Dampening systems in offset printers

Abstract: An abnormality detector for a nozzle-type dampening system (DN) of an offset printer comprises a plurality of electrically conductive members (10) disposed between the nozzles (132) and a roller (112) of the printer. A solution detection means (30) is connected to the members (10) and the nozzles (132) to sense the electrical resistance therebetween which is representative of the amount of solution passing the members (10). If this amount falls below a predetermined threshold (TH), an error signal is generated.

Description: This invention relates to an abnormality detector for nozzle-type dampening systems, and particularly for such systems of the type used in offset printers. In offset printing, a printing plate having a substantially smooth surface is used which has lipophilic image area and a hydrophilic "non-image" area. Dampening solution, which chiefly consists of water, and oily ink are fed onto the plate surface. Because of the mutual repellence of water and oil, after a short time the ink is only present on the image area. Dampening systems commonly employ one of two types of mechanism. In one, a roller train is provided which extends from a reservoir of dampening solution to the plate surface; one rotating roller is partly dipped into the solution and carries it to the plate surface via an adjacent roller. This allows the dampening solution to be fed in a thin film over the entire circumferential roller surface. With this mechanism, however, it is difficult to change the amount of dampening solution fed onto different axial areas of the roller. In addition, ink may be carried from the plate surface to the dampening solution reservoir via the continuous roller train and thereby contaminate the dampening solution. In the second type of mechanism, the above problems are overcome by separating the roller train from the dampening solution source and spraying the dampening solution towards the rollers. The amount of solution applied to different areas of the rollers can be varied using this type of mechanism. An example of this mechanism is a nozzle-type dampening arrangement in which the dampening solution is ejected through nozzles, as disclosed in Published Unexamined Japanese patent Application Nos. Sho-51(1976)-59511, Hei-1(1989)-110146 and Hei-5(1993)- 330009, for example. Published Unexamined Japanese Patent Application No. Sho-51(1976)-59511 discloses a dampening arrangement in which the amount of dampening solution fed to each of a plurality of nozzles is regulated by a metering pump; the dampening solution being atomized by a rapid air stream from a blower. The metering pump is operated at a desired speed corresponding to the speed of the printer by controlling its drive motor. Published Unexamined Japanese Patent Application No. Hei-1(1989)-110146 discloses a dampening arrangement which comprises a pump unit for feeding dampening solution, nozzles for ejecting the solution fed from the pump unit, and a controller for controlling the ejection of the solution from the nozzles in accordance with the printing speed of the printer. The ejection of the solution is controlled by opening the nozzles for a predetermined time at intervals calculated from a basic preset and stored value, an adjustment value that is set in accordance with the printing elements corresponding to each of the ejection nozzles, and a corrected value that is selected from a set of preset and stored values which are related to different printing speeds of the printer, by relating them to the rotation of a plate cylinder of the printer. That is, control is effected so that a predetermined amount of the dampening solution is ejected at intervals dependent on the rate of rotation of the plate cylinder. Though specific details are not disclosed, it is stated in this Patent Application that the duration of injection, that is, the amount of dampening solution ejected may be controlled by changing the timing of the nozzle opening, the ejection pressure, and/or the extent to which respective shutter members provided in front of each of the nozzles are opened. Published Unexamined Japanese Patent Application No. Hei-5(1993)-330009 discloses a dampening arrangement comprising means for detecting the printing speed of a printer, a memory for storing values representing the amount of dampening solution to be fed depending on the printing conditions and the printing speed of the printer, an injecting means having a plurality of nozzles connected by tubing to a source of dampening solution, an air source for continuously ejecting dampening solution onto the plate surface (or the surface of the roller coming in contact with the plate surface) which is atomized by a rapid air stream, and a pressure control means provided in the tubing connecting the source of dampening solution and the ejecting means for controlling the feeding pressure of dampening solution to the ejecting means in accordance with the feed rate value stored in the memory. The amount of dampening solution ejected is controlled by setting the feed rate of dampening solution in accordance printing speed and printing conditions, such as humidity and temperature, and keeping the pressure of dampening solution delivered to the downstream side of the pressure control means constant by comparing the set feed of dampening solution with the pressure value in the tubing on the downstream side of the pressure control means. A needle valve is provided on the upstream side of each nozzle to control finely the feed of the dampening solution. Prior art pertaining to monitoring the feeding of the dampening solution includes Published Unexamined Japanese Patent Application Nos. Sho-57(1982)-18255, Sho-58(1983)-49252, and Hei-4(1992)-74910. Published Unexamined Japanese Patent Application Nos. Sho-57(1982)-18255 and Sho-58(1983)-49252 disclose arrangements wherein a roller is provided in a roller train for feeding dampening solution to the plate surface, which has a water-containing layer on its outer surface and electrodes in contact with the inner surface of the water-containing layer. The dampening solution content of the water-containing layer of the roller is checked by measuring the electrical resistance of the layer or the impedance across the electrodes. Published Unexamined Japanese Patent Application No. Hei-4(1992)-74910 discloses a technique for measuring the thickness of dampening solution on a plate surface by shining parallel light onto the surface, and the reflected light is received and converted into a voltage value representative of the thickness of the solution. In the monitoring arrangements discussed above, the quantity of dampening solution is measured on a roller on the mid-stream and downstream side of the dampening arrangement, or on the plate surface to which the dampening solution is fed, but not directly after ejection from the nozzle on the upstream side of the dampening arrangement. In the nozzles of a nozzle-type dampening arrangement, dampening solution is normally ejected through an extremely narrow opening to form a mist. The dampening solution is usually water or water to which a surface tension decreasing agent has been added, which tends to include impurities through the generation of salts due to chemical reactions of metallic ions in the solution, the generation of organisms such as bacteria or fine algae, or infiltration of solid particles such as paper powder. Although the dampening solution is generally fed under a slight pressure by a pump from the reservoir to the nozzles via tubing in which an appropriate filter is provided, some insoluble matter nevertheless tends to pass through the filter to the nozzle opening, resulting in the clogging of the nozzle opening, thus preventing the dampening solution from being ejected. This leads to poor printing quality due to inadequate feeding of dampening solution. The present invention is directed at an abnormality detector for a dampening system in a printing apparatus, the system comprising a plurality of nozzles for selectively applying dampening solution onto an adjacent surface, and a nozzle controller for generating a nozzle control signal to initiate ejection of dampening solution from the nozzles. It is also directed at a dampening system comprising such an abnormality detector. According to the invention, the detector comprises electrically conductive members disposed between said nozzles and said adjacent surface; solution detection means connected to said nozzles and the members for sensing the electrical resistance therebetween and generating a flow signal representative of said resistance; and analysis means for receiving said control signal and said flow signal, analysing said signals and outputting an error signal when said analysis indicates abnormality in the operation of the system. The invention also provides a method for detecting abnormality in the operation of a dampening system of a printing apparatus; the system comprising a plurality of nozzles for selectively applying dampening solution onto an adjacent surface, and a nozzle controller for generating a nozzle control signal to initiate ejection of dampening solution from the nozzles. According to the invention, the method comprises the steps of sensing the electrical resistance between said nozzles and electrically conductive members disposed between the nozzles and said adjacent surface; generating a flow signal representative of said resistance and therefore the amount of solution passing the members; comparing the flow signal with a predetermined threshold value and outputting a resultant signal dependent on the result of the comparison; commencing output of an intermediate signal on receipt of a said nozzle control signal and ceasing said output a receipt of a said resultant signal; and outputting an error signal if a predetermined nozzle control signal is generated whilst the intermediate signal in being outputted. With an abnormality detector of the invention the feeding of dampening solution on the upstream side of the dampening system is checked for each nozzle to detect any abnormality quickly, thereby substantially reducing the likelihood of the print quality deteriorating due to incorrect ejection of dampening solution. The detector may be relatively small and durable as it can be implemented using semiconductor components, such as a comparator/amplifier, a flip-flop circuit and an AND circuit. The invention will be described by way of example and with reference to the accompanying drawings wherein: Figure 1 is a circuit diagram for a dampening apparatus according to an embodiment of the present invention; Figure 2 is a front elevation of a nozzle and detecting member configuration; Figure 3 is a side view of the configuration of Figure 2; Figure 4 illustrates signals which may be generated in the circuit of Figure 1; Figure 5 is a perspective view of a lithographic printer including a dampening apparatus of the invention; and Figure 6 illustrates the distribution of dampening solution injected through the nozzles of the dampening apparatus of Figure 5 on the surface of the roller 112. A lithographic printer incorporating a dampening apparatus of the invention is shown in Figures 5 and 6. It uses a plate (not shown) consisting of a lipophilic printing image area and a surrounding hydrophilic "non-image" area mounted on a plate cylinder PC. An appropriate amount of ink is fed by an inking device IN and also an appropriate amount of dampening solution is applied by a dampening system DN. The image is printed onto material on a web W threaded between a blanket cylinder BC and an impression cylinder IC via the surface of a blanket (not shown) fitted to the blanket cylinder (BC). The dampening system DN1 comprises a roller means 110, and a nozzle means 130. The roller means 110 has a downstream roller 111 which rotates in contact with the downstream roller, both rollers disposed in substantially parallel alignment. The dampening solution is applied through the nozzle means 130 onto the cylindrical surface of the roller 111. The nozzle means 130 includes an elongate tubular member 131 which is substantially parallel with the axis of the upstream roller 112, and a plurality of nozzles 132 are provided on the tubular member 131 at equal intervals. A dampening solution feeding means 150 is connected to the tubular member 131 to feed dampening solution in a pressurized state. The nozzles 132 may configured to eject dampening solution in a jet having a generally elliptical cross-section, and disposed such that the major axes of the elliptical distribution areas 132a on the surface area of the upstream roller 112 are oriented obliquely with respect to the axis of the upstream roller 112 and parallel with each other, as shown in Figure 6. Dampening solution passes from the tubular member 131, via an inlet, into each nozzle 132, and is then emitted through an injection hole towards the surface of the upstream roller 112. A valve member (not shown) is biased to close the injection hole by a spring (not shown), and the hole is opened by excitation of a solenoid (not shown). The solenoid is energised by a nozzle opening signal generated by a nozzle controller 200 with predetermined timing. Symbol MM refers to a printer drive means, and PG to a means for generating a signal related to the work rate of the drive means for input into the nozzle controller. An abnormality detector 1 has at least one electrically conductive member 10 between the nozzle 132 and the upstream roller 112 in the path of the ejected dampening solution. Electrical continuity can be established between the nozzle 132 and the electrically conductive member 10 via the dampening solution, and dampening solution detecting means 30 then generates dampening solution detection signal. The voltage of the signal corresponds to the intensity of current between the nozzles 132 and the electrically conductive member(s) 10. A comparing means 50 receives the dampening solution detection signal, and outputs a secondary detection signal when the magnitude of the received signal level exceeds that of the reference voltage level of a predetermined threshold value L. Intermediate processing means 70 receives the aforementioned nozzle opening signal and the secondary detection signal. It outputs a signal when the nozzle opening signal is received, and then ceases output when the secondary detection signal is received. Abnormality judgment means 90 receives any output signal from the intermediate processing means 70 and the nozzle opening signal. It produces an abnormality judgment signal when both the signals are received simultaneously. Figures 1, 2 and 3 show an abnormality detector 1 corresponding to one nozzle 132. The electrically conductive member 10 is in the form of rectangular loop. It is disposed with its long sides 11 and 12 substantially perpendicular to the axis of the upstream roller 112 at a distance D away from the tip of the nozzle 132. The centres of the long sides 11 and 12 are aligned with the centre of the dampening solution spray cross-section. Preferably, the distance D is fixed by mounting the electrically conductive member 10 on an outer cover 133, for example. An inner cover 134 may be movable relative to the outer cover 133. The conductive member 10 is insulated from the outer and inner covers 133 and 134. As shown in Figure 1, dampening solution detecting means 30 comprises a current bridge circuit 31 formed by connecting resistors VR1, R1, R2, R4 and R3 sequentially to form a loop and a voltage supply portion for supplying positive voltage between the resistors VR1 and R3. One end of each of the resistors R2 and R4 is grounded, and an electrically conductive member 10 is connected between the resistors R1 and R2. The input of a voltage follower 32 is connected between the resistors R4 and R3. A nozzle 132 is connected between the resistors R2 and R4, and the electrically conductive member is also connected to the input of another voltage follower 33. The outputs of the voltage followers 32 and 33 are applied to respective inputs of a comparator/amplifier 34. The output of the comparator/amplifier 34 is in turn connected to an input of an amplifier 36 via a differentiation circuit 35 comprising a capacitor C1, and resistors R9 and R10. Comparing means 50 comprises a comparator/amplifier 51, one input of which is connected to an output of the amplifier 36 of the dampening solution detecting means 30, the other input being connected to a negative voltage supply portion via resistors R15 and VR2 which set the threshold voltage level L. Intermediate processing means 70 comprises a flipflop circuit 71. Its clear CL input is connected to an output of the comparator/amplifier 51 and its clock-pulse CP input is connected to a nozzle opening signal output end of nozzle controller 200. Abnormality judgement means 90 comprises an AND circuit 91, one input end of which connected to an output end Q of the flipflop circuit 71, and the other input thereof being connected to a nozzle opening signal output end of the nozzle controller 200. The operation of the aforementioned abnormality detector 1 is described below. When the printer commences operation, the nozzle controller 200 outputs a nozzle opening pulse signal ((1) in Figure 4) at a predetermined rate. The nozzle controller 200 simultaneously commences output of a solenoid excitation signal ((2) in Figure 4) of predetermined duration counted by a timer in the nozzle controller 200. The solenoid in a nozzle 132 is energised, opening an injection hole of the nozzle to eject the dampening solution in mist form. The dampening solution spray establishes continuity between the nozzle 132 and the electrically conductive member 10, and current flows between the nozzle 132 and the electrically conductive member 10. A finite resistance is thus formed across X and Y terminals of the current bridge circuit 31. Assuming the resistance value of the resistor R2r, and the resistance value between the nozzle 132 and the electrically conductive member 10 is ROr, the resistance value Rr across the X and Y terminals in the current bridge circuit 31 is Rr = (ROr x R2r)/(ROr + R2r) where ROr corresponds with changes in conductivity dependent upon changes in the state of the dampening solution mist ejection through the nozzle 132. As the dampening solution is ejected through the nozzle 132, the voltage applied to the voltage follower 32 remains unchanged, and as a result, the output voltage of the voltage follower 32 remains constant, while voltage applied to the voltage follower 33 changes, and as a result, the output voltage of the voltage follower 33 also changes, depending on the state of the ejected dampening solution. The comparator/amplifier 34 compares and amplifies the voltages of signals entered from the voltage followers 32 and 33, and outputs a voltage signal shown in (3) of Figure 4, for example. In Figure 4, waveforms P1 and P2 represent the output when the ejection of dampening solution is normal, while a waveform P3 represents the state of output when the ejection is abnormal and a waveform P4 represents the state of output when there is no ejection. The output signal of the comparator/amplifier 34 is entered into the amplifier 36 via the differentiation circuit 35. The amplifier 36 outputs dampening solution detection signal (voltage signal (4) of Figure 4) in response to an input signal. This dampening solution detection signal is entered into one input of the comparator/amplifier 51, whilst a reference voltage adjusted and set by the resistors R15 and VR2 is entered into the other input end thereof. The comparator/amplifier 51 outputs a secondary detection signal, shown as (5) of Figure 4 for example, when the voltage of the dampening solution detection signal entered from the amplifier 36 exceeds the threshold value TH, that is, when the level of the dampening solution detection signal is less than the reference voltage level of the threshold value TH. The output end Q of the flipflop circuit 71 is reversed from the "LOW" state to the "HIGH" state (hereinafter referred to as L and H respectively) with the "fall" of the pulse of the nozzle opening signal ((6) in Figure 4), and this H signal is entered into the AND circuit 91. The flipflop circuit 71 is reversed from the H state to the L state and released when the secondary detection signal generated by the comparator/amplifier 51 is received. The AND circuit 91 outputs an abnormality judgment signal ((7) in Figure 4, for example) when a nozzle opening signal and an H signal at the output end Q of the flipflop circuit 71 are applied thereto simultaneously. When the dampening solution is ejected normally, continuity is established between the nozzle 132 and the electrically conductive member 10, and the amplifier 36 of the dampening solution detecting means 30 outputs an dampening solution detection signal. The voltage level of this signal is well above the preset threshold value TH, that is, below the reference voltage level of the threshold value TH in Figure 4. The comparator/amplifier 51 therefore outputs a secondary detection signal. The preceding nozzle opening signal is entered into the flipflop circuit 71 of the intermediate processing means 70 and the AND circuit of the abnormality judgement means 90. Then, the output end Q of the flipflop circuit 71 of the intermediate processing means 70 is turned to the H state with the "fall" of the pulse of the nozzle opening signal. The H signal at the output end Q of the flipflop circuit 71 is entered into the AND circuit 91 of the abnormality judgement means 90, lagging slightly behind the preceding nozzle opening signal. The aforementioned secondary detection signal is entered into the flipflop circuit 71, which releases the H state at the output end Q of the flipflop circuit 71. This release stops the inputting of signals from the flipflop circuit 71 to the AND circuit 91. When the preceding nozzle opening signal is entered, the AND circuit 91 of the abnormality judgement means 90 does not output any abnormality judgment signal because the H signal of the flipflop circuit 71 of the intermediate processing means 70 is not applied thereto at the same time. Then a succeeding nozzle opening signal is generated by the nozzle operation controller 200 to perform the same sequence of operations as described above. When the ejection of dampening solution from the nozzle 132 is not normal, the nozzle operation controller 200 operates in the following manner. When a solenoid excitation signal is generated by the preceding nozzle opening signal generated by the nozzle operation controller 200, the nozzle 132 is opened. However, if dampening solution is not injected normally from the opened nozzle 132, continuity is not established between the nozzle 132 and the electrically conductive member 10. Accordingly, no dampening solution detection signal is produced by the amplifier 36, or even if some continuity is established between the nozzle 132 and the electrically conductive member 10, only a small current flows. Thus, the amplifier 36 of the dampening solution detecting means 30 outputs dampening solution detection signal of a level that cannot be higher than the reference voltage level of the threshold value TH. That is, the dampening solution detection signal does not fall below the threshold level TH in Figure 4. Consequently, the comparator/amplifier 51 does not output any secondary detection signal. The preceding nozzle opening signal is entered into the flipflop 71 of the intermediate processing means 70 and the AND circuit 91 of the abnormality judgement means 90. Then, the output end Q of the flipflop circuit 71 of the intermediate processing means 70 is turned to the H state with the "fall" of the pulse of the nozzle opening signal. The H signal at the output end Q of the flipflop circuit 71 is entered into the AND circuit 91 lagging slightly behind the preceding nozzle opening signal. Since the comparator/amplifier 51 does not output secondary detection signal the output end Q of the flipflop circuit 71 stays in a H state. The H signal is entered from the flipflop circuit 71 into the abnormality judgement means 90. However, the AND circuit 91 does not output any abnormality judgement signal because the input from the flipflop circuit 71 was not HIGH at the time when the preceding nozzle opening signal was entered. Next, a succeeding nozzle opening signal is generated by the nozzle operation controller 200, and the solenoid excitation signal is generated by this signal to open the nozzle 132, while the nozzle opening signal is entered into the flipflop circuit 71 and the AND circuit 91. The H signal has meanwhile been maintained in the flipflop circuit 71 and does not change even if a succeeding nozzle opening signal is entered. Therefore, the AND circuit 91 outputs an abnormality judgement signal as both the nozzle opening signal and the H signal of the flipflop circuit 71 are entered simultaneously. In this way, as the state of ejection of the dampening solution from the nozzle 132 deviates from the normal state based on the threshold value TH set by the resistors VR2 and R15 of the comparing means 50, an abnormality judgement signal indicating abnormality is generated from the AND circuit 91. Circuits other than those shown in Figure 1 may be used to implement the injection dampening solution detecting means 30, the comparing means 50, the intermediate processing means 70 and the abnormality judgement means 90. As described above, an abnormality detector according to this invention can positively detect the abnormal ejection of dampening solution from each nozzle located upstream of the dampening system, by detecting abnormality in the voltage generated in accordance with the state of ejection of the dampening solution. Consequently, this invention has a wide range of applications, and can alert an operator to any abnormality in the ejection of dampening solution by an appropriate visual or audible signal. Defective printed matter can therefore be detected and destroyed, thus maintaining reliable printing quality, substantially reducing waste, improving the operating efficiency of the printer, and providing peace of mind for the operator.

Claims: An abnormality detector for a dampening system (DN) in printing apparatus of the type comprising a plurality of nozzles (132) for selectively applying dampening solution onto an adjacent surface, and a nozzle controller (200) for generating a nozzle control signal to initiate ejection of dampening solution from the nozzles, which detector comprises: electrically conductive members (10) disposed between said nozzles (132) and said adjacent surface (112); solution detection means (30) connected to said nozzles and the members (10) for sensing the electrical resistance therebetween and generating a flow signal representative of said resistance; and analysis means (50,70,90) for receiving said control signal and said flow signal, analysing said signals and outputting an error signal when said analysis indicates abnormality in the operation of said system. A detector according to Claim 1 wherein the analysis means comprises comparator means (50) for comparing said flow signal with a predetermined threshold value (TH) and outputting a signal dependent on the result of the comparison; intermediate means (70) for commencing output of an intermediate signal on receiving a said nozzle control signal and ceasing said output on receiving a predetermined comparator output signal; and judgement means (90) for outputting a said error signal if a said nozzle control signal is received when the intermediate means (70) is outputting said intermediate signal. A detector according to Claim 2 wherein the intermediate means (70) comprises a flipflop circuit (71) the output of which is set on receiving a said nozzle control signal and reset on receiving a said comparator output signal, the judgement means (90) being adapted to output a said error signal if a said nozzle control signal is received when the flipflop circuit output is set. A dampening system (DN) including an abnormality detector according to any preceding Claim, the dampening system comprising a plurality of nozzles (132) for selectively applying dampening solution onto an adjacent surface, and a nozzle controller (200) for generating a nozzle control signal to initiate ejection of dampening solution from the nozzles. A system (DN) according to Claim 4 wherein the nozzle controller (200) is adapted to generate a nozzle control signal to open individual nozzles for predetermined periods of time. A system (DN) according to Claim 4 or Claim 5 wherein said adjacent surface is defined by a roller (112) and the plurality of nozzles (132) is disposed along a line substantially parallel with the rotational axis of the roller. A system (DN) according to Claim 6 wherein the conductive members (10) are each disposed between a respective nozzle (132) and the roller (112). A system (DN) according to Claim 7 wherein the conductive members (10) are fixedly fitted to individual nozzles (132). A method of detecting abnormality in the operation of a dampening system (DN) in a printing apparatus of the type comprising a plurality of nozzles (132) for selectively applying dampening solution onto an adjacent surface (112), and a nozzle controller (200) for generating a nozzle control signal to initiate ejection of dampening solution from the nozzles, which method comprises the steps of sensing the electrical resistance between said nozzles (132) and electrically conductive members (10) disposed between the nozzles (132) and the adjacent surface (112); generating a flow signal representative of said resistance and therefore the amount of solution passing the members (10); comparing the flow signal with a predetermined threshold value (TH) and outputting a resultant signal dependent on the result of the comparison; commencing output of an intermediate signal on receipt of a said nozzle control signal and ceasing said output a receipt of a predetermined resultant signal; and outputting an error signal if a said nozzle control signal is generated whilst the intermediate signal is being outputted.

Inventors: FUJIO NOBORU, OHKAWARA TAMAKI, OHNO KINICHIRO, FUJIO, NOBORU, OHKAWARA, TAMAKI, OHNO, KINICHIRO
Applicants: TOKYO KIKAI SEISAKUSHO LTD, TOKYO KIKAI SEISAKUSHO LTD.
Application year: 1995
Main classification: B41F33
IPC codes: B41F33, B41F7