| 摘要 |
<p>When laser engraving flexographic and rotary screen printing cylinders half-tones are generated by providing a series of regularly spaced cells (1) including dots or pits (3) which collectively are referred to as a screen. By varying the ratio between the areas of the dots or pits (3) and the cells (1) the intensity of the half-tone is varied. A laser engraving machine for engraving such printing cylinders includes variable timing means (37, 38) which are triggered by their engraving head (13) crossing into each cell (1) and which generate a variable time delay (D) to determine the start time of the laser engraving in each pass of each cell (1) and also to determine the duration (P) of the time for which the laser is turned ON to engrave material during each pass of each cell.</p> |
