Abstract:
A direct-to-press system using a seamless cylinder coated on the press with a curable polymer. After curing, the surface of the polymer is modified by selective laser radiation to change its affinity to printing ink. The cylinder is used in place of a plate cylinder in an offset press, either conventional or waterless. Since only the surface is selectively modified by the laser, coating thickness and uniformity are of little concern. After printing, the cylinder is cleaned on the press using a cleaning station similar to blanket washers used on presses. Cleaning does not have to be complete since the surface of the cylinder is never exposed to the ink. The system is compatible with existing press designs as it can be mounted in the space currently used to access the plate cylinder for plate changing.

Description:
RELATED APPLICATION 
     This application is a continuation-in-part of U.S. patent application Ser. No. 08/438,817, filed May 11, 1995, now abandoned. 
    
    
     FIELD 
     The present invention relates to direct-to-press imaging lithographic printing presses in which printing images are formed on a printing cylinder used in printing operations. 
     BACKGROUND 
     In accordance with conventional lithographic printing practices, ink in desired image patterns is formed on a plate that is clamped to a plate cylinder. The inked image pattern is ordinarily transferred from the surface of the plate to a blanket cylinder having a compressible surface. From the blanket cylinder the image is impressed onto paper. The printing plate carries a differentiated image on a dimensionally stable substrate such as an aluminum sheet. The imaged aluminum plate is secured to the plate cylinder by a mechanical lock-up mechanism which defines positional registration between the plate and the surface of the cylinder. When new images are to be printed, the mechanical lock-up system is released so that the printing plate carrying the old image can be removed and discarded and a newly imaged printing plate can be positioned and locked into place for the next print run. 
     In the past, press-ready lithographic printing plates have been prepared off-press by forming the required ink receptive image areas (and water receptive non-image areas) on suitable printing plate surfaces in a manner similar to photographic development. Preparation can be by means of hand operation or by readily available automatic developing and processing machines. Once having been imaged, printing plates generally are hand carried to the vicinity of the printing press and attached to the printing cylinder by press operators using a lock-up mechanism built into the cylinder itself. Although the attachment of the printing cylinder is generally a manual operation, robotic means have been developed for positioning and securing the printing plates. 
     Operations involving off-press imaging and manual mounting of printing plates are relatively slow and cumbersome. On the other hand, high speed information processing technologies are in place today in the form of pre-press composition systems which can electronically handle all the data required for directly generating the images to be printed. Almost all large scale printing operations currently utilize electronic pre-press composition systems that provide the capability for direct digital proofing, using video displays and visible hard copies produced from digital data, text and digital color separation signals stored in computer memory. These pre-press composition systems can also be used to express page-composed images to be printed in terms of rasterised, digitized signals. Consequently, conventional imaging systems in which the printing images are generated off-press on a printing plate that must subsequently be mounted on a printing cylinder present inefficient and expensive bottle-necks in printing operations. 
     On-press imaging is a method of generating the required image directly on the plate or printing cylinder. Existing on-press imaging systems can be divided into two types. In the first type a blank plate is mounted on the press and imaged once, thus requiring a new plate for each image. An example of this technology is the well known Heidelberg Model GTO-DI, manufactured by the Heidelberg Druckmaschinen AG (Germany) which is described in detail in U.S. Pat. No. 5,339,737. The major advantage compared to off-press plate making is much better registration between printing units when printing color images. A drawback of this method is the need to use a new plate for each image, thus increasing the cost of printing. 
     With press imaging systems that use plates, whether imaged off-press or on-press, the mounting cylinder is split so that clamping of the ends of the plate can be effected by a clamping means that passes through a gap in the cylinder and a slit between the juxtaposed ends of the plate. The gap in the mounting cylinder causes the cylinder to lose its rigidity and to make it susceptible to deformation and vibration. The vibration causes noise and wears out the bearings. The gap in the ends of the plate also leads to paper waste. 
     The second type of on-press imaging system uses the difference in the affinity to ink and water between two surfaces to define an image. One of these surfaces can be the surface of a cylinder (which could be treated to enhance certain properties) and a thin layer deposited on the cylinder having properties different than those of the cylinder surface. An example of this type is disclosed in U.S. Pat. No. 4,718,340 which teaches a hydrophilic cylinder coated with a thin layer of a hydrophobic liquid. The hydrophobic liquid is subsequently selectively ablated by a laser to generate the desired image pattern which is subsequently coated with ink. Alternatively, a hydrophobic cylinder may be coated by a hydrophilic layer and the latter layer selectively ablated. A disadvantage of this system is the low durability of the liquid layer. A refinement of this idea which overcomes the low durability of a liquid layer, is disclosed in U.S. Pat. No. 5,129,321 which teaches a method of coating a hydrophilic cylinder with a hydrophobic substance. The substance is fused to the cylinder to increase its durability. The fusing can take place before or after selectively ablating the substance coating the cylinder in accordance with digital data. After printing, the cylinder is cleaned, re-coated and re-imaged. While the latter method overcomes the limitation of U.S. Pat. No. 5,339,737 and avoids the requirement for a new plate each time the image is changed, it has not been commercially successful due to the great difficulty in completely cleaning off the substance coating the cylinder before application of a new coating. In any system which relies on the cylinder surface and the coating having opposite properties, such as hydrophilic and hydrophobic, the slightest trace of the coating left after cleaning will render the surface of the cylinder non-functional. Moreover, any cleaning based on wiping with solvents results in dissolved coating contaminating the solvent. Cleaning with this contaminated solvent results in a thin layer of the coating being left on the cylinder after cleaning. Since the properties affecting chemical affinity are related to surface properties, even a monolayer (a layer of one molecule in thickness) left behind will display opposite properties than a perfectly clean cylinder. 
     A second cleaning method disclosed in U.S. Pat. No. 4,718,340, which uses the imaging laser to clean the surface of the cylinder by ablation, is not practical. As the cylinder is typically made of metal or a ceramic material to withstand repeated laser exposure, it has a high heat conductivity. When the remaining thickness of the coating approaches a monolayer, most of the heat of the laser is absorbed directly in the cylinder and the surface temperature is not sufficient to drive off the coating. Adding light absorbing dye to the coating is of little use since all dyes are almost transparent at such thin layers. 
     It is an object of this invention to provide a reusable surface without the need for perfect cleaning, by using a method tolerant to any residual coating left on the cylinder after cleaning. It is another object of the present invention to use a seamless cylinder in order to reduce vibration and torsional resonances, achieve a faster changeover when changing from one image to another, to achieve better registration, to print at higher speeds and to eliminate the printing gap on paper caused by the separation between ends of the plate when using imaged printing plates. It is yet another object of the invention to reduce the time it takes to clean and re-image the surface of the cylinder. It is yet another object of the present invention to produce a compact design requiring access only to a small part of the cylinder circumference, thus making the invention easy to integrate into existing printing press designs. Other objects and advantages of the present invention will become apparent by considering the following description in conjunction with the drawings. 
     SUMMARY OF THE INVENTION 
     The invention relies on the fact that lithographic printing depends only on the surface properties of the printing surfaces. It is for this reason that it is necessary to modify the hydrophilic or hydrophobic properties of only an exceedingly thin layer on the surface in order to selectively attract printing ink. This enables less than perfect cleaning of a re-usable printing surface as long as the top layer of the newly applied coating is not contaminated. In conventional printing plate technology the substrate, typically anodized aluminum, has to be cleaned of any trace of the hydrophobic layer after plate processing. It is possible, however, to modify the hydrophobic properties of a polymer surface using the intense heat of laser radiation. The polymer layer can be applied on top of a partially cleaned surface without degrading the surface quality. The method can be used for waterless offset as well, using polymers capable of repelling ink, such as silicones, but which are still capable of being cleaned. 
     According to the invention there is provided a direct-to-press imaging lithographic printing press for imaging a pattern on the press, which includes an imaging cylindrical surface upon which image patterns are formed, a coating unit mounted proximate the imaging cylindrical surface so as to traverse the cylindrical surface and coat the surface with a thin layer of convertible material whose surface is or portions thereof are convertible from one having a first water responsive property to one having an opposite water responsive property. A convertible material curing unit is mounted proximate the imaging cylindrical surface for curing the convertible material coating after initial application to the imaging cylinder. A convertible material imaging means is used for imaging the convertible material coating over the imaging cylindrical surface so as to form a desired pattern of ink carrying areas and ink repellent areas thereon. Means for coating the convertible material coating with ink are provided. A blanket cylinder is in contact with the imaging cylinder so as to transfer the inked image pattern on the imaging cylinder to the blanket cylinder. A cleaning unit is mounted proximate the imaging cylindrical surface and is operative to clean off the cylinder after transfer of the image pattern to the blanket cylinder. 
     Preferably, the convertible material is a polymer and the polymer imaging means may be a radiation source. The radiation source may be a multi-beam laser exposure unit. The water responsive property may be one of hydrophobic and hydrophilic. 
     The polymer may be a tetra hydro pyranyl modified methyl acrylate diluted by a ketone solvent. 
     Advantageously, there may be included means for coating the polymer coating with a fountain solution after conditioning but before inking. 
     The radiation source may be mounted proximate the cylindrical surface and be movable so as to traverse the cylindrical surface as it rotates. 
     The polymer curing means may use heat, U.V. radiation or a combination of both. The imaging unit may use a multi-channel near infra-red source. Both the curing means and the imaging unit are mounted together on an assembly traversing the length of the cylinder while the cylinder rotates. This makes it possible to do the coating, curing and imaging at the same time, covering the whole surface of the cylinder in a spiral pattern. The cleaning station can be added to this assembly to clean the cylinder in a spiral fashion, however the use of conventional cleaning units, known as &#34;blanket washers&#34; may be an advantage. The advantage of a narrow cleaning unit co-mounted with the coating unit is the ability to selectively modify parts of the printing cylinder, for example clean, re-coat and re-image a single page out of an 8 page signature. This mode is valuable when only part of the data needs changing. Other features and advantages of the invention will become apparent when considering the preferred embodiment in conjunction with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as other features and advantages thereof, will be best understood by reference to the description which follows read in conjunction with the accompanying drawings wherein: 
     FIG. 1 is a perspective view of the printing section of a lithographic press, modified according to the invention; and 
     FIG. 2-a to 2-e illustrates the steps in re-imaging the surface of the printing cylinder in order to print a new signature. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In FIGS. 1 and 2-a to 2-e, like reference numbers refer to like parts. Referring now to FIG. 1 representing the printing section of a lithographic press, paper 1 (either in sheet or web form) is compressed between impression cylinder 2 and blanket cylinder 3. Blanket cylinder 3 is in contact with image cylinder 4 which replaces the plate cylinder in a conventional press. The main difference is that image cylinder 4 is a seamless cylinder, thus being able to run faster and with no vibration compared to a plate cylinder having an elongated gap along the length of the image cylinder (not shown) for clamping the plate. The image cylinder 4 is being inked by a water/ink system using fountain solution rolls 5 and ink rolls 6. Rolls 5 and 6 will be merged in some inking systems known as &#34;integrated&#34; inking chain. Alternatively, the press can operate in waterless offset (also known as &#34;dry offset&#34;) mode in which fountain solution rolls 5 are not used. Up to this point the press is conventional and well known. A cleaning unit 7 is mounted near image cylinder 4 and is capable of washing off most of the ink, water and imaged layer used on a previous print run. The cleaning unit is similar to the well known &#34;blanket washer&#34; units employed in modern presses to clean the blanket cylinder between print runs, with the exception that extra solvents may have to be added to dissolve most of the imaged layer. Additional cleaning units can be used to clean blanket cylinder 3 and other cylinders in accordance with modern press design. 
     A linear track 9 is rigidly mounted parallel to image cylinder 4. A traveling carriage 8 is traversing image cylinder 4 under the control of motor 11 and lead screw 10. The motion of image cylinder 4 and motor 11 are synchronized using shaft encoders in a manner similar to all drum imaging devices. Drum imaging devices are well known and have been commercially available for many years. Thus, no further details of the synchronization and handling of the image data will be given. A coating unit 12, curing unit 13 and imaging unit 14 are mounted on carriage 8 and capable of traversing the full width of image cylinder 4. Coating unit 12 sprays a polymer solution onto image cylinder 4, after the image cylinder 4 has been cleaned. Alternatively the polymer can be applied by a roll, similar to ink application. As the polymer has to be imaged a short time after its deposition (typically less than one minute), the curing of the liquid polymer into a solid is accelerated. The curing is accelerated by heat, either radiant or hot air, generated by curing unit 13. Use of U.V. light to accelerate curing is also possible but less desirable as U.V. curing generates cross-linked polymers which are harder to clean by cleaning unit 7. The thickness of the polymer layer is from 1 to 10 microns typically, thus the amount of material to be cured is small and energy required for curing is low, even for rapid curing. 
     After curing the polymer surface is imaged by a multi-channel laser head 14. In order to image the complete surface of image cylinder 4 in a short time (in the order of one or two minutes) a large number of beams are required as well as a relatively high power. Multi-beam laser imagers are well known. By the way of example, a laser array is described in U.S. Pat. No. 4,743,091 which is incorporated herein by reference. The number of beams required depends on the required imaging time, power, and the maximum rotational speed of the image cylinder 4. While the cleaning, coating and imaging is done, the press is in the &#34;impression off&#34; mode. In this mode the image cylinder 4 does not touch any of the other cylinders (same as a plate cylinder in &#34;impression off&#34; mode). After imaging the press is switched to &#34;impression on&#34; mode and the image cylinder 4 is inked in the conventional or waterless offset manner. A detailed explanation of the steps is shown in FIG. 2-a to FIG. 2-e. 
     Referring now to FIG. 2-a the old image, consisting of polymer coating 17 which is cured and covered with ink 19 and, in conventional offset, water 20, is cleaned by a conventional automatic blanket washer 7 (normally used to clean blanket cylinders). The blanket washer consists of a renewable wiping material 15, usually fed from one roll to another, and a solvent 16 used to wet the roll. Since the cylinder itself is immune to solvents and, typically made of metal, any suitable solvent capable of dissolving the old ink and the imaged polymer can be used. The cleaning need not be perfect and a very thin layer of residual polymer coating 18 mixed with some ink is assumed to stay on the image cylinder 4. Repeated cleaning can reduce the thickness of residual polymer coating 18 but not eliminate it, as the solvent is contaminated with the ink and polymer. Fortunately, a very thorough cleaning is not required and as long as residual polymer coating 18 is much thinner than original layer of cured polymer coating 17 the process can be used repeatedly without build-up of the layers. 
     Referring now to FIG. 2-b, a new polymer coat 17 is applied over the residue of the residual polymer coating 18 by a coating unit 12 which is equipped with a spray nozzle. Alternatively the coat can be applied with a roller or any other of the common methods. Typically the polymer is a thermoplastic material and is diluted with a solvent. Curing unit 13 uses radiative heat to evaporate the solvent and harden polymer coating 17. In the alternative, hot air or U.V. radiation can be used to cure polymer coating 17. Either linear or cross-linked polymers can be used, the disadvantage of cross-linked polymers is the relative difficulty of cleaning. When waterless printing is used most of the effective oil repelling materials are cross-linked. The thickness of polymer coating 17 is typically from 2 to 10 microns but layers as thin as 1 micron can be used if their durability is sufficient. 
     Referring now to FIG. 2-c, the cured polymer is selectively addressed by a multi-channel laser head 14 according to the pre-press data files 23. Preferably, the reaction is purely thermal, so that any type of laser can be used. Laser diodes operating in the near infra-red are the preferred source. Typically the cylinder is imaged at a resolution of 2400 DPI. Lowering the resolution does not reduce the imaging time in most cases as the process is limited by the amount of energy required, not the data rate. For conventional printing the polymers tested required between 0.1 J/cm 2  to 0.2 J/cm 2  while for waterless printing, energy requirements were 0.4 J/cm 2  to 0.8 J/cm 2 . When a laser supplying 10 W is used on an 8 page press (80 cm×100 cm signature size) the time required to expose the image varies from (80 cm×100 cm×0.1 J/cm 2 ) : 10 W=80 sec for the best case, up to (80 cm×100 cm×0.8 J/cm 2 ): 10 W=640 sec for the worst case, using the lowest sensitivity waterless printing polymer. Thus, these times can only be shortened by having a more powerful head and typically are not limited by the data rate of the pre-press system. The laser beam 22 modifies the polymer from hydrophobic to hydrophilic and/or from non-soluble in water to soluble in water. The polymer is mixed with a large amount of carbon black or laser absorbing dye in order to absorb most of the laser energy in a thin layer, typically 1-2 micron. The temperature in this layer reaches easily 600° C. and sometimes higher thus the chemical composition is easily modified. The modified surface, layer 21 has as different an affinity to ink and water as possible from the unmodified polymer coating 17. To print, the press is switched to &#34;impression on&#34; mode, causing the image cylinder to engage the blanket cylinder and the inking system. Referring now to FIG. 2-d, fountain solution roll 5 applies fountain solution 20 (water) to the hydrophilic areas followed by ink rolls 6 applying ink 19 to the hydrophobic areas. Since the polymer coating 17 is not very durable in the modified areas 21, when it wears through to the image cylinder 4 the surface of the image cylinder 4 has to have a hydrophilic surface such as anodized aluminum. 
     In the alternate embodiment, waterless offset, the fountain solution and rolls 5 are not used. A second alternate embodiment uses integrated inking. In an integrated inking system an ink/water emulsion is applied. From that point on, the printing proceeds in a conventional manner until the printed material has to be changed. For multi-color printing, multiple press units are used. The on-press imaging has much improved color registrations as all registration errors caused by plate mounting are eliminated. 
     Referring to FIG. 2-e, the modified parts of the polymer coating 17 can be made soluble in the fountain solution 20 while still being hydrophilic, so that the fountain solution rapidly dissolves the polymer underneath it until the fountain solution extends down to the surface of the seamless image cylinder 4. Additives can be added to the fountain solution to enhance its ability to dissolve the modified polymer. Provided the material of the cylinder itself is hydrophilic, water will coat the surface of the cylinder and remain interspersed between hydrophobic areas of the polymer. Materials which are hydrophilic and suitable for the seamless cylinder are anodized aluminum, chrome, nickel, steel, and ceramics such as alumina (Al 2  O 3 ) and zirconia (ZrO 2 ). Zirconia is particularly desirable as it is very durable, hydrophilic and refractory while having a low thermal conductivity. Low thermal conductivity minimizes the amount of laser energy required to heat up polymer coating 17 in order to induce the chemical transformation. Since the modified polymer is hydrophilic and the cylinder surface is hydrophilic, no change in printing is noticeable as the coating wears away. 
     Many different polymer compositions can be used since most polymers are hydrophobic and many turn hydrophilic (or less hydrophobic) when heated to a temperature sufficient to decompose them. Most polymers do not absorb well in the infra-red spectrum and a suitable absorbing dye, or carbon black, has to be mixed with the polymer. A suitable selection of absorbing dyes is given in U.S. Pat. No. 5,126,760 which is incorporated herein by reference. When a longer wavelength laser, such as a CO 2  laser, is used the absorbing dyes are not required. 
     In the preferred embodiment the polymer is a tetra hydro pyranyl modified methyl acrylate. It is available from the 3M corporation (Minn.). The polymer switches from hydrophobic to hydrophilic when heated up by a laser and becomes water soluble at the same time. A detailed discussion on the composition and properties of these polymers is presented in U.S. Pat. Nos. 5,102,771, 5,225,316 and 5,314,785 which are also incorporated herein by reference. The polymer is soluble in many organic solvents, particularly ketones. 
     Example: Conventional Offset Press Application 
     The polymer used is 3M thermal plate coating available from the printing plate division of 3M (Minn., U.S.A.). This coating is already pre-mixed with the infrared absorbing dye and the solvent. It is a tetra hydro pyranyl modified methyl acrylate diluted by a ketone solvent. Acetone is added to the blanket washer solvent. The imaging head used is a Creo Thermal Head (from Creo Products Inc., B.C. Canada). The head is a 240 channel head having an output power of 18 W. The imaging sensitivity for a print run length of over 50,000 impressions was found to be 0.15 J/cm 2 . Imaging time was about 1 minute for a 80 cm×100 cm area. (80 cm circumference, 100 cm length). 
     The resolution was 2400 DPI and the data rate about 15 MB/sec. The imaging drum rotated at about 400 RPM during imaging. 
     While a laser is the preferred source, broad area light sources such as arc lamps can be used, in conjunction with light valves, to supply the required radiation. In this case the photonic effect of the radiation can be used to an advantage beyond the heating effect. By way of example, ultra violet light may be used to activate cross-linking of a polymer to render the ink carrying areas non-soluble. 
     Whether the polymer is made soluble or not, perfect cleaning is not required as the new layer will be contaminated very slightly by the old layer. For example, the layer left behind after using a blanket washer is 1 to 5 nm thick while the thickness of the new coat is 1 to 10 microns. Thus, the contamination is under 0.1%, which does not materially affect the properties of the layer. 
     While the preferred coating is a polymer, any convertible material capable of rapid curing and whose surface can be selectively converted from either hydrophilic or hydrophobic to either hydrophobic or hydrophilic, respectively, would work. 
     Accordingly, while this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to this description. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as fall within the true scope of the invention.