GB2038084A - Improvements in or relating to methods of operating an electron gun - Google Patents

Abstract

In a method of operating an electron gun for processing a gravure cylinder 8, the electron gun comprises an evacuable processing chamber 6, an evacuable beam- guiding chamber 5 and an evacuable beam-generating chamber 1, a replaceable foil member 14 being installed between the chambers 5 and 6 effectively to seal the two chambers from one another and enable replacement of parts and cleaning, whereafter, when the operating vacuum levels are reached, the electron beam is directed into its operating direction to pierce the foil 14 to form an optimally dimensioned and central aperture forming a vacuum restrictor between the processing chamber and the beam- guiding chamber. <IMAGE>

Description

SPECIFICATION Improvements in or relating to methods of operating an electron gun The present invention relates to methods of operating an electron gun for the processing of material, in particular for engraving gravure cylinders. Such guns normally include an evacuable chamber in which the actual processing takes place an electron-beam, guiding section and a beam-generating section.

In workshops for processing material by means of electron beams, it is frequently desirable whilst changing the workpiece, not to vent the whole evacuated electron gun but only the actual processing chamber. Constructional arrangements are known, which for this -purpose incorporate valves of many different designs between the processing chamber and the remainder of the gun, which are actuated via vacuum-tight passage by mechanical, electrical or pneumatic means and opened when the operating vacuum is reached in the processing chamber after a workpiece exchange, so that the electron beam may pass through-- see for example US Patent Specification No. 3,426,173 and US Patent specification no. 3,588,574. Apart from their complex structure, valves of this kind are affected by a series of disadvantages.As a rule, the actual valve unit must be located in soft resilient seals. These are poor electrical and thermal conductors, without exception. This has the result that these seals acquire a charge during operation, caused by electrons originating from the marginal or boundary radiation of the principal beam, or by secondary electrons. For their part, the electrical fields of charges of this nature affect the electron beam in undesirable manner. Moreover, it is hardly possible to prevent stray electrons striking the valve unit and heating the same. A heat built-up which may lead to troublesome deformations of the valve unit by thermal expansion, is caused by the poor thermal conduction of the seals. Furthermore, it is hardly possible to prevent the valve unit being struck by spatter, vapour or ions, in particular at the side facing towards the workpiece which is to be processed.Precipitations of this kind result in operational disturbances. Higher demands are made on the systems for producing printing formes, in particular for the engraving of gravure cylinders by means of electron beams, than required for most other electron beam processing methods such as welding, drilling, fusing and so on.

In the case of gravure, the requirement lies in working up to 10,000 dots per cm2 into the forme surface in rapid sequence in extremely precise even pattern (screen), the individual dots representing mutually separate minute pits or depressions whose volume, i.e.

depth and/or aperture, is a function of the gray scale value which is to be reproduced.

If it is visualised that the smallest minute pit or depression should have a depth of no more than say 1 to 2 microns, whereas the largest should have a depth of say 50 microns, that approximately 1 20 different gray scale values should be reproducible within this range of depths, that furthermore several tens of thousands such pits or depressions should be produced per second, and that intermittent or sudden alterations in the regularity of the pit pattern should amount to less than 1% because of the sensitivity of the human eye to such alterations, the difficulties prevailing as compared to the less complex processing methods applying electron beams are recognised right-away.

Stringent requirements are applied in respect of precision to the electron gun in particular. A gun of this nature commonly comprises three consecutively arranged sections, namely the beam generator comprising a thermionic cathode, a modulating electrode and an anode, the beam guiding section containing the essential elements for forming, masking-out, timing, focussing and correcting the electron beam, and the processing chamber which is joined to the gravure cylinder revolving in front of the gun by means of a sliding and sealing mouthpiece.

The object of so complex an engraving technique is to fulfil the demands of the printing trade for the production of formes in more rapid, more satisfactory, more reliable and environmentally more compatible manner.

The graving of formes by means of electron beams has greater requirements in these respects than the processes customary until now, namely that of chemical etching and also that of mechano-electronic graving.

The technological advance of electron beam gravure may evidently be exploited fully only if the advantage of the substantially shortened engraving periods is not nullified again by protracted setting-up periods, and if the reliablility of the process may be raised to so high a level that the error quotient remains lower than that of the forme production processes current at present.

In the course of development, it was discovered that a series of fundamental measures is required to fulfil these requirements: The highly stressed therminic cathode must be replaced in each case after the engraving of a cylinder. A burn-out, or fluctuating emission during the next graving operation would cause scrapping. Devices have been developed which render it possible to perform the replacement and precise positioning of the cathode strip in approximately 2 minutes as described in US Patent Specification No.

3,478,244.

The material removed may collect only within the region of the processing chamber.

Droplets, gas and ions should be kept out of the beam guiding section and out of the beam generator because they lead to considerable disturbances therein. At the very least, the comtamination of these sections should remain so small that cleaning and maintenance jobs become necessary after a plurality of graving cycles aiming at short setting-up periods.

The replacement of the large and heavy printing cylinder and the removal of material deteritus from the processing chamber represent the greatest proportion of the setting-up period. To this end, the processing chamber has to be opened at the end thereof next adjacent the cylinder when a cylinder is not present in the machine.

The individual gun sections have different rerequirements as regards the vacuum needed. 10-5 mbar must prevail in the beam generator for operation, approximately 10-4 mbar must prevail in the beam guiding section, and finally no more than say 10-2 mbar should prevail in the processing chamber.

These spaces are consequently evacuated separately and are separated by vacuum restrictors of sufficiently small aperture, which are traversed by the electron beam.

The present invention is based on the problem inherent in establishing a method which combines the measures required to comply with the demands specified.

Accordingly, the invention consists in a method of operating an electron gun for processing a gravure cylinder, which gun comprises an evacuable processing chamber, an evacuable beam-guiding chamber and an evacuable beam-generating chamber, wherein a replaceable foil member is installed between the beam-guiding chamber and the processing chamber so that said foil effectively seals the two chambers from one another in vacuumtight manner to enable the beam-guiding and beam-generating chambers to be evacuated and placed in operation again after replacement of any desired part of the beam-generating chamber, the processing chamber being still opened for cleaning, the processing chamber being evacuated after it has been cleaned and after the gravure cylinder has been changed, and after the operating vacuum level is reached in the processing chamber, and the electron beam being directed into its operating direction to pierce said foil, whereby an optimally dimensioned and central aperture forming a vacuum restrictor is formed between the processing chamber and the beam-guiding chamber.

In order that the invention may be more clearly understood, reference will now be made to the accompanying drawing which shows an axial section through an electron gun and part of a forme cylinder by way of example, and in diagrammatic form.

Referring now to the drawing, an electron gun for processing formes, in particular gravure cylinders, as already stated normally comprises three consecutively arranged chambers, namely a beam-generating chamber 1 comprising a thermionic cathode, 2, a modulating electrode 3 and an anode 4, a beamguiding chamber 5 which contains the usual elements intended for forming, masking-out, timing, focussing and correcting the electron beam, and a processing chamber 6 which is connected to a gravure cylinder 8 caused to revolve in front of the gun by means of a sliding vacuum-tight mouthpiece 7. Since these usual elements are well-known to the man-skilled in the art, they are not further described here. Each of said chambers of the electron gun is evacuated but they all require different vacuum levels for operation.The beam-generating chamber 1 has the highest requirement of approximately 10-5 mbar. The beam-guiding chamber 5 requires approximately 10-4 mbar, and the processing chamber only approximately 10-2 mbar. The individual chamber are consequently evacuated separately by means of vacuum pumps 9, 10, 11 respectively and must be internally separated from each other by means of vacuum restrictors of minimum aperture. The anode 4 is constructed as a vacuum restrictor, between the beam-generating chamber 1 and the beam-guiding chamber 4. For practical operation, it is necessary to replace the cathode 2 in each case after the engraving of one cylinder.Whilst replacing the cathode 2 after an engraving operation, a foil 14, conveniently of thin copper sheet, is installed in vacuum-tight manner in a partition 1 3 between the processing chamber 6 and the beam-guiding chamber 5 after opening a mouthpiece cover 1 2 of the processing chamber 6 on the cylinder side thereof. The advantage of this system is that the beam-generating chamber 1 and the beam-guiding chamber 5 which set the greatest requirements in respect of vacuum may already be evacuated again by means of their separate vacuum pumps 9 and 10 in readiness for the next engraving operation, immediately after replacing the cathode. After reaching the operating vacuum, the cathode 2 is reenergised and the high voltage is applied, with the processing chamber 6 open. If control over the beam is performed by means of a deflection system 1 5 which does not cancel the electron beam during the cadence intervals but directs it into a cooled "beam absorption sump" within the beam-guiding chamber 5, the electron beam is then switched on at full power in advance even when the processing chamber 6 is open.

This results in the great advantage that stable conditions in respect of thermal and electrical balance are already established in the two already evacuated critical chambers of the electron gun during the comparatively protracted period for changing the cylinder and cleaning the processing chamber 6. The advantage consequently consists not only in the saving on setting-up time, but drift phenomena are also reliably prevented upon starting the engraving operation, by the procedure described.

After cleaning the processing chamber 6 and inserting another cylinder 8 which shuts off the mouthpiece 7 of the processing chamber 6, the latter is evacuated to its operating pressure by the corresponding vacuum pump 11, which occurs relatively quickly. In accordance with the invention, the electron beam is then directed into its operating position towards the cylinder 8 by activating the electron beam or rather by appropriate actuation of the system 1 5. The beam than produces a hole in the sealing foil 14, thereby clearing the beam path towards the cylinder 8 for engraving the latter.Apart from the advantages hereinabove referred to concerning the saving of time changing a cylinder and the stabilisation of the conditions in the beam-generating and beam-guiding chambers 1 and 5, the residue of the requirements cited in the foregoing is also fulfilled in advantageous manner by this procedure: the hole produced in the foil 1 4 by the electron beam represents an optimally dimensioned aperture for the beam, which bars removed particles of material, droplets, gas and ions from access into the inside of the upper gun chambers 1 and 5. The centring of this aperture with respect to the beam is optimised automatically. Any mechanical aperture centring is therefore no longer necessary.Another advantage is that this hole simultaneously acts as an optimally dimensioned vacuum restrictor between the processing chamber 6 and the beam-guiding chamber 5, because the minimum possible restrictor diameter results automatically. In advantageous manner, it may be accomplished by appropriate timing of the beam and/or slight defocussing, that no accidental action is performed as yet on the cylinder 8 whilst perforating the foil 1 4. Tests have shown that this may be accomplished even with the cylinder 8 at rest, because the cylinder 8 may draw the heat off from the point of impingement in view of the short firing periods and long timing intervals. By contrast, the foil 1 4 does not dissipate the heat, due to its small thickness, even if it consists of satisfactorily thermally conductive material, and is perforated precisely with the diameter of the beam at the locus of the aperture. The foil 1 4 is advantageously produced as a cheap and rapidly replaceable element. With appropriate dimensioning, the entire perforating action lasts no more than a few milliseconds. An appropriate control system may enable this procedure to evolve automatically prior to starting the engraving operation.

Claims (3)

1. A method of operating an electron gun for processing a gravure cylinder, which gun comprising an evacuable processing chamber, an evacuable beam-guiding chamber and an evacuable beam-generating chamber, wherein a replaceable foil member is installed between the beam-guiding chamber and the processing chamber so that said foil effect effectively seals the two chambers from one another in vacuum-tight manner to enable the beamguiding and beam-generating chambers to be evacuated and placed in operation again after replacement of any desired part of the beamgenerating chamber, the processing chamber being still opened for cleaning, the processing chamber being evacuated after it has been cleaned and after the gravure cylinder has been changed and after the operating vacuum level is reached in the processing chamber, and the electron beam being directed into its operating direction to pierce the foil, whereby an optimally dimensioned and central aperture forming a vacuum restrictor is formed between the processing chamber and the beamguiding chamber.

2. A method according to claim 1, wherein the beam intensity or its focussed position are so controlled that no undesired action on the cylinder occurs.

3. A method of operating an electron gun, substantially as hereinbefore desired with reference to the accompanying drawing.