GB2028722A

GB2028722A - Attaching printing plates to supports

Info

Publication number: GB2028722A
Application number: GB7928048A
Authority: GB
Original assignee: TETRA PAK INT; Tetra Pak International AB
Current assignee: TETRA PAK INT; Tetra Pak AB
Priority date: 1978-08-21
Filing date: 1979-08-13
Publication date: 1980-03-12
Also published as: SE7808789L; DE2933458A1; FR2434035A1; FR2434035B1

Abstract

Flexible plate 14 is retained on printing cylinder 1 by vacuum produced in a vacuum chamber 2 arranged between the underside of the plate and the cylinder surface and the end edges of the plates are maintained in tight contact against the cylinder surface by means of a holding strip mechanically attached to the cylinder, and bearing against the end edges of the plate. The strip may, as shown be attached to register pins 11,12 cooperating with holes in the plate ends. In this case, resilient seal 19 is provided. Alternatively, the strip may be attached to the cylinder 1 between the plate ends. In this case, the strip and plate ends preferably have complementary bevelled surfaces. <IMAGE>

Description

SPECIFICATION Improvements in and relating to plate fastening in a rotary printing press This invention relates to fastening means for the attachment of the facing end edges of a printing plate to a vacuum printing cylinder.

Rotary printing by means of flexible printing plates which are retained on the rotary printing cylinder by vacuum is at present in general use in printing. The vacuum printing cylinders used in the process are each cylindrical with a surface largely covered by recesses in the form of grooves, knurled surfaces, countersunk areas etc. When a flexible printing plate is applied to the cylinder surface one or more closed chambers are formed between the smooth underside of the plate and the recesses which, through channels drilled into the cylinder, can be connected to a vacuum source. The vacuum is maintained during the whole printing process and holds the printing plate to the cylinder surface.When the printing plate is to be removed again, after the printing has been completed, the connection between vacuum source and evacuated chamber(s) is broken and the latter is connected instead to the surrounding atmosphere, whereupon the printing plates can be simply removed from the cylinder surface or replaced by other plates.

The areas of the cylinder surface which form the vacuum chambers are generally given a shape which corresponds to the shape of the printing plate which is to be used and a size which is a iittle smaller than the size of the printing plate. By surrounding each vacuum chamber with a non-recessed edge area, a tight seal is created when the plate rests against the surface, which prevents the surrounding air from penetrating into the vacuum chamber. To facilitate the application of the printing plates in the correct position on the cylinder surface, a number of guide pins are usually provided for each printing plate on the cylinder surface. Corresponding guide holes are provided in the two end edges of the printing plate, that is to say, the end edges which are opposite to one another when the printing plate has been applied to the cylinder.The guide holes are placed at some distance from the respective plate edge and the pin locations are such that a distance of only a few millimetres appears between the end edges facing each other when the plates are applied.

The printing plates are usually made of a material of good flexibility (e.g. a photopolymer), but it has been found nevertheless on working with the arrangements described that there is a danger of the end edges of the printing plates not retaining their correct positions in contact with the cylinder surface.

This is true in particular when cylinders of a relatively small diameter are used, and to avoid this danger it is customary to fasten the end edges as well as the side edges to the cylinder by means of double-faced adhesive tape. The tape also contributes to an increased safety against vacuum leakage, but implies at the same time an extra stage of operation which sometimes can cause difficulties, since the tape is relatively awkward to handle.

Moreover, when the plates are to be removed a certain stretching of the edge portions of the plates occurs which, if it persists, can deform the plate pattern to such a degree that repeated use of the plates is rendered impossible.

It is an object of the present invention to provide an arrangement which allows the rapid and secure fixing of a printing plate in the correct position on a vacuum printing cylinder without being subject to the abovementioned disadvantages.

It is a further object of the present invention to provide an arrangement which in a simple manner makes possible a secure attachment of the end edges of a printing plate in tight contact against the vacuum cylinder without making it necessary to use double-faced adhesive tape or similar fixing agents for the purpose.

It is a further object of the present invention to provide a fastening arrangement which is simple and cheap and which allows repeated application of the fastening arrangement as well as of the printing plate.

These and other objects have been achieved in accordance with the invention in that a fastening arrangement for the fixing of the end edges of a printing plate facing each other to a vacuum printing cylinder is characterised by a holding strip mechanically jointed to the cylinders, which lies against and retains the end edges of the printing plate in contact with the cylinder.

In a preferred embodiment of the fastening arrangement in accordance with the invention the strip extends axially over the cylinder and covers the two end edges of the printing plate as well as the space between them.

The side of the strip facing from the cylinder surface may be at a lower level than the printing surface of the printing plate.

The portion of the strip situated between the end edges of the printing plate may be provided with fixing elements to co-operate with corresponding fixing elements in the cylinder surface.

A strip used on the type of vacuum cylnder which is provided with guide pins for locating the plate ends in correct position on the cylinder, may be adapted to be joined to the cylinder via the guide pins.

The guide pins on their sides facing one another may have grooves which by co-operating with the longitudinal edges of the strip retain the strip in such a position that it lies against the two edges of the underlying printing plate and covers them.

The angle between the surfaces of the strip facing the end edges of the printing plate and the cylinder surface may be an acute angle. The said angle is preferably between 30 and 60 .

The edges of the printing plate may rest against sealing bodies arranged in the cylinder surface.

Preferred embodiments of the invention will now be described in detail with reference to the accompanying schematic drawings which only show the details required for the understanding of the invention, and in which Figure 1 represents a side view of a vacuum cylinder of known type to which a printing plate has been applied using the customary fastening means described in the introduction Figure 2 shows on a larger scale a section through a part of the surrounding surface of a vacuum cylinder of known type and shows the area where the two end edges of a printing plate facing one another are fixed to the cylinder surface by means in accordance with the invention: and Figure 3 corresponds to Figure 2, but shows a second fastening embodiment in accordance with the invention.

Figure 1 shows a conventional vacuum printing cylinder 1 whose substantially cylindrical surface is divided into two vacuum chambers 2 situated side by side. Only one of these is visible in Figure 1, since the other is covered by a printing plate 3 applied to the cylinder. The two vacuum chambers are of the same design and each consists in principle of a partially countersunk area of the surface, the dimensions of which area substantially correspond to the length and width of the printing plate which is to be retained on the cylinder. Within the said area there are a number of recesses which can be placed under vacuum, so that a printing plate covering the area is retained in position on the cylinder surface.The vacuum is created by means of an external vacuum source which through axial channels (not shown) through the axle journals 4 of the cylinder and the cylinder, as well as through radial channels 5 ending in the cylinder surface can be connected to the vacuum chamber.

The vacuum printing cylinder 1, more particularly, is provided with vacuum chambers of a design which for distribution of the vacuum over the side of the printing plate fully facing the cylinder makes use on the one hand of grooves 6 present in the cylinder surface, which extend helically round the cylnder, on the other hand of a screenlike pattern of indentations and projections covering the whole of the vacuum area. The grooves 6 as well as the indentations of the pattern 7 are connected respectively in an indirect manner to the vacuum source through the radial ducts 5 ending in the grooves 6 and the channels (not shown) passing axially through the cylinder.

The width of the vacuum chamber 2 is somewhat smaller than the width of the printing plate which is to be applied, so that the lateral edge areas of the printing plate will rest against two annular sealing section 8,9 of the surrounding surface, which delimit the vacuum chamber 2 in lateral direction. The two annular sections 8, 9 are connected to each other by a further sealing section 10 which extends axially over the vacuum chamber between the two annular sealing sections 8,9 and serves for the sealing of the two end edges of a printing plate applied to the cylinder.

One on each side of the axial sealing section 10 there are two guide pins 11, 12 projecting radially from the cylinder surface, which in co-operating with corresponding guide holes in the printing plate facilitate the locating of the plate in the correct position on the enveloping surface.

The printing plate 3 is applied over an area on the cylinder surface corresponding to the vacuum chamber 2 with the help of the guide pins 11, 12 and the corresponding guide holes in the plate in such a way that its lateral edges lie against the annular sealing sections delimiting the vacuum chamber 2 at the same time as the end edges of the printing plates facing one another lie against the axial sealing section situated between the guide pins.In order to ensure that the end edges of the flexible printing plate facing each other are retained in accurate tight contact against the axial sealing section a doublefaced adhesive tape 13 is used which prior to the application of the printing plate to the cylinder surface is affixed not only along the end edges of the plate but frequently also along its lateral edges so as to improve the seal between the printing plate and the cylinder and thus ensure that a sufficiently strong vacuum can be built up in the vacuum chamber.

As mentioned earlier, the abovementioned type of fixing of the printing plate to the cylinder surface with the help of double-faced adhesive tape applied along the end edges of the printing plate, as used up to now, has a number of disadvantages, such as a great risk of the printing plate being damaged when it is removed from the vacuum cylinder.

The fastening arrangement in accordance with the present invention eliminates this disadvantage and in Figures 2 and 3 different preferred embodiments of the fastening arrangement are shown in section.

Figure 2 shows on an enlarged scale a radial section through a part of a vacuum printing cylinder where use has been made of a first embodiment of the fastening arrangement in accordance with the invention. The part of the surface of the vacuum printing cylinder 1 shown is provided, like the vacuum printing cylinder shown in Figure 1, with a vacuum chamber consiting of a number of indentations and grooves, the vacuum chamber, for the sake of clarity, being indicated only by broken lines. The cylinder 1 is also provided with an axial sealing section 10 and with radial guide pins 11 and 12 respectively arranged on either side of the same. To the outer surface of the vacuum printing cylinder is applied a printing plate 14, which printing plate in a known manner is provided with raised, printing portions 14' and with lower, non-printing, portions 14".Centrally on the sealing section extending axially is a steel strip 15 which is mechanically joined to the cylinder, e.g. by spot-welding. The strip 15 is thus situated in the middle between the two guide pins 11, 12 and has lateral surfaces 16 facing towards them which are inclined outwards towards the guide pins, that is to say, the angle between the lateral surfaces of the strip and the cylinder surface is an acute one. The acute angle may be between 30 and 60 , but is preferably 45 , the two end edges of the printing plate being given a corresponding slope so as to co-operate with the lateral surfaces 16 of the strip 15. The height of the strip 15, like the height of the guide pins 11 and 12, must be limited so that they do not project higher than any printing surface 14' of the printing plate 14. Thanks to the design shown the function of the fastening arrangement remains satisfactory, however, if the height of the strip 15 is less than the non-printing surface 14" of the printing plate.

When the embodiment shown in Figure 2 is used for fixing the end edges of a printing plate facing one another, one end of the printing plate is applied first by inserting its end edge, suitably bevelled off e.g. at an angle of 45 , between the guide pins 12 and the strip 15 so that the end edge comes to rest against the lateral edge 16 of the strip, whereupon the end of the plate is lowered so that the guide pins 12 engage the corresponding guide holes of the plate. Subsequently, the plate is wound round the cylinder and its opposite end edge is inserted in a similar manner under the strip 15 so that it rests against the strip surface 16, whereupon the end of the plate is lowered so that the guide pins engage the guide holes.The two end edges of the printing plate are thus fixed by means of the strip 15 and the guide pins 11 and 12 respectively and the printing plate ends rest against the surfaces 16 of the strip 15 as well as against the underlying axial sealing section 10. The lateral edges of the printing plate accordingly rest against the two annular sealing sections on each side of the vacuum chamber, and when the pressure in the vacuum chamber is reduced the printing plate is firmly retained in its correct position on the surface of the printing cylinder.

The space existing between the end edges of the printing plate is covered and completely filled by the strip 15 extending axially between the end of the cylinder and serves also for the attachment of the strip to the cylinder surface, or more particularly to the axial sealing section 10. This attachment, as mentioned previously, may be done e.g. by spotwelding of the strip to the surface, since the strip normally does not have to be removed when the printing plates are changed, since their flexibility allows the insertion of the end part of the printing plate underneath the strip whilst the same is in position.Howeve, if it is desirable, e.g. for the purpose of regrinding of the cylinder, that it should be possible to detach the strip and reinstall it again, this can be done by providing the portion of the strip situated between the end edges of the printing plate with fastening elements which are adapted to cooperate with corresponding fastening elements in the cylinder surface. In their simples form such fastening elements may be screws and threaded holes provided in the cylinder surface, but it is also possible to attach the strip by means of fastening elements which are especially adapted for attach ment in the grooves 6 in the cylinder surface.

A further embodiment of fastening arrangements in accordance with the invention is shown in Figure 3, which shows, as does Figure 2, on an enlarged scale a section through a vacuum printing cylinder 1.

The surface of the vacuum printing cylinder is provided with a vacuum chamber 2 which is indi cated by means of broken lines. The surface of the vacuum cylinder is again provided with an axial sealing section 10 and with guide pins 11 and 12. On the cylinder surface is present a printing plate 14 with printing and non-printing surfaces 14' and 14" respectively. In contrast to the fastening arrange ments shown earlier the two end edges of the printing plate 14 are retained in contact against the axial sealing section 10 by a detachable strip 17 which is joined to the cylinder via the guide pins.The strip 17 is in the form of ah elongated, relatively thin flat band which in its active position is fixed by being inserted axially along the cylinder between the guide pins, its two longitudinal edges running in the grooves 18 which are present in the sides of the guide pins facing one another and which are substantially at a level above the cylinder surface which corresponds to the thickness of the non-printing portion 14" of the printing plate 14. In Figure 3 is also shown a sealing strip 19 of flexible material, e.g.

rubber, which is inserted in the axial sealing section 10 so as to contribute to an even better seal between the end edges of the printing plate and the cylinder.

Corresponding sealing strips may be provided in all the embodiments of the fastening arrangement according to the invention along the whole circumference of the printing plate. The sealing strip 19 preferably projects a few tenths of a millimetre above the cylinder surface.

The fitting of a printing plate to a vacuum printing cylinder comprising a fastening arrangement such as shown in Figure 3 is effected in the following manner. When the strip 17 has been removed from the space between the guide pins, one end edge of the printing plate is placed at an equal level with the axial sealing section and in such a position that the guide holes situated in the vicinity of this end edge are directly opposite the guide pins 12. The plate ends are then lowered so that they lie against the cylinder surface and the printing plate is wound round the cylinder.Since the length of the printing plate corresponds to the circumference of the cylinder (apart from the space shown between the end edges which amounts to no more than a few millimetres), the guide holes at the free end of the printing plate become located in the correct position over the guide pins 11,so that this end of the plate too can be lowered and placed in contact with the cylinder surface. After both plate ends have been placed in correct position, the strip 17 is inserted between the guide pins 11, 12 so that its two edges slide into the grooves 18 in the guide pins.The strip 17 has a length which corresponds to the width of the printing plate and when the strip has been placed in the correct position it thus covers and retains the two end edges of the printing plate in contact against the sealing section 10 extending axially over the cylinder, or as shown in Figure 3, against the band 19 of flexible sealing material sunk into the said sealing section.

The embodiment of the invention described in connection with Figure 3 offers maximum safety against unintentional detachment of the end edges of the printing plate during the printing process as well as against leakage along the limiting surface of the vacuum chamber. The fastening arrangement is simple to apply and does not require any adaptation of printing plates such as are in use at present and which are attached in accordance with the method described in connection with Figure 1,that is to say, by means of guide pins and double-faced adhesive tape. The embodiment of the invention shown in Figure 2 on the other hand does require the bevelling of the end edges of the printing plate mentioned, but offers in return a relatively secure attachment and seal without any movable components having to be fitted.

The fastening embodiments in accordance with the invention thus make it possible in a simple and secure manner to attach facing end edges of the printing plate to a vacuum printing cylinder without having to use tape, glue or the like. As a result the printing plates are not subjected to wear or deformation caused by the fastening arrangement, so that it is made possible by repeated dismantling and re-installation to use the printing plate a number of times.

Claims

1. A method of attaching a vacuum-held printing plate to a rotary printing cylinder wherein the adjacent free edges of the plate are held down by a strip member extending between the plate edges and temporarily or permanently attached to the cylinder.

2. A fastening arrangement for the attachment of end edges of a printing plate facing each other to a vacuum printing cylinder, characterized in that a holding strip (15, 17) mechanically attached to the cylinder (1) lies against and retains the end edges of the printing plate (14) in contact with the cylinder.

3. A fastening arrangement in accordance with Claim 2, characterized in that the strip (15, 17) extends axially over the cylinder (1) and covers the two end edges of the printing plate (14) as well as the space between them.

4. A fastening arrangement in accordance with Claim 2 or 3, characterized in that the face of the strip (15, 17) facing away from the cylinder surface (1) is at a lower level than the printing surface (14') of the printing plate (14).

5. Afastening arrangement in accordance with Claim 4, characterized in that the portion of the strip situated between the end edges of the printing plate is provided with fixing elements to co-operate with corresponding fixing elements in the cylinder surface.

6. A fastening arrangement as claimed in any of Claims 2-4, characterized in that a strip (17) used on the type of vacuum cylinder (1 ) which is provided with guide pins (11,12) for locating the plate ends in correct position on the cylinder, is adapted to be attached to said guide pins.

7. A fastening arrangement in accordance with Claim 6, characterized in that the guide pins (11,12) on their sides facing one another have grooves (18) which by co-operating with the longitudinal edges of the strip (17) retain the strip in such a position that it bears against the two end edges of the underlying printing plate (14) and covers them.

8. Afastening arrangement as claimed in any of Claims 2-4, characterized in that the strip (15) is spot-welded to the cylinder surface.

9. A fastening arrangement as claimed in any preceding claim, characterized in that the angle between the surfaces of the strip (15) facing the end edges of the printing plate and the cylinder surface is an acute angle.

10. A fastening arrangement in accordance with Claim 9, characterized in-that the said angle is between 30" - 60".

11. A fastening arrangement as claimed in any preceding claim characterized in that the edges of the printing plate (14) rest against a sealing attachment (19) in the cylinder surface.

12. A method of attaching a vacuum-held printing plate to a cylindrical rotary printing cylinder, substantially as described herein with reference to the accompanying drawings.

13. Means for attaching a vacuum-held rotary printing cylinder, substantially as described herein with reference to the accompanying drawings.