in Vol. 10 - May Issue - Year 2009
Cleaning with CO2 snow before laser welding
Cleanliness Gives The Best Connections Increasing productivity, reliability and environmental compatibility whilst at the same time reducing operating costs and maintenance costs – these are fundamental requirements in the construction machine industry. Helping to achieve these aims, the Volvo Construction Equipment AB also develops innovative gears. The aggregates are produced by the Swedish manufacturer using modern technologies such as laser welding. A CO2 snow-jet cleaning system has been integrated into the production line to ensure the level of cleanliness required for the welding sites.
Volvo Construction Equipment AB (Volvo CE AB) is one of the world’s largest manufacturers of construction machinery. The range of products comprises over 100 different models of wheel loaders, backhoe loaders, wheeled excavators, motor graders and other similar machines. The gears and powered axes for these vehicles are developed and produced by the company’s component division in Eskilstuna in Sweden. “In order to constantly improve our machines, we are always working on new solutions in this field” stated Lennart Larsson, project manager of investments at Volvo CE AB, Component Division. One of these innovative solutions is a new gear made up of components which have been connected for the first time using laser-welding. The advantage of this joining technique is that the connections are much more accurate and warp to a much lesser extent. However, a pre-requisite for this is that the welding sites must be absolutely clean and free of grease. After their manufacture, the cogs and other gear components pass through an aqueous cleaning process with a final rinse containing an anti-corrosive agent. “In order to obtain an optimum welded joint, this protective layer has to be reliably removed at the welding sites”, said the project manager.
Big demands – not just regarding the cleaning result
During the development phase of the gear, Lennart Larsson started looking around for a suitable cleaning method. The most important requirement was for a "dry" technique that gave reproducible results and a high degree of process reliability. Further demands included high system availability, low space requirements and easy integration into the manufacturing process. Larsson found what he was looking for with the special CO2 snow-jet technology developed by acp GmbH. The technology of the Wiernsheim-based (Germany) company is also used to finely clean defined areas on functional surfaces.
Comprehensive cleaning tests performed together with the manufacturer of the laser welder
Talks showed that the cleaning technique was suitable as a cleaning process for use before laser welding. "First of all, we carried out cleaning tests in the company's test centre. When these showed promising results, we then went to the manufacturer of the laser welder - the Trumpf company in Ditzingen - together with the original gear components and a mobile cleaning system made by acp and continued the tests there", recalled the project manager. In order to visually assess the quality of the laser-welded joint, the welded gear components were sawed apart and the weld seams inspected. Torque tests were also performed to assess the resilience of the weld seams. Once the tests had been successfully completed, Volvo CE opted for the cleaning module made by acp.
Effective cleaning with liquid carbon dioxide
The snow-jet cleaning process has been developed by acp in close cooperation with the Fraunhofer Institute for Manufacturing Engineering and Automation (IPA). The technique uses liquid carbon dioxide as a blasting agent and is fed constantly to the module by way of cylinders at Volvo CE AB. The non-toxic, non-flammable CO2 snow removes solid and filmy contamination through a combination of mechanical, chemical and thermal effects in a dry-cleaning process without leaving any residues. The snow crystals liquefy on impacting on the surface to be cleaned and then sublimate. The resulting sublimation impulse causes the fine particulate contamination present to detach and be removed. In the liquid phase, the environmentally-neutral carbon dioxide functions as a solvent and removes filmy layers or contaminants such as anti-corrosive oil. A further advantage of this process is the low degree of hardness of the tiny snow crystals; this ensures that the surfaces of the welding sites do not become damaged and impair the quality of the weld seam.
The basis of the good cleaning effect of the snow crystals is formed by the cleaning head which is a supersonic two-component ring nozzle. The liquid carbon dioxide expands on exiting the nozzle and forms a mixture of snow and gas which makes up the core jet. Compressed air is also fed to the cleaning head as a jacketed jet which accelerates the CO2 snow crystals to supersonic speed. In comparison with single nozzle systems, the acceleration with compressed air makes the cleaning process much more effective and also reduces carbon dioxide requirements.
The cleaning module supplied to Volvo CE AB also contains a media preparation unit. This filters out any fine contamination present in the liquid carbon dioxide or compressed air before it reaches the cleaning head. The cleaning medium is thus always available in a consistently high quality, ensuring a constant and reproducible cleaning result.
Fully-automated cleaning process integrated into the production line
The main reasons for deciding on the CO2 snow-jet cleaning system from acp were not only the cleaning result but also the support provided by the company to integrate the cleaning process in an optimum way. "acp even helped us to choose the right interfaces which enabled us to optimally incorporate the cleaning system into the production line" reported Lennart Larsson.
The CO2 snow-jet cleaning module is part of a complete system equipped with four robots. Not only are the fully-automated cleaning and laser-welding processes carried out, but one robot also brushes off residues from the welding process. "The system was put into operation in December 2006 and has been running smoothly ever since. At the moment, it looks as though we’ll have to extend capacities. We’re definitely going to be investing in a second cleaning system from acp because we’re so pleased with this solution" explained the project manager responsible for investments.