JP5896111B2 - Recording method and recording apparatus using ultraviolet curable ink - Google Patents

Description

  The present invention relates to a recording method and a recording apparatus using ultraviolet curable ink.

  Conventionally, white ink has been used for dimming interior design and label base for the purpose of making the appearance beautiful. When printing on a colored print medium, white may be printed as a base. In some cases, printing is performed on the back side of a transparent printing medium, and white is printed as a base when an image is viewed through the printing medium. In these cases, as a white color material, a white pigment having a large particle size such as titanium oxide is used as a white color material so that the back side is not transparent. However, since the specific gravity and particle size of such a white pigment are very large, it is difficult to obtain an ink having good storage stability and ejection stability. Therefore, various efforts have been made to overcome this difficulty.

  For example, Patent Document 1 discloses a white ink coating film in which the average transmittance in the visible light region wavelength of 400 to 800 nm is 0.5% or less and the average transmittance in the wavelength range of 190 to 400 nm is higher than that in the visible light region. It is disclosed that, when formed on a transparent printing medium, the color bleed does not occur when color ink is printed on a white ink coating film with excellent visibility.

  For example, Patent Document 2 discloses an ink jet apparatus that improves concealability and curability by controlling the inclination of a nozzle and preventing sedimentation by using a white organic pigment.

  For example, Patent Document 3 uses a UV-curable ink jet recording composition having a good storage stability and ejection stability, in which a low-boiling point solvent is blended in an UV-curable clear ink, and has a low boiling point during curing. It discloses that the concealability of the ink film is improved because the solvent evaporates to form a porous coating film.

JP 2007-161847 A JP 2008-1072 A JP 2005-298757 A

  However, all of the ink coating films disclosed in Patent Documents 1 to 3 have room for improvement in terms of embedding property and concealing property in recorded matter.

  Accordingly, an object of the present invention is to provide a recording method using an ultraviolet curable ink excellent in embedding property and concealing property in a recorded matter.

  In order to solve the above-mentioned problems, the inventors of the present application have conducted intensive studies. As a result, a process of ejecting an ultraviolet curable ink containing at least a pigment from a head toward a printing medium, and an ultraviolet ray attached to the printing medium. A step of curing the ultraviolet curable ink by irradiating the curable ink with ultraviolet rays, and performing a unit recording operation including at least a plurality of unit recording operations on the surface of the printing medium facing the head. The present inventors have found that the above problems can be solved by forming a cured product having a roughness (Rq) in a predetermined range, and have completed the present invention.

That is, the present invention is as follows.
[1]
A step of discharging an ultraviolet curable ink containing at least a white pigment from a head toward a printing medium, and irradiating the ultraviolet curable ink attached to the printing medium with ultraviolet rays, thereby the ultraviolet curable ink. performed a plurality of times and curing, including at least unit recording operation, said a region facing the head of the print medium, the surface roughness (Rq) is Ri 1.5 to 5.0 [mu] m der, the A recording method using an ultraviolet curable ink, comprising forming a cured product obtained by curing the ultraviolet curable ink.
[2]
In one unit recording operation, there are pixels that form dots and pixels that do not form dots for one raster line, and one raster line is formed by performing a plurality of unit recording operations. Or at least one of the raster lines for forming dots in another unit recording operation exists in the sub-scanning direction of the raster lines for forming dots in one unit recording operation. A recording method using the ultraviolet curable ink according to [1].
[3]
In one unit recording operation, there are pixels that form dots and pixels that do not form dots for one raster line, one raster line is formed by performing unit recording operations a plurality of times, and The ultraviolet curable ink according to [1], wherein in one unit recording operation, a raster line for forming dots in another unit recording operation exists between the sub-scan directions of the raster lines for forming dots. Recording method using
[4]
The recording method using the ultraviolet curable ink according to any one of [1] to [3], wherein the ultraviolet curable ink is a white ink.
[5]
The recording method using the ultraviolet curable ink according to [2] or [3], wherein the pixels that perform dot formation include pixels that perform dot formation by performing unit recording operations on a single pixel a plurality of times. .
[6]
The recording method using the ultraviolet curable ink according to any one of [1] to [5], wherein the conversion rate of the ultraviolet curable ink adhered and cured in each unit recording operation is in the range of 20 to 90%. .
[7]
The recording method using the ultraviolet curable ink according to any one of [1] to [6], wherein the ultraviolet curable ink contains titanium oxide as the white pigment .
[8]
The recording method using the ultraviolet curable ink according to [7], wherein the average particle diameter D50 of the titanium oxide is in a range of 150 to 500 nm.
[9]
The recording method using the ultraviolet curable ink according to [7] or [8], wherein a content of the titanium oxide is 10 to 30% by mass with respect to a total mass of the ultraviolet curable ink.
[10]
The recording method using the ultraviolet curable ink according to any one of [1] to [9], wherein the adhesion amount of the ultraviolet curable ink to the printing medium is 5 to 16 mg / inch ² .
[11]
Means for discharging ultraviolet curable ink containing at least a white pigment from a head toward a printing medium; and irradiating the ultraviolet curable ink attached to the printing medium with ultraviolet rays, thereby the ultraviolet curable ink. means for curing the unit recording operation consisting of is performed a plurality of times, the a region facing the head of the print medium, the surface roughness (Rq) is Ri 1.5 to 5.0 [mu] m der, the A recording apparatus using an ultraviolet curable ink, in which a cured product is formed by curing the ultraviolet curable ink.

1 is a block diagram illustrating an overall configuration of a printer. FIG. 3 is a schematic view around the head 31 of the printer 1. 3A and 3B are cross-sectional views of the printer 1. It is a figure for demonstrating the overlap printing in embodiment of this invention.

Hereinafter, embodiments for carrying out the present invention will be described in detail.
In this specification, the “unit recording operation” means an operation for forming one image over the entire printing medium, and is also referred to as pass or main scanning hereinafter. The n-th pass is referred to as “pass n”. “Image printing” means one or more unit recording operations over the entire print medium. “Head scanning” means one operation of ejecting ink while moving the head in a predetermined direction relative to the printing medium in forming an image.

  In this specification, “pixel” means a minimum recording unit area corresponding to the recording resolution. A “raster line” means a row (dot row) in which pixels are arranged in a row in the main scanning direction.

  In the present specification, the “conversion rate” means a rate at which a polymerizable compound contained in an ultraviolet curable ink is converted into a cured product, and can be translated into the degree of curing of the ink by ultraviolet irradiation. The conversion rate in this specification is a value measured using an infrared spectrophotometer (NEXUS470, manufactured by Thermo Nicolet Corp.) capable of real-time measurement.

  Further, in this specification, “temporary curing” means ink temporary fixing (pinning), and refers to curing before main curing in order to prevent dot bleeding and color mixing. The conversion rate in the curing is lower than the conversion rate in the main curing performed after the temporary curing. “Main curing” refers to curing the dots formed on the recording medium to a curing state necessary for using the recorded material. In this specification, the energy required for the main curing is referred to as irradiation energy.

  Further, in this specification, “surface roughness” means a surface roughness defined based on JIS B0601, that is, an Rq value (square root height). Here, JIS B0601 corresponds to ISO 4287, “Geometrical Product Specifications (GPS) -Surface texture: Profile” (Geometrical Product Specifications (GPS) -Surface texture: Profile) method-Terms, definitions and surface texture parameters). Among these, the root mean square height (Rq) represents the root mean square height in the reference length, and means the standard deviation of the surface roughness.

  In this specification, “recorded material” refers to a material in which ink is recorded on a printing medium to form a cured product. In addition, the hardened | cured material in this specification means the hardened | cured substance containing a cured film (coating film).

  In the present specification, “curability” refers to a property of curing in response to light. “Filling property” is also referred to as filling property, and refers to a property in which a printed medium as a base cannot be seen when a recorded material is viewed from a side on which a cured product (image) is formed. “Concealment” refers to a property in which a cured product formed by recording ink does not easily transmit light having a wavelength of 400 to 800 nm.

  In the present specification, “(meth) acrylate” means at least one of acrylate and methacrylate corresponding thereto, and “(meth) acryl” means at least one of acrylic and methacryl corresponding thereto. To do.

[Recording method using ultraviolet curable ink]
The first embodiment of the present invention relates to a recording method using ultraviolet curable ink (hereinafter also simply referred to as “ink”). Hereinafter, a recording apparatus (printer) for carrying out the recording method will be described in detail.

[Recording device configuration]
FIG. 1 is a block diagram illustrating a configuration of the printer 1. FIG. 2 is a schematic view around the head of the printer 1. 3A and 3B are cross-sectional views of the printer 1. 3A corresponds to the AA cross section of FIG. 2, and FIG. 3B corresponds to the BB cross section of FIG.

  The printer 1 according to the present embodiment discharges ultraviolet curable ink that is cured by irradiation with ultraviolet rays (UV) toward a printing medium such as paper (hereinafter, also simply referred to as “medium”), to the medium. An apparatus for printing an image. Here, the printer 1 of the present embodiment can print an image using inks of various colors. For example, the printer 1 can print an image using four colors of CMYK, or can use white ink. For example, a base that gives excellent concealability to the medium is formed.

  The printer 1 includes a transport unit 10, a carriage unit 20, a head unit 30, an irradiation unit 40, a detector group 50, and a controller 60. The printer 1 that has received print data from the computer 110 that is an external device controls each unit (the transport unit 10, the carriage unit 20, the head unit 30, and the irradiation unit 40) by the controller 60. The controller 60 controls each unit based on the print data received from the computer 110 and prints an image on a medium. The situation in the printer 1 is monitored by the detector group 50, and the detector group 50 outputs the detection result to the controller 60. The controller 60 controls each unit based on the detection result output from the detector group 50.

  The transport unit 10 is for transporting a medium such as paper in a predetermined direction (hereinafter referred to as “transport direction” or “sub-scanning direction”). The transport unit 10 includes a paper feed roller 11, a transport motor (not shown), a transport roller 13, a platen 14, and a paper discharge roller 15. The paper feed roller 11 is a roller for feeding the medium inserted into the paper insertion slot into the printer. The transport roller 13 is a roller that transports the medium fed by the paper feed roller 11 to a printable area, and is driven by a transport motor. The platen 14 supports the medium being printed. The paper discharge roller 15 is a roller for discharging the medium to the outside of the printer, and is provided on the downstream side in the transport direction with respect to the printable area.

  The carriage unit 20 causes the head 31 to remain stationary in the printing area while ejecting ink while intersecting the transport direction (sub-scanning direction) (hereinafter referred to as “moving direction” or “main scanning direction”). This is a moving mechanism that moves, that is, scans. The carriage unit 20 includes a carriage 21 and a carriage motor (not shown). In addition, the carriage 21 detachably holds an ink cartridge that stores ultraviolet curable ink. The carriage 21 is reciprocated along the guide shaft 24 by a carriage motor while being supported by a guide shaft 24 that intersects a conveyance direction described later.

The head unit 30 is for ejecting ultraviolet curable ink onto a medium. The head unit 30 includes a head 31 having a plurality of nozzles. Since the head 31 is provided on the carriage 21, when the carriage 21 moves in the movement direction, the head 31 also moves in the movement direction. Then, by intermittently discharging the ultraviolet curable ink while the head 31 is moving in the moving direction, a dot row (raster line) along the moving direction is formed on the medium.
In the movement of the head 31, the ultraviolet curable ink is ejected while moving from one end side to the other end side in FIG. 2, but the ultraviolet curable ink is discharged while moving from the other end side to the one end side. Ink is not ejected.

  The irradiation unit 40 cures the ultraviolet curable ink by irradiating the ultraviolet curable ink attached (landed) on the medium with ultraviolet rays. The dots formed on the medium are cured by being irradiated with ultraviolet rays from the irradiation unit 40 to form a cured product. The irradiation unit 40 of the present embodiment includes provisional curing irradiation units 42 a and 42 b and a main curing irradiation unit 43 on the downstream side in the transport direction of the head unit 30.

  Note that one unit recording operation is realized by one main scanning by the carriage unit 20, the head unit 30, and the irradiation unit 40. In this embodiment, this unit recording operation is performed a plurality of times.

  The detector group 50 includes a linear encoder (not shown), a rotary encoder (not shown), a paper detection sensor 53, an optical sensor 54, and the like. The linear encoder detects the position of the carriage 21 in the moving direction. The rotary encoder detects the amount of rotation of the transport roller 13. The paper detection sensor 53 detects the position of the leading edge of the paper (medium) being fed. The optical sensor 54 detects the presence / absence of a medium by a light emitting unit and a light receiving unit attached to the carriage 21. The optical sensor 54 can detect the position of the end of the medium while being moved by the carriage 21 to detect the width of the medium. The optical sensor 54 also has a leading end (an end on the downstream side in the transport direction and also called an upper end) and a rear end (an end on the upstream side in the transport direction and also called a lower end) depending on the situation. Can also be detected.

  The controller 60 is a control unit (control unit) for controlling the printer 1. The controller 60 includes an interface unit 61, a CPU 62, a memory 63, and a unit control circuit 64. The interface unit 61 transmits and receives data between the computer 110 that is an external device and the printer 1. The CPU 62 is an arithmetic processing unit for controlling the entire printer 1. The memory 63 is for securing an area for storing a program of the CPU 62, a work area, and the like, and includes storage elements such as a RAM and an EEPROM. The CPU 62 controls each unit via the unit control circuit 64 in accordance with a program stored in the memory 63.

  When performing printing, the controller 60 alternately repeats a dot forming operation for ejecting ultraviolet curable ink from the head 31 moving in the movement direction and a conveyance operation for conveying paper in the conveyance direction, as will be described later. An image composed of a plurality of dots is printed on paper.

As described above, the recording apparatus using the ultraviolet curable ink forms a cured product in a region facing the head 31 of the medium. At this time, when the surface roughness (Rq) of the cured product is 1.5 to 5.0, the embedding property and the concealing property of the recorded material can be improved. Further, in order to further improve the embedding property and the concealing property, the surface roughness (Rq) is preferably 1.5 to 5.0, more preferably 1.5 to 4.5, and 2.0 to 4 .5 is more preferred.
Note that the surface roughness (Rq) within the preferable range is a predetermined number which will be described later, such as the number of passes (number of main scans), the driving amount, the pigment (type, content, average particle diameter), recording resolution, and nozzle density of the head. It has been found by the present inventors that this can be realized by setting the above range.

[Printing operation]
In the recording method of the present embodiment, a step of ejecting an ultraviolet curable ink containing at least a pigment from a head toward a printing medium (hereinafter, also simply referred to as “ejection process”) and the recording medium adhered to the printing medium. A unit recording operation (step) including at least a step of curing the ultraviolet curable ink by irradiating the ultraviolet curable ink with ultraviolet rays (hereinafter, also simply referred to as “curing step”) is performed a plurality of times. It is characterized by that. More specifically, the recording method performs printing by alternately performing the unit recording operation (process) and the transport operation (transport process) for transporting the print medium. Therefore, at the time of printing, the printing medium is not conveyed and is held by the platen 14 located in the printing area. In this way, a cured product having the above-described desired surface roughness can be formed in a region of the printing medium facing the head. In the following, first, the unit recording operation in the recording method of the present embodiment will be described in detail.

(Discharge process)
In the ejection step, ink is ejected from the head toward the printing medium, and this ink adheres (lands) on the printing medium. The amount of ink deposited per unit area (printing amount) on the printing medium is 5-16 mg / in order to provide excellent embedding and concealing properties and to prevent wasteful use of ink. Inch ² is preferable, and 7-14 mg / inch ² is more preferable.

The amount of ink deposited per unit area (printing amount) varies depending on the recording resolution and the amount of ink applied per recording unit area (pixel) defined by the recording resolution, but the recording resolution (printing resolution) is In terms of “resolution in the sub-scanning direction × resolution in the direction crossing the sub-scanning direction (main scanning direction)”, 300 dpi × 300 dpi to 1500 dpi × 1500 dpi is preferable. Then, it is preferable to adjust the nozzle density and driving amount of the head according to the recording resolution.
In addition, the ink ejection amount per pixel is preferably 2 to 200 ng / pixel, and more preferably 3 to 160 ng / pixel. The nozzle density (distance between nozzles in the nozzle row) is preferably 180 to 720 dpi, and more preferably 300 to 720 dpi.

The viscosity of the ink during ejection is preferably 20 mPa · s or less, and more preferably 10 mPa · s or less. If the ink viscosity is as described above with the ink temperature set to room temperature or the ink not heated, the ink may be discharged at room temperature or without heating the ink. At that time, the temperature of the ink at the time of ejection is preferably 25 ° C. On the other hand, the ink may be discharged with a preferable viscosity by heating the ink to a predetermined temperature. The heating temperature when heating the ink is preferably 50 ° C. or lower. In this way, good discharge stability is realized.
The ultraviolet curable ink used in the present embodiment has a higher viscosity than that of a normal water-based ink, and therefore has a large viscosity fluctuation due to a temperature fluctuation during ejection. Such a variation in the viscosity of the ink has a great influence on the change in the dot size and the change in the dot discharge speed, and can cause image quality deterioration. Therefore, it is preferable to keep the temperature of the ink during ejection as constant as possible.

(Curing process)
In the curing step, the ultraviolet curable ink ejected and adhered onto the printing medium is cured by irradiation with ultraviolet rays (light). This is because the photopolymerization initiator that can be contained in the ink is decomposed by irradiation with ultraviolet rays to generate initiation species such as radicals, acids, and bases, and the polymerization reaction of the polymerizable compound is accelerated by the function of the initiation species. It is to be done. Alternatively, the polymerization reaction of the (photo) polymerizable compound is started by irradiation with ultraviolet rays. At this time, if the sensitizing dye is present together with the photopolymerization initiator in the ink, the sensitizing dye in the system absorbs ultraviolet rays to be in an excited state, and promotes decomposition of the photopolymerization initiator by contacting with the photopolymerization initiator. And a more sensitive curing reaction can be achieved.

  As an ultraviolet ray source, a mercury lamp, a gas / solid laser or the like is mainly used, and as a light source used for curing an ultraviolet curable ink, a mercury lamp and a metal halide lamp are widely known. On the other hand, there is a strong demand for mercury-free from the viewpoint of environmental protection, and replacement with a GaN-based semiconductor ultraviolet light-emitting device is very useful industrially and environmentally. Furthermore, ultraviolet light-emitting diodes (UV-LEDs) and ultraviolet laser diodes (UV-LDs) are small, have a long life, have high efficiency, and are low in cost, and are expected as light sources for photo-curable ink jets. Among these, UV-LED is preferable.

Further, the conversion rate of the ultraviolet curable ink adhered and cured in each unit recording operation is preferably 20 to 90%, more preferably 30 to 80% in order to further improve the embedding property and the concealing property. Irradiation energy in each of the unit recording operation is preferably ^{3~200mJ / cm 2, 10~180mJ / cm} 2 is more preferable.

Further, when a UV-LED having an emission peak wavelength of preferably 350 to 400 nm is used, it is preferably 500 mJ / cm ² or less, more preferably 300 mJ / cm ² or less, and even more preferably 50 to 300 mJ / cm ^2. It is preferable to use an ultraviolet curable ink that can be cured with the irradiation energy of the recording method of the present embodiment. In this case, it is easy to increase the output of the LED, and low-cost printing and a high printing speed can be realized. In addition, said irradiation energy is total irradiation energy also including the irradiation energy for main curing.
Such an ink can be obtained by including at least one of a photopolymerization initiator that decomposes upon irradiation with ultraviolet rays in the wavelength range and a polymerizable compound that starts polymerization upon irradiation with ultraviolet rays in the wavelength range.

  Note that the unit recording operation in the present embodiment may include at least the above-described ejection step and curing step, and may further include other steps as appropriate.

(Overlap printing)
Next, the recording method of the present embodiment is performed by so-called overlap printing. Here, overlap printing in the present embodiment will be described. Overlap printing is a printing method in which one dot row, that is, a raster line is formed by two or more nozzles. More specifically, as a first example, in one unit recording operation by one nozzle, there are pixels that form dots and pixels that do not form dots for one raster line (in the main scanning direction). , Dot rows are formed intermittently every few dots.) Then, in one unit recording operation by other nozzles, one raster line (dots) is formed so as to complement the pixels that do not form the dots (intermittent dot rows that have already been formed). As a second example, in one unit recording operation, there is a raster line in which dots are formed in another unit recording operation in the sub-scanning direction of the raster line in which dots are formed.
In this way, by performing unit recording operations a plurality of times (in the above case, twice), one raster line is formed almost intermittently. That is, according to the overlap printing that requires a plurality of unit recording operations, the number of passes (the number of main scans) is two times or more, and the embedding property and the concealing property are excellent.

  The recording method of the present embodiment preferably adopts at least one unit recording operation pattern of the first example and the second example, and the unit recording operation patterns of both the first example and the second example are used. It is more preferable to adopt.

Here, the number of passes described above is “the number of main scans required to form dots at each pixel in the sub-scanning direction corresponding to the recording resolution in the sub-scanning direction” and “main scanning corresponding to the recording resolution in the main-scanning direction”. It is represented by the product of “the number of main scans required to form dots at each pixel in the direction”.
The above “the number of times of main scanning required to form dots in each pixel in the main scanning direction corresponding to the recording resolution in the main scanning direction” is 2 or more in one unit recording operation (main scanning direction). In other words, there are pixels that form dots and pixels that do not form dots for one raster line, and in other words, one raster line is formed by performing unit recording operations a plurality of times. On the other hand, the fact that the “number of times of main scanning required to form a dot in each pixel in the sub-scanning direction corresponding to the recording resolution in the sub-scanning direction” is 2 or more means that in one unit recording operation, In other words, there is a raster line in which dots are formed in another unit recording operation between the raster lines to be formed in the sub-scanning direction.
Thus, it is preferable that both the number of main scanning operations and the number of main scanning operations of the latter are two or more, that is, four or more passes by the product of two or more. In this case, the embedding property and the concealing property can be further improved.

  FIG. 4 is a diagram for explaining overlap printing in the present embodiment. FIG. 4 shows the positions of the heads 31 (nozzle rows) and the pre-curing irradiation unit 42a in pass 1 to pass 8 and how dots are formed.

  The left side of FIG. 4 shows the position of the head 31 (nozzle row) in pass 1 to pass 8. The nozzles indicated by black circles in the figure are nozzles that can eject ink. On the other hand, nozzles indicated by white circles are nozzles that cannot eject ink. For convenience of explanation, the head 31 (nozzle row) is depicted as moving with respect to the medium, but the medium is actually moved (conveyed) in the transport direction.

  Further, the right side of FIG. 4 shows dots formed on the medium by the pass. A dot indicated by a black circle is a dot formed in the last pass, and a dot indicated by a white circle is a dot formed in a previous pass. That is, in this figure, white circles are dots formed by passes 1 to 7, and black circles are dots formed by pass 8.

  Continue explaining overlap printing. In overlap printing, each time the medium is transported at a constant transport amount F in the transport direction, each nozzle intermittently forms dots every few dots. In another pass, by forming dots so that the intermittent dots already formed by other nozzles are complemented (filling between the dots), one raster line is formed by a plurality of nozzles. It is formed. When one raster line is formed in M passes in this way, it is defined as “overlap number M”.

  In FIG. 4, since each nozzle intermittently forms dots every other dot, dots are formed in odd-numbered pixels or even-numbered pixels for each pass. Since one raster line is formed by two nozzles, the overlap number M = 2.

  In overlap printing, in order to perform printing with a constant conveyance amount, (1) N / M is an integer, (2) N / M is relatively prime to k, (3) The condition is that the carry amount F is set to (N / M) · D.

  In FIG. 4, the nozzle row has eight nozzle rows in the transport direction. However, since the nozzle pitch k of the nozzle row is 4, not all the nozzles can be used to satisfy “N / M and k are relatively prime”, which is a condition for performing overlap printing. Therefore, overlap printing is performed using six of the eight nozzles. In addition, since six nozzles are used, the medium is transported by a transport amount of 3 · D. As a result, for example, dots are formed on the medium at a dot interval of 720 dpi (= D) using a nozzle row having a nozzle pitch of 180 dpi (4 · D).

  When one raster line is formed by M nozzles, k × M passes are required to complete a raster line for the nozzle pitch. For example, in FIG. 4, since one raster line is formed by two nozzles, eight passes are required to complete four raster lines. According to FIG. 4, a continuous raster line has a dot interval upstream of the raster line formed by the # 4 nozzle in pass 3 and the # 1 nozzle in pass 7 (raster line indicated by an arrow in the figure) in the transport direction. D is shown to be formed.

  In the case of FIG. 4, in pass 1, each nozzle forms dots in odd pixels, in pass 2, each nozzle forms dots in even pixels, in pass 3, each nozzle forms dots in odd pixels, and in pass 4, Each nozzle forms dots at even pixels. That is, in the first four passes, dots are formed in the order of odd pixels, even pixels, odd pixels, and even pixels. In the latter four passes (pass 5 to pass 8), dots are formed in the reverse order of the first four passes, and dots are formed in the order of even pixels, odd pixels-even pixels-odd pixels. The dot formation order after pass 9 is the same as the dot formation order from pass 1.

  For example, in the raster line indicated by the arrow in FIG. 4, dots are formed in odd pixels by the # 4 nozzle in pass 3, and dots are formed in even pixels by the # 1 nozzle in pass 7. In the raster line below, dots are formed in even pixels by the # 5 nozzle in pass 2, and dots are formed in odd pixels by the # 2 nozzle in pass 6.

  In the case of FIG. 4, not only a difference in the total irradiation amount of ultraviolet rays occurs between dots in a dot row (raster line) aligned in the transport direction, but also a difference in the total irradiation amount of ultraviolet rays occurs between dots in each raster line. become.

(Dot formation on one pixel by multiple unit recording operations)
In the recording method of this embodiment, dots can be formed in one pixel by one or a plurality of unit recording operations. In particular, it is preferable to form dots in one pixel by a plurality of unit recording operations. For example, in the recording method described above, the number of nozzles in one nozzle row is doubled, the nozzle density is not changed, and the head in which the distance in the sub-scanning direction of the nozzle row is doubled is used for one sub-scan. By performing the same recording method without changing the sub-scanning distance in, dots can be formed in each pixel by two unit recording operations. Alternatively, the number of unit recording operations in which a nozzle faces one raster line is doubled by halving the sub-scan distance in one sub-scan without changing the number of nozzles in the nozzle row. Even so, it is possible to form dots in each pixel by two unit recording operations. By further increasing the number of nozzles or further shortening the sub-scanning distance, dots may be formed in one pixel by a unit recording operation three times or more. Further, a pixel in which dot formation is performed by one unit recording operation per pixel and a pixel in which dot formation is performed by one or more unit recording operations may be mixed. In other words, it is only necessary that the pixel for forming dots has a pixel for forming dots by performing unit recording operations for one pixel a plurality of times. At that time, it is preferable that the number of pixels in which dots are formed by performing the unit recording operation a plurality of times for one pixel among the pixels in which dots are formed is preferably 10% or more, and more preferably 50% or more. More preferably, it is more preferably 90% or more. Further, it is preferable that at least one unit recording operation pattern of the first example and the second example described above is adopted, and dots are formed in one pixel by a plurality of unit recording operations. Thereby, the embedding property and the concealing property can be further improved.

As described above, according to the present embodiment, it is possible to provide a recording method using an ultraviolet curable ink excellent in embedding property and concealing property in a recorded matter, and a recording apparatus using the same.
[UV curable ink]
In addition, an embodiment of the present invention relates to an ultraviolet curable ink that can be used in the above-described recording method and recording apparatus. Hereinafter, additives (components) that are included in the ink of the present embodiment or that may be included as desired will be described.

[Polymerizable compound]
The polymerizable compound contained in the ink of this embodiment can be polymerized at the time of ultraviolet irradiation by the action of a photopolymerization initiator described later, and the printed ink can be cured.

  As the polymerizable compound, conventionally known various monomers and oligomers such as monofunctional, bifunctional, and trifunctional or more polyfunctional can be used. Examples of the monomer include unsaturated carboxylic acids such as (meth) acrylic acid, itaconic acid, crotonic acid, isocrotonic acid and maleic acid, salts or esters thereof, urethane, amide and anhydride thereof, acrylonitrile, styrene, various types Unsaturated polyesters, unsaturated polyethers, unsaturated polyamides, and unsaturated urethanes. Examples of the oligomer include oligomers formed from the above monomers such as linear acrylic oligomers, epoxy (meth) acrylates, oxetane (meth) acrylates, vinyl ether group-containing (meth) acrylates, and aliphatic urethanes (meth). Examples include acrylates, aromatic urethane (meth) acrylates, and polyester (meth) acrylates.

  Among those listed above, esters of (meth) acrylic acid, that is, (meth) acrylates are preferred. Hereinafter, this (meth) acrylate will be described in detail.

(Monofunctional (meth) acrylate)
Examples of the monofunctional (meth) acrylate include, but are not limited to, phenoxyethyl (meth) acrylate, isoamyl (meth) acrylate, stearyl (meth) acrylate, lauryl (meth) acrylate, octyl (meth) acrylate, and decyl. (Meth) acrylate, isomyristyl (meth) acrylate, isostearyl (meth) acrylate, 2-ethylhexyl-diglycol (meth) acrylate, 2-hydroxybutyl (meth) acrylate, 4-hydroxybutyl (meth) acrylate, butoxyethyl (Meth) acrylate, ethoxydiethylene glycol (meth) acrylate, methoxydiethylene glycol (meth) acrylate, methoxypolyethylene glycol (meth) acrylate, methoxy Lopylene glycol (meth) acrylate, tetrahydrofurfuryl (meth) acrylate, isobornyl (meth) acrylate, 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, 2-hydroxy-3-phenoxypropyl (meth) Acrylate, lactone-modified flexible (meth) acrylate, t-butylcyclohexyl (meth) acrylate, dicyclopentanyl (meth) acrylate, dicyclopentenyloxyethyl (meth) acrylate, benzyl (meth) acrylate, ethoxylated nonylphenyl (Meth) acrylate, alkoxylated nonylphenyl (meth) acrylate, p-cumylphenol EO-modified (meth) acrylate, and vinyl ether group represented by the following general formula (I) Available (meth) acrylic acid esters.

_{^{CH 2 = CR 1 -COOR 2 -O}} -CH = CH-R 3 ··· (I)
(In the formula, R ¹ is a hydrogen atom or a methyl group, R ² is a divalent organic residue having 2 to 20 carbon atoms, and R ³ is a hydrogen atom or a monovalent organic residue having 1 to 11 carbon atoms. Group.)

  Among these, phenoxyethyl (meth) acrylate and vinyl ether group-containing (meth) acrylic acid esters represented by the above general formula (I) are preferable, and phenoxyethyl (meth) acrylate is more preferable.

When the ink contains phenoxyethyl (meth) acrylate, the viscosity of the ink can be reduced, and the curability, abrasion resistance, adhesion, and solubility of the photopolymerization initiator are all excellent. Can be.
The content of phenoxyethyl (meth) acrylate is preferably 10 to 40% by mass, more preferably 10 to 30% by mass, and more preferably 15 to 30% by mass with respect to the total mass (100% by mass) of the ink. % Is more preferable. When the content is 10% by mass or more, the solubility of the photopolymerization initiator is further improved in addition to the curability. On the other hand, when the content is 40% by mass or less, in addition to curability, the adhesion is further improved.

In addition, the ink contains the vinyl ether group-containing (meth) acrylic acid ester, whereby the viscosity of the ink can be reduced, and the curability, abrasion resistance, adhesion, and photopolymerization initiator Any of the solubilities can be excellent. Here, the reason why the photopolymerization initiator is excellent in solubility is that the compatibility between the polymerizable compound and additives such as the photopolymerization initiator is improved.
Furthermore, rather than using a compound having a vinyl ether group and a compound having a (meth) acrylic acid ester group (a kind of radical polymerizable group) separately, the vinyl ether group and the (meth) acrylic acid ester group are contained in one molecule. It is preferable to use a compound having both of these in order to further improve the curability of the ink.

In the above general formula (I), the divalent organic residue having 2 to 20 carbon atoms represented by R ² is a linear, branched or cyclic substituted having 2 to 20 carbon atoms. May be an alkylene group, an optionally substituted alkylene group having an oxygen atom by an ether bond and / or an ester bond in the structure, and an optionally substituted divalent alkylene group having 6 to 11 carbon atoms. Aromatic groups are preferred. Among these, C2-C6 alkylene groups such as ethylene group, n-propylene group, isopropylene group, and butylene group, oxyethylene group, oxy n-propylene group, oxyisopropylene group, and oxybutylene group An alkylene group having 2 to 9 carbon atoms having an oxygen atom due to an ether bond in the structure is preferably used.
In the above general formula (I), the monovalent organic residue having 1 to 11 carbon atoms represented by R ³ is a linear, branched or cyclic substituted having 1 to 10 carbon atoms. Suitable alkyl groups and optionally substituted aromatic groups having 6 to 11 carbon atoms are preferred. Among these, C6-C2 aromatic groups, such as a C1-C2 alkyl group which is a methyl group or an ethyl group, a phenyl group, and a benzyl group, are used suitably.

  When each of the organic residues is an optionally substituted group, the substituent is divided into a group containing a carbon atom and a group not containing a carbon atom. First, when the substituent is a group containing a carbon atom, the carbon atom is counted in the carbon number of the organic residue. Examples of the group containing a carbon atom include, but are not limited to, a carboxyl group and an alkoxy group. Next, examples of the group not containing a carbon atom include, but are not limited to, a hydroxyl group and a halo group.

  Examples of the vinyl ether group-containing (meth) acrylic acid esters include, but are not limited to, for example, 2-vinyloxyethyl (meth) acrylate, 3-vinyloxypropyl (meth) acrylate, and 1-methyl (meth) acrylate. 2-vinyloxyethyl, 2-vinyloxypropyl (meth) acrylate, 4-vinyloxybutyl (meth) acrylate, 1-methyl-3-vinyloxypropyl (meth) acrylate, 1-vinyloxymethyl (meth) acrylate Propyl, 2-methyl-3-vinyloxypropyl (meth) acrylate, 1,1-dimethyl-2-vinyloxyethyl (meth) acrylate, 3-vinyloxybutyl (meth) acrylate, 1-methyl- (meth) acrylate 2-vinyloxypropyl, 2-vinyloxybutyl (meth) acrylate, (meth) a 4-vinyloxycyclohexyl silylate, 6-vinyloxyhexyl (meth) acrylate, 4-vinyloxymethylcyclohexylmethyl (meth) acrylate, 3-vinyloxymethylcyclohexylmethyl (meth) acrylate, (meth) acrylic acid 2-vinyloxymethylcyclohexylmethyl, p-vinyloxymethylphenylmethyl (meth) acrylate, m-vinyloxymethylphenylmethyl (meth) acrylate, o-vinyloxymethylphenylmethyl (meth) acrylate, (meth) 2- (vinyloxyethoxy) ethyl acrylate, 2- (vinyloxyisopropoxy) ethyl (meth) acrylate, 2- (vinyloxyethoxy) propyl (meth) acrylate, 2- (vinyloxy) (meth) acrylate Ethoxy) isopropyl, (meth) acrylic acid 2- Vinyloxyisopropoxy) propyl, 2- (vinyloxyisopropoxy) isopropyl (meth) acrylate, 2- (vinyloxyethoxyethoxy) ethyl (meth) acrylate, 2- (vinyloxyethoxyisopropoxy) (meth) acrylate ) Ethyl, 2- (vinyloxyisopropoxyethoxy) ethyl (meth) acrylate, 2- (vinyloxyisopropoxyisopropoxy) ethyl (meth) acrylate, 2- (vinyloxyethoxyethoxy) propyl (meth) acrylate 2- (vinyloxyethoxyisopropoxy) propyl (meth) acrylate, 2- (vinyloxyisopropoxyethoxy) propyl (meth) acrylate, 2- (vinyloxyisopropoxyisopropoxy) propyl (meth) acrylate, (Meth) acrylic acid 2- (vinylo) Xylethoxyethoxy) isopropyl, (meth) acrylic acid 2- (vinyloxyethoxyisopropoxy) isopropyl, (meth) acrylic acid 2- (vinyloxyisopropoxyethoxy) isopropyl, (meth) acrylic acid 2- (vinyloxyisopropoxy) Isopropoxy) isopropyl, (meth) acrylic acid 2- (vinyloxyethoxyethoxyethoxyethoxy) ethyl, (meth) acrylic acid 2- (vinyloxyethoxyethoxyethoxyethoxy) ethyl, (meth) acrylic acid 2- (isopropenoxyethoxy) ) Ethyl, (meth) acrylic acid 2- (isopropenoxyethoxyethoxyethoxy) ethyl, (meth) acrylic acid 2- (isopropenoxyethoxyethoxyethoxy) ethyl, (meth) acrylic acid 2- (isopropenoxyethoxyethoxyethoxy) D ) Ethyl, and (meth) Polyethylene glycol monovinyl ether acrylate, and (meth) acrylic acid polypropylene glycol monovinyl ether.

  Among these, since the viscosity of the ink can be further lowered, the flash point is high, and the curability of the ink is further improved, 2- (vinyloxyethoxy) ethyl (meth) acrylate, that is, 2- (vinyloxyacrylate) At least one of (roxyethoxy) ethyl and 2- (vinyloxyethoxy) ethyl methacrylate is preferable, and 2- (vinyloxyethoxy) ethyl acrylate is more preferable. In particular, 2- (vinyloxyethoxy) ethyl acrylate and 2- (vinyloxyethoxy) ethyl methacrylate each have a simple structure and a small molecular weight, so that the viscosity of the ink can be further reduced. Examples of 2- (vinyloxyethoxy) ethyl (meth) acrylate include 2- (2-vinyloxyethoxy) ethyl (meth) acrylate and 2- (1-vinyloxyethoxy) ethyl (meth) acrylate. Examples of 2- (vinyloxyethoxy) ethyl acrylate include 2- (2-vinyloxyethoxy) ethyl acrylate and 2- (1-vinyloxyethoxy) ethyl acrylate. Incidentally, 2- (vinyloxyethoxy) ethyl acrylate is superior in terms of curability compared to 2- (vinyloxyethoxy) ethyl methacrylate.

A vinyl ether group containing (meth) acrylic acid ester may be used individually by 1 type, and may be used in combination of 2 or more type.
The content of the vinyl ether group-containing (meth) acrylic acid esters is preferably 10 to 70% by mass in order to further improve the curability with respect to the total mass (100% by mass) of the ink.

The method for producing the vinyl ether group-containing (meth) acrylic acid esters is not limited to the following, but is a method of esterifying (meth) acrylic acid and a hydroxyl group-containing vinyl ether (Production Method B), (meth) acrylic acid halide. And esterification of (meth) acrylic anhydride and hydroxyl group-containing vinyl ether (production method D), esterification of (meth) acrylic acid ester and hydroxyl group-containing vinyl ether Method of exchange (Production method E), Method of esterifying (meth) acrylic acid and halogen-containing vinyl ether (Method of production F), Method of esterifying alkali (earth) metal salt of (meth) acrylic acid and halogen-containing vinyl ether (Production G), hydroxyl group-containing (meth) acrylic acid ester and carboxylic acid vinyl ester How to vinyl exchange and Le (Procedure H), hydroxyl group-containing (meth) How to ether exchange and acrylic acid ester and an alkyl vinyl ether (Process I).
Among these, since the desired effect can be further exhibited in this embodiment, the production method E is preferable.

The above monofunctional (meth) acrylate may be used individually by 1 type, and may be used in combination of 2 or more type.
The monofunctional (meth) acrylate may be contained in the ink as long as the desired effect is not impaired in the present embodiment. The content (total) of the monofunctional (meth) acrylate is preferably 70% by mass or less with respect to the total mass (100% by mass) of the ink.

(Bifunctional (meth) acrylate)
Examples of the bifunctional (meth) acrylate include, but are not limited to, triethylene glycol di (meth) acrylate, tetraethylene glycol di (meth) acrylate, polyethylene glycol di (meth) acrylate, and dipropylene glycol di (meth). Acrylate, tripropylene glycol di (meth) acrylate, polypropylene glycol di (meth) acrylate, 1,4-butanediol di (meth) acrylate, 1,6-hexanediol di (meth) acrylate, 1,9-nonanediol di (Meth) acrylate, neopentyl glycol di (meth) acrylate, dimethylol tricyclodecane di (meth) acrylate, EO (ethylene oxide) adduct di (meth) acrylate of bisphenol A, bisphenol PO Nord A (propylene oxide) adduct di (meth) acrylate, hydroxypivalic acid neopentyl glycol di (meth) acrylate, and polytetramethylene glycol di (meth) acrylate.

The above bifunctional (meth) acrylate may be used individually by 1 type, and may be used in combination of 2 or more type.
Bifunctional (meth) acrylate is good to be contained in an ink in the range which does not impair the desired effect in this embodiment. The content of the bifunctional (meth) acrylate is preferably 40% by mass or less with respect to the total mass (100% by mass) of the ink.
When the content of the bifunctional (meth) acrylate is within the above range, it is possible to prevent the ink from increasing in viscosity and to ensure good flexibility of the recorded matter.

(Trifunctional or higher (meth) acrylate)
The trifunctional or higher functional (meth) acrylate is not limited to the following. For example, trimethylolpropane tri (meth) acrylate, EO-modified trimethylolpropane tri (meth) acrylate, pentaerythritol tri (meth) acrylate, ethoxylated isocyanuric Acid triacrylate, pentaerythritol tetra (meth) acrylate, dipentaerythritol hexa (meth) acrylate, ditrimethylolpropane tetra (meth) acrylate, glycerin propoxytri (meth) acrylate, cowprolactone modified trimethylolpropane tri (meth) acrylate , Pentaerythritol ethoxytetra (meth) acrylate, and caprolactam-modified dipentaerythritol hexa (meth) acrylate

Trifunctional or higher functional (meth) acrylates may be used alone or in combination of two or more.
Trifunctional or higher functional (meth) acrylates may be included in the ink as long as the desired effects of the present embodiment are not impaired. The content of the trifunctional or higher (meth) acrylate is preferably 40% by mass or less with respect to the total mass (100% by mass) of the ink.
When the content of the trifunctional or higher (meth) acrylate is within the above range, it is possible to prevent the ink from increasing in viscosity and to ensure good flexibility of the recorded matter.

(Polymerizable compounds other than the above)
In addition to the above-mentioned vinyl ether group-containing (meth) acrylic acid esters, monofunctional (meth) acrylates, and polyfunctional (meth) acrylates, various conventionally known monofunctional and polyfunctional monomers and oligomers are also used. It is possible (hereinafter referred to as “other polymerizable compounds”). Examples of the monomer include unsaturated carboxylic acids such as itaconic acid, crotonic acid, isocrotonic acid and maleic acid, salts or esters thereof, urethanes, amides and anhydrides thereof, acrylonitrile, styrene, various unsaturated polyesters, unsaturated monomers. Examples include saturated polyethers, unsaturated polyamides, and unsaturated urethanes. Moreover, as said oligomer, the oligomer formed from said monomer is mentioned, for example.

Another polymeric compound may be used individually by 1 type, and may be used in combination of 2 or more type.
Other polymerizable compounds may be included in the ink as long as the desired effects of the present embodiment are not impaired.

  Although it is possible to omit the addition of a photopolymerization initiator by using a photopolymerizable compound as the polymerizable compound, the use of a photopolymerization initiator can easily adjust the start of polymerization. Can be preferred.

(Photopolymerization initiator)
The ink of this embodiment may contain a photopolymerization initiator. The photopolymerization initiator is used to form a print by curing the ink present on the surface of the printing medium by photopolymerization by ultraviolet irradiation. By using ultraviolet light (UV) among light, it is excellent in safety and the cost of the light source lamp can be suppressed. There is no limitation as long as it generates active species such as radicals and cations by the energy of ultraviolet rays and initiates the polymerization of the polymerizable compound, but a photoradical polymerization initiator or a photocationic polymerization initiator should be used. Among them, it is preferable to use a photo radical polymerization initiator.

  Examples of the photo radical polymerization initiator include aromatic ketones, acyl phosphine oxide compounds, aromatic onium salt compounds, organic peroxides, thio compounds (thioxanthone compounds, thiophenyl group-containing compounds, etc.), hexaarylbiphenyls, and the like. Examples include imidazole compounds, ketoxime ester compounds, borate compounds, azinium compounds, metallocene compounds, active ester compounds, compounds having a carbon halogen bond, and alkylamine compounds.

  Among these, since it is possible to further improve the curability of the ink, it is preferable to use at least one of an acyl phosphine oxide compound and a thioxanthone compound, and it is more preferable to use an acyl phosphine oxide compound. More preferably, an acylphosphine oxide compound and a thioxanthone compound are used in combination.

  Specific examples of the photo radical polymerization initiator include acetophenone, acetophenone benzyl ketal, 1-hydroxycyclohexyl phenyl ketone, 2,2-dimethoxy-2-phenylacetophenone, xanthone, fluorenone, benzaldehyde, fluorene, anthraquinone, triphenylamine. , Carbazole, 3-methylacetophenone, 4-chlorobenzophenone, 4,4′-dimethoxybenzophenone, 4,4′-diaminobenzophenone, Michler's ketone, benzoin propyl ether, benzoin ethyl ether, benzyldimethyl ketal, 1- (4-isopropylphenyl) ) -2-Hydroxy-2-methylpropan-1-one, 2-hydroxy-2-methyl-1-phenylpropan-1-one, thioxanthone, diethyl Oxanthone, 2-isopropylthioxanthone, 2-chlorothioxanthone, 2-methyl-1- [4- (methylthio) phenyl] -2-morpholino-propan-1-one, bis (2,4,6-trimethylbenzoyl) -phenyl Phosphine oxide, 2,4,6-trimethylbenzoyl-diphenyl-phosphine oxide, 2,4-diethylthioxanthone, and bis- (2,6-dimethoxybenzoyl) -2,4,4-trimethylpentylphosphine oxide Can be mentioned.

  Examples of commercially available photo radical polymerization initiators include IRGACURE 651 (2,2-dimethoxy-1,2-diphenylethane-1-one), IRGACURE 184 (1-hydroxy-cyclohexyl-phenyl-ketone), DAROCUR 1173. (2-hydroxy-2-methyl-1-phenyl-propan-1-one), IRGACURE 2959 (1- [4- (2-hydroxyethoxy) -phenyl] -2-hydroxy-2-methyl-1-propane- 1-one), IRGACURE 127 (2-hydroxy-1- {4- [4- (2-hydroxy-2-methyl-propionyl) -benzyl] phenyl} -2-methyl-propan-1-one}, IRGACURE 907 (2-Methyl-1- (4-methylthiophenyl) -2-morphol Linopropan-1-one), IRGACURE 369 (2-benzyl-2-dimethylamino-1- (4-morpholinophenyl) -butanone-1), IRGACURE 379 (2- (dimethylamino) -2-[(4- Methylphenyl) methyl] -1- [4- (4-morpholinyl) phenyl] -1-butanone), DAROCUR TPO (2,4,6-trimethylbenzoyl-diphenyl-phosphine oxide), IRGACURE 819 (bis (2, 4,6-trimethylbenzoyl) -phenylphosphine oxide), IRGACURE 784 (bis (η5-2,4-cyclopentadien-1-yl) -bis (2,6-difluoro-3- (1H-pyrrole-1-) Yl) -phenyl) titanium), IRGACURE OXE 01 (1.2-octanedione, 1- [4- (phenylthio)-, 2- (O-benzoyloxime)]), IRGACURE OXE 02 (ethanone, 1- [9-ethyl-6- (2-methylbenzoyl) -9H-carbazol-3-yl]-, 1- (O-acetyloxime)), IRGACURE 754 (oxyphenylacetic acid, 2- [2-oxo-2-phenylacetoxyethoxy] ethyl ester and oxyphenylacetic acid, 2- (Mixture of (2-hydroxyethoxy) ethyl ester) (above, trade name manufactured by BASF), KAYACURE DETX-S (2,4-diethylthioxanthone) (trade name, manufactured by Nippon Kayaku Co., Ltd.) ), Speedcure TPO (2,4,6-trimethylbenzoyl-diphenyl-phosphine oxide), Spe dcure DETX (2,4-diethylthioxanthen-9-one) (Lambson's product name), Lucirin TPO, LR8883, LR8970 (above, BASF's product name), and Ubekrill P36 (UCB's product name) ) And the like.

A photoinitiator may be used individually by 1 type and may be used in combination of 2 or more type.
The content of the photopolymerization initiator can improve the ultraviolet curing rate and have excellent curability, and in order to avoid undissolved photopolymerization initiator and coloring derived from the photopolymerization initiator, It is preferably 20% by mass or less with respect to the total mass (100% by mass) of the ink.

In particular, when the photopolymerization initiator includes an acyl phosphine oxide compound, the content thereof is more preferably 5 to 15% by mass, and 7 to 13% by mass with respect to the total mass (100% by mass) of the ink. % Is more preferable. When the content is not less than the above lower limit, the curability is further improved. More specifically, the curability is further improved because a sufficient curing speed can be obtained particularly when curing with an LED (preferable emission peak wavelength: 350 nm to 420 nm). On the other hand, when the content is not more than the above upper limit value, the solubility of the photopolymerization initiator is further improved.
Here, when the content of the phenoxyethyl (meth) acrylate is within the above-mentioned preferable range and the content of the acylphosphine oxide compound is within the above-mentioned preferable range, the solubility of the acylphosphine oxide compound is further increased. This is very preferable because it is excellent, the curability of the ink is further improved, and the viscosity of the ink can be further reduced. Thus, there is a correlation between the content of phenoxyethyl (meth) acrylate and the content of the acylphosphine oxide compound.

  Moreover, when a photoinitiator contains a thioxanthone compound, it is preferable that the content is 1-5 mass% with respect to the total mass (100 mass%) of an ink.

[Pigment]
The ink of this embodiment includes a pigment as a color material.

  In the present embodiment, any of inorganic pigments and organic pigments can be used as the pigment.

  As the inorganic pigment, carbon black (CI pigment black 7) such as furnace black, lamp black, acetylene black and channel black, iron oxide, and titanium oxide can be used.

  Organic pigments include insoluble azo pigments, condensed azo pigments, azo lakes, chelate azo pigments, etc., phthalocyanine pigments, perylene and perinone pigments, anthraquinone pigments, quinacridone pigments, dioxane pigments, thioindigo pigments, isoindolinone pigments, quinophthalone pigments, etc. Polycyclic pigments, dye chelates (eg basic dye chelates, acid dye chelates, etc.), dye rakes (basic dye rakes, acid dye rakes), nitro pigments, nitroso pigments, aniline black, daylight A fluorescent pigment is mentioned.

Among the inks of the present embodiment, white ink is preferable because a desired surface roughness can be easily obtained in the present embodiment. The reason why white ink, particularly white ink containing a white pigment is preferable will be described in more detail. White ink tends to have a significantly higher pigment content than other color inks. And when there is notably much pigment content, desired surface roughness will become easy to be obtained.
As the white pigment used as the white ink, for example, a white inorganic pigment, a white organic pigment, or white hollow resin particles can be used. Examples of white inorganic pigments include alkaline earth metal sulfates such as barium sulfate, alkaline earth metal carbonates such as calcium carbonate, silicas such as fine silicate and synthetic silicate, calcium silicate, alumina, and alumina. Examples thereof include metal compounds such as hydrate, titanium oxide, and zinc oxide, and talc and clay.

  Examples of white organic pigments include organic compound salts disclosed in JP-A No. 11-129613 and alkylene bismelamine derivatives disclosed in JP-A Nos. 11-140365 and 2001-234093. Specific examples of the white organic pigment include Shigenox OWP, Shigenox OWPL, Shigenox FWP, Shigenox FWG, Shigenox UL, and Shigenox U (all trade names manufactured by Hackol Chemical Co., Ltd.). Resin particles can be used as the white organic pigment, and white hollow resin particles can be used as the resin particles. Examples of the white hollow resin particles include particles which are disclosed in US Pat. No. 4,089,800 and which are substantially made of an organic polymer and exhibit thermoplasticity.

  The pigment used in the white ink is not limited to the following. I. Pigment White 1 (basic lead carbonate), 4 (zinc oxide), 5 (mixture of zinc sulfide and barium sulfate), 6 (titanium oxide), 6: 1 (titanium oxide containing other metal oxides), 7 (Zinc sulfide), 18 (calcium carbonate), 19 (clay), 20 (titanium mica), 21 (barium sulfate), 22 (natural barium sulfate), 23 (gloss white), 24 (alumina white), 25 (gypsum) ), 26 (magnesium oxide / silicon oxide), 27 (silica), and 28 (anhydrous calcium silicate).

  Among these, the ink of the present embodiment preferably contains titanium oxide, which is a white inorganic pigment, because it has excellent concealability, colorability, and dispersed particle size, and good visibility (brightness) is obtained.

  Among the titanium oxides, rutile type titanium oxide, which is a general white pigment, is preferable. This rutile type titanium oxide may be produced by itself or may be commercially available. As an industrial production method for producing rutile-type titanium oxide (in powder form) by itself, conventionally known sulfuric acid method and chlorine method can be mentioned.

  Examples of commercially available rutile titanium oxide include Tipaque (registered trademark) CR-60-2, CR-67, R-980, R-780, R-850, R-980, R-630, R- 670, PF-736, etc. (named above, trade names manufactured by ISHIHARA SANGYO KAISHA, LTD.).

  150-500 nm is preferable and, as for the 50% average particle diameter (henceforth "D50") of titanium oxide, 150-300 nm is more preferable. If D50 is within the above range, the ink curability and the rub resistance of the cured ink are excellent, and the recorded image can have excellent visibility. An image can be formed.

Here, “50% average particle diameter of titanium oxide” in this specification means D50 of titanium oxide existing in the ink, not D50 of titanium oxide before the ink is prepared. Further, “50% average particle diameter” in the present specification means a 50% average particle diameter in terms of a sphere by a dynamic light scattering method, and is a value obtained as follows.
The particles in the dispersion medium are irradiated with light, and the generated diffracted scattered light is measured by detectors arranged in front, side, and rear of the dispersion medium. Using the measured values obtained, assuming that particles that are originally indefinite are spherical, a cumulative curve is obtained with the total volume of the particle population converted to a sphere equal to the volume of the particles as 100%. The point at which the cumulative value at that time is 50% is defined as “50% average particle diameter (D50) in terms of sphere by dynamic light scattering method”.

  The content of titanium oxide is excellent in curability, hardly settles, and has excellent concealability and color reproducibility (especially on a black print medium), so that the total mass (100% by mass) of the ink is achieved. On the other hand, 5-30 mass% is preferable, 10-30 mass% is more preferable, 15-25 mass% is further more preferable.

  The said pigment may be used individually by 1 type, and may use 2 or more types together.

[Dispersant]
When the ink of this embodiment contains a pigment, it may further contain a dispersant in order to improve pigment dispersibility. Although it does not specifically limit as a dispersing agent, For example, the dispersing agent currently used for preparing pigment dispersion liquids, such as a polymer dispersing agent, is mentioned. Specific examples include polyoxyalkylene polyalkylene polyamines, vinyl polymers and copolymers, acrylic polymers and copolymers, polyesters, polyamides, polyimides, polyurethanes, amino polymers, silicon-containing polymers, sulfur-containing polymers, fluorine-containing polymers, and epoxies. The thing which has 1 or more types of resin as a main component is mentioned. Commercially available polymer dispersants include Ajinomoto Fine Techno Co., Ltd. Ajisper series (trade name), Solsperse series available from Avecia Co. (Solsperse 32000, 36000, etc. [above, trade name]) , BYK Chemie's Disperbic series (trade name) and Enomoto Kasei's Disparon series (trade name).

  A dispersing agent may be used individually by 1 type, and may be used in combination of 2 or more type. The content of the dispersant is not particularly limited, and a preferable amount may be added as appropriate.

(Polymerization inhibitor)
The ink of this embodiment may further contain a polymerization inhibitor. When the ink contains a polymerization inhibitor, the polymerization reaction of the polymerizable compound before curing can be prevented.

  Although it does not restrict | limit especially as a polymerization inhibitor, For example, a phenol type polymerization inhibitor is mentioned. Examples of the phenol-based polymerization inhibitor include, but are not limited to, for example, p-methoxyphenol, cresol, t-butylcatechol, di-t-butylparacresol, hydroquinone monomethyl ether, α-naphthol, 3,5-di- t-butyl-4-hydroxytoluene, 2,6-di-t-butyl-4-methylphenol, 2,2'-methylene-bis (4-methyl-6-t-butylphenol), 2,2'-methylene -Bis (4-ethyl-6-butylphenol) and 4,4'-thio-bis (3-methyl-6-tert-butylphenol).

  Examples of commercially available phenolic polymerization inhibitors include p-methoxyphenol (trade name, p-methoxyphenol, manufactured by Tokyo Chemical Industry Co., Ltd.), non-flex MBP (Seiko Chemical Co., Ltd. ( Seiko Chemical Co., Ltd., trade name, 2,2′-methylene-bis (4-methyl-6-tert-butylphenol)), BHT Swanox (trade name, 2,6-di-t, manufactured by Seiko Chemical Co., Ltd.) -Butyl-4-methylphenol).

  A polymerization inhibitor may be used individually by 1 type, and may be used in combination of 2 or more type. The content of the polymerization inhibitor is not particularly limited, and a preferable amount may be added as appropriate.

[Surfactant]
The ink of this embodiment may further contain a surfactant. The surfactant is not particularly limited. For example, polyester-modified silicone or polyether-modified silicone can be used as the silicone-based surfactant, and polyether-modified polydimethylsiloxane or polyester-modified polydimethylsiloxane can be used. Particularly preferred. Examples of commercially available slip agents include BYK-347, BYK-348, BYK-UV3500, 3510, 3530, and 3570 (manufactured by BYK).

  Surfactant may be used individually by 1 type and may be used in combination of 2 or more type. The content of the surfactant is not particularly limited, and a preferable amount may be added as appropriate.

[Other additives]
The ink of this embodiment may contain additives (components) other than the additives listed above. Such components are not particularly limited, and may include, for example, conventionally known polymerization accelerators, penetration enhancers, wetting agents (humectants), and other additives. Examples of the other additives include conventionally known fixing agents, antifungal agents, preservatives, antioxidants, ultraviolet absorbers, chelating agents, pH adjusting agents, and thickeners.

  Thus, according to the present embodiment, it is possible to provide a recording method using an ultraviolet curable ink that is excellent in embedding property and concealing property in a recorded matter.

[Printed media]
The printing medium used in the recording method of the above embodiment is a non-ink-absorbing or low-absorbing printing medium.

  Among the above-mentioned printing media, examples of the non-ink-absorbing printing media include substrates such as plastic films and paper that are not surface-treated for inkjet recording (that is, have no ink absorbing layer formed). Examples include those coated with plastic or those having a plastic film adhered thereto. Examples of the plastic herein include polyvinyl chloride (vinyl chloride), polyethylene terephthalate (PET), polycarbonate (PC), polystyrene (PS), polyurethane (PU), polyethylene (PE), polypropylene (PP), and the like. Examples of the ink-absorptive printing medium include printing paper such as art paper, coated paper, and matte paper.

  Hereinafter, the present embodiment will be described more specifically with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples.

[material]
The materials used in the examples and comparative examples are as shown below.
[Pigment]
・ Titanium oxide:
PT501R (trade name manufactured by Ishihara Sangyo Co., Ltd., for D50 = 150 nm), CR60 (trade name manufactured by Ishihara Sangyo Co., Ltd., for D50 = 200 nm), CR93 (trade name manufactured by Ishihara Sangyo Co., Ltd., for D50 = 300 nm), KR380 (Titanium Industry Co., Ltd.) Product name, D50 = 400nm)
Titanium oxides with D50 for 150 nm, 200 nm, 300 nm, and 400 nm were prepared. Preparation of titanium oxide having a D50 of 500 nm and a pigment dispersion will be described later.

[Polymerizable compound]
VEEA (2- (2-vinyloxyethoxy) ethyl acrylate, a product name manufactured by Nippon Shokubai Co., Ltd., hereinafter abbreviated as VEEA)
-Biscote # 192 (phenoxyethyl acrylate, trade name of OSAKA ORGANIC CHEMICAL INDUSTRY LTD., Abbreviated as PEA below)
A-9300 (ethoxylated isocyanuric acid triacrylate, manufactured by Shin-Nakamura Chemical Co., Ltd., hereinafter abbreviated as A-9300)
(Polymerization inhibitor)
P-methoxyphenol (trade name, p-methoxyphenol, manufactured by Tokyo Chemical Industry Co., Ltd., abbreviated as “MEHQ” below)
[Surfactant]
BYK-UV3500 (polyether-modified polydimethylsiloxane, manufactured by BYK) (abbreviated as “BYK3500” in the table)
(Photopolymerization initiator)
IRGACURE 819 (trade name, manufactured by BASF, solid content: 100%, hereinafter abbreviated as “819”)
Speedcure TPO (trade name, manufactured by Lambson, hereinafter abbreviated as “TPO”)
Speedcure DETX (trade name, manufactured by Lambson, hereinafter abbreviated as “DETX”)
[Dispersant]
Solsperse 32000 (Avicia brand name)

[Example 1-1 to Example 16-5]
(Preparation of pigment dispersion)
40% by mass of each of the above titanium oxides, 44% by mass of PEA, and 16% by mass of a dispersant were mixed and stirred to obtain a mixture. Using a sand mill (manufactured by Yaskawa Seisakusho Co., Ltd.), the mixture was dispersed together with zirconia beads (diameter 1.5 mm). Then, the pigment dispersion used for an ink composition was obtained by isolate | separating a zirconia bead with a separator. Here, both titanium oxides for D50 of 400 nm and 500 nm were obtained by controlling the dispersion treatment of KR380. That is, during the dispersion treatment of the mixture using KR380, a pigment dispersion liquid having a D50 of 500 nm was obtained by carrying out dispersion treatment together with zirconia beads (diameter 5.0 mm).
Thus, pigment dispersions (hereinafter also referred to as “dispersions”) 1, 2, 3, 4, and 5 were prepared. At that time, the concentration of titanium oxide was set to 40% by mass in any of the pigment dispersions.

[Preparation of UV curable ink]
Inks 1 to 7 were prepared by mixing materials with the compositions shown in Table 1 below. In Table 1, the unit of numerical values is mass%.

[Evaluation of sedimentation characteristics]
Each ink was added to the screw tube to a height of 10 cm and left for 1 month. After standing, the screw tube was inverted and the state of the precipitate was observed. The evaluation criteria are as follows. The results are shown in Table 1 below.
A: No precipitate adhered to the bottom surface of the screw tube.
B: The deposit adhered to less than 1/4 of the bottom area of the screw tube.
C: Precipitate adhered to 1/4 or more of the bottom area of the screw tube.

  From Table 1, it was found that the sedimentation characteristics depend particularly on the average particle diameter of the pigment. That is, when the average particle size of the pigment is small, the pigment is difficult to precipitate, whereas when the average particle size of the pigment is large, the pigment is likely to precipitate and easily becomes a cake. The ink 4 and the ink 5 can be used for recording because the precipitation of the pigment can be prevented by increasing the cleaning frequency.

Hereinafter, the recording method in each example will be described.
[Examples 1-1 to 1-5, Examples 2-1 to 2-5, Examples 3-1 to 3-5, Examples 4-1 to 4-5, and Examples 5-1 to 5-5]
A transparent film (Lumirror S10 [trade name] manufactured by Toray Co., Ltd.) was subjected to main scanning while discharging ink from a head mounted on a carriage of an ink jet printer, and was printed. A head with one nozzle row was used.
At the time of the main scanning, the UV curable ink landed on and adhered to the film by the main scanning was temporarily cured for each pass (main scanning) with an LED having a top peak at a wavelength of 395 nm mounted on the carriage. At this time, Firefly (illuminance of 1,000 mW / cm ² ) was used as the LED mounted on the carriage. Moreover, the irradiation energy in 1 main scanning irradiation adjusted the irradiation time by changing the light emission width | variety of the main scanning direction of LED for every branch number of each example, and was changed in the range of 5-200 mJ / cm < ² >. For example, the amount of irradiation energy can be increased by a factor of two by increasing the light emission width (lighting row) of the LEDs in the main scanning direction by a factor of two.
Next, after carrying out sub-scanning in which the film was transported in the sub-scanning direction intersecting with the main scanning direction, the next main scanning was carried out, and the main scanning and sub-scanning were repeated alternately.
After the printing was finished, the UV curable ink was irradiated with a light source (the same type as that mounted on the carriage) provided separately from the carriage to completely cure the uncured ink. The irradiation energy at this time was 200 mJ / cm ² .

Here, the recording conditions (printing conditions) will be supplemented. The recording conditions were in accordance with FIG. 4 and the description thereof except for the following points. The ink used in each example is shown in Tables 2 to 4 below.
First, the number of main scans was 4 × 2 = 8 passes. More specifically, the number of main scans required to form dots at each pixel in the sub-scanning direction corresponding to the recording resolution in the sub-scanning direction is set to 4. On the other hand, the number of times of main scanning required to form dots at each pixel in the main scanning direction corresponding to the recording resolution in the main scanning direction is set to 2.
The driving amount was 7.26 mg / inch ² . At that time, the driving amount per pixel was 14 ng / pixel. The recording resolution (sub-scanning direction × main scanning direction) was 720 dpi × 720 dpi. Furthermore, the nozzle density of the head used was 180 dpi.

[Examples 6-1 to 6-5]
Recording was performed in the same manner as in Example 1-1 to Example 5-5 except that the number of main scans was changed to 4 × 3 = 12 passes. That is, printing was performed so that the number of times the nozzles face the same raster line is increased by shortening the sub-scanning distance as compared with the printing methods of Examples 1-1 to 5-5. The ink used in each example is shown in Table 4 below.

[Examples 7-1 to 7-5]
Recording was performed in the same manner as in Examples 1-1 to 5-5 except that the number of main scans was changed to 4 × 4 = 16 passes. That is, compared with the recording methods of Examples 1-1 to 5-5, the nozzles are arranged on the same raster line by shortening the sub-scanning distance (further than in the case of Examples 6-1 to 6-5). Recording was performed so as to increase the number of times of facing each other. The ink used in each example is shown in Table 5 below.

[Examples 8-1 to 8-5]
Recording was performed in the same manner as in Example 1-1 to Example 5-5 except that the number of main scans was changed to 4 × 1 = 4 passes. That is, as shown in FIG. 10 of Japanese Patent Application Laid-Open No. 2010-167777, dots were formed by one main scan on one raster line, and recording was performed. The ink used in each example is shown in Table 5 below.

[Examples 9-1 to 9-5]
Recording was performed in the same manner as in Examples 1-1 to 5-5 except that the nozzle density of the head was changed to 720 dpi and the number of main scans was changed to 1 × 2 = 2 passes. That is, four heads used in the recording methods of Examples 1-1 to 5-5 are prepared, and the nozzles are displaced by shifting the position of each head in the sub-scanning direction by a quarter of the nozzle pitch distance. A head having a density of 720 dpi was used. Then, under the recording resolution of 720 dpi in the sub-scanning direction, the distance of one sub-scan is half the height of the nozzle row of the head, so that the nozzles face the raster line twice. In this way (the number of main scans is 1 × 2 = 2 passes), recording was performed (recording over the recording area was performed in a manner similar to the POL area shown in FIG. 6B of JP 2010-162766 A). went.). The ink used in each example is shown in Table 6 below.

[Examples 10-1 to 10-5]
Recording was performed in the same manner as in Examples 1-1 to 5-5, except that the amount of implantation was changed to 10 ng / pixel (5.18 mg / inch ² ). The ink used in each example is shown in Table 6 below.

[Examples 11-1 to 11-5]
Recording was performed in the same manner as in Example 1-1 to Example 5-5, except that the driving amount was changed to 20 ng / pixel (10.37 mg / inch ² ). The ink used in each example is shown in Table 7 below.

[Examples 12-1 to 12-5]
As shown in Table 7, ink 6 was used, and recording was performed in the same manner as in Examples 1-1 to 5-5.

[Examples 13-1 to 13-5]
As shown in Table 8, ink 7 was used, and recording was performed in the same manner as in Examples 1-1 to 5-5.

[Examples 14-1 to 14-5]
Recording was performed in the same manner as in Examples 1-1 to 5-5 except that the nozzle density of the head was changed to 360 dpi and the number of main scans was changed to 2 × 2 = 4 passes. That is, two heads used in the recording methods of Examples 1-1 to 5-5 are prepared, and nozzles are shifted by shifting the position of each head in the sub-scanning direction by a half of the nozzle pitch distance. A head having a density of 360 dpi was used. Then, the distance of one sub-scan is made longer than in the case of Examples 1-1 to 5-5, and the raster line that forms dots in one main scan is positioned at an intermediate position in the sub-scan direction. In the next main scanning, the nozzles are opposed to each other, and the nozzle is opposed twice to one raster line under the recording resolution of 720 dpi in the sub-scanning direction (the number of main scanning is 2 × 2 = 4). It was a pass.) The ink used in each example is shown in Table 8 below.

[Examples 15-1 to 15-5]
Recording was performed in the same manner as in Example 1-1 to Example 5-5 except that the nozzle density of the head was changed to 720 dpi and the number of main scans was changed to 1 × 1 = 1 pass. That is, the heads in Examples 9-1 to 9-5 above were used, and the distance of one sub-scan was defined as the height distance of the nozzle row of the head. Then, recording was performed by forming dots at 720 dpi in both the sub-scanning direction and the main-scanning direction in a region corresponding to the height of the nozzle row in one main scanning (a figure shown in Japanese Patent Laid-Open No. 2010-162766). The recording was performed over the recording area by a method similar to the dot forming method of No. 6 normal area).
In this example 15, recording is completed in one main scan in a recording area facing the nozzle row, and recording is performed in a plurality of main scans in the recording area facing the nozzle row of the head. Absent. Therefore, Example 15 corresponds to a comparative example for the present invention. The ink used in each example is shown in Table 9 below.

[Examples 16-1 to 16-5]
As a head, a head in which the number of nozzles in one nozzle row is doubled compared to the heads used in Examples 1-1 to 1-5 (nozzle density is 180 dpi, and the distance in the sub-scanning direction of the nozzle row is double), This nozzle row was filled with ink 1 in the same manner as in Examples 1-1 to 1-5. The length of the two irradiation units mounted on the carriage is also twice the distance in the sub-scanning direction than that used in Examples 1-1 to 1-5, and is arranged beside the nozzle array in the main scanning direction. It was. Further, the ejection amount ejected to one pixel in one unit recording operation is half of Examples 1-1 to 1-5 (7 ng / pixel). Except for the above points, printing was performed in the same manner as in Examples 1-1 to 1-5. Examples 16-1 to 16-5 continued on the dots formed on the film by the same recording method as in Examples 1-1 to 1-5 by using the nozzle rows on the upstream side in the sub-scanning direction of the nozzle rows. The dots are formed by the same recording method as in Examples 1-1 to 1-5 by using half of the nozzle rows on the downstream side in the sub-scanning direction. Although dot formation is performed by one unit recording operation for one pixel, the final driving amount per unit area of the recording medium and the final driving amount per pixel are examples 1-1 to 1-5. Is the same.

[Evaluation / Measurement Items]
[Conversion rate]
For the conversion rate at the time of temporary curing, a sample in which printing was interrupted when main scanning was performed once was separately prepared. The curing rate was measured using an infrared spectrophotometer (NEXUS 470 [trade name], manufactured by Thermo Nicolet Corp.) capable of real-time measurement as a measuring device. The measurement results are shown in Tables 2 to 9 below.

〔Surface roughness〕
As described above, the surface roughness is obtained by performing recording according to the recording conditions shown in each example for the above transparent film using a laser microscope VK-9710 (manufactured by KEYENCE) in accordance with the provisions of JIS B0601. The surface roughness Rq (square root height) of the recorded material was measured. The magnification at the time of measurement was 100 times. The measurement results are shown in Tables 2 to 9 below.

[Concealment]
About the ink coating film obtained by the recording of the said Example 1-1-Example 15-5, the average value of the transmittance | permeability in the wavelength range of 400-800 nm was computed. The evaluation criteria are as follows. The results are shown in Tables 2 to 9 below.
AA: less than 1% A: 1% or more and less than 2% B: 2% or more and less than 3% C: 3% or more When the evaluation result is AA to B, the average transmittance in the wavelength range of 190 to 400 nm is Since it is lower than the average transmittance in the wavelength range of 400 to 800 nm (the visible light region wavelength of the white ink cured film), it can be evaluated that it is excellent in concealability.

[Fillability]
The ink cured film in the above hiding evaluation was observed with a magnifying glass (10 times). The evaluation criteria are as follows. The results are shown in Tables 2 to 9 below.
A: The substrate could not be seen by visual observation or observation with a magnifying glass.
B: Filling was confirmed visually, but spots that were not filled with a loupe were observed.
C: Spots or streaks on the ground were visually observed (not buried).

In Tables 2 to 9 below, “D50” represents a 50% average particle diameter of titanium oxide, which is a white pigment, and its unit is nm. The unit of “irradiation energy (−)” is mJ / cm ² . The unit of “conversion” is%.

  From the above results, the following became clear. First, from Examples 1 to 5 in which D50 of titanium oxide was changed, it was found that the greater the D50, the better the concealability. Further, from Example 1-1 to Example 5-5 (comparison between branch numbers in each example), when the surface roughness is less than 1.5, the surface of the ink cured film (cured product) is uniform and buried. It was found that it was inferior in concealment, though it was excellent. The reason why the concealability is inferior when the surface roughness is small is presumed to be that the irregular reflection on the surface of the ink coating film is reduced. On the other hand, when the surface roughness exceeds 5.0, it is estimated that the embedding property is poor, and thus the concealing property is inferior. Hereinafter, the recording conditions in Examples 1-1 to 5-5 will be considered as a reference.

  Next, when the number of main scans (the number of passes) is 8 passes (= 4 × 2) or more (Example 6-1 to Example 7-5), the filling is further improved and the concealment property is also excellent. I understood. On the other hand, when the number of passes for forming dots in the main scanning direction is 1 pass (Examples 8-1 to 8-5), it was found that the surface roughness tends to be low and the concealability is slightly inferior. It was also found that the amount of irradiation energy needs to be increased in order to make the surface roughness 1.5 or more.

  Next, from Examples 9-1 to 9-5, attention was paid to the number of passes for forming dots in the sub-scanning direction. When the nozzle density of the head is increased fourfold from 180 dpi to 720 dpi and the number of passes for forming dots in the sub-scanning direction is one pass, the surface roughness is similar to the results of Examples 8-1 to 8-5. Tended to be low, and it was found that the concealability was slightly inferior. It was also found that the amount of irradiation energy needs to be increased in order to make the surface roughness 1.5 or more. On the other hand, from Examples 14-1 to 14-5, the nozzle density of the head is doubled from 180 dpi to 360 dpi, and the number of passes is changed from 8 passes (= 4 × 2) to 4 passes (= 2 × 2). When the number of passes for forming dots in the main scanning direction and the number of passes for forming dots in the sub-scanning direction are both two passes, the results of Examples 1-1 to 1-5 It was found that the filling is good and the concealing property is excellent.

  Next, from Examples 10-1 to 10-5 and Examples 11-1 to 11-5, it was found that the surface roughness tends to decrease when the driving amount is increased or decreased. Accordingly, the lower the conversion rate, the smaller the surface roughness, and thus a tendency to adversely affect the embedding and concealing properties was observed.

  Next, from Examples 12-1 to 12-5 and Examples 13-1 to 13-5, when the content of titanium oxide was increased or decreased, the embedding and concealing properties were further improved particularly when the content was increased. I found out that it would be a thing.

  Next, in Examples 15-1 to 15-5, when the number of passes is increased from 180 dpi to 720 dpi and the number of passes is set to 1 pass (one unit recording operation), the surface roughness tends to decrease. Yes, it was found to be inferior in concealment. The purpose of Example 15 is to show the difference in the effect of the recording method after aligning the recording resolution and the shot amount with Example 1. The recording conditions of Examples 15-1 to 15-5 are the same as the recording conditions of the line printer.

  Next, Examples 16-1 to 16-5 are recording methods similar to those of Examples 1-1 to 1-5, but dots are formed in one pixel by two unit recording operations. Compared with Examples 1-1 to 1-5, the embedding property was better and the concealing property was also excellent.

DESCRIPTION OF SYMBOLS 1 ... Printer, 10 ... Conveyance unit, 11 ... Feed roller, 13 ... Conveyance roller, 14 ... Platen, 15 ... Discharge roller, 20 ... Carriage unit, 21 ... Carriage, 24 ... Guide shaft, 30 ... Head unit, 31 ... head, 40 ... irradiation unit, 42a, 42b ... provisional curing irradiation unit, 43 ... main curing irradiation unit, 50 ... detector group, 53 ... paper detection sensor, 54 ... optical sensor, 60 ... controller, 61 ... interface Part 62 ... CPU 63 ... memory 64 ... unit control circuit 110 ... computer.

Claims (11)

A step of discharging an ultraviolet curable ink containing at least a white pigment from a head toward a printing medium, and irradiating the ultraviolet curable ink attached to the printing medium with ultraviolet rays, thereby the ultraviolet curable ink. And a unit recording operation including at least a step of curing,
Wherein in a region facing the head of the print medium, the surface roughness (Rq) Ri is 1.5 to 5.0 [mu] m der includes the ultraviolet curable ink to form a cured product obtained by curing, ultraviolet Recording method using curable ink. In one unit recording operation, there are pixels that form dots and pixels that do not form dots for one raster line, and one raster line is formed by performing a plurality of unit recording operations. Or in one unit recording operation, there is a raster line for forming dots in another unit recording operation between the sub-scanning directions of the raster lines for forming dots,
The recording method using the ultraviolet curable ink according to claim 1, which is at least one of the above. In one unit recording operation, there are pixels that form dots and pixels that do not form dots for one raster line, one raster line is formed by performing unit recording operations a plurality of times, and
2. The ultraviolet curable ink according to claim 1, wherein in one unit recording operation, a raster line for forming dots in another unit recording operation exists between the sub-scan directions of the raster lines for forming dots. 3. Recording method using   The recording method using the ultraviolet curable ink according to claim 1, wherein the ultraviolet curable ink is a white ink.   The recording method using ultraviolet curable ink according to claim 2 or 3, wherein the pixel for forming dots has a pixel for forming dots by performing unit recording operations for one pixel a plurality of times.   The recording method using the ultraviolet curable ink according to claim 1, wherein the conversion rate of the ultraviolet curable ink adhered and cured in each unit recording operation is in a range of 20 to 90%. . The recording method using the ultraviolet curable ink according to claim 1, wherein the ultraviolet curable ink contains titanium oxide as the white pigment .   The recording method using the ultraviolet curable ink according to claim 7, wherein the titanium oxide has an average particle diameter D50 in a range of 150 to 500 nm.   The recording method using the ultraviolet curable ink according to claim 7 or 8, wherein a content of the titanium oxide is 10 to 30% by mass with respect to a total mass of the ultraviolet curable ink. The recording method using the ultraviolet curable ink according to claim 1, wherein an adhesion amount of the ultraviolet curable ink to a printing medium is 5 to 16 mg / inch ² . Means for discharging ultraviolet curable ink containing at least a white pigment from a head toward a printing medium; and irradiating the ultraviolet curable ink attached to the printing medium with ultraviolet rays, thereby the ultraviolet curable ink. A unit recording operation consisting of a means for curing
Wherein in a region facing the head of the print medium, the surface roughness (Rq) is 1.5 to 5.0 [mu] m der is, one in which the cured product of the ultraviolet curable ink is cured is formed, A recording apparatus using ultraviolet curable ink.