JP2008137758A - Inkjet printer - Google Patents

Abstract

An inkjet printer capable of securing an electrostatic attraction force to a conveyance belt of a print medium even when suppressing a surface potential on the side opposite to the conveyance belt of the print medium.
The surface of a conveyor belt is charged to a potential having a polarity opposite to that in the belt moving direction, and a print medium is adsorbed and conveyed to the surface by electrostatic force. When performing printing by ejecting ink droplets, a printing medium charging roller 9 that contacts the surface of the printing medium 2 is provided, and the printing medium 2 is controlled so that the potential of the surface of the printing medium 2 opposite to the conveying belt 1 is suppressed. The voltage applied to the charging roller 9 is controlled, and when the resistance value of the printing medium 2 is small, the voltage change rate at the rising and falling of the applied voltage is reduced to reduce the voltage applied to the printing medium charging roller 9. The amount of charge leakage from the conveyance belt 1 when the polarity is switched can be reduced, and the electrostatic attraction force of the print medium 2 to the conveyance belt 1 can be ensured.
[Selection] Figure 12

Description

  The present invention conveys a printing medium such as printing paper or an OHP sheet, and, for example, discharges fine particles (dots) of a plurality of colors of liquid ink onto the printing medium to draw predetermined characters and images. The present invention relates to a printer, and is particularly suitable for transporting a print medium attracted to the surface of a transport belt by electrostatic force.

Such an inkjet printer is generally inexpensive and can easily obtain a high-quality color printed matter. Accordingly, along with the widespread use of personal computers and digital cameras, it has become widespread not only in offices but also in general users.
In such an ink jet printer, generally, a moving body called a carriage or the like, which is integrally provided with an ink cartridge and an ink discharge head (generally called an ink jet head), runs on a print medium in a direction intersecting the transport direction. By driving the actuator while reciprocating, fine particles (ink droplets) of liquid ink are ejected from the nozzles formed on the ink jet head, thereby forming minute ink dots on the print medium, thereby forming predetermined characters and A desired printed matter is created by drawing an image. The carriage is equipped with four color (yellow, magenta, cyan) ink cartridges including black (black) and an inkjet head for each color, so that not only monochrome printing but also full-color printing combining each color is easy. (Furthermore, 6 colors, 7 colors, or 8 colors in which light cyan, light magenta, etc. are added to these colors are also put into practical use).

  In addition, in an ink jet printer that performs printing while reciprocating the ink jet head on the carriage in the direction intersecting the print medium transport direction (width direction of the print medium), the entire page is printed neatly. Therefore, it is necessary to reciprocate the inkjet head several tens of times to 100 times or more. On the other hand, in an ink jet printer of a type in which a long ink jet head (not necessarily integral) having the same dimensions as the width of the print medium is disposed and the carriage is not used, the ink jet head is moved in the width direction of the printing paper. There is no need, and so-called one-pass printing is possible, so that high-speed printing is possible. The former inkjet printer is generally referred to as a “multi-pass printer”, and the latter inkjet printer is generally referred to as a “line head printer”.

  By the way, in this type of line head type ink jet printer, a configuration in which a print medium is adsorbed and conveyed on a conveyance belt for conveyance is often used. In order to efficiently attract the print medium to the conveyor belt, the surface of the conveyor belt is alternately charged to a reverse polarity potential, and the electrostatic force is equivalent between the adjacent reverse polarity potential of the conveyor belt and the print medium. It is desirable to configure a circuit to adsorb the print medium. Generally, a charging roller is brought into contact with the surface of the conveyance belt, and a so-called alternating voltage is used in which the voltage applied to the charging roller is alternately reversed in polarity. As a result, the surface of the transport belt is charged to a potential having a polarity opposite to that of the transport belt.

In such electrostatic force adsorption to the conveyance belt of the print medium, dielectric polarization occurs in the print medium, and charges having a polarity opposite to the charge potential of the conveyance belt are collected on the back side of the conveyance belt side of the print medium. Charges of the opposite polarity are collected on the surface side opposite to the belt. That is, the surface of the print medium is also charged. However, when the surface of the printing medium is charged, there is a possibility that ink droplets ejected from the inkjet head cannot be ejected (also referred to as landing) to the target position due to the influence of the electric field. Therefore, in the ink jet printer described in Patent Document 1 below, a pressure roller that applies a voltage having a polarity opposite to the charging potential of the conveying belt charged at a constant period, as a slow-charging means for removing the charge on the surface of the printing medium. And a conductive brush. Specifically, a pressure roller or a conductive brush, which is a slow-charging means, is disposed on the downstream side of the conveying belt charging roller by being separated by a conveying belt moving distance corresponding to 1.5 times the conveying belt charging cycle. The same alternating voltage signal is supplied (the partial voltage is supplied to the slow voltage means to reduce the potential). In other words, the voltage applied to the print medium by the pressure roller and the conductive brush is also reverse in polarity at a constant cycle.
JP 2005-324877 A

By the way, the resistance value of the print medium differs depending on the temperature and humidity as well as the type of the print medium. When the resistance value of the print medium is different, the adsorption force of the print medium is different even if the conveyance belt is in the same charging state. Specifically, the electrostatic attraction force of the print medium to the conveyance belt is due to the electric charge accumulated between the print medium and the conveyance belt (gap), but if the resistance value of the print medium is small, the print medium The charge flows from the charged conveying belt that is in direct contact with the toner to the gradual charging means (leaks), and the amount of charge that should be accumulated between the printing medium and the conveying belt (gap) is reduced. As a result, there is a problem that the electrostatic attraction force of the print medium to the conveyance belt becomes weak.
The present invention has been made paying attention to the above-described problems, and ensures electrostatic attraction to the conveyance belt of the print medium even when the surface potential on the side opposite to the conveyance belt of the print medium is suppressed. An object of the present invention is to provide an ink jet printer that can perform the above-described operation.

  [Invention 1] In order to solve the above-described problem, an ink jet printer according to Invention 1 has a charging roller in contact with the surface of the conveyance belt, and the surface of the conveyance belt is alternately reversed in the moving direction of the conveyance belt. In an ink jet printer that is charged with an electric potential, adsorbs and conveys the print medium to the surface of the charged conveyance belt by electrostatic force, and ejects ink droplets from the ink jet head onto the print medium, and the resistance value of the print medium The print medium resistance value detecting means for detecting the print medium, the print medium charging roller in contact with the surface of the print medium opposite to the conveyance belt, and the printing medium so that the potential on the surface opposite to the conveyance belt of the print medium is suppressed. Printing medium surface potential control means for controlling the voltage applied to the medium charging roller, and the printing medium surface potential control means is the printing medium resistance value detection means. It is characterized in that controlling the rate of change of voltage rise and fall of the issued voltage applied to the printing medium charging roller based on the resistance value of the printing medium.

  In the ink jet printer according to the first aspect of the invention, the charging roller is brought into contact with the surface of the conveyance belt, and the surface of the conveyance belt is alternately charged to a potential having a polarity opposite to the moving direction of the conveyance belt. When the print medium is attracted to the surface of the transport belt by electrostatic force and transported, and ink droplets are ejected from the inkjet head onto the print medium for printing, the print medium is charged against the surface of the print medium opposite to the transport belt. A roller is provided to control the voltage applied to the print medium charging roller so that the potential on the surface opposite to the conveyance belt of the print medium is suppressed, and based on the detected resistance value of the print medium For example, when the resistance value of the printing medium is small, the printing medium charging roller is configured to control the voltage change rate of the rising voltage and the falling voltage applied to the charging roller. By reducing the rate of voltage change between the rising and falling of the applied voltage, the amount of charge leakage from the conveying belt when the polarity of the applied voltage to the printing medium charging roller is switched is reduced to the conveying belt for the printing medium. The electrostatic attraction force can be ensured.

[Invention 2] The ink jet printer of Invention 2 is the ink jet printer of Invention 1, wherein the print medium surface potential control means has a small resistance value of the print medium detected by the print medium resistance value detection means. It is characterized in that the rate of voltage change at the rise and fall of the voltage applied to the print medium charging roller is reduced.
According to the ink jet printer according to the second aspect of the present invention, the voltage change rate of the rising and falling of the voltage applied to the printing medium charging roller is reduced when the detected resistance value of the printing medium is small. It is possible to reduce the amount of charge leakage from the conveyance belt when the polarity of the voltage applied to the roller is switched, and to secure the electrostatic adsorption force of the print medium to the conveyance belt.

[Invention 3] The ink jet printer according to Invention 3 is the ink jet printer according to Invention 1 or 2, wherein the printing medium resistance value detecting means applies a constant current to a predetermined interval on the surface of the printing medium opposite to the conveying belt. In this case, the resistance value of the print medium is detected from the voltage value generated at the predetermined interval.
According to the ink jet printer according to the third aspect of the present invention, a constant current is passed through a predetermined interval on the surface opposite to the conveyance belt of the print medium, and the resistance value of the print medium is detected from the voltage value generated at the predetermined interval at that time. Because of the configuration, if the voltage value is detected by bringing the probe into contact with the surface of the print medium at a predetermined interval, the resistance value of the print medium can be detected simply and reliably simply by bringing the probe into contact with the surface of the print medium. be able to.

[Invention 4] In addition, in the inkjet printer of Invention 4, in the inkjet printer of Inventions 1 to 3, the print medium resistance value detecting means detects the resistance value of the print medium before starting printing and immediately after replacing the printing medium. It is characterized by.
According to the ink jet printer according to the fourth aspect of the present invention, since the resistance value of the print medium is detected before starting printing and immediately after replacing the print medium, it is possible to accurately ensure the electrostatic adsorption force of the print medium to the conveyance belt. it can.

Next, an embodiment of an inkjet printer according to the present invention will be described with reference to the drawings.
FIG. 1 is a schematic configuration diagram of an ink jet printer according to the present embodiment. Reference numeral 1 in the figure denotes an endless conveying belt for conveying a printing medium 2 such as printing paper. As described above, the print medium 2 is a medium / high resistance sheet-like member such as printing paper or an OHP sheet. Further, the transport belt 1 has a surface (outer peripheral surface) made of an insulating resin such as PET, polyimide, or fluorine resin, and a back surface (inner peripheral surface) of low to medium resistance (1 × 10 ¹⁰ Ω / □ or less). This is a so-called two-layer belt made of the above resin. The conveyor belt 1 is wound around a driving roller 3 disposed at the left end in the figure, a driven roller 4 disposed at the right end in the figure, and a tension roller 5 disposed below the central part thereof. It has been turned. The drive roller 3 is rotationally driven in the direction of the arrow in the figure by a drive source (not shown), and the print medium 2 is sucked from the right side of the figure in a state where the print medium 2 is adsorbed to the conveyance belt 1 charged by the charging roller described later. Transport to the left, that is, in the direction of the arrow. The driven roller 4 is grounded so as to apply a voltage with the conveying belt 1 being sandwiched between the driven roller 4 and a contact portion of a charging roller described later. The tension roller 5 is urged downward by a spring (not shown), thereby applying tension to the transport belt 1.

  A conveying belt charging roller 7 as a charging unit is in contact with the conveying belt 1 so as to face the driven roller 4. As will be described later, the conveying belt charging roller 7 has a conveying belt charging control unit 8. The alternating voltage signal having a predetermined potential is applied from the above so-called AC charging. The conveyor belt charging controller 8 sets the potential of the conveyor belt AC charging pattern signal to the conveyor belt charging roller 7 in accordance with a command from the system controller 6. The arrangement of the conveying belt charging roller 7 corresponds to the position immediately before the feeding position of the printing medium 2. In AC charging, a conveying belt AC charging pattern signal (alternating voltage signal) is applied to the conveying belt charging roller 7 to alternately charge the surface of the conveying belt 1 to different polarities, and the printing medium 2 is generated by an electric field between the different polarities. Dielectric polarization is generated, and electrostatic force is generated by the charge of the print medium 2 and the charge of the surface of the transport belt 1 due to the dielectric polarization, and the print medium 2 is attracted to the surface of the transport belt 1.

  Above the driven roller 4, a print medium charging roller 9 also serving as a paper presser is disposed. The print medium charging roller 9 is biased downward by a spring (not shown) and has a function of pressing the print medium 2 against the conveyance belt 1 on the driven roller 4. As described above, when the print medium 2 is mounted on the outer peripheral surface of the charged transport belt 1 and the print medium charging roller 9 presses the print medium 2 against the transport belt 1, the print medium 2 is moved to the outer periphery of the transport belt 1 by electrostatic force. Adsorbed on the surface. Further, an alternating voltage signal having a potential with a predetermined period is applied to the printing medium charging roller 9 from the printing medium charging control unit 18. As described above, the printing medium charging roller 9 suppresses the potential of the surface of the printing medium 2 due to dielectric polarization generated by the charged conveying belt 1 and allows flying of ink droplets ejected from an inkjet head described later. In order to prevent interference, a voltage signal having a polarity opposite to the charging potential of the conveying belt 1 is applied in principle. However, in the present embodiment, as will be described later, the surface resistance value of the printing medium 2 is measured, and the rising and falling voltage change rates of the applied voltage when the polarity is switched are controlled according to the surface resistance value.

  On the upstream side of the print medium charging roller 9 in the conveyance direction of the print medium 2, a feed roller 13 for feeding the print medium 2 of the paper feed unit and a guide 15 for supporting the fed print medium are disposed. . In addition, a print medium detection sensor 10 that detects the leading end position of the print medium 2 in the conveyance direction is provided on the downstream side of the feed roller 13 in the conveyance direction of the print medium 2, and the print medium charging roller 9 has a downstream side in the conveyance direction of the print medium 2. An encoder sensor 12 that detects the positions of the conveyance belt 1 and the print medium 2 from a linear scale attached to the conveyance belt 1 is provided.

Furthermore, the print medium 2 is stored in the paper feed unit on the most upstream side in the conveyance direction of the print medium 2, and the surface resistance sensor 14 is in contact with the upper surface of the uppermost print medium 2. The surface resistance sensor 14 is in contact with the positive electrode probe P _P and a negative electrode probe P _N the predetermined distance apart in the surface of the print medium 2 as shown in FIG. 3, the voltage value when a constant current from the positive electrode to the negative electrode The surface resistance value of the printing medium 2 is detected by measuring and dividing the measured voltage value by a constant current value. In addition, the resistance in a figure shows typically the resistance which the printing medium 2 has.

  An inkjet head 11 is provided downstream of the encoder sensor 12 in the conveyance direction of the print medium 2. The ink-jet head 11 is arranged so as to be shifted in the transport direction of the printing medium 2 for each of the four colors of yellow (Y), magenta (M), cyan (C), and black (K). Ink is supplied to each inkjet head 11 from an ink tank of each color (not shown) via an ink supply tube. Each inkjet head 11 is formed with a plurality of nozzles in a direction intersecting with the conveyance direction of the print medium 2, and by ejecting a necessary amount of ink droplets from these nozzles to a necessary location at the same time, the print medium 2. A minute ink dot is formed and output on the top. By performing this for each color, it is possible to perform so-called one-pass printing by passing the print medium 2 adsorbed on the conveyor belt 1 once. That is, the area where the inkjet head 11 is disposed corresponds to a printing area.

  As a method for discharging and outputting ink from each nozzle of the ink jet head, there are an electrostatic method, a piezo method, a film boiling ink jet method, and the like. In the electrostatic system, when a drive signal is given to the electrostatic gap, which is an actuator, the diaphragm in the cavity is displaced, causing a pressure change in the cavity, and ink drops are ejected from the nozzle by the pressure change. It is. In the piezo method, when a drive signal is given to a piezo element that is an actuator, the diaphragm in the cavity is displaced to cause a pressure change in the cavity, and ink droplets are ejected from the nozzle by the pressure change. . In the film boiling ink jet method, there is a minute heater in the cavity, the ink is instantaneously heated to 300 ° C. or more, the ink becomes a film boiling state, bubbles are generated, and ink droplets are ejected from the nozzle by the pressure change. That's it. Any ink output method can be applied to the present invention.

  A specific configuration of the ink jet head of the present embodiment will be described with reference to FIG. The ink jet head 11 uses a piezoelectric actuator, and includes a diaphragm 21, a piezoelectric actuator 22 that displaces the diaphragm 21, and ink inside that is a liquid inside. And a nozzle (24) communicating with the cavity (23) and ejecting ink as droplets by increasing / decreasing the pressure in the cavity (23).

  More specifically, the inkjet head 11 includes a nozzle substrate 25 on which nozzles 24 are formed, a cavity substrate 26, a vibration plate 21, and a laminated piezoelectric actuator 22 in which a plurality of piezoelectric elements 27 are laminated. Yes. The cavity substrate 26 is formed in a predetermined shape as shown in the figure, whereby a cavity 23 and a reservoir 28 communicating with the cavity 23 are formed. The reservoir 28 is connected via an ink supply tube 29 to an ink cartridge 33 that is an ink reservoir. The piezoelectric actuator 22 includes comb-shaped electrodes 31 and 32 arranged opposite to each other, and piezoelectric elements 27 arranged alternately with the comb teeth of the electrodes 31 and 32. The piezoelectric actuator 22 is joined to the vibration plate 21 at one end side through the intermediate layer 30.

  In the piezoelectric actuator 22 having such a configuration, the drive pulse from the drive pulse source applied between the first electrode 31 and the second electrode 32 expands and contracts in the vertical direction as indicated by arrows in FIG. The mode to use is used. Therefore, in the piezoelectric actuator 22, for example, when a drive pulse as shown in FIG. 2 is applied, the diaphragm 21 is displaced, the pressure in the cavity 23 changes, and an ink droplet is ejected from the nozzle 24. It has become. Specifically, the volume of the cavity 23 is enlarged to draw ink, and then the volume of the cavity 23 is reduced to eject ink droplets.

  Returning to FIG. 1, the transport belt charging control unit 17 sets the potential of the charging roller charging pattern signal to the transport belt charging roller 7 in accordance with a command from the system control unit, and the print medium charging control unit 18 is a system control unit. 6, the potential of the print medium charging pattern signal to the print medium charging roller 9 is set. For example, as shown in FIG. 4, the system control unit 6 controls a printing apparatus, a sheet feeding apparatus, and the like based on print data input from a host computer 60 such as a personal computer or a digital camera, so that the printing medium is printed. A printing process is performed. An input interface unit 61 that receives print data input from the host computer 60, a control unit 62 configured by, for example, a microcomputer that executes print processing based on the print data input from the input interface unit 61, A carriage motor driver 63 for driving and controlling the feed roller motor 41, a driving roller motor driver 64 for driving and controlling the driving roller motor 51 of the conveying belt 1, a head driver 65 for driving and controlling the inkjet head 11, and the drivers 63 and 64 , 65 output signals are converted into control signals used by the external feed roller motor 41, drive roller motor 51, and inkjet head 11, and output, or command signals from the control unit 62 are transferred to the conveyor belt charging control unit 8 or printing. Control to the medium charging control unit 18 The signal is converted into a signal and output, or the surface resistance value of the printing medium 2 detected by the surface resistance sensor 14 and the output signal of the encoder sensor 12 and the printing medium detection sensor 10 are converted into an input signal to the control unit 62. And an interface 67.

  The control unit 62 temporarily stores a CPU (Central Processing Unit) 62a that executes various processes such as a print process, and print data input through the input interface 61 or various data when the print data print process is executed. A ROM (Read-Only ROM) comprising a RAM (Random Access Memory) 62c that temporarily stores an application program such as print processing or the like, and a non-volatile semiconductor memory that stores a control program executed by the CPU 62a Memory) 62d. When the control unit 62 obtains print data (image data) from the host computer 60 via the interface unit 61, the CPU 62a performs a predetermined process on the print data and ejects ink droplets from any nozzle. Alternatively, print data indicating how many ink droplets are ejected is output, and a control signal is output to each of the drivers 63 to 65 based on the print data and input data from various sensors. When the control signals are output from the drivers 63 to 65, these are converted into drive signals by the interface unit 67, and the actuators 22, feed roller motors 41, drive roller motors 51 corresponding to the plurality of nozzles 24 of the inkjet head 11 are provided. The conveyance belt charging control unit 8 and the printing medium charging control unit 18 are operated to execute printing processing on the printing medium. Each component in the control unit 62 is electrically connected through a bus (not shown).

  Next, the charging waveform generation circuit 34 built in the conveyance belt charging control unit 8 and the print medium charging control unit 18 will be described with reference to FIG. The conveyor belt charging waveform generation circuit 34 is configured to generate a conveyor belt charging pattern signal composed of an alternating voltage signal having a predetermined potential for the conveyor belt charging roller 7 for charging the conveyor belt 1 and the printing medium charging roller 9 for charging the printing medium 2. Alternatively, a print medium charging pattern signal is generated, and when a charging pattern signal amplitude instruction value Vp for instructing an amplitude value (potential) of the charging pattern signal is input as digital data from the system control unit 6 described above, D / It is converted into an analog value by the A converter and output as a DC voltage value from the buffer B1. Since this output is grounded through the resistor R1, the charging pattern signal Sp consisting of an alternating voltage signal corresponding to the charging pattern signal amplitude instruction value Vp is supplied from the amplifier AMP1 to the conveying belt charging roller with reference to the ground GND (= 0V). 7 or the print medium charging roller 9.

  Next, a method for setting a printing medium charging pattern signal to the printing medium charging roller 9 will be described. FIG. 6 a schematically shows a contact state between the conveyance belt 1 and the print medium 2 at the print medium charging roller 9 and an electrical equivalent circuit. In the figure, reference numeral 1a is an insulating layer made of an insulating resin formed on the front surface side of the conveyor belt 1 as described above, and reference numeral 1b is a low to medium resistance resin formed on the back surface side of the conveyor belt 1. Is a conductive layer made of As described above, since the printing medium charging roller 9 suppresses the surface potential of the printing medium 2, a voltage E having a polarity opposite to the surface potential of the printing medium 2, that is, the surface potential of the conveying belt 1 is applied. Is done. The charged conveyor belt 1 is also in contact with the printing medium 2 with a voltage E or through a gap.

For example, the print medium 2 is in contact with the conveyance belt 1 at a portion where the print medium 2 is pressed against the conveyance belt 1 by the print medium charging roller 9, and there is a gap between the print medium 2 and the conveyance belt 1 at the adjacent portion. As shown, an equivalent circuit between the print medium charging roller 9, the print medium 2, and the conveyor belt 1 is extracted in FIG. 6b. Only the resistance R _P of the print medium 2 in a portion where the print medium 2 and the conveyor belt 1 is in contact is interposed, and the resistance R _P of the print medium 2 in a portion where the print medium 2 and the conveyor belt 1 is not in contact It is assumed that the capacitance C of the gap is interposed in series and connected to the power source 2E in parallel. If the amount of charge stored in the capacitance C in the gap between the print medium 2 and the conveyance belt 1 is large, the adsorption force of the print medium 2 to the conveyance belt 1 increases.

Originally, if the humidity of the environment is high, that is, if the humidity of the gap between the printing medium 2 and the conveyance belt 1 is high, the dielectric constant of the air is large, so that the amount of stored charge increases. As a result, the conveyance belt of the printing medium 2 The adsorption power to 1 should increase. However, if the printing medium 2 absorbs high humidity in the environment, the resistance value of the resistance R _P of the printing medium 2 becomes smaller. Therefore, the current flowing through the circuit outside the figure in which only the resistance R _P is interposed. , The amount of charge stored in the capacity C of the gap between the print medium 2 and the transport belt 1 is reduced, and even the charge stored in the capacity C of the transport belt 1 is discharged through the circuit outside the figure ( Leakage), the voltage applied to the gap decreases, and as a result, the adsorption force of the print medium 2 to the transport belt 1 decreases. FIG. 7 shows the relationship between the humidity in the air and the surface resistance value of the print medium 2. As is clear from the figure, the surface resistance value of the print medium 2 decreases as the humidity in the air increases.

  Therefore, in the present embodiment, the surface resistance of the print medium 2 indicates the rate of change of the voltage with respect to time at the rise and fall of the print medium charge pattern signal, which should be a voltage signal whose polarity is switched in a rectangular wave shape, that is, the polarity change portion. When it is small, make it small. FIG. 8A shows a print medium charging pattern signal composed of a voltage signal whose polarity changes in a rectangular wave shape, and FIG. 8B shows a print medium charging pattern signal whose voltage change rate at the rising and falling of the voltage at the polarity switching portion is small. Show. The fact that the voltage change rate of the voltage rise and fall at the polarity switching part is small, it can be said that the voltage rise and fall are delayed, so hereinafter the voltage rise and fall at the polarity switching part. A printing medium charging pattern signal with a reduced voltage change rate is referred to as a delayed printing medium charging pattern signal, and a rectangular wave printing medium charging pattern signal is referred to as a rectangular wave printing medium charging pattern signal.

  FIG. 9A shows the change over time of the current value when a rectangular wave printing medium charging pattern signal is applied to the printing medium charging roller 9. FIG. 9B shows the application of the delayed printing medium charging pattern signal to the printing medium charging roller 9. The change over time in the current value is shown. In either case, the switching time of the polarity of the current value substantially coincides with the switching time of the voltage polarity of the print medium charging pattern signal. As is clear from FIG. 9a, when a rectangular wave print medium charging pattern signal is applied, a large current flows immediately after the voltage polarity is switched. This is because if the resistance value of the printing medium 2 is large when the voltage polarity of the rectangular wave printing medium charging pattern signal is switched, the charge stored in the capacitance C in the gap between the printing medium 2 and the conveyance belt 1 increases. If the resistance value of the print medium 2 is small, the charge charged on the conveyance belt 1 is consumed (leaked) through the resistance of the recording medium 2, and the charge stored in the capacitance C in the gap between the print medium 2 and the conveyance belt 1 is It means to decrease. On the other hand, in the delayed printing medium charging pattern signal, a large charge / discharge current is suppressed from flowing when the voltage polarity is switched.

For example, although the polarity is opposite to the surface potential VB of the conveying belt 1 whose polarity is alternately switched as shown in FIG. 10, the voltage change rate at the rising and falling at the time of polarity switching is different, that is, the first having a different delay time. Fifth delayed printing medium charging pattern signals P _{1 to} P ₅ are prepared, and the same printing medium type, the same environmental temperature, and when the environmental humidity is 60% and 20%, respectively, the printing medium charging roller 9 and the adsorption force of the printing medium 2 to the conveyor belt 1 was measured. FIG. 11a shows the measurement result when the environmental humidity is 60%, and FIG. 11b shows the measurement result when the environmental humidity is 20%. Time t ₁ ~t ₅ in the figure, respectively, and corresponds to the delay time of the first to fifth delay of the print medium charge pattern signal P ₁ to P _5, therefore, for example, suction force of the time t ₁ is the This means the adsorption force of the print medium to the conveyor belt 1 when the 1-delayed print medium charging pattern signal P ₁ is applied.

In any case, the attracting force that is asymptotic (or saturated) over time is the original attracting force at the humidity. As described above, when the environmental humidity is 60%, since the dielectric constant of air is large, the capacity C itself of the gap between the printing medium 2 and the conveyance belt 1 is large, and a sufficient charge is charged as time passes. If this is done, the attractive force will also increase. On the other hand, when the environmental humidity is 20%, since the dielectric constant of air is small, the charge to be charged is small and the adsorption power is small. Therefore, an applied voltage value necessary to obtain a sufficient adsorption force is set regardless of whether the humidity is low or high. Here, in the fourth delayed printing medium charging pattern signal P ₄ or the fifth delayed printing medium charging pattern signal P _{5 having} a large delay time, that is, a small rate of voltage change between rising and falling at the time of polarity switching, the environmental humidity Is large, the voltage change rate of the applied voltage is small, so that the leakage current is suppressed and the capacitance C of the gap is gradually charged. As a result, the adsorption force of the print medium 2 to the transport belt 1 increases. On the other hand, when the environmental humidity is low, the resistance value of the resistance R _P of the printing medium 2 is large, and the value of the current flowing in the capacity C of the gap between the printing medium 2 and the conveying belt 1 is small. Furthermore, if the voltage change rate of the applied voltage is small, the value of the current flowing through the capacitance C of the gap is suppressed, and the amount of charge charged within a certain period is restricted. small.

On the other hand, in the first delayed printing medium charging pattern signal P ₁ or the second delayed printing medium charging pattern signal P _{2 having} a small delay time, that is, a large rate of voltage change between rising and falling at the time of polarity switching, Like the rectangular wave print medium charging pattern signal, a large current flows when the polarity is switched. For example, when the environmental humidity is 20%, the suction force is large at the times (delay times) t ₁ and t ₂ because the large amount of electric current has a sufficient charge amount for the capacity C of the gap between the print medium 2 and the conveyance belt 1. This is because it is charged. On the contrary, when the environmental humidity is 60%, the adsorption force is small at the times (delay times) t ₁ and t ₂ because the electric charge is generated from the capacitance C of the gap between the printing medium 2 and the conveying belt 1 due to this large current. This is because discharge (leakage) occurs. Therefore, in the first delayed printing medium charging pattern signal P ₁ or the second delayed printing medium charging pattern signal P _{2 having} a small delay time, that is, a large rate of change in the rising and falling voltage when switching the polarity, the environmental humidity is When it is large, the adsorption force of the print medium 2 to the conveyance belt 1 is small, and when the environmental humidity is small, the adsorption force of the print medium 2 to the conveyance belt 1 is large.

Next, a calculation process for setting a print medium charge pattern signal performed by the print medium charge control unit 18 will be described with reference to the flowchart of FIG. This arithmetic processing is executed, for example, before starting printing and immediately after replacing the printing medium. First, in step S1, charging conditions such as the type of printing medium, environmental temperature, and environmental humidity are read.
Next, the process proceeds to step S2, and for example, data of a conveying belt charging pattern signal to the conveying belt charging roller 7 set by the conveying belt charging controller 8 according to the charging condition read in step S1, for example, a charging cycle. Read the length.

Next, the process proceeds to step S3, and the output start timing and polarity switching timing of the printing medium charging pattern signal to the printing medium charging roller 9 are set based on the data of the conveying belt charging pattern signal read in step S2.
In step S4, the surface resistance value of the printing medium 2 is measured by the surface resistance sensor 10.

Next, the process proceeds to step S5, where it is determined whether or not the surface resistance value of the print medium 2 measured in step S4 is equal to or greater than a predetermined value, and when the surface resistance value of the print medium 2 is equal to or greater than the predetermined value. Shifts to step S6, otherwise shifts to step S7.
In step S6, the voltage change rate at the rise and fall when the polarity of the print medium charging pattern signal is switched is set large, and then the process proceeds to step S8.

In step S7, the voltage change rate at the rise and fall when the polarity of the print medium charging pattern signal is switched is set small, and then the process proceeds to step S8.
In step S8, the output start timing and polarity switching timing of the printing medium charging pattern signal to the printing medium charging roller 9 set in step S3 and the polarity switching of the printing medium charging pattern signal set in step S6 or step S7. The print medium charging pattern signal is set using the rising and falling voltage change rates, and then the process returns to the main program.

  According to this calculation process, the data of the conveying belt charging pattern signal to the conveying belt charging roller 7 set by the conveying belt charging control unit 8 is read, and the printing medium charging roller 9 is charged based on the data. When the pattern signal output start timing and polarity switching timing are set, and the surface resistance value of the printing medium 2 measured by the surface resistance sensor 10 is equal to or greater than a predetermined value, the polarity of the printing medium charging pattern signal at the time of polarity switching is set. When the rise and fall voltage change rates are set large and the surface resistance value of the print medium 2 is less than a predetermined value, the rise and fall voltage change rates when the polarity of the print medium charge pattern signal is switched are reduced. The output start timing and polarity switching of the print medium charging pattern signal to the set print medium charging roller 9 is set. For example, when the resistance value of the print medium 2 is large by setting the print medium charge pattern signal using the rising and falling voltage change rates when the polarity of the timing and the print medium charge pattern signal is switched, the print medium 2 When a sufficient charge is charged in the gap C between the belt and the conveyor belt 1 to secure an adsorption force and the resistance value of the print medium 2 is small, the gap C between the print medium 2 and the conveyor belt 1 It is possible to prevent the electric charge from being discharged and to secure the adsorption power.

  As described above, according to the ink jet printer of this embodiment, the conveyance belt charging roller 7 is brought into contact with the surface of the conveyance belt 1 so that the surface of the conveyance belt 1 is alternately reversed in the moving direction of the conveyance belt 1. When the printing medium 2 is charged with an electric potential, and the printing medium 2 is adsorbed and conveyed to the surface of the charged conveying belt 1 by electrostatic force and ink droplets are ejected from the inkjet head 11 to the printing medium 2, the printing medium 2 is printed. The printing medium charging roller 9 that contacts the surface opposite to the conveying belt 1 is disposed, and the printing medium charging roller 9 is applied so that the potential on the surface opposite to the conveying belt 1 of the printing medium 2 is suppressed. For example, the voltage is controlled and the rate of change in the voltage rise and fall of the voltage applied to the print medium charging roller 9 is controlled based on the detected resistance value of the print medium 2. When the resistance value of the print medium 2 is small, the voltage change rate of the rise and fall of the voltage applied to the print medium charging roller 9 is reduced, so that the conveyance belt 1 when the polarity of the voltage applied to the print medium charging roller 9 is switched. The amount of charge leakage from the toner can be reduced, and the electrostatic attraction force of the print medium 2 to the conveyor belt 1 can be ensured.

In addition, when the detected resistance value of the print medium 2 is small, the voltage change rate of the rise and fall of the voltage applied to the print medium charging roller 9 is reduced, so that the voltage applied to the print medium charging roller 9 is reduced. The amount of charge leakage from the conveyance belt 1 when the polarity is switched can be reduced, and the electrostatic attraction force of the print medium 2 to the conveyance belt 1 can be ensured.
In addition, since a constant current is passed through a predetermined interval on the surface of the print medium 2 opposite to the conveying belt 1, the resistance value of the print medium 2 is detected from the voltage value generated at the predetermined interval. If the probes P _P and P _N are brought into contact with the surface of 2 at predetermined intervals to detect the voltage value, the print medium can be simply and surely brought into contact with the surface of the print medium 2 simply by contacting the probes P _P and P _N. A resistance value of 2 can be detected.

In addition, since the resistance value of the print medium 2 is detected before the start of printing and immediately after replacement of the print medium, the electrostatic adsorption force of the print medium 2 to the transport belt 1 can be accurately ensured.
In the above embodiment, only an example in which the ink jet printer of the present invention is applied to a so-called line head type ink jet printer has been described in detail. However, the ink jet printer of the present invention can be applied to all types of ink jet including multi-pass type. Applicable to printers.

1 is a front view illustrating a schematic configuration of a first embodiment of an inkjet printer according to the present invention. It is a longitudinal cross-sectional view which shows an example of the inkjet head of the inkjet printer of FIG. It is a block diagram of the surface resistance sensor of FIG. FIG. 2 is a block diagram of a system control unit of the ink jet printer of FIG. 1. FIG. 2 is a block diagram of a charging waveform generation circuit built in the conveyance belt charging control unit and the printing medium charging control unit in FIG. 1. It is explanatory drawing of the typical equivalent circuit of a conveyance belt, a printing medium, and a printing medium charging roller. It is explanatory drawing of the relationship between environmental humidity and the surface resistance value of a printing medium. It is explanatory drawing of the printing medium charging pattern signal which adjusted the voltage change rate of the rise at the time of polarity switching, and a fall. It is explanatory drawing which shows the time-dependent change of an electric current value when the printing medium charging pattern signal of FIG. 8 is applied. It is explanatory drawing of the printing medium charging pattern signal which changed the voltage change rate of the rise at the time of polarity switching, and a fall. It is explanatory drawing of the adsorption | suction force to the conveyance belt of a printing medium when the printing medium charging pattern signal of FIG. 10 is applied. 3 is a flowchart illustrating a calculation process for setting a print medium charging pattern signal performed in a print medium charging control unit in FIG. 1.

Explanation of symbols

  1 is a conveyance belt, 2 is a printing medium, 3 is a driving roller, 4 is a driven roller, 5 is a tension roller, 6 is a system control unit, 7 is a conveyance belt charging roller, 8 is a conveyance belt charging control unit, and 9 is a printing medium Charging roller, 10 is a print medium detection sensor, 11 is an inkjet head, 12 is an encoder sensor, 13 is a feed roller, 14 is a surface resistance sensor, 15 is a guide, and 34 is a charging waveform generation circuit.

Claims (4)

  A charging roller is brought into contact with the surface of the conveyor belt, and the surface of the conveyor belt is alternately charged to a potential having a polarity opposite to the moving direction of the conveyor belt, and the print medium is electrostatically applied to the surface of the charged conveyor belt. In an ink jet printer that picks up and conveys ink and ejects ink droplets from the ink jet head onto the print medium, printing medium resistance value detecting means for detecting the resistance value of the print medium and the opposite side of the print medium conveyance belt A printing medium charging roller that contacts the surface of the printing medium, and a printing medium surface potential control unit that controls a voltage applied to the printing medium charging roller so as to suppress a potential of the surface of the printing medium opposite to the conveyance belt. The print medium surface potential control means sets the voltage applied to the print medium charging roller based on the resistance value of the print medium detected by the print medium resistance value detection means. Inkjet printer and controlling the rate of change of voltage rising and falling.   The print medium surface potential control means reduces the rate of voltage change at the rise and fall of the voltage applied to the print medium charging roller when the resistance value of the print medium detected by the print medium resistance value detection means is small. The inkjet printer according to claim 1.   The printing medium resistance value detecting means applies a constant current to a predetermined interval on the surface of the printing medium opposite to the conveyance belt, and detects a resistance value of the printing medium from a voltage value generated at the predetermined interval at that time. The ink jet printer according to claim 1, wherein the ink jet printer is an ink jet printer.   4. The ink jet printer according to claim 1, wherein the print medium resistance value detection unit detects a resistance value of the print medium before starting printing and immediately after replacing the print medium. 5.

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