Patent Publication Number: US-10308019-B2

Title: Charge control-type inkjet printer and printing method using same

Description:
TECHNICAL FIELD 
     The present invention relates to an inkjet printer that performs printing by charging ink particles, and a printing method using the same, and more particularly to a charge control-type inkjet printer having a function of controlling the charging of ink particles and a printing method using the same. 
     BACKGROUND ART 
     Regarding the charge control-type inkjet printer related to the present invention, the abstract of Patent Document 1 describes, as a solution for easily determining an optimal nozzle driving electric voltage for forming ink particles without skill, “the controlling unit does not energize a deflection electrode until the ink particles enter a gutter after giving an electric charge to the ink particles from an electrically charged electrode, arbitrarily set a plurality of times of nozzle driving electric voltages, and gives an electrical charge to the ink particles by an optional electrically charged electric voltage. The amount of the electric charge given to the ink particles is detected by a sensor for the amount of the electric charge, and when the electrically charged electric voltage is proportional to the amount of the electric charge of the ink particles, it is determined to be normal, and the central value of the nozzle driving electric voltage within a range where it is determined to be normal is set as the nozzle driving voltage used for printing.” 
     CITATION LIST 
     Patent Document 
     Patent Document 1: JP 2010-17981 A 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     As an example of an inkjet printer, there is a charge control-type inkjet printer as described in Patent Document 1, which is a device for printing a manufacturing number, an expiration date and the like on a product to be printed. 
     Hereinafter, an operation principle of the charge control-type inkjet printer will be described based on the configuration described in FIG. 1 of Patent Document 1. 
     In the configuration described in FIG. 1 of Patent Document 1, the ink filled in a main body is pressurized by a pump and pressure-fed to a printing head. The ink pressure-fed to a nozzle in the printing head is vibrated by vibration of several ten kilohertz by a piezoelectric element  23  in the nozzle. An ink flow (inkjet) spouted from a discharge port of several ten microns of the nozzle is regularly cut by surface tension and vibration of the ink to be granulated (formed into droplet). 
     Since the ink particles (droplet) are cut from the ink flow in a charged state during the granulation, the ink particles are charged according to a size of a character. The charged ink particles are subjected to electrostatic force by a deflection electrode during flight and are deflected. The deflected ink particles adhere to a printed matter moving with respect to the printing head to form a character (a matrix character). 
     The ink for the charge control-type inkjet printer is basically formed of a coloring agent, a resin, a conductive agent, an additive, and a solvent. The ink is vibrated by ultrasonic vibration, spouted in an ink column shape from the discharge port, and granulated while being charged, whereby the ink flies as the ink particles. At this time, the charged state of the ink particles varies depending on how the ink column and the ink particles are cut off. When the charging state is not good, the particles may not be charged sufficiently with a designated amount of charge. Therefore, an accurate printing cannot be performed. 
     In order to obtain an optimal ink particle state, selection of the aforementioned material, first of all, is important. Besides that, a condition under which the ink is spouted and a condition under the ink is charged are important. Specifically, these conditions include an ink viscosity, an excitation frequency, an excitation voltage, and a discharge pressure. In addition, optimal conditions of them change depending on an operating environment such as a temperature and a humidity, and even on a deterioration state of the ink. 
     So far, optimal printing conditions could not be controlled in real time while the charge control-type inkjet printer is in operation and have been managed based on a relationship between a temperature and a viscosity by software that was created earlier. Therefore, when the printer is operated in an unpredicted environment, the software cannot follow up, resulting in a poor printing. 
     Further, when designing an ink including the aforementioned resin, coloring agent, additive (conductive material, leveling agent and the like) and solvent, there is a problem in which printing conditions (ink viscosity, excitation frequency, excitation voltage, discharge pressure, etc.) need to be individually set and examined without being determined based on a particle shape unconditionally, which takes time. 
     The present invention provides a charge control-type inkjet printer and a printing method using the same which is capable of solving the above-mentioned problem to enable real-time control during an operation of the charge control-type inkjet printer and maintaining optimal printing conditions while the charge control-type inkjet printer is in operation. 
     Solutions to Problems 
     In order to solve the above-mentioned problems, according to the present invention, there is provided a charge control-type inkjet printer which includes a printing head including a nozzle unit that discharges ink, a pressure reduction valve that adjusts a pressure of the ink supplied to the nozzle unit of the printing head, and an ink container that accommodates the ink to be supplied to the nozzle unit of the printing head, and the charge control-type inkjet printer further includes an imaging unit that images the ink discharged from the nozzle unit being in a particulate state. 
     In addition, in order to solve the above-described problems, the present invention provides a printing method using a charge control-type inkjet printer where the ink accommodated in an ink container is spouted as the particle from a nozzle unit of a printing head via a pressure reduction valve to be printed on a printing target. In the printing method, printing is performed while acquiring an image of the ink spouted from the nozzle unit and being in a particulate state by imaging the ink spouted from the nozzle unit and being in a particulate state with a camera. 
     Effects of the Invention 
     According to the present invention, it is possible to determine whether the shape of the ink particle is optimal during printing in the charge control-type inkjet printer, and it is possible to print in a stable state all the time by controlling the shape of the ink particle to be optimal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view illustrating a configuration of a charge control-type inkjet printer according to an embodiment of the present invention. 
         FIG. 2  is a block diagram illustrating a schematic configuration of an ink circulation system of a charge control-type inkjet printer according to an embodiment of the present invention. 
         FIG. 3  is a block diagram schematically illustrating a detailed configuration of a printing head unit, a detection unit, and a control unit of a charge control-type inkjet printer according to an embodiment of the present invention, and a state in which ink is made into particles in the printing head unit. 
         FIG. 4  is a block diagram illustrating a configuration of an image processing unit of a charge control-type inkjet printer according to an embodiment of the present invention. 
         FIG. 5  is a flowchart illustrating a flow of processing in an image processing unit of a charge control-type inkjet printer in an embodiment of the present invention. 
         FIG. 6  is an image illustrating both inspection machines in an image acquired by a particle observation unit of a charge control-type inkjet printer in an embodiment of the present invention. 
         FIG. 7  is a table showing a list of images of ink particles stored in a database for classifying images acquired by a particle observation unit of a charge control-type inkjet printer according to an embodiment of the present invention. 
         FIG. 8  is a table stored in a database illustrating a relationship between a result of classifying images acquired by a particle observation unit of a charge control-type inkjet printer and a control region and a control amount of the apparatus according to an embodiment of the present invention. 
         FIG. 9  is a table stored in a database illustrating a relationship between a difference between a reference value and a feature amount of an ink particle image acquired by a particle observation unit of a charge control-type inkjet printer in an embodiment of the present invention and a control region and a control amount of the apparatus. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     The present application is a charge control-type inkjet printer which is capable of observing a shape of an ink particle in real time, estimating an electrification property, and controlling under a printing condition which is optimal in all conditions. 
     Features of the present invention include: 
     (1) an adjustment mechanism of observing shapes of an ink column and ink particles coming out from a discharge port and mainly adjusting an excitation frequency, an excitation voltage, and an ink pressure with a fluid control unit so that the observed shape becomes an optimal shape, when performing printing with a charge control-type inkjet printer, 
     (2) a mechanism of recognizing whether an optimal particle shape matches optimal shape data of the ink column and the ink particles in the charge control-type inkjet printer including the adjustment mechanism, 
     (3) a mechanism for verifying whether or not charged state is suitable by measuring electric charge is provided in the charge control-type inkjet printer including a charge state observation unit, in addition to the above features (1) and (2), 
     (4) in addition, a mechanism capable of accumulating image data of a newly acquired particle shape and charged state measurement data and verifying a correlation between both data is provided in the charge control-type inkjet printer. 
     Hereinafter, embodiments of the present invention will be described with reference to the drawings. 
       FIG. 1  is a perspective view illustrating a charge control-type inkjet printer  100  according to the present embodiment. The charge control-type inkjet printer  100  includes a main body  1  including an operation display unit  5 , a printing head  2 , and a detection unit  3 . The main body  1  and the printing head  2  are connected by a conduit  4 . 
     Further,  FIG. 2  illustrates a simplified ink circulation system of the charge control-type inkjet printer  100  according to the present embodiment. An ink container  10  stored in the main body  1  of the charge control-type inkjet printer  100  stores ink, and the ink is sucked out from the ink container  10  by the pump  13  and is pressure-fed to the printing head  2  through the conduit  4 . The pressure-fed ink is filtered by a filter  12  and adjusted to a predetermined pressure by a pressure reduction valve  14 . 
     Viscosity of the ink sucked out from the ink container  10  by the pump  13  is measured with a viscometer (not shown), and a solvent is sucked out from a solvent tank  110  by a pump  11  so that the viscosity of the ink is maintained constant and is supplied to the ink container  10 . 
     The filter  12  and the pressure reduction valve  14  may be provided in the main body  1  or on the side of the printing head  2 . 
     When a valve  15  provided in a conduit of the circulation system is opened, the ink is delivered to a nozzle  16  of the printing head  2  and ejected from an orifice (not shown) which is a discharge port of the nozzle  16 . 
     The printing head  2  includes a nozzle  16 , a charging electrode  17 , a deflection electrode  18  formed by disposing a pair of electrode plates facing each other, and a gutter  19  for collecting ink particles not contributing to printing, and is controlled by a control unit  30  on the side of the main body  1 . In addition, a detection unit  3  is attached to the printing head  2 . 
     The nozzle  16  is connected to an excitation unit  22 . The excitation unit  22  applies a predetermined excitation voltage to a piezoelectric element  23  (see  FIG. 3 ) attached to the nozzle  16 , and vibrates the piezoelectric element  23  attached to the nozzle  16  at a predetermined frequency. 
     The ink adjusted to a predetermined pressure by the pressure reduction valve  14  is supplied to the printing head  2  and is spouted as an ink column from the nozzle  16 . At this time, the piezoelectric element  23  attached to the nozzle  16  is driven by the excitation unit  22  to vibrate at a predetermined frequency, the vibration is transmitted to the nozzle  16 , and the excited ink is spouted from the nozzle  16 . The columnar ink (ink column) spouted from the nozzle  16  in the excited state is separated into particles at a point apart by a predetermined distance from an outlet of the nozzle  16  to become ink particles  20 . The state in which particles are formed from the columnar ink (ink column) to become the ink particles  20  is observed and controlled by the control unit  30 . 
     When the ink particles  20  are separated from the columnar ink while passing between a pair of charging electrodes  17  constituting a charging part, the ink particles  20  are charged by the charging electrode  17  constituted by arranging a pair of electrode plates to face each other such that each of the ink particles  20  has a controlled charge amount. 
     A charging circuit (not shown) for controlling the charging electrode  17  changes a charging voltage of the charging electrode  17  according to a size of a symbol such as a character or a bar code to be printed on a printing object  130 , and changes the charging amount of the ink particles  20 . 
     The ink particles  20  charged while passing between the pair of electrode plates of the charging electrode  17  reaches a space between the pair of electrode plates forming the deflection electrode  18  with a momentum spouted from the nozzle  16 . 
     One of the two electrode plates constituting the deflection electrode  18  is grounded, and the other is supplied with a high voltage. Thus, an electrostatic field is formed between the electrodes. The ink particles  20  charged to a predetermined charge by the charging electrode  17  which has reached the space between the pair of electrode plates forming the deflection electrode  18  are deflected according to their charge amount. 
     The ink particles  20  deflected by the deflection electrode  18  jump out from the printing head  2 , and land on the printing object  130  moving away from the printing head  2  by a predetermined distance. 
     Although the ink particles  20  landed on the printing object  130  are points, a plurality of points gather to form a character symbol. 
     The ink particles  20  not used for printing are captured by a gutter  19  provided inside the printing head  2 . The captured ink particles  20  are sucked by a recovery pump  21  and returned to the ink container  10  through an ink recovery pipe of the ink circulation system. 
     Next,  FIG. 3  illustrates a schematic configuration of a charge control-type inkjet printer  100  having a function of adjusting so as to achieve an optimal particle shape according to the present embodiment. 
       FIG. 3  schematically illustrates a configuration of the control unit  30  and a state in which the ink column  117  spouted from the nozzle  16  is made into particles, in addition to a detailed configuration of the detection unit  3  described with reference to  FIG. 2 . The same components as those described in  FIG. 2  are denoted by the same reference numerals. 
     The detection unit  3  includes a particle observation unit  31  and a charge amount observation unit  33 . The control unit  30  includes a control unit  32  and a fluid control unit  34 . The particle observation unit  31  and the charge amount observation unit  33  constituting the detection unit  3  are attached to the printing head  2 . 
     The particle observation unit  31  includes a stroboscope  311  and a camera  312 , and illuminates the ink column  117  spouted from the nozzle  16  by the stroboscope  311 . In such an illumination (stroboscopic illumination) by the stroboscope  311 , a frequency of the stroboscopic illumination is adjusted so that the ink spouted from the nozzle  16  is stopped and observed. The ink spouted from the nozzle  16  adjusted in the manner described above and illuminated by the stroboscopic illumination is imaged by enlarging a field of view of the camera  312  to such an extent that shapes of a plurality of ink particles can be sufficiently discriminated by the camera  312  or that several ink particles illuminated by the stroboscopic illumination enter the field of view of the camera  312 . 
     For the sake of explanation,  FIG. 3  illustrates a configuration in which the particle observation unit  31  is disposed on the charging electrode  17  and the ink spouted from the nozzle  16  is imaged by the camera  312  from an upper side of the charging electrode  17 . However, in practice, the particle observation unit  31  is disposed at a position where the field of view is not blocked by a pair of electrode plates of the charging electrode  17  in a direction rotated by 90 degrees with respect to the charging electrode  17 , that is, in a lateral direction of the charging electrode  17 , and images the ink particles passing through the charging electrode  17 . 
     The ink illuminated with the stroboscope and imaged by the camera  312  is observed to have such a shape that, immediately after a tip portion  118  is separated into  119 ,  120 ,  121 ,  122  and  123  from the state of the ink column  117  immediately after being spouted, a tailed elongated shape ( 119 ) gradually changes to a round particle shape ( 123 ). Image data of the shapes of the ink particles  119  to  123  observed by the camera  312  of the particle observation unit  31  is sent from the particle observation unit  31  to the control unit  32 . 
     The control unit  32  includes an image processing unit  321  and a mechanism control unit  322 . The image processing unit  321  compares image data of the shapes of the ink particles  119  to  123  imaged by the camera  312  with reference image data stored in advance to extract a difference between the observed shapes of the ink particles  119  to  123  and reference shape data at respective positions. The mechanism control unit  322  controls the fluid control unit  34  so that the difference extracted by the image processing unit  321  becomes small. 
     Based on the control signal output from the control unit  32 , the fluid control unit  34  controls a voltage applied to the excitation unit  22  that controls an amplitude of vibration of the piezoelectric element  23  (not shown) that applies ultrasonic vibration to the nozzle  16 , a pressure of the pressure reduction valve  14  that adjusts a pressure of the ink supplied to the nozzle  16 , and a supply amount of the solvent that dissolves the ink stored in the solvent tank  110  to the ink container  10  by the pump  11 . 
     The ink whose pressure is adjusted by the pressure reduction valve  14  is discharged from the nozzle  16  ultrasonically vibrated by the piezoelectric element  23  driven by the excitation unit  22  to become the ink column  117 . Then, the tip portion  118  of the ink column  117  changes into the ink particles  119  and then deformed into the ink particles  120 ,  121  and  122  gradually to become a charged ink particle  123 . This charging is performed by passing between the pair of electrode plates on which an electric field of the charging electrode  17  is formed. 
     The ink particles  123  charged by passing through the charging electrode  17  are deflected by the electric field formed by the deflection electrode  18  formed by a pair of electrode plates. 
     In this case, it was found that a chargeability of the ink particle  123  depends on the shape of the ink particle  123 , particularly, the shape when the tip portion  118  of the ink column  117  changes to the ink particle  119 . 
     Therefore, it is possible to examine (predict) the chargeability of the ink particles by observing the shape at this time, that is, the shape when the tip portion  118  of the ink column  117  changes to the ink particle  119  with the use of the particle observation unit  31 . 
     As described above, the shapes of the ink particles  119  to  123  formed by the ink discharged from the nozzles  16  may be changed by adjusting the excitation frequency and the excitation voltage of the excitation unit  22  and a pressure-feeding power (an ink pressure) by the pressure reduction valve  14  with the use of the fluid control unit  34  controlled by the control unit  32 . 
     In view of this, in the present embodiment, the particle observation unit  31  observes the shape of the ink particle immediately after spouted from the nozzle  16  until it enters the deflection electrode  18 , and the control unit  32  controls the fluid control unit  34  such that the shape conforms to an optimal particle shape for each position input in advance. As a result, printing can be stably performed even under operation conditions of various charge control-type inkjet printers. 
     The chargeability is checked by checking a charge amount of the ink particles recovered by the gutter  19 , which is an ink particle recovery unit, with the use of the charge amount observation unit  33 . At this time, if a voltage is applied to the deflection electrode  18 , the charged ink particle  123  is deflected. For this reason, no voltage is applied to the deflection electrode  18 . 
     Further, a relationship between the image data imaged by the camera  312  of the particle observation unit  31  and the charge amount measured by the charge amount observation unit  33  is stored as data and fed back to the control unit  32 . 
     Next, a configuration of the image processing unit  321  of the control unit  32  is illustrated in  FIG. 4 . The image processing unit  321  includes an inspection area setting unit  3211  for setting an inspection area from the image imaged by the camera  312  of the particle observation unit  31 , a pattern matching unit  3212  that performs a pattern matching of images of ink particles in an area set by the inspection area setting unit  3211  to images stored in advance to extract images similar to the images of the ink particles, and a control information acquisition unit  1 : 3213  for acquiring information to be controlled by the mechanism control unit  322  from information of the images of the ink particles stored in advance which is extracted by the pattern matching unit  3212 . 
     Further, the image processing unit  321  includes a particle area specifying unit  3214  for specifying a region of ink particles within the area set by the inspection area setting unit  3211 , a foreign substance determination unit  3215  for determining whether foreign matter is contained in the ink particle area specified by the particle area specifying unit  3214 , an image feature amount extraction unit  3216  for extracting an image feature amount of the ink particles from the images of the ink particles determined to include no foreign matter by the foreign substance determination unit  3215 , and a control information acquisition unit  2 : 3217  for acquiring information to be controlled by the mechanism control unit  322  from the image feature amount of the ink particle extracted by the image feature amount extraction unit  3216 . 
     Next, a process flow of an ink particle shape control method in present embodiment will be described with reference to the flow chart of  FIG. 5 . 
     First, a stroboscopic light is irradiated from the stroboscope  311  to the ink spouted from the nozzle  16  in a state where the piezoelectric element  23  is driven by the excitation unit  22  under an initial setting condition and the nozzle  16  is vibrated at high frequency (S  501 ). At this time, an emission frequency of the stroboscopic light is adjusted so that the ink particles irradiated with the stroboscopic light are observed to be stationary. 
     In this way, the camera  312  of the particle observation unit  31  images the ink particles which are irradiated with the stroboscopic light and observed to be stationary (S 502 ), and acquires images of the ink particles. The images of the ink particles imaged and acquired by the camera  312  are sent from the particle observation unit  31  to the control unit  32 , and input to the image processing unit  321 . 
     The images input to the image processing unit  321  are binarized by the inspection area setting unit  3211 , and then an inspection area  601  is set as illustrated in  FIG. 6  (S 503 ). 
     Subsequently, a region  602  including the ink particles in the left end of the inspection area  601 , that is, a region  602  including an image of the ink particle which is observed first is extracted, and an image that most closely matches the image of the region  602  is selected from images of the ink particles whose relationship between shapes of the ink particles stored in advance by a pattern matching and control information by the mechanism control unit is known, thus specifying the shape of the particle (S 504 ). For example, as illustrated in a table  701  of  FIG. 7 , the image whose shape is known may include images obtained by classifying the ink particles for classifying the images acquired by the particle observation unit and stored in a database, as A, B, C and D. 
     Next, control information by the mechanism control unit is acquired from a relationship between the shapes of the ink particles stored in advance and the control information by the mechanism control unit (S 505 ). Here, the relationship between the shapes of the ink particles stored in advance and the control information by the mechanism control unit may include a relationship between a result of classifying the images acquired by the particle observation unit and stored in a database and a control region and a control amount of the apparatus, as illustrated in a table  801  of  FIG. 8 . 
     For example, if the image of the ink particle is determined to closest to the shape A in  FIG. 7  as a result of the pattern matching in S 504 , it is determined from the table of  FIG. 8  that there is no place to be controlled by a storage control unit and the current status is okay, as it is, in S 505 . On the other hand, if it is determined in S 504  that the image of the ink particle is closest to the shape B in  FIG. 7 , a control command is selected from the table of  FIG. 8  so as to control an ink viscosity adjustment mechanism to adjust the viscosity of the ink to be decreased in S 505 . 
     On the other hand, a particle region is specified from the image of the inspection area  601  set in S 503  (S 506 ). In the image illustrated in  FIG. 6 , an image  603  of a small area including the fourth particle from the right side in the image of the inspection area  601  is specified. 
     Subsequently, it is checked whether foreign matter such as minute ink particles is included in an image of the small area specified in S 506  (S 507 ). As a result of the check, if it is determined that a foreign object is included in the image (in the case of NO), the process proceeds to S 505 , where the control information by the mechanism control unit is selected from the ink particle image information stored in the database. 
     On the other hand, if it is determined in S 507  that foreign matter is not included in the image (in the case of YES), the feature amount of the image of the ink particle in the particle region specified in S 506  is extracted (S 508 ). Here, the feature amount of the image of the ink particle to be extracted may include dimensions in the X direction or the Y direction of the image of the ink particle, a ratio between the dimension in the X direction and the dimension in the Y direction, and the like. 
     Subsequently, the feature amount of the image of the ink particle extracted in S 508  is compared with a feature amount of a reference image data, and control information of a mechanism unit is acquired from a relationship between an image feature amount difference value obtained by storing a difference value between the feature amount of the image of the extracted ink particle and the feature amount of the reference image data in the database and a mechanism unit control amount is obtained (S 509 ). An example of the relationship between the image feature amount difference value stored in the database and the mechanism unit control amount may be summarized by a relationship between a result of classifying the images acquired by the particle observation unit and stored in a database and a control region and a control amount of the apparatus, as illustrated in a table  901  of  FIG. 9 . 
     Whether the control information is acquired in S 505  and whether the control information is acquired in S 509  are determined (S 510 ). If both are YES (when the particle shape and the image feature amount difference value are within ranges stored in the database), the respective control information are output to the mechanism control unit  322  (S 511 ). 
     The mechanism control unit  322  that has input the control information from the image processing unit  321  in step S 511  sends a control signal based on the input control information to the fluid control unit  34 , and adjusts an excitation frequency or an excitation voltage of the excitation unit  22 , an ink pressure by the pressure reduction valve  14 , and viscosity of the ink by driving the pump  11  to supply the solvent from the solvent tank  110  to the ink container  10 . 
     On the other hand, if it is determined as NO in S 511  (when control information cannot be acquired in at least one of S 505  and S 509 ), a warning is issued (S 512 ) to stop the printing (S 513 ), and the information determined as NO is displayed on a screen of the operation display unit  5  of the main body  1  (S 514 ). 
     According to the present embodiment, it is possible to quantitatively monitor the shape of the ink particle immediately after being spouted from the nozzle  16  while printing and adjust each mechanism unit, thus performing the printing continuously with a constant quality maintained. In addition, since the shape of the ink particle immediately after being spouted from the nozzle  16  is monitored quantitatively, even if an ink material changes, it is possible to continue printing while maintaining a constant printing quality without taking much time for adjustment. 
     REFERENCE SIGNS LIST 
       1 : main body,  2 : printing head,  3 : detection unit,  4 : conduit,  10 : ink container,  14 : pressure reduction valve,  16 : nozzle,  17 : charging electrode,  18 : deflection electrode,  19 : gutter,  22 : excitation unit,  30 : control unit,  31 : particle observation unit,  32 : control unit,  321 : image processing unit,  322 : mechanism control unit,  33 : charge amount observation unit,  34 : fluid control unit,  100 : charge control-type inkjet printer.