GB2348397A

GB2348397A - Lithographic plate production

Info

Publication number: GB2348397A
Application number: GB9827250A
Authority: GB
Inventors: Matthew Snook
Original assignee: PRINTING TECH EQUIPMENT LIMITE
Current assignee: PRINTING TECH EQUIPMENT LIMITE
Priority date: 1998-12-10
Filing date: 1998-12-10
Publication date: 2000-10-04
Also published as: GB9827250D0

Abstract

An apparatus (10) for sensing the surface temperature of a lithographic plate (30), during manufacture or processing thereof, comprises a support (40) for supporting the plate (30) and at least one infra-red temperature detector (12) arranged to receive infra-red radiation from the plate surface. The invention also comprises a method of monitoring the temperature of a lithographic plate and a system for controlling the temperature of a lithographic plate, which system may be automated.

Description

LITHOGRAPHIC PLATE PRODUCTION This invention relates generally to lithographic plate production, and more particularly to a method and apparatus for monitoring the temperature of lithographic plates during processing thereof.

Lithography is the process of printing from a plane surface (such as a smooth stone or metal plate) on which the image to be printed is ink-receptive and the blank area ink-repellent.

In the present day, the lithographic technique utilises metal plates which are coated with a photo-polymer, for example, which in turn is coated with a protective layer, such as poly-vinyl alcohol (PVA). The image to be printed is transferred to the plate by laser, for example, by the laser removing (or etching) the photo-polymer protective layer to expose the photo-polymer itself beneath. The exposed areas of photo-polymer are ink-receptive, whereas the areas which are not laser-treated, i. e. non-image areas, are ink-repellent.

In order for the laser transferred images to be receptive to development the laser etched plates need to be pre-heated. The temperature to which the plates should be heated is dependent upon the particular photo-polymer coating used, but is typically of the order of 105 C. If the temperature of the plate is too low before developing begins, this can drastically reduce the working life of the plate.

Since laser image transfer onto photo-polymer coated lithographic plates lends itself to automated printing procedures, such as newspaper printing, loss of plate life can be exceedingly costly. For example, when a correctly prepared plate would be expected to produce approximately 250,000 copies, an inadequately heated plate may be good for only 5,000 copies or so. This means that, for the popular newspapers, in a typical print run approximately only eight copies of each laser etched image will be required, but if a plate run is reduced, more will be needed.

The heating step of the lithographic plate is also usually part of an automated system which can handle approximately 150 plates an hour. If the heating step is ineffective to bring the plates up to their optimum working temperature, and this is not realised until it is too late, a considerable number of plates with substandard print-run life can be introduced into production.

The amount of time required to rectify the situation so that all plates in a production run are up to their working temperature can be considerable, involving wasted man hours, but more importantly loss of production, which in turn may lead to production deadlines being missed, with all the attendant problems that may ensue.

A print plate heated to lower than optimum working temperature is virtually indistinguishable to the naked eye from a correctly heated plate.

Accordingly, there are no visual clues to those working the print production line that anything is wrong.

To date efforts in the industry to detect plate temperature have concentrated in two areas. Firstly, indirect means of sensing the plate temperature have been adopted. In one means, the temperature of the heating element in the plate heater is used as an indication of the plate temperature. In another means, the temperature above the surface of the heated plate is"sampled"using a conventional thermocouple, or the like. These indirect means for sensing plate temperature are not ideal, because under operating conditions the temperature indicated may not accurately reflect the actual temperature at the plate surface.

For example, newspaper production lines may be housed in huge warehouse-like facilities, and draughts may cause a greater than usual disparity between actual plate temperature and detected plate temperature. Furthermore, the method of detecting plate temperature using a thermocouple is designed to detect heat radiated from a heated plate. However, if the thermocouple is subject to convection heat currents, or reflected heat from the plate heating source, the temperature detected at the thermocouple may not accurately reflect the actual temperature of the plate.

In a second approach, the plate surface temperature is detected directly by applying adhesive heat-sensitive labels to a dummy plate. These labels are designed to change colour at particular temperatures. In practice, every two or three days a dummy plate is fed through the automated system to check that the plate temperature is substantially at an optimum throughout the system.

Adjustments can be made to the heating mechanisms if the plate is found to be below temperature at any point in the system. However, the conditions experienced by a single plate in a dummy run of the automated system do not always equate with those experienced by actual plates when the system is in full use. Furthermore, heating efficiency may alter between two test runs. Thus, whilst a dummy plate may show adequate heating capacity throughout the automated system during one test run, and a test run two days later shows that the heating capacity of the system is still adequate, this is no guarantee that heating capacity in the system has not dipped below optimum levels in the meantime.

The dummy plate is wasted at the end of the test run, and cannot be used because it has the adhesive strips placed all over it damaging the delicate coating. Additionally, the procedure is time consuming, as a large number of strips must be applied to check uniform heat distribution. The colour of the strips needs to be checked regularly throughout the automated system. The whole procedure is thus time consuming and labourious.

Accordingly, there is a need in the art for a means of more accurately detecting surface temperature of a lithographic plate for use in, for example, an automated printing system.

We have now found that this can be achieved by using an infra-red temperature detector arranged to sense the infra-red wavelength radiation emitted from the plate surface and so give an indication of its temperature. We have found that this technique is accurate and reliable and overcomes many of the problems associated with prior known procedures.

The invention thus provides a method of monitoring the temperature of a lithographic plate which comprises exposing surface of the plate to an infrared temperature detector.

The invention also includes apparatus for sensing the surface temperature of a lithographic plate which comprises a support for supporting the plate and an infra-red temperature detector arranged to receive infra-red radiation from the plate surface.

The infra-red temperature detector (or detectors where more than one is used) provide an output indicative of temperature. Means can be provided to raise an alarm when a measured plate temperature is not within prescribed limits.

In this way, when a plate has not been heated enough, or has been overheated, it can be detected and removed. Control systems can be provided whereby the temperature measurement can be used to vary a heater output or conveyor speed, to adjust the exposure of a plate to heat thereby to modify its temperature to be at a required level.

The lithographic plates for use in the present invention may be metal plates, for example, and will usually include a coating of a photo-polymer.

The apparatus of the present invention can be used in an automated printing process. The temperature can be displayed on a computer screen display of a central console for controlling the operation of an automated printing system.

If the temperature drops to below a preset level, a warning message or signal will flash up on the screen so that the appropriate remedial action may be taken. In a particularly preferred embodiment, the warning signal is part of an automatic feedback system in which the detected temperature of the lithographic plate is used to increase or decrease the rate of heating of a lithographic plate heating means to provide optimal heating of the plate.

The infra-red detectors for use in the present invention may be provided for detecting the plate temperature on either side thereof. That is to say, the infra-red detector may be provided to detect the temperature on the coated face of the plate or on the other face (or both if desired).

The apparatus of the invention may include one or more infra-red detectors for detecting the temperature of each plate. Alternatively, in a print system capable of handling two or more plates at the same time, for example in an automated system, the apparatus may provide one or more infra-red detectors per plate path.

In a further aspect of the invention, there is provided a system for heating a lithographic plate, which system includes a lithographic plate support, a lithographic plate heater and a lithographic plate temperature sensor. Preferably the system is controlled by a computer. The support may include a first roller. A second roller may oppose the first roller defining a lithographic plate path therebetween. The lithographic plate temperature sensor may be an infra-red sensor, for example.

The system may include one or more infra-red detectors arranged along the lithographic plate path.

In a further aspect, the invention provides a method of handling a lithographic plate in an automated printing operation including the steps of heating the lithographic plate; sensing the temperature of the heated plate; and generating a warning signal if the temperature of the heated plate is below a preset figure.

In an alternative embodiment the infra-red detector may comprise part of a drive roller defining the lithographic plate path, the arrangement being such that the infra-red detector means is brought substantially into contact with the lithographic plate.

In order that the invention may be more fully understood, an embodiment of the invention will now be described by reference to the accompanying drawings, in which: Figure 1 is a schematic side elevation of a device according to the present invention; and Figure 2 is a plan view of the device shown in Figure 1.

Referring to Figure 1, this shows a device 10 for detecting surface temperature of a lithographic plate according to the invention. The device 10 includes an infra-red sensor 12, and a cable 14 for connecting the sensor to a display unit 16. The display unit 16 shown is a digital display, but the infra-red sensor may be linked to a computer console, which console is for operating an automatic printing system of which the device 10 is a part. The automatic printing system may further include a heating unit 20 for pre-heating a laser etched lithographic plate 30. Opposing drive rollers 40 are provided, which rollers may be described as lithographic plate handling means. The space 42 between the opposing drive rollers is part of what can be described as a lithographic plate path.

Figure 2 shows that two or more lithographic plates 30 may be handled by the system of the invention. In the embodiment shown, individual infra-red sensors are provided for each lithographic plate path.

Claims

CLAIMS: 1. A method of monitoring the temperature of a lithographic plate, during manufacture or processing thereof, which method comprises exposing a surface of the plate to an infra-red temperature detector.
2. A method according to claim 1, further comprising the step of heating the plate and using the measured temperature to control the heating step.
3. An apparatus for sensing the surface temperature of a lithographic plate, during manufacture or processing thereof, which apparatus comprises a support for supporting the plate and at least one infra-red temperature detector arranged to receive infra-red radiation from the plate surface.
4. An apparatus according to claim 3, further comprising a means to raise an alarm when a measured plate temperature is not within prescribed limits.
5. An apparatus according to claim 3 or 4, wherein at least one infrared temperature detector is provided on one side of the plate.
6. An apparatus according to claim 3,4 or 5, wherein the one side of the plate is coated.
7. An apparatus according to claim 3,4,5 or 6, wherein at least one infra-red temperature detector is provided on both sides of the plate.
8. An apparatus according to any of claims 3 to 7, wherein the support comprises at least one roller and at least one roller comprises an infra-red temperature detector whereby the detector is brought substantially into contact with the plate.
9. An apparatus according to claim 8, wherein the roller is a drive roller.
10. An apparatus according to claim 9, wherein the drive roller is one of a pair of opposing drive rollers whereby a lithographic plate path is defined therebetween.
11. A system for controlling the temperature of a lithographic plate, during manufacture or processing thereof, which system includes a conveyor comprising a support for supporting the plate, at least one infra-red temperature detector arranged to receive infra-red radiation from the plate surface and a heater whereby the measured temperature of the plate is used to vary the heater output and/or the conveyor speed to adjust the exposure of a plate to heat thereby to modify its temperature to a predetermined level.
12. A system according to claim 11 controlled by a computer.
13. A system according to claim 12, wherein the detected temperature is displayed on a computer screen.
14. A system according to claim 12 or 13, wherein a warning message or signal is displayed on a computer screen.
15. A system according to any of claims 11 to 14, wherein the conveyor defines a plate path and one or more infra-red detectors are arranged along the path.
16. A system according to any of claims 11 to 15, wherein it is capable of handling two or more plates simultaneously.
17. A system according to any of claims 11 to 16, wherein it is automated.
18. A system according to claim 17, wherein the system automation is such that it includes negative or positive feedback to control the heat exposure to the plate.
19. A method of monitoring the temperature of a lithographic plate, during manufacture or processing thereof, substantially as described herein with reference to the accompanying drawings.
20. A system for controlling the temperature of a lithographic plate, during manufacture or processing thereof, substantially as described herein with reference to the accompanying drawings.
21. An apparatus substantially as described herein with reference to the accompanying drawings.