A DEVICE FOR TREATING A BODY WITH A BEAM OF RAYS

Abstract: A device (1) for programmed spot treatment of a body with a beam of rays, particularly a laser beam (7), using a lens system (4), in which the point of impact of the beam (7) describes a predetermined pattern on the surface of the body particularly a lacquer sealed rotary screen printing stencil 3a, as a result of a relative movement of the body with respect to the device. According to the invention the device (1) has means for maintaining a constant predetermined distance between the centre of the lens system (4) and the centre of the point of impact of the beam (7) of rays with the surface of the body. Said means for maintaining a constant distance comprise distance-determining means, preferably a scannner of the optical, inductive or capacitive type, for determining the distance of the surface of the body from a lens system (4) located between a light source and the body, displacement means (11,12,12′,14,14′,15), preferably electromagnets cooperating with anchoring plates and spring packets, for displacing the lens system, and control means, preferably a microprocessor, for controlling the displacement means (11,12,12′,14,14′,15) accor­ ding to the difference between a set value and the measured value of the distance between the lens system and the surface of the body. The stencil 3a is supported by a roller 3.

Description: The invention relates primarily to a device for programmed spot treatment of a body with a beam of rays, using a lens system, in which the point of impact of the beam of rays describes a predetermined pattern on the surface of the body as a result of a movement which the body and the device carry out relative to each other. An example of such a device is described in EP-A-0 179 939. This reference discloses a device for the treatment of a cylindrical clamped registration material with a light beam. Thus colour separations are manufactured by using a film material, which is treated with the light beam according to a predetermined pattern. During the treatment a drum, to which the film material is secured, is rotated, while the lightbeam is displaced axially relative to the drum. However, the invention is not limited to the type of device as described in the above-mentioned reference. In general, the invention relates to devices in which a particular treatment can be carried out on a body with a beam of rays. Examples are: exposure of a clamped screenprinting stencil, comprising a perforated sheet or a stencil gauze, the perforations of same being sealed by means of a sealing agent such as a lacquer, said stencil being treated in a programmed manner with a laser beam, so that the perforations being struck by the laser beam are stripped of their lacquer, and a cylindrical printing plate is formed in this way; photo-curing of a clamped photo-polymer material etc. The body on which the beam acts will often be cylindrical; but a flat shape can also occur. Such known devices have the disadvantage that the effect of their action is very sensitive to even very minor change in the distance of the lens to the surface of the body, since only the optimum action of a laser beam burning away, for example, lacquer is obtained if the focal point of the lens used remains on the surface of the body to be treated with radiation, or so far below said surface as is desirable in connection with the effect to be obtained. A consistently correct distance is, however, not simple to achieve, due to faulty settings and inaccuracies which may occur, including rotational and surface displacement inaccuracies. The object of the invention is to provide a solution to the above problem. According to the invention, a device of the type indicated is characterized in that the device has means for maintaining a constant predetermined distance between the centre of the lens system and the centre of the point of impact of the beam of rays with the surface of the body, said means comprising distance-determining means for determining the distance of the surface of the body from a lens system located between light source and the body; displacement means for displacing the lens system, and control means for controlling the displacement means according to the difference between a set value and the measured value of the distance between the lens system and the surface of the body. By including the means according to the invention with which the distance between the centre of the lens system and the centre of the point of impact of the beam of rays with the surface of the body can be kept constant, it can be ensured that the focus of the lens remains on the surface of the body, the result of which is that any patterns to be formed can be made extremely sharp. The accuracy with which, for example, a screen printing stencil is made has a very great influence on the final result on the material to be printed; by patterning a screen printing stencil with a device according to the invention, a very accurate pattern formation is obtained and with great reproducibility. Scanners are generally used for determining the distance between the lens system and the body to be treated. Said scanners can be of different types such as: optical, inductive or capacitive scanners. The lens system in the device according to the invention is advantageously accommodated in a housing which is also provided with means as described in the characterizing part of Claim 4. The housing in the embodiment described acts as the fixed point relative to which the position of the lens in relation to the surface of the body, such as a rotary roller, is measured and kept constant. The electronic means present are preferably in the form of a microprocessor. In a preferred embodiment of the device according to the invention the beam of rays used is a laser beam, by means of which the device can be used to open perforations in a metal screen printing stencil which are sealed with a masking agent, and the opened perforations are arranged in groups which together form a predetermined pattern. If the device is designed in the form of a laser engraving device, it is constructed as follows. The body has a drum with a stencil clamped thereon, the perforations of which are sealed. By means of a laser beam the perforations can be opened in a programmed manner by burning away the sealing. Very good results are obtained through the use of special lacquers and lasers of suitable wavelength and power. The invention will now be explained with reference to the drawing, in which: Fig. 1 shows schematically a top view of a device for treating a rotary screen printing stencil whose perforations are sealed by means of a lacquer with a laser beam; Fig. 2 shows schematically a cross-sectional view of a housing, a lens system and a cylindrical roller which form part of the device in Fig. 1; Fig. 3 shows a mirror, a housing and a lens system in the device of Fig. 1; and Fig. 4 shows the housing with the lens system on an enlarged scale. Fig. 1 indicates by reference number 1 a device according to the invention; in this case a laser engraving device for forming a pattern of opened perforations using a laser beam in a lacquer-sealed rotary screen printing stencil 3a. A suitable rotary screen printing stencil is, for example, a seamless nickel stencil with a fineness between 80 and 500 mesh or more (80-500 perforations per linear inch= 25.4 mm) and a thickness of 75-200 µm. The device is provided with a roller 3 which is firmly connected at one side to a drive in a housing 9, and at the other side rests on a galvanizable support 2. The roller 3 is capable of accommodating a stencil 3a for treatment in such a way that the stencil 3a is centred round the shaft of the roller 3. The laser beam 7 from the laser 6 is guided with the mirrors 8 to a lens or lens system 4 inside a housing 5. During the operation of the device the roller 3, with a stencil 3a (not shown separately) fitted thereon is turned about its axis at a constant angular speed, while the housing 5 with the lens 4 is moved, often by means of a spindle, at a fixed distance from and parallel to the shaft of the roller. The point of impact of the laser beam focused through the lens 4 then describes a cylindrical or spiral pattern on the surface of the stencil clamped on the roller. Fig. 2 shows schematically the mode of operation of the means for maintaining a constant distance between the lens system or the lens 4 and the surface of a stencil 3a clamped on the roller 3. A dotted line shows that the roller 3 exhibits an eccentricity. The focus of the lens 4 is indicated by 10. The lens is accommodated inside the housing 5. As a result of the occurring eccentricity of the roller during rotation, the lens must be displaced as indicated by the dotted line, in order to keep the focus 10 on the surface of the roller. Fig. 3 shows more clearly how the laser beam 7 arrives on a reflection means in the form of a mirror 8, as a result of which the laser beam is deflected and reaches the lens system 4 through a connecting device 17. During the burning process a scanner (not shown further) records the distance of the surface of the roller from the housing, and another scanner 16 records the distance of the lens system 4 relative to the housing 5. By an arithmetical operation, the electronic means present calculate the distance between the lens system and the surface of the roller. This distance is compared with a set value and according to the difference between this set value and the measured value the electromagnets 11 are energized. When the electromagnets 11 are energized the result is that the lens system moves in the direction of the retaining plate 12 as a result of the lens mounting in the form of anchor plate 15 working in concert with the electromagnets. During this movement of the lens in the direction of the retaining plate 12 spring packets between the lens mounting and the anchor plate are compressed round the guides 14 which return the lens mounting to its original position when the electromagnets are switched off. This device therefore permits continuous focusing in which the device has correction circuits and/or damping circuits if desired for synchronization of the lens adjustment and measured roller eccentricity. Fig. 4 shows another example of an embodiment. In this example the lens mounting is fitted in an anchor plate 12 which is connected by means of rods to a plate 12′. Plate 15 is connected to the housing. Spring packets (14, 14′) are again provided between the plate 15 and the parts 12 and 12′. In this example of an embodiment the movement of the lens relative to the housing is carried out as follows. As a result of its design, the lens if capable of being moved in two opposite directions by the electromagnets, depending on the direction of current. In one case the lens is moved with anchor plate 12 in the direction of the plate 15, and after the electromagnet is switched off is moved back to the centre position again by the spring packet 14, while in another case the lens is moved in the opposite direction, and after the electromagnet is switched off is moved back again to the centre position, but this time by the spring packet 14′. The result of this is that the lens system can be corrected in two opposite directions relative to the surface of the roller. Apart from these examples of embodiments, there are, of course, other possibilities for the correction movement of the lens system relative to the housing, such as: a threaded lens system which is driven by a motor with gear wheel etc. As a result of this autofocusing, the maximum revolution speed of the roller can be greatly increased, since the device is extremely well equipped to compensate for the associated eccentricity, resulting in the disadvantages discussed above.

Claims: A device (1) for programmed spot treatment of a body (3a) with a beam (7) of rays, using a lens system (4) located between the light source of said beam of rays and said body, in which the point of impact of the beam describes a predetermined pattern on the surface of said body as a result of a movement which said body and said device carry out relative to each other, characterized in that the device has means for maintaining a constant predetermined distance between the centre of said lens system (4) and the centre of the point of impact of the beam of rays on the surface of said body, said means comprising distance-determining means for determining the distance between the surface of said body and said lens system (4); displacement means (11,12,12′,14,14′,15) for displacing said lens system (4), and control means for controlling said displacement means (11,12,12′,14,14′,15) according to the difference between a set value and the measured value of the distance between said lens system (4) and the surface of said body. A device according to Claim 1, characterized in that the distance-determining means is constituted by a scanner. A device according to Claim 2, characterized in that said scanner is of the optical, inductive or capacitive type. A device according to any one of Claims 1-3 characterized in that said lens system (4) is accommodated in a housing (5) which comprises, on the one hand, a body scanner which measures the distance from a fixed point on the housing to the surface of said body and, on the other, a lens scanner (16) which measures the distance from a fixed point on the housing to said lens system (4), and electronic means are present in order in an arithmetical operation to calculate from the distance values measured by the scanners the distance between said lens system (4) and the surface of said body, which electronic means can also be used to control the energization of said displacement means (11,12,12′,14,14′,15) for displacing said lens system (4) relative to said housing (5) according to the difference between a preselected set value and the thus measured value for the distance between said lens system (4) and the surface of said body. A device according to Claim 4, characterized in that said electronic means is constituted by a microprocessor. A device according to one or more of Claims 1-5, characterized in that said beam of rays is a laser beam (7), and said device can be used to open performations in a metal screen printing stencil (3a) which are sealed with a masking agent, and the opened perforations are arranged in groups which together form a predetermined pattern.

Inventors: RUCKL, SIEGFRIED
Applicants: SCHABLONENTECHNIK KUFSTEIN GESELLSCHAFT M.B.H.
Application year: 1989
Main classification: G02B7
IPC codes: B41C1, G02B7, G03F7