Printing – Planographic – Lithographic plate making – and processes of making or using...
Reexamination Certificate
2002-04-26
2004-05-11
Colilia, Daniel J. (Department: 2854)
Printing
Planographic
Lithographic plate making, and processes of making or using...
C101S401100, C101S467000
Reexamination Certificate
active
06732653
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to positive-working lithographic printing plates. More particularly, it relates to methods for avoiding the need to remove unwanted, unexposed areas left on the finished plates due to shading of sections of the plate precursors by platesetter clamps or other plate-holding elements.
BACKGROUND OF THE INVENTION
In lithographic printing, ink-receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink-receptive regions accept the ink and repel the water. The ink is then transferred to the surface of a material upon which the image is to be reproduced. Typically, in a method known as “offset”, this is done indirectly by first transferring the ink to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
A class of imageable elements called printing plate precursors, useful for preparing lithographic printing plates, comprises a layer applied over the surface of a hydrophilic substrate. The layer includes one or more radiation-sensitive components, which may be dispersed in a suitable binder. Alternatively, or in addition, the binder itself may be radiation-sensitive. The layer is commonly applied as a coating, using a solvent.
If after exposure to radiation the exposed regions of the coating are removed in the developing process, revealing the underlying hydrophilic surface of the substrate, the plate precursor is referred to as “positive-working”. Conversely, if the unexposed regions are removed by the developing process and the exposed regions remain, the plate precursor is called “negative-working”.
In both cases, the regions of the radiation-sensitive layer (i.e., the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water, typically a fountain solution, and repel ink.
An alternative way of achieving the same result is to begin with a hydrophilic surface upon which, after imagewise exposure and developing, an ink-receptive pattern representing the image is obtained. If the unexposed areas become ink receptive, the plate precursor is “positive-working”, while if the exposed areas become ink receptive, it is “negative-working”.
Recent developments in the field of printing plate precursors deal with radiation-sensitive compositions that can be imagewise exposed by means of lasers or laser diodes. This type of exposure, known as digital imaging, does not require films as intermediate information carriers since lasers can be controlled by computers.
High-performance lasers or laser diodes that are typically used in commercially available exposure devices (known as platesetters) emit light in the wavelength ranges of either 800 to 850 nm or 1060 to 1120 nm. Therefore printing plate precursors, or initiator systems contained therein, which are to be imagewise exposed by means of such platesetters, have to be sensitive in the near infrared range. They are not however typically very sensitive to visible light. Such printing plate precursors can therefore basically be handled under daylight conditions, which significantly facilitates their production and processing.
Thermally imageable elements useful as lithographic printing plate precursors, exposable by infrared lasers or laser diodes as described above, are becoming increasingly important in the printing industry. After imagewise thermal exposure, the rate of removal of the exposed regions by a developer in positive-working elements is greater than the rate of removal of the unexposed regions, so that the exposed regions are removed by the developer to form an image. Such systems are disclosed in, for example, Parsons, WO 97/39894 and U.S. Pat. No. 6,280,899; Nagasaka, EP 0 823 327; Miyake, EP 0 909 627; West, WO 98/42507 and U.S. Pat. No. 6,090,532; and Nguyen, WO 99111458 and U.S. Pat. No. 6,060,217.
Printing plate precursors are also in use which are imageable by ultraviolet radiation, as are types that are imageable by visible radiation.
Imaging of digital, thermally imageable precursors is typically done using platesetters, where the plate precursor is mounted either
i). on a rotatable drum (external drum), typically using clamps, or
ii). in a drum (internal device), in which case the plate precursors are held in place with compressed air or with clamps, which may be magnetic.
When a positive-working lithographic printing plate precursor is imaged on a platesetter employing clamps, the clamping device prevents the successful exposure of the coating immediately under the clamp. After development, this unexposed area of coating accepts ink. Unless this section of coating is removed manually (a time-consuming process), it will cause an unwanted image on the press. The problem is particularly troublesome for web presses, where ink is wasted and unwanted inked image areas can transfer to the back of paper stocks.
Rather than using clamps, some platesetters employ suction cups and powerful vacuums. On mounting a plate precursor on such a platesetter, however, at least one edge of the plate precursor is typically inserted into a crevice in the drum, where it is shaded from the imaging radiation. In such systems, the presence of unwanted, remaining image areas is therefore still not avoided. Thus there remains a need for ways of avoiding the time-consuming step of removing such unwanted image areas after plate development.
SUMMARY OF THE INVENTION
This need is addressed by the present invention. In one aspect, the invention is a method for eliminating an unwanted ink-receptive section of a printing plate precursor present after treatment of an unwanted unexposed section with a developer, the unwanted ink-receptive section being shaded by a plate-holding element from exposing radiation during an imagewise exposure of the plate precursor and therefore remaining unexposed to the exposing radiation following the imagewise exposure, the section comprising a heat-sensitive layer, the method comprising heating the unwanted unexposed section prior to developing the plate precursor to a temperature such that following development the section is hydrophilic.
In another aspect, the invention is a method for preparing a printing plate comprising the steps of
(a) providing a positive-working printing plate precursor comprising a radiation-sensitive region on a front surface of the precursor, the radiation-sensitive region comprising a heat-sensitive layer;
(b) mounting the precursor on an exposure device with a plate-holding element wherein the element overlaps a section of the radiation-sensitive region of the precursor;
(c) imagewise exposing the radiation-sensitive region of the precursor to exposing radiation, the radiation selected to render the heat-sensitive layer exposed to the radiation soluble or dispersible in a developer;
(d) heating the section of the radiation-sensitive region overlapped by the element in a manner sufficient to render the overlapped section soluble or dispersible in the developer;
(e) removing the precursor from the exposure device; and
(f) developing the precursor in the developer to form the plate.
In yet another aspect, the invention is a method of avoiding the formation of an unwanted unexposed section in a positive-working printing plate precursor due to shading by a plate-holding element from exposing radiation during an imagewise exposure of the precursor and therefore remaining unexposed to the exposing radiation following the imagewise exposure, the method comprising employing a plate-holding element to secure the precursor during imagewise exposure, wherein the plate-holding element is substantially transparent to the exposing radiation.
In still another aspect, the invention is a method for preventing the generation of ink receptivity in an unwanted unexposed section of a developed plate prepared from a positive-working printing plate precursor, the section bei
Felker Melanie A.
Ray Kevin
Shimazu Ken-ichi
Colilia Daniel J.
Culler Jill E.
Kodak Polychrome Graphics LLC
RatnerPrestia
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