JP2008068594A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem wherein a droplet discharging failure is generated because mist flows back and adheres to the droplet discharging surface of a recording head when an image is formed by the droplet discharging while a medium is electrostatically atracted and fed. <P>SOLUTION: The image forming apparatus is constituted of: a nozzle plate 103 and a diaphragm 102, each formed of a polyimide; a flow path plate 101 formed of DFR; a piezoelectric element 121 bonded to the rear surface of the diaphragm 102; a supporting substrate 122 formed of PTFE supporting the piezoelectric element 121; a resin frame member 130; and an FPC cable 126 connected to the piezoelectric element 121. Thus, the droplet discharging surface is made up of an insulating body, and its outer peripheral part is also made up of an insulating body. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus including a recording head that discharges droplets and a transport unit that transports a recording medium with electrostatic attraction force.

  As an image forming apparatus such as a printer, a facsimile machine, a copying machine, or a multifunction machine of these, for example, a liquid (e.g., a liquid ejecting apparatus) including a recording head composed of a liquid ejecting head for ejecting liquid droplets of a recording liquid (liquid) is used. Hereinafter, although it is also referred to as “paper”, the material is not limited, and a recording medium as a liquid (hereinafter, referred to as “recording medium”, “recording medium”, “transfer material”, “recording paper” and the like is also used synonymously). Some of them perform image formation (recording, printing, printing, and printing are also used synonymously) by attaching the ink to the paper.

  The image forming apparatus means an apparatus for forming an image by discharging a liquid onto a medium such as paper, thread, fiber, fabric, leather, metal, plastic, glass, wood, ceramics, etc. The term “not only” means not only giving an image having a meaning such as a character or a figure to a medium but also giving an image having no meaning such as a pattern to the medium. Further, the liquid is not limited to the recording liquid and ink, and is not particularly limited as long as it is a liquid capable of forming an image. Further, the liquid ejection apparatus means an apparatus that ejects liquid from a liquid ejection head, and is not limited to an apparatus that forms an image.

In an image forming apparatus using such a liquid ejecting apparatus, it is necessary to improve the landing position accuracy of a droplet on a sheet in order to improve image quality. In addition, the transport belt that transports the paper is charged uniformly and positively, and the paper is attracted by the electrostatic force to keep the distance between the recording head and the paper constant, and the paper feed is accurately controlled. It is known that the paper is prevented from being misaligned and the paper is prevented from being lifted to prevent jamming and dirt due to the contact between the paper and the recording head.
JP-A-4-201469 JP-A-9-254460

  However, when the transport belt is uniformly charged positively and the paper is attracted by the suction force, the droplet ejected from the recording head is affected by the electric field, and the landing position of the droplet on the paper is not shifted. It is known that liquid mist flows back to the recording head side as it occurs.

In order to prevent the deviation or backflow of the landing positions of the liquid droplets, as disclosed in Patent Document 3, the upstream side of the conveyance direction of the recording head is applied to the conveyance belt charged with the same charge on the surface. By applying a charge of the opposite polarity to the transport belt to the surface of the paper, the potential of the paper surface is weakened so that the ejected ink droplets are not affected by the electric field, and from the surface of the paper to the transport belt surface. It is also known that the paper and the transport belt are further attracted by the attracting force by weakening the potential of the same polarity.
Japanese Patent No. 3224528

Further, as described in Patent Document 4, by using a conveyance belt in which a conductive pattern for generating an electrostatic adsorption force is embedded, a voltage applied to the conductive pattern of the conveyance belt is controlled. Some prevent the generation of an electric field between the head and the paper.
Japanese Patent No. 3604894

Further, as a conventional liquid discharge head, in Patent Document 5, a piezoelectric element is attached to the upper surface of the nozzle plate and a position facing the pressurizing chamber through a diaphragm, and the piezoelectric of the cover lay of the flexible printed cable. A notch portion having a larger area than the electrode surface of the piezoelectric element is provided at a position to face the element, and a terminal portion which is a part of the conductive pattern is located in the notch portion, and the terminal portion is smaller than the electrode surface. When the conductive adhesive is dropped on the electrode surface and the notch is pressed onto the electrode surface, the electrode surface and the terminal part are in direct contact within the notch, and the adhesive surrounds the area. The liquid discharge head is described in which the conductive efficiency is increased and the usage fee of the adhesive can be reduced by joining together, and the cause of the short circuit is eliminated because the adhesive does not flow out to the surface of the coverlay.
JP-A-8-156252

Patent Document 6 describes a head in which a water repellent film made of an inorganic compound having a dielectric constant lower than that of a nozzle plate body is provided at least around the nozzle opening on the surface opposite to the flow path forming substrate of the nozzle plate body. .
Japanese Patent Laid-Open No. 2003-237086

In Patent Document 7, the ink jet head is provided with a nozzle cover made of a conductive member that covers the peripheral portion of the nozzle forming member and the side surface of the head, and has a charge eliminating means for electrically grounding the nozzle cover of the ink jet head in a non-printing area. An apparatus for providing is described.
JP-A-10-016240

    As described above, when the paper is attracted and held by the electrostatic force, an electric field is generated between the surface of the paper and the recording head, and the droplets ejected from the recording head are polarized and fly by the influence of the electric field. There is a problem in that good recording cannot be performed due to the disturbance, and ink mist generated by the flight flows back and adheres to the vicinity of the head discharge portion as a result of polarization.

  To solve this problem, as disclosed in Japanese Patent Laid-Open No. 2004-228561, a charge having the opposite polarity to the conveyance belt is applied to the conveyance belt, the surface of which is charged with the same charge, upstream of the conveyance direction of the recording head. The mist backflow cannot be prevented only by the configuration applied to the surface. Further, as disclosed in Patent Document 4, in the configuration using the conveyance belt having the conductive pattern embedded therein, there is a problem that the configuration of the belt becomes complicated and the voltage application control for the conductive pattern becomes complicated.

  SUMMARY An advantage of some aspects of the invention is that it provides an image forming apparatus capable of stably forming a high-quality image while ensuring stable conveyance performance with a simple configuration. .

  In order to solve the above problems, an image forming apparatus according to the present invention includes a recording head that discharges droplets onto a recording medium, and a transport unit that electrostatically attracts and transports the recording medium. The droplet discharge surface is made of an insulator, and has a structure that does not have a conductor in the outer peripheral portion of the recording head and facing the recording medium via the insulator.

  Here, the recording head can be configured such that portions other than the transmission means for transmitting the drive signal are made of an insulator. In addition, the recording head has a plurality of layers made of different main constituent materials in a direction perpendicular to the droplet discharge surface, and the dielectric constant of the main constituent material of the layer closest to the droplet discharge surface is the other main configuration. The structure can be lower than the dielectric constant of the substance. Further, it is possible to employ a configuration in which a mechanism for wiping the droplet discharge surface of the recording head with a wiper member made of a grounded conductive material is provided.

  According to the image forming apparatus of the present invention, the droplet discharge surface of the recording head is made of an insulator, and the conductor is disposed on the outer peripheral portion of the recording head and facing the recording medium via the insulator. Since the configuration does not include, the backflow of the mist with respect to the droplet discharge surface of the recording head is reduced, and a high-quality image can be stably formed with a simple configuration while ensuring a stable conveyance performance.

Embodiments of the present invention will be described below with reference to the accompanying drawings. First, an example of an image forming apparatus according to the present invention will be described with reference to FIGS. FIG. 1 is an explanatory side view for explaining the overall configuration of the image forming apparatus, and FIG. 2 is an explanatory plan view of a main part of the apparatus.
In this image forming apparatus, a carriage 3 is slidably held in a main scanning direction by a guide rod 1 and a guide rail 2 which are guide members horizontally mounted on left and right side plates (not shown), and a driving pulley 6A is driven by a main scanning motor 4. And the driven pulley 6B are moved and scanned in the direction indicated by the arrow (main scanning direction) via a timing belt 5 stretched between them.

  The carriage 3 includes, for example, four recording heads 7y, 7c, 7m, which are liquid ejection heads that eject ink droplets of yellow (Y), cyan (C), magenta (M), and black (K), respectively. A plurality of ink ejection openings are arranged in a direction crossing the main scanning direction, and the ink droplet ejection direction is directed downward.

  The liquid discharge head constituting the recording head 7 includes a piezoelectric actuator such as a piezoelectric element, a thermal actuator that uses a phase change caused by liquid film boiling using an electrothermal transducer such as a heating resistor, and a metal phase change caused by a temperature change. A shape memory alloy actuator using an electrostatic force, an electrostatic actuator using an electrostatic force, and the like provided as pressure generating means for generating a pressure for discharging a droplet can be used. In addition, the configuration is not limited to an independent head for each color, and may be configured with one or a plurality of liquid ejection heads having a nozzle row composed of a plurality of nozzles that eject droplets of a plurality of colors.

  The carriage 3 is also equipped with sub-tanks 8 for each color for supplying ink of each color to the recording head 7. Ink is supplied to the sub tank 8 from a main tank (ink cartridge) (not shown) via an ink supply tube 9.

  On the other hand, as a paper feeding unit for feeding paper 12 stacked on a paper stacking unit (pressure plate) 11 such as a paper feeding cassette 10, a half-moon roller (for separating and feeding the paper 12 one by one from the paper stacking unit 11) A separation pad 14 made of a material having a large friction coefficient is provided facing the sheet feeding roller 13 and the sheet feeding roller 13, and the separation pad 14 is urged toward the sheet feeding roller 13 side.

  In order to convey the sheet 12 fed from the sheet feeding unit below the recording head 7, a conveyance belt 21 for electrostatically attracting and conveying the sheet 12 and a guide 15 from the sheet feeding unit. A counter roller 22 for transporting the sheet 12 fed between the conveyor belt 21 and the conveyor belt 21, and for shifting the sheet 12 fed substantially vertically upward by approximately 90 ° to follow the conveyor belt 21. A conveyance guide 23 and a pressing roller 25 urged toward the conveyance belt 21 by a pressing member 24 are provided. In addition, a charging roller 26 as a charging unit for charging the surface of the transport belt 21 is provided.

  Here, the conveyance belt 21 is an endless belt, is stretched between the conveyance roller 27 and the tension roller 28, and the conveyance roller 27 rotates from the sub-scanning motor 31 via the timing belt 32 and the timing roller 33. By doing so, it is configured to circulate in the belt conveyance direction (sub-scanning direction) of FIG. A guide member 29 is disposed on the back side of the conveying belt 21 corresponding to the image forming area by the recording head 7. The charging roller 26 is disposed so as to come into contact with the surface layer of the transport belt 21 and rotate following the rotation of the transport belt 21.

  The transport belt 21 may be a single-layer belt as shown in FIG. 3 or a multi-layer (two or more layers) belt as shown in FIG. In the case of the conveyance belt 21 having a one-layer structure, the entire layer is formed of an insulating material because it contacts the paper 12 and the charging roller 26. In the case of the transport belt 21 having a multilayer structure, the side that contacts the paper 12 and the charging roller 26 is formed of an insulating layer 21A, and the side that does not contact the paper 12 and the charging roller 26 is formed of a conductive layer 21B. Is preferred.

Examples of the insulating material for forming the single-layered transport belt 21 and the insulating material for forming the insulating layer 21A of the multi-layered transport belt 21 include resins or elastomers such as PET, PEI, PVDF, PC, ETFE, and PTFE. It is preferable that the material does not contain a conductivity control material, and the volume resistivity is 10 ¹² Ωcm or more, preferably 10 ¹⁵ Ωcm. In addition, as a material for forming the conductive layer conductive layer 21B of the transport belt 21 having a multilayer structure, the resin or elastomer is made to contain carbon so that the volume resistivity is 10 ⁵ to 10 ⁷ Ωcm. preferable.

  Here, an encoder wheel 34 is attached to the shaft of the conveyance roller 27, and an encoder sensor 35 that detects a slit of the encoder wheel 34 is provided. The amount of movement of the conveyance belt 21 by the encoder wheel 34 and the encoder sensor 35, A rotary encoder 36 for detecting the moving speed is configured. Further, a linear encoder 41 for detecting the movement amount and movement speed of the carriage 3 is constituted by an encoder scale 42 arranged along the movement direction of the carriage 3 and an encoder sensor 43 comprising a photo sensor provided on the carriage 3. .

  Further, as a paper discharge unit for discharging the paper 12 recorded by the recording head 7, a separation claw 51 for separating the paper 12 from the transport belt 21, a paper discharge roller 52 and a paper discharge roller 53, and a discharge And a paper discharge tray 54 for stocking the paper 12 to be printed.

  A double-sided paper feeding unit 61 is detachably mounted on the back. The double-sided paper feeding unit 61 takes in the paper 12 returned by the reverse rotation of the transport belt 21, reverses it, and feeds it again between the counter roller 22 and the transport belt 21.

  Further, a maintenance / recovery mechanism 56 for maintaining and recovering the nozzle state of the recording head 7 is disposed in the non-printing area on one side of the carriage 3 in the scanning direction. The maintenance / recovery machine 56 includes a cap 57 for capping each nozzle surface of the recording head 7, a wiper blade 58 which is a wiper member (blade member) for wiping the nozzle surface, and a thickened recording liquid. An empty discharge receptacle 59 for receiving droplets when performing empty discharge for discharging droplets that do not contribute to recording in order to be discharged is provided.

  In the image forming apparatus configured as described above, the sheets 12 are separated and fed one by one from the sheet feeding unit, and the sheet 12 fed substantially vertically upward is guided by the guide 15, and includes the transport belt 21 and the counter roller 22. The leading end is guided by the conveying guide 23 and pressed against the conveying belt 21 by the pressing roller 25, and the conveying direction is changed by approximately 90 °.

  At this time, an alternating voltage that alternately repeats positive and negative is applied to the charging roller 26 from an AC bias supply unit, which will be described later, and the charging voltage pattern that alternates the conveying belt 21, that is, in the sub-scanning direction that is the circumferential direction, is added. It is charged with a pattern in which negative and negative are alternately repeated with a predetermined width. When the paper 12 is fed onto the charged transport belt 21, the paper 12 is attracted to the transport belt 21 by electrostatic force, and the paper 12 is transported in the sub-scanning direction by the circular movement of the transport belt 21.

  Therefore, by driving the recording head 7 according to the image signal while moving the carriage 3 in the forward and backward directions, ink droplets are ejected onto the stopped paper 12 to record one line. After transporting a predetermined amount, the next line is recorded. Upon receiving a recording end signal or a signal that the trailing edge of the paper 12 has reached the recording area, the recording operation is finished and the paper 12 is discharged onto the paper discharge tray 54.

  In the case of double-sided printing, when recording on the front surface (surface to be printed first) is completed, the recording belt 12 is fed into the double-sided paper feeding unit 61 by rotating the conveyor belt 21 in the reverse direction. The paper 12 is reversed (with the back surface being the printing surface), fed again between the counter roller 22 and the transport belt 21, controlled in timing, and transported onto the transport bell 21 as described above. After recording on the back side, the sheet is discharged onto the discharge tray 54.

  During printing (recording) standby, the carriage 3 is moved to the maintenance / recovery mechanism 56 side, the nozzle surface of the recording head 7 is capped by the cap 57, and the nozzles are kept in a wet state. To prevent. In addition, the recording liquid is sucked from the nozzle in a state where the recording head 7 is capped by the cap 57, and a recovery operation is performed to discharge the thickened recording liquid and bubbles, and the ink adhered to the nozzle surface of the recording head 7 by this recovery operation. Wiping is performed with a wiper blade 58 in order to remove the cleaning. In addition, an idle ejection operation for ejecting ink not related to recording is performed before the start of recording or during recording. Thereby, the stable ejection performance of the recording head 7 is maintained.

  Next, an example of the liquid discharge head constituting the recording head 7 will be described with reference to FIGS. 5 is a cross-sectional explanatory view along the longitudinal direction of the liquid chamber of the head, and FIG. 6 is a cross-sectional explanatory view of the head along the lateral direction of the liquid chamber (nozzle arrangement direction).

  The liquid discharge head is formed by bonding and laminating a flow path plate 101, a vibration plate 102 bonded to the lower surface of the flow path plate 101, and a nozzle plate 103 bonded to the upper surface of the flow path plate 101. A nozzle communication path 105 that is a flow path through which a nozzle 104 that discharges droplets (ink droplets) communicates, a liquid chamber 106 that is a pressure generation chamber, and an ink for supplying ink to the liquid chamber 106 through a fluid resistance portion (supply path) 107. An ink supply port 109 and the like communicating with the common liquid chamber 108 are formed.

  In addition, two rows (only one row is shown in FIG. 6) of stacked piezoelectric elements as electromechanical conversion elements that are pressure generating means (actuator means) for pressurizing ink in the liquid chamber 106 by deforming the diaphragm 102. An element 121 and a base substrate 122 to which the piezoelectric element 121 is bonded and fixed are provided. Note that a column portion 123 is provided between the piezoelectric elements 121. This support portion 123 is a portion formed simultaneously with the piezoelectric element 121 by dividing and processing the piezoelectric element member. However, since the drive voltage is not applied, the support portion 123 becomes a simple support. The piezoelectric element 121 is connected with an FPC cable 126 as a transmission means on which a drive circuit (drive IC) (not shown) for supplying a drive signal is mounted.

  The peripheral edge of the diaphragm 102 is joined to a frame member 130, and the frame member 130 serves as a through-hole 131 and a common liquid chamber 108 that house an actuator unit composed of the piezoelectric element 121 and the base substrate 122. A recess and an ink supply hole 132 for supplying ink from the outside to the common liquid chamber 108 are formed. The frame member 130 is formed by injection molding with a thermosetting resin such as an epoxy resin or polyphenylene sulfite, for example.

  The nozzle plate 103 forms a nozzle 104 having a diameter of 10 to 30 μm corresponding to each liquid chamber 106 and is bonded to the flow path plate 101 with an adhesive. A water repellent layer is formed on the nozzle plate 103 on the droplet discharge surface 7a side.

  In this embodiment, the ink in the liquid chamber 106 is pressurized using the displacement in the d33 direction as the piezoelectric direction of the piezoelectric element 121. However, the pressure in the d31 direction is used as the piezoelectric direction of the piezoelectric element 121. The ink in the liquid chamber 106 may be pressurized. Alternatively, a structure in which one row of piezoelectric elements 121 is provided on one substrate 122 may be employed.

  In the liquid discharge head having such a configuration, for example, by lowering the voltage applied to the piezoelectric element 121 from the reference potential, the piezoelectric element 121 contracts, and the diaphragm 102 descends to expand the volume of the liquid chamber 106. Then, the ink flows into the liquid chamber 106, and then the voltage applied to the piezoelectric element 121 is increased to extend the piezoelectric element 121 in the stacking direction, and the diaphragm 102 is deformed in the direction of the nozzle 104, so that the volume / volume of the liquid chamber 106 is increased. By contracting the volume, the recording liquid in the liquid chamber 106 is pressurized, and droplets of the recording liquid are ejected (jetted) from the nozzle 104.

  Then, by returning the voltage applied to the piezoelectric element 121 to the reference potential, the diaphragm 102 is restored to the initial position, and the liquid chamber 106 expands to generate a negative pressure. The recording liquid is filled in 106. Therefore, after the vibration of the meniscus surface of the nozzle 104 is attenuated and stabilized, the operation proceeds to the next droplet discharge.

  Note that the driving method of the head is not limited to the above example (pulling-pushing), and it is also possible to perform striking or pushing depending on the direction to which the driving waveform is given.

Next, an outline of the control unit of the image forming apparatus will be described with reference to the block diagram of FIG.
In the control unit 200, the CPU 211 that controls the entire apparatus, the ROM 202 that stores programs executed by the CPU 211 and other fixed data, the RAM 203 that temporarily stores image data and the like, and the power supply of the apparatus are cut off. A rewritable non-volatile memory 204 for holding data in between, an image processing for performing various signal processing and rearrangement on image data, and an ASIC 205 for processing input / output signals for controlling the entire apparatus. ing.

  The control unit 200 also includes an I / F 206 for transmitting and receiving data and signals to and from the host, data transfer means for driving and controlling the recording head 7, and drive waveform generation means for generating a drive waveform. A print control unit 207; a head driver (driver IC) 208 for driving the recording head 7 provided on the carriage 3 side; a motor driving unit 210 for driving the main scanning motor 4 and the sub-scanning motor 31; An AC bias supply unit 212 that supplies an AC bias to the roller 34, I / O 213 for inputting detection signals from encoder sensors 43 and 35, detection signals from various sensors such as a temperature sensor that detects environmental temperature, and the like It has. The control unit 200 is connected to an operation panel 214 for inputting and displaying information necessary for the apparatus.

  The control unit 200 also detects a speed detection value and a position detection value obtained by sampling a detection pulse from the encoder sensor 43 constituting the linear encoder, and a speed target value and a position target obtained from a previously stored speed / position profile. The drive output value (control value) for the main scanning motor 4 is calculated based on the value and the main scanning motor 4 is driven via the motor driving unit 210. Similarly, based on the speed detection value and position detection value obtained by sampling the detection pulse from the encoder sensor 35 constituting the rotary encoder, and the speed target value and position target value obtained from the previously stored speed / position profile. Then, a drive output value (control value) for the sub-scanning motor 31 is calculated, and the sub-scanning motor 31 is driven via the motor driver 210 and the motor driver.

  Here, the control unit 200 transmits image data or the like from an image processing program including an information processing apparatus such as a personal computer, an image reading apparatus such as an image scanner, and a printer driver on the host side such as an imaging apparatus such as a digital camera. The data is received by the I / F 206 via the net.

  The CPU 201 of the control unit 200 reads and analyzes the print data in the reception buffer included in the I / F 206, performs necessary image processing, data rearrangement processing, and the like in the ASIC 205, and prints the image data. The data is transferred from the unit 207 to the head driver 208.

  The print control unit 207 transfers the above-described image data to the head driver 208 as serial data, and transmits a transfer clock, a latch signal, a droplet control signal (mask signal), and the like necessary for the transfer of the image data and confirmation of the transfer. In addition to the output to the head driver 208, a drive waveform generator and a head driver composed of a D / A converter, a voltage amplifier, a current amplifier, etc. for D / A conversion of drive signal pattern data stored in the ROM Drive waveform selection means for generating a drive waveform composed of one drive pulse (drive signal) or a plurality of drive pulses (drive signal) and outputting the drive waveform to the head driver 208.

  The head driver 208 selectively selects droplets of the recording head 7 based on image data corresponding to one line of the recording head 7 input serially, and forms a driving signal provided from the print control unit 207. The recording head 7 is driven by applying it to a driving element (for example, a piezoelectric element as described above) that generates energy to be discharged. At this time, by selecting a driving pulse constituting the driving waveform, for example, dots having different sizes such as large droplets (large dots), medium droplets (medium dots), and small droplets (small dots) can be distinguished. it can.

Next, charging control for the conveying belt 21 in the image forming apparatus will be described with reference to FIG.
As described above, the amount of rotation is detected by the encoder 36 provided at the end of the conveying roller 27 that drives the conveying belt 21, and the sub-scanning motor 31 is driven and controlled from the control unit 200 in accordance with the detected amount of rotation. The output of an AC bias supply unit (high voltage power source) 212 that applies a high voltage (AC bias) to the charging roller 26 is controlled.

  The AC bias supply unit 212 controls the cycle (application time) of the positive and negative voltages applied to the charging roller 26, and at the same time, the control unit 200 controls the driving of the conveyance belt 21, thereby The positive and negative charges can be applied to the electrode with a predetermined charging cycle length. Here, the “charging cycle length” is a width (distance) in the transport direction per cycle of the applied voltage of the positive and negative electrodes as shown in FIG.

  Here, as described above, when printing is started, the transport roller 27 is rotationally driven by the sub-scanning motor 31 to rotate the transport belt 21 clockwise in FIG. 1, and at the same time from the AC bias supply unit 212 to the charging roller 26. A square wave of positive and negative electrodes is applied to. As a result, the charging roller 26 is in contact with the insulating layer 21A of the conveyor belt 21, so that positive and negative charges are transferred to the insulating belt 21A of the conveyor belt 21 as shown in FIG. Are applied alternately in the transport direction (band-shaped positive charge areas 301 and negative charge areas 302 are alternately formed), and an unequal electric field is generated on the transport belt 21 as shown in FIG. The

As described above, the insulating layer 21A of the conveying belt 21 to which the positive and negative charges are applied is formed so that the volume resistivity is 10 ¹² Ωcm or more, preferably 10 ¹⁵ Ωcm. The charged positive and negative charges can be prevented from moving at the boundary, and the positive and negative charges applied to the insulating layer 21A can be held.

  Here, when the paper 12 is fed to the conveyor belt 21 where an unequal electric field is generated by the formation of positive and negative charges on the insulating layer 21A, the paper 12 is instantly polarized along the direction of the electric field. To do. As shown in FIG. 10, due to the unequal electric field, the charge that forms an attractive force with the conveyance belt on the conveyance belt side of the sheet becomes dense, and the charge that forms a repulsive force with the conveyance belt 21 that appears on the opposite sheet surface becomes sparse. Due to this charge difference, the sheet 12 is instantaneously adsorbed to the transport belt 21. Further, since the sheet 12 has a finite resistance, at the same time, a true charge is induced on the adsorption surface of the sheet 12 and the opposite side.

The positive and negative true charges induced on the suction surface side of the paper 12 stably attracts by attracting the charges applied on the conveyor belt 21, but the positive and negative true charges induced on the opposite side of the sheet 12. The charge is unstable. Since the sheet 12 has a finite resistance value of 10 ⁷ Ω / □ to 10 ¹³ Ω / □, the true charge induced on the surface opposite to the suction surface side of the sheet 12 is charged. The charge of adjacent positive and negative electrodes attracts and moves as time passes, and decreases while being neutralized.

  As a result, the charge on the conveyor belt 21 balances with the true charge induced on the suction surface side of the paper 12 and the electric field is closed, and the true charge induced on the opposite side of the suction surface of the paper 12 is medium as described above. It is summed to close the electric field. That is, the electric field toward the recording head 7 decreases. In addition, since the charge applied to the surface of the transport belt 21 and the charge of the transport belt 21 and the charge forming a weak force decrease from the surface of the paper 12, the adsorption force of the paper 12 to the transport belt 21 increases with time.

  Here, the amount of decrease in the surface potential on the surface of the paper 12 and the time until the charge disappears vary depending on the resistance value of the paper 12 and the charging cycle length, and the higher the resistance of the paper 12, the more the surface of the paper 12 (as opposed to the conveyance belt). The amount of charge induced on the side surface) per unit time is reduced, and it takes time for the surface charge to be neutralized. Further, the longer the charging cycle length, the farther the induced positive and negative charges are separated from each other, so that the substantial resistance when the charges move increases. Furthermore, since the potential acting between the positive and negative charges also decreases in inverse proportion to the distance, it similarly takes time for the surface charges to be neutralized.

  Therefore, if the resistance value of the paper 12 is the same and the charge amount per unit area applied on the transport belt 21 is the same, the charge disappearance time on the paper surface (surface opposite to the transport belt) is , Which is proportional to the square of the charging cycle length.

  Next, the behavior of the liquid droplet when there is an electric charge on the paper when the liquid droplet is ejected from the recording head 7 onto the paper 12 adsorbed on the conveyance belt 21 will be described with reference to FIG.

  As shown in FIG. 10A, the droplet 401A ejected from the recording head 7 104 is affected by the electric field generated by the surface potential on the paper 12 adsorbed to the transport belt 21, and is shown in FIG. ), A true charge is induced in the droplet 401A, and the ink droplet 401A is split into a main droplet 402A and a mist (follower droplet) 403A. At this time, as shown in FIG. 6C, the mist 403A is often charged with the same polarity as the paper 12, and thus repels the charge on the paper 12 having the same polarity, and the mist 403A shown in FIG. As shown, the liquid flows backward to the droplet discharge surface 7 a side of the recording head 7 and adheres to the vicinity of the droplet discharge surface 7 a of the recording head 7.

  As described above, when the mist adheres to the droplet discharge surface 7a of the recording head 7, the water repellency of the droplet discharge surface 7a is impaired, resulting in nozzle down (discharge failure such as inability to discharge due to clogging or bending in the discharge direction). It tends to occur. When nozzle down occurs, it appears as a white streak on the printed image, and the image quality is significantly impaired.

Therefore, in the present invention, the structure itself of the liquid discharge head is configured to reduce the backflow of mist.
First, the structure / material for suppressing the electric field strength between the head and the medium was examined by electrostatic field analysis simulation. As a calculation condition, the model structure of the head is configured to include an actuator unit 501, a support substrate unit 502, and an electrode unit 503 as shown in FIG. The actuator unit 501 is a part that performs electric-pressure conversion by a piezoelectric element, an electrothermal conversion element, or the like, and has a nozzle surface 504 on which a discharge port (nozzle) is formed. The support substrate portion 502 is a portion that supports or fixes the actuator portion 501. The electrode part 503 is a part that transmits a drive signal to the actuator part 501.

  Further, it is assumed that the head 500 is mounted on a carriage 505, and the head 500 is opposed to a medium 507 adsorbed by an electrostatic force to a conveyance belt 506 charged with positive and negative electrodes at a constant cycle.

  When simulation was performed under the above conditions, it was first found that there was a great difference in the electric field strength between the head and the medium when the head 500 was made of an insulator and when it was made of a conductor. However, only the electrode portion 503 for transmitting the drive signal must be a conductor in any case.

Here, the head configuration is as follows.
Head A: When all the constituent members of the head 500 are made of a conductor.
Head B: When the constituent members excluding the electrode portion 503 of the head 500 are made of an insulator, and the electrode portion 500 is 90% of the total area of the nozzle surface 504 (FIG. 13).
Head C: When the constituent members excluding the electrode portion 503 of the head 500 are made of an insulator, and the electrode portion 500 is 80% of the total area of the nozzle surface 504 (FIG. 14).
Head D: When the constituent members excluding the electrode portion 503 of the head 500 are made of an insulator, and the electrode portion 500 is 100% of the total area of the nozzle surface 504. Note that the electrode portion 503 is a nozzle other than 100%. It is assumed that it is arranged at the center of the surface 504.

  FIG. 15 shows the absolute value of the electric field intensity near the head surface. The head position on the horizontal axis corresponds to between P1 and P2 in FIG. The electric field strength of the head A is indicated by a solid line A, the electric field strength of the head B is indicated by a broken line B, the electric field strength of the head C is indicated by a one-dot chain line C, and the electric field strength of the head D is indicated by a two-dot chain line D.

From this result, all the constituent members of the head 500 are made of a conductor, the head A in which the droplet discharge surface is made of a conductor, and the insulator that forms the droplet discharge surface including the outer periphery of the head. The head D, which is configured by disposing an electrode portion serving as a conductor in a portion facing the recording medium via the head, is configured such that the electric field strength between the head and the medium forms a droplet discharge surface on the outer periphery of the head. It can be seen that the electric field strength is relatively higher than that of the heads B and C in which there is no electrode portion serving as a conductor in a portion facing the recording medium via the insulator.

  In other words, if the droplet discharge surface is made of an insulator and the ratio of the electrode portions to the nozzle surface (droplet discharge surface) is lowered, there is an effect of suppressing the electric field strength between the head and the medium. I understand. In other words, it can be seen that a structure in which an electrode made of a conductor is not disposed at the end of the nozzle surface is preferable because it suppresses charging mist (back flow of mist).

  In this manner, the droplet discharge surface of the recording head is made of an insulator, and the outer periphery of the recording head is configured to have no conductor at a portion facing the recording medium via the insulator. In addition, it is possible to suppress the backflow of mist and improve the image quality.

Next, the main constituent materials in the head having a structure having a plurality of layers in the direction perpendicular to the droplet discharge surface as in the head configuration described above will be described.
Here, the investigation was conducted by paying attention to the dielectric constant of the main constituent members. The result is shown in FIG. When the dielectric constant of the member on the nozzle surface 504 side from the electrode portion 503 is εb and the dielectric constant of the member on the opposite side to the nozzle surface 504 is εa, the electric field strength at each position of the head is a broken line when εa = εb. In the case of E and εa <εb, the solid line F, and in the case of εa> εb, the dot-and-dash line G indicates.

  That is, if a material having a low dielectric constant is used for a component on the nozzle surface 504 side of the electrode portion 503 and a material having a higher dielectric constant is used for a component on the opposite side of the nozzle surface 504 than the electrode portion 503, the head- It was found that there is an effect of suppressing the electric field strength between the recording media. It can be said that the use of a so-called thermal type head using an electrothermal conversion element as pressure conversion means is suitable.

  Examples of members to be used include fluororesins such as PTFE (polytetrafluoroethylene) and ETFE (tetrafluoroethylene and ethylene copolymer), which have a certain degree of hardness and are easy to process, For the members such as the chamber member and the nozzle plate, PE (polyethylene) which is a material having a relatively low dielectric constant in addition to the above characteristics is suitable.

  Thus, when having a plurality of layers made of different main constituent materials in the direction perpendicular to the droplet discharge surface, the dielectric constant of the main constituent material of the layer closest to the droplet discharge surface is the dielectric constant of the other main constituent materials. By making the configuration lower than the ratio, the electric field strength between the head and the recording medium can be suppressed.

  Further, when the head is configured with an insulator as described above, static electricity due to movement of the carriage or the like may occur on the head surface. Such static electricity may also cause a minute electric field near the nozzle surface and cause mist adhesion to the nozzle surface. By wiping the nozzle surface with a wiper member made of a conductive material, wiping can be performed at the same time as discharging of the nozzle surface.

Here, an example of a thermal head in which the energy generating means (pressure generating means) for discharging droplets is an electrothermal conversion element will be described with reference to FIG.
In this liquid discharge head, a flow path plate 604 constituting a side wall of a flow path (liquid chamber) 603 is laminated on a support substrate (actuator substrate) 602 having an electrothermal conversion element 601, and a nozzle is formed on the flow path plate 604. A nozzle plate 606 on which 605 is formed is laminated. In this head, by supplying a drive signal to the electrothermal transducer 601, bubbles are generated in the ink due to film boiling, the ink in the flow path 603 is pressurized, and the droplets are ejected from the nozzles 605.

Specific examples will be described below.
(Example 1)
The support substrate 122 was made of PTFE (polytetrafluoroethylene), and the laminated piezoelectric element 121 as a pressure converting means was joined to the support substrate 121 with an anaerobic adhesive. Next, the piezoelectric element was grooved using a dicing saw. An FPC cable 126 as a current-carrying member was joined to the side surface of the grooved piezoelectric element using solder to form an actuator portion. Next, the nozzle plate 103 made of polyimide, the vibration plate 102, and the flow path plate 101 formed of DFR were positioned and laminated with high accuracy, and the respective interfaces were joined with an epoxy adhesive. The upper surface of the piezoelectric element 121 of the actuator part manufactured previously is joined to the back surface of the vibration plate 102 with an epoxy adhesive, and finally the resin frame member 130 is anaerobic on the back surface of the vibration plate 102 and the end surface of the support substrate 122. Bonding with an adhesive produced a piezo ink jet head.

  When this ink jet head was mounted on a printer and a printing durability test was conducted using an in-house standard chart, 100 sheets could be continuously printed without cleaning the nozzle surface.

  For comparison, when an ink jet head using SUS430 as a support substrate was manufactured and similarly mounted on a printer and a print test was performed, nozzle down occurred at about the 60th sheet.

  Therefore, in this case, it can be said that the nozzle down due to the ink mist can be suppressed by configuring the support substrate with an insulator.

(Example 2)
The support substrate 602 was made of a fluororesin, and an electrothermal conversion element 601 was provided as pressure conversion means, and an FPC was joined as an energizing member to the side surface of the electrothermal conversion element 601 using solder to form an actuator portion. Next, the nozzle plate 606 and the flow path plate 604 were formed of PE (polyethylene), and the respective interfaces were joined with an epoxy adhesive. The flow path plate 604 and the support substrate 602 of the actuator unit were bonded with an anaerobic adhesive to produce a thermal ink jet head.

  When this ink jet head was mounted on a printer and a print durability test was conducted using an in-house standard chart, 80 sheets could be continuously printed without cleaning the nozzle surface.

  For comparison, when an ink jet head having a nozzle plate formed of polyimide and a flow path plate formed of DFR was manufactured and similarly mounted on a printer and a print test was performed, the nozzle down occurred at about the 40th sheet.

  Therefore, in this case, it can be said that the nozzle down due to the ink mist can be suppressed by forming the nozzle surface side of the electrode with a low dielectric constant material.

  In this way, the droplet discharge surface of the recording head is made of an insulator, and the outer periphery of the recording head is configured so as not to have a conductor at a portion facing the recording medium via the insulator. Therefore, mist adhesion can be reduced, stable droplet ejection can be performed, and image quality can be improved. In this case, since the recording head is composed of an insulator other than the transmission means for transmitting the drive signal, mist adhesion can be reduced with a simple configuration.

  Further, as described above, the recording head has a plurality of layers made of different main constituent materials in a direction perpendicular to the droplet discharge surface, and the dielectric constant of the main constituent material of the layer closest to the droplet discharge surface is By making the structure lower than the dielectric constant of the layer made of another main constituent material, it is possible to more effectively reduce the adhesion of mist. For example, in the first embodiment, the nozzle plate and the diaphragm are layers made of the same main constituent material, and the flow path member is a layer made of a different main constituent material, and the dielectric constant of the main constituent material of the nozzle plate is the flow path. Lower than the dielectric constant of the main constituent material of the member (the diaphragm is the same material as the nozzle plate, so it is not another main constituent material) In the configuration, Example 2, the nozzle plate and the flow path plate are the same main constituent material And the support substrate is a layer made of a different main constituent material, and the dielectric constant of the main constituent material of the nozzle plate is lower than the dielectric constant of the main constituent material of the support substrate (the flow path plate is the same material as the nozzle plate) Therefore, it is not another main constituent material.)

  The image forming apparatus according to the present invention can be applied to a facsimile machine, a copying machine, a printer / fax / copier multifunction machine, etc. in addition to a printer configuration. Furthermore, the present invention can also be applied to recording liquids and liquids other than ink, such as resist, and image forming apparatuses that discharge DNA samples in the medical field. Further, in the above-described embodiment, an example in which the present invention is applied to a serial type image forming apparatus has been described, but the present invention can be similarly applied to a line type image forming apparatus including a line type liquid discharge head.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an explanatory side view illustrating an overall configuration showing an example of an image forming apparatus according to the present invention. It is principal part plane explanatory drawing of the apparatus. It is a typical explanatory view explaining an example of a conveyance belt in the same device. It is a typical explanatory view explaining other examples of a conveyance belt similarly. FIG. 4 is a cross-sectional explanatory view along the longitudinal direction of the liquid chamber showing an example of a liquid discharge head constituting the recording head in the apparatus. FIG. 6 is a cross-sectional explanatory view along the lateral direction of the liquid chamber of the liquid discharge head. It is a block explanatory drawing explaining the outline | summary of the control part of the apparatus. FIG. 2 is a schematic explanatory diagram for explaining a portion related to charging control of the apparatus. It is explanatory drawing with which it uses for description when the conveyance belt is charged. FIG. 6 is an explanatory diagram for explaining when a sheet comes into contact with the conveyance belt. It is explanatory drawing with which it uses for description of the charging of a conveyance belt, and the reverse flow of mist. It is a typical explanatory view of a head configuration used for explanation of simulation of electric field strength between a head and a medium. It is explanatory drawing with which it uses for description of the relationship between the electrode part of a head used for the simulation, and a nozzle surface. It is explanatory drawing with which it uses for description of the relationship between the electrode part of a head used for the simulation, and a nozzle surface. It is explanatory drawing with which it uses for description of the simulation result of the electric field strength between a head and a medium. It is explanatory drawing with which it uses for description of the simulation result of the dielectric constant and electric field strength of a head structural member. FIG. 10 is a schematic explanatory diagram for explaining another example of the liquid discharge head.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Apparatus main body 3 ... Carriage 4 ... Recording head (liquid discharge head)
12 ... paper 21 ... conveying belt 101 ... channel plate (channel member)
DESCRIPTION OF SYMBOLS 102 ... Diaphragm 103 ... Nozzle plate 104 ... Nozzle 106 ... Liquid chamber 121 ... Piezoelectric element 126 ... FPC cable (transmission means)

Claims (4)

In an image forming apparatus comprising: a recording head that discharges droplets onto a recording medium; and a conveying unit that electrostatically attracts and conveys the recording medium.
The droplet discharge surface of the recording head is made of an insulator, and the outer periphery of the recording head does not have a conductor at a portion facing the recording medium via the insulator. Image forming apparatus   2. The image forming apparatus according to claim 1, wherein the recording head includes a portion other than a transmission unit that transmits a drive signal.   3. The image forming apparatus according to claim 1, wherein the recording head has a plurality of layers made of different main constituent materials in a direction perpendicular to the droplet discharge surface, and is closest to the droplet discharge surface. An image forming apparatus, wherein a dielectric constant of a main constituent material of a layer is lower than a dielectric constant of another main constituent material.   4. The image forming apparatus according to claim 1, further comprising a mechanism for wiping the droplet discharge surface of the recording head by a wiper member made of a grounded conductive material. Image forming apparatus.

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