Patent Publication Number: US-8967795-B2

Title: Printing apparatus

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates to a printing apparatus. 
     2. Related Art 
     Printing apparatuses such as ink jet recording apparatuses, as disclosed in, for example, JP-A-2010-260341, are conventionally used for printing performed by applying ink on a flexible sheet-type recording or printing medium. The printing apparatus disclosed in JP-A-2010-260341 includes a transfer mechanism that transfers a printing medium, a plate-shaped platen, including a platen cover, that supports the transferred printing medium from the rear side (lower side), a head unit that applies an ink to the printing medium supported by the platen, and a heater that heats the printing medium and the ink deposited on the printing medium together to help the ink adhere to the printing medium. In this printing apparatus, the transfer mechanism includes a plurality of transfer rollers arranged in a direction intersecting the transfer direction of the printing medium, immediately upstream of the platen in the transfer direction. 
     The platen has a plurality of through holes that are open at the upper surface thereof and arranged in a staggered manner. 
     Printing media are expanded when heated in a printing apparatus. In the printing apparatus of the above cited patent document, the thermally expanded portion of the printing medium is likely to be bent and trapped between the transfer rollers. Unfortunately, this causes creases in the printing medium, resulting in some problem such as unclear printing or printing failure. Although the through holes of the platen suck the printing medium to flatten it, the bending or creases of the printing medium are not completely suppressed. 
     SUMMARY 
     An advantage of some aspects of the invention is that it provides a printing apparatus that can certainly prevent unclear printing caused by thermal expansion of the printing medium. 
     According to an aspect of the invention, a printing apparatus is provided which includes a transfer device that transfers a flexible printing medium, a platen including a plate having a support surface that supports thereon the printing medium transferred by the transfer device, a head unit including a liquid ejection head that prints on the printing medium by ejecting an ink onto the printing medium on the support surface, and a heating device that heats the printing medium on which the ink has been ejected. The transfer device is disposed immediately upstream of the platen in the transfer direction, and has a plurality of rollers arranged in a direction intersecting the transfer direction. The platen has a plurality of suction holes that are open at the support surface and suck the printing medium on the support surface. The support surface includes first portions extending in the transfer direction corresponding to extension lines extending in the transfer direction from the portions between each of the rollers, and second portions other than the first portions. The first portions are high-density regions where the suction holes are arranged at a high density, and the second portions are low-density regions where the suction holes are arranged at a low density. 
     A recording medium is likely to be expanded by being heated, and the thermally expanded portion may crease. In the printing apparatus, however, the suction holes of the first portions, which are high-density regions having suction holes with a high density, suck the creases to eliminate. Thus, the thermally expanded portion of the printing medium is prevented from creasing in the high-density regions, and is stretched from the high-density regions and preferentially on the second portions or in the low-density regions. In the printing apparatus, the function of the high-density regions to remove creases and the function of the low-density regions to stretch portions likely to crease produce a synergistic effect of flattening the printing medium on the platen. Consequently, unclear printing resulting from thermal expansion of the printing medium can be certainly prevented. 
     Preferably, the printing medium will be thermally expanded preferentially on the second portions when heated by the heating device. A recording medium is likely to be expanded by being heated, and the thermally expanded portion may crease. In the printing apparatus, however, the suction holes of the first portions, which are high-density regions having suction holes with a high density, suck the creases to eliminate. Thus, the thermally expanded portion of the printing medium is prevented from creasing in the high-density regions, and is stretched from the high-density regions and preferentially on the second portions or in the low-density regions. In the printing apparatus, the function of the high-density regions to remove creases and the function of the low-density regions to stretch portions likely to crease produce a synergistic effect of flattening the printing medium on the platen. Consequently, unclear printing resulting from thermal expansion of the printing medium can be certainly prevented. 
     In each first portion, preferably, two or more of the suction holes are aligned in a line in the transfer direction. Such suction holes of the first portion sufficiently suck the thermally expanded printing medium. 
     Preferably, each second portion has at most one of the suction holes. Consequently, the thermally expanded portion of the printing medium can be certainly stretched in the low-density regions. 
     Preferably, the first portions have the suction hole located most upstream in the transfer direction. Consequently, creases formed in the thermally expanded portion of the printing medium are preferentially and certainly sucked, and the printing medium is thus prevented from creasing. 
     Preferably, the transfer device includes a plurality of holders arranged in a direction intersecting the transfer direction. Each holder holds at least two of the rollers. One or more of the first portions lie corresponding to any of the extension lines extending in the transfer direction from the portions between each of the holders, and have suction holes with a higher density than the other first portions. The printing medium on the platen is more likely to crease in the portions corresponding to the extension lines extending in the transfer direction from the portions between each holder. However, the portions of the first portions on which the printing medium is more likely to crease have suction holes with a higher density than the other first portions, and accordingly preferentially suck creases to eliminate the creases certainly. 
     The printing apparatus may further include a head moving device that transfers the head unit in a direction intersecting the transfer direction. The support surface has a printing region across the first portions and the second portions, over which the head unit prints while being transferred by the head moving device. The suction holes are arranged such that the printing region has suction holes with a higher density than a region of the support surface downstream of the printing region in the transfer direction. 
     The printing medium is thus certainly stabilized by the suction of the suction holes in the printing region. Consequently, the printing apparatus can perform accurate printing on the printing medium on the printing region. The thermally expanded portion of the printing medium is stretched in the transfer direction from the portion adjacent to the printing region, preferentially on the region of the support surface downstream of the printing region. Consequently, the portion to be printed of the printing medium is flattened, and thus, unclear printing on the printing medium can be certainly prevented. 
     Preferably, the density of the suction holes is reduced in the transfer direction. The printing medium is thus certainly stabilized by the suction of the suction holes in the printing region. Consequently, the printing apparatus can perform accurate printing on the printing medium on the printing region. The thermally expanded portion of the printing medium is stretched in the transfer direction from the portion adjacent to the printing region, preferentially on the region of the support surface downstream of the printing region. Consequently, the portion to be printed of the printing medium is flattened, and thus, unclear printing on the printing medium can be certainly prevented. 
     Preferably, the density of the suction holes in each first portion is 1.1 to 10 times as high as the density of the suction holes in each second portion. Consequently, a synergistic effect is markedly produced by eliminating creases in the high-density regions and stretching the portion likely to crease in the low-density region. 
     Preferably, the suction holes are circular in shape when viewed from above and have an average diameter of 4 mm or less. Such suction holes can adequately suck the printing medium. 
     The transfer device may further includes a driving roller that rotates the plurality of the rollers and transfers the printing medium while pinching the printing medium with the plurality of rollers therebetween. The combined use of the driving roller and the plurality of rollers ensures the transfer of the printing medium. 
     Preferably, the heating device includes a heater opposing the support surface with the liquid ejection head therebetween. This structure helps the ink applied on the printing medium to dry, thus certainly drying the ink. 
     According to another aspect of the invention, a printing apparatus is provided which includes a head unit that ejects an ink onto a printing medium, a platen having a support surface that supports the printing medium thereon, and a plurality of rollers that transfer the printing medium in the transfer direction. The support surface has a plurality of suction holes that suck the printing medium on the support surface. The support surface includes first portions extending in the transfer direction corresponding to extension lines extending in the transfer direction from the portions between each of the rollers, and second portions other than the first portions. Each of the first portions has suction holes with a higher density than each of the second portions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
         FIG. 1  is a perspective view of a printing apparatus according to an embodiment of the invention. 
         FIG. 2  is a schematic cross sectional view of the printing apparatus taken along line II-II in  FIG. 1 . 
         FIG. 3  is a plan view of the printing apparatus viewed in the direction indicated by arrow III in  FIG. 2 . 
         FIG. 4  is a plot showing the relationship between the suction power of the platen shown in  FIG. 3  and the number of suction holes in the platen. 
         FIGS. 5A and 5B  are plots showing the reduction of cockling (creases) in a printed portion of a printing medium printed by the printing apparatus shown in  FIG. 1 . 
         FIGS. 6A and 6B  are plots showing the reduction of unevenness (creases) in the entirety of a printing medium printed by the printing apparatus shown in  FIG. 1 . 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     A printing apparatus according to an embodiment of the invention will now be described in detail with reference to the drawings.  FIG. 1  is a perspective view of a printing apparatus of an embodiment of the invention.  FIG. 2  is a schematic cross sectional view of the printing apparatus taken along line II-II in  FIG. 1 .  FIG. 3  is a plan view viewed from the position of arrow II in the direction indicated by arrow II in  FIG. 2 .  FIG. 4  is a plot showing the relationship between the suction power of the platen shown in  FIG. 3  and the number of suction holes in the platen.  FIGS. 5A and 5B  are plots showing the reduction of cockling (creases) in a printed portion of a printing medium printed by the printing apparatus shown in  FIG. 1 .  FIGS. 6A and 6B  are plots showing the reduction of unevenness (creases) in the entirety of a printing medium printed by the printing apparatus shown in  FIG. 1 .  FIGS. 1 to 3  each show three axes orthogonal to each other: x axis, y axis, and z axis for the sake of convenience. The x axis extends in one direction (depth direction of the printing apparatus) on a horizontal plane. The y axis extends in a direction perpendicular to the x axis (longitudinal direction of the printing apparatus) on the horizontal plane. The z axis extends in a vertical direction. In the drawings, each arrow of the x, y and z axes points the positive (+) direction, and the starting point of the arrow is located at the negative (−) side. The upper side in  FIGS. 1 and 2  is the upper side of the printing apparatus, and the lower side in the figures is the lower side of the printing apparatus. 
     As shown in  FIG. 1 , the printing apparatus  1  includes an apparatus body  2 , legs (stands)  3 , and a curing unit  4 , and applies an ink onto a printing medium  100 , thus performing color printing by an ink jet technique. The components of the printing apparatus will be described below. 
     The ink and the printing medium  100  will first be described. In the present embodiment, “latex ink” is used as ink. A cartridge of an ink set is mounted in the printing apparatus  1 . The ink set includes a first ink and a second ink, each having a predetermined composition in either case (A) or case (B) described later. 
     The first ink contains a coloring material, resin particles, a first moisturizing agent, and an aprotic polar solvent. The second ink contains a coloring material with a higher content than the coloring material content of the first ink, resin particles with a lower content than the resin particle content of the first ink, and a second moisturizing agent, and an aprotic polar solvent. Also, the first and second inks do not substantially contain an alkylpolyol having a boiling point of 280° C. or more. Thus the load of drying operation can be reduced. 
     The phrase “not substantially contain” used above implies that the ink does not contain, for example, 1.0% by mass or more of a substance. Preferably the content of the substance is less than 0.5%, more preferably less than 0.1% by mass, still more preferably less than 0.01%, and further preferably less than 0.001% by mass, relative to the total mass (100% by mass) of the ink. 
     The ink set includes the first ink and the second ink, and may further include any other ink. If the ink set includes an ink other than the first and second inks, that ink may contain an alkylpolyol having a boiling point of 280° C. or more. 
     Possible constituents of the inks of the ink set will now be described. In the following description, the first and second inks and optionally added other inks of the ink set may be collectively referred to as the ink(s). 
     The constituents and their contents of one of the first and second inks are selected in view of the properties of substances independently of the other ink. The ink set may include a single first ink and a singly second ink, or may include a plurality of first inks or a plurality of second inks. If the ink set includes a plurality of first inks, or if the ink set includes a plurality of second inks, the constituents and their contents of each ink are selected in view of the properties of substances independently of the other inks. When the ink set includes a plurality of first inks, the content of a substance in the first ink refers to the average content of the substance in each first ink. The same applies to a plurality of second inks. 
     Moisturizing Agent 
     Each of the first and second inks contains a moisturizing agent. In the description herein, a “first moisturizing agent” refers to the moisturizing agent contained in the first ink, and a “second moisturizing agent” refers to the moisturizing agent contained in the second ink. A set of the first moisturizing agent and the second moisturizing agent is in either case (A) or (B). Cases (A) and (B) will be described below. 
     First, case (A) will be described. In case (A), the first moisturizing agent is: a solvent (a1) containing 1,2-alkanediol and any other solvent; or a solvent (a2) other than 1,2-alkanediol. The solvent other than 1,2-alkanediol has a boiling point of 200 to 260° C. Hence, the first moisturizing agent contains a solvent other than 1,2-alkanediol having a boiling point in a specific range irrespective of whether or not the first moisturizing agent contains 1,2-alkanediol. 
     When the first moisturizing agent has a boiling point of 160° C. or more, the printing apparatus can satisfactorily perform intermittent printing. Also, when the first moisturizing agent has a boiling point of 260° C. or less, the first ink does not contains glycerol and accordingly can dry rapidly. Consequently, the resulting printed article exhibits good rub fastness. The solvent other than 1,2-alkanediol having a boiling point of 200 to 260° C. may be, but is not limited to, a glycol ether or 1,α-alkanediol (α: number other than 2). 
     Examples of the glycol ether include, but are not limited to, diethylene glycol, dipropylene glycol, dibutylene glycol, and other polyalkylene glycols. Exemplary 1,α-alkanediols other than 1,2-alkanediol include, but are not limited to, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, and 1,7-heptanediol. Polyalkylene glycols include alkylene glycol monoalkyl ethers. Exemplary alkylene glycol monoalkyl ethers include ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monoisopropyl ether, ethylene glycol monobutyl ether, ethylene glycol monohexyl ether, ethylene glycol monophenyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, triethylene glycol monobutyl ether, tetraethylene glycol monomethyl ether, tetraethylene glycol monoethyl ether, tetraethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, dipropylene glycol monomethyl ether, and dipropylene glycol monoethyl ether. Polyalkylene glycols include alkylene glycol dialkyl ethers. Exemplary alkylene glycol dialkyl ethers include ethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol dibutyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol dibutyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, tetraethylene glycol dibutyl ether, propylene glycol dimethyl ether, propylene glycol diethyl ether, dipropylene glycol dimethyl ether, and dipropylene glycol diethyl ether. Poly alkylene glycols have superior moisture-retaining properties and are accordingly advantageous. Thus, the solvent other than 1,2-alkanediol is preferably selected from the glycol ethers and 1,α-alkanediols (α: number other than 2). These moisturizing agents can impart appropriate moisture-retaining properties. 
     When the first moisturizing agent is a solvent (a1) containing 1,2-alkanediol and any other solvent, the mass ratio of the total content of first moisturizing agents in the first ink to the content of aprotic polar solvent described later, that is, first moisturizing agent content/aprotic polar solvent content, is preferably 0.6 to 2.6. Such a mass ratio leads to good adhesion. 
     In case (A), the boiling point of the first moisturizing agent is higher than that of the second moisturizing agent. If the first moisturizing agent contains two or more solvents, the boiling point of the first moisturizing agent refers to the average of the boiling points of the solvents. The same applies to the boiling point of the second moisturizing agent. 
     When the above conditions are satisfied, the second moisturizing agent is also: a solvent (a3) containing 1,2-alkanediol and any other solvent; or a solvent (a4) other than 1,2-alkanediol. In the second moisturizing agent, the solvent other than 1,2-alkanediol preferably has a boiling point of 160 to 240° C. Hence, the second moisturizing agent contains a solvent other than 1,2-alkanediol having a boiling point in a specific range irrespective of whether or not the second moisturizing agent contains 1,2-alkanediol. 
     When the second moisturizing agent has a boiling point of 160° C. or more, the printing apparatus can satisfactorily perform intermittent printing. In addition, when the second moisturizing agent has a boiling point of 240° C. or less, the load of drying the ink can be reduced effectively. The solvent of the second moisturizing agent other than 1,2-alkanediol is not limited as long as the boiling point is in the range of 160 to 240° C. and lower than the boiling point of the first moisturizing agent. For example, glycol ethers are advantageously used from the viewpoint of easy drying. 
     Case (B) will now be described. In case (B), the first and second moisturizing agents are each dipropylene glycol. In addition, the dipropylene glycol content in the first ink is higher than the dipropylene glycol content in the second ink. The dipropylene glycol content in the first ink is preferably in the range of 3% to 30% by mass, more preferably 5% to 15% by mass, relative to the total mass (100% by mass) of the first ink. The dipropylene glycol content in the second ink is preferably in the range of 3% to 30% by mass, more preferably 5% to 15% by mass. When the dipropylene glycol contents in the first and second inks are in the above ranges, the load of drying the ink can be reduced effectively. When the ink set further includes an ink other than the first and second inks, the ink other than the first and second inks may contain any of the above-described moisturizing agents. 
     Coloring Material 
     Each of the first ink and the second ink contains a coloring material. The color material is selected from among pigments and dyes. 
     1. Pigment 
     Pigments are not only insoluble or difficult to dissolve in water, but are also not easily discolored by light or gases. Accordingly, printed articles prepared by printing with an ink containing a pigment are resistant to water, gases, weather and light, and can be stably stored. 
     The pigment may be an inorganic pigment or an organic pigment. Preferably, the pigment exhibits high color developability, and has such a low specific gravity that the pigment particles do not easily settle when dispersed. Exemplary inorganic pigments include, but are not limited to, carbon black, iron oxide, and titanium oxide. 
     Examples of the carbon black include, but are not limited to, furnace black, lamp black, acetylene black, and channel black (C. I. Pigment 7). Commercially available carbon blacks include Nos. 2300 and 900, MCF 88, No. 20B, No. 33, No. 40, No. 45, No. 52, MA 7, MA 8, MA 100, and No. 2200B (each produced by Mitsubishi Chemical); Color Black series FW1, FW2, FW2V, FW18, FW200, 5150, 5160 and 5170, Pritex series 35, U, V and 140U, and Special Black series 6, 5, 4A, 4 and 250 (each produced by Degussa AG); Conductex SC, and Raven series 1255, 5750, 5250, 5000, 3500, 1255 and 700 (each produced by Columbian Carbon); Regal series 400R, 330R and 660R, Mogul L, Monarch series 700, 800, 880, 900, 1000, 1100, 1300 and 1400, and Elftex 12 (each produced by Cabot). 
     Examples of the organic pigment that can be used in the first and second inks include, but are not limited to, quinacridone pigments, quinacridonequinone pigments, dioxazine pigments, phthalocyanine pigments, anthrapyrimidine pigments, anthanthrone pigments, indanthrone pigments, flavanthrone pigments, perylene pigments, diketopyrrolopyrrole pigments, perinone pigments, quinophthalone pigments, anthraquinone pigments, thioindigo pigments, benzimidazolone pigments, isoindolinone pigments, azomethine pigments, and azo pigments. More specific examples of the organic pigment are as below. 
     Pigments that can be used in a cyan ink include C. I. Pigment Blues 1, 2, 3, 15, 15:1, 15:2, 15:3, 15:4, 15:6, 15:34, 16, 18, 22, 60, 65 and 66, and C. I. Vat Blues 4 and 60. 
     Pigments that can be used in a magenta ink include C. I. Pigment Reds 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 14, 15, 16, 17, 18, 19, 21, 22, 23, 30, 31, 32, 37, 38, 40, 41, 42, 48(Ca), 48(Mn), 57(Ca), 57:1, 88, 112, 114, 122, 123, 144, 146, 149, 150, 166, 168, 170, 171, 175, 176, 177, 178, 179, 184, 185, 187, 202, 209, 219, 224, 245, 254 and 264, and C. I. Pigment Violets 19, 23, 32, 33, 36, 38, 43 and 50. 
     Pigments that can be used in a yellow ink include C. I. Pigment Yellows 1, 2, 3, 4, 5, 6, 7, 10, 11, 12, 13, 14, 16, 17, 24, 34, 35, 37, 53, 55, 65, 73, 74, 75, 81, 83, 93, 94, 95, 97, 98, 99, 108, 109, 110, 113, 114, 117, 120, 124, 128, 129, 133, 138, 139, 147, 151, 153, 154, 155, 167, 172, 180, 185 and 213. 
     For other color inks, such as a green ink and an orange ink, known pigments can be used. A pigment may be used singly, or two or more pigments may be used in combination. 
     2. Dye 
     The coloring material may be a dye. Examples of the dye include, but are not limited to, acid dyes, direct dyes, reactive dyes, and basic dyes. Exemplary dyes include C. I. Acid Yellows 17, 23, 42, 44, 79 and 142, C. I. Acid Reds 52, 80, 82, 249, 254 and 289, C. I. Acid Blues 9, 45 and 249, C. I. Acid Blacks 1, 2, 24 and 94, C. I. Food Blacks 1 and 2, C. I. Direct Yellows 1, 12, 24, 33, 50, 55, 58, 86, 132, 142, 144 and 173, C. I. Direct Reds 1, 4, 9, 80, 81, 225 and 227, C. I. Direct Blues 1, 2, 15, 71, 86, 87, 98, 165, 199 and 202, C. I. Direct Blacks 19, 38, 51, 71, 154, 168, 171 and 195, and C. I. Reactive Reds 14, 32, 55, 79 and 249, and C. I. Reactive Blacks 3, 4 and 35. 
     A dye may be used singly, or two or more dyes may be used in combination. The coloring material content in the second ink is higher than the coloring material content in the first ink. Accordingly, in view of the density of the coloring material, the first ink and the second ink may be called a light ink and a deep ink respectively. The coloring material content in the second ink is preferably 1% to 7% by mass relative to the total mass (100% by mass) of the second ink. The coloring material content in the first ink is preferably 0.1% to 2% by mass relative to the total mass (100% by mass) of the first ink. When the ink set further includes an ink other than the first and second inks, the ink other than the first and second inks may contain any of the above-cited coloring materials. 
     Resin Particles 
     Each of the first ink and the second ink contains resin particles. By adding resin particles to the first and second inks, the resulting printed article can exhibit good rub fastness. The content of the resin particles in the second ink is lower than the content of the resin particles in the first ink. Thus the inks of the ink set can have the same viscosity. The contents of the resin particles in the first ink and the second ink will be described later. The material of the resin particles may be, but is not limited to, a resin binder, or a wax such as paraffin wax or polyolefin wax. 
     1. Binder Resin 
     The binder resin used as the resin particles will form a resin coating when the printing medium  100  is heated for ink jet printing. The resin coating helps the ink adherer to the printing medium  100 , thus enhancing the rub fastness of the printed article. Accordingly, the binder resin is preferably thermoplastic. If an ink containing a binder resin is used on an ink-non-absorbent or ink-low-absorbent printing medium  100 , the resulting printed article can exhibit good rub fastness effectively. 
     The binder resin is present in a state of emulsion in the ink. The use of the binder resin in an emulsion state in the ink makes it easy to control the viscosity of the ink in a proper range, and enhances the storage stability and ejection stability of the ink. The term “ejection stability” used herein refers to a characteristic that ink droplets can be constantly ejected stably without clogging nozzles. 
     Examples of the binder resin include, but are not limited to, homopolymers and copolymers of (meth)acrylic acid, (meth)acrylic acid ester, acrylonitrile, cyanoacrylate, acrylamide, olefin, styrene, urethane, vinyl acetate, vinyl chloride, vinyl alcohol, vinyl ether, vinyl pyrrolidone, vinyl pyridine, vinyl carbazole, vinyl imidazole and vinylidene chloride, fluororesins, and natural resins. Preferably, the binder resin contains at least either a (meth)acrylic resin or a styrene-(meth)acrylic acid copolymer, more preferably either an acrylic resin or a styrene-acrylic acid copolymer, and still more preferably a styrene-acrylic acid copolymer. If a copolymer is used, the copolymer may be a random copolymer, a block copolymer, an alternating copolymer, or a graft copolymer. In the description herein, “(meth)acrylate” refers to at least either an acrylate or the corresponding methacrylate, and “(meth)acrylic” compound refers to at least either an acrylic compound or the corresponding methacrylic compound. 
     The binder resin may be prepared using known materials by a known method, or a commercially available binder resin may be used. Commercially available binder resins include Micro Gel E-1002 and Micro Gel E-5002 (each produced by Nippon Paint Co., Ltd.), VONCOAT 4001 and VONCOAT 5454 (each produced by DIC), SAE 1014 (produced by Zeon Corporation), Saivinol SK-200 (produced by Saiden Chemical Industry Co., Ltd.), and JONCRYL 7100, JONCRYL 390, JONCRYL 711, JONCRYL 511, JONCRYL 7001, JONCRYL 632, JONCRYL 741, JONCRYL 450, JONCRYL 840, JONCRYL 74J, JONCRYL HRC-1645J, JONCRYL 734, JONCRYL 852, JONCRYL 7600, JONCRYL 775, JONCRYL 537J, JONCRYL 1535, JONCRYL PDX-7630A, JONCRYL 352J, JONCRYL 352D, JONCRYL PDX-7145, JONCRYL 538J, JONCRYL 7640, JONCRYL 7641, JONCRYL 631, JONCRYL 790, JONCRYL 780 and JONCRYL 7610 (each produced by BASF). 
     If the binder resin is prepared by a known method, the method is not particularly limited and, for example, the following methods may be applied, in combination if necessary. A polymerization catalyst (polymerization initiator) and a dispersant may be mixed to a monomer forming a desired resin to polymerize (emulsion polymerization). A solution of a resin having a hydrophilic portion in a water-soluble organic solvent may be mixed with water and then the water-soluble organic solvent is removed by vaporization. A solution of a resin in a water-insoluble organic solvent and a dispersant may be mixed in water. 
     A dispersant may be used for dispersing the binder resin to prepare an emulsion of the binder resin. Examples of the dispersant include, but are not limited to, anionic surfactants, such as sodium dodecylbenzenesulfonate, sodium lauryl phosphate and polyoxyethylene alkyl ether ammonium sulfate; and nonionic surfactants, such as polyoxyethylene alkyl ether, polyoxyethylene alkyl ester, polyoxyethylene sorbitan fatty acid ester and polyoxyethylene alkylphenyl ether. These dispersants may be used singly or in combination. 
     The average particle size of the binder resin is preferably in the range of 5 to 400 nm, more preferably 20 to 300 nm, from the viewpoint of enhancing the storage stability and ejection stability of the ink. The average particle size mentioned herein is a value measured by dynamic light scattering. 
     The content of the binder resin (in terms of solid) in each ink is preferably in the range of 0.5% to 5% by mass, more preferably 0.5% to 1.5% by mass, relative to the total mass (100% by mass) of the ink. Such a binder resin content leads to enhanced rub fastness. 
     2. Paraffin Wax 
     Paraffin wax contained in the ink imparts a slip property to the resulting printed article and thus enhances the rub fastness of the printed article. In addition, since paraffin wax is repellent to water, the resulting printed article can exhibit high water fastness. 
     The term “paraffin wax” used herein refers to a wax prepared from petroleum, and is a mixture of hydrocarbons having weight average molecular weights of about 300 to 500, containing mainly a linear paraffin (normal hydrocarbon) having a carbon number of about 20 to 30, and a small amount of isoparaffin. 
     The paraffin wax is present in a state of emulsion in the ink. Such a paraffin wax makes it easy to adjust the ink to a viscosity suitable for ink jet printing, and helps enhance the storage stability and ejection stability of the ink. 
     The melting point of the paraffin wax is preferably 110° C. or less from the view point of forming a harder coating for the resulting printed article, and further enhancing the rub fastness of the printed article. On the other hand, the lower limit of the melting point of the paraffin wax is preferably 60° C. or more from the viewpoint of preventing the printed article from having a sticky surface by being dried. Still more preferably, the melting point of the paraffin wax is 70 to 95° C. from the viewpoint of further enhancing the ejection stability of the ink. 
     The average particle size of the paraffin wax is preferably in the range of 5 to 400 nm, more preferably 50 to 200 nm, from the viewpoint of making the paraffin wax a stable emulsion and further enhancing the storage stability and ejection stability of the ink. A commercially available paraffin wax may be used as it is. The commercially available paraffin wax may be, but is not limited to, AQUACER 537 or AQUACER 539 (each produced by BYK). The paraffin wax content (in terms of solid) in each ink is preferably in the range of 0% to 1.5% by mass, more preferably 0.25% to 0.75% by mass, relative to the total mass (100% by mass) of the ink. 
     3. Polyolefin Wax 
     By adding polyolefin wax to the ink, the rub fastness of the printed article can be further enhanced. The polyolefin wax may be, but is not limited to, polyethylene wax or polypropylene wax. Preferably, a polyethylene wax is used. 
     A polyethylene wax can be produced by polymerizing ethylene, or by thermally decomposing ordinary polyethylene into low-molecular weight components. The resulting polyethylene wax is oxidized so that a carboxy group or a hydroxy group is added, and is then emulsified with a surfactant to yield a stable aqueous emulsion of the polyethylene wax. 
     A commercially available polyolefin wax may be used as it is. The commercially available polyolefin wax may be, but is not limited to, NOPCOTE PEM 17 (produced by Sannopco Limited), CHEMIPEARL W4005 (produced by Mitsui Chemicals), or AQUACER 515 or AQUACER 593 (each produced by BYK). The average particle size of the polyolefin wax is preferably in the range of 5 to 400 nm, more preferably 50 to 200 nm, from the viewpoint of further enhancing the storage stability and ejection stability of the ink. 
     The polyolefin wax content (in terms of solid) in each ink is preferably in the range of 0% to 1.5% by mass, more preferably 0.25% to 0.75% by mass, relative to the total mass (100% by mass) of the ink. In order to further enhance the rub fastness of the printed article, the resin particles contain at least either polyolefin wax or paraffin wax. 
     The resin particles may further contain other wax. The wax other than polyolefin wax and paraffin wax has the function of imparting a slip property to the resulting printed article to enhance the rub fastness of the printed article. Such wax is preferably present in an emulsion state in the ink. The emulsions of waxes in the ink make it easy to adjust the ink to a viscosity suitable for ink jet printing, and help enhance the storage stability and ejection stability of the ink. When the ink set further includes an ink other than the first and second inks, the ink other than the first and second inks may contain any of the above-cited resin particles. 
     Aprotic Polar Solvent 
     Each of the first ink and the second ink contains an aprotic polar solvent. Since the aprotic polar solvent can dissolve the resin particles in the ink, the ink can be prevented from clogging nozzles during ink jet printing. 
     The aprotic polar solvents contained in the first and second inks may be the same. Exemplary aprotic polar solvents include, but are not limited to, pyrrolidones, lactones, sulfoxides, imidazolidinones, sulfolanes, urea derivatives, dialkylamides, cyclic ethers, and amide ethers. These may be used singly or in combination. 
     Exemplary pyrrolidones include 2-pyrrolidone, N-methyl-2-pyrrolidone, and an N-ethyl-2-pyrrolidone. Exemplary lactones include γ-butyrolactone, γ-valerolactone, and ε-caprolactone. Exemplary sulfoxides include dimethyl sulfoxide and tetramethylene sulfoxide. An example of the imidazolidinone may be 1,3-dimethyl-2-imidazolidinone. Exemplary sulfolanes include sulfolane and dimethyl sulfolane. Exemplary urea derivatives include dimethylurea and 1,1,3,3-tetramethylurea. Exemplary dialkylamidoes include dimethylformamide and dimethylacetamide. Exemplary cyclic ethers include 1,4-dioxane and tetrahydrofuran. An example of the amide ethers may be the compound expressed by the following general formula (1): 
     
       
         
         
             
             
         
       
     
     In general formula (1), R1 is preferably an alkyl group having a carbon number of 1 to 4. The alkyl group having a carbon number of 1 to 4 may be linear or branched, and examples of the alkyl croup include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, and tert-butyl. The solvent expressed by general formula (1) in which R1 represents an alkyl group having a carbon number of 1 to 4 can impart proper pseudoplasticity to the ink. Consequently, the ink can be stably ejected. Also, such a solvent can dissolve resin and is thus advantageous. 
     The HLB (Hydrophile-Lipophile Balance) value of the solvent expressed by general formula (1) is preferably in the range of 10.5 to 20.0, more preferably 12.0 to 18.5. A solvent expressed by general formula (1) having an HLB value in these ranges is more advantageous in imparting proper pseudoplasticity to the ink and in interaction with the resin component. 
     The HLB value of the solvent expressed by general formula (1) refers to a value obtained from the following equation using the ratio of the inorganic value (I) of the solvent to the organic value (O) of the solvent according to an organic conceptual diagram, and may be simply referred to as I/O value.
 
HLB value=(inorganic value( I )/organic value( O ))×10
 
     More specifically, the I/O value can be calculated according to any of the books: A. Fujita, Systematic Organic Qualitative Analysis for Mixtures (in Japanese), Kazama Shobo, (1974); H. N. Kuroki, Theoretical Chemistry for Dyeing (in Japanese), Maki Shoten, (1966); and H. Inoue, Method for Separating Organic Compounds (in Japanese), Shokabo Publishing, (1990). Preferably, the aprotic polar solvent is selected from among pyrrolidones, lactones, sulfoxides and amide ethers from the viewpoint of enhancing the fixity of the ink to the printing medium  100 . 
     The aprotic polar solvent preferably has a boiling point in the range of 200 to 260° C. Such an aprotic polar solvent may be, but is not limited to, 2-pyrrolidinone. The aprotic polar solvents in the first and second inks may be the same or different, and may be a single solvent or a mixture of two or more solvents. 
     The aprotic polar solvent content in each of the first and second inks is preferably in the range of 3% to 30% by mass, more preferably 8% to 20% by mass, relative to the total mass (100% by mass) of the ink. When the ink set further includes an ink other than the first and second inks, the ink other than the first and second inks may contain any of the above-cited aprotic polar solvents. 
     Surfactant 
     Each ink of the ink set may contain a surfactant. The surfactant may be, but is not limited to, a nonionic surfactant. Nonionic surfactants help the ink spread uniformly on the printing medium  100 . Accordingly, when an ink containing a nonionic surfactant is used for ink jet printing, high-definition images can be formed without bleeding. Examples of the nonionic surfactant include, but are not limited to, acetylene glycol-based surfactants, silicone surfactants, polyoxyethylene alkyl ethers, polyoxypropylene alkyl ethers, polycyclic phenyl ethers, sorbitan derivatives, and fluorochemical surfactants. These surfactants may be used singly or in combination. The surfactant content in each ink can be in the range of 1.5% by mass or less relative to the total mass (100% by mass) of the ink. 
     Water 
     Each ink of the ink set may contain water. Particularly when the ink is aqueous, the water acts as the main medium of the ink and will be evaporated by heating the printing medium  100 . 
     The water may be pure water or ultra-pure water from which ionic impurities have been removed as much as possible. Examples of such water include ion exchanged water, ultrafiltered water, reverse osmosis water, and distilled water. Sterile water prepared by, for example, UV irradiation or addition of hydrogen peroxide can prevent, for a long time, the occurrence of mold or bacteria in the ink. 
     Other Constituents 
     Each ink of the ink set may further contain an organic solvent other than the above described solvents, a pH adjuster, a preservative and a fungicide, a rust preventive, a chelating agent, and other additives. The printing medium  100  to which the ink will be deposited is flexible, and a roll of the printing medium  100  is loaded in the printing apparatus  1 . The printing medium  100  may be an ink-absorbent medium, or an ink-non-absorbent or ink-low-absorbent medium. 
     The ink-absorbent printing medium  100  may be, but is not limited to, ink jet printing paper, such as plain paper, high-quality paper, or glossy paper. The ink-low-absorbent printing medium  100  may be book printing paper, such as art paper, coated paper, or matte paper. The ink-non-absorbent printing medium  100  may be, but is not limited to, a plastic film not surface-treated for ink jet printing (not having an ink-absorbing layer), or a paper sheet or any other material coated or bonded with a plastic film. The plastic film may be made of, but not limited to, polyvinyl chloride, polyethylene terephthalate, polycarbonate, polystyrene, polyurethane, polyethylene, or polypropylene. 
     The printing apparatus  1  will now be described. The printing apparatus  1  of an embodiment of the invention includes an apparatus body  2 , legs  3 , and a curing unit  4 , as described above with reference to  FIG. 1 . As shown in  FIG. 2 , the apparatus body  2  includes a transfer device  7 , a head unit (head assembly)  5 , a head moving device  22 , a platen  8 , a preheater  25 , a drying heater (heating unit)  26 , a blowing fan  27 , a suction fan  28 , and a housing  29 . The housing  29  is a case containing together the transfer device  7 , the head unit  5 , the head moving device  22 , the platen  8 , the preheater  25 , the drying heater  26 , the blowing fan  27 , and the suction fan  28 . The housing  29  (apparatus body  2 ) has a shape long in the y direction. 
     The transfer device  7  transfers the printing medium  100 . The direction in which the printing medium  100  is transferred is referred to as the transfer direction. The transfer device  7  includes a single driving roller  71 , a plurality of driven rollers  72  rotated by the rotation of the driving roller  71 , and one or more holders  73  each holding three of the driven rollers  72 . 
     The driving roller  71  is connected to a motor with a deceleration mechanisms such as a gear therebetween The driving roller  71  is rotated by the operation of the motor. The driven rollers  72  oppose the driving roller  71  above the driving roller  71 . The driving roller  71  has such a length as to oppose all the driven rollers  72  at one time. 
     As shown in  FIG. 3 , the driven rollers  72  are aligned in a direction intersecting the transfer direction, that is, in the y direction. The printing medium  100  pinched between the driving roller  71  and the driven rollers  72  can be certainly transferred by the rotation of the rollers. 
     The holders  73  are aligned in the y direction as with the driven rollers  72 . Each holder  73  holds three driven rollers  72 . In the present embodiment, one holder  73  and three driven rollers  72  constitute a roller unit  74 . 
     The holder  73  has a body  731  in the form of a block and four ribs  732  projecting in one direction from the body  731 . The body  731  is fixed to the housing  29 . The four ribs  732  are arranged at regular intervals in the y direction. The driven rollers  72  are disposed, one each, between the ribs  732  so as to be held for rotation. Although the number of driven rollers  72  that the holder  73  holds is three in the embodiment shown in  FIG. 3 , it may be two or four or more without being limited. 
     The driving roller  71 , the driven rollers  72  and the holders may be made of any material without particular limitation. For example, a metal or a resin may be used. The preheater  25  heats the printing medium  100  before the printing medium  100  is printed. The preheater  25  includes a preheater housing  252  having a contact surface  251  in contact with the rear side of the printing medium  100 , and a heating element  253  in the housing  252 . 
     The contact surface  251  is curved into an arch shape. The printing medium  100  comes into contact with the contact surface  251  in the course of transfer by the transfer device  7 . At this time, heat from the heating element  253  is transmitted to the printing medium  100  through the contact surface  251 . Thus, the printing medium  100  is heated. Preferably, the preheater heats the printing medium  100  so that the surface temperature of the printing medium  100  becomes 5° C. or more higher than the surface temperature of the printing medium  100  on the platen  8 . 
     Preferably, the curvature of the contact surface  251  is gradually reduced in the transfer direction, that is, in the positive x direction. The preheater housing  252  may be made of, but is not limited to, aluminum or an aluminum alloy, or stainless steel. The heating element  253 , which generates heat by receiving electric power, is made of a metal having a relatively high electric resistance, such as nichrome wire. The platen  8  is disposed downstream of the preheater  25  in the transfer direction. The platen  8  keeps the printing medium  100  as flat as possible when an ink is applied to the printing medium  100 . 
     As shown in  FIG. 3 , the platen  8  includes a rectangular plate in a position long in the y direction when viewed from above. The rectangular plate of the platen  8  has an upper flat surface serving as a support surface  81  that supports the printing medium  100  transferred by the operation of the transfer device  7 , from the rear side (lower side) of the printing medium  100 , that is, the side opposing the upper surface of the platen  8 . The support surface  81  keeps the printing medium  100  as flat as possible during printing. 
     The support surface  81  has a plurality of suction holes  82  that suck the printing medium  100  supported on the support surface  81 . Each suction hole  82  may be, for example, circular, oval (ellipsoidal), or rectangular in shape when viewed from above, and is preferably circular. 
     Under the platen  8 , a suction fan  28  is disposed. The printing medium  100  on the platen  8  is sucked through the suction holes  82  by the operation, or rotation, of the suction fan  28 . Thus the position of the printing medium  100  is stabilized so that the ink can be accurately applied onto desired points on the printing medium  100 . 
     The suction fan  28  can be selected from among various type of fan without particular limitation, and, for example, may be a multiblade fan such as a sirocco fan. The platen  8  may be made of the same material as the housing  252 . 
     As shown in  FIG. 2 , the head unit  5  is an assembly including a liquid ejection head  23  and a carriage  6 . The liquid ejection head  23  is disposed over the platen  8 . The liquid ejection head  23  has a plurality of nozzle apertures (not shown) that are open downward. The liquid ejection head  23  ejects droplets of an ink through the nozzles onto the printing medium  100  supported on the support surface  81  of the platen  8 . Thus, the printing medium  100  is printed. 
     Each of the nozzle apertures communicates with the ink set (cartridge) through a tube  231 . Thus, the inks are supplied to the nozzle apertures. The carriage  6  holds the liquid ejection head  23 . The carriage  6  is connected to the head moving device  22 . 
     The head moving device  22  reciprocally transfers the head unit  5  in the y direction, or a direction intersecting (perpendicular to) the transfer direction of the printing medium  100 . The head moving device  22  includes, for example, a motor, a ball screw connected to the motor, and a linear guide disposed parallel to the boll screw. While the head moving device  22  reciprocally transfers the liquid ejection head  23 , the liquid ejection head  23  ejects the inks onto the printing medium  100  being transferred in the positive x direction, thus printing the printing medium  100  with the inks. 
     The drying heater  26  opposes the support surface  81  of the platen  8  with the head unit  5  (liquid ejection head  23 ) therebetween. The drying heater  26  heats the printing medium  100  and the ink deposited on the printing medium  100 . More specifically, while ink is ejection onto the printing medium  100 , the drying heater  26  irradiates the ink to infrared radiation so as to help the ink dry. 
     The drying heater  26  includes a tube  261  running in the y direction, and a heating element  262  passing through the tube  261 . The tube  261  is made of a metal, and is preferably made of iron. The length, in the y direction, of the tube  261  is preferably larger than the width of the printing medium  100  in the y direction so that the entirety of the ink on the printing medium  100  transferring under the tube  261  (drying heater  26 ) can be irradiated with infrared radiation. 
     The heating element  262 , which generates heat by receiving electric power, is made of, for example, an electrically-heated wire such as nichrome wire. The tube  261  is heated by heat generated by the heating element  262 , thus emitting infrared radiation. Thus, the water in the ink can be certainly removed, so that the ink is dried. The heating temperature at which the tube  261  is heated is for example in the range of 400 to 800° C., and is preferably 700° C. or less. 
     The ink on the printing medium  100  can be dried by heating the rear side of the printing medium  100 , using a platen having a structure capable of functioning as a heating plate. However, this method may cause the ink to form a coating due to the nature of the ink, and the coating is likely to inhibit the evaporation of water in the ink. It is therefore preferable that the ink be heated from a position over the printing medium  100 . 
     The blowing fan  27  is disposed upstream in the transfer direction at an upper portion of the apparatus body  2 . The blowing fan  27  sends air  271  along the transfer of the printing medium  100 . The blowing air  271  purges the vapor generated by heating the ink from the apparatus body  2 . Thus, the condensation of the liquid ejection head  23  can be prevented. As with the suction fan  28 , the blowing fan  27  can be selected from among various type of fan without particular limitation, and, for example, may be a multiblade fan. 
     The apparatus body  2  is supported upward by legs  3 , as shown in  FIG. 1 . The legs  3  include a frame  31 , four casters  32 , and two adjuster feet (fixtures)  33 . The frame  31  is an assembly including a plurality of bar members  311  that are appropriately connected and fixed to each other. 
     The casters  32  are fixed with spaces therebetween to the bottom of the frame  31 . The printing apparatus  1  thus can be transferred. The adjuster feet  33  are also fixed to the bottom of the frame  31 . The adjuster feet  33  are each disposed near either of the two of the four casters  32  located at the negative side of the x direction. The adjuster feet  33  can stop the printing apparatus  1  by being brought into contact with the floor after transfer of the printing apparatus  1 . 
     The curing unit  4  is disposed downstream of the apparatus body  2  in the transfer direction. As shown in FIG.  2 , the curing unit  4  includes a curing heater  41 , a cooling fan  42 , and a housing  43 . The housing  43  is a case containing together the curing heater  41  and the cooling fan  42 . The housing  43  of the curing unit  4  has a shape long in the y direction, having a length smaller than the housing  29  of the apparatus body  2 . 
     The housing  43  of the curing unit  4  is provided with a passage  432  through which the printing medium  100  passes. The end of the passage  432  is the ejection port  433  from which the printing medium  100  is ejected. The curing heater  41  is located at a position over the passage  432  so as to oppose the upper surface of the printing medium  100  passing through the passage  432 , on which ink has been applied. The curing heater  41  heats and cure the ink by irradiating the ink on the printing medium  100  with infrared radiation. Thus, the ink is fixed to the printing medium  100  tightly. 
     As shown in  FIG. 2 , the curing heater  41  includes a tube  411  extending in the y direction, and a heating element  412  passing through the tube  411 . The tube  411  is made of a metal, and is preferably made of iron. The length, in the y direction, of the tube  411  is preferably larger than the width of the printing medium  100  in the y direction so that the entirety of the ink on the printing medium  100  transferring under the tube  411  (curing heater  41 ) can be irradiated with infrared radiation. 
     The heating element  412 , which generates heat by receiving electric power, is made of, for example, an electrically-heated wire such as nichrome wire. The tube  411  is heated by heat generated by the heating element  412 , thus emitting infrared radiation. Thus, the curable component of the ink is cured. Consequently, the resulting printed article, including the printing medium  100  and the cured ink on the printing medium  100 , can exhibit good weather fastness and rub fastness. 
     The surface temperature of the printing medium  100  being heated is for example in the range of 60 to 120° C., preferably 80 to 100° C. The surface temperature of the printing medium  100  can be measured with, for example, an infrared (IR) sensor. For controlling the surface temperature of the printing medium  100  in these ranges, the power of the curing heater  41  may be switched according to the result of the measurement of the IR sensor. 
     The cooling fan  42  is disposed downstream of the curing heater  41  in the transfer direction. The cooling fan  42  blows on the printing medium  100  heated with the curing heater  41  to cool the printing medium  100 . As with the blowing fan  27  and the suction fan  28 , the cooling fan  42  can be selected from among various type of fan without particular limitation, and, for example, may be a multiblade fan. 
     The support surface  81  of the platen  8  has a plurality of suction holes  82 , as described above. Also, the support surface  81  is divided into first portions  811  and second portions  812 , as shown in  FIG. 3 . The first portions  811  are strip-shaped portions in plan view corresponding to the extension lines  75  extending in the transfer direction from the portions between each driven roller  72 . The second portions  812  are portions of the support surface  81  other than the first portions  811 . The second portions  812  each have larger width than the first portion  811 . The suction holes  82  are arranged at different densities between the first portions  811  and the second portions  812 . More specifically, each first portion  811  has suction holes  82  with a high density to define a high-density region  83 , and each second portion  812  has suction holes  82  with a low density to define a low-density region  84 . The high density regions  83  are divided into first high-density regions  831  and second high-density regions  832 , as will be described later. 
     In the high-density regions  83 , as shown in  FIG. 3 , the suction holes  82  are aligned in a line in the transfer direction, or the positive x direction, at some (2 to 5 in the embodiment shown in  FIG. 3 ) intervals. The intervals between the suction holes  82  may be the same or different. The suction holes  82  located most upstream in the transfer direction (hereinafter referred to as most upstream suction hole(s)  82   a ) are arranged in the high-density regions  83 . Although the most upstream suction holes  82   a  may be arranged one for each high-density region  83 , it is preferable that the most upstream suction holes  82   a  be alternately arranged in the y direction in the high-density regions  83 . 
     The low-density regions  84  each have a small number of suction holes  82 , more specifically, at most one suction hole  82 . In the low-density regions  84  having one suction hole  82 , the suction hole  82  is located at the same position in the x direction of the low-density regions  84 . Each of the suction holes  82  in the low-density regions  84  is disposed at a position staggered in the x direction with respect to the positions of the suction holes  82  in the high-density region  83 , that is, at a different position in the x direction from the suction holes  82  in the high-density region  83 . 
     Preferably, the density of the arrangement of the suction holes  82  in each high-density region  83  is 1.1 to 10 times, preferably twice to 5 times, as high as that in the low-density region  84 . The support surface  81  has a portion that will be overlaid with the head unit  5  (liquid ejection head  23 ) being transferred by the head moving device  22 . This portion is a printing region  85  where the transferring head unit  5  (liquid ejection head  23 ) prints on the printing medium  100 . The printing region  85  is across the high-density regions  83  and the low-density regions  84  in the y direction. 
     The printing medium  100  on the platen  8  is headed with the drying heater  26 , as described above. At this time, the printing medium  100  is expanded by heat (thermal expansion). The driven rollers  72  of the roller unit  74  are arranged immediately upstream of the platen  8 . The thermally expanded portion of the printing medium  100  is not pressed by the driven rollers  72 . Therefore this thermally expanded portion is likely to be bent and trapped between the driven rollers  72 . This printing medium  100  is kept bent (the bent state of the printing medium  100  is referred to as a crease) even on the first portions  811 . 
     However, the suction holes  82  arranged in the first portions  811 , which are the high-density region  83  having more suction holes  82  than the second portions  812 , preferentially suck creases, thus eliminating the creases. Creases are formed in the thermally expanded portion of the printing medium  100 , as described above. This portion is prevented from creasing in the high-density regions  83 , and is stretched from the high-density regions  83  in directions of arrows p shown in  FIG. 3 , and preferentially in the low-density regions  84 . 
     In the printing apparatus  1 , the function of the high-density regions  83  to remove creases and the function of the low-density regions  84  to expand portions likely to crease produce a synergistic effect of flattening the printing medium  100  on the printing region  85 , as shown in  FIGS. 6A and 6B . Consequently, unclear printing resulting from thermal expansion of the printing medium  100  can be certainly prevented. 
     The high density regions  83  are divided into first high-density regions  831  and second high-density regions  832 , as shown in  FIG. 3 . The first high-density regions  831  lie corresponding to the extension lines  76  extending in the transfer direction from the portions between the holders  73  when viewed from above. The second high-density regions  832  are high-density regions  83  other than the first high-density regions  831 . The density of the suction holes  82  in the first high-density regions  831  is higher than that in the second high-density regions. In the embodiment shown in  FIG. 3 , each first high-density region  831  has five suction holes  82 , and the second high-density region  832  has two to four suction holes  82 . 
     The printing medium  100  is more likely to crease in the portions corresponding to the extension lines  76  extending in the transfer direction from the portions between the holders  73  among the first portions  811 . However, in the portions of the first portions  811  more likely to cause creases, which are the first high-density regions  831 , creases are preferentially sucked, thus more certainly being eliminated. 
     The printing region  85  of the support surface  81  has more suction holes  82  with a higher density than the other region, or non-printing region  86 , downstream of the printing region  85  in the transfer direction. Hence, the density of the suction holes  82  in the support surface  81  is reduced in the transfer direction. The printing medium  100  is certainly stabilized by the suction of the suction holes  82  in the printing region  85 . Consequently, the printing apparatus can perform accurate printing on the printing medium  100  on the printing region  85 . 
     The printing medium  100  is thermally expanded from the portion adjacent to the printing region  85  in the direction indicated by arrows q shown in  FIG. 3 , preferentially in the non-printing region  86 . Consequently, the portion to be printed of the printing medium  100  is flattened so that creases are removed as shown in  FIGS. 5A and 5B , and thus the unclear printing on the printing medium  100  can be certainly prevented. Preferably, the suction holes  82  have an average diameter of 4 mm or less, more preferably 2 to 4 mm. 
     The total suction power of the suction holes  82  to suck the printing medium  100  depends on the size of the printing medium  100 , particularly on the width in the y direction of the printing medium  100 , and preferably, the number of the suction holes  82  and the width of the printing medium  100  satisfy the relationship as shown  FIG. 4 . By satisfying such a relationship, the printing medium  100  can be adequately sucked according to the size thereof. 
     While the invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, and that the components of the printing apparatus of the invention, at least in part, may be replaced with equivalents having the same function. Any other component may be added. Although the number of the suction holes in the high-density region is 2 to 5 in the above embodiment, it is not limited to these numbers, and may be 6 or more. 
     The entire disclosure of Japanese Patent Application No. 2012-244363, filed Nov. 6, 2012 is expressly incorporated reference herein.