Abstract:
A method for forming a tubular printing blanket includes the steps of applying a application layer to a base, applying a polymer over the application layer so as to form as flexible inner tubular sleeve, at least one of the application layer and the polymer being an innermost layer of the tubular sleeve, and applying a print layer over the tubular sleeve. The interior of the flexible tubular sleeve can collapse to permit better storage of the blankets.

Description:
BACKGROUND INFORMATION 
     The present invention relates to the offset printing blankets, and more particularly, to tubular offset lithographic printing blankets and methods for manufacturing the same. 
     A web offset printing press typically includes a plate cylinder, a blanket cylinder and an impression cylinder supported for rotation in the press. The plate cylinder carries a printing plate having a rigid surface defining an image to be printed. The blanket cylinder typically carries a printing blanket having an outer print layer, for example of rubber, which contacts the printing plate at a nip between the plate cylinder and the blanket cylinder. A web to be printed moves through a nip between the blanket cylinder and the impression cylinder. Ink is applied to the surface of the printing plate on the plate cylinder. An inked image is picked up by the printing blanket at the nip between the blanket cylinder and the plate cylinder, and is transferred from the printing blanket to the web at the nip between the blanket cylinder and the impression cylinder. The impression cylinder can be another blanket cylinder for printing on the opposite side of the web. 
     A conventional printing blanket is manufactured as a flexible flat sheet. Such a printing blanket is mounted on a blanket cylinder by wrapping the sheet around the blanket cylinder and by attaching the opposite ends of the sheet to the blanket cylinder in an axially extending gap in the blanket cylinder. The adjoining opposite ends of the sheet define a gap extending axially along the length of the printing blanket. The gap moves through the nip between the blanket cylinder and the plate cylinder, and also moves through the nip between the blanket cylinder and the impression cylinder, each time the blanket cylinder rotates. 
     When the leading and trailing edges of the gap at the printing blanket move through the nip between the blanket cylinder and an adjacent cylinder, pressure between the blanket cylinder and the adjacent cylinder is relieved and established, respectively. The repeated relieving and establishing of pressure at the gap causes vibrations and shock loads in the cylinders and throughout the printing press. Such vibrations and shock loads detrimentally affect print quality. For example, at the time that the gap relieves and establishes pressure at the nip between the blanket cylinder and the plate cylinder, printing may be taking place on the web moving through the nip between the blanket cylinder and the impression cylinder. Any movement of the blanket cylinder or the printing blanket caused by the relieving and establishing of pressure at that time can smear the image which is transferred from the printing blanket to the web. Likewise, when the gap in the printing blanket moves through the nip between the blanket cylinder and the impression cylinder, an image being picked up from the printing plate by the printing blanket at the other nip can be smeared. The result of the vibrations and shock loads caused by the gap in the printing blanket has been an undesirably low limit to the speed at which printing presses can be run with acceptable print quality. 
     In response to these deficiencies in conventional flat printing blankets, gapless tubular printing blankets were developed by the assignee of the present invention. These gapless tubular printing blankets are described, for example, in U.S. Pat. Nos. 5,768,990, 5,553,541, 5,440,981, 5,429,048, 5,323,702, and 5,304,267. These tubular blankets however have required that the print layer and compressible layers be supported by a stiff inner sleeve, for example made of nickel. The tubular blankets thus were not flexible, in that the inner surface of the sleeve could not contact itself or collapse without damaging the layers of the blanket. The tubular blankets thus need to be stored in a tubular shape, taking up valuable space in a press room or print shop. 
     U.S. Pat. No. 5,654,100 discloses an offset rubber-blanket sleeve with rubber as a base material, reinforced by layer inserts embedded therein, such as a spiral winding to provide similar strength to a fiberglass or metallic sleeve. The sleeve is thus not collapsible. 
     SUMMARY OF THE INVENTION 
     Commonly-assigned U.S. Pat. No. 6,257,140, which is hereby incorporated by reference herein, describes gapless tubular printing blankets produced continuously and cut to length as desired. The sleeve and print layer are “continuously” formed in that the sleeve forming station continues to form an additional portion of the sleeve while the print layer forming station applies the print layer to the previously formed portion of the sleeve. Wound tapes or cross-head extruders are used to apply various layers. 
     Commonly-assigned U.S. Pat. No. 6,538,970 discloses a machine for winding a sleeve, and is also incorporated by reference herein. 
     Commonly-assigned U.S. patent application Ser. No. 09/716,696, which is hereby incorporated by reference herein, provides for ribbon casting of materials to form various layers of a tubular printing blanket. “Ribbon casting” occurs when a liquid material is deposited from a stationary source onto a rotating and translating substrate or that a liquid is deposited from a rotating source onto a translating substrate. A continuous ribbon of liquid material thus can be placed on the substrate. Urethane is used in the ribbon casting process. The urethane sets after a certain time. 
     All of the sleeves in the above-mentioned patent applications were designed to be stiff, typically being made of metal. 
     The present invention provides a device for manufacturing a flexible continuous printing blanket comprising: 
     a base; 
     an application layer located directly on the base; 
     a polymer applicator applying a polymer layer over the application layer so as to define a flexible sleeve layer, at least one of the application layer and the polymer layer being an innermost layer of the sleeve layer; and 
     a print layer applicator applying a print layer over the flexible sleeve layer. 
     The flexible sleeve of the present invention permits for better storage of blankets. 
     Preferably, a compressible layer applicator is located between the polymer layer and print layer applicators. The compressible layer applicator preferably applies a radiation-curing polymer that is a compressible liquid polymer, such as urethane mixed with microspheres, carbon dioxide, a blowing agent or water, for example. 
     Preferably, the radiation-curing polymer is polyurethane, and the radiation source is ultraviolet light. An electron beam also may be used for curing the polymer. 
     The sleeve preferably is made of urethane, for example a self-cure or radiation-curing urethane. A polyurethane layer with a hardness of at least 70 Shore A and most preferably a hardness of about 70 Shore D is preferred as the sleeve material. 
     The flexible application layer may be part of the sleeve, and may be made of a pre-fabricated tape that wraps around the rotating base. The tape may be a polyurethane film with a hardness of at least 70 Shore A and most preferably a hardness of about 70 Shore D is preferred as the sleeve material. 
     Alternately, the flexible application layer may be a release layer separating the base from the polymer applied by the polymer applicator. 
     The release layer may be for example a TEFLON tape which is removed from the flexible sleeve layer. 
     The present device preferably includes a rotation device for rotating the base, and the base and rotation device may be similar to the base devices used to form blankets in incorporated-by-reference U.S. Pat. Nos. 6,257,140 and 6,538,970 and U.S. application Ser. No. 09/716,696. These devices as a plurality of slats which push the sleeve so as permit a continuous manufacture. 
     Optional surface finishers for smoothing the surface may be located after the various applicators. 
     The sleeve may be formed continuously, so that a cutting device may be provided to cut the sleeve when a desired sleeve length is reached. 
     The present invention also provides a method for forming a tubular printing blanket comprising the steps of: 
     applying an application layer to a base; 
     applying a polymer over the application layer so as to form as flexible tubular sleeve, at least one of the application layer and the polymer being an innermost layer of the tubular sleeve; and 
     applying a print layer over the flexible tubular sleeve. 
     The method preferably includes forming a compressible layer over the flexible tubular sleeve and under the print layer. 
     The method may include removing the flexible application layer. 
     The applying of the flexible application layer may include winding a tape around the rotating base. 
     The method preferably further includes rotating the base. 
     The compressible layer may be a radiation curable polymer of a compressible material, for example UV-curable urethane. A curing step then preferably takes place in a few seconds, although times up to 5 minutes are possible. 
     A smoothing step may be provided both after and before the curing step. 
     The flexible sleeve can be pre-manufactured, and then used in a separate process to make the blanket. Alternately, the blanket can be made in a single continuous process. 
     Preferably, the print layer, compressible layer and flexible sleeve are made of urethane, and a reinforcing layer is provided between the compressible layer and the print layer. The reinforcing layer is also preferably made of urethane. 
     The reinforcing layer preferably is made of a high durometer urethane of greater than 70 shore A, most preferably about 70 shore D. 
     The print layer preferably is made of a urethane with a durometer of less than 80 shore A and most preferably of about 60 shore A. 
     The method of the present invention preferably includes compressing the printing blanket so that two different circumferential points of an inner surface of the sleeve when round contact each other. This permits for example storage of the sleeve. Preferably, most of the inner surface contacts itself. Various fold shapes are possible, depending on the storage area available. 
     The method also includes sliding the printing blanket over a cantilevered cylinder of an offset lithographic printing press. 
     The present invention also provides an offset printing blanket comprising: 
     a flexible and collapsible inner sleeve made of polymeric material, the sleeve being the innermost layer of the blanket; and 
     a print layer disposed over the flexible inner sleeve. 
     Preferably, a compressible layer is disposed between the print layer and the inner sleeve, and a reinforcing layer is disposed over the compressible layer and under the print layer. 
     The flexible inner sleeve may be made of urethane. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is described in more detail with reference to the following figures, in which: 
     FIG. 1 shows a device for manufacturing a tubular printing blanket according to the present invention; 
     FIG. 2 shows more detail of one embodiment of the sleeve-forming station of FIG. 1; 
     FIG. 3 shows more detail of the sleeve forming device in FIG. 2, with a preformed tape at a wider spacing; 
     FIG. 4 shows an alternate embodiment of the sleeve-forming station of FIG. 1; and 
     FIG. 5 shows a flexible blanket according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a device for manufacturing a preferred lithographic continuous-process gapless tubular flexible printing blanket  10 . In this regard, the term “continuous process” indicates that the process creates a continuous tubular blanket of undetermined axial length. 
     A sleeve forming station  20  forms a flexible sleeve  18 . The sleeve forming station  20  includes a rotation and translation device or base  22 , for example one having a series of axially-translating and rotating slats, as described in the incorporated-by-reference U.S. patent application Ser. No. 09/716,696, for example. 
     In a first embodiment shown in FIG.  1  and in more detail in FIG. 2, sleeve forming station  20  includes a flexible polymer tape  24 , for example made of urethane, which is wound over slates of the rotation and translation device so as to form a flexible application layer  25 . 
     On the application layer  25  is deposited a polymer using a liquid applicator  26 , which may be for example a spraying device. At a location  27 , the deposited polymer is still in a flowable form, and at a location  28  the liquid has cured so as to harden. The polymer may be a self-cure polyurethane, for example, or a UV-cure polyurethane, in which case UV light is applied to the outer surface of the sleeve  18 . 
     In this embodiment, the flexible application layer  25  and the polymer together form sleeve  18 . Preferably, both the layer  25  and the polymer have a hardness of at least 70 Shore A and most preferably of about 70 Shore D. 
     As shown in FIG. 3, tape  24  need not align perfectly, and some of the polymer can flow into interstices  29  of layer  25 . This is advantageous, since alignment of the tape can be difficult. 
     FIG. 4 shows an alternate embodiment of sleeve-forming station  20 . A release tape  124 , with for example a TEFLON outer coating slides over the outer surface of rotating and translation device  22 . On top of the application layer  125  formed by the TEFLON tape is deposited a polymer by a liquid applicator  26 , the polymer preferably being urethane. The polymer then cures, for example using UV light, while still on application layer  125 . The cured polymer thus forms tubular sleeve  18 . The release tape  124  can be pulled out the front end of the sleeve forming station  20 , as shown by arrow  126 . 
     An alternate to the tape  124  for application layer  125  is a release agent, for example dried TEFLON spray, for example 0.0001″ in thickness. This layer then can remain as part of sleeve  18 , or can remain part of rotating and translating device  22 . Application layer  125  also could be a permanent coating on the rotating and translating device  22 , such as TEFLON-impregnated nickel. 
     As shown in FIG. 1, over sleeve  18  is applied a compressible layer  16  of, for example, UV-curing urethane, commercially available from the Bomar Specialties Company of Conn., for example. The urethane may be applied for example in liquid form from a polymer liquid applicator  30 , which for example may be a spraying device. The radiation-curing urethane may be premixed before application, and then blown with a blowing agent or carbon dioxide for example to add compressibility. 
     A smoothing station  32 , for example a doctor blade or planing device, can reduce undulations in the applied compressible layer  16 . 
     The layer  16  is then cured using a radiation source  40 , for example a UV light source. An electron beam or other radiation could be used depending on the type of polymer to be cured. Layer  16  then cures to form the compressible layer of blanket  10 . 
     A second smoothing station  36  then may contact the urethane layer  16  to smooth layer  16  to reduce imperfections such as undulations. Smoothing station  36  may be, for example, a grinding device or surface planer. 
     Over the compressible layer  16  after grinding device may deposited, for example by a liquid applicator device, a reinforcing layer  14  (FIG.  5 ). The durometer of the reinforcing layer, which also may be urethane, preferably is greater than  70  shore A, and preferably about  70  shore D, similar to that of the sleeve  18 . 
     A second liquid applicator  50  similar to device  30  then forms a print layer  12  over the compressible layer  16 . The urethane of the print layer may have a shore A durometer value of about 60, for example. The deposited print layer forms a seamless and gapless layer when it sets. If desired, a scraper and/or a grinding device may be used to correct or reduce any imperfections such as undulations in the print layer. Both the print layer  12  and the reinforcing layer may be made from radiation-curing polymers, for example, and a radiation source may be provided after the respective applicators. Ribbon casting can also provide the print layer  12  and reinforcing layer. 
     Once the print layer  12  is complete, the blanket continues moving in the direction of arrow  5  until a desired length is reached, at which time the blanket is cut, for example by a rotating cutter or saw. 
     FIG. 5 shows a cross-sectional view of the blanket  10  when compressed by an outside force, the blanket having a sleeve  18 , compressible layer  16 , reinforcing layer  14  and print layer  12 . 
     As shown an innermost surface  19  of the printing blanket  10  can collapse, so that the surface contacts itself along circumferential sections that normally would not be in contact when the sleeve is round. Cardboard inserts can be provided in spaces  160  so that pinching of the blanket is prevented. Due to the sleeve construction, the blanket returns to its tubular shape when not compressed. 
     The compressible layer  16  may be made compressible in any manner known in the art, including for example, through the use of microspheres, blowing agents, foaming agents, or leaching. Examples of such methods are disclosed for example in U.S. Pat. Nos. 5,768,990, 5,553,541, 5,440,981, 5,429,048, 5,323,702, and 5,304,267. 
     As used herein, the term print layer, or printing layer refers to an polymeric material such as urethane which is suitable for transferring an image from a lithographic printing plate or other image carrier to web or sheet of material, with such print quality as the particular printing application requires. 
     It should be understood that a blanket in accordance with the present invention might also include multiple compressible layers, multiple build up layers, or multiple reinforcing layers. 
     The reinforcing layer also may be formed by winding fabric or plastic tape, cords or threads around the work piece.