JP2006306076A - Liquid ejection apparatus and image formation apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a liquid ejection apparatus and an image formation apparatus which can find out the conditions for obtaining a negative pressure sufficient to tear a liquid column, even if the liquid is highly viscous, and, based on that, eject the highly viscous liquid as fine liquid droplets. <P>SOLUTION: The liquid ejection apparatus of this invention uses a drive waveform including, at least, as a drive signal 100 of an actuator which changes the volume of a pressure chamber, a first pull drive waveform component 102 which increases the volume of the pressure chamber, a push drive waveform component 106 which reduces the volume of the pressure chamber after the first pull drive waveform component 102 so as to eject the liquid from nozzles, and a second pull drive waveform component 110 which increases the volume of the pressure chamber again after the push drive waveform component 106, so as to divide the column of the ejected liquid. The amount (V<SB>SPL</SB>) pulled by the second pull drive waveform component 110 is equal to or more than the amount (V<SB>ph</SB>) pushed by the push drive waveform component 106. The time T2 from pushing to pulling is equal to or less than the time T1 which is from the first pulling to pushing. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a liquid ejecting apparatus and an image forming apparatus, and more particularly to a liquid ejecting apparatus suitable for an application for ejecting a high viscosity liquid as fine droplets and an image forming apparatus such as an ink jet recording apparatus using the liquid ejecting apparatus.

As a driving method for ejecting minute droplets from an inkjet head, the ink volume is first pushed out from the nozzle when the internal volume of the ink chamber is increased and then returned to the original state. A method of cutting the ink from the nozzle by increasing the internal volume of the ink chamber again before being cut is proposed (Patent Document 1).
Japanese Patent Laid-Open No. 2-184449

  Patent Document 1 does not mention the viscosity of the ink. However, if the above-described cutting operation is not performed when high-viscosity ink is ejected, the pressure decays quickly due to the high viscosity, and the liquid is ejected. Since a negative pressure for tearing off the liquid does not occur after the liquid is applied, the liquid column extends long and is difficult to form a fine droplet. For this reason, in order to discharge a high-viscosity liquid as fine droplets, it is desirable to add a waveform (waveform for cutting ink) that tears the liquid column.

  However, although Patent Document 1 discloses the ink cutting operation, it does not clearly indicate the timing of increasing the internal volume effective for the cutting operation. In high-viscosity liquids, the pressure decays quickly, and a sufficient negative pressure cannot be obtained unless a drawing operation is performed at a specific timing in order to produce a negative pressure that tears the liquid column.

  The present invention has been made in view of such circumstances, and elucidates the conditions under which a negative pressure sufficient to tear a liquid column can be obtained even with a high-viscosity liquid, and based on this, a high-viscosity liquid can be discharged as fine droplets. An object of the present invention is to provide a liquid ejecting apparatus and an image forming apparatus using the same.

  In order to achieve the above object, a liquid ejection apparatus according to claim 1 includes a nozzle for ejecting liquid, a pressure chamber that communicates with the nozzle and is filled with liquid to be ejected from the nozzle, An actuator for changing the volume to apply a pressure change to the liquid in the pressure chamber and discharging the liquid from the nozzle; and a drive signal generating means for generating a drive signal for driving the actuator. A first pulling drive waveform element that drives the actuator to increase the volume of the pressure chamber, and a volume of the pressure chamber is decreased after the first pulling drive waveform element to discharge liquid from the nozzle. The drive drive waveform element for driving the actuator, and the volume of the pressure chamber is increased again after the push drive waveform element to cut the liquid column of the discharge liquid A second pulling drive waveform element that drives the actuator, and the volume that is increased again by the second pulling drive waveform element is greater than or equal to the volume that is decreased by the push drive waveform element. It is characterized by.

The discharge driving method according to the present invention first performs a first pulling operation to increase the volume of the pressure chamber, and then performs a pushing operation to decrease the volume of the pressure chamber to push out the liquid from the nozzle. In this method, a second pulling operation for increasing the volume of the pressure chamber is performed again, and the liquid column is broken (cut) to form a fine droplet. In such a pull-push-pull drive, the amount pulled by the second pulling drive waveform element is equal to or greater than the amount pushed by the push drive waveform element, so that a sufficient negative pressure for breaking the liquid column is obtained. Can be generated. As a result, the liquid column of the discharge liquid can be reliably divided (teared), and it is possible to discharge the fine droplets of the high-viscosity liquid.

  The drive signal includes a pull holding waveform element that holds the state of the pressure chamber expanded by the first pull driving waveform element, and a push holding waveform element that holds the state of the pressure chamber contracted by the push driving waveform element. Also good. That is, the drive signal may have a triangular voltage waveform composed of a first pull drive waveform element, a push drive waveform element, and a second pull drive waveform element, You may have the trapezoidal voltage waveform which consists of a pull holding | maintenance waveform element, a push drive waveform element, a push hold | maintain waveform element, and a 2nd pull drive waveform element.

According to a second aspect of the present invention, there is provided the liquid ejection device according to the first aspect, wherein when the natural vibration period of the liquid volume velocity in the nozzle is Te, the first pulling drive waveform element is first drawn. The time T1 from the time of pressing to the pressing drive waveform element is expressed by the following equation (Te / 2) -1 [μs] ≦ T1 ≦ (Te / 2) +1 [μs]
It is characterized by satisfying.

  Assuming discharge by the drive method (pull push drive) that only pulls and pushes first, considering the conditions for the fastest discharge of the liquid (conditions for the time T0 until it is pressed after pulling), this T0 is the liquid at the nozzle. If it is a half cycle of the natural vibration period Te of the volume velocity, an effect (superposition effect) in which the pressure waveform by the pulling operation and the pressure waveform by the pushing operation are intensified is produced, so that the maximum discharge force can be obtained.

  Therefore, in the pull-push-pull drive system of the present invention, the time T1 from the first pulling by the first pulling drive waveform element to the pressing by the push driving waveform element is the discharge (pull-push drive) only by pulling and pushing. It is preferable that the time is substantially equal to the time T0 (that is, 1/2 of the natural vibration period Te) at which the liquid is discharged most quickly. According to the experiment, an allowable range of about ± 1 μs is recognized, and if the range satisfies (Te / 2) −1 [μs] ≦ T1 ≦ (Te / 2) +1 [μs], good discharge is possible. is there.

  The invention according to claim 3 relates to an aspect of the liquid ejecting apparatus according to claim 1 or 2, wherein the time T2 until the second pulling drive waveform element is pulled after being pushed by the push drive waveform element is The time period from the first pulling by the first pulling drive waveform element to the pressing by the push driving waveform element is less than T1.

  Preferably, the difference between T2 and T1 is made smaller, and more preferably, T2 and T1 are made identical (T2 = T1). Thereby, discharge can be performed using a resonance phenomenon. In addition, by setting the application timing of the second pulling drive waveform element so that the pressure waveform generated by the pulling operation and the pressure waveform generated by the second pulling operation are intensified, even a highly viscous liquid with severe pressure attenuation can be obtained. Sufficient negative pressure sufficient to tear the liquid column can be obtained, and a high-viscosity liquid can be ejected as fine droplets.

  The invention according to claim 4 relates to an aspect of the liquid ejecting apparatus according to any one of claims 1 to 3, wherein the pressure signal is volume-increased by the second pulling drive waveform element. A pull holding waveform element that holds the expanded state, and a static recovery waveform element that reduces the volume of the pressure chamber held in the expanded state by the pull holding waveform element to return to the steady state. It is characterized by that.

Refilling is facilitated during this period by maintaining the expanded state of the pressure chamber after the pulling action to tear the liquid column. In addition, after ensuring the required refill time, the pressure chamber is returned to the static state to prepare for the next discharge operation.

  The invention according to claim 5 relates to an aspect of the liquid ejecting apparatus according to claim 4, in which the time T3 for holding the expanded state by the pull holding waveform element is first drawn by the first pull driving waveform element. It is longer than the time T1 from when the pressing drive waveform element is pressed to when it is pressed.

  Thus, by ensuring a necessary and sufficient refill period, refilling can be performed quickly, and as a result, the discharge cycle can be shortened (discharge frequency can be increased).

  The invention according to claim 6 relates to an aspect of the liquid ejection apparatus according to claim 4 or 5, wherein the slope of the waveform of the static recovery waveform element is gentler than the slope of the waveform of the push drive waveform element. It is characterized by that.

  According to this aspect, it is possible to prevent a situation in which liquid is ejected from the nozzle during the pressure chamber contraction operation by the static recovery waveform element.

  The invention according to a seventh aspect relates to an aspect of the liquid ejection apparatus according to any one of the first to sixth aspects, wherein the viscosity of the liquid is 10 cP to 50 cP.

  The liquid ejecting apparatus according to the present invention is particularly suitable as a means for ejecting a highly viscous liquid with severe pressure attenuation as fine droplets.

  The invention according to an eighth aspect relates to an aspect of the liquid ejecting apparatus according to any one of the first to seventh aspects, wherein the length of the nozzle is ln, the area of the nozzle is An, and the pressure chamber is provided with the pressure chamber. The flow path length of the supply port for supplying the liquid is ls, the area of the supply port is As, the volume of the pressure chamber is V, the density of the liquid is ρ, the viscosity of the liquid is ν, the speed of sound in the liquid c, where P is the generated pressure of the actuator when the excluded volume is X, and X is the excluded volume of the actuator when the generated pressure is P,

It is characterized by satisfying.

  The above inequality ([Equation 2]) indicates that the pressure attenuation is so severe that the pressure fluctuation of the liquid oscillates and the second extreme value of the function indicating the pressure fluctuation is 1 / e or less of the first extreme value. This is a condition derived from the above condition. By using the pull push-pull control drive signal according to the present invention for the liquid discharge head configured to satisfy the above equation ([Equation 2]), it is possible to discharge micro droplets.

  The invention according to claim 9 provides an image forming apparatus that achieves the object. That is, an image forming apparatus according to a ninth aspect includes the liquid ejecting apparatus according to any one of the first to eighth aspects, and forms an image on a recording medium with the ink liquid ejected from the nozzle. And

According to such an image forming apparatus, for example, color conversion and halftoning processing are performed based on image data (print data) input via an image input unit, and ejection data corresponding to the ink color is generated. Based on this ejection data, the drive of the actuator corresponding to each nozzle of the liquid ejection head is controlled, and ink droplets are ejected from the nozzles. In order to realize high-resolution image output, a droplet discharge element (ink liquid chamber unit) including a nozzle (discharge port) for discharging ink liquid, a pressure chamber corresponding to the nozzle, and an actuator A mode in which a liquid discharge head (printing head) in which a large number of liquid crystals are arranged at high density is preferable.

  As a configuration example of such a liquid discharge head for printing, a full line type head having a nozzle row in which a plurality of discharge ports (nozzles) are arranged over a length corresponding to the entire width of the recording medium can be used. In this case, a combination of a plurality of relatively short ejection head modules having a nozzle row less than the length corresponding to the entire width of the recording medium, and connecting them together, the nozzle having a length corresponding to the entire width of the recording medium as a whole There is an aspect that constitutes a column.

  A full-line type head is usually arranged along a direction perpendicular to the relative feeding direction (relative conveyance direction) of the recording medium, but has a certain angle with respect to the direction perpendicular to the conveyance direction. There may be a mode in which the head is arranged along the oblique direction.

  “Recording medium” is a medium (which can be referred to as a printing medium, an image forming medium, a recording medium, an image receiving medium, a discharged medium, or the like) that receives adhesion of ink discharged from the discharge port of the liquid discharge head. Various media are included regardless of material or shape, such as continuous paper, cut paper, sealing paper, resin sheets such as OHP sheets, printed boards on which films, cloths, wiring patterns, etc. are formed.

  The transporting means for moving the recording medium and the liquid discharge head relative to each other includes a mode for transporting the recording medium to the stopped (fixed) head, a mode for moving the head relative to the stopped recording medium, or a head And a mode in which both the recording medium and the recording medium are moved. When a color image is formed using an inkjet print head, a print head may be arranged for each of a plurality of colors of ink (recording liquid), or a plurality of colors of ink are ejected from one print head. It is good also as a possible structure.

  According to the present invention, a negative pressure that tears the liquid column can be generated even with a high-viscosity liquid, so that the high-viscosity liquid can be discharged as fine droplets.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[Overall configuration of inkjet recording apparatus]
FIG. 1 is an overall configuration diagram of an ink jet recording apparatus according to an embodiment of the present invention. As shown in the figure, the inkjet recording apparatus 10 includes a print unit 12 having a plurality of print heads 12K, 12C, 12M, and 12Y provided for each ink color, and each print head 12K, 12C, 12M, An ink storage / loading unit 14 for storing ink to be supplied to 12Y, a paper feeding unit 18 for supplying recording paper 16, a decurling unit 20 for removing curling of the recording paper 16, and a nozzle of the printing unit 12 A suction belt transport unit 22 that is disposed to face a surface (ink ejection surface) and transports the recording paper 16 while maintaining the flatness of the recording paper 16, and a print detection unit 24 that reads a printing result by the printing unit 12, A paper discharge unit 26 that discharges printed recording paper (printed matter) to the outside.

In FIG. 1, a magazine for rolled paper (continuous paper) is shown as an example of the paper supply unit 18, but a plurality of magazines having different paper widths, paper quality, and the like may be provided side by side. Further, instead of the roll paper magazine or in combination therewith, the paper may be supplied by a cassette in which cut papers are stacked and loaded.

  When multiple types of recording paper are used, an information recording body such as a barcode or wireless tag that records paper type information is attached to the magazine, and the information on the information recording body is read by a predetermined reader. Therefore, it is preferable to automatically determine the type of paper to be used and perform ink ejection control so as to realize appropriate ink ejection according to the type of paper.

  The recording paper 16 delivered from the paper supply unit 18 retains curl due to having been loaded in the magazine. In order to remove this curl, heat is applied to the recording paper 16 by the heating drum 30 in the direction opposite to the curl direction of the magazine in the decurling unit 20. At this time, it is more preferable to control the heating temperature so that the printed surface is slightly curled outward.

  In the case of an apparatus configuration that uses roll paper, a cutter (first cutter) 28 is provided as shown in FIG. 1, and the roll paper is cut into a desired size by the cutter 28. The cutter 28 includes a fixed blade 28A having a length equal to or greater than the conveyance path width of the recording paper 16, and a round blade 28B that moves along the fixed blade 28A. The fixed blade 28A is provided on the back side of the print. The round blade 28B is disposed on the printing surface side with the conveyance path interposed therebetween. Note that the cutter 28 is not necessary when cut paper is used.

  After the decurling process, the cut recording paper 16 is sent to the suction belt conveyance unit 22. The suction belt conveyance unit 22 has a structure in which an endless belt 33 is wound between rollers 31 and 32, and at least portions facing the nozzle surface of the printing unit 12 and the sensor surface of the printing detection unit 24 are horizontal ( Flat surface).

  The belt 33 has a width that is greater than the width of the recording paper 16, and a plurality of suction holes (not shown) are formed on the belt surface. As shown in FIG. 1, a suction chamber 34 is provided at a position facing the nozzle surface of the print unit 12 and the sensor surface of the print detection unit 24 inside the belt 33 spanned between the rollers 31 and 32. Then, the suction chamber 34 is sucked by the fan 35 to be a negative pressure, whereby the recording paper 16 on the belt 33 is sucked and held. In place of the suction adsorption method, an electrostatic adsorption method may be adopted.

  When the power of a motor (not shown in FIG. 1, not shown in FIG. 5) is transmitted to at least one of the rollers 31 and 32 around which the belt 33 is wound, the belt 33 rotates in the clockwise direction in FIG. , And the recording paper 16 held on the belt 33 is conveyed from left to right in FIG.

  Since ink adheres to the belt 33 when a borderless print or the like is printed, the belt cleaning unit 36 is provided at a predetermined position outside the belt 33 (an appropriate position other than the print area). Although details of the configuration of the belt cleaning unit 36 are not shown, for example, there are a method of niping a brush roll, a water absorbing roll, etc., an air blow method of blowing clean air, or a combination thereof. In the case where the cleaning roll is nipped, the cleaning effect is great if the belt linear velocity and the roller linear velocity are changed.

  Although a mode using a roller / nip conveyance mechanism instead of the suction belt conveyance unit 22 is also conceivable, if the roller / nip conveyance is performed in the print area, the image easily spreads because the roller contacts the printing surface of the sheet immediately after printing. There is a problem. Therefore, as in this example, suction belt conveyance that does not bring the image surface into contact with each other in the print region is preferable.

A heating fan 40 is provided on the upstream side of the printing unit 12 on the paper conveyance path formed by the suction belt conveyance unit 22. The heating fan 40 heats the recording paper 16 by blowing heated air onto the recording paper 16 before printing. Heating the recording paper 16 immediately before printing makes it easier for the ink to dry after landing.

  The printing unit 12 is a so-called full line type head in which line type heads having a length corresponding to the maximum paper width are arranged in a direction (main scanning direction) orthogonal to the paper feed direction (see FIG. 2). Although a detailed structural example will be described later (FIGS. 3 to 4), each of the print heads 12K, 12C, 12M, and 12Y is a recording paper of the maximum size targeted by the inkjet recording apparatus 10 as shown in FIG. The line head includes a plurality of ink discharge ports (nozzles) arranged over a length exceeding at least one side of 16.

  The print heads 12K, 12C, 12M, and 12Y are arranged in the order of black (K), cyan (C), magenta (M), and yellow (Y) from the upstream side along the feeding direction of the recording paper 16, The print heads 12K, 12C, 12M, and 12Y are fixedly installed so as to extend along a direction substantially orthogonal to the conveyance direction of the recording paper 16.

  A color image can be formed on the recording paper 16 by ejecting different color inks from the print heads 12K, 12C, 12M, and 12Y while conveying the recording paper 16 by the suction belt conveyance unit 22.

  Thus, according to the printing unit 12 in which the full line head that covers the entire area of the paper width is provided for each ink color, the operation of relatively moving the recording paper 16 and the printing unit 12 in the sub-scanning direction is performed once. An image can be recorded on the entire surface of the recording paper 16 only by performing it (that is, in one sub-scan). Thereby, it is possible to perform high-speed printing as compared with a shuttle type head in which the print head reciprocates in the main scanning direction, and productivity can be improved.

  In this example, the configuration of KCMY standard colors (four colors) is illustrated, but the combination of ink colors and the number of colors is not limited to this embodiment, and light ink, dark ink, and special colors are used as necessary. Ink may be added. For example, it is possible to add an ink jet head that discharges light ink such as light cyan and light magenta. Also, the arrangement order of the color heads is not particularly limited.

  As shown in FIG. 1, the ink storage / loading unit 14 has tanks that store inks of colors corresponding to the print heads 12K, 12C, 12M, and 12Y, and each tank is connected via a conduit (not shown). The print heads 12K, 12C, 12M, and 12Y communicate with each other. Further, the ink storage / loading unit 14 includes notifying means (display means, warning sound generating means) for notifying when the ink remaining amount is low, and has a mechanism for preventing erroneous loading between colors. ing.

  The print detection unit 24 includes an image sensor for imaging the droplet ejection result of the print unit 12, and functions as a means for checking nozzle clogging and other ejection defects from the droplet ejection image read by the image sensor.

  The print detection unit 24 of this example is composed of a line sensor having a light receiving element array having a width wider than at least the ink ejection width of each print head 12K, 12C, 12M, 12Y. The line sensor includes an R sensor row in which photoelectric conversion elements (pixels) provided with red (R) color filters are arranged in a line, a G sensor row provided with green (G) color filters, The color separation line CCD sensor is composed of a B sensor array provided with a blue (B) color filter. Instead of the line sensor, an area sensor in which the light receiving elements are two-dimensionally arranged can be used.

The print detection unit 24 reads the test pattern printed by the print heads 12K, 12C, 12M, and 12Y for each color, and detects the ejection of each head. The ejection determination includes the presence / absence of ejection, measurement of dot size, measurement of dot landing position, and the like.

  A post-drying unit 42 is provided following the print detection unit 24. The post-drying unit 42 is means for drying the printed image surface, and for example, a heating fan is used. Since it is preferable to avoid contact with the printing surface until the ink after printing is dried, a method of blowing hot air is preferred.

  When printing on porous paper with dye-based ink, the weather resistance of the image is improved by preventing contact with ozone or other things that cause dye molecules to break by pressurizing the paper holes with pressure. There is an effect to.

  A heating / pressurizing unit 44 is provided following the post-drying unit 42. The heating / pressurizing unit 44 is a means for controlling the glossiness of the image surface, and pressurizes with a pressure roller 45 having a predetermined surface uneven shape while heating the image surface to transfer the uneven shape to the image surface. To do.

  The printed matter generated in this manner is outputted from the paper output unit 26. It is preferable that the original image to be printed (printed target image) and the test print are discharged separately. The ink jet recording apparatus 10 is provided with a sorting means (not shown) that switches the paper discharge path so as to select the print product of the main image and the print product of the test print and send them to the discharge units 26A and 26B. Yes. Note that when the main image and the test print are simultaneously formed in parallel on a large sheet, the test print portion is separated by a cutter (second cutter) 48. The cutter 48 is provided immediately before the paper discharge unit 26, and cuts the main image and the test print unit when the test print is performed on the image margin. The structure of the cutter 48 is the same as that of the first cutter 28 described above, and includes a fixed blade 48A and a round blade 48B.

  Although not shown in FIG. 1, the paper output unit 26A for the target prints is provided with a sorter for collecting prints according to print orders.

[Head structure]
Next, the structure of the print head will be described. Since the structures of the print heads 12K, 12C, 12M, and 12Y provided for the respective ink colors are common, the print heads are represented by reference numeral 50 in the following.

  FIG. 3 (a) is a plan perspective view showing an example of the structure of the print head 50, and FIG. 3 (b) is an enlarged view of a part thereof. 3C is a plan perspective view showing another example of the structure of the print head 50, and FIG. 4 is a cross-sectional view showing the three-dimensional configuration of the ink chamber unit as a droplet discharge element unit (in FIG. 3A). FIG. 4 is a cross-sectional view taken along line 4-4. In order to increase the dot pitch printed on the recording paper surface, it is necessary to increase the nozzle pitch in the print head 50. As shown in FIGS. 3A to 3C and FIG. 4, the print head 50 of this example includes a plurality of inks including nozzles 51 from which ink droplets are ejected and pressure chambers 52 corresponding to the nozzles 51. The chamber units 53 have a structure in which the chamber units 53 are two-dimensionally arranged in a staggered matrix, thereby achieving an increase in the apparent nozzle pitch density.

  That is, in the print head 50 according to this embodiment, as shown in FIGS. 3A and 3B, a plurality of nozzles 51 that eject ink are arranged in a direction substantially orthogonal to the feeding direction of the print medium (recording paper 16). This is a full line head having one or more nozzle rows arranged over a length corresponding to the entire width (printable width) of the print medium.

The form in which one or more nozzle rows are configured over a length corresponding to the entire width of the recording paper 16 in a direction substantially orthogonal to the feeding direction (sub-scanning direction) of the recording paper 16 is not limited to this example. For example, instead of the configuration of FIG. 3 (a), short head modules 50 ′ in which a plurality of nozzles 51 are arranged in a two-dimensional manner are arranged in a zigzag manner and connected as shown in FIG. 3 (c). A line head having a nozzle row having a length corresponding to the entire width of the recording paper 16 may be configured.

  The pressure chamber 52 provided corresponding to each nozzle 51 has a substantially square planar shape (see FIG. 3 (b)), and the outlet to the nozzle 51 is provided at one of the diagonal corners. The other side is provided with an inlet (supply port) 54 for supply ink. The shape of the pressure chamber 52 is not limited to this example, and the planar shape may have various forms such as a quadrangle (rhombus, rectangle, etc.), a pentagon, a hexagon and other polygons, a circle, and an ellipse.

  As shown in FIG. 4, the pressure chamber 52 communicates with the common channel 55 through the supply port 54. The common channel 55 communicates with an ink tank (not shown in FIG. 4, equivalent to the ink storage / loading unit 14 described in FIG. 1) as an ink supply source, and the ink supplied from the ink tank is a common flow. Distribution is supplied to each pressure chamber 52 via a passage 55.

  An actuator 58 (pressurizing means) provided with individual electrodes 57 is joined to a pressurizing plate (vibrating plate also serving as a common electrode) 56 constituting a part of the pressure chamber 52 (the top surface in FIG. 4). Has been. By applying a driving voltage to the individual electrode 57, the actuator 58 is deformed to change the volume of the pressure chamber 52, and ink is ejected from the nozzle 51 due to the pressure change accompanying this. The actuator 58 is preferably a piezoelectric element using a piezoelectric material such as lead zirconate titanate or barium titanate. After the ink is ejected, when the displacement of the actuator 58 returns, the pressure chamber 52 is refilled with new ink from the common flow channel 55 through the supply port 54.

  As shown in FIG. 4, the nozzle 51 provided in the print head 50 has a configuration including a nozzle opening 51 </ b> A, a discharge-side flow path 51 </ b> B that communicates the pressure chamber 52 and the nozzle opening 51 </ b> A, The diameter of the nozzle opening is Dn, the diameter of the discharge side channel 51B is Dn (same as the diameter of the nozzle opening 51A), and the channel length is ln. Note that the vicinity of the nozzle opening 51A may be formed in a tapered shape, or the discharge-side flow path 51B may be combined with a plurality of flow paths (pipe lines) having different diameters.

  Further, the supply port 54 for communicating the pressure chamber 52 and the common channel 55 has a cylindrical shape with a diameter Ds and a channel length ls as shown in FIG.

  As shown in FIG. 3B, the multiple ink chamber units 53 having such a structure are arranged in a row direction along the main scanning direction and an oblique column direction having a constant angle θ that is not orthogonal to the main scanning direction. The structure is arranged in a grid pattern with a constant arrangement pattern along the line. With a structure in which a plurality of ink chamber units 53 are arranged at a constant pitch d along a certain angle θ with respect to the main scanning direction, the pitch PN of the nozzles projected so as to be aligned in the main scanning direction (substantially in the main scanning direction) The typical nozzle interval is d × cos θ.

  That is, in the main scanning direction, each nozzle 51 can be handled equivalently as a linear array with a constant pitch PN. With such a configuration, it is possible to realize a high-density nozzle configuration in which 2400 nozzle rows are projected per inch (2400 nozzles / inch) so as to be aligned in the main scanning direction.

  In implementing the present invention, the nozzle arrangement structure is not limited to the illustrated example, and various nozzle arrangement structures such as an arrangement structure in which nozzles are arranged in a row direction along the main scanning direction and a column direction along the sub-scanning direction can be applied.

  In the present embodiment, a full line head having one or more nozzle rows arranged over a length corresponding to the entire width of the print medium in a direction substantially orthogonal to the print medium feed direction is shown. The serial type (which forms a dot row along the width direction of the print medium while moving a short head having a length shorter than the entire width of the print medium in the main operation direction orthogonal to the feed direction of the print medium) It can also be applied to a shuttle scan type head.

  In the present embodiment, a single-layer piezoelectric element having one piezoelectric layer is shown (FIG. 4). However, the present invention may be applied to a multilayer piezoelectric element in which two or more piezoelectric layers are laminated. .

  Furthermore, in this embodiment, although the press plate 56 was used as a common electrode, the press plate 56 and the common electrode may be provided separately. In an aspect in which the pressure plate 56 and the common electrode are separately provided, an insulating layer is provided between the pressure plate 56 and the common electrode when a conductive material such as a metal material is used for the pressure plate 56.

[Explanation of control system]
FIG. 5 is a principal block diagram showing the system configuration of the inkjet recording apparatus 10. The inkjet recording apparatus 10 includes a communication interface 70, a system controller 72, an image memory 74, an image memory 74, a motor driver 76, a heater driver 78, a print control unit 80, an image buffer memory 82, a head driver 84, and the like.

  The communication interface 70 is an interface unit that receives image data sent from the host computer 86. As the communication interface 70, a serial interface such as USB, IEEE 1394, Ethernet (registered trademark), a wireless network, or a parallel interface such as Centronics can be applied. In this part, a buffer memory (not shown) for speeding up communication may be mounted. Image data sent from the host computer 86 is taken into the inkjet recording apparatus 10 via the communication interface 70 and temporarily stored in the image memory 74.

  The image memory 74 is a storage unit that temporarily stores an image input via the communication interface 70, and data is read and written through the system controller 72. The image memory 74 is not limited to a memory made of a semiconductor element, and a magnetic medium such as a hard disk may be used.

  The system controller 72 includes a central processing unit (CPU) and its peripheral circuits, and functions as a control device that controls the entire inkjet recording apparatus 10 according to a predetermined program, and also functions as an arithmetic device that performs various calculations. . That is, the system controller 72 controls each part such as the communication interface 70, the image memory 74, the motor driver 76, the heater driver 78, etc., performs communication control with the host computer 86, read / write control of the image memory 74, etc. A control signal for controlling the motor 88 and the heater 89 of the transport system is generated.

  The ROM 75 stores programs executed by the CPU of the system controller 72 and various data necessary for control. The ROM 75 may be a non-rewritable storage means, or may be a rewritable storage means such as an EEPROM. The image memory 74 is used as a temporary storage area for image data, and is also used as a program development area and a calculation work area for the CPU.

The motor driver 76 is a driver that drives the motor 88 in accordance with instructions from the system controller 72. The heater driver 78 is a driver that drives the heater 89 such as the post-drying unit 42 in accordance with an instruction from the system controller 72.

  Under the control of the system controller 72, the print control unit 80 performs various processes such as processing and correction for generating a droplet ejection control signal from image data (multi-value input image data) in the image memory 74. It has a discharge data generation unit 92 having a signal processing function, and functions as discharge drive control means for supplying the generated discharge data (dot data) to the head driver 84.

  The print control unit 80 includes an image buffer memory 82, and image data, parameters, and other data are temporarily stored in the image buffer memory 82 when image data is processed in the print control unit 80. In FIG. 5, the image buffer memory 82 is shown in a form associated with the print control unit 80, but it can also be used as the image memory 74. Also possible is an aspect in which the print controller 80 and the system controller 72 are integrated and configured with one processor.

  An overview of the flow of processing from image input to print output is as follows. Image data to be printed is input from the outside via the communication interface 70 and stored in the image memory 74. At this stage, for example, RGB multivalued image data is stored in the image memory 74.

  In the ink jet recording apparatus 10, a pseudo continuous tone image is formed by changing the droplet ejection density and dot size of fine dots with ink (coloring material) to the human eye. It is necessary to convert to a dot pattern that reproduces the gradation (shading of the image) as faithfully as possible. Therefore, the original image (RGB) data stored in the image memory 74 is sent to the print control unit 80 via the system controller 72, and a dither method or error diffusion method is performed in the discharge data generation unit 92 of the print control unit 80. Is converted into dot data for each ink color by a halftoning process using the above.

  That is, the print control unit 80 performs processing for converting the input RGB image data into dot data of four colors K, C, M, and Y. Thus, the dot data generated by the print control unit 80 is stored in the image buffer memory 82. The dot data for each color is converted into CMYK droplet ejection data for ejecting ink from the nozzles of the print head 50, and the ink ejection data to be printed is determined.

  The print control unit 80 also includes a drive waveform generation unit 94 that generates a drive signal waveform for driving the actuator 58 of the print head 50, and a signal (drive waveform) generated by the drive waveform generation unit 94. ) Is supplied to the head driver 84. The signal output from the drive signal generation unit 94 may be digital waveform data or an analog voltage signal.

  The head driver 84 is based on ejection data (that is, dot data stored in the image buffer memory 82, CMYK droplet ejection data, or ink ejection data to be printed) and a drive waveform signal supplied from the print control unit 80. A drive signal for driving the actuator 58 corresponding to each nozzle 51 of the print head 50 is output according to the print contents. The head driver 84 may include a feedback control system for keeping the head driving conditions constant.

In this way, when the drive signal output from the head driver 84 is applied to the print head 50, ink is ejected from the corresponding nozzle 51. An image is formed on the recording paper 16 by controlling ink ejection from the print head 50 in synchronization with the conveyance speed of the recording paper 16. That is, the combination of the drive waveform generator 94 and the head driver 84 in FIG. 5 corresponds to the “drive signal generator” of the present invention. The characteristics of the drive signal output from the head driver 84 will be described later.

  As described above, the ejection amount and ejection timing of the ink droplets from each nozzle are controlled via the head driver 84 based on the dot data and the drive signal waveform generated through the required signal processing in the print control unit 80. Done. Thereby, a desired dot size and dot arrangement are realized.

  As described with reference to FIG. 1, the print detection unit 24 is a block including an image sensor, reads an image printed on the recording paper 16, performs necessary signal processing, etc. Variation, optical density, etc.) and the detection result is provided to the print controller 80. It should be noted that other discharge detection means (corresponding to discharge abnormality detection means) may be provided instead of or in combination with the print detection unit 24.

  As another discharge detection means, for example, a pressure sensor is provided in or near each pressure chamber 52 of the print head 50, and a detection signal obtained from the pressure sensor when ink is discharged or when an actuator for pressure measurement is driven. Using an optical detection system consisting of a light source such as a laser light emitting element and a light receiving element, and irradiating the liquid droplets ejected from the nozzle with light such as laser light. There may be a mode (external detection method) in which flying droplets are detected based on the amount of transmitted light (light receiving amount).

  The print control unit 80 performs various corrections to the print head 50 based on information obtained from the print detection unit 24 or other discharge detection means (not shown) as necessary, and performs preliminary discharge, suction, wiping, etc. as necessary. To perform the cleaning operation (nozzle recovery operation).

[Explanation of drive signal]
Next, drive signals applied to the inkjet recording apparatus 10 according to the present embodiment will be described.

  In the ink jet recording apparatus 10 of this example, a high viscosity ink having a higher viscosity (about 10 to 50 cP) than a commonly used ink is used (where 1 cP = 0.001 Pa.s), and a very small amount of high viscosity ink is used. It is configured to discharge at a short discharge cycle (that is, a high discharge frequency, for example, 40 kHz).

  FIG. 6 is a waveform diagram showing an example of a voltage waveform of a drive signal given to the actuator 58. The waveform of this drive signal 100 is the first pull (in which the actuator 58 is operated so that the pressure plate 56 shown in FIG. pull) driving waveform element 102, first pull holding waveform element 104 that holds the expanded state of the pressure chamber 52 expanded by the first pulling driving waveform element 102, and the pressure plate 56 are deformed in the direction of the nozzle 51. The actuator 58 is operated to cause the pressure chamber 52 to contract, the push drive waveform element 106, the push holding waveform element 108 holding the contracted state of the pressure chamber 52 contracted by the push drive waveform element 106, and the pressure chamber again. The second pulling drive waveform element 110 that expands the pressure chamber 52 and the second pulling holding waveform that maintains the expanded state of the pressure chamber 52 expanded by the second pulling drive waveform element 110. A waveform element 112, a second pull holding waveform element 112 settling recovery waveform element 114 a pressure chamber 52 which is held in an expanded state is contracted back to be static state by, composed.

The waveform of the drive signal 100 is configured to include at least a waveform element corresponding to a pulling operation (pull), then pushing (push), and further pulling (pull).
Ink in the pressure chamber 52 by pull-push-pull drive that combines a pull operation (pulling operation) and a subsequent pressing operation (push operation) followed by a second pulling operation (pull operation). Is discharged from the nozzle 51. Note that the second pulling operation corresponds to an operation of cutting the ink column coming out of the nozzle 51 by the push operation (an operation of tearing the liquid column to form a fine droplet).

  Characteristics 1 to 5 of the waveform of the drive signal 100 for performing this ejection operation are listed below.

[Characteristic 1]: The time T1 from the first pulling to the pressing is Tp when the time when the liquid is discharged fastest by pulling and pushing only (pull push drive) is
Tpp−1 [μs] ≦ T1 ≦ Tpp + 1 [μs]
Shall be satisfied. If the error is about ± 1 μs with respect to Tpp, it has been experimentally confirmed that ejection is performed. This Tpp will be described later.

   [Characteristic 2]: The time T2 from when the button is pressed to when it is pulled is set to be equal to or shorter than the time T1 from when the button is first pulled to when it is pressed (T2 ≦ T1). Preferably, T2 is in the vicinity of T1 (the difference between T2 and T1 is reduced), and more preferably, T2 and T1 are the same.

[Characteristic 3]: The amount of pulling (second pulling amount) V _sPL after pulling and pressing is equal to or greater than the pressing amount V _Ph when pressing (V _sPL ≧ V _Ph ).

FIG. 6 shows a waveform in which the second pulling amount (corresponding to the potential difference V _sPL ) is larger than the pressing amount (corresponding to the potential difference V _Ph ). However, as shown in the waveform example in FIG. A _{mode in} which V _sPL is equal to the pressing amount V _Ph is also possible. In FIG. 7, elements that are the same as or similar to those in FIG. 6 are given the same reference numerals, and descriptions thereof are omitted.

   [Characteristic 4]: The time T3 for maintaining the state pulled for the second time is longer than the time T1 from the first pulling to the pressing. Refilling is promoted during this period T3.

   [Characteristic 5]: The inclination of the static recovery waveform element 114 is set to be gentler than the inclination of the push drive waveform element 106 during ejection. This is to prevent liquid from being discharged by the static recovery waveform element 114.

[Detailed conditions of ink chamber unit in print head]
Next, the structure of the ink chamber unit 53 described in FIG. 4 will be described in detail.

  The print head 50 resonates with the compressibility (compliance) of the pressure chamber 52 and the inertia (inertance) of a supply-side flow path (hereinafter simply referred to as the supply port 54) including the nozzle 51 and the supply port 54. By performing pulling (pull-push-pull driving) by using, a predetermined discharge frequency is obtained.

  FIG. 8 shows a lumped constant model (lumped constant circuit) 300 when the function (characteristic) of the ink chamber unit 53 shown in FIG. 4 is replaced with an equivalent electric circuit.

  In the lumped constant model 300 shown in FIG. 8, Mn is the inertance of the nozzle 51, Rn is the resistance of the nozzle 51, Ms is the inertance of the supply port 54, Rs is the resistance of the supply port 54, Ci is the compliance of the pressure chamber 52, Ca Is the compliance of the actuator 58, and is expressed by the following equations.

  Where ρ is the ink density, ν is the ink viscosity, ln is the flow path length of the nozzle 51 (shown in FIG. 4), An is the area (cross-sectional area) of the nozzle 51, and rn is the radius of the nozzle 51 (FIG. 4). Is the diameter Dn of the nozzle 51) Is is the flow path length of the supply port 54 (shown in FIG. 4), An is the area (cross-sectional area) of the supply port 54, V is the volume of the pressure chamber 52, and c is in the ink. Is the generated pressure of the actuator 58 when the excluded volume is X, and X is the excluded volume of the actuator 58 when the generated pressure is P.

  The compliance Cn due to the surface tension of the nozzle 51 is sufficiently smaller than the compliance Ci of the pressure chamber 52 and is a capacity component that is almost negligible. Therefore, Cn is omitted in the system shown in the lumped constant model 300 shown in FIG. The excluded volume X referred to here is the sum of the volume V1 of ink ejected from the nozzle 51 to the outside, the volume V2 of ink returning to the supply side, and the volume V3 of ink compressed by pressurization. (V1 + V2 + V3).

In FIG. 8, un represents the volume velocity of the ink flowing in the nozzle 51, ui represents the volume velocity of the ink flowing in the pressure chamber 52, us represents the volume velocity of the ink flowing in the supply port 54, and .PHI. This is the pressure applied to the entire system.

  The differential equation of the lumped constant model 300 shown in FIG. 8 is expressed by the following equation.

  The pressure Pn applied to the nozzle 51 (the force obtained by the action of the inertance Mn of the nozzle 51 and the fluid resistance Rn of the nozzle 51) is expressed by the following equation.

  By solving the above simultaneous differential equation with the input waveform as a rectangular push wave (step function 400 as shown in FIG. 9), the volume velocity un of the ink in the nozzle 51 is expressed by the following equation ([Equation 13]). The pressure Pn of the nozzle portion is expressed by the following formula ([Equation 14]). In addition, since the solution method of a differential equation is well-known, the description is abbreviate | omitted in this specification.

  Here, C, D, and E are represented by the following equations, respectively.

  However, M and R shown above are constants that satisfy the following expression.

P ₀ represents the magnitude (absolute value) of the pressure (Φ in FIG. 8) applied to the entire system when the actuator is driven.

  When Pn (t) shown in the equation ([Formula 14]) is differentiated and the time change is examined, it can be seen that an extreme value is obtained at t = n × (π / E) (where n is an integer).

  Considering the case of n = 0, 1, each is expressed by the following equations.

  The fact that the pressure decay is severe means that the pressure (absolute value) at t = π / E is small. This specific amount can be defined as a case where the second extreme value (absolute value) is 1 / e or less of the first extreme value (absolute value). Therefore, when this condition is expressed by an expression,

It becomes. When rewritten using the equations ([Equation 20], [Equation 21]), the following equation is obtained.

  This can be rewritten as:

  When rewritten using the relationship of the equations ([Equation 16], [Equation 17]), the following equation is obtained.

  On the other hand, since there is a condition that D and E are positive as a condition for the pressure fluctuation to oscillate, the following expression is obtained.

  When the above conditions are arranged, the following equation is obtained.

  From the above, by designing a head that satisfies the above, it is possible to discharge micro droplets with a drive waveform as in the present invention.

  Here, when the input waveform is a rectangular pull-push waveform as shown in FIG. 10, the conditions under which the maximum ejection force can be obtained will be considered.

  From the equation of volume velocity at step input ([Equation 13]), the volume velocity in the pull-push waveform 410 as shown in FIG. 10 is expressed by the following equation.

Since the discharge area is t ≧ t ₀ , considering this range, the following expression is obtained.

In contrast to the volume velocity function (−un (t)) generated by the pull waveform element 412 in FIG. 10, the volume velocity function (un (t−t ₀ )) generated by the push waveform element 414 is the natural vibration period Te. If the phase is shifted by a half period (π / E) of 2π / E, the two are strengthened to obtain the maximum discharge force. Therefore, in the formula ([Equation 29]), t ₀ = π / E may be set. That is, the time Tpp at which the liquid is discharged the fastest by discharging only by pulling and pushing (rectangular wave pull-push driving) is a half period of the natural vibration period Te, that is, t ₀ = π / E described above.

[Example of head design]
FIG. 11 shows a print head design example. The pressure chamber had a square planar shape with a side of 300 μm and a height of 125 μm. The supply port had a long cross-sectional area of 60 μm, a short side of 30 μm, and a length (the flow path length of the supply port) of 200 μm. The nozzle was a circular tube with a diameter of 20 μm and a length of 20 μm. As for the characteristics of the actuator, the generated pressure when the drive voltage was applied (applied voltage peak value: 30 V) was 2 MPa, the excluded volume was 20 pl (picoliter), and the resonance point was 1 MHz. The ink used had a viscosity of 20 cP and a surface tension of 30 mN / m.

  A 1.8 pl (picoliter) droplet is ejected at 7 m / s so that the head designed under the above conditions is ejected using the drive signal 100 illustrated in FIGS. 6 and 7. Is done.

  The illustrated conditions are examples, and the present invention is not limited to these conditions when implementing the present invention. In the illustrated head design, the range of the viscosity that can be discharged with high viscosity is 17 to 35 cP. For example, as a numerical range of the head design at a viscosity of 20 cP, the height of the pressure chamber can be changed within a range of 100 to 125 μm when the illustrated head design value is used as a base. Moreover, one side with a long cross-sectional area of a supply port can be changed in the range of 60-80 micrometers.

  Note that when changing the value of the pressure chamber height or the long side of the cross-sectional area of the supply port from the numerical value shown in the figure, it is acceptable to change one of the values, and changing both values at the same time makes discharge difficult. It becomes.

  Further, as a head design at a viscosity of 15 cP, for example, a pressure chamber: 300 × 300 × 100 μm, a supply port: 40 × 30 μm, and a length of 200 μm can be considered (other parameters are the same as in the example of FIG. 11). . As an example of the change design in this case, the height of the pressure chamber can be changed in the range of 100 to 125 μm, and the long side of the cross-sectional area of the supply port can be changed in the range of 30 to 40 μm.

  In the above description, the ink jet recording apparatus is illustrated as an example of the liquid ejecting apparatus. However, the application range of the present invention is not limited to this, and various kinds of liquid ejected onto the ejected medium and adhered onto the ejected medium. The present invention can be applied to a liquid discharge apparatus (image forming apparatus, coating apparatus, coating apparatus, spraying apparatus, wiring forming apparatus, etc.) for use.

1 is an overall configuration diagram of an inkjet recording apparatus according to an embodiment of the present invention. FIG. 1 is a plan view of a main part around a printing unit of the ink jet recording apparatus shown in FIG. Plane perspective view showing the configuration of the print head of the ink jet recording apparatus shown in FIG. The figure which shows the three-dimensional structure of the print head shown in FIG. Main block diagram showing the system configuration of the inkjet recording apparatus of this example Waveform diagram showing an example of drive signal Waveform diagram showing another example of drive signal The figure which shows the lumped constant model of the print head shown in FIG.3 and FIG.4 The figure which shows the example of the input pressure at the time of calculating | requiring the volume velocity of the ink in a nozzle The figure which shows the other example of the input pressure at the time of calculating | requiring the volume velocity of the ink in a nozzle Chart showing head design examples

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Inkjet recording device, 12 ... Printing part, 12K, 12C, 12M, 12Y ... Print head, 16 ... Recording paper, 50 ... Print head, 51 ... Nozzle, 52 ... Pressure chamber, 54 ... Supply port, 55 ... Common flow Path: 58 ... Actuator 72 ... System controller 80 ... Print controller 85 ... Drive signal generator 100 ... Drive signal 102 ... First pull drive waveform element 104 ... First pull hold waveform element 106 ... push drive waveform element, 108 ... push hold waveform element, 110 ... second pull drive waveform element, 112 ... second pull hold waveform element, 114 ... static definite recovery waveform element

Claims (9)

A nozzle for discharging liquid;
A pressure chamber filled with a liquid communicating with the nozzle and discharged from the nozzle;
An actuator for changing the volume of the pressure chamber to change the pressure of the liquid in the pressure chamber and discharging the liquid from the nozzle;
Drive signal generating means for generating a drive signal for driving the actuator;
With
The drive signal includes a first pull drive waveform element that drives the actuator to increase the volume of the pressure chamber;
A push drive waveform element that drives the actuator to reduce the volume of the pressure chamber after the first pull drive waveform element to discharge liquid from the nozzle;
And a second pulling drive waveform element that drives the actuator to increase the volume of the pressure chamber again after the push driving waveform element and to cut the liquid column of the discharge liquid,
The liquid ejecting apparatus according to claim 1, wherein the volume increased again by the second pulling drive waveform element is equal to or larger than the volume decreased by the push drive waveform element. Assuming that the natural vibration period of the liquid volume velocity at the nozzle is Te, the time T1 from the first drawing by the first pulling drive waveform element to the pushing by the push driving waveform element is given by the following formula (Te / 2) − 1 [μs] ≦ T1 ≦ (Te / 2) +1 [μs]
The liquid ejecting apparatus according to claim 1, wherein:   The time T2 from the first pulling drive waveform element to the second pulling drive waveform element after being pushed by the push driving waveform element is the time T1 from the first pulling by the first pulling drive waveform element to the pushing by the push driving waveform element. The liquid discharge apparatus according to claim 1, wherein:   The drive signal includes a pull holding waveform element that holds the expanded state of the pressure chamber increased in volume by the second pull driving waveform element, and a volume of the pressure chamber that is held in the expanded state by the pull holding waveform element. 4. The liquid ejection apparatus according to claim 1, further comprising a static recovery waveform element that decreases the pressure to return to a static state. 5.   The time T3 for holding the expanded state by the pull holding waveform element is longer than the time T1 from the first pulling by the first pull driving waveform element to the pressing by the push driving waveform element. Item 5. The liquid ejection device according to Item 4.   6. The liquid ejecting apparatus according to claim 4, wherein the inclination of the waveform of the static recovery waveform element is gentler than the inclination of the waveform of the push drive waveform element.   The liquid ejection apparatus according to claim 1, wherein the liquid has a viscosity of 10 cP to 50 cP. The length of the nozzle is ln, the area of the nozzle is An, the flow path length of the supply port for supplying the liquid to the pressure chamber is ls, the area of the supply port is As, and the volume of the pressure chamber is V The density of the liquid is ρ, the viscosity of the liquid is ν, the speed of sound c in the liquid is P, the generated pressure of the actuator when the excluded volume is X, and the excluded volume of the actuator when the generated pressure is P X, where

The liquid ejecting apparatus according to claim 1, wherein:   9. An image forming apparatus comprising the liquid ejecting apparatus according to claim 1, wherein an image is formed on a recording medium by an ink liquid ejected from the nozzle.