Lithographic printing typically involves the use of a so-called printing master such as a printing plate which is mounted on a cylinder of a rotary printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-abhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the image-wise exposure and processing of an imaging material called plate precursor. In addition to the well known photosensitive, so-called pre-sensitized plates, which are suitable for UV contact exposure through a film mask, also heat-sensitive printing plate precursors have become very popular in the late 1990s. Such thermal materials offer the advantage of daylight stability and are especially used in the so-called computer-to-plate method wherein the plate precursor is directly exposed, i.e. without the use of a film mask. The material is exposed to heat or to infrared light and the generated heat triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by crosslinking of a polymer, heat-induced solubilization, or particle coagulation of a thermoplastic polymer latex.
Although some of these thermal processes enable plate making without wet processing, the most popular thermal plates form an image by a heat-induced solubility difference in an alkaline developer between exposed and non-exposed areas of the coating. The coating typically comprises an oleophilic binder, e.g. a phenolic resin, of which the developer solubility is either reduced (negative working) or increased (positive working) by the image-wise exposure. During processing, the solubility differential leads to the removal of the non-image (non-printing) areas of the coating, thereby revealing the hydrophilic support, while the image (printing) areas of the coating remain on the support. Typical examples of such plates are described in EP-As 625728, 823327, 825927, 864420, 894622 and 901902.
The industrial manufacturing of printing plate precursors involves the steps of unwinding a coil of the support material in web form which is typically aluminum, coating one or more layers on the web, drying the coating by blowing hot air on the web and finally rewinding the coated web on a core or immediately cutting the coated web in sheets which are then stacked and packaged. On an industrial scale, all these steps are carried out “on-line”, i.e. on a moving web in a single continuous operation without any intermediate storage.
A specific problem associated with thermal plate precursors comprising phenolic resins is that their sensitivity is not stable over time because the coating gradually becomes more resistant against the developer and therefore more heat needs to be applied during the image-wise exposure for triggering the imaging mechanism. Typically a high sensitivity, e.g. less than 100 mJ/cm2, is obtained just after coating and then slowly decreases to reach an equilibrium value of e.g. 250 mJ/cm2. The aging period that is required to arrive at a stable sensitivity may take several months after coating. In order to reduce the aging period, WO 99/21715 proposes a heat treatment by leaving the material shortly after coating in an oven at 40 to 90° C. for an extended period, which is at least 4 hours and most preferably at least 48 hours. U.S. Pat. No. 6,251,559 disclosed that a controlled slow cooling after the heat treatment provides additional improvements. According to the latter document, “controlled slow cooling” means that heat is lost from the precursor more slowly than if it is cooled under ambient conditions. Examples of such a cooling method include insulating the material after the heat treatment or leaving it in an oven which progressively cools to lower temperature. Such a cooling process lasts several hours and can only be carried out “off-line”, i.e. a coil or a stack of sheets is placed in an oven and left there during the required time. Off-line storage however is to be avoided for several reasons. Besides additional cost and logistic implications, it is quite clear that a coil or stack cannot be cooled uniformly since the interior of the coil or stack will go through a different temperature profile than the exterior. Therefore, there is a need for a method that provides an effective cooling step which can be implemented on-line, before winding the web on a coil or cutting the web into sheets.