GB2457902A

GB2457902A - Seal release apparatus and method for removing printing plate from plate cylinder using compressed air jet.

Info

Publication number: GB2457902A
Application number: GB0803577A
Authority: GB
Inventors: Tony Hollings; Bob Goodman
Original assignee: Goss Graphic Systems Ltd
Current assignee: Goss Graphic Systems Ltd
Priority date: 2008-02-27
Filing date: 2008-02-27
Publication date: 2009-09-02
Also published as: GB0803577D0; WO2009106614A1; CN101970234A; JP2011513091A; EP2282892A1

Abstract

A method of releasing a seal formed between a used printing plate and a surface of a plate cylinder of a printing press prior to removal of said printing plate from said surface comprises directing a jet of compressed air towards the surface of the plate cylinder between adjacent printing plates.

Description

A method of, and apparatus for, releasing a seal formed between a used printing plate and a surface of a plate cylinder of a printing press

Description

The present invention relates to a method of releasing a seal formed between a used printing plate and a surface of a plate cylinder of a printing press prior to removal of said printing plate from said surface so as to facilitate the removal of said plate. The invention also relates to apparatus for achieving the method of the invention and to a printing press comprising said apparatus.

A web offset printing press comprises a number of printing units. Each unit is designed to print matter onto a continuous web of paper as it travels through the print unit. The printed web, emerging from each print unit, come together in a folding unit which orientates, folds and cuts each web to produce the finished article such as a newspaper or magazine. Each unit contams at least one pair of cylinder groups or print couples comprising a rotatably mounted plate cylinder, to which one or more printing plates are attached for printing and, a rotatably mounted blanket cylinder. The printing unit may incorporate a shaft less drive system in which each cylinder group is driven by its own drive motor that directly drives one of the cylinders of a group via a belt or gear drive and the other cylinder of that group is mechanically coupled to the driven cylinder. An inking system associated with each print couple is operable to feed ink onto the printing plates attached to the plate cylinder as the plate cylinder rotates. As the cylindrical surfaces of the plate and blanket cylinder are in rolling contact, an inked image is transferred from the printing plates onto the blanket cylinder and from the blanket cylinder onto the medium to be printed.

In large scale high volume presses used, for example, in the production of newspapers, multi-colour printing is achieved by providing each print unit with a number of printing couples arranged in pairs which are mounted vertically above one another in a stack so that the paper web travels in a vertically upward direction between each pair of print couples. A unit having four print couple pairs, i.e. eight print couples, is able to print up to four colours on each side of the paper web and is often referred to as a "four-high" unit.

When a press is prepared for a print run, at least one printing plate carrying the image to be printed must be mounted to the plate cylinder of each print couple. Generally, each plate cylinder includes at least one recess or lock-up slot extending longitudinally along the surface of the plate cylinder into which is received the leading and trailing edge of a printing plate. A mechanism located in the recess engages the leading and trailing edges of an inserted printing plate and firmly holds the printing plate in position during a print run. The number of printing plates that must be attached to the plate cylinder of each print couple depends on the width of the press and whether the plate cylinder carries one or two printing plates around its circumference, i.e. whether it is a "one around" or "two around" plate cylinder. For example, in a double width one-around press, the plate cylinder may carry four printing plates across its width and one printing plate extending circumferentially around its cylindrical surface. Therefore, there could be as many as eight printing plates in a single printing couple pair and thirty-two printing plates in a four high printing unit all of which must be replaced before a new print run can be initiated. When the plate cylinder is of the "two-around" type, the number of printing plates is doubled accordingly. Therefore, it will be appreciated that even in the production of just one newspaper issue, a very large number of printing plates will be required.

In a conventional printing machine, the press operator manually removes used printing plates from the plate cylinders, although automatic printing plate removal systems are also known. One such system is known from the Applicant's own earlier application W006/120171 Al. One method of removing plates manually from a printing cylinder is for a press operator to attach a plate gripping element like a suction cup to the surface of the printing plate to be removed. The plate gripping element has a handle by which the plate is pulled off the plate cylinder surface. Similarly, in an automatic plate removal system, plate gripping elements depending from a plate unloading head grip the surface of the printing plate to be removed, with the intention of pulling the plate off the surface of the plate cylinder when the plate loading head is raised away from the surface of the plate cylinder.

Irrespective of whether the operator manually removes printing plates from the plate cylinder or an automatic plate removal process is employed, it is often difficult to quickly and easily remove a used printing plate from a plate cylinder. This can substantially increase the time taken to carry our a plate-change operation before the press, or at least individual print units, can be put back into service, thereby having a significantly detrimental affect on the overall productivity of the press.

Printing plates tend to adhere to a plate cylinder, especially when they have been bedded down onto the plate cylinder by printing pressures applied to it during a print run. This is largely because a solution of water and gum arabic is applied to both front (j)rinting surface) and back surfaces of the plate to form a protective film that remains in-situ after the water has evaporated. The gum arabic prevents oxidation of the aluminium printing plate in the interval between the processing of the plate and its use on the printing press. The gum arabic film remaining on the back of the printing plate acts as a mild adhesive that sticks or seals the printing plate to the surface of the plate cylinder. This effect is made worse by the printing pressure and heat that is generated during a print run that tends to make the gum arabic film even more tacky and effectively "bakes" the printing plate in place on the plate cylinder. This problem is compounded by the presence of residual gum arabic that has built up within the lock-up slots.

Even without the problems caused by the gum arabic film, the printing forces also act to force air out from between the printing plate and plate cylinder during printing, thereby creating a vacuum seal between the printing plate and plate cylinder. When the effect of this seal and the seal caused by the presence of gum arabic film are combined with the forces required to overcome the lock-up clamping or frictional spring force that holds the leading and trailing edges of the printing plate within their respective lock-up slots, the difficulties experienced in removing used printing plates from a plate cylinder of a printing press are exacerbated.

Due to the aforementioned issues, the force which the press operator must apply to the plate gripping element to pull a printing plate off the plate cylinder may be excessive and/or beyond his ability. Alternatively, the force required may exceed the force generated between the plate gripping element and the surface of the plate cylinder causing the grip to be broken without lifting the printing plate. Even if an automatic plate loading system is used, the gripping force applied must be high, otherwise they simply detach from the plate when the plate loading head is lifted.

The present invention seeks to provide a method and apparatus that substantially overcomes or alleviates the problems discussed above and enables easier and faster removal of printing plates from the surface of a plate cylinder of a printing press.

According to the invention, there is provided a method of releasing a seal formed between a used printing plate and a curved surface of a plate cylinder of a printing press to facilitate removal of said printing plate from said surface, the method including the step of directing a jet of compressed air towards the surface of the plate cylinder adjacent to an edge of a printing plate mounted on said cylinder.

The edge of the printing plate is preferably the longer edge that extends in a circumferential direction around the curved surface of the plate cylinder.

In a preferred embodiment, the method includes the step of directing said jet of compressed air towards the surface of the plate cylinder between edges of adjacent printing plates.

The plate cylinder may have a trailing edge lock-up slot in which is received the trailing edge of a printing plate, and the method preferably includes the step of directing said jet of compressed air towards the surface of the plate cylinder between adjacent printing plates at location circumferentially spaced away from said trailing edge lock-up slot.

In one embodiment, the method includes the step of directing the jet of compressed air in a direction towards the plate cylinder at an angle which is substantially perpendicular to the axis of the plate cylinder. However, in an alternative embodiment, the method includes the step of directing the jet of compressed air in a direction towards the plate cylinder at an angle which is between 0 and 90 degrees to the axis of the plate cylinder.

The method may involve directing multiple pulses of compressed air towards the surface of the plate cylinder.

In one embodiment, the printing press comprises a plate removal apparatus and at least one nozzle mounted to the plate removal apparatus connected to a supply of compressed air through which the jet is emitted. In this case, the method may include the step of moving the plate removal apparatus into a position in which the or each nozzle is positioned above the surface of the plate cylinder between adjacent printing plates.

The plate removal apparatus may comprise a plurality of plate gripping elements movable into contact with the surface of a printing plate to be removed from the plate cylinder. If so, the method preferably includes the step of directing a jet of compressed air towards the surface of the plate cylinder between adjacent printing plates prior to moving said plate gripping elements into contact with the surface of a printing plate.

It is possible for there to be multiple nozzles mounted to the plate removal apparatus. The method may then include the step of firing said nozzles consecutively or simultaneously.

The method may include the step of directing a jet of compressed air towards the surface of a plate cylinder for a controlled period of time of typically one second or less.

According to another aspect of the invention, there is provided apparatus for releasing a seal formed between a used printing plate and a surface of a plate cylinder of a printing press to facilitate removal of said printing plate from said surface, said apparatus comprising a nozzle for directing a jet of compressed air towards the surface of a plate cylinder adjacent to an edge of a printing plate.

Preferably, said edge of the printing plate is an edge that extends in a circumferential direction around the curved surface of the plate cylinder.

In a preferred embodiment, the nozzle is configured to direct said jet of compressed air towards the surface of the plate cylinder between edges of adjacent printing plates.

In one embodiment, the plate cylinder has a trailing edge lock-up slot in which is received the trailing edge of a printing plate, the nozzle being positioned relative to a plate cylinder so as to direct a jet of compressed air towards the surface of said plate cylinder between adjacent printing plates at location circumferentially spaced away from said trailing edge lock-up slot.

The nozzle may be configured to direct a jet of compressed air in a direction towards a plate cylinder at an angle, said angle being substantially perpendicular to the axis of said plate cylinder.

The apparatus may include means to control the nozzle to direct multiple pulses of compressed air towards the surface of a plate cylinder.

According to another aspect of the invention, there is provided a printing press comprising apparatus for releasing a printing plate according to the invention.

The printing press can include a plate cylinder and apparatus for removing printing plates from said plate cylinder, the nozzle being mounted to the plate removal apparatus such that the nozzle is moved into a position in which the or each nozzle is positioned above the surface of the plate cylinder between adjacent printing plates when the plate removal apparatus is moved into a plate removal position adjacent to a plate cylinder of the printing press.

In one embodiment, the plate removal apparatus comprises a plurality of printing plate gripping elements movable into contact with the surface of a printing plate to be removed from the plate cylinder after a jet of compressed air has been directed towards the surface of the plate cylinder between adjacent printing plates.

The printing press may comprise multiple nozzles mounted to the plate removal apparatus, each nozzle being located above the plate cylinder between adjacent printing plates when the plate removal head is moved into a plate removal position above the plate cylinder.

In a modified embodiment, the plate cylinder comprises a groove or scalloped surface that extends from beneath a printing plate towards an adjacent printing plate mounted on said plate cylinder, said groove being configured to direct the jet of compressed air from said nozzle under the printing plate that extends over said groove.

Embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings, in which: FIGURE 1 illustrates a side sectional view of a printing plate unloading head disposed above the surface of a plate cylinder of a printing press to which a printing plate is mounted; FIGURE 2 illustrates a top view of the plate cylinder shown in Figure 1 having four printing plates mounted thereto in side-by-side relation, but with the plate loading head omitted; FIGURE 3 illustrates a section along line X-X in Figure 2; FIGURE 4 illustrates an alternative section, showing a modified plate cylinder, along line X-X in Figure 2; and FIGURE 5 illustrates a section along line X-X of Figure 2 using angled compressed air nozzles.

Although an embodiment of the invention is primarily described with reference to printing plate removal using an automatic plate unloading system, it will be appreciated that the invention is equally applicable to manual plate removal.

Referring now to the drawings, there is shown in Figure 1 a portion of a plate cylinder 1 and blanket cylinder 2 forming a single print couple of a printing unit which is equipped with an automatic plate removal apparatus comprising a plate removal head 3 that extends across the width of the plate cylinder 1. It will be appreciated that the plate removal head 3 may also be used in an automatic or semi-automatic plate loading procedure to mount unused printing plates in position on the plate cylinder of the press prior to initiation of a print run. The plate removal head 3 can be moved using compressed air between a raised position, in which it is spaced away from the plate cylinder I when no plate unloading or loading operations are being performed and the printing unit is in use and, a lowered position for carrying out printing plate loading or unloading operations on the plate cylinder 1.

The plate removal head 3 mcludes an elongate support frame 5 that extends substantially across the entire width of the plate cylinder 1. An array of plate gripping elements 6 are mounted to and depend from the support frame 5 so as to face the curved cylindrical surface I a of the plate cylinder I. When the plate gripping elements 6 are activated, they are operable to engage the surface of a printing plate 7, or set of printing plates 7 placed side-by-side across the width of the plate cylinder I (see Figure 2), to enable the printing plates 7 to be manipulated by the plate removal head 3. The plate removal head 3 may also include a pusher element 8 that is independently movable relative to the support frame 5 and to the plate gripping elements 6 in a direction towards and away from the surface I a of the plate cylinder 1. The pusher element 8 is generally only used in a plate loading procedure and so no further reference to it will be made here.

When a printing plate 7, or a set of printing plates 7, such as those shown in side-by-side relation in Figure 2, are to be removed from the plate cylinder 1, the plate cylinder I is indexed into a position in which the trailing edges 7a of the printing plates 7 are proximal to the plate removal head 3. This preferred position is as shown in Figure 1.

In a conventional plate removal procedure, such as that described in the Applicant's own earlier PCT application published as W006/120171 Al, the plate removal head 3 is lowered so that the plate gripping elements 6 contact the surface of the printing plates 7 adjacent to their trailing edges 7a and the plate gripping elements 6 are then activated to grip the printing plates 7. The plate removal head 3 is then lifted (in the direction of arrow in Figure 1), with the intention of pulling or lifting the printing plates 7 off the surface 1 a of the plate cylinder 1 and the trailing edges 7a of the printing plates 7 out of their lock-up slot 9 in the plate cylinder 1. Instead of lifting the entire print head 3, it is also possible for the plate gripping elements 6 to be retractably mounted to the print head 3 so that they retract relative to the print head 3 to lift the trailing edges 7a of the printing plates 7 out of the lock-up slot 9.

Once removal of the plate trailing edge 7a has been achieved, the plate cylinder 1 is rotated (in the direction of arrow "Y" in Figure 1) until the leading edge (not shown) is located beneath the plate removal head 3 for removal, or at least partial removal, of said leading edge using the suction elements 6. If the leading edge is only partially removed from its lock up slot, the plate cylinder I is rotated once again so that the leading edge is fully removed from its lock up slot by a plate extraction element (not shown) that contacts the partially removed leading edge as the plate cylinder I rotates and leaves the printing plates 7 hanging by their leading edges on the extraction element.

As previously described, complications occur due to the difficulty of initially lifting the printing plates 7 off the surface of the plate cylinder I to cause the trailing edge 7a to slide out of its lock-up slot 9 due to the presence of arabic gum, which has effectively caused the printing plates 7 to adhere to the surface of the plate cylinder 1 during a print run and due to the vacuum that is created as a result of the plates 7 bedding in -10 -against the plate cylinder 1. However, the Applicant's have devised a way of overcoming this problem which will now be described.

The plate loading head 3 is provided with a number of compressed air nozzles 10 spaced along its length (only one of which is shown in Figure 1). These nozzles 10 are located on the plate removal head 3 so that, when the plate cylinder 1 has been indexed into its trailing edge loading position, as shown in Figure 1, the compressed air nozzles are positioned directly above the circumferentially extending long edges 7b of two adjacent printing plates 7 close to, but preferably not directly above the trailing edge lock-up slot 9. Ideally, the nozzles 10 are spaced in a circumferential direction away from the trailing edge 7a of the plates 7 by a short distance in the region of between 30 to 200mm.

During a plate removal procedure and prior to lowering of the plate removal head 3 and activation of the plate gripping elements 6, a jet of compressed air (indicated by "J" in Figure 1) is emitted from the nozzles 10 towards the printing plates 7 and impinges on the surface Ia of the plate cylinder 1 directly between the long edges 7b of the plates 1. It has been found that a short blast of compressed air directed towards the plate cylinder 1 in this way has the effect of breaking the seal between each adjacent plates 7 and the plate cylinder surface I a caused by, for example, the presence of arabic gum and/or a vacuum caused by bedding down of the plates 7 onto the surface I a of the plate cylinder 1. The compressed air jet need only last a fraction of a second to fulfil its purpose of loosening the plates 7 so that, when the plate gripping elements 6 are subsequently activated, they can easily lift the plates 7 off the surface 1 a of the plate cylinder I and draw the trailing edges 7a out of their lock-up slots 9. Although it is envisaged that compressed air will be used, it is also possible that some other compressed gas could be fired from the nozzles 10 instead. It is also possible to direct a jet of compressed air into the lock-up slot, rather than towards the plate cylinder at an orientation spaced from the lock-up slot.

Figure 2 shows a plan view of the plate cylinder I shown in Figure 1, but with the plate loading head 3 omitted for clarity. The nozzles 10 are positioned such that the air jet -11 - impinges the plate cylinder I between adjacent plates 7 at the points marked "A". It has been found that, in a press having four printing plates 7 extending in side-by-side relation across the width of the plate cylinder 1, two nozzles 10, positioned with one between the two left hand plates and one between the two right hand plates, as shown in Figure 2, is sufficient to break the seal between each printing plate 7 and the plate cylinder 1, although it will be appreciated that any number of nozzles 10 may be employed.

As shown more clearly in Figure 3, the jet of compressed air emitted by each nozzle 10 impinges against the plate cylinder I in the circumferential gap 11 between printing plates. Although most of that air blast will be re-directed, as indicated by arrows "C", back out of the gap 11, at least some of that air forces its way under the printing plates 7 (in the direction of arrow "D"). This air flows under the plates 7 and thereby loosens their adherence to the plate cylinder 1.

With reference to Figure 4, the effectiveness of the air jet can be improved if small grooves or scallops 12 are formed in the surface of the plate cylinder 1 to re-direct the air jet up against the underside of the printing plates 7, as shown by arrows "E" in Figure 4. However, it has been found that the air jet is sufficent to release the plates even if no modification to the plate cylinder 1 is made. It has also been found that an air jet positioned as described above, so as to be close to one corner of each of two adjacent plates 7 is sufficient to achieve the desired release of the plates 7 from the surface of the plate cylinder 1.

It is envisaged that only a single pulse of compressed air from a nozzle 10 is necessary to release the plates 7. However, it is also possible to provide a sequence of pulses of compressed air to each nozzle 10 and/or to rotate the plate cylinder 1 whilst the jet is operational. It is also possible to supply compressed aix to all the nozzles 10 simultaneously or, one after the other so as to release the plates 7, or pairs of plates, in sequence.

-12 -As mentioned above, the present invention is also applicable to manual plate removal.

In a manual procedure, an operator may simply pass a compressed air hose across the surface of the printing plates 7 so that a jet of compressed air is directed in roughly the same place as the jet of air emitted from the nozzle 10 attached to the plate removal head 3, prior to placing a plate gripping elements 6 on each printing plate 7 and pulling the plate off the surface of the plate cylinder 1.

Although it is envisaged that the jet of compressed air will be fired in a substantially radial direction towards the surface of the plate cylinder and printing plates, i.e. so that the air jet impinges at an angle of substantially 90 degrees to the surface of the plate cylinder I immediately below the nozzle 10, as shown in Figures 3 and 4, it is possible for the nozzles 10 to be angled so that the air jet impinges against the surface at an angle of less than 90 degrees, as shown in Figure 5. If the nozzles 10 are angled in this way, the jet is more closely directed towards the edge of a printing plate 7 where it meets the surface of the plate cylinder 1. However, it will be appreciated that a second nozzle 10 is then required which is angled towards the edge of the adjacent printing plate 7, when an automatic plate removal apparatus is used.

Many modifications and variations to the invention falling within the scope of the appended claims will be apparent to those skilled in the art and the foregoing description should be regarded as a description of the preferred embodiments only.

Claims

Claims 1. A method of releasing a seal formed between a used printing plate and a curved surface of a plate cylinder of a printing press to facilitate removal of said printing plate from said surface, the method including the step of directing a jet of compressed air towards the surface of the plate cylinder adjacent to an edge of a printing plate mounted on said cylinder.
2. A method according to claim 1, wherein said edge of the printing plate is an edge that extends in a circumferential direction around the curved surface of the plate cylinder.
3. A method according to claim I or claim 2, wherein the method includes the step of directing said jet of compressed air towards the surface of the plate cylinder between edges of adjacent printing plates.
4. A method according to any of claims 1 to 3, wherein the plate cylinder has a trailing edge lock-up slot in which is received the trailing edge of a printing plate, the method including the step of directing said jet of compressed air towards the surface of the plate cylinder between adjacent printing plates at location circumferentially spaced away from said trailing edge lock-up slot.
5. A method according to any preceding claim, including the step of directing the jet of compressed air in a direction towards the plate cylinder at an angle which is substantially perpendicular to the axis of the plate cylinder.
6. A method according to any of claims I to 4, including the step of directing the jet of compressed air in a direction towards the plate cylinder at an angle which is between 0 and 90 degrees to the axis of the plate cylinder.
7. A method according to any preceding claim, including the step of directing multiple pulses of compressed air towards the surface of the plate cylinder.
8. A method according to any preceding claim, wherein the printing press comprises a plate removal apparatus and at least one nozzle mounted to the plate removal apparatus connected to a supply of compressed air through which the jet is emitted, the method including the step of moving the plate removal apparatus into a position in which the or each nozzle is positioned above the surface of the plate cylinder between adjacent printing plates.
9. A method according to claim 8, wherein the plate removal apparatus comprises a plurality of plate gripping elements movable into contact with the surface of a printing plate to be removed from the plate cylinder, the method including the step of directing a jet of compressed air towards the surface of the plate cylinder between adjacent printing plates prior to moving said plate gripping elements into contact with the surface of a printing plate.
10. A method according to claim 8 or claim 9, wherein there are multiple nozzles mounted to the plate removal apparatus and the method includes the step of firing said nozzles consecutively.
11. A method according to any preceding claim, including the step of directing a jet of compressed air towards the surface of a plate cylinder for a controlled period of time typically of one second or less.
12. Apparatus for releasing a seal formed between a used printing plate and a surface of a plate cylinder of a printing press to facilitate removal of said printing plate from said surface, said apparatus comprising a nozzle for directing a jet of compressed air towards the surface of a plate cylinder adjacent to an edge of a printing plate.-
13. Apparatus according to claim 12, wherein said edge of the printing plate is an edge that extends in a circumferential direction around the curved surface of the plate cylinder.
14. Apparatus according to claim 13, wherein the nozzle is configured to direct said jet of compressed air towards the surface of the plate cylinder between edges of adjacent printing plates.
15. Apparatus according to claim 14, wherein the plate cylinder has a trailing edge lock-up slot in which is received the trailing edge of a printing plate, the nozzle being positioned relative to a plate cylinder so as to direct a jet of compressed air towards the surface of said plate cylinder between adjacent printing plates at location circumferentially spaced away from said trailing edge lock-up slot.
16. Apparatus according to claim 14 or claim 15, wherein the nozzle is configured to direct a jet of compressed air in a direction towards a plate cylinder at an angle, said angle being substantially perpendicular to the axis of said plate cylinder.
17. Apparatus according to any of claims 14 to 16, including means to control the nozzle to direct multiple pulses of compressed air towards the surface of a plate cylinder.
18. A printing press comprising apparatus according to any of claims 14 to 17, including a plate cylinder and apparatus for removing printing plates from said plate cylinder, the nozzle being mounted to the plate removal apparatus such that the nozzle is moved into a position in which the or each nozzle is positioned above the surface of the plate cylinder between adjacent printing plates when the plate removal apparatus is moved into a plate removal position adjacent to a plate cylinder of the printing press.

-16 -
19. A printing press according to claim 18, wherein the plate removal apparatus comprises a plurality of printing plate gripping elements movable into contact with the surface of a printing plate to be removed from the plate cylinder after a jet of compressed air has been directed towards the surface of the plate cylinder between adjacent printing plates.
20. A printing press according to claims 18 or 19, comprising multiple nozzles mounted to the plate removal apparatus, each nozzle being located above the plate cylinder between adjacent printing plates when the plate removal head is moved into a plate removal position above the plate cylinder.
21. A printing press according to any of claims 18 to 20, wherein the plate cylinder comprises a groove or scalloped surface that extends from beneath a printing plate towards an adjacent printing plate mounted on said plate cylinder, said groove being configured to direct the jet of compressed air from said nozzle under the printing plate that extends over said groove.
22. A method substantially as hereinbefore described.
23. Apparatus substantially as hereinbefore described with reference to the accompanying drawings.