered to make sure it meets the desired production requirements. We can get around this by using multiple robots or multiple weld heads or part indexing, but each application and customer requirement is unique.”
Another significant factor can be the potential added cost of robotic equipment. “The upfront cost can sometimes be greater than dedicated machines,” Brunssen points out. “But, robotic sonic welding is not necessarily more expensive. There are a lot of factors that determine the cost compared to traditional types of assembly equipment. [It’s important] to develop a system that can meet budget requirements without sacrificing future flexibility.”
In recent years, the price of robots has decreased, while the variety of products and features has increased. “The benefit comes from the end user’s ability to reuse the robot for a new application,” says Gourley. The advantage of a robotic cell is that it can be easily retooled for future applications. Manufacturers can run several programs in a cell vs. several dedicated cells.
“If robotic ultrasonic welding were used for a dedicated product, then it could be more expensive,” adds Naumovski. “The reason to justify it would be cycle time. If the assembly process were tied to a molding machine operation of say 45 seconds, and there were six welds to be accomplished, if the robot could accomplish the task in that time frame, then it’s justifiable. If the welds take longer to accomplish, then a different process should be considered.”
Payload capacity is another important consideration. A 20-kilohertz welding stack can be quite heavy. As the weight of end-of-arm tooling goes up, the cost of a robot typically increases.
“End-of-arm quick-change tooling can be used to solve this problem, but this adds cost and complexity to the machine,” warns Michael Johnston, vice president of sales and marketing at Dukane Corp. “Ideally, the end-of-arm tool should include a small-stroke air cylinder. This provides for significant improvement of weld performance and machine setup.
“In an effort to reduce end-of-arm tooling weight, a special nodal-mount, high-gain horn [allows engineers to eliminate] a booster,” adds Johnston. “This can reduce end-of-arm tool weight by as much as 30 percent.”
Other factors that engineers should consider are programming, controls and fixturing. They must ensure correct alignment of the weld points, perpendicularity of the weld horn to the part, and consistent application of force by the robot. A
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