WO1989001866A1

WO1989001866A1 - Printing plate

Info

Publication number: WO1989001866A1
Application number: PCT/GB1988/000724
Authority: WO
Inventors: David Godden
Original assignee: Drg (Uk) Limited
Priority date: 1987-09-04
Filing date: 1988-09-02
Publication date: 1989-03-09
Also published as: GB8720815D0; GB9004893D0; JPH06503043A; EP0380512A1; AU2303388A; KR890701366A; GB2228895A; ES2010820A6

Abstract

A printing plate (2) which is inherently insufficiently rigid to be locked up on a printing cylinder (4) by the conventional means is provided with stiffeners (14; 114) at both the leading and trailing edges. One stiffened edge portion may be located in a slot (11) in a reel bar (12), while the other engages between the reel bar (12) and an internal surface (or in a second reel bar). Locking-up of the plate (2) may be enhanced by frictional engagement between one stiffened (and, generally, thickened) edge region with the opposite edge region, possibly where this too is stiffened and thickened.

Description

PRINTING PLATE The present invention relates to a printing plate which can be "locked up" on a printing press, and in particular to a plate comprising a floppy sheet for carrying an image. It also relates to a plate cylinder assembly; and to a method of mounting a plate to a cylinder. It is mainly concerned with web-fed presses, which may have very narrow lock-up gaps, placing severe constraints on the techniques employable. Lithographic and planographic printing plates have, conventionally been made of metal e.g. of aluminium. Such a plate is flexible enough to be wrapped around a plate cylinder, but is sufficiently rigid (i.e. not floppy) to permit the end portions to retain required shapes into which they are bent. As shown in Fig. 1 of the accompanying drawings, a litho-plate 2 of this type is adapted to be mounted on a plate cylinder having reel bar lock-up. Thus the plate 2 has opposite edge portions bent before mounting so that the leading edge 6 of the plate 2 is bull-nosed to fit over the leading edge δ of a plate cylinder 4, while the trailing edge 10 of the plate forms an L whose free arm is to be inserted into a slot 11 in the reel bar 12. When the reel bar 12 is turned in an anticlockwise direction as shown by the arrow in Fig. 1 the plate is tightened against the surface of the plate cylinder and is held securely in position by the hook effect of the pre-bent edges. Additional security is provided by the leading 8 and trailing 10 edges of the plate coming close to, or into contact with, each other near the opening of the slot 11. The bull-nose cannot disengage without straightening, which is prevented by contacting the other edge, which also provides frictional resistance.

Metal printing plates are suited to mounting by this method because they will hold bends or folds and stay in place on the cylinder while the reel bar is rotated to tighten the plate. Furthermore, they may be sufficiently thick for their ends to jam together in the restricted space between the reel bar and the plate cylinder when the reel bar is rotated, so that frictional engagement can result if desired. Floppy printing plates (typically of generally non- metallic materials) are gradually replacing metal ones. But they cannot simply be used with the conventional narrow-gap lock-up described above because, on rotating the reel bar, any folds made in the plates would come out, and the plate would come off the cylinder. Also, non-metal printing plates are often thinner than their metal counterparts, and so the possibility of the frictional effect is not there.

EP-A-194111 discloses means permitting a floppy plate to be mounted, but they are rather elaborate. They require: a plastic under-packing to which the plate must be releasably adhered; a stiffener at the "bull-nose" end; and a double crimp in the bull nose end portion of the plate such that it can enclose the stiffener. It is the under- packing which is locked up permanently bonded to the cylinder, possibly by use of a reel bar lock-up. The plate is secured by hooking the stiffened bull-nose portion over the bull nose of the cylinder, and adhering its other end to the under-packing using repositionable adhesive. Thus (1) a plastic under-packing is required, and (2) the plate is adhered at one end region to the cylindrical surface of the under-packing. This adhesion has to be non-permanent, and there may be a danger of slippage in use. Perhaps more seriously, the presence of the layer of adhesive is likely to degrade printing quality seriously by producing a bulge in the printing surface. A printing plate embodying the present invention can be simply mounted by a lock-up method on a plate cylinder of a printing press.

The present invention is primarily concerned with substantially non-metallic plates, this term being used to include plates which have some metal content, e.g. laminates of metal foils with paper or plastics.

According to a first aspect of the present invention there is provided a printing plate for locking up on a cylinder of a printing press, the plate comprising a floppy sheet for carrying an image to be printed; the sheet having a pair of opposite edge portions for engagement with printing cylinder mounting means, each of said edge portions being provided with stiffening means. The stiffening means may be substantially flat. They may extend inwardly from the edges of the plate. They are preferably thicker than the main body of the sheet. Preferably, at least one of the stiffening means comprises a metal strip attached to the sheet along the length of the edge.

A stiffening means may be a metal strip secured to an edge of the sheet e.g. by adhesion and/or by folding the strip over the edge of the sheet. In a particularly preferred embodiment the stiffening means each comprise a flattened-spiral sandwich of metal strip and printing plate. A metal strip is folded twice around the singly folded edge of a plate to give a multi-layer structure (metal-plate-metal-plate-metal).

A printing plate embodying the invention may be usable on a plate cylinder which is also usable with conventional metal plates, substantially without modification.

According to a second aspect of the invention there is provided a plate cylinder assembly comprising: a plate cylinder; a reel bar rotatably mounted within the plate cylinder; a floppy printing plate mountable around the plate cylinder, said plate being as described above, wherein a first stiffened edge portion of the plate is insertable through a longitudinally extending opening in the cylinder surface into a space between the reel bar and an inner surface of the cylinder; and the second stiffened edge portion is insertable through said opening into a slot in the reel bar and, by virtue of its stiffness, is retainable in the slot on rotation of the bar; so that on rotation of the reel bar to bring the slot towards the first edge portion, the second edge portion is retained within the slot, and plate surfaces may be brought into contact in the space between the reel bar and the plate cylinder, thus locking the plate on the cylinder. The stiffened first edge portion helps prevent the first edge being drawn off the cylinder because it is retained in the narrow gap between reel bar and plate cylinder.

The invention also provides a method of mounting a printing plate having a floppy sheet on a cylinder, particularly of a web-fed printing press.

Embodiments of the invention will now be discusssed by way of example with reference to the figures, of which:

Fig. 1 illustrates a conventional lock-up mechanism for mounting a metal printing plate on a plate cylinder;

Fig.2 illustrates lock-up according to one embodiment of the present invention;

Fig. 3 is a schematic view to show one way in which a printing plate and a metal strip may be secured to facilitate lock-up using the method of the present invention; Fig. 4 is a like view of another edge arrangement; and Fig. 5 is a view like Fig. 2 but showing an embodiment with two lock-up bars.

Fig. 2 shows an embodiment of the present invention in comprising a printing plate 2a of a material which will not reliably hold a fold when bent (i.e. is floppy). Since the arrangement illustrated in Fig. 1 depends on the use of folds to hold the plate 2 in position on the cylinder 4, while the plate 2 is secured, it is unsuitable for non- metallic plates, for example of paper or polyester, or for plates which include metal, but in an amount which is insufficient to give the required stiffness, e.g. paper- aluminium foil laminates.

As shown, respective metal strips 14 are secured at the leading and trailing edges of the plate 2a to provide stiff portions. The width of the metal strips 14 is approximately equal to the depth of the slot 11 in the reel bar 12, and each strip 14 extends along a respective plate edge. The leading edge of the plate 2a is tucked round the leading edge 8 of the plate cylinder 4 and the metal strip

14 at the trailing edge of the printing plate is loaded into the slot 11 in the reel bar 12. When the reel bar 12 is cranked in an anti-clockwise direction, as the plate 2a is tightened one stiffened end remains in the slot 11 while the other stiffened end remains in the gap adjacent the reel bar 12. Were it not stiffened it would simply be pulled out, straightening as it was drawn past the bull nose of the cylinder gap. But with the stiffening, as soon as the edge of the stiffening strip reaches the bull-nose it endeavours to pivot. This is restrained by abutment with the opposed edge of the plate. Thus it is effectively hooked in place. Depending on the size of the gap between the bar 12 and the internal wall of the cylinder, and the thickness of the strips, friction may also contribute markedly to the lock-up. The locked-up plate can be maintained in substantially constant and uniform tension around the cylinder 4, even during printing.

The metal strip 14 may be secured to the plate 2 in any convenient way. For example, an adhesive can be used to attach a metal strip 14 on one surface of the plate 2a or the strip 14 may be folded over the edge of the plate 2a and held there with adhesive. In the latter case, the folded width of the strip 14 is about the same as the depth of the slot 11 in the reel bar 12.

A particularly preferred arrangement for securing an edge of the printing plate 2a and its metal strip 14 is shown at Fig. 3 (in which thicknesses are much exaggerated for clarity). In this arrangement, the edge of the printing plate 2a is doubled over one long edge of the metal strip 14. The overlap is approximately the same as the depth of the slot 11 in the reel bar 12. The metal strip 14 is then folded back on itself, over the edge of the plate 2a, and back on itself again, so that it lies in contact with one surface of the plate 2a and extends along the plate surface. This arrangement is similar to that used for securing metal strips to the paper sheets of a calendar. The strip and plate can be locked together by crimping, without the need for adhesive. One advantage of this arrangement is that a fairly thick (and therefore stiff) portion is produced at each end of the plate, since each folded edge comprises two thicknesses of printing plate 2a and three of the metal strip 14. So, even with a thin plate 2a and metal strip 14, satisfactory edges can be produced.

Fig. 4 shows an alternative to the arrangement shown in Fig. 2 in which the metal strip 114 is wider than the depth of the slot 11 in the reel bar 12. It can then be engaged with the plate 102a and folded at the trailing edge in the same way as a conventional metal plate 2 as seen in Fig. 1 is folded, so that thickened portions at both the leading and trailing edge abut each other and provide a good grip. Since the metal stiffened portion will usually be of the same width at both the leading and trailing edges of the plate 2a it is important to ensure that it is not so wide that it can become folded back over the surface of the plate cylinder 4 at its leading edge 8, since this would interrupt the printing surface.

A cylinder may first have an underlay applied, before a plate embodying the invention is applied over it.

In making conventional (metal) lithographic plates, pin registration is commonly used. This requires an array of openings adjacent an edge of the plate, by which it engages pins to ensure accurate registration for photographic processing. With the present invention, a floppy plate can be provided with such registration openings and processed, then the stiffening can be applied over the openings. Of course, it is easier to provide openings in a floppy film than in metal.

The present invention enables plates to be located on cylinders quickly and accurately. They can be removed and reused. In contrast, procedures requiring adhesion are slow and unlikely to permit removal without damage.

A floppy plate may be made much longer than the circumference of the cylinder, and the reel bar 12 can then be used to wind it up. (The gap between the bar and the cylinder interior would have to be great enough to accommodate several thicknesses of plate.)

There may be a pair of reel bars 12, so that both the leading edge and the trailing edge can be wound up. This is shown schematically in Fig. 5. If the bars 12 are close together, there may be frictional engagement between a plate portion wound upon one bar 12 and the other bar 12 or a plate portion wound thereon. Alternatively (particularly if lock-up employs rotation in the opposite senses to those indi c a ted by the arrows A ) , there may be f ri c t i on al engagement with inner surfaces B of the cylinder.

A cylinder may be designed to receive a plurality of plates around its periphery, having a conresponding plurality of narrow gaps providing lock-up points; and/or to receive a plurality across its periphery.

Claims

CLAIMS :

1. A floppy printing plate (2) for locking up a cylinder (4) of a printing press, the plate (2) having leading and trailing edge portions for engagement with printing cylinder lock-up means (8, 12), the leading edge portion being provided with stiffening means (14; 114) characterised in that the trailing edge portion is also provided with stiffening means (14; 114).

2. A printing plate according to claim 1 wherein at least one said stiffening means (14) is substantially flat.

3. A printing plate according to claim 2 wherein one stiffening means (14) is substantially flat and the second stiffening means (114) has an outer portion extending at an angle to an inner portion from which the plate extends.

4. A printing plate according to any preceding claim wherein at least one stiffening means comprises a metal strip (14; 114) attached to the printing plate (2) along the length of the respective edge.

5. A printing plate according to any preceding claim wherein at least one stiffening means comprises a flattened metal spiral of metal strip (14; 114) and printing plate (2).

6. A printing plate according to claim 5 wherein the metal strip (14; 114) is folded twice around the singly folded edge of the plate (2).

7. A printing plate according to any one of the preceding claims wherein at least one stiffening means (14; 114) is secured by means of adhesive.

8. A printing plate according to any one of the preceding claims wherein at least one stiffening means (14; 114) is secured by being crimped over the plate; and optionally also by adhesion,

9. A plate cylinder assembly comprising; a plate cylinder (4); a printing plate according to any one of the preceding claims mountable on the plate cylinder (4); wherein the assembly includes lock-up means (8, 12) in the cylinder (4) which lock the plate (2) onto the cylinder (4), the stiffening means (14) serving in use to retain the plate (2) in engagement with the lock-up means,

10. An assembly according to claim 9 wherein the lock-up means comprises a reel bar (12) mounted in an opening in the surface of the cylinder (4), and an edge (8) of that opening, the stiffening means of one edge portion being adapted to contact an internal surface adjacent the mouth of the opening, and the stiffening means (14) of the second edge portion being insertable into a longitudinal slot (11) in the reel bar (12), and by virtue of its stiffness, retainable in the slot (11) on rotation of the reel bar.

11. An assembly according to claim 10 arranged so that, on rotation of the reel bar (12) to bring the slot (11) towards the edge (8) of the opening, the plate surfaces adjacent each edge portion are brought into mutual contact.

12. An assembly according to claim 10 or 11 wherein the printing plate is according to claim 3. the stiffening means being dimensioned so that the outer portion of said second stiffening means (114) can be received in the slot (11) in the reel bar (12), and the inner portion abuts the first stiffening means (14), urging it against the internal surface in the opening.

13. An assembly according to claim 9 wherein the lock-up means comprises two reel bars mounted in an opening in the surface of the cylinder (4), the stiffening means (14; 114) of each edge portion of the plate (2) being insertable into a longitudinal slot in each reel bar (12), and by virtue of its stiffness, retainable in the respective slot (11) on rotation of the respective reel bar (12).