OFFSET ROTARY SCREEN PRINTER

BACKGROUND

In rotary screen printing with conventional screens and machines, there are several restrictions on the type of image that can be produced. The image master which is formed on the rotary screen has a maximum length which is equal to the circumference of the screen cylinder. The printing machine will print a repeating image on a

5 continuous web of substrate, the repeat length being equal to the circumference of the screen cylinder. Generally, the image master on conventional rotary screens is formed around the full circumference of the cylinder as a continuous image without joins. The repeating image which is printed on the substrate is therefore also continuous and without joins. It is possible that the image master on the rotary screen could be shorter

10 than the circumference of the screen cylinder, but this will normally leave an unprinted gap between each image on the substrate. There are rotary screen printing machines in which the web substrate can be stopped and started periodically to avoid this gap, but for several colours the mechanics become complex. The second point is the registration between colours. On a rotary screen printer with

15 more than one colour, which is usual, the cylinders are placed sequentially in the machine, and the web passes under and comes into contact with each cylinder in turn. The registration accuracy between subsequent colours depends therefore on the continued accurate positioning of the substrate as it moves through the printer. This can present problems, particularly for web materials which are not stable

20 geometrically. This is particularly true in the type of printer in which the web substrate can be stopped and started periodically, and the image length and repeat length is less than or different from the screen circumference.

STATEMENT OF INVENTION

The present invention is a rotary screen printing system which combines the benefits of variable repeat length and image size with accurate registration between two or more

25 colours. This is implemented by printing the image in two or more colours on a transfer medium such as a continuous belt, and then transferring the complete image to the substrate. The contact between the transfer medium and the substrate may be

intermittent so that the repeat length on the substrate is not the same as the repeat length on the transfer medium which is set by the diameter of the printing cylinders. In this way the registration is not affected by the dimensional stability of the substrate to be printed, and the repeat length on the substrate may be varied.

INTRODUCTION TO DRAWINGS

5 Figure 1 shows a side view of a schematic layout of the invention with two screens, showing the rotary printing screens 2, 3, the transfer belt 10 and the substrate to be printed 14.

Figure 2 shows a side view of the machine as in figure 1, but with the substrate not in contact with the transfer belt. 10 Figure 3 shows a schematic partial end view of a type of rotary screen printing drum in which the image screen 24 may be generated separately and is replaceable.

Figure 4 shows the arrangement of the image screen on the type of printing drum shown in figure 3.

Figure 5 shows a partial side view of the invention with a transfer belt with a clearance 15 region 29 which may be used with the type of printing drum shown in figure 3.

Figure 6 shows a side view of a 4 colour machine with screen printing drums of the type as shown in figure 3, with a transfer belt which is not cut away for clearance of print drum components, but with impression rollers 6, 7, 8 and 9, which may be moved away from the print drums for this purpose. 20 Figure 7 shows a side view of a printing machine with three colours, with ink drying or curing units 35, 36, 37 for each colour, and a substrate preheating unit 39.

Figure 8 shows a detail of the transfer belt to substrate printing section, showing a conformal pressure roller 40 to increase the contact time between the transfer belt and substrate.

DESCRIPTION

25 A schematic arrangement of the invention is shown in figure 1. The printer consists of two or more screen printing cylinders 2 and 3, which have an image screen on the circumference of at least part of the circumference of the cylinders. These printing cylinders would also have internal ink delivery systems. The ink passes through the

screen apertures in the usual way. The printing cylinders 2, 3 as shown in figure 1 rotate in an anticlockwise direction, as shown by the dashed arrows 41, such that the ink is forced through the screens by the squeegees 13. The printing cylinders 2, 3 are in contact with a transfer medium such as an endless transfer belt 10, along the longitudinal line of the printing cylinders at which printing occurs onto the transfer belt, and the transfer belt moves as shown by the dashed arrows 42. The speed of the transfer belt is such that the surface in contact with the printing cylinders is the same as the surface speed of the printing cylinders, to produce a sharp image on the transfer belt. The impression rollers 6 and 7 maintain contact and produce the necessary pressure between the transfer belt and the printing cylinders 2 and 3. Clearly this will have the effect of printing the image from each cylinder onto the transfer belt. Both the printing cylinders and the transfer belt may move at uniform speed, and are not required to stop and start during the printing process. The rotation of the two or more printing cylinders is synchronised and they will move at the same speed and are within normal technical limits, of the same diameter. As the transfer belt moves as shown in figure 1 it will pass under and come into contact with the sequence from left to right as shown, of screen printing cylinders 2 and 3. There may be four or more cylinders with impression rollers, but only two are shown for the purpose of illustration. The phase relationship of the printing cylinders is such that the image from the second cylinder 3 will print in registration with the image from the first cylinder 2, and so on for all the cylinders. It is thus possible to print a combined image of several colours on the transfer belt. Because the transfer belt is an integral part of the printing machine it can be sufficiently stable to obtain a high degree of registration accuracy between the sequence of images. Because the transfer belt does not have to stop and start with each image, it can be more easily, simply and accurately synchronised with the rotation of the printing cylinders. Further, because of the absence of any stop start motion, the separation between the printing cylinders in the horizontal direction as shown, is not governed by, and does not have to be changed with the size of the image on the screen printing cylinders or the repeat length between each image.

It can be imaged that after passing under and in contact with the several screen printing cylinders, the transfer belt will have a combined image printed on its outer surface. The spacing between consecutive images, the repeat length, on the transfer belt will clearly be the circumference of the printing cylinders. If the length of the images on the rotary screens is less than the circumference of those screens, then the repeat length on the final medium to be printed may need to be less than or different from this circumference.

The combined image on the surface of the transfer belt needs to be transferred to the surface of the actual medium to be printed, the substrate 14. The substrate 14 is a continuous web, and may be paper, textile, plastic, metal foil etc, or any printable material which is sufficiently flexible to be transported through the machine. The substrate has a transport mechanism, which can consist for example, of pairs of pinch and drive rollers, 15 and 16, and 17 and 18. The transport mechanism is controlled such that it can move the substrate 14 so that the printable surface of the substrate moves at the same speed as the surface of the transfer belt 10, and such that it can also stop and start and independently move the substrate as required. The substrate 14 passes close to the transfer belt 10, and the printable surface of the substrate may be held in contact with the transfer belt 10 by a pressure roller 19. The pressure roller 19 may also be part of the web transport mechanism, that is, it may be used in conjunction with other rollers or by itself to drive the web substrate 14. There is a backing roller 11, often called an impression roller in other forms of printing, which enables sufficient pressure to exist between the transfer belt 10 and the substrate 14. This impression roller may conveniently be part of the transport mechanism of the transfer belt 10 as shown in figure 1, but this could also be a separate roller on the inner perimeter of the transfer belt, located in some position after the last and before the first screen printing cylinder in terms of transfer belt motion. To transfer the printed combined image on the transfer belt 10 to the substrate 14, the pressure roller 19 maintains the substrate in contact with the transfer belt over at least the length of each printed image, and the transport mechanism moves the substrate at synchronised speed with the transfer belt, in the direction shown by the dashed arrow 43. This will transfer the ink on the transfer belt 10 to the substrate 14, so that the substrate is printed. The

substrate can move away from the transfer belt so that the surfaces of the transfer belt and the substrate are not in contact, by movement of the pressure roller 19 or the impression roller 11, or both. Figure 2 shows the arrangement whereby the pressure roller 19 is moved away from the impression roller 11, so that the transfer belt and substrate are not in contact. While the printing cylinders and the transfer belt continue to move the motion of the substrate can be changed. It would be possible to halt or to move the substrate forwards or backwards independently during this period of no contact. When the printing cylinders have images which are shorter than the circumference of those cylinders, there will be unprinted gaps on the transfer belt, as already explained. In normal circumstances it is these unprinted gaps which will pass through the transfer belt to substrate print region while the substrate is not in contact. The transport motion of the substrate may then be restarted at synchronised speed and the substrate again brought into printing contact with the transfer belt, such that the next combined image on the transfer belt is printed on the substrate. Because of the intermittent contact between the substrate and the transfer belt, the repeat length for images on the substrate is independent of, and can be more than or less than, the repeat length on the transfer belt, which is set by the circumference of the printing cylinders. After the print operation of the substrate as described above, the substrate can pass in the vicinity of a drier 38, which dries the ink on the substrate. For inks which cure rather than dry, 38 would be the ink curing unit for the substrate. In this manner the machine combines screen printing with accurate registration, with the benefit of variable image size and repeat length. The variable motion of the substrate, which is necessary when the repeat length is not set by the printing cylinders, need only be operated at one print position. Further, the variable motion of the substrate does not effect registration accuracy between colours, since this is controlled only by the operation of the printing cylinders and the transfer belt.

It is unlikely that for the simple design shown in figure 1, there will be 100% transfer of the ink from the transfer belt to the substrate when the substrate is printed. This would leave a sequence of feint images on the transfer belt as it leaves the substrate print area. It would then be beneficial to make the length of the external periphery of the transfer belt an integer number of repeat lengths, i.e. printing cylinder

circumferences. In this way, when the remaining ink from an image on the transfer belt again reaches the printing cylinders, the newly printed image on the transfer belt will coincide with the previously printed image. This avoids the generation of a series of slightly displaced feint images, ghost images, on the transfer belt and as a result, similar ghost images on the substrate. It would further be possible to synchronise the motion of the transfer belt with that of the printing cylinders to ensure exact registration with any remnant of the previously printed images. Since it is clear from the above description that a printer of this type will be able to operate with screen images which are shorter in length than the circumference of the printing cylinders, it is therefore possible to use screen printing cylinders of a type in which the image screen is shorter than the circumference of the printing cylinders. Such printing cylinders, henceforth called print drums, are used in several types of small office printers, and the essentials of a typical design is shown in figure 3. The print drum has a stationary frame 20 upon which is mounted a hollow cylinder 21 which is able to rotate. The direction of rotation for printing is shown by the dashed arrow 41. Ink is delivered by some means to the inside of the cylinder 21 and is distributed along the length of the cylinder. The print drum will have any of several conventional means of applying ink under pressure to the internal cylindrical surface of the cylinder 21. Figure 3 shows a print drum with a metering roller 22 and an ink roller 23. The cylinder 21 is generally impermeable to ink, but an area, which is shorter than the circumference of the cylinder and less wide than the length of the cylinder is made easily permeable to the ink. This is the print region of the print drum, and is indicated in figure 4 by 27. This area can have for example multiple small holes through the surface of the cylinder 21, or an open mesh, or a combination of these or other means. As the cylinder rotates and under the influence of the internal pressure on the ink, the ink will pass through the cylindrical surface within this print region. The image to be printed by the print drum is developed on a separate plastic film, by making multiple small holes in the film which correspond to the image. The holes are of a sufficient size for the ink from the print drum to pass through the plastic film. The plastic film 24 is wrapped around the cylinder 21 in close contact with the surface, with the developed image area on the plastic film wholly within the cylinder surface

region through which ink may pass. The ink, having passed through the general area of the cylinder may then pass through only the specific image area of the plastic film. The print drum is then capable of screen printing the image which has been developed on the plastic film. It can be seen that for the print drum shown in figure 3, there is a clamping mechanism 25 which holds the leading edge of the plastic film 24 in place. This clamping mechanism is shown as protruding above the surface of the cylinder. This construction allows for an uninterrupted surface of constant radius on the inside of the cylinder 21, which is simpler for the ink delivery mechanism inside the print drum. Figure 4 shows an unrolled view of the surface of the cylinder 21 to explain the above. The external cylindrical surface is shown as a rectangle 26, where the top and bottom edges as shown would be along the surface of the cylinder, and the vertical edges would be the cylindrical ends. The clamp mechanism 25 is along the cylinder, and the area of the cylinder surface which is permeable to ink, the print region, is shown as a rectangle 27. The plastic film 24 with the image 28 is held along one edge by the clamp 25, and at least covers the ink permeable region 27. The image to be printed is fixed in the plastic film 24 as a pattern of holes 28, through which the ink may pass. The placement of the film 24 is such that image area is made to coincide with the ink permeable area 27 on the surface of the cylinder. For the type of print drum shown in figure 3 to be used in the place of the conventional screen printing cylinders shown in figure 1, some modification of the machine is necessary. The first requirement is that the print drum must be able to turn freely without impediment by the transfer belt, which must not touch the clamping mechanism 25 and protruding sections of the screen 24, shown in figure 3. One solution to this is the modification to the transfer belt 30 as shown in figure 5. In this the transfer belt 30 has been modified to include the addition of multiple regions 29 which are below the printing surface of the transfer belt. The depth of the depressions 29 is greater than the protrusion of the film clamping mechanisms 25 on the print drums. The depressions 29 are wide enough across the transfer belt to clear all of the clamping mechanism, and are greater in length than the circumferential length of the clamping mechanisms and the end of the plastic film 24 which protrudes on the print drums. The depressions 29 are spaced at the repeat length of the image on the transfer

belt, that is , at a distance equal to the effective circumference of the print drums. Since the transfer belt 30 is an integer number of repeat lengths, the depressions 29 are regularly spaced. This effective circumference is that of a simple cylinder of the same diameter as a print drum of this type without the clamping mechanism. When a machine for these print drums is assembled, the position of the transfer belt 30 is set up so that the depressions 29 in the transfer belt will coincide with the clamping mechanisms 25 on the print drums as the machine operates, so that the transfer belt does not impede the rotation of the print drums, or damage the plastic film 24. This is shown in figure 5, in which the clamping mechanism 25 on the second drum 32 is at the bottom, and coincides with the transfer belt depression 29. The first drum 31 is at a different angular position, and there is no depression in the transfer belt. It is clear that this synchronisation must be maintained by some means during continuous operation of the machine. In order to enhance smooth operation of the machine, the depressions in the transfer belt can be made longer than the absolute minimum required, and chevron shaped on the surface.

An alternative to the modification of the transfer belt, described above, is to provide translational motion to the impression rollers. An arrangement with four print drums is shown in figure 6. In this arrangement the impression rollers 6, 7, 8 and 9 under the print drums 31, 32, 33 and 34, are positioned so that the transfer belt 10 is carried along a sequence of segments, with some curvature around the impression roller at each roller. This applies also to the first roller 6 because of the position and size of the transfer belt roller 12, and the last roller 9 because of the position and size of the transfer belt roller 11. This condition is for the position of the impression rollers when the transfer belt is in contact with the printing surface of the print drum. As the machine operates and the print drums rotate, the clamping mechanism 25 on each drum will reach the proximal position to the transfer belt, that is, at the bottom of the print drums as shown. The impression roller may be moved away from the corresponding print drum so that no parts of the print drum, including the clamping mechanism, touch the transfer belt. When an impression roller 6 to 9 is moved down, it is assumed that the transfer belt 10 will follow the movement sufficiently, for example, by means of tension in the transfer belt and the segmental path of the belt.

For the printing operation from a print drum to the transfer belt, the impression roller will keep the transfer belt in contact with the cylindrical printing surface of the print drum. In the arrangement shown in figure 6, with four print drums, it can be seen that the impression roller 6 holds the transfer belt 10 in contact with the first drum 31, and the belt can be printed. The second print drum 32 is at a different rotational phase to the first, corresponding to the length of the transfer belt between the two. For this drum 32, the impression roller 7 is moved down, and the transfer belt with it. It can be seen that the clamping mechanism on drum 32 is close to the bottom of the drum, but that the transfer belt 10 is clear of the drum and the clamping mechanism. The next, third, drum 33 is in contact with the transfer belt because the impression roller 8 is in the printing position. The phase of the fourth drum 34 is such that the clamping mechanism is just passed the lowest position, and the drum is not in contact with the belt. The impression roller 9 for this drum would be raised before the start of the image area on this drum. For any screen printer it is necessary to be able to change the screens, since these hold the image to be printed. For conventional screen printers the rotary screens may often be lifted out vertically, and for conventional rotary screens in this machine, the same method of screen removal may be used. In this case the transfer belt may be arranged as shown in figures 1 and 2, with the upper printable surface in a straight line, and there will be no need to provide for translational movement of the impression rollers under the screen printing cylinders. The type of print drum as shown in figure 3 however is often inserted and withdrawn from the printer horizontally. The print drum could be removed and inserted into the machine vertically, as for conventional screens, but this may be complex mechanically. For this type of drum as shown in figure 3 to be inserted and removed horizontally, the cylindrical surface of the print drum must not be in contact with the transfer belt, as this will damage the film screen, and there may be further protruding mechanical parts on the ends of the print drum which inhibit removal. Several options are possible. Firstly, for the option above in which the transfer belt 30 has clearance depressions 29, a print drum may only be removed when the machine is stopped and a depression 29 in the transfer belt coincides with the print drum to be removed, such as the right hand print drum 32 in figure 5. The depression

in the transfer belt must be sufficiently wide and deep to clear the mechanical parts of the print drum, as well as eliminating contact between the printing surface of the drum 32 and the transfer belt 30. .The second option is for a machine with a transfer belt 10 with a smooth surface, as in figures 1, 2 and 6, and a segmental geometry as in figure 6. In this case one or more of the impression rollers 6 to 9, may be lowered away from the print drum, as they are during a phase of the printing operation, so that the transfer belt moves clear of the print drum. If the translational motion of the impression rollers can be controlled independently from the printing phase, a print drum may be removed at any rotational position of that drum. This may be desirable. It would of course be possible to combine the two options above, with a transfer belt with suitable depressions for printing clearance, and impression rollers which could be lowered for print drum removal.

Because of the change in conditions in this machine, i.e., that the ink is printed first onto a transfer belt and then transferred to the substrate, it may be beneficial to add further processes to control image degradation and assist final transfer of the image to the web substrate.

The first possible addition is to provide some degree of drying of the image between print screens or drums to limit set off on subsequent cylinders. An arrangement for this is shown in figure 7 for conventional print cylinders, with driers 35, 36 and 37 over the transfer belt after each print cylinder. The last drier 37 may not be needed since transfer to the substrate follows, but if it is present, it would produce a more uniform image consistency. This last drier 37 can also be used to maintain an elevated image temperature to assist the subsequent transfer to the substrate. The same arrangement is applicable to print drums as in figure 3, with plastic film screens. It is likely to be beneficial to transfer the image from the transfer belt to the substrate with the assistance of heat, depending on the type of ink and the nature of the substrate. If the ink used has a thermoplastic condition after printing on the transfer belt, then it can be softened by heat for the transfer to the substrate. For substrates which have a mechanically open surface, such as many types of paper and textiles, the pressure applied for this transfer will force the plastic ink film into the substrate surface. In this case, ink flow may be improved by increasing the temperature of the

substrate before the transfer, such as by a heater positioned as 39 in figure 7. Adhesion to the substrate surface would be increased if, for a thermoplastic ink, the ink is chilled during the transfer, on the substrate side of the ink film. This could be implemented by preheating the substrate with a heater such as 39, and chilling the substrate by cooling the pressure roller 19. The amount of ink transferred to the substrate will depend on the relative magnitudes of the adhesive force to the transfer belt and to the substrate. The cohesive forces within the image must be sufficient during transfer to maintain the image integrity. Ideally the adhesive force between the image and the transfer belt is reduced to a very small value for maximum transfer. The transfer belt may be heated for transfer by conduction of heat from the impression roller 11. Again, depending on the ink, the outer surface of the transfer belt could be heated by induction very quickly if the surface of the transfer belt is made to be suitably electrically conductive. The time required to transfer the image from the transfer belt to the substrate may be a critical factor. However, the contact time between the transfer belt surface and the substrate during the image transfer for this machine can be extended by using a relatively soft pressure roller 40 behind the substrate, which will conform to the surface of the impression roller. This is illustrated in figure 8 which shows a detailed view of this part of the machine with this option.