Abstract:
Inkjet printing is provided onto rigid panels such as office partitions, which have surfaces that are contoured, textured or made of another three-dimensional material, or are otherwise differently spaced from the plane of the panel such that the distance between a printing element and the point on the surface on which ink is to be deposited is not always the same or exactly predictable. Preferably, three dimensional covered panels are printed using ink jet printing, preferably using ultraviolet (UV) light curable ink, which is first, at least partially cured with UV light and then subjected to heating to more completely cure and dry the ink to remove, by evaporation, further curing or otherwise, the uncured monomers. The panel surface may be contoured by quilting or molding processes. Print head to panel spacing is adjustable to maintain a predetermined constant distance from the printing element to the surface of the panel where the ink is to be applied. Each of a plurality of print heads is provided and independently moveable to control the spacing of the print heads from the substrate surface. Sensors on the print head carriage measure the shape, or vertical position of, the print heads. The position or focal length of the UV light curing head may also be varied to maintain focus of the UV light on the ink on a contoured surface of the substrate.

Description:
[0001]    application Ser. No. 09/070,948, filed May 1, 1998, now U.S. Pat. No. 5,873,315, all of which are hereby expressly incorporated by reference herein. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to printing onto textured, contoured or other three-dimensional substrates. The invention is particularly related to the printing onto such substrates as those having textile fabric surfaces or molded objects, rigid panels such as office partitions, automobile interior panels and other contoured objects, and to such printing using ink jet printing techniques.  
         BACKGROUND OF THE INVENTION  
         [0003]    Applying ink to a substrate by ink jet printing requires a proper spacing between the ink jet nozzles and the surface of the substrate to which the printing is applied. Normally, this spacing must be set to within one or two millimeters to maintain effective printing by an ink jet process. If the distance from the nozzles to the surface being printed is too great, deviations from ideal parallel paths of the drops from different nozzles become magnified. Further, the longer the flight path of the drops from the print head to the substrate, the more dependent the accuracy of the printing becomes on the relative speed between the print head and the substrate. This dependency limits the rate of change in print head to substrate velocity, including changes in direction. Also, the velocity of the drops moving from the print head nozzles to the substrate declines with the distance traveled from the nozzles, and the paths of such drops become more greatly affected by air currents and other factors with increased nozzle to substrate distance. Additionally, droplet shape changes the farther the drop moves from the nozzle, which changes the effects of the drop on the substrate. Accordingly, variations in the distance from the print head to the substrate can cause irregular effects on the printed image.  
           [0004]    In addition to problems in jetting ink onto contoured surfaces, the curing of UV inks that requires sharply focused UV energy to deliver sufficient curing energy to the ink is difficult to achieve where the surface is contoured.  
           [0005]    For the reasons stated above, inkjet printing has not been successful on contoured materials and other three-dimensional substrates, particularly printing with UV curable inks in ink jet printing processes.  
         SUMMARY OF THE INVENTION  
         [0006]    An objective of the present invention is to provide for the printing onto three-dimensional substrates, particularly onto highly textured fabrics, tufted or irregular fabrics and other materials, contoured surfaces such as quilts, and mattress covers, and onto molded, stamped and otherwise shaped rigid or semi-rigid materials, and other three-dimensional surfaces. A particular objective of the invention is to print onto such surfaces with ink jet or digital printing processes. One more particular objective of the invention is to print onto such substrates with UV curable inks.  
           [0007]    According to the principles of the present invention, printed images are applied to three-dimensional substrates with printing elements that are moveable relative to the plane of the substrate being printed. In certain embodiments, the invention provides a wide-substrate ink jet printing apparatus with print heads that move toward and away from the plane of a substrate to maintain a fixed distance between the nozzles of the printhead and the surface onto which the ink is being jetted. The variable distance over the plane of the substrate allows a controlled and uniform distance across which the ink is jetted.  
           [0008]    In one preferred embodiment of the invention, the printing element is an ink jet print head set having a plurality of heads, typically four, each for dispensing one of a set of colors onto the substrate to form a multi-colored image. To maintain the constant distance or to otherwise control the distance, one or more sensors is provided to measure the distance from the print head or from the print head carriage track to the point on the substrate on which ink is to be projected. The sensors generate reference signals that are fed to a controller that controls a servo motor on the print head carriage. The print head is moveably mounted to the carriage, for example on a ball screw mechanism, and is moveable toward and away from the plane of the substrate by operation of the servo motor.  
           [0009]    In a preferred embodiment of the invention, each print head of a set of four different color print heads is separately moveable relative to a common print head carriage, and is connected to one of a set of four servo motors by which its position relative to the plane of the substrate is capable of control relative to the positions of the other print heads. The print heads of the set are preferably arranged side by side in the transverse direction on the carriage so that one head follows the other across the width of the substrate as the carriage scans transversely across the substrate. Each head has a plurality of ink jet nozzles thereon for dispensing a given color of ink in a corresponding plurality of dots, for example 128 in number, that extend in a line transverse to the carriage, which is in a longitudinal direction perpendicular to the scan direction of the carriage. Two laser or optical sensors are provided on the carriage, one on each side of the heads, so that a distance measurement of the surface to the substrate can be taken ahead of the print heads when the heads are scanning in either direction. The controller records the contour of the substrate ahead of the print heads and varies the position of each print head, toward and away from the substrate plane, as each print head passes over the points at which the measurements were taken, so that each of the independently moveable heads follows the contour and maintains a fixed distance from the surface being printed.  
           [0010]    While it is preferred to adjust the position of the print head or nozzle thereof relative to the substrate which is fixed on a printing machine frame, the substrate surface can alternatively be positioned relative to a print head that is maintained at a fixed vertical position on the frame.  
           [0011]    Preferably, UV ink is printed onto material and the cure of the ink is initiated by exposure to UV light. UV curing lights may be mounted on the print head carriage, one on each side of the print head set, to expose the printed surface behind the heads. With or following the exposure to the UV light, the printed textile substrates or other textured or porous fabric is subjected to heat, preferably by blowing heated air onto the material downstream of the printing station, which extends the UV light initiated curing process and removes uncured components of the ink. With quilted bedding fabric materials, UV curable ink is jetted onto the fabric and the jetted ink is exposed to UV curing light to cure the ink preferably to about 90 to 97% polymerization, with the fabric bearing the partially cured, jetted ink then heated in a hot air blower curing oven at which the UV light initiated polymerization continues, uncured monomers are vaporized, or both, in order to produce a printed image of UV ink that contains a low quantity of uncured monomer or other ink components, for example, less than 0.01%.  
           [0012]    Where UV ink is jetted onto a highly textured fabric such as a mattress cover ticking material, the ink is jetted at a dot density of from about 180×254 dots per inch per color to about 300×300 dots per inch per color. For certain common UV inks, four colors of a CMYK color palette are applied, each in drops or dots of, for example, about 75 picoliters, or approximately 80 nanograms, per drop, utilizing a UV ink jet print head. A UV curing light head is provided, which moves either with the print head or independent of the print head and exposes the deposited drops of UV ink with a beam of about 300 watts per linear inch, applying about 1 joule per square centimeter, thereby producing at least a 90% UV cure. The fabric on which the jetted ink has been thereby partially UV cured is then passed through an oven where it is heated to about 300° F. for from about 30 seconds up to about three minutes. Forced hot air is preferably used to apply the heat in the oven, but other heating methods such as infrared or other radiant heaters may be used. Similar parameters may be used for cloth covered rigid panels such as office partitions.  
           [0013]    When printing onto contoured material, the distance from the print heads to the substrate where the ink is to be deposited can be determined by measuring the distance from a sensor to the substrate ahead of the print heads and mapping the location of the surface. For bidirectional print heads that move transversely across the longitudinally advancing fabric, providing two distance measuring sensors, one on each of the opposite sides of the print heads, are provided to measure the distance to the contoured fabric surface when the print heads are moving in either direction. For some inks and for sufficiently rigid materials, a mechanical rolling sensor may be used, for example, by providing a pair of rollers, with one roller ahead of, and one head behind, the print head so that the average distance between the two rollers and a reference point on the print head can be used to control the distance of the print head from the plane of the substrate. To achieve this, one or more print heads can be mounted to a carriage having the rollers on the ends thereof so that the mechanical link between the rollers moves the print head relative to the plane of the substrate. In most cases, a non-contact sensor, such as a laser or photo eye sensor, is preferred in lieu of each roller. The outputs of two sensors on opposite sides of the print heads can be communicated to a processor, to measure the distance from the heads to the fabric ahead of the bidirectional heads, to drive a servo motor connected to the print head to raise and lower the head relative to the substrate plane so that the print heads move parallel to the contoured surface and jet ink onto the fabric across a fixed distance.  
           [0014]    These and other objects of the present invention will be more readily apparent from the following detailed description of the preferred embodiments of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a perspective view of one embodiment of an apparatus embodying principles of the present invention in which ink jet printing is applied to panels of rigid office partitions that are covered with textured or contoured textile material or fabric.  
         [0016]    [0016]FIG. 1A is a perspective view, similar to FIG. 1, of another embodiment of an apparatus embodying principles of the present invention in which ink jet printing is applied to rigid panels.  
         [0017]    [0017]FIG. 2 is a cross-sectional view along line  2 - 2  of FIG. 1 showing structure for maintaining print head to substrate distance where a substrate is more highly contoured.  
         [0018]    [0018]FIG. 2A is a cross-sectional view similar to FIG. 2 showing alternative structure for maintaining print head to substrate distance.  
         [0019]    [0019]FIG. 3 is a cross-sectional view along line  3 - 3  of FIG. 1A showing structure for maintaining print head to substrate distance on a contoured substrate. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0020]    [0020]FIG. 1 illustrates a machine  10  for printing onto rigid panels. The machine  10  includes a stationary frame  11  with a longitudinal extent represented by an arrow  12  and a transverse extent represented by an arrow  13 . The machine  10  has a front end  14  into which is advanced a rigid panel  15 , such as that of which an office partition may be formed. The panel  15  may include a metal or wooden frame  17  on which is stretched a facing material that forms the surface  16  to be printed. The surface  16  may also be a flat but highly textured fabric, a molded material such as a foam or some other contoured or variable surface. Panels  15  are carried longitudinally on the machine  10  by a conveyor or conveyor system  20 , formed of a pair of opposed pin tentering belt sets  21  which extend through the machine  10  and onto which the panels  15  are fed at the front end  14  of the machine  10 . The belt sets  21  retain the panels  15  in a precisely known longitudinal position on the belt sets  21  to carry the panels  15  through the longitudinal extent of the machine  10 , preferably with an accuracy of {fraction (1/4)} inch. The longitudinal movement of the belts  21  of the conveyor  20  is controlled by a conveyor drive  22 . The conveyor  20  may take alternative forms including, but not limited to, opposed cog-belt side securements, longitudinally moveable positive side clamps that engage the panels  15  or other securing structure for holding the panels  15  fixed relative to the conveyor  20 .  
         [0021]    Along the conveyor  20  are provided three stations, including an ink jet printing station  25 , a UV light curing station  24 , and a heated drying station  26 . The printing station  25  includes an ink jet carriage having one or more ink jet printing heads  30  thereon. The carriage of the print heads  30  is shown as transversely moveable on the front of a cross bar  28  that extends transversely across the frame  11  and may, but not necessarily, also be longitudinally moveable on the frame  11  under the power of a transverse servo drive motor  31  and an optional longitudinal drive  32 . Alternatively, the heads  30  may extend across the width of the web  15  and be configured to print an entire transverse line of selectable points simultaneously onto the panel  15 .  
         [0022]    The ink jet printing heads  30  are configured to jet UV ink, for example, at 75 picoliters, or approximately 80 nanograms, per drop, and may do so for each of four colors according to a CMYK color pallette. The dots are preferably dispensed at a resolution of about 180 dots per inch by about 254 dots per inch. The resolution may be higher or lower as desired, but the 180×254 resolution is preferred. If desirable for finer images or greater color saturation, 300×300 dots per inch is preferable. The drops of the different colors can be side-by-side or dot-on-dot. Dot-on-dot (sometimes referred to as drop-on-drop) produces higher density.  
         [0023]    The print heads  30  are provided with controls that allow for the selective operation of the heads  30  to selectively print designs of one or more colors onto the surface of the panel  15 . The drive  22  for the conveyor  20 , the drives  31 , 32  for the print head  30  and the operation of the print heads  30  are program controlled to print patterns  33  at known locations on the panel  15  by a controller  35 , which includes a memory  36  for storing programmed patterns, machine control programs and real time data regarding the nature and longitudinal and transverse location of printed designs  33  on the panel  15  and the relative longitudinal position of the panel  15  in the machine  10 .  
         [0024]    The UV curing station  24  includes a UV light curing head  23  that may move with the print heads  30  or, as is illustrated, move independently of the print heads  30 . The UV light curing head  23  is configured to sharply focus a narrow, longitudinally extending beam of UV light onto the printed surface of the fabric. The UV curing head  23  is provided with a transverse drive  19  which is controlled to transversely scan the printed surface of the fabric to move the light beam across the fabric.  
         [0025]    Preferably, the curing head  23  is intelligently controlled by the controller  35  to selectively operate and quickly move across areas having no printing and to scan only the printed images with UV light at a rate sufficiently slow to UV cure the ink, thereby avoiding wasting time and UV energy scanning unprinted areas. If the head  23  is included in the printing station  25  and is coupled to move with the print heads  30 , UV curing light can be used in synchronism with the dispensing of the ink immediately following the dispensing of the ink.  
         [0026]    The UV curing station  24 , in the illustrated embodiment, is preferably located either immediately downstream of the printing station  25 , or on the print head carriage to the sides of the print heads, so that the fabric, immediately following printing, is subjected to a UV light cure. In theory, one photon of UV light is required to cure one free radical of ink monomer so as to set the ink. In practice, one joule of UV light energy per square centimeter of printed surface area is supplied by the UV curing head  23 . This is achieved by sweeping a UV beam across the printed area of the fabric at a power of 300 watts per linear inch of beam width. This is sufficient to produce a UV cure of at least 90%. Increasing the UV light power up to 600 watts per linear inch can be done to achieve a 97% or better cure. Alternatively, if fabric thickness and opacity are not too high, curing light can be projected from both sides of the fabric to enhance the curing of the UV ink. Using power much higher can result in the burning or even combustion of the fabric, so UV power has an upper practical limit.  
         [0027]    The heat curing or drying station  26  may be fixed to the frame  11  downstream of the UV light curing station or may be located off-line. With 97% UV cure, the ink will be sufficiently colorfast so as to permit the drying station to be off-line. When on-line, the drying station should extend sufficiently along the length of fabric to adequately cure the printed ink at the rate that the fabric is printed. When located off-line, the heat curing station can operate at a different rate than the rate of printing. Heat cure at the oven or drying station  26  maintains the ink on the fabric at about 300° F. for up to three minutes. Heating of from 30 seconds to three minutes is the anticipated advantageous range. Heating by forced hot air is preferred, although other heat sources, such as infrared heaters, can be used as long as they adequately penetrate the fabric to the depth of the ink.  
         [0028]    A quilting station may be located on-line with the printing station or offline, and either before or after the printing station. Locating a quilting station downstream of the oven  26  is advantageous in the case of quilted comforters and mattress covers and where quilting is to be applied and registered with printing on the fabric. A single-needle quilting station may be used, such as is described in U.S. Pat. No. 5,832,849, to Kaetterhenry et al. entitled “Web-fed Chain-stitch Single-needle Mattress Cover Quilter with Needle Deflection Compensation”, which is expressly incorporated by reference herein. Other suitable single-needle type quilting machines with which the present invention may be used are disclosed in U.S. Pat. Nos. 5,640,916 and 5,685,250, respectively, both entitled “Quilting Method and Apparatus”, expressly incorporated by reference herein. Such a quilting station may also include a multi-needle quilting structure such as that disclosed in U.S. Pat. No. 5,154,130, also expressly incorporated by reference herein.  
         [0029]    Where quilting, molding or other contouring of a substrate is carried out before the printing onto the substrate, registration of the printing to the preapplied contouring will usually be desired. To registerthe printing to pre-applied contours, the location of the contour pattern can be calculated in relation to a reference point on the substrate that can be sensed by sensors at the printing station. The location of the pattern can be directly sensed with a sensor  40  mounted on the print head  30 , as illustrated respectively as  40   a ,  40   b  in FIGS. 2 and 2A. The print head  30  includes a nozzle or ink jet nozzle array  41  that is directed downward toward the upwardly facing surface  16  of a substrate such as the panel  15 . The panel  15  may have, for example, depressions or channels  43  on its surface  16  that have been formed by stitching or molding, as illustrated in FIG. 2. The sensor  40  measures the distance from the nozzle  41  to the surface  16 . Information from the sensor  40  can be communicated to the controller  35  and correlated with the longitudinal and transverse position information of the print head  30  and interpreted to determine the location of the contoured pattern so that the printed image can be applied to the surface  16  in registration with the pre-applied contour pattern.  
         [0030]    In the embodiment of FIG. 2, the sensor  40  is a mechanical sensor  40   a  that includes a wheeled carriage  45 . The nozzle  41  is mounted at the midpoint of the carriage  45 , which is, in turn, pivotally connected to the print head  30  about a longitudinal axis  46  through the center of the carriage  45 . The carriage  45  has left and right sensing wheels  47 ,  48 , respectively, that ride on the surface  16  of the panel  15  and follow the contour. The carriage  45  moves vertically relative to the print head  30  and follows the contour of the surface  16 . The nozzle  41 , being midway between the wheels  47 ,  48 , will be positioned vertically at the average of the vertical positions of the wheels  47 ,  48 . In this way, the nozzle  41  is passively positioned at a controlled distance relative to the surface  16  of the panel  15  in response to the detected location of the surface  16  of the panel  15  as determined by the carriage  45  as the wheels  47 ,  48  ride on the surface  16 .  
         [0031]    The distance between the UV head  23  and the fabric is preferably also controllable so that the curing light is always precisely focused onto the printed contoured surface of the fabric. This distance may controlled by mounting the UV curing head to move with the print heads, such as by communicating the UV light through optic fibers adjacent the print heads, forexample, one fiberon each side of the print heads, or by mounting the UV curing head  23  on a separate carriage and providing it with a separate distance adjusting servo motor. Separate control of the UV curing head  23  can be in response to the sensors used to measure print head distance or in response to separate sensors provided to measure curing head distance. Where the print head sensors are used to control curing head to fabric distance, a memory can be used to store a map of the surface or portion of the surface while a controller retrieves the correct distance information from the memory that corresponds to the position of the curing head overthe fabric. Alternatively, the UV curing head can be fixed and the focal length of the UV light from the source automatically varied.  
         [0032]    Whether the panel  15  has a contoured pattern on its surface  16  or merely a textured material, print quality is maintained by maintaining precise spacing between the nozzle  41  and the surface  16  of the panel  15 . FIG. 2A illustrates a rigid panel  15  having its outer upwardly facing surface  16  covered with a coarse woven or textured fabric. As the print head  30  moves transversely on the cross bar  28 , the vertical position, relative to the print head  30 , of the point on the surface  16  of the panel  15  at which the nozzle  41  is directed varies, often one or more millimeters. To measure such distance variations, an optical or laser sensor  40   b  is provided either on the print head  30  or on the carriage at a fixed height from the plane of support of the fabric. The sensor  40   b  instantaneously measures the distance from the nozzle  41  to the surface  16  of the panel  15  and communicates the measurement to the controller  35 . The nozzle  41  is mounted on an output actuator  51  of a servo motor  50  mounted in the print head  30 . The controller  35  sends a control signal to the servo motor  50  to move the nozzle  41  on the print head  30  vertically in response to the distance measurement from the sensor  40   b  to maintain a constant distance from the nozzle  41  to the surface  16  of the panel  15 .  
         [0033]    Printing on rigid panels, even where the surface is not textured or contoured, can benefit from the sensing and adjustment of the distance from print nozzle to surface of the panel since the rigid frame of the panel and the thickness of the panel when supported on the frame of a printing apparatus makes the position of the upper surface of the panel unpredictable.  
         [0034]    [0034]FIG. 1A illustrates an alternative embodiment  100  of the machine  10  described above. The machine embodiment  100  includes a stationary frame  111  with a longitudinal extent represented by an arrow  112  and a transverse extent represented by an arrow  113 . The machine  100  has a front end  114  into which the rigid office partition panel  15  may be loaded onto a belt  121  of a conveyor system  120  having one or more flights which carry the panel  15  longitudinally through the machine  100 . The belt  121  of the conveyor  120  extends across the width of the frame  111  and rests on a smooth stainless steel vacuum table  105 , which has therein an array of upwardly facing vacuum holes  106  which communicate with the underside of the belt  121 . The belt  121  is sufficiently porous that the vacuum from the table  105  communicates through the belt  121  to the underside of the rigid panel  15  to assist gravity in holding the panel  15  in place against the top side of the belt  121 . Preferably, the belt  121  has a high friction rubber-like surface  108  to help prevent a horizontal sliding of a panel resting on it, through which an array of holes  109  is provided to facilitate communication of the vacuum from the table  105  to the substrate.  
         [0035]    The top surface of the belt  121  of the conveyor  120  is such that it provides sufficient friction between it and the underside of the panel  15  to keep the panel  15  from sliding horizontally on the conveyor  120 . The conveyor  120  is further sufficiently non-elastic so that it can be precisely advanced. To this end, the belt  121  has a non-elastic open weave backing  107  to provide dimensional stability to the belt while allowing the vacuum to be communicated between the holes  106  of the table  105  and the holes  109  in the surface of the belt  121 . The forward motion of the panel  15  on the frame  111  is precisely controllable by indexing of the belt  121  by control of a servo drive motor  122  with signals from the controller  35 . The belt  121  thereby retains the panels  15  in a precisely known longitudinal position on the belt  121  so as to carry the panels through the longitudinal extent of the machine  100 . Such indexing of the belt  121  should be controllable to an accuracy of about 0.0005 inches where used to move the panel  15  relative to a print head on a fixed bridge (which embodiment is not shown).  
         [0036]    In the embodiment  100  illustrated in FIG. 1A, the longitudinal movement of the belt  121  of the conveyor  120  is controlled by the conveyor drive  122  to move the panel into printing position and then to advance it downstream after it is printed. One or more additional separately controllable drives  132  may be provided to control the downstream flights, if any, of the conveyor  120 .  
         [0037]    Along the length of travel of the conveyor  120  are provided three stations, including an ink jet printing station  125  and one or more curing or drying stations, which may include UV light curing stations  124  and/or a heating station  126 . The printing station  125  includes a bridge  128 . Where the belt  121  is operable to precisely index the panel  15  relative to the bridge  128 , the bridge may be fixed to the frame  111  and extend transversely across it. A printhead carriage  129  is transversely moveable across the bridge  128  and has one or more sets  130  of ink jet printing heads thereon. The carriage  129  is preferably fixed to the armature of a linear servo motor  131  which has a linear array of stator magnets extending transversely across the bridge  128 , so that the carriage  129  is transversely moveable across the bridge  128  by positioning and drive control signals sent to the servo  131  by the controller  35 , described above.  
         [0038]    In the illustrated embodiment, the bridge  128  is mounted to the moveable armatures  133   a , 134   a  that ride on longitudinal tracks  133   b , 134   b  of linear servo motors  133 , 134  at each side of the conveyor  120 . Once a panel  15  is positioned under the bridge  128  by movement of the belt  121 , the bridge  128  is indexed in the longitudinal direction as transverse bands of an image are printed in successive scans of print heads  130 , described below. This indexing should be as accurate as needed to insure that the scans register one with another and can be interlaced, as required, to produce the desired print quality and resolution. Such accuracy is preferred to be about 0.0005 inches. Lower resolution, and thus less accuracy, is acceptable for printing on textile surfaces than on smoother surfaces such as vinyl.  
         [0039]    [0039]FIG. 3 illustrates a set  130  of four ink jet printing heads  130   a - 130   d  configured to respectively apply the four colors of a CMYK color set. The ink jet printing heads  130   a -d each include a linear array of one hundred twenty-eight (128) ink jet nozzles that extend in the longitudinal direction relative to the frame  111  and in a line perpendicular to the direction of travel of the carriage  129  on the bridge  128 . The nozzles of each of the heads  130  are configured and controlled to simultaneously but selectivelyjet UV ink of one of the CMYK colors side by side across the substrate  15 , and to do so in a series of cycles as the nozzles scan the substrate  15 . The heads  130   a - d  of a set are arranged side-by-side to print consecutively across the same area of the substrate  15  as the carriage  129  moves across the bridge  128 , each depositing one of the four colors sequentially on each dot position across the substrate  15 .  
         [0040]    Each of the heads  130   a - d  is moveably mounted to the carriage to individually move vertically, or perpendicular to the plane of the substrate  15 . The distance of each head  130   a - d  from the plane of the substrate  15  is controlled by a respective one of a set of servos  137   a - d  mounted to the carriage  129  to follow one behind the other over the same contour of the substrate  15 . The servos  137   a - d  are responsive to signals from the controller  35  which controls the positions of the heads  130   a - d  to maintain each a controlled distance from the surface of the substrate  15  where the surface  16  of the substrate  15  is contoured.  
         [0041]    Usually, it is desirable to maintain the heads a fixed distance from the surface  16  on which they are to print. This is achieved by providing optical sensors  138   a , 138   b  on the opposite transverse sides of the carriage  129 . The printhead set  130  is bidirectional and prints whether moving to the right or to the left. As the print head carriage  129  moves on the bridge  128 , the leading one of the sensors  138   a  or  138   b  measures the distance from the sensor  138  and the surface  16  of the substrate  15  at a point directly in line with, typically directly below, the sensor  138 . This measurement is communicated to the controller  35 , which records the measured distance and the coordinates on the surface  16  of the substrate  15  at which the measurement was taken. These coordinates need only include the transverse position on the substrate  15  where the information is to be used in the same pass or scan of the carriage in which the measurement was taken. However, the controller  35  may also record the longitudinal coordinate by taking into account the position of the panel  15  on the frame  111  relative to the bridge  128 .  
         [0042]    In response to the measurements, the controller  35  controls the servos  137  to vertically position the each of the heads  130  to a predetermined distance from the contoured surface  16  of the substrate  15  as the respective head arrives at the transverse coordinate on the substrate  15  at which each measurement was taken. As a result, the nearest of the heads  130  to the leading sensor  138 , which are spaced a distance B from the sensor  138 , follows the contour of the fabric at a delay of V/B seconds after a given measurement was taken, where V is the velocity of the carriage  129  on the bridge  128 . Similarly, the heads  130  are spaced apart a distance A and will each sequentially follow the same contour as the first head at V/A seconds after the preceding head.  
         [0043]    The extent of the heads  130  in the longitudinal direction determines the accuracy with which the heads can follow the contours of the substrate  15 . Greater accuracy can be maintained, and more variable contours can be followed, by using narrower heads, for example, of 64 or 32 jets per head in the longitudinal direction. Accordingly, multiple sets of heads  130  can be arranged in a rectangular or other array on the carriage  129 , with heads of the different sets being arranged side-by-side across the carriage  129  in the longitudinal direction of the substrate  15  and frame  111 . For example, two sets of heads having  64  jets per head each or four sets of heads having  32  jets per head each will produce the same  128  dot wide scan, but with greater ability to maintain spacing from head to substrate where the contours vary in the longitudinal direction on the substrate  15 .  
         [0044]    Where UV curable ink is used, the UV curing station  124  is provided as illustrated in FIG. 1A. It may include a printhead  23  transversely moveable independently of the print heads  130  across the downstream side of the bridge  128  or otherwise located downstream of the printing station  125 , and/or may include UV light curing heads  123   a  and  123   b  mounted on the carriage  129 . As the carriage  129  moves transversely on the bridge  128 , only the curing head  123   a , 123   b  that trails the print heads  130  is operated so that the UV light exposes ink after its deposition onto the substrate  15 . The curing heads  123   a , 123   b  may also be moveable toward and away from the plane of the substrate  15  and controllable by servos  139   a , 139   b , respectively, to maintain their spacing from the surface  16 , as illustrated in FIG. 3. Proper curing of UV ink requires that the UV light be focused on the surface bearing the ink. Therefore, moving the UV heads  123   a ,  123   b  to maintain a constant spacing from the surface  16  maintains the focus of the curing UV light. UV light curing heads are typically configured to sharply focus a narrow, longitudinally extending beam of UV light onto the printed surface. Therefore, instead of physically moving the UV light curing heads or sources  123   a , 123   b , the focal lengths of the light curing heads  123   a ,  123   b  may be varied to follow the contours of the substrate  15 . The light curing head  123 , where used, may similarly be configured to move perpendicular to the surface  16  of the substrate  15 .  
         [0045]    The heat curing or drying station  126  may be fixed to the frame  111  downstream of the printing station  125  and the UV light curing station, if any, may be located off-line. Such a drying station  126  may be used to dry solvent based inks with heated air, radiation or other heating techniques. It may also be used to further cure or dry UV inks.  
         [0046]    Printing on rigid panels, even where the surface is not textured or contoured, can benefit from the sensing and adjustment of the distance from print nozzle to surface of the panel since the rigid frame of the panel and the thickness of the panel when supported on the frame of a printing apparatus makes the position of the upper surface of the panel unpredictable.  
         [0047]    The above description is representative of certain preferred embodiments of the invention. Those skilled in the art will appreciate that various changes and additions may be made to the embodiments described above without departing from the principles of the present invention. Therefore, the following is claimed: