In one application, ultrasonic was tasked with welding aluminum-coated insulation panels for the engine compartment. The material combination was a killer: Aluminum-coated polyurethane (PU) acoustic mats had to be bonded to glass-fiber reinforced polypropylene (PP) panels. If that wasn’t enough of a challenge, six different parts had to be welded.
This application represented a real challenge for permanent and safe joining of the components. Adhesives would not work due to cost considerations and the combination of materials. Clamping elements or screws were too risky, because they could become loose during motor operation. None of the other thermal joining methods, such as laser welding, infrared welding or vibration welding, were suitable.
Initial tests using a handheld ultrasonic welder proved positive. A tight joint could be created between the PU mat and the injection-molded PP part, despite the intervening aluminum layer. It was possible for ultrasonic vibrations to penetrate the aluminum coating and energize the molecules in the PP part.
The task then became how to obtain repeatable weld results and ensure the customer’s tensile strength requirement of at least 50 newtons per weld spot. Wear-resistant hardened steel proved to be the right material for the sonotrodes. Herrmann engineers determined the weld spot locations and designed the complex fixtures, including the clamping system and part scanning sensors.
The sonotrodes were mounted to pneumatic actuators attached to six-axis robots. Six different fixtures were designed to match the part shapes and to support the weld spots.
Assembly takes place on rotary indexing tables. In two positions, assembly is done by two robots. At a third position, another robot automatically loads and unloads the parts.
Valuable time—up to one second per weld spot—was saved by preloading the pneumatic cylinder that extends and retracts the ultrasonic stack. Preloading means that the cylinder has already been extended via an intelligent pneumatic system before the robot brings the weld tool in contact with the weld spot.
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