Inkjet image forming apparatus

Abstract

To provide an inkjet image forming apparatus that does not reduce dryness while maintaining adhesion between white ink and a printing base material.SOLUTION: An inkjet image forming apparatus 1 includes an image forming part 9 that ejects inks of a plurality of colors including white onto a printing base material M conveyed along a conveyance direction X, a precoating unit 5 that applies a precoating liquid to the printing base material M, a pre-drying part 7 that pre-dries the precoating liquid applied to the printing base material M, and a control part 51 that controls the pre-drying part 7. The control part 51 controls the pre-drying part 7 to achieve W1<W2, where W1 (g / m2) is an amount of the precoating liquid applied to the printing base material M at the time of reaching the image forming part 9 when a print job for ejecting white ink is performed, and W2 (g / m2) is an amount of the precoating liquid applied to the printing base material M at the time of reaching the image forming part 9 when a print job for not ejecting white ink is performed.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an inkjet type image forming apparatus that forms an image on a printing substrate by an inkjet method.

Background Art

[0002] In an inkjet type image forming apparatus, a precoat treatment for applying a precoat liquid to a printing substrate may be performed in order to improve the adhesion between the printing substrate and the ink.

[0003] For example, in the image forming apparatus described in Patent Document 1, a pretreatment liquid (corresponding to a precoat liquid) having an application amount determined based on the type of ink and the adhesion amount per unit area of the ink ejected onto the recording medium is applied.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] By the way, generally, it is known that white ink has lower adhesion to a printing substrate than other color inks. This is because in order to increase the hiding power of white, it is necessary to increase the particle size of the pigment. In that case, however, the pigment cannot be completely coated with the binder resin, resulting in poor adhesion to the printing substrate. In particular, this tendency becomes stronger in the case of a printing substrate having a smooth surface such as a plastic film. For this reason, when performing a printing job of ejecting white ink, the ejection amount of white ink may be increased compared to the ejection amount of other color inks.

[0006] However, when the ink ejection amount is increased in this way, in the drying process after ink ejection, the ink (solvents such as water contained in the ink) may not dry sufficiently. If the ink does not dry sufficiently after the drying process, when the printing substrates are stacked one on top of the other, a problem may occur in that the ink on the lower printing substrate transfers to the back surface of the upper printing substrate. When precoat treatment is performed, since the solvent contained in the applied precoat liquid is also included, the drying property further deteriorates. In the image forming apparatus described in Patent Document 1 mentioned above, such drying property is not considered.

[0007] Therefore, in view of the above circumstances, an object of the present invention is to provide an inkjet type image forming apparatus that maintains the adhesion between the white ink and the printing substrate and does not reduce the drying property.

Means for Solving the Problems

[0008] To achieve the above object, an inkjet type image forming apparatus of the present invention includes an image forming unit that ejects inks of a plurality of colors including white onto a printing substrate conveyed along a predetermined conveyance direction to form an image, a precoat treatment unit disposed upstream of the image forming unit in the conveyance direction, that applies a fixed amount of precoat liquid to the printing substrate, a pre-drying unit disposed downstream of the precoat treatment unit in the conveyance direction, that pre-dries the precoat liquid applied to the printing substrate before ink is ejected from the image forming unit, and a control unit that controls the pre-drying unit to adjust the degree of drying of the precoat liquid applied to the printing substrate. The control unit sets the amount of the precoat liquid applied to the printing substrate at the time of reaching the image forming unit when performing a printing job of ejecting white ink as W1 (g / m 2 ) and sets the amount of the precoat liquid applied to the printing substrate at the time of reaching the image forming unit when performing a printing job of not ejecting white ink as W2 (g / m 2 ), and is characterized by controlling the pre-drying unit so that W1 < W2.

[0009] In the present invention, the image forming unit may include a head unit provided for each color of ink and configured to discharge the ink, the precoat processing unit may include a head unit configured to discharge the precoat liquid, and the two head units may have the same structure.

[0010] In the present invention, the head unit for white ink may be arranged on the upstream side in the conveying direction rather than the head unit for other colors.

[0011] In the present invention, the pigment of the white ink may be titanium oxide.

[0012] In the present invention, the binder resin of the white ink may be a polyurethane resin.

[0013] In the present invention, the printing substrate may be a plastic film.

[0014] In the present invention, when the viscosity of the white ink at 25 degrees is ρw (mPa·s) and the viscosity of the ink of colors other than white at 25 degrees is ρc (mPa·s), it may be characterized in that ρw < ρc.

[0015] In the present invention, when the average particle diameter of the pigment of the white ink is Dw (nm) and the average particle diameter of the pigment of the ink of colors other than white is Dc (nm), it may be characterized in that Dw > Dc.

Effects of the Invention

[0016] According to the present invention, in a printing job in which white ink is discharged, the amount of a solvent such as water on the printing substrate can be reduced to such an extent that it can be completely dried while maintaining the effect of enhancing the adhesion of the ink by the precoat treatment.

Brief Description of the Drawings

[0017]

Figure 1

Figure 2

Figure 3A

Figure 3B

Figure 3C

Figure 4

Mode for Carrying Out the Invention

[0018] Hereinafter, an inkjet type image forming apparatus according to an embodiment of the present invention will be described with reference to the drawings.

[0019] With reference to FIG. 1, the overall configuration of the inkjet type image forming apparatus 1 will be described. FIG. 1 is a front view schematically showing the internal configuration of the inkjet type image forming apparatus 1.

[0020] The inkjet type image forming apparatus 1 includes a supply roller 3 around which a long printing substrate M is wound, a precoat treatment unit 5 that performs a precoat treatment of applying a precoat liquid to the printing substrate M, a pre-drying unit 7 that pre-dries the precoat liquid applied to the printing substrate M, an image forming unit 9 that forms an image on the printing substrate M by an inkjet method, a main drying unit 11 that dries the image formed on the printing substrate M, a transport plate 13 disposed between the supply roller 3 and the main drying unit 11, and a take-up roller 15 that takes up the printing substrate M.

[0021] First, the supply roller 3 will be described. A long printing substrate M is wound around and attached to the supply roller 3. By rotating the supply roller 3, the printing substrate M is fed out in a predetermined conveyance direction X.

[0022] Next, the precoat treatment unit 5 will be described. The precoat treatment unit 5 is disposed on the downstream side of the supply roller 3 in the conveyance direction X, and performs a precoat treatment of applying a certain amount of precoat liquid to the printing substrate M fed out from the supply roller 3. The precoat treatment unit 5 includes a precoat head unit 21 that discharges the precoat liquid. The precoat head unit 21 is electrically connected to the control unit 51.

[0023] Referring to FIG. 2, the precoat head unit 21 will be described. FIG. 2 is a plan view showing the precoat head unit 21. The precoat head unit 21 includes three print heads 23 and a plate 25 that supports the three print heads 23. The three print heads 23 are arranged in a staggered manner (line head method) along the width direction Y so as to be approximately the same as the width of the printing substrate M (the length along the width direction Y intersecting the conveyance direction X), and are supported by the plate 25. The precoat liquid is supplied to the three print heads 23 from a precoat liquid supply source.

[0024] The print head 23 includes a large number of nozzles and a piezoelectric element provided in each nozzle. The discharge port of the nozzle opens on the lower surface of the print head 23. By applying a voltage to the piezoelectric element, the piezoelectric element deforms, and the precoat liquid in the nozzle is discharged downward from the discharge port.

[0025] The components of the precoat liquid are, for example, 10% of polyester resin (trade name: Pesresin A - 640, manufactured by Takamatsu Resin Co., Ltd.), 0.04% of surfactant (trade name: Surfynol 440, manufactured by Nissin Chemical Industry Co., Ltd.), 25% of propylene glycol, and 64.96% of water.

[0026] Next, the pre-drying unit 7 will be described with reference to FIG. 1. The pre-drying unit 7 is disposed on the downstream side in the conveyance direction X of the pre-coating treatment unit 5, and evaporates solvents such as water contained in the pre-coating liquid applied to the printing substrate M, and pre-dries the applied pre-coating liquid to a semi-dry state. The pre-drying unit 7 includes a heater that heats air and a fan that blows the air heated by the heater toward the printing substrate M. The pre-drying unit 7 is electrically connected to the control unit 51.

[0027] Next, the image forming unit 9 will be described with reference to FIG. 1. The image forming unit 9 is disposed on the downstream side in the conveyance direction X of the pre-drying unit 7, and forms an image on the pre-dried printing substrate M by an inkjet method. The image forming unit 9 includes five image forming head units 31W, 31B, 31C, 31M, and 31Y (collectively referred to as the image forming head unit 31) corresponding to five colors (white, black, cyan, magenta, and yellow) of ink. The five image forming head units 31W, 31B, 31C, 31M, and 31Y are arranged in order along the conveyance direction X. The five image forming head units 31 are electrically connected to the control unit 51.

[0028] The image forming head unit 31 has the same configuration as the pre-coating head unit 21 of the pre-coating treatment unit 5 shown in FIG. 2, and includes three print heads 33 and a plate 35 that supports the three print heads 33. Each of the three print heads 33 of the five image forming head units 31 is supplied with white, black, cyan, magenta, and yellow ink, respectively.

[0029] The print head 33 includes a large number of nozzles and a piezoelectric element provided in each nozzle. The discharge port of the nozzle opens on the lower surface of the print head 33. By applying a voltage to the piezoelectric element, the piezoelectric element deforms, and the ink in the nozzle is discharged downward from the discharge port. Each ink contains a pigment and a binder resin.

[0030] The pigment of the white ink is titanium oxide, and the binder resin is a polyurethane resin. Also, the viscosity ρw (mPa·s) of the white ink at 25 degrees is less than the viscosity ρc (mPa·s) of the inks of colors other than white at 25 degrees. Furthermore, the average particle diameter Dw (nm) of the pigment of the white ink is larger than the average particle diameter Dc (nm) of the pigment of the inks of colors other than white.

[0031] Next, the present drying unit 11 will be described with reference to FIG. 1. The present drying unit 11 is disposed on the downstream side in the conveyance direction X from the image forming unit 9, and dries the image formed on the printing substrate M by the image forming unit 9. The present drying unit 11 includes a heat drum 41, and upstream and downstream tension rollers 43 and 45 disposed on the upstream side and the downstream side of the heat drum 41. The printing substrate M that has passed through the image forming unit 9 is wound around the heat drum 41 between the upstream and downstream tension rollers 43 and 45. The heat drum 41 is heated to dry the printing substrate M conveyed along the surface thereof.

[0032] Next, the conveyance plate 13 will be described with reference to FIG. 1. The conveyance plate 13 is disposed below the precoat processing unit 5, the pre-drying unit 7, and the image forming unit 9 (five head units 31) between the supply roller 3 and the present drying unit 11. The upper surface of the conveyance plate 13 is formed flat. Also, as shown in FIG. 3, the width of the conveyance plate 13 is wider than the width of the printing substrate M and all the head units 31. A heat source (not shown) is provided below the conveyance plate 13 to heat the conveyance plate 13 to a predetermined temperature. The conveyance plate 13 is made of, for example, metal.

[0033] Next, the take-up roller 15 will be described with reference to FIG. 1. The take-up roller 15 is disposed on the downstream side in the conveyance direction X from the present drying unit 11. The end of the printing substrate M is fixed to the take-up roller 15. The take-up roller 15 is connected to a motor (not shown) and rotates. By rotating the take-up roller 15 in a predetermined direction at a predetermined rotational speed by the motor, the printing substrate M is fed out from the supply roller 3.

[0034] The fed printing substrate M passes below the precoat processing unit 5, the pre-drying unit 7, and the image forming unit 9 above the conveying plate 13, is wound around the heat drum 41 in the main drying unit 11, and then is wound up by the winding roller 15.

[0035] Next, the control unit 51 will be described with reference to FIG. 1. Image data of the image to be printed is input to the control unit 51. The control unit 51 determines whether the printing job to be executed is a printing job that ejects white ink or a printing job that does not eject white ink based on the input image data.

[0036] Also, the control unit 51 controls the temperature and air volume of the warm air blown out in the pre-drying unit 7 to adjust the drying degree (the evaporation degree of solvents such as water) of the precoat liquid applied to the printing substrate M based on the printing job. Specifically, in the case of a printing job that does not eject white ink, the drying degree is set to the reference level. The reference level is the level at which the solvent of the precoat liquid evaporates to such an extent that the precoat treatment effect is sufficiently exerted, and the applied precoat liquid dries to a state of being slightly damp. On the other hand, in the case of a printing job that ejects white ink, the drying degree is set to a strong level higher (more likely to dry) than the reference level. However, even at the strong level, the applied precoat liquid is not completely dried so that the precoat treatment effect is exerted, and it is dried to a degree slightly drier than the slightly damp state of the reference level.

[0037] Furthermore, the control unit 51 is also electrically connected to the inkjet head 21 of the precoat processing unit 5 to control the inkjet head 21 so as to apply a certain amount of precoat liquid. Further, the control unit is also electrically connected to the five head units 31 of the image forming unit 9 to control the ejection of ink from each head unit 31 based on the input image data.

[0038] In the inkjet type image forming apparatus 1 having the above configuration, the image forming operation on a non-absorbent base material (plastic film) will be described. At the start of the image forming operation, the control unit 51 determines whether the input image data is a print job that ejects white ink or a print job that does not eject white ink. First, the take-up roller 15 is driven by a motor to rotate, and the printing base material M is fed out from the supply roller 3. The fed-out printing base material M is sequentially conveyed along the upper surface of the conveying plate 13 to the precoat processing unit 5, the pre-drying unit 7, and the image forming unit 9.

[0039] The precoat processing unit 5 applies a certain amount of precoat liquid to the printing base material M. Thereafter, the printing base material M to which the precoat liquid has been applied is conveyed to the pre-drying unit 7. The pre-drying unit 7 dries the precoat liquid applied to the printing base material M at a reference level when the print job is a print job that does not eject white ink. On the other hand, when the print job is a print job that ejects white ink, the precoat liquid applied to the printing base material M is dried at a strong level. The printing base material M on which the precoat liquid has been pre-dried in this way is conveyed to the image forming unit 9.

[0040] In the image forming unit 9, based on the image data, ink of a predetermined color is ejected from the corresponding head unit 31, and an image is formed on the printing base material M. The printing base material M on which the image has been formed in this way is conveyed to the main drying unit 11. In the main drying unit 11, the ink is completely dried. Thereafter, the printing base material M is wound up by the take-up roller 15.

[0041] As described above, according to the present invention, in the case of a print job in which white ink is ejected, the degree of drying of the precoat liquid (degree of evaporation of the solvent) by the pre-drying unit 7 is higher than in the case of a print job in which white ink is not ejected. As a result, the amount of the precoat liquid applied to the printing base material M when reaching the image forming unit 9 is W1 (g / m 2 ) in the case of a print job in which white ink is ejected, and W2 (g / m 2 ) in the case of performing a print job in which white ink is not ejected. Then, W1 < W2.

[0042] In the case of a printing job in which white ink is ejected, the amount of ink ejected by the image forming unit 9 increases by the amount of white ink. Also, the white ink is ejected in a larger amount than other color inks in order to enhance the hiding property. For this reason, the total amount of ink ejected onto the printing substrate M, that is, the amount of solvent contained in the printing substrate M, becomes larger than in the case of a printing job in which white ink is not ejected. Then, there is a possibility that the solvent may not be completely dried in the main drying unit 11.

[0043] Therefore, in a printing job in which white ink is ejected, the amount of solvent contained in the precoat liquid applied to the printing substrate M can be reduced to such an extent that the effect of enhancing the ink adhesion by the precoat treatment can be maintained and complete drying can be achieved in the main drying unit 11. Also, by passing through the pre-drying process, the amount of solvent in the precoat liquid decreases and the viscosity increases, so that the precoat treatment effect can be enhanced.

[0044] Also, the pigment of the white ink is titanium oxide and the binder resin is a polyurethane resin. Titanium oxide is known as a pigment with good hiding property. Also, since the polyurethane resin has good compatibility with the printing substrate, the adhesion between the white ink and the printing substrate M can be enhanced. Note that the white ink generally contains water and a high-boiling-point material having a boiling point higher than that of water. Comparing water and the high-boiling-point material, water is easier to dry. Therefore, the amount of the contained high-boiling-point material is reduced and the amount of moisture is increased accordingly. As a result, the viscosity of the white ink appears to be low.

[0045] Also, in order to increase the hiding property of white, it is preferable that the size (average particle diameter) of the pigment of the white ink is large. In the present embodiment, the hiding property can be enhanced by lowering the viscosity of the pigment of the white ink and further increasing the average particle diameter.

[0046] In addition, in order to reduce the amount of the precoat liquid applied to the printing substrate M when it reaches the image forming unit 9 in the case of a printing job in which white ink is ejected as compared to the case of a printing job in which no white ink is ejected, for example, it is conceivable to reduce the amount of the precoat liquid applied by the precoat processing unit 5 in the case of a printing job in which white ink is ejected as compared to the case of a printing job in which no white ink is ejected. In an inkjet type image forming apparatus, there is a viscosity range of ink suitable for the print head. Therefore, in the precoat processing unit 5, the materials necessary for fixing the ink are diluted with a solvent such as water to an appropriate viscosity. Therefore, when the application amount of the precoat liquid is reduced, the amount of the materials necessary for fixing is insufficient, and a sufficient precoat result cannot be obtained.

[0047] Next, a test for evaluating the adhesion of ink in the inkjet type image forming apparatus 1 of the present embodiment will be described.

[0048] As Comparative Example 1, a printing substrate M on which a solid cyan image was printed at 600 dpi without precoat processing was prepared. As Comparative Example 2, a printing substrate M on which a solid white image was printed at 600 dpi without precoat processing was prepared. On the other hand, as an Example, a printing substrate M was pre-dried after precoat processing and a solid white image was printed at 600 dpi was prepared. The printing substrate M is an OPP film, the width of the printing substrate M is 210 mm, and the conveyance speed of the printing substrate M is 3 m / min. The components of the precoat liquid in the Example are 10% of a polyester resin (trade name: Pes resin A-640, manufactured by Takamatsu Resin Co., Ltd.), 0.04% of a surfactant (trade name: Surfynol 440, manufactured by Nissin Chemical Industry Co., Ltd.), 25% of propylene glycol, 64.96% of water, and the application amount of the precoat liquid is 3 g / m 2 is. The temperature of the warm air in the pre-drying unit 7 is 80 degrees, the wind speed is 10 m / s, and the amount of the precoat liquid when reaching the image forming unit 9 is 2 g / m 2 is.

[0049] Adhesion was evaluated by a peel test based on JIS K5600 (no cross-cut was formed). The tape (product name CT18, manufactured by Nichiban Co., Ltd.) was firmly rubbed against the image with a fingertip, and within 5 minutes, the tape was peeled off, or the edge of the tape was grasped at an angle close to 60 degrees and reliably separated within 0.5 to 1.0 seconds.

[0050] Photographs of the images after peeling the tape for Comparative Examples 1 and 2 and the Example are shown in FIGS. 3A to 3C. FIG. 3A shows Comparative Example 1, FIG. 3B shows Comparative Example 2, and FIG. 3C shows the Example. In each photograph, the left side shows the printed substrate from which the tape has been peeled, and the right side shows the peeled tape.

[0051] As shown in FIG. 3A, in the case of cyan, no peeling of the image was observed. However, as shown in FIG. 3B, in the case of a white solid image, peeling of the image was observed. On the other hand, as shown in FIG. 3C, in the Example, no peeling of the white image was observed. From these results, it was confirmed that the precoat treatment and the pre-drying process are necessary, particularly when forming a white image.

[0052] Next, a test for evaluating the adhesion, hiding property, and white drying property of the ink in an image containing white in the inkjet type image forming apparatus 1 of the present embodiment will be described.

[0053] As Examples 1 to 6, after performing a precoat treatment on the printing substrate M, pre-drying was performed so that the amount of the precoat liquid at the time of reaching the image forming unit (referred to as the remaining amount of the precoat liquid) was less than the remaining amount of the precoat liquid for an image that does not contain white, and an image containing white was printed at 600 dpi. Examples 1 to 4 show that the viscosity of the white ink is lower than the viscosity of the inks other than white, and the average particle diameter of the white ink is larger than the average particle diameter of the inks other than white. Example 5 shows that the viscosity of the white ink is equal to the viscosity of the inks other than white. Example 6 shows that the average particle diameter of the white ink is equal to the average particle diameter of the inks other than white. On the other hand, as Comparative Example 1, under the same conditions of viscosity and average particle diameter as in Example 1, after performing a precoat treatment on the printing substrate M, pre-drying was performed so that the remaining amount of the precoat liquid was the same as the remaining amount of the precoat liquid for an image that does not contain white, and an image containing white was printed at 600 dpi. The printing substrate M is an OPP film, the conveyance speed is 25 m / min, and the coating amount of the precoat liquid is 3 g / m 2 is.

[0054] The results of evaluating the adhesion, hiding power, and white drying property of Examples 1 to 6 and the comparative example are shown in the table of FIG. 4.

[0055] The adhesion was evaluated by the image density of the tape peeled off in the above-described peeling test. Specifically, an unused tape and a peeled-off tape were attached to copy paper (product name: C2, manufactured by Fujifilm Business Innovation Co., Ltd.), and the image density was measured with an image densitometer (product name: FD-5, manufactured by Konica Minolta Inc.). Those with a difference in image density between the unused tape and the peeled-off tape of less than 0.05 were marked as ○, those less than 0.15 were marked as Δ, and those 0.15 or more were marked as ×.

[0056] The hiding power was evaluated by the following method. The image formed on the printing substrate was placed on the black portion of a hiding power rate test paper (manufactured by TP Giken Co., Ltd.), and the density of black was measured using a spectrocolorimeter (product name: CM-25cG, manufactured by Konica Minolta Inc.). Those with an image density of less than 0.2 were marked as ○, those less than 0.3 were marked as Δ, and those 0.3 or more were marked as ×.

[0057] The white drying property was evaluated by the following method. An OPP film was placed on the image of the printed substrate 3 seconds after the main drying. If the ink offset, it was marked as ○; if the ink offset after 3 seconds, it was marked as △; if the ink offset after 5 seconds, it was marked as ×.

[0058] As shown in Examples 1 to 4, when the viscosity of the white ink was lowered and the average particle diameter was increased, good results were obtained for adhesion, hiding property, and white drying property by performing precoat treatment and pre-drying.

[0059] On the other hand, as shown in Example 5, when the viscosity of the white ink was increased, sufficient adhesion could not be obtained. This is because the viscosity of the ink was high and it could not be dried sufficiently. Also, as shown in Example 6, when the particle size of the white ink was decreased, sufficient hiding property could not be obtained. In this example, it is preferable that the viscosity of the white ink is less than 6.0 (mPa·s) and the average particle diameter is 200 (nm) or more.

[0060] Also, as shown in Comparative Example 1, when pre-drying was performed so that the remaining amount of the precoat liquid was the same as that of the precoat liquid for an image without white, it was found that the adhesion and hiding property were good, but the white drying property was poor.

[0061] In the above embodiment, the inkjet head 31W for white ink is arranged on the upstream side in the conveyance direction X compared to the inkjet heads for other colors. Generally, a flexible package printed matter is formed by laminating a plurality of films on a printed substrate on which an image is formed. For this reason, a step of laminating the films is included. In this case, it is preferable in terms of productivity to perform surface printing after the laminating step. In the case of surface printing, since white is required for the lowermost layer, it is preferable that the inkjet head 31W for white ink is arranged on the upstream side in the conveyance direction X compared to the inkjet heads for other colors.

[0062] In the above embodiment, a five-color head unit 31 is used. However, the present invention is not limited to five colors and may have more or fewer colors than five. Also, the head unit 31 is not limited to the line head method. However, the line head method is preferable because it can increase the printing speed. Further, a long plastic film (PET film) is used as the printing substrate M, but a cut film may also be used.

[0063] Although the present invention has been described with respect to specific embodiments, the present invention is not limited to the above embodiments. Those skilled in the art can modify the above embodiments without departing from the scope and gist of the present invention.

Explanation of Reference Numerals

[0064] 1 Inkjet type image forming apparatus 5 Precoat processing unit 7 Pre-drying unit 9 Image forming unit 21, 31 Head unit 51 Control unit

Claims

1. An image forming unit that discharges inks of a plurality of colors including white onto a printing substrate conveyed along a predetermined conveyance direction to form an image; A precoat processing unit disposed upstream of the image forming unit in the conveyance direction, and applying a fixed amount of precoat liquid to the printing substrate; A pre-drying unit disposed downstream of the precoat processing unit in the conveyance direction, and pre-drying the precoat liquid applied to the printing substrate before ink is discharged from the image forming unit; A control unit that controls the pre-drying unit to adjust the degree of drying of the precoat liquid applied to the printing substrate, and comprising: When the control unit performs a printing job for discharging white ink, the amount of the precoat liquid applied to the printing substrate at the time of reaching the image forming unit is W1 (g / m ２ ), and when performing a printing job for not discharging white ink, the amount of the precoat liquid applied to the printing substrate at the time of reaching the image forming unit is W2 (g / m ２ ), an inkjet type image forming apparatus characterized by controlling the pre-drying unit so that W1 < W2.

2. The image forming unit includes a head unit provided for each color of ink and discharging ink; The precoat processing unit includes a head unit that discharges the precoat liquid; The inkjet image forming apparatus according to claim 1, wherein both of the head units have the same structure.

3. The inkjet image forming apparatus according to claim 2, wherein the head unit for white ink is disposed upstream of the head unit for other colors in the conveyance direction.

4. The inkjet image forming apparatus according to claim 1, wherein the pigment of the white ink is titanium oxide.

5. The inkjet image forming apparatus according to claim 1, wherein the binder resin of the white ink is a polyurethane resin.

6. The inkjet image forming apparatus according to claim 1, wherein the printing substrate is a plastic film.

7. The inkjet image forming apparatus according to claim 1, wherein when the viscosity of the white ink at 25 degrees is ρw (mPa·s) and the viscosity of the ink of a color other than white at 25 degrees is ρc (mPa·s), ρw < ρc.

8. The inkjet image forming apparatus according to claim 1, wherein when the average particle diameter of the pigment of the white ink is Dw (nm) and the average particle diameter of the pigment of the ink of a color other than white is Dc (nm), Dw > Dc.

Images