PRINTING MEMBER ENGRAVING

Abstract: A method of engraving a printing member (1) using a digitally controlled engraving head (6, 12) comprising generating a predetermined calibration pattern on a region of the printing member (1) away from the main printing region of the printing member (1), illuminating the printing member in the region of the calibration pattern, detecting light from the region of the calibration pattern, comparing the total light intensity with an expected value, and automatically modifying the laser engraving parameters for the main printing region of the printing member (1) in accordance with the results of the comparison.

Description: PRINTING MEMBER ENGRAVING TECHNICAL FIELD The present invention relates to a method and apparatus for engraving printing members such as rotary screens. BACKGROUND ART Conventional screens, such as those used for screen printing textiles, are formed with perforations which are coated with a photosensitive lacquer. The photosensitive lacquer is exposed through a transparent sheet carrying an opaque pattern - usually a photographic negative - and subsequently developed. After development the unexposed areas of lacquer are removed to leave areas of open perforations forming an ink transmitting pattern. Recently there has been a trend toward the use of digitally controlled engraving heads to expose the screen, rather than using exposure through a photographic negative. The present applicant's earlier European application EP-A-0320137 discloses one example of an engraving machine used for preparing a rotary printing screen. A laser engraving head traverses the screen with the laser turned on to ablate the lacquer from regions of the screen that are required to transmit ink. The control data for the engraving head is primarily determined by the characteristics of the pattern, often made of dots, which is required on the screen. However, the appropriate control parameters for the engraving head to produce a desired output will also depend upon the characteristics of the lacquer, the ink to be used in printing, the nature of the substrate and the performance characteristics of the engraving and printing machines. In order to obtain the desired output the engraving parameters, such as the timing and duration of the pulses from the laser, should be adjusted to reflect these variables. SUMMARY OF THE INVENTION According to the present invention, a method of engraving a printing member using a digitally controlled engraving head is characterised by generating a predetermined calibration pattern on a region of the printing member away from the main printing region of the printing member, illuminating the printing member in the region of the calibration pattern, detecting light from the region of the calibration pattern, comparing the total light intensity with an expected value, and modifying engraving parameters for the main printing region of the printing member in accordance with the results of the comparison. The present invention provides a method of engraving having a control loop which effectively compensates for many of the variables discussed above. By measuring the return of light from a predetermined calibration pattern a value can be determined which, in the example of a perforated nickel sleeve, will correspond directly to the total area of open perforations. By comparing this with the expected value for the predetermined pattern an error value is derived which is then used to correct the control parameters as the main portion of the screen is engraved. This correction may be effected by manual intervention of the operator to set particular control parameters, but preferably is carried out automatically and more preferably as part of a closed control loop. A source of light and a light detector may be arranged on the same side of the printing member, and the light detected after scattering or reflection from the calibration pattern. Preferably, however, the printing member is a perforated, hollow, cylindrical screen and the light illuminating the calibration pattern is transmitted from one side of the screen and detected on the other side of the screen. Preferably the light detector is optically restricted to minimise sensitivity to ambient illumination. The screen may be supported on a mandrel extending along the length of the screen, but preferably the screen is supported without a mandrel by end ring assemblies positioned at either end and the light source or light sensor inside the printing screen is positioned between the end-ring assemblies. Preferably the calibration pattern includes regions corresponding to 0% and 100% transmission and these regions are used to calibrate the light source and detector. When the screen is to be used for solid colour or line work engraving then preferably the calibration pattern includes lines having a predetermined mark-space ratio. Preferably the engraving parameters modified in accordance with the result of the comparison include one or more of the laser intensity and the delay in turning the laser on and in turning the laser off. According to a further aspect of the present invention an engraving machine for preparing printing members is characterised in that it comprises an engraving head, means for generating a predetermined calibration pattern for engraving by the head on a region of the printing member away from a main printing region, a light source for illuminating the printing member in the region of the calibration pattern, a light sensor for detecting light from the region of the calibration pattern, means for comparing the total light intensity with an expected value, and means for modifying engraving parameters for the main portion of the printing member in accordance with the results of the comparison. Preferably the light sensor is optically restricted so that it only accepts light over a small range of solid angle. The use of an optically restricted sensor also minimises sensitivity to ambient illumination. Further immunity from ambient illumination may be achieved by introducing a filter over the detector with a narrow pass characteristic for the illumination spectrum. The light used to illuminate the calibration pattern need not be in the visible part of the spectrum: ultra violet or other appropriate types of radiation may be used. BRIEF DESCRIPTION OF DRAWINGS 5 A method and apparatus in accordance with the present invention will now be described in detail with reference to the accompanying drawings in which: Figure 1 is a schematic cross-sectional view of an engraving apparatus; 0 Figure 2 is a longitudinal section through the engraving apparatus of Figure 1; Figure 3 shows three alternative calibration patterns; Figure 4 shows a further alternative calibration pattern; 5 Figure 5 is a block diagram showing schematically the control system for the apparatus of Figure 1; Figure 6 is a sectional view of an alternative embodiment; and Figure 7 is a schematic view of one example of the 0 arrangement of the light source and light sensor of the engraving apparatus. DETAILED DESCRIPTION OF EXAMPLES A printing screen 1 comprising a hollow perforated 5 nickel sleeve is mounted and supported by a mandrel 2 in a laser engraving machine. The screen 1 is supported towards its ends by end ring assemblies 3. A rotary drive is applied to the mandrel 2 to turn the screen 1 about its longitudinal axis. As the screen 1 turns a laser engraving 0 head 6 traverses the screen 1 in the longitudinal direction. The laser engraving head is turned ON to ablate lacquer from those regions of the screen 1 where ink transmissive areas are required. The image of the required _ . screen - that is the required arrangement of transmissive 5 and non-transmissive areas - is stored in a computer 8 which translates the image into appropriate control signals to turn ON and OFF a laser 7 in the engraving head. o The arrangement of the engraving head and the screen, and the mechanical and control mechanisms associated wi*h the movement of the engraving head relative to the screen, are conventional in nature and generally similar to that described in the present applicant's above cited European application. As described in that application, the engraving head 6 is mounted on a carriage which is moved progressively from one end of the screen to the other as the screen is rotated. Using the method of the present invention, when the engraving head is at the far end of the screen, prior to its moving into the region of the screen to be used for printing, it engraves on the screen a predetermined calibration pattern. Examples of t ' -ee calibration patterns are shown in Figure 3, which t <ws from top to bottom patterns for regular half-tone, horizontal lines and vertical lines respectively. In this preferred embodiment a series formed by repeating one or more of these patterns is engraved, with the pr _,rtion of open perforations in each pattern successively increased. In this manner a step wedge of different intensities from 0% to 100% is produced, as shown in Figure 4. Alternatively, or in addition, the calibration pattern may include a plurality of thin lines having a known predetermined mark-space ratio. The data for generating the calibration patterns is stored in the control computer 8 and is output to the laser engraving head at the start of the engraving of each screen, irrespective of the nature of the image subsequently to be engraved in the main printing region of the screen. As described in further detail below, a light source and light detector are then used to compare a parameter dependent on the total area of open cells with an expected predetermined value, also programmed in the computer 8 and on this basis a control signal to modify the engraving parameters is derived. In the example shown in Figures 1 and 2 the support mandrel 2 extends along the length of the screen. At the end of the screen in the region where the calibration pattern is engraved a light source 5 is mounted in a recess 9. The source 5 is mounted so as to be held stationary as the mandrel 2 rotates and is positioned so as to clear the portion 10 of the mandrel bridging the recess 9. The light source 5 directs light onto the screen 1 in the region of the calibration pattern. Light passing through the screen is detected by a sensor 4 mounted on the outside of the screen 1 opposite the light source 5. A mask may be provided between the source 5 and sensor 4 to ensure that the sensor 4 only receives light from a known precisely determined area. In Figure 7 the light source 5 includes optics 19 to collimate the light. The light then enters a beam expander 17 before transmission to the screen 1 so that only a fixed area of the screen is illuminated independent of its exact position between the illumination source 5 and the sensor 4. A focusing lens 18 is shown situated on the opposite side of the screen 1 to concentrate the transmitted light onto the light sensor 4. The light sensor 4 may be optically restricted so that it only accepts light over a small range of solid angle. This also minimises sensitivity to ambient illumination. The sensor 4 may be a conventional photo-electric cell arranged to produce an output proportional to the intensity of light falling on it across its entire surface. The light source 5 need not be chosen to be blocked by the screen lacquer. Light transmitted by the lacquer is deflected because the surfaces of the lacquer film are not parallel. Once light is deflected from its original direction it cannot reach the sensor 4. As the screen is rotated the light source 5 illuminates different regions of the calibration pattern and the intensity of light reaching the sensor 4 varies according to the total area of open cells. The output from the sensor 4 provides a parameter dependent on the total open area of a region of the calibration pattern. In this preferred example, a separate determination is made for each of the steps of the step wedge. This produces a series of parameters corresponding to different intensities. A corresponding series of expected values is stored in the computer. Comparison of the measured and expected values produces an error signal which is used in the computer 8 to modify the data sent to the engraving head as it moves further along the screen to engrave the image in the main printing portion of that screen. Whenever, for example, it is found that the transmissive areas in the calibration pattern, as measured with the light source and sensor, are less than the expected predetermined value then the control data for the laser may be modified to increase the amplitude of the laser and/or to increase the duration of each pulse of the laser. This first correction in particular is appropriate when the calibration pattern includes thin lines and it is found that the mark-space ratio for circumferential lines is lower than expected. Increasing the duration of the laser pulse is appropriate where the mark-space ratio in the horizontal direction is too low. The error coefficient derived by comparison of the predetermined and measures values may be used in a closed loop control system, with a measured correction being applied to the engraving parameters supplied to the laser, calculated in accordance with the magnitude of the error i.e. an iterative control system. Alternatively, an open loop control system may be used. In this case, after a single engrave and measure sequence, the achieved results are interpolated and possibly extrapolated to produce a response curve. The inverse of this curve is applied to the engraving signal to achieve the required transmission. Although described above in relation to the preparation of open perforated screens the present invention is applicable to many different screen types. For example, it may be used for engraving galvano screens in which acid resistant lacquer is ablated by the laser from a solid copper or nickel surface. For use with such a screen both the light source and sensor are arranged on the same side of the screen when the measurement made on the basis of the different reflectivities of the ablated and non-ablated cells. In a further embodiment of the present invention shown in Figure 6 the light source 5 and sensor 4 are incorporated in an engraving machine generally similar to the type described in EP-A-0320137. This machine includes a laser 11, an engraving head 12 mounted on a movable carriage 13 and end ring assemblies 14. Rather than using a solid support mandrel this machine relies upon the end ring assemblies 14 at each end of the screen being driven in synchronism by motor 15. A movable support ring 16 tracks with the engraving head 12 to support the screen 1 locally. In this case, since there is free space inside the screen immediately adjacent the end rings the mounting of the light source is considerably simplified. When a support mandrel is used, then the source and sensor can be fitted and serviced when the mandrel is stationary, avoiding the need for an axial opening to provide access.

Claims: CLAIMS 1. A method of engraving a printing member (1) using a digitally controlled engraving head (6, 12) characterised by generating a predetermined calibration pattern on a region of the printing member (1) away from the main printing region of the printing member (l) , illuminating the printing member (1) in the region of the calibration pattern, detecting light from the region of the calibration pattern, comparing the total light intensity with an expected value, and modifying engraving parameters for the main printing region of the printing member (1) in accordance with the results of the comparison. 2. A method according to claim 1, in which the comparison of the total light intensity detected from the illuminated calibration pattern with the expected value generates an error signal corresponding to the magnitude of the measured error. 3. A method according to claim 2, in which the error signal is used in a closed loop control system to apply corrections to the engraving parameters of the laser (7, 11). 4. A method according to claim 2, in which the error signal is used in an open loop control system to apply corrections to the engraving parameters of the laser (1 , 11). 5. A method according to any one of the preceding claims, in which the engraving parameters include one or more of the laser intensity and the delay between the laser (1 , 11) being turned or -and being turned off. 6. A method according to any one of the preceding claims, in which the light illuminating the calibration pattern is transmitted from one side of the printing member (1) and detected on the other side of the printing member (1). 7. A method according to any one of the preceding claims, in which the light illuminating the calibration pattern is both transmitted and detected on the same side of the printing member (1) . 8. A method according to any one of the preceding claims, in which a series of calibration patterns are generated adjacent each other, the calibration patterns having incrementally varied intensities, so as to form a step wedge. 9. A method according to any one of the preceding claims, in which the calibration pattern includes a plurality of lines having a predetermined mark-space ratio. 10. An engraving machine for use in preparing printing members (1) comprising an engraving head (6, 12), means (8) for generating a predetermined calibration pattern for engraving by the head (6, 12) on a region of the printing member (1) away from a main printing region, a light source (5) for illuminating the printing member (1) in the region of the calibration pattern, a light sensor (4) for detecting light from the region of the calibration pattern, means (8) for comparing the total light intensity with an expected value, and means (8) for modifying engraving parameters for the main portion of the printing member (1) in accordance with the results of the comparison. 11. An engraving machine according to claim 10, in which the printing member (1) comprises a perforated, hollow, cylindrical screen. 12. An engraving machine according to claim 11, in which the screen (1) is supported on a mandrel (2) extending along the length of the screen (1) . 13. An engraving machine according to claim 11, in which the screen is supported by ring assemblies (14) positioned at either end. 14. An engraving machine according to claims 11 to 13, in which the light source (5) or light sensor (4) is positioned inside the screen. 15. An engraving machine according to claims 11 to 13, in which both the light source (5) and light sensor (4) are on the same side of the screen. 16. An engraving machine according to claims 11 to 15, in which the light sensor (4) is optically restricted so that it only accepts light over a small range of solid angle.

Inventors: ZOLLMAN PETER MARTIN, ZOLLMAN, PETER, MARTIN
Applicants: ZED INSTR LTD, ZED INSTRUMENTS LIMITED
Application year: 1991
Main classification: B41C1
IPC codes: B41C1