JP4347824B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus including a liquid discharge head that discharges droplets of a recording liquid.

  As various image forming apparatuses such as printers, facsimiles, copying apparatuses, plotters, printer / fax / copier multifunction machines, etc., recording heads (printing heads) constituted by liquid discharging heads that discharge recording liquid (for example, ink) droplets Is mounted on a carriage, and the carriage is used to transport a recording medium (hereinafter referred to as “paper”, but the material is not limited to paper, and is also referred to as recording medium, recording paper, transfer material, etc.). In addition to serial scanning in a direction orthogonal to the direction, the recording medium is intermittently conveyed according to the recording width, and an image is formed on the recording medium by alternately repeating conveyance and recording (recording, printing, printing) In this case, there is a serial type image forming apparatus or a line type image forming apparatus equipped with a line type recording head.

  In such an image forming apparatus using a liquid discharge head, in order to perform gradation printing (gradation printing), droplets having different sizes, for example, large droplets, medium droplets, and small droplets, can be discharged from the liquid discharge head. It is known that it is possible to form dots of different sizes.

For example, Patent Document 1 discloses that a first drive pulse for ejecting a first ink droplet within one printing cycle and a second ink droplet having a different size from the first ink droplet are ejected. An apparatus for selectively forming dots of four or more gradations by combining the first and second drive pulses using a drive waveform including the second drive pulse is described.
Japanese Patent No. 3264422

In the apparatus described in Patent Document 1, print data of 1 channel (CH) 1 bit (1 bit / CH) is transferred to the head driver for each drive pulse constituting the drive waveform. In this case, since ON / OFF can be selected for each drive pulse, in principle, the gradation can be increased to 2 ⁿ (n = the number of drive pulses in one printing cycle). One channel corresponds to one nozzle, a liquid chamber corresponding to the nozzle, and pressure generating means.

In Patent Document 2, a storage means (shift register) and a translation means are provided from a common drive waveform consisting of a plurality of drive pulses within one printing cycle, and the operation of the switch means is controlled based on the decoded print data. Thus, it is described that the data transfer within one printing cycle is 2 bits / CH (in the case of 4 gradations) once, and the amount of data transfer and the number of signal lines are reduced by providing a translation means. .
Japanese Patent No. 3219241

  In this data transfer method, data transfer can be performed once in one printing cycle regardless of the drive pulse interval, so that data transfer corresponding to the number of channels and the printing speed can be performed. The burden is also reduced.

Further, Patent Document 3 describes a method of transferring gradation data and selecting a part of the common drive waveform with a control signal corresponding to the gradation.
JP 2003-1817 A

  These Patent Documents 2 and 3 differ in the method of switching the analog switch ON / OFF for each gradation, but the gradation data is transferred once within one printing cycle, and based on this, in the common drive waveform The selection of the waveform to be applied is the same.

  By the way, in the image forming apparatus, there are non-interlaced and one-pass printing as printing modes for performing high-speed printing. In this print mode, an image is formed by one scan, so high-speed printing can be performed.

  In this non-interlaced printing, the resolution in the sub-scanning direction is the same as the nozzle pitch of the head. Therefore, in order to be able to print a solid image, the size of the largest droplet (large droplet) is an ejection that fills the solid. It is necessary to have a drop volume.

  However, particularly in a head using an electromechanical transducer (piezoelectric element), the machining accuracy becomes the limit of the nozzle pitch, so the resolution cannot be increased so much. Currently, there is a limit of about 360 dpi in a single-row 180 dpi, 2-row staggered arrangement. In this case, in order to fill the solid in one scan, an ejection droplet amount of 30 to 50 pl is required depending on the paper type / ink type.

  On the other hand, from the pursuit of high image quality, smaller droplets (small droplets) used in the high image quality mode are required to be smaller. The discharge amount may exceed 2 pl, reach 1.5 pl, and possibly 1 pl in the future.

  As described above, in an image forming apparatus using a recording liquid, there is an increasing demand for changing the droplet amount in a wider range.

  However, when changing the discharge amount over a wide range, it is very difficult to realize by simply controlling the discharge of one drop, and in many cases, a large drop is formed by discharging a plurality of drops. . In addition, with the expansion of the range in which the discharge amount must be varied, the discharge amount Mj per droplet tends to be decreased and the number of discharged droplets further increased.

  However, increasing the number of droplets within one printing cycle shortens the discharge interval (pulse interval). Therefore, in the method of transferring print data for each drive pulse as in Patent Document 1 described above, data transfer must be completed in the cycle of the drive pulse with the shortest pulse width that needs to be switched ON / OFF. This makes it impossible to transfer data, for example, by increasing the clock frequency of data transfer. In addition, in order to increase the printing speed, the number of channels of the head tends to increase, and thus the pulse width of the drive pulse tends to shorten, so that the data transfer time will limit the printing speed. There is.

  Therefore, as described in Patent Documents 2 and 3 described above, by transferring gradation data of 2 bits or more and providing a translation unit, a selection unit, etc., the data can be obtained regardless of the drive pulse interval. Since the transfer may be performed once per printing cycle, data transfer corresponding to the number of channels and the printing speed can be performed, and the burden on the data transfer unit is reduced.

  However, only 2 gradations (large, medium, small, and none) can be obtained with 2 bits / CH. In this case, the number of gradations can be increased by increasing the number of bits (for example, 8 gradations for 3 bits / CH), but the number of circuit elements in the head driver increases, leading to an increase in cost. Increasing the number of bits also causes a problem that the amount of data transfer increases.

  On the other hand, a method is known in which the resolution of main scanning is different from the resolution of sub scanning, for example, an image is formed at 600 dpi for main scanning and 300 dpi for sub scanning. In other words, when the nozzle arrangement direction is the sub-scanning direction in the serial type, the sub-scanning resolution is defined by the nozzle pitch, but the main-scanning resolution is determined by the carriage speed and printing cycle, so adjust the carriage speed. Can be changed relatively easily, and only the main scanning resolution can be increased.

  However, if the printing speed is kept the same as the printing speed of 300 dpi × 300 dpi, the driving frequency must be doubled. For example, rather than ejecting a droplet with a droplet amount of 40 pl at a driving frequency of 10 kHz, In terms of control, it is more difficult to eject the droplets at a driving frequency of 20 kHz.

  When a large number of droplets are ejected, the meniscus is raised by the ejection of the preceding droplet, so that the subsequent droplet is larger. For example, when discharging 40 pl with 6 drops, the total of the first 3 drops does not reach 20 pl. Furthermore, if only the resolution on the main scanning side is increased, filling in the sub-scanning direction with ink becomes a restriction, and even if the main scanning resolution is doubled, the required discharge amount is not halved, so 300 dpi per unit time. Compared to the case of x300 dpi, a larger amount of liquid must be discharged, and the difficulty level becomes higher.

  However, if the resolution is increased only in main scanning on the image, the image quality can be improved, for example, the degree of freedom of image processing increases, the density range that can be expressed increases, and the character quality improves.

  Note that in order to form large droplets that fill a solid image, it is necessary to strictly control the droplet ejection interval as the number of droplets ejected in one printing cycle is increased. As the number of ejected droplets increases, it is impossible to wait until the pressure in the pressurized liquid chamber is sufficiently attenuated. Therefore, it is possible to discharge efficiently according to the pressure vibration in the pressurized liquid chamber and to prevent runaway pressure vibration. It is necessary to control the pressure, and for these reasons, it is necessary that the waveform forming the large droplets is controlled in time and voltage.

  The present invention has been made in view of the above-described problems, and an object thereof is to provide an image forming apparatus capable of achieving both high-speed printing and high image quality.

In order to solve the above problems, an image forming apparatus according to the present invention provides:
A liquid discharge head for discharging recording liquid droplets is mounted on the carriage and moved in the main scanning direction, the recording medium is moved in the sub-scanning direction, an image is formed on the recording medium, and the resolution of the sub-scanning A serial type image forming apparatus having a mode for forming an image having a higher resolution of main scanning than
Via drive waveform generating means for generating a drive waveform including two or more drive signals within one printing cycle, and switch means for inputting gradation data and opening / closing according to the gradation data from among the drive waveforms A driving unit that selects a required driving signal and applies the selected driving signal to the liquid ejection head; and a unit that switches the gradation data within one printing cycle .
The drive waveform includes a drive signal for forming one dot by a large droplet and a drive signal for forming at least one of two or more dots by a small droplet and two or more dots by a medium droplet in one printing cycle. And the drive signal for forming one dot by the large droplet is a signal having a period longer than a half period of the one printing cycle,
In the mode, the resolution in the main scanning direction and the sub-scanning resolution are the same only for the formation of one dot by the large droplet.
The configuration .

Here, it is preferable that a plurality of droplets forming one dot by a large droplet are ejected and combined before the plurality of droplets land to form the one large droplet . Et al of the drive signal for forming one dot by the large droplet comprises a plurality of drive signals, and, on the plurality of driving signals, one dot by the drive signals and droplet to form one dot of medium droplets It is preferable to include at least one of drive signals to be formed.

Further, it is preferable to form an image noninterlaced in said mode.

  Furthermore, it is preferable that the recording liquid contains a pigment as a coloring material and has a viscosity at 23 ° C. of 5 mPa · s or more.

According to the image forming apparatus of the present invention, in the mode for forming an image having a main scanning resolution higher than the sub-scanning resolution , the resolution in the main scanning direction and the sub-scanning resolution are the same only for the formation of one dot by a large droplet. Because it has a certain configuration, even if the resolution in the main scanning direction and the resolution in the sub-scanning direction are different, large droplets can be ejected according to the resolution in the sub-scanning direction, resulting in high-speed printing and high image quality. Can be achieved.

  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 structure of the mechanism section of the image forming apparatus, and FIG. 2 is a plan view for explaining the mechanism section.

  In this image forming apparatus, a carriage 4 is slidably held in a main scanning direction by a guide rod 2 and a stay 3 which are guide members horizontally mounted on left and right side plates 1A and 1B constituting a frame 1, and a main scanning motor. 5 is moved and scanned in the direction indicated by the arrow (main scanning direction) in FIG. 2 via the timing belt 7 spanned between the driving pulley 6A and the driven pulley 6B.

  The carriage 4 includes, for example, four droplet ejection heads 11k, 11c, 11m, and 11y that eject ink droplets of each color of yellow (Y), cyan (C), magenta (M), and black (Bk). The recording head 11 is mounted with the nozzle row of the nozzle surface 11a on which a plurality of ink discharge ports (nozzles) are formed arranged in a direction (sub-scanning direction) orthogonal to the main scanning direction, and the ink discharging direction is directed downward. . Although an independent droplet discharge head is used here, a configuration in which one or a plurality of heads having a plurality of nozzle rows that discharge droplets of recording liquid of each color can be used. Further, the number of colors and the arrangement order are not limited to this.

  Here, as described above, the recording head 11 is composed of four separate heads 11k, 11c, 11m, and 11y that discharge droplets of the respective colors, and the heads 11k, 11c, 11m, and 11y are shown in FIG. As described above, the nozzle array is formed by arranging two nozzles n for discharging droplets side by side (each nozzle array is referred to as nozzle arrays NA and NB). Not limited to this, as shown in FIG. 4, two nozzle rows 11 kA, 11 kB, 11 cA, 11 cB, 11 mA, 11 mB, 11 yA, and 11 yB that discharge droplets of each color are arranged in one recording head 11. It can also be constituted by a head having one or a plurality of nozzle rows that eject black ink and a head having one or a plurality of nozzle rows for each color that ejects color ink.

  As the ink jet head constituting the recording head 11, a head provided with a piezoelectric actuator such as a piezoelectric element as pressure generating means (actuator means) for generating a pressure for discharging a droplet is used as will be described later. .

  A driver IC is mounted on the recording head 11 and connected to a control unit (not shown) via a harness (flexible print cable: FPC cable) 12.

  In addition, the carriage 4 is equipped with a sub tank 15 for each color for supplying ink of each color to the recording head 11. Each color sub-tank 15 is supplementarily supplied with ink of each color from the ink cartridge 10 of each color mounted in the cartridge loading unit 9 via the ink supply tube 16 of each color. The cartridge loading 9 is provided with a supply pump unit 17 for feeding ink in the ink cartridge 10, and the ink supply tube 16 is engaged with the rear plate 1C constituting the frame 1 in the middle of turning. It is held by a stop member 18.

  On the other hand, as a paper feed unit for feeding the paper 22 stacked on the paper stacking unit (pressure plate) 21 of the paper feed tray 20, a half-moon roller (feed) that feeds the paper 22 from the paper stacking unit 21 one by one. A separation pad 24 made of a material having a large friction coefficient is provided opposite to the sheet roller 23 and the sheet feeding roller 23, and the separation pad 24 is biased toward the sheet feeding roller 23 side.

  In order to feed the paper 22 fed from the paper feeding unit to the lower side of the recording head 11, a guide member 25 for guiding the paper 22, a counter roller 26, a conveyance guide member 27, and a tip pressure roller. And a holding belt 28 as a conveying means for electrostatically attracting the fed paper 22 and conveying it at a position facing the recording head 11.

  The conveyance belt 31 is an endless belt, is configured to wrap around the conveyance roller 32 and the tension roller 33 and circulate in the belt conveyance direction (sub-scanning direction). 34 is charged (charged).

  The transport belt 31 may be a single-layer belt or a multi-layer (two or more layers) belt. In the case of the conveyance belt 31 having a one-layer structure, the entire layer is formed of an insulating material because it contacts the paper 32 and the charging row 34. In the case of the transport belt 31 having a multi-layer structure, the side that contacts the paper 22 and the charging roller 34 is preferably formed of an insulating layer, and the side that does not contact the paper 22 and the charging roller 34 is preferably formed of a conductive layer. .

As an insulating material for forming the transport belt 31 having a single-layer structure or an insulating material for forming the insulating layer of the transport belt 31 having a multi-layer structure, for example, a conductive material such as PET, PEI, PVDF, PC, ETFE, or PTFE is used. It is preferable that the material does not contain a control material, and the volume resistivity is 10 ¹² Ωcm or more, preferably 10 ¹⁵ Ωcm. Further, as a material for forming the conductive layer of the transport belt 31 having a multi-layer structure, it is preferable that carbon is contained in the resin or elastomer so that the volume resistivity is 10 ⁵ to 10 ⁷ Ωcm.

The charging roller 34 is disposed so as to come into contact with an insulating layer (in the case of a multilayer belt) that forms the surface layer of the transport belt 31 and to rotate following the rotation of the transport belt 31. It is over. The charging roller 34 is formed of a conductive member having a volume resistivity of 10 ⁶ to 10 ⁹ Ω / □. As will be described later, for example, a 2 kV positive / negative AC bias (high voltage) is applied to the charging roller 34 from an AC bias supply unit (high voltage power supply). The AC bias may be a sine wave or a triangular wave, but a square wave is more preferable.

  In addition, a guide member 35 is disposed on the back side of the conveyance belt 31 so as to correspond to a printing area by the recording head 11. The guide member 35 has an upper surface that protrudes toward the recording head 11 from the tangent line of the two rollers (the conveyance roller 32 and the tension roller 33) that support the conveyance belt 31, thereby maintaining high-precision flatness of the conveyance belt 31. Like to do.

  The transport belt 31 rotates in the belt transport direction of FIG. 2 when the transport roller 32 is rotationally driven by the sub-scanning motor 36 via the drive belt 37 and the timing roller 38. Although not shown, an encoder wheel having a slit is attached to the shaft of the conveying roller 32, and a transmission type photosensor for detecting the slit of the encoder wheel is provided, and the wheel encoder is configured by the encoder wheel and the photosensor. It is composed.

  Further, as a paper discharge unit for discharging the paper 22 recorded by the recording head 11 to the paper discharge tray 40, a separation claw 41 for separating the paper 22 from the transport belt 31, a paper discharge roller 42, and paper discharge The roller 43 is provided.

  A duplex unit 51 is detachably mounted on the back surface of the apparatus body 1. The duplex unit 51 takes in the paper 22 returned by the reverse rotation of the transport belt 31, reverses it, and feeds it again between the counter roller 26 and the transport belt 31. The upper surface of the duplex unit 51 is a manual feed tray 52.

  Further, a maintenance / recovery mechanism 61 for maintaining and recovering the state of the nozzles of the recording head 11 is disposed in a non-printing area on one side in the scanning direction of the carriage 4. The maintenance / recovery mechanism 61 includes cap members (hereinafter referred to as “caps”) 62 a to 62 d (hereinafter referred to as “caps 62” when not distinguished) and nozzles for capping the nozzle surfaces 11 a of the recording head 11. A wiper blade 63 that is a blade member for wiping the surface 11a, an empty discharge receiver 64 that receives liquid droplets when performing empty discharge for discharging liquid droplets that do not contribute to recording in order to discharge the thickened recording liquid, and the like It has. Here, the cap 62a is a suction and moisture retention cap, and the other caps 62b to 62d are moisture retention caps.

  Further, in the non-printing area on the other side of the carriage 4 in the scanning direction, there is an empty space for receiving a liquid droplet when performing an empty discharge for discharging a liquid droplet that does not contribute to recording in order to discharge the recording liquid thickened during recording or the like. A discharge receiver 68 is disposed, and the idle discharge receiver 68 is provided with an opening 69 along the nozzle row direction of the recording head 11.

  Further, as shown in FIG. 1, a density sensor 71 including an infrared sensor (a type of sensor is not limited to the infrared sensor) which is a medium detecting unit for detecting the presence or absence of the paper 22 in the carriage 4. Is provided. The density sensor 71 is provided on the recording area (image forming area) side (conveying belt 31 side) side of the carriage 4 at the home position and upstream of the recording head 11 in the paper conveying direction. .

  Further, an encoder scale 72 having slits is provided on the front side of the carriage 4 along the main scanning direction, and an encoder sensor 73 including a transmission type photosensor for detecting the slits of the encoder scale 72 is provided on the front side of the carriage 4. These components constitute a linear encoder 74 for detecting the position of the carriage 4 in the main scanning direction.

  Next, an example of a droplet discharge head constituting the recording head in this image forming apparatus 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 droplet discharge head includes a flow channel plate 101 formed by anisotropic etching of a single crystal silicon substrate, a vibration plate 102 formed by nickel electroforming, for example, bonded to the lower surface of the flow channel plate 101, and a flow plate. A nozzle plate 103 bonded to the upper surface of the path plate 101 is bonded and stacked, and a nozzle communication path 105, a liquid chamber 106, and a liquid chamber, which are channels through which the nozzles 104 that discharge liquid droplets (ink droplets) communicate with each other. An ink supply port 109 communicating with a common liquid chamber 108 for supplying ink to 106 is 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.

  Further, an FPC cable 12 equipped with a drive circuit (drive IC) (not shown) is connected to the piezoelectric element 121.

  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.

  Here, the flow path plate 101 is formed by, for example, subjecting the single crystal silicon substrate having a crystal plane orientation (110) to anisotropic etching using an alkaline etching solution such as an aqueous potassium hydroxide solution (KOH), so that the nozzle communication path 105, Although a recess or a hole serving as the liquid chamber 106 is formed, the invention is not limited to a single crystal silicon substrate, and other stainless steel substrates, photosensitive resins, and the like can also be used.

  The vibration plate 102 is formed from a nickel metal plate, and is manufactured by, for example, an electroforming method (electroforming method). Alternatively, a metal plate or a joining member between a metal and a resin plate may be used. it can. The piezoelectric element 121 and the support post 123 are bonded to the diaphragm 102 with an adhesive, and the frame member 130 is further bonded with an adhesive.

  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. The nozzle plate 103 is formed by forming a water repellent layer on the outermost surface of a nozzle forming member made of a metal member via a required layer. The surface of the nozzle plate 103 is the nozzle surface 34a described above.

  The piezoelectric element 121 is a stacked piezoelectric element (here, PZT) in which piezoelectric materials 151 and internal electrodes 152 are alternately stacked. An individual electrode 153 and a common electrode 154 are connected to each internal electrode 152 drawn out to different end faces of the piezoelectric element 121 alternately. 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 droplet discharge head configured as described above, for example, by lowering the voltage applied to the piezoelectric element 121 from the reference potential, the piezoelectric element 121 contracts, and the vibration plate 102 descends to expand the volume of the liquid chamber 106. As a result, 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 to change the volume of the liquid chamber 106. / 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.

  In the image forming apparatus configured as described above, the sheets 22 are separated and fed one by one from the sheet feeding unit, and the sheets 22 fed substantially vertically upward are guided by the guide 25, and the conveyance belt 31 and the counter roller 26, and the leading end is guided by a conveying guide 27 and pressed against the conveying belt 31 by a leading end pressing roller 29, and the conveying direction is changed by approximately 90 °.

  At this time, a positive output and a negative output are alternately and repeatedly applied to the charging roller 34 from an AC bias supply unit, which will be described later, that is, an alternating voltage is applied, and a charging voltage pattern in which the conveying belt 31 alternates, that is, a loop In the sub-scanning direction, which is the direction, plus and minus are alternately charged in a band shape with a predetermined width. When the paper 32 is fed onto the conveying belt 31 that is alternately charged with plus and minus, the paper 22 is attracted to the conveying belt 31 by electrostatic force, and the paper 22 is conveyed in the sub-scanning direction by the circular movement of the conveying belt 31. Is done.

  Therefore, by driving the recording head 11 according to the image signal while moving the carriage 4, ink droplets are ejected onto the stopped paper 22 to record one line, and after the paper 22 is conveyed by a predetermined amount, Record the next line. Upon receiving a recording end signal or a signal that the trailing edge of the paper 22 has reached the recording area, the recording operation is finished and the paper 22 is discharged onto the paper discharge tray 40.

  In the case of double-sided printing, when recording on the front surface (surface to be printed first) is completed, the recording belt 32 is fed into the double-sided paper feeding unit 51 by rotating the conveyor belt 31 in reverse. The paper 22 is reversed (with the back surface being the printing surface), fed again between the counter roller 26 and the conveyor belt 31, controlled in timing, and conveyed by the conveyor bell 31 as described above. After recording on the back surface, the paper is discharged onto the paper discharge tray 40.

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 includes an I / F 206 for transmitting and receiving data and signals to and from the host side, a head drive control unit 207 including a data transfer unit for driving and controlling the recording head 11, and a carriage 4 side. A head driver (driver IC) 208, which is a head driving device for driving the provided recording head 11, a main scanning motor driving unit 210 for driving the main scanning motor 5, and a sub scanning motor 36 are driven. A sub-scanning motor drive unit 211, an AC bias supply unit 212 that supplies an AC bias to the charging roller 34, detection pulses from the linear encoder 74 and the wheel encoder 236, a detection signal from the temperature sensor 215 that detects the environmental temperature, and An I / O 213 for inputting detection signals from other various sensors is provided. The control unit 200 is connected to an operation panel 214 for inputting and displaying information necessary for the apparatus.

  Here, the control unit 200 transmits print data from the host side such as an information processing device such as a personal computer, an image reading device such as an image scanner, an imaging device such as a digital camera, etc. via an I / F 206 via a cable or a network. Receive.

  Then, 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 this image data is head-driven. The data is transferred from the control unit 207 to the head driver 208. The dot pattern data for image output may be generated by storing font data in the ROM 202, for example, or the image data is developed into bitmap data by a host-side printer driver and transferred to this apparatus. You may do it.

  The head drive control unit 207 transfers the above-described image data as serial data, and outputs a transfer clock, a latch signal, a control signal, and the like necessary for transferring the image data and confirming the transfer to the head driver 208. Includes a drive waveform generation unit including a D / A converter and an amplifier that D / A converts drive pulse pattern data stored in the ROM 202 and read out by the CPU 201. One drive pulse or a plurality of drive pulses A drive waveform composed of (drive signal) is output to the head driver 208.

  The head driver 208 selectively selects the drive pulse constituting the drive waveform supplied from the head drive control 207 based on the image data corresponding to one line of the print head 11 inputted serially (actuator means of the print head 11 ( In the head configuration described above, the head 11 is driven by being applied to the piezoelectric element 121).

  The main scanning motor driving unit 210 calculates a control value based on a target value given from the CPU 201 side and a speed detection value obtained by sampling a detection pulse from the linear encoder 74, and performs main scanning via an internal motor driver. The motor 5 is driven.

  Similarly, the sub-scanning motor drive control unit 211 calculates a control value based on a target value given from the CPU 101 side and a speed detection value obtained by sampling a detection pulse from the wheel encoder 136 to determine an internal motor driver. And the sub-scanning motor 36 is driven.

Next, an example of the configuration of the head drive control unit 207 and the head driver 28 will be described with reference to FIG.
As described above, the head drive control unit 207 generates and outputs a drive waveform (common drive waveform) composed of a plurality of drive pulses (drive signals) within one printing cycle, and printing. A data transfer unit 302 that outputs 2-bit image data (tone signals 0 and 1) corresponding to an image, a clock signal, a latch signal (LAT), and control signals M0 to M3 is provided.

  The control signal is a 2-bit signal that instructs each drop to open and close an analog switch 317, which will be described later, of the head driver 208. The control signal is a waveform that should be selected in accordance with the printing cycle of the common drive waveform. The state transitions to (ON), and when not selected, the state transitions to L level (OFF).

  The head driver 208 receives a transfer clock (shift clock) and serial image data (gradation data: 2 bits / CH) from the data transfer unit 302, and each register value of the shift register 311 by a latch signal. A latch circuit 312 for latching, a decoder 313 that decodes gradation data and control signals MN0 to MN3 and outputs the result, and a logic level voltage signal of the decoder 313 is converted to a level at which the analog switch 315 can operate. Level shifter 314, and an analog switch 316 that is turned on / off (opened / closed) by the output of the decoder 313 provided via the level shifter 314.

  The analog switch 316 is connected to the selection electrode (individual electrode) 154 of each piezoelectric element 121, and the common drive waveform from the drive waveform generation unit 301 is input thereto. Accordingly, when the analog switch 316 is turned on in accordance with the result of decoding the serially transferred image data (gradation data) and the control signals MN0 to MN3 by the decoder 313, a required drive signal constituting the common drive waveform is obtained. Passing (selected) is applied to the piezoelectric element 121.

Next, drive waveforms in the first embodiment of the present invention will be described with reference to FIG.
The drive waveform shown in FIG. 7A is a waveform including eight drive pulses (drive signals) P1 to P8 in one printing cycle, and driving for forming one dot by a large droplet in one printing cycle. Pulses P1 to P6 (excluding the drive pulse P3), drive pulses P1 and P2 and drive pulses P7 and P8 for forming two dots by medium droplets, and a drive pulse P1 and drive pulses for forming two dots by small droplets P7 is included. The drive pulse P3 is a fine drive pulse that causes the meniscus to vibrate without ejecting droplets (non-ejection).

  That is, in one printing cycle, one dot is formed by a large droplet, and two dots each by a medium droplet and a small droplet are formed. The driving pulses P1 and P2 for forming one dot with a large droplet also serve as driving pulses for forming one dot with a medium droplet and a small droplet. Hereinafter, the small droplet ejected by the driving pulse P1 is “small droplet 1”, the small droplet ejected by the driving pulse P7 is “small droplet 2”, and the middle droplet ejected by the driving pulses P1 and P2 is “medium droplet 1”. ”, The medium droplet ejected by the drive pulses P7 and P8 is referred to as“ medium droplet 2 ”.

  In addition, the control signal MN0 shown in FIG. 7B is a signal for selecting the drive pulse P3, and the control signal MN1 is a drive pulse P1 to P6 (a drive pulse for forming a large droplet, as described above, the drive pulse 3 is a droplet. 5 droplets are ejected since no ejection is performed.) And a drive pulse P7 (a drive pulse for forming a droplet 2) is selected, and the control signal MN2 is a drive pulse P1. This is a signal for selecting (driving pulse for forming small droplet 1) and selecting driving pulses P7 and 8 (driving pulse for forming medium droplet 2), and the control signal MN3 is driving pulses P1, P2 (medium droplet 1). This is a signal for selecting a drive pulse for formation.

  The grayscale data transferred from the data transfer unit 302 to the shift register 311 is switched at the timing Ta between the pulses P6 and P7 of the drive waveform, and the control signals MN1 and MN2 are large droplets as described above. The signal is switched from selection to droplet selection and from droplet selection to medium droplet selection.

Since such a configuration, by the tone data (2 bits / CH) to give the data transfer unit 30 2 to the head driver 208, in the first half portion extending in the timing Ta, the one dot by large droplets when the selected control signal M1 When the control signal M2 is selected, one dot by the small droplet 1 is formed, and when the control signal M3 is selected, one dot by the medium droplet 1 is formed. Then, by switching the gradation data at the timing Ta, one dot by the small droplet 2 is selected when the control signal M1 is selected and one dot by the medium droplet 2 is selected when the control signal M2 is selected in the latter half portion after the timing Ta. Is formed.

  Thus, within one printing cycle, as shown in FIG. 8A, a combination of large-nothing, medium-medium, small, no, small-medium, small, no, no-medium, small, no, As shown in FIG. 8B, dots can be formed by combinations of large-medium, small, no, medium-medium, small, no, small-medium, small, no, no-medium, small, and no. .

  For example, as shown in FIG. 9, large droplets are ejected so as to form dots at a resolution of 300 dpi, and medium and small droplets can be ejected twice within one printing cycle. Can be discharged. That is, it is possible to realize a printing mode in which the resolution in the main scanning direction is matched with the resolution in the sub-scanning direction only for dot formation with large drops.

  Thus, the drive waveform forms at least one of a drive signal for forming one dot by a large droplet and two or more dots by a small droplet and two or more dots by a medium droplet in one printing cycle. Even when a print mode that changes the resolution of main scanning and sub-scanning is realized, the resolution in the main scanning direction is the same as the resolution in the sub-scanning direction for large drops. It is possible to form a large droplet drive signal whose time and voltage are controlled, and a solid image can be realized. As a result, both high-speed printing and high image quality can be achieved.

  In this case, a large image can form a good image by combining a plurality of droplets during flight (before landing on the paper) and landing as a single droplet. That is, as shown in FIG. 10, a plurality of droplets I are sequentially ejected from the nozzle 104 and landed on the paper 22 in a state of being merged (merged) into one droplet IM during flight. Good dots Da can be formed as shown in a). On the other hand, when a plurality of droplets are landed without merging before landing, as shown in FIG. 11B, the dot Db in which the landing position of each droplet I is shifted forms a good dot. I can't.

  Further, by ejecting four or more droplets forming one dot by large droplets, it becomes easy to include a drive signal for forming medium droplets or small droplets in a drive signal for forming large droplets. In this way, the drive signal for forming one dot by a large droplet is composed of a plurality of drive signals, and the plurality of drive signals include a drive signal for forming one dot by a medium droplet and one dot by a small droplet. By including at least one of the drive signals forming the time, the time of the entire drive waveform can be shortened.

  Furthermore, by switching the gradation data within one printing cycle, it is possible to eject medium droplets and small droplets a plurality of times within one printing cycle and to increase the number of gradations. Further, as described above, the image forming apparatus has a mode for forming an image having a higher main scanning resolution than the sub scanning resolution. In this mode, only the formation of one dot by a large droplet has a resolution in the main scanning direction and a sub scanning resolution. By using the same, it is possible to achieve both high-speed printing and high image quality, and in this mode, it is possible to achieve higher speed by forming an image by a non-interlace method.

  In this embodiment, a resolution of 600 dpi × 300 dpi is taken as an example, but the resolution is not limited to this. Further, the drive waveforms and the drive signals (drive pulse shapes) constituting the drive waveforms are not limited to those shown in FIG.

Next, driving waveforms in the second embodiment of the present invention will be described with reference to FIG.
The driving waveform of this embodiment is a waveform that repeats the waveform constituted by the driving pulses P11 to P13 a plurality of times, here twice, within one printing cycle. Here, the driving pulses P11 to P13 are driving signals for ejecting small droplets (for example, only the driving pulse P11) and medium droplets (for example, driving pulses P11 to P13), respectively, and these driving pulses P11 to P13 are repeated twice. A drive signal is formed which forms a large droplet as a whole.

  Therefore, in this case, according to the gradation data (2 bits / CH), as shown in FIG. 8B, within one printing cycle, large-medium, small, none, medium-medium, small, no, A dot can be formed by a combination of small-medium, small, no, no-medium, small, and nothing. Note that the combination shown in FIG. 5A is not possible because a large droplet uses all the drive pulses that make up the drive waveform.

  As in the first embodiment, this also allows large droplets to be ejected so as to form dots at a resolution of 300 dpi, and medium and small droplets can be ejected twice within one printing cycle, as shown in FIG. The ink can be ejected so as to form dots with a resolution equivalent to 600 dpi which is doubled. That is, it is possible to realize a printing mode in which the resolution in the main scanning direction is matched with the resolution in the sub-scanning direction only for dot formation with large droplets, and both high-speed printing and high image quality can be achieved.

  Here, the resolution of 600 dpi × 300 dpi is taken as an example, but the resolution is not limited to this. Further, the drive waveform and the drive signal (drive pulse shape) constituting the drive waveform are not limited to those shown in FIG.

  Next, the recording liquid (ink) to be used will be described. The ink contains a pigment as a colorant and has a viscosity at 23 ° C. of 5 mPa · s or more, so that there is little bleeding and high color developability. High water resistance can be achieved.

  Although there is no limitation in particular as a pigment used here, For example, the pigments listed below are used suitably. Moreover, you may use these pigments in mixture of multiple types. For example, organic pigments include azo, phthalocyanine, anthraquinone, quinacridone, dioxazine, indigo, thioindigo, perylene, isoindolenone, aniline black, azomethine, rhodamine B lake pigment, carbon black, etc. Is mentioned.

  Examples of inorganic pigments include iron oxide, titanium oxide, calcium carbonate, barium sulfate, aluminum hydroxide, barium yellow, bitumen, cadmium red, chrome yellow, and metal powder.

  The carbon black used in the black pigment ink is carbon black produced by the furnace method and the channel method, the primary particle size is 15 to 40 millimicrons, the specific surface area by the BET method is 50 to 300 square meters / g, The DBP oil absorption is preferably 40 to 150 ml / 100 g, the volatile content is 0.5 to 10%, and the pH value is 2 to 9. As such a thing, for example, no. 2300, no. 900, MCF88, No. 40, no. 52, MA7, MA8, no. 2200B (above, manufactured by Mitsubishi Kasei), RAVEN1255 (made in Colombia), REGAL400R, REGAL660R, MOGUL L (above, manufactured by Kibobot), Color Black FW1, Color Black FW18, Color Black S170, Color Black S150, Printex 35, Printex U ( As described above, commercially available products such as Degussa) can be used.

1 is an explanatory side view illustrating an overall configuration of a mechanism unit of an image forming apparatus according to the present invention. It is a plane explanatory view of the mechanism part. FIG. 3 is an explanatory cross-sectional view along the longitudinal direction of the liquid chamber showing an example of a liquid discharge head that constitutes the recording head of the image forming apparatus. It is a cross-sectional explanatory drawing along a liquid chamber short direction similarly. FIG. 2 is a block explanatory diagram illustrating an overview of a control unit in the image forming apparatus. FIG. 3 is a block explanatory diagram illustrating an example of a head drive control unit and a head driver in the control unit. It is explanatory drawing explaining the drive waveform in 1st Embodiment of this invention. It is explanatory drawing with which it uses for description of the combination of the dot in the same embodiment. It is explanatory drawing with which it uses for description of the drive waveform in the same embodiment, the droplet to be discharged, and the dot formation position. It is explanatory drawing with which it uses for description of the merge of several droplets. It is explanatory drawing with which it uses for description of the dot shape when not merging before landing similarly. It is explanatory drawing explaining the drive waveform in 2nd Embodiment of this invention. It is explanatory drawing with which it uses for description of the drive waveform in the same embodiment, the droplet to be discharged, and the dot formation position.

Explanation of symbols

4 ... Carriage 5 ... Main scanning motor 11 ... Recording head 22 ... Recording medium (paper)
DESCRIPTION OF SYMBOLS 31 ... Conveyance belt 32 ... Conveyance roller 36 ... Sub scan motor 121 ... Piezoelectric element 200 ... Control part 207 ... Head drive control part 208 ... Head driver 301 ... Drive waveform generation part 302 ... Data transfer part

Claims (5)

A liquid discharge head for discharging recording liquid droplets is mounted on the carriage and moved in the main scanning direction, the recording medium is moved in the sub-scanning direction, an image is formed on the recording medium, and the resolution of the sub-scanning A serial type image forming apparatus having a mode for forming an image having a higher resolution of main scanning than
Via drive waveform generating means for generating a drive waveform including two or more drive signals within one printing cycle, and switch means for inputting gradation data and opening / closing according to the gradation data from among the drive waveforms A driving unit that selects a required driving signal and applies the selected driving signal to the liquid ejection head; and a unit that switches the gradation data within one printing cycle .
The drive waveform includes a drive signal for forming one dot by a large droplet and a drive signal for forming at least one of two or more dots by a small droplet and two or more dots by a medium droplet in one printing cycle. And the drive signal for forming one dot by the large droplet is a signal having a period longer than a half period of the one printing cycle,
The image forming apparatus according to claim 1, wherein the resolution in the main scanning direction and the sub scanning resolution are the same only in the formation of one dot by the large droplet in the mode .   2. The image forming apparatus according to claim 1, wherein a plurality of droplets forming one dot by the large droplets are ejected, and are combined to form the one large droplet before the plurality of droplets land. An image forming apparatus.   3. The image forming apparatus according to claim 1, wherein the driving signal for forming one dot by the large droplet includes a plurality of driving signals, and one dot by the medium droplet is formed in the plurality of driving signals. An image forming apparatus comprising at least one of a drive signal and a drive signal for forming one dot by a small droplet. 4. The image forming apparatus according to claim 1 , wherein an image is formed by a non-interlace method in the mode. The image forming apparatus according to any one of claims 1 to 4, wherein the pigment the recording liquid as a coloring material, and an image forming apparatus, wherein the viscosity at 23 ° C. is 5 mPa · s or more.

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