Automaker improves weld seam cleaning results with switch to waterjet

Situation

Electric and hybrid vehicles are changing how vehicles are built in ways that go far beyond the drive technology. For example, because of the weight of the battery, integral subframes now need a higher load-bearing capacity. The redesign of these axle components also is prompting many vehicle manufacturers to reassess their production processes, such as the cleaning step to remove impurities such as silicate islands, scale, and smoke residues formed during welding. These can cause major quality problems in the subsequent cathodic dip painting process and lead to corrosion damage.

This was the situation for a Germany-based automobile manufacturer. The automaker was using the traditional method of blasting parts with a solid medium, such as corundum. Then to remove dust and abrasive residues from the parts before cathodic dip painting, it would send the parts to a separate machine for aqueous cleaning.

These conventional solid media blasting processes with the additional aqueous cleaning step proved to be wear-intensive and costly, however. So when designing a new production line for integral subframes, the automaker’s project planner sought information about ways to optimize this cleaning process. His aim was to clean weld seams with the same or better results as the conventional methods, but at a lower cost and with less maintenance and use of resources. He also wanted to improve employees’ working conditions by eliminating the dust pollution caused by fragmented abrasives from the company’s solid-state blasting machines.

Resolution

The project planner found what he was looking for with Ecoclean's EcoCbooster technology. The patented process employs medium-pressure water between 300 and 700 bar without the addition of abrasives. Through the use of ultrasound, the prestressed column of water turns into an ultrahigh-frequency, pulsating waterjet after exiting the nozzle. This imparts a high kinetic energy to the jet so that it can reliably and reproducibly eliminate silicate and weld spatter, as well as scale and smoke residues, from the surfaces in a clearly defined operating window. Feed rates of up to 800 mm per second are possible, with a robot moving either the workpiece or the nozzle tool.

The part is cleaned at the same time the welding residues are removed, eliminating the need for the additional aqueous cleaning step and corresponding machine. The process water is mixed with a corrosion protection medium as well so that it can be recirculated for a long time for resource efficiency.

The effective range of the waterjet can be adapted to the application by using different nozzle geometries, including fan, solid jet, and lance. This enables selective cleaning of both the outer and inner contours of the integral subframes.

These process parameters were precisely adjusted for the automaker during a special test setup. To validate the new waterjet process, integral subframes were coated in the cathodic dip painting line and then subjected to a lengthy salt spray test. The weld seams were cleaner after treatment with the pulsating waterjet than after blasting with abrasive media, and any discoloration from the welding process still visible on the surface had no effect on the quality of the paint finish.

For the automaker to implement the developed process in a fully automated machine concept, it had to jet, clean, and dry an integral subframe within 50 seconds. To meet this requirement, the cleaning machine has three independently functioning jetting modules with an integrated manipulator, a hot air dryer, one robot each for loading/unloading, and a bath treatment unit for the process water. Pallet cages loaded with several integral subframes to be processed are transported to the machine. The machine’s loading robot picks up the parts and places them on the transfer stations of the jet modules. The manipulator of the respective module picks up the subframe and moves it to the jet tool, and then places the cleaned part on a clean surface. The loading robot then takes the subframe to the drying step, after which it is picked up by the unloading robot and hooked into the rack for painting.

The vehicle manufacturer has been using the new cleaning system in series production since mid-2020. Because of the optimizations achieved, the automaker now is considering integrating the new cleaning system into more production lines for integral subframes.