JP2012056084A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus that can achieve reduction of strike-through density when using a recording medium having small basis weight (thin thickness) and a short air permeability and reduction of power consumption when using a recording medium having large basis weight (thick thickness) and a long air permeability.SOLUTION: There is provided the image forming apparatus for forming an image by discharging ink to the recording medium by a recording head. A preheating part is arranged at an upstream side in a conveying direction of the recording medium with respect to the recording head, a postheating part is arranged at a downstream side in the conveying direction of the recording medium and modes can be switched between using the preheating and the post heating and using only the postheating. The switch of the modes is executed based on at least one condition of the basis weight and the thickness of the recording medium.

Description

  The present invention relates to an ink jet image forming apparatus, and more particularly to drying and fixing of ink.

  In recent years, in an ink jet image forming apparatus, for example, in Patent Document 1, the amount of ink bleed on a recording surface in an area where recording has not yet been performed, prior to an area where recording is performed by ejecting ink on a recording medium. And an ink application process for applying ink onto a recording surface in which the amount of ink bleeding is controlled. In this ink application step, the amount of bleeding is controlled by controlling the temperature of the recording medium by heating (preheating) or by controlling the water content in the recording medium. Further, after the ink application process, there is a fixing process (post heating) for fixing the ink applied on the recording medium to the recording medium.

  Patent Document 1 does not describe mode switching between preheating and postheating depending on the basis weight, thickness, and type of the recording medium, or using only postheating. In addition, there is no description about the pre-heating increasing the viscosity of the ink and reducing the permeability to the recording medium, and there is a problem in that the required amount of heat in post-heating is increased due to the decrease in permeability.

  First, in a recording medium with good ink permeability from the surface layer to the back layer (e.g. plain paper), the pre-heating increases the viscosity of the ink to reduce the back-through density. Is higher than the allowable value, and there is a problem in print quality.

  Further, the plain paper has a higher print stain density as the paper temperature when passing through the recording head is increased by preheating. That is, preheating is effective for reducing the back-through density, but has a problem of deteriorating the fixing property (printing stain). This is because in order to fix the ink which has been difficult to penetrate into the recording medium because the ink is thickened by preheating, the ink cannot be fixed unless the evaporation of the ink moisture is promoted.

  Next, in a recording medium (for example, glossy paper) having poor ink permeability from the surface layer to the back layer, the back-through density is constant even if the paper temperature when passing the recording head portion is increased by preheating. This is because the ink does not easily penetrate into the recording medium in order to increase the viscosity of the ink, but in the gloss paper, since the ink penetrates only to the ink receiving layer on the surface layer, preheating does not affect the back-through density. It is.

  Also, the glossiness of the glossy paper increases as the paper temperature when passing through the recording head is increased by preheating. For this reason, when the paper temperature when passing the recording head is increased by preheating, there is a problem that it is necessary to increase the amount of heat required for fixing (power consumption) in post heating.

  Therefore, when the paper temperature when passing the recording head is increased by preheating, there is a problem that the heat amount (power consumption) required for fixing by post heating must be increased. Further, when the printing speed is increased, the amount of heat that can be supplied per unit time is limited, so that there is a problem that the apparatus size needs to be increased in order to secure the heating time.

  Accordingly, the present invention has been made in view of the above-mentioned problems, and it is possible to reduce the back-through density when using a recording medium having a small basis weight (thin thickness) and a short air permeability, and a recording medium. It is an object of the present invention to provide an image forming apparatus capable of reducing power consumption when using a recording medium having a large basis weight (thickness) and a long air permeability.

  In order to solve the above problems, an image forming apparatus according to the present invention is an image forming apparatus that forms an image by ejecting ink onto a recording medium by a recording head, and is upstream of the recording head in the recording medium conveyance direction. A pre-heating unit is arranged on the side, and a post-heating unit is arranged on the downstream side in the recording medium conveyance direction, and a mode using pre-heating and post-heating and a mode using only post-heating can be switched. The mode switching is performed according to at least one condition of basis weight and thickness of the recording medium.

  According to the present invention, when the recording medium has a small basis weight (thickness is thin) and a recording medium with a short air permeability is used, the back-through density is reduced, and the recording medium has a large basis weight (thickness is large). It is possible to provide an image forming apparatus capable of reducing power consumption when using a recording medium having a long temper.

1 is a schematic configuration diagram of an image forming apparatus according to a first embodiment of the present invention. FIG. 5 is a diagram illustrating a situation (through plain paper) of a back-through density with respect to an increase in paper temperature when passing through a recording head portion due to preheating. FIG. 6 is a diagram illustrating a state of printing stain density (plain paper) with respect to a rise in paper temperature when passing through a recording head portion due to preheating. FIG. 6 is a diagram illustrating a state of back-through density (gloss paper) with respect to a rise in paper temperature when passing through a recording head portion due to preheating. FIG. 6 is a diagram illustrating a state of printing stain density (gloss paper) with respect to an increase in paper temperature when passing through a recording head portion due to preheating. It is a figure which shows the printing stain reduction effect (plain paper) by a heating system. It is a figure which shows the printing stain reduction effect (gloss paper) by a heating system.

  Preferred embodiments of the present invention will be described below in detail with reference to the drawings.

  FIG. 1 is a schematic configuration diagram of an image forming apparatus according to an embodiment of the present invention. The image forming apparatus 10 mainly includes a recording medium 1, a suction unit 2, a preheating unit 3, a recording head unit 4, a post heating unit 5, an outfeed roller 6, and a nip roller 7.

  The recording medium 1 of the image forming apparatus 10 is conveyed to the pre-heating unit 3 through the suction unit 2 that adsorbs the recording medium 1, and the recording head unit 4 arranged along the conveyance direction of the recording medium 1 and post-heating. It passes through the section 5 and is discharged out of the apparatus by the outfeed roller 6 and the nip roller 7. That is, the recording medium 1 is conveyed in the X direction in FIG.

  A plurality of guide rollers 8 (not shown) are provided on the conveyance path of the recording medium 1.

  The recording head unit 4 includes at least a colorant, a wetting agent, and a permeability improver that are dispersed in water, and a viscosity increase rate (mPa · s /%) due to water reduction is 30% by weight of water with respect to the total weight of the ink. Up to 5.0 or less, and an ink configured to have a point where the rate of increase in viscosity exceeds 50 during a moisture reduction rate of 30-45% is ejected toward the recording medium to form an image. .

  The recording medium 1 is given an appropriate tension between the suction portion 2, the outfeed roller 6, and the nip roller 7, so that the fluttering of the recording medium 1 in the Y direction can be suppressed. The pre-heating unit 3 and the post-heating unit 5 are provided with a plurality of infrared heaters so that the temperature of the recording medium 1 can be controlled.

FIG. 2 includes at least a colorant, a wetting agent, and a permeability improver that are dispersed in water, and the viscosity increase rate (mPa · s /%) accompanying the water reduction is 5. An ink having a point where the viscosity increase rate exceeds 50 while the moisture reduction rate is 30 to 45% is used, and plain paper (basis weight: 81 g / m) is used as the recording medium 1. ^{2 When} the recording medium 1 heated by the preheating 3 when printing (paper thickness: 100 μm) and air permeability: 18 seconds) passes through the recording head unit 4 and the back-through density (on the printing surface) It is a figure which shows the relationship of the phenomenon which can see through when ink sees from a non-printing surface.

From FIG. 2, it can be seen that the back-through density decreases as the paper temperature when passing through the recording head unit 4 is increased. That is, the preheating 3 promotes the evaporation of the moisture of the ink that is flying from the recording head 4 toward the recording medium surface 1 or the evaporation of the moisture of the ink immediately after landing on the recording medium, thereby reducing the moisture of the ink. As a result, the ink is thickened, so that it is difficult for the ink to permeate the recording medium 1, and the effect of reducing the back-through density is produced. Further, when preheating 3 is not performed, on the plain paper (basis weight: 81 g / m ² (paper thickness: 100 μm), air permeability: 18 seconds), the back-through density is higher than the allowable value, which causes a problem in print quality. I understand that there is.

  3 shows the same ink and recording medium (plain paper) as in FIG. 2 described above, and the temperature when the recording medium 1 heated by the preheating 3 passes through the recording head unit 4 and the ink printed on the paper. It is a figure which shows the relationship with the stain | concentration density | concentration (henceforth a printing stain | contamination density | concentration) of a printing surface by transferring to the nip roller 7, and retransferring this transfer ink to a paper.

It can be seen from FIG. 3 that the print stain density increases as the paper temperature when passing through the recording head unit 4 is increased.
That is, the preheating 3 is effective for reducing the back-through, but deteriorates the fixing property (printing stain). This cause will be described by a technique for fixing the ink to the recording medium 1. First, there are two methods for fixing the ink to the recording medium 1; (A) penetrating the recording medium 1 and (B) evaporating the water. In order to fix the ink which has become difficult to permeate to the recording medium 1, it is necessary to fix the ink to the recording medium 1 mainly by evaporating water, and the ink cannot be fixed unless more ink water is evaporated. Fixing properties deteriorate. For this reason, when the paper temperature at the time of passing through the recording head 4 is increased by the preheating 3, it is necessary to increase the amount of heat required for fixing (power consumption) in the postheating 5. Further, when increasing the printing speed, there is a limit to the amount of heat that can be supplied per unit time, so that it is necessary to increase the size of the apparatus in order to ensure the heating time.

4 includes at least a colorant, a wetting agent, and a permeability improver that are dispersed in water, and the viscosity increase rate (mPa · s /%) due to moisture reduction is 5. The recording medium 1 is glossy paper (basis weight: 81 g / m) using an ink that has a point where the viscosity increase rate exceeds 50 while the water reduction rate is 30-45%. ² shows the relationship between the temperature when the recording medium 1 heated by the preheating 3 passes through the recording head unit 4 and the back-through density when printing on (paper thickness: 100 μm) and air permeability: 6000 seconds). FIG.

  FIG. 4 shows that the back-through density is constant even when the paper temperature when passing through the recording head is increased. That is, the preheating 3 promotes the evaporation of the moisture of the ink that is flying from the recording head 4 toward the recording medium 1 or the evaporation of the moisture of the ink immediately after landing on the recording medium. Since the ink is thickened, it is difficult for the ink to penetrate into the recording medium 1. However, in the gloss paper, since the ink penetrates only to the ink receiving layer on the surface layer, the preheating 3 does not affect the through-through density. . It can be seen that the back-through density is lower than the allowable value without preheating, and there is no problem in print quality.

  FIG. 5 is a diagram showing the relationship between the temperature when the recording medium 1 heated by the preheating 3 passes through the recording head unit 4 and the print stain density.

  FIG. 5 shows that the stain density on the printing surface increases as the paper temperature when passing through the recording head unit 4 is increased. That is, even if there is no change in the back-through density, the ink permeation into the ink receiving layer on the surface of the gloss paper is inhibited by the ink thickening by the preheating 3. For this reason, it is understood that when the paper temperature when passing through the recording head 4 is increased by the preheating 3, it is necessary to increase the amount of heat necessary for fixing (power consumption) in the postheating 5. Even if the viscosity increase tendency of the ink is different, the same problem as described above will occur if an ink that thickens due to a decrease in water content is used.

  In addition, the ink contains at least a colorant, a wetting agent, and a permeability improver that are dispersed in water, and the viscosity increase rate (mPa · s /%) accompanying the water reduction is 5. In the case of using an ink having a point where the viscosity increase rate exceeds 50 while the water decrease rate is 30-45%, the ink lands on the surface of the recording medium and the moisture is applied to the recording medium. Absorption increases the viscosity abruptly, so there is little problem of bleeding on the surface of the recording medium, but this does not cause a problem when the recording medium is thin.

  FIG. 6 is a diagram for explaining the printing stain reduction effect by the heating method when printing on plain paper. FIG. 7 is a diagram for explaining the printing stain reduction effect by the heating method when printing on glossy paper. A detailed description of FIGS. 6 and 7 will be described later.

(First embodiment)
The preheating unit 3 and the post heating unit 5 can be switched between a mode in which the preheating unit 3 + post heating unit 5 is used and a mode in which only the post heating unit 5 is used. The selection conditions for each mode will be described in detail below.

<Mode using pre-heating unit 3 + post-heating unit 5>
Recording medium thickness (t): t ≦ 120 μm or recording medium basis weight (g): g ≦ 105 g / m ² or recording medium air permeability (s): s ≦ 25 seconds <Post-heating unit Mode using only 5>
Recording medium thickness (t): 120 μm <t or recording medium basis weight (g): 105 g / m ² <g or recording medium air permeability (s): 25 seconds <s

An example in which plain paper (paper basis weight 81 g / m ² (paper thickness: 100 μm), air permeability: 18 seconds) is printed with the configuration of the image forming apparatus 10 shown in FIG. The description will be given with reference.

  In this example, drying / fixing is performed using the pre-heating unit 3 and the post-heating unit 5 together under the mode selection conditions described above.

  FIG. 2 shows the situation of back-through density (plain paper) with respect to the paper temperature when the pre-heating unit 3 passes through the recording head unit 4. In the pre-heating unit 3, the back-through density can be suppressed to an allowable value. In this way, heating is performed so that the paper temperature when passing through the recording head unit 4 is A ° C.

  FIG. 6 is a diagram showing the status of printing stains (plain paper) with respect to the paper temperature. (A) to (c) in the figure show (a) the effect of deteriorating dirt by pre-heating, (b) the effect of reducing dirt by pre-heating and post-heating, and (c) the effect of reducing dirt only by post-heating. ing.

  In FIG. 6 (a), due to the effect of deteriorating dirt due to preheating, when the paper temperature is set to A ° C., the ink printed on the paper when the post heating 3 is not performed is transferred to the nip roller 7, and the transferred ink is transferred to the paper The smear density on the printed surface (hereinafter abbreviated as “printed surface smudge density”) due to retransfer to (b) is b [D].

  6 (b), the post heating 5 heats the paper to C [° C.] that can reduce b [D] to 0 [D]. The pre-heating 3 and the post-heating 5 according to the above description enable drying and fixing without smearing on the printing surface while keeping the back-through density at an allowable value.

  On the other hand, when the paper temperature is not increased by preheating 3, the back-through density is not an allowable level, but a [D] is set to 0 [D] due to the stain reduction effect only by (c) post heating in FIG. B [° C.] that can be reduced to a low level enables printing without print stains, and an additional amount of heat corresponding to the paper temperature Δt1 = B [° C.] − C [° C.] is required to suppress the back-through density to an acceptable level. ing.

  In the present embodiment, the mode using the preheating unit 3 + post heating unit 5 and the mode using only the post heating unit 5 are switched and used under the above-described conditions. When the back-through density is acceptable without performing heating 3, since preheating 3 is not used, it is possible to reduce the amount of heat by which the paper temperature is increased by Δt1, and when the paper basis weight and thickness are small. It is possible to reduce the amount of heat loss when the paper has a large basis weight and thickness.

(Second Embodiment)
Next, an example of printing gloss paper (paper basis weight 81 g / m ² (paper thickness: 100 μm), air permeability: 6000 seconds) with the configuration of the image forming apparatus 10 shown in FIG. 4 and FIG.

  Drying and fixing are performed only by the post heating 5 according to the mode selection conditions described above.

  FIG. 4 shows the temperature and back-through density (glossy paper) when the recording medium 1 heated by the preheating 3 passes through the recording head unit 4, and the situation with plain paper shown in FIG. 2. Thus, there is no effect of reducing the back-through concentration due to the preheating 3, and the back-through concentration is less than the allowable value even if the preheating is not performed.

  FIG. 7 is a diagram illustrating a state of printing stain (gloss paper) with respect to the paper temperature. (A) to (c) in the figure show (a) the effect of deteriorating dirt by pre-heating, (b) the effect of reducing dirt by pre-heating and post-heating, and (c) the effect of reducing dirt only by post-heating. ing.

  According to the mode selection conditions described above, drying / fixing is performed only with the post-heating 5, so the paper temperature is set by post-heating in order to set the stain density: c [D] to 0 [D] without the pre-heating 3. D [° C.] is set, and drying / fixing without printing stains is possible while keeping the back-through density below an allowable value under the drying / fixing conditions.

  In this embodiment, since the mode using the pre-heating unit 3 + post-heating unit 5 and the mode using only the post-heating unit 5 are switched under the above-described conditions, printing stains due to pre-heating with gloss paper are used. It is possible to reduce the amount of heat for increasing the paper temperature necessary to prevent the occurrence of Δt2 = E [° C.] − D [° C.].

  Since E [° C.] in FIG. 7 according to the above description is higher than C [° C.] in FIG. 6, only mode switching depending on the type of paper (for example, plain paper, glossy paper, matte paper) and air permeability. But it is effective in reducing the amount of heat.

  In the embodiment shown in FIG. 1, an infrared heater is used for the heating unit. However, the present invention is not limited to this, and the heating drum type heating is configured so as to have a halogen heater or the like inside the hollow drum. The same result can be obtained.

  In FIG. 1, the printing surface of the recording medium is heated, but the same effect can be obtained by heating the back surface.

<Operation and effect of this embodiment>
According to the above embodiment, the first means of the present invention includes at least a colorant, a wetting agent, and a permeability improver that are dispersed in water, and has a viscosity increase rate (mPa · s /%) that accompanies water reduction. Up to 30% by weight of the ink based on the total weight of the ink is 5.0 or less, and the viscosity increase rate exceeds 50 between 30% to 45% by weight of the ink or Ink configured to increase in viscosity with decrease is ejected onto the recording medium 1 by the recording head 4 to form an image, and an upstream heating unit (pre-pressed in the conveying direction of the recording medium 1) is formed on the recording head 4. In the printing apparatus in which the heating 3 and the downstream heating unit (post heating) 5 are arranged, the mode using the preheating 3 + post heating 5 and the mode using only the post heating 5 can be switched. Since the replacement is performed based on the basis weight or thickness of the recording medium, post-heating due to a decrease in permeability to the recording medium due to ink thickening in the preheating 3 when the basis weight or thickness of the recording medium 1 is large. Thus, it is possible to achieve both an increase in the amount of heat of 5 and a reduction in the back-through density when the basis weight and thickness of the recording medium 1 are small.

Further, according to the above embodiment, the second means of the present invention, when switching the mode with the basis weight (g) of the recording medium,
Mode using preheating 3 + postheating 5: g ≦ 105 g / m ²
Mode in which only post-heating 5 is used: 105 g / m ² <g
And when switching the mode by the thickness (t) of the recording medium,
Mode using preheating 3 + postheating 5: t ≦ 120 μm
Mode in which only post heating 5 is used: 120 μm <t
Thus, when the basis weight and thickness of the recording medium 1 are large, the heat amount of the post heating 5 accompanying the decrease in permeability to the recording medium due to ink thickening in the preheating 3 and the basis weight and thickness of the recording medium 1 are increased. It is possible to achieve both reduction in the back-through density when the value is small.

  According to the above embodiment, the third means of the present invention includes at least a colorant, a wetting agent, and a permeability improver dispersed in water, and a viscosity increase rate (mPa · s /%) due to a decrease in water content. Is less than 5.0 up to a water reduction rate of 30 wt% with respect to the total weight of the ink, and the ink or water is configured so that the viscosity increase rate exceeds 50 between the water reduction rates of 30 to 45 wt% The recording head 4 ejects ink configured to increase in viscosity with the decrease in the recording head 4 to form an image, and forms an image with respect to the recording head 4 in the upstream of the transport direction of the recording medium 1 (pre- In the printing apparatus in which the heating 3 and the downstream heating unit (post heating) 5 are arranged, the mode using the preheating 3 + post heating 5 and the mode using only the post heating 5 can be switched. By changing the recording medium 1 depending on the type of the recording medium 1 (difference in ink permeability from the surface layer to the back layer) and the air permeability, the recording medium 1 has a poor ink permeability from the surface layer to the back layer or the air permeability. When the temperature is long, the heat amount of the post-heating 5 increases due to the decrease in the permeability to the recording medium due to the ink thickening in the preheating 3, and the ink permeability from the surface layer to the back layer of the recording medium 1 is good. It is possible to achieve both reduction of the back-through density when the length is short.

According to the above embodiment, the fourth means of the present invention provides the mode switching when the type of the recording medium 1 is used.
Mode using pre-heating 3 + post-heating 5: recording medium with good ink permeability from the surface layer to the back layer (eg plain paper)
Mode that uses only post-heating 5: Recording medium with poor ink permeability from the surface layer to the back layer (eg gloss paper, matte paper)
And when performing with the air permeability (s) of the recording medium,
Mode using preheating 3 + postheating 5: s ≦ 25 seconds Mode using only postheating 5: 25 seconds <s
Thus, when the recording medium 1 has poor ink permeability from the surface layer to the back layer or has a long air permeability, the post-heating 5 accompanying the decrease in permeability to the recording medium due to ink thickening in the preheating 3. The increase in the amount of heat and the reduction in the penetration density when the recording medium 1 has good ink permeability from the surface layer to the back layer or when the air permeability is short can be achieved.

  The above-described embodiment is a preferred embodiment of the present invention, and the scope of the present invention is not limited to the above-described embodiment alone, and various modifications are made without departing from the gist of the present invention. Implementation is possible.

In the image forming apparatus according to the present invention, when the mode switching is performed based on the basis weight of the recording medium, the preheating and the post heating are used when the recording medium weight is 105 g / m ² or less. When the recording medium weighing exceeds 105 g / m ² , only the post-heating is used. When the mode is switched by the thickness of the recording medium, the pre-heating is performed when the recording medium thickness is 120 μm or less. And post-heating mode, and when the recording medium thickness exceeds 120 μm, only the post-heating mode is used.

  The image forming apparatus according to the present invention is characterized in that the mode switching is performed according to at least one condition of ink permeability from a surface layer to a back layer of the recording medium and air permeability of the recording medium. .

  In the image forming apparatus according to the present invention, when the mode switching is performed by ink permeability from the surface layer to the back layer of the recording medium, the recording medium having good ink permeability from the surface layer to the back layer is the pre-printed medium. A mode in which heating and the post heating are used, a recording medium having poor ink permeability from the surface layer to the back layer is in a mode in which only the post heating is used, and the mode switching is performed based on the air permeability of the recording medium. In this case, when the air permeability of the recording medium is 25 seconds or less, the preheating and the post heating are used. When the air permeability of the recording medium exceeds 25 seconds, only the post heating is used. It is characterized by.

  In the image forming apparatus according to the present invention, the recording medium having good ink permeability is plain paper, and the recording medium having poor ink permeability is glossy paper and matte paper.

  The image forming apparatus according to the present invention is characterized in that the ink is an ink configured to increase in viscosity with a decrease in moisture.

  In the image forming apparatus according to the present invention, the ink includes at least a colorant dispersed in water, a wetting agent, and a permeability improver, and the rate of increase in viscosity (mPa · s /%) due to water reduction is The ink is configured to have a point where the viscosity increase rate exceeds 50 between the moisture reduction rate of 30-45 wt% and the moisture reduction rate of 30 wt% with respect to the weight is 5.0 or less. To do.

  INDUSTRIAL APPLICABILITY The present invention can be applied to applications such as an image forming apparatus and an apparatus that requires control of the penetration depth of the liquid into the absorber layer when spraying a liquid whose viscosity changes by heating to the absorber.

DESCRIPTION OF SYMBOLS 1 Recording medium 2 Suction part 3 Pre heating part 4 Recording head part 5 Post heating part 6 Out feed roller 7 Nip roller 8 Guide roller 10 Image forming apparatus

Japanese Patent Laid-Open No. 6-23977

Claims (7)

An image forming apparatus for forming an image by ejecting ink onto a recording medium by a recording head,
A pre-heating unit is disposed on the upstream side in the recording medium conveyance direction with respect to the recording head, and a post-heating unit is disposed on the downstream side in the recording medium conveyance direction,
A mode that uses pre-heating and post-heating, and
The mode that only uses post-heating can be switched,
The mode switching is performed according to at least one condition of basis weight and thickness of the recording medium. When switching the mode with the basis weight of the recording medium,
When the recording medium weighing is 105 g / m ² or less, the pre-heating and the post-heating are used.
In the case where the recording medium weighing exceeds 105 g / m ² , only the post heating is used.
When switching the mode by the thickness of the recording medium,
When the recording medium thickness is 120 μm or less, the pre-heating and the post-heating are used.
2. The image forming apparatus according to claim 1, wherein when the recording medium thickness exceeds 120 [mu] m, only the post heating is used.   The image forming apparatus according to claim 1, wherein the mode switching is performed according to at least one condition of ink permeability from a surface layer to a back layer of the recording medium and an air permeability of the recording medium. When switching the mode with ink permeability from the surface layer to the back layer of the recording medium,
The recording medium having good ink permeability from the surface layer to the back layer is a mode in which the pre-heating and the post-heating are used,
The recording medium with poor ink permeability from the surface layer to the back layer is a mode that uses only the post-heating,
When switching the mode according to the air permeability of the recording medium,
When the air permeability of the recording medium is 25 seconds or less, the pre-heating and the post-heating are used.
The image forming apparatus according to claim 3, wherein when the air permeability of the recording medium exceeds 25 seconds, only the post heating is used.   5. The image forming apparatus according to claim 4, wherein the recording medium having good ink permeability is plain paper, and the recording medium having poor ink permeability is gloss paper and matte paper.   The image forming apparatus according to claim 1, wherein the ink is an ink configured to increase in viscosity with a decrease in moisture.   The ink contains at least a colorant dispersed in water, a wetting agent, and a penetrability improver, and the viscosity increase rate (mPa · s /%) accompanying water reduction is 5 up to a water reduction rate of 30 wt% with respect to the total weight of the ink. The image forming apparatus according to claim 6, wherein the ink is configured to have a point where the viscosity increase rate exceeds 50 while the water decrease rate is 30 to 45 wt%. .

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