Patent GB-1129675-A (GB)
Title: Improvements in etching devices for printing cylinders
IPC: B41C1/00
Filed: 1965-12-08
Abstract: 1,129,675. Etching. SOC. NATIONALE D'EDITIONS ARTISTIQUES. 8 Dec., 1965 [14 Dec., 1964], No. 52096/65. Heading B6J. A device for etching the surface of a printing cylinder 1 comprises a tank 3 containing liquid etchant, means for supporting and rotating the surface in the etchant about a horizontal axis, a pump 8 for circulating the etchant, through suction pipe 6 and delivery pipe 10, and means for gradually modifying one of the factors for varying the rate of etching; the device further comprising, programming means capable of producing a plurality of times during etching, an indication as to the degree of etching, scheduled to-have occured at said times according to a predetermined programme; means for detecting at each of said times, the actual' state of etching; means for comparing at each of said times the scheduled degree with the actual degree of etching; and means for modifying at each of said times at least one of the factors capable of varying the rate of etching e.g. the dilution of the etchant, to bring the actual state of etching at said given time into accord with the stateof etching scheduled in respect of this given time. In a preferred embodiment, Fig. 1, the detecting means comprises a series of areas 26 of decreasing permeability to the etchant extending along one of the generatrices of the masking sheet 2 covering the cylinder 1 and carrying the design to be etched. The areas 26 which produce, in the cylindrical surface, a series of recesses of different depthsare scanned by microscope 27 moved along slideways 29 by motor 33 according to the required etch rate such that if etching is proceeding according to programme no angular displacement of microscope 27 from its position shown in full lines will be required for an operator to observe therethrough the area 26 where etching is just beginning, as indicated by a reflectivity charge in that area. If etching is not proceeding according to plan the operator needs to displace the microscope, e.g. as shown in dotted lines, toobservethe area where etching is just beginning, and this displacement, represented as a potential difference by means of a potentiometer serves to control pumps 25 and 12 to dilute or concentrate the etchant as required. Alternatively manual control of the composition of the etchant may be effected. In a further embodiment variations, in reflectivity of areas 26 are directly recorded by a photo-electric cell which automatically follows the progress of etching from area to area in the group 26, the actual position of the cell on its carriage at any given time being compared with at position calculated if etching is proceeding according to plan and the result of this comparison being used to vary the concentration of theetchant, if necessary, to restore the programme. Alternatively a photo-electric, cell which automaticallylines up with the area where etching is just beginning may replace microscope 27. In a further embodiment a photo-electric cell is employed to automatically follow the progress of etching along areas 26 and the reflectivity of these areas is comparedwith an external source of illumination varying in accordance with the etching programme theresult of the comparison being used to control the etch rate. The corrections applied in any of the above embodiments may additionally be the pressure of the etchant on the surface being etched, the speed of displacement of the surface with respect to jets of etchant projected thereonto or the temperature of theetchant. The etchant may be an aqueous solution of iron perchloride, in which case the etch rate may be reduced by supplying, through pump 25, concentrated iron perehloride, and. the rate may be increased by supplying water through pump 12.