Electric heating – Metal heating – By arc
Patent
1993-04-02
1995-05-16
Paschall, Mark H.
Electric heating
Metal heating
By arc
21912169, 21912175, 21912182, 372 13, B23K 2600
Patent
active
054162980
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND TO THE INVENTION
The present invention relates to the preparation of cylinders for use in printing. It has particular relevance to the engraving of anilox rollers but is also applicable to other processes involving the engraving or exposure of cells on a rotating medium.
In conventional systems for preparing anilox cylinders, a ceramic-coated roller is rotated and a laser engraving head is moved along the roller parallel to its axis. The engraving head may, for example, include a plane mirror and lens which focuses light from a laser onto the surface of the roller. The laser is pulsed and the combined effect of the rotation of the roller and the lateral movement of the engraving head is to produce a helical pattern of cells with one cell corresponding to each pulse from the laser. By appropriate synchronisation of roller rotation, the speed of the engraving head, and the laser pulsing, the cells can be arranged in hexagonal, square or other patterns.
Whilst conventional engraving techniques, such as that described above, are generally satisfactory at lower speeds, a number of problems arise as the rate at which the cells are engraved increases. When engraving separated cells, for example, it is found that at high rotational speeds each cell tends to be elongated in the direction of rotation of the roller, producing a cell which is oval in form. When engraving contiguous cells at high speeds the wall between adjacent cells tends to break down, an effect known as "channelling". In order to avoid such effects, known techniques have in practice been limited to the use of engraving frequencies of less than 10 kHz. A typical cell density is 22000/cm.sup.2 and a typical roller size is greater than 10000 cm.sup.2 At a rate of 10 kHz, such a roller takes at least six hours to engrave. Significant advantages would be realised by any system making possible an increase in the engraving frequency.
SUMMARY OF THE INVENTION
According to the present invention, an apparatus for preparing a medium for use in a printing process comprises means to support the medium for rotation about an axis and a head mounted for movement parallel to the axis including means to direct a laser beam onto the surface of the medium to engrave or expose a series of cell sites around the medium characterised by an acousto-optic modulator positioned in the path of the laser beam and arranged to deflect the beam substantially in the direction of rotation of the medium.
The present inventor has found that the use of an acousto-optic modulator (also called AOM or acousto-optic Bragg cell) to deflect the beam in the direction of rotation of the medium makes possible the engraving of cells at a significantly increased rate, at frequencies greater than 10 kHz, whilst avoiding the disadvantages encountered with conventional techniques at such high engraving rates. It is known from GB-A-2009558 to use an acousto-optical cell in a scanning apparatus to correct for any deviation of a scanned beam in the direction transverse to the scanning direction.
Preferably the medium is a cylinder for use in a printing process.
Preferably the acousto-optic modulator is arranged to deflect the beam so that it tracks the surface of the cylinder at a speed matched to the medium to keep the beam stationary on a cell site while that site is being engraved or exposed.
The use of the acousto-optic modulator makes it possible to track cell sites and thereby to avoid elongation of the cells even when engraving at high rotational speeds.
Preferably the laser is a continuous wave (CW) laser and the acousto-optic modulator is arranged to deflect the beam between cell sites at a speed very much greater than the speed of rotation of the medium. Alternatively, a pulsed laser may be used in order to provide the high instantaneous power need, for example, in order to engrave deep cells.
In this preferred aspect of the invention, after the engraving of each cell site there is a "flyback" in which the beam is moved at high speed relative to the surface of the cylinder to t
REFERENCES:
patent: 4336439 (1982-06-01), Sasnett et al.
patent: 4547023 (1990-08-01), Minamida et al.
patent: 4547651 (1985-10-01), Maruyama
patent: 5109149 (1992-04-01), Leung
Paschall Mark H.
Zed Instruments Limited
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