Patent Publication Number: US-6339443-B1

Title: Thermal head, thermal printer and thermal printing method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a thermal head, a thermal printer and a thermal printing method. More particularly, the present invention relates to a thermal head, a thermal printer and a thermal printing method in which surface gloss of recording material is prevented from being lowered after the printing operation at the thermal head. 
     2. Description Related to the Prior Art 
     In a color direct thermal recording method of an optical fixation type, a color thermosensitive recording material is used, and includes yellow (Y), magenta (M) and cyan (C) coloring layers overlaid one on another. The coloring layers are heated to develop colors, to record a full-color image. Each of the coloring layers includes micro capsules, coupler and binder. The micro capsules have a sub-micron size, and include a diazonium salt compound as precursor of azo dye as a coloring substance. The coupler and the binder quicken the color development of the micro capsules. In the color development, each coloring layer is heated by a thermal head, to change partitions between the micro capsules to the light-transmitting state, so that the coupler is introduced to the micro capsules to develop the color. In the heating for the magenta, the yellow is prevented from being colored. In the heating for the cyan, the magenta is prevented from being colored. For this prevention, the precursor of the coloring substance of each color is decomposed by application of ultraviolet rays, near ultraviolet rays or visible violet rays, so that each upper coloring layer is kept from being further colored while a coloring layer being next underlaid is heated with relative high heat energy. 
     In FIG. 25, curves of coloring characteristics of the recording material are illustrated in a graph. The curves represent relationships between coloring density of each of the coloring layers and coloring heat energy generated by heating elements while the thermal head is pressed against the recording material. As is understood from the graph, it is necessary in the color direct thermal recording to use the dynamic range of the coloring heat energy without overlapping between three coloring layers disposed to lie in respective depths in the recording material. If it is desired to set the thermosensitivity of the coloring layers nearly equal to that of other printing methods such as thermal wax transfer printing, then it is required to set a range of the coloring heat energy three times as great as a range of the coloring heat energy according to the thermal wax transfer method. However a range of the coloring heat energy for the three colors is actually set 1.5 or less times as great as a range of the coloring heat energy according to the thermal wax transfer method. This is due to limited heat resistance of the recording material. 
     Ink ribbon used in the thermal wax transfer method as recording material is discarded after the printing operation. It is possible to construct the ink ribbon only in view of high suitability to thermal printing without considering its final appearance after the printing operation. In contrast final appearance of the recording material for the color direct thermal recording is important after the printing operation, because the recording material should become a print as a finished product in a manner similar to an image receiving sheet used in the thermal wax transfer method. Consequently the recording material must have sufficiently high rigidity and heat capacity. In general it is difficult to contact the recording material being rigid and including paper in a state of readily conducting heat. As is known in the art, the color direct thermal recording requires heat control with higher precision than other thermal printing methods. Furthermore, the color direct thermal recording is associated with a heat contacting condition more difficult than that of other thermal printing methods. It follows in the color direct thermal recording that more stable heat contact should be effected than other thermal printing methods. 
     In the printer of the color direct thermal recording, the thermal head having partial glaze formed locally in a ridge-shape is used to stabilize heat contact between the thermal head and the recording material, for the purpose of strengthen a head touch of the recording material. The heating elements of the thermal head are arranged on the partial glaze to heighten pressure in the contact by pressing the recording material by a platen roller. The thermal head known in the prior art has the heating elements of which the center is positioned at the top of protruded shape of the partial glaze. Disposition of the platen roller, a pressing condition and a material conveying condition are determined in consideration of stabilized contacting condition of the recording material with the thermal head. 
     Irrespective of states in which the recording material is pressed against the protruding portion of the partial glaze by the platen roller, there is tension applied to the recording material in a system where a pair of conveyor rollers convey the recording material by drawing it from between the thermal head and the platen roller. In a range downstream from the top of the partial glaze in the conveying direction of the recording material, the tension causes a downstream portion of the recording material to come away from the partial glaze. It is likely that the contacting condition between the recording material and the heating elements at the glaze top is influenced by changes in the tension, irregularity in rotation of the platen roller, and changes in pressure. The contacting state becomes unstable, to change coloring density in an unstable manner. 
     The type of the recording material is a direct recording medium, of which its recording surface directly heated by the thermal head at high temperature becomes a finally image-reproducing surface. Influence of heat application remains on the surface of the obtained print in a conspicuous manner in comparison with thermal printing with the ink ribbon or the like. Among the coloring layers, the coloring heat energy of the highest value is required to color the cyan coloring layer underlying the lowest of the three. If the cyan is colored at its maximum density, the thermal head becomes as hot as 200 degrees centigrade. If the recording material comes away from the thermal head immediately after passage of the heating elements, the pressure to the recording material abruptly comes down despite the state of the high temperature of the surface and the inside of the recording material. Gas is likely to occur inside the recording material to create blisters or bubbles. The surface of the recording material is likely to be roughened. The surface gloss of the recording material will be lowered. 
     In any of known methods, it is impossible in the color direct thermal recording to discharge heat from the recording material after passing the recording material. In the range downstream from the top of the partial glaze and upstream from a sheet outlet of the printer, the contact between the thermal head and the recording material is unstable. No good gloss on the recording material is obtainable. 
     There are various smoothing methods as disclosed in JP-A 2-215569, JP-A 2-233281 and U.S. Pat. No. 5,179,391 (corresponding to JP-A 3-21460), in which the recording material provided with minute protrusions and recesses on its surface is smoothed in a post-process to heighten its gloss. However those require an additional device and additional material for the post-process of glossing separately succeeding to the thermal printing process. The post-process requires manual operation, to complicate the operation of the entirety to a somewhat great extent. It is certain that the device for the post-process could be incorporated in a thermal printer. However the printer thus constructed would be excessively large and expensive. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing problems, an object of the present invention is to provide a thermal head, a thermal printer and a thermal printing method in which surface gloss of recording material is prevented from being lowered after the printing operation at the thermal head. 
     Another object of the present invention is to provide a thermal head, a thermal printer and a thermal printing method in which surface irregularity is prevented on recording material from occurring after the printing operation at the thermal head. 
     In order to achieve the above and other objects and advantages of this invention, a thermal printer includes a conveyor for conveying thermosensitive recording material in a predetermined conveying direction. A thermal head applies heat to the recording material being conveyed, to record an image to the recording material. The thermal head incorporates plural heating elements, arranged in an array crosswise to the conveying direction, for generating the heat. The thermal head includes a contact region predetermined for pressing the recording material, a center of the contact region being positioned down stream from a center of the heating elements with reference to the conveying direction. 
     The contact region includes a heating surface, disposed on an outside of the heating elements, for conducting the heat to the recording material. A cooling surface is disposed downstream adjacent to the heating surface in the conveying direction, for cooling the recording material. 
     A platen member is disposed opposite to the thermal head, for supporting a back of the recording material pressed by the thermal head. 
     The contact region further includes a pre-contact surface disposed upstream adjacent to the heating surface in the conveying direction, and the thermal head satisfies a condition of: 
     
       
         LCRL&gt;UCRL 
       
     
     where UCRL is a length of the pre-contact surface with reference to the conveying direction, and LCRL is a length of the cooling surface with reference to the conveying direction. 
     The platen member is disposed upstream offset from the center of the heating elements in the conveying direction. 
     The thermal head includes a base plate. A partial glaze is disposed to project from the base plate in a ridge shape with smooth convexity, the heating elements being arranged on the partial glaze, the partial glaze pressing the recording material. The heating elements are disposed upstream offset from a center of the partial glaze in the conveying direction. 
     In another preferred embodiment, the platen member is disposed upstream offset from a center of the partial glaze in the conveying direction. 
     In still another preferred embodiment, a rise surface is disposed between the heating surface and an upstream distal end of the partial glaze with reference to the conveying direction, and at least partially curved at a first radius of curvature. The cooling surface is flat or curved at a predetermined radius of curvature, the predetermined radius being greater than the first radius. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objects and advantages of the present invention will become more apparent from the following detailed description when read in connection with the accompanying drawings, in which: 
     FIG. 1 is a cross section illustrating an arrangement of a thermal head, thermosensitive recording material and a platen roller; 
     FIG. 1A is a cross section illustrating the recording material with layers; 
     FIG. 2 is a graph illustrating changes in temperature of the recording material and heating elements with time; 
     FIG. 3 is a graph illustrating the same as FIG. 2 but in enlargement in the axial direction of the time; 
     FIG. 4 is a graph illustrating a simulated relationship between the offset distance OSL 1  of the heating elements and changes in temperature of the cyan coloring layer; 
     FIG. 5 is a perspective, partially in section, illustrating another preferred thermal head; 
     FIG. 6 is a plan illustrating the thermal head; 
     FIG. 7 is an explanatory view in elevation, illustrating a thermal printer including the thermal head; 
     FIG. 8 is a cross section illustrating an arrangement of the thermal head, the recording material and a platen roller; 
     FIG. 9 is an explanatory view in elevation, illustrating the thermal printer; 
     FIG. 10 is a cross section illustrating another preferred arrangement of a thermal head, the recording material and a platen roller; 
     FIG. 10A is a cross section illustrating an arrangement of the thermal head, the recording material and the platen roller in a manner opposite to FIG. 10; 
     FIG. 11 is a graph illustrating an experimented relationship between occurrence of a blister and the offset distance OSL 4 , together with the coloring density; 
     FIG. 12 is a cross section illustrating still another preferred embodiment in which the center P 2  of heating elements is downstream offset from the center P 1  of a contact region; 
     FIG. 13 is a cross section illustrating another preferred embodiment similar to that of FIG. 12 but in which a thermal head contacts the recording material in a different manner; 
     FIG. 14 is a cross section illustrating a preferred embodiment in which a straight line CL passing through the heating element center TCP of heating elements and the roller center RC of a platen roller is inclined with reference to a direction AL of conveyance of the recording material; 
     FIG. 15 is a perspective, partially in section, illustrating a preferred thermal head in which partial glaze is shaped asymmetrically; 
     FIG. 16 is an explanatory view in cross section, illustrating the thermal head with indications of dimensions; 
     FIG. 17 is an explanatory view in elevation, illustrating a thermal printer including the thermal head; 
     FIG. 18 is an explanatory view in cross section, illustrating an arrangement of the thermal head, the recording material and the platen roller; 
     FIG. 19 is a graph illustrating a relationship between the cooling surface length LCRL and surface gloss; 
     FIG. 20 is an explanatory view in cross section, illustrating another preferred thermal head; 
     FIG. 21 is an explanatory view in cross section, illustrating yet another preferred thermal head; 
     FIG. 22 is an explanatory view in cross section, illustrating another preferred thermal head; 
     FIG. 23 is an explanatory view in elevation, illustrating a preferred thermal head of a corner edge type; 
     FIG. 24 is an explanatory view in cross section, illustrating a preferred thermal head of an end face type; and 
     FIG. 25 is a graph illustrating curves of coloring characteristics of the recording material. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE PRESENT INVENTION 
     FIG. 1 illustrates a model of a relationship of a thermal head and a thermosensitive recording material as considered experimentally in thermal analysis according to the finite element method. A thermosensitive recording material  2  is placed on a heating element array  3  or ridge-shaped head component. The ridge-shaped head component  3  includes a partial glaze  4  and plural heating elements  5  arranged on the partial glaze  4 . In the experimental model, the heating elements  5  have a length TL=220 μm as viewed in a material conveying direction being horizontal. The thermal head is constructed to contact the recording material  2  in a range of a contact region CR, which has a contact region length CRL=500 μm as viewed in the conveying direction. Let RC be a roller center of a platen roller  6 . Let PCL be a center or vertical center line on which the roller center RC lies. Let TCL be a heating element center or vertical center line on which a center of the heating elements  5  lies. Let OSL 1  be an offset distance between the center PCL and the heating element center TCL. In simulation, the offset distance OSL 1  was set at values of 120 μm, 60 μm, 0 μm, −60 μm and −120 μm. Thermal changes of a cyan coloring layer of the recording material  2  were calculated assuming that the cyan coloring layer is heated with exactly sufficient heat for cyan maximum density. Note that, with the offset distance OSL 1 , the positive sign “+” means an upstream side in the conveying direction. The negative sign “−” means a downstream side in the conveying direction. In the drawing, the arrow indicates the conveying direction of the recording material  2 . 
     For characteristics of the recording material  2 , see FIG.  25 . The recording material  2  has a layer structure in FIG. 1A, and includes a support  2   a,  a cyan coloring layer  2   b,  a magenta coloring layer  2   c,  a yellow coloring layer  2   d,  and a protective layer  2   e.    
     Simulation of the printing was conditioned as follows: 
     Recording material: A4-G50 (trade name) manufactured by Fuji Photo Film Co., Ltd. 
     Thermal printer: NC-501 (trade name) manufactured by Fuji Photo Film Co., Ltd. 
     Printing speed: 10 mm/sec. 
     Period of powering: 10 msec. 
     Duration of powering: 6 msec. 
     Electric power: 0.11 W per one dot. 
     FIGS. 2 and 3 illustrate characteristics of cooling of the recording material and the thermal head conditioned as described above. In both of FIGS. 2 and 3, a horizontal axis is taken for time, and a vertical axis is taken for temperature in centigrade. Changes in the temperature with reference to the time in the course of the cooling are indicated for both the recording material and the thermal head. As the speed in the change of the temperature is extremely different between the recording material and the thermal head, it is difficult in a single drawing to depict two curves in an apparently curved manner. In FIG. 2, the time on the horizontal axis is expressed in seconds. In FIG. 3, the time on the horizontal axis is expressed in milliseconds or msec. Room temperature was 27° C. The thermal head is constructed with extremely rapid changeability in temperature, and is cooled down to the room temperature even after 10 milliseconds. In contrast, the recording material is slow in the change of its temperature, and is cooled down to the room temperature only after 35 seconds as a result of an experiment. 
     The result obtained from the simulation in the thermal analysis is depicted in FIG. 4, that is a graph illustrating a relationship between the offset distance OSL 1  of the heating elements  5  and changes in temperature of the cyan coloring layer. The horizontal axis is taken for the distance x in μm or a relative on the recording material. The vertical axis is taken for the temperature in ° C. Curves A-E indicate results of setting the offset distance OSL 1  at the five different values: Curve A for OSL 1 =120 μm, Curve B for OSL 1 =60 μm, Curve C for OSL 1 =0 μm, Curve D for OSL 1 =−60 μm, and Curve E for OSL 1 =−120 μm. It was observed that the temperature after passage of the heating elements changed and depended upon the changes in the offset distance OSL 1  of the heating elements. 
     In FIG. 1, the contact region CR where the thermal head contacts the recording material  2  is split into a pre-contact surface UCR, a heating surface MCR and a post-contact surface LCR. The heating surface MCR is located outside the heating elements  5  to apply heat to the recording material  2 . The pre-contact surface UCR is located upstream from the heating surface MCR. The post-contact surface LCR is located downstream from the heating surface MCR. As a result of the experiment, a post-contact surface length LCRL of the post-contact surface LCR with reference to the conveying direction was found to have a greater effect to changes in the density and surface smoothness or gloss than the length of the heating surface MCR or that of the pre-contact surface UCR. 
     When the post-contact surface length LCRL was changed, the density was changed. The smaller the post-contact surface length LCRL was, the more dependent the density was upon the post-contact surface length LCRL. It is concluded that changes in the density were reduced by enlarging the post-contact surface length LCRL in comparison with that according to the prior art. When the post-contact surface length LCRL was sufficiently greater, a separating point PP, where the heating elements start separating from the recording material, was the farther from the heating elements  5 . Even when the separating point PP changed due to changes in tension of the recording material  2 , there was only reduced influence due to the changes in the separating point PP, so that changes the density were reduced. Moreover, the post-contact surface LCR was smoothed and flat to press and cool the recording material. A surface of the recording material  2  was provided with high quality and much gloss. With the post-contact surface LCR maintained long in the conveying direction, there were only small influence of changes in the tension of the recording material  2 . Only small changes in pressure occurred even with the changes in the tension. Consequently the changes in density due to the changes in the pressure were reduced. It was possible to stabilize the contacting condition between the recording material  2  and the ridge-shaped head component  3 , reduce changes in the density, and avoid lowering the gloss. The recording material  2  is cooled by pressure of the post-contact surface LCR, to heighten the gloss of the recording material  2 . 
     Note that the post-contact surface LCR is hereinafter referred to as cooling surface LCR. 
     To determine the cooling surface length LCRL greater than that in the prior art, the thermal head is pressed against the recording material by setting the center of the contact region CR downstream from the heating element center TCL of the heating elements. To condition the thermal head in this manner, it is possible to position a center of the platen roller downstream from the heating element center TCL. 
     In the thermal head in which the heating elements are arranged on the partial glaze, it is possible to dispose the heating elements in an upstream offset position from the center of the partial glaze for the purpose of ensuring greatness of the cooling surface length LCRL. A platen member such as a platen roller is disposed in an upstream offset position from the center of the partial glaze. 
     In the foregoing description, the heating elements are offset relative to the partial glaze. Alternatively it is possible in the present invention to utilize a known thermal head in which the center of heating elements is positioned at the top of the partial glaze. With such a thermal head, the heating elements and the platen roller are so disposed that a virtual straight line passing both the center of the heating elements and the rotational center of the platen roller is determined with inclination to intersect the conveying direction of the recording material. Consequently the cooling surface length LCRL is determined greater than that according to any manner of thermal printing known so far conventionally. 
     In FIG. 5, a thermal head of a heating-element-offset-type is depicted. There is an alumina base plate  11 , having a surface on which a flat glazed layer  12  is flatly disposed. A partial glaze  13  is protruded from the flat glazed layer  12  and shaped as a ridge or a portion of a cylinder. To form the flat glazed layer  12 , a coating of glass paste is applied to the base plate  11 , and heated, melted and cooled to become the flat glazed layer  12 . For forming the partial glaze  13 , the flat glazed layer  12  in the initially flat shape is used. The flat glazed layer  12  is etched, heated again, melted and formed for the shape of the partial glaze  13 . The flat glazed layer  12  is 35 μm thick. The partial glaze  13  has a maximum thickness of 70 μm. Of course it is possible to change those thicknesses as suitable for any used type of recording medium and recording system. A preferable range of the thickness of the flat glazed layer  12  is 20-2,000 μm. A preferable range of the maximum thickness of the partial glaze  13  is 50-2,050 μm. A preferable range of a radius of a curved surface of the partial glaze  13  is 1-8 mm. 
     There are a resistor membrane or resistor film  15  and electrodes  16  and  17  disposed on surfaces of the partial glaze  13  and the flat glazed layer  12 . A protective layer  18  of glass is layered to cover the resistor membrane  15  and the electrodes  16  and  17 , to obtain a heating element array  20  or ridge-shaped head component. The resistor membrane  15  consists of thin membrane of heat emitting resistor, and deposited on the surfaces of the flat glazed layer  12  and the partial glaze  13  in accordance with the sputtering method, the vacuum deposition method, the CVD method or other suitable methods. Preferred examples of the heat emitting resistor membrane are Ni—Cr, Ta 2 N, Ta—SiO 2 , Ta—Si, Ta—Si—C, Cr—Si—O, ZrN, Ta—SiC, poly—Si and the like. 
     In FIG. 6, the ridge-shaped head component  20  is depicted in enlargement. The electrodes  16  and  17  have shapes of teeth of a comb. Portions of the resistor membrane  15  between the electrodes  16  and  17  respectively constitute heating elements  21 . Each of the heating elements  21  has a width of 78 μm in a main scan direction M and a length of 225 μm in a sub scan direction S. The electrodes  16  and  17  are arranged in such positions that the heating element center TCL passing the center of the heating elements  21  is offset from an offset distance OSL 2 =180 μm in the upstream direction from a glaze center GCL or center line of the top of the partial glaze  13 . See FIG.  1 . Of course it is possible to change the size of the heating elements  21  as suitable for any used type of recording medium and recording system. The offset distance OSL 2  is 180 μm, but may be changed as suitable in various manners. The electrodes  16  and  17  are formed of aluminum (Al), gold (Au) or the like, and deposited in accordance with the sputtering method, the vacuum deposition method, the CVD method or other suitable methods. In the present embodiment, slits  19  are formed in the resistor membrane  15  to etch the resistor membrane  15  in form split between the heating elements  21 . Alternatively the heating elements  21  may be disposed without slitting the resistor membrane  15 , simply with the electrodes  16  and  17 . 
     In FIG. 7, disposition of a thermal head  25  and a platen roller  26  is illustrated. The base plate  11  or head chip with the ridge-shaped head component  20  is fixedly disposed on a head support plate  27  of metal, to constitute the thermal head  25 . In the head support plate  27 , there are an integrated circuit (IC) board  28  and an integrated circuit (IC) cover  29  for protecting the IC board  28 . The IC board  28  selectively drives the respective heating elements. The thermal head  25  is secured to a chassis of a thermal printer by head support brackets (not shown). 
     Under the ridge-shaped head component  20  is disposed the platen roller  26 . There are a pair of conveyor rollers  30  disposed downstream from the thermal head  25  in the conveying direction. The conveyor rollers  30  are constituted by a drive roller  30   a  and a nip roller  30   b,  which nip the recording material  2  to convey it by drawing it from the thermal head  25 . A head shifter mechanism  32  is disposed on the head support brackets, and shifts the thermal head  25  between a recording position and a retracted position. The thermal head  25 , when shifted in the recording position, is pressed toward the platen roller  26 , and when shifted in the retracted position, is away from the platen roller  26 . A printing mechanism  31  is constituted by the conveyor rollers  30 , the thermal head  25 , the platen roller  26  and the head shifter mechanism  32 . Note that, instead of shifting the thermal head  25 , the platen roller  26  may be shifted while the thermal head  25  is supported in a stationary manner. 
     In FIG. 8, the platen roller  26  is disposed in such a manner that its roller center RC is offset by an offset distance OSL 3 =400 μm from the glaze center GCL of the partial glaze  13  in the upstream orientation as viewed in the conveying direction of the recording material. The platen roller  26  is pushed by the ridge-shaped head component  20 . Note that the platen roller  26  is a rubber roller including a core and a rubber roll  26   a  fitted thereabout. It is possible to adjust the offset distance OSL 3  as suitable for a diameter of the platen roller and the shape of the ridge-shaped head component  20  in a range of keeping the cooling surface length LCRL sufficiently great. 
     In FIG. 9, a full-color thermal printer is illustrated. The recording material  2  in the roll form is drawn and un-wound by feeder rollers  40 , and sent to a yellow printing station  41 , a magenta printing station  42  and a cyan printing station  43  in the order listed. The yellow printing station  41  has a yellow printing unit  44  and the printing mechanism  31 . The magenta printing station  42  has a magenta printing unit  45  and the printing mechanism  31 . The cyan printing station  43  has a cyan printing unit  46  and the printing mechanism  31 . Yellow, magenta and cyan images are recorded to each of single recording domains on the recording material  2  to record a full-color image. There is a yellow fixing unit  47  disposed between the yellow printing station  41  and the magenta printing station  42 , for applying near-ultraviolet rays of a wavelength peaking at 420 nm to a recording domain after the yellow recording. There is a magenta fixing unit  48  disposed between the magenta printing station  42  and the cyan printing station  43 , for applying ultraviolet rays of a wavelength peaking at 365 nm to the recording domain after the magenta recording. After the recording in the printing stations  41 - 43  and the fixation of the fixing units  47  and  48 , the recording material  2  is cut by a cutter  49  frame from frame to obtain a full-color print. 
     An experiment was conducted to check the effect of the present embodiment. In FIG. 10, a platen roller  50  was disposed with a change, to change a position of heating elements  52  in the contact region CR of contact between the recording material  2  and a heating element array  51  or ridge-shaped head component. An offset distance OSL 4  was defined as a distance as viewed in the conveying direction and between the roller center RC of the platen roller  50  and the heating element center TCL or vertical line passing through the center of the heating elements  52  and vertical to the conveying direction. The offset distance OSL 4  was defined positive (+) when the heating element center TCL was located upstream from the roller center RC, and negative (−) when the heating element center TCL was located downstream from the same. A relationship between the offset distance OSL 4  and the coloring density was observed. To the heating elements  52 , heat energy changing stepwise in four values was applied. 
     The printing experiment was conditioned as follows: 
     Thermal printer: full-color thermal printer NC-1 (trade name) manufactured by Fuji Photo Film Co., Ltd. 
     Recording material: P20 (trade name) manufactured by Fuji Photo Film Co., Ltd. 
     Printing speed: 10 mm/sec. 
     Period of powering: 14 msec. 
     Duration of powering: 11 msec or less. 
     Electric power: 0.234 W per one dot. 
     The ridge-shaped head component  51  known in the prior art was used, in which the center of the heating elements  52  was located at the center of a partial glaze  53 . The heating elements  52  was 360 μm long in the conveying direction. A diameter of the platen roller was 50 mm. 
     A result of the experimental printing is illustrated in FIG.  11 . The hatched area A 1  designates a blister creating condition creating blister on the recording material. When the sign of the offset distance OSL 4  was positive (+) then the heating elements were offset upstream from the platen roller  50 . No blister was created even when the high heat energy was applied to the recording material. When the sign of the offset distance OSL 4  was negative (−), then the heating elements were offset downstream from the platen roller  50 . Considerable blister was created even the low heat energy was applied to the recording material. Consequently the cooling surface length LCRL was determined greater by lengthening the offset distance OSL 4  in the positive manner. It was possible that the cooling surface LCR pressed the recording material  2  and cooled it sufficiently for the purpose of avoiding occurrence of blister on the recording material. Note that the four curves in the drawing respectively correspond to the four values of the heat energy. 
     It is also to be noted that, as depicted in FIG. 11, the offset distance OSL 4  has a desirable range of OSL 4 ≧−0.2 mm. The offset distance OSL 4  may be negative but must not be smaller than −0.2 mm as a lower limit. 
     In the above embodiments, the heating elements in the thermal head are offset. FIGS. 12-14 illustrate other embodiments in which the center of the contact region is determined downstream from the center of the heating elements in the conveying direction. Head touch conditions for the same effects are obtained by offsetting the center of the platen roller downstream from the center of the heating elements. The contact region, where each of heating element arrays  55 ,  56  and  57  or ridge-shaped head components is contacted with the recording material  2 , has a range equal to or longer than the range of the heating elements as viewed in the conveying direction. 
     In FIG. 12, another preferred embodiment is depicted, in which a thermal head is contacted in such a manner that a cooling surface length LCRL 1  is greater than a pre-contact surface length UCRL 1 . A contact region length CRL 1  is greater than a length TL 1  of heating elements  58  in the conveying direction. A contact region center P 1  is offset downstream with the heating elements  58  entirely kept in contact with the recording material  2 . The cooling surface length LCRL 1  and the pre-contact surface length UCRL 1 , therefore, meet LCRL 1 &gt;UCRL 1 . If the contact region center P 1  is further offset downstream, an upstream end of the heating elements  58  comes out of the contact region. This is still effective in preventing occurrence of irregularity in density, blister, and surface roughening, in spite of a partial waste of the heat energy from the heating elements  58 . 
     In FIG. 13, a preferred embodiment is depicted, in which the ridge-shaped head component  56  is contacted in such a manner that a contact region length CRL 2  is determined equal to a length TL 2  of heating elements  59 . To keep a cooling surface length LCRL 2  sufficiently great, the contact region center P 1  is downstream from a heating element center P 2  of the heating elements  59 . Also in the present thermal head, it is possible to prevent occurrence of irregularity in density, blister, and surface roughening. 
     In FIGS. 12 and 13, the lengths TL 1  and TL 2  of the heating elements  58  and  59  are equal to or smaller than the length CRL of the contact region CR between the recording material  2  and the ridge-shaped head components  55  and  56  with reference to the conveying direction. Alternatively a length of heating elements in the conveying direction may be greater than a contact region length CRL, namely TL≧CRL. Of course the heating elements can be offset upstream to keep the cooling surface length LCRL greater than that according to the prior art. 
     Also a thermal head  63  of FIG. 14 can be used, in which the cooling surface length LCRL is set greater than that of the prior art while the center of heating elements  60  is set nearly at the center of a partial glaze  62 . The ridge-shaped head component  57  and a platen roller  64  are disposed such that a straight line CL passing through the heating element center TCP of the heating elements  60  and the roller center RC of the platen roller  64  intersects a direction AL parallel to the conveyance of the recording material  2 , and is inclined with reference to the direction AL. This is similar in operation to the above embodiments in which the heating elements are offset upstream from the top of the partial glaze. The contacting condition between the ridge-shaped head component  57  and the recording material  2  is stabilized to prevent the density from changing and gloss from lowering. Note that the thermal head  63  having the conventional contour is likely to interfere the recording material due to the inclined disposition. It is preferable to cut away a small portion of an edge of the thermal head, namely the downstream edge of the thermal head nearer to the recording material  2 . 
     A further preferred embodiment is described now, in which a thermal head as viewed in cross section is asymmetrical. In the above embodiments of FIGS.  5  and  12 - 14 , the thermal head has a partially cylindrical shape on the periphery of the partial glaze. If the cooling surface length LCRL is enlarged, a radius of curvature of the cylindrical shape must be greater, to lower pressure to the recording material. Irregularity in density is likely to occur. In the present embodiment of FIG. 15 in contrast, a contour of partial glaze is shaped asymmetrically. To be precise, a surface of a downstream half of the partial glaze has a radius of curvature greater than a surface of an upstream half of the partial glaze, the downstream and upstream halves being defined with reference to the center of the partial glaze. The heating elements are disposed in a position offset upstream from the center of the partial glaze, so that a cooling surface can press and cool the recording material after being heated with the cooling surface length LCRL kept great. Accordingly the pressure between the recording material and the heating elements is kept sufficiently high. The surface smoothness or gloss of the recording material is heightened, to keep the printing quality high. 
     In FIG. 15, another preferred thermal head is depicted, in which a heating element array  70  or ridge-shaped head component is asymmetrical in the conveying direction. Element similar to those in FIGS. 5 and 6 are designated with identical reference numerals. The most distinct feature of the present embodiment lies in the shape of a partial glaze  71 . For forming the partial glaze  71 , the flat glazed layer  12 , which has the flat hardened shape, is etched, heated again, melted and formed for the shape asymmetrical and curved. Preferable ranges of thicknesses of the flat glazed layer  12  and the partial glaze  71  are the same as those of FIG.  5 . Disposition of the electrodes and the offset distance OSL 2  are also the same as those of FIG.  6 . 
     In FIG. 16, the ridge-shaped head component  70  is asymmetrical as viewed in section. An upstream half of the ridge-shaped head component  70  with reference to the glaze center GCL or center line of the ridge-shaped head component  70  is constituted by a rise surface CF 1 , which is shaped in an arc having a curvature radius R 1 =5 mm. A flat section FF 1  is downstream adjacent to the rise surface CF 1 . Geometrically the flat section FF 1  has a curvature radius R 2  of infinity. A curved section CF 2  is downstream adjacent to the flat section FF 1 , and is shaped in an arc having a curvature radius R 3 =3 mm. The thermal head structure has the height h 1 =65 μm. In the present thermal head, the cooling surface LCR is constituted of the flat section FF 1  and a portion of the curved section CF 2  for contact between the ridge-shaped head component  70  and the recording material  2 . The cooling surface length LCRL of the cooling surface LCR is determined so as to cool the recording material down to temperature equal to or lower than glass transition point of the protective layer  2   e  of the recording material  2  upon separation of the recording material  2  from the cooling surface LCR. The cooling surface length LCRL is 600 μm in the present embodiment, and may be preferably 500 μm or more. Accordingly the recording surface of the recording material can be sufficiently cooled, reliably to prevent occurrence of irregularity in density, blister, and surface roughening. 
     A curvature radius R 1  of the rise surface CF 1  is 2-8 mm, and more preferably 2.5-7 mm. A curvature radius R 3  of the curved section CF 2  may be any value in comparison with the curvature radius R 1  of the rise surface CF 1 . In FIG. 16, the curvature radius R 3  of the curved section CF 2  is smaller than that of the rise surface CF 1 . In FIGS. 21 and 22 to be described later, heating element arrays  76  and  77  or ridge-shaped head components respectively have a curved section CF 5 , of which a curvature radius R 5  is greater than that of a rise surface CF 6 , and a curved section CF 9 , of which a curvature radius R 9  is greater than that of a rise surface CF 6 . However it is more preferable that the curvature radius R 3  of the curved section CF 2  is smaller than the curvature radius R 1  of the rise surface CF 1 , for example 1 mm≦R 3 ≦6 mm. Consequently it is possible to reduce a changeable range of the separating point PP where the recording material  2  comes away from the ridge-shaped head component  70  even if the change occurs due to changes in the tension of the recording material  2 . See FIG.  18 . Note that, in FIG. 15, the protective layer  18  is formed with a regular thickness relative to the partial glaze  13 . A surface shape of the partial glaze  13  is substantially similar to that of the ridge-shaped head component  20  in terms of geometry. The size and contour of the partial glaze are determinable according to the deduction of thicknesses of the protective layer, resistor membrane and the electrodes from a size and contour of the heating element array or ridge-shaped head component. 
     FIG. 17 illustrates a relationship between the platen roller  26  and a thermal head  73  including the ridge-shaped head component  70 . In FIG. 17, elements similar to those in FIG. 7 are designated with identical reference numerals. Unlike FIG. 7, the thermal printer of FIG. 17 has the thermal head  73  disposed under the platen roller  26 . One feature of the present embodiment lies in that a nipping position of the conveyor rollers  30  is offset in a direction toward the alumina base plate of the thermal head away from the heating elements for contact with the recording material. Consequently it is possible to enlarge an amount of contact of the recording material  2  to the surface of the thermal head. The cooling surface length LCRL can be great. 
     The platen roller  26  is shifted by a roller shifter mechanism  78 . The thermal head  73 , when the platen roller  26  is shifted in the recording position, is pressed toward the platen roller  26 . The platen roller  26 , when shifted in the retracted position, is away from the thermal head  73 . 
     In FIG. 18, the platen roller  26  is so disposed that its roller center RC is offset by the offset distance OSL 3 =400 μm from the glaze center GCL or center line vertical to the partial glaze  71  at its center. The periphery of the platen roller  26  is pressed against the ridge-shaped head component  70 . Note that the offset distance OSL 3  is sufficient if the cooling surface length LCRL is sufficient. The offset distance OSL 3  may be changed according to a diameter of the platen roller  26  and a shape of the ridge-shaped head component  20 . 
     An experiment was conducted to check the effect of the present embodiment. The thermal head was given the cooling surface length LCRL set at values of 100 μm, 300 μm and 500 μm, to observe a relationship between the cooling surface length LCRL and the surface gloss. Note that the characteristic of the surface gloss herein was obtained by measurement with a gloss measuring device VG-2000 (trade name) manufactured by Nippon Denshoku Kogyo Co., Ltd. and at a measuring angle of 20 degrees. 
     The printing experiment was conditioned as follows: 
     Thermal printer: full-color thermal printer NC-501 (trade name) manufactured by Fuji Photo Film Co., Ltd. 
     Recording material: A4-G50 (trade name) manufactured by Fuji Photo Film Co., Ltd. 
     Printing speed: 10 mm/sec. 
     Period of powering: 10 msec. 
     Duration of powering: 6 msec. 
     Electric power: 0.08 W per one dot. 
     The heating elements  52  had the cooling surface length LCRL of 100 μm, 300 μm and 500 μm, and had an entire length of 360 μm in the conveying direction. 
     A result of the experiment is shown in FIG.  19 . When LCRL=100 μm, the surface gloss was 55%. When LCRL=300 μm, the surface gloss was approximately 60%. When LCRL=500 μm, the surface gloss was 65%. The greater the cooling surface length LCRL was, the higher the surface gloss was. The cooling surface LCR pressed and sufficiently cooled the recording material after being heated. The gloss of the recording material was heightened. Occurrence of irregularity in density, blister, and surface roughening was prevented. 
     Note that, instead of the flat section FF 1  of FIG. 16, a heating element array  75  or ridge-shaped head component of FIG. 20 can be used, in which a curved section CF 3  has a curvature radius R 4  greater than the curvature radius R 1  of the heating surface MCR. The cooling surface length LCRL is 500 μm or more. Also the ridge-shaped head component  76  in FIG. 21 can be used, in which a flat section length FF 2 L of a flat section FF 2  adjacent to the heating surface MCR is set smaller than a flat section length FF 1 L of FIG. 16, the curved section CF 5  is adjacent to the flat section FF 2 , and the curvature radius R 5  of the curved section CF 5  is set greater than a curvature radius R 6  of the rise surface CF 6 . Furthermore the ridge-shaped head component  77  in FIG. 22 can be used, in which curved sections CF 7  and CF 8  and the curved section CF 9  are used and have radii of curvature R 7 , R 8  and R 9  greater than the curvature radius R 6  of the heating surface MCR, to lengthen the cooling surface LCR. In FIGS.  16  and  20 - 22 , the phantom lines indicate the conventional thermal head with the cylindrical ridge contour, for the purpose of comparison with the present invention. 
     It is to be noted that, in FIG. 22, the curvature radii R 6 -R 9  have a relationship of R 6 &lt;R 9 &lt;R 8 &lt;R 7 . Of course curvatures of the curved sections CF 7 -CF 9  may have other relationships between them in various preferable manners. Furthermore, curves different from the arcs of circles may be used, for example elliptic and parabolic curves, and combinations of those and/or arcs. 
     Moreover, it is possible in the present invention to use a thermal head  82  of FIG.  23 . The thermal head  82  is a corner edge type, and has a base plate  80  and a heating element array  81  or ridge-shaped head component disposed on an end  80   a  of the base plate  80 . Also in FIG. 24, an end face type of thermal head  85  may be used. The thermal head  85  includes an upright base plate  83  and a heating element array  84  or ridge-shaped head component disposed on a top end face  83   a  of the upright base plate  83 . If a glaze etching is used, it is possible to leave a downstream half of the partial glaze without being etched, for the purpose of forming a flat section on the thermal head. A combination of the flatly remaining glaze and the partial glaze constitutes the heating element array or the ridge-shaped head component. 
     Note that the above thermal printer is a one-pass three-head type, in which the recording material is moved for one time past the thermal heads, and subjected to the three-color frame-sequential recording method to obtain a full-color image. Alternatively the present invention may be used in a three-pass one-head type of thermal printer in which the recording material is moved back and forth for three times past one thermal head, and subjected to the three-color frame-sequential recording method to obtain a full-color image. Also the thermal printer in the present invention may be a platen drive type, in which a platen shaft is rotated to convey recording material on a platen roller or platen drum. Again the gloss of the recording surface of the recording material can be heightened. 
     In the above embodiments, the thermal head is used in a direct full-color thermal printer of which thermosensitive sheet material is heated to obtain a printed material directly. Alternatively a thermal head of the present invention may be a thermal melt type or thermal wax transfer type, so that a contacting state between an ink ribbon and the thermal head can be kept stable. In the above embodiments, the partial glaze is ridge-shaped and partially cylindrical. Of course a partial glaze may be shaped in a quadrangle or trapezoid as viewed in cross section. The contour of the surface of the ridge shape are constituted of straight lines or arcs, but may be constituted of lines of curves of any form in any combination. The present invention may be used in a monochromatic thermal printer for use with monochromatic recording material, in which only one coloring layer is formed. 
     Although the present invention has been fully described by way of the preferred embodiments thereof with reference to the accompanying drawings, various changes and modifications will be apparent to those having skill in this field. Therefore, unless otherwise these changes and modifications depart from the scope of the present invention, they should be construed as included therein.