Abstract:
Certain types of lithographic printing plates are activated by preheating while certain other types are post heated to harden the coating. A platen having a large mass compared to the mass of a printing plate is heated and maintained in the exact temperature range to which the printing plate is to be heated. A printing plate is brought into heat exchange contact with the platen for the period of time required to heat the printing plate to the temperature of the platen. The printing plate may be heated while resting in a fixed position on the platen or while the printing plate is carried over the platen by a continuous thin metal conveyor belt in intimate contact with both the platen and printing plate.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the heating of digitally imageable lithographic printing plates and more particularly to the preheating of a plate in preparation for imaging or to the post heating of a plate after imaging. 
     Present thermal printing plate imaging technology requires the preheating of certain types of plates prior to imaging while post heating or baking is used with other types of plates after imaging. The preheating step is required for those plates requiring heating to activate the plate so that it is responsive to the laser imaging process. An example is a negative working plate where the preheat serves to form a Bronsted acid in the coating, thereby making the coating imageable by subsequent exposure with a thermal laser which insolubilizes the coating in those areas struck by the laser. Post baking is often used to further harden a coating for improved run length. This post baking process is very well known for positive-working plates with coatings containing phenolic resins. In both situations, the plate must be heated to a specific temperature for the process to work successfully. 
     The present technique for heating such plates uses convection ovens with the oven being set at some temperature well above the desired plate temperature. The expectation is that the dynamics of the heat transfer process can be controlled such that the plate achieves and is maintained at the desired temperature as it is conveyed through the oven. There are several drawbacks to this technique: 
     1. The heat transfer is non-uniform across the plate. As the plate travels through the oven, different areas of the plate are subjected to different degrees of heating depending on relative proximity to the heat source and airflow dynamics. The leading edge of the plate enters the oven at ambient temperature, whereas the trailing edge is preheated by conduction within the sheet prior to entering the oven. Obviously both edges have the same dwell within the oven. 
     2. There is the potential to overheat or to underheat the plate if any variable in the heating process changes. For example, if the initial plate temperature or ambient air temperature varies, the resultant temperature of the heated plate in the oven will give rise to a different end point temperature. 
     3. The oven set point needs to be varied for different size plates or different plate thicknesses. The mass that needs to be brought to the desired temperature will be different for 0.008″ and 0.012″ plates. 
     4. There are substantial heat losses to the room environment. This necessitates a larger investment in energy costs to maintain both the oven temperature and the room temperature. 
     When the plates are not brought to the correct temperature, performance problems arise. For example, for negative-working plates, if insufficient Bronsted acid is formed due to underheating, the plate will not respond properly to the imaging laser radiation. This may be localized due to non-uniform heating that can result in hot or cold spots on the plate. Conversely, if the preheat is too high, it may begin to convert the coating to an insolubilized state, thus causing toning in non-laser imaged areas. 
     For the positive-working plates with a post-baking process, underheating results in insufficient hardening of the coating. The coating will then suffer from premature wear, and the press run length will be shortened. The convection ovens are often run at settings near the annealing point of the aluminum. Hot spots or overheating may result in distortion of the aluminum sheet. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide apparatus and a method for heating a printing plate either before or after imaging whereby precise plate temperature control can be achieved. More particularly, a platen of sufficient mass and heat capacity is heated and maintained at the desired printing plate temperature. The printing plates, which are of low mass compared to the platen, are loaded onto and maintained in heat transfer contact with the platen for the period of time required to heat the printing plates to the temperature of the platen. The printing plates are then unloaded from the platen and processed according to the type of plate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic process diagram generally illustrating an imaging process line with the heating platen of the present invention for a preheating operation. 
     FIG. 2 is a similar process diagram for a post-heating operation. 
     FIGS. 3 and 4 are more detailed drawings of various features of the platen and associated equipment. 
     FIG. 5 is a cross-section view of another embodiment of the invention employing a conveyor for transporting the printing plate over the heated platen. 
     FIG. 6 is a plan view of the conveyor of FIG.  5 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 of the drawings relates to a preheating operation for printing plates which require heating to activate the coating on the plate so that the coating is responsive to the digital laser imaging process. As previously indicated, an example is a negative-working printing plate where the laser renders the coating insoluble in the imaged areas. A specific example is where the preheat serves to form a Bronsted acid in the coating which makes the coating imageable by the subsequent exposure to thermal laser radiation. In this FIG. 1, individual lithographic printing plates are periodically fed by the conveyor  12  onto the heated platen  14 . The individually loaded plates slide down the sloped top surface  16  of the platen by gravity and come to rest against the plate stops  18 . 
     The platen  14 , which will be described in more detail later, is provided with a plurality of channels  20  extending therethrough parallel to the top surface  16  and containing electrical elements for heating the platen. After the printing plate has been heated to the required activating temperature on the platen, the stops are released and the printing plate slides off of the platen onto the conveyor  22  and is delivered to the conventional digital laser imager generally indicated at  24 . The imaged plate is then delivered by the conveyor  26  to the processor  28  where the non-laser imaged areas are removed leaving the ink-receptive coating. 
     FIG. 2 relates to the post-heating operation for the hardening of the positive imaged coating as previously described. In this operation, a plate is fed by the conveyor  30  into the digital laser imager  24  and then fed by the conveyor  34  into the processor  28 . From the processor  28 , the plate containing the positive polymer image is fed by the conveyor  35  onto the heated platen  14 , heated to the required temperature and carried away as a finished printing plate on the conveyor  36 . 
     The platen, as seen in more detail in FIG. 3, contains a plurality of small holes  38  extending through the platen from the top surface  16  to the bottom surface  40 . On the bottom surface  40  of the platen is a vacuum chamber  42  connected to the vacuum pump  43 . The vacuum which is drawn through the holes  38  pulls the printing plate into intimate heat exchange contact against the top surface  16  of the platen  14 . The spacing of the holes  38  is selected to provide this intimate contact over the entire area of the printing plate. Means are also provided, such as the roller  44 , to help guide the plate onto the platen so that it slides down the platen in full contact with the surface  16 . In order to assure the maintenance of the temperature and prevent the dissipation of heat to the surroundings, the platen arrangement includes the housing  46  which may be insulated. Openings  47  are provided for the printing plate to enter and exit. This FIG. 3 also shows one of the stops  18  including a solenoid  48  for lowering and raising the stop. The solenoid  48  as well as the vacuum pump  43  are connected to the control unit discussed later to synchronize the lowering of the stops and turning of the vacuum to permit the plate to slide off of the platen. 
     As shown in FIG. 4 which includes features not shown in FIG. 3 for purposes of clarity, the top surface  16  of the platen is preferably coated at  49  to assure a smooth surface. One reason is to provide a surface which permits the plates to slide down the slope easily. The other reason is to assure a flat surface for the intimate heat exchange contact with the plate. The coating may, for example, be a chrome plating or a Teflon coating. 
     The platen is provided with a plurality of channels  20  extending therethrough parallel to the top surface  16 . These channels  20  contain electrical heating elements  50 . These electrical heating elements  50  are connected by the wiring  52  to the control unit  54  which may control each of the heating elements individually or in selected groups. The platen is also provided with temperature sensing devices  56  such as thermocouples. These sensors  56  are also connected individually to the control unit  54  by the wires  58 . These sensors are connected in the control unit  54  to activate and deactivate the adjacent heating elements to accurately control the temperature over the entire platen. The temperature of the platen and the plate are usually controlled to ±2 to 5° F. and preferably to 1 to 2° F. Since the platen, which is perhaps 1 or 2 inches thick, has such a large mass as compared to the mass of the plate, which is usually 0.008 to 0.012 inches thick, loading the plate onto the platen will have a negligible effect on the platen temperature and the plate will rapidly heat to the platen temperature. The heating of the plate only takes about 20 to 30 seconds. 
     Another embodiment of the present invention is shown in FIGS. 5 and 6. In this embodiment, the printing plate being heated is moved across the platen as it is being heated rather than having the plate stationary on the platen as in the previously discussed embodiment. Referring to FIG. 5, there is a platen  60  shown in cross-section. In this embodiment, the platen can be horizontal as illustrated since the plate will be conveyed across the platen as will become apparent. Also, in this embodiment, the electrical heating elements are illustrated as strip heaters  62  attached at intervals and extending across the bottom of the platen. The specific type and location of the electrical heating elements is a matter of design choice as long as the platen can be evenly heated and the temperature controlled. Temperature sensing devices and a control unit are not shown in this FIG. 5 but they would be included just as in the previous embodiment. 
     The printing plate in this FIG. 5 embodiment is carried across the platen  60  by means of a heat conductive belt conveyor which comprises the conveyor drive rollers  64  and  66  and a continuous thin metal conveyor belt  68 . The belt  68  is metal and is thin in order to effectively transfer the heat from the platen through the belt to the plate. The preferred belt material is stainless steel with a thickness of about 0.005 inches. 
     The platen and belt conveyor are surrounded by the enclosure  70  which is a heat shield structure formed from or including insulation to retain the heat and maintain the proper temperature. For example, the enclosure  70  can be one-inch thick calcium silicate insulation board. The printing plate  72  is fed by the feed rollers  74  and  76  through the feed opening  78  in the enclosure  70  onto the feed shelf  80 . This feed shelf  80  directs the plate onto the belt  68  for transport across the heated platen  60 . The heated plate is then guided by the discharge shelf  82  out through the discharge opening  84  in the enclosure  70 . 
     If it is required or desirable to provide means for holding the belt firmly in contact with the platen and/or the plate firmly in contact with the belt, the platen  60  can have vacuum holes and a vacuum chamber, such as shown in the FIG. 3 embodiment. Likewise, the belt  68  can have holes  86  as shown in FIG. 6 to apply the vacuum through the belt to the plate. Also, if it is desired or necessary to assure the free movement of the belt across the surface of the platen, the platen can be coated such as with Teflon. 
     The heating of the plate by conduction according to the present invention is a fast and energy efficient heat transfer process. Since the platen is maintained at the desired plate temperature, there is no underheating problem as long as the plate is given sufficient time in contact with the platen. Likewise, there is no overheating problem since the maximum temperature of the plate cannot exceed the platen temperature no matter how long the plate remains on the platen. Since the mass of the platen is large compared to the plate, the impact of varied plate sizes or thicknesses has a negligible impact on the ultimate plate temperature. Even if a small region of the plate, such as a plate corner, is not in intimate contact with the platen, the high heat conductivity of the plate itself will rapidly elevate the temperature of that area to the set point. An advantage of one embodiment of the invention is that the heating is static with the plate in a fixed position. The result is that the heating means of that embodiment of the invention requires considerably less space.