METHOD FOR MOUNTING PRINTING PLATES ON SLEEVES AND FOR MOUNTING THE RESULTANT SLEEVES ON FLEXOGRAPHIC PRINTING MACHINE CYLINDERS

Abstract: The printing plate mounting method comprises the following stages: applying those plates relative to a certain colour to a sleeve formed from a flexible transparent polymer film which is substantially indeformable in the plane of the film, this sleeve constituting the development of all or part of the surface of the relative plate cylinder; then applying the sleeve to the relative cylinder.

Description: METHOD FOR MOUNTING PRINTING PLATES ON SLEEVES AND FOR MOUNTING THE RESULTANT SLEEVES ON FLEXOGRAPHIC PRINTING MACHINE CYLINDERS. This invention relates to a method for mounting printing plates of substantially transparent polymer on the plate cylinders of flexographic printing machines. As is well known to an expert of the art, when a flexographic printing machine is to be used to print writing, markings or designs on a polymer strip which are to be repeated along the strip, various operations are required, and in particular: the polymer printing plates must be prepared and their completeness checked by comparison with the sample or sketch; the various plates must be mounted on the relative flexographic machine cylinders (one per colour), this mounting being achieved by double-adhesive tape of a width sufficient to cover a whole plate or a group of plates; the plate cylinders must be mounted on the relative flexographic printing machine; the various colours must be centered and the printing pressures adjusted to obtain exact position of all the colours; the printing machine must then be started to achieve the required production. The polymer printing plates can be prepared and mounted on the plate cylinders either manually or by means of appropriate devices known as "plate assemblers", for example of the electronic or so- called "point" type. However the operations involved in mounting the plates on their cylinders are extremely repetitive, whether they are mounted manually or by means of plate assemblers. In addition, each time a given client's order is to be repeated, even if the printing has the same pitch as that underway, the plate cylinders have to be removed from the printing machine, the plates have to be mounted on the various cylinders and, when the production batch is complete, the plates have to be removed from the cylinders and be returned to store to remain there until the next order. As will be apparent, said operations are complicated, lengthy, repetitive and significantly affect the production cost. Moreover the current market tendency (based on the "just in time" concept) is to reduce the volume of an individual production batch and increase the order frequency, to consequently increase the number of times a certain production has to be repeated. At each production repeat, it is necessary in practice to replace the said double-adhesive tape by which the plates are applied to the cylinders, this tape being of considerable width and of consequent high cost per unit area. In removing the polymer plates from the relative cylinders to store them until their next use, it not infrequently happens that the plates stretch and deform or even tear, with consequent involvement of cost for their replacement. Furthermore, as is well known to the expert of the art, each time the plates are applied to the cylinders there is a very high risk of error consequent on forgetfulness (one or more plates are not mounted) or on inaccurate measurement of the printing pitch. When the production batch is complete the cylinders have to be changed on the printing machine, with consequent machine down-time required to; a) halt the machine; b) withdraw the plate cylinder from the relative impression cylinder, this operation being required for each colour; c) remove the caps from the pivots of the relative plate cyl inder; d) manually or automatically remove each plate cylinder from the printing machine; e) arrange the plate cylinders on appropriate supports in a parking region provided for this purpose; f) remove the plates applied to the cylinders and mount on the relative cylinders the plates of the new batch to be worked; g) manually or automatically pick up the cylinders to which the new plates have been applied and transport them to the printing machine; h) mount the plate cylinders on the printing machine; i) centre the various colours and set the pressures; j) restart the machine. From the aforegoing it is apparent that the aforedescribed operations are substantially repetitive. It should however be noted that some of these operations are also dangerous for the operator (in particular the removal, transport and remounting of plate cylinders). In any event the relative machine stoppage significantly affects production cost, even if this cost cannot be easily quantified precisely. However, to give an idea, reference will be made to a batch of about 2000 kg of polyethylene film of width 100 centimetres and thickness 50 microns printed with six colours. It is printed at a rate of about 120 metres per minute, with a resultant production time normally of about 6 hours, also taking account of possible machine down-times. The time required for changing the work, on the assumption that the new work also requires six colours, is 70-80 minutes, representing a certainly not negligible percentage of the overall batch production time. An object of this invention is to provide a method for mounting printing plates on the relative cylinders of flexographic printing machines which enables the plant down-time involved in mounting or changing the plates to be substantially reduced. A further object is to provide a plate mounting method which substantially reduces operator danger. A further object of the invention is to provide a method of the aforesaid type which drastically reduces the possibility of error in applying the plates to the cylinders. Said objects are attained according to this invention by a plate mounting method comprising the following stages: applying those plates relative to a certain colour to a sleeve formed from a flexible transparent polymer film which is substantially indeformable in the plane of the film, ie which cannot be elongated on any side, this sleeve constituting the development of all or part of the surface of the relative plate cylinder; and applying the sleeve to the relative cylinder. The fact of being able to apply the individual plates not to the curved surface of a cylinder but to said flexible polymer sleeve, which can be extended over a flat working surface, conveniently a drawing board, enables the plates to be applied in a more simple and comfortable manner for the operator, under conditions which certainly allow greater accuracy. The subsequent application of the sleeve to the cylinder is extremely simple if, conveniently, suitable reference markings are applied to the sleeve with which to make the reference markings conventionally present on plate cylinders of flexographic printing machines coincide. The sleeve can be conveniently applied to the relative cylinder by previously applying strips of double-adhesive tape to the sleeve and then utilizing the said reference markings. This requires considerably less time than that involved in applying the various plates to the cylinder, with the result of also achieving greater accuracy of application. In addition, having terminated the printing of the batch concerned, the sleeve can be easily removed from the cylinder and stored ready for subsequent use, because the plates remain applied to it. At most, only the strips of double-adhesive tape for applying the sleeve to the relative plate cylinder will need to be replaced. The use of said sleeves enables any changes requested in the meantime by the client and involving replacement of individual plates to be comfortably, easily and quickly accommodated. It should be noted that with the traditional method, to print two orders having the same pitch (ie using the same cylinder format), all the steps involved in the above points a) to i) have to be carried out, whereas with the method of the invention the work is firstly programmed for all those production batches requiring a determined cylinder format, then after steps a) and b) the relative sleeves are changed without it being necessary to remove the plate cylinders from the printing machine, but simply by removing the relative sleeve from the cylinders and applying the new sleeve to each cylinder. The time saving is evident, and is truly considerable. To give an idea, by applying the method of this invention, the machine down-time necessary for changing the sleeves is reduced to 2-3 minutes per colour (making a total of 12-18 minutes for six colours), against a time of about 2 hours with the traditional method requiring the removal and remounting of the cylinders, resulting in a truly significant cost saving. Even if plate cylinders have to be changed because the subsequent production involves a different printing pitch, there is the significant advantage of being able to previously apply the sleeves to the relative different-pitch cylinders at ground level, the cylinders hence being ready to mount on the printing machine as soon as the current batch production has ended and the relative cylinders have been removed from the printing machine, with significant time saving. It has been found that it can be convenient to protect the squaring carried out on the sleeves (which as stated can be conveniently done with a pen able to write on plastic sheets) with transparent adhesive tape, especially in those regions in which the sleeve is handled by the operator. This prevents the lines drawn on them on the sleeve from being able to be erased with the passage of time. Production costs are also reduced by the fact that the method of the invention requires the printing plates to be mounted on the sleeves only once, whereas in the traditional method they are mounted on the relative cylinder each time a certain production has to be repeated. Again, as the individual plates no longer have to be removed from relative cylinder, there is no longer any danger of tearing or deforming them, nor the need to change the double-adhesive tape (of very large width type) used for applying the individual plates, given that these always remain applied to the sleeve. At most, only those strips of double-adhesive tape (of narrow width type) used to apply the sleeves to the cylinders may need to be changed. Finally, the method of the invention results in lesser handling of the plate cylinders, it being no. longer necessary to remove them if the next production batch has the same printing pitch, so reducing the risk to the operator. The individual sleeves conveniently consist of rectangular sheets which, as already stated, represent the development of all or part of the surface of a plate, cylinder. In practice these sleeves are spread on a drawing board, preferably of the luminous millimetre- graduated type, so as to be able to accurately reproduce on said sleeves (for example by a pen of the type suitable for writing on polyester) the same longitudinal and circumferential reference lines (but developed in plan) as reproduced on each plate cylinder (and which in the traditional method are used as reference markings to arrange the various plates in their correct position). Then when applying the sleeve to the relative plate cylinder it becomes very easy to make the reference markings on the sleeve coincide with the same reference markings present on the cylinder. However before doing this, the various plates have to be applied to the sleeve in the correct position relative to said reference lines. This operation is made very simple and accurate for the operator by using the reference markings on the millimetre- graduated paper while positioned on the luminous surface of the drawing board rather than on the curved surface of a cylinder. Conveniently the aforesaid operations are carried out in practice in the following manner: 1) preparing a sleeve of a format suitable for the format of the plate cylinder to which it is to be applied, the sleeve being cut from a film of polyester or other transparent material undeformable when pulled, preferably a rectangle of such dimensions as to enable the relative plates to be applied to it; 2) squaring each polymer plate or group of plates, this operation being conventional and consisting of reproducing on each plate or group of plates two perpendicular axes (this operation being conveniently performed on said luminous millimetre-graduated drawing board); 3) squaring a first sleeve, consisting of reproducing on it all or part of of the reference lines present on the plate cylinder on which the sleeve is to be applied (this operation being conveniently carried out on said luminous millimetre-graduated drawing board); 4) plate position squaring effected on the same sleeve (preferably carried out on the same aforesaid luminous millimetre- graduated drawing board), consisting of reproducing on the sleeve, in the exact position for enabling the required printing result to be achieved, two perpendicular lines corresponding to said perpendicular axes reproduced on the individual sleeves or groups of sleeves (ie the axes of the preceding point 2); 5) superimposing on the first sleeve (preferably while still on the luminous millimetre-graduated drawing board) the other sleeves, one at a time, copying on each superimposed sleeve the underlying sleeve squarings of the preceding points 3 and 4 (the number of sleeves equals the number of colours to be printed); 6) applying under each plate either a single piece of double- adhesive tape extending across the entire plate, or strips of narrower double-adhesive tape (decidedly less costly, but having proved suitable for the purpose); 7) positioning each plate or group of plates on the relative sleeve, done by simply superimposing the drawn axes of each plate (in accordance with the preceding point 2) on the plate positioning axes reproduced on the sleeve (in accordance with the preceding point 4); 8) applying a double-adhesive tape (in one piece or, less costly, in strips) to the rear of each sleeve; 9) applying the sleeve to the relative plate cylinder, by simply superimposing on the reference lines present on the cylinder the corresponding reference lines reproduced on the sleeve.

Claims: Cl aims : 1. A method for applying printing plates of substantially transparent polymer to the plate cylinders of a flexographic printing machine, characterised by comprising the following stages: applying those plates relative to a certain colour to a sleeve formed from a flexible transparent polymer film which is substantially indeformable in the plane of the film, this sleeve constituting the development of all or part of the surface of the relative plate cylinder; and applying the sleeve to the relative cylinder. 2. A method as claimed in claim 1, wherein the operation by which the plates are applied to the relative sleeve is carried out on a surface on which the sleeve is spread. 3. A method as claimed in claim 2, wherein the surface is a luminous millimetre-graduated surface. 4. A method as claimed in claim 1, wherein the sleeve consists of a rectangular sheet. 5. A method as claimed in claim 1, wherein the sleeve consists of a polyester sheet. 6. A method as claimed in claim 1, wherein the reference markings present on the relative plate cylinder are previously reproduced wholly or partly on each sleeve. 7. A method as claimed in claim 1, wherein the plate centering reference markings present on the plates are previously reproduced on each sleeve in the relative position. 8. A method as claimed in claim 1, wherein each sleeve is applied to the relative cylinder by means of a double-adhesive tape involving the entire sleeve or only part of it, and previously applied to the sleeve. 9. A method as claimed in claim 1, wherein each plate or group of plates is applied to the relative sleeve by a double- adhesive tape involving the entire plate or group of plates or only part of it, and previously applied to the plate or group of plates. 10. A method as claimed in claim 6 and/or 7, wherein the reference markings to be reproduced on each sleeve are produced by an instrument able to draw on the transparent polymer with which the sleeve is made. 11. A method as claimed in claim 10, wherein the reference markings reproduced on each sleeve are protected by a transparent adhesive tape.

Inventors: CACCHI GABRIELLA, CACCHI, GABRIELLA
Applicants: CACCHI GABRIELLA, CACCHI, GABRIELLA
Application year: 1998
Main classification: B41F27
IPC codes: B41F27, B41N6, G03F1