Patent Publication Number: US-6701601-B2

Title: Metal body painting system and method

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to painting of fabricated items, and more particularly to a method for manufacturing a painted body-in-white for an automotive vehicle. 
     BACKGROUND 
     As is known, one of the processes involved in the manufacture of automotive vehicles is the painting of what is termed the body-in-white. A body-in-white is the body shell for automotive vehicles prior to any further assembly of the vehicle, such as installation of the powertrain and interior. Typically, the body-in-white is assembled from metal parts. After the body-in-white is assembled, it is painted. Once painted, the remainder of the vehicle assembly operations, such as installation of the powertrain and the interior of the vehicle, are completed. 
     The painting process used to paint the body-in-white is one of the critical processes in the manufacture of an automotive vehicle. This painting process gives the automotive vehicle its color and also much of its corrosion protection and resistance. If the painting process is flawed, the vehicle&#39;s color may fade over time, corrosion may occur more quickly, or the vehicle experiences other known painting process related problems. 
     The painting systems, usually termed paint shops, used to paint automotive vehicles are large, complex and expensive. Moreover, in the modem paint shops used to paint the body-in-white for automotive vehicles, controlling the emissions of volatile organic compounds and reducing the energy used is an ongoing challenge. Additionally, the number of paint layers have been increased to meet more stringent requirements, resulting in considerable additional complexity and expense in building and operating a paint shop. 
     Referring to FIG. 1, a generic manufacturing process  10  for manufacturing a painted body-in-white for an automotive vehicle is described. A vehicle body  11  is manufactured in a body shop  13  by fabricating body parts (not shown) at step  14  from galvanized and/or uncoated steel  12 , usually in coil form and then assembling the vehicle body  11  by welding, fastening or adhesive bonding these parts at operation  16 . Vehicle body  11  is now at the body-in-white stage. 
     After it is assembled, vehicle body  11  enters paint shop  19 . Once entering the paint shop  19 , vehicle body  11  is cleaned at cleaning station  20 , which is typically one or more cleaning baths or spray tanks. After being cleaned, vehicle body  11  is phosphated by passing it through a phosphating bath or spray operation  22  where zinc phosphate is applied to vehicle body  11 . As is known, the cleaning and phosphating operations are critical steps in the prior art paint processes because how well the body-in-white is cleaned and the quality of the zinc phosphate application significantly impact the corrosion protection that the painting process imparts to the automotive vehicle body as well as how well the paint adheres to the vehicle body. It is also well known that the cleaning and phosphating steps may consist of up to 10 or 12 individual treatment stations. 
     After the phosphating operation  22 , the vehicle body  11  is passed through an e-coat operation  24  where e-coat is applied to the vehicle body  11 . As is known, the e-coat application provides significant corrosion protection to the vehicle body  11 . After the e-coat application, the e-coat is cured  26  by passing the vehicle body  11  through an e-coat oven  26 . Next, the vehicle body  11  may be sanded and repaired at operation  28  and its seams sealed at seam sealer operation  30  by the application of a sealer to the seams of the vehicle body  11 . The sealer is cured by passing the vehicle body  11  through a sealer oven  32 . A primer is then applied to the vehicle body in primer paint booth  34 . This primer can be a liquid primer or a powder coat primer. The primer is then cured by passing the vehicle body through a primer oven  36 . 
     After the primer is cured, the primer may be sanded and/or repaired at operation  38  prior to the application of the color base coat. The color base coat is applied to the vehicle body  11  in base coat paint booth  40 . In most cases, the color base coat is a liquid paint that emits volatile organic compounds which must be collected and processed with appropriate emission control systems. In some cases, such as in the painting of the metal part of the vehicle body for the Smart car manufactured by DaimlerChrysler A.G., a powder base coat is used. However, the use of powder base coats presents certain problems that aren&#39;t presented by the use of liquid base coats, as will be discussed below. In cases where the base coat used is a waterborne paint, the vehicle body  11  is next dried by passing the vehicle body  11  through a base coat flash-off oven  42 . 
     A clear coat is next applied to the vehicle body  11  in clear coat paint booth  44 . After the clear coat is applied, the base coat and the clear coat are fully cured by baking the vehicle body  11  in a top coat oven  46 . The color base coat and clear coat are often referred to collectively as the top coat. After the top coat is cured, the painted vehicle body  11  is inspected and any necessary finesse operations (touch up, etc.) carried out at operation  48 . The painted vehicle body  11  is then ready for use in subsequent assembly operations to complete the assembly of a vehicle. Painted vehicle body  11  at the stage where it exits paint shop  19  and before any subsequent assembly operations is referred to as a painted body-in-white. 
     The color base coat, clear coat, and inspection and finesse operations typically consume more time than the steps preceding them. Consequently, many paint shops provide multiple paths for these operations, as shown in FIG. 1, to maximize the capacity of paint shop  19 . 
     As can be seen, the painting process used to paint automotive vehicle bodies is complex. Further, the paint shops used to implement this process are often large and expensive. The cost of a paint shop used in the manufacture of high volume automotive can easily exceed several hundred million dollars. 
     It is an object of this invention to simplify the process used to manufacture painted metal body shells, such as a painted body-in-white, during the original equipment manufacture of the product in which the metal body is used, such as an automotive vehicle. 
     It is a further object of this invention to simplify the paint shop used to paint the body-in-white for an automotive vehicle. 
     It is another object of this invention to reduce the volatile organic compounds emitted during the painting of a vehicle body-in-white. 
     It is another object of this invention to reduce the amount of energy used during the painting of a vehicle body-in-white. 
     It is another object of this invention to reduce the number of paint layers applied in the paint shop. 
     It is another object of this invention to reduce the number of cleaning and pretreatment stations required. 
     SUMMARY OF THE INVENTION 
     A method of painting automotive vehicle bodies in accordance with this invention A method of manufacturing a body, preferably an automotive vehicle body, to have a plurality of coating layers, starts with assembling the body out of parts made from pre-coated metal, such as pre-coated galvanized steel. The galvanized steel may be produced either by the hot dip galvanized process or by the electrogalvanized process. Additionally the galvanized may be either a zinc or zinc alloy material. 
     After cleaning and pretreatment, a powder paint base coat is then applied onto the pre-coated metal of the vehicle body. The pre-coat layer is one of the plurality of coating layers and the powder paint base coat is another one of the plurality of coating layers. 
     In another embodiment, the powder paint base coat is applied in two steps, a first powder paint base coat and a finished color powder paint base coat. The first powder paint base coat is applied to a first portion of the vehicle body and the finished color powder paint base coat is applied to a second portion of the vehicle body. Unused first powder paint base coat and unused finished color powder paint base coat are both reclaimed, mixed, and the mixture used for at least some of the first powder paint base coat applied to the first portion of the vehicle body. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Additional features and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of a preferred embodiment exemplifying the best mode of carrying out the invention as presently perceived. The detailed description particularly refers to the accompanying figures in which: 
     FIG. 1 is a process flow chart of a prior art vehicle body manufacturing process, including the painting process; 
     FIG. 2 is a process flow chart of the manufacturing process to produce a painted metal body in accordance with this invention; 
     FIG. 3 is a cross-section of a piece of pre-primed steel; 
     FIG. 4 is a chart showing a comparison between the prior art paint layering system and the paint layering system achieved with the method of this invention; and 
     FIG. 5 is a process flow chart of the underbody, base coat and powder reclaim steps preferably used in the painting process of this invention 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2 illustrates a process flow of a preferred embodiment of the manufacturing process of this invention. A vehicle body  103  is manufactured in a body shop  101 . Parts (not shown) for vehicle body  103  are cut and stamped from pre-primed or pre-coated steel  100 , preferably pre-primed galvanized steel in coil form, at operation  102 . Pre-primed or pre-coated steel, which terms are used interchangeably herein, is known in the art and is steel to which a thin coating of paint, such as paint primer, has been applied prior to the fabrication of parts from the steel. 
     FIG. 3 shows in cross-section a piece of pre-primed galvanized coil steel stock  200 . Pre-primed steel  200  has a central layer of steel  202  with a galvanized layer  204  on each side. On top of each galvanized layer  204  is an optional pretreatment layer  206 , such as provided by a dry-in-place conversion coating operation. On top of each pretreatment layer  206  is a paint pre-primer layer  208 . Pre-primer layer  208  is a thin layer of paint, such as paint primer, typically having a thickness of 2 microns to 15 microns. Pre-primed steels that can be illustratively used in the in the inventive process are the pre-primed steels made by PPG Industries, Inc. One PPG Place, Pittsburgh, Pa., 15272 and marketed under the trade names Bonazinc 3000R and Bonazinc 3001 and the pre-primed steel made by Henkel Surface Technologies, 32100 Stephenson Highway, Madison Heights, Mich. 48071 and marketed under the tradenames Granocoat S, Granocoat ZE and Granocoat LC. It should be understood that the inventive process is not limited to these specific pre-primed steels and other pre-primed steels formulated to have the characteristics required for use in automotive bodies, which characteristics are known in the art, can also be used in the inventive process. 
     Vehicle body  103  is next assembled at operation  104  from the parts (not shown) fabricated in operation  102  (which those familiar with automotive vehicle body part fabrication understand to involve multiple operations) and after the vehicle body  103  is assembled, its seams are sealed at seam sealer operation  108 , such as by use of a caulking type of seam sealer. The seam sealer is cured by passing vehicle body  103  through an optional sealer oven  108 . Alternatively, the sealer operation may be placed after the drying oven  116  and before the underbody paint booth  120 , thereby utilizing the sinter bake oven  124  to cure or partially cure the seam sealer. 
     The vehicle body  103  now enters paint shop  110  where it is cleaned in cleaning operation  112  and pretreated in pretreatment operation  114 . Pretreatment operation  114  is a surface conversion or treatment to promote paint adhesion and that may also retard corrosion depending on the type of surface conversion operation, such as a zinc or iron phosphate pretreatment. However, since the body-in-white is manufactured from pre-primed steel  103  which provides corrosion protection and adhesion of the subsequent paint layers, the pretreatment step  114  is principally to treat any bare metal in vehicle body  103  due to fabrication operation  102  and body build operation  104 , such as might be caused by metal finishing of surface defects, the attachment of miscellaneous uncoated parts and fasteners, cut edges of stamped parts, or welds areas. As such, the pretreatment operation of step  114  can be a more abbreviated zinc phosphate or other pretreatment operation than zinc phosphate operation  22  used in the prior art automotive vehicle body painting processes illustrated by the process of FIG.  1 . Pretreatment operation  114  may also be a passivation rinse coat type of surface conversion operation, an autodeposition paint coating operation, an organic coating containing complex metal oxides treatement, or other comparable pretreatment operation capable of promoting paint adhesion on bare metal whilst providing additional corrosion protection. 
     After being pre-treated, vehicle body  103  is next dried in a drying oven  116 . Then, vehicle body  103  may be touched-up at operation  118  where any bare spots in the pre-coat primer that may have been caused by the vehicle body parts fabrication and vehicle body build operations  102 ,  104  are touched-up with a primer and vehicle body  103  is otherwise prepared for the application of the paint base coat. 
     The paint base coat, usually referred to as the base coat, is preferably applied to the vehicle body  103  in two steps, in underbody paint booth  120  where the base coat is applied to the underbody and all other surfaces of vehicle body  103  that are not visible when vehicle body  103  is assembled into a completed vehicle and in color base coat paint booth  122  where the finished color base coat is applied to the exterior surfaces of the vehicle body  103  that are visible when vehicle body  103  is assembled into a completed vehicle. In a preferred embodiment of the invention, the base coat applied to the underbody and the base coat applied to the remainder of the vehicle are both powder paint coats, as will be described in more detail below. 
     After the base coats are applied, they are sintered by passing vehicle body  103  through a sinter bake oven  124 . After sintering, a clear coat is applied to vehicle body  103  in clear coat paint booth  126 . Preferably, the clear coat is a powder clear coat. After the clear coat is applied, the clear coat and base coats are cured in bake oven  128 . Next, vehicle body  103  goes through inspection and finesse operation  130 . The painted vehicle body  103  prior to any further assembly is referred to as a painted body-in-white. 
     FIG. 4 is a chart showing the comparison of a typical paint layering system of the prior art and of the inventive process. Referring to FIG. 4, it can be seen that the prior art paint layering system has five layers of coatings totaling almost 175 microns. Further four of the coatings, the e-coat, powder primer, water basecoat, and clear coat, are applied when the vehicle body is painted during the original equipment (OE) manufacturing of the vehicle. In contrast, the paint layering system resulting from the present invention has four layers totaling less than 125 microns. Moreover, only two layers, the powder basecoat and clear coat, are applied when the vehicle body is painted during the original equipment (OE) manufacturing of the vehicle. 
     The inventive process significantly reduces both the amount of volatile organic compounds emitted and energy consumed during the painting of a vehicle body-in-white. As discussed above, the zinc phosphating operation if used for pre-treatment operation  114  is a more abbreviated and the amount of bare surface treated is significantly less than the zinc phosphating operation  22  used in the prior art painting process shown in FIG.  1 . Consequently, lower amounts of sludge and hazardous waste materials are produced in the zinc phosphating operation that can be used for pre-treatment operation  114 . Moreover, other types of surface conversion treatments can be used for pre-treatment operation  114  thus reducing further the amount of hazardous waste materials produced by pre-treatment operation  114  and, depending on the surface conversion treatment used, reducing the amount of hazardous waste in pre-treatment operation  114  to zero. 
     Also, the inventive process eliminates the need for e-coat operation  24  (FIG.  1 ), thus eliminating the VOC&#39;s emitted and energy consumed by e-coat operation  24 . By eliminating the e-coat operation, the e-coat primer surfacer operation can also be eliminated. In the case where the primer surface is a liquid paint there is a further elimination of the VOC&#39;s emitted and yet another reduction in energy consumption. The inventive process also preferrably uses powder paint for the paint base coat. As is known, powder paint does not emit VOC&#39;s and requires less energy to paint in contrast to liquid paints. 
     One of the problems with using a powder color coat for the base coat is that powder painting like liquid painting is relatively inefficient, with only about sixty-five percent of the paint ending up on the vehicle body. Since powder paint is expensive, the excess powder is typically reclaimed and reused. However, in order to avoid one color contaminating another, powder color paint systems have either had separate reclaim systems for each color or separate paint booths for each color. Additionally, using reclaimed powder contributes to painting defects. 
     Referring to FIG. 5, an inventive powder paint reclaim system  300  and method for use in powder painting systems is shown. Powder paint reclaim system  300  is described as it is implemented in the painting system of FIG.  2  and like elements will be identified with the same reference numerals. As discussed with reference to FIG. 2, the underbody and other “non-visible” surfaces of vehicle body  103  are painted with a powder paint in underbody paint booth  120 . Excess powder from underbody paint booth  120  is reclaimed into reclaim tank  302 . The exterior “visible” surfaces of vehicle body  103  are then painted with a color powder paint in base/color paint booth  122 . The excess powder paint from base/color paint booth  122  is also reclaimed into reclaim tank  302  where it mixes with the powder paint reclaimed from underbody paint booth  120 . The reclaimed powder in reclaim tank  302  is then provided to underbody paint booth  120  for use in painting the underbody and other “non-visible” surfaces of vehicle body  103 . 
     Since the underbody and other “non-visible” surfaces cannot readily be seen after vehicle body  103  is assembled into a complete vehicle, the color of the underbody and other “non-visible” surfaces of vehicle body  103  need not be any particular color nor have the uniformity and appearance characteristics that the exterior “visible” surfaces of vehicle body  103  must have. Consequently, base/color paint booth  122  can be used for multiple colors as the excess color powder is reclaimed into reclaim tank  302  and used to paint the underbody and other “non-visible” surfaces of vehicle body  103  in paint booth  120 . If it is desired that the color of the underbody and other “non-visible” surfaces of vehicle body  103  be more consistent, then a pigmented powder, hereinafter referred to as pigment, can be added to reclaim tank  302 . The color and amount of this pigment can be varied depending on the color and amount of the powder paint being reclaimed from paint booths  120  and  122  to result in the desired powder paint color of the powder paint provided to underbody paint booth  120 . Also, depending on the amount of powder paint reclaimed from underbody paint booth  120  and base/color paint booth  122 , it might be necessary to add additional powder paint to reclaim tank  302  or remove paint powder from reclaim tank  302 . 
     Although the invention has been described in detail with reference to certain preferred embodiments and specific examples, variations and modification exist within the scope and spirit of the invention as described and as defined in the following claims.