A method of manufacturing a painted motor-vehicle body

Abstract: The method of manufacturing a painted motor-vehicle body comprising drawn sheet-metal components, comprises the steps of prepainting the sheet metal with at least one anchoring coat (primer) and at least one covering paint coat, drawing the prepainted sheet metal to the desired shapes and assembling the prepainted, drawn components. To advantage, the sheet metal is prepainted by a continuous process with the application of a polyurethane paint cross-linked by the permeation of a vaporised amine which can cause the cross-linking of the paint.

Description: The present invention relates to a method of manufacturing motor-vehicle bodies comprising drawn sheet metal components having an ornamental pattern. In the conventional method of manufacturing motor-vehicle bodies, the pieces of sheet metal are deep-drawn to the desired shapes and the drawn components are then assembled. The body thus produced is painted by the application of a primer coat and a covering or base coat, followed by the application of a further, finishing paint coat. From US-A-3 891 471 it is known a method for manufacturing a metal sheet body, wherein the metal sheets are pre-painted with a primer coat and a covering pain prior to being drawn to the desired shape. An object of the present invention is to provide a method of manufacturing a motor-vehicle body which allows to obtain by industrial mass production a motor-vehicle body having a characterising ornamental pattern. This object is achieved by means of a method as defined in claim 1 and appended claims. The deep-drawing of pre-painted pieces of sheet metal provides many advantages. In particular, it is not necessary to apply a lubricant to the metal sheet before drawing. Moreover, the sheet metal pre-painted with the polymeric covering coat shows an improved behaviour in respect of creeping during the drawing operation so that the thickness of the sheet metal can be reduced or, alternatively, poorer-quality sheet metal can be used for a given drawing depth. Typically, in the method according to the invention, the primer coat is applied in thicknesses of between 5 and 10 microns, preferably 6-7 microns; in the case of galvanised sheet metal, the primer is typically acrylic. In view of the subsequent deep-drawing operation, it is advantageous for the covering or base paint coat to be applied to a total thickness of the order of 50-80 microns, which generally requires two or more successive applications of paint. The paint used for the covering coat is preferably of a cross-linkable polyurethane type. To advantage, in order to industrialise, and make economic, the method of pre-painting strips of the sheet metal wound in rolls for use in the automotive sector by the application of the method described above, the polyurethane paint is cross-linked by the "vapour permeation curing" technique. According to the invention, preferably, in order to paint the strips of sheet metal wound in rolls, the sheet metal strip is supplied continuously to one or more paint baths for the application of one or more covering coats of cross-linkable polyurethane paint and each covering coat is cross-linked by the continuous supply of the strip output from each paint bath to an environment saturated with a vaporised amine, preferably a tertiary aliphatic amine, which can cause the polyurethane paint to cross link at a temperature of generally between 60 and 80 o C. Further characteristics and advantages of the invention will become clear from the detailed description which follows with reference to the appended drawing which shows, purely by way of non-limiting example the layout of a plant for prepainting metal sheets intended for subsequent drawing operations. With reference to the drawing, a strip of sheet metal L is supplied continuously from an unwinding reel 1 to a preliminary degreasing stage 2 and then to a painting stage 3 for the application of a coat of primer, generally constituted by an acrylic resin, from 5 to 10 microns thick. The coat of primer, applied to both sides of the sheet metal, is then dried by conventional methods in an oven 4; since the primer coat is quite thin, the drying times are quite short in this stage. From the drying oven, the strip L is supplied continuously to one or more painting stages 5, 6, 7 for the application of a covering coat of polyurethane resin. At the output of each painting stage, the polyurethane paint coat is cross-linked in stages 8, 9 and 10 by the "vapour permeation curing" technique (VPC). The VPC technique is known and is described, for example, in U.S. Patent No. 4,396,647, but is not used for the continuous painting of sheet metal. In this technique, the polyurethane paints contain a blocked catalyst which is activated to cause the cross-linking of the resin following its permeation by an amine vapour, preferably a tertiary aliphatic amine. The stages 8, 9 and 10 shown in the drawing are each constituted by a vacuum chamber, either vertical or inclined, in which an aliphatic amine vapour is brought into contact with the painted surface causing it to dry quickly at quite low temperatures, generally between 60 and 80 o C. The chambers 8, 9 and 10 are operated under vacuum in order to prevent the amine vapour from escaping and may be provided with interspaces for preventing the entry of dust which would damage the surfaces of the sheet metal . The method of painting with VPC cross-linking enables several coats of paint to be applied within a single sheet-handling operation with advantages as regards painting costs. This method is particularly advantageous for the prepainting of sheet metal for drawing since the thickness of the coating layer required makes several passes necessary for its deposition. The periods of time spent in the VPC chamber are of the order of 10-15 minutes and are much shorter than the periods necessary for conventional, thermal cross-linking. Moreover, with the use of this method, the successive paint coats can be deposited without the need to wait for each individual coat to dry completely. Moreover, the polyurethane paints used in the method have a greater polymerisation density. The sheet metal output from stage 10 may then be supplied to a stage 11 for the deposition of a lubricant. The application of a lubricant is not, however, strictly necessary for the subsequent drawing. The sheet metal output from stage 11 may be subjected to a final drying stage in an oven 12 and is then rewound into rolls 13. Within the scope of the method of manufacturing a motor-vehicle body, the pre-painted sheet metal strip is then deep-drawn to produce components of the desired shapes. The drawn and pre-painted components are then assembled. The pre-painted and drawn components can be connected permanently to each other or to other parts of the motor-vehicle body by welding and gluing. The conventional spot-welding of pre-painted sheet metal involves operative problems since the presence of the polymeric coating film, which has insulating properties, prevents the passage of current. It is therefore necessary first to remove the coating from the contact surfaces to be interconnected. The coat can be removed mechanically by milling or, conveniently, by the application of a concentrated heat load by means of a laser beam to cause the sublimation of the paint layer from a limited area little larger than that necessary for the spot weld. According to the method of the present invention, painted bodies are produced with a predetermined, decorative, ornamental pattern, which cannot be applied industrially by the conventional method of assembly. For this purpose, according to the invention, pre-painted metal sheets are used which carry an ornamental pattern applied to the sheet metal itself by a transfer technique. An ornamental pattern is applied, with heat and by a transfer technique, to the sheet metal strip which has already been coated with a primer and at least one first covering paint, with the use of a transfer film or sheet such as that described, for example, in DE-U-88 07 848. When the ornamental pattern is applied, the sheet metal is preferably painted with a primer coat, a coat of polyamide paint about 30 to about 50 microns thick and a coat of polyurethane paint between 15 and 25 microns thick. The application of the pattern by the transfer technique does not result in an increase in the thickness of the coating; it is, however, necessary to apply a protective coat of polyester lacquer, about 5-8 microns thick, after the body has been assembled.

Claims: A method of manufacturing a motor vehicle body comprising at least a drawn metal sheet component having an ornamental pattern thereon, characterised in that it comprises the steps of: pre-painting the metal sheet in the sheet form with a primer coat and a covering paint coat; applying to said pre-painted metal sheet an ornamental pattern by means of a transfer technique; drawing the pre-painted metal sheet having said ornamental pattern to the desired shape; and assembling the motor vehicle body including said component. A method according to Claim 1, wherein the covering paint coat comprises a first polyamide coat and a polyurethane paint coat. A method according to Claim 2, wherein the polyamide coat has a thickness of from 30 to 50 microns and the polyurethane paint coat has a thickness of from 15 to 25 microns. A method according to Claim 1, wherein the primer coat consists of a coat of acrylic primer having a thickness of from 5 to 10 microns. A method according to any of Claims 2 to 4, wherein the polyurethane coat consists of a cross-linkable polyurethane and is applied to the sheet metal in the sheet form by supplying the sheet to one or more paint baths and each polyurethane coat is cross-linked by supplying the sheet exiting from the paint bath to an environment saturated with vapourised aliphatic amine. A method according to Claim 5, in which the polyurethane coat is cross-linked at a temperature of between 60 and 80°C.

Inventors: DA RE MARIO, DA RE', MARIO
Applicants: FIAT AUTO SPA, FIAT AUTO S.P.A.
Application year: 1990
Main classification: B05D7
IPC codes: B05D5, B05D3, B05D7, B05D1