Inkjet printing device

Abstract

To stabilize and mute a hue of ultraviolet curable ink irradiated with ultraviolet, in a shorter time.SOLUTION: An inkjet printing device 1 comprises: inkjet heads 23 that discharge ultraviolet curable ink toward media M; an irradiation lamp 25 that irradiates the ultraviolet curable ink discharged toward the media M with ultraviolet; and heaters 37 that heat the ultraviolet curable ink irradiated with ultraviolet by the irradiation lamp 25 and stabilizes coloring of the ultraviolet curable ink, where the ultraviolet curable ink irradiated with the ultraviolet is heated at least at 35°C.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to inkjet printing devices. [Background technology]

[0002] Patent Document 1 discloses an inkjet printer that uses ultraviolet-curable ink. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2005-96277 A

[0004] UV-curable ink hardens in a short time when exposed to UV rays, making it suitable for printing information such as images and text (hereinafter referred to as images) on a variety of media. Summary of the Invention [Problem to be solved by the invention]

[0005] It is known that when an image is printed using ultraviolet-curable ink, the color of the image immediately after printing deviates from the intended color, but the color of the image changes over time and eventually settles into the intended color. Therefore, color matching such as proofreading of printed matter, calibration performed at the start of printing, and emulation when printing with a different inkjet printer requires waiting until the color of the image changes over time and settles. However, there is a need to proofread printed matter as soon as possible after printing. Therefore, there is a demand for a method to settle the color of an image in a shorter time. [Means for solving the problem]

[0006] The present invention relates to An inkjet head that ejects ultraviolet-curable ink toward a medium; an ultraviolet ray irradiation unit that irradiates the ultraviolet ray curable ink ejected onto the medium with ultraviolet rays; and a heating unit for heating the ultraviolet curable ink after the ultraviolet irradiation by the ultraviolet irradiation unit, thereby stabilizing the color development of the ultraviolet curable ink.

[0007] The inventors of the present invention have discovered that when ultraviolet-curable ink is heated after exposure to ultraviolet light, the color of the printed image settles to the desired color in a very short time compared to when the ink is not heated. Inkjet printing devices that use ultraviolet curable inks do not require a heater, unlike inkjet printing devices that use water-based or solvent-based inks. By intentionally providing a heating means to heat the ultraviolet curable ink after ultraviolet irradiation, the color development of the ultraviolet curable ink can be stabilized in a shorter time. This allows the colors in the image to settle in a shorter time.

[0008] The present invention, the second invention, The heating means is provided at a position downstream in the transport direction of the medium from the position where the ultraviolet light is irradiated.

[0009] UV-curable ink turns yellow immediately after exposure to UV rays, and develops a yellowish color. Over time, the yellowness fades and settles back into its original color. The dot gain of the ink droplets that land on the medium is fixed by being irradiated with ultraviolet light, but changes before being irradiated with ultraviolet light, so there is a possibility that the dot gain will be affected if the ink droplets are heated before being irradiated with ultraviolet light. With the above-mentioned configuration, the heating means is disposed at a position where it can appropriately heat only the ultraviolet curable ink after it has been irradiated with ultraviolet rays. This means that heating does not affect the dot gain of ink droplets that land on the media, which is expected to improve print quality.

[0010] The present invention, the third invention, The heating means is configured to heat the ultraviolet-curable ink at a downstream position in the transport direction of the medium from the heating means so as to eliminate any change in hue of the ultraviolet-curable ink, thereby stabilizing the color development of the ultraviolet-curable ink.

[0011] UV-curable ink turns yellow immediately after being exposed to UV light, and takes on a yellowish color. When yellowed UV-curable ink is heated, the yellowness fades and the ink returns to its original color in a shorter time than when it is not heated. Therefore, by determining the timing to end heating by the heating means based on the hue change of the ultraviolet curable ink after ultraviolet irradiation, the heating time of the ultraviolet curable ink after ultraviolet irradiation can be optimized. Furthermore, since the color development of the ultraviolet curable ink can be stabilized at a downstream position immediately after heating by the heating means, proofreading of the printed matter can be performed at an early stage after printing.

[0012] The present invention, the fourth invention, The lower limit of the heating temperature of the ultraviolet curable ink by the heating means is 35°C.

[0013] By setting the heating temperature by the heating means to 35° C. or higher, the ultraviolet curable ink after ultraviolet irradiation can be heated until there is no change in the hue of the ultraviolet curable ink. Therefore, it is possible to easily incorporate the heating means into the media transport path of the current printer device. In other words, the ultraviolet curable ink can be heated until there is no change in hue after ultraviolet irradiation without having to lengthen the transport path of the media to ensure a heating time for the ultraviolet curable ink after ultraviolet irradiation. Therefore, there is no need to increase the size of the inkjet printing device.

[0014] The fifth invention is as follows: The medium used is one in which the contact angle of the ultraviolet curable ink with respect to the medium when irradiated with ultraviolet light by the ultraviolet irradiating means is less than 55°.

[0015] In this way, even for media with a contact angle of less than 55°, i.e., media on which the contact angle of the landed ultraviolet-curable ink is small, by heating the ultraviolet-curable ink immediately after irradiation with ultraviolet rays, the color of the ultraviolet-curable ink can be stabilized in a shorter time and the color of the printed image can be settled in a shorter time.

[0016] The sixth invention is as follows: The media is PVC or PET.

[0017] Even with media with a small contact angle, such as PVC and PET, the color of the UV-curable ink can be stabilized in a shorter time, and the color of the printed image can be settled in a shorter time. Effect of the Invention

[0018] According to the present invention, the color tone of a printed image can be settled in a shorter time. [Brief description of the drawings]

[0019] [Figure 1] FIG. 1 illustrates an inkjet printing device. [Diagram 2] FIG. 1 illustrates an inkjet printing device. [Diagram 3] FIG. 13 is a diagram illustrating the behavior of color difference after ultraviolet irradiation. [Figure 4] FIG. 13 is a diagram illustrating the difference in color difference behavior after ultraviolet irradiation depending on whether or not heating is performed. [Diagram 5] FIG. 1 is an estimated diagram of a change in color over time due to a change in initiator radicals over time. [Figure 6] FIG. 13 is a graph showing the change over time in color difference ΔE of ultraviolet curable ink. [Figure 7] FIG. 11 is a diagram for explaining the relationship between different media and color difference ΔE. [Figure 8] FIG. 11 is a diagram illustrating the relationship between different media and the contact angle of ultraviolet curable ink. [Figure 9]FIG. 13 is a graph showing the relationship between the presence or absence of heating, the difference in heating temperature, and the change in color difference ΔE over time. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Hereinafter, an embodiment of the present invention will be described. 1 and 2 are diagrams illustrating an inkjet printing apparatus 1. FIG. Fig. 1 is a perspective view of the inkjet printing device 1 as seen obliquely from above. Fig. 2(a) is a side cross-sectional view of the inkjet printing device 1. Fig. 2(b) is a diagram for explaining the arrangement of the inkjet heads 23 in the carriage 22, and is a schematic diagram showing the carriage 22 as seen from the sub-scanning direction.

[0021] 1, the inkjet printing device 1 has a guide rail 21 that is oriented along the main scanning direction. A carriage 22 on which a plurality of inkjet heads 23 are mounted is provided on the guide rail 21 so as to be movable in the longitudinal direction of the guide rail 21 (the main scanning direction).

[0022] As shown in FIG. 2(b), the inkjet heads 23 are arranged in the carriage 22 in the main scanning direction, and the inkjet heads 23 are provided with a plurality of ejection nozzles (not shown) on the lower surface thereof facing the platen 31. The carriage 22 is provided with an ultraviolet ray irradiation lamp 25 (ultraviolet ray irradiation means) on at least one side in the main scanning direction.

[0023] 1, the guide rail 21 on which the carriage 22 is provided is housed in a box-shaped case 24. Both sides of the case 24 in the longitudinal direction (main scanning direction) are fixed to the upper part of the support column 111 of the support leg 11. A platen 31 is located below the case 24. The platen 31 is provided along the main scanning direction, and is supported by pillars 111, 111 on both sides in the longitudinal direction.

[0024] Legs 112 extending in the front-rear direction (sub-scanning direction) of the inkjet printing apparatus 1 are provided at the bottom of the support pillars 111, and the bottom surfaces of the legs 112 are placed on a mounting surface G (see FIG. 2(a)) of the inkjet printing apparatus 1. A case 24 of the inkjet printing apparatus 1 is provided horizontally above the mounting surface G.

[0025] As shown in FIG. 2(a), in the front-rear direction of the inkjet printing apparatus 1, an upstream support portion 35 is provided behind the platen 31, and a downstream support portion 36 is provided in front of it. The platen 31 and the upstream support portion 35 are provided with a gap in the front-to-rear direction of the inkjet printing device 1, and a grid roller 14 and a pinch roller 15 are provided vertically side by side between the platen 31 and the upstream support portion 35.

[0026] In a cross-sectional view, the upstream support portion 35 is formed in an arc shape, and the tension bar 16 supported by the rotation support portion 18 is located below the upstream support portion 35. In a cross-sectional view, the downstream support part 36 is formed in an arc shape. The downstream support part 36 is formed with a smaller radius of curvature than the upstream support part 35. The tension bar 17 supported by the rotation support part 19 is located below the downstream support part 36.

[0027] The tension bars 16, 17 are rotatably supported at the tips of rotation support parts 18, 19. The base ends of the rotation support parts 18, 19 are supported by a support beam 113 provided on the support leg 11 so as to be rotatable about axes X18, X19 along the main scanning direction. In the support leg 11, the support beam 113 extends from the region between the leg portion 112 and the case 24 in the front-rear direction of the inkjet printing apparatus 1 (the sub-scanning direction).

[0028] The payout roller 12 is rotatably supported at the rear end of the support beam 113, and the take-up roller 13 is rotatably supported at the front end of the support beam 113. The payout roller 12 and the take-up roller 13 are rotatable around axes X12 and X13, respectively, that extend along the main scanning direction.

[0029] A strip-shaped medium M before printing is set on the pay-out roller 12. The medium M is supplied in a state where it is wound around the outer periphery of a core material. The medium M pulled out from the pay-out roller 12 is wound around a tension bar 16, passes through an upstream support section 35, a platen 31, and a downstream support section 36 in that order, is wound around a tension bar 17, and is then wound around a core material set on the winding roller 13.

[0030] The grid roller 14 holds the medium M between itself and the pinch roller 15. When the grid roller 14 rotates, the medium M held between itself and the pinch roller 15 is transported in one of the sub-scanning directions determined according to the rotation direction of the grid roller 14. At this time, the tension bars 16 and 17 apply tension to the media M, so that the media M is pressed against the upper surfaces of the platen 31, the upstream support portion 35, and the downstream support portion 36. This prevents wrinkles and the like from occurring in the area of ​​the media M located below the carriage 22 (the area of ​​the media M that is printed).

[0031] The inkjet printing apparatus 1 according to this embodiment is a printing apparatus that uses ultraviolet curable ink. The ultraviolet-curable ink printed on the medium M is solidified by exposure to ultraviolet light and fixed to the medium. Therefore, unlike inkjet printing devices that use water-based or solvent-based inks, there is no need for a heater (heating means) to volatilize the solvent and fix the ink.

[0032] It is known that when an image printed using ultraviolet-curable ink is irradiated with ultraviolet light, the image takes on a stronger yellowish hue, and thereafter the color of the printed image changes over time, eventually settling on the desired color; this is known as the photobleaching phenomenon. The time it takes for the yellowed color to settle into the desired color varies depending on the type of ink and media, and can range from a few hours to a few days.

[0033] As a result of extensive research with a view to shortening the time it takes for the yellowed color to settle into the desired color, the inventors of the present invention have discovered that by heating the ultraviolet-curable ink after ultraviolet irradiation, the time it takes for the color to settle can be shortened compared to when the ink is not heated. Therefore, a heater 37, which is not essentially required for a printing device that uses ultraviolet curable ink, is provided in the inkjet printing device 1, and the ultraviolet curable ink after ultraviolet irradiation is heated by the heater 37 downstream of the position where the ultraviolet ray was irradiated in the transport direction of the medium M, until there is no change in the hue of the ultraviolet curable ink. As a result, the time required for the yellowish tinge to disappear from the ultraviolet curing ink that has turned yellow due to irradiation with ultraviolet rays and for the color to settle is shorter than the time required for the ultraviolet curing ink to be heated after irradiation with ultraviolet rays.

[0034] In this embodiment, a heater 37 is provided inside the downstream support portion . 2(a), the heater 37 is provided downstream of the carriage 22 in the transport direction of the medium M in the inkjet printing device 1. This is to heat the ultraviolet-curable ink printed on the medium M after printing and after irradiation with ultraviolet rays. In the case of FIG. 2(a), the heater temperature is set to 35 to 70° C., and the length of the heater 37 in the transport direction of the medium M is 300 mm to 400 mm, preferably 350 mm.

[0035] In the case of (a) of Figure 2, the heater 37 directly heats the downstream support part 36, and the medium M placed on the downstream support part 36 and the ultraviolet-curable ink on the medium M are heated by radiant heat. Moreover, instead of the heater 37, a metal halide lamp may be used to heat the ultraviolet curable ink after it has been irradiated with ultraviolet light.

[0036] Here, the earlier the heating of the ultraviolet curable ink starts after irradiation of the ultraviolet ray to the ultraviolet curable ink, the shorter the total time from printing to heating can be. Therefore, the heater 37 may be installed at a position directly below the area through which the carriage 22 moving in the main scanning direction passes (see the phantom line in the figure). That is, the heater 37 only needs to be provided in a range that includes a position directly below the area through which the carriage 22 moving in the main scanning direction passes, and is downstream of the position directly below in the transport direction of the medium M during printing.

[0037] Also, a hot air heater 38 may be disposed above the downstream support portion 36, and hot air may be blown onto the ultraviolet curable ink on the medium M to heat the ultraviolet curable ink. Furthermore, the hot air heater 38 and the heater 37 may be combined to heat the ultraviolet curable ink. In other words, any heating means may be used as long as it can heat the ultraviolet curable ink after ultraviolet irradiation. Furthermore, the location of the heating means is not limited to the above-mentioned locations.

[0038] The change in color tone after ultraviolet irradiation and the stabilization of the color tone by heating will be described below. FIG. 3 is a diagram illustrating the behavior of color difference after ultraviolet irradiation. FIG. 4 is a diagram illustrating the difference in color difference behavior after ultraviolet irradiation depending on whether or not heating is performed.

[0039] [Hue change] A cyan ultraviolet-curable ink (product name "LUS-120C", manufactured by Mimaki Engineering Co., Ltd.) was coated onto a substrate (photo paper: PL-5000L) using a bar coater (#6), and then irradiated with ultraviolet light using a JFX-200 lamp. And the hue (L * a * b * The change over time in the color (value) was confirmed using a color difference meter.

[0040] As shown in Figure 3, when the coated UV-curable ink is irradiated with UV light, the color of the UV-curable ink turns yellow (see symbol α → β in the figure). The yellowness of the yellowed UV-curable ink then decreases over time, and the color eventually stabilizes (see symbol β → γ in the figure). In the case of Figure 3, the color difference (ΔE) before and after UV irradiation is 9 (ΔEαβ=9), and the color difference (ΔE) until the yellowness is reduced and the color stabilizes is 6 (ΔEβγ=6). As a result, the color difference (ΔE) between the color before UV irradiation and the color stabilized after the yellowness is removed is 4 (ΔEαγ=4).

[0041] In this way, when ultraviolet-curable ink is irradiated with ultraviolet rays, the color difference ΔE is maximum immediately after irradiation, and thereafter, the value of the color difference ΔE decreases over time, and finally, the color difference ΔE converges (see the dashed line in Figure 4).

[0042] The inventors of the present invention presumed that, based on the change in color difference ΔE after UV irradiation, the change in color difference as shown in Figure 4 occurs due to the influence of a component that increases immediately after UV irradiation and then decreases over time. They also presumed that the change in color over time is due to the initiator contained in the UV-curable ink.

[0043] FIG. 5 is an estimated diagram of the change in color over time due to the change in initiator radicals over time. The initiator is cleaved by irradiation with ultraviolet light to generate initiator radicals. The generated initiator radicals react with the ultraviolet-curable resin (monomer), causing the monomers to bond together sequentially to polymerize, curing the ultraviolet-curable ink. Since the initiator radicals are unstable, among the initiator radicals generated immediately after the ultraviolet light, the unreacted initiator radicals decompose over time.

[0044] The ultraviolet curable ink contains an initiator in excess of the ultraviolet curable resin in order to ensure curing after irradiation with ultraviolet rays. Therefore, immediately (at time t1) after irradiation with ultraviolet light (at time t0), a large amount of unreacted initiator radicals appears, and the color changes to yellow due to the unreacted initiator radicals that appear. It was estimated that the yellow color fades due to the consumption of initiator radicals as the polymerization progresses and the decrease in the initiator radicals due to decomposition over time (time t1 → time t2 → time t3), and finally the color stabilizes (after time t3).

[0045] Therefore, the inventors of the present invention presumed that the color tone of the ultraviolet-curable ink could be stabilized in a shorter time after ultraviolet irradiation by promoting the volatilization or decomposition of the unreacted initiator radicals, and investigated heating the ultraviolet-curable ink after ultraviolet irradiation in order to promote the decomposition of the unreacted initiator radicals.

[0046] As a result, as shown in Figure 4, it was confirmed that when the UV-curable ink was heated after UV irradiation, the color difference ΔE decreased in a shorter time (the color became stable and settled) compared to when it was not heated. It was confirmed that the time from time t0, when ultraviolet light was irradiated, until the color difference ΔE fell below the threshold color difference (threshold ΔE), at which it is determined that the color has stabilized and settled, was shorter when heated (tb-t0) than when not heated (ta-t0).

[0047] Here, the threshold color difference (threshold ΔE) is set to a value at which the color is virtually indistinguishable from the hue before UV irradiation. In this embodiment, the threshold color difference (threshold ΔE) is set to 3.0. This threshold color difference (threshold ΔE) is not limited to this value, but can be changed as appropriate depending on the required color accuracy.

[0048] Fig. 6 is a diagram showing the change over time in color difference ΔE of ultraviolet curable ink when printing and heating are performed using an inkjet printing device (UCJV300) and a heater (JV300). Note that the change over time in color difference ΔE in Fig. 6 is the result when the heater 37 is installed inside the downstream support part 36 described above. The printing with the inkjet printer (UCJV300) was performed under the following conditions: Print speed: Fastest 300 x 900 dpi / 12 passes / reciprocating printing. FIG. 6(a) is a diagram showing the relationship between the presence or absence of heating, the heating time, and the color difference ΔE immediately after heating, and is a diagram illustrating the case where OPP (Oriented Polypropylene: biaxially stretched polypropylene film) is used as the base material. FIG. 6(b) is a diagram showing the relationship between the presence or absence of heating, the heating time, and the color difference ΔE immediately after heating, and is a diagram illustrating the case where PVC (Polyvinyl Chloride) is used as the base material.

[0049] FIG. 7 is a diagram illustrating the relationship between different media (substrates) and color difference ΔE when no heating is performed after ultraviolet irradiation. Figure 7(a) shows the change over time in color difference ΔE when solid printing was performed on different media (OPP, PVC, PET) using a cyan UV-curable ink (product name "LUS-120C", manufactured by Mimaki Engineering Co., Ltd.) and an inkjet printer (UCJV300) and heater (JV300). FIG. 7(b) is a conceptual diagram illustrating the difference in dot diameter when solid printing is performed on different media (OPP, PVC, PET).

[0050] Figure 8 is a diagram explaining the relationship between differences in media (substrates) and the contact angle of ultraviolet-curable ink. The contact angle was measured under the following conditions using a cyan ultraviolet-curable ink (product name "LUS-120C", manufactured by Mimaki Engineering Co., Ltd.) and a fully automatic contact angle meter (DM-701: manufactured by Kyowa Interface Science Co., Ltd.). Syringe diameter: 18G, Droplet volume: 1μL, Measurement temperature: 25°

[0051] As shown in (a) and (b) of Figure 6, when comparing no heating (room temperature: rt) with heating (40°C, 50°C, 60°C, 70°C), the higher the heating temperature, the smaller the color difference ΔE immediately after heating. Furthermore, when comparing the heating times (1 min, 2 min, 5 min, 10 min), it was confirmed that the color difference ΔE immediately after heating became smaller as the heating time became longer. In addition, in the case of no heating, the color difference ΔE is measured at the time points (1 min, 2 min, 5 min, 10 min) that have elapsed since immediately after ultraviolet irradiation.

[0052] In addition, when comparing different media (base materials), it was confirmed that OPP could reduce the color difference ΔE with a shorter heating time than PVC. As shown in FIG. 7, without heating, OPP has a smaller color difference ΔE immediately after ultraviolet irradiation than PVC or PET (Polyethylene terephthalate). In any case of OPP, PVC, or PET, the color difference ΔE decreases and stabilizes as the time elapsed from the ultraviolet irradiation increases.

[0053] As shown in Figure 7(b), the dot diameter of the printed matter immediately after UV irradiation is smallest for OPP (53.7 μm). The dot diameters of PVC (91.3 μm) and PET (99.2 μm) are at roughly the same level. As shown in Figure 8, in the case of OPP, the wettability of UV-curable ink is lower (the contact angle is larger) than that of PVC or PET, so the dot diameter of the ink droplets is smaller. Therefore, the exposed surface ratio of the substrate in the printed matter is higher.

[0054] Here, the yellowing of the ultraviolet curable ink changes depending on the amount of initiator radicals contained in the ultraviolet curable ink. In the case of OPP, the dot diameter of the UV-curable ink is smaller and the exposed proportion of the base material is greater, so even if yellowing occurs, the proportion of the media occupied by the yellowed ink area is lower than with PVC or PET. As a result, the degree to which the color of the printed image is less likely to be yellowish is reduced, and it is thought that OPP has a smaller color difference than PVC and PET.

[0055] That is, the higher the wettability of the ultraviolet curable ink with respect to the medium (substrate) and the smaller the contact angle, the greater the degree to which the color of the printed image immediately after irradiation with ultraviolet rays becomes yellowish.

[0056] As shown in Table 1 and Figure 8 below, the contact angle of UV-curable ink with respect to media is larger when the media (substrate) is OPP than when it is PVC or PET. Furthermore, when checking the change in the contact angle of UV-curable ink over time, it was confirmed that the contact angle when the media (substrate) is OPP is maintained without decreasing in a short period of time compared to when the media is PVC or PET.

[0057] [Table 1]

[0058] In Table 1, 300 ms corresponds to the time from when the ink droplet lands on the medium M until it is irradiated with ultraviolet light. 10,000 ms corresponds to the time until the change over time in the contact angle of the ink droplet that has landed on the medium M settles down. Here, "the change in the contact angle over time settles down" refers to a state in which the amount of change in the contact angle of the ink droplet per unit time becomes less than a threshold value.

[0059] Therefore, considering the results of Figures 7 and 8, even in the case of a medium (substrate) with a small contact angle of less than 54.7° for the UV-curable ink, by heating the UV-curable ink after UV irradiation, the color of the image after UV irradiation can be settled in a shorter time than if it is not heated.

[0060] Here, as shown in FIG. 8, the contact angle of the ink droplets that land on the medium M differs depending on the material of the medium M (substrate). PVC has a smaller contact angle than OPP, and PET has an even smaller contact angle than PVC. The smaller the contact angle, the easier it is for ink droplets to wet and spread after landing on the medium M. And the greater the dot gain of the ink droplets due to wetting and spreading, the greater the effect of photobleaching.

[0061] For this reason, in general-purpose printers that can use a variety of media, the heating conditions (temperature and overall length) of the heater are designed to suppress the effects of photobleaching even when printing at the fastest speed using media with a small contact angle. That is, the heating temperature of heater 37 in FIG. 2(a) is set to 35 to 70° C., and the length is set to 300 to 400 mm, because this is a size that can be easily incorporated into the transport path of a general-purpose printer, and because by printing at near the fastest speed, the color difference ΔE immediately after heating by the heater can be suppressed to below a threshold value even under severe drying conditions. The heating temperature and length of time by the heater 37 are not particularly limited, and may be changed as appropriate as long as the color difference ΔE immediately after heating by the heater can be suppressed to a threshold value or less.

[0062] FIG. 9 is a graph showing the relationship between the presence or absence of heating, the difference in heating temperature, and the change in color difference ΔE over time. As shown in Figure 9, when comparing the results without heating (room temperature: rt) with those with heating (40°C, 50°C, 60°C, 70°C), it was confirmed that the color ultimately settled into the same hue regardless of whether or not heating was used and regardless of the heating temperature.

[0063] As a result, even if the ultraviolet-curable ink is heated after ultraviolet irradiation, the final color does not change significantly. Furthermore, when the ultraviolet-curable ink is heated after ultraviolet irradiation, the color of the printed image is stabilized in a shorter time than when the ink is not heated. For this reason, the inventors of the present invention have provided the inkjet printing apparatus 1 with a heater 37 (heating means), which is not actually required, in order to heat the ultraviolet-curable ink after it has been irradiated with ultraviolet rays. This makes it possible to stabilize the color of the ultraviolet curable ink by settling it within a color difference range (less than a threshold color difference) based on the target color in a shorter time.

[0064] In the case of OPP, the color immediately after heating can be made less than the threshold color difference (threshold ΔE) by heating at least 40°C for 2 minutes after UV irradiation, and in the case of PVC, the color immediately after heating can be made less than the threshold color difference (threshold ΔE) by heating at least 50°C for 5 minutes after UV irradiation.

[0065] Taking the above into consideration, although it differs depending on the media, in the inkjet printing device 1, by heating the ultraviolet light after irradiation to at least 40°C, it is possible to make the color less than the threshold color difference (threshold ΔE) in a shorter time than if no heating was used. As a result, the time required for the color to stabilize, which would have taken several hours to several days without heating, can be reduced to just a few minutes to a few tens of minutes by heating. Therefore, in an inkjet printing device 1 that uses ultraviolet-curable ink, the color of the image after ultraviolet irradiation can be settled in a shorter time, which meets the need to proofread the printed matter as soon as possible after printing.

[0066] As described above, the inkjet printing apparatus 1 according to this embodiment has the following configuration. (1) The inkjet printing device 1 is an inkjet head 23 that ejects ultraviolet-curable ink toward the medium M; an irradiation lamp 25 (ultraviolet ray irradiation means) that irradiates ultraviolet rays onto the ultraviolet ray curable ink discharged onto the medium M; and a heater 37 (heating means) for heating the ultraviolet curable ink after it has been irradiated with ultraviolet light by the irradiation lamp 25, thereby stabilizing the color development of the ultraviolet curable ink.

[0067] The inventors of the present invention have discovered that when ultraviolet-curable ink is heated after exposure to ultraviolet light, the color of the printed image settles to the desired color in a very short time compared to when the ink is not heated. The ultraviolet-curable ink is solidified by irradiation with ultraviolet rays and fixed to the medium M. Therefore, in the case of an inkjet printing device 1 that uses ultraviolet-curable ink, a heater that is provided in an inkjet printing device that uses water-based or solvent-based ink is not required. By providing the heater 37 (heating means), which is not actually required, and heating the ultraviolet curable ink, the color of the ultraviolet curable ink can be stabilized in a shorter time. This allows the color of the image to settle in a shorter time.

[0068] The inkjet printing apparatus 1 according to this embodiment has the following configuration. (2) The heater 37 is provided at a position downstream in the medium transport direction from the position where the ultraviolet light is irradiated.

[0069] UV-curable ink turns yellow immediately after UV irradiation, and the yellowness fades over time until it settles back into its original color. Therefore, heating the ink before UV irradiation does not help stabilize the color. With the above configuration, the heater 37 is disposed at a position where it can appropriately heat only the ultraviolet curable ink after it has been irradiated with ultraviolet light. This makes it possible to limit the position at which the heater 37 is disposed, thereby suppressing an increase in the manufacturing cost of the inkjet printing device 1 due to the addition of the heater 37.

[0070] In addition, the dot gain of ink droplets that land on the media is fixed by UV irradiation and changes before UV irradiation. Therefore, by heating after UV irradiation, the effect on dot gain can be reduced, which is expected to improve print quality.

[0071] The inkjet printing apparatus 1 according to this embodiment has the following configuration. (3) The heater 37 (heating means) heats the ultraviolet curable ink so that there is no change in hue of the ultraviolet curable ink at a downstream position of the heater 37 in the medium transport direction, thereby stabilizing the color development of the ultraviolet curable ink.

[0072] UV-curable ink turns yellow immediately after being exposed to UV light, and takes on a yellowish color. When yellowed UV-curable ink is heated, the yellowness fades and the ink returns to its original color in a shorter time than when it is not heated. Therefore, by determining the timing to end heating by the heater 37 based on the hue change of the ultraviolet curable ink after ultraviolet irradiation, it is possible to optimize the heating time of the ultraviolet curable ink after ultraviolet irradiation. Furthermore, since the color development of the ultraviolet curable ink can be stabilized at a downstream position immediately after heating by the heater 37, it is possible to perform proofreading of the printed matter at an early stage after printing.

[0073] The inkjet printing apparatus 1 according to this embodiment has the following configuration. (4) The lower limit of the heating temperature of the ultraviolet-curable ink by the heater 37 after ultraviolet irradiation is 35°C.

[0074] By setting the heating temperature by the heater 37 to 35° C. or higher, the ultraviolet curable ink can be heated after ultraviolet irradiation until there is no change in the hue of the ultraviolet curable ink. Therefore, the heater 37 can be easily incorporated into the media transport path of the current printer device. In other words, the ultraviolet curable ink after ultraviolet irradiation can be heated until there is no change in hue of the ultraviolet curable ink, even if the transport path of the medium M is not lengthened to ensure a heating time for the ultraviolet curable ink after ultraviolet irradiation. Therefore, there is no need to increase the size of the inkjet printing device 1.

[0075] Furthermore, when the heating temperature is lower, it takes longer for the color difference, which is the difference between the current color of the ultraviolet curable ink and the target color, to reach less than the target threshold color difference (threshold ΔE). When the lower limit of the heating temperature is preferably 35° C., more preferably 40° C., the time required for the color difference to reach a value less than the target color difference can be sufficiently shortened compared to the case where no heating is performed. This allows the color to settle in a shorter time than if no heating was used, meeting the need to proofread printed materials as soon as possible after printing.

[0076] The upper limit of the heating temperature is not particularly limited, but should be a temperature determined according to the heat resistance temperature of the media to be printed, and should not affect the characteristics of the media or the characteristics of the printed image (strength, abrasion resistance, etc.). If the media is heated to a temperature that affects its characteristics, it may not be possible to satisfy the performance requirements of the printed matter.

[0077] The inkjet printing apparatus 1 according to this embodiment has the following configuration. (5) The medium M is a medium M for which the contact angle of the ultraviolet-curable ink on the medium M when irradiated with ultraviolet rays by the irradiation lamp 25 is less than 55°.

[0078] Even for media with a contact angle of less than 55°, i.e., media on which the contact angle of the landed ultraviolet-curable ink is small, by heating the ultraviolet-curable ink immediately after irradiation with ultraviolet rays, the color development of the ultraviolet-curable ink can be stabilized in a shorter time, and the color of the printed image can be settled in a shorter time. Media M with a contact angle of less than 55° with ultraviolet curable ink has high wettability with ultraviolet curable ink, so the color of the printed image immediately after irradiation with ultraviolet light tends to be yellowish to a large extent. When printing on such a medium M (substrate), if the ultraviolet-curable ink is heated immediately after irradiation with ultraviolet rays, the color of the ultraviolet-curable ink can be stabilized in a shorter time, and the color of the printed image can be settled in a shorter time.

[0079] The inkjet printing apparatus 1 according to this embodiment has the following configuration. (6) The medium M used in the inkjet printing device 1 is PVC or PET.

[0080] Even with media with a small contact angle, such as PVC and PET, the color of the UV-curable ink can be stabilized in a shorter time, and the color of the printed image can be settled in a shorter time.

[0081] The inkjet printing apparatus 1 according to this embodiment has the following configuration. (7) It is preferable that the lower limit of the heating temperature of the ultraviolet curable ink by the heater 37 after ultraviolet irradiation is 40° C., and the lower limit of the heating time by the heater 37 is 2 minutes.

[0082] By configuring it in this manner, the heating of the ultraviolet-curable ink after ultraviolet irradiation can be controlled under optimal conditions suited to the media, so that the color can be made less than the threshold color difference (threshold ΔE) in a shorter time than if no heating was applied.

[0083] In the above embodiment, the inkjet printing apparatus 1 is a roll machine (vertical machine) that prints on a medium wound in a roll. The present invention can also be suitably applied to a flatbed type inkjet printing device that prints on a sheet-like medium placed on a table. In this case, for example, it is possible to heat the ultraviolet curable ink after irradiation with ultraviolet rays by providing a heater on the table or on the carriage.

[0084] Although the embodiments of the present invention have been described above, the present invention is not limited to the aspects shown in these embodiments. Appropriate modifications are possible within the scope of the technical concept of the invention. [Explanation of symbols]

[0085] 1 Inkjet printing device 11 Support legs 12 Payout roller 13 Winding roller 14 Grid Roller 15 Pinch roller 16, 17 Tension bar 18, 19 Rotation support part 21 Guide rail 22 Carriage 23 Inkjet head 24 cases 25 Irradiation lamp 31 Platen 35 Upstream support part 36 Downstream support section 37 Heater 38 Hot air heater G Installation surface M Media ΔE color difference

Claims

1. An inkjet head that ejects ultraviolet-curable ink toward a medium; an ultraviolet ray irradiation unit that irradiates ultraviolet rays onto the ultraviolet curable ink ejected onto the medium; and a heating means for heating the ultraviolet curable ink after the ultraviolet irradiation by the ultraviolet irradiation means, thereby stabilizing the color development of the ultraviolet curable ink.

2. 2. The inkjet printing apparatus according to claim 1, wherein the heating means is provided at a position downstream in a transport direction of the medium from a position where the ultraviolet light is irradiated.

3. 3. The inkjet printing device according to claim 2, wherein the heating means heats the ultraviolet-curable ink at a downstream position in the transport direction of the medium from the heating means so that there is no change in hue of the ultraviolet-curable ink, thereby stabilizing the color development of the ultraviolet-curable ink.

4. 4. The inkjet printing apparatus according to claim 1, wherein a lower limit of a heating temperature of the ultraviolet curing ink by the heating means is 35[deg.] C.

5. 4. The inkjet printing apparatus according to claim 1, wherein the medium is a medium in which a contact angle of ultraviolet curable ink with respect to the medium when irradiated with ultraviolet light by the ultraviolet light irradiation means is less than 55 degrees.

6. 6. The inkjet printing apparatus of claim 5, wherein the media is PVC or PET.

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