METHOD AND APPARATUS FOR DAMPENING PLATE CYLINDER OF OFFSET PRESS

Abstract: A novel dampening unit for an offset press is proposed, comprising a nozzle (20) with a discharging mouth (21) in the form of a slit directed substantially at a tangent to an outer surface of the plate (6a) and extending in the axilal direction of the plate cylinder (6), which discharging mouth (21) is disposed near the plate surface. The interior of the nozzle (21) is divided into upper and lower chambers (20a, 20b) from the innermost area thereof to the discharging mouth (21) by a partition (22) extending in the direction of an air stream over the whole length of the nozzle (20). A device for generating an air stream, such as a fan (23) is disposed within the upper chamber (20a) and a device for generating a mist such as an ultrasonic generator (27) is disposed within the lower chamber (20b). According to this structure, the mist sucked out from the lower chamber (20b) can be stably and effectively held on the plate surface while sandwiched between the plate surface and an air film formed by the air stream ejected from the upper chamber (20a).

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an offset press, more particularly, to a novel method and apparatus for dampening a plate cylinder thereof. 2. Description of the Related Art In general, in a printing process utilizing an offset press, a water film is formed on part of a plate surface on which there is no image to be printed so that an ink imparted to the plate surface will not stick to the non-image area but will adhere only to an image-carrying area of the plate surface. The water film must be thin and reproductive, and the film thickness must be varied in accordance with other conditions, such as printing speed, distribution of the image-carrying area and the non-image area, material of the plate, and property of the ink used. The operation of a dampening unit for the offset press will be described below with reference to Fig. 1, in which part of an offset press provided with the most widely used dampening unit is schematically illustrated. Water in a pan 1 is lifted by rotation of a water pan roller 2, and adhered to the surface thereof. A water ductor roller 3 is rotatably secured on one end of a lever 3a and the other end of the lever 3a is pivoted so as to swing between a first position where a surface of the water ductor roller 3 touches that of the water pan roller 2 and a second position where a surface of the water ductor roller 3 touches that of a chrome roller 4. The water ductor roller 3 intermittently transfers the water film formed on the pan roller 2 to the chrome roller 4 by to this swing motion. Further, the chrome roller 4 is always in contact with a water form roller 5 disposed above the chrome roller 4, and reciprocates along the length thereof so that the water film carried on the surface of the chrome roller 4 is transferred onto the surface of the water form roller 5 as a uniform film. The water form roller 5 is always in contact with a plate 6a carried on the surface of a plate cylinder 6, and thus the water film on the water form roller 5 is transferred to a non-image area of the surface of the plate 6a. Ink is fed from an ink fountain (not shown) to a vibration roller 7 and then to a pair of ink form roller 8. An ink film formed on the ink form roller 8 is then transferred to the surface of the plate 6a rotating integrally with the plate cylinder 6 while in contact with the surface of the ink form roller 8. During this transfer, the ink selectively adheres to the image carrying area of the plate 6a because the oily ink cannot adhere to the rest of the plate surface, i.e., the non-image area, because of the water film carried thereon. The ink thus selectively adhered to the plate 6a is transferred to the surface of a constantly rotating rubber blanket cylinder 9 which is brought into contact with the plate 6a, and then is transferred to a paper 11 through a nip gap between the rubber blanket cylinder 9 and an impression cylinder 10 as by the rotation of the cylinders 9, 10, and thus, a predetermined print out is obtained. The conventional offset press provided with the above described dampening unit however, has the following drawbacks: First, since the dampening unit uses a plurality of rollers for providing water to the surface of the plate 6a, it is very difficult to determine the most preferable conditions for forming a proper water film on the plate 6a. Accordingly, frequent trial-and-error operations, with the accompanying waste of time and paper, are necessary before the most favorable stable conditions have been established at the beginning of or a change-over during the printing operation. Further, since the water form roller 5 is in constant rotation and in contact with the surface of the plate 6a, to which a residual ink film is still adhered, this ink film is liable to be transferred to the respective rollers of the dampening unit and thus contaminate the same. Accordingly, these rollers must be removed from the unit and cleaned when color of the ink is to be changed. In addition, as the thickness of the ink film on the plate 6a becomes thicker, the ink is liable to be forced out of the predetermined image area due to the pressure of the water form roller 5 and so-called "ink threading" generates. This, of course, leads to a lowering of the printing quality. Alternatives to the above dampening unit have been proposed. One such alternative is a "Dahlgren" type dampening unit. In this unit, a metal roller in stable contact with the surfaces of both the water pan roller 2 and the plate cylinder 6 is provided instead of the water ductor roller 3, and alcohol is added to the water to improve the adhesivity thereof to the roller. Another alternative is "Beko" type dampening unit, which eliminates the water pan 1 and the water pan roller 2 but is provided with a rotor disc having a water conduit. The water is atomized to form a mist which is jetted onto the surface of the water form roller and transferred to the plate via a series of rollers similar to those of the aforesaid unit. These alternatives are advantageous in that the water feeding rate is easily adjustable in accordance with the variance of the printing speed, even in a high speed operation. However, since in these apparatuses the water form roller is always in contact with the surface of the plate, the previously described drawback caused by the water form roller still remains. SUMMARY OF THE INVENTION Thus, a general object of the present invention is to eliminate the above drawbacks in the prior art dampening unit to be assembled with the offset press. A first object of the present invention is to provide a dampening unit having a simple structure in which a water form roller always in contact with a plate cylinder is omitted, and which minimizes the cost and time required for maintenance and cleaning of the unit. A second object of the present invention is to provide a method for dampening a plate with a lower water consumption and without contamination from the environment. US-A-3139028 describes an apparatus for supplying to press plates moisture and such chemical ingredients as are required to produce high class printed matter for extended periods of time. The apparatus described includes a dispenser housing containing a plurality of atomizers and having a discharge end adapted to fit into close proximity to the cylinder or roller with which it cooperates. A suction passage is provided adjacent the cylinder or roller and the discharge end of the dispenser housing to prevent excess mists created by the atomizers from escaping from the apparatus into the room in which the printing press is located. FR-2569611 describes a dampening apparatus for use with an offset printing machine in which the diffusion of a liquid spray produced by an atomizer is confined by means of air jets. According to a first aspect of the present invention there is provided an apparatus for use with an offset press for dampening a plate carried by a plate cylinder comprising a nozzle with a discharging mouth in the form of a slit and extending in an axial direction of the plate cylinder, the discharging mouth being disposed in the vicinity of the surface of the plate, and the interior of the nozzle being divided into upper and lower chambers from the innermost area thereof to the discharging mouth by a partition extending in the lengthwise direction of the nozzle; and means for generating a mist disposed within the lower chamber characterised in that means are provided within the upper chamber for generating and forcing an air stream through the upper chamber to eject from the discharging mouth, the discharging mouth being directed substantially at a tangent to an outer surface of the plate. Preferably, the means for generating an air stream is a fan, a blower or a jet connected to a pressurized air source. Advantageously, the means for generating a mist comprises a water bath and an ultrasonic generator fitted on the bottom of the water bath. According to a second aspect of the present invention there is provided a method of dampening a plate of an offset press including the steps of generating an air stream within the vicinity of the surface of the plate and generating a mist of water characterised by the additional steps of: directing the air stream substantially tangentially toward the plate so that the air stream flows along part of an outer surface thereof forming an air film; and sucking the mist of water into a secondary stream adjacent the air stream and directed in substantially the same direction so that the mist of water is sandwiched and held between the surface of the plate and the air film formed therealong. According to the above method, the mist of water is pressed onto the plate surface by the air film which prevents a repulsion of the mist from the plate surface, so that the water is effectively held and adhered to the plate surface to form a thin film without being scattered out to the environment. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be described in more detail with reference to the drawings illustrating the preferred embodiments, wherein: Fig. 1 is a diagrammatic side view of part of an offset press provided with a conventional dampening unit; Fig. 2 is a diagrammatic side view of part of an offset press in which a dampening unit according to the present invention is assembled; and Fig. 3 is an enlarged side view of the dampening unit shown in Fig. 2. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 2 illustrates a main part of an offset press provided with a dampening unit according to the present invention. In this drawing, the same reference numerals are used as in Fig. 1 except for those relating to the dampening unit. A description of parts common to Fig. 1 is omitted since they are substantially similar to the aforementioned conventional parts. As illustrated, the dampening unit according to the present invention is not provided with rollers for transferring a water film onto a plate 6a carried by a plate cylinder 6. Instead, a nozzle 20 for ejecting air over a broad width area along the axis of the plate roller 6 is arranged in the vicinity of the surface of the plate 6a with a mouth 21 of the nozzle 20 being directed onto the surface of the plate 6a so that the air stream ejected from the mouth 21 follows the rotation of the plate cylinder 6, i.e., impinges on the surface thereof substantially at a tangent thereto. The mouth 21 preferably has an inner height of between 3 to 5 millimeters, and a gap between the mouth and the surface of the plate 6 of 1 to 3 millimeters. The interior of the nozzle 20 is substantially separated into two parts, an upper chamber 20a and a lower chamber 20b, by a partition 22 extending in the lengthwise direction of the nozzle 20 parallel to the axis of the plate cylinder 6. The partition 22 also reaches the outer end of the mouth 21 so that first and second jets 21 a and 21b are formed corresponding to the respective chambers 20a, 20b. Inside the upper chamber 20a is arranged a blower fan 23 having a rotational axis extending in the axial direction of the plate cylinder 6. The fan 23 is driven by a motor 24 to generate an air stream directed to the firs jet 21a. The lower chamber 20b is provided with a water bath 25 in the bottom wall thereof, in which a predetermined amount of water is filled through a conduit 26 connected to a water source (not shown). A plurality of ultrasonic generators 27 are arranged in series in the lengthwise direction of the nozzle 20 underneath the bottom wall of the water bath 25. A water mist is generated by the action of the ultrasonic generator 27 which vibrates the water in the bath 25 at a high frequency, and is fulfilled within the lower chamber 20b. In Figure 2, reference numeral 28 designates an air intake for the lower chamber 20b. Next, the operation of the dampening unit having the above structure will be described. In the normal operation of the offset press, the plating cylinder 6 rotates in the arrowed direction as shown in Fig. 1. The fan 23 is driven to generate an air stream in the upper chamber 20a, to be directed onto the surface of the plate 6a carried by the plate cylinder 6. The air stream is ejected from the first jet 21a to form a stable air film on the surface of the plate 6a. This air stream ejected from the first jet 21a causes the air pressure in the vicinity of the outer end of the second jet 21b disposed directly beneath the first jet 21a to be depressurized. The air within the second chamber 20b is sucked out of the interior of the lower chamber 20b through the second jet 21b to form a secondary air stream and ejected onto the plate 6a together with the primary air stream from the first jet 21a. At the same time, the water mist 30 (in Fig. 3) filled in the second chamber 20b is also made to flow out of the second chamber 20b through the second jet 21b and sandwiched between the air film formed by the primary air stream and the surface of the plate 6a. In this sandwiched condition, the water mist 30 is stably and effectively held on the plate surface without scattering to the environment. On the contrary, if the primary air stream is eliminated and only an air stream mixed with the water mist is forcibly ejected onto the surface of the plate 6a, the mist is liable to be repulsed and scattered by the latter and a uniform formation of the water film thereon cannot be expected. Further, the mist is liable to adhere to the partition 22 and the inner wall of the nozzle 20, which causes a decrease in and fluctuations of the effective amount of mist to be delivered to the plate 6a. According to the present invention, the mist supplied to the non-image area is pressed onto the plate surface by the air film formed by the primary air stream supplied from the first jet 21a, whereby the water film is held in place without being scattered and lost in the atmosphere. These conditions are diagrammatically illustrated in Fig. 3. According to the present invention, the particle size of the mist is also important for the adherence of the mist to the plate surface. The particle size in a range of from 15 µm to 18 µm is the most preferable. Therefore, the ultrasonic generator should be adjusted to form a water mist having such a particle size. In the above embodiment, a fan is used to generate a first air stream, but of course, a known blower or an air jet connected to a pressurized air source can be used instead. Also, the water mist can be generated by a known spray or a rotary atomizer. The thickness of the water film on the plate surface is adjustable by control of the fan rotation (adjustment of the blow rate) or control of the vibration of the ultrasonic generator (adjustment of mist generation). Moreover, in the above embodiment, in which a plurality of ultrasonic generators are arranged in series along the length of the water bath, if the respective generator is selectively operated by a known control means, the mist density in the lower chamber can be varied in the lengthwise direction of the water bath so that the supply of the water mist on the plate surface can be controlled in accordance with a ratio of the image carrying area to the non-image area in the widthwise direction of the plate. Features of the present invention will be made more apparent by the following example: Example Various printing tests were conducted on an offset press having the structure illustrated in Fig. 2, in which a plate FNS-A marketed by FUJI FILM K.K., Japan, four ultrasonic generators NB-83D marketed by TDK K.K., Japan, and a crossflow fan having a blow rate capacity of 1.6 m³/min and a pressure of 1.5 mm Aq, are used. The tests were carried out while varying a printing speed from 5 m/min to 200 m/min, and a supply rate of an ink SCR-TM087 BLACK HC marketed by TOYO INK K.K., Japan, to determine the upper limit of ink thickness printed on the paper without defects such as ink threading. Further the number of trial-and-error tests required before determination of the optimum dampening conditions of the plate when the printing conditions were varied, was estimated. As a result, the upper limit of the ink thickness was represented by a D value of 1.62 measured by a Color Densiometer B318 marketed by X-RITE Co. Ltd, Grand Rapids Mi., U.S.A. Conversely, when using the conventional offset press shown in Fig. 1, the D value was 1.09, which means that more ink is carried on the plate compared to the conventional dampening unit according to the present invention. In this regard, the principle of this measurement is disclosed in U.S. Patent 4080075. Ten trial-and-error tests were sufficient to determine the optimum dampening conditions, which were much less than the 100 needed for the conventional dampening unit. Moreover, the consumption of water for dampening the plate was less in the present invention relative to the conventional unit. This means that the thickness of the water film on the plate surface is minimized according to the present invention.

Claims: An apparatus for use with an offset press for dampening a plate carried by a plate cylinder comprising a nozzle (20) with a discharging mouth (21) in the form of a slit and extending in an axial direction of the plate cylinder, the discharging mouth (21) being disposed in the vicinity of the surface of the plate, and the interior of the nozzle being divided into upper and lower chambers (20a,20b) from the innermost area thereof to the discharging mouth by a partition (22) extending in the lengthwise direction of the nozzle (20); and means (25, 27) for generating a mist disposed within the lower chamber (20b) characterised in that means (23) are provided within the upper chamber (20a) for generating and forcing an air stream through the upper chamber (20a) to eject from the discharging mouth (21), the discharging mouth being directed substantially at a tangent to an outer surface of the plate. An aparatus in accordance with claim 1, wherein the means (23) for generating an air stream is selected from a fan, a blower, and a jet connected to a pressurized air source. An apparatus in accordance with claim 1 or claim 2, wherein said means (25,27) for generating a mist comprises a water bath (25) and an ultrasonic generator (27) secured on the bottom of the water bath (25). A method of dampening a plate of an offset press including the steps of generating an air stream within the vicinity of the surface of the plate and generating a mist of water characterised by the additional steps of: directing the air stream substantially tangentially toward the plate so that the air stream flows along part of an outer surface thereof forming an air film; and sucking the mist of water into a secondary stream adjacent the air stream and directed in substantially the same direction so that the mist of water is sandwiched and held between the surface of the plate and the air film formed therealong. A method in accordance with claim 4, wherein the mist of water comprises droplets having a size in a range of from 15 µm to 18 µm.

Inventors: MARUTA, KAZUO
Applicants: MARUTA, KAZUO
Application year: 1987
Main classification: B41F7
IPC codes: B41F7, B41N3