Polyester based lithographic printing and mounting thereof on a web press

Abstract

 The present invention provides a method for mounting a lithographic printing plate carrying a lithographic image on a polyester film support having a thickness of 0.1mm to 0.35mm and a Young modulus of at least 4300N/mm², on a plate cylinder of a web press, said plate cylinder having a channel for receiving the lead edge of the lithographic printing plate and said method comprising the steps of:

(1) bending the lithographic printing plate along a line parallel to registration-line openings near its lead edge and perpendicular to a direction of orientation of said polyester film wherein the Young modulus is at least 4300N/mm²;

(2) applying an adhesive to the backside of the lithographic printing plate at least near its lead edge and trailing edge;

(3) inserting the lead edge in a longitudinal channel of the plate cylinder and;

(4) wrapping the lithographic printing plate around the plate cylinder while pressing the adhesive against the plate cylinder.

The present invention further provides a lithographic printing plate carrying a lithographic image on a polyester film support and having a lead edge and a trailing, said lithographic printing plate being bended perpendicular to a direction of orientation of said polyester film wherein the Young modulus is at least 4300N/mm² and parallel and adjacent its lead edge under an angle of less than 90° and wherein the backside of said lithographic printing plate is provided with an adhesive at least near the lead edge and trailing edge.

Description

1. Field of the invention

[0001] The present invention relates to a method for mounting a lithographic printing plate carrying a lithographic image on a polyester film support on a plate cylinder of a web press. The present invention further relates to a polyester based lithographic printing plate that is appropriately prepared for mounting on a web press. Finally, the present invention concerns a device for preparing a polyester based lithographic printing plate for mounting on a plate cylinder of a web press.

2. Background of the invention

[0002] As illustrated in Fig. 1, a typical web press has a plate cylinder 10 including a reel bar 120. Before being mounted, a metal lithoplate 14 is bent so that its lead edge fits over the leading edge or "bullnose" 16 of the plate cylinder and its trailing edge fits into a slot 18 of the reel bar 120. The reel bar is then cranked clock-wise (as seen in Fig. 1) until the plate fits tightly against the surface of the plate cylinder. To remove the lithoplate requires counter clockwise cranking of the reel bar. Typically, a skilled operator will be able to remove one plate and mount another in about one minute. Printing presses of this type are available from Harris Graphics, American Type Founders. Geo. Hantscho CO. Inc., and Timpsons Ltd..

[0003] Generally, this type of printing press is used to print with metal plates having a thickness of about 0.3mm. To the underside of the plates base may be adhered a paper or plastic underpacking, the thickness of which is selected to provide the desired interference between the printing plate and the blanket cylinder, usually about 0.1mm.

[0004] There exists however a desire to replace the metal plates with less expensive polyester based lithographic printing plates at least for the short-run print jobs. Unfortunately, mounting of polyester based lithographic printing plates on a web press as described above is difficult and it is often seen that the plate loosens during printing which often involves high rotation speeds of the plate cylinder. As a consequence the printing endurance of the plate is limited and moreover printing with such plates becomes unreliable.

[0005] US-A-4.643.093 describes a polyester based lithographic printing plate that is double bended near its lead edge and that has been provided with an adhesive at the backside of the plate near its trailing edge . Between the two bends, a stiffener is fitted. The bended portion and stiffener are inserted in the longitudinal channel of the plate cylinder and the plate is wrapped around the plate cylinder. The trailing edge is adhered to the plate cylinder by pressing the trailing edge towards the plate cylinder.

[0006] A disadvantage of the mounting procedure as disclosed in US-A-4.643.093 is that it requires a substantial amount of manipulation of the printing plate before it can be mounted. Moreover, the bends have to be very precise in order to assure a firm mounting of the printing plate on the plate cylinder. Finally, with this type of mounting the printing endurance will still be limited because of image stretching that occurs during printing.

3. Summary of invention.

[0007] It is an object of the present invention to provide a method for mounting a polyester based lithographic printing plate on a web press in a convenient way and that allows for a high printing endurance of the printing plate.

[0008] It is also an object of the present invention to provide a polyester based lithographic printing plate that is appropriately prepared for mounting on a web press.

[0009] Further objects of the present invention will become clear from the description hereinafter.

[0010] The different objects of the present invention are accomplished by a solution as set out in the claims.

4. Brief description of the drawings

[0011] The present will be described in more detail with reference to the following drawings that are provided only for illustrating the invention. There is however no intention to limit the invention to these drawings or the accompanying description.

Fig. 1 is a schematic cross-sectional view of a plate cylinder of a conventional web press with a metal-based lithographic printing plate mounted in a conventional manner as practiced in the prior art.

Fig. 2 is a schematic cross-sectional view of a bender in accordance with the present invention;

Fig. 3 is a schematic side view of a lithographic printing plate in accordance with the present invention and that is ready for mounting on a web press.

Fig. 4 is a schematic cross-sectional view of a plate cylinder mounted with a lithographic printing plate in accordance with the present invention.

5. Detailed description of the present invention.

[0012] In accordance with the method for mounting a lithographic printing plate carrying a lithographic image on a polyester film support (also called polyester based printing plate) having a thickness of 0.1mm to 0.35mm and a Young modulus of at least 4300N/mm² on a web press, the printing plate is first bended near its lead edge parallel to registration-line openings. According to a highly preferred embodiment in connection with the present invention, the printing plate is also trimmed to an appropriate length of the printing plate between bend and trailing edge that is between a size corresponding to the circumference of the plate cylinder minus the length of the arc corresponding to the mounting channel and a size corresponding to the circumference of the plate cylinder.

[0013] According to an aspect of the present invention, bending and trimming is performed in an accurate and convenient way using a bender in accordance with the present invention. Refering now to Fig. 2, there is shown a schematic cross-sectional view of a bender according to a particular embodiment of the present invention. In using the bender, the polyester based lithographic printing plate 25 is put on the surface of bender bars 61 and 62 and is pushed against a pair of stops 12. Conveniently, the printing plate has notches in which stops 12 can be fitted. Stops 12 each have a micrometer adjustment in the direction of movement of the bender bar 62. Thus, stops 12 are mounted on a slide bar 70 movable in an inverted T-shaped guideway in the bender bar 62. A fixed barrel 71 has an opening to receive a projection from the bar 70 which is joined to the end of the adjusting screw 73 of the micrometer. A set screw 72 will set the position of the bar 70 once established. Subsequently, the printing plate is cut to an appropriate length by moving knife 40 downwardly. Preferably, the bender includes a bar 41 to hold the printing plate firmly down during cutting. Bender bars 61 and 62 are at an elavated temperature, typically around 65°C. An elongated blade 60 is moved downwardly to force the printing plate between the bender bars against bender bar surface 64. Immediately thereupon, bender bar 62 is moved towards bender bar 61 to create a force concentrated in the region of the bend. The bender bar is then retracted and blade 60 is moved back upwardly. A bend with a desired angle, typically about 45° results.

[0014] Subsequent to cutting and bending a band of adhesive, typically between 1 and 10cm but preferably between 1 and 5cm, is provided on the back side, i.e. the side opposite to that containing the lithographic image, of the printing plate near the leading edge as well as near the trailing edge. A side view of such a printing plate is shown in Fig. 3. Thus, Fig. 3 shows two bands of adhesive 22 and 23 respectively near the lead edge having a bend 90 and near the trailing edge of the printing plate. The short edge 91 of bend 90 preferably has a length between 1 and 3cm. Preferably, the adhesive to be used in connection with the present invention will be a low-tack repositionable pressure-sensitive adhesive such as e.g. SPRAY-MOUNT™ or Crown adhesive 8091N™.

[0015] Now the printing plate is ready for mounting on a web press. In Fig. 4 there is shown a printing plate mounted in accordance with the present invention. As illustrated, the lead edge 91 of the bend is inserted into the longitudinal channel of the plate cylinder 10 and fitted over the leading edge or "bullnose" 16 thereof. Adhesive bands 22 and 23 are provided between the printing plate and the surface of the plate cylinder which is preferably provided with an underpacking (not shown). It can further be seen that reel bar 120 is not used.

[0016] It was experimentally found that a polyester based lithographic printing plate in accordance with the present invention mounted on a web press as described above could reliably be printed and could print an exceptionally high number of copies for a polyester based printing plate of upto 100000. Moreover, the preparation of the printing plate for mount on a web press proved to be convenient and fast and only a small number of copies had to be disposed of during start of printing.

[0017] The polyester film support of the lithographic printing plate in connection with this invention has a thickness of 0.1mm to 0.35mm and an Young modulus of at least 4300N/mm².

[0018] The Young-modulus also called E-modulus in connection with the present invention can be measured according to method A of the ANSI-D882-91 standard.

[0019] In accordance with the present invention, at least one of the two main directions of orientation of the polyester film, i.e. the machine direction (longitudinal direction) or the direction perpendicular thereto (transverse direction), should have an E-modulus of at least 4300 N/mm².

[0020] According to the most preferred embodiment of the present invention the polyester film support is isotropic in the E-modulus i.e. the ratio of E-modulus in longitudinal direction to the transverse direction is between 0.8 and 1.3. The use of an isotropic polyester film support offers the advantage that when the plate is turned 90° the same printing properties are obtained so that manufacturing of different sizes of the printing plates out of a master roll is simplified.

[0021] Such isotropic supports also offer an advantage when the imaging element is to be used in an image-setter for high intensity short time exposure. In this application, several images on one printing plate may be oriented parallel and/or perpendicular to the axis of the printing cylinder in the printing machine. When an isotropic polyester film support is used it will be much easier to obtain a steady print.

[0022] On the other hand, when an anisotropic polyester film support is used a larger E-modulus can be obtained because a large E-modulus in one direction generally goes to the detriment of the E-modulus in the direction perpendicular thereto. In order to obtain the benefit of an anisotropic polyester film support, such printing plate should be mounted with its E-modulus of more than 4300 N/mm² perpendicular to the axis of the printing cylinder of the printing press.

[0023] The polyester film in accordance with the present invention is preferably a polyethylene terephthalate film although other polyester films may also be used in accordance with the present invention. The polyester film in accordance with the present invention may be uniaxially oriented but is preferably biaxially oriented. The desired E-modulus may be obtained by setting the degree of orientation of the polyester film. Preferably, the degree of orientation is obtained by stretching the polyester film in the longitudinal and/or transverse direction with a factor between 1:2 to 1:4.

[0024] The E-modulus may further be influenced by adding substances to the polyester film such as e.g. low molecular weight compounds, voiding agents such as e.g. BaSO₄, polypropylene and opacifying agents such as e.g. TiO₂.

[0025] The polyester film support in accordance with the present invention is preferably coated with a layer improving the adhesion of the hydrophilic layers.

[0026] Examples of lithographic printing plates carrying a lithographic image on a polyester support can be produced from imaging elements that may be photosensitive or that may be imaged in other ways such as e.g. by means of heat. A particular example of such an imaging element comprises on a polyester film support in the order given a silver halide emulsion layer (photosensitive layer) and a layer containing physical development nuclei. After image-wise exposure to e.g. visible light or infrared light, the imaging element is developed in a processing liquid according to the silver salt diffusion transfer process to obtain a lithographic printing plate.

[0027] Such type of printing plates are known as photodirect printing plates because the paste-up can immediately be copied onto the imaging element without the need for intermediate films. A particular interesting variant of this type of printing plates is produced by exposing the imaging element with a laser or laser-diode under the control of a computer.

[0028] The latter exposure is generally performed by mounting the imaging element on a drum in image setter. The laser beam then scans over the drum and writes the digital image-data received from a computer while the drum rotates.

Claims

1. A method for mounting a lithographic printing plate carrying a lithographic image on a polyester film support having a thickness of 0.1mm to 0.35mm and a Young modulus of at least 4300N/mm², on a plate cylinder of a web press, said plate cylinder having a channel for receiving the lead edge of the lithographic printing plate and said method comprising the steps of:

(1) bending the lithographic printing plate along a line parallel to registration-line openings near its lead edge and perpendicular to a direction of orientation of said polyester film wherein the Young modulus is at least 4300N/mm²;

(2) applying an adhesive to the backside of the lithographic printing plate at least near its lead edge and trailing edge;

(3) inserting the lead edge in a longitudinal channel of the plate cylinder and;

(4) wrapping the lithographic printing plate around the plate cylinder while pressing the adhesive against the plate cylinder.

2. A method according to claim 1 wherein the length of said lithographic printing plate between the bend and the trailing edge is between a size corresponding to the circumference of said plate cylinder minus the arc corresponding to the opening of the mounting channel and a size corresponding to the circumference of the plate cylinder.

3. A method according to claim 1 wherein said adhesive is a low-tack repositionable pressure-sensitive adhesive.

4. A method according to claim 1 wherein said adhesive is applied near the leading edge and trailing edge of the lithographic printing plate in a band having a width between 1 and 5cm.

5. A method according to claim 1 wherein said bend has an angle between 30° and 60°.

7. A lithographic printing plate carrying a lithographic image on a polyester film support having a thickness of 0.1mm to 0.35mm and a Young modulus of at least 4300N/mm² and having a lead edge and a trailing, said lithographic printing plate being bended perpendicular to a direction of orientation of said polyester film wherein the Young modulus is at least 4300N/mm² and parallel and adjacent its lead edge under an angle of less than 90° and wherein the backside of said lithographic printing plate is provided with an adhesive at least near the lead edge and trailing edge.

8. A lithographic printing plate according to claim 7 wherein the bend has an angle between 30° and 60°.

9. A lithographic printing plate according to claim 7 wherein said adhesive is a low-tack repositionable pressure-sensitive adhesive.

10. A bender for bending a lithographic printing plate carrying a lithographic image on a polyester film support, said bender comprising bend bars 30 and 31, an elongated blade and a positioning stop 10, said bender further comprising cutting means 40 for cutting said lithographic printing plate to an appropriate length.

Drawing