Abstract:
A flexographic press of conventional design is used to print on a container, with the container to be printed upon replacing the web and the impression roll of the conventional press. In order to maintain the registration between the print stations, the container is placed into a carrier and stays registered to the carrier until all colors are printed. The carrier is moved between the different print stations and is registered to each print station independently. All print stations are set up to print in exactly the same place relative to the carrier, thus registration is achieved.

Description:
REFERENCE TO RELATED APPLICATION  
       [0001]     This is a division of Application Ser. No. 10/689,087 filed on 21 Oct. 2003 entitled “Flexographic Printing on Containers”. 
     
    
     TECHNICAL FIELD  
       [0002]     The invention pertains to printing and, more specifically, to an apparatus for directly printing multi-color images on containers such as bottles and cans.  
       BACKGROUND  
       [0003]     When printing multi-color images, accurate registration is required between colors. Since most containers have neither accurate reference features nor stiffness, it is difficult to print multi-color images on them. Such printing normally requires multiple printing units (one for each color). Registration is difficult to maintain when a container is transferred between successive printing units. For this reason, most color images on bottles are done by applying a pre-printed label to the bottle, increasing production costs over direct printing. In some cases, such as when printing drinking cups or unfilled cans, a mandrel may be inserted into the container to achieve stiffness and registration (see for example U.S. Pat. Nos. 5,193,456 and 3,661,282). In the great majority of cases, the insertion of a mandrel to fill the container and allow registration is not possible, as inserting a mandrel requires that the container have an opening at least as large as its largest cross-section.  
         [0004]     Flexographic printing is an ideal process for printing on thin-walled containers, as flexographic printing requires almost no pressure. Accordingly, a method and apparatus for flexographic printing on containers is highly desirable. A typical flexographic press comprises an ink supply (also referred to as an “ink fountain”), and a metering roll in contact with the ink supply. The metering roll transfers an accurately-metered amount of ink to the plate (which is mounted on a plate cylinder). The flexographic press prints on a material to be printed, usually in the form of a web, and includes an impression cylinder used to support the web. The most common form of metering roll is known as an anilox roll. An anilox roll is a hard cylinder engraved with a continuous pattern of small pits. Excess ink is removed by a doctor blade or a reverse roll, leaving ink only in the recessed areas. The flexographic plate operates in a manner similar to the common rubber stamp: the elevated areas are inked and this ink is transferred to the web. The plate is usually mounted on a thin layer of cushioning foam.  
         [0005]     There is a need for practical systems for printing monochrome and color images directly onto containers, such as plastic and glass bottles, cans, cups, jars and the like. There is a particular need for such systems which can maintain registration between images applied by different printing units in a manner compatible with present flexographic press design.  
       SUMMARY OF INVENTION  
       [0006]     This invention provides apparatus for printing on containers which are not cylindrical. The apparatus includes a number of flexographic printing stations. The container to be printed replaces the web and the impression roll. To maintain registration between the print stations, the container is placed into a carrier. Registration with the carrier is maintained until all of the colors are printed. The carrier is moved between the different print stations and is registered to each print station independently. All print stations are set up to print in exactly the same place relative to the carrier, thereby ensuring registration. Because of the slight shape variations between containers (even among ones from the same batch) a thicker and softer cushioning foam is used. In order to automate the process, a number of such carriers can be mounted on a conveyor belt, which moves the carriers from one print station to the next.  
         [0007]     Registration may be performed while both the conveyor belt and the press are in operation, thus eliminating the need to stop and register. Performing the registration while in motion greatly increases throughput. The carriers are designed such that containers can be clamped and released (after printing is completed) while the carriers are in motion. This allows a high throughput continuous process, which is desirable for printing on high volume items, such as cans and bottles. The apparatus can be made to print on any shape of container that a regular label can be used on, such as, but not limited to, cylindrical, oval, conical and conical with oval cross section.  
         [0008]     The invention and its objectives will become more clear by studying the preferred implementation in conjunction with the drawings. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0009]     In drawings which illustrate non-limiting embodiments of the invention:  
         [0010]      FIG. 1  is an isometric view of a printing system according to a particular embodiment of the invention;  
         [0011]      FIG. 2  is an isometric view of the carrier of the  FIG. 1  system;  
         [0012]      FIG. 3  is a cross section of the  FIG. 2  carrier;  
         [0013]      FIG. 4  is a top view of the conveyor belt system, showing loading and unloading of containers from the carriers;  
         [0014]      FIG. 5  is an isometric view of the mechanism for registering the carrier to a printing unit;  
         [0015]      FIGS. 6   a ,  6   b ,  6   c  and  6   d  show schematically the sequence of a carrier passing through a printing unit; and  
         [0016]      FIG. 7  is an isometric view of printing on an oval container, with the sidewalls of the printing unit removed for clarity. 
     
    
     DESCRIPTION  
       [0017]     Referring to  FIG. 1 , a flexographic printing press  6  comprises a plurality of printing units. Each unit prints one color. Typically, the number of printing units on such a press ranges from 4 to 10 units. An endless conveyor belt  2  moves carriers  3  past the printing units. The containers  1  (bottles in some preferred embodiments) are supplied by an infeed tray  4  and are unloaded to an output tray  5 . The conveyor belt  2  is powered by a shaft  8 , which may be driven by a separate motor (not shown) or may be connected mechanically to the motor of press  6 . If a separate motor is used, it must be synchronized to the speed of press  6  using the well-known principles of servo systems (also known as “shaftless” systems in printing presses).  
         [0018]     At both the infeed and unload positions of conveyor belt  2 , means  9  are provided to open carrier  3  in order to accept a container  1  (at infeed tray  4 ) and release the container  1  (at output tray  5 ). The details of mechanism  9  are discussed later with reference to  FIG. 3  and  FIG. 4 . Each printing unit also has a registration means  7  to register carrier  3  to the printing unit, and thereby to the printing plate mounted on the printing cylinder of the printing unit as the carrier  3  passes through it. The cylinder and plate are described below in more detail with reference to  FIGS. 5 and 7 .  
         [0019]      FIG. 2  shows a preferred embodiment of carrier  3 . Carrier  3  is loosely attached to conveyor belt  2  via guides  17 . Guides  17  allow some slippage between carrier  3  and conveyor belt  2 , in order for carrier  3  to be able to align itself with each print unit. A stop  20  limits the range over which carrier  3  can move relative to belt  2 . An alternative embodiment uses an elastic attachment, for example a spring, to attach carrier  3  to conveyor belt  2 . Container  1  is held from two of its ends, similar to a workpiece held in a lathe. At one end, a chuck  16  is shaped to fit the container; at the other end, a tapered plug  10  fits into the opening of the container and is held there by the force of spring  12 . Shaft  11  can be retracted by pulling on ball bearing  13 . When shaft  11  is retracted, container  1  can be inserted and removed. As described further below, ball bearings  14 A and  14 B are used to align carrier  3  to the printing unit. In this description, reference numerals ending in the letters “A” and “B” refer to similar components located on the left and right hand sides of press  6 , in the orientation shown in  FIG. 1 .  
         [0020]     In some cases, for example when printing on thin-walled containers, it is desirable to pressurize the inside of the container via an air hole  15 . Referring now to  FIGS. 2 and 3 , it can be seen that air hole  15  is connected to a hole in shaft  11  and plug  10 . This allows air to be fed into container  1  for the short duration which container  1  is in contact with the printing unit.  
         [0021]     The mechanism to retract shaft  11  can be as simple as a wedge  9 , which is placed in the path of carrier  3 . As bearing  13  rolls against the edge of wedge  9 , shaft  11  is pulled out.  FIG. 4  shows the placement of such wedges  9  at both the infeed position (at or near infeed tray  4 ) and the unload position (at or near output tray  5 ) of conveyor belt  2 .  
         [0022]     Returning to  FIGS. 2 and 3 , different sizes and shapes of chuck  16  and plug  10  may be provided for each size and shape of container. When printing on cans, the shape of plug  10  may be similar to chuck  16 . Means for removing chuck  16  are shown schematically as a setscrew  33 . It has been found that the pressure of spring  12  is sufficient to keep container  1  in place during printing if the inside of chuck  16  is coated with a high friction material  36  such as silicone rubber or polyurethane rubber.  
         [0023]     Shafts  11  and  30  can rotate freely in bearings  32  and  31 . In some applications, for example when printing on rectangular or oval containers, container  1  should be prevented from rotating during printing. In some other applications, such as printing all around cylindrical containers, container  1  may be allowed to rotate, but should return to a known orientation. This is accomplished via detent  18  and spring loaded pin  19 . When printing covers the full circumference of container  1 , chuck  16  will return to the detent position.  
         [0024]     If printing is not required to cover the full circumference of container  1 , the printing plate may be continued as a narrow non-inked strip in order to complete the rotation of container  1 . More details on this subject are provided later in this disclosure. It should be noted that registration is required in both the circumferential direction (achieved by detent  18 ) and in the axial direction. Therefore, shaft  30  should be free from any axial play and the shoulders  35  of bearing  14 B should fit the mating part (item  7 B in  FIG. 5 ) accurately. In one preferred embodiment, belt  2  is a timing belt, bearings  13  and  14  are shielded ball bearings, bearings  31  and  32  are sintered bronze bushings, and carrier body  3  is made of aluminum.  
         [0025]      FIG. 5  depicts the mechanism for registering carrier  3  to a printing unit. The  FIG. 5  mechanism has four functions: 
        1. locating carrier  3  axially relative to printing plate  25 . In this disclosure, the axial direction is the direction of the axis of container  1  and of printing cylinder  22 ;     2. locating the axis of container  1  in an orientation that is parallel to the axis of printing cylinder  22 ;     3. bringing container  1  into contact with printing plate  25  at the correct circumferential point and ensuring contact is sufficient for a complete rotation (for round containers); and     4. locating container  1  in the vertical direction to achieve the correct impression pressure via the correct compression of the foam backing  24  of printing plate  25 .          
         [0030]     As conveyor belt  2  brings carrier  3  closer to printing press  6 , arms  7 A and  7 B engage bearings  14 A and  14 B. It is desirable to make arm  7 B with a tapered tip, i.e. the thickness of the arm in the axial direction at the tip is less than the thickness at the position of normal engagement during printing. This helps with guiding arm  7 B between the shoulders  35  of bearing  14 B (see  FIG. 3 ). The sequence of the engagement between a bearing  14  and its corresponding arm  7  is shown in  FIG. 6   a  to  6   d.    
         [0031]     As shown in  FIG. 5 , arms  7 A and  7 B are coupled by a sturdy shaft  28  which runs parallel to the axis of the plate cylinder  22 . Arms  7 A and  7 B therefore force the axis of container  1  to be parallel to the axis of plate cylinder  22 . The elevation of carrier  3  during printing, and therefore the compression of foam layer  24  under plate  25 , is determined by guide plates  26 A and  26 B (see also  FIG. 7  for greater clarity). Guide plates  26  should be adjusted for an average compression of about 0.5 mm in foam layer  24 . Foam layer  24  is made of dense closed cell foam, about 2-4 mm in thickness. The standard foam tape used for mounting flexographic printing plates is too thin for this purpose (but can be used to attach plate  25  to foam layer  24 ). It has been found that, under these conditions, very good dot reproduction 5%-95%) of fine screens (up to 80/cm) may be achieved even with a container run-out of 1 mm. Obviously, the compression of foam layer  24  should be such as to allow contact with container  1  even at the worst run-out to be encountered. Too much compression degrades print quality, too little compression may cause loss of contact. The optimum elevation of guide plate  26  may be found by carefully experimenting during a trial run.  
         [0032]     In order to achieve circumferential registration between container  1  and plate  25  and between the image and the index position of container  1 , the angular position of plate cylinder  22  is measured by shaft encoder  23  ( FIG. 5 ). At the correct position of cylinder  22 , actuators  27  push carrier  3  into contact with plate cylinder  22 . In the illustrated embodiment, actuator  27  is a servomotor, coupled to arm  7 B by a gear. An alternative coupling is via a timing belt. The details of connecting an output of shaft encoder  23  to the servomotor actuator  27  are not shown or described, as they follow standard procedures of servo systems well known in the art of printing press design. Because actuators  27  may momentarily stop carrier  3  from moving while conveyor belt  2  keeps moving, some relative motion should be possible between carrier  3  and belt  2 . In the illustrated embodiment, there is a sliding fit, which may be a friction fit, between them.  
         [0033]     Bearing  14 B is shaped to allow part of the bearing to ride on guide plate  26 B, while the other part engages arm  7 B (see  FIGS. 3 and 7  for more detail). Together bearing  14 B and arm  7 B provide axial registration between carrier  3  and printing plate  25 . An alternative to using bearing  14 B for axial registration is to use a vertical guide plate to guide bearing  14 B in the axial direction, similar to the guidance provided by plates  26  in the vertical direction. There should be only minimal play (i.e. gap) between arms  7 A and  7 B and corresponding bearings  14 A and  14 B, as any play will tend to cause axial mis-registration.  
         [0034]     When container  1  touches plate  25 , it starts rotating because of friction (overcoming the detent action of detent  18  in  FIG. 3 ). At the same time, arms  7  move carrier  3  and container  1  slowly to the other side of plate cylinder  22  until container  1  stops touching plate  25 . By adjusting the speed and amount of travel of arms  7 , container  1  will complete one rotation as it travels from one side of plate cylinder  22  to the other. A slight variation (a few %) will not matter, as container  1  will be pulled into the reference position by the action of detent  18 . The detent action of carrier  3  is also important when containers are loaded at a specific orientation, in order to avoid printing on the seam or other defects. Containers may be loaded at a random orientation and additional hardware may be used to orient them to a reference position. This is common practice in current label applicators.  
         [0035]     Clearly, the motion of arms  7  must be slower than the circumferential velocity of plate cylinder  22 , otherwise container  1  will not complete a full rotation during the time that it travels from one side of plate cylinder  22  to the other. In those cases where it is not desired to print the full circumference of container  1 , a “dummy” portion  29  of plate  25  is left to complete the rotation. This portion  29  is aligned with chuck  16  and is not inked by anilox roll  21 , as its only function is to serve as a friction drive for container  1 . Accidental inking, however, is not detrimental. Anilox roll  21  can be made narrower than plate cylinder  22  to avoid inking of strip  29 . No further details of press  6  are provided in this description, as the rest is conventional in construction and well known in the art of flexographic printing presses.  
         [0036]      FIG. 7  shows printing on an oval container  1 . Similar techniques to those shown in  FIG. 7  may also be used to print on rectangular containers. For clarity, the side walls of the press are not shown in  FIG. 7 . For oval or rectangular container shapes, it is preferable to prevent container  1  from rotating by using a firmer pressure of pin  19  against the detent hole in chuck  16 . Container  1  is moved into printing position by arm  7  and actuator  27 , but from the point that plate  25  touches container  1 , actuator  27  should not force container  1  across plate  25 . Container  1  should move at a velocity determined by plate cylinder  22 . This is required as container  1  is no longer free to rotate to find the correct circumferential velocity. This condition can be achieved by disconnecting actuator  27  at the moment that plate  25  touches container  1 , or by programming a velocity profile in actuator  27  to match the traverse speed imparted by plate cylinder  22 . As in  FIG. 5 , a section  29  of “dummy plate” may be left to engage container  1  before printing starts and to push it past plate cylinder  22  at the end of the printed area. It is desirable, but not mandatory, not to ink this “dummy” section as it comes into contact with chuck  16 .  
         [0037]     To print the other side of an oval container, a second print station may be used, or container  1  may be raised and rotated 180 degrees within one print cycle. The latter option requires the use of a more complex guide plate  26 .  
         [0038]     A more complex case arises when the container is tapered, or both tapered and oval. In such a case, it is best to use a tapered plate cylinder (not shown) that matches the taper of the container. Such a tapered plate cylinder will have some slippage relative to anilox roll  21 , but such slippage is not detrimental to image quality. On the other hand, any slippage of printing plate  25  relative to the container will ruin the printed image. In the most generic case, each of arms  7 A and  7 B should have its own actuator  27  rather than a coupling shaft  28 . This allows handling of containers with a high degree of taper or taper and ovality, as each end of the container can be moved at a different speed to maintain line contact with the plate  25 .  
         [0039]     The embodiments described above use mainly mechanical means to bring containers into registration with the plate. It is well known that any mechanical linkage such as a gear, lever, clutch or the like can be replaced by an electronic linkage performing the same function. Many modern flexographic presses no longer use gears to synchronize the cylinders; instead, they rely on electronic servo systems. Such presses are described by the general term “shaftless”. It is considered to be obvious to one skilled in the art that the mechanical components in the above-described embodiments can be replaced with their electronic equivalents (or any other equivalent system, such as hydraulic). It is also clear that all the functions that are shown as purely mechanical in the embodiments described here can be performed with servo systems; thus items such as guide plates, detents, friction drive and the like can all be implemented using servo systems if so desired.  
         [0040]     The current description should therefore be read in the broadest sense. For example, when a mechanical actuator such as a lever is shown, it is considered to be obvious that the lever can be replaced by an electrical actuator such as a solenoid or a motor or by a hydraulic cylinder. Similarly, while an endless belt type conveyor system is shown here to bring the carriers to the press, any other method of moving the carriers between the print units can be utilized. Examples of some well-known alternate techniques for moving carriers between print units include: 
        1. robotic arms to transport carriers between print units;     2. a rigid arrangement of carriers at the periphery of a large wheel; and     3. carriers linked together to form a linked belt (similar to a bicycle chain).        
 
         [0044]     There have thus been outlined the important features of the invention in order that it may be better understood, and in order that the present contribution to the art may be better appreciated. Those skilled in the art will appreciate that the conception on which this disclosure is based may readily be utilized as a basis for the design of other methods and apparatus for carrying out the several purposes of the invention. It is most important, therefore, that this disclosure be regarded as including such equivalent methods and apparatus as do not depart from the spirit and scope of the invention.