Ultrasonic welder uses pneumatic innovation

Design Engineering reports on a new ultrasonic welder for bumper assembly that has been developed with pneumatics technology

When Luton-based ultrasonic welding and joining specialist, Herfurth was commissioned to develop a precision welding system for the manufacture of vehicle bumper assemblies, the company took an unusual route. To provide an inexpensive solution to the design challenge, Herfurth decided to incorporate a new-generation of pneumatic handling equipment in the design.

Established in 1984, Herfurth UK is a manufacturer of specialist ultrasonic welding equipment and the only ultrasonic company manufacturing these advanced systems in Britain. All bespoke ultrasonic systems are designed in-house, with the transducers and generator modules manufactured exclusively for the company under a Herfurth licence. Herfurth also specialises in radio frequency, hotplate and spin-welding technologies.

The contract called for the design of a bumper welding machine to the prime contractor of a global car manufacturer. The application itself required ultrasonic `staking’ to secure various moulded sub-assemblies into the final bumper form. On the customer’s production line, the machine was required to operate with a floor to floor cycle time of 45s.

`Using the bumper specification and required cycle time as a basis, we proposed a highly robust but precise solution,’ says Brian Smith, managing director of Herfurth. `The welding design requirements necessitated that a four phase manufacturing process was used: phases one and four are the load and unload activities with phases two and three being the twin table positions for the two-stage weld.’

The Herfurth tender, along with the design concept, was favourably received and, fortunately, Herfurth was brought in at an early stage by the prime contractor and was able to contribute to the design for the moulding tools for the bumper in order to assist ease of access for the welding heads.

`Inevitably, form followed function,’ Herfurth design manager Colin Coles points out. `We started with the layout and profile sizes of the ultrasonic heads, deploying advanced spring-loaded transducers mounted on non-rotating spline bearing shafts from IKO-Nippon Thompson for optimum weld consistency. Then we needed to decide how best to automate the process. Of all our designs across our markets, this machine had the most moving elements!’

The Herfurth designers looked at the options for automation and were unsure whether conventional electronic robots would prove robust or reliable enough – particularly for the unwieldy handling and transport operation moving the bumper assembly into the ultrasonic work stage.

Having approached many pick and place companies offering a variety of solutions, Herfurth worked with Festo to develop a robust pneumatic handling system. `We were not immediately convinced that pneumatics was the right way to go. But after investigation, we managed to prove the viability of the process and found that the cost was surprisingly attractive. In fact, we were able to reduce the cost over all other solutions we had investigated,’ says Smith.

Festo’s new HMP range of cylinders, designed expressly for handling applications, was specified to load the bumper assembly into a split location nest and, after the welding stages, unload the bumper for final finishing. Sensors on the arm ensured that all the subassemblies were in place before the bumper was placed into the nest.

Selected for their rigidity and ruggedness, the HMP cylinders feature a substantial guideshaft which gives powerful but adjustable interconnection between the X and Z movement axes to ensure the demanding operating repeatability required by Herfurth.

A sliding table, mounted on IKO linear bearings and powered by a servo-electric drive, moves the location nest between the two welding stages. At each stage, the weld heads, mounted on vertical or angled platens (also using IKO linear bearings), are pneumatically driven into position, staking a total of six components to the bumper. These devices offer precision control over end positions, velocity and motion profile. In the event of a drop in air pressure, locking cylinders on the vertical welding platens prevent the weld heads from dropping and causing injury or damage to the tools.

The modular welding machine uses a Fieldbus system to reduce the amount of wiring; it also saves time and materials costs – especially in installation and during commissioning. The specification also called for a Siemens PLC, which was interfaced with the Festo system via Profibus.

A BUMPER MACHINE

Herfurth’s final bumper welding machine uses 39 ultrasonic heads activating 70 weld points on the bumper assembly. The system uses Herfurth’s proprietary ultrasonic switching unit, which enables several welding transducers to be operated from one generator. The machine will run 20 hours per day for two to three years – based on the expected life of the given vehicle bumper before being superseded by a facelift variant. In addition, it is likely to continue after this time producing spare bumpers.

`The new technologies – the bus system, servo-pneumatic drives, and servo-electric table drive – were all new departures for Herfurth,’ explains Coles. Despite the learning curve, he has been surprised at the ease with which the system was integrated: `The wiring was simple; `clipping’ on the modules was simple; the PLC software programming was straightforward; and the machine looks very clean in its design.

`Pneumatic pick and place handling now offers our customers and us a real alternative to electrical servo-driven robot solutions. It gives the same levels of control in positioning, speed and motion profile but without the complexity and cost – which makes Herfurth systems even more competitive,’ concludes Coles.

{{Herfurth Tel: 01582 436000FESTO Tel: 01252 775000IKO NIPPON THOMPSON Tel: 01908 566144}}