Patent Description:
Patent Literature <NUM> discloses an inkjet printer using UV curable ink.

UV curable ink is solidified by irradiation with ultraviolet rays in a short time. Therefore, information such as images and letters (hereafter referred to as images) can be suitably printed on a variety of media. The publication <CIT> discloses a method of digitally printing UV-curable ink with a direct-to-textile printing system onto a textile, the method comprising: positioning the textile in the direct-to-textile printing system; applying UV-curable ink to the textile with the direct-to-textile printing system according to a digital design file; and at least partially curing the UV-curable ink applied to the textile with UV light. In embodiments, after the step of curing the UV- curable ink applied to the textile with UV light, heat is applied to the textile, which comprises heat pressing the textile for about <NUM> seconds at about <NUM>. In one embodiment, the UV-curable ink is partially cured with the UV light, and is fully cured by applying heat to the textile. The publication <CIT> discloses a method of printing on fabric comprising the steps of: jetting an ink composition containing a UV curable ink component and a dye component onto a fabric; then substantially curing at least the UV curable jetted ink component on the fabric by exposing the UV curable ink component to UV light, the curing resulting in a substantially cured UV ink component on the fabric containing uncured monomers of the UV curable ink and the dye component; then heating the fabric having the substantially cured UV cured ink component thereon and thereby reducing the level of the uncured monomers of the UV curable ink component on the fabric and effecting the dyeing of the fabric with the dye component. The publication <CIT> discloses an image forming method which can decide a drying condition in a simple method in forming an image on a recording medium using an aqueous active energy curing ink and can obtain an image having image flatness without luster unevenness. In the image forming method which is provided with a drying process after a printing process for printing on a recording medium by ejecting an active ray curing ink on the recording medium with an inkjet recording head, a luster sensor is used to detect a kind of the recording medium, allowing drying temperature or wind force to be controlled corresponding to the kind of the recording medium.

It has been known that when an image is printed using UV curable ink, the color tone of the image immediately after printing deviates from the target color tone, but the color tone of the image changes over time and is eventually settled to the target color tone.

Therefore, color matching in calibration at correcting printed matters or at the start of printing, emulation for printing with different inkjet printers, and the like needs to wait until the color tone of the image changes over time and is settled. However, there is a need that it is desired to correct the printed matter as early as possible after printing. In view of this, it has been requested to settle the color tone of the image in a shorter time.

An inkjet printing apparatus according to an aspect of the present invention includes: a UV curable ink; an ink-jet head that ejects the UV curable ink to a medium; an ultraviolet ray irradiation unit that irradiates the UV curable ink ejected to the medium with ultraviolet rays; and a heating unit that heats the UV curable ink after the ultraviolet ray irradiation by the ultraviolet ray irradiation unit and thereby stabilizes color fixation of the UV curable ink; the initiator being cleaved by the ultraviolet ray radiation to generate the initiator radicals that react with the UV curable resin monomers causing the monomers to bond to each other sequentially and be polymerized, so that the UV curable ink is cured; the UV curable ink containing an excessive amount of the initiator relative to the UV curable resin to ensure curing after the ultraviolet ray radiation; wherein the heating unit is adapted to heat to the extent that initiator radicals are decomposed or volatilized; and to heat without a need for volatilization of a solvent to fix the ink.

The present inventor has found that when the UV curable ink is heated after the ultraviolet ray irradiation, the color tone of the printed image is settled to the target color tone in a very short time compared to the case without heating.

The inkjet printing apparatus that uses the UV curable ink does not require a heater, unlike other inkjet printing apparatuses that use water-based or solvent-based ink. By providing a heating unit on purpose to heat the UV curable ink after the ultraviolet ray irradiation, the color fixation of the UV curable ink can be stabilized in a shorter time.

Thus, the color tone of the image can be settled in a shorter time.

In a second aspect of the present invention, the heating unit is provided downstream of a position irradiated with the ultraviolet rays in a conveying direction of the medium.

The UV curable ink turns yellow immediately after the ultraviolet ray irradiation, resulting in a yellowish color tone, and thereafter the yellowish color decreases over time and the color settles to the original color.

Here, the dot gain of the ink droplet that lands on the medium is fixed by the ultraviolet ray irradiation and changes before the ultraviolet ray irradiation. Therefore, heating the UV curable ink before the ultraviolet ray irradiation may affect the dot gain.

With the structure as described above, the heating unit is disposed at a position where the UV curable ink can be properly heated after the ultraviolet ray irradiation.

Thus, the heating will not have any influence on the dot gain of the ink droplet that has landed on the medium and accordingly, the improvement of the printing quality can be expected.

In a third aspect of the present invention, the heating unit is configured to heat the UV curable ink so that the UV curable ink stops changing in hue at a position downstream of the heating unit in the conveying direction of the medium and thereby stabilizes the color fixation of the UV curable ink.

The UV curable ink turns yellow immediately after the ultraviolet ray irradiation, resulting in a yellowish color tone. When the yellowed UV curable ink is heated, the yellowish color tone is reduced and the color tone settles to its original color in a shorter time than when heating is not performed.

Therefore, the heating time of the UV curable ink after the ultraviolet ray irradiation can be optimized by determining the end timing of heating by the heating unit on the basis of the hue change of the UV curable ink after the ultraviolet ray irradiation. Furthermore, the color fixation of the UV curable ink can be stabilized at the downstream position immediately after heating by the heating unit, so that proofreading of printed matters, etc. can be performed at an early stage after printing.

In a fourth aspect of the present invention, the lower limit of heating temperature of the UV curable ink by the heating unit is <NUM>.

By setting the heating temperature by the heating unit to <NUM> or higher, the UV curable ink after the ultraviolet ray irradiation can be heated until the UV curable ink stops changing in hue.

This makes it possible to integrate the heating unit into the medium conveyance path of the existing printer apparatus without difficulty.

In other words, the UV curable ink after the ultraviolet ray irradiation can be heated until the UV curable ink stops changing in hue without the necessity of extending the medium conveyance path in order to secure the time for heating the UV curable ink after the ultraviolet ray irradiation. Therefore, there is no need to make the inkjet printing apparatus particularly large.

In a fifth aspect of the present invention, the medium is a medium that makes a contact angle of the UV curable ink less than <NUM>° with respect to the medium at the time of ultraviolet ray irradiation by the ultraviolet ray irradiation unit.

Thus, even in the case of the medium with a contact angle of less than <NUM>°, that is, the medium with the small contact angle of the UV curable ink that has landed thereon, heating the UV curable ink immediately after the ultraviolet ray irradiation can stabilize the color fixation of the UV curable ink in a shorter time and settle down the color tone of the printed image in a shorter time.

In a sixth aspect of the present invention, the medium is PVC or PET.

Even in the case of the medium with the small contact angle of the ink droplet of the UV curable ink that has landed thereon, such as PVC or PET, the color fixation of the UV curable ink can be stabilized in a shorter time and the color tone of the printed image can be settled down in a shorter time.

According to the present invention, the color tone of printed images can be settled down in a shorter time.

An embodiment of the present invention is hereinafter described.

<FIG> and <FIG> are diagrams for describing an inkjet printing apparatus <NUM>.

<FIG> is a perspective view in which the inkjet printing apparatus <NUM> is viewed obliquely from above. In <FIG> is a side cross-sectional view of the inkjet printing apparatus <NUM>. In <FIG> is a diagram for describing the arrangement of ink-jet heads <NUM> in a carriage <NUM>, schematically illustrating the carriage <NUM> viewed from a sub scanning direction.

As illustrated in <FIG>, the inkjet printing apparatus <NUM> has a guide rail <NUM> provided in a direction along a main scanning direction. In the guide rail <NUM>, the carriage <NUM> on which the ink-jet heads <NUM> are mounted is provided movably in a longitudinal direction of the guide rail <NUM> (main scanning direction).

As illustrated in (b) in <FIG>, the ink-jet heads <NUM> are aligned in the main scanning direction in the carriage <NUM>. A plurality of ejection nozzles (not illustrated) are provided on a lower surface of the ink-jet heads <NUM> on a platen <NUM> side.

In the carriage <NUM>, an ultraviolet ray irradiation lamp <NUM> (ultraviolet ray irradiation unit) is provided on at least one side in the main scanning direction.

As illustrated in <FIG>, the guide rail <NUM> to which the carriage <NUM> is provided is housed in a box-shaped case <NUM>. Both sides of the case <NUM> in the longitudinal direction (main scanning direction) are fixed to the top of columns <NUM> of support legs <NUM>.

The platen <NUM> is located on the lower side of the case <NUM>. The platen <NUM> is provided along the main scanning direction. Both sides of the platen <NUM> in the longitudinal direction are supported by the columns <NUM> and <NUM>.

At the bottom of each column <NUM>, a leg part <NUM> that extends in the front-rear direction (sub scanning direction) of the inkjet printing apparatus <NUM> is provided. A lower surface of the leg part <NUM> is placed on an installation surface G of the inkjet printing apparatus <NUM> (see (a) in <FIG>). The case <NUM> of the inkjet printing apparatus <NUM> is provided horizontally above the installation surface G.

As illustrated in (a) in <FIG>, an upstream supporting unit <NUM> is provided behind (in the drawing, on the right side of) the platen <NUM> in the front-rear direction of the inkjet printing apparatus <NUM>. Ahead of (in the drawing, on the left side of) the platen <NUM>, a downstream supporting unit <NUM> is provided.

The platen <NUM> and the upstream supporting unit <NUM> are provided with a gap therebetween in the front-rear direction of the inkjet printing apparatus <NUM>. Between the platen <NUM> and the upstream supporting unit <NUM>, a grid roller <NUM> and a pinch roller <NUM> are arranged vertically.

In the cross-sectional view, the upstream supporting unit <NUM> is formed to have an arc-like shape. Below the upstream supporting unit <NUM>, a tension bar <NUM> supported by a rotation supporting unit <NUM> is provided.

In the cross-sectional view, the downstream supporting unit <NUM> is formed to have an arc-like shape. The downstream supporting unit <NUM> is formed with a smaller radius of curvature than the upstream supporting unit <NUM>. Below the downstream supporting unit <NUM>, a tension bar <NUM> supported by the rotation supporting unit <NUM> is provided.

The tension bars <NUM> and <NUM> are rotatably supported at the ends of the rotation supporting units <NUM> and <NUM>. The base ends of the rotation supporting units <NUM> and <NUM> are supported by a support beam <NUM> provided to the support leg <NUM>, enabling rotation around axial lines X18 and X19 along the main scanning direction.

In the support leg <NUM>, the support beam <NUM> extends from the area between the leg part <NUM> and the case <NUM> in the front-rear direction (sub scanning direction) of the inkjet printing apparatus <NUM>.

At a rear end portion of the support beam <NUM>, a feed roller <NUM> is rotatably supported. At a front end portion of the support beam <NUM>, a take-up roller <NUM> is rotatably supported. The feed roller <NUM> and the take-up roller <NUM> can rotate around axial lines X12 and X13 along the main scanning direction, respectively.

To the feed roller <NUM>, a band-shaped medium M before printing is set. The medium M is supplied in a state of being wound around an outer periphery of a core material.

The medium M drawn from the feed roller <NUM> is wound around the tension bar <NUM>, and then passes through the upstream supporting unit <NUM>, the platen <NUM>, and the downstream supporting unit <NUM> in order. After being wound by the tension bar <NUM>, the medium M is wound around the core material set on the take-up roller <NUM>.

The medium M is gripped between the grid roller <NUM> and the pinch roller <NUM>. As the grid roller <NUM> rotates, the medium M gripped between the grid roller <NUM> and the pinch roller <NUM> is conveyed. The medium M is conveyed to one side in the sub scanning direction that is determined in accordance with the rotating direction of the grid roller <NUM>.

In this process, the tension bars <NUM> and <NUM> apply tension to the medium M, and accordingly, the medium M is pressed against upper surfaces of the platen <NUM>, the upstream supporting unit <NUM>, and the downstream supporting unit <NUM>. Thus, generation of creases and the like is prevented in an area of the medium M that exists below the carriage <NUM> (the area of the medium M to be printed).

The inkjet printing apparatus <NUM> according to the present embodiment is a printing apparatus that uses UV curable ink.

The UV curable ink printed on the medium M is solidified by irradiation with ultraviolet rays and is thereby fixed to the medium M.

Therefore, unlike another inkjet printing apparatus that uses water-based or solvent-based ink, a heater (heating unit) to volatilize the solvent to fix the ink is not necessary.

Here, irradiating an image, which is printed with UV curable ink, with ultraviolet rays results in the deep yellow color tone. It has been known that, subsequently, the color tone of the printed image changes over time and eventually settles to the target color tone, which is a so-called photobleaching phenomenon.

The time it takes for the yellowed color tone to settle to the target color tone depends on the type of ink and media, and ranges widely from several hours to several days.

The present inventor has made a diligent study with a view to shortening the time required for the yellowed color tone to settle down to the target color tone. As a result, it has been found that heating the UV curable ink after the ultraviolet ray irradiation can shorten the time required to settle the color tone compared to the case without heating.

In view of this, a heater <NUM>, which is originally unnecessary in the printing apparatus that uses the UV curable ink, is provided on purpose to the inkjet printing apparatus <NUM>. The UV curable ink after the ultraviolet ray irradiation is heated by the heater <NUM> downstream of the position irradiated with ultraviolet rays in the conveying direction of the medium M. Heating with the heater <NUM> is continued until the UV curable ink stops changing in hue.

This allows the yellow tint to be removed from the UV curable ink that has yellowed due to the ultraviolet ray irradiation, and the time required to settle the color tone becomes shorter than when the UV curable ink is not heated after the ultraviolet ray irradiation.

In this embodiment, the heater <NUM> is installed inside the downstream supporting unit <NUM>.

In the case of (a) in <FIG>, the heater <NUM> is installed downstream of the carriage <NUM> in the conveying direction (sub scanning direction) of the medium M in the inkjet printing apparatus <NUM>. Therefore, the heater <NUM> heats the UV curable ink that is printed on the medium M after printing and that is after the ultraviolet ray irradiation.

In the case of (a) in <FIG>, the heating is performed with a heater temperature of <NUM> to <NUM>, and the length of the heater <NUM> in the conveying direction of the medium M is <NUM> to <NUM>, preferably <NUM>.

In the case of (a) in <FIG>, the heater <NUM> directly heats the downstream supporting unit <NUM>. The medium M placed on the downstream supporting unit <NUM> and the UV curable ink on the medium M are heated by the radiation heat.

Instead of the heater <NUM>, a metal halide lamp (MH lamp) may be used to heat the UV curable ink after the ultraviolet ray irradiation.

Here, starting to heat the UV curable ink sooner after the ultraviolet ray irradiation to the UV curable ink can shorten the total time from printing to heating. Therefore, the heater <NUM> may be installed at a position directly below the area through which the carriage <NUM> moving in the main scanning direction passes (see a virtual line in the drawing).

In other words, it is only necessary that the heater <NUM> is provided in the range that includes the position directly below the area through which the carriage <NUM> moving in the main scanning direction passes, as well as downstream of the position directly below the position in the conveying direction of the medium M during printing.

A warm air heater <NUM> may be placed above the downstream supporting unit <NUM> to heat the UV curable ink by blowing warm air to the UV curable ink on the medium M. In addition, a combination of the warm air heater <NUM> and the heater <NUM> may be used to heat the UV curable ink.

In other words, any heating unit that can heat the UV curable ink after the ultraviolet ray irradiation may be employed. Moreover, the location of the heating unit is not limited to only the location mentioned above.

Next, the change of the color tone of the UV curable ink after the ultraviolet ray irradiation and the stabilization of the color tone by heating are hereinafter described.

<FIG> is a diagram for describing the color difference behavior after the ultraviolet ray irradiation.

<FIG> is a diagram for describing the difference of the color difference behavior after the ultraviolet ray irradiation depending on the presence or absence of heating.

A base material (photo paper: PL-<NUM>) was coated with cyan-based UV curable ink (product name "LUS-120C", manufactured by MIMAKI ENGINEERING CO. ) using a bar coat (#<NUM>), and then irradiated with ultraviolet rays using a JFX-<NUM> lamp.

The change over time in hue (L*a*b* value) before and after the ultraviolet ray irradiation was then checked using a colorimeter.

As illustrated in <FIG>, irradiating the UV curable ink after the coating with ultraviolet rays changes the color tone of the UV curable ink into yellow (see signs α to β in the drawing). The yellow tint of the yellowed UV curable ink decreases over time, and finally the color tone becomes stable (see signs β to γ in the drawing).

In <FIG>, the color difference (ΔE) before and after the ultraviolet ray irradiation is <NUM> (ΔEαβ = <NUM>), and the color difference (ΔE) until the yellow tint decreases and the color tone is stabilized and settled is <NUM> (ΔEβγ = <NUM>). This results in a color difference (ΔE) of <NUM> (ΔEαγ = <NUM>) between the color tone before the ultraviolet ray irradiation is performed and the color tone after the yellow tint is removed and the color tone is stabilized.

Thus, irradiating the UV curable ink with ultraviolet rays maximizes the color difference ΔE immediately after the irradiation. Thereafter, the value of color difference ΔE decreases over time, and finally, the color difference ΔE converges (see <FIG>, single-dot chain line).

The present inventor has presumed from the change behavior of the color difference ΔE after the ultraviolet ray irradiation that the change behavior of the color difference as expressed in <FIG> is caused by the influence of a component that increases immediately after the ultraviolet ray irradiation and then decreases over time. Moreover, it has been presumed that the change of the color tone over time is due to the initiator contained in the UV curable ink.

<FIG> is a diagram for presuming the change of the color tone over time due to the change of the initiator radicals over time.

The initiator is cleaved by the ultraviolet ray irradiation to generate the initiator radicals. The generated initiator radicals react with UV curable resin (monomers), causing the monomers to bond to each other sequentially and be polymerized, so that the UV curable ink is cured.

Since the initiator radicals are unstable, unreacted initiator radicals among the initiator radicals generated immediately after the ultraviolet rays decompose and volatilize over time.

The UV curable ink contains an excessive amount of initiators relative to the UV curable resin to ensure curing after the ultraviolet ray irradiation.

Therefore, at the time (time t1) immediately after the ultraviolet ray irradiation (time t0), a large amount of unreacted initiator radicals appear, and the unreacted initiator radicals that have appeared make the color yellow.

It is presumed that the yellow tint fades (time t1, time t2, and time t3) due to the consumption of the initiator radicals along with the progress of polymerization and the decrease of the initiator radicals due to decomposition over time, and finally the color tone is stabilized (after time t3).

Therefore, the present inventor has presumed that the color tone of the UV curable ink can be stabilized in a shorter time after the ultraviolet ray irradiation by promoting volatilization or decomposition of the unreacted disclosing agent radicals, for example. Then, to accelerate the decomposition and volatilization of the unreacted disclosing agent radicals, the present inventor considered heating the UV curable ink after the ultraviolet ray irradiation.

As a result, as illustrated in <FIG>, it has been confirmed that heating the UV curable ink after the ultraviolet irradiation reduces the color difference ΔE (stabilizes and settles the color tone) in a shorter time than when heating is not performed.

The period from the time t0 of irradiation with ultraviolet rays until the color difference ΔE becomes less than or equal to the threshold color difference (threshold ΔE) at which it is determined that the color tone is stabilized and settled is shorter in the case with heating (tb - t0) than in the case without heating (ta - t0).

Here, the threshold color difference (threshold ΔE) is set to a value at which the color tone is almost indistinguishable visually when the hue is compared to the hue before the ultraviolet ray irradiation. The threshold color difference (threshold ΔE) is set to <NUM> in this embodiment. This threshold color difference (threshold ΔE) is not limited to this value only, and can be changed as appropriate depending on the required accuracy of the color tone.

<FIG> is a diagram expressing the change over time of the color difference ΔE of the UV curable ink when printing and heating are performed using the inkjet printing apparatus (UCJV300) and the heater (JV300). The change over time of the color difference ΔE in <FIG> is the result in the case where the heater <NUM> is installed inside the downstream supporting unit <NUM> as described above.

Printing was performed in the inkjet printing apparatus (UCJV300) under the following conditions. Printing speed: <NUM> x <NUM> dpi at the fastest/<NUM> Pass/Reciprocating printing.

In <FIG> is a diagram expressing the relation between the presence or absence of heating and the heating time, and the color difference ΔE immediately after heating, and illustrates the case where oriented polypropylene (OPP, biaxially oriented polypropylene film) is used as the base material.

In <FIG>, (b) is a diagram expressing the relation between the presence or absence of heating and the heating time, and the color difference ΔE immediately after heating, and illustrates the case where polyvinyl chloride (PVC) is used as the base material.

<FIG> is a diagram for describing the relation between the difference in medium (base material) and the color difference ΔE in the case without heating after the ultraviolet ray irradiation.

In <FIG> is a diagram expressing the change over time of the color difference ΔE when solid printing is performed on different media (OPP, PVC, PET) using cyan-based UV curable ink (product name "LUS-120C", manufactured by MIMAKI ENGINEERING CO. ), the inkjet printing apparatus (UCJV300), and the heater (JV300).

In <FIG> is a conceptual diagram for describing the difference in dot diameter when solid printing is performed on different media (OPP, PVC, PET).

<FIG> is a diagram for describing the relation between the difference in medium (base material) and the contact angle of the UV curable ink. The contact angle was measured using the cyan-based UV curable ink (product name: LUS-120C, manufactured by MIMAKI ENGINEERING CO. ) and a fully automatic contact angle meter (DM-<NUM>, manufactured by Kyowa Interface Science Co. , Ltd) under the following conditions. Syringe diameter: <NUM>, Droplet volume: <NUM>µL, Measurement temperature: <NUM>.

As expressed in (a) and (b) in <FIG>, when comparing the color difference ΔE without heating (room temperature: r. t) and with heating (<NUM>, <NUM>, <NUM>, <NUM>), the higher heating temperature results in the smaller color difference ΔE immediately after heating.

Then, when comparing the color difference ΔE according to the heating time (<NUM>, <NUM>, <NUM>, <NUM>), it has been confirmed that the longer heating time results in the smaller color difference ΔE immediately after heating.

In the case without heating, the color difference ΔE corresponds to the color differences at the elapsed time (<NUM>, <NUM>, <NUM>, <NUM>) immediately after the ultraviolet ray irradiation, respectively.

When comparing the color difference ΔE according to the different media (base materials), it has been confirmed that OPP can reduce the color difference ΔE in a shorter heating time than PVC.

As expressed in <FIG>, in the case without heating, OPP has the smaller color difference ΔE immediately after the ultraviolet ray irradiation than PVC or PET (polyethylene terephthalate).

In all cases of OPP, PVC, and PET, the color difference ΔE decreases and becomes stable as the time passes from the ultraviolet ray irradiation.

As expressed in (b) in <FIG>, OPP has the smallest dot diameter (<NUM>) on the printed matter immediately after the ultraviolet ray irradiation. The dot diameter of PVC (<NUM>) and the dot diameter of PET (<NUM>) are at substantially the same level.

As expressed in <FIG>, OPP has lower wettability (larger contact angle) of the UV curable ink than PVC or PET, resulting in the smaller dot diameter of an ink droplet. Therefore, the proportion of the exposed surface of the base material in the printed matter is large.

Here, the yellowing of the UV curable ink changes depending on the amount of initiator radicals contained in the UV curable ink.

In the case of OPP, the dot diameter of the UV curable ink is smaller and the proportion of the exposed surface of the base material is larger. Therefore, even when yellowing occurs, the proportion of the ink area that has yellowed in the medium is smaller in OPP than in PVC or PET.

As a result, it is considered that since the yellowing degree of the printed image is suppressed, the color difference is smaller in OPP than in PVC or PET.

In other words, as the wettability of the UV curable ink on the medium (base material) is higher and the contact angle is smaller, the yellowing degree of the printed image immediately after the ultraviolet ray irradiation is higher.

As expressed in Table <NUM> below and <FIG>, the contact angle of the UV curable ink on the medium is larger when the medium (base material) is OPP than when the medium is PVC or PET. Then, as a result of checking the change over time of the contact angle of the UV curable ink, it has been confirmed that the contact angle when the medium (base material) is OPP is retained without decreasing in a short time compared to when the medium is PVC or PET.

In Table <NUM>, <NUM> corresponds to the time after the ink droplet lands on the medium M and before the ultraviolet ray irradiation is performed. Moreover, <NUM> corresponds to the time required for the change over time of the contact angle of the ink droplet that has landed on the medium M to settle down.

Here, "the change over time of the contact angle settles down" means the state in which the amount of change of the contact angle of the ink droplet per unit time becomes less than a threshold.

Therefore, based on the results in <FIG> and <FIG>, even in the case of the media (base materials) with a small contact angle of the UV curable ink of less than <NUM>°, heating the UV curable ink after the ultraviolet ray irradiation can settle the color tone of the image after the ultraviolet ray irradiation in a shorter time than in the case without heating.

Here, as expressed in <FIG>, the ink droplet that lands on the medium M has different contact angles depending on the difference in material of the medium M (base material).

PVC has a smaller contact angle than OPP, and PET has an even smaller contact angle than PVC. The ink droplet with a smaller contact angle wets and spreads more easily after landing on the medium M. The ink droplet, which has spread and had larger dot gain, has a larger influence of photobleaching.

Therefore, in general-purpose printers that can use a variety of media, the heating conditions (temperature and total length) of the heater are designed to suppress the influence of photobleaching even when the fastest printing is performed using the medium with the small contact angle.

That is to say, the heating temperature by the heater <NUM> in (a) in <FIG> is <NUM> to <NUM>, and the length in the conveying direction of the medium M is set to <NUM> to <NUM>. This is the setting for the size that can be reasonably integrated into the conveyance path of the general-purpose printer, in consideration of printing at the fastest or similar speed. This setting can suppress the color difference ΔE immediately after the heating by the heater to be less than or equal to the threshold, even under conditions that are severe to dryness.

The heating temperature and the length of the heater <NUM> are not particularly limited, and may be changed as appropriate as long as the color difference ΔE immediately after the heating by the heater can be suppressed to be less than or equal to the threshold.

<FIG> is a diagram expressing the relation between the presence or absence of the heating and the difference in heating temperature, and the change over time of the color difference ΔE.

As expressed in <FIG>, a comparison of the case without heating (room temperature: r. t) and the case with heating (<NUM>, <NUM>, <NUM>) indicates that the color tone eventually settles to the same hue regardless of whether heating is performed and regardless of the difference in heating temperature.

Thus, heating the UV curable ink after the ultraviolet ray irradiation does not cause a large difference in the final color tone. Then, heating the UV curable ink after the ultraviolet ray irradiation stabilizes the color tone of the printed image in a shorter time than in the case without heating.

For this reason, the present inventor provides the heater <NUM> (heating unit), which is originally unnecessary, in the inkjet printing apparatus <NUM> on purpose to heat the UV curable ink after the ultraviolet ray irradiation.

Thus, the color fixation of the UV curable ink can be settled down in a shorter time within the color difference range (less than threshold color difference) based on the target color tone, and the color tone can be stabilized.

As expressed in <FIG>, in the case of OPP, heating at least at <NUM> for <NUM> minutes after the ultraviolet ray irradiation can make the color tone immediately after heating less than the threshold color difference (threshold ΔE). In the case of PVC, heating at least at <NUM> for <NUM> minutes after the ultraviolet ray irradiation can make the color tone immediately after heating less than the threshold color difference (threshold ΔE).

In light of the above, the inkjet printing apparatus <NUM> heats the UV curable ink at least at <NUM> after the ultraviolet ray irradiation, although this varies depending on the media. This allows the inkjet printing apparatus <NUM> to make the color tone less than the threshold color difference (threshold ΔE) in a shorter time than in the case of not heating the UV curable ink.

In the case of not heating the UV curable ink, it used to take several hours to several days to stabilize the color tone. By heating the UV curable ink, the inkjet printing apparatus <NUM> can reduce the time it takes to stabilize the color tone to be in the range of several minutes to several tens of minutes.

Therefore, in the inkjet printing apparatus <NUM> that uses the UV curable ink, the color tone of the image after the ultraviolet ray irradiation can be settled in a shorter time, so that the apparatus can meet the need of those who want to proofread printed matters as early as possible after printing.

As described above, the inkjet printing apparatus <NUM> in this embodiment has the following structure.

The UV curable ink is solidified by the ultraviolet ray irradiation and is thereby fixed to the medium M. Therefore, the inkjet printing apparatus <NUM> that uses the UV curable ink does not require a heater included in other inkjet printing apparatuses that use water-based or solvent-based ink.

By providing the heater <NUM> (heating unit), which is originally unnecessary, on purpose to heat the UV curable ink, the color fixation of the UV curable ink can be stabilized in a shorter time. Thus, the color tone of the image can be settled in a shorter time.

The inkjet printing apparatus <NUM> according to this embodiment has the following structure.

(<NUM>) The heater <NUM> is provided downstream of the position irradiated with ultraviolet rays in the conveying direction of the medium M.

The UV curable ink turns yellow immediately after the ultraviolet ray irradiation, resulting in a yellowish color tone, and thereafter the yellowish color decreases over time and the color settles to the original color. Therefore, heating before the ultraviolet ray irradiation does not contribute to stabilization of the color fixation.

With the structure as described above, the heater <NUM> is disposed at the position where the UV curable ink can be properly heated after the ultraviolet ray irradiation.

Thus, the position to place the heater <NUM> can be limited, so that the increase in manufacturing cost for the inkjet printing apparatus <NUM> due to the addition of the heater <NUM> can be suppressed.

Moreover, the dot gain of the ink droplet that lands on the medium is fixed by the ultraviolet ray irradiation and changes before the ultraviolet ray irradiation. Therefore, heating after the ultraviolet ray irradiation can reduce the influence on the dot gain, so that the improvement of the printing quality can be expected.

(<NUM>) The heater <NUM> (heating unit) heats the UV curable ink so that the UV curable ink stops changing in hue at a position downstream of the heater <NUM> in a conveying direction of the medium M and thereby stabilizes the color fixation of the UV curable ink.

Therefore, the heating time of the UV curable ink after the ultraviolet ray irradiation can be optimized by determining the end timing of heating by the heater <NUM> on the basis of the hue change of the UV curable ink after the ultraviolet ray irradiation. Furthermore, the color fixation of the UV curable ink can be stabilized at the downstream position immediately after heating by the heater <NUM>, so that proofreading of printed matters, etc. can be performed at an early stage after printing.

(<NUM>) The lower limit of the heating temperature of the UV curable ink by the heater <NUM> after the ultraviolet ray irradiation is <NUM>.

By setting the heating temperature by the heater <NUM> to <NUM> or higher, the UV curable ink after the ultraviolet ray irradiation can be heated until the UV curable ink stops changing in hue.

This makes it possible to integrate the heater <NUM> into the medium conveyance path of the existing printer apparatus without difficulty.

In other words, the UV curable ink after the ultraviolet ray irradiation can be heated until the UV curable ink stops changing in hue without the necessity of extending the conveyance path of the medium M in order to secure the time for heating of the UV curable ink after the ultraviolet ray irradiation. Therefore, there is no need to make the inkjet printing apparatus <NUM> particularly large.

As the heating temperature becomes lower, it takes longer for the color difference, which is the difference between the color fixation of the UV curable ink at the current time point and the target color fixation, to become below the target threshold color difference (threshold ΔE).

In the inkjet printing apparatus <NUM> according to the embodiment, the lower limit of the heating temperature is preferably <NUM>, and more preferably <NUM>. The inkjet printing apparatus <NUM> can sufficiently shorten the time until the color difference becomes less than the target color difference compared to the case without heating.

Thus, since the color fixation can be settled in a shorter time than in the case without heating, it is possible to meet the need of those who want to proofread printed matters as early as possible after printing.

The upper limit of the heating temperature is not limited in particular, but should be the temperature that is determined according to the heat resistance temperature of the medium to be printed, and that does not affect the characteristics of the medium or the characteristics of the printed image (strength, abrasion resistance, etc.).

This is because heating at temperatures that affect the characteristics of the medium may fail to satisfy the required performance of the printed matters.

(<NUM>) The medium M is a medium M that makes the contact angle of the UV curable ink less than <NUM>° with respect to the medium M at the time of irradiation with ultraviolet rays by the irradiation lamp <NUM>.

Even in the case of the medium with a contact angle of less than <NUM>°, that is, the medium with the small contact angle of the UV curable ink that has landed thereon, heating the UV curable ink immediately after the ultraviolet ray irradiation can stabilize the color fixation of the UV curable ink in a shorter time and settle down the color tone of the printed image in a shorter time.

The medium M with a contact angle of UV curable ink of less than <NUM>° has high wettability with the UV curable ink, resulting in a high degree of yellowish color tone of the printed image immediately after the ultraviolet ray irradiation.

In printing on such a medium M (base material), heating the UV curable ink immediately after the ultraviolet ray irradiation can stabilize the color fixation of the UV curable ink in a shorter time and settle the color tone of the printed image in a shorter time.

(<NUM>) The medium M used in the inkjet printing apparatus <NUM> is PVC or PET.

Even in the case of the medium with the small contact angle of the droplet of the UV curable ink, such as PVC or PET, the color fixation of the UV curable ink can be stabilized in a shorter time and the color tone of the printed image can be settled down in a shorter time.

(<NUM>) In the case of OPP, it is preferable that the lower limit of the heating temperature of the UV curable ink after the ultraviolet ray irradiation by the heater <NUM> is <NUM> and the lower limit of the heating time by the heater <NUM> is <NUM> minutes.

This structure allows control of the heating of the UV curable ink after the ultraviolet ray irradiation under optimal conditions for the medium. The inkjet printing apparatus <NUM> can reduce the color tone to be less than the threshold color difference (threshold ΔE) in a shorter time than when the UV curable ink is not heated.

In the example described in the above embodiment, the inkjet printing apparatus <NUM> prints on the medium rolled in a roll shape by a roll machine (vertical machine).

The present invention can also be suitably used for a flood-bed type inkjet printing apparatus that prints on a sheet-shaped medium placed on a table.

In this case, it is possible to heat the UV curable ink after the ultraviolet ray irradiation by installing a heater on the table or on the carriage, for example.

Claim 1:
An inkjet printing apparatus (<NUM>) comprising:
a UV curable ink;
an ink-jet head (<NUM>) that ejects the UV curable ink to a medium (M);
an ultraviolet ray irradiation unit (<NUM>) that irradiates the UV curable ink ejected to the medium (M) with ultraviolet rays; and
a heating unit (<NUM>,<NUM>) that heats the UV curable ink after the ultraviolet ray irradiation by the ultraviolet ray irradiation unit (<NUM>) and thereby stabilizes color fixation of the UV curable ink;
the initiator being cleaved by the ultraviolet ray radiation to generate the initiator radicals that react with the UV curable resin monomers causing the monomers to bond to each other sequentially and be polymerized, so that the UV curable ink is cured;
the UV curable ink containing an excessive amount of the initiator relative to the UV curable resin to ensure curing after the ultraviolet ray radiation;
characterised in that
the heating unit (<NUM>, <NUM>) is adapted to heat to the extent that initiator radicals are decomposed or volatilized; and to heat without a need for volatilization of a solvent to fix the ink.