Inkjet image forming apparatus

Abstract

To provide an inkjet-type image forming apparatus that can reduce the occurrence of blurring and streaks. [Solution] The inkjet image forming apparatus 1 comprises a head unit 11 for white ink, a head unit 15 for color ink located downstream of the head unit 11 in the transport direction, a first heating unit 17 located upstream of the head unit 11 in the transport direction to heat the printing substrate before the white ink is ejected, and a second heating unit 19 located downstream of the head unit 11 in the transport direction and upstream of the head unit 15 in the transport direction to heat the printing substrate before the color ink is ejected. If the temperature of the printing substrate heated by the first heating unit 17 is Tw (°C) and the temperature of the printing substrate heated by the second heating unit 19 is Tc (°C), then Tc

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] An inkjet type image forming apparatus includes a heating unit that heats a printing substrate to evaporate a solvent (such as water) contained in the ink in order to prevent bleeding of the ink or the like.

[0003] For example, the drying device (corresponding to the heating unit) described in Patent Document 1 includes a first drying unit that evaporates a first liquid component contained in a liquid, and a second drying unit that evaporates a second liquid component having a higher boiling point than the first liquid component. The first drying unit is a flat platen in which a heating wire is arranged inside.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] Generally, when forming a white image on a printing substrate such as a film, a high hiding power (opacity, the degree to which the image cannot be seen through) is required. For this reason, the discharge amount of white ink is larger than that of color ink. As the discharge amount increases, the amount of solvent such as water contained in the ink also increases, making it difficult for the ink to dry. To completely dry the ink, it is necessary to lengthen the drying time, resulting in a decrease in workability. In particular, this tendency becomes stronger in the case of a printing substrate having a smooth surface such as a plastic film.

[0006] In addition, if the heating temperature is increased to accelerate the drying of the ink, streaks along the conveyance direction are likely to occur in the image. On the other hand, when the heating temperature is low, bleeding is likely to occur. Therefore, it is necessary to set the heating temperature in consideration of the balance between streaks and bleeding.

[0007] In the drying device of Patent Document 1 described above, since such a difference in discharge amount is not considered, bleeding or streaks may occur depending on the heating temperature.

[0008] Therefore, an object of the present invention is to provide an inkjet type image forming apparatus that can reduce the occurrence of bleeding and streaks in consideration of the above circumstances.

Means for Solving the Problems

[0009] To achieve the above object, an inkjet type image forming apparatus according to the present invention includes a white ink head unit that discharges white ink onto a print substrate conveyed in a predetermined conveyance direction, a color ink head unit that is disposed downstream of the white ink head unit in the conveyance direction and discharges color ink other than white ink onto the print substrate, a first heating unit that is disposed upstream of the white ink head unit in the conveyance direction and heats the print substrate before white ink is discharged, and a second heating unit that is disposed downstream of the white ink head unit in the conveyance direction and upstream of the color ink head unit in the conveyance direction and heats the print substrate before color ink is discharged. When the temperature of the print substrate heated by the first heating unit is Tw (°C) and the temperature of the print substrate heated by the second heating unit is Tc (°C), Tc < Tw.

[0010] In the present invention, the first heating unit and the second heating unit may include a conveyance plate having a conveyance surface on which the print substrate is conveyed, and a heating body that heats the conveyance plate from the back side of the conveyance surface.

[0011] In the present invention, the temperature of the printing substrate heated by the first heating unit and the second heating unit may be 80°C or lower.

[0012] In the present invention, the difference between the temperature Tc and the temperature Tw may be characterized by being 20°C.

[0013] The present invention may further be characterized by comprising a pre-drying section for heating the printing substrate, which is located downstream of the white ink head unit in the transport direction.

[0014] In the present invention, the viscosity of the white ink may be 5.5 to 6.5 mPa·S, and the viscosity of the color ink may be 5.0 to 5.0 mPa·S.

[0015] In the present invention, the appropriate amount of the white ink may be 0.5 to 5 pL, and the appropriate amount of the color ink may be 2 to 10 pL.

[0016] In the present invention, the printing substrate may be characterized by being a plastic film. [Effects of the Invention]

[0017] According to the present invention, by heating the printing substrate at different temperatures for white ink and colored ink, bleeding and streaking can be reduced. [Brief explanation of the drawing]

[0018] [Figure 1] This is a schematic front view showing the internal configuration of an inkjet image forming apparatus according to one embodiment of the present invention. [Figure 2] This is a plan view showing the head unit in an inkjet image forming apparatus according to one embodiment of the present invention. [Figure 3] This table shows the evaluation results for blurring and streaking in an inkjet image forming apparatus according to one embodiment of the present invention. [Figure 4]In an inkjet type image forming apparatus according to an embodiment of the present invention, it is a table showing the components of color ink and white ink.

Embodiments for Carrying Out the Invention

[0019] Hereinafter, an inkjet type image forming apparatus according to an embodiment of the present invention will be described while referring to the drawings.

[0020] 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.

[0021] The inkjet type image forming apparatus 1 includes a supply roller 3 around which a long printing substrate M is wound, an image forming unit 5 that forms an image on the printing substrate M in an inkjet manner, a drying unit 7 that dries the image formed on the printing substrate M, and a take-up roller 9 that takes up the printing substrate M.

[0022] 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.

[0023] Next, the image forming unit 5 will be described. The image forming unit 5 is disposed above the printing substrate M fed out from the supply roller 3. The image forming unit 5 includes, in order from the upstream side in the conveyance direction X, a white ink head unit 11, a hot air blowing nozzle 13, a color ink head unit 15, and first heating units 17 and second heating units 19 disposed below these.

[0024] First, the white ink head unit 11 will be described. The white ink head unit 11 discharges white ink onto the white image forming area of the printing substrate M fed out from the supply roller 3.

[0025] The head unit 11 for white ink will be described with reference to Figure 2. Figure 2 is a plan view showing the head unit 11 for white ink. The head unit 11 for white ink comprises three print heads 21 and a plate 23 that supports the three print heads 21. The three print heads 21 are arranged in a staggered pattern along the width direction Y (line head method) so as to be approximately the same as the width of the printing substrate M (length along the width direction Y intersecting the transport direction X), and are supported by the plate 23. White ink is supplied to the three print heads 21.

[0026] The white ink contains a pigment and a binder resin. The pigment is titanium dioxide, and the binder resin is polyurethane resin. The viscosity of the white ink at 25 degrees Celsius is preferably 5.0 to 6.0 mPa·s, and most preferably 5.5 to 5.75 mPa·s. The appropriate amount of white ink is preferably 2 to 10 pL, and most preferably 3 to 7 pL.

[0027] The print head 21 comprises numerous nozzles and piezoelectric elements provided in each nozzle. The nozzle ejection ports open on the underside of the print head 21. When voltage is applied to the piezoelectric elements, the elements deform, causing the white ink inside the nozzles to be ejected downwards from the ejection ports.

[0028] Next, the hot air blowing nozzle 13 will be described with reference to Figure 1. The hot air blowing nozzle 13 blows hot air toward the printing substrate M that has passed below the white ink head unit 11. As a result, solvents such as water contained in the white ink evaporate, and the discharged white ink is pre-dried. The hot air blowing nozzle 13 is an example of a pre-drying section that heats the printing substrate M in the present invention.

[0029] Next, the color ink head unit 15 will be described with reference to Figure 1. The color ink head unit 15 includes four head units 15B, 15C, 15M, and 15Y (also simply called the color ink head unit 15) that eject four color inks (black, cyan, magenta, and yellow) into the color image forming area of ​​the printing substrate M. The four color ink head units 15 are arranged in order along the transport direction X.

[0030] Each color ink head unit 15 has the same configuration as the white ink head unit 11 shown in Figure 2, and includes three print heads 31 and a plate 33 that supports the three print heads 31. The three print heads 31 of the four color ink head units 15 are supplied with black, cyan, magenta, and yellow ink, respectively.

[0031] The color ink contains a pigment and a binder resin. The binder resin may be the same as that of the white ink, or it may be different. The viscosity of the color ink at 25 degrees Celsius is higher than that of the white ink, preferably 5.5 to 6.5 mPa·s, and most preferably 5.75 to 6.0 mPa·s. The appropriate amount of color ink is less than that of the white ink, preferably 0.5 to 5 pL, and most preferably 0.75 to 2 pL.

[0032] Next, the first heating unit 17 will be described with reference to Figure 1. The first heating unit 17 is located below the white ink head unit 11 and the hot air blowing nozzle 13, and heats the printing substrate M before the white ink is discharged from the white ink head unit 11. The first heating unit 17 includes a transport plate 41, a rubber heater 43, a thermocouple 45, and a temperature control unit 47.

[0033] The transport plate 41 extends upstream in the transport direction X from below the white ink head unit 11 and the hot air blowing nozzle 13. The upper surface of the transport plate 41 is the transport surface on which the printing substrate M is transported, and is formed flat. The rubber heater 43 is attached to the lower surface of the transport plate 41 (the surface opposite to the transport surface) using heat-resistant double-sided tape. The thermocouple 45 is attached to the rubber heater 43.

[0034] The temperature control unit 47 is connected to the rubber heater 43 via lead wires. The temperature measured by the thermocouple 45 is also input to the temperature control unit 47. The temperature control unit 47 controls the rubber heater 43 so that the temperature measured by the thermocouple 45 becomes a predetermined temperature Tw (°C). The predetermined temperature Tw is preferably 80°C or less, and more preferably 60°C. By controlling the rubber heater 43 in this way by the temperature control unit 47, the temperature of the conveying surface of the conveying plate 41 is adjusted to approximately temperature Tw (°C). The rubber heater 43 is an example of a heating element that heats the conveying plate 41 from the back side of the conveying surface.

[0035] Next, the second heating unit 19 will be described with reference to Figure 1. The second heating unit 19 is located below the color ink head unit 15 and heats the printing substrate M before the color ink is ejected from the color ink head unit 15. The second heating unit 19, like the first heating unit 17, includes a transport plate 51, a rubber heater 53, a thermocouple 55, and a temperature control unit 57.

[0036] The transport plate 51 extends from below the color ink head unit 15 to the upstream side in the transport direction X. The upper surface of the transport plate 51 is the transport surface on which the printing substrate M is transported, and is formed to be flat. The rubber heater 53 is attached to the lower surface of the transport plate 51 (the surface opposite to the transport surface) using heat-resistant double-sided tape. The thermocouple 55 is attached to the rubber heater 53.

[0037] The temperature control unit 57 is connected to the rubber heater 53 via lead wires. The temperature measured by the thermocouple 55 is also input to the temperature control unit 57. The temperature control unit 57 controls the rubber heater 53 so that the temperature measured by the thermocouple 55 becomes a predetermined temperature Tc (°C). The predetermined temperature Tw is preferably 80°C or lower, and more preferably 60°C, which is lower than temperature Tw. By controlling the rubber heater 53 in this way by the temperature control unit 57, the temperature of the conveying surface of the conveying plate 51 is adjusted to approximately temperature Tc (°C). The rubber heater 53 is an example of a heating element that heats the conveying plate 51 from the back side of the conveying surface.

[0038] Next, the drying section 7 will be described with reference to Figure 1. The drying section 7 dries the image formed on the printing substrate M in the image forming section 5. The drying section 7 comprises a heat drum 61 and upstream and downstream tension rollers 63 and 65 positioned upstream and downstream of the heat drum 61. The printing substrate M that has passed through the image forming section 5 is wrapped around the heat drum 61 between the upstream and downstream tension rollers 63 and 65. The heat drum 61 dries the printing substrate M as it is conveyed along its surface by being heated.

[0039] Next, the winding roller 9 will be described with reference to Figure 1. The end of the printing substrate M is fixed to the winding roller 9. The winding roller 9 is connected to a motor (not shown) and rotates. By rotating the winding roller 9 in a predetermined direction at a predetermined rotational speed using the motor, the printing substrate M is fed out from the supply roller 3.

[0040] The printed substrate M that is sent out passes below the image forming unit 5 along the upper surfaces of the transport plate 41 of the first heating unit 17 and the transport plate 51 of the second heating unit 19, is wrapped around the heat drum 61 in the drying unit 7, and is then wound onto the winding roller 9.

[0041] In the inkjet type image forming apparatus 1 having the above-described configuration, the image forming operation on a non-absorbent printing substrate M (plastic film) will be described. The image is assumed to include a white image and a color image other than white. The transport surface of the transport plate 41 of the first heating unit 17 and the transport surface of the transport plate 51 of the second heating unit 19 are heated to a predetermined temperature Tw (°C) and temperature Tc (°C), respectively. First, the take-up roller 9 is driven by a motor to rotate, and the printing substrate M is fed out from the supply roller 3 and transported below the image forming unit 5.

[0042] The printing substrate M is first transported along the transport surface of the transport plate 41 of the first heating unit 17 (a portion upstream of the white ink head unit 11). Since the transport plate 41 is heated to the temperature Tw (°C), the printing substrate M is also heated to a temperature close to Tw (°C). Thereafter, the printing substrate M is transported along the transport surface of the transport plate 41 below the white ink head unit 11, and white ink is ejected from the white ink head unit 11 into the white image forming area (an area where the white image of the image is formed).

[0043] Thereafter, the printing substrate M is transported along the transport surface of the transport plate 41 below the hot air blowing nozzle 13. The hot air blown out from the hot air blowing nozzle 13 and the heated transport plate 41 pre-dry the solvent and the like contained in the white ink. Here, it is preferable that the white ink is completely dried.

[0044] Thereafter, the printing substrate M is transported along the transport surface of the transport plate 51 of the second heating unit 19 (a portion upstream of the color ink head unit 15). Since the transport plate 51 is heated to the temperature Tc (°C) (<Tw), the printing substrate M is also heated to a temperature close to Tc (°C). The printing substrate M is very thin and has a small heat capacity. Therefore, although the printing substrate M is heated in the first heating unit 17, it returns to room temperature due to the surrounding atmosphere (room temperature) while being transported from the first heating unit 17 to the second heating unit 19. Thereafter, the printing substrate M is heated to a temperature close to Tc (°C) by being transported along the transport surface of the transport plate 51 of the second heating unit 19.

[0045] Subsequently, the printing substrate M is transported along the transport surface of the transport plate 51 to below the color ink head unit 15, and color ink is ejected from the color ink head unit 15 into the color image forming area. While the printing substrate M is transported below the color ink head unit 15 in this manner, the image is dried by the transport plate 51.

[0046] Subsequently, the printed substrate M on which the image has been formed is transported to the drying section 7. In the drying section 7, the ink is completely dried. After that, the printed substrate M is wound onto the winding roller 9.

[0047] As explained above, according to the present invention, the temperature of the printing substrate M when white ink is ejected is higher than the temperature of the printing substrate M when color ink is ejected. As mentioned above, the white ink is used in a larger quantity and has lower viscosity than the color ink in order to increase opacity (hiding ability). Therefore, the total amount of ink ejected onto the printing substrate M, that is, the amount of solvent contained in the printing substrate M, is greater than in the case of color ink.

[0048] Therefore, in the white image formation area, the heating temperature of the printing substrate M is increased to promote the drying of the white ink and prevent ink bleeding. On the other hand, in the color image formation area, the heating temperature of the printing substrate M is lowered to prevent the occurrence of streaks. As mentioned above, bleeding is more likely to occur when the heating temperature is low, but by allowing the ink on both sides of the area where streaks occur to bleed appropriately, the occurrence of streaks can be suppressed. In this way, by setting different heating temperatures for the printing substrate M for white ink and color ink, a balance can be struck between bleeding and the occurrence of streaks.

[0049] Furthermore, pre-drying with the hot air blowing nozzle 13 reduces the amount of solvent in the white ink and increases its viscosity, allowing it to be thoroughly dried in the drying section 7.

[0050] Furthermore, the pigment in the white ink is titanium dioxide, and the binder resin is polyurethane resin. Titanium dioxide is known as a pigment with good opacity. Also, since polyurethane resin has good compatibility with the printing substrate, it can improve the adhesion between the white ink and the printing substrate M. Generally, white ink contains water and a high-boiling point material that has a higher boiling point than water. Comparing water and high-boiling point materials, water dries more easily. Therefore, the amount of high-boiling point material included is reduced, and the amount of water is increased accordingly. As a result, the viscosity of the white ink appears to be low.

[0051] Next, the test results for bleeding and drying time in the inkjet image forming apparatus 1 of this embodiment will be described with reference to Figures 3 and 4. Figure 3 is a table showing the results of evaluating bleeding and drying time for the example and comparative example. Figure 4 is a table showing the components of the white ink, color ink, and pre-coat liquid.

[0052] The blurring and drying time of test images, in which a solid cyan image and a solid magenta image were adjacent, were evaluated under the conditions of Examples 1 and 2 and Comparative Examples 1 and 2, as shown in Figure 3. The heating temperature refers to the temperature of the transport surface of the transport plate 41 of the first heating unit 17 and the transport surface of the transport plate 51 of the second heating unit 19. Example 1 shows a temperature Tw(°C) of 60°C and a temperature Tc(°C) of 40°C, while Example 2 shows a temperature Tw(°C) of 80°C and a temperature Tc(°C) of 50°C. Comparative Example 1 shows a temperature Tw(°C) of 50°C and a temperature Tc(°C) of 50°C, while Comparative Example 2 shows a temperature Tw(°C) of 60°C and a temperature Tc(°C) of 60°C.

[0053] For bleeding, after the ink had dried, the boundary between the two solid images was observed under a microscope (product name MM-800 (Nikon Corporation)). Images with a boundary width of less than 10 μm were marked with ○, and those with a boundary width of 10 μm or more were marked with ×. For streaks, after the ink had dried, the images were observed visually. Images with no streaks within a specified range were marked with ○, images with two or fewer streaks were marked with △, and images with three or more streaks were marked with ×.

[0054] As shown in Examples 1 and 2, no bleeding was observed when the temperature Tw was lower than the temperature Tc. In Example 2, streaks were observed. In Example 2, the temperatures Tw and Tc were higher than those in Example 1. In this case, it is presumed that the ink dried more, making bleeding less likely and causing streaks. From these results, it is preferable that the temperature Tw be 60°C and the temperature Tc be 40°C.

[0055] On the other hand, bleeding and streaking were observed in Comparative Example 1. In Comparative Example 1, it is presumed that bleeding occurred because the ink did not dry sufficiently due to the low temperatures Tw and Tc. In Comparative Example 2, bleeding was not observed, but streaking was. In Comparative Example 2, it is presumed that bleeding did not occur because the ink dried sufficiently due to the high temperatures Tw and Tc. However, it is presumed that the high temperatures Tw and Tc caused the ink to dry more quickly, making it less likely to bleed, and thus resulting in the appearance of streaking.

[0056] In addition to rubber heaters 43 and 53, halogen heaters, ceramic heaters, induction heating (IH), hot water, etc., can be used as heating elements for the first heating section 17 and the second heating section 19.

[0057] In the above embodiment, a head unit 15 for four color inks (black, cyan, magenta, and yellow) was used, but the present invention is not limited to four colors, and may have more or fewer colors. Also, the head units 11 and 15 are not limited to a line head system. However, a line head system is preferred because it can increase the printing speed. In addition, a long plastic film (PET film) was used as the printing substrate M, but a cut film may also be used.

[0058] Although the present invention has been described in terms of 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 spirit of the present invention. [Explanation of symbols]

[0059] 1. Inkjet image forming apparatus 11. Head unit for white ink 13. Hot air outlet nozzle (front drying section) 15. Head unit for color inks 17. First heating section 19. Second heating section 41, 51 Conveyor board 43, 53 Rubber heater (heating element)

Claims

1. A head unit for white ink ejects white ink onto a printing substrate being transported in a predetermined transport direction, A color ink head unit is positioned downstream of the white ink head unit in the transport direction and ejects color inks other than white ink onto the printing substrate. A first heating unit is positioned upstream of the white ink head unit in the transport direction and heats the printing substrate before the white ink is ejected. It comprises a second heating unit positioned downstream of the white ink head unit in the transport direction and upstream of the color ink head unit in the transport direction, which heats the printing substrate before the color ink is ejected, An inkjet image forming apparatus characterized in that, if the temperature of the printing substrate heated by the first heating unit is Tw (°C) and the temperature of the printing substrate heated by the second heating unit is Tc (°C), then Tc < Tw.

2. The first heating unit and the second heating unit are A conveying plate having a conveying surface on which the printed substrate is conveyed, The inkjet image forming apparatus according to claim 1, further comprising a heating element for heating the transport plate from the back side of the transport surface.

3. The inkjet image forming apparatus according to claim 1, characterized in that the temperature of the printing substrate heated by the first heating unit and the second heating unit is 80°C or lower.

4. The inkjet image forming apparatus according to claim 1, characterized in that the difference between the temperature Tc and the temperature Tw is 20°C.

5. The inkjet image forming apparatus according to claim 1, further comprising a pre-drying section for heating the printing substrate, which is located downstream of the white ink head unit in the transport direction.

6. The inkjet image forming apparatus according to claim 1, characterized in that the viscosity of the white ink is 5.5 to 6.5 mPa·s and the viscosity of the color ink is 5.0 to 5.0 mPa·s.

7. The inkjet image forming apparatus according to claim 1, characterized in that the appropriate amount of the white ink is 0.5 to 5 pL, and the appropriate amount of the color ink is 2 to 10 pL.

8. The inkjet image forming apparatus according to claim 1, characterized in that the printing substrate is a plastic film.

Images