Abstract:
A system for effectively defrosting a plastic window includes a transparent plastic panel, a heater grid having a plurality of grid lines that are integrally formed with the plastic panel, and equalizing means for equalizing the electrical current traveling through each of the grid lines.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. application Ser. No. 11/645,851 filed Dec. 27, 2006, which claims priority to U.S. Provisional Application Ser. No. 60/754,966 filed on Dec. 29, 2005, the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to a conductive heater grid for use in defrosting plastic and glass panels, such as windows in vehicles. 
         [0004]    2. Related Technology 
         [0005]    Plastic materials, such as polycarbonate (PC) and polymethylmethyacrylate (PMMA), are currently being used in the manufacturing of numerous automotive parts and components, such as B-pillars, headlamps, and sunroofs. Automotive rear window (backlight) systems represent an application for these plastic materials due to their many identified advantages, particularly in the areas of styling/design, weight savings, and safety/security. More specifically, plastic materials offer the automotive manufacturer the ability to reduce the complexity of the rear window assembly through the integration of functional components into the molded plastic system, as well as the ability to distinguish their vehicles by increasing overall design and shape complexity. Being lighter in weight than conventional glass backlight systems, their incorporation into the vehicle may facilitate both a lower center of gravity for the vehicle (and therefore better vehicle handling &amp; safety) and improved fuel economy. Further, enhanced safety is realized, particularly in a roll-over accident because of a greater probability of the occupant or passenger being retained in a vehicle. 
         [0006]    In order to be used as a rear window or backlight on a vehicle, the plastic material must be compatible with the use of a defroster or defogging system, better known as a heater grid. For commercial acceptance, a plastic backlight must meet the performance criteria established for the defrosting or defogging of glass backlights. One difference between glass and plastics panels is related to the electrical conductivity exhibited by the heater grid. This difference in conductivity manifests itself in poor defrosting characteristics exhibited by the plastic window, as compared to the glass window. This difference in conductivity manifests itself in the inefficient heating of portions of the defroster, such as the busbar, that provides very little to no benefit to defrosting the overall window. 
         [0007]    In addition to the previously mentioned drawbacks, the amount of electrical current traveling through each of the grid lines of the heater grid may vary. This variance causes grid lines with a less restrictive conductive path to heat up faster, leaving both defrosted and frosted portions of the plastic panels. 
         [0008]    Therefore, there is a need for a system that will effectively defrost a plastic window with performance characteristics similar to that of a conventional glass window. 
       SUMMARY 
       [0009]    In satisfying the above need, as well as overcoming the enumerated drawbacks and other limitations of the known technology, the present invention provides a system that effectively defrosts a plastic window with performance characteristics similar to that of a conventional glass window. The system includes a transparent plastic panel, a heater grid having a plurality of grid lines that are integrally formed with the plastic panel, and equalizing means for equalizing the amount of electrical current traveling trough each of the grid lines. 
         [0010]    The equalizing means typically includes a first and second busbar connected to positive and negative terminals, respectively, of a power supply. The plurality of grid lines extend between the first and second busbars. In order to equalize the current traveling through the grid lines, the busbars may be made of a material that is more conductive than the material used to make the grid lines. Additionally or alternatively, the busbars may be made thicker than the grid lines, thereby allowing current to travel more freely from the power supply to the grid lines. 
         [0011]    The equalizing means may also include additional highly conductive material placed along the lengths of the busbars. By so doing, current will travel more freely from the power supply to the grid lines, thereby equalizing the current traveling through the grid lines. This highly conductive material may be in the form of a metallic insert or may be a portion of a metallic tape. 
         [0012]    Finally, the equalizing means may also include a plurality of connections on each busbar to the power supply. By having a plurality of connections to the busbars, current is more equally distributed to the busbars, resulting in a more equal distribution in the current traveling through the grid lines. 
         [0013]    Further objects, features and advantages of this invention will become readily apparent to persons skilled in the art after a review of the following description, with reference to the drawings and claims that are appended to and form a part of this specification. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a plastic window assembly incorporating a defrosting grid with busbars embodying the principles of the present invention; 
           [0015]      FIG. 2  is a more detailed view of a portion of the window assembly of  FIG. 1 ; 
           [0016]      FIG. 3  is a plastic window assembly similar to  FIG. 1  having a black out ink border; 
           [0017]      FIG. 4  is a plastic window assembly similar to  FIG. 1  having conductive strips along a portion of the length of the busbars embodying the principles of the present invention; 
           [0018]      FIG. 5  is a plastic window assembly similar to  FIG. 1  having more than one electrical connection per busbar embodying the principles of the present invention; 
           [0019]      FIG. 6  is a chart showing the temperature profile of busbars and grid lines of a heater grid with one electrical connection per busbar; 
           [0020]      FIG. 7  illustrates the temperature profile of busbars and grid lines of a heater grid with two electrical connections per busbar; 
           [0021]      FIG. 8  illustrates the temperature profile of busbars and grid lines of a heater grid with one electrical connection per busbar; and 
           [0022]      FIG. 9  illustrates the temperature profile of busbars and grid lines of a heater grid with two electrical connections per busbar. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    Referring to  FIG. 1 , window defroster assembly  10  generally includes a defroster  12  provided on a panel  14 . The panel  14  may be made of a thermoplastic resin including, but not limited to, polycarbonate resins, acrylic resins, polyarylate resins, polyester resins, and polysulfone resins, as well as copolymers and any combination thereof. Preferably, the panel  14  is transparent. The panel  14  may further comprise a protective coating system that lies on the surface of the thermoplastic resin and upon which the defroster  12  is applied. The protective coating system may comprise a weather resistant coating, an abrasion resistant coating, or both. An example of a panel  14  that comprises a plastic resin, a weather resistant coating, and an abrasion resistant coating upon which a defroster can be applied is the Exatec® 900 glazing system. This glazing system comprises a polycarbonate resin, an SHP9X &amp; SHX weather resistant coating, and a glass-like abrasion resistant coating. 
         [0024]    The defroster  12  includes a heater grid  16  having a series of grid lines extending between generally opposed busbars  20 ,  22 . The heater grid may include grid lines of the same dimensions or it may include major grid lines  24 ,  26  with minor grid lines  28 ,  30 ,  32  located there between. The major and minor grid lines  24 ,  26 ,  28 ,  30 ,  32  are described in U.S. Pat. No. 7,129,444, the entirety of which is hereby incorporated by reference. 
         [0025]    While illustrated with three minor grid lines, it should be understood that there may be any number of minor grid lines between the major grid lines  24 ,  26 . Furthermore, the minor grid lines  28 ,  30 ,  32  may be replaced by a conductive film or coating between the major grid lines  24 ,  26 . In this illustrated embodiment, the heater grid  16  includes seventeen major grid lines and forty-eight minor grid lines. The present invention contemplates additional major and/or minor grid lines. The major grid lines  24 ,  26  and minor grid lines  28 ,  30 ,  32  may be made of a conductive ink, such as silver ink. 
         [0026]    The busbars  20 ,  22  are respectively designated as positive and negative busbars. The busbars  20 ,  22  have electrical connectors  34 ,  36  and are connected respectively to positive and negative leads  35 ,  37  of a power supply  38 . The power supply  38  may be the electrical system of an automobile vehicle. Upon the application of a voltage across the heater grid  16 , current will flow through the grid lines  16 , from the positive busbar  20  to the negative busbar  22  and, as a result, the grid lines  16  will heat up via resistive heating. In this type of design, it has been observed that the major grid lines  24 ,  26  exhibit a temperature between 10-15° C. higher than the minor grid lines  28 ,  30 ,  32 . 
         [0027]    In one embodiment, the busbars  20 ,  22  generally have a width W 1  of about 19 mm and have a length H 1  of about 704 mm. However, width W 1  and length H 1  may be any suitable dimension. Reference lines  40  and  42  divide the heater grid  16  into a first zone  43 , a second zone  45  and a third zone  47 . The first zone  42  is the portion of the heater grid  16  between the lines  40 ,  42 . The second zone  45  is the portion of the heater grid  16  between reference line  40  and the right busbar  20 . Finally, the third zone  47  is the portion of the heater grid  16  between reference line  42  and the left busbar  22 . In the above implementation, zone  43  has a length W 2  of about 650 mm, while the second and third zones  45 ,  47  have lengths W 3  of about 27 mm. It should be understood that width W 2  and width W 3  may be any suitable dimension. In the first zone  43 , the major grid lines  24 ,  26  and minor grid lines  28 ,  30 ,  32  may have a width of about 0.85 mm and 0.25 mm, respectively. In the second and third zones  45 ,  47 , the major grid lines  24 ,  26  and minor grid lines  28 ,  30 ,  32  may have a width of about 2.00 mm and 0.40 mm, respectively. Of course, the width of the major grid lines  24 ,  26  and minor grid lines  28 ,  30 ,  32  may be any suitable dimension. 
         [0028]    Referring to  FIGS. 1 and 2 , the further dimensions of the heater grid  16  are shown.  FIG. 2  is a close up view of a portion of the window defroster assembly  10  as with the reference circle  41 . The distance D 1  between the major grid lines  24 ,  26  may be about 25 mm. The distance D 2  between minor grid lines  28 ,  32  and major grid lines  24 ,  26  may be about 13.5 mm. The distance D 3  between minor grid lines  28 ,  32  and minor grid line  30  may be about 8.5 mm. Of course, the distances D 1 , D 2  and D 3  may be any suitable dimension. 
         [0029]    The resistive heating of a busbars  20 ,  22  is highly dependent upon the amount of electrical voltage applied and the volume of conductive ink through which the electrical current flows. Thus, increasing the volume of conductive ink by adding additional conductive ink to the busbars  20 ,  22  through a second printing process, decreases the resistive heating of the busbars  20 ,  22 . The volume of conductive ink deposited during the initial printing of the entire heater grid  16  can also be increased in the busbars  20 ,  22 . Volume control by the use of various techniques is generally known to screen printing manufacturers. This technique can increase the emulsion thickness on the screen localized around busbars  20 ,  22 , thereby increasing the print thickness of the busbars  20 ,  22  in comparison to the print thickness of the heater grid  16 . Other printing techniques, such as dispensing, can increase the amount of ink deposited, and thus the volume for each busbar by controlling printing parameters, such as flow rate, transverse speed, etc. 
         [0030]    Another way of reducing the resistive heating of the busbars  20 ,  22  is to make the busbars  20 ,  22  out of a different material than the heater grid  16 . More specifically, this different material should exhibit a conductivity that is greater than the conductivity associated with the heater grid  16 . In this respect, busbars  20 ,  22  could be made of a metallic tape or a metallic insert. The conductive tape or panel may be positioned underneath or on top of the heater grid  16  in order to establish sufficient electrical connection between the busbars  20 ,  22  and the heater grid  16 . The metallic tape or panel can be attached to the panel  14  after the panel  14  is formed through the use of an adhesive or during the forming of the window as an insert (e.g., film insert molding, etc.). 
         [0031]    Referring to  FIG. 3  another embodiment of the window defroster assembly  10 ′ is shown. The window defroster assembly  10 ′ is similar to the embodiment shown in  FIG. 1 ; however, the window defroster assembly  10 ′ further includes areas of opacity, such as a black-out border  44 . Such borders  44  are typically used for aesthetic reasons, such as masking fit and finish imperfections and concealing mounting structures or functional components such as the busbars  20 ,  22 . The blackout border  44  can be applied to the panel  14  by printing an opaque ink onto the surface of the panel  14  or through the use of known in mold decorating techniques, including insert film molding. 
         [0032]    Referring to  FIG. 4  another embodiment of the window defroster assembly  10 ′ is shown. This embodiment is similar to the embodiment illustrated in  FIG. 1 ; however, conductive inserts  21 ,  23  are in electrical communication with busbars  20 ,  22 , respectively. Generally, the conductive inserts  21 ,  23  run along at least a portion of the length of the busbars  20 ,  22 . The electrical connectors  34 ,  36  are connected to conductive inserts  21 ,  23 , respectively. The electrical connectors  34 ,  36  are also connected to positive and negative leads  35 ,  37  of a power supply  38 , thereby providing a voltage to the busbars  20 ,  22  via the conductive inserts  21 ,  23 , respectively. Generally, the conductive inserts  21 ,  23  are highly conductive and may be a conductive metallic tape or highly conductive trace. 
         [0033]    As further discussed below, the use of conductive inserts may reduce the temperature of the busbars  20 ,  22  as a voltage is applied to the heater grid  16  via the busbars  20 ,  22 . In other words, for two electrical connectors spaced 5 inches apart is equivalent to using one electrical connection to a 5 inch metallic insert or tape positioned on the printed busbar. 
         [0034]    Referring to  FIG. 5  another embodiment of the window defroster assembly  10 ″′ is shown. The window defroster assembly  10 ″′ is similar to the embodiment shown in  FIG. 1 ; however, the busbars  20 ,  22  are connected in a different manner to the power supply  38 . More specifically, the assembly  10 ″′, the busbars  20 ,  22  are each connected to power supply  38  thorough at least two connections. For example, a pair of electrical connectors  46 ,  48  and electrical connectors  50 ,  52 , are connected to busbars  20  and  22 , respectively. Of course, the present invention contemplates additional electrical connectors. 
         [0035]    Referring to  FIG. 6 , a chart displaying the temperature profile of the busbars with one electrical connector per busbar is shown. The inventors have discovered that one electrical connection per busbar  34 ,  36  as shown in  FIG. 1 , printed on the panel  14  through the use of a conductive ink will cause the busbars  34 ,  36  to significantly increase in temperature. The single electrical connectors on busbars present in the heater grid is seen to increase in temperature within several minutes to about 80-100° C. In the chart shown in  FIG. 6 , busbars  34 ,  36  with one electrical connection are observed to exhibit a greater amount of resistive heating than the major grid lines  24 ,  26  grid lines in the associated heater grid. The grid lines are shown to exhibit a temperature of between 40-50° C. The resistive heating of the busbars is observed to occur either over the entire length of the busbars to certain portions of the busbars or localized to an area near the electrical connectors. 
         [0036]    Referring to  FIG. 7 , a chart displaying the temperature profile of the busbars with two electrical connections per busbar, such as shown in  FIG. 5 , is shown. The heater grid  16  having busbars with two electrical connectors  34 ,  36  per busbar was tested. The electrical connectors on each of the busbars were spaced about 6 inches apart from each other. With this construction, the busbars  20 ,  24  were found to exhibit very little resistive heating, stabilizing at a temperature of about 40° C., while the major grid lines  24 ,  26  were observed to heat to 60-70° C. 
         [0037]    Referring to  FIGS. 8 and 9 , the same effect is observed to occur, if the heater grid  16  is printed onto a thin film of plastic and then insert molded into a window. As shown in  FIGS. 8 and 9 , one connection to each busbar causes a different heating of the busbar ( FIG. 8 ), while two connections per busbar allows the heater grid to function as designed ( FIG. 9 ). 
         [0038]    The inventors have discovered that the average temperature of the busbar can be sustained below the average temperature of the grid lines when the electrical connections to each busbar in a defroster printed on a plastic panel are provided at about three inches (74 mm) apart. However, when the electrical connectors are in this close position, there will still be some localized heating of the busbar in that the maximum temperature exhibited by the busbar is above the average temperature exhibited by the grid lines. Thus, it is preferred that the electrical connectors be positioned more than three inches apart and more preferably about five inches (125 mm) or greater apart. In this case, the average and maximum temperature exhibited by the busbar will be equal to or less than the average temperature exhibited by the grid lines. 
         [0039]    The inventors have further discovered that greater than about five inches (125 mm) spacing between the electrical connectors is necessary for a defroster printed on a thin sheet and incorporated into a window via film insert molding (FIM). As shown in Table 1, the spacing of greater than five inches is necessary to ensure that the average and maximum temperatures exhibited by the busbars are equal to or less than the average temperature exhibited by the grid lines when voltage is applied to the defroster. 
         [0000]    
       
         
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
             
               
                   
                 Temperature (° C.) 
               
             
          
           
               
                   
                 Connections/busbar 
                 Distance 
                 Time 
                 Grid lines 
                 Connections 
               
             
          
           
               
                 Trial 
                 # 
                 Location 
                 (mm) 
                 (minutes) 
                 Min 
                 Max 
                 Average 
                 Min 
                 Max 
                 Average 
               
               
                   
               
             
          
           
               
                 IMD design printed on 730 PC plaque: 
               
             
          
           
               
                 1 
                 1 
                 Center 
                   
                 10 
                 55 
                 62 
                 58.5 
                 72 
                 78 
                 75 
               
               
                 2 
                 1 
                 Diagonally (Top left 
                   
                 2 
                 44 
                 45 
                 44.5 
                 85 
                 113 
                 99 
               
               
                   
                   
                 to bottom right) 
               
               
                 3 
                 2 
                 Lines 7-8 and 8-9 
                 44 
                 10 
                 57 
                 60 
                 58.5 
                 65 
                 75 
                 70 
               
               
                 4 
                 2 
                   
                 74 
                 10 
                 55 
                 58 
                 56.5 
                 27 
                 71.7 
                 49.35 
               
               
                 5 
                 2 
                   
                 100 
                 10 
                 53 
                 68 
                 60.5 
                 27 
                 74 
                 50.5 
               
               
                 6 
                 2 
                 Lines 6-7 and 9-10 
                 132 
                 10 
                 56 
                 68 
                 62 
                 31 
                 61 
                 48 
               
               
                 7 
                 2 
                 Lines 5-6 and 10-11 
                 220 
                 10 
                 57 
                 64 
                 60.5 
                 30 
                 51 
                 40.5 
               
             
          
           
               
                 IMD film 
               
             
          
           
               
                 1 
                 1 
                 Center 
                   
                 2 
                 44 
                 67 
                 55.5 
                 83 
                 99 
                 91 
               
             
          
           
               
                 2 
                 1 
                 Diagonally (Top left 
                   
                 2 
                 low 30 s 
                   
                 &gt;100 
               
               
                   
                   
                 to bottom right) 
               
             
          
           
               
                 3 
                 2 
                 Lines 7-8 and 8-9 
                 44 
                 2 
                 59 
                 63 
                 61 
                 80 
                 92 
                 86 
               
               
                 4 
                 2 
                 Lines 6-7 and 9-10 
                 132 
                 10 
                 55 
                 60 
                 57.5 
                 67 
                 75 
                 71.5 
               
               
                 5 
                 2 
                 Lines 5-6 and 10-11 
                 220 
                 5 
                 62 
                 68 
                 65 
                 49 
                 60 
                 54.5 
               
               
                   
               
             
          
         
       
     
         [0040]    As a person skilled in the art will readily appreciate, the above description is meant as an illustration of implementation of the principles this invention. This description is not intended to limit the scope or application of this invention in that the invention is susceptible to modification, variation and change, without departing from spirit of this invention, as defined in the following claims.