Method and means for printing with a variable repeat length

Abstract

 A method of printing a succesion of images on to an elongate web (10) comprises feeding the web past a rotating print cylinder (12), the relative motion of the cylinder and the web being controlled to vary the repeat distance between adjacent images. The print cylinder (12) rotates at constant speed and the movement of the web (10) is periodically interrupted, thus controlling the repeat distance.

Description

[0001] The invention relates to printing, and particularly to the printing of a succession of images on to an elongate web.

[0002] Such printing is commonly done by feeding the web past a rotating print cylinder which bears a printing plate. However when this method is used, the repeat distance, i.e. the distance,between any point on one image and the identical point on the succeeding image, is of course dependent on the circumference of the print cylinder. Thus if one wishes to vary the repeat distance, for example in order to change from the printing of a succession of images of one size to the printing of a succession of larger or smaller images, it is necessary to change the cylinder.

[0003] Changing of the print cylinder is commonly carried out in practice, but because of the time taken to change the cylinder, it is normally only economic to employ this method of printing when long print runs are involved. With short print runs, which would involve frequent changing of the cylinder, it has been more economic up to now to employ a different printing technique altogether, for example by printing on to separate sheets using a hand-fed machine.

[0004] We have now appreciated that it is possible to overcome or reduce the problems caused by the changing of the print cylinder.

[0005] The invention provides a method of printing a succession of images on to an elongate web, comprising feeding the web past a rotating print cylinder, characterised in that the relative motion of the cylinder and the web is controlled to vary as desired the repeat distance between adjacent images.

[0006] Preferably the print cylinder is rotated at a constant speed and the movement of the web past the print cylinder is periodically interrupted, the length of the interruptions controlling the repeat distance.

[0007] The interruptions are preferably controlled by switching means associated with the print cylinder.

[0008] For example switch means may be mounted adjacent to the print cylinder, there being switch actuating means on the print cylinder. There may be two actuating devices spaced circumferentially apart around the print cylinder, the angular distance between the switch actuating devices controlling the length of the interruptions and the angular distance between the two actuating devices being adjustable.

[0009] Alternatively, means may be provided to control the length of web which is fed past the print cylinder between successive interruptions.

[0010] Preferably the means comprise drive means for the web and means to control the movement of the drive means.

[0011] The drive means may comprise at least one roller for driving the web, and means operable to permit the roller to rotate by a preselectable amount.

[0012] A rotatable switch device may be provided which is arranged to rotate by a predetermined amount, e.g. one revolution, between interruptions, the roller being connected to the switch device by a variable speed gearbox, so that the amount by which the roller rotates during the predetermined rotation of the switch device can be controlled as desired.

[0013] Alternatively means may be provided to generate signal pulses as the roller rotates, there being means to stop rotation of the roller after a predetermined number of pulses has been generated.

[0014] For example the roller may be rotatable with a disc having a series of holes spaced apart circumferentially around the disc, a generator and receiver being respectively arranged at opposite sides of the disc so that each time a hole passes the generator and receiver a signal is generated.

[0015] If it is desired to print one or more subsequent colours on to an initial set of images, requiring one or more further runs of the web past the print cylinder, interruption of the movement of web during subsequent runs is preferably carried out by sensing the position of the initial images, for example using a light sensitive device.

[0016] The invention includes a printing machine comprising a rotatable print cylinder, means for feeding an elongate web past a print cylinder when rotating to receive a succession of images, and means to control relative movement between the print cylinder and the web to vary the repeat distance between adjacent images.

[0017] By way of example, specific embodiments of.. the invention will now be described, with reference to the accompanying drawings, in which:-

Figure I is a schematic side view of part of one embodiment of printing machine according to the invention;

Figure 2 is a schematic side view of part of a second embodiment of printing machine according to the invention;

Figure 3 is a plan view of a drive roller of the embodiment shown in Figure 2; and

Figure 4 is a plan view of a drive roller of another embodiment.

[0018] The embodiments of the invention described are concerned with a reel to reel printing machine for applying printed matter to a succession of plastics (e.g. polythene) or paper bags whilst the bags are still in web form. A portion of web is illustrated at 10 in Figure 1 and the machine has a pair of nip rollers 11 for driving the web from right to left as viewed in the Figure. The payout reel and the takeup reel of the machine, together with their associated equipment, are conventional and are not therefore.shown or described in detail.

[0019] Printing is carried out by means of a rotatable print cylinder 12 and a co-operating impression cylinder 13. Printing blocks such as 14 can be attached to the print cylinder 12 so that as the web 10 is moved past the rotating cylinder 12 the printing plate 14 applies an image to the web 10.

[0020] According to the invention it is only necessary to use one size of print cylinder 12, even if different lengths of bag are being printed.

[0021] If a printing plate is being used which extends around substantially the entire circumference of the print cylinder 12 to print the greatest length of bag with which the machine will cope, the print cylinder 12 and nip rollers 11 are run continuously.

[0022] If the machine is being used to print bags which have a length shorter than that of the maximum with which the machine will cope, a smaller printing plate is used, such as that shown in Figure 1 at 14, and after each bag has been printed, movement of the web 10 is interrupted, by stopping the nip rollers 11, until the printing plate 14 travels around to the point where it is ready to print the next succeeding bag.

[0023] The machine is shown in Figure 1 at the start of printing a first bag. The web 10 is stationary and the leading edge 14a of the printing plate 14 is approaching the web.

[0024] In order to operate the machine according to the invention it is necessary to provide means for intermittently driving the nip rollers 11.

[0025] A proximity switch 15 is positioned at a fixed point adjacent the print cylinder 12 and as shown in Figure 1 an activating device 16, fixed to the print cylinder, is approaching the switch 15.

[0026] The activating device 16 reaches the switch 15 slightly before the leading edge 14a of the printing plate reaches the web 10. The effect of the device 16 on the switch 15 is to commence the rotation of the nip rollers 11 which then drive the web 10 past the print cylinder 12 thus causing the printing plate 14 to make contact with the web and print one bag.

[0027] As the trailing edge 14b of the printing plate 14 leaves the web another activating device, in the form of the end 17 of an arm 18 passes a second proximity switch 20 and the signal generated is used to stop the nip rollers 11. The web 10 thus waits until the leading edge 14a of the printing plate 14 comes around again, whereupon the cycle repeats itself.

[0028] Thus any length of bag, up to the maximum, can be printed without removing the print cylinder 12. The angular position of the arm 18 with respect to the device 16 is adjustable so that the length of time for which movement of the web is interrupted can be adjusted to suit different sizes of printing plates.

[0029] If it is desired to run the web past the print cylinder for one or more further runs, in order to print one or more additional colours, then with a view to ensuring synchronisation of each subsequent colour image with the associated first colour image, interruption of the movement of the web is controlled not by the arm 18, but by a light sensitive device 19 positioned in the path of the web. The device 19 senses the leading edge of one of the images and stops the web. Re-starting is carried out as before, the device 16 actuating the switch 15 as the leading edge 14a of the printing plate approaches the web. The position of the device 19 is adjustable in the direction of movement of the web to suit different lengths of image but fine . adjustment is carried out by adjusting the position of the switch 15.

[0030] In order to ensure that the device 19 responds only to a leading edge, it is only energised momentarily, and this energisation can be carried out by arranging a third switch 21 adjacent the path which is followed by the end 17 of the arm 18. When the arm passes the switch 21 it momentarily energises the device 19.

[0031] The cylinders 12 and 13 can be continuously driven from the main drive shaft (not shown) via chains (not shown). In the second embodiment of the invention shown in Figure 2, the main drive shaft is also used to transmit drive continuously by means of a chain (not shown) to a conventional clutch brake device 22. The device 22 is connected to the lower nip roller 11 by means of a chain 23 and the lower roller 11 drives the upper roller 11 via a gear device which is not visible in Figure 2.

[0032] A proximity switch 15 is positioned at a fixed point adjacent to the print cylinder 12 and as shown in Figure 2 an activating device 16, fixed to the print cylinder, is approaching the switch 15.

[0033] The activating device 16 reaches the switch 15 somewhat before the leading edge 14a of the printing plate reaches the web 10. The effect of the device 16 on the switch 15 is to engage the clutch 22 to transmit drive to the nip rollers 11. They then drive the web 10 past the print cylinder 12 thus causing the printing plate 14 to make contact with the web and print one bag.

[0034] As the trailing edge 14b of the printing plate 14 leaves the web the clutch of the device 22 is disengaged and the brake is engaged to stop the nip rollers 11.

[0035] The web 10 thus waits until the leading edge 14a of the printing plate 14 comes round again, whereupon the effect of device 16 on switch 15 causes the cycle to repeat itself.

[0036] To closely control the rotation of the nip rollers 11 between interruptions, and compensate for any slip that may occur when the clutch of the device 22 is first engaged, one of the rollers 11 is connected to a rotatable switch device 24 via a variable speed gear box 25. The device 24 comprises a disc with a switch 26 adjacent its periphery. The device is such that once rotation of . the disc 24 is commenced the disc will perform one revolution and then emit a signal to stop the nip rollers. By adjusting the variable speed gear box using control 27, the extent to which the nip rollers 11 rotate, for one rotation of the disc 24, can be adjusted as desired. When the machine is initially set up to carry out a print run for a particular size of bag, the control 27 can be used to adjust the extent to which the nip rollers 11 revolve during one cycle, and hence the length of web that is fed during one cycle, to make the length of web which is fed suit the size of the printing plate 14. The measurement of the length of web that is fed is thus carried out in effect by the disc 24 and as rotation of this disc does not commence until rotation of the nip rollers 11 commences, any slip of the clutch of the device 22 does not effect the length of web that is fed.

[0037] Figure 4 illustrates an alternative embodiment in which one of the nip rollers 11 is rotatable with a disc 28 which has a plurality of holes 29 therethrough, the holes being spaced apart around the periphery of the disc. On opposite sides of the disc are mounted a transmitter 30 and receiver 31. They may for example comprise a photo cell and light source, or an infra-red transmitter and receiver, or the like. Thus whenever a hole 29 passes the transmitter and receiver, a signal pulse is generated. A digital counter (not shown) is provided which is operable to stop the nip rollers 11 after a preselected number of pulses has been generated.

[0038] If it is desired to run the web past the print cylinder for one or more further runs, in order to print one or more additional colours, then the devices shown in Figures 3 and 4 are only required for the printing of the first image. Thereafter, movement of the web can be controlled by sensing the leading edges of the first images as described in the first embodiment of the invention.

[0039] The invention is not restricted to the details of the foregoing embodiments.

Claims

1. A method of printing a succession of images on to an elongate web (10), comprising feeding the web past a rotating print cylinder (12), characterised in that the relative motion of the cylinder and the web is controlled to vary as desired the repeat distance between adjacent images.

2. A method as claimed in Claim 1, in which the print cylinder (12) is rotated at a constant speed and the movement of the web (10) past the print cylinder is periodically interrupted, the length of the interruptions controlling the repeat distance.

3. A method as claimed in Claim 2, in which the interruptions are controlled by switching means (15) associated with the print cylinder (12).

4. A method as claimed in Claim 3, in which switch means (15) are mounted adjacent to the print cylinder (12), there being switch actuating means (16) on the print cylinder.

5. A method as claimed in Claim 4, in which two actuating devices (16, 17) are spaced circumferentially apart around the print cylinder (12), the angular distance between the switch actuating devices (16, 17) controlling the length of the interruptions and the angular distance between the two actuating devices being adjustable.

6. A method as claimed in any one of Claims 2 to 5, in which means are provided to control the length of web (10), which is fed past the print cylinder (12) between successive interruptions.

7. A method as claimed in Claim 6, in which the means comprise drive means (11, 22, 23) for the web (10) and means to control the movement of the drive means.

8. A method as claimed in Claim 7, in which the drive means comprise at least one roller (11) for driving the web (10), and means operable to permit the roller to rotate by a preselectable amount.

9. A method as claimed in Claim 8, in which a rotatable switch device (24) is provided which is arranged to rotate by a predetermined amount between interruptions, the roller (11) being connected to the switch device by a variable speed gear box (25), so that the amount by which the roller rotates during the predetermined rotation of the switch device can be controlled as desired.

10. A method as claimed in Claim 8, in which means are provided to generate signal pulses as the roller (11) rotates, there being means to stop rotation of the roller after a predetermined number of pulses have been generated.

11. A method as claimed in Claim 10, in which the roller (11) is rotatable with a disc (28) having a series of holes (29) spaced apart circumferentially around the disc, a generator (30) and receiver (31) being respectively arranged at opposite sides of the disc so that each time a hole (29) passes the generator (30) and receiver (31) a signal is generated.

12. A method as claimed in any one of Claims 2 to 11 when used to print one or more subsequent colours on to an initial set of images, requiring one or more further runs of the web (10) past the print cylinder (12), interruption of the movement of the web during subsequent runs being carried out by sensing the position of the initial images.

13. A method as claimed in Claim 12, in which the sensing of the position of the initial images is by means of a light sensitive device (19).

14. A printing machine comprising a rotatable print cylinder (12), means for feeding an elongate web (10) past a print cylinder when rotating to receive a succession of images, and means to control relative movement between the print cylinder and the web to vary the repeat distance between adjacent images.

Drawing