GB980471A - Improvements in printing presses - Google Patents

Abstract

980,471. Automatic registration control. LINOTYPE & MACHINERY Ltd. Jan. 30,1963 [Jan. 30,1962], No. 3533/62. Heading G3R. A method of registering sheets for a second or subsequent colour printing thereon in a printing press includes scanning a sheet by means of at least two scanning heads each responsive to a given colour registration mark printed on a sheet, controlling information fed to a computer by means of said scanning heads to obtain a computation of front and/or side sheet registration error, or of sheet skew error, and actuating front and/or side lay controlling mechanism under the control of a feedback system to cancel out the sheet registration or skew error entered in the computer. Registration marks 1 to 4 and 9 are printed along one side edge and registration mark 10 along the opposite side edge during the first colour printing operation, the lines 1, 2 being at 45 degrees and the lines 3, 4 at 135 degrees to the direction of travel of the sheet. The sheet is checked temporarily by front and side lays prior to the second printing operation to register the sheet in position. Further registration marks 7, 8,11 and 12, are printed along the side edges at the second colour printing operation. The distances (a - b) + (c - d) and (ab) -(c -d) give a measure of the front and side register errors, respectively, and the distance (e -f) is proportional to the sine of the angle at which the sheet is skewed to the direction of travel. Two pairs of photo-electric cells are provided for scanning the side edges of the sheet. The two cells of each pair are responsive to different colours. In operation, the passage of the mark 11 below the appropriate cell closes a circuit and a train of digit pulses are fed from a pulse generator into a computer until the mark 9 is sensed and the pulse train cut off. The detection of mark 12 inverts the circuit (e.g. of a "Dekatron" (Registered Trade Mark) and the computer starts to subtract from the digit pulses counted over the distance e, the digit pulses counted over the distance f. The result is stored in the computer. This operation is repeated for five sheets (say), the marks 1 to 4, 7 and 8, being ignored, to obtain the skew error for five sheets. Then reversible motors are switched on to move the front lays to correct for sheet skew error, whereby the succeeding sheets are registed squarely prior to the second pringint operation. A contact arrangement of the lays feeds back to the computer a train of pulses proportional to the sine of the angle of correction to cancel out the sheet registration error stored in the computer. When the error has been eliminated, that is, when the computer store is empty, a pulse is emitted to switch off the motors and switch the computer to a state ready to calculate the space in between the registration marks 1, 7; 7, 2; 3, 8; and 8, 4; and hence the side and/or front register errors. These errors are calculated as above by means of two "Dekatrons" for five sheets (say), and then reversible motors are switched on to correct the front and/or side lays. The motors move the front and/or side lays in the correct sense and feed back digit pulses to be subtracted from those held in the store. Since the number of pulses stored are for five sheets, the lay correcting device will emit five pulses for each unit of correction applied to the lays. When the error is eliminated the computer is switched back to detect for skew. Two front lays and one side lay incorporate sheet sensors, each in the form of a pick-up stylus, which generate a signal when touched by a sheet. If one of the styli fails to detect a sheet, a gate associated with the sensor (Fig. 10 not shown) will close and a pulse will be transmitted to the computer, switching it off for one sheet feed cycle. If a predetermined number of such sheets are fed in succession the printing press is stopped and a warning alarm is actuated. Blocks of colour 17, 18 printed at the second and first printing operations respectively, activate the appropriate photo-electric cells to prepare the computer to receive information. The computer may be utilized to compute errors between actual ink film thickness and ink film thickness as required, as described in Specification 974,565 the computations alternating between sheet registration and ink film thickness corrections.