Method And Apparatus For The Seamless Engraving Of Half-tone Printing Plates

Abstract

Method and apparatus is provided for point by point and line by line production of half-tone printing cylinders which are seamless in a direction perpendicular to the direction of the lines from a cylindrical master having a seam in that direction. The surfae of the master, including the seam, is scanned to produce a selected number of engraving signals per revolution of the master and printing cylinders. Of these, a certain small number will correspond to the seam on the master and these are eliminated on the printing cylinder and the engraved image on the printing cylinder is "stretched" a corresponding number of engraving points by repeating the engraving of a number of scanning points on the master image.

Parent Case Text

CROSS-REFERENCE TO RELATED APPLICATION

This is a Continuation-In-Part of parent application Ser. No. 25,978 filed Apr. 4, 1970 and now abandoned.

Description

BACKGROUND OF THE INVENTION

In the production of cylindrical half-tone printing plates there is the difficulty that the master disposed on a cylinder has a seam extending transversely to the line direction and situated where the edges of the fitted master abut each other, thus forming a joint. The master is therefore interrupted over some distance, so that a printing plate engraved from this master would also have a seam. This is undesirable, however, because with continuous printing from such a half-tone printing plate the product would repeatedly be interrupted by seams. Various attempts have already been made to reduce the seam in the master to a very small size, but these attempts have not yielded the desired result.

BRIEF SUMMARY OF THE INVENTION

The invention provides apparatus and method for the production of a seamless half-tone printing plate wherein a number of points are repeatedly engraved in the line direction. The number of points thus engraved is so chosen that the seam is entirely bridged. Preferably, the points repeatedly engraved on a line are staggered--in the line direction in relation to the points repeatedly engraved on the adjacent lines, as disclosed in copending application Ser. No. 185,946 filed Oct. 4, 1971. This prevents the repeatedly engraved points from being disposed on a straight line, which could cause a blemish in a print made with this half-tone printing plate.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 is a combined view showing principal portions of the apparatus and including circuit components for carrying out the invention; and

FIGS. 2 and 3 are diagrammatic time diagrams illustrating certain aspects of the invention.

The master 2 disposed on the rotatable cylinder 1 has a seam 3 which extends approximately at right angles to the line direction in which the master 2 is scanned and which seam occupies a small but finite number of total scan points per revolution of the cylinder. The scanning of the master rotating with the cylinder 1 is performed pointwise by means of the scanning element 4, which measures the intensity of the light which is emitted by the lamp 5 and reflected by the master. The number of scan points per revolution of the cylinder is controlled by a screen generator 24 deriving its signals from the shaft which drives the cylinder and providing a fixed number of control signals per revolution. The screen generator 24 provides these output signals at the conductor 45 which are applied to shift the registers 18-23 and these signals are also used to control the output of the converter 17, as indicated by the conductor 46, and the engraving control converter 32, as well, is controlled in accord with the screen generator frequency as symbolically indicated by the conductor 47. Details of the screen generator and the converters are disclosed in copending applications Ser. Nos. 185,946 and 185,947, filed Oct. 4, 1971 and incorporated herein by reference.

The engraving element 6 engraves the plate 8 disposed on the cylinder 7 and thereby produces a half-tone printing plate. The cylinders 1 and 7 are disposed on the same shaft and rotate synchronously. Two signal transmitters 9 and 10 are disposed on the same shaft as the cylinders 1 and 7. The signal transmitters 9 and 10 each comprise a cylindrical part 11 and 12 formed with a number of apertures, a lamp 13 and 14 respectively on the inside of the cylindrical part 11 and 12 respectively, and a photo-electric cell 15 and 16 respectively on the outside of the cylindrical part. At the instant when, during the rotation of the cylindrical part, an aperture in the cylindrical wall is situated between the lamp and the photo-electric cell, a signal is generated. The cylindrical parts 11 and 12 rotate synchronously with the cylinders 1 and 7. The analogue information scanned by the scanning element 4 is converted into digital information in the circuit 17. From the circuit 17 the digital information can be fed to the first (18) of a number of memory cells 18-23. These memory cells 18-23 are connected as a serial shift register. The control pulses for shifting the information through the shift register are derived with the aid of the circuit 24 from the screen frequency. Associated with the respective memory cells 18-23 are the AND-gates 25-30. The output of each AND-gate is connected to the input of an OR-gate 31. The output of the OR-gate 31 is connected to the circuit 32 which converts the information obtained from the OR-gate 31 into control signals for the engraving element 6. At any particular time one and only one of the AND-gates 25-30 is open for passing on the contents of the associated memory cell to the OR-gate 31. The AND-gate which is open is determined by the contents of the 3-bits register comprising the bistables 33, 34 and 35, the output of each of which is connected to the decoding circuit 36. The decoding circuit 36 has six outputs 37-42. Output 37 is connected to an input of the gate 25, output 38 is connected to an input of gate 26, etc. Of the six outputs 37-42, one and only one is energized at any particular time to enable a respective AND gate. Each time a pulse is emitted by the photo-electric cell through the agency of the circuit 43, the contents of the 3-bits register 33, 34 and 35 are increased by "1". During each revolution of the cylinders 1 and 7 the bistables 33, 34 and 35 and the memory cells 18-23 are reset by a pulse from the photoelectric cell 15 through the agency of the circuit 44.

The apparatus functions as follows. When the aperture in the cylindrical part 11 passes the photo-electric cell 15, the circuit 44 emits a pulse which resets the two registers 33, 34, 35 and 18-23. During the continuing rotation of the cylinders 1 and 7 the information relating to the scanned point of the master 2 from the analogue-digital converter 17 is fed, at the screen frequency, into the memory cell 18. When the next point in the line direction is scanned, the information relating to the previously scanned point is transferred to the memory cell 19, while the information relating to the present point is again fed into the memory cell 18. When six points on a line have consecutively been scanned, the entire register 18-23 is filled with information relating to the six points which have been scanned. When the seventh point is scanned, the information relating to the first point, which information was in the memory cell 23, is lost. During this time, the contents of the register 33, 34, 35 is such that the output 36 is energized, so that the gate 25 is open and the information is fed from the memory cell 18 through the gate 25 and the gate 31 to the circuit 32 which emits the control signals for the engraving element 6. Therefore up to the instant when the contents of the register 33, 34, 35 change, the point last scanned is always engraved. When an aperture in the cylindrical part 10 passes the photo-electric cell 16, a pulse is generated which is fed through the circuit 43 to the register 33, 34, 35 whose contents are consequently increased by "1". Since the register 33, 34, 35 now contains the value "1", the output 38 is energized and the gate 26 is opened, while the gate 25 closes. Now therefore the information relating to the point which is in the memory cell 19 is engraved. The first point which is now engraved is the same point as the one last engraved from the memory cell 18, since the information in the memory cell 18 has been shifted to memory cell 19. The point concerned is therefore engraved twice in succession. Thereafter the point penultimately scanned is engraved each time. This continues in this way until the next pulse from the photo-electric cell 16 causes the contents of the register 33, 34, 35 to be increased by "1". Then the memory cell 20 is selected and a corresponding point is engraved twice.

To illustrate the principles according to the present invention, let it be assumed that the screen generator 24 generates three hundred control signals per revolution of the cylinder 1 and that the disc 9 has its opening so set as to create a reset signal for the registers 18-23 and for the registers 33, 34 and 35 just after the scanning head 4 has passed the seam 3 but before the first scanning point on the cylinder 1 beyond the seam is reached. Let it also be assumed that the seam 3 occupies a width on the cylinder 1 corresponding to 5 scanning points. Let it further be assumed that the disc 10 contains five openings so that there are five signals to step from the gate 25 to the gate 26; from the gate 26 to the gate 27 and so on until the last step from the gate 29 to the gate 30, and that these stepping pulses occur just after the 30th, 90th, 150th, 210th and 270th scanning points, the first scanning point being the first point just after the seam 3. Under these conditions, the 30th, 89th, 148th, 207th, and 266th scanning points will be engraved twice and, moreover, all five scanning points corresponding to the seal 3 will be skipped.

To illustrate how this occurs, reference is now had to FIG. 2 wherein the registers 18-23 contain, at the 300th scanning point which corresponds to the last point on the seam 3 just before the image on the cylinder 1 again begins, all five of the scanning points corresponding to the seam 3, that is the 296th-300th scanning points will be contained in the registers 15-22 whereas the last scanning point on the image prior to the seam 3, the 295th scanning point, will be in the register 23 and will be read out, as indicated, because the gate 30 is enabled.

If there were no resetting of the registers, the next scanning point after the situation of FIG. 2 would be as depicted in FIG. 3, wherein the various registers 18-23 contain the scanning points as noted in FIG. 3. Of course, the registers have been reset just prior to the first scanning point so that, in actuality, the situation of FIG. 3 is accurate only as to the first register 18, the remaining registers 19-23 being empty.

Thus, by cycling back from the last register 23 as indicated in FIG. 2 back to the register 18 as indicated in FIG. 3, all five scanning points containing the seam scans will be skipped. Thus, it will be seen that the number of scanning points which may be skipped is one less than the number of gates 25-30 and that the number of openings in the disc 10 is not more than this number although, as is evident it could be less. That is to say, the number of scanning points on the entire circumference of the cylinder 1 which will be skipped and which are chosen to correspond to the seam area 3 is equal to the number of openings in the disc 10 since, for each opening in the disc 10, one of the scanning points on the image portion of the cylinder, exclusive of the seam 3 will be engraved twice and, to accommodate for this, there must be a number of gates 25-30 which is one greater than the number of scanning points which are engraved twice.

It is further to be understood that the analog-to-digital converter 17, although shown with only one output conductor 48 issuing therefrom, in reality will contain a number of output conductors each having the bit of a binary word thereon. Preferably, the binary words from the converter 17 are five-bit binary words so as to allow a maximum of 32 scanning levels to be accommodated for and, correspondingly, there will be five sets of registers 18-23 and five corresponding sets of AND gates 25-30 and five OR gates 31, all feeding into the digital-to-analog converter 32 to provide in each instance a single analog output signal for actuating the engraving head 6.

Ordinarily, the width of the seam on the master will be in the order of five or six scanning points so that the capability of eliminating five scanning points on the seam as specifically disclosed will be entirely adequate. That is to say, it is not essential that every scanning point of the seam be eliminated, material reduction in its width as engraved may be sufficient under some circumstances. If a seam, on the other hand, were to be of substantially less width-say only two scanning points wide-the use of a disc 10 having only two openings is sufficient completely to eliminate it, in which case only three of the gates 25-30 would be functioning for this purpose.

Claims

What Is claimed Is;

1. In a system for producing half-tone screen printing cylinders:

a master cylinder having an image portion interrupted circumferentially by a seam of known circumferential width;

a printing cylinder upon which the image on said master cylinder, at least substantially exclusive of said seam, is to be engraved;

means for circumferentially scanning said master cylinder to produce a selected number of engraving signals per complete circumferential scan cycle and including a number of certain engraving signals corresponding to the circumferential width of said seam;

means for circumferentially engraving said printing cylinder in accord with said engraving signals during each engrave cycle, and

means for causing a selected number of said engraving signals corresponding to different parts of said image portion from throughout said image to be engraved twice and a corresponding selected number of said certain engraving signals corresponding to said seam to be skipped during each circumferential engrave cycle.

2. In a system for producing half-tone screen printing cylinders as defined in claim 1 wherein said selected number of engraving signals is substantially equal to said certain number of engraving signals corresponding to said width of the seam.

3. In a system for producing half-tone screen printing cylinders as defined in claim 1 wherein the last mentioned means comprises a plurality of storage members connected as a serial shift register and gate means for passing signals from one storage member at a time to said means for engraving, said gate means including a plurality of gates associated with said storage members and control means for periodically stepping from one storage member to the next as the output to said means for engraving.

4. In a system for producing half-tone screen printing cylinders as defined in claim 3 wherein said plurality of gates is equal to one more than said selected number of engraving signals to be skipped.

5. In a system for producing half-tone screen printing cylinders as defined in claim 4 wherein said engraving signals are connected to the first of said series of storage members and the serial shift register comprised of said storage members is shifted at the same rate as the engraving signals are generated.

6. Apparatus for producing a half-tone image on a cylindrical printing cylinder which occupies the entire circumference of the printing cylinder along a certain length thereof and which is derived from an image on a master cylinder which occupies slightly less than the full circumference thereof due to the presence of a longitudinal seam, said apparatus comprising:

means for rotating said master cylinder and said printing cylinder in unison;

means for scanning said master cylinder point-by-point and line-by-line over the longitudinal length of the image thereon, and for producing a selected number of point-by-point engraving signals per complete revolution of said master cylinder and said printing cylinder whereby certain of said engraving signals per revolution correspond to the presence of said seam;

means for engraving said printing cylinder point-by-point and line-by-line in accord with said engraving signals and to produce, per complete revolution of said printing cylinder, a number of engraving points equal to said selected number of engraving signals; and

means for eliminating at least substantially all of said certain engraving signals per revolution which correspond to said seam and replacing them by repeated engraving signals generated from different portions of said image from throughout said image on the master cylinder.

7. Apparatus according to claim 6 wherein the last means comprises serial shift register means receiving said engraving signals, control means for shifting out said shift register means in parallel, and gate means for selecting one of the parallel outputs at a time to control said means for engraving.

8. Apparatus according to claim 7 wherein said gate means includes a number of serially selected gates equal in number to one more than the number of engraving signals which are eliminated per revolution.

9. The method of producing a half-tone image on a cylinder printing cylinder which occupies the entire circumference of the printing cylinder along a certain length thereof and which is derived from an image on a master cylinder which occupies slightly less than the full circumference thereof due to the presence of a longitudinal seam, which comprises the steps of:

a. rotating the master and printing cylinders in unison while scanning the surface of the master cylinder point-by-point and line-by-line at a selected scanning point rate per revolution;

b. engraving the printing cylinder point-by-point and line-by-line at the same rate per revolution as the scanning of step (a); and

c. during each revolution in step (b), skipping those scanning points corresponding to at least a major portion of the seam and substituting therefor repetitively engraved scanning points derived from different portions of the image from throughout said image on the master cylinder.

10. The method of producing a half-tone image as defined in claim 9 wherein the repetitively engraved scanning points of step (c) are distributed throughout the image on the printing cylinder.

11. The method of producing a half-tone image as defined in claim 10 wherein each repetitively engraved scanning point is engraved twice in succession.

12. Apparatus for producing a half-tone image on a cylindrical printing cylinder which occupies the entire circumference of the printing cylinder along a certain length thereof and which is derived from an image on a master cylinder which occupies slightly less than the full circumference thereof due to the presence of a longitudinal seam, said apparatus comprising:

means for rotating said master cylinder and said printing cylinder in unison;

means for scanning said master cylinder point-by-point and line-by-line over the longitudinal length of the image thereon, and for producing a selected number of point-by-point engraving signals per complete revolution of said master cylinder and said printing cylinder whereby certain of said engraving signals per revolution correspond to the presence of said seam;

means for engraving said printing cylinder point-by-point and line-by-line in accord with said engraving signals and to produce, per complete revolution of said printing cylinder, a number of engraving points equal to said selected number of engraving signals;

means for eliminating substantially all of said certain engraving signals per revolution which correspond to said seam; and

means for repeating a sufficient number of engraving signals generated from different portions of said image, from throughout said image on the master cylinder remote from said seam as to replace the eliminated engraving signals.