Printing – Printing members – Rolling contact
Reexamination Certificate
2001-03-15
2003-03-11
Gordon, Raquel Yvette (Department: 2853)
Printing
Printing members
Rolling contact
C101S377000, C101S378000
Reexamination Certificate
active
06530321
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to an image transfer blanket construction to be used principally in offset printing systems, and more particularly to an image transfer blanket which is flexible and yet which has a non-extensible backing.
In offset lithography, a rotary cylinder is covered with a printing plate which normally has a positive image area which is receptive to oil-based inks and repellent to water and a background area where the opposite is true. The printing plate is rotated so that its surface contacts a second cylinder covered with a rubber-surfaced ink-receptive printing blanket (sometimes also called a printer's blanket, or broadly an image transfer blanket). One common type of printing blanket is one which is manufactured as a flat, fabric reinforced sheet having an elastomeric, ink-receptive surface. The ink present on the image surface of the printing plate transfers, or offsets, to the surface of the blanket. Paper or other sheet stock to be printed is then passed between the blanket-covered cylinder and a rigid back-up cylinder to transfer the image from the surface of the blanket to the paper.
During the step in which the inked image is transferred from the plate to the blanket and the step where the image is transferred from the printing blanket to the paper, it is important to have intimate contact between the two contacting surfaces. An exact amount of interference pressure is required so that the blanket contacts and removes ink from the printing plate and transfers the inked image to a proper depth into the paper. This is ordinarily achieved by positioning the blanket-covered cylinder and the supporting cylinder it contacts so that there is a fixed interference between the two and so that the blanket is compressed throughout the run to a fixed depth, typically approximately 0.05 to 0.10 mm (0.002 to 0.004 inches). It is important that this compression be maintained uniformly over the entire surface of the blanket.
Within the current state of the art, all conventional printing blankets lose thickness (i.e., lose gauge or “sink”) when they are initially tensioned and installed, and further lose thickness as the blanket is repeatedly exposed to the interference pressures at the nips between the printing cylinder and blanket-covered cylinder and the blanket-covered cylinder and rigid back-up cylinder, respectively. Blankets can fail catastrophically due to blanket smash, a permanent deformation in a portion of the entire blanket surface, or from a gradual deterioration of blanket gauge over time due to the repeated cycling of the interference pressures on the blanket's surface. When the thickness of a blanket recedes beyond the limits of press adjustment, the print pressure becomes insufficient to cause transfer of the inked image from the print cylinder to the blanket or the blanket to the paper, or both. Thus, for a typical blanket, a permanent loss of thickness of as little as 0.05 to 0.10 mm (0.002 to 0.004 inches) may require a press stoppage. Such problems are even more severe at or near the gap in the cylinder because there is a tendency of the blanket to “fall off” into the gap (i.e., lose the thickness needed to offset the image to the web).
Conventionally, the fixed interference described above is accomplished by inserting one or more thin layers of paper or the like between the blanket and the surface of the blanket cylinder to build up the thickness of the blanket. This process is known as packing a blanket. Once the gauge loss of the blanket reaches a certain amount, as described above, additional thickness must be supplied under the blanket. This involves stopping the press, demounting the blanket and original packing, repacking, and then remounting and retensioning the blanket.
The packing process presents problems however in that the procedure is time consuming, resulting in down time for the printing equipment. Typically, press downtime can cost from several hundred to over a thousand dollars per hour. It may take over 30 minutes to pack or repack a blanket. Further time is lost as the system is retuned to optimum settings. Additionally, once positioned on the cylinder, the packing paper tends to slide, slip, and/or fold which may render the blanket surface nonuniform and result in poor printing results.
To avoid some of the problems associated with packed blankets, some press operators, and in particular news press operators, have used blankets which do not require packing. So-called “no pack” blankets have been developed to provide a fixed interference without the need to pack the blanket. No pack blankets are manufactured to very precise gauges so that they can be installed directly onto a blanket cylinder with the correct amount of interference. These blankets have the advantage of a one-piece construction which requires no positioning of packing paper beneath the blanket. This results in less down time for the printing equipment when an old blanket is removed and replaced with a new blanket.
Such no pack blankets, like most printing blankets, are normally composed of a base material which gives the blanket dimensional stability. Woven fabrics are preferred. The base typically includes two or more layers of such fabric adhered together. The working surface of the blanket which contacts the ink is typically an elastomeric layer of natural or synthetic rubber which is applied over the base layer or layers. The base layers and working surface are laminated together using suitable adhesives. Again, such blankets exhibit some gauge loss upon initial tensioning and installation and continue to lose thickness over time during use. However, once the gauge (thickness) loss on a no pack blanket exceeds the limits of press adjustment, the blanket becomes unusable without recourse and must be replaced by a new blanket.
An important goal in offset printing is to increase the operating speeds of printing presses in order to maximize production. Typically, conventional flat printing blankets are manufactured so that their ends can be mounted and secured into a relatively wide gap or groove in the blanket cylinder. The gap runs in the axial direction, and the leading and trailing ends of the blanket are inserted into the gap and secured by any of a number of techniques including lock-up mechanisms and clamps. Typically, the leading and trailing ends of the blanket are generally reinforced with strips of metal known as blanket bars to stiffen the blanket ends and facilitate insertion of the blanket into the lock-up mechanism.
However, the need for a gap in the blanket cylinder has resulted in problems when the speed of the cylinder is increased, as the cylinder is unbalanced (i.e., weight is unevenly distributed), and the blanket itself is subjected to increased stresses. This can result in vibrations and shock loading of the blanket, reducing print quality. Newer higher speed presses have appeared which have addressed these problems by providing a smaller gap in the blanket cylinder, sometimes known as “mini-gap” presses. Thus, shock loading can be reduced by making the cylinder gap as narrow as possible. Conventional cylinder gap widths, i.e., for use with fabric backed blankets, range from about 5 mm to about 10 mm in width. To address the need for narrow gap blanket cylinders, newer types of printing blankets have been developed. Such blankets are known in the art as metal-backed blankets (see, e.g., International Publication No. WO 93/01003 of Pinkston et al.) which rest upon and are supported by, a thin metal sheet. Metal-backed blankets can be mounted on cylinders with gaps that are less than 3 mm wide. Blanket cylinders having these much-narrower gaps can operate at high speeds with a reduced incidence of shock loading. A further advantage of such narrow gap cylinders is that there is less web area wasted in printing as the print can extend to the narrow gap.
A metal-backed printing blanket typically comprises a base layer of a thin, flat, flexible sheet of metal and a top layer comprising an el
Andrew Robert L.
Dzierzynski Edward P.
Kopecky Dawn
Loyer Philip K.
McLean Michael E.
Day International Inc.
Do An H.
Gordon Raquel Yvette
Killworth, Gottman Hagan & Schaeff LLP
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