Abstract:
A method of printing on a surface of a three-dimensional object with a printer having a print surface and a print head, wherein a jig is used for positioning the object on the print surface in a position in which the surface to be printed on faces the print head, comprising by the steps of:
       providing a three-dimensional digital model of the jig;   using the printer for printing the jig on the print surface; and   placing the object against the jig.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. § 119(a) to Application No. 16180095.8, filed in Europe on Jul. 19, 2016, the entire contents of which is hereby incorporated by reference into the present application. 
       BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
       [0002]    The invention relates to a method of printing on a surface of a three-dimensional object with a printer having a print surface and a print head, wherein a jig is used for positioning the object on the print surface in a position in which the surface to be printed on faces the print head. 
         [0003]    In this specification, the term “three-dimensional object” shall designate any physical object that is extended in three dimensions, including plate-like objects with a relief surface, which objects are sometimes also termed 2.5-dimensional objects. 
       2. Background of the Invention 
       [0004]    What is being printed on the surface of the three-dimensional object is usually an image. The three-dimensional object already has a specific three-dimensional shape. In such cases it might suffice that the image is a two-dimensional image of coloured pixels or shapes. However, the image might as well include a relief, wherein the pixels or shapes in the image not only have a colour, but also a height above a reference surface (usually the surface being printed on). In an extreme case, the pixels or shapes in the image all have the same colour and the image merely defines a relief. Relief images are typically printed by printing multiple layers of ink on top of each other. For the sake of brevity, in the remainder of this document, the term image will refer to image in the broad scope as described above, i.e. it may refer to a two-dimensional image, or to a relief image in one or more colours. 
         [0005]    While the image receiving surface, i.e. the surface of the object on which an image is to be printed, should be approximately flat in order for the print head to be able to deposit a marking material such as ink on that surface with sufficient accuracy, the surface of the object on the side opposite to the image receiving surface may be non-parallel with the image receiving surface and may not even be flat. In such cases, it is difficult to stably align and accurately position the object on the print surface in a position and posture suitable for forming the image as intended. It is therefore known to place the object into or onto one or more jigs which have shapes suitable for stably supporting the object on the print surface. When the printer is a flat bed printer wherein the print surface is formed by a perforated suction plate, a flat bottom side of the jig may also be used for securely holding the jig and, therewith, the object on the suction plate. 
         [0006]    However, providing the jigs with suitable shapes and accurately positioning them on the print surface frequently turns out to be a cumbersome task. 
       SUMMARY OF THE INVENTION 
       [0007]    It is therefore an object of the invention to facilitate this process and to permit an improved position accuracy in printing an image on the image receiving surface. 
         [0008]    In order to achieve this object, the method according to the invention comprises the steps of:
       providing a three-dimensional digital model of the jig;   using the printer for printing the jig on the print surface; and   placing the object against the jig.       
 
         [0012]    Thus, according to the invention, the printer that is used for forming the image on the object is also used for forming the jig or jigs by means of a 3D (or 2.5D) printing technique. This has the advantage that not only the shape but also the position of the jig on the print surface can already be determined in the process of printing the jig, so that, since the jig determines the position of the object to be printed on, the information on the exact position of the image receiving surface of the object is readily available in the printer control software when the image is to be printed. As a consequence, not only the process of providing a jig with the correct shape but also the process of correctly positioning the jig on the print surface is greatly facilitated. 
         [0013]    Useful optional details of the invention are indicated in the dependent claims. 
         [0014]    In a preferred embodiment, the printer is an ink jet printer. The same print head or print heads may be used for printing the jig and for forming the image on the object. In another embodiment, different print heads may be used for these purposes, but the print heads may be provided on a common carriage which is arranged to scan the print surface. 
         [0015]    As the jigs require some rigidity, an ink is preferred that forms rigid structures when dried or cured. Therefore, radiation curable inks, such as UV curable inks, are suitable for forming jigs. 
         [0016]    In addition to the information on the shape of the jig, information on the shape of the object should be available at least to the extent that the height of the image receiving surface relative to the print surface can be determined for appropriately adjusting the height of the print head. 
         [0017]    Frequently, a three-dimensional model of the object to be printed on will be available or can be obtained by means of a 3D-scanner. In that case, the control software of the printer may be configured such that three-dimensional models of suitable jigs are calculated automatically from the three-dimensional model of the object. 
         [0018]    When the print surface of the printer is a suction plate, a sheet-like substrate may be placed onto the suction plate, and the jig or jigs may be printed onto that sheet. Then, the suction plate is preferably kept active for immobilizing the substrate while the object or objects are placed on the jigs. 
         [0019]    In another embodiment, the jig may be printed directly onto the print surface of the printer. In that case, the ink that is used for forming the jig should preferably have a consistency that permits to easily remove the jig from the print surface when the images on the objects have been formed and the printer shall be used for a different task. As an alternative, a special print head may be provided for printing a primer layer onto the print surface in those positions in which the jigs will then be formed on the primer layer. In that case, only the ink that is used for the primer layer needs to have a consistency that permits to easily remove the primer from the print surface. 
         [0020]    In a further embodiment for printing an image on the image receiving surface of the object, the height of the image receiving surface relative to the print surface of the printer is automatically based on the three-dimensional model of the object and, if relevant, the three-dimensional digital model of the jig. The height of the print head is automatically adjusted based on the determined height of the image receiving surface. If the object has on an opposite side a surface substantially parallel to the image receiving surface, the jig only serves to fix the position of the object and consequently does not contribute to the height of image receiving surface. As only the object contributes to this height in this case, only the three-dimensional model is needed to determine the height of the image receiving surface and subsequently the height of the print head. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    Embodiment examples will now be described in conjunction with the drawings, wherein: 
           [0022]      FIG. 1  is a schematic sectional view of a flat bed ink jet printer while being used for printing onto an object in accordance with the method according to the invention; 
           [0023]      FIG. 2  shows the printer of  FIG. 1  in a condition ready for practicing the method according to the invention; 
           [0024]      FIG. 3  shows the printer of  FIGS. 1 and 2  in an intermediate state while practicing the invention; 
           [0025]      FIG. 4  illustrates another embodiment example of the method according to the invention; 
           [0026]      FIGS. 5 and 6  illustrate other embodiment examples to be performed with another type of printer; 
           [0027]      FIG. 7  is a cross-sectional view of a jig and an object to be printed on; and 
           [0028]      FIG. 8  shows the jig and the object of  FIG. 7  in a longitudinal section along the line VIII-VIII in  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0029]      FIG. 1  shows a flat bed ink jet printer having a print surface  12  constituted by a perforated suction plate  14  that forms the top surface of a suction chamber  16 . A carriage  18  is guided along guide rails  20  so as to be movable across the print surface  12  in a main scanning direction. The guide rails  20  form part of a height-adjustable gantry that is movable in a sub-scanning direction, normal to the plane of the drawing in  FIG. 1 . 
         [0030]    A print head assembly  22  is provided on the bottom side of the carriage  18  and comprises a plurality of print heads  24  arranged for printing with different inks and facing the print surface  12 . 
         [0031]    A sheet-like substrate  26  has been placed onto the print surface  12  such that two adjacent edges of the substrate  26  are aligned at two orthogonal alignment ledgers  28 . Note that aligning the sheet-like substrate  26  to the alignment ledgers is only necessary if it is intended to remove the sheet-like substrate  26  after use and reuse it with the printed jigs  36  at a later time again. 
         [0032]    In this example, an object  30  to be printed on is constituted by a smartphone cover that has been placed on the substrate  26  with a concave side facing downwards. A flat top side of the object  30  constitutes an image receiving surface  32  on which an image is to be formed with the print heads  24 . 
         [0033]    Since the concave side of the object  30  is facing the suction plate  14 , it would not have been possible to immobilize the object by placing it directly on the print surface  12 , because the object  30  does not close-off a substantial number of perforations of the suction plate, and ambient air could enter into the cavity of the smartphone cover through openings  34  in the edges thereof, so that no suction pressure could be built-up to hold the object. 
         [0034]    For this reason, in order to securely hold the object  30  in position on the substrate  26 , two jigs  36  have been fixed on the substrate  26  inside the cavity of the object  30  so as to engage two opposite edges of this object from inside. The jigs  36  have external contours that are complementary to the contour of the cavity in the object  30 , so that the object is safely held in position. Further, the jigs have extensions that provide a large bottom surface area for adhesion on the substrate  26 . 
         [0035]    In the condition shown in  FIG. 1 , the printer is ready for forming an image on the image receiving surface  32  of the object. To that end, the gantry with the guide rails  20  will be lowered into a position in which the nozzle faces of the print heads  24  form only a narrow gap with the image receiving surface  32 , and then the carriage  18  will scan the print surface in order to print the image in a specified position. The operations of the print heads and the movements of the carriage  18  are controlled by an electronic controller  38 . 
         [0036]      FIG. 2  shows the printer  10  without the substrate  26 , the jigs  36  and the object  30 . The controller  38  includes a digital processor  40 , a memory  42  for storing data that specify the shape of the object  30 , and another memory  44  for storing a three-dimensional model of the jigs  36 . 
         [0037]    In this example, it shall be assumed that the memory  42  stores a three-dimensional model of the object  30 , and the processor  40  is programmed to calculate, from that model, the three-dimensional model of the jigs  36  specifying the shapes of the jigs  36  as well as their positions on the print surface  12 . The result of this calculation is then stored in the memory  44 . 
         [0038]    Note that in a preferred embodiment the data that specifies the shape of the object  30  is stored in a memory in separate workstation. The separate workstation will use its internal processor to calculate, from the shape data, the three-dimensional model of the jigs  36 . 
         [0039]    Further, in order to prepare the printer for printing on the image receiving surface of the object  30 , the substrate  26  is placed on the print surface, and the printer is started for printing the jigs  36  onto the substrate  26  on the basis of the data stored in the memory  44 . This process has been illustrated in  FIG. 3 , where the topmost layers of the jigs  36  have not been completed jet. 
         [0040]    When the jigs have finally been completed, the vacuum in the suction chamber  16  is maintained so as to securely hold the substrate  26  in position, the object  30  is placed over the jigs as shown in  FIG. 1 , and the printer is started again for now printing the image on the image receiving surface  32 . 
         [0041]    When the image has been printed, the object  30  may be removed from the substrate  26  and the jigs  36  while the substrate and the jigs remain in position. Then, when more objects  30  of the same type need to be decorated, they may be placed onto the jigs  36  in the same manner, and the image printing process may be repeated. 
         [0042]    When the printer  10  has to be reconfigured for printing onto a different type of objects, the substrate  26  with the jigs  36  adhering thereto is removed, the three-dimensional data for the new object are loaded into the memory  42 , a new substrate sheet is placed onto the print surface, and the printer is started for printing suitable jigs for the new object. 
         [0043]    Optionally, the substrate  26  with the jigs  36  formed thereon may be stored and may be used again when objects of the same type are to be processed. In that case, the alignment ledgers  28  will help to correctly align the substrate  26  and the jigs on the print surface. 
         [0044]      FIG. 4  illustrates an example where objects  30 ′ of a different type are to be processed on the same printer  10 . In this example, the objects  30 ′ are cone- or pyramid-shaped bodies, and the image receiving surface  32  is the base surface of the pyramid or cone. As illustrated in this example, the image receiving surface  32  needs not to be perfectly flat but may also be gently curved, as long as the height differences are not significantly larger than the gap between the print heads and the object. Further, in this example, a plurality of jigs  36 ′ have been printed onto the substrate  26  so as to process a plurality of objects  30 ′ in a single print process for enhanced efficiency. The jigs  36 ′ have been printed in a shape complementary to the tip of the pyramid or cone, so that the image receiving surfaces  32  are held in the correct posture. The height of the carriage  18  over the print surface  12  is automatically adapted on the basis of the known geometries of the jigs  36 ′ and the objects  30 ′. 
         [0045]    The objects  30 ′ may also have different shapes, as long as their image receiving surfaces ( 32 ) are level with one another. 
         [0046]      FIG. 5  illustrates a modified embodiment utilizing a printer  10 ′ of a different type. This printer does not have a suction chamber with a suction plate for fixing the print substrates, and the print surface  12  is simply formed by a plate. In this embodiment, jigs such as the jigs  36 ′ are formed by printing directly on the print surface  12  so that the jigs will adhere to the print surface. In the condition shown in  FIG. 6 , the objects  30 ′ have not yet been inserted, so that the cavities for accommodating the tips of the cones are visible. 
         [0047]    The ink that has been used for forming the jigs  36 ′ has a consistency that assures on the one hand that the jigs will adhere to the print surface  12  with sufficient strength when the ink has cured, but on the other hand the jigs can still be removed from the print surface  12  without substantial effort and without leaving any residues on the print surface. Thus, when the printer  10  has to be reconfigured for a different type of objects, the jigs  36  can easily be removed and new jigs can be printed. 
         [0048]      FIG. 6  illustrates a variant in which the printer  10 ′ is first used for printing primer layers  46  onto the print surface  12  in those positions where the jigs  36 ′ are to be formed. The jigs  36 ′ are then printed on top of the primer layers  46 . When the jigs are to be removed again, the primer layers  46  assure that the jigs can easily be separated from the print surface. It will be understood that, in this embodiment, one of the print heads  24  will be dedicated for printing the primer layers. 
         [0049]      FIG. 7  is a cross-sectional view of an object  30 ″ which is constituted by a profile member having curved top and bottom surfaces, except for a flat image receiving surface  32  on the top side. A corresponding jig  36 ″ has been printed in a shape complementary to the bottom side of the object  30 ″. However, as is shown in a longitudinal section in  FIG. 8 , a reference lug  48  is formed at one end of the jig  36 ″, so that the object  30 ″ may be placed onto the jig such that one end thereof abuts the reference lug  48  for precisely defining the position of the object in lengthwise direction of the profile, thus assuring that an image can be printed, for example, right in the center of the image receiving surface  32 . 
         [0050]    Note that it is not strictly necessary for the shape of the jig to be complementary to the bottom side of the object to print on. It is sufficient if the jig is shaped such that the object to print on is stably held in a known position and posture. Furthermore, the jig may be comprised of more than one part. For example, the smartphone cover in  FIG. 1  may be held stably in a known position and posture by four distinct parts that form corners that engage with the sides of the four corners of the smartphone cover.