Patent Publication Number: US-11046089-B2

Title: Thermal transfer sheet, printing sheet, and thermal transfer printing apparatus

Description:
FIELD OF THE INVENTION 
     The present invention relates to a thermal transfer sheet, a printing sheet, and a thermal transfer printing apparatus. 
     DESCRIPTION OF RELATED ART 
     Thermal transfer printers have been widely used which print letters, characters, images, and the like onto an image receiving sheet or any other body to be transferred by using a thermal transfer sheet (ink ribbon). The thermal transfer sheet includes a ribbon (support layer), which is a long strip, and a dye layer disposed on the ribbon, and, optionally, a protective layer and a hot-melt ink layer. 
     In the known thermal transfer sheet, dye layers for three colors of yellow, magenta, and cyan and a protective layer are sequentially arranged in a plane direction, and, optionally, a detection mark is formed of an ink containing a pigment, such as carbon black or aluminum, between each dye layer or between a dye layer and the adjacent protective layer. A thermal transfer printing apparatus reads the detection mark of the thermal transfer sheet loaded therein to determine a print start position and identify the type and the size of the thermal transfer sheet. However, securing regions where detection marks are formed between the dye layers increases the full length of the thermal transfer sheet, accordingly increasing the amount of the substrate to be used and increasing manufacturing cost. In addition, when detection marks are formed on a base film by printing, scattered ink may be printed at unwanted positions, leading to defects in thermal transfer images. 
     PTL 1 discloses a thermal transfer sheet including dye layers for two or more colors that are sequentially arranged in a plane direction, wherein any of the dye layers has a two-layer structure and one layer of the two-layer structure forms a detection mark having a difference in color from the adjacent portion. However, since the step of forming a further detection layer (dye layer) is required for the detection mark, the manufacturing cost increases. In addition, when a high-resolution image is printed, the color properties of the image may vary. 
     PTLs 2 and 3 each disclose a thermal transfer dye sheet including a yellow dye layer, a magenta dye layer, and a cyan dye layer, wherein the yellow dye layer has a print region (detection mark) for a binary code or the like producing a difference in optical density capable of being detected by a printer, the print region being formed by varying the thickness of the yellow dye. However, since the thickness of the dye layer is varied so as to produce a difference in optical density, color properties in high-resolution printing may vary. 
     PTLs 4 and 5 each disclose a thermal transfer sheet including dye layers for one or more colors sequentially arranged in a plane direction and a detection layer disposed between the substrate and the dye layers or between the substrate and a rear surface layer. However, since the step of forming the detection layer is required, the manufacturing cost increases. 
     PTL 1: Japanese Patent No. 5799525 
     PTL 2: European Patent No. 1872960 
     PTL 3: European Patent No. 2035233 
     PTL 4: Japanese Patent No. 5760763 
     PTL 5: Japanese Patent Application Publication No. 2013-1047 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention takes account of such circumstances and an object of the present invention is to provide a thermal transfer sheet capable of being identified by a thermal transfer printing apparatus, as well as being capable of preventing color property changes in high-resolution printing and reducing production cost. Also, it is an object of the present invention to provide a printing sheet capable of being identified by a thermal transfer printing apparatus. Furthermore, it is an object of the present invention to provide a thermal transfer printing apparatus configured to identify the thermal transfer sheet or a printing sheet loaded therein and perform printing operation. 
     According to the present invention, a thermal transfer sheet includes a dye layer and a protective layer disposed on one surface of a substrate, wherein the protective layer contains an invisible light absorbing material and is provided with an identification mark having at least one of a recessed portion and a protruding portion. 
     According to one aspect of the present invention, the identification mark has a protruding strip or a recessed strip. 
     According to one aspect of the present invention, the protruding strip or the recessed strip extends in a transverse direction of the sheet. 
     According to one aspect of the present invention, the identification mark is located at a periphery of the protective layer that is not transferred to printing paper. 
     According to the present invention, a thermal transfer printing apparatus includes a thermal head and a platen roll and in which the thermal head heats the thermal transfer sheet according to the present invention to transfer a dye onto a printing paper while the thermal transfer sheet and the printing paper, lying one on the other, are transported between the thermal head and the platen roll, thus forming an image on the printing paper and transferring the protective layer onto the image. The thermal transfer printing apparatus includes a detector disposed between a feeder feeding the thermal transfer sheet and the thermal head, the detector detecting the identification mark, a memory storing a table in which a type of the thermal transfer sheet and a pattern of the identification mark are associated with each other, and an identification unit referring to the table and identifying the thermal transfer sheet fed from the feeder based on the pattern detected by the detector. 
     According to one aspect of the present invention, the pattern of the identification mark is represented by the number of strips or portions, a width, a shape or a position of the identification mark. 
     According to the present invention, a thermal transfer printing apparatus includes a thermal head and a platen roll and in which the thermal head heats a thermal transfer sheet including a dye layer and a protective layer containing an invisible light absorbing material to transfer a dye onto a printing paper while the thermal transfer sheet and the printing paper, lying one on the other, are transported between the thermal head and the platen roll, thus forming an image on the printing paper and transferring the protective layer onto the image. The thermal transfer printing apparatus includes a detector disposed between a feeder feeding the thermal transfer sheet and the thermal head, the detector applying invisible light to the protective layer and measuring an intensity of invisible light transmitted through or reflected from the protective layer, a memory storing a table in which a type of the thermal transfer sheet and the intensity are associated with each other, and an identification referring to the table and identifying the thermal transfer sheet fed from the feeder based on a measurement result of the detector. 
     According to one aspect of the present invention, the table contains printing conditions associated with each type of the thermal transfer sheet, and printing operation is performed under the printing conditions associated with the type of the thermal transfer sheet identified by the identification unit. 
     According to the present invention, a printing sheet includes a substrate, an intermediate layer disposed on the substrate, and a receiving layer disposed on the intermediate layer. The intermediate layer contains an invisible light absorbing material and is provided with an identification mark including at least one of a recessed portion and a protruding portion. 
     According to one aspect of the present invention, the identification mark includes a protruding strip or a recessed strip. 
     According to the present invention, a thermal transfer printing apparatus includes a thermal head and a platen roll and in which the thermal head heats the thermal transfer sheet according to the present invention to transfer a dye onto the printing sheet according to the present invention while the thermal transfer sheet and the printing sheet, lying one on the other, are transported between the thermal head and the platen roll, thus forming an image on the printing sheet and transferring the protective layer onto the image. The thermal transfer printing apparatus includes a first detector disposed between a feeder feeding the thermal transfer sheet and the thermal head, the first detector detecting a first identification mark provided in the protective layer, a second detector detecting a second identification mark provided in the intermediate layer, a memory storing a table in which a type of the thermal transfer sheet and a pattern of the first identification mark are associated with each other and a table in which a type of the printing sheet and a pattern of the second identification mark are associated with each other, and an identification unit referring to the tables, identifying the type of the thermal transfer sheet based on the pattern detected by the first detector, and identifying the type of the printing sheet based on the pattern detected by the second detector. 
     According to one aspect of the present invention, the thermal transfer printing apparatus further includes a light source applying invisible light to the thermal transfer sheet and the printing sheet. The printing sheet is irradiated with invisible light transmitted through the protective layer, the first detector receives light from the protective layer, and the second detector receives light from the printing sheet, the light having been transmitted through the protective layer. 
     According to one aspect of the present invention, the protective layer of the thermal transfer sheet contains an ultraviolet light absorbing material, and the intermediate layer of the printing sheet contains a fluorescent brightening agent. 
     ADVANTAGEOUS EFFECTS OF INVENTION 
     The present invention enables a thermal transfer printing apparatus to identify thermal transfer sheets, as well as to prevent color property changes in high-resolution printing and reduce production cost. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of the structure of a thermal transfer printing apparatus according to an embodiment of the present invention. 
         FIG. 2  is a plan view of a thermal transfer sheet according to the embodiment. 
         FIG. 3  is a sectional view taken along line III-III shown in  FIG. 2 . 
         FIGS. 4 a  and 4 b    each show the section of a protective layer. 
         FIGS. 5 a  and 5 b    are each a plan view of a protective layer. 
         FIGS. 6 a  and 6 b    are each a plan view of a protective layer. 
         FIGS. 7 a  and 7 b    are each a plan view of a protective layer. 
         FIGS. 8 a  and 8 b    are each a plan view of a protective layer. 
         FIG. 9  is a plan view of a protective layer. 
         FIG. 10  is a representation of some plan views of protective layers. 
         FIG. 11  is a plan view of a thermal transfer sheet. 
         FIG. 12  is a schematic diagram of the structure of a thermal transfer printing apparatus according to another embodiment. 
         FIG. 13  is a plan view of a printing sheet. 
         FIGS. 14 a  and 14 b    are each a sectional view taken along line XIV- XIV shown in  FIG. 13 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a schematic diagram of a thermal transfer printing apparatus according to an embodiment of the present invention,  FIG. 2  is a plan view of a thermal transfer sheet  5  used in the thermal transfer printing apparatus, and  FIG. 3  is a sectional view of the thermal transfer sheet  5 . 
     The thermal transfer sheet  5  includes: dye layers  52  containing a dye and a binder resin and a transfer protective layer (hereinafter referred to as a protective layer  54 ) that are repetitively and sequentially arranged in a plane direction on one surface of a substrate  50 ; and a rear surface layer  57  on the other surface of the substrate  50 . The dye layers  52  include yellow (Y) dye layers, magenta (M) dye layers and cyan (C) dye layers that are sequentially arranged in a plane direction. A dye primer layer may be disposed between the substrate  50  and the arrangement of the dye layers  52  and the protective layers  54 . Also, a rear primer layer may be disposed between the substrate  50  and the rear surface layer  57 . 
     The thermal transfer printing apparatus includes a thermal head  1  configured to sublimate and transfer Y, M and C onto a printing sheet  7  (printing paper, image-receiving paper) with the thermal transfer sheet  5 , thus printing an image and forming a protective layer over the image. 
     A feeder  3  formed by winding the thermal transfer sheet  5  thereon is disposed downstream from the thermal head  1 , and a collecting unit  4  is disposed upstream from the thermal head  1 . The thermal transfer sheet  5  fed from the feeder  3  passes the thermal head  1  and is taken up by the collecting unit  4 . 
     A rotatable platen roll  2  is disposed below the thermal head  1 . A printing unit  40 , which includes the thermal head  1  and the platen roll  2 , pinches the printing sheet  7  and the thermal transfer sheet  5  and heats the thermal transfer sheet  5  to transfer dyes onto the printing sheet  7 , thus forming an image. 
     The printing unit  40  also heats the protective layer  54  to transfer the protective layer onto the image. By increasing transfer energy for forming the protective layer (printing energy for printing by the printing unit  40 ), the surface of the protective layer becomes matt and has a low glossiness; by reducing the transfer energy, the surface of the protective layer becomes glossy and has a high glossiness. 
     A rotatable capstan roller  9 a for transporting the printing sheet  7  and a pinch roller  9 b for pressing the printing sheet  7  on the capstan roller  9 a are disposed upstream from the thermal head  1 . 
     The printing sheet  7  is wound on a printing paper roll  6  and fed from the printing paper roll  6 . The printing sheet  7  may be a known one. A driving section  30 , which includes the printing paper roll  6 , the capstan roller  9   a , and the pinch roller  9   b , feeds the printing sheet  7  (transports the printing sheet forward) and takes up the printing sheet (transports the printing sheet backward). 
     The printing sheet  7  that has been subjected to image formation and transfer of the protective layer in the printing unit  40  is cut into a printed cut sheet  7   a  with a cutter  8  on the downstream side. The printed cut sheet  7   a  is ejected through an ejection port (not shown). 
     In the present embodiment, the protective layer  54  of the thermal transfer sheet  5  contains an invisible light absorbing material. The invisible light absorbing material may be, for example, a fluorescent brightening agent, an ultraviolet light absorbing material, or an infrared light absorbing material. A detector  20  suitable for a type of the invisible light absorbing material is disposed in the vicinity of the feeder  3 . 
     If the protective layer  54  contains a fluorescent brightening agent, a fluorescence sensor is used as the detector  20 , and the protective layer  54  is irradiated with ultraviolet light. The detector  20  receives fluorescence emitted from the protective layer  54  to measure the fluorescence intensity. If the protective layer  54  contains an ultraviolet light absorbing material or an infrared light absorbing material, an ultraviolet sensor or an infrared sensor is used as the detector  20 , and the protective layer  54  is irradiated with ultraviolet light or infrared light. The detector  20  measures the intensity (reflectance or transmittance) of light reflected from or transmitted through the protective layer  54 . Ultraviolet light refers to a radiation having a maximum absorption wavelength (λmax) range of 280 nm or more and 400 nm or less. Infrared light refers to a radiation having a maximum absorption wavelength (λmax) range of 780 nm or more and 1 mm or less. The wavelength range of visible light is from more than 400 nm to less than 780 nm. 
     The protective layer  54  has an identification mark  55  therein, and the measurement value of the detector  20  corresponding to the portion of the identification mark  55  is different from the measurement value of the detector  20  corresponding to the region other than the portion of the identification mark  55 . 
     For example, the identification mark  55  may be defined by a recessed portion having a thickness smaller than the region other than the portion of the identification mark  55 , as shown in  FIG. 3  and  FIG. 4 a   . Alternatively, the identification mark  55  may be defined by a protruding portion having a thickness larger than the region other than the portion of the identification mark  55 , as shown in  FIG. 4   b.    
     For example, the identification mark  55  may be defined by a protruding or recessed strip (line pattern) extending in the width direction (the transverse direction (short length direction) of the sheet perpendicular to the longitudinal direction of the sheet) of the thermal transfer sheet. In this instance, when the detector  20  irradiates the protective layer  54  of the thermal transfer sheet  5  fed and transported from the feeder  3  with ultraviolet light or infrared light and scans the protective layer  54  in the longitudinal direction, the measurement value varies at an edge of the identification mark  55 . The detector  20  thus detects the pattern of the identification mark  55  represented by the number of strips or portions, the width, the shape, the position or the like of the mark. 
     For example, in the case of the protective layer  54  containing a fluorescent brightening agent, the position at which the detector  20  starts receiving fluorescence corresponds to the front edge of the protective layer  54 . Subsequently, the position at which the fluorescence intensity increases (decreases) corresponds to the edge of one of the ends of the identification mark  55 , and then, the position at which the fluorescence intensity decreases (increases) corresponds to the edge of the other end of the identification mark  55 . The position from which the detector  20  no longer receives fluorescence corresponds to the rear edge of the protective layer  54 . 
     Plural types of thermal transfer sheet  5  may be loaded in the thermal transfer printing apparatus. The type of thermal transfer sheet  5  and the pattern (the number of strips or portions, the width, the shape, or the position) of the identification mark  55  are recorded in association with each other in a table T in a memory  12  that will be described later herein. For example, the number of strips of the identification mark  55  may vary depending on the type of thermal transfer sheet  5 , as shown in  FIGS. 5 a  and 5 b   . For example, the width of the identification mark  55  denoted by w 1  or w 2  may vary depending on the type of thermal transfer sheet  5 , as shown in  FIGS. 6 a  and 6 b   . For example, the position of the identification mark  55  in the longitudinal direction of the thermal transfer sheet may vary depending on the type of thermal transfer sheet  5 , as shown in  FIGS. 7 a  and 7 b   . For example, the identification mark  55  may be formed on a part in the transverse direction of the sheet, and the position of the identification mark  55  in the transverse direction of the sheet may vary depending on the type of thermal transfer sheet  5 , as shown in  FIGS. 8 a  and 8 b   . The type of thermal transfer sheet  5  may be represented by a combination of the number of strips or portions, the width, the shape, the position and the like of the identification mark  55 . 
     The identification mark  55  defined by a protruding strip or a recessed strip may extend in the longitudinal direction of the sheet, as shown in  FIG. 9 . The identification mark  55  is not necessarily in the shape of a straight line but may be in the shape of a wavy line. The identification mark  55  is not limited to a line pattern and may be a checkered pattern or a pattern in a shape of hart, star, spade or the like, as shown in  FIG. 10 . 
     A control device  10  controls the operation of members or components of the thermal transfer printing apparatus and operates for identification of the thermal transfer sheet  5  and printing. The control device  10  is a computer including a CPU (central processing unit) and a memory  12  including a flash memory, a ROM (Read-only Memory), and/or a RAM (Random Access Memory). The memory  12  stores a control program and the above-mentioned table T. The CPU executes the control program to implement an identification unit  11 . 
     The identification unit  11  refers to the table T and identifies the type of thermal transfer sheet  5  from the detection result of the detector  20  for the identification mark  55 . In the table T, suitable printing conditions (printing speed, energy applied for printing), the type of printing sheet  7  to be used, and other information may be recorded in association with each type of thermal transfer sheet  5 . If the type of the printing sheet  7  loaded in the thermal transfer printing apparatus is not suitable for the type of the identified thermal transfer sheet  5 , the control device  10  may output an alarm sound or a warning sign or may cancel the printing operation. 
     The structure of the thermal transfer sheet  5  will now be described. 
     [Substrate] 
     The substrate  50  used for the thermal transfer sheet  5  may be any known thermal transfer sheet, provided that it is resistant to heat to some extent and has some strength. Examples of such a substrate include polyethylene terephthalate films, 1,4-polycyclohexylenedimethylene terephthalate films, polyethylene naphthalate films, polyphenylene sulfide films, polystyrene films, polypropylene films, polysulfone films, aramid films, polycarbonate films, polyvinyl alcohol films, cellulose derivatives, such as cellophane and cellulose acetate, polyethylene films, polyvinyl chloride films, nylon films, polyimide films, ionomer films, and other resin films. 
     The thickness of the substrate  50  is generally approximately 0.5 μm or more and 50 μm or less and is preferably approximately 3.0 μm or more and 10 μm or less. The substrate  50  may be subjected to surface treatment to improve the adhesion to the layer to come into contact with the substrate  50 . The surface treatment may be corona discharge treatment, flame treatment, ozone treatment, ultraviolet treatment, radiation treatment, surface roughening treatment, chemical treatment, plasma treatment, grafting treatment, or any other known treatment for improving the surface of the resin. One or two or more of surface treatment techniques may be applied. 
     Among those surface treatment techniques, corona discharge treatment or plasma treatment are advantageous for low-cost production. Optionally, the substrate  50  may be provided with an undercoat layer on one or both of the surfaces thereof. Primer treatment for forming the undercoat layer may be performed by applying a primer liquid onto the unstretched plastic film extruded from a melt extruder and stretching the film. A rear primer layer (adhesive layer) may be formed between the substrate  50  and the rear surface layer  57  by coating. The rear primer layer may be formed of, for example, polyester-based resin, polyacrylate-based resin, polyvinyl acetate-based resin, polyurethane-based resin, styrene acrylate-based resin, polyacrylamide-based resin, polyamide-based resin, polyether-based resin, polystyrene-based resin, polyethylene-based resin, polypropylene-based resin, vinyl-based resin, such as polyvinyl chloride resin, polyvinyl alcohol resin, and polyvinylidene chloride resin, polyvinyl acetal-based resin, such as polyvinyl acetoacetal and polyvinyl butyral, and cellulose-based resin. 
     [Dye Layer] 
     Preferably, materials prepared by melting or dispersing a sublimable dye in a binder resin are used for the dye layers  52 . Examples of the sublimable dye include diarylmethane-based dyes; triarylmethane-based dyes; thiazole-based dyes; merocyanine dyes; pyrazolone dyes; methine-based dyes; indoaniline-based dyes; azomethine-based dyes, such as acetophenoneazomethine, pyrazoloazomethine, imidazolazomethine, imidazoazomethine, and pyridoneazomethine; xanthene-based dyes; oxazine-based dyes; cyanostyrene-based dyes, such as dicyanostyrene and tricyanostyrene; thiazine-based dyes; azine-based dyes; acridine-based dyes; benzene azo-based dyes; azo dyes, such as pyridone azo, thiophene azo, isothiazole azo, pyrrole azo, pyrazole azo, imidazole azo, thiadiazole azo, triazole azo, and disazo; spiropyran-based dyes; indolinospiropyran-based dyes; fluorane-based dyes; rhodamine lactam-based dyes; naphthoquinone-based dyes; anthraquinone-based dyes; and quinophthalone-based dyes. 
     The sublimable dye content in each dye layer is 5% by mass or more and 90% by mass or less, preferably 20% by mass or more and 80% by mass or less, relative to the total solid content of the dye layer. By controlling the content of the sublimable dye to be used, a preferred print density can be achieved, and degradation in storability can be reduced. 
     The binder resin used to hold the dye is, in general, resistant to heat and appropriately compatible with the dye. Examples of the binder resin include cellulose-based resins, such as ethyl cellulose, hydroxyethyl cellulose, ethyl hydroxy cellulose, hydroxypropyl cellulose, methyl cellulose, cellulose acetate, and cellulose butyrate; vinyl-based resins, such as polyvinyl alcohol, polyvinyl acetate, polyvinyl butyral, polyvinyl acetoacetal, and polyvinylpyrrolidone; acrylic resins, such as poly(meth)acrylates and poly(meth)acrylamide; polyurethane-based resins; polyamide-based resins; and polyester-based resins. Among these binder resins, cellulose-based resins, vinyl-based resins, acrylic resins, urethane-based resins, polyester-based resins, and the like are preferred in terms of, for example, heat resistance and dye transferability. Vinyl-bases resins are more preferred, and polyvinyl butyral, polyvinyl acetoacetal, and the like are particularly preferred. 
     The dye layers  52  may contain an additive, such as a release agent, inorganic particles, or organic particles. The release agent may be silicone oil, phosphoric acid ester, or the like. The inorganic particles may be particles of carbon black, aluminum, molybdenum disulfide, or the like. The organic particles may be polyethylene wax particles or the like. 
     The dye layers  52  may be formed by applying a coating liquid, which is prepared by dissolving or dispersing any of the above-cited dyes, the binder resin, and optionally added additives in an appropriate organic solvent or water, onto one of the surfaces of the substrate  50  by a known method, such as gravure printing, screen printing, or reverse roll coating using a gravure plate, and drying the applied coating liquid. 
     The organic solvent may be toluene, methyl ethyl ketone, ethanol, isopropyl alcohol, cyclohexanone, dimethylformamide [DMF], or the like. Each dye layer, when dried, has a thickness of approximately 0.2 μm or more and 6.0 μm or less, preferably approximately 0.2 μm or more and 3.0 μm or less. 
     [Protective Layer] 
     The protective layer  54  is made of a resin conventionally used for forming a protective layer, to which a fluorescent brightening agent, an ultraviolet light absorbing material, or an infrared light absorbing material is added. Examples of the resin used for forming a protective layer include polyester resin, polystyrene resin, acrylic resin, polyurethane resin, acryl urethane resin, vinyl chloride-vinyl acetate copolymer, silicone-modified resins of these resins, and mixtures of these resins. 
     Examples of the fluorescent brightening agent include fluorescein-based compounds, thioflavin-based compounds, eosin-based compounds, rhodamine-based compounds, coumarin-based compounds, imidazole-based compounds, oxazole-based compounds, triazole-based compounds, carbazole-based compounds, pyridine-based compounds, imidazolone-based compounds, naphthalic acid derivatives, stilbenedisulfonic acid derivatives, stilbenetetrasulfonic acid derivatives, and stilbenehexasulfonic acid derivatives. 
     Examples of the ultraviolet light absorbing material include organic ultraviolet light absorbing materials, such as benzotriazole-based compounds, triazine-based compounds, benzophenone-based compounds, and benzoate-based compounds, and inorganic ultraviolet light absorbing materials, such as titanium oxide, zinc oxide, cerium oxide, iron oxide, and barium sulfate. In particular, benzotriazole-based compounds are preferably used. 
     Examples of the infrared light absorbing material include diimonium-based compounds, aluminum-based compounds, phthalocyanine-based compounds, dithiol-based organic metal complexes, cyanine-based compounds, azo-based compounds, polymethine-based compounds, quinone-based compounds, diphenylmethane-based compounds, triphenylmethane-based compounds, and oxol-based compounds. 
     The protective layer  54  may be formed by, for example, gravure printing application of a coating liquid containing the above-described resin to which an above-described fluorescent brightening agent, ultraviolet light absorbing material or infrared light absorbing material is added, followed by drying. The plate cylinder used in the gravure printing has very small recesses called cells in the surface thereof. The recesses are filled with the coating liquid, and the coating liquid in the recesses is applied onto the substrate  50 . In the present embodiment, the protective layer  54  having a recessed or protruding portion (identification mark  55 ) having a varied thickness is formed by adjusting the recess or protrusion formed at the surface of the plate cylinder. 
     The thickness of the protective layer  54  (region other than the portion of the identification mark  55 ), when dried, is preferably 0.1 μm or more and 2.0 μm or less. The thickness of the portion of the identification mark  55  is preferably 65% or more and 80% or less or 125% or more and 150% or less relative to the thickness of the region other than the portion of the identification mark  55 . 
     For a recessed identification mark  55 , when the thickness of the portion of the identification mark  55  is 80% or less relative to the thickness of the region other than the portion of the identification mark  55 , the values, detected by the detector  20 , of the identification mark  55  and the other region have a sufficient difference, and thus the identification mark  55  is easily detected. Also, when the thickness of the portion of the identification mark  55  is 65% or more relative to the thickness of the region other than the portion of the identification mark  55 , the recess or protrusion of the identification mark  55  will be inconspicuous on the printed cut sheet  7   a  having a thermally transferred image. 
     For a protruding identification mark  55 , when the thickness of the portion of the identification mark  55  is 125% or more relative to the thickness of the region other than the portion of the identification mark  55 , the values, detected by the detector  20 , of the identification mark  55  and the other region have a sufficient difference, and thus the identification mark  55  is easily detected. Also, when the thickness of the portion of the identification mark  55  is 150% or less relative to the thickness of the region other than the portion of the identification mark  55 , the recess or protrusion of the identification mark  55  will be inconspicuous on the printed cut sheet  7   a  having a thermally transferred image. 
     [Rear Surface Layer] 
     The thermal transfer sheet  5  includes the rear surface layer  57  on the surface of the substrate  50  opposite the dye layers  52  and the protective layers  54 . The rear surface layer  57  is disposed on that surface of the substrate  50  to increase the runnability for the thermal head  1  during printing, as well as heat resistance. 
     The rear surface layer  57  is made of a material appropriately selected from the known thermoplastic resins and the like. Examples of such a thermoplastic resin include polyester-based resins, polyacrylate-based resins, polyvinyl acetate-based resins, styrene acrylate-based resins, polyurethane-based resins, polyolefin-based resins, such as polyethylene-based resins and polypropylene-based resins, polystyrene-based resins, polyvinyl chloride-based resins, polyether-based resins, polyamide-based resins, polyimide-based resins, polyamide-imide-based resins, polycarbonate-based resins, polyacrylamide resin, polyvinyl chloride resin, polyvinyl acetal resins such as polyvinyl butyral resin and polyvinyl acetoacetal resin, and silicone-modified forms of these thermoplastic resins. 
     A curing agent may be added to the thermoplastic resin. The curing agent may be selected from the known polyisocyanate resins without particular limitation, and it is desirable to use an aromatic isocyanate adduct. Examples of such an aromatic polyisocyanate include 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, a mixture of 2,4-toluene diisocyanate and 2,6-toluene diisocyanate, 1,5-naphthalene diisocyanate, tolidine diisocyanate, p-phenylene diisocyanate, trans-cyclohexane-1,4-diisocyanate, xylylene diisocyanate, triphenylmethane triisocyanate, and tris(isocyanatophenyl) thiophosphate. In particular, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, and a mixture of 2,4-toluene diisocyanate and 2,6-toluene diisocyanate are preferable. Such a polyisocyanate resin causes the hydroxy group of the above-described hydroxy-including thermoplastic resin to form crosslinks, thus increasing the strength and the heat resistance of the coating film for the rear surface layer  57 . 
     In addition to the thermoplastic resin, the rear surface layer  57  may contain wax, a higher fatty acid amide, a phosphate ester compound, metal soap, silicone oil, a surfactant or any other release agent, fluororesin powder or any other organic powder, inorganic particles of silica, clay, talc, calcium carbonate, or the like, and other additives to increase slip properties. 
     The rear surface layer  57  is formed by applying a coating liquid, which is prepared by, for example, dispersing or dissolving the above-cited thermoplastic resin and optional additives in an appropriate solvent, onto the surface of the substrate  50  opposite the dye layers  52  and the protective layers  54  by a known method, such as gravure printing, screen printing, or reverse roll coating using a gravure plate, and drying the applied coating liquid. The thickness of the rear surface layer, when dried, is preferably 3 μm or less, more preferably 0.1 μm or more and 2 μm or less, from the viewpoint of increasing heat resistance or the like. 
     In printing operation using the thermal transfer sheet  5 , the printing sheet  7  and the Y layer of the dye layers  52  are first caused to correspond in position to each other, and the thermal head  1  is brought into contact with the platen roll  2  with the printing sheet  7  and the thermal transfer sheet  5  interposed therebetween. Then, the printing sheet  7  and the thermal transfer sheet  5  are transported backward by driving for rotation of the capstan roller  9 a and the collecting unit  4 . During this operation, the region of the Y layer is selectively heated by the thermal head  1  on the basis of image data, so that Y is sublimated and transferred onto the printing sheet  7  from the thermal transfer sheet  5 . 
     After the sublimation transfer of Y, the thermal head  1  rises to separate from the platen roll  2 . Subsequently, the printing sheet  7  and the M layer are caused to correspond in position to each other. In this instance, the printing sheet  7  is transported forward by a distance corresponding to the print size, while the thermal transfer sheet  5  is transported backward by a distance corresponding to the margin between the Y layer and the M layer. 
     M and C are sublimated to be transferred one after the other onto the printing sheet  7  on the basis of image data in a manner similar to the sublimation transfer of Y, thus forming an image on the printing sheet  7 . 
     After the image formation, the printing sheet  7  and the protective layer  54  are caused to correspond in position to each other, and the protective layer  54  is heated by the thermal head  1 , thus transferred from the thermal transfer sheet  5  onto the printing sheet  7  so as to cover the image. In the protective layer  54 , the portion of the identification mark  55  has a thickness of 65% or more and 80% or less or 125% or more and 150% or less relative to the thickness of the region other than the portion of the identification mark  55 . Accordingly, the identification mark  55  in the protective layer after being transferred cannot be perceived by the naked human eye, not affecting the finished quality of the resulting printed item. 
     From the viewpoint of preventing the appearance of the printed item from being affected by an unwanted change of the portion of the identification mark  55  caused due to the storage period or the storage environment of the printed item, the identification mark  55  may be formed at the periphery of the protective layer  54  that is outside the print region so as not to be transferred to the printing sheet  7 . Also, from the viewpoint of reducing the effect on the appearance of the printed item, a linear identification mark  55  may be positioned only at the periphery of the printed item. 
     In the present embodiment, since the identification mark  55  is formed in the protective layer  54  but not in the dye layers  52 , color properties are not affected. In gravure printing, since a coating liquid containing an invisible light absorbing material for forming the protective layer can be applied after adjusting the recess or protrusion at the surface of the plate cylinder, the number of application process steps for forming the identification mark  55  does not increase, and, accordingly, production cost does not increase. The identification mark  55  may have either a recessed portion or a protruding portion or both in combination. 
     Although the above-described embodiment has described an example in which the pattern (the number of strips or portions, the width, the shape, the position, or the like) of the identification mark  55  formed in the protective layer  54  is varied for each type of thermal transfer sheet  5 , the content (added concentration) of the invisible light absorbing material relative to the resin for forming the protective layer may be varied for each type of thermal transfer sheet  5  (without varying the thickness of the protective layer  54 ). In this instance, the value detected by the detector  20  varies depending on the type of thermal transfer sheet  5 . The type of thermal transfer sheet  5  and the intensity of transmitted light or reflected light are recorded in association with each other in the table T in the memory  12 . 
     Also, the invisible light absorbing material added to the resin for forming the protective layer may be changed for each type of thermal transfer sheet  5 . In this instance, ultraviolet or infrared light absorption wavelength varies depending on the type of thermal transfer sheet  5 . In the table T in the memory  12 , absorption wavelength is recorded in association with each type of thermal transfer sheet  5 . 
     As shown in  FIG. 11 , the identification mark  55  may be located at a back end of the protective layer  54  in the longitudinal direction of the sheet so as to be used as a detection mark to determine the position of the following dye layer  52  (Y layer). The identification mark  55  may be located in a region not transferred to the printing sheet  7 , for example, in the vicinity of the Y layer. 
     The above-described embodiment has described an example in which the type of thermal transfer sheet  5  is identified by providing the identification mark  55  (first identification mark) formed in the protective layer  54 . As with the thermal transfer sheet  5 , the printing sheet  7  may be provided with an identification mark (second identification mark) to identify the type of the printing sheet. 
       FIG. 12  is a schematic diagram of the structure of a thermal transfer printing apparatus configured to identify also the type of printing sheet  7 ,  FIG. 13  is a plan view of a printing sheet  7 , and  FIGS. 14 a  and 14 b    are each a sectional view of a printing sheet  7 . While the thermal transfer printing apparatus shown in  FIG. 1  is provided with a detector  20  (first detector), the thermal transfer printing apparatus shown in  FIG. 12  is different in that it is provided with the detector  20  (first detector) and a detector  60  (second detector). 
     The printing sheet  7  has a receiving layer  71  on one surface of a substrate  70  and a rear surface layer  72  on the other surface. The substrate  70  and the receiving layer  71  are provided with an intermediate layer  73  interposed therebetween to increase adhesion between the substrate  70  and the receiving layer  71 . The printing sheet  7  may include further layers. 
     The intermediate layer  73  contains an invisible light absorbing material. The invisible light absorbing material may be, for example, a fluorescent brightening agent, an ultraviolet light absorbing material, or an infrared light absorbing material. The invisible light absorbing material in the intermediate layer  73  is different from the invisible light absorbing material in the protective layer  54 . The detector  20  is suitable for the type of the invisible light absorbing material contained in the protective layer  54 , and the detector  60  is suitable for the type of the invisible light absorbing material contained in the intermediate layer  73 . 
     If the intermediate layer  73  contains a fluorescent brightening agent, a fluorescence sensor is used as the detector  60 . The sensor irradiates the printing sheet  7  with ultraviolet light and receives fluorescence emitted from the printing sheet  7 , thus measuring the fluorescence intensity. If the intermediate layer  73  contains an ultraviolet light absorbing material or an infrared light absorbing material, an ultraviolet sensor or an infrared sensor is used as the detector  60 , and the printing sheet  7  is irradiated with ultraviolet light or infrared light, thus measuring the intensity (reflectance or transmittance) of reflected light or transmitted light. 
     If the detector  20  and the detector  60  are disposed close to each other as shown in  FIG. 12 , a light source for ultraviolet irradiation may be shared with the detectors. Ultraviolet light emitted from the light source is transmitted through the protective layer  54 , and the intermediate layer  73  is irradiated with the ultraviolet light. The ultraviolet light reflected from the intermediate layer  73  or the fluorescence emitted from the intermediate layer  73  is transmitted through the protective layer  54  and detected by the detector  60 . 
     The detector  60  may be disposed between the printing unit  40  and the printing paper roll  6 . 
     The intermediate layer  73  of the printing sheet  7  has an identification mark  75  therein, and the measurement value of the detector  60  by measuring the portion of the identification mark  75  is different from the measurement value of the detector  60  by measuring the region other than the portion of the identification mark  75   
     For example, the identification mark  75  in the intermediate layer  73  may be defined by a recessed portion having a thickness smaller than the region other than the portion of the identification mark  75 , as shown in  FIG. 14 a   . Alternatively, the identification mark  75  in the intermediate layer  73  may be defined by a protruding portion having a thickness larger than the region other than the portion of the identification mark  75 , as shown in  FIG. 14   b.    
     For example, the identification mark  75  may be defined by a protruding or recessed strip (line pattern) extending in the width direction (the transverse direction (short length direction) of the sheet perpendicular to the longitudinal direction of the sheet) of the printing sheet  7 . In this instance, when the detector  60  irradiates the printing sheet  7  fed and transported from the printing paper roll  6  with ultraviolet light or infrared light and scans the printing sheet  7  in the longitudinal direction, the measurement value varies at an edge of the identification mark  75 . The detector thus can detect the pattern of the identification mark  75  represented by the number of strips or portions, the width, the shape, the position and the like of the mark. Identification marks  75  are provided at regular intervals. 
     For example, in the case of the intermediate layer  73  containing a fluorescent brightening agent, the position at which the intensity of fluorescence received by the detector  60  increases (decreases) corresponds to the edge of one of the ends of the identification mark  75 , and then, the position at which the fluorescence intensity decreases (increases) corresponds to the edge of the other end of the identification mark  75 . 
     Plural types of printing sheet  7  may be loaded in the thermal transfer printing apparatus. The type of printing sheet  7  and the pattern (the number of strips or portions, the width, the shape, and the position) of the identification mark  75  are recorded in association with each other in the table T in the memory  12 . For example, the number of strips, the width, the position or the like of the identification mark  75  varies depending on the type of printing sheet  7 . 
     The identification unit  11  refers to the table T and identifies the type of printing sheet  7  from the detection result of the detector  60  for the identification mark  75 . 
     In the table T, preferred combinations between thermal transfer sheets  5  and printing sheets  7  may be registered. If the type of thermal transfer sheet  5  and the type of printing sheet  7  that have been identified by the identification unit  11  do not correspond to any of the registered combinations, the control device  10  may output an alarm sound or a warning sign or may cancel the printing operation. 
     After printing operation in the printing unit  40 , a cutter  8  cuts the printing sheet  7  in the width direction at the boundary between a region for a printed cut sheet and a region for a margin. The region for a printed cut sheet is ejected as the printed cut sheet  7   a  through the ejection port. On the other hand, the region for a margin is cut off as a margin piece and collected in a collection container (not shown) disposed right under the cutter  8 . 
     The image is printed slightly larger than the region for the printed cut sheet. Thus, the image is printed over the entire surface of the printed cut sheet  7   a  even if the cutting position of the cutter  8  is slightly shifted. 
     The above-cited identification mark  75  may be formed in the region of a margin that will be collected as a margin piece. 
     The substrate  70  of the printing sheet  7  may be high-quality paper, coated paper, resin-coated paper, art paper, cast-coated paper, paperboard, synthetic paper (polyolefin-based paper, polystyrene-based paper), synthetic resin or emulsion-impregnated paper, synthetic rubber latex-impregnated paper, synthetic resin internally added paper, cellulose fiber paper, or the like. The thickness of the substrate  70  may be, but is not limited to, approximately 10 μm or more and 300 μm or less. 
     The receiving layer  71  contains a binder resin and a release agent. The binder resin may be a known resin material that can easily receive the dyes contained in the dye layers of the thermal transfer sheet. The release agent is intended to facilitate easy release of the thermal transfer sheet from the dye layers and may be silicone oil, polyethylene wax, amide wax, or a fluorine-based or phosphate ester-based surfactant, or the like. 
     The rear surface layer  72  may be a layer having a desired function, appropriately selected in accordance with the use of the printing sheet  7 . For example, a rear surface layer  72  having a function to facilitate the transfer of the printing sheet  7  or a function to prevent curling is preferably used. 
     For the intermediate layer  73 , an invisible light absorbing material is added to a known resin functioning as a good adhesive between the substrate  70  and the receiving layer  71 . Examples of such a resin include polyurethane resin, acrylic resin, polyethylene resin, polypropylene resin, and epoxy resin. 
     The thickness of the intermediate layer  73  (region other than the portion of the identification mark  75 ), when dried, is preferably 0.1 μm or more and 2.0 μm or less. The thickness of the portion of the identification mark  75  is preferably 65% or more and 80% or less or 125% or more and 150% or less relative to the thickness of the region other than the portion of the identification mark  75 . 
     For a recessed identification mark  75 , when the thickness of the portion of the identification mark  75  is 80% or less relative to the thickness of the region other than the portion of the identification mark  75 , the values, detected by the detector  60 , of the identification mark  75  and the other region have a sufficient difference, and thus the identification mark  75  is easily detected. Also, when the thickness of the portion of the identification mark  75  is 65% or more relative to the thickness of the region other than the portion of the identification mark  75 , the recess or protrusion of the identification mark  75  will appear inconspicuously at the surface of the receiving layer  71 . If the identification mark  75  is formed in a region for a margin, the recess or protrusion will not appear at the surface of the printed cut sheet  7   a.    
     For a protruding identification mark  75 , when the thickness of the portion of the identification mark  75  is 125% or more relative to the thickness of the region other than the portion of the identification mark  75 , the values, detected by the detector  60 , of the identification mark  75  and the other region have a sufficient difference, and thus the identification mark  75  is easily detected. Also, when the thickness of the portion of the identification mark  75  is 150% or less relative to the thickness of the region other than the portion of the identification mark  75 , the recess or protrusion of the identification mark  75  will be inconspicuous on the printed cut sheet  7   a  having a thermally transferred image. If the identification mark  75  is formed in a region for a margin, the recess or protrusion will not appear at the surface of the printed cut sheet  7   a , as described above. 
     The identification mark  75  may have either a recessed portion or a protruding portion or both in combination. 
     Although an example in which the pattern (the number of strips or portions, the width, the shape, the position, and the like) of the identification mark  75  is varied for each type of printing sheet  7 , the content of the invisible light absorbing material in the intermediate layer  73  may be varied for each type of printing sheet  7  (without varying the thickness of the intermediate layer  73 ). In this instance, the value (received light intensity) detected by the detector  60  varies depending on the type of printing sheet  7 . Types of printing sheet  7  and detection values are recorded in association with each other in the table T in the memory  12 . 
     When the detector  20  and the detector  60  are disposed close to each other so as to share the ultraviolet emission light source with each other and detect the identification mark  55  and the identification mark  75  in a state where the protective layer  54  and the printing sheet  7  lie one on the other, as shown in  FIG. 12 , the identification mark  55  and the identification mark  75  may be detected simultaneously or independently. 
     For simultaneously detecting the identification mark  55  and the identification mark  75 , if the invisible light absorbing material in the protective layer  54  and the invisible light absorbing material in the intermediate layer  73  are the same, it is difficult to determine which the identification mark  55  or the identification mark  75  has produced a change in light intensity detected by the detector. 
     For simultaneously detecting the identification mark  55  and the identification mark  75 , it is therefore preferable that the invisible light absorbing material contained in the protective layer  54  and the invisible light absorbing material contained in the intermediate layer  73  be different from each other. Particularly in view of the quality of printed items (printed cut sheets  7   a ) to be produced, it is preferable that the protective layer  54  contains an ultraviolet light absorbing material, while the intermediate layer  73  contains a fluorescent brightening agent. 
     The intermediate layer  73  is irradiated with ultraviolet light transmitted through the identification mark  55  of the protective layer  54 . From the viewpoint of reducing the attenuation of ultraviolet light by the transmission through the identification mark  55  so that the intermediate layer  73  can be irradiated with ultraviolet with a sufficient intensity, the identification mark  55  is preferably defined by a recessed form. 
     The present invention is not limited to the above described embodiments as they are, and the elements thereof may be modified without departing from the scope and spirit of the invention when the invention is implemented. Also, appropriate combinations of the elements or components disclosed in the above-described embodiments can lead to various inventions. For example, some of the elements used in the embodiments may be omitted. Furthermore, some elements used in an embodiment may be combined with elements used in another embodiment as required. 
     The present application is based on Japanese Patent Application No. 2017-148112 filed on Jul. 31, 2017 and Japanese Patent Application No. 2018-008302 filed on Jan. 22, 2018, the entirety of which is incorporated herein by reference. 
     REFERENCE SIGNS LIST 
       1  thermal head 
       2  platen roll 
       3  feeder 
       4  collecting unit 
       5  thermal transfer sheet 
       7  printing sheet 
       10  control device 
       11  identification unit 
       12  memory 
       20  detector (first detector) 
       40  printing unit 
       50  Substrate 
       52  dye layer 
       54  protective layer 
       55  identification mark 
       60  detector (second detector) 
       75  identification mark