Recorders – Printing – dotting – or punching marker – Ink transfer support or moving means
Patent
1992-12-28
1994-07-05
Reinhart, Mark J.
Recorders
Printing, dotting, or punching marker
Ink transfer support or moving means
358299, H04N 121, B41J 2435
Patent
active
053271670
DESCRIPTION:
BRIEF SUMMARY
When laser engraving flexographic and rotary screen printing cylinders half-tones are generated by providing a series of regularly spaced dots or pits which collectively are referred to as a screen. When printing in color it is conventional to arrange for an axis of the dot or pit patterns which is usually referred to as the screen angle, to be different for each different color separation. This helps to avoid the generation of Moire defects.
A typical laser engraving machine includes a motor to rotate a printing cylinder about its longitudinal axis, a laser, an engraving head which focuses a beam from the laser onto the surface of the printing cylinder to ablate selected portions of the printing cylinder, a drive to move the engraving head along the printing cylinder parallel to its longitudinal axis, a laser controller arranged to control operation of the laser engraving head, and position encoders coupled to both the printing cylinder and the drive for the engraving head to provide data for the laser controller corresponding to the current location of the engraving head with respect to the printing cylinder to enable it to control the operation of the laser.
In laser engraving the size and angle arrangement of the screens is controlled by software input into the laser engraving machine so that it is as if the printing cylinder to be engraved has a layout of imaginary lines defining the boundaries of a large number of identical cells arranged in a regular array around its surface. By reading out the angular position of the printing screen from the encoders the laser engraving machine can determine at any instant when its engraving head notionally crosses a boundary of one of the cells as it moves in a helicoidal fashion over the surface of the printing cylinder. The present invention is concerned primarily with how the laser engraving machine responds after its engraving head notionally crosses the boundaries of one of these cells to remove the required amount of material from inside that cell to give a dot or pit of the required size for the final half-tone required on printing from that printing cylinder.
At present the surface of the printing cylinder is notionally divided into a number of pixels defined in one dimension by the output of the position encoder coupled to the printing cylinder and in the other dimension by the pitch of the helix engraved by the laser engraving machine. The laser engraving machine is then arranged to ablate material from one or more of the pixels in each cell to provide the required half-tone image. There are therefore only a limited number of discrete grey levels that can be achieved corresponding to the number of pixels per cell. With fine screens i.e. those with small cells, this problem increases resulting in only a coarse control of the intensity of a resulting half-tone. At present the only way in which this can be improved is by providing higher precision encoders that can resolve the angular position of the printing cylinder to a greater accuracy. At present the best that can be achieved is a pixel having a circumferential extent of around 10 microns.
In accordance with this invention such a laser engraving machine includes variable timing means which are triggered by the engraving head crossing into each cell and which generate a variable time delay to determine the start time of the laser engraving in each pass of each cell and also to determine the duration of the time for which the laser is turned ON to engrave material during each pass of each cell.
By triggering the variable timing means at the start of each cell and then controlling the engraving by varying the delay before switching ON the laser and by controlling the duration for which the laser is maintained ON provides a more accurate control which provides a higher resolution and thus more finely controlled half-tone than can be achieved merely by clocking the turning ON and OFF of the laser from the angular position encoder associated with the printing cylinder.
Taking a simple example where the timin
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Marsh Terence E. J.
Pollard Brendan T.
Pollard Brendon T.
Reinhart Mark J.
Zed Instruments Limited
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