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Oven-ready

Remote laser welding and cutting are not typically used to produce ovens. Electrolux decided to break with tradition.

Three welded metal sheets and an enamel coating — surely that’s all you really need to produce an oven ? Bernd Ebert, Director, Global Manufacturing Engineering at Electrolux, laughs: “If it’s that simple, why is the Electrolux plant in Rothenburg one of the most advanced household appliance factories in Europe? An oven cavity may look like a fairly basic box, but producing it involves many processes — from clamping and welding to enameling and baking.”

The Rothenburg plant produces some 700,000 built-in ovens each year for markets around the globe. In 2009, the company switched to the latest generation of Apollo ovens. The quality standards required for these ovens led to the development of a fully automated laser welding and cutting line — the first of its kind in the oven manufacturing industry.

The specifications called for an oven with a volume of 65 liters, available in various heights, that could be equipped with standardized components such as a heating element and oven lamp. “The Apollo range of ovens comprises 500 models, including features such as steam cooking and pyrolytic self-clean functions,” says Ebert, an engineer and business economist. “Our vision was to feed just one type of each basic component into the system. At that point the process steps should define which variant is to be produced.”

The team was also planning a big step forward in quality, aiming to eliminate the annoying edges that typically appear on the enameled surface. This was one of the main reasons for breaking with tradition: to use laser welding instead of resistance welding and laser cutting instead of punching. This new approach threw tried-and-tested processes into question and plenty of people within the company harbored reservations about the new technology, but Electrolux was determined to press on. The project involved the three European Electrolux production sites as well as integration experts called in from the company FFT EDAG Produktionssysteme (PS).

Automated to the max
The Rothenburg plant took on a pioneering role when it started using the new process for mass production in February 2010. The automated assembly line produces oven cavities at a rate of one every 12 seconds, with the top, bottom and cavity wrapper components being fabricated from 0.6 mm-thick, low carbon steel. The cavity wrapper is formed into a C-shape in a bending unit and transferred to a pallet conveyor. The top and bottom panels then enter the line via a “paternoster” and are also transferred to the pallet conveyor, at which point everything disappears through the tunnel into the welding booth.

“We use an electronic Kanban procedure that shows the machine operator which ovens are required and forwards the production program to the system. The plant control system displays all the active and ongoing production orders,” says Christian Ehninger, an engineer who works closely with Ebert. A steerable camera zooms in to reveal the complex inner workings of the welding booth. It contains two welding robots, each of which is equipped with a TRUMPF TruDisk 5302, and four cutting robots, each of which features a TRUMPF TruFiber 400. In the initial step, high-precision clamping chucks are used to hold the components together on the tool rack.

The Z-edg of the top and bottom panels serves as a limit stop. It must be at exactly the same level all the way round so that the overlapping part of the wrapper can be pushed up against it. Two small angle brackets are welded on, followed by a three-meter-long weld seam. Next, the oven cavities are picked up by six-axis robots and placed in the cutting pallets where the TRUMPF cutting lasers produce the complex hole patterns using special optically controlled cutting techniques.

The ovens then pass through a cleaning stage before being powder coated with a fog-like spray to create an enameled surface which is between 120 and 150 micrometers thick. Finally, the ovens are baked at over 800 degrees Celsius, carefully inspected, and then placed in a holding bay ready to enter one of the nine final assembly lines. A three-hour buffer gives the necessary leeway to cope with any machine downtime.

Ebert attributes the success of the project to an iterative approach and their conviction that they were on the right track. He also credits the open tendering process. The team used a matrix to measure up the welding concepts and processes against the required oven specifications. “The tender specifications cited the three-meter-long weld seam — plus the fact that it had to be invisible under the enamel coating — as a key challenge, but we deliberately avoided specifying a particular process,” explains Ebert.

The machine suppliers bidding for the contract produced prototypes using a range of different methods which Electrolux then subjected to intensive testing in Rothenberg. FFT EDAG PS was the only provider to recommend laser technology. “Roll seam welding would generally be the standard choice in this context,” admits Jean Heussner, project manager at FFT EDAG PS. “But using seam welding in this particular application caused problems in the corners — and it would have required multiple machines to meet Electrolux’s 12-second output rate.” FFT EDAG PS drew on its experience with joining processes in the automotive industry, and Ebert says that the test results clearly supported their final choice: “The critical weld seam is perfectly burr-free.”

From laboratory to factory
A vibration-free foundation was laid in preparation for the installation of the welding cell, next to the basement area containing the power generator. The building modifications were worthwhile, because they allowed the operator to achieve a new level of process excellence. With their ability to handle flexible welding parameters and geometries, the TRUMPF laser systems only needed minor adjustments to fit in with the process requirements.

Before delivering the new production system to the end customer, FFT EDAG PS and TRUMPF jointly ran a series of laboratory trials to define the equipment and validate the processes. “We had no difficulty integrating the machines in the production line”, reports Jean Heussner of FFT EDAG PS. The major emphasis lay on end-to-end process validation, including defining the geometries, testing for accessibility, and process reliability testing — “essential prerequisites for reproducible quality in a series-production environment.”

The integration of cutting and welding processes in a single production system has two tangible benefits. Firstly, there is no interruption in the flow of data at the transfer points between different components — a factor that leads to greater process stability. And secondly, it enables new variants and product modifications to be implemented rapidly and cheaply.

Changes to cutting and welding geometries can be user programmed in a single operation for both machining stages, because lasers produce the cutouts and drill the holes. There is no need to produce and modify new punching tools — another factor that saves time and money. Moreover, the results fulfill the main objective of a product (in this case a baking oven) that fully meets the customer’s criteria for visual and haptic quality.

The Electrolux Group’s experience of the new manufacturing system at its Rothenburg plant in Germany was transferred to other European sites in 2011. It is currently being introduced at the Electrolux appliance plants in Forlì, Italy, and Swidnica, Poland. The laser welding and cutting cells for these plants were also supplied by FFT EDAG PS. Meanwhile, the Electrolux plant in the United States has also expressed an interest, which gives confidence to Ebert and his team: “We are proud of this exciting project — and the advanced know-how it has generated secures Rothenburg’s future in the international group.”

Contact
Bernd Ebert, Director, Global Manufacturing Engieering,
Electrolux Rothenburg GmbH
Phone +49 (0) 9861 694670,
bernd.ebert@electrolux.de

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