TECHNICAL FIELD

This invention relates to an apparatus and method for preparing a cylindrical medium for use in a printing process, and more particularly relates to a system suitable for the formation of surface cavities or cells on an anilox or gravure roller.

BACKGROUND ART

Conventionally, an anilox roller is prepared for use in a printing press by focusing a beam of pulsed laser light onto a small area on the roller to engrave a cell. The roller is rotated and the focused laser beam is moved along the roller parallel to its axis to produce a helical pattern of cells with each cell being formed by one pulse from the laser. By synchronising the rotation of the roller, the frequency of the laser pulses, and the movement of the focused laser beam, the cells can be arranged in patterns, for example at the corners of hexagons, diamonds, or parallelograms, the latter required for gravure printing rollers. The preparation of a roller requires a large number of cells to be engraved on the surface of the roller. The cells should be deep and have smooth sides, and the area of unengraved land between adjacent cells should be minimised. When a hexagonal array of cells is engraved on the roller, it is possible to engrave the cells closely together. For diamond arrays the cells cannot be engraved as densely without the walls between adjacent cells breaking down. This is known as channelling because it allows ink to channel between cells. This is undesirable. As the cells in a diamond array cannot be arranged as densely as in a hexagonal array, the area of unengraved land for a diamond pattern is higher than for a hexagonal pattern.

In conventional laser engraving apparatus, the laser beam remains focused at a fixed position in space, however the roller rotates and therefore the cells tend to be elongated circumferentially. Further, the circumferential elongation of the cells, whilst reducing the area of unengraved land, is likely to lead to channelling. The laser pulse has a rapid change in intensity at its leading edge, and a gradual change in intensity at its trailing edge. The gradual intensity change anneals the cell giving the desired smooth finish. However, due to the rotation of the roller, the leading edge of the cell is not irradiated by the trailing edge of the laser pulse, and so is not annealed, but has a rough surface. To overcome this problem, our earlier International Patent Applications PCT/GB 93/01235 and PCT/GB 91/01829 describe an engraving apparatus which deflects the laser beam in correspondence with the rotation of the roller to give zero relative surface speed between the roller and the laser beam during engraving. The cells produced by this system are circular, deep and have smooth sides. As there is no danger of channelling, the cells may be engraved at a higher rate than is possible with conventional apparatus.

DISCLOSURE OF INVENTION

According to the present invention, a method of engraving or exposing a pattern of cells on a cylindrical medium including the steps of rotating the medium about its longitudinal axis, and irradiating the surface of the medium with a focused laser beam to form a cell, the focused laser beam being movable in a direction parallel to the longitudinal axis of the medium to allow a pattern of cells to be formed on the medium, is characterised by modifying the cross-section of the laser beam so that the cells are substantially oval.

By forming substantially oval cells, the area of unengraved land is reduced compared to circular cells. This is particularly beneficial when the cells are arranged in a diamond pattern. The oval cells may be formed e.g., by forming a focused image of an appropriately shaped cut out. It is much preferred however that the lens should produce a diffraction-limited region of illumination and that the beam should be modified to provide an effective aperture prior to the lens having a greater extent in one direction then in an orthogonal direction. This then avoids any need to focus the beam prior to the final focusing lens, and consequent problems with the high power density of the focused beam.

Preferably, the focused laser beam tracks the surface of the medium during cell formation so that there is no relative movement between the focused laser beam and the surface of the medium during cell formation. This ensures that the cells formed are annealed, and reduces the risk of channelling. The tracking may be provided by an acousto- optic deflector.

The cross-section of the laser beam is preferably modified by clipping part of the beam so that the beam has a larger dimension in one direction than in an orthogonal direction. This clipping may be achieved using an optical element disposed between the laser beam source and a focusing lens, the optical element having an oval aperture in a plane normal to the laser beam. In this case, the optical element used to modify the cross-section of the laser beam may have a circular aperture, and be inclined with respect to the normal plane. The surface of the optical element may either be opaque, in which case part of the beam is blocked to give the required shape, or may be reflective so that the excess part of the beam is reflected.

Alternatively, the cross-section of the laser beam may be modified by a variable width slit which is adjusted to block some of the laser beam, and thereby remove the circular symmetry.

Further still, the beam can be made to overfill the width of the acousto-optic deflector where the deflector is included in the system, as determined by the width of the acoustic waves travelling through the interaction medium. By overfilling, the input laser beam has a dimension greater than the width of the acoustic waves propagated through the crystal and therefore only the central part of the beam is refracted as it passes through the crystal. This method restricts ovality to a narrowing in a plane parallel to the roller axis since the acoustic waves must travel perpendicular to this for tracking around the roller. The resulting cells have their minor axis parallel to the direction of rotation.

Another method of modifying the cross-section of the laser beam is by expanding or condensing the beam in one plane. This may be achieved by introducing an astigmatism into the beam, for example using a cylindrical focusing optic. With this system, the resulting ovality varies with focus position. Positioning the focused laser beam on the roller at or near the beam waist is normally used as a variable controlling the aspect ratio of opening to depth of the cell. Using focus position to set ovality loses this variable for setting the aspect ratio. Control of ovality can be achieved by varying the degree of astigmatism introduced by the cylindrical optic. However the focus position must be adjusted with any change in astigmatism to achieve the right combination of ovality and aspect ratio.

A further way in which the beam can be modified is by anamorphic compression by passing the beam through a plurality of prisms. In this way, the beam is compressed

in one plane, or expanded in one plane. Preferably four prisms are used as this brings the outgoing beam onto the same axis as the ingoing beam. The prisms should have anti-reflective coatings to ensure good transmission efficiency, and the orientation of the beam with respect to the prisms should be selected to enhance the transmission efficiency which depends on the refraction which in turn depends on the polarization.

According to a second aspect of the present invention, an optical head for a laser engraving machine for engraving or exposing a pattern of cells on a cylindrical medium comprises a focusing lens for focusing a laser beam onto the surface of the medium by a focusing lens to form a pattern of cells on the medium, characterised by a beam modifying means arranged in the path of the laser beam to modify the cross-sectional shape of the laser beam so that the cells formed on the medium are substantially oval.

Preferably, the optical head is provided with a means to deflect the focused laser beam during cell formation to track the rotation of the medium so that there is no relative movement between the focused laser beam and the surface of the medium during cell formation.

The beam modifying means may be an optical element having an oval aperture in a plane normal to the laser beam. In this case, the oval aperture may be formed from a circular aperture, which is inclined with respect to the normal plane. The optical element may have either an opaque surface to block the laser beam, or a reflective surface to reflect the laser light.

The beam modifying means may alternatively be a variable width slit which is adjusted to block off some of the laser beam, and thereby remove the circular symmetry,

or may be the acousto-optic deflector which is overfilled with the laser beam.

Alternatively, the beam modifying means may be a cylindrical focusing optic, for example a lens or curved mirror which introduces an astigmatism into the beam, or a plurality of prisms which compress the laser beam to the required cross-section. In these cases the beam is compressed or expanded rather than being clipped.

The laser beam should have a high instantaneous power and so may be a pulsed laser beam, or may be a continuous wave (CW) laser with an associated means to operatively deflect the beam away from the cell site, or block the beam when it is not required to expose a cell.

According to a third aspect of the present invention, an apparatus for engraving or exposing a pattern of cells on a cylindrical medium comprises a motor for rotating the medium about its longitudinal axis, and a laser beam source, and is characterised by further comprising the optical head of the second aspect.

According to a fourth aspect of the present invention, a cylindrical medium manufactured by the method, or using the apparatus of the first or second aspect, includes substantially oval cells arranged in a pattern with their major axes lying generally parallel to the longitudinal axis of the medium, or circumferentially on the surface of the medium. Any other orientation of cells is possible. Preferably, the cells are formed in a diamond, or parallelogram pattern.

DESCRIPTION OF DRAWINGS

An apparatus in accordance with the present invention will now be described in detail and contrasted with the

prior art with reference to the accompanying drawings, in which: -

Figure 1 is an apparatus for engraving oval cells on a cylindrical medium; Figure 2 shows the convergence of a light beam by a focusing lens;

Figure 3 and 4 show the diffraction effect at the focal point of a focusing lens;

Figure 5 shows an acousto-optic deflector; Figure 6 shows a first example of a beam modifying means;

Figures 7 and 8 show a second example of a beam modifying means;

Figure 9 shows the cross-section of a laser beam modified by the beam modifying means of Figures 7 and 8;

Figure 10 shows a third example of a beam modifying means;

Figure 11 shows the cross-section of a laser beam modified by an overfilled acousto-optic deflector; Figures 12a to 12c show cell patterns on an anilox roller; and

Figure 13 shows an aperture and image system for engraving oval cells.

Figure 1 shows an apparatus for engraving substantially oval cells C on an anilox roller 2. The apparatus includes a motor 1 which rotates the roller 2 about its longitudinal axis. A laser engraving head 6 is provided on a movable carriage 7, the carriage 7 being movable along a plane parallel to the longitudinal axis of the roller 2. The laser engraving head 6 includes a focusing lens L which focuses a laser beam from a beam source 4 onto the surface of the roller 2. When the focused beam is incident on the surface of the roller 2, a cell C is engraved on the roller 2. By controlling the angular position of the roller 2, and the focus point of

the focused laser beam along the roller 2, a pattern of cells C are engraved on the roller 2.

The laser engraving head 6 includes a mirror 11 which reflects the laser beam towards the roller 2. The reflected beam is passed through a beam modifying means 13, an acousto-optic deflector AOD, and the focusing lens L.

The laser beam emitted from the laser source 4 has a generally circular cross-section, and is a parallel beam. When this beam is passed through the beam modifying means 13, the circular symmetry of the cross-section of the beam is removed, thereby giving a laser beam having a larger dimension in one direction than in an orthogonal direction, such as shown in Figures 9 and 11. This modified beam is focused by the focusing lens 11.

As shown in Figure 2, where a laser beam is focused by a focusing lens, the beam converges towards a focus point FP. As shown more clearly in Figures 3 and 4, at the focus point FP the beam converges to a "waist". The dimension of this waist d., d ₂ is determined by the wavelength λ of the light, and the convergence angle 0., 0 ₂ . The dimension is given by the equation _d _ 1.22λ θ

As shown in Figure 3, where the beam diameter D ₁ is small, the angle of convergence 0. is also small, and accordingly the dimension of the waist d ₁ at the focal point will be large. As shown in Figure 4, where the diameter D ₂ is large, the angle of convergence θ _z will also be large, and accordingly the dimension of the waist d ₂ will be small. As the beam modifying means 13 modifies the input laser beam so that its dimension in one direction is greater than its dimension in an orthogonal direction, when the beam is focused by the focusing lens L, the large

dimension of the beam will have a large angle of convergence, and accordingly the resulting focused beam at the focal point will have a small dimension as shown in Figure 4. Conversely, the smaller dimension of the beam will have a smaller angle of convergence, and accordingly, by diffraction, the focused beam will have a larger dimension in this direction. It will therefore be seen that the focusing lens L produces a diffraction limited region of illumination which is substantially oval with its major axis rotated by 90° with respect to the larger dimension of the modified laser beam. Accordingly the engraved cell will be substantially oval.

The apparatus includes an encoder 3 which monitors the angular position of the roller 2. The encoder supplies signals to a driver 5 which drives the laser source 4 to produce the pulsed laser beam. The driver also supplies a signal 12 to an acousto-optic deflector AOD provided on the laser engraving head. As described in our earlier applications PCT/GB 91/01829, and PCT/GB 93/01235, and as shown in Figure 5 of the present application, the acousto- optic deflector AOD includes a germanium crystal 43, on one surface of which is provided at least one piezo-electric transducer 44, and on the opposite side an acoustic absorber 45. The transducer 44 is provided with an RF voltage, which creates acoustic waves which pass through the crystal 43. The acoustic waves passing through the crystal 43 introduce stress into the crystal 43 which changes the refractive index. Accordingly, an incoming laser beam will be deflected as it passes through the crystal 43. In this way, the acousto-optic deflector is able to deflect the laser beam to prevent this from being focused onto the surface of the anilox roller 2, and therefore no cell is engraved.

The beam modifying means 13 can be any means which allows the beam to be modified to remove the circular

symmetry of the beam, and provide a beam having a dimension in one direction being greater than a dimension in an orthogonal direction. One such system is shown in Figure 6. The beam modifying means 13 is formed from a plate 14 having a central aperture 15. The aperture has a smaller dimension than the incoming laser beam, and therefore the laser beam "overfills" the aperture. By inclining the plate 14 with respect to the axis of the incoming laser beam, the beam can be modified to have a substantially oval cross-section. Light which hits the plate 14, and does not pass through the aperture 15, is either absorbed by the plate where this has an opaque surface, or is reflected where the plate 14 has a reflective surface. With a beam modifying means of this type, the orientation of the modified laser beam is easily adjustable, by rotation of the plate 14 about a generally vertical axis in the orientation shown in Figure 6. When the modified laser beam is focused by the focusing lens L, the beam will be rotated about 90° as previously described, and therefore the direction of the major axis of the cells engraved on the roller 2 are controllable.

An alternative system is shown in Figures 7 and 8. In this case, rather than passing the laser beam through an oval aperture, two opposed jaws 21, 22 are provided, both of which are rotatable about pivot points 23. By rotating the jaws 21, 22, they can be brought within the diameter of the laser beam. Figure 7 shows the jaws 21, 22 just clipping the laser beam, and Figure 8 shows the jaws 21, 22 further within the laser beam. Laser light which is incident upon the jaws 21, 22 is reflected, and is absorbed by the walls of the laser engraving head 6.

The jaws 21, 22 are rotated by the pressure applied by a thrust tube 24. The thrust tube is in turn controlled by a system of gears (not shown) . Typically, the width of the beam H in one direction is reduced from 8mm to about 3mm.

The resulting beam transmitted through the beam modifying means will not have an oval shape, but the top and bottom of the beam will be removed, producing a beam substantially as shown in Figure 9. This beam has a larger width W, than height H, and therefore when this beam is focused by the focusing lens L, the focused laser beam at the focal point will have a substantially oval cross-section. Due to diffraction effects, the straight sides of the modified laser beam will become blurred as the beam is focused, hence giving the substantially oval cross-section.

A further method of modifying the laser beam is shown in Figure 10. This system uses a plurality of prisms 30-33 which are used to compress the laser beam anamorphically to produce a modified laser beam having a substantially oval cross-section. Figure 10 shows four prisms 30-33 which are used to ensure that the output laser beam is coaxial with the input laser beam, however a similar effect can be achieved with only three prisms.

A final method of modifying the laser beam is to use the acousto-optic deflector AOD. Acoustic waves are only set up in the germanium crystal 43 immediately below the transducer 44. Accordingly, in the acousto-optic deflector AOD shown in Figure 5, in the direction transverse to the incoming beam, acoustic waves will only be set up in the middle part of the crystal, and not at the edge portions giving the AOD a limited effective width W _e . If the incoming circular beam has a diameter larger than the effective width W _e of the transducer 44, the acousto-optic deflector AOD acts as a slot, clipping the edges of the incoming beam. As shown in Figure 11, the beam output from the acousto-optic deflector when the incoming beam "overfills" the acousto-optic deflector AOD, will have a width W smaller than its height H. When this modified beam is focused by the focusing lens L, the focused spot on the roller 2 will be rotated by 90°, and accordingly the cells

C will have a substantially oval shape with their major axis lying generally parallel to the longitudinal axis of the roller.

By controlling the deflection of the acousto-optic deflector AOD, and the pulsing of the laser source 4, it is possible to engrave patterns of cells on the roller. The oval cells C engraved on the roller 2 reduce the area of unengraved land. This is clearly shown in Figures 12A-12C. Figure 12A shows circular cells engraved in a diamond pattern, namely a cell is engraved at each corner of a diamond. Figure 12B shows the same arrangement of cells, however each of the cells has a substantially oval cross- section. In this case, the area of unengraved land is reduced. Figure 12C shows oval cells arranged in a parallelogram pattern. Again, the area of unengraved land is reduced by the use of oval cells. In Figures 12A-12C, the cells are engraved with their major axis lying in the circumferential direction of the roller 2. It is possible for the cells to be engraved at any other orientation, for example in a direction parallel to the longitudinal axis of the roller 2.

Typically, for the above examples, the laser wavelength λ is e.g., 10.6μm, the numerical aperture NA is 0.3, the beam waist is 43μm wide, and the engraved cells are 32μm wide.

Other methods are available for engraving substantially oval cells on an anilox roller. By using two lens and an aperture, it is possible to focus a laser beam onto a shaped aperture, and image this onto the surface of the roller 2. This is shown in Figure 13.