FLEXOGRAPHIC PRINTING CAMERA AND METHOD OF OPERATION

Abstract: A method of preparing a flexographic printing plate to compensate for the distortion imposed on the images on the plate when it is fitted to a printing roll incorporating the steps of producing an intermediate positive or negative with an intermediate image which has a predetermined percentage distortion in one direction, and using the intermediate positive or negative to produce the final printing plate incorporating the final image to be printed, where the same percentage distortion is applied to the intermediate image, but in the inverse direction. The percentage distortion is dependent on the number of teeth (and thereby the diameter) of the printing roller on which the plate is to be used. The invention also provides a suitable camera (10) for producing the plates using this two stage method. The camera (10) has a fixed focal length lens (30) selectively movable along its focal axis (39) to apply a predetermined percentage distortion to an image on a copy or transparency mounted on a copyboard (14) and transmitted onto a photographic film mounted on a vacuum board (45) movable in predetermined steps along two mutually perpendicular horizontal axes.

Description: TITLE: "Flexographic Printing Camera and Method of Operation" BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a flexographic printing camera and a method of operating the camera to produce a flexographic printing plate. Description of the Prior Art Flexographic printing is used inter alia, for the printing of labels to be fitted to jars, bottles or other containers. An image is formed on a photo polymer plate which is wrapped around a plate cylinder or roller, the cylinder or roller being driven by a gear wheel having a fixed number of teeth for that size cylinder or roller. The plate may be arranged to print 12 labels for each revolution of the roller, where the images are arrange ed in 3 rows and 4 columns. If the roller gear has 69 teeth, the plate would be known as a 69 tooth 3 across 4 around (12 up) plate. The photo polymer plates are thick, while the cylinders or rollers have very small diameters. When the plates are wrapped around the cylinders or rollers, the images are distorted in the direction around the roller (but not across it) . For example, squares will appear as oblongs and circles as ellipses. The percentage distortion is known as tabulated for each roller size and smaller diameter cylinders or rollers will produce greater distortion than larger ones. For example, a 48 tooth roller may have a percentage distortion of 93.50%, whereas a 69 tooth roller has a percentage distortion of 95.48% and an 84 tooth roller a percentage distortion of 96.29%. (The smaller the number, the greater the actual distortion of the image.) This problem of distortion is particularly significant where artwork with two or more colours and/or designs is to be printed onto a label. For example, where the artwork includes a circle of blue inside, and engaging a OMPI circle or red, care must be taken when printing to ensure the two circles touch but do not overlap, otherwise the aesthetic effect is spoilt. As more and more colours and or designs are added to the label, the margin for error is increased and the demands on the printer become almost overwhelming. BRIEF SUMMARY OF THE INVENTION It is an object of the present invention to provi a method of producing a flexographic printing plate where the image distortion is compensated for. It is a further object to provide a flexographic printing camera for carrying out the method. It is a preferred object to provide a camera wher the distortion on each plate, for a given roller and number images across and around the plate, will be equal (or substantially equal so that any error is insignifican It is a further preferred object to provide a cam which can be easily and accurately operated in a dark roo and where positive stops or markings are provided to eliminate, or markedly reduce, any operator error. It is a still further preferred object to provide camera which can produce reduced images on the printing plate. It is a still further preferred object to provice a camera which can produce printing plates from positive or negative transparencies and/or opaque copies. Other preferred objects of the present invention will become apparent to the skilled addressee from the following description. in one aspect, the present invention resides in a method of preparing a flexographic printing plate characterized by the steps of: (a) mounting a copy or transparency incorporating an image to be printed in a first mounting means i a flexographic camera; (b) exposing the image onto a first photographic film, mounted in a second mounting means in the flexographic camera, through a lens system to form an intermediate image having a predetermined percentage distortion oriented in a first direction; (c) processing the first photographic film to produce an intermediate positive or negative incorporating the intermediate image; (d) mounting the intermediate positive or negative in the first mounting means and a second photograph¬ ic film in the second mounting means; (e) exposing the intermediate image onto the second photographic film through the lens system to form a final image to which the predetermined percentage distortion has been applied in the inverse direc¬ tion; and (f) processing the second photographic film to produce the printing plate incorporating the final image. Where a plurality of final images are to be formed on the printing plate, the method may be further characterized by the steps of: (el) after step (e) , moving the second mounting means a predetermined distance along at least one horizontal axis and repeating step (e) ; (e2) repeating step (el) until the required number of images are formed down or across the second photographic film; and (e3) carrying out step (f) to produce the multiple image printing plate. In a second aspect, the present invention resides in a flexographic printing camera for effecting the method, wherein the camera is characterized by: a frame; a first mounting means mounted on the frame; means to illuminate a copy, transparency, positive or negative mounted in the first mounting means; a second mounting means mounted in the frame for movement along at least one axis; and a lens system interposed between the first- and second mounting means which applies a predetermin percentage distortion to an image transmitted through the lens system. Preferably the lens system incorporates a lens of fixed focal length selectively movable along its focal ax from a fixed datum point to enable the percentage distortion applied to the image to be varied; visual indicator means indicating the percentage distortion applied to the image by the lens. Preferably the lens system can be set to reduce t size of the image to be printed, the reduction between th original image and final image being carried out in two equal steps. Preferably the first mounting means includes a sheet glass copyboard, register pins on the copyboard and removable aling ent sheet adapted to enable the copy, transparency, positive or negative to be accurately positioned on the copyboard. Preferably the means to illuminate the copy, transparency, positive and negative includes a pair of lights in front of, and one behind, the copyboard. Preferably the second mounting means includes a first pair of parallel horizontal shafts mounted on the frame, an intermediate carriage slidably mounted on the first pair of shafts, a second pair of parallel horizonta shafts on the intermediate carriage substantially perpend ular to the first pair of shafts, a plate carrier slidabl mounted on the second pair of shafts, a pair of register pins on the plate carrier to enable the photographic plat to be accurately positioned on the plate carrier, and mea to apply a vacuum to the plate carrier to releasably secu a photographic film thereto, prepunched strips being releasably connectable to the plate carrier and intermedi carriage respectively for engagement with respective pins the frame to enable accurate selective movement of the pl carrier relative to the frame and the copy board along tw perpendicular horizontal axes. Preferably the angular relationship between the copyboard and the plate carrier is maintained as a constant during any movement of the plate carrier relative to the frame. To enable the invention to be fully understood, a preferred embodiment will now be described with reference to the accompanying drawings, in which: FIG. 1 is a side view of the camera, parts being broken away for clarity; FIG. 2 is a top view taken on line 2-2 on FIG. 1; and FIG. 3 is a sectional plan view taken on line 3-3 on FIG. 1. The camera is substantially rectangular in plan and has an open-fronted housing 10 supported by four legs 11 above a base 12. An inclined glass plate 13 is spring-mounted in an inclined peripheral copyboard frame 14 mounted on the upper rear portion of the housing 10. An open-fronted external light box 15 is hingedly mounted (via hinges 15a) on the copyboard frame 14 to enable access to the glass plate 13. A fluorescent tube 16 is mounted on the rear wall 17 of the light box and may be operated by the operator to illuminate a positive or negative transparency mounted on the glass plate 13. - An internal light box 18 is provided in the housing 10 to enable an opaque copy mounted on the copyboard to be illuminated. The side walls 19 and the rear wall 20 of the housing provide the same function for the internal light box 18 but the front wall 21 of the light box is spaced rearwardly of the front end of the housing 10. An aperture 22 is formed in the forward portion of the light box floor 23. A pocket 24 at each rear corner of the light box contains a light 25 directed to illuminate the front face of the copyboard 14. An adjustable lens system 26 is mounted underneat the forward portion of the internal light box 18 and is provided with a bellows 27 which surrounds the aperture 2 The other end of the bellows 27 is connected to a plate 2 which has an aperture 29. A fixed focal length lens 30 is mounted in a removable plate 31 mounted on blocks 31a on a plate 32 mounted for longitudinal movement on the underside of the plate 28. A threaded shaft 33 is rotatably connected at one end to the removable plate 31 and passes through a knurled adjustment nut 34 which may be rotated to travers the lens 30 across the aperture 29 in the plate 28. A di gauge 35 provides a visual indication of the relative position of the lens 30 in the aperture 29. The plate 28 is supported by a vertical arm 36 mounted on bushes 37 slidably mounted on a shaft 38, the axis of the shaft 38 being parallel with the optical axis of the lens 30. Brackets 40 on the housing 10 support ea end of the shaft 38 and a parallel screw threaded spindle having a knurled wheel 42. A nut 43 on the end of the plate 28 is screw threadedly engaged on the spindle 41 so that rotation of the latter moves the lens 30 along its optical axis 39. Movement of the lens 30 from a datum point is indicated by a micrometer dial gauge 44. When the image to be printed is to be reduced, th removable plate 31 is inverted so that the lens 30 is directed below it to reduce the size of the image by approximately 70%. As the final printing plate is produc by a two—step process, the printing image is of approximately 50% reduction of the original image on the original copy or transparency. A vacuum board 45 is provided to support the photographic film onto which the copy or transparency is be photographed. The vacuum board 45 is hollow and is connected via a suitable hose 46 to a vacuum pump (not shown) . The upper face of the board 45 is provided with suction holes 47 and a pair of register pins 48 to locate and secure the photographic film in position, The vacuum board 45 is mounted for longitudinal movement in the camera via bearings 49 sliding on parallel shafts 50 on an intermediate carriage 51. This carriage 51 in turn, can be moved transversely to the camera and is mounted on parallel transverse shafts 52 fixed in blocks 53 on the base 12 via slidable bearings 54. The position¬ ing of the vacuum board 45 in the camera is controlled as follows. For each roller, e.g. a 69 tooth roller, and for each number of labels around the roller, e.g. 3, there is a corresponding first strip with a number of holes punched along its length at predetermined intervals. Similarly, for a particular number of labels across any roll, e.g. 4, there is a second corresponding strip with holes punched at predetermined distances, e.g. 50 mm. equal to the distance centre-to-centre between each label. The first strip 55 is releasably connected to ' the carriage 51 and has a series of spaced holes 56 which can be engaged by a pin 57 on a finger 58 pivotally mounted on the vacuum board 45 via hinge 59. Therefore the vacuum board 45 can be moved along the shafts 50 and fixed at specific interval steps relative to the carriage 51. Similarly, the second strip 60 is releasably connected to the base 12 and has a series of equally spaced holes 61 which can be engaged by a pin 62 on a finger 63 pivotally mounted on the carriage 51 via hinge 64, so that the carriage 51 can be traversed along the shafts 52 at selected intervals relative to the base 12. To accurately position the copy or transparency in the camera, the copyboard frame 14 which surrounds the glass plate 13 is provided with a pair of register pins 65. For a particular roller (and thereby a particular distortion) , a particular lining up sheet is provided marked with the number of teeth on the roller, e.g. 69 teeth, and the reading on the dial gauge 44 which shows the setting of the lens 30 for that distortion. The lining up sheet has a vertical line and a series of cross-lines as a datum for lining up the copy or transparency. When the copy or transparency is correctly positioned, the lining up sheet is removed. As stated above, each roller has a particular lining up sheet of its own. The operation of the camera will now be described with reference to producing a plate for a 69 tooth roller having three prints around the roller, and 4 prints across the roller (i.e. 12 up) . The operator tilts the light box 15 on its hinges 15a to gain access to the copyboard frame 14 and glass plate 13. He places the opaque copy or transparency on th glass plate 13 and places the lining up sheet for the 69 tooth roller over it, registering the lining up sheet on the pins 68 on the copyboard frame 14. He then positions the copy or transparency on the glass plate 13 with reference to the vertical line and cross lines on the lini up sheet, secures the copy or transparency in position, e. by adhesive tape, removes the lining up sheet and closes the light box 15. By rotating the spindle 41, via knurled wheel 42, he then moves the lens 30 along its optical axis 39 to the desired setting on the dial gauge 30 for the distortion fo the 69 tooth roller as shown on the lining up sheet. The vacuum pump is set in operation to evacuate the vacuum board 45. The lights in the darkroom are switched off and either a red light is switched on, or no lights, depending on the photographic film to be used. A photographic film is removed from its light proof box and registry holes are punched in one end. The film is laid on the vacuum board 45 in register with the pins 48. The operator switches on the front light 25 (for a opaque copy) or the back light 16 (for a transparency) to illuminate the copy or transparency and to project the image of the label thereon to the photographic film. O The intermediate film is processed to form an intermediate negative or positive (which bears an image having a distortion applied by the lens 30 in one direction) . The intermediate positive or negative is then placed on the copyboard frame 14 (in register with the pins 65) and glass plate 13 in the manner described above, except that it is inverted to give the required distortion in the resultant image on the second film to be exposed. The operator again darkens the dark room as required and places a second photographic film on the vacuum board 45 as described above. When exposed and processed, this film will be used to prepare the printing plate. The operator has previously selected and fitted the 69 tooth 3 around first strip 55 and 4 across second strip to the carriage 51 and base 12 respectively. He moves the vacuum board 45 along the shafts 50 until the pin 51 or finger 58 engages the first hole 56 on the first strip 55, and then moves the carriage 51 along shafts 52 until the pin 62 on the finger 63 engages the first hole 61 on the second strip 60. The operator then switches on the fluorescent light 16 to illuminate the intermediate image on the intermediate negative or positive which is projected via lens 30 to expose a portion of the second film for the first label. The carriage 51 is then traversed until the pin 62 on finger 63 engages the second hole 61 in the second strip 60 and the second portion of film is exposed. This process is repeated until the 4 sections across the film corresponding to the first row of labels has been exposed. The vacuum board 45 is then moved along the shafts 50 until the pin 57 on finger 58 engages the second hole 54 of the first strip 55. The section of film is exposed and then the process is repeated with the carriage 51 being traversed back across the camera in steps to complete the second row. The vacuum board 45 is advanced a second time. and the carriage 51 traversed again to complete the third row. The photographic film has now been exposed in 12 sections and the final image on each section has the percentage distortion tabulated for a 69 tooth 3 around plate. In the manner described above, the intermediate image on the intermediate negative or positive is exposed in turn onto the second photographic film, which will be used to produce the printing plate on the press in the known manner. Where a second colour is to be printed, the first process is carried out to produce a new intermediate negative for this second colour, and the images on this new intermediate negative are then exposed onto another piece of film on which the images will be in correct registry with the images on the film produced for the firs colour. As the original copy or transparency, the first intermediate negative or positive and the second film have always been accurately and reproducably positioned in the the vacuum board 45 and the carriage 51 have always been moved in fixed steps relative to the optical axis 39, and lining up sheet, dial gauge 44 and first- and second strip 55, 60 for a particular roller and layout have been employ the process of producing the printing plate can be accurately repeated and the resultant labels are of extremely high quality. Should amendments be required at a later time, new intermediate negative or positive incorporating the amend¬ ments can be produced by the first process and then expose onto a second film which has been prepared which has the images of the original label exposed on it. This apparatu and method described above will ensure that the amendments are accurately positioned on the new label with the correc percentage distortion for the roller to be used. Where the image on the copy or transparency is to be reduced by 50%, the removable plate 31 is rotated so th the lens 30 lies underneath it. The intermediate image on G-.-rPi " - li ¬ the intermediate positive or negative is reduced by approximately 70% as the distance between the lens and the film is less than the distance between the lens 30 and the copy or transparency on the glass plate 13. The final image on the second film taken from the intermediate image of the intermediate positive or negative is similarly reduced approximately 70%, to achieve the desired 50% reduction. In practice, only a limited number of rollers are used, e.g. ten. Therefore, a person operating the camera will be supplied with the following: (1) a chart setting out the percentage distortion for each of the rollers; (2) a lining up sheet for each roller, marked with the corresponding dial gauge reading; (3) a set of first strips 55 for each roller, where the labels are to be, e.g. laid up as 1 around, 2 around, 3 around etc. (this means: for 10 rollers, where the labels may be laid 1 around, 2 around, or 3 around, 30 first strips would be supplied) ; and (4) a set of second strips 60, with the holes punched at selected centres, e.g. 50mm., 55mm., 60mm., so that the labels can be laid, e.g. 1, 2, 3 or 4 across the rollers. As discussed previously, the lining up sheet has a vertical line and a series of horizontal lines, one of the horizontal lines is a datum line. If the image on the copy or transparency is centred on the intersection of the vertical line and horizontal datum line, the desired percentage distortion will be applied to the images projected through the lens. If the image is placed above the datum line, the projected image will be reduced in width, while the projected image will be reduced in height if the image is placed below the datum line. This enables the operator to adjust the shape of the projected image if he so desires. By noting which line is used on the lining up sheet, the operator can accurately reproduce the projected image. By traversing the lens 30 across the aperture 29 in the plate 28, the operator can move the lens 30 across its focal axis to determine the best position for the len to project the image onto the film on the vacuum board 45. The dial gauge 55 ensures this adjustment can be readily and accurately reproduced when plates are being produced for printing rollers having different numbers of teeth. While this camera has been described with respect to the printing of labels, it can be used to prepare the plates for the flexographic printing of any other material Various changes and modifications may be made to the embodiment described without departing from the scope of the present invention.

Claims: CLAIMS : 1. A method of preparing a flexographic printing plate characterized by the steps of: (a) mounting a copy or transparency incorporating an image to be printed in a first mounting means (14) in a flexographic camera (10) ; (b) exposing the image onto a first photographic film, mounted in a second mounting means (45) in the flexographic camera (10) , through a lens system (26) to form an intermediate image having a predetermined percentage distortion oriented in a first direction; (c) processing the first photographic film to produce an intermediate positive or negative incorporating the intermediate image; (d) mounting the intermediate positive or negative in the first mounting means (14) and a second photographic film in the second mounting means (45) ; (e) exposing the intermediate image onto the second photographic film through the lens system (26) to form a final image to which the predetermined percentage distortion has been applied in the inverse direction; and (f) processing the second photographic film to produce the printing plate incorporating the final image. 2. A method as claimed in Claim 1, further characterized in that: the lens system (26) incorporates a lens (30) of fixed focal length selectively movable along its focal axis (39) from a fixed datum point to vary the percentage distortion applied to the intermediate- and final images, the percentage distortion being indicated by a visual indicator means (44) . 3. A method as claimed in Claim 2, further characterized in that: the percentage distortion applied to the inter- '71 ., viro - Im ¬ mediate and final images is determined by the number of teeth on the roller of the flexographic printing machine on which the printing plate is to be used. 4. A method as claimed in any one of Claims 1 to 3, further characterized in that: the second mounting means (45) is mounted for movement along at least one horizontal axis in the camera the angular relationship between the first- and second mounting means (14, 45) being maintained at a constant during any movement of said second mounting mean 5. A method as claimed in Claim 4 where a plurality final images are to be formed on the printing plate, characterized by the steps of: (el) after step (e) , moving the second mounting means (45) a predetermined distance along at leas one horizontal axis and repeating step (e) ; (e2) repeating step (el) until the required numb of images are formed down or across the second photographic film; and (e3) carrying out step (f) to produce the multip printing plate. 6. A flexographic printing camera for effecting the method claimed in Claim 1, said camera (10) being characterized by: a frame (12) ; a first mounting means (14) mounted on the frame (12) ; means (16, 25} to illuminate a copy, transparency, positive or negative mounted in the first mounting means (14); a second mounting means (45) mounted in the frame (12) for movement along at least one axis; and a lens system (26) interposed between the first- second mounting means which applies a predetermined percentage distortion to an image transmitted through the lens system (26) . 7. A camera as claimed in Claim 6 and further characterized in that: the lens system (26) incorporates a lens (30) of fixed focal length selectively movable along its focal axis (39) from a fixed datum point to enable the percentage distortion applied to the image to be varied; visual indicator means (44) indicating the percent¬ age distortion applied to the image by the lens. 8. A camera as claimed in Claim 7 and further characterized in that: the lens system (26) includes reduction means operable to reduce the image transmitted thrmugh the lens. 9. A camera as claimed in any one of Claims 6 to 8 and further characterized in that: the first mounting means (14) includes a sheet glass copyboard (13) , register pins (65) on the copyboard (13) and a removable alignment sheet adapted to enable the copy, transparency, positive or negative to be accurately positioned * on the copyboard (13) ; and the second mounting means (45) includes a first pair of parallel horizontal shafts (52) mounted on the frame (12) , an intermediate carriage (51) slidably mounted on the first pair of shafts (52) , a second pair of parallel horizontal shafts (50) on the intermediate carriage (51) substantially perpendicular to the first pair of shafts (52) , a plate carrier (45) slidably mounted on the second pair of shafts (50) , a pair of register pins (48) on the plate carrier (45) to enable the photographic plate to be accurately positioned on the plate carrier (45) , and means (47) to apply a vacuum to the plate carrier to releasably secure a photographic film thereto, prepunched strips (60 / 55) being releasably connectable to the frame (12) and intermediate carriage (51) respectively for engagement with respective pins (62, 57) on the intermediate carriage (51) and the plate carrier (45) to enable accurate selective movement of the plate carrier (45) relative to the frame (12) and the copy board (14) along two perpendicular horizontal axes . 10. A camera as claimed in Claim 9 and further characterized in that: the angular relationship between the copyboard (1 and the plate carrier (45) is maintained as a constant during any movement of the plate carrier (45) relative to the frame (12) .

Inventors: KURPERSHOEK H
Applicants: KURPERSHOEK H
Application year: 1981
Main classification: G03B27
IPC codes: G03B27, G03F1, G03F7