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Products and solutions, 2001-09-11, 02:00 AM

Prepainted steel sheet in automobile construction

Cleaning, chemical pre-treatment, dip priming, primer baking, filler spraying, drying, spraying top coat, flashing off, applying clear coat finish and renewed baking: completing the paintwork on a car body involves a large number of operations performed on expensive equipment. And when it comes to add-on parts, the process is even more complicated, frequently involving manual painting in separate facilities. Automotive suppliers are facing increasing industry demand to supply such parts primed or prepainted. Investing in expensive painting equipment would not pay off, and suppliers are therefore forced to turn to toll processors, incurring significant additional logistical expense.

So it is hardly surprising that for some time the automobile, automotive supply and steel industries have been investigating new ways of shortening the corrosion protection and painting process using organic coatings. Thyssen Krupp Stahl AG`s solution is "Finish First - Fabricate Later": as far as possible, corrosion protection and paintwork are applied to the steel strip on the company`s coil coating lines, combining cleaning, pre-treatment and painting in one continuous process. This method delivers extremely even coating layers in consistently high quality.

The aim is to supply carmakers and module suppliers with prepainted sheet of the required color. 75 percent of ThyssenKrupp Stahl`s cold strip production already features metallic or organic coatings, and the company has invested heavily in the development of its surface engineering capabilities. The Dortmunder OberflächenCentrum set up by TKS is home to Europe`s biggest flat steel surface coating research center, working on areas such as improving the scratch resistance of clear coatings, new joining methods for prepainted parts or the development of new die surfaces to prevent damage to prepainted blanks. Automakers and automotive suppliers are involved in this development work, carrying out intensive tests and component trials using prepainted steel sheet from ThyssenKrupp Stahl. Current indications are that initially these materials could be put to use primarily by module suppliers.

The success already achieved using organically precoated materials in auto manufacture shows that fully prepainted automotive sheet is a realistic target. For example, ThyssenKrupp Stahl is one of the biggest suppliers of film-laminated sheet for the automobile industry. A 2 to 4 micrometer thick organic layer applied by coil coating to galvanized sheet provides the corrosion protection required in areas inaccessible to painting operations, such as beam cavities or overlaps on welding flanges. In the past, special cavity or seam sealing was required to prevent rusting in these zones. Organic precoating now renders such secondary anti-corrosion measures virtually superfluous. TKS is currently developing film-laminated sheet with even greater corrosion protection which will allow automakers to extend their warranties.

Automobile manufacturers and their suppliers should also welcome a material developed by TKS which substitutes the electropainting process otherwise used to prime components. ThyssenKrupp Stahl offers preprimed galvanized sheet with a 25 micrometer thick film on each side. Prospects for this material are brightest in the manufacture of add-on parts, for which present priming procedures are costly but no additional painting is required. Often referred to as black parts, these components include engine compartment parts, brake disc covers and dash panels. Preprimed steel sheet will also simplify the production of spares and thus reduce costs.

Numerous fabrication trials have been performed on preprimed sheet under production conditions to prove the material`s forming potential. Parts can be assembled by clinching, riveting or bonding; in the latter case the adhesive must be carefully matched to the composition of the organic coating. Tools with special surfaces are required for the mechanical joining methods to ensure that the organic coating is not damaged in the join zone.

For further information contact:
ThyssenKrupp AG
Trade Press
Bernd Overmaat
Telephon: +49 (0)211 824-36012
e-mail overmaat@tk.thyssenkrupp.com

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