Abstract:
An apparatus for printing a conductive ink onto a plastic panel including an articulatable arm having an end that opposes a surface of the panel. A nozzle is mounted via a nozzle height actuator to the end of the arm, and the nozzle is coupled to a source of conductive ink. A flow regulator, coupled to the ink source, regulates the flow rate of ink out of the nozzle and is controlled by the controller. A height sensor is configured to output a height signal relative to the surface and the controller, which is coupled to the arm, the flow regulator, the nozzle height actuator and the sensor, is configured to control the arm, flow regulator, nozzle height actuator, and speed of nozzle movement such that a conductive trace of predetermined height and width is applied to the substrate.

Description:
BACKGROUND 
     1. Field of the Invention 
     This invention relates to an apparatus and method of printing a conductive heater grid design on plastic or glass glazing panels, such as those used as backlights in vehicles. 
     2. Related Technology 
     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. 
     Although there are many advantages associated with implementing plastic windows, these windows are not without limitations that represent technical hurdles that must be addressed prior to wide-scale commercial utilization. Limitations relating to material properties include the stability of plastics during prolonged exposure to elevated temperatures and the limited ability of plastics to conduct heat. Regarding the latter, in order to be used as a backlight in a vehicle, the plastic material must be compatible with the use of a defroster or defogging system (hereafter just referred to as a “defroster”). For commercial acceptance, a plastic backlight must meet the performance criteria established for the defrosting or defogging of glass backlights. 
     The difference in material properties between glass and plastics becomes quite apparent when considering heat conduction. The thermal conductivity of glass (T c =22.39×10 −4  cal/cm-sec-° C.) is approximately 4-5 times greater than that exhibited by a typical plastic (e.g., T c  for polycarbonate=4.78×10 −4  cal/cm-sec-° C.). Thus, a defroster designed to work effectively on a glass window may not necessarily be efficient at defrosting or defogging (hereafter just “defrosting” or “defrost”) a plastic window. The lower thermal conductivity of the plastic may limit the dissipation of heat from the heater grid lines across the surface of the plastic window. Thus, at a similar power output, a heater grid on a glass window may defrost the entire viewing area, while the same heater grid on a plastic window may only defrost those portions of the viewing area that are close to the grid lines. 
     A second difference between glass and plastics that must be overcome is related to the electrical conductivity exhibited by a printed heater grid. The thermal stability of glass, as demonstrated by a relatively high softening temperature (e.g., T soften &gt;&gt;1000° C.), allows for the sintering of a metallic paste on the surface of the glass window to yield a substantially inorganic frit or metallic wire. Since the softening temperature of glass is significantly greater than the glass transition temperature of a typical plastic resin (e.g., polycarbonate T g =145° C.), a metallic paste cannot be sintered onto a plastic panel. Rather, it must be cured on the panel at a temperature lower than the T g  of the plastic resin. 
     A metallic paste typically consists of metallic particles dispersed in a polymeric resin that will bond to the surface of the plastic to which it is applied. The curing of the metallic paste provides a conductive polymer matrix having closely spaced metallic particles dispersed throughout a dielectric layer. The presence of the dielectric layer (e.g., polymer) between dispersed conductive particles leads to a reduction in the conductivity, or an increase in resistance, of the cured heater grid lines, as compared to dimensionally similar heater grid lines sintered onto a glass substrate. This difference in conductivity manifests itself in poor defrosting characteristics exhibited by the plastic window, as compared to the glass window. 
     With the above in mind, it is clear that controlling the quality of the heater grid printed onto the panel is important to maximizing the efficiency and effectiveness of any defroster used with that panel. Various parameters affect the quality of the printed heater grid and these parameters include any variances in the width, height and straightness of the grid lines. The more variances that exist in width and height, the greater the negative impact on the effectiveness of the defroster. This is a result of unequal resistances in various sections of the grid line and busbars resulting in unequal resistive heating in various sections of the defroster. With regard to straightness, this is mainly an aesthetic concern that becomes more of an issue because of the ability of plastic window assemblies to have greater design flexibility and curvature. 
     A defroster may be printed directly onto the surface inner or outer of a panel, or on the surface of a protective layer, using a conductive ink or paste and various methods known to those skilled in the art. Such methods include, but not limited to, screen-printing, ink jet printing and automatic dispensing. Automatic dispensing includes techniques known to those skilled in the art of adhesive application, such as drip &amp; drag, streaming, and simple flow dispensing. In each of the above instances, the shape of the panel impacts the quality of the printed lines, i.e. screen printing becomes very difficult on non-planer panels, and the speed at which printing is done both the width and height of the grid lines. Slower speeds and higher flow for the ink or paste rates can result in wider and higher grid lines. Conversely, higher speeds and slower flow rates can result in slimmer and lower grid lines. With screen printing in particular, the height of the grid line is not readily variable. 
     From the above, it is seen that there is a need in the industry for an apparatus and method that can effectively control the quality and consistency with which grid lines are printed onto a panel. 
     SUMMARY OF THE INVENTION 
     In satisfying the above need, as well as overcoming the enumerated drawbacks and other limitations of the related art, the present invention provides an apparatus for printing grid lines formed from a conductive ink onto a plastic substrate or panel. The apparatus includes a support bed adapted to support the panel and an articulatable arm positioned relative the support bed such that an end of the arm opposes a surface of the panel to be printed. A dispensing nozzle is carried by the arm and mounted thereto at the end of the arm; the nozzle being coupled to a source of conductive ink and to a nozzle height actuator that mounts the nozzle to the arm. Finally, a flow regulator is coupled to the ink source and the nozzle whereby the flow rate of conductive ink out of the nozzle is regulated. The apparatus also includes a height sensor that is configured to output a height signal relative to the surface of the panel. A controller, coupled to the arm, the flow regulator, the nozzle height actuator and the height sensor, is configured to articulate the arm so as to move the nozzle in a predetermined pattern about the surface of the panel. In addition, the controller is configured to control at least one of the flow regulator and the nozzle height actuator as a function of the speed at which the nozzle is moved, the height signal from the height sensor and/or the flow rate of conductive ink out of the nozzle, such that a conductive trace of predetermined height and width is applied to the panel. 
     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 
         FIG. 1  is a schematic sectional view of four alternative embodiments of a window assembly according to the present invention; 
         FIG. 2  is a perspective view of a robot arm traversing a dispensing head over a panel of a window assembly; 
         FIG. 3  is a partial front view of the robot arm and dispensing head over the panel; and 
         FIG. 4  is a close up, cross sectional view of a heater grid line disposed on the panel. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings and as seen in  FIG. 1 , a defroster or heater grid  16  may be positioned near the external surface  18  of a plastic window assembly  20  (schematic A), on an internal surface  22  of the plastic window assembly  20  (schematic B and C), or encapsulated within the plastic panel (Schematic D) itself. Each of the possible positions for the heater grid  16  offers different benefits in relation to overall performance and cost. Positioning the heater grid  16  near the external surface  18  (schematic A) of the window assembly  20  is preferred so as to minimize the time necessary to defrost the window assembly  20 . Positioning the heater grid  16  on the internal surface  22  (Schematic B and C) of a plastic panel  24  of the window assembly  20  offers benefits in terms of ease of application and lower manufacturing costs. 
     The transparent plastic panel  24  itself may be constructed of any thermoplastic polymeric resin or a mixture or combination thereof. Appropriate thermoplastic resins include, but are not limited to, polycarbonate resins, acrylic resins, polyarylate resins, polyester resins, and polysulfone resins, as well as copolymers and mixtures thereof. The panels  24  may be formed into a window through the use of any of the various known techniques, such as molding, thermoforming, or extrusion. The panels  24  may further include areas of opacity applied by printing an opaque ink on the panel  24  in the form of a black-out border  26  or molding a border using an opaque resin. 
     The heater grid  16  may be printed directly onto the inner surface  28  or outer surface  30  of the plastic panel  24 . Alternatively, it may be printed on the surface of one or more protective layers  32 ,  34 . In either construction, printing is affected using a conductive ink. 
     In its final construction, the plastic panel  24  may be protected from such natural occurrences as exposure to ultraviolet radiation, oxidation, and abrasion through the use of a single protective layer  32  or additional, optional protective layers  34 , both on the exterior side and/or interior side of the panel  24 . As the term is used herein, a transparent plastic panel  24  with at least one protective layer  32  is defined as a transparent plastic glazing panel. 
     The protective layers  32 ,  34  may be a plastic film, an organic coating, an inorganic coating, or a mixture thereof. The plastic film may be of the same or different composition as the transparent panel. The film and coatings may comprise ultraviolet absorber (UVA) molecules, rheology control additives, such as dispersants, surfactants, and transparent fillers (e.g., silica, aluminum oxide, etc.) to enhance abrasion resistance, as well as other additives to modify optical, chemical, or physical properties. Examples of organic coatings include, but are not limited to, urethanes, epoxides, and acrylates and mixtures or blends thereof. Some examples of inorganic coatings include silicones, aluminum oxide, barium fluoride, boron nitride, hafnium oxide, lanthanum fluoride, magnesium fluoride, magnesium oxide, scandium oxide, silicon monoxide, silicon dioxide, silicon nitride, silicon oxy-nitride, silicon oxy-carbide, silicon carbide, tantalum oxide, titanium oxide, tin oxide, indium tin oxide, yttrium oxide, zinc oxide, zinc selenide, zinc sulfide, zirconium oxide, zirconium titanate, or glass, and mixtures or blends thereof. 
     The protective coatings applied as protective layers  32 ,  34  may be applied by any suitable technique known to those skilled in the art. These techniques include deposition from reactive species, such as those employed in vacuum-assisted deposition processes, and atmospheric coating processes, such as those used to apply sol-gel coatings to substrates. Examples of vacuum-assisted deposition processes include but are not limited to plasma enhanced chemical vapor deposition, ion assisted plasma deposition, magnetron sputtering, electron beam evaporation, and ion beam sputtering. Examples of atmospheric coating processes include but are not limited to curtain coating, spray coating, spin coating, dip coating, and flow coating. 
     As an illustrative example, a polycarbonate panel  24  comprising the Exatec® 900 automotive window glazing system with a printed defroster  16  generally corresponds to the embodiment of schematic C of  FIG. 1 . In this particular case, the transparent polycarbonate panel  24  is protected with a multilayer coating system (Exatec® SHP-9X, Exatec® SHX, and a deposited layer of a “glass-like” coating (SiO x C y H z ) that is then printed with a heater grid  16  on the exposed surface of the protective layer  34  facing the interior of the vehicle. As a further alternative construction, a heater grid  16  may be placed on top of a layer or layers of a protective coating or coatings  32 ,  34 , and then over-coated with an additional layer or layers of a protective coating or coatings. For instance, a heater grid  16  may be placed on top of a silicone protective coating (e.g., AS4000, GE Silicones) and subsequently over-coated with a “glass-like” film. 
     Turning now to the present invention,  FIG. 2  illustrates a machine  40 , which may be a robotic arm or other device, having active z-axis control for dispensing conductive ink upon the panel  24 , resting on a support  38 , to form a series of heater grid lines  54 . The machine  40  illustrated in the figure is comprised of a robot arm  42 , mounted in a stationary manner to a support surface, and a dispensing head  44  attached to the end of the robot arm  42 . A controller  45  is electrically coupled to the robot arm  42 , the dispensing head  44  and a flow regulator  47  fluidly coupled to a conductive ink source  49 . The robot arm  42  is articulatable and capable of moving the dispensing head  44  to any point on the surface  22  of the panel  24 . In a preferred operation, the robot arm  42  moves the dispensing head  44  in a linear direction across the panel  25  and the dispensing head dispenses the conductive ink from the source  49  onto the panel  25  in lines, forming the heater grid lines  54 , only some of which are shown in  FIG. 2  for clarity. While this is an exemplary embodiment, other examples may dispense the heater grid lines  54  in any other pattern, such as curves. 
     Looking more closely at the dispensing head  44 , it is primarily composed of a base  46  supported by the robot arm  42 . Coupled to the base  46  is a sensor  50  and an actuator  52 , to which a nozzle  48  is mounted and further coupled to the conductive ink source  49  and flow regulator  47 . The flow regulator  47  may be any device capable of controlling the flow rate of ink from the ink source  49  to the nozzle  48 . During operation, by means of the flow regulator, the conductive ink is dispensed through the nozzle  48 , onto the internal surface  22  of the panel  24 . The flow regulator  47  may include but not be limited to a means of positively displacing the fluid, such as that known to occur via an auger, a piston, or a gear mechanism. 
     To ensure the ink is dispensed in a manner to form a grid line  54  of the desired predetermined width and height, the sensor  50 , directly or indirectly, measures the distance of the dispensing head  48  from the surface  22  of the panel  24 . As a result, the controller  45 , while controlling the robot arm  42  to move the dispensing head  44  to a desired position over the surface  22 , actively controls a z-axis position of the nozzle  48  using the actuator  52  based on input from the sensor  50 . The actuator  52  translates the position of the nozzle  48  to within a precise height  56  along the z-axis, (see  FIG. 2 ), that lies preferably within 0-3 mm, but more typically between 0.5-1 mm, from the surface  22 , depending on the desired characteristics of the grid lines  54 . While the actuator  52  is a linear motor, alternative embodiments may use any electric, hydraulic, pneumatic, piezoelectric, electromagnetic, or other actuator  52  capable of similar precision and response time. 
     The sensor  50  is any sensor capable of measuring a height  56  from the surface  22  of the panel and must be capable of measuring relative to a semi-reflective and/or transparent surface. In the example shown, the sensor  50  comprises a triangulation laser arrangement made up of an emitter  58  and a receiver  60 . To measure the distance of the nozzle  48  from the internal surface  22 , laser light is projected from the emitter  58  and either directed or reflected onto the surface  22 . The light is then reflected back to the receiver  60  and, based on the relative positions of the emitter  58  to the receiver  60 , the sensor  50  calculates, by triangulation, the distance of the surface  22  from a reference point of the sensor  50 . The height  56  is then calculated by the controller  45  based on the signal from the sensor  50  and a known position of the actuator  52  and the nozzle  48 . As a result, the controller  45  may command the actuator  52  to raise or lower the nozzle  48  along the z-axis to compensate for variations in the surface of the panel  24  and maintain a predetermined height  56  above the surface  22 . 
     While the exemplary sensor  50  is a laser triangulation sensor, any other non-contact sensor  50  could also be used, for example, a photonic sensor (i.e. measures the intensity of the reflected light), an air pressure sensor, an ultrasonic sensor, a magnetic sensor, or any other sensor. Additionally, contact sensors with appropriate means contacting the surface  22  in an appropriate manner (i.e. rolling contacts, sliding contacts, etc.) are also anticipated as being applicable with the present invention. 
     As a result, this arrangement allows for the precise control of the characteristics of the heater grid lines  54  by varying (increasing or decreasing) the height  56  (h) of the dispensing head  44  relative to the panel  24  and the flow rate (r) at which the ink is dispensed, based on the speed at which the dispensing head is being moved across the panel. Therefore, by precisely adjusting the height of the nozzle  48  relative to the contour of the panel  24 , and/or adjusting the flow rate of conductive ink from the nozzle  48 , the apparatus  40  is able to dispense the ink in extremely straight lines of consistent width  64  and height  66  (see  FIG. 4 ). Furthermore, by varying one or more of the height  56  (h), the speed (s) and flow rate (r) of ink, the width  64  and height  66  of the heater grid lines may be varied depending on the technical and aesthetic requirements of a particular application. By varying the height of the gridlines  54 , and therefore the cross sectional area of grid lines  54 , the resistivity in that section of the grid line can be varied without altering the visible aesthetics of the line (e.g. the line shows a constant width). Thus one benefit of the present invention is that the time consuming scanning and mapping of the entire surface contour of the panel prior to initiating the printing of the grid lines  54  is avoided. 
     While the present embodiment compensates for variations in the z-axis, alternate embodiments may also compensate for variations in the x and y axes in order to keep the nozzle  48  normal to the surface  22  at all times as it traverses the panel  24 . This configuration (not shown) may be achieved using a plurality of sensor&#39;s  50  and actuator&#39;s  52  to manipulate the nozzle accordingly. In one embodiment, at least two additional sensor&#39;s  50  would measure the positions (x &amp; y axes) of the surface  22  to determine curvature in the panel. Based on inputs from these sensors, the controller  45  would command the robot arm  42  and/or additional actuator&#39;s to precisely rotate the nozzle  48  about the x-axis and y-axis, in addition to translating along the z axis. As a result, the controller  45  may keep the nozzle  48  normal to the surface  22  at all times as it translates across the panel  24 . 
     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.