The challenge was attaching multiple leaf springs to an insert molded plastic part with electrical conductors. Riveting these leaf springs in place was not an option. Therefore, the company wondered if ultrasonic welding could meet volume manufacturing requirements and quality standards.
According to Rick Winfree, president, "Insert molding accounts for about 70 percent of our work. It is a highly specialized and rapidly growing business. Auto manufacturers are looking for ways to reduce part counts, cut assembly costs and improve quality. Insert molded parts are a little like irregularly-shaped, three-dimensional circuit boards. But unlike board manufacturers who literally print a low-voltage circuit on a flat surface, we insert copper carriers into precision plastic components, leaving the terminals exposed."
Winfree continues, "In traditional welding systems, stronger is better. Little or no attention is paid to weld area deformation. However, this project required precise dimensional control. Resistance welding was ruled out because of the amount of deformation. That left riveting or ultrasonic welding as possibilities."
"Rivets are notorious for loosening when sub-jected to constant vibration and provide a less than ideal electrical connection," Winfree adds.
Ultrasonic welding offered a possible solution. However, the company did not automatically go out and buy ultrasonic welding equipment. It had to be shown that ultrasonic welding would meet both the quality standards and the demands of volume manufacturing.
The company chose Stapla Ultrasonics to design a precision manufacturing cell incorporating ultrasonic welding equipment that would meet these exacting needs. Winfree says, "We realized that 'over welding' could deform the leaf springs and impair their movement. That's why it was essential to provide strict dimensional control over the welding process. This requires a high level of precision where each of the ultrasonic welding parameters--energy, welding force, time and amplitude--are controlled within strict limits."
"Fortunately, these ultrasonic welders have a controller that maintains preset welding parameters. The challenge was to find and refine the right settings and then work with QSI Automation (Churubusco, IN), the systems integrator, to build a fully automated manufacturing cell," Winfree concludes.
The new system is currently making 100,000 welds a week. The manufacturing cell utilizes dual indexing tables with pneumatic and servomotor operated arms to position each component for welding. Positioning pins align each leaf in the correct position. After the welds are made, the finished component undergoes electrical testing.
Ultrasonic metal welding uses motion and kinetic energy to join metal parts by creating a permanent molecular bond. As the molecules in the two parts intermingle with one another, they unite the two materials. If a thin aluminum sheet is ultrasonically welded to a thin copper sheet, copper particles appear at the end of a predetermined bond region in the aluminum sheet. Aluminum particles appear at the end of the copper sheet's bond region, showing that the materials have penetrated each other. Because no oxides are left in the interface, a true metallurgical bond is achieved.
This is an advanced process where horizontally introduced mechanical vibrations at 20 to 40 kilohertz produce both shear and static forces. Static force is introduced at right angles to the working force. Deformation isn't a problem as long as the forces inside the workpieces are below the limit of linear elasticity.
The shear force breaks down contamination on the surface of the workpieces, while diffusion occurs at the atomic level in the contact area. As a result, the metal recrystallizes into a fine-grained homogeneous structure having the properties of a cold-worked metal. The entire process takes place during a fraction of a second. Heat build-up is minimal.
Pressure, amplitude and welding time must be controlled within tight parameters for this fusion to occur. The temperature profile during this process is somewhat dependent and, within limits, can be influenced and held to 10 percent to 15 percent of the melting temperature of the metal being welded.
For more information on automated assembly systems, call QSI Automation at 219-693-1500, visit www.qsiautomation.com or Reply 1.
For more information on ultrasonic welding systems, call Stapla Ultrasonics Corp. at 978-658-9400, visit www.staplaultrasonics.com or Reply 2.