Inkjet type image formation apparatus

Abstract

To provide an inkjet type image formation apparatus that can improve the adhesion between white ink and a printing substrate.SOLUTION: An inkjet type image formation apparatus 1 comprises: head units 41W, 41B, 41C, 41M, 41Y that are provided for each ink of a plurality of colors including white, and that discharge ink onto a printing substrate M being conveyed along a prescribed conveyance direction; a corona processing unit 5 which is disposed at an upstream side in the conveyance direction beyond the head units 41W, 41B, 41C, 41M, 41Y, and which applies corona discharge to the printing substrate M; and a controller 31 for controlling a discharge amount of the corona discharge performed by the corona processing unit 5. The controller 31 controls a discharge amount so that W1≥W2 is established, in the case that W1 (W min / m2) denotes a discharge amount of the corona discharge when conducting a printing job by discharging white ink, and W2 (W min / m2) denotes a discharge amount of the corona discharge when conducting a printing job by not discharging white ink.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, as described in Patent Documents 1 and 2, corona treatment for corona discharging a printing substrate may be performed in order to improve the adhesion between the printing substrate and the ink.

[0003] In the printing apparatus described in Patent Document 1, corona treatment is performed to improve the pinning property of the ink after landing. In the inkjet recording apparatus described in Patent Document 2, corona treatment is performed to suppress a decrease in color development due to the ink that lands later being buried in the image formed by the ink that landed earlier.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] By the way, generally, it is known that white ink is likely to peel off from a printing substrate. 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, and the adhesion to the printing substrate deteriorates. In particular, this tendency becomes stronger in the case of a printing substrate having a smooth surface such as a plastic film.

[0006] Therefore, in view of the above circumstances, an object of the present invention is to provide an inkjet type image forming apparatus capable of improving the adhesion between white ink and a printing substrate.

Means for Solving the Problems

[0007] To achieve the above object, an inkjet type image forming apparatus of the present invention includes a head unit provided for each of a plurality of colors of ink including white, and discharging the ink onto a printing substrate conveyed along a predetermined conveyance direction, and a corona treatment unit disposed upstream of the head unit in the conveyance direction and performing corona discharge on the printing substrate, and a control unit for controlling the discharge amount of the corona discharge performed by the corona treatment unit, wherein the control unit sets the discharge amount of the corona discharge when performing a printing job of discharging white ink as W1 (W·min / m 2 ), and sets the discharge amount of the corona discharge when performing a printing job of not discharging white ink as W2 (W·min / m 2 ), and is characterized in that the discharge amount is controlled so that W1≧W2.

[0008] In the present invention, the head unit for discharging white ink may be disposed upstream of the head unit for discharging other colors of ink in the conveyance direction.

[0009] In the present invention, the pigment of the white ink may be titanium oxide.

[0010] In the present invention, the binder resin of the white ink may be a polyurethane resin.

[0011] In the present invention, the printing substrate may be a plastic film.

[0012] 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.

[0013] 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 a color other than white is Dc (nm), it may be characterized in that Dw > Dc.

Advantages of the Invention

[0014] According to the present invention, in the case of a printing job for discharging white ink, by subjecting the printing substrate to corona treatment at a higher discharge amount, even white ink can enhance the adhesion to the printing substrate, and peeling of the image or the like can be prevented.

Brief Description of the Drawings

[0015]

Figure 1

Figure 2

Figure 3

Figure 4A

Figure 4B

Figure 4C

Figure 5

Embodiments for Carrying Out the Invention

[0016] Hereinafter, an inkjet type image forming apparatus according to an embodiment of the present invention will be described with reference to the drawings.

[0017] Referring 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.

[0018] The inkjet type image forming apparatus 1 includes a supply roller 3 around which a long printing substrate M is wound, a corona treatment unit 5 that performs a corona treatment for corona discharging the printing substrate M supplied from the supply roller 3, an image forming unit 7 that forms an image on the printing substrate M subjected to the corona treatment by an inkjet method, a drying unit 9 that dries the image formed by the image forming unit 7, and a take-up roller 11 that takes up the printing substrate M dried by the drying unit 9.

[0019] First, the supply roller 3 will be described. The 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.

[0020] Next, the corona treatment unit 5 will be described with reference to FIG. 2. FIG. 2 is a diagram schematically explaining the corona treatment unit 5. The corona treatment unit 5 is disposed on the downstream side in the conveyance direction X from the supply roller 3. The corona treatment unit 5 includes a corona discharge electrode 21 and a counter roller 23.

[0021] The corona discharge electrode 21 is made of, for example, a rod-shaped ceramic that is long in the width direction Y of the printing substrate M or an aluminum cylinder coated with ceramic. The corona discharge electrode 21 is covered with an electrode cover 25 having an open bottom surface. The corona discharge electrode 21 is connected to a high-frequency power supply 29 via a high-voltage transformer 27. The high-frequency power supply 29 generates a high-frequency alternating current. The high-frequency alternating current is, for example, 5.5 kV to 6.5 kV. The higher the discharge power, the higher the discharge voltage (discharge amount). However, since the discharge voltage varies depending on the atmospheric pressure, temperature, humidity, etc., it is set according to the environmental conditions, etc. when actually performing the corona discharge treatment. The high-voltage transformer 27 amplifies the generated high-frequency alternating current, converts it into discharge power, and applies it to the corona discharge electrode 21. The high-frequency power supply 29 is electrically connected to a control unit 31 and generates a high-frequency alternating current based on an instruction from the control unit 31.

[0022] The opposing roller 23 is made of, for example, a stainless steel cylinder and is grounded. The opposing roller 23 is arranged at a predetermined interval from the corona discharge electrode 21.

[0023] Next, the image forming unit 7 will be described with reference to FIG. 1. The image forming unit 7 is arranged on the downstream side in the conveyance direction X of the corona treatment unit 5. The image forming unit 7 includes five head units 41W, 41B, 41C, 41M, 41Y (collectively referred to as the head unit 41) corresponding to five colors (white, black, cyan, magenta, yellow) of ink, and a conveyance plate 43 along which the printing substrate M is conveyed.

[0024] The head unit 41 will be described with reference to FIG. 3 as well. FIG. 3 is a plan view showing the head unit 41. Each head unit 41 includes three print heads 45 and a plate 47 that supports the three print heads 45.

[0025] As shown in FIG. 3, the three print heads 45 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, and are supported by the plate 47. The print head 45 is provided with a large number of nozzles (not shown). The discharge ports of the nozzles are open on the lower surface of the print head 45.

[0026] The five head units 41W, 41B, 41C, 41M, and 41Y are arranged in order along the conveyance direction X, and white, black, cyan, magenta, and yellow inks are supplied thereto. The supplied ink is discharged downward from the discharge ports of the nozzles of each print head 45. Each ink contains a pigment and a binder resin.

[0027] 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 smaller than the viscosity ρc (mPa·s) of the inks of colors other than white at 25 degrees. Further, the average particle diameter Dw (nm) of the pigment of the white ink is larger than the average particle diameter Dc (nm) of the pigments of the inks of colors other than white.

[0028] As shown in FIG. 1, the conveyance plate 43 is disposed below the five head units 41. The upper surface of the conveyance plate 43 is formed flat. Also, as shown in FIG. 3, the width of the conveyance plate 43 is wider than the widths of the printing substrate M and all the head units 41. A heat source (not shown) is provided below the conveyance plate 43 to heat the conveyance plate 43 to a predetermined temperature. The conveyance plate 43 is made of, for example, metal.

[0029] Next, the drying unit 9 will be described. The drying unit 9 is arranged on the downstream side in the conveyance direction X from the image forming unit 7. The drying unit 9 includes a heat drum 51, and upstream and downstream tension rollers 53 and 55 arranged on the upstream side and the downstream side of the heat drum 51. The printed substrate M that has passed through the image forming unit 7 is wound around the heat drum 51 between the upstream and downstream tension rollers 53 and 55. The heat drum 51 is heated to dry the printed substrate M conveyed along its surface.

[0030] Next, the take-up roller 11 will be described. The take-up roller 11 is arranged on the downstream side in the conveyance direction X from the drying unit 9. The end of the printed substrate M is fixed to the take-up roller 11. The take-up roller 11 is connected to a motor (not shown) and rotates. By rotating the take-up roller 11 in a predetermined direction at a predetermined rotational speed by the motor, the printed substrate M is fed out from the supply roller 3.

[0031] The fed-out printed substrate M passes between the corona discharge electrode 21 of the corona treatment unit 5 and the counter roller 23 and is conveyed to the image forming unit 7. In the image forming unit 7, the printed substrate M is conveyed along the upper surface of the conveyance plate 43 under the five head units 41 toward the drying unit 9. Then, after being wound around the heat drum 51 in the drying unit 9, it is wound around the take-up roller 11.

[0032] Next, the control unit 31 will be described. As shown in FIG. 2, the control unit 31 controls the high-frequency power supply 29 of the corona treatment unit 5 to create a high-frequency current of a predetermined intensity. Further, the control unit 31 is also electrically connected to the head unit 41 and controls the head unit 41 based on the input image data.

[0033] Further, data of an image to be printed is input to the control unit 31. The control unit 31 determines, based on the input image data, whether the print job to be executed is a print job that ejects white ink or a print job that does not eject white ink. Further, the control unit 31 controls the discharge amount discharged from the corona discharge electrode 21 of the corona treatment unit 5.

[0034] In the inkjet type image forming apparatus 1 having the above configuration, the image forming operation on a non-absorbent substrate (plastic film) will be described. During the image forming operation, the control unit 31 controls the corona treatment unit 5 to execute corona treatment. As a result, corona discharge occurs between the corona discharge electrode 21 and the counter roller 23 in the corona treatment unit 5. Here, the control unit 31 determines whether the print job ejects white ink or not, and sets the discharge amount of corona discharge when performing a print job that ejects white ink to W1 (W·min / m 2 ) and sets the discharge amount of corona discharge when performing a print job that does not eject white ink to W2 (W·min / m 2 ). Here, W1≧W2. Note that W2 may be zero. When the printing substrate M is not non-absorbent, corona treatment by the corona treatment unit 5 is not performed.

[0035] Thereafter, the motor is driven and the take-up roller 11 rotates, and the printing substrate M is sent out from the supply roller 3 to the corona treatment unit 5. In the corona treatment unit 5, corona discharge occurs at the set discharge amount between the corona discharge electrode 21 and the counter roller 23, and corona treatment is performed on the upper surface of the printing substrate M.

[0036] Thereafter, the printing substrate M is conveyed below the head unit 41, and based on the image data, ink of a predetermined color is ejected from the corresponding head unit 41, and an image is formed on the printing substrate M. The printing substrate M on which the image is formed in this way is conveyed to the drying unit 9. In the drying unit 9, the ink is completely dried. Thereafter, the printing substrate M is wound around the take-up roller 11.

[0037] As described above, according to the present invention, the discharge amount in the corona treatment in the case of a printing job in which white ink is ejected is equal to or greater than the discharge amount in the corona treatment in a printing job in which white ink is not ejected. As described above, in order to increase the hiding power of white ink, it is necessary to increase the particle size of the pigment. In that case, the pigment cannot be completely coated with the binder resin, and the adhesion to the printing substrate M deteriorates. Therefore, by subjecting the printing substrate M to corona treatment, the adhesion between the printing substrate M and the ink can be enhanced. That is, when the printing substrate M is subjected to corona treatment, fine irregularities are formed on the surface of the printing substrate M, and the ink easily enters these irregularities (anchor effect). Further, hydrophilic polar groups such as carbonyl groups, carboxyl groups, and nitrides are formed on the surface of the printing substrate M. Such polar groups increase the wettability of the surface of the printing substrate M. Therefore, due to such physical and chemical actions by corona discharge treatment, the compatibility (adhesion) between the printing substrate M and the ink is improved.

[0038] Therefore, in the case of a printing job in which white ink is ejected, by subjecting the printing substrate M to corona treatment with a higher discharge amount, the adhesion between the white ink and the printing substrate M can be enhanced, and peeling of the image and the like can be prevented. Also, the adhesion between inks other than white and the printing substrate M is increased.

[0039] 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 power. 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.

[0040] Furthermore, in order to improve the hiding property of white, a large amount of white ink is ejected. However, when the amount of ink ejected in this way increases, the drying property of the ink after ejection decreases. White ink generally contains water and a high-boiling-point material whose boiling point is higher than that of water. Comparing water and the high-boiling-point material, water is easier to dry. Therefore, the amount of the high-boiling-point material contained is reduced, and the amount of water is increased accordingly. As a result, the viscosity of the white ink appears to be lower.

[0041] Also, in order to improve 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, by reducing the viscosity of the pigment of the white ink and further increasing the average particle diameter, the hiding property can be enhanced.

[0042] Next, a test for evaluating the adhesion of ink in the inkjet type image forming apparatus 1 of the present embodiment will be described with reference to FIGS. 4A to 4C. The adhesion was evaluated by a peeling test.

[0043] The peeling test was performed as follows. First, after the printing substrate M was corona-treated by the corona treatment unit 5, cyan and white solid images were printed on the printing substrate M at 600 dpi. 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 discharge power of the corona treatment unit 5 is 100 W (discharge voltage 6.3 kV).

[0044] The printed image was subjected to a peeling test based on JIS K5600 (no cross-cut was formed). A tape (product name CT18, manufactured by Nichiban Co., Ltd.) was firmly rubbed on 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.

[0045] FIG. 4A is a photograph showing a wet image of cyan after the tape is peeled off, and FIG. 4B is a photograph showing a wet image of white after the tape is peeled off. FIG. 4C is a photograph of a wet image of white without corona treatment as a comparative example after the tape is peeled off. In each figure, the left side shows the printed substrate from which the tape is peeled off, and the right side shows the peeled tape.

[0046] As shown in FIG. 4A, when corona treatment was performed, no image peeling was observed in the wet image of cyan. As shown in FIG. 4B, no image peeling was observed in the wet image of white either. On the other hand, as shown in FIG. 4C, when corona treatment was not performed, peeling of the white image was observed.

[0047] Next, a test for evaluating the ink adhesion and hiding power in the inkjet type image forming apparatus 1 of the present embodiment will be described with reference to the table in FIG. 5.

[0048] The adhesion was evaluated by the image density of the peeled tape in the above-described peeling test. Specifically, an unused tape and a peeled tape were attached to copy paper (trade name C2, manufactured by Fujifilm Business Innovation Co., Ltd.), and the image density was measured with an image densitometer (trade name FD-5, manufactured by Konica Minolta Inc.). Those with a difference in image density between the unused tape and the peeled 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 ×.

[0049] The hiding power was evaluated by the following method. The image formed on the printed substrate was placed on the black portion of the hiding power test paper (manufactured by TP Giken Co., Ltd.), and the black density was measured using a spectrocolorimeter (trade 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 ×.

[0050] As shown in Examples 1 to 5, when the viscosity of the white ink was lowered and the average particle size was increased, good results were obtained in both adhesion and hiding properties by performing corona treatment. In this case, the corona discharge amount was larger than or equal to the discharge amount of inks other than the white ink. In this example, the corona discharge amount for the white ink is preferably about 1.125 to 1.25 times (or 90 W·min / m 2 or more) of the corona discharge amount for inks other than white.

[0051] On the other hand, as shown in Example 6, 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 7, 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 size is 200 (nm) or more.

[0052] Also, as shown in Comparative Example 1, when the viscosity of the white ink is low and the average particle size is large, if the corona discharge amount is low, the adhesion becomes insufficient (see also Fig. 4C).

[0053] Generally, a flexible package printed matter is formed by laminating a plurality of films on a printed base material on which an image is formed. For this reason, a process of laminating the films is included. In this case, it is preferable in terms of productivity to perform surface printing after the laminating process. In the case of surface printing, since white is required for the lowermost layer, it is preferable that the head unit 41W that discharges the white ink is arranged upstream of the head units that discharge inks of other colors in the conveyance direction X.

[0054] In the above embodiment, the drying unit 9 was arranged on the downstream side of the five head units 41. However, a drying unit for white may be provided separately on the downstream side of the head unit 41W that discharges the white ink.

[0055] In the above-described embodiment, a five-color head unit 41 is used. However, the present invention is not limited to five colors and may have more or fewer colors than five. Further, the head unit 41 is not limited to the line head method. However, the line head method is preferable because the printing speed can be increased. Also, although a long plastic film (PET film) is used as the printing substrate M, a cut film may also be used.

[0056] 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

[0057] 1 Inkjet type image forming apparatus 5 Corona treatment unit 31 Control unit 41 Head unit

Claims

1. A head unit provided for each of a plurality of colors of ink including white, and discharging the ink onto a printing substrate conveyed along a predetermined conveyance direction; A corona treatment unit disposed upstream of the head unit in the conveyance direction and performing corona discharge on the printing substrate; A control unit that controls the discharge amount of the corona discharge performed by the corona treatment unit, comprising: When the control unit performs a printing job of discharging white ink, the discharge amount of the corona discharge is set to W1 (W·min / m ２ ), and when performing a printing job of not discharging white ink, the discharge amount of the corona discharge is set to W2 (W·min / m ２ ), the inkjet type image forming apparatus is characterized in that the discharge amount is controlled so that W1≥W2.

2. The head unit that discharges white ink is disposed upstream of the head unit that discharges other colors of ink in the conveyance direction, according to the inkjet type image forming apparatus according to claim 1.

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

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

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

6. 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), ρw<ρc, according to the inkjet type image forming apparatus according to claim 1.

7. In the inkjet image forming apparatus according to claim 1, 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), characterized in that Dw > Dc.

Images