US2870253A - Production of printing surfaces - Google Patents

Description

Jan. 20, 1959 c os D ETAL 2,870,253

PRODUCTION OF PRINTING SURFACES Filed July 26. 1954 F G. my log 9 7 United States Patent PRODUCTION OF PRINTING SURFACES John Fothergill Crosfield and Gordon Stanley James Allen, London, England, assignors to J. F. Crosfield Limited, London, England, a British company Application July 26, 1954, Serial No. 445,736

Claims priority, application Great Britain August 11, 1953 9 Claims. (Cl. 1786.6)

This invention relates to the production of printing surfaces including printing cylinders and fiat plates. The invention is particularly applicable to the production of cylinders and flat plates for intaglio printing, but is of general application, including the production of colour plates and monochrome, and can, of course, be employed in making printing plates for letter press and similar forms of printing.

At the present time, the method of making printing cylinders for intaglio printing which is commonly in use, involves the etching of a copper surface of a cylinder to varying depths, or, in some cases, to varying depths and areas. However, other methods are known, including the use of a mechanical cutting tool to vary the depth of cut in accordance with the tones required over different parts of the plate, and another method involves the use of a heated stylus, in which case the effect is produced by varying the depth of penetration. These last two methods are used for producing letter-press plates, but either principle could be applied to intaglio plates. However, they cannot be used for making high definition printing sur faces in a short period of time because there is a limit to the possible acceleration of parts possessing mass under control.

It is also known in the preparation of stencils to make perforations through sheets of prepared paper or other material by means of a spark discharge. In this case, the density of the tone on the stencil is varied by varying the number of holes per unit area. Although the scanning is effected with high definition in this case, the stencil itself of necessity is always of quite low definition because, in this case, the resolved element is not asingle hole but an aggregate of holes. Thus, this system is only suitable in the case of plates for small low definition pictures, whereas the present invention aims at providing large printing cylinders and plates of high definition such as are required for high quality monochrome and colour reproductions.

In intaglio printing wherein ink is retained in cavities or recesses in the printing surface, the density of tone in the reproduction depends upon the quantity of ink held in the recesses or cavities, and thus it depends upon the volume of the latter, and the present invention produces printing surfaces which contain a large number of equally spaced cavities while the characteristics of the individual cavities are varied to produce the variations in density of the tone.

Thus, according to the present invention, a printing plate is produced by causing an electric discharge to pass through a partially electrically-conducting layer which is to form the printing surface and which is made to consist of small areas of different characteristics by removing varying amounts of the material at different points over the surface by setting up variations in a parameter associated with the electrical discharge. The variables associated with the discharge that can be usefully controlled for the purposes of the present invention, bearing in mind that the discharge takes place between a needle-shaped electrode and a flat or cylindrical surface, with the partially conducting layer between them, are the amplitude of the individual pulses, the duration of the individual pulses, the shape of the pulses, the resistance connected in series with the electrodes, and an electric or magnetic field for focussing the discharge and, therefore, associated with it. At present, the most convenient variable to deal with is either the amplitude of the pulse or the duration of the pulse.

The characteristics of the individual recesses in the printing surface are either variable depth with constant area, constant depth and variable area, or simultaneous variation of area and depth. The last of these is to be preferred, since it gives possibilities for a larger range of tone. Incidentally, variation of depth of constant area has the advantage of eliminating what is known as the moir effect which will be referred to further below.

In order to obtain these three different types of characteristic, the procedure may be as follows: First to obtain simultaneous variation of depth and area, the electrode may have a diameter less than the smallest recess required and the thickness of the partially conducting material is greater than the range of depth required. Variation of the amplitude of the pulse or its duration or of resistance in series with the electrode, will then give the simultaneous variation in depth and area. -The electric discharge is limited to a value less than that required to perforate the partially conducting layer. Secondly, to produce recesses varying in depth but of constant area, an electrode is used of the diameter of the recesses required and the pulse is varied in amplitude or duration, or a series resistance is varied, over the lower end of the power range, that is, over a range of power in which the power is not sufiicient to make a hole of larger diameter than that of the electrode. Thirdly, if the partially conducting layer is thin and all the recesses are holes passing through the material, then the areas of the recesses are varied by variation of the amplitude or duration of the pulse or of series resistance, in which case the constant depth is the thickness of the material. The remaining two variables, that is to say shape of the pulse or the use of a focusing field, modifies the above conditions somewhat, but both of these factors can be used in addition as controlled variables. Thus, if an electromagnetic or electrostatic field is used for focussing, it modifies the shape of the lines of force between the electrodes, and the form of the discharge then depends upon the resulting shape of the field. In order to vary the shape of the recesses by this method, all the other parameters may be fixed and a variable current passed through a coil having its axis coincident with the axis of the wire electrode.

The moir effect already mentioned, introduces a very considerable problem in the production of printing cylinders for multi-colour printing by means of electrical engraving. Thus, when a colour reproduction is formed of a combination of small dots of variable area, if the dots of each colour are printed in substantially parallel lines, an interference pattern results which gives rise to an entirely undesirable reproduction; for example, a neutral grey in the picture may appear as a tartan. In ordinary relief and offset printing the usual method of preventing this effect as used by colour printers, is to arrange that the lines of different colours shall not lie parallel to one another but shall be set at relative pre-determined angles usually about 15 degrees. However, in producing a colour printing cylinder electrically, the dots of each colour will generally lie in continuous helices on the surface of the cylinder, and these helices for the different colours are parallel. It has already been mentioned that vanation of depth with constant area carried out in accordance with the invention does eliminate this moir effect. In that case, the area of the cavities is made a maximum and only the depth is variable, while a mobile grade of ink such as the normal kind of ink used in intaglio printing may be employed and then the colour effect is produced by applying to the paper thin superimposed layers of ink. As an alternative, the area of the dots may be varied, but the dots for the different colours may be produced by slightly displacing the commencement of each convolution of the helix on the cylinder by a different amount for each colour and, if necessary, by using electrical discharges of different repetition rates for the different colours.

The invention can be carried out by use of relatively simple apparatus consisting of a long cylinder with the orlgmal to be copied wrapped around one end and the layer of partially conducting material to form a printing surface applied to the other. This material may be a plastic material such as polyvinyl chloride, cellulose acetate, nylon, or a natural or artificial rubber. It is necessary to impregnate such a material with fine electrically conducting particles such as carbon or a finely ground metal powder in order that holes may be produced 1n the printing surface close together without the danger of the discharge forming one hole tracking down through an adiacent hole. However, a metal compound or alloys of hi h resistance may be used as the layer and may be applied, at least in the case of an alloy by electro-plating on to a metal cylinder. The cylinder is rotated and the electrode which may take the form of a fine tungsten wire and the scanning unit which may. for example, consist of a lamp, a lens system and a photo-electric cell are moved axially along the cylinder either by a lead screw or a rotary cam so that the electrode and the scanning unit describe identical helices on the cylindrical surfaces. However. such a simple device suffers from two serious disadvantages. First. that only flexible ori inals can be used. and secondly the en raved surface will always be of the same size as the original. There are, of course, alternatives. for instance, the use of an optical-mechanical system such as a rotating mirror system. In such a system, a small lamp having a high luminous intensity is placed at the focus of a lens which forms the light into a beam of parallel rays. This beam is directed on to a rotating mirror drum which reflects it through a second lens which concentrates the light to a spot image at its focus. Thus as the mirror drum rotates, the spot image produced by the second lens is traversed along a straight line. By moving the lamp with respect to the first lens, the spot is moved in a direction at right angles to the line image so as to produce a complete raster which may be caused to scan a photograph.

However, the preferred system consists in employing a flying spot raster on a cathode ray tube. Such a system can be used in conjunction with either a rotating cylinder or a fiat bed scanned in successive lines by the electrode. The flying spot serving as a source of light is of constant intensity and the resulting beam, after passing through the original, is concentrated on a photo-electric cell, the output of which, after amplification, is employed to provide variable pulses applied to the discharge electrode.

The advantage of working by intaglio printing lies in the fact that only about 20 percent of the material is removed as compared with 80 percent in the case of offset or relief printing.

In order that the invention may be more clearly understood and readily carried into effect, one example of the novel system will now be described with reference to the accompanying drawings, in which:

Figure 1 shows, in purely diagrammatical fashion, the

ample, of turbine blade steel.

elements of the system and the electrical connections shown in block fashion, and

Figure 2 is a diagrammatical representation of one section of the printing surface and one electrode.

Referring first to Figure 2, the partially conducting material 1 is applied to the outer surface of a metal cylinder 2 which is earthed, and the electrode 3 is a tungsten wire or a wire of other suitable metal, for ex- The layer 1 is of one of the materials mentioned above, but in this example it may be polyvinyl chloride mixed with powdered carbon to form an homogeneous plastic sheet 0.007 inch thick. The electrode 3 is a tungsten wire of a diameter 0.0025 inch. The pulses applied to form the discharge between the electrode 3 and the metal cylinder 2 have an amplitude of 220 volts and a duration of 25 micro seconds.

However, good results may be obtained with different dimensions. Thus, for example, the partially conducting layer 1 may have a thickness over the range from 0.002 inch to 0.020 inch. The amplitude of the pulses may range from 200 to 1000 volts, the duration of the pulses may range from 10 micro seconds to 3 milliseconds, and the electrode may have a diameter of from 0.0015 inch to 0.008 inch.

The system shown in Figure 1 comprises the following elements: a cathode-ray tube 4 is energized to produce on the screen a flying spot raster 5 similar L0 that used in television, but of uniform brightness. A lens 6 is provided to project the raster 5 on to the original picture 7 which, in this case, is a three-colour transparency so that the latter is scanned by the flying spot as shown at 8. The original 7 could, of course, be a set of separation negatives or a negative in monochrome. A. collector lens 9 is provided to focus the light passing through the transparency 7 on to three photo multiplier cells 10g, 102* and 10b. For this purpose there are two partly silvered mirrors 11, 12 and the light to the cell 1017 is reflected by the mirror 11 and part of the light passing through the mirror 11 is reflected by the mirror 12 to the cell 10g, and the remainder of the light passes directly to the cell 101'. Colour filters 13g, l3;- and 131) are interposed in the respective paths. Instead of the partly silvered mirrors and filters, dichroic mirrors could, of course, be used. I

The outputs from the three cells 10g, 101' and 1011 are voltages proportional to the transmission of particular colours in the transparency 7 and these voltages are supplied by conductors 14 to three amplifiers and modifying units 15g, 15r and 15b. These units form no part of the invention, but may modify the voltages in various ways. For example, since the transparency may be either a positive or a negative, it is desirable to provide an inverting circuit in the units 15g, 151' and 15b. Since, as mentioned above, the signals from the cells 10g, 101' and 10!) are proportional to the transmission of the different colours in the transparency 7, while the signals to be utilised are proportional to the density of the transparency, logarithmic attenuator circuits are included in the units 15g, 151- and 15b.

Again, it may be desirable to change the tone range law, by including suitable non-linear circuits. Also some simple colour correction may be carried out to compensate for the fact that the printing inks available cannot have a spectral response which is the reciprocal of the colour filters 13g, 131- and 13b. These and other variations may be effected in the units 15g, 151- and 15b. The outputs from these units 15g, 15;- and 15b are respecitvely fed to three modulators 16g, 161' and 16b which produce outputs of suitable form to vary one of the parameters associated with the discharge between the electrode 3 and the cylinder 2 such as the pulse amplitude or pulse length. Variation of pulse amplitude can be carried out by means of any conventional modulator circuit. For example, for that case, it is possible to use the well known four-diode bridge circuit in which during the times that the discharge is not required, the four diodes are rendered conductive and connect the signal point direct to earth, thus ensuring zero voltage across the electrodes 3, 2. When the bi-assing voltage of the diodes is removed, a pulse of an amplitude proportional to the signal is applied across those electrodes.

Variation of the time of the pulses may be effected by connecting the signal voltage to a single-pulse multivibrator circuit by which the time duration is made to depend upon the signal voltage.

The output pulses from the modulators 16g, 16iand 16b appearing in a low resistance circuit are applied to the respective tungsten wire electrodes 3g, Br and 3b which co-operated with the partially conducting layer 1 on the three sections of the cylinder 2 so that the holes or recesses are formed below the three electrodes. Therefore, while the flying spot on the cathode-ray tube 4 crosses an element on the transparency 7, the recesses are formed on the corresponding parts of the printing surface 1 for the three colours. In Figure l the cylinder 2 is shown as having a screw-threaded spindle 17 passing through tapped bores in two bearings 18 so that, at each revolution of the cylinder 2, the latter is moved axially by the pitch of the screw 17. ln practice, of course, this axial movement could be produced by an end cam with a step which causes a slight axial movement at one point in each revolution. It will be realised that the cylinder 2 makes one revolution each time the flying spot 5 makes one sweep across the oathode ray tube 4 and, as the flying spot 5 moves from one line to the next, the cylinder 2 is moved one step along its axis. These are various possible alternatives to the system. For example, although three channels, a blue, red, and green channel, are illustrated, a fourth channel could be added for the black printer. There are also two more significant alternatives. The printing surfaces 1 could be on a fiat bed instead of on a cylinder, with the electrodes 3g, 31' and 3!) moving rapidly across the flat bed and moving with a slow linear movement in a directioh at right angles to correspond to the flying spot 5 of the cathode-ray tube 4. Again, the whole process of colour correction could be carried out while the signals are in the form of electrical voltages. In other words, as an example, the signal output from a scanner as described in patent application Ser. No. 451,466, filed August 23, 1954 which is already in the form of amplitude-modulated pulses of a suitable kind, could be fed directly into the three modulators 16g, 161' and 16b in Figure 1, and the outputs from those modulators applied to the electrodes as already described With reference to Figure 1.

Without affecting the essential mode of operation of the apparatus, it is possible, at any stage at which the signals are in the form of electrical voltages, to transm'i: them from one point to another by way of wire or radio transmission.

We claim:

1. A method of producing a printing surface on plates formed of a rigid electrically-conducting backing and a surface layer of non-flammable, heat-removable, partially electrically-conducting material permanently attached thereto, which method comprises passing an electric discharge transversely through said partially electrically- -conducting layer in regularly recurring pulses, scanning the surface of said layer with said electric discharge and varying a parameter controlling said electric discharge during the scanning to effect removal of varying amounts of the surface material of said layer to different depths at different points to form a plurality of small depressed areas having different depths, and limiting said discharge to a value less than that required to perforate said layer.

2. A method according to claim 1, wherein the said parameter which is varied from point to point over the printing surface, is the amplitude of the individual pulses constituting said discharge.

3. A method according to claim 1, wherein the said parameter which is varied from point to point over the printing surface is the duration of the individual pulses constituting said discharge.

4. A method of producing a printing surface on plates formed of a rigid backing and a layer of non-flammable, heat-removable, partially electrically-conducting material permanently attached thereto, which method comprises passing an electric discharge through said partially electrically-conducting layer in regularly recurring pulses from an electrode spaced from the surface of said layer and having a diameter smaller than the smallest recess required in said printing surface, said layer being thicker than the required range of depths of the recesses required in said printing surface, scanning the surface of said layer by said electrode and varying the amplitude of the individual pulses constituting said discharge during the scanning so as thereby to form recesses at the surface of said layer varying both in area and in depth but limited to a depth less than the thickness of said layer.

5. A method of producing a printing surface on plates formed of a rigid backing and a layer of non-flammable, heat-removable, partially electrically-conducting material permanently attached thereto, which method comprises passing an electric discharge through said partially electrically-conducting layer in regularly recurring pulses from an electrode spaced from the surface of said layer and having a diameter smaller than the smallest recess required in said printing surface, said layer being thicker than the required range of depths of the recesses required in said printing surface, scanning the surface of said layer by said electrode and varying the duration of the individual pulses constituting said discharge during the scanning so as thereby to form recesses at the surface of said layer varying both in area and in depth but limited to a depth less than the thickness of said layer.

6. A method of producing a printing surface on plates formed of a rigid backing and a layer of non-flammable, heat-removable, partially electrically-conducting material permanently attached thereto, which method comprises passing an electric discharge through said partially electrically-conducting layer in regularly recurring pulses from an electrode spaced from the surface of said layer and having a diameter smaller than the smallest recess required in said printing surface, said layer being thicker than the required range of depths of the recesses required in said printing surface, scanning the surface of said layer by said electrode and varying a resistance in series with said electrode during the scanning so as thereby to form recesses at the surface of said layer varying both in area and in depth but limited to a depth less than the thickness of said layer.

7. A method according to claim 2, wherein in addition to the variation of the amplitude of the pulses, a focussing field of adjustable value is applied to the discharge during the scanning as an additional controlled variable.

8. A method according to claim 3, wherein in addition to the variation of the duration of the pulses, a focussing field of adjustable value is'applied to the discharge during the scanning as an additional controlled variable.

9. A method of producing a printing surface on plates formed of a rigid backing and a surface layer of heatremovable partially electrically-conducting material permanently attached thereto, said layer being formed of insulating plastic material impregnated with fine, electrically-conducting particles, which method comprises passing an electric discharge through said partially electrically-conducting layer, scanning the surface of said layer by said electric discharge and varying a parameter controlling said electric discharge during the scanning to effect removal of varying amounts of the surface material of said layer to different depths at different points to form a plurality of small cavities having different depths, and

References Cited in the file of this patent UNITED STATES PATENTS Cooley Feb. 19, 1929 8 Finch June 1, 1937 Hackenbcrg Apr. 18, 1944 Dalton June 5, 1951 Bedford Feb. 3, 1953 Faus July 28, 1953 Dalton Dec. 29, 1953 Dalton Dec. 29, 1953

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