Dampening fluid supply system

Abstract: Dampening fluid supply system for lithographic press utilises a device for detecting presence or absence of an excess of dampening fluid in the nip between adjacent rollers and controlling supply of dampening fluid accordingly to limit volume in the nip. Suitably the device has a plurality of sensors 4 spaced along one roller 2 grouped in a plurality of zones. The group sensors 4 in each zone is arranged to control supply via a supply manifold 13 of dampening fluid to that zone.

Description: FIELD OF THE INVENTION This invention relates to a new and improved fountain solution (dampening fluid) supply system for use on lithographic printing presses. Generally speaking, the invention relates to a new and improved system for supplying fountain solution to Dampening systems of the contact or two-way type. More specifically, the invention relates to a new and improved fountain solution supply system for continuous contact type dampening systems which results in improved performance in the areas of reduced ink buildup on dampening system rollers, sharper printing, and less ink contamination of components in the fountain solution supply system. BACKGROUND OF PRIOR ART IN THE FIELD OF THE INVENTION It is well known that the lithographic offset printing plate is treated chemically so that there are printing and non-printing areas so that the printing area is receptive to ink. The non-printing area, on the other hand, is hydrophilic and accepts moistening fluid. In order to achieve the desired printing, a film of moistening fluid is applied to the surface of the plate which is retained by the hydrophilic area but which beads up on the printing area thereby allowing the printing area to receive the printing ink. The non-printing area, thus, is separated and isolated from the inking rollers by the film of moistening fluid. In this manner only the printing or image area receives ink which is then transferred to the blanket cylinder and onto the paper on which the image is printed. The purpose of lithographic dampening systems is to feed moistening or dampening fluid to the printing plate. One method of categorizing dampening systems is in terms of whether or not a return path for fountain solution exists between the plate cylinder on the printing press and the metering elements of the dampening system. Dampening systems of the brush and spray types contain a physical gap between the means for metering the dampening fluid, or fountain solution, and the plate cylinder and, as a result, fountain solution can travel in one direction only, that is toward the plate. Thus, dampening systems in this category are referred to as the non-contact or one-way type. In contrast, dampening systems of the contact type do not contain a physical gap in the path of the fountain solution, thus making is possible for fountain solution to travel to and fro between the metering means in the dampening system and the plate cylinder. Thus, this second general class of dampening systems has also been referred to as the two-way or contact type dampening system Contact or two-way type dampening systems are further divided in continuous types and conventional or ductor types. The vase majority of modern contact type dampeners are of the continuous type and generally contain either three or four rollers; hence this class of dampeners are often referred to as either a three-roller or a four-roller design. Figure 1 illustrates a typical modern non-contact type dampener which uses a rotary brush to flick off fine droplets of fountain solution from a pan roller in a pan and to propel the droplets across a gap toward a chrome surfaced vibrating roller in the dampening system. Metering of the amount of dampening fluid that is delivered to the vibrating roller is accomplished by varying the speed at which the pan roller delivers fountain solution to the point where it is flicked away by the bristles of the rotating brush. A major advantage of this type of dampener is that no ink is fed back to contaminate the metering elements and the fountain solution supply system by virtue of the physical gap which exists in the path from the pan to the plate cylinder. There are, however, two major drawbacks to this type of dampening system design. First, there is no easy way to control the rate of flow in the lateral direction along the vibrating roller. Second, and perhaps more importantly, when overfeeding of dampening fluid to the plate occurs i.e. an excess of dampening fluid, the system cannot automatically compensate by returning some of the excess dampening fluid back to the supply system This can cause a degradation in print quality and can result in more waste. As a result, brush systems require more operator time and skill to achieve good quality printing. Spray type dampeners meter damPening solution by the use of plurality of atomizing spray nozzles which direct a pulsed flow of fine drops of fluid across a physical gap onto a dampening or inking roller. The use of many nozzles makes it possible to control flow laterally and the one-way character of the design eliminates the problem of ink contamination in the dampening fluid supply system. Spray systems, however, retain the drawback of one-way designs in that there is no automatic compensation for overfeeding. Thus print quality often suffers when using spray type dampeners. A typical example of a spray dampener is illustrated in U.S. Patent Number 4,469,024 to Schwartz et al. issued Sept. 4, 1984. U.S. Patent Number 4,724,764 to MacPhee et al issued February 18, 1988 illustrates various embodiments of the three-roller continuous contact type dampening system. U.S. Patent Number 4,777,877 to Lemaster issued October 18, 1988, illustrates one embodiment of the four-roller continuous contact type dampening system. In both three and four-roller designs metering of the dampening fluid is accomplished by a pair of rollers squeezed together so as to limit the amount of fluid which passes through their junction or nip. More specifically, the amount of fluid metered is adjusted by varying either the speed, pressure setting, or hardness of the rollers. This scheme requires that an excess amount of fountain solution be fed to the inlet side of this metering nip. This is normally accomplished by partially immersing one of the rollers in a pan containing fountain solution, so that an excess of dampening fluid is carried by the roller from the pan to the metering nip, with excess fluid automatically flowing back down into the pan. In order to avoid slinging of dampening fluid, the metering rollers are normally limited to speeds in the range of 150 to 200 feet per minute. Thus most three and four-roller continuous contact the dampening systems manufactured today are characterized by the existence of a slip nip in the fluid path between the metering or squeeze rollers and the plate cylinder. Slip nips are characterized as nips formed by rollers travelling at significantly different surface speeds. For example, in a modern web offset press, the plate cylinder and most rollers on the press may travel at a speed of about 450m per minute (1,500 feet per minute) whereas the pair of squeeze rollers in the dampening system may be driven separately at a speed one tenth that or about 45m per minute (150 feet per minute). Thus a slip nip must exist. One other unique characteristic of most continuous contact three-roller and four-roller type dampening systems is that the metering or squeeze roller pair is arranged so that any fluid contained in the metering nip will drain out in a circumferential direction over one of the two roller surfaces, whenever the rollers stop turning or if excess fluid is supplied to the nip. An additional feature normally found in such dampening systems is a fountain solution circulating system, consisting basically of a pump and tank or reservoir for maintaining a constant value of fluid in the pan. Because fountain solution is free to flow back and forth between plate cylinder and metering rollers, overfeeding of fountain solution to the plate is automatically compensated for by an increase in the back flow from plate cylinder to the metering rollers. This feature, plus the ability to vary lateral flow by skewing of one of the metering rollers, makes this class of dampeners very user friendly and capable of producing very high print quality. A drawback of this type of dampening system, however, is that the large inventory of fountain solution contained in the pan and the circulating system often rapidly becomes contaminated with ink, which results in degradation of print quality, buildup of emulsified ink on dampening system rollers, and the need to periodically clean the circulating system. Various attempts to solve these problems by installing filters to remove the offending ink particles from the dampening fluid have been largely unsuccessful. Another drawback of existing fountain supply systems for continuous contact type dampeners is that the inventory of fountain solution must be replaced periodically with fresh solution. Due to feedback from the press, contaminants build up in the fountain solution, and these contaminants have an adverse affect on printing. The problem is especially acute when using alcohol substitutes in the fountain solution. The need to periodically replace fountain solution in the supply system oftentimes necessitates shutting down the press, which results in lost production time and lost printed product. In addition, disposal of the contaminated or waste fountain solution is becoming increasingly expensive because of ever stricter environmental regulations governing disposal of such wastes. A continuous contact type dampener system equipped with a spray-type fluid supply system is disclosed in Marcum Patent 4,481,855 entitled "Dampening Unit For Printing Press" dated June 27, 1989. The purpose of the spray-type supply is to prevent pick-up of lint and debris that may collect in the pan. Thus no attempt was made to minimize either the volume in the metering nip or the amount draining away from the metering nip. Another variation of a continuous contact type dampening system is disclosed in Loudon U.S. Patent 4,455,938 entitled "Dampening Apparatus for Lithographic Press" dated June 26, 1984. The unique features of this design are that only two rollers are used and that both metering or squeeze rollers travel at press speed. GB-A-1,439,002 discloses a sensor device for detecting the amount of water present on the surface of a lithographic printing plate by measuring changes in electrical resistance between electrodes spaced apart by a monitoring zone. OBJECTS OF THE INVENTION It is, therefore, an object of this invention to provide a new and improved dampening fluid supply system for use in conjunction with two-way or contact type dampening systems. It is another object of this invention to eliminate and/or reduce ink contamination of components in the dampening fluid supply system. A further object of this invention is to eliminate and/or reduce the need for filters to remove ink fed back into the dampening fluid supply system by two-way or contact type dampening systems. Another object of this invention is to provide a new and improved dampening fluid supply system which improves the print quality on presses equipped with contact or two-way type dampening systems. A still further object of this invention is to reduce or eliminate the buildup of ink on the rollers of two-way or contact type dampening systems. An object of this invention is to impart to contact type dampening systems the advantages of non-contact types, while still retaining all of the advantages inherent in the former. A still further object of this invention is to provide a new and improved dampening fluid supply system for use in conjunction with contact or two-way dampening systems which is less expensive to manufacture. Another object of this invention is to greatly reduce the volume of fountain solution that must be disposed of as waste, should it be necessary or desirable to refresh the fountain solution supply due to deterious buildup of contaminants within the supply. A still further object of this invention is to minimize, or reduce to zero, the volume of fountain solution generated as waste due to leakage from the fountain solution supply system. Additional objects and advantages of the invention will be set forth in the description which follows and, in part, will be obvious from the description; the objects and advantages being realized and obtained by means of the instrumentation, parts apparatus, systems, steps and procedures particularly pointed out in the appended claims. BACKGROUND DESCRIPTION OF THIS INVENTION The invention herein is particularly useful for use with contact type dampening systems. A large majority of the contact type dampening systems manufactured today use the squeeze roll principle to meter out a thin film of dampening fluid, which is then further thinned before being transported and applied to the plate cylinder on the press. In this method of metering, a hard surfaced roller and a compliant surfaced roller are forced into contact with one another and one of the rollers is partially immersed in a pan or tray containing dampening fluid. This roller pair is geared together and connected to a motor drive which causes the two rollers to turn in counter rotating directions. As shown by the three-roller designs in Figure 2, two different configurations for a contact type dampening system are used. Thus as shown, Configuration A illustrates a pan roller in engagement with a transfer and metering roller which contacts a form roller. Configuration B shows a pan roller in engagement with a metering roller and a form roller. However, the two configurations possess common metering nip characteristics. That is, the roller immersed in the pan carries an excess of fluid to the metering nip, which results in the nip becoming flooded and in the excess fluid falling back into the pan. Figure 2 also shows the location of the slip nip that is normally present as a consequence of driving the metering or squeeze roller pair at a lower surface speed than the plate to prevent slinging of dampening fluid. The volume or inventory contained in the pan is typically about 3.6 litres (a gallon) or more, depending on the size of the press The total inventory of dampening fluid is increased further by as much as a factor of five or more by the use of additional components in the fluid supply system for circulating, cooling, and filtering the dampening fluid. This inventory of dampening fluid often becomes contaminated with ink fed back from the plate via the dampening system and the form roller which is in contact with the plate cylinder. These contaminants are the cause of many problems as a result of their deposition on various components of the dampening and fluid supply systems. The invention disclosed here resulted from the discovery that only a small volume or inventory of fluid is needed to maintain the proper metering performance of a pair of squeeze rollers. More specifically, it was discovered during initial printing tests that proper dampening system performance could be achieved by draining the pan in which one of the rollers is normally immersed and by keeping the entrance of the metering nip filled with the aid of a hand operated spray bottle, similar to the spray bottle used to clean windows. It was also discovered that a volume of dampening fluid large enough to sustain normal printing operations for a period of 10 to 20 seconds could be stored in the nip entrance without overflowing, i.e., draining back down the lower of the two rollers. In subsequent printing tests, it was discovered that improved printing performance resulted when the volume of fluid in the supply system was reduced to a small quantity in this way. Although the reasons for this improvement are not fully understood at this time, it is theorized that the improvement is due to the corresponding reduction in mean fluid residence time. Mean fluid residence time is defined as the average time a particle of fluid resides in the fluid supply system before it is carried into the metering nip formed by the dampening system squeeze rollers. For example, the mean fluid residence time in a conventional supply system may be 90 minutes or longer. In contrast, the mean fluid residence time in the second series of press tests was less than one half minute or shorter by a factor of over 200. Although the upper limit on mean fluid residence time may vary depending on such factors as press speed and dampening system configuration, it is probable that it should not exceed five (5) minutes in order to realize the benefits of this invention. It was further discovered that when an excess of fluid was fed into the metering nip, overflowing or back-flowing occurred in the form of rivulets, where there is uniform ink coverage as shown in Figure 3. Initially, it was thought that overflowing could then be determined by simply sensing the presence of a single rivulet anywhere along the length of the roller and that the metering nip could then be refilled or replenished by periodically feeding fresh dampening fluid uniformly along the nip length. However, during subsequent printing tests it was discovered that this was not correct and that lateral zones on the printing plate which contain larger image areas (i.e., heavier ink coverage) require more dampening fluid, with the result that fluid in the corresponding lateral zones of the metering nip is consumed faster. This results in starved or depleted sections of the metering nip, as shown in Figure 4. There it can be seen that the zone of heavy coverage does not result in the formation of excess rivulets of dampening fluid. Based on these experiments, it became evident that an overflow sensor should be provided but divided into zones so that independent feed means corresponding to the sensor zones can be provided. In addition, within each zone further subdivisions of the sensors are needed to avoid blinding of a sensor by overflows in an adjacent zone. That is, a starved region can be supplied by fluid flowing into it laterally along the nip from an adjacent flooded region. Experience has shown that a starved region of a length of up to about 10cm (four inches) or more can be so supplied. Thus, each sensor should not cover a nip length of more than about 7.5 or 10cm (three or four inches) to insure that a starved region of longer than 7.5 or 10cm (three or four inches) cannot exist. For example if 23cm (nine inch) wide sensors were used it would be possible for a 18 or 20cm (seven or eight inch) long starved region to exist undetected in the zone covered by a given sensor, since the given sensor could be erroneously detecting fluid that had flowed laterally into the edges of its range, from an adjacent region. Further printing tests disclosed that the volume of waste fountain solution generated during a given period of press operation could be reduced even further by utilizing an embodiment in which there was no leakage of fountain solution out of the supply system whenever the press was stopped. The invention is capable of utilizing certain devices and sensors known in the art. For example, various sensing techniques, familiar to those skilled in the art, can be used to sense when overflowing or overfilling of the nip occurs. These include passive listening devices, as described in U.S. Patent 4,505,154, ultrasonic ranging sensors as described in U.S. Patent 4,479,433 and sensors which respond to changes in capacitance. BRIEF DESCRIPTION OF INVENTION Briefly described, the present invention relates to an improved dampening fluid supply system used in conjunction with a two-way type dampening system in which the volume or inventory of dampening fluid, that can come in contact with the dampening system rollers, is very small. The invention thus takes advantage of the discovery that the nip between adjacent contacting dampening feed rollers contains sufficient fluid for printing. This is accomplished by providing a sensor to determine at the nip when makeup dampening fluid is necessary and should be fed and then only feeding enough fresh dampening fluid to flood the metering nip in the dampening system. The invention provides a dampening system for minimizing contamination of dampening fluid in a lithographic press in accordance with claim 1 of the appended claims. In the preferred embodiment overflowing dampening fluid is collected in a shallow trough where its level is monitored by a sensing means. Whenever it is detected by the sensing means that the small volume of dampening fluid in the trough decreases below a predetermined prescribed level, a signal is generated to a feeding means which causes a small volume of dampening fluid to be fed to the trough so as to restore the level of dampening fluid to the predetermined predescribed level. The accompanying drawings which are incorporated in and constitute a part of this specification illustrate an embodiment of the invention and together with the detailed description serve to explain the principles of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an example of a prior art brush type dampener. Figure 2 is a diagram which illustrates prior art squeeze roll metering systems in Configuration A and Configuration B. Figure 3 is a sketch of a test done in making the invention in which rivulets flow back down the fountain pan roller when feed to the metering nip is excessive. Figure 4 is a sketch of a test done in making the invention showing non-uniformity of rivulets flow caused by zones of heavy coverage. Figure 5 is a view partially in section showing a sensor bar assembly described for better understanding of the present invention. Figure 5A is a sectional view taken along line A-A of Figure 5. Figure 6 is a schematic to assist understanding of the invention showing various block diagrams of instrumentation. Figure 7 is a schematic view of one embodiment of the invention. Figure 8 is a schematic view to assist understanding of the invention showing various block diagrams of instrumentation. Figure 9 is a schematic view to assist understanding of the invention. Figure 10 is a top view of a fountain pan used in another preferred embodiment of the invention. Figure 11 is a cross-sectional view of the fountain pan shown in Figure 10 taken through line 10'-10'. Figure 12 is a cross-sectional view of a variation of the fountain pan shown in Figures 10 and 11, wherein the manifold is located in the back portion of the pan. DETAILED DESCRIPTION OF INVENTION As shown in Figure 6, there is a pan roller 2 extending along the width of the press and a plurality of manifolds 13 extending substantially along the length of the pan roller. A plurality of sensors 4 are spaced along the length of the roller 2 which may be of the type described with reference to Figure 5 and Figure 5A although the invention is not limited thereto. The sensors 4 send signals to the multiplexer driven by the multiplexer driver circuits in a manner known to those skilled in that art. The multiplexer receives the signals from the sensors which in turn signals the threshold detectors which signal the output timers and valve devices. Each output timer and valve driver include a timing mechanism and a signal capable of opening a selected valve for a predetermined period of time to provide dampening fluid to the manifold 13 at the appropriate time and for a predetermined time period. It will be understood that the hardware referred to herein, except as specifically described, are known to those skilled in the art and the details thereby are not shown or necessary to the practice of this invention. Figure 6 is a schematic of the complete system used to supply fluid to a dampening system on a 38 inch wide press. As shown there is a pan roller 2 extending along the width of a lithographic press which is the position of the usual pan roller shown in Figure 2. The pan roller 2 forms a nip area with another roller of the type shown in Figure 2 but not shown in Figure 6 for purposes of clarity. Thus, the nip is found or formed between the pan roller 2 and the transfer metering roll (Configuration A) or between the pan roll 2 and the metering roll (Configuration B). Arrayed along the axis of the roller 2 to be supplied with dampening fluid are a plurality of sensors 4 arranged in groups. As illustrated, there are twelve (12) sensors in groups of three so as to provide four zones, identified as Zone 1, Zone 2, Zone 3 and Zone 4, of control. The output leads 4a from the sensors 4 shown by arrows are connected through cables to a multiplexer referred to by the block diagram. The multiplexer may be a 4066 CMOS type integrated circuit which is a generic device available from several U.S. manufacturers. The purpose of the multiplexer is to sample or connect one sensor at a time in the group to a threshold detector to determine if an overflow condition exists at that sensor location. When a conductivity type sensor is utilized, the threshold detector may consist of a wheatstone bridge of conventional type connected to a type 3130 operational amplifier, which is a BIMOS integrated circuit device manufactured and sold by Harris Semiconductor. The threshold detector generates no control action at its output if there is an excess of dampening fluid at the position of the sensor 4 which is connected to it by the multiplexer. Conversely, if flooding is not detected, by the sensor connected to it, the threshold detector will generate a signal at its output to initiate a control action. This is accomplished by connecting the sensor being sampled through the multiplexer to one leg of the wheatstone bridge. The null points of the bridge are connected to the input terminals of the operational amplifier in such a way that if flooding is detected, as evidenced by a low state of resistance between the parallel plates as explained in reference to Figure 5, no control action is initiated by the system and no dampening fluid is supplied to the nip. If, however, overflowing is not sensed, as evidenced by a high state of resistance between the parallel plates, then a feed of dampening fluid in the zone being sampled is initiated. This is done by generating a signal from the threshold detector which starts the output timer assigned to the given control zone. The timer in turn is connected to a valve driver which energizes a valve identified as valves #1, #2, #3 and #4 for supplying pressurized dampening fluid to the zone manifold 13, thereby feeding dampening fluid to the roller in the region of the zone being sensed. The duration of the feed (valve open time) is governed by the corresponding timer which can be set in the conventional manner to maintain the valve open for a predetermined time period. The details of the manifold are conventional and consist in general of a hollow tube 13 with openings therein to direct the dampening fluid to the gap when appropriate. A separate manifold is provided for each zone. Considerable latitude exists in selecting a sampling rate and a valve open time satisfactory for a given press, and those skilled in the art will have no difficulty in doing so depending on the situation. Similarly all of the control circuitry shown in Figure 6 is commonplace and can readily be designed and built by a person skilled in the art. Thus, the units identified as multiplexer driver circuits, multiplexer, threshold detectors and output timers and valve drives are not the invention per se and can be built and/or obtained by those skilled in the art. Sensor means is provided for determining the presence or absence of an overflow condition of dampening fluid at the nip. The sensors 4 are groups of parallel conductivity probes 20, located along the axis of the pan roller, so as to intersect the overflow rivulets of the type shown in Figure 4 (not shown in Figure 5) at right angles. Each conductivity sensor consists of a pair of parallel electrically conducting plates 22, 24 having a width of about 7cm (2-3/4 inches) spaced about 0.6cm (1/4 inch) apart and mounted approximately (0.06cm (.025") away from the surface of the roller 2, as shown in Figure 5. The plates are encased in plastic insulating material 21 of any suitable type, also as shown in Figure 5. Because dampening fluid is a relatively good electrical conductor, an overflow rivulet which contacts both plates can be detected by the presence of an electrical current flowing in the circuit formed by a convenient voltage source connected to the two plates. On the other hand, if there is no rivulet resulting from overflow the circuit is open. For a typical 96.5cm (38 inch) wide modern web press, the following system parameters can be used for determining the sequence and timing of the operation. By way of an illustration, the term "high sensor state" refers to a state when no water is present so that there is a high resistance. The term "low sensor state" refers to a state when water is present so that the circuit will be closed. "Sampling rate" refers to the time period for determining whether water is present in the nip. The term "valve open time" refers to the fact that the valve is in the open position. High Sensor State (No Water in Nip) A resistance between the parallel plates which exceed a value R which is in the range of 20 - 50 thousand ohms. Lower Sensor State (Water in Nip) A resistance between the parallel plates which equals or is less than the above value R. Sampling Rate A suitable range for the sampling rate is between once every six seconds to once every eighteen seconds. Valve Open Time A suitable range for valve open time is 0.5 to 2.5 seconds. The exact value will depend on the demands of the press and the design of the fluid supply system, designated by the symbol "S" in Figure 6. The relationship between minimum valve open time, usage rate, sampling rate, and the feedrate provided by the manifold and fluid supply system is as follows: Example: A press where the maximum usage rate per plate cylinder is 2 litres (0.5 U.S. gallons) per hour, per zone, or about 0.03 litres (1.0 fluid ounce) per minute. If the available feedrate is 0.3 litre (10 fluid ounces) per minute and the sampling rate is once per 6.0 seconds, then the minimum valve open time is 0.6 seconds. In such a case, the valve open time should be set somewhat longer, e.g. 1.0 seconds, to provide a safety margin in the average feedrate. Other design requirements recognized by those skilled in the art are that the voltage source applied to the parallel plates should be A.C. and that all sensors should be connected to ground when not being sampled. It is also necessary to disable the control system and stop feeding whenever the roller drive is turned off. To accomplish this a proximity sensor (not shown) is mounted adjacent to one of the roller drive gears and generates an enabling signal when motion is detected. As shown in Figure 7 means is provided for supplying dampening fluid to the nip at the pan roller wherein a collection trough means capable of being replenished with dampening fluid is maintained so that the pan roller can deliver dampening fluid to the nip in a manner that prevents contamination. This means includes a pan roller 2 having a nip 40 at junction with another roller and means for controlling the supply of fountain solution available for delivery to the pan roller nip 40. In Figure 7 there is shown sensing means used to determine the necessity of additional dampening fluid supply. As shown this means 30 consists of a single conductivity probe for detecting the presence of dampening fluid in a collection trough 32. The collection trough is formed by a conforming rail 34 extending along the length of the roller 2. The conforming rail includes an inclined surface 31 which forms the collection trough in cooperation with the surface of the pan roller. Extending from the inclined surface 31 is a curved surface 33. The curved surface 33 is on a radius substantially equal to the radius of the pan roll. As will be discussed, the surface 33 is spaced a predetermined distance from the adjacent surface of pan roll 2. As long as overflowing of the metering nip 40 occurs, the trough 32 will contain fluid along its entire length. It will be noted that the conforming rail 34 is adjacent to but spaced from the pan roll 2 with a gap 36 between the conforming rail and the pan roll 2. The length of the gap between the conforming rail 34 and the pan roller is referred to as the sector length 37. However, when overflowing decreases or stops, the level in the trough will recede raising the risk of nip starvation. Means is provided to supply fluid to the trough to prevent dampening fluid starvation. To affect this, the conductivity probe 30 is used to detect the drop in level and to initiate a fluid feed through a feed line/valve combination from the supply system to replenish the nip so that the trough is again filled. Thus, the sensor 30 signals the controller 39 to control valve 38 which can open or close dampening fluid supply line 43. In accordance with this invention means is provided to permit the trough to maintain a supply of fluid. As embodied both the sector length of the conforming trough and the clearance or gap between it and the roller are critical to successful operation. If the sector length 37 is too short and/or the gap 36 too large, fluid will leak out of the trough at a rate faster that can be maintained by the viscous pumping action of the moving roller surface. This pumping action is a result of the rotation of the pan roll in the counter clockwise direction which is against the force of gravity. On the other hand, if the gap 36 is too small, it may become plugged with ink globules causing the roller surface to pick up ink. It has been found that the minimum practical gap dimension is about 0.06cm (0.025 inches) with the result that the minimum sector length 3.8cm (1-1/2 inches). Longer sector lengths can be utilized with corresponding wider gaps. In fact, if the sector length is increased so that the gap covers the lower half of the roller, the gap width can be increased without limit, but this is not considered desirable. Although the reasons why beneficial effects are achieved with this invention are not fully understood, it is theorized that they are due primarily to the very short mean fluid residence times which result in reducing the volume of fluid held by the metering roller pair. As an example, consider a press having a fluid consumption rate per plate cylinder of 7.6 litres (2.0 U.S. gallons) per hour An existing fluid supply system has a storage volume ranging from three to five gallons, which results in a mean residence time of 90 to 150 minutes. In contrast, if only the metering nip is used for storage, as in the preferred embodiment, the mean residence time is only 1/3rd of a minute, or a factor of at least 250 lower than in an existing system. In the embodiment shown in Figure 7, a gap thickness of 0.06cm (0.025 inches) and a sector length of 3.8cm (1 1/2 inches) will add approximately 0.2 minutes to the residence time while a relatively thick gap of 0.3cm (1/8 inch) covering the bottom half of a 9cm (3 1/2 inch) diameter pan roller would add over 3 1/2 minutes to the residence time, i.e. increase it over that in the preferred embodiment by a factor of ten. Thus, while thicker gaps and longer sectors can be utilized, it is preferred to use the minimum values in order to minimize fluid residence time. It should be noted that this alternate embodiment of the invention is most suitable for use with hard surfaced pan rollers because the gap dimensions cannot be maintained with rubber pan rollers because they are not dimensionally stable. This is because the diameter of a rubber roller can and does vary due to heating and chemical changes caused by interactions with inks and wash-up solvents. However, when it can be used this alternate embodiment possesses the advantages of greater simplicity and lower cost. Another advantage is that cooling of the dampening system can be achieved by providing passages 44 in the conforming rail for the flow of a suitable coolant. Figures 10 and 11 show another embodiment wherein no fountain solution is allowed to leak or drain away from the supply system whenever the press is stopped and/or pressure is released between the metering roller pair. This embodiment also helps to minimize contamination of the supply of fountain solution, thereby reducing the need to periodically replace the supply with with fresh solution. The fountain pan 60 includes a sheet metal trough 61 angled upwardly from pan bottom 98, and is equipped with watertight end pieces 62 which can also be used to locate the pan 60 in an accurate and close relationship to the fountain roller 71 (shown in phantom in Figure 11). A supply manifold 63 having, for example, a trapezoidally-shaped cross section, extends along the length of pan 60 and is securely fixed to it. A rectangularly-shaped groove 64 may be machined or formed through manifold 63, and together with pan bottom 98, the groove 64 defines an enclosed fluid conducting channel running along the length of pan 60. The groove 64 is connected via a fitting 65 disposed through pan bottom 98 to a fluid supply 96 for feeding fresh fountain solution to the supply manifold 63. Flow passages 66 are formed through manifold 63. The flow passages 66 are in fluid communication with the groove 64 and are spaced at intervals along the length of the manifold, so that the front pan region 75 that is located between the surface 120 of manifold 63 and the fountain roller 71 can be filled with fountain solution along the entire length of the pan whenever a feed of fountain solution is initiated. The pan 60 is located with respect to the fountain roller 71 to define two radial lines 90A, 90B passing through the longitudinal central axis 110 of roller 71, which lines 90A, 90B are perpendicular to the surfaces of pan bottom 98 and trough 61, respectively. As illustrated, a pair of clearances 90 are established, one at the lower surface of pan 60 (between the surface of roller 71 and the pan bottom 98) and the other at the back pan region 76 (between the surface of roller 71 and trough 61). These clearances 90 are measured along lines 90A and 90B, respectively. The clearances 90 from both the sheet metal trough 61 and from the pan bottom 98 should be small enough to insure that any debris carried from the flooded metering nip 73 into the back pan region 76 of pan 60 will not remain in the back pan region 76, but will instead be carried forward into the front pan region 75 by the action of fountain roller 71. In this manner, the debris will not accumulate in the pan 60, and will instead be carried back up into the roller system. In practice, it has been found that the maximum clearance 90 should be not more than about 0.08cm (.030 inches). Accordingly, contamination of the inventory of fountain solution in the pan is minimized or substantially reduced, thereby alleviating the need to periodically replace the inventory with fresh solution. The size of the pan 60 and manifold 63 should be selected to minimize the volume of fountain solution stored in the front pan region 75. However, if the front pan region is made too small, surface tension effects will prevent the fountain solution from distributing itself uniformly, via axial flow through front pan region 75, along the length of roller 71. In this regard, it has been found that the placement of pan 60 (and consequently, surface 120 of manifold 63) with respect to roller 71 should define a front pan region 75 having cross-section dimensions of no less than about 0.6cm by 0.6cm (1/4 inch by 1/4 inch). A conductivity sensor assembly 67 is provided in order to maintain a proper fountain solution level 74 in the pan, thereby avoiding fountain solution starvation at metering nip 73, and preventing overflow of the fountain solution from the pan. The sensor assembly 67 includes an insulating block 68, into which is mounted one or more electrodes 69 which jut downwards into a "bay" of the fountain solution that is accumulated within a U-shaped cut-out 92 formed in the manifold 63. The electrodes 69 are used to detect a drop in fountain level 74 and to initiate a feed of fountain solution through the fluid supply 96 connected to the fitting 65. To insure that the electrodes 69 do not become fouled with debris, two additional flow passages 70 are drilled through manifold 63 and communicate with groove 64. The passages 70 are oriented so that the surfaces of electrodes 69 that are closest to the insulating block 68 will be sprayed and thereby cleaned every time a feed of fountain solution is initiated. Figure 12 illustrates a variation of the pan embodiment illustrated in Figures 10 and 11. Here, for example, owing to considerations of press design, it is sometimes necessary or desirable to locate sensor assembly 67 and the manifold 63 adjacent the back pan region 76. For certain press designs, this arrangement improves accessibility and serviceability of the manifold and sensor assembly. As shown, the volume of dampening fluid contained within front pan region 75 is determined by the positioning of the trough 61 relative to the surface of roller 71. In addition, the clearance 90 at the back pan region 76 is governed by placement of the manifold 63 (and its surface 120) relative to the roller 71. As shown, the back clearance 90 is measured along radial line 90B, which runs through central axis 110 and is perpendicular to manifold surface 120. Otherwise, the cross-section dimensions of front region 75, and the widths of clearances 90, are governed in the same way as set forth as described for Figures 10-11. Here, as before, the clearances are selected to ensure that debris carried from metering nip 73 will not remain in back pan region 76, but will pass to front pan region 75 to be carried back up into the roller system. Moreover, as before, the cross-section dimensions of front pan region 75 are established to promote uniform distribution of dampening fluid along the length of roller 71. As with the embodiment of Figures 10-11, the preferred clearances 90 are no more than 0.030 inches, while the preferred cross-section of front pan region 75 is no less than about 0.6cm by 0.6cm (1/4 inch by 1/4 inch). The supply of dampening fluid may be controlled by the speed of the press. A controller of the type described in U.S. Patent 4,469,024 for a spray dampener is used to affect the flow of fluid through the valves and manifolds as schematically shown in Figure 6. However, in this embodiment, instead of controlling dampening fluid supply by sensing overflowing of the metering nip, the duration between feeds and the length of feed is governed primarily by a program within the controller which increases the valve open time and/or decreases the interval between feeds in proportion to increases in press speed. The program is as in Figures 7A, 7B and 7C of U.S. Patent 4,469,024 except that it is revised, and the press speed affects the controller as described in U.S. Patent 4,469,024 with reference to numeral 26 which is the sensor that produces a signal proportional to press speed. Referring to Figure 8, there is shown a press speed signal generated by a sensor described above which is directed to controller for a spray type dampener of the type described in the United States Patent 4,469,024. The controller signals valves #1, #2, #3 and #4 which in turn are connected to the manifold 13 which direct dampening fluid to the pan roller. An additional feature of an embodiment of this type of controller is that a precise feedrate versus speed-curve can be entered into the program by the press operator. Also the controller front panel has adjustments (e.g. control knobs) which allow the press operator to vary the feedrate in each zone by an amount equal to plus or minus 50% or more of the programmed amount. Following installation on the press, the controller is programmed to deliver approximately twice the feedrate judged to be necessary by the pressman when printing a form with average ink coverage. Thus ample margin in feedrate will exist even when a heavy coverage form is run. This of course means that overflowing will occur at all times, with the excess fluid dripping into the pan. However this excess flow is very small and can be returned to the supply system by placing filter material inside the pan and collecting the fluid which draws therefrom. As a result the time between filter changes will be increased by a factor of several hundred over that in existing contact type dampeners. In addition, this excess flow will also act to reduce the mean fluid residence time. Further improvement in this regard can be realized by instructing the pressman to trim back feedrate, on each job run, in accordance with his visual observation of overflowing. In Figure 9 metering roller pair is rearranged wherein the metering nip is such that dampening fluid cannot drain away in a circumferential direction when the rollers stop moving. Thus the metering nip constitutes a reservoir capable of being replenished with dampening fluid so that the metering rollers can deliver dampening fluid in a manner that prevents contamination. As shown in Figure 9, this means includes a pair of metering rollers 50 and 51, having a nip 52 at their junction and means for controlling the supply fountain solution available for delivery to the metering nip 52. In Figure 9 there is shown sensing means used to determine the necessity of additional dampening fluid supply. This means 53 consists of a single conductivity probe for detecting the presence of dampening fluid in the reservoir formed by the metering nip 52. Means is provided to supply fluid to the nip to prevent dampening fluid starvation. To affect this, the conductivity probe 53 is used to detect the drop in level and to initiate a fluid feed through a feed line/valve combination from the supply system to replenish the nip so that the reservoir is again filled. Thus, the sensor 53 signals the controller 54 to control valve 55 which can open or close dampening fluid supply line 56. It should be noted that this is most suitable for use on new printing presses because of the relative ease of rearranging rollers, compared to the task on existing presses. It will be apparent that other and further forms of the invention may be devised without departing from the scope of the appended claims.

Claims: A dampening system for minimizing contamination of dampening fluid in a lithographic press by minimizing the residence time of the dampening fluid in the dampening system, comprising: a) a rotating plate cylinder on the lithographic press, b) dampening fluid applying means for applying dampening fluid to said plate cylinder at a predetermined rate, c) said dampening fluid applying means consisting of a pair of rotating metering rollers (2 or 71,72) which rotate at a slower speed than said plate cylinder, said pair of rollers (2 or 71,72) defining a nip (40,73) therebetween. d) a fluid collection pan (32,60) positioned so that one of said pair of rollers (2,71) is at least partially immersed within a volume of fluid in said fluid collection pan (32,60), e) means for rotating of said partially immersed roller which acts to cause dampening fluid flow to said nip (40,73) and flooding of said nip (40,73) sufficient to supply said pre-determined rate of fluid, and f) dampening fluid supply means (13,39,38,43,63,96,67) characterised in that: (i) said dampening fluid supply means supplies a predetermined supply of dampening fluid at said predetermined rate to said collection pan (32,60); (ii) said fluid collection pan (32,60) contains a volume of dampening fluid (74) which is a small fraction of the volume of dampening fluid which is consumed by said press at said predetermined rate over one hour and said fluid collection pan (32,60) has communicating back and front regions (75,76), and said dampening system further comprises: iii) sensor means (30,53,67) extending into said fluid collection pan (32,60), said sensor means (30,53,67) determining the proper level of fluid within said collection pan (32,60) to avoid nip starvation at said nip (40,73); and adapted to control said fluid supply means; iv) means for maintaining a pre-determined distance relationship between the surface of said partially immersed roller (2,71) and at least two surfaces of said fluid collection pan (32,60), said predetermined distance being the minimum necessary to permit contaminating ink particles to be moved from said back (76) to said front (75) regions of said collecting pan (32,60) for removal by said partially immersed roller (2,71). A dampening system as defined in claim 1 characterised in that said pair of rotating rollers (2 or 71,72) is adapted to supply dampening fluid to said plate cylinder at a pre-determined rate in an amount not in excess of the amount necessary. A dampening system as defined in claim 1 characterised in that said sensor means (30,53,67) determines the proper level of fluid within said collection pan (32,60) to avoid nip starvation at said nip (40,73) while preventing overflow of fluid. A dampening system as defined in claim 1 characterised in that said fluid supply means is activated by said sensor means (30,53,67) for maintaining the proper level of fluid within said collection pan (32,60) so that fluid is supplied to said fluid collection pan (32,60) at said predetermined rate as to minimize the residence time of any given particle of dampening fluid within said collection pan (32,60). A dampening system as defined in any one of the preceding claims characterised in that said pre-determined distance is determined by the distance taken along a radial line (90a,90b) of said partially immersed roller (2,71) which is perpendicular to the surface of the fluid collection pan (32,60) and is no more than about 0.08 cm (0.03 inches). A dampening system as defined in claim 5 characterised in that said front region (75) of said fluid collecting pan (32,60) is about 0.64 cm (0.25 of an inch) by 0.64 cm (0.25 of an inch). A dampening system as defined in any one of claims 1 to 4 characterised in that the volume contained in said collection pan (32,60) is less than the volume consumed by said press over five minutes at said predetermined rate. A dampening system as defined in claim 4 so constructed and arranged that the mean residence time of a particle of the dampening fluid in the pan is one-half a minute or less. A dampening system as defined in claim 1 wherein said pair of rotating metering rollers (2 or 71,72) are arranged so as to prevent drain of dampening fluid when the rollers (2 or 71,72) are at rest. A dampening system as defined in claim 1 comprising a manifold means (13,63) for supplying said dampening fluid into said fluid collection pan (32,60). A dampening system as claimed in claim 10, wherein said manifold means (63) comprises: a) a manifold defining a U-shaped cutout adapted (92) for insertion of said sensor means (35,53,67) and further defining a fluid conducting channel running along the length of said fluid collection pan (32,60); b) a fitting (65) in communication with said fluid conducting channel (64) for supplying said dampening fluid from a fluid supply (96); and c) one or more flow passages (66) in communication with said fluid conducting channel (64) and the interior of said pan (32,60) for supplying said dampening fluid to said pan (32,60). A dampening system as claimed in claim 10 characterised in that said pre-determined distance is no more than 0.08 cm (0.03 inches). A dampening system as claimed in claim 10, wherein said one of said nip-forming rollers (2,71) is placed within said fluid collection pan (32,60) to define a front fluid-filled region (75) in front of said nip-forming roller (2,71) and a back fluid-filled region (76) in back of said nip-forming roller (2,71). A dampening system as claimed in claim 13, wherein said front fluid-filled region (75) has a cross-section dimension of no less than 0.64 cm by 0.64 cm (one fourth (1/4) inch by one-fourth (1/4) inch). A dampening system as claimed in claim 11, wherein said manifold (13,63) further defines one or more flow passages (66) in communication with said fluid conducting channel (64) and disposed through said U-shaped cutout (92) for spraying dampening fluid onto said sensor means (30,53,67).

Inventors: MACPHEE JOHN, MACPHEE, JOHN
Applicants: BALDWIN GRAPHIC SYSTEM INC, BALDWIN GRAPHIC SYSTEMS, INC.
Application year: 1992
Main classification: B41F7
IPC codes: B41F7, B41F33, B41N3