Abstract:
An apparatus for printing designs or images on a small or constrained area of a substrate. The apparatus includes a screen assembly, a print head, and a guide assembly. A squeegee and a flood bar of the print head are positioned at a predetermined angles and an actuator facilitates switching positions of the flood bar and squeegee through actuation of a toggle mechanism during the print cycle.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. provisional application Ser. No. 60/882,302 filed Dec. 28, 2006. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention generally relates to screen printing. More particularly, the invention relates to a printing apparatus, which is small in size for printing images on small areas of objects that may have a surface characterized as flat, curved, or constrained by the presence of a frame, lip, rib, or some other feature that exhibits a change in step height. 
         [0003]    Screen printing is a conventional printing technique known for applying an image to relatively flat surfaces. The compatibility of the inks used in screen printing has been well established for a variety of substrates, such as textiles, ceramics, metal, wood, paper, glass, composites, and plastic. As a result, screen printing is used in many different industries, ranging from clothing to circuit board printing. 
         [0004]    Conventional screen printing devices usually employ a print mechanism in which the print cycle includes an initial vertical downward movement of the flood bar, then a horizontal movement of the print head during a flood stroke followed by an upward movement of the flood bar and a downward movement of the squeegee. The subsequent print stroke is another horizontal movement, followed by an upward squeegee movement at the end of the cycle. These simple steps require a number of actuators, typically air cylinders for the vertical movements and a timing belt for the horizontal strokes, which limit the minimum size of the printing equipment. The large size of the equipment leads to difficulty in printing images with great clarity on small substrates, as well as on substrates whose surface is curved or constrained by a feature exhibiting a change in step height. Accordingly, it would be advantageous to have a printing apparatus with reduced size and a simplified mechanism that is adaptable to print images with great clarity on small curved or constrained surfaces. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    In overcoming the drawbacks and limitations of conventional screen printing, an image printing apparatus is disclosed. Accordingly, in one aspect, the present invention provides an image printing apparatus for printing images on small areas. 
         [0006]    The printing apparatus generally includes a screen assembly with a printing screen and a flexible screen frame mounted onto a frame holder. The screen frame may be made of flat spring steel, which will allow it to be shaped according to the curvature of the substrate. 
         [0007]    Another component of the printing apparatus is a squeegee assembly that is a combined holder for both a squeegee and flood bar, which are positioned at a predetermined angle and supported by a movable print head. The print head is positioned vertically above both the squeegee and the flood bar and is movable in a horizontal direction parallel to the screen for printing. In the present invention, the printing apparatus has no means to support the substrate. The substrate is held stationary by a separate fixture. 
         [0008]    In one embodiment of the present invention, the print mechanism of the image printing apparatus is controlled by a guide assembly comprising at least one guide rod and at least one pneumatic actuator or air cylinder. The squeegee assembly is connected to the air cylinder by a pivoting means used for varying the angle of the squeegee assembly relative to the surface of the substrate. During the print cycle, the reciprocating movement of the guide assembly along the guide rod toggles the pivoting motion of the squeegee and the flood bar. This pivoting motion is actuated by an air cylinder. The design of the squeegee assembly, the pivot point, and the actuator location is in such a way that the toggling results in a downward movement of the squeegee and the flood bar thereby applying pressure onto the screen frame. 
         [0009]    In another embodiment of the present invention, the guide assembly is mounted on a print guide track flanked on both sides by guide belts. A push rod, mounted on the side wall of the printing apparatus, actuates a pivot mechanism that allows the squeegee assembly to present either the flood bar or the squeegee as the print head moves back and forth along the print direction during the print cycle, thereby pressing the ink through the screen with sufficient pressure to apply a printed image to the substrate. 
         [0010]    In yet another embodiment of the present invention, a printing apparatus is provided that is small in size. The reduced size of the printing apparatus has a two-fold advantage over conventional screen printing. First, the printing apparatus has the ability to print images on small areas of substrates. Examples of such images include logos, trademarks, pictures, and fractal antennas. Second, the printing apparatus can be moved by a transporting means, more particularly, by a robot, to a stationary or fixed substrate, such as an automotive plastic window. In an adaptation to this embodiment, the fixed substrate may be moved along by a conveyor belt, while the robot in its “line tracking” mode keeps the printing apparatus in a constant position relative to the fixed substrate. Alternatively, the printing apparatus can be made stationary with the substrate being moved by the robot. 
         [0011]    A further aspect of the present invention is to provide an image printing apparatus that is capable of printing close to a change in step height or surface transitions formed in an object by the presence of design features, such as a frame, lip, or rib. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The present invention will be better understood from the detailed description given herein below in conjunction with the accompanying drawings, which are given by way of illustration only and are not intended to limit the scope of the present invention, wherein: 
           [0013]      FIG. 1  is an illustration of the printing apparatus, embodying the principles of the present invention, attached to a robot that is in the process of applying an image onto the surface of a substrate; 
           [0014]      FIG. 2  is a cross-sectional diagrammatic view of the printing apparatus including the print head with squeegee and flood bar assemblies; 
           [0015]      FIG. 3  is a cross-sectional diagrammatic view of an alternative embodiment for the print head in a non-linear track; and 
           [0016]      FIG. 4  is a progressive side view of the movement of the flood bar during the flood stroke and the squeegee during the print stroke of a print cycle. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    Hereinafter, there will be described in detail an image printing apparatus to which the present invention is applied by reference to the drawings. 
         [0018]    Referring now to  FIG. 1 , a printing system incorporating the printing apparatus of the present invention is shown. In this printing system, the printing apparatus  10  is attached to a robot arm  6  via a top plate  15 . The movement of the robot arm  6  allows the printing apparatus  10  to come into close proximity with the surface of substrate  3  as it is held stationary by a fixture  7 . This close proximity being referred to herein as a predetermined off-contact distance. As will be fully described in the following discussion, the printing apparatus  10  is capable of printing an image anywhere on the substrate&#39;s surface  5  even within close proximity to a substantial change in the curvature or step-height of the surface. Such a step-height change is shown for the illustrated substrate  3  as the interface between surfaces  4  and  5 . Although the substrate  3  is shown to be an injection molded window having a transparent plastic surface  5  and an opaque plastic border  4 , the present invention is equally applicable to printing an image on other substrates. 
         [0019]    In  FIG. 2  the printing apparatus  10  is shown to generally comprise a screen assembly  18 , a squeegee assembly  20 , and a guide assembly  40 . The screen assembly  18  may include a screen  12  either embedded or secured to a screen frame  11 . Preferably, the screen frame  11  is conformable to the curvature of the surface of the substrate upon which the image is to be printed. The screen frame is flexible in at least one direction. In a preferred embodiment, the screen  12  is a high tension, low elongation material capable of receiving and transferring a pigment containing material, such as printing ink. One example of a screen  12  material is a porous and flexible mesh of polyester or polyamide fibers, or a combination of both. Obviously, other materials known to someone skilled-in-the-art of screen printing, such as stainless steel, could alternatively be used. The screen frame  11  is constructed so as to enable tensioning of the screen  12 , while at the same time providing a degree of flexibility to the screen  12 . The screen frame  11  is mounted in a screen frame holder (not shown), which may be constructed of metal, such as but not limited to spring steel, as well as any other desired material. Additionally, the screen assembly  18  may optionally include a gasket  13  on to its top surface, as is shown in  FIG. 1  and  FIG. 2 . This gasket  13  forms a seal between the screen assembly  18  and the frame in which the squeegee assembly  20  and guide assembly  40  are mounted. 
         [0020]    In accordance with present invention, the squeegee assembly  20  includes a flood bar  27  to spread a printing medium, such as ink  1 , onto the screen  12  during a flood stroke; a squeegee  28 , to force the ink  1  through the screen  12  to form an image on the substrate  5  during the print stroke and a print head  25 , which provides a holding means for both the flood bar  27  and the squeegee  28 , and for positioning them at a predetermined angle. The flood bar  27  and squeegee  28  are further attached to the lower portion of the print head  25  to maintain a desired off-contact distance at which the squeegee  28  is held relative to the screen  12 . The flood bar and the squeegee are constructed of a material commonly used by someone skilled-in-the-art of screen printing for the construction of squeegees, which include, among other materials, various rubbers and elastomers. In a preferred embodiment, the flood bar  27  and squeegee  28  are both made of a polyurethane material. 
         [0021]    The guide assembly  40  comprises the means through which the squeegee assembly  20  is caused to interact with and move perpendicularly across the screen assembly  18 . In one embodiment of the present invention, the guide assembly  40  comprises at least one guide rod  60  and at least one pneumatic actuator or air cylinder  22 . The pneumatic actuator or air cylinder  22  may be mounted to the guide rod  60  by a carriage plate  21  with a pivot point  24  being projected downward from the carriage plate  21 . 
         [0022]    The air cylinder  22  may be any pneumatic actuator known in the art including those comprising a rod attached to a piston, as well as those defined as being rod-less, wherein magnets couple the cylinder&#39;s piston with a mobile carrier. However, a dual acting actuator is preferred due to the movement in opposite directions during the flood stroke and print stroke in the print cycle. 
         [0023]    In another embodiment of the present invention, the guide assembly  40  may utilize in place of the air cylinder  22 , at least one push rod  35  mounted on the side wall of the printing apparatus  10  and located generally perpendicular to the squeegee assembly  20 , as shown in  FIG. 3 . Preferably, such a push rod  35  would be mounted on each side wall that represents the end of the print stroke and flood stroke in the print cycle. In this embodiment of the present invention, the printing apparatus  10  further comprises a supporting means through which the guide assembly  40  and the squeegee assembly  20  are attached. Such supporting means may be a threaded rod  30  and a squeegee adjustable knob  33 , which are designed in such a way that they assist the vertical adjustment of the squeegee assembly  20  as desired by the shape of the screen assembly  18  with respect to the surface  5  of the substrate  3 . 
         [0024]    For either embodiment, during the horizontal movement of the printing apparatus  10  in the print cycle along the guide rod  60 , the pneumatic actuator  22  or push rod  35  actuates a pivot mechanism in the print head  25  resulting in a pivoting movement, an upward and downward shifting of the squeegee  28  and flood bar  27 , of the squeegee assembly  20 . Preferably, the pivoting means is via the interaction of the pneumatic actuator  22  and the pivot point  24  projected downward from carrier plate  21  to the print head  25 . When the push rod  35  is utilized instead of the pneumatic actuator  22 , the pivoting means is in the form of a universal joint (U-joint)  23  or similar construction. In either embodiment of the present invention, the pivoting means allows for about a 30 degree back and forth movement of the squeegee  28  and flood bar  27  at either end of the guide rod  60  during the print cycle. 
         [0025]    The guide rod  60  preferably comprises the cylinder body of a rod-less pneumatic actuator or a guide track to which the piston rod of the pneumatic actuator is attached. However, as shown in the embodiment of the present invention depicted in  FIG. 3 , the guide rod  60  may act as a guide track flanked by guide belts  50  on either side, along which a print head bar  38  makes a horizontal movement in a direction parallel to the screen frame  11 . The print head bar  38  is connected to the guide rod or track  60  and guide belts  50 , by an adjustable means, which is represented in  FIG. 3  by a pair of guide wheels  31 . One advantage represented by this embodiment of the present invention is that the guide rod and guide belts can be linear or curved, thereby, allowing the printing apparatus  10  to maintain a constant off-contact distance and be relatively parallel to the surface of a curved substrate. 
         [0026]    During a print cycle as depicted in  FIG. 4 , the reciprocating movement of the guide assembly  40  toggles the squeegee  28  and the flood bar  27  in the squeegee assembly  20  through a pivoting motion. As noted above, this pivoting motion is actuated or driven by the push rod  35  or air cylinder  22 , which facilitates a downward presentment of the flood bar  27  against the screen  12  during a flood bar stroke and the squeegee  28  against the screen  12  during a print stroke. Similar to conventional screen printing, the flood bar stroke lightly spreads or floods the screen with a layer of ink  1 , while the print stroke causes the ink  1  to be pushed through the screen, thereby, depositing the desired printed image on to the substrate. 
         [0027]    The printing apparatus  10  of the present invention can be used to print images on a variety of substrates. Some of the more common substrates include fabrics, metals, glass, plastics, paper, composites, and paperboard. The substrate on which the images are printed may be planar, or may have different shapes and curvatures, as well as various design features, such as frames, lips, ribs, or a change in step height. A change in step height represents a transition between two areas on a substrate that are not in the same geometric plane. An example of such as step height is illustrated in  FIG. 1  for substrate  3  as the interface between surface  4  and surface  5 . 
         [0028]    In a preferred embodiment, the substrate  3  is an injection molded automotive plastic window or panel. Such a substrate may be comprised of any thermoplastic or thermoset polymeric resin. Typically, a plastic window is substantially comprised of a transparent region  5 , but may contain opaque regions  4 , such as, but not limited to, an opaque frame or border. The polymeric resins may include, but are not limited to, polycarbonate, acrylic, polyarylate polyester, polysulfone, polyurethane, silicone, epoxy, polyamide, polyalkylenes, and acrylonitrile-butadiene-styrene (ABS), as well as copolymers, blends, and mixtures thereof. The preferred transparent, thermoplastic resins include, but are not limited to, polycarbonate, acrylic, polyarylate, polyester, and polysulfone, as well as copolymers and mixtures thereof. 
         [0029]    The printing apparatus  10  of the present invention may be used to apply a printed image directly on to the surface of the substrate  3  or on to the surface of any coating or film that may be applied to the substrate  3 . Such a coating or film may be present to protect the substrate against weathering, abrasion, or any other means of being damaged or degraded. 
         [0030]    The substrate  3  is placed into a fixture  7  and held stationary during the print cycle. The substrate  3  may be held in the fixture by any mechanical or other means known to someone skilled in the art of printing. Examples of mechanical means for holding the substrate in place include, but are not limited to, clamps, hooks, pins, suction cups, and fasteners. A preferred method of holding the substrate in place during the print cycle is through the use of vacuum or negative pressure applied to the substrate opposite to the side upon which the printed image will be applied. Such negative pressure may be established by any means known to someone skilled in the art of printing, including but not limited to the use of an air venturi vacuum system. Suction cups may be utilized in conjunction with a vacuum system to provide additional support for holding the substrate. 
         [0031]    The loading of the substrate  3  into a fixture  7  may be done manually, for example by an operator. Alternatively, the loading can be performed automatically by a transporting means such as a suitably programmed robot arm or other machine. A substrate may similarly be unloaded in the same manner as the loading of the substrate. For example, the substrate may, after it has been printed on, be removed from the printer either manually or automatically. In a preferred embodiment, the substrate includes articles of the automotive industry, such as an automotive glazing panel, wherein the substrate is moved by a robot in line with the cycle time established by the injection molding of the substrate, where printing can take place after molding and before coating or storage. 
         [0032]    Similarly, a suitably programmed robot arm  6  may transport the printing assembly  10  to the substrate  3  held in a fixture  7 . The robot arm  6  may manipulate the position of the printer in relation to the substrate in order to establish an appropriate off-contact distance necessary to apply a printed image with great clarity. The robot arm may further manipulate the position of the printer to apply multiple printed images on to the substrate. In an adaptation to this embodiment, the fixed substrate may be moved along by a conveyor belt, while the robot, in its “line tracking” mode, keeps the printing apparatus in a constant position relative to the fixed substrate. Although not shown, another embodiment of the present invention is to maintain the printing apparatus in a stationary position and allow the substrate to be held in a fixture that can be robotically manipulated or moved during a print cycle. 
         [0033]    The nature of the printed image applied to the substrate  3  by the printing apparatus  10  depends upon that desired pattern. Examples, of such patterns include but are not limited to logos, script, pictures, trademarks, and fractal antennas, as well as borders or frames comprised of lines, dots, or other geometric images. 
         [0034]    Preferably, the printing apparatus  10  further comprises at least one means (not shown) for locating and delivering the printing ink  1  adjacent to the screen  12 . The ink  1  should be delivered in a pre-determined quantity to the surface of the screen  12  near the squeegee assembly  20  at a predetermined time before the screen  12  is initially deformed to initiate the flood bar stroke. The flood bar  27  evenly distributes or spreads the printing ink  1  across the upper surface of the screen  12  during the flood bar stroke. Then during the print stroke, the printing ink  1  is forced through the screen  12  by the squeegee  28  to form an image on the surface  5  of the substrate  3 . 
         [0035]    The printing apparatus  10  of the present invention provides many advantages over conventional screen printing. For example, when a printed image is applied to substrate  3  while the substrate is at an elevated surface temperature, conventional screen printing is severely hampered by accelerated evaporation of the solvent in the ink resulting in the drying of the ink in the screen. An elevated surface temperature of the substrate  3  would occur if the printing was performed immediately after the injection molding of the substrate. The printing apparatus  10  of the present invention overcomes this issue by enclosing the area above the screen  12  via the presence of a top plate  15 . This top plate serves as a “lid” on top of the printer, which inherently trap the solvent present in the ink  1  inside the printing apparatus. The saturation of the air volume within the printing apparatus  10  with a solvent-rich mixture assists in reducing the possibility that the ink  1  could dry in the screen. 
         [0036]    In addition to the above advantage, the printing apparatus  10  of the present invention represents a printer that is significantly smaller in size than a conventional screen printer. The printing apparatus  10  is especially suitable for printing small features with image sizes typically in the range of about 5×10 mm to about 50×150 mm in dimension. 
         [0037]    The process for printing an image on a substrate  3  using the printing apparatus  10  of the present invention may further be elaborated upon as described below. In general, the printing apparatus  10  comprises a guide assembly  40  whose movement is along the guide rod  60  in a generally horizontal direction, and a squeegee assembly  20  which provides the necessary downward pressure on the screen  12 . The basic printing mechanism of the printing apparatus  10  comprises placing the substrate  3  in a position with respect to the screen frame  11  and screen assembly  18 , engaging the flood bar  27  with the printing ink on the print screen  12 , moving the flood bar  27  along the screen  12  to distribute the ink, lowering the screen  12  to a predetermined off contact distance from the surface of the substrate  3  such that the screen  12  conforms to the shape of the substrate  3 , moving the squeegee  28  along the screen  12  to impart ink to the substrate  3 , and raising the print screen  12  to remove the substrate  3  after being printed with the desired image. 
         [0038]    During the printing process according to the present invention, the screen frame  11  comprises the inverse of the image pattern that is to be printed on the substrate. The screen frame  11  conforms to the shape of the substrate by a tensioning mechanism operated by the screen assembly  18 , when the printer and substrate are brought in contact with each other. The substrate  3  on which the image is printed is held in position by a separate fixture  7 . 
         [0039]    The print cycle, in general, comprises a flood stroke and a print stroke, which involves the movement of the print head  10  along the guide rod  60 . During the flood stroke, the flood bar  27  is swept over the surface of the screen  12  in a direction along the substrate  3  on which the image is printed, thereby engaging the screen assembly  18  to evenly distribute the printing ink  1  as illustrated in  FIG. 4 . This is followed by a print stroke, wherein the squeegee  28 , is drawn across the screen  12  to effect printing on the substrate  3  also shown in  FIG. 4 . As the guide assembly  40  moves horizontally along the guide rod  60 , the pneumatic actuator  22  or push rod  35  located on the side of the printer actuates the pivoting motion of the squeegee assembly, switching the flood bar and squeegee for presentment against the screen  12 . After printing, the squeegee  28  is raised from the screen frame  11  and the printed object is removed either manually or by a robotic means. 
         [0040]    It is to be understood that the present invention may be embodied with other changes, modifications, variations, and improvements, such as those described in the above description, which may occur to a person skilled in the art, without departing from the spirit and scope of the invention defined by the appended claims.