Patent Publication Number: US-8524330-B2

Title: Method and apparatus for paint curing

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/157,928, filed on Mar. 6, 2009, which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure is related to automotive paint application and automotive paint curing. 
     BACKGROUND 
     The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
     During the assembly of an automobile, it is desirable to provide the automobile body a high quality finish. The quality of the finish improves the marketability of the automobile as well as protects the automobile body from elements. 
     The paint baking process during automobile assembly is a major energy consuming process in an automotive assembly paint shop. A typical topcoat oven used for paint baking has three major functions: (1) controlling volatile organic compound (VOC) emissions and solvent odors by driving out paint solvents or water; (2) achieving appearance quality where the top coat oven helps paint flow and level during film formation; and (3) providing durability by promoting cross-linking to cure the paint. However, topcoat ovens are large, ranging in size to about 470 feet long, thus increasing manufacturing costs and limiting space in the automotive assembly paint shop. Additionally, operation of a topcoat oven is associated with a high energy consumption rate per year. It is recognized that operation of topcoat ovens are second only to spray booths in the highest consumption of energy at the automobile paint shop. A typical automotive assembly paint shop utilizes two to three topcoat ovens. 
     SUMMARY 
     A method for curing a paint coating applied to a workpiece includes applying radiant light energy to cure the paint coating on surfaces of the workpiece within a line of sight of a radiant light energy source, and applying ambient air to the workpiece to cure paint coating on surfaces of the workpiece not within the line of sight of the radiant light energy source. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       One or more embodiments will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  schematically illustrates a paint application process in accordance with an exemplary embodiment of the present disclosure; 
         FIG. 2  schematically illustrates the chemical composition of a paint coating that can be cured by both efficient radiant light energy and low bake systems in accordance with the present disclosure; 
         FIG. 3  illustrates a graphical depiction of an electromagnetic spectrum in order of increasing wavelength in accordance with the present disclosure; 
         FIG. 4  illustrates a graphical depiction illustrating energy emissions of near infrared light, short wavelength infrared light and medium wavelength infrared light in accordance with the present disclosure; 
         FIGS. 5   a - 5   d  illustrate pictorial diagrams of the chemical reactions during the curing of a workpiece utilizing various curing methods that include near infrared light, ultraviolet light, medium-wave infrared light and induction heating in accordance with the present disclosure; and, 
         FIG. 6  illustrates a pictorial diagram of the chemical reaction during the curing of a workpiece utilizing ambient air at an ambient cure station in accordance with the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, wherein the showings are for the purpose of illustrating certain exemplary embodiments only and not for the purpose of limiting the same,  FIG. 1  schematically illustrates a paint application process  100  in accordance with an exemplary embodiment of the present disclosure. The exemplary paint application process  100  includes a coating station  10 , a heat flash station  12 , a curing process  20  and an inspection station  18 . The curing process  20  includes a radiation cure station  14  and an ambient cure station  16 . In operation, an unfinished workpiece  2  is presented to the coating station  10  where a fresh coat of paint is applied to the workpiece  2 . Upon exiting the coating station  10 , the painted workpiece  2  is first presented to the heat flash station  12  and then to the radiation cure station  14  and the ambient cure station  16  of curing process  20  to substantially cure the workpiece  2 . Upon completion of the curing process  20 , the substantially cured workpiece  2  is examined at the inspection station  18 . 
     An exemplary coating station  10  includes a paint spray booth where a fresh coat of paint is applied to the workpiece  2 . An exemplary workpiece  2  is an automobile wherein a fresh coat of paint is applied to interior and exterior surfaces of the automobile. However, the workpiece  2  is not limited to automobiles. The fresh coat of paint includes a paint material having a chemical composition enabling the paint coating to be cured by both efficient radiant light energy (i.e., the radiation cure station  14 ) and low bake systems (i.e., the ambient cure station  16 ). It is desirable that the paint coating be substantially resistant to scratches and chips, meet appearance and exposure standards and be adaptable to existing application processes (i.e., a spray booth). 
     Referring to  FIG. 2 , the chemical composition of an exemplary paint coating  200  is illustrated in accordance with an exemplary embodiment of the present disclosure. The paint coating  200  can be cured or hardened by both efficient radiant light energy (i.e., the radiation cure station  14 ) and low bake systems (i.e., the ambient cure station  16 ). Efficient radiant light energy can include ultraviolet light, near infrared (NIR) light, and conventional infrared light having short, medium and long wavelengths. Likewise, low bake systems can include ambient air at ambient temperature or can additionally blow warm or hot air to help facilitate the curing process and decrease tack free times. The paint coating  200  cross-links polymer segments  204  and silica segments  202 , wherein each end of each polymer segment  204  is linked to a silica segment  202  utilizing a cross-linking material  206 . The silica segments  202  are hard segments that provide scratch resistance, whereas the polymer segments  204  are soft and flexible segments that provide structural integrity while substantially preventing cracking during the curing process  20 . It should be appreciated that the exemplary paint coating  200  not be limited to a chemical composition including the cross-linking of polymer and silica segments  204  and  202 , respectively, but can include any chemical composition capable of being cured by both low bake systems and efficient radiation energy. 
     As mentioned above, after a fresh coat of paint is applied to the workpiece  2  at the coating station  10 , the workpiece  2  is sent to the heat flash station  12 . The heat flash station  12  includes a heated flash process to drive out solvents and water from the paint coating  200 . Driving out solvents and water from the paint coating substantially reduces volatile organic compound (VOC) emissions and solvent odors from the paint coating  200  before curing at the radiation cure station  14  and the ambient cure station  16 . Heated flash stations  12  are known in the art and will not be discussed in great detail herein. 
     As discussed above, topcoat ovens can be impractical due to size and cost constraints as well as the high energy consumption required for operating topcoat ovens. Many ideas and concepts have emerged to try to reduce or eliminate the need for paint ovens. These ideas generally fall into two categories: (1) low bake paint systems and (2) efficient radiant light energy cure systems. However, low bake paint systems and efficient radiant light energy cure systems used alone to cure a workpiece have shortfalls that prevent these systems and processes from replacing the topcoat oven. For example, low bake paint systems eliminate the need for a topcoat oven, however, exterior surfaces may attract airborne dust during a longer than desirable cure time and tack-free time. Radiant light energy cure systems allow for a fast cure time, however, reaching surfaces not in the line of sight of a radiant light energy source providing the radiant light energy requires the use of additional equipment or steps such as robotic arms and plasma chambers to reach surfaces not in the line of sight of the radiant light energy source. The exemplary curing process  20  illustrated in  FIG. 1 , and disclosed herein, utilizes the radiation cure station  14  (i.e., radiant light energy cure systems) and the ambient cure station  16  (i.e., low bake paint systems) to substantially cure the workpiece  2  without encompassing the drawbacks associated with only utilizing one of the of the systems discussed above. 
     Referring to  FIG. 3 , a graphical depiction of an electromagnetic spectrum  300  is illustrated in order of increasing wavelength (λ). The electromagnetic spectrum includes gamma rays  30 , x-rays  32 , ultraviolet radiation  34 , visible light  36 , infrared (IR) light  38  and radio waves  40 . Ultraviolet light  34  includes a wavelength range between 10 nanometers and 0.38 microns. Near infrared (NIR) light  42  having a wavelength between 0.8 and 1.5 microns, overlaps portions of the visible light spectrum  36  and the IR light spectrum  38 . Whereas the IR light spectrum  38  includes short and medium wavelengths  44  and  46 , respectively, having wavelengths in the ranges of 1.2 and 2.0 microns, respectively. It is appreciated that short-wave IR light  44  overlaps into the visible light  36  spectrum at wavelengths between 1.0 and 1.2 microns. 
     Referring to  FIG. 4 , a graphical depiction illustrating energy emissions versus wavelength of NIR light  42 , short-wave IR light  44  and medium-wave IR light  46  are illustrated in accordance with the present disclosure. The axis of ordinate denotes energy emissions (MW/μm*m 2 ) and the axis of abscissa denotes wavelength (μm). It is appreciated that NIR light  42  emits a higher amount of energy than short-wave IR light  44  and medium-wave IR light  46 , and as will become apparent, the cure time is substantially shorter when utilizing NIR light  42  (or ultraviolet light  34 ) than it is for short- and medium-wave IR lights  44  and  46 , respectively. 
     As will be discussed in greater detail herein, when radiant light energy (i.e., ultraviolet light  34  or NIR light  42 ) is applied to the surface of a paint coated (i.e., paint coating  200  shown in  FIG. 2 ) workpiece  2 , molecules within the paint are cross-linked during a chemical reaction and thereby achieve a hardened and substantially cured state. Radiant energy in the form of light (i.e., ultraviolet light  34  or NIR light  42 ) is particularly advantageous over topcoat ovens for curing a workpiece  2  surface because light energy provides for reduced energy consumption, while attaining very high gloss levels in the paint coating. The entire cross-linking of the paint coated (i.e., paint coating  200  shown in  FIG. 2 ) workpiece  2  takes place in seconds when utilizing ultraviolet light  34  or NIR light  42 , as opposed to minutes or hours in the thermal baking processes (i.e., topcoat oven). Cross-linking of the paint coated workpiece  2  takes place in minutes when utilizing shortwave IR  44  or medium-wave IR  46 . In addition to reduced energy consumption, a lead benefit to the fast cure times produced by utilizing ultraviolet light energy  34  or NIR light energy  42 , is the elimination or drastic reduction in airborne dust collection associated with slow tack free times of the painted workpiece  2  prior to being substantially cured. 
     Referring to  FIGS. 5   a - 5   d , pictorial diagrams illustrating the chemical reactions during the curing of a workpiece  2   a - 2   d  utilizing various curing technology methods to cure the painted workpiece  2   a - 2   d  is shown, in accordance with the present disclosure. The curing technologies illustrated include NIR light  42  ( FIG. 5   a ), ultraviolet light  34  ( FIG. 5   b ), medium-wave IR light  46  ( FIG. 5   c ) and induction heating ( FIG. 5   d ). 
     Referring to  FIG. 5   a , NIR light  42  is projected from a NIR lamp  542  onto a paint coating  29   a  applied to a substrate surface  52   a  of a workpiece  2   a . The paint coating  29   a  includes a plurality of paint molecules  204   a  disposed therein. The NIR lamp  542  projects NIR light  42  in a straight line to surfaces within the line of sight  50   a  of the NIR lamp  542 . In an exemplary example, the NIR lamp  542  is shaped and sized to cure a workpiece  2  the size of a full automobile. In an alternative embodiment, a plurality of NIR lamps  542  can be utilized to cure the workpiece  2   a , wherein each NIR lamp  542  can be configured to cure a portion of the workpiece  2   a . As shown, radiation within the NIR light  42  is substantially absorbed by the paint coating  29   a . The absorption of the NIR light  42  provides for fast and homogenous penetration of the NIR light  42  into the paint coating  29   a  to substantially cure a surface of the workpiece  2   a  in the line of sight  50   a  of the NIR lamp  542  without heating the substrate surface  52   a  as in the case of conventional infrared light radiation (i.e., medium-wave IR light  46  shown in  FIG. 5   c ). As demonstrated by the high energy emissions in  FIG. 4 , the bandwidth of NIR light  42  can accomplish cure times at or near 70 seconds. It is appreciated that the paint coating  29   a  can include the chemical composition of the paint coating  200  (see  FIG. 2 ) that can be cured or hardened by both NIR light  42  and low bake systems (i.e., the ambient cure station  16 ). 
     Referring to  FIG. 5   b , ultraviolet light  34  is projected from an ultraviolet lamp  534  onto a paint coating  29   b  applied to a substrate surface  52   b  of a workpiece  2   b . The paint coating  29   b  includes a plurality of paint molecules  204   b  and a plurality of photo initiators  205   b  disposed therein. The ultraviolet lamp  534  projects ultraviolet light  34  in a straight line to surfaces within the line of sight  50   b  of the ultraviolet lamp  534 . In an exemplary embodiment, the ultraviolet lamp  534  is shaped and sized to cure a workpiece  2   b  the size of a full automobile. In an alternative embodiment, a plurality of UV lamps  534  can be utilized to cure the workpiece  2   b , wherein each UV lamp  534  can be configured to cure a portion of the workpiece  2   b . When the paint coating  29   b  receives the ultraviolet light  34 , the plurality of photo initiators  205   b  disposed within the paint coating  29   b  initiate a chemical chain reaction to promote cross-linking between the plurality of paint molecules  204   b  and thereby substantially cure a surface of the workpiece  2   b  in the line of site  50   b  of the UV lamp  534 . This chemical chain reaction within the paint coating  29   b  can accomplish cure times in seconds. It is appreciated that the paint coating  29   b  can include the chemical composition of the paint coating  200  (see  FIG. 2 ) that can be cured or hardened by both ultraviolet light  34  and low bake systems (i.e., the ambient cure station  16 ). 
     Referring to  FIG. 5   c , medium-wave IR light  46  is projected from an IR lamp  546  onto a paint coating  29   c  applied to a substrate surface  52   c  of a workpiece  2   c . The paint coating  29   c  includes a plurality of paint molecules  204   c  disposed therein. The IR lamp  546  projects the medium-wave IR light  46  in a straight line to surfaces within the line of sight  50   c  of the IR lamp  546 . In an exemplary embodiment, the IR lamp  546  is shaped and sized to cure a workpiece the size of a full automobile. In an alternative embodiment, a plurality of IR lamps  546  can be utilized to cure the workpiece  2   c , wherein each IR lamp  546  can be configured to cure a portion of the workpiece  2   c . Additionally, the substrate surface  52   c  is heated via conduction and only the top surface of the paint coating  29   c  is heated by the medium-wave IR light  46 . Heating the top surface of the paint coating  29   c  and the substrate surface  52   c  via conduction can accomplish cure times in the paint coating  29   c  at or near 25 minutes. It is appreciated that the paint coating  29   c  can include the chemical composition of the paint coating  200  (see  FIG. 2 ) that can be cured or hardened by both medium-wave IR light  46  and low bake systems (i.e., the ambient cure station  16 ). 
     NIR light  42  and ultraviolet light  34  are preferred methods of curing a surface within the line of sight of the radiant light energy source (i.e., lamps  542  or  534 ) due to decreased cure and tack free times compared to medium-wave IR light  46 . 
     Referring to  FIG. 5   d , induction heating is applied to cure a paint coating  29   d  applied to a metallic substrate surface  52   d  of a workpiece  2   d . The paint coating  29   d  includes a plurality of paint molecules  204   d  disposed therein. The substrate surface  52   d  is electromagnetically heated by a plurality of induction coils  54  around the substrate surface  52   d , wherein the heat is absorbed by the paint coating  29   d  to substantially cure the paint coating  29   d . The workpiece  2   d  can be substantially cured in seconds. In an example, induction heating can be utilized to substantially cure a paint coating applied to a roll-bar for application on a vehicle, wherein the roll-bar is electromagnetically heated by induction coils and the paint coating absorbs the heat so substantially cure the paint coating. 
     Referring back to  FIG. 1 , the workpiece  2  enters the radiation cure station  14  of the exemplary curing process  20  upon exiting the heat flash station  12 . Exemplary embodiments envisioned of the radiation cure station  14  include the application of ultraviolet light  34  or NIR light  42  discussed by methods described in  FIGS. 5   a  and  5   b . Alternative forms of radiant light energy contemplated to cure the workpiece include shortwave and medium-wave IR  44  and  46 , respectively; however these forms of radiant light energy are less preferred due to increased tack free and cure times. In addition to radiant light energy, alternative forms of energy to cure the workpiece  2  include induction heating ( FIG. 5   d ), hydrogen bombardment and electron beams. It should be appreciated that any combination of the above forms of energy may be used in combination to assist in the curing of the workpiece  2 . 
     As discussed above, both ultraviolet and NIR light energy  34  and  42 , respectively are limited to curing surfaces of a workpiece  2  that are within the line of sight of the radiant light energy source (i.e., UV lamp  534  or NIR lamp  542 ) because light travels in a straight line. For example, interior surfaces of an automobile that include door frames or the back side of a trunk lid cannot be cured if the radiant light energy (i.e., ultraviolet light  34  or NIR light  42 ) is blocked by other panels of the automobile. It is known to mount lamps for projecting ultraviolet light  34  or NIR light  42  on robotic arms or to utilize plasma ultraviolet light  34  chambers to reach interior or hidden surfaces of the workpiece  2 . However, these solutions can increase cost and slow down process cycle time for substantially curing the workpiece  2 . The exemplary curing process  20  disclosed herein utilizes the radiant cure station  14  to promote cross-linking on a surface of the painted workpiece  2  by projecting radiant light energy (i.e., ultraviolet light  34  or NIR light  42 ) on exterior surfaces of the workpiece  2 , and thus, achieving reduced energy consumption and fast cure times on the exterior surfaces of the workpiece  2 . Whereas, the exemplary curing process  20  additionally utilizes the ambient curing station  16  to cure interior surfaces, or surfaces not in the line of sight of the radiant light energy source (i.e., UV lamp  534  or NIR lamp  542 ), to cure the workpiece  2 . It is appreciated that slow tack free times associated with ambient curing are less susceptible to airborne dust collection on interior surfaces of the painted workpiece  2  as opposed to exterior surfaces. 
     Once exterior surfaces of the workpiece  2  within the line of sight of the radiant light energy source (i.e., NIR lamp  542  or UV lamp  534  shown in  FIGS. 5   a  and  5   b , respectively) are substantially cured at the radiant cure station  14 , the workpiece  2  enters the ambient cure station  16 . Utilizing ambient air at ambient temperature, the ambient cure station  16  cures surfaces of the workpiece  2  that were not cured at the radiation cure station  14 . Curing the workpiece  2  at ambient temperature is advantageous because interior surfaces and other surfaces that were not accessible at the radiation cure station  14  get cured while avoiding the use of expensive equipment (i.e., robotic arms and plasma chambers). In an alternative embodiment, the ambient cure station  16  can blow warm or hot air to help facilitate the curing process and decrease tack free times. 
     Referring to  FIG. 6 , a pictorial diagram of the ambient cure station  16  illustrating the chemical reaction during the curing of a workpiece  2   e  utilizing ambient air  60  is shown, in accordance with the present disclosure. Paint coating  29   e  applied to a substrate surface  52   e  of the workpiece  2   e  is cured by cross-linking the plurality of paint molecules  204   e  with the ambient air  60  over a period of time. For example, full cure of the paint coating  29   e  can occur in about 12 to 16 hours utilizing ambient air  60 . Tack free time is established at or near 20 to 30 minutes. However, because interior surfaces are not directly exposed to airborne dust, the workpiece  2   e  is not as susceptible to having dirt-in-paint defects. It is appreciated, that the paint coating  29   e  can include the chemical composition of the paint coating  200  (see  FIG. 2 ) capable of being cured or hardened by both efficient radiant light energy (i.e., the radiation cure station  14 ) and ambient air  60  at the ambient cure station  16 . 
     Referring to  FIGS. 1 ,  5  and  6 , it is appreciated that the exemplary curing process  20  in association with the paint coating  200  (see  FIG. 2 ) enables exterior surfaces of a workpiece  2   a - 2   d  to be cured within seconds, and surfaces not easily accessible (i.e., interior surfaces) at the radiant cure station  14  to be cured by ambient air  60  at the ambient cure station  16 . Thus, the exemplary curing process  20  eliminates or substantially reduces the collection of airborne dust and dirt-in paint on appearance critical exterior surfaces due to slow tack free time, while the ambient cure system  16  eliminates the need for expensive equipment and additional steps to cure paint on less-appearance critical interior surfaces or other surfaces not within the line of sight of the radiant light energy source (i.e., UV lamp  534  or NIR lamp  542 ). 
     Upon exiting the exemplary curing process  20 , the substantially cured workpiece  2  enters the inspection station  18 . At the inspection station  18 , the substantially cured workpiece  2  is inspected for scratches, blemishes and defects in the workpiece  2 . If the finish of the workpiece  2  meets industry standards the workpiece  2  exits the paint application process  100 . If the finish of the workpiece  2  does not meet industry standards (i.e., defects are found in the finish of the workpiece  2  or workpiece is not substantially cured), the workpiece  2  may be sent back to the coating station  10 , the heat flash station  12 , the radiation cure station  14  or the ambient cure station  16  to fix any defects found in the finish of the workpiece  2  at the inspection station  18 . For example, the finished workpiece  2  can be an automobile where it is determined that portions of the inside door frame were not painted. The unpainted portions of the inside door frame can be touched up and left to cure in the ambient cure station  16  until being substantially cured. 
     The disclosure has described certain preferred embodiments and modifications thereto. Further modifications and alterations may occur to others upon reading and understanding the specification. Therefore, it is intended that the disclosure not be limited to the particular embodiment(s) disclosed as the best mode contemplated for carrying out this disclosure, but that the disclosure will include all embodiments falling within the scope of the appended claims.