Abstract:
A system and method for developing a defect-free or reduced-defect electrophoretic coating on an electrically conductive substrate involves vibrating the substrate to remove residual paint solids after the substrate has been removed from the electrophoretic deposition bath and before the electrophoretically deposited coating is cured.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not applicable. 
       FIELD OF THE DISCLOSURE 
       [0002]    This disclosure relates to methods and apparatuses for coating an electrically conductive surface, and more particularly to such methods and apparatuses that produce electrophoretic coatings with fewer or less frequent defects. 
       BACKGROUND OF THE DISCLOSURE 
       [0003]    Electrophoretic coating has become a preferred method of applying primer, protective and/or aesthetic coatings to electrically conductive surfaces of automobile and truck body components, and is often used on tractors and other farm equipment; cranes, bulldozers, earthmovers, graders and other heavy equipment; appliance components such as refrigerator condensers; metal furniture; switch gear; and other products. For example, electrophoretic primer coatings are applied to components of substantially all mass produced trucks and cars. Electrophoretic deposition (EPD) has proven to provide durable, corrosion resistant aesthetically pleasing finishes at a lower cost than other alternatives, such as powder coating, in a variety of applications. Recognized advantages include very uniform coating thickness, low porosity, and the ability to coat objects having a complex shape. Additionally, electrophoretic coating processes are easy to control and offer high product throughput, with modern EPD processes being significantly more environmentally friendly than many other coating techniques. 
         [0004]    Typical EPD processes generally include a surface pre-treatment to enhance adhesion between the surface of a component and the coating, submerging the pre-treated component in an electrophoretic deposition bath and applying a direct current through the bath, rinsing the coated component to remove undeposited material from the EPD bath, and baking or curing the coated component to effect crosslinking of the paint film. 
         [0005]    Although EPD processes provide very uniform coating thickness, there can be a tendency for paint solids (resin and/or pigments) that are not bonded to the surface being coated to be retained on the surface by residual water drops that can adhere to certain surfaces of an electrophoretically coated surface. When such drops remain on the coated surface during baking or curing, the paint solids in the drops can become cured to and strongly adhered to the surface of the coated article creating a coating defect. As a consequence, repair booths are a common feature at the end of an EPD production line, where defects are removed and polished using abrasives. Because such repairs are not currently amenable to robotics, there is a significant amount of labor associated with such necessary repairs. Additionally, the repair booths require a significant amount of production floor space adding further to costs associated with EPD processes. While EPD processes are already cost effective, it is desirable to improve the process and further reduce costs by eliminating or reducing the incidence of coating defects requiring repair by reducing or eliminating residual drops on the surface of an EPD coated article. 
         [0006]    Residual drops can be removed from an EPD coating using blowers or compressed air. However, blowers and compressed air tend to propel dust and other small particles that can be embedded into the coating degrading desirable aesthetic and functional properties of the coating; and have therefore been deemed unsatisfactory. 
         [0007]    A technique that has achieved a small or marginal amount of success from a cost to benefit perspective involves tilting the coated article in one or more directions after it has left the EPD bath to cause residual drops to run off certain surfaces (e.g., normally upwardly facing surfaces). A more efficient and cost effective solution is desired for many applications. 
         [0008]    Another technique for removing residual drops from an EPD coated article is to dry the coated article before baking or curing the coating. These techniques can be effective for removing residual drops from the surface of the coated article. However, these techniques are not particularly cost effective because they require an enormous amount of floor space, particularly if ambient air drying is used, and can require a significant amount of energy consumption, such as when a preheat oven (at a temperature below the baking or curing temperature) is used. 
       SUMMARY OF THE DISCLOSURE 
       [0009]    Disclosed is a process for forming a cured electrophoretically deposited coating on an article having an electrically conductive surface by submerging the surface of the article in an electrophoretic deposition bath while maintaining an electrical potential through the bath between the electrically conductive surface, which acts as one of the electrodes, and a counter-electrode located in the bath in spaced relation to the electrically conductive surface of the article; removing the article after it has been electrophoretically coated; and vibrating the coated article after it has been removed from the electrophoretic deposition bath to remove residual bath drops from the coated surface before the coating is cured by heating in an oven. The step of vibrating the coated article before oven curing removes residual bath drops that might otherwise be present on the surface during curing, thereby eliminating or substantially reducing coating defects requiring repair, while requiring significantly less floor space than is needed for conventional techniques for mitigating coating defects caused by residual bath drops retained on coated surfaces during curing. The use of a vibrator to remove residual bath drops before curing also requires substantially less energy than the known use of a pre-curing oven. 
         [0010]    Also disclosed is a system for forming a cured electrophoretically deposited coating on an article having an electrically conductive surface that includes an electrophoretic deposition bath; an overhead conveyor for transporting the article through the electrophoretic deposition bath; at least one vibrator adapted to directly contact the article or to directly contact a support structure for the article; and an oven for curing an electrophoretically deposited coating on the article. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a schematic diagram showing a system in accordance with this disclosure for forming a cured electrophoretically deposited coating on an electrically conductive surface of an article. 
           [0012]      FIG. 2  is an elevational side view of an electrophoretically coated automobile body temporarily supported on a structure for vibrating the automobile body before passing it through an oven to cure the coating. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    The disclosed process and systems for forming a cured electrophoretically deposited coating on an electrically conductive surface of an article can be advantageously employed in a variety of EPD applications in which coating defects are caused by residual bath drops retained on coating surfaces during curing. Additionally, the disclosed process and systems are expected to expand the applicability of EPD processes to articles that were not amenable to an EPD process due to excessive coating defects. 
         [0014]    The term “electrophoretic deposition” is intended to include a variety of processes in which charged colloidal solids from a bath are deposited onto an electrically conductive surface of an article submerged or immersed in the bath by virtue of an electrical potential imposed between the electrically conductive surface and a counter-electrode located in the bath in spaced relation to the electrically conductive surface of the article. Electrophoretic deposition processes generally encompass electrocoating, e-coating, cathodic electrodeposition, anodic electrodeposition, electrophoretic coating, and electrophoretic painting. 
         [0015]    The electrophoretic deposition bath is a colloidal suspension that can generally comprise about 80% to 90% water and about 10% to 20% paint solids (weight basis). The paint solids are comprised of resins and optional pigments that are capable of carrying a charge and having a colloidal particle size (i.e., approximately 1 to 1000 nanometers in diameter). The solids can sometimes include other materials such as dyes, ceramics and/or metals capable of carrying a charge. The term “colloidal suspension” means that the water and solid particles form a homogenous mixture in which the behavior of the dispersed solids is predominately determined by the surface chemistry of the colloidal suspension, such that settling either does not occur or only appreciably occurs after a very long time. Examples of crosslinkable polymer resins that can carry a charge, and are therefore useful for EPD processes include epoxy resins such as diglycidal ethers of bisphenol A, and acrylic resins such as polymers of acrylic acid esters or methacrylic acid esters. 
         [0016]    Suitable bath components, chemistries, bath temperatures, electrical potentials, counter-electrode materials and geometry, bath vessel geometry, residence times, and other EPD processing parameters are well known in the art and/or can be determined without undue experimentation using routine testing and optimization procedures, and do not constitute novel aspects of this disclosure. 
         [0017]      FIG. 1  diagramatically illustrates an electrophoretic deposition system  10  in accordance with certain aspects of this disclosure. System  10  includes an overhead conveyor  12  that transports non-coated articles  14  having a surface  16  that is to be coated from a pretreatment station (not shown) to an electrophoretic deposition bath  18  contained in a vessel or tank  20 , through bath  18 , and optionally through a rinse station  22  comprising one or more spray nozzles  24 , one or more collection tanks  26 , associated pumps  28 , piping  30 , and rinse collection surfaces  32  for recirculating rinse water from the tanks, to the nozzles, on to coated articles  34 , and back to the tanks. 
         [0018]    Pretreatment typically involves cleaning the surfaces  14  of article  12  that are to be clear coated or painted (dyed and/or pigmented) to remove dirt, oil, grease, etc.; and applying a phosphate conversion coating (e.g., a zinc phosphate, manganese phosphate, or iron phosphate) such as by immersion or spraying in order to improve corrosion resistance, lubricity and adhesion with the subsequently applied coating. Suitable detergent or cleaning solutions, techniques, and equipment are well known and commercially available, and do not constitute a novel aspect of this disclosure. 
         [0019]    An ultrafiltration heat exchanger unit  36  is typically used to recover paint solids (e.g., colloidal resin and pigment particles) that are removed from coated articles  34  at the rinse station  22  and return the paint solids to the bath  18 , and to control the temperature of the bath. 
         [0020]    A power supply  38  supplies a direct current (DC) electric charge to the bath to induce movement of the charged paint solids toward the surface  16  that is to be coated and binding of the paint solids to the surface  16 . 
         [0021]    In accordance with certain novel aspects of this disclosure, the coated articles  34  are transported to a vibration station  40  to remove residual bath drops from the coating surface. It is difficult to rinse all of the paint solids that are loosely adhered to the coating, but which are not bonded to the underlying electroconductive interface  16  of the article, particularly with larger articles and/or articles having complex surface geometry. By vibrating articles  34  before transporting the articles to a curing oven  42 , a substantial reduction in residual paint solids and associated coating defects can be economically achieved. The articles  34  can be vibrated while they are suspended from overhead conveyor  12  such as by directly contacting the vibrator with each article or a hanger  44  from which the article is suspended. Alternatively, the articles  34  can be transferred to a floor conveyor and vibrated while supported on the floor conveyor, such as by directly contacting the vibrator with each article or with a portion of the conveyor structure supporting the article. 
         [0022]      FIG. 2  illustrates another alternative vibration station  50  in which the article  34  (e.g., an automobile body) is supported on jacks  52 ,  54  during a step of vibrating article  34  to remove residual paint solids. Article  34  is shown lifted so that it is not supported by conveyor carrier  56  of conveyor  58 . Vibrators  60  are rigidly secured to a vertical member of each of jacks  52  and  54 . After article  34  has been raised of off conveyor carrier  56 , vibrators  60  are energized for a short period of time (typically a matter of seconds) sufficient to substantially reduce residual solids on the article. In the illustrated vibration station  50 , pneumatic vibrators are used. Alternatively, electric or hydraulic vibrators can be used. Suitable vibrators include those of the type conventionally used for material handling equipment as conveyors, bins, hoppers, chutes, etc. Vibrators can be selected to provide a stroke length or amplitude of about 0.25 inches (6 mm) to about 0.5 inches (12 mm) at a frequency of about 600 to about 800 RPM. However, different stroke lengths and frequencies outside of these ranges are possible. 
         [0023]    While the present invention is described herein with reference to illustrated embodiments, it should be understood that the invention is not limited hereto. Those having ordinary skill in the art and access to the teachings herein will recognize additional modifications and embodiments within the scope thereof. Therefore, the present invention is limited only by the claims attached herein.