Abstract:
A method of applying a powder coating to a medium density fiberboard (MDF) or particleboard door includes attaching a first hanger to the door, providing an electrical source to the door through the first hanger, and conveying the door, via the first hanger, into a coating booth. The door is then powder-coated in the coating booth while being electrostatically charged. The powder-coated door is then cured to cause the powder to adhere to the door.

Description:
The present invention relates to powder coating and, more particularly to, an improved method of powder coating interior and exterior passage doors made from medium density fiberboard or particleboard. 
     BACKGROUND OF THE INVENTION 
     New construction put in place during August 2001 was estimated at a seasonally adjusted annual rate of $845.5 billion according to the U.S. Commerce Department&#39;s Census Bureau. Additionally, expenditures for improvements and repairs of residential properties in the first quarter of 2001 were at a seasonally adjusted annual rate of $169.3 billion. The cost of building materials is a significant portion of the total cost of new construction and home improvement. Consequently, building material manufacturers are continuously searching for new methods of improving their products and reducing their costs to gain a larger portion of this enormous market for building materials. 
     Door manufacturers, for example, have developed many manufacturing processes to improve their doors for the consumer. Innovative manufacturing techniques to produce hollow core doors reduce the expense of the door as compared to a solid door while maintaining functional features such as noise reduction and structural integrity. Alternative door materials such as medium density fiberboard (MDF) and particleboard may also reduce the cost of the door while maintaining functional features. Building doors with alternative materials or by innovative techniques, however, may result in an unfinished door that lacks the aesthetic qualities of an unfinished wood door, which are important for appealing to the consumer. 
     Many manufacturers apply a primer or finish coating to the door to appeal to the consumer. Primed or finished doors save the consumer time and effort and also have a more attractive appearance to some builders and consumers. Primer and paint coatings may be applied to the doors in an automated fabrication process. These coatings, however, typically require specialized equipment such as paint sprayers, separate paint booths, ventilation equipment, and driers to apply an attractive or useful coating to the doors. This specialized equipment consequently increases the cost of the finished door. Priming and painting also produces a variable finish quality according to the amount of material sprayed onto the door. Overspray may result in drips and wasted primer and paint while color variations may result from using an inadequate volume of primer and paint. Additionally, primers and paints typically contain volatile organic compounds (VOC&#39;s) that are vented to the environment. Permits to vent the VOC&#39;s and customer perception of the environmental impact of manufacturing processes may also increase the cost or consumer acceptance of the door. 
     Powder coating has been used as an alternative to priming and painting many metallic surfaces of building materials and components such as mailboxes, water heaters, window frames, and HVAC equipment, for example. The unique application characteristics of powder coatings provide superior consistency and uniformity of finish without drips, runs, or bubbles. Powder coating provides tough, durable films, enhancing the quality appearance of building materials. In general, performance properties such as impact resistance, flexibility, and corrosion resistance, of powder coatings are better than liquid paints. Powder coated building materials resist cracking, peeling, and marring during handling and normal service use. 
     Powder coating frequently costs less than priming and painting a product. Production costs are conserved through energy savings, labor costs, rework costs, reduced material usage, reduced waste disposal costs, and an increase in overall line efficiency. One advantage to manufacturers and consumers is that powder coated building materials have a superior quality finish at a reasonable cost. 
     Powder coating building materials is also more environmentally sound than painting. Environmental issues are a significant interest to the government and consumers. Unlike many primers and paints, powder coatings comply with environmental regulations. Primers and paints often contain VOC&#39;s, which can contribute to air pollution and, in some cases, ozone depletion. Powder coatings contain minimum amounts of VOC&#39;s and, therefore, may be perceived by the consumer to be environmentally friendly. Wet painting processes may also generate sludge, which must be disposed in hazardous waste landfills. This disposal increases the cost of the building products. Properly formulated powder coatings generate no hazardous waste. Unused or excess powder may be reclaimed and recycled, which also reduces overall product cost. 
     It would, therefore, be desirable to have an improved method of finishing passage doors that does not produce an unsightly finish. It would also be desirable to have an improved method of finishing passage doors that is not as costly as conventional finishing methods. Further, it would be desirable to have an improved method of finishing passage doors that is not detrimental to the environment. 
     SUMMARY OF THE INVENTION 
     The present invention is a system for powder coating a passage door. The system has a coating booth hanger to electrically charge and suspend the passage door from an overhead conveyor within the coating booth. A powder dispenser distributes powder to the passage door. A curing device cures the powder that is distributed on the passage door. 
     In one embodiment of the invention, a method of applying a powder coating to a door has the step of hanging a door in a coating booth. Powder is distributed to the coating booth where the door is grounded to attract electrostatically charged powder. Then the powder is cured to adhere to the door. 
     In another embodiment of the invention, a method of manufacturing a powder coated door has the step of coating a first door half and a second door half with a powder. The powder on the first door half and the second door half is then cured. The first door half is then joined to the second door half to form a complete door. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, including its features and advantages, reference is now made to the detailed description of the invention, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a schematic view of a powder coating conveyor that depicts an embodiment of the present invention; 
         FIG. 2  is detail view of a door hanger that depicts an embodiment of the present invention; 
         FIG. 3  is a schematic view of a powder coating conveyor that depicts an embodiment of the present invention; and 
         FIG. 4  is a schematic view of a horizontal powder coating conveyor that depicts an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the making and using of various embodiments of the present invention is discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention, and do not delimit the scope of the invention. 
     A method of powder coating passage doors according to one embodiment of the present invention has many desirable features that were previously unavailable to those in the business of powder coating. Although methods for powder coating wood were known, economically powder coating doors presented several problems such as the difficulty of hanging the passage door on a powder coating conveyor. Powder coating objects typically requires that a hangar ground the object so that the object and the powder have opposite electrical charges. This opposite charge uniformly attracts the positively charged powder to surface of the object where the powder may be cured. Curing the powder results in a desirable finish being formed on the object. In the case of passage doors, however, both sides of the finished door must be attractively finished. Therefore, hanging the passage door from either face may leave an unattractive hole in the door that must be patched. Patching holes increases manufacturing costs and does not guarantee that consumers will be satisfied with the overall appearance of the door. As will be described in more detail with reference to the figures, one embodiment of the present invention produces doors without the added expense of patching holes on the visible surfaces of the door. 
     Referring now to the figures and, in particular to  FIG. 1 , a conveyor  10  is configured to convey a door  12  through a powder coating booth  14 . A powder dispenser  16  may be located within the powder coating booth  14  and dispenses a powder  18  to the door  12 . The conveyor  10  conveys the door  12  through the powder coating booth  14  in the direction indicated by arrow  20 . A hanger  22  may support the door  12  by one or more edges  24  as the door  12  is conveyed through the powder coating booth  14 . A curing device  26  cures the powder  18  after the powder  18  has been applied to the door  12 . 
     The conveyor  10  may be a typical conveyor for powder coating operations that is known to those having ordinary skill in powder coating. The conveyor  10  conveys one or more doors  12  through the powder coating booth  14 . The conveyor  10  may be configured to support the weight of several doors  12  to improve the efficiency of a manufacturing operation. Multiple doors  12  may be loaded onto the conveyor  10  and conveyed through the powder coating booth  14  at a relatively high conveyor speed to continuously apply the powder  18  to the doors  12 . Those having ordinary skill in powder coating will be aware that conveyor speed may be limited by the speed at which a desired volume of powder  18  may be deposited or applied to the door  12 . Additionally, the conveyor speed may be limited by how fast the applied powder  18  may be cured. 
     The powder coating booth  14  may be a typical powder coating booth for powder coating operations that is known to those having ordinary skill in powder coating. The powder coating booth  14  must be large enough to accept one or more doors  12  for powder coating. The powder dispenser  16  may be a typical dispenser known to those having ordinary skill in powder coating, such as the corona method dispenser or the triboelectric dispenser, for example. Other methods of applying powder  18  to the door will be apparent to those having ordinary skill in powder coating. 
     The powder  18  may be, for example, powder of the type described in U.S. Pat. Nos. 5,907,020 or 6,077,610 to Correll et al., U.S. Pat. No. 5,721,052 to Muthiah et al, or U.S. Pat. No. 5,714,206 to Daly et al. These patents are all assigned to Morton International, Inc. and disclose powder coating systems that are suitable for coating wood-based products. The powder  18  may be configured to produce a smooth or a textured finish on the door  12 . In some instances, a wood grain texture, for example, may be produced by configuration of the powder. Other desirable finish characteristics will be apparent to those having ordinary skill in powder coating wood-based products. 
     The curing device  26 , which may incorporate an infrared, ultraviolet, or thermal curing process, for example, cures the powder  18  after the powder  18  is applied to the door. The powder  18  is configured to cure at temperatures that will not adversely affect the door  12 . As discussed above with reference to coating speed, the conveyor speed may also be limited by the rate that the curing device  26  is capable of curing the powder  18  on the door  12 . 
     The door  12  may be made from a variety of wood or wood-based materials such as particleboard, MDF, and the like. The door  12  may also be made from a combination of these materials. The powder coating method described herein may be used for interior and exterior passage doors, cabinet doors and other doors  12 . The door  12  may be preheated before being conveyed to the powder coating booth  14  to enhance the adhesion of the coating onto the door  12 . 
     Because both inside and outside surfaces of many doors  12  are visible in normal installations, the hanger  22  holds the door by the edge  24 . Holding the door  12  by the edge  24  reduces potential blemishes that may remain on the surface of the door  12  if the door  12  were held by another method. The hanger  22  may hold the edge  24  in more than one location to increase the stability of the door  12  as it is conveyed through the powder coating process. As depicted in  FIG. 1 , for example, the door  12  is suspended at two points on opposite edges  24 . The edges  24  may be on the sides, top or bottom of the door depending on the dimensions of the door  12  or a desired orientation. The orientation of the door  12  may be varied according to the configuration of the conveyor  10 , the powder coating booth  14 , or the curing device  26 . Additionally, the orientation of the door  12  may be adjusted or changed during the powder coating process. 
     Several different interfaces may be used to securely hold and electrically ground the door  12  during the powder coating process. In a typical powder coating process, a product to be powder coated is hung from a conductive rod. A round hole in the product accepts the rod, which is slightly smaller in diameter than the hole, and secures the product during the process. After the powder coating process, however, the hole in the product remains and must be filled unless the hole is not located on a visible surface. 
     Turning now to  FIG. 2 , different interfaces for hanging the door  12  are depicted. The edge  24  of the door  12  has round holes  28  that are slightly larger in diameter than the hanger  22 . After the door  12  is powder coated, the holes  28  may be filled with dowels or wood filler, for example, to improve the aesthetics of the door  12 . Alternatively, the holes  28  may be left open because the edge  24  may not be visible after the door  12  is installed. If, for example, the holes  28  are located on the top or bottom edges  24  of the door  12 , the holes  28  may not be visible. Alternatively, if the holes  28  are located in an edge  24  on the side of the door  12 , for example, hinges, locksets, or other hardware may cover the holes  28 . In this particular example, the hanger  22  supports the edge  24  of the door in two locations to increase the stability and secure the door  12  during the powder coating process. Other methods of hanging the door  12  from the edge  24  will be apparent to those having ordinary skill in powder coating. 
     Referring now to  FIG. 3 , another embodiment of the present invention is depicted. The door  12  may be manufactured in two separate door sections  38 . The door sections may be made from particle board, MDF, wood-based material, or the like. The hanger  22  may hang the door sections  38  from a hole  28  in a back surface  40 . A generally teardrop-shaped hole may also be formed in the back surface  40  to facilitate automatically loading the door section  38  onto the hanger  22 . After the front surface  42  of the individual door sections  38  are powder coated, the back surfaces  40  of the door sections  38  are joined to form a complete door  12 . Consequently, the hole  28  does not require filling because it will not be visible when the door  12  is assembled. 
     Turning now to  FIG. 4 , an alternative embodiment of powder coating a door  12  is depicted. The door  12  may be placed onto a horizontal conveyor  50 . Contact between the door  12  and the horizontal conveyor  50  electrically grounds the door  12  for powder coating. The horizontal conveyor  50  is configured to convey the door  12  through the powder coating booth  14 . The powder dispenser  16  may be located within the powder coating booth  14  and dispenses a powder  18  to the door  12 . The conveyor  50  conveys the door  12  through the powder coating booth  14  in the direction indicated by arrow  20 . The curing device  26  cures the powder  18  after the powder  18  has been applied to the door  12 . 
     The horizontal conveyor  50  conveys one or more doors  12  through the powder coating booth  14 . The horizontal conveyor  50  may be configured to convey several doors  12  in an efficient layout, such as side-by-side or end-to-end, for example, according to the most efficient manufacturing layout. Multiple doors  12  may be loaded onto the horizontal conveyor  50  and conveyed through the powder coating booth  14  at a relatively high conveyor speed to continuously apply the powder  18  to the doors  12 . Those having ordinary skill in powder coating will be aware that conveyor speed may be limited by the speed at which a desired volume of powder  18  may be deposited or applied to the door  12 . Additionally, the conveyor speed may be limited by how fast the applied powder  18  may be cured. 
     Whereas the invention has been shown and described in connection with the preferred embodiment thereof, it will be understood that many modifications, substitutions and additions may be made which are within the intended broad scope of the appended claims. There has therefore been shown and described an improved powder coating system that accomplishes at least all of the above stated advantages.