JP4928328B2 - Liquid ejection device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve discharge stability by keeping the concentration of a solvent in ink in the vicinity of a discharge opening by a minimum consumption of the solvent in accordance with a variation in the solvent concentration for every discharge opening with respect to the thickening of the ink in the vicinity of the discharge opening. <P>SOLUTION: The liquid discharge device has an ink channel 60 communicating with a discharge opening 51 and having a discrete channel portion 62 and a common channel portion 61 provided with an actuator 58, a solvent channel 70 (71 and 72) connected to the discrete channel portion 62 of the ink channel 60 and a semitransparent membrane 73 provided in the connection between the discrete channel portion 62 and the solvent channel 70 of the ink channel 60. <P>COPYRIGHT: (C)2009,JPO&amp;INPIT

Description

  The present invention relates to a liquid ejection device that ejects liquid.

  Various types of liquid ejection apparatuses having a liquid ejection head that ejects ink and having a function of preventing non-ejection are provided.

  Japanese Patent Application Laid-Open No. H10-228667 describes a device in which a solvent holding member made of a porous material impregnated with an ink solvent is pressed against a nozzle surface (discharge port forming surface) of a liquid discharge head.

  Japanese Patent Application Laid-Open No. 2004-228688 describes a liquid discharge head nozzle (discharge port) that is immersed in a solvent container provided adjacent to the liquid discharge head.

  Patent Document 3 describes a nozzle plate that is made of a porous material, and ink is supplied to the nozzles through the porous material.

Patent Documents 4 and 5 also describe an ink tank in which recovered ink and ink solvent are brought into contact with each other through a semipermeable membrane.
JP 2004-188682 A JP 2006-136770 A JP 2003-191470 A JP-A-9-29995 JP-A-9-58013

  In the invention described in Patent Document 1, since the solvent holding member made of a porous material cannot be pressed against the nozzle surface of the liquid discharge head unless the printing is in a maintenance state or a printing storage state such as a nozzle storage state, in the printing state, discharge is difficult. There is a problem that the outlet cannot be prevented from drying.

  In the invention described in Patent Document 2, since the nozzle of the liquid discharge head cannot be immersed in the solvent container unless it is in a printing pause state such as a maintenance state or a nozzle storage state, as in the invention described in Patent Document 1, There is a problem that the discharge port cannot be dried in the printing state.

  In the invention described in Patent Document 3, when ink is supplied to a nozzle plate made of a porous material, ink is supplied to the nozzle regardless of the thickened state of ink for each nozzle and the driving state for each nozzle. Therefore, it is true that if the ink is supplied excessively, the ink density is maintained in all the nozzles, but there is a problem that the amount of supplied ink becomes excessive. Further, when the solvent is supplied to the nozzle plate made of a porous material instead of the ink, there is a problem that the ink density of the nozzle becomes thinner than a desired density.

  Note that the inventions described in Patent Documents 4 and 5 cannot be expected to have an effect on the ink thickening phenomenon that occurs locally at the nozzles of the liquid ejection head, even if the ink concentration can be set in the ink tank. Therefore, as conventionally performed, the ink in the liquid discharge head is discarded by so-called purging or the like, thereby preventing nozzle non-discharge.

  The present invention has been made in view of such circumstances, and in contrast to the ink thickening in the vicinity of the ejection port, the ink in the vicinity of the ejection port can be consumed with the minimum solvent consumption according to the change in the solvent concentration for each ejection port. An object of the present invention is to provide a liquid ejection apparatus that can maintain the solvent concentration and improve the ejection stability.

In order to achieve the object, the invention according to claim 1 is common to a plurality of individual flow path portions respectively connected to a plurality of discharge ports and provided with actuators, and a plurality of the individual flow path portions. A first flow path having a common flow path section, a second flow path connected to the individual flow path section of the first flow path, the individual flow path section of the first flow path, and the A semipermeable membrane provided in a connection portion with the second flow path, a first liquid supply source for supplying the first liquid discharged from the discharge port to the first flow path, and the first liquid. A second liquid that includes at least one solvent that is included and that passes through the semipermeable membrane and that has a higher concentration of the solvent than the first liquid. 2 liquid supply sources and the back pressure of the second flow path by controlling the back pressure of the second flow path. Pressure setting means for setting a pressure difference from the pressure, wherein the pressure setting means sets the back pressure of the first flow path to P1, and the solvent in the first liquid is not volatilized from the discharge port in the initial stage. If the osmotic pressure generated by the semipermeable membrane in the state is Π0, the back pressure of the second flow path is set to (P1 （Π0) in the nonvolatile state where the solvent does not volatilize from the discharge port, and the discharge port In the volatile state in which the solvent volatilizes, the back pressure of the second flow path is set lower than the back pressure P1 of the first flow path and higher than (P1−Π0), and around the discharge port A liquid ejection apparatus characterized by changing the back pressure of the second flow path in accordance with a change in the solvent vapor pressure of the liquid to balance the volatilization of the solvent and the supply of the solvent from the second flow path. I will provide a.

According to a second aspect of the present invention, in the first aspect of the invention, the pressure setting unit includes a vapor pressure sensor that detects a solvent vapor pressure around the discharge port, and the solvent is volatilized from the discharge port. In the volatilized state, a liquid ejection apparatus is provided that changes the back pressure of the second flow path in accordance with a change in solvent vapor pressure around the ejection port detected by the vapor pressure sensor .

According to a third aspect of the present invention, in the first or second aspect of the invention, the individual flow path portion of the first flow path includes a pressure chamber in which the actuator is provided, and the discharge from the pressure chamber. And a discharge port communication path leading to an outlet, wherein the second flow path is connected to the discharge port communication path.

The invention according to claim 4 is the invention according to claim 1 or 2 , wherein the individual flow path portion of the first flow path includes at least a flow path portion where the actuator is provided, The second flow path is connected to a position between the position where the actuator is provided and the discharge port in the individual flow path section of the first flow path. Providing equipment.

  According to the present invention, with respect to ink thickening in the vicinity of the ejection port, the solvent concentration in the ink in the vicinity of the ejection port is maintained with minimum solvent consumption according to the change in the solvent concentration for each ejection port, thereby improving ejection stability. Can be improved.

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

  First, the osmotic pressure used in the present invention will be described.

  The osmotic pressure is a pressure that works so that the solvent moves from a liquid with a high solvent concentration to a liquid with a low solvent concentration through a semipermeable membrane (also referred to as “permeation membrane”) that transmits only small molecules.

  For example, when a semipermeable membrane that allows the solvent to pass freely but does not allow the solute to pass through is fixed in the water tank, a solution consisting of the solvent and the solute is in contact with one surface of the membrane, and only the solvent is in contact with the other surface. Part of the solvent penetrates through the semipermeable membrane into the solution, and the pressure relationship reaches equilibrium on both sides of the semipermeable membrane. The pressure difference between the two sides of the semipermeable membrane at this time is the osmotic pressure. Osmotic pressure causes solvent migration.

  It is known that the osmotic pressure can be expressed by the following Phanto-Hoff equation, where C is the molar concentration of the solution, T is the absolute temperature, and R is the gas constant.

[Equation 1]
Π = CRT
For example, in an aqueous solution having a solute concentration of 0.6 kmol / m ³ at 25 ° C., the osmotic pressure is 2.4 megapascals (approximately 24 atmospheres).

  As such a semipermeable membrane, for example, a precipitated film of copper ferrocyanide, a collodion film, and a bladder film are well known, but regenerated cellulose (cellophane), polyacrylonitrile, cellulose acetate, which are easy to handle industrially, Aromatic polyamide, polyvinyl alcohol, polysulfone, and Teflon (registered trademark) are often used. Furthermore, in order to maintain the strength of the film, a combination of these is also used. For example, a combination of aromatic polyamide and polyvinyl alcohol, a combination of aromatic polyamide and polysulfone, or the like. Specific examples of preferred semipermeable membranes used in the present invention will be described later.

  Next, the term “back pressure” used in this specification will be explained.

  The term “back pressure” of a flow path refers to the pressure of a liquid (solution or solvent) in the flow path, which is applied from the outside of the flow path.

  A pressure lower than the atmospheric pressure is referred to as “negative pressure”. For example, in FIG. 3, the pressure Pb and the pressure Pc are lower than the pressure Pa, and changing from the pressure Pb to the pressure Pc is expressed as lowering the pressure.

  By the way, in a liquid discharge head (hereinafter simply referred to as “head”), an actuator constituted by a piezoelectric element such as a piezo or a heating element (electrothermal conversion element) is driven to discharge ink to communicate with the discharge port. By applying pressure to the pressure chamber, ink is ejected from the ejection port, and the pressure change at that time is a very fast phenomenon. For example, a maximum of 20,000 times per second is performed in one pressure chamber.

  On the other hand, the “back pressure” in the present specification is a pressure that is normally maintained at a substantially constant value. Even in the case of intentionally changing, the time required for the change is about 0.1 seconds at the earliest. It is a thing different from the pressure change in said discharge.

  Further, “back pressure” described later in this specification is the pressure of the liquid in the flow channel during non-ejection. In addition, in a head using a piezoelectric element as an actuator, in order to suppress thickening in the vicinity of the ejection port during non-ejection, there is a case where a slight vibration that does not eject ink, which is called meniscus fluctuation, is given to the ink. In such a slight vibration, the average pressure is set to “back pressure”.

  Next, the term “volatilization” used in this specification will be explained.

  The term “volatilization” of the solvent refers to vaporization of the solvent at the ambient temperature where the head is used.

  The vaporization of water is generally called “evaporation”, but hereinafter, it is referred to as “volatilization” regardless of whether the solvent is water or not, and the volatile solvent contains water. Further, the environmental temperature in which the head is used is not particularly limited to a so-called normal temperature (generally 15 ° C. to 25 ° C.).

  Hereinafter, the liquid ejection apparatus according to the present invention will be described in various embodiments.

[First Embodiment]
FIG. 1 is a plan perspective view conceptually showing a main part of a liquid ejection apparatus 10 according to the present invention. FIG. 2 is a cross-sectional view taken along line 2-2 of FIG.

  In FIG. 1, an ink supply source 31 is constituted by a tank (for example, a sub tank) that stores ink, and is disposed in an ink flow path 60 (61, 62) of a liquid discharge head 50a (hereinafter simply referred to as “head”). Supply ink. The ink contains a volatile solvent such as water that volatilizes in the air atmosphere (hereinafter simply referred to as “solvent”) and a solute such as a coloring material. The ink in the ink supply source 31 is set to a solvent concentration suitable for ejection from the head 50a. The solvent supply source 32 is constituted by a tank for storing a solvent, and supplies only the solvent contained in the ink to a solvent flow path 70 (71, 72) of the head 50a described later. The ink flow path pressure source 33 sets a back pressure of an ink flow path 60 (61, 62) described later in the head 50a. The solvent flow path pressure source 34 sets a back pressure of a solvent flow path 70 (71, 72) described later in the head 50a. Examples of the ink channel pressure source 33 and the solvent channel pressure source 34 include a pump, a pressure source using a height difference (water head difference), a pressure source using a piston, and the like. The ink flow path back pressure detector 35 is a known pressure sensor, and detects the back pressure of an ink flow path 60 (61, 62) to be described later in the head 50a. The solvent flow path back pressure detection unit 36 includes a known pressure sensor, and detects the back pressure of a solvent flow path 70 (71, 72) described later in the head 50a.

  In the present embodiment, the head 50a includes an ink common channel 61 communicating with the ink supply source 31 as shown in FIG. 1, and a throttle 53 and a pressure chamber 52 with respect to the ink common channel 61 as shown in FIG. The ink individual flow path 62 and the actuator (58 in FIG. 2) communicate with each other in the order of the ejection port communication path 512 and the ejection port 51 (also referred to as “nozzle”). The ink common flow path 61 and the individual ink flow path 62 constitute an ink flow path 60. As the actuator (58 in FIG. 2), a known one such as a piezoelectric element, a heating element (electrothermal conversion element), or an electrostatic actuator is used. Ink is ejected from the ejection port 51 by changing the volume of the pressure chamber 52 by the actuator 58 or by generating water bubbles by vaporizing water in the ink. The discharge port communication path 512 is a flow path portion disposed between the pressure chamber 52 and the discharge port 51. The throttle 53, the pressure chamber 52, the discharge port communication path 512, the discharge port 51, and the actuator 58 are combined to form a liquid discharge element, and the head 50a is formed by a plurality of liquid discharge elements. The arrangement of the discharge ports 51 is drawn in a one-dimensional array in FIG. 1 for convenience of illustration, but in this example, the arrangement of the discharge ports 51 is a two-dimensional matrix. In the present invention, the arrangement of the discharge ports 51 is not particularly limited, and may be another known arrangement such as a linear shape or a staggered shape.

  The head 50a is formed with a common solvent flow path 71 communicating with the solvent supply source 32 and a separate solvent flow path 72 extending from the common solvent flow path 71 to the discharge port communication path 512 of each liquid discharge element. ing. The solvent common channel 71 and the individual solvent channel 72 constitute a solvent channel 70 separated from the ink channel 60.

  Note that the individual solvent flow path 72 of this example is connected to the ejection port communication path 512 in the individual ink flow path 62, but in the present invention, the connection point between the individual solvent flow path 72 and the individual ink flow path 62. Is not particularly limited to the discharge port communication path 512, and includes, for example, the case where the solvent individual flow path 72 is connected to the pressure chamber 52.

  As the solvent supplied from the solvent supply source 32, for example, a substance (solid particles, dissolved and dispersed in ink supplied from the ink supply source 31 to the head 50a (that is, ink discharged from the discharge port 51)). A pure solvent having a purity of approximately 100% from which ions and gases are removed is used.

  If the main solvent (first solvent) in the ink is pure and the viscosity is high, the solvent cannot be allowed to flow through the solvent flow path 70 alone. Therefore, the corresponding solvent (second solvent) in the ink is used. Then, it is mixed with a solvent that does not easily volatilize in the air atmosphere and supplied to the solvent flow path 70. For example, the concentration of the first solvent is higher than the concentration of the first solvent in the ink, and the concentration of the second solvent is equal to the concentration of the second solvent in the ink. A liquid containing the second solvent is supplied to the solvent flow path 70.

  At least one solvent contained in the ink supplied from the ink supply source 31 permeates the boundary between the individual ink flow path 62 and the individual solvent flow path 72, while other than the solvent contained in the ink. A semipermeable membrane 73 having a property of not transmitting a predetermined component (specifically, a solute such as a coloring material) is provided. That is, the solvent individual flow path 72 is connected to the ink individual flow path 62 in the vicinity of the ejection port 51 through the semipermeable membrane 73.

  Suitable materials for the semipermeable membrane 73 include semipermeable membranes such as regenerated cellulose (cellophane), polyacrylonitrile, cellulose acetate, aromatic polyamide, polyvinyl alcohol, polysulfone, Teflon (registered trademark), and combinations thereof. It is. In particular, a combination of aromatic polyamide and polyvinyl alcohol, or a combination of aromatic polyamide and polysulfone having high strength is preferable.

  The ink channel pressure source 33, the solvent channel pressure source 34, the ink channel back pressure detector 35, the solvent channel back pressure detector 36, and the controller 38 shown in FIG. Pressure setting means for setting a pressure difference from the back pressure of the solvent flow path 70 is configured.

  The control unit 38 includes a microcomputer and its peripheral circuits. The controller 38 sets the back pressure of the ink channel 60 using the ink channel pressure source 33 based on the back pressure of the ink channel 60 detected by the ink channel back pressure detector 35. Further, the control unit 38 is based on the back pressure of the ink channel 60 detected by the ink channel back pressure detection unit 35 and the back pressure of the solvent channel 70 detected by the solvent channel back pressure detection unit 36. By using the solvent flow path pressure source 34 (or by using both the ink flow path pressure source 33 and the solvent flow path pressure source 34), the back pressure of the ink flow path 60 and the back pressure of the solvent flow path 70 are reduced. Set the pressure differential. There are various pressure setting modes by the controller 38, and these various pressure setting modes will be described in detail later.

  In FIG. 2, the head 50 a includes a discharge port plate 41 in which a discharge port 51 is formed, a discharge port communication channel plate 42 that is fixed to the discharge port plate 41 and in which a discharge port communication channel 512 is formed, The pressure chamber plate 43 is fixed to the discharge port communication passage plate 42 and has the pressure chamber 52, the throttle 53 and the ink common flow path 61. The pressure chamber plate 43 is fixed to the top of the pressure chamber 52. The diaphragm 44 includes a surface, and an actuator 58 fixed to a position corresponding to the pressure chamber 52 on the diaphragm 44. Further, in this example, the discharge port communication path plate 42 constitutes the solvent flow path plate 421 in which the solvent flow path 70 (the solvent common flow path 71 and the solvent individual flow path 72) is formed, and the bottom surface of the pressure chamber 52. The pressure chamber bottom plate 422 is configured. Thus, the individual solvent flow path 72 is formed on the plate closest to the discharge port plate 41 in which the discharge port 51 is formed, and the semipermeable membrane 73 is formed in the vicinity of the discharge port 51 formed in the discharge port plate 41. Is preferably arranged.

  Next, various modes of pressure setting that keep the concentration of the solvent (solvent concentration) contained in the ink near the ejection port 51 constant using the semipermeable membrane 73 will be described.

  In various aspects, as described above, the liquid ejection device 10 includes the ink flow path 60 and the solvent flow path 70 connected to the individual ink flow path 62 that is the individual flow path portion of the ink flow path 60. A semipermeable membrane 73 is provided at a connection portion between the individual ink flow path 62 and the solvent flow path 70. The liquid ejection device 10 includes a first liquid supply source (ink supply source 31) that supplies ink ejected from the ejection port 51 to the ink flow path 60, and at least one solvent contained in the ink. A second liquid supply source (solvent supply source 32) that supplies the solvent flow path 70 with a liquid that contains a solvent that passes through the semipermeable membrane 73 and that has a higher concentration of the solvent than the ink, and the ink flow path 60. Pressure setting means (33, 34, 35, 36, 38) for setting the pressure difference between the back pressure of the solvent and the back pressure of the solvent flow path 70 is provided.

  In the following, the pressure of the ink in the ink flow path 60 is equal in the common flow path portion (ink common flow path 61) and the individual flow path portion (ink individual flow path 62), and the solvent flow path 70. It is assumed that the pressure of the solvent in the inside is equal between the common flow path part (solvent common flow path 71) and the individual flow path part (solvent individual flow path 72). Hereinafter, the individual ink flow path 62 is also referred to as a “first flow path”, and the individual solvent flow path 72 is sometimes referred to as a “second flow path”. A state in which the solvent (for example, water) in the ink is volatilized from the ejection port 51 is referred to as a “volatile state”, and a state in which the solvent (for example, water) in the ink is not volatilized from the ejection port 51 is referred to as a “nonvolatile state”.

  FIG. 4A shows a fresh state (hereinafter referred to as “initial state”) in which the solvent in the ink has not yet volatilized from the ejection port 51 by simplifying the head structure. In such an initial state, that is, in a state where the solvent concentration of the ink in the vicinity of the ejection port 51 is an initial value, the back pressure P1 of the first flow path 62 and the meniscus in the ejection port 51 are expressed as shown in the following equation (2). Is balanced with the surface tension ST.

[Equation 2]
P1 = ST
Further, as shown in the following Equation 3, the back pressure P2 of the second flow path 72 is set lower than the back pressure P1 of the first flow path 62 by the osmotic pressure 0.

[Equation 3]
P2 + Π0 = P1
Here, Π0 is an osmotic pressure generated by the semipermeable membrane 73 in an initial state where the solvent in the ink has not yet evaporated from the ejection port 51.

  In the state shown in Equation 3, the back pressure P1 of the first flow path 62 is equal to the sum of the back pressure P2 of the second flow path 72 and the osmotic pressure ２0 (P2 + Π0). No movement of the solvent through 73 occurs.

  FIG. 4B represents the volatile state by simplifying the head structure. In such a volatile state, that is, in a state where the solvent concentration of the ink in the vicinity of the ejection port 51 is reduced from the initial value due to the solvent volatilization indicated by the arrow 301 in FIG. The pressure increases from Π0 to Π1 (Π0 <Π1).

[Equation 4]
P2 + Π1> P1
In the state shown in Equation 4, the back pressure P1 of the first flow path 62 is lower than the sum of the back pressure P2 of the second flow path 72 and the osmotic pressure Π1 (P2 +） 1). As indicated by an arrow 302 in B), the solvent moves from the second channel 72 to the first channel 62 through the semipermeable membrane 73. Then, when the pressure shown in Equation 3 is balanced, the movement of the solvent stops. As a result, the solvent concentration of the ink in the vicinity of the ejection port 51 is kept substantially constant.

  In the above-described aspect (first pressure setting aspect), in short, the back pressure P2 of the second flow path 72 is higher than the back pressure P1 of the first flow path 62 in both the nonvolatile state and the volatile state. The pressure is set to a lower pressure (P1-Π0) by the osmotic pressure Π0 in the non-volatile state (initial state).

  Such pressure setting is based on the back pressure of the first flow path 62 detected by the two back pressure detection sections (the ink flow path back pressure detection section 35 and the solvent flow path back pressure detection section 36) in FIG. Based on the back pressure of the second flow path 72, the control unit 38 of FIG. 1 uses the solvent flow path pressure source 34 (or both the ink flow path pressure source 33 and the solvent flow path pressure source 34) to This is done by setting the pressure difference between the back pressure of the flow path 62 and the back pressure of the second flow path 72 to Π0.

  The present invention is not particularly limited to the first pressure setting mode described above. The back pressure of the second flow path 72 in the volatile state may be set to be lower than the back pressure P1 of the first flow path 61 and higher than (P1−１0). Such an aspect (second pressure setting aspect) will be described below.

  As shown in Equation 1 above, basically, the osmotic pressure does not depend on the composition and thickness of the semipermeable membrane 73, but the speed at which the solvent passes through the semipermeable membrane 73 is affected by these effects. Receive. Briefly, the semipermeable membrane 73 should be as thin as possible, but it needs a certain thickness in terms of manufacturing limitations and strength. However, it takes time for the solvent to permeate through the thick semipermeable membrane 73. Therefore, in consideration of the time for the solvent to pass through the semipermeable membrane 73, when the solvent starts to volatilize in the vicinity of the discharge port 51, the second flow is set so that the solvent easily moves through the semipermeable membrane 73. The back pressure of the path 72 is set slightly higher than the back pressure P2 (= P1-Π0) of the second flow path 72 in the initial state (the absolute value is reduced as a negative pressure).

  The pressure relationship between the back pressure P2 'of the second flow path 72 and the back pressure P1 of the first flow path 62 in the volatilized state in the second pressure setting mode is expressed by the following equation (5).

[Equation 5]
P2 '+ Π0> P1
That is, the back pressure P2 of the second flow path 72 in the non-volatile state is lower than the back pressure P1 of the first flow path 62 by the osmotic pressure の 0 in the initial state as described using Equation 3 above. While setting (P1-Π0), the back pressure P2 ′ of the second flow path 72 in the volatile state is lower than the back pressure P1 of the first flow path 62 and higher than (P1-Π0). Set.

  Even when the pressure is set as shown in Equation 5, in the volatile state, the back pressure P1 of the first flow path 62 is the sum of the back pressure P2 ′ of the second flow path 72 and the actual osmotic pressure 2 (P2 Therefore, the solvent moves from the second flow path 72 to the first flow path 62 through the semipermeable membrane 73.

  Such pressure setting is based on the back pressure of the first flow path 62 detected by the two back pressure detection sections (the ink flow path back pressure detection section 35 and the solvent flow path back pressure detection section 36) in FIG. Based on the back pressure of the second flow path 72, the control unit 38 of FIG. 1 uses the solvent flow path pressure source 34 (or both the ink flow path pressure source 33 and the solvent flow path pressure source 34) to This is performed by setting a pressure difference between the back pressure of the flow path 62 and the back pressure of the second flow path 72.

  Considering not only the speed at which the solvent passes through the semipermeable membrane 73 but also the concentration gradient of the solvent from the meniscus of the discharge port 51 to the semipermeable membrane 73, the back pressure of the second flow path 72 is set. Also good. Such a pressure setting mode (third pressure setting mode) will be described below.

  In FIG. 5A, since the moving speed of the solvent in the ink is finite, if the solvent is constantly volatilized from the ejection port 51, the solvent concentration in the ink from the meniscus of the ejection port 51 to the semipermeable membrane 73 is obtained. A gradient occurs. FIG. 5B shows the concentration of the solvent in the ink in time series in the order of the first curve 411, the second curve 412, and the third curve 413 with respect to the straight line 410 indicating the initial solvent concentration. It shows how the gradient changes. 5B, the horizontal axis indicates the position on the curve 401 from the meniscus of the discharge port 51 to the semipermeable membrane 73 shown in FIG. 5A as the meniscus of the discharge port 51 being “0”. The vertical axis indicates the solvent concentration at each position on the curve 401. As a result of such a concentration gradient, the ink of the meniscus at the discharge port 51 has a reduced solvent concentration and an increased viscosity.

  Therefore, taking into account the solvent concentration gradient, the back pressure of the second flow path 72 in the volatile state is set to be higher than the aforementioned P2 '(the absolute value of the negative pressure is small). A pressure relationship between the back pressure P2 ″ of the second flow path 72 and the back pressure P1 of the first flow path 62 in the volatilized state in the third pressure setting mode is expressed by the following equation (6).

[Equation 6]
P2 "+ Π0> P1
That is, the back pressure P2 of the second flow path 72 in the non-volatile state is lower than the back pressure P1 of the first flow path 62 by the osmotic pressure の 0 in the initial state as described using the above equation 3. On the other hand, the back pressure of the second flow path 72 in the volatile state is set to P2 ″ lower than the back pressure P1 of the first flow path 62 and higher than the P2 ′. (P2 ″> P2 ′).

  As a result, as shown in the solvent concentration gradient curve 420 in FIG. 5C, the ink solvent concentration at the position closer to the ejection port 51 than the semipermeable membrane 73 (specifically, the ejection port communication path 512) is set to the initial value. The solvent concentration 410 is slightly higher than the above, and the viscosity of the meniscus at the discharge port 51 is somewhat lowered. In addition, it is possible to keep the average density of ink at the position closest to the ejection port 51 at a normal value (initial value, design value).

  The state shown in Equation 6 is a state in which the volatilization of the solvent from the discharge port 51 and the supply of the solvent from the second flow path 72 are balanced. Therefore, in a non-volatile state in which the solvent in the ink does not volatilize from the ejection port 51, such as when the ejection port surface of the head 50a is capped, control is performed to return P2 ″ to P2 (= P1−。0). 6 is obtained as shown in Fig. 6. In Fig. 6, P2 is the back pressure of the second flow path 72 in the capped state (non-volatile state), and P2 "is in the uncapped state (volatile) The back pressure of the second flow path 72 in the state).

By the way, the solvent concentration is determined by the composition of the ink, the composition and thickness of the semipermeable membrane, the back pressure of each flow path, and the size of the ejection port 51. Here, when the ink ejection characteristics are determined, the composition of the ink (the ratio of the solvent and the solute, the viscosity of the ink, the surface tension, etc. are determined), the back pressure of the first flow path 62 (the ink is discharged from the ejection port 51). In order to prevent leakage, in general, a negative pressure of about −10 to −50 mmH ₂ O is used, and the size of the ejection port 51 (for example, 15 to 25 μm) Influence) is determined by some desired value. Therefore, the composition and thickness of the semipermeable membrane 73 and the back pressure of the second flow path 72 are parameters that can be changed, and among these changeable parameters, the head 50a can be dynamically changed during use. What can be done is the back pressure of the second flow path 72. Therefore, by controlling the back pressure of the second flow path 72 as shown in FIG. 6, the solvent concentration in the ink near the ejection port 51 can be kept substantially constant.

  As described above, in the configuration shown in FIG. 1, when the solvent in the ink volatilizes from the ejection port 51 and the ink in the vicinity of the ejection port 51 becomes highly viscous (that is, when the solvent concentration decreases), the solvent in the ink. The partial pressure decreases, and a partial pressure difference of the solvent is generated between the first flow path 62 and the second flow path 72 with the semipermeable membrane 73 interposed therebetween. As a result, the solvent molecules move from the second flow path 72 to the first flow path 62 through the semipermeable membrane 73, and the solvent concentration in the ink near the ejection port 51 is kept constant. That is, with respect to the ink thickening in the vicinity of the ejection port 51, the solvent concentration in the ink in the vicinity of the ejection port 51 is kept constant with the minimum solvent consumption corresponding to the change in the solvent concentration for each ejection port 51, and the ejection stability. Can be improved.

  Note that the back pressure of the second flow path 72 may be reduced before reuse under conditions where the ink at the discharge port 51 portion becomes somewhat thick even after capping, such as after long-term storage. Is set higher than (P1-Π0) (the absolute value becomes smaller as the negative pressure). In this way, even in a state where the thickening has progressed, the ink in the vicinity of the ejection port 51 can be reduced in viscosity and maintenance can be facilitated.

  The case where only the solvent in the ink flows through the second flow path 72 has been described above as an example. However, the present invention is not limited to such a case, and the liquid flowing through the second flow path 72 is discharged from the discharge port. 51 includes at least one solvent (which is a solvent that volatilizes from the ejection port 51) contained in the ink (first liquid) ejected from the ink 51, and the concentration of the solvent is higher than that of the ink (first liquid). In the case of the second liquid, the characteristic effect of the present invention can be obtained.

  Next, an example of a method for forming the semipermeable membrane 73 will be described. This method is particularly effective when a thin semipermeable membrane 73 is formed.

  First, as shown in FIG. 7A, the discharge port plate 41 and the discharge port communication path plate 42 (the solvent flow path plate 421 and the pressure chamber bottom plate 422) are joined to each other. A semipermeable membrane forming substrate 74 for forming the semipermeable membrane 73 is prepared in the solvent individual flow channel 72 facing the discharge port communication channel 512. The semipermeable membrane forming substrate 74 is formed in a portion where the individual solvent flow path 72 is connected to the discharge port communication path 512. Such a semipermeable membrane forming substrate 74 can be formed using a photolithography technique.

  When the individual solvent flow path 72 is formed in a donut shape so as to surround the discharge port communication path 512, the semipermeable membrane forming substrate 74 has a donut shape surrounding the discharge port communication path 512. . Such a shape is desirable because the effective area of the semipermeable membrane 73 is increased.

  Next, as shown in FIG. 7B, a resin component is applied to the surface of the semipermeable membrane forming substrate 74 facing the discharge port communication path 512 to form a thin film as the semipermeable membrane 73 and dried. . Thereafter, the semipermeable membrane forming substrate 74 is removed. 7C, the discharge port 51, the discharge port communication path 512, and the solvent individual flow path 72 are provided, and the semipermeable membrane 73 is formed at the boundary between the solvent individual flow path 72 and the discharge port communication path 512. A structure 412 having is formed. The semipermeable membrane forming substrate 74 can be easily removed by using a water-soluble resin, a resin that dissolves in a specific acid or alkali, a metal, or the like.

  Thereafter, the structure 412 in FIG. 7C is joined to the pressure chamber plate (43 in FIG. 2).

  The semipermeable membrane 73 may be formed after the discharge port communication passage plate 42 and the discharge port plate 41 are joined to the pressure chamber plate (43 in FIG. 2).

  When the semipermeable membrane 73 to be formed may be relatively thick, the semipermeable membrane forming substrate 74 in FIG. 7A may be formed of the material of the semipermeable membrane 73. However, it is preferable to make the semipermeable membrane 73 thin in order to shorten the time for the solvent to move through the semipermeable membrane 73, and to prevent the occurrence of partial chipping of the semipermeable membrane 73, In view of improving the close contact of the flow path walls, it is desirable to form the semipermeable membrane 73 by the formation method shown in FIGS.

[Second Embodiment]
FIG. 8 is a cross-sectional view showing a head 50b as a main part of the liquid ejection apparatus according to the second embodiment. This sectional view shows a section taken along line 2-2 of FIG. In FIG. 8, the same components as those of the head 50a in the first embodiment shown in FIGS. 1 and 2 are denoted by the same reference numerals, and the description of the contents already described is omitted. To do.

  In FIG. 8, a vapor pressure sensor 74 is disposed on the formation surface (discharge surface) of the discharge port 51 of the head 50b. The vapor pressure sensor 74 detects the solvent vapor pressure around the discharge port 51 (“humidity” when the solvent is water). In this example, the vapor pressure sensor 74 is disposed on the ejection surface of the head 50b, but the present invention is not particularly limited to such a case, and is provided at other positions of the head 50b or outside the head 50b. It may be.

  In the present embodiment, the control unit (38 in FIG. 1) uses the solvent flow path pressure source (34 in FIG. 1) according to the change in the solvent vapor pressure around the discharge port 51 detected by the vapor pressure sensor 74. Thus, the pressure difference between the back pressure of the ink flow path 60 and the back pressure of the solvent flow path 70 is set by performing control to change the back pressure of the solvent flow path 70 (71, 72). Here, the amount of change in the back pressure of the solvent channel 70 is set approximately equal to the amount of change in the solvent vapor pressure around the discharge port 51. The back pressure of the solvent flow path 70 is set to be lower than the back pressure P1 of the ink flow path 60 and higher than the above-described (P1−Π0) (the absolute value of the negative pressure is small). Here, Π0 is the osmotic pressure by the semipermeable membrane 73 in the initial state where the solvent is not volatilized from the discharge port 51.

  In a state where the solvent is likely to volatilize from the discharge port, as described in the first embodiment, the solvent concentration gradient from the semipermeable membrane 73 to the discharge port 51 becomes large, so that the change in the solvent vapor pressure around the discharge port 51 is greater. Also, the pressure change of the back pressure of the solvent channel 70 is set large. Since this amount depends on the structure and dimensions of the head 50 and the physical properties of the ink, it cannot be determined unconditionally and is determined by experiment, but the solvent concentration in the ink near the ejection port 51 is maintained in a state that does not hinder ejection. To control.

[Third Embodiment]
FIG. 9 is a cross-sectional view showing a head 50c as a main part of the liquid ejection apparatus according to the third embodiment. This sectional view shows a section taken along line 2-2 of FIG. In FIG. 9, the same components as the components of the head 50a in the first embodiment shown in FIGS. 1 and 2, and the same components as the components of the head 50b in the second embodiment shown in FIG. Are denoted by the same reference numerals, and description of the contents already described is omitted.

  The head 50a of the first embodiment and the head 50b of the second embodiment have a configuration in which the throttle 53, the pressure chamber 52, the discharge port communication path 512, and the nozzle 51 are connected to the ink common flow path 61 in this order. However, in the present embodiment, as shown in FIG. The present invention is effective even with such a configuration.

  A heating element is used as the actuator 58 shown in FIG. 9, and the head 50c of this example is a so-called thermal ink jet head that instantaneously vaporizes and discharges ink by heating.

  10A and 10B, even when a piezoelectric element such as a piezo is used as the actuator 58, the dimensions of the actuator 58 and the pressure chamber 52 are increased, and the diaphragm 53 shown in FIGS. 2 and 8 does not exist. It is good. The present invention is effective even with such a configuration.

[Image forming apparatus]
FIG. 11 is a block diagram showing an example of an image forming apparatus 100 to which the liquid ejection apparatus according to the present invention is applied.

  11, the image forming apparatus 100 mainly includes a liquid ejection head 50, a communication interface 101, a system controller 102, memories 103a and 103b, a conveyance motor 104, a conveyance driver 105, a print control unit 106, a head driver 107, and A liquid supply unit 108 is included.

  Here, the liquid supply unit 108 includes the ink supply source 31, the solvent supply source 32, the ink channel pressure source 33, the solvent channel pressure source 34, the ink channel back pressure detection unit 35, and the solvent channel back pressure shown in FIG. The detector 36 is configured to be included. The print control unit 106 includes the control unit 38 shown in FIG. Further, the liquid discharge head 50 is provided with the vapor pressure sensor 74 shown in FIGS.

  The image forming apparatus 100 includes a total of four liquid ejection heads 50 for each color of K (black), C (cyan), M (magenta), and Y (yellow).

  The communication interface 101 is an image data input unit that receives image data transmitted from the host computer 190. A wired or wireless interface can be applied to the communication interface 101. Image data taken into the image forming apparatus 100 by the communication interface 101 is temporarily stored in the first memory 103a for storing the image data.

  The system controller 102 includes a microcomputer and its peripheral circuits, and is main control means for controlling the entire image forming apparatus 100 according to a predetermined program. That is, the system controller 102 controls each unit such as the communication interface 101, the conveyance driver 105, the print control unit 106, and the like.

  The conveyance motor 104 applies power to a roller, a belt, or the like for conveying an ejection medium such as paper. By the transport motor 104, the medium to be ejected and the liquid ejection head 50 are relatively moved. The transport driver 105 is a circuit that drives the transport motor 104 in accordance with an instruction from the system controller 102.

  The print control unit 106 includes a microcomputer and its peripheral circuits, and the liquid discharge head 50 discharges (drops) the liquid toward the discharge target medium based on image data input to the image forming apparatus 100. And generating dot data (also referred to as “droplet ejection data”) necessary for forming dots on the medium to be ejected. That is, the print control unit 106 performs image processing such as various processes and corrections for generating droplet ejection dot data from image data in the first memory 103a according to the control of the system controller 102. And the generated dot data is supplied to the head driver 107.

  The print controller 106 is accompanied by a second memory 103b, and dot data and the like are temporarily stored in the second memory 103b during image processing in the print controller 106.

  In FIG. 11, the second memory 103b is shown in a mode associated with the print control unit 106, but it can also be used as the first memory 103a. Also possible is an aspect in which the print control unit 106 and the system controller 102 are integrated to form a single processor.

  The head driver 107 outputs ejection drive signals to the piezoelectric elements 58 of the liquid ejection head 50 based on dot data (actually, dot data stored in the second memory 103b) given from the print control unit 106. Is output. By supplying the ejection drive signal output from the head driver 107 to the piezoelectric element 58 of the liquid ejection head 50, the liquid (droplet) is ejected from the nozzle 51 of the liquid ejection head 50 toward the ejection medium. .

  The present invention is not limited to the examples described in the present specification and the examples illustrated in the drawings, and various design changes and improvements may be made without departing from the scope of the present invention. is there.

Plane perspective view conceptually showing a main part of the liquid ejection apparatus according to the first embodiment. Sectional view along line 2-2 in FIG. Explanatory drawing used to explain back pressure (A) is explanatory drawing used for description of the force relationship in an initial state, (B) is explanatory drawing used for description of the force relationship in a volatile state. (A) And (B) is explanatory drawing used for description of the solvent concentration gradient of the discharge port vicinity in a volatile state, (C) is used for description when the solvent concentration of the discharge port vicinity is raised by the back pressure setting in a volatile state. Illustration Explanatory drawing used for explanation of control for switching back pressure between a non-capped capped state and a non-capped volatile state Process diagram used to explain the method of forming a semipermeable membrane Sectional drawing of the liquid discharge apparatus which concerns on 2nd Embodiment of this invention. Sectional drawing of an example of the liquid discharge apparatus which concerns on 3rd Embodiment of this invention. Sectional drawing of the other example of the liquid discharge apparatus which concerns on 3rd Embodiment of this invention. 1 is a block diagram of an example of an image forming apparatus to which a liquid ejection apparatus according to the present invention is applied.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Liquid discharge apparatus, 31 ... Ink supply source, 32 ... Solvent supply source, 33 ... Ink flow path pressure source, 34 ... Solvent flow path pressure source, 35 ... Ink flow path back pressure detection part, 36 ... Solvent flow path back Pressure detecting unit, 38 ... control unit, 41 ... discharge port plate, 42 ... discharge port communication path plate, 43 ... pressure chamber plate, 44 ... vibrating plate, 50 ... liquid discharge head, 51 ... discharge port, 52 ... pressure chamber, 58 ... Actuator, 60 ... Ink channel, 61 ... Ink common channel (common channel part of ink channel), 62 ... Ink individual channel (individual channel part of ink channel), 70 ... Solvent channel, 71 ... Solvent common channel (common channel part of solvent channel), 72 ... Individual solvent channel (individual channel part of solvent channel), 73 ... Semipermeable membrane, 74 ... Vapor pressure sensor, 512 ... Discharge port Communication path

Claims (4)

A first flow path having a plurality of individual flow path portions respectively connected to the plurality of discharge ports and provided with actuators; and a common flow path portion common to the plurality of individual flow path portions;
A second flow path connected to the individual flow path portion of the first flow path;
A semipermeable membrane provided in a connection portion between the individual flow path portion of the first flow path and the second flow path;
A first liquid supply source for supplying the first liquid discharged from the discharge port to the first flow path;
A second liquid that is at least one solvent contained in the first liquid and that passes through the semipermeable membrane and has a higher concentration of the solvent than the first liquid; A second liquid supply source for supplying the channel;
Pressure setting means for setting a pressure difference between the back pressure of the first channel and the back pressure of the second channel by controlling the back pressure of the second channel;
The pressure setting means, when the back pressure of the first flow path is P1, and the osmotic pressure generated by the semipermeable membrane in the initial state where the solvent in the first liquid is not volatilized from the discharge port is Π0, In the non-volatile state where the solvent does not volatilize from the discharge port, the back pressure of the second flow path is set to (P1-Π0), and in the volatile state where the solvent volatilizes from the discharge port, The back pressure is set lower than the back pressure P1 of the first flow path and higher than the (P1-Π0), and the back pressure of the second flow path is changed according to the fluctuation of the solvent vapor pressure around the discharge port. A liquid ejecting apparatus , wherein the pressure is changed to balance the volatilization of the solvent and the supply of the solvent from the second flow path . The pressure setting means includes a vapor pressure sensor that detects a solvent vapor pressure around the discharge port, and a solvent around the discharge port detected by the vapor pressure sensor in a volatile state in which the solvent volatilizes from the discharge port. The liquid ejection apparatus according to claim 1, wherein a back pressure of the second flow path is changed according to a change in vapor pressure. The individual flow path portion of the first flow path includes at least a pressure chamber in which the actuator is provided, and a discharge port communication path extending from the pressure chamber to the discharge port,
Said second flow path, the liquid ejecting apparatus according to claim 1 or 2, characterized in that it is connected to the discharge port communicating passage. The individual flow path portion of the first flow path is configured to include at least a flow path portion in which the actuator is provided,
The said 2nd flow path is connected to the position between the position from which the said actuator is provided among the said separate flow path parts of the said 1st flow path to the said discharge outlet. The liquid ejection device according to 1 or 2 .

Images