Patent Publication Number: US-6222571-B1

Title: Color thermal printer and color thermal printing method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a color thermal printer and color thermal printing method. More particularly, the present invention relates to a color thermal printer and color thermal printing method in which the entire surface of color thermosensitive recording material after recording can be fixed and bleached. 
     2. Description Related to the Prior Art 
     A direct type of a color thermal printer is known in the field of thermal recording, and is operated to record a full-color image in a three-color frame-sequential method. A full-color thermosensitive recording material of a direct recording type is used, which includes a support, a cyan thermosensitive coloring layer, a magenta thermosensitive coloring layer and a yellow thermosensitive coloring layer. The yellow coloring layer among those has the highest heat sensitivity. The cyan coloring layer has the lowest heat sensitivity. Three-color images are recorded in the order of yellow, magenta and cyan. In order to avoid further coloring of the yellow coloring component in the course of the magenta recording, a yellow fixer lamp emanates visible violet rays or near ultraviolet rays in a wavelength range peaking at the wavelength of 420 nm, and fixes the yellow coloring layer of the recording material. In order to avoid further coloring of the magenta coloring component in the course of the cyan recording, the magenta fixer lamp emanates ultraviolet rays in a wavelength range peaking at the wavelength of 365 nm, and fixes the magenta coloring layer. As unrecorded regions on the recording material defined around the full-color image has a very light yellow-greenish color, the magenta fixer lamp is driven after the cyan recording to apply rays to all the unrecorded portions to bleach them. 
     The direct type of the thermal recording has an advantage in that no ink ribbon is used, in contrast to the melt-type or wax transfer type of thermal transfer printer. The direct type does not have any mechanism for feeding ink ribbon, and is mechanically simpler. However the set of the fixer lamps is required for fixation of the yellow and magenta coloring layers. 
     If the thermal printer has the fixer lamps disposed higher than the recording material, the entire surface of the recording material can receive application of fixing rays. But if a thermal printer has the fixer lamps disposed lower than the recording material, the thermal printer requires guide members for supporting the recording material in contact with its lower surface. The guide members have shapes for supporting only lateral sides of the recording material as viewed in the width direction, for sufficient fixation. Unfixed regions in a belt-shape remain on the lateral sides of the recording material after the recording material is fixed in passage through the guide members. If it is desired to bleach the unfixed regions, an additional ultraviolet lamp must be used for bleaching the recording material having been conveyed out of the guide members. The requirement of the additional lamp is unadvantageous, as it is inconsistent to the small cost and the structural simplicity. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing problems, an object of the present invention is to provide a color thermal printer and color thermal printing method in which the entire surface of color thermosensitive recording material after recording can be fixed and bleached. 
     Another object of the present invention is to provide a color thermal printer and color thermal printing method in which a user can be caused to recognize a finish of the printing operation. 
     Still another object of the present invention is to provide a color thermal printer and color thermal printing method in which a margin of the recording sheet is fixed and bleached with increased efficiency. 
     In order to achieve the above and other objects and advantages of this invention, a thermal printer is used with thermosensitive recording material, the recording material including a support, and at least one thermosensitive coloring layer, formed on the support, colorable with heat, and fixable with electromagnetic rays, the recording material having a recording region, and first and second lateral sides between which the recording region is disposed. The thermal printer includes a feeder unit for conveying the recording material along a conveying path, the first and second lateral sides being set to extend in a direction of the conveying path. A thermal head applies the heat to the coloring layer during conveyance of the recording material along the conveying path, to record an image. A fixer applies the electromagnetic rays to the recording material being colored during conveyance of the recording material along the conveying path, to fix the coloring layer. At least two guide tooth portions, being first and second, are disposed between the fixer and the recording material, for guiding respectively the first and second lateral sides along the conveying path, there being first and second gaps respectively defined adjacent to the first and second guide tooth portions, the first and second gaps allowing the electromagnetic rays to pass between the fixer and the recording material, to fix the first and second lateral sides, 
     In a preferred embodiment, the first and second guide tooth portions are transparent. 
     The thermal head and the fixer are disposed under the conveying path, and the recording material is conveyed with the support upwards oriented. 
     The recording material includes first and second margins, the recording region being surrounded by the first and second lateral sides and the first and second margins. The feeder unit conveys the recording material by advancing either of the first and second margins along the conveying path. Furthermore a controller (i.e., means for controlling) controls the feeder unit and the fixer, wherein the controller causes the feeder unit to convey the recording region past the fixer, and while the recording region is conveyed, causes the fixer to apply the electromagnetic rays to the recording region for fixing the recording region, and then causes the feeder unit to convey the first margin past the fixer, and while the first margin is conveyed, causes the fixer to apply the electromagnetic rays to the first margin for fixing the first margin, thereby the first and second lateral sides being fixed entirely through the first and second gaps. 
     The at least one coloring layer comprises at least first to third coloring layers colorable in colors different from each other, the first coloring layer being disposed at a recording surface, the third coloring layer being disposed deepest from the recording surface, and the first and second coloring layers being fixable with the electromagnetic rays of respectively first and second wavelength ranges. The fixer includes a first fixer lamp for emanating the electromagnetic rays of the first wavelength range, to fix the first coloring layer. A second fixer lamp emanates the electromagnetic rays of the second wavelength range, to fix the second coloring layer, the second fixer lamp applying the electromagnetic rays of the second wavelength range to the recording region and afterwards to the first margin, thereby the first margin being fixed and bleached. 
     Each of the first and second guide tooth portions includes plural teeth arranged in a comb shape, and there are gaps defined between the plural teeth to constitute the first and second gaps. 
     The controller causes the feeder unit to convey the recording region at a first speed past the fixer, for fixation of the recording region, and causes the feeder unit to convey the first margin at a second speed smaller than the first speed, past the fixer, for fixing and bleaching the first margin. 
     The controller causes the feeder unit to convey the first margin intermittently past the fixer, for fixing and bleaching the first margin. 
     The thermal printer further includes a printer body. An entrance opening is formed in the printer body, for allowing the recording material to move into and out of the printer body. The feeder unit supplies the printer body with the recording material by advancing the first margin, and after recording and fixation of the recording material, ejects the recording material from the entrance opening by advancing the second margin. 
     The conveying path includes a supply passageway for supplying the recording material in advancement of the first margin is advanced therethrough. A print passageway is formed to extend downstream from the supply passageway, and has the thermal head, the fixer and the first and second guide tooth portions, the feeder unit conveying the recording material back and forth therethrough. An ejection passageway is formed to extend from the print passageway in a position between the supply passageway and the print passageway, for ejecting the recording material in advancement of the second margin after the recording material is conveyed through the print passageway. 
     In still another preferred embodiment, the controller, after the electromagnetic rays are applied to the recording region, causes the feeder unit to confront the first margin with the fixer, causes the feeder unit to convey the recording material back and forth within a predetermined stroke range, and while the recording material is conveyed back and forth, causes the fixer to apply the electromagnetic rays to the first margin to fix the first margin. 
     In another preferred embodiment, the controller, after the electromagnetic rays are applied to the recording region, causes the feeder unit to confront the first margin with the fixer, causes the feeder unit to convey the recording material intermittently, and while the recording material is conveyed intermittently, causes the fixer to apply the electromagnetic rays to the first margin to fix the first margin. 
     In an additional preferred embodiment, the controller, before the heat is applied to the coloring layer, stops the feeder unit and the recording material with the first margin confronted with the fixer, and while the first margin is confronted with the fixer, causes the fixer to apply the electromagnetic rays to the first margin to fix the first margin. 
    
    
     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 a color thermal printer; 
     FIG. 1A is a cross section illustrating a structure of layers of a color thermosensitive recording sheet; 
     FIG. 2 is a perspective illustrating a combination of a supply unit and a print unit; 
     FIG. 3 is an exploded perspective illustrating the supply unit and the print unit separated from each other; 
     FIG. 4 is a top plan illustrating the supply unit; 
     FIG. 5 is a rear elevation illustrating the supply unit; 
     FIG. 6 is a cross section taken on line VI—VI of FIG. 4, illustrating a guide mechanism of the supply unit; 
     FIG. 7 is an exploded perspective illustrating an assembling state of a supply plate; 
     FIG. 8 is an explanatory view in section taken on line VIII—VIII of FIG. 4, illustrating a supply roller and a guide mechanism; 
     FIG. 9 is a left side elevation illustrating the supply unit; 
     FIG. 10 is a side elevation illustrating a clutch of the supply unit; 
     FIG. 11 is a right side elevation illustrating a state where a motor rotates a platen lifter cam; 
     FIG. 12 is a cross section taken on line XII—XII of FIG. 4, illustrating a state of incorporating a sheet supply cassette; 
     FIG. 13 is an explanatory view in section, illustrating a state of supply of the recording sheet; 
     FIG. 14 is an exploded perspective illustrating a print unit; 
     FIG. 15 is an explanatory view in perspective, illustrating a rail unit having rail tooth portions; 
     FIG. 16 is a cross section illustrating a stopped state for bleaching a rear margin; 
     FIG. 17 is an explanatory view in section taken on line XVII—XVII of FIG. 16, illustrating the rail unit with a fixer; 
     FIG. 18 is a block diagram schematically illustrating circuits of the thermal printer; 
     FIGS. 19A and 19B are a flow chart illustrating operation of the thermal printer; 
     FIG. 20 is a flow chart illustrating a bleaching process for the rear margin; 
     FIG. 21 is an explanatory view in section, illustrating an accidental state where two overlapped sheets are supplied; 
     FIG. 22A is a side elevation illustrating another preferred rail unit having rectangular teeth; 
     FIG. 22B is a side elevation illustrating still another preferred rail unit having rod-shaped teeth; 
     FIG. 22C is a side elevation illustrating another preferred rail unit having teeth in a triangular prism shape; 
     FIG. 23 is a flow chart illustrating another preferred bleaching process for the rear margin; 
     FIG. 24 is an explanatory view in elevation, illustrating another preferred color thermal printer, having a state of a sheet supply; 
     FIG. 25 is an explanatory view in elevation, illustrating a state of a start of thermal recording; 
     FIG. 26 is a flow chart illustrating the operation of the thermal printer; 
     FIG. 27 is an explanatory view in elevation, illustrating a state of a start of fixation of a recording region; 
     FIG. 28 is an explanatory view in elevation, illustrating a state of a start of fixation of a front margin; 
     FIG. 29 is an explanatory view in elevation, illustrating a state of continuing fixation of the front margin; 
     FIG. 30 is an explanatory view in elevation, illustrating another preferred thermal printer in which the recording sheet is conveyed with an inclination; 
     FIG. 31 is an explanatory view in elevation, illustrating still another preferred color thermal printer, having a state of a sheet supply; 
     FIG. 32 is an explanatory view in elevation, illustrating a state of a start of thermal recording; 
     FIG. 33 is an explanatory view in elevation, illustrating a state of a start of fixation of a recording region; 
     FIG. 34 is an explanatory view in plan, illustrating the recording sheet; 
     FIG. 35 is a flow chart, illustrating the operation of the thermal printer; 
     FIG. 36 is an explanatory view in elevation, illustrating a state of continuing fixation of the front margin; 
     FIG. 37 is an explanatory view in elevation, illustrating a state of ejection the recording sheet; and 
     FIG. 38 is an explanatory view in elevation, illustrating another preferred thermal printer in which the recording sheet is conveyed with an inclination. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE PRESENT INVENTION 
     In FIGS. 1,  2  and  3 , a color thermal printer  9  of the present invention is constituted by a sheet supply unit  10 , a print unit  11  and a cabinet or printer body  12 . The supply unit  10  is mounted on a supply side of the print unit  11  in a unified manner. The supply unit  10  and the print unit  11  are incorporated in the printer body  12 . One color thermosensitive recording sheet  13  is manually set on the supply unit  10 . Also a supply cassette  14  can be used instead of manual insertion of the recording sheet  13 . The supply cassette  14  contains a plurality of recording sheets to cause them to be automatically supplied for the print unit  11 . 
     The recording sheet  13  is a full-color thermosensitive recording sheet of a direct recording type having optical fixability, and known in the art. In FIG. 1A, the recording sheet  13  includes a support  13   d , a cyan thermosensitive coloring layer  13   g , a magenta thermosensitive coloring layer  13   f  and a yellow thermosensitive coloring layer  13   e . The yellow coloring layer  13   e  among those has the highest heat sensitivity. The cyan coloring layer  13   g  has the lowest heat sensitivity. The coloring ability of the yellow coloring layer  13   e  is destroyed when visible violet rays or near ultraviolet rays with a wavelength of 420 nm is applied to it. The coloring ability of the magenta coloring layer  13   f  is destroyed when ultraviolet rays with a wavelength of 365 nm is applied to it. 
     In FIGS. 2-4, the supply unit  10  is constituted by a supply frame  15 , a supply plate  16 , a supply roller  17 , a supply guide unit  18  and an ejector  19 . The supply plate  16  operates as a supply passageway included in a conveying path. The supply frame  15  is constituted by a pair of side plates  15   a  and  15   b  and a base plate  15   c  for connecting them to one another. 
     In FIGS. 4-6, the supply plate  16  is disposed in the supply frame  15  in a manner pivotal about a support shaft  16   a . The supply plate  16  is biased up by a coil spring  20 , and presses the recording sheet  13  against the supply roller  17 . In FIG. 7, the support shaft  16   a  of the supply plate  16  is so kept that its center is located on a plane surface of the supply plate  16  for supporting the recording sheet. Consequently an upper half of the support shaft  16   a  protrudes over the plane surface of the supply plate  16 . The support shaft  16   a  has a rod shape being cylindrical. Note that the support shaft  16   a  may have a semi-cylindrical shape, which would consist of only a lower half of the rod shape as depicted. 
     The support shaft  16   a  is received in an axial hole  15   d  formed in the supply frame  15  in a rotatable manner. However an axial receiving portion may be formed with the supply frame  15  for receiving the support shaft  16   a  in a rotatable manner. The axial receiving portion may have any suitable shape, for example cylindrical or semi-cylindrical. The disposition of the support shaft  16   a  on the plane of the supply plate  16  for supporting the recording sheet makes it possible that the recording sheet can be pressed on the supply roller reliably even when a small number of recording sheets remain on the supply plate  16 . The recording sheet can be prevented from dropping away from the supply roller. 
     The supply roller  17  is disposed over an end of the supply plate  16 . The supply roller  17  is constituted by a roller shaft  17   a  and two rubber rolls  17   b  and  17   c , which are disposed on the supply roller  17  and away from each other. The supply roller  17  rotates in contact with an uppermost one of the plural recording sheets placed on the supply plate  16 , and conveys the uppermost one toward the supply guide unit  18 . 
     In FIGS. 4 and 5, the supply guide unit  18  is constituted by five guide plates  22 - 26  disposed erectly inside the supply frame  15 . Top end faces of the guide plates  22 - 26  are guide faces  22   a - 26   a , which contact the recording sheet  13  to guide it. The guide plates  22 - 26  include the central guide plate  22 , the intermediate guide plates  23  and  24  and the peripheral guide plates  25  and  26 . The central guide plate  22  lies on an extension of a central line of the supply plate  16 . The intermediate guide plate  23  lies between the central guide plate  22  and the peripheral guide plate  25 . The intermediate guide plate  24  lies between the central guide plate  22  and the peripheral guide plate  26 . 
     In FIG. 6, a stopper block  30  is secured to the central guide plate  22 . The stopper block  30  is disposed in a position to receive a margin line of the recording sheet  13  from the supply plate  16 . The stopper block  30  is made of rubber material and formed in a generally rectangular shape. A top end  30   b  of the stopper block  30  is protruded over the guide face  22   a  of the central guide plate  22  to orient a regulating face  30   a  in a crosswise inclined manner to the guide face  22   a . The stopper block  30  is inserted in a cutout  22   b  formed in the central guide plate  22 . In FIG. 4, a support bracket  31  keeps the stopper block  30  from dropping out of the cutout  22   b . In the present embodiment, the regulating face  30   a  is formed on the stopper block  30  with an inclined shape. Alternatively the stopper block  30  may have an entirely rectangular shape and may be positioned on the central guide plate  22  in such a manner that its top face may be oriented with an inclination with reference to the guide face  22   a.    
     In FIG. 4, the intermediate guide plates  23  and  24  are disposed respectively under the rubber rolls  17   b  and  17   c  of the supply roller  17 . In FIG. 8, let G be an interval between the guide face  23   a  of the intermediate guide plate  23  and the supply roller  17 . Let t be a thickness of the recording sheet  13 . The interval G is determined as: 
     
       
           t≦G&lt; 2 t    
       
     
     namely one or more time as much as the thickness of the recording sheet  13 , and under two times as much as the thickness of the recording sheet  13 . An end face  23   b  of the intermediate guide plate  23  nearer to the supply plate hinders a second uppermost one of the recording sheets  13 , and enables only the uppermost one of them to advance. This avoids occurrence of overlapped advancement of recording sheets. In the present embodiment, a corner  23   c  disposed between the guide face  23   a  and the end face  23   b  is faceted at an angle of 45 degrees. An amount C 1  of the faceting is 1.5 times as much as the thickness t of the recording sheet  13  (C 1 =1.5t). Therefore the recording sheet  13  can be transferred from the supply plate to the intermediate guide plate  23 . Note that this faceted shape may be omitted. The faceting amount C 1  may be suitably changed, and set over or under 1.5t. 
     The peripheral guide plates  25  and  26  respectively have the guide faces  25   a  and  26   a  in the same shape as that of the central guide plate  22 . Unlike the central guide plate  22 , the stopper block  30  does not exist on either of the peripheral guide plates  25  and  26 . The peripheral guide plates  25  and  26  do not have the cutout  22   b  or the support bracket  31 . The central guide plate  22  and the peripheral guide plates  25  and  26  have a smoothly curved face being rounded off for smoothing movement of the recording sheet  13  from the supply plate  16  to them. The central guide plate  22  and the peripheral guide plates  25  and  26  do not have the structure of t≦G&lt;2t as depicted in FIG.  8 . In FIG. 4, the end of the supply plate  16  has cutouts  16   c  for receiving insertion of the guide plates  22 - 26 , so that the smoothness in the movement of the recording sheet  13  from the supply plate  16  is increased. 
     In FIG. 6, the ejector  19  is constituted by an ejector roller  33  and a path selector mechanism  34 . The ejector roller  33  is disposed beside, and higher than, the supply roller  17 . In FIGS. 4 and 9, a drive gear  35  is located on an ejector roller shaft  33   a . The drive gear  35  is associated with a drive gear  38  of the print unit  11  by engagement via intermediate gears  36  and  37 . The drive gear  35  is frictionally coupled with the ejector roller shaft  33   a  by a coil spring  39 . When load over a predetermined amount is applied to the ejector roller  33 , the drive gear  35  rotates about the ejector roller shaft  33   a.    
     In FIGS. 10 and 11, a gear  40  is secured to an axial end of the roller shaft  17   a  of the supply roller  17 . A drive shaft  42   a  of a supply motor  42  is coupled with the gear  40  via a planetary clutch  41  or planetary gear mechanism. A worm  43  is fixedly coupled with the drive shaft  42   a  of the supply motor  42 . A worm wheel  44  is meshed with the worm  43 . An intermediate gear  45  is secured on a worm wheel shaft  44   a . A sun gear  46  of the planetary clutch  41  is meshed with the intermediate gear  45 . 
     The planetary clutch  41  is constituted by a swing arm  47  and a planet gear  48 . The swing arm  47  is coaxial with the sun gear  46 . The planet gear  48  is rotatable on the swing arm  47  and meshed with the sun gear  46 . The swing arm  47  is coupled with the sun gear  46  in a weakly frictional manner, of which friction causes the swing arm  47  to swing in a rotational direction of the sun gear  46 . When the swing arm  47  contacts either one of stoppers  53  after the planet gear  48  is rotated by the sun gear  46  in one of two rotational directions, then the swing arm  47  stops swinging despite further rotation of the sun gear  46 , due to the frictional coupling. To be precise, when the sun gear  46  rotates counterclockwise in the supply direction, then the swing arm  47  rotates clockwise to engage the planet gear  48  with the gear  40  to transmit rotation of the supply motor  42  to the supply roller  17 , which is rotated in the supply direction. When, in contrast, the sun gear  46  rotates clockwise in reverse to the supply direction, then the swing arm  47  rotates clockwise to engage the planet gear  48  with a drive gear  49  to transmit rotation of the supply motor  42  to a platen lifter cam  50 , which is included in a shifter mechanism. 
     In FIG. 3, a cam follower  85  of a swing frame  68  of the central guide plate  22  is in contact with the platen lifter cam  50 . The swing frame  68  is included in the shifter mechanism, and swung up and down by the platen lifter cam  50  to move a platen roller  51  as a platen mechanism, which is described later in detail. The platen roller  51  is shifted between a press position and a retracted position, and when in the press position, causes an array  52   a  of heating elements of a thermal head  52  to press the recording sheet  13  as indicated by the solid lines in FIG.  1 . The platen roller  51 , when in the retracted position, releases the recording sheet  13  from being pressed on the thermal head  52 , as indicated by the phantom lines in FIG.  1 . 
     In FIG. 6, the path selector mechanism  34  is mounted pivotally about the ejector roller shaft  33   a . A cutout  34   a  is formed in the path selector mechanism  34 , and receives each of upper ends of the guide plates  22 - 26  of the supply guide unit  18 . After the supply of the recording sheet  13 , in turn the recording sheet  13  is conveyed in a print direction reverse to the supply direction of the arrow B. While the recording sheet  13  is conveyed in the print direction, in FIG. 12, a rear margin region of the recording sheet  13  on the supply guide unit  18  is guided by the path selector mechanism  34  toward the ejector roller  33 . The path selector mechanism  34  is biased by a bias bracket  55  to a guiding position for the recording sheet  13 . Or the path selector mechanism  34  is kept by the bias bracket  55  in the guiding position in a manner resistant to swinging. 
     An end of the bias bracket  55  is retained on the roller shaft  17   a . In the course of the sheet supply, a portion of a margin region of the recording sheet  13  pushes open the path selector mechanism  34  upwards, so that the recording sheet  13  can advance to the print unit  11 . See FIG.  13 . Note that the path selector mechanism  34  may be biased not by the bias bracket  55  but by a coil spring. Furthermore gravity may keep the path selector mechanism  34  in the guiding position without the use of the bias bracket  55 . 
     In FIG. 12, the supply frame  15  has an ejector guide unit  56  secured to an exit side of the ejector roller  33  and located over the supply plate  16 . The ejector guide unit  56  defines an ejection passageway  57  included in the conveying path. A bottom plate  56   a  of the ejection passageway  57  operates as an upper guide wall at the time of manual insertion of a recording sheet. 
     If a great number of recording sheets  13  are used in the automated supply, the supply cassette  14  is inserted and set in an entrance opening  61 . The supply cassette  14  has a shape of a rectangular tube, and can contain  20  recording sheets at most. Of course the supply cassette  14  may be shaped in a different size. The supply cassette  14  contains the recording sheets  13  in a light-tight manner for the purpose of keeping them from application of ultraviolet rays which would lower their coloring ability. A cap (not shown) is secured to the supply cassette  14 , and when the supply cassette  14  is removed from the supply unit  10 , keeps the recording sheets  13  from ambient light. 
     In FIG. 14, the print unit  11  is generally constituted by a printed circuit board  63 , a chassis  64 , a drive unit  65 , a head support plate  66 , a conveyor frame  67  or print passageway, the swing frame  68 , a head cover  69  and a sheet guide unit  70 . The conveyor frame  67  is included in the conveying path. 
     In FIG. 1, the printer body  12  has a box shape with a small height. An operation panel  71  is disposed on a top face of the printer body  12 . The operation panel  71  includes a display window and a keyboard as an input unit. 
     The printed circuit board  63  is mounted on the bottom of the inside of the printer body  12 . The printed circuit board  63  includes circuits for control of relevant components in the thermal printer  9 , and has IC, transistors, resistors, capacitors and other various electrical parts. Yellow and magenta fixer lamps  72  and  73  are disposed on the printed circuit board  63 . To cover the fixer lamps  72  and  73  on the printed circuit board  63 , the chassis  64  is secured. 
     The yellow fixer lamp  72  emanates visible violet rays or near ultraviolet rays in a wavelength range peaking at the wavelength of 420 nm, and fixes the yellow coloring layer  13   e  of the recording sheet  13 , in order to avoid further coloring of the yellow coloring component in the course of the magenta recording. The magenta fixer lamp  73  emanates ultraviolet rays in a wavelength range peaking at the wavelength of 365 nm, and fixes the magenta coloring layer  13   f , in order to avoid further coloring of the magenta coloring component in the course of the cyan recording. In FIG. 14, the fixer lamps  72  and  73  have a substantially straight tubular shape, and emanate rays at such distribution that amounts of rays are constant in central portions between ends  72   a  and  73   a , but not constant in the ends  72   a  and  73   a . The central portions are used for fixing the recording sheet  13 . Note that, if lengths of the ends  72   a  and  73   a  were greater, the fixer lamps  72  and  73  would have an excessive length to increase a width of the printer body  12 . This would be inconsistent to an advantageous reduction in the size of the printer body  12 . It is however preferable that the fixer lamps  72  and  73  with a great length are shaped in a bent form like a letter L, in which the ends  72   a  and  73   a  are bent vertically. Consequently it is possible to keep the printer width small while the length of the central portions of the fixer lamps  72  and  73  are increased. 
     The chassis  64  is formed by bending a single flat metal plate in a channel shape. A ray applying opening  64   a  is formed in the chassis  64  for passage of rays from the fixer lamps  72  and  73 . The chassis  64  includes support brackets  64   b  and  64   c  located beside the ray applying opening  64   a.  Also the chassis  64  includes support brackets  64   d  and  64   e , which are respectively located on the supply side and the downstream side. 
     A set of feeder rollers  75  in a conveyor unit and the platen roller  51  are respectively rotatable on the conveyor frame  67 . The feeder roller set  75  is constituted by a capstan roller  75   a  and a nip roller  75   b . A gear (not shown) is fixed on an axial end of the capstan roller  75   a , and is in mesh with a roller drive gear  78  of the drive unit  65 , which causes the capstan roller  75   a  to rotate in the supply direction of the arrow A, and in the print direction of the arrow B reverse to the supply direction. Note that the supply direction is herein referred to as a first direction A, the print direction being referred to as a second direction B. 
     A rail unit or a pair of paper guides  80  are secured to the conveyor frame  67  and located on the supply side of the feeder roller set  75 . The paper guides  80  support lateral sides  13   a  of the recording sheet  13  defined in its length direction, and guides the recording sheet  13  toward the feeder roller set  75 . In FIG. 15, the paper guides  80  as viewed in cross section have a channel shape inside which a guide channel  80   a  is formed. A rail tooth portion or a rail bottom  81  of the paper guides  80  is formed in a comb shape, and includes a plurality of ray applying gaps  82  arranged at one pitch. In FIGS. 16 and 17, the lateral sides  13   a  of the recording sheet  13  guided by the paper guides  80  are optically fixed and bleached through the ray applying gaps  82 . The lateral sides  13   a  do not remain unbleached in belt shapes. In FIG. 16, rail teeth  81   a  of the rail bottom  81  include facets  81   b  which are defined by cutting upper corners of a rectangular shape. The facets  81   b  enable the recording sheet  13  to receive a sufficient amount of rays from the magenta fixer lamp  73 . 
     Among portions of the paper guides  80 , at least the rail bottom  81  is formed from transparent material, for the purpose of increasing an effect of rays applied to the lateral sides  13   a.    
     In FIG. 14, the platen roller  51  is supported in a manner movable up and down by means of a slot  67   a  formed in the conveyor frame  67 . An axial end of the platen roller  51  is rotatable on the swing frame  68 . One end of the swing frame  68  is secured to an end of the conveyor frame  67  by use of a support shaft  67   b  in a pivotal manner. A coil spring  83  is secured to another end of the swing frame  68 , and biases the swing frame  68  down. In FIG. 2, another end of the coil spring  83  is secured to a spring receiving bracket  84 , which extends from the supply frame  15  of the supply unit  10 . In FIG. 3, the cam follower  85  is formed with the swing frame  68  to project downwards and in a flexed shape, and is contacted by the platen lifter cam  50 . 
     In FIGS. 1 and 14, the thermal head  52  is secured to the chassis  64  by means of the head support plate  66  in a position of contact with the platen roller  51 . The head support plate  66  has heat dissipator fins  86  for dissipating heat generated from the thermal head  52 . The head cover  69  is associated with the thermal head  52 , and guides the recording sheet  13 . The sheet guide unit  70  is located above the fixer lamps  72  and  73 , and is secured to the conveyor frame  67  in a removable manner. A sensor base plate  89  is mounted on the sheet guide unit  70 . On the sensor base plate  89  are disposed a lamp irradiance sensor, a rear margin detecting sensor for the recording sheet, and a sensor for detecting rotations of the capstan roller. 
     The drive unit  65  is constituted by a gear train  93 , a conveyor motor  94 , and a drive frame  92  incorporating the coil spring  83  and the conveyor motor  94 . The gear train  93  includes the roller drive gear  78  and the drive gear  38 . The roller drive gear  78  is meshed with the gear of the capstan roller  75   a  and rotates it. The drive gear  38  is meshed with the intermediate gear  37  for rotating the ejector roller  33 . See FIG.  9 . The gear train  93  transmits rotation of the conveyor motor  94  to the capstan roller  75   a  and the ejector roller  33  in such a manner that peripheral speed of the capstan roller  75   a  and the ejector roller  33  is kept constant, so that the capstan roller  75   a  and the ejector roller  33  are rotated in the first direction A for the supply and in the second direction B for the printing. See FIGS. 1 and 14. 
     The swing frame  68  and the platen lifter cam  50  constitute a lifter mechanism  97 . See FIG.  18 . The lifter mechanism  97  shifts the platen roller  51  between the press position and the retracted position. In the press position, the recording sheet  13  is pressed by the heating element array  52   a  of the thermal head  52 . The heating element array  52   a  includes a great number of heating elements arranged in a direction parallel with an axis of the platen roller  51 . For the thermal printing, each heating element is driven according to image data, to record a full-color image to the recording sheet  13  in the three-color frame sequential method. 
     The fixer lamps  72  and  73  are disposed near to each other, for the purpose of making them arranged in a compact space. In FIG. 18, an irradiance sensor  98  is disposed between the fixer lamps  72  and  73 , and measures their irradiance. A signal from the irradiance sensor  98  is sent to a lamp inverter circuit  99 , which adjusts voltage applied to the fixer lamps  72  and  73  to keep the irradiance unchanged. 
     In FIG. 18, electrical circuits of the thermal printer  9  are depicted. A controller  100  consists of a microcomputer well-known in the art, and receives image data from a video instrument  101  or any of a digital still camera, a video recorder, a television set, a personal computer, and the like. After a print start signal is generated by operation of the operation panel  71  and after a detection signal is generated by a margin sensor  102 , the controller  100  operates for printing a full-color image on to the recording sheet  13  by controlling drivers  103  and  104 , the lifter mechanism  97 , a print controller  105 , the lamp inverter circuit  99  and a counter  107 . 
     The controller  100  causes the driver  103  to rotate the supply motor  42  in the forward or backward direction. In the forward rotation, the planet gear  48  of the planetary clutch  41  is in mesh with the gear  40  to rotate the supply roller  17  in the supply direction. In the backward rotation, the planet gear  48  comes in mesh with the drive gear  49 , to set the platen lifter cam  50  in the platen press position or platen retreat position. 
     The controller  100  causes the driver  104  to rotate the conveyor motor  94  in a forward or backward direction. In the forward rotation, the capstan roller  75   a  is rotated in the first direction A to convey the recording sheet  13  in a supply direction. In the backward rotation, the capstan roller  75   a  is rotated in the second direction B to convey the recording sheet  13  in a print direction. While the recording sheet  13  is conveyed by the conveyor motor  94 , the controller  100  causes the counter  107  to count pulses from a rotating amount sensor  108  which detects a rotating amount of the roller. This counting operation is started upon occurrence of a rear margin detecting signal generated by the margin sensor  102 . The pulses are counted up when the recording sheet  13  is conveyed in the print direction, and counted down when the recording sheet  13  is conveyed in the supply direction. It is possible to detect a relationship between positions of the recording sheet  13  and the heating element array  52   a  of the thermal head  52  during the conveyance, and to determine various positions, including a conveyance stop position, a conveyance start position, a press start position of the recording sheet relative to the heating element array, a press release position of the recording sheet relative to the heating element array, a print start position, and a print finish position. The margin sensor  102  is disposed between the intermediate gears  36  and  37  and near to the intermediate gear  37 , and detects passage of the first or rear margin line and the second or front margin line of the recording sheet  13 . 
     Of course the rotating amount sensor  108  generates one pulse each time that the capstan roller  75   a  rotates by a unit rotating amount. 
     The print controller  105  is constituted by a head driver and a memory, which stores three-color image data of one frame. The print controller  105  drives each of the heating elements of the heating element array  52   a  in accordance with image data of the respective three colors. The heating elements develop heat associated with the respective colors and image data of the colors, to color the recording sheet  13  at desired density. The lamp inverter circuit  99  controls the fixer lamps  72  and  73  at a constantly regulated amount of rays in accordance with a detection signal from the irradiance sensor  98 . Note that the memory does not require capacity enough to store data of one frame. The memory may have only small capacity enough to store data of a number of lines within one frame. For such a memory, image data is consecutively sent to the thermal printer  9  by the video instrument  101  by a unit amount of several lines and in accordance with the sequential steps for the printing. 
     The operation of the above construction is described with reference to FIGS. 19A,  19 B and  20 . A user manually operates the operation panel  71  to enter a command of printing. Responsively the supply motor  42  and the conveyor motor  94  are caused to rotate in the supply direction. If the recording sheet  13  does not exist on the supply plate  16 , a message for instructing a recording sheet to be supplied is indicated in a display window of the operation panel  71 . If the recording sheet  13  is not automatically inserted, the recording sheet  13  is manually set on the supply plate  16  and inserted between it and the supply roller  17 . Then the recording sheet  13  is sent to the printer body  12  by rotation of the supply roller  17 . 
     When the second or front margin line of the recording sheet  13  is moved past the feeder roller set  75 , the supply motor  42  is stopped. For detection of the passage of the second or front margin line at the feeder roller set  75 , at first the second or front margin line being moved is detected by the margin sensor  102 . Upon this detection, elapsed time starts being measured. The measured elapsed time is evaluated. When the measured elapsed time becomes a predetermined value, the passage of the second or front margin line at the feeder roller set  75  is recognized. It is to be noted that an additional sensor may be disposed near to the feeder roller set  75  for directly detecting the second or front margin line. Afterwards the feeder roller set  75  conveys the recording sheet  13  in the supply direction. When the first or rear margin line is conveyed past the margin sensor  102 , the conveyor motor  94  is stopped, to stop the sheet supply. 
     The conveyor motor  94  is caused to rotate in the print direction to effect the printing operation. According to the counted number of the counter  107 , the controller  100  determines the press start position of the thermal head  52 , the print start position, the print finish position, the retreat start position of the thermal head  52 , and the conveyance stop position of the recording sheet  13 . At first the press start position is determined. In FIG. 11, the supply motor  42  is rotated in reverse to the supply direction. This reverse rotation brings the planet gear  48  of the planetary clutch  41  in mesh with the drive gear  49 , so that the platen lifter cam  50  is rotated. The platen lifter cam  50  is rotationally shifted from the lifting position to the lowering position. The swing frame  68  is lowered to set the platen roller  51  in the press position. The planetary clutch  41  is stopped when the platen roller  51  comes to the press position. 
     When a front edge or start position of a recording region of the recording sheet  13  comes to the heating element array  52   a  of the thermal head  52 , the yellow color starts being thermally recorded. In the yellow recording, the yellow fixer lamp  72  is kept turned on to fix the yellow coloring layer  13   e , which will be prevented from developing further color in the course of the magenta or cyan recording. Upon the finish of the yellow recording, the platen roller  51  comes to the press release position. Then the supply motor  42  rotates in the direction reverse to the supply direction, so that the platen lifter cam  50  makes half a rotation. The platen roller  51  is set in the retreat position. The conveyor motor  94  is stopped from rotating in the print direction. After this, the conveyor motor  94  is rotated in the supply direction to return the recording sheet  13  to the print start position. 
     The magenta and cyan colors are similarly printed in the three-color frame-sequential method of thermal recording, to obtain a full-color image. The magenta fixer lamp  73  is turned on during the magenta recording, to fix the magenta coloring layer  13   f  optically. Again the magenta fixer lamp  73  is turned on during the cyan recording, to bleach unrecorded portions of the recording sheet, which would remain with a yellow-greenish color if not bleached. The recording sheet  13  after the finish of the cyan recording is sent to conveyed to an exit  62 . See FIG.  1 . 
     When the first or rear margin line of the recording sheet  13  comes to the ray applying region of the magenta fixer lamp  73 , the rear margin region of the recording sheet  13  is bleached. See FIG.  20 . For the bleaching, it is detected at first whether the first or rear margin line of the recording sheet  13  has come into the ray applying region of the magenta fixer lamp  73  during the ejection of the recording sheet  13 . Upon the finish of the cyan recording, an amount of conveying the recording sheet  13  starts being measured. It is checked whether the conveying amount has become an amount, which is predetermined as such an amount by which the recording sheet has been conveyed until the first or rear margin line has normally come to the ray applying region. Upon the reach of the first or rear margin line to the ray applying region of the magenta fixer lamp, the conveyor motor is stopped in a temporary manner. During this stop the magenta fixer lamp  73  emanates rays to bleach the rear margin region of the recording sheet. 
     Then the recording sheet  13  is conveyed by a length equal to a width of the ray applying gaps  82  as viewed in the conveying direction, before the recording sheet  13  is stopped. This is the intermittent conveyance. Portions of the lateral sides  13   a  of the recording sheet  13  masked by the rail teeth  81   a  during the previous stop of the recording sheet  13  are moved to positions of the ray applying gaps  82 , and are bleached in a reliable manner. After the stop, the conveyor motor rotates by a predetermined amount, to eject the recording sheet. Note that the stops for the two times are continued until rays from the fixer lamp come up to an amount enough for the bleaching. 
     If a user desires to obtain a plurality of printed sheets, the supply cassette  14  is set in the entrance opening  61 . See FIG.  12 . The supply cassette  14  contains the recording sheets  13  in a light-tight state. The operation of sheet supply is started when the supply motor  42  is rotated in the supply direction. In the supply, the guide faces  23   a  and  24   a  of the intermediate guide plates  23  and  24  and the supply roller  17  are positioned in such a manner that 
     
       
           t≦G&lt; 2 t    
       
     
     where G is the thickness of the space and t is the thickness of the recording sheet  13 . See FIGS. 8 and 12. Recording sheets lower than the uppermost sheet are hindered by the guide faces  23   a  and  24   a  from advancement. It is possible to prevent two or more sheet from being conveyed in an overlapped manner. 
     It is conventionally likely that two overlapped recording sheets  13  are conveyed to the side of the sheet guide, typically because the recording sheets  13  in use have a smaller thickness than is determined according to the specification of the thermal printer  9 . However the second or front margin line of the second uppermost recording sheet is contacted by the regulating face  30   a  of the stopper block  30 , and hindered from advancing. See FIG.  21 . It is possible to allow only one recording sheet to advance at one time. 
     Note that positions of the thermal head  52  and the feeder roller set  75  are not limited to the above embodiment, but may be modified as desired. The printing operation is not limited to the above embodiment, but may be modified suitably. For example, the yellow coloring layer  13   e  may be fixed during the conveyance in the supply direction after the yellow recording. Also it is possible to fix the yellow coloring layer  13   e  both during the conveyance in the print direction associated with the yellow recording, and during the conveyance in the supply direction after the yellow recording. In the above embodiment, the platen roller  51  is moved up and down. Alternatively the thermal head  52  may be moved up and down whereas the platen roller  51  may be disposed in a stationary manner. 
     In the above embodiment, the rotation of the capstan roller  75   a  is detected by the rotating amount sensor  108 . Alternatively the position of the recording sheet  13  in the printing unit may be detected differently. For example the conveyor motor  94  may be a stepping motor supplied with drive pulses. The drive pulses may be counted, to recognize the position of the recording sheet  13 . In the above embodiment, the supply plate is swung and supports a great number of recording sheets. Alternatively a movable supply plate may be used and may be moved straight in a parallel manner. 
     In the above embodiment, the width of the rail teeth  81   a  of the paper guides  80  is set equal to that of the ray applying gaps  82  with respect to the conveying direction. However the rail teeth  81   a  and the ray applying gaps  82  may have any suitably determined widths. For example the width of the rail teeth  81   a  of the paper guides  80  may be greater than that of the ray applying gaps  82  with respect to the conveying direction. Efficiency in bleaching the lateral sides  13   a  will be increased. Also the shape of the rail teeth  81   a  may be modified suitably. In FIG. 22A, a pair of paper guides or rail unit  121  has rail teeth  120  having a rectangular shape as viewed in cross section, like a thin plate. In FIG. 22B, a pair of paper guides or rail unit  123  has rail teeth  122  having a rod shape or cylindrical shape. In FIG. 22C, a pair of paper guides or rail unit  125  has rail teeth  124  having a shape of a triangular prism. 
     In the above embodiment, the recording sheet  13  is stopped in a temporary manner to bleach a first or rear margin region  13   b . FIG. 23 depicts another preferred embodiment, in which the speed of the conveyor motor is decreased when the rear margin region  13   b  comes to the ray applying region of the magenta fixer lamp  73 , to bleach the rear margin region  13   b  while the recording sheet  13  is conveyed at the decreased speed. The rear margin region  13   b  and the lateral sides  13   a  are bleached by application of rays at an amount enough to bleach. After a rear margin line  13   c  of FIG. 15 is moved past the ray applying region to finish the bleaching, the speed of the conveyor motor to set again at its initial speed, before the recording sheet  13  is ejected. 
     Note that the resetting of the speed of the conveyor motor may be omitted, so that the recording sheet  13  may be ejected at the same decreased speed. 
     In the above embodiment, the support shaft  16   a  of the supply plate  16  is positioned at a level of an upper surface for placing the recording sheet. Alternatively it is possible to dispose the support shaft  16   a  in any suitable position at the rear end of the supply plate  16 , namely higher or lower than the upper surface for placing the recording sheet. In the above embodiment, the supply plate  16  is disposed pivotally about the rotational shaft. Alternatively the supply plate  16  may be disposed in a manner shiftable up and down by use of rails or other guiding structures. 
     Another preferred embodiment is described now by referring to FIGS. 24-30, in which a user can be caused to recognize a finish of the printing operation. 
     There is a suggestion of a color thermal printer of JP-A (Japanese Patent Laid-open Publication No.) 8-174883, which is characterized by a reduced size of the printer body with a simplified structure of a sheet feeder mechanism. A portion of the recording sheet inevitably appears outside the printer body. While a full-color image is being printed, the user is informed of the printing operation by the apparent movement of the recording sheet. However the recording sheet is stopped in the course of bleaching the margin region. The stopped appearance may cause the user to misunderstand the condition. He may forcibly pull out the recording sheet during the bleaching even when the recording sheet has not finished being treated. 
     This problem is solved in a color thermal printer  202  of FIG. 24. A printer body  203  incorporates various parts of the thermal printer  202 . A front panel of the thermal printer  202  has an entrance opening  205  through which a color thermosensitive recording sheet  204  being unused is inserted manually, and is exited from the printer body  203  after being colored. A rear panel of the thermal printer  202  opposite to the entrance opening  205  has an auxiliary opening  206  through which a portion of the recording sheet  204  appears externally in the course of the printing operation. At least one portion of the recording sheet  204  is caused to appear outside the printer body  203  in the course of the printing, so that the conveying passageway of the recording sheet  204  in the printer body  203  is shortened. The side of the thermal printer  202  is reduced. 
     In a position inward from the entrance opening  205 , there is a reflection type of photo sensor  208  for detecting a line of first margin  204   a  of the recording sheet  204  inserted into the entrance opening  205 . The inside of the printer accommodates a first set of conveyor rollers or ejector roller set  209  as a conveyor unit, an optical fixer  210 , a second set of conveyor rollers or feeder roller set  211 , a thermal head  212  and a platen roller  213 . 
     The ejector roller set  209  squeezes the recording sheet  204  from the entrance opening  205  and supplies the printer body  203  with the recording sheet  204 . Also after the printing operation, the ejector roller set  209  ejects the recording sheet  204  through the entrance opening  205 . The ejector roller set  209  is constituted by a capstan roller  215  to be driven directly, and a nip roller  216  rotated together with the capstan roller  215 . A spring (not shown) having a small bias force keeps the nip roller  216  in contact with the capstan roller  215 . 
     The feeder roller set  211  conveys the recording sheet relative to the thermal head  212  during the thermal printing, and is constituted by a capstan roller  218  and a nip roller  219 , which has a smaller diameter than the capstan roller  218 , and is rotated by rotation of the capstan roller  218 . The nip roller  219  is constantly kept in contact with the capstan roller  218  by a spring (not shown) having a strong biasing force, and conveys the recording sheet  204  in a slip-free state. 
     The capstan roller  215  of the ejector roller set  209  and the capstan roller  218  of the feeder roller set  211  are driven by a stepping motor  221 . Rotation of the stepping motor  221  is transmitted to the capstan rollers  215  and  218  by cooperation of pulleys  222 ,  223  and  224  and belts  225  and  236  disposed on their periphery. 
     The optical fixer  210  is disposed between the ejector roller set  209  and the feeder roller set  211 . The optical fixer  210  is constituted by a yellow fixer lamp  226 , a magenta fixer lamp  227  and a reflector  228 . The yellow fixer lamp  226  emanates visible violet rays or near ultraviolet rays in a wavelength range peaking at the wavelength of 420 nm, and fixes the yellow coloring layer. The magenta fixer lamp  227  emanates ultraviolet rays in a wavelength range peaking at the wavelength of 365 nm, and fixes the magenta coloring layer. The reflector  228  covers tops and sides of the fixer lamps  226  and  227 , and reflects the rays toward the recording sheet  204 . 
     The thermal head  212  includes an array of heating elements  212   a  which are arranged in line perpendicular to the conveying direction of the recording sheet  204 . The thermal head  212  is disposed in a stationary manner. The heating element array  212   a  generates heat energy to the coloring layers of the recording sheet  204 . 
     The thermal head  212  is supported by a support lever  231 , which is rotated about a shaft  230  by a rotating mechanism (not shown). In the thermal printing, the thermal head  212  is rotated upwards as depicted in FIG. 25, and squeezes the recording sheet  204  between it and the heating element array  212   a  of the thermal head  212 . 
     The recording sheet  204  includes a support, a cyan thermosensitive coloring layer, a magenta thermosensitive coloring layer and a yellow thermosensitive coloring layer. It is possible to add a black thermosensitive coloring layer to the three coloring layers of the recording sheet  204 . 
     The operation of the present embodiment is described with reference to FIG.  26 . The thermal printer  202  is connected with a video instrument, for example a computer. When a user enters a print start command through the computer, print data is sent by the computer to the thermal printer  202 , which is caused to stand by for printing operation. 
     When the thermal printer  202  stands by for the printing operation, an insertion instructing lamp (not shown) on the front of the printer body  203  is caused to blink to inform a user of the standby condition and requirement of the recording sheet  204 . 
     In FIG. 24, the recording sheet  204  is inserted into the entrance opening  205 . The first margin  204   a  of the recording sheet  204  pushes the ejector roller set  209 . In the course of insertion of the recording sheet  204 , the line of the first margin  204   a  is detected by the photo sensor  208 . Upon the detection of the first margin  204   a , a detection signal from the photo sensor  208  causes the stepping motor  221  to rotate in a forward direction, which is clockwise in the drawing, so that the capstan roller  215  is caused to rotate clockwise by means of the pulleys  222 - 224  and the belts  225  and  236 . The nip roller  216  in contact with the capstan roller  215  is rotated in the counterclockwise direction, to nip the first margin  204   a  of the recording sheet  204 . 
     The recording sheet  204  conveyed by the ejector roller set  209  is moved under the optical fixer  210 , until the first margin  204   a  becomes nipped by the feeder roller set  211 . The recording sheet  204  is caused by the first and second conveyor roller sets  209  and  211  to come between the thermal head  212  and the platen roller  213 , and becomes protruded out of the auxiliary opening  206 . See FIG.  25 . The stepping motor  221  stops while a second margin  204   b  of the recording sheet  204  remains squeezed by the feeder roller set  211 . 
     When the recording sheet  204  stops being conveyed, the rotating mechanism (not shown) rotates the support lever  231  to move the platen roller  213  to the thermal head  212 . The recording sheet  204  is squeezed between the platen roller  213  and the heating element array  212   a  of the thermal head  212 . 
     When the stepping motor  221  rotates in reverse, the feeder roller set  211  conveys the recording sheet  204  by advancing the second margin  204   b . The platen roller  213  is caused to rotate by movement of the recording sheet  204 . When a start position of a recording region comes to the heating element array  212   a  during the conveyance, the heating elements generates heat energy according to respective pixels of a yellow image, which is recorded to the yellow coloring layer one line after another. 
     The recording sheet  204  after the yellow recording is nipped by the ejector roller set  209  again, and partially protruded through the entrance opening  205 . Upon the finish of the yellow recording to the recording sheet  204 , the thermal head  212  stops being driven. The platen roller  213  rotates to the retracted position away from the thermal head  212 . In FIG. 27, the stepping motor  221  is stopped in a state with the first margin  204   a  of the recording sheet  204  nipped by the feeder roller set  211 . 
     Immediately after the stepping motor  221  stops, the yellow fixer lamp  226  is turned on. Again the stepping motor  221  starts rotating forwards, so as to convey the recording sheet  204  toward the auxiliary opening  206  by advancing the first margin  204   a . During the conveyance the yellow fixer lamp  226  emanates near ultraviolet rays of 420 nm to the recording sheet  204 , to fix the yellow coloring layer for avoidance of further yellow coloring in the course of the magenta recording. In the drawing, a letter L designates a margin region on the recording sheet  204 . The yellow fixer lamp  226  does not apply rays to the margin region L in the course of the yellow fixation. 
     A front edge of a recording region of the recording sheet  204  is conveyed to come again to the heating element array  212   a . The stepping motor  221  is stopped to turn off the yellow fixer lamp  226 . In FIG. 25, the platen roller  213  is pushed to the thermal head  212  again. The stepping motor  221  is rotated backwards, to convey the recording sheet  204  by advancing the second margin  204   b . The thermal head  212  applies heat energy to the magenta coloring layer to record the magenta color to the recording sheet. 
     After the magenta image is recorded to the end position of the recording region short of the first margin  204   a  of the recording sheet  204 , the platen roller  213  is shifted again to the retreat position, to release the recording sheet  204  from pressure. See FIG.  27 . The stepping motor  221  stops. Then the magenta fixer lamp  227  is turned on. Again the stepping motor  221  starts rotating forwards, to convey the recording sheet  204  toward the auxiliary opening  206  by advancing the first margin  204   a . During the conveyance the magenta fixer lamp  227  emanates ultraviolet rays of 365 nm to the recording sheet  204 , to fix the magenta coloring layer for avoidance of further magenta coloring in the course of the cyan recording. 
     The front edge of the recording region of the recording sheet  204  is conveyed to the heating element array  212   a . The platen roller  213  is pushed to the thermal head  212  again for the thermal recording. The stepping motor  221  is rotated backwards, to convey the recording sheet  204  by advancing the second margin  204   b . The thermal head  212  records the cyan color to the recording sheet. 
     After thermal recording to all the three coloring layers is finished, the platen roller  213  moves back to the retreat position. In FIG. 28, the stepping motor  221  further rotates in the reverse direction, to protrude a portion of the recording sheet  204  from the entrance opening  205 , before the margin region L is confronted with the magenta fixer lamp  27  and stopped. 
     In FIG. 29, the magenta fixer lamp  227  is turned on immediately after the stop of the stepping motor  221 . Then the stepping motor  221  is caused to rotate alternately in the forward and backward directions in such a range that the margin region L of the recording sheet  204  does not come out of a ray applying range of the magenta fixer lamp  227 . Therefore the margin region L of the recording sheet  204  is optically fixed and bleached. 
     The user recognizes the finished state of the printing operation because of the back-and-forth movement of the recording sheet  204 . There is no stop of the recording sheet  204  during the bleaching of the margin region L. The recording sheet  204  is kept from being pulled out of the entrance opening  205 , because the user does not misunderstand the bleaching operation. The margin region L can be fixed and bleached appropriately. 
     After the back-and-forth conveyance of the recording sheet  204  for fixation of the margin region L, then the stepping motor  221  continuously rotates in the backward direction. The recording sheet  204  is ejected out of the entrance opening  205 . 
     In the above embodiment, the recording sheet  204  is conveyed horizontally. Alternatively the recording sheet  204  may be conveyed erectly with an inclination as depicted in FIG. 30. A color thermal printer  240  has an entrance opening  241  through which the recording sheet  204  is supplied. A thermal head  242  prints the three colors to the recording sheet  204 , which is fixed by an optical fixer  243 . A portion of the recording sheet  204  is caused to appear externally through an auxiliary opening  245 . After the printing operation the recording sheet  204  is ejected up through the entrance opening  241 . 
     In the above embodiment, the recording sheet  204  is conveyed back and forth within the small range for the purpose of bleaching the margin region L. Alternatively the recording sheet  204  may be conveyed intermittently within the same small range by a unit length being predetermined still smaller, for the purpose of bleaching the margin region L. 
     In the above embodiments, the thermal printer is a three-pass one-head type in which a single thermal head is used for three-time thermal recording. Of course a thermal printer in the present invention may be a one-pass three-head type in which three thermal heads are used for one-time thermal recording of one full-color image. In the above embodiments, the thermal printer is a color printer for recording a full-color image. Alternatively a thermal printer in the present invention may be a monochromatic printer. 
     In the above embodiment, the recording sheet as a single piece is used in the printing operation. Alternatively a continuous recording sheet of a roll may be used in another type of thermal printer in the present invention. The thermal printer may be supplied successively with the continuously sheet. The continuous recording sheet after the printing operation may be cut into each piece of sheet. 
     Still another preferred embodiment is illustrated in FIGS. 31-37, in which a margin of the recording sheet is fixed and bleached before the thermal recording and immediately after the sheet supply. 
     In a position inward from an entrance opening  305 , there is a reflection type of photo sensor  308  for detecting a line of a first margin  304   a  of a color thermosensitive recording sheet  304  inserted into the entrance opening  305 . The inside of the printer accommodates a first set of conveyor rollers or supply roller set  309  as a conveyor unit, an optical fixer  310 , a micro switch  314 , a second set of conveyor rollers or feeder roller set  311 , a thermal head  312  and a platen roller  313 . A color thermal printer  302  is controlled by a controller  322  which consists of a computer. When the photo sensor  308  detects the line of the first margin  304   a  of the recording sheet  304 , it outputs a detection signal which is sent to the controller  322 . 
     A capstan roller  315  of the supply roller set  309  and a capstan roller  318  of the feeder roller set  311  are driven by a stepping motor  321 . The stepping motor  321  is driven by a driver  323 , which is controlled by the controller  322 . 
     In FIG. 32, the recording sheet  304  is conveyed by advancing a second margin  304   b  to the left as viewed in the drawing, while the thermal head  312  thermally records three-color images. In FIG. 33, the recording sheet  304  is conveyed by advancing the first margin  304   a  to the right as viewed in the drawing, while the optical fixer  310  optically fixes the coloring layers. In FIG. 34, a full-color image is recorded on a recording region located in the center about which blank spaces are formed without any image. A margin region L formed on the first margin  304   a  of the recording sheet  304  does not become confronted with the optical fixer  310  during the printing operation. Although a region L 1  included in the margin region L is fixed by reflected rays from a reflector  328 , yet a region L 2  within the margin region L remains unfixed, because nipped by the feeder roller set  311 . 
     The micro switch  314  includes a plunger  314   a , which is contacted by the line of the first margin  304   a  of the recording sheet  304 . The contact of the first margin  304   a  turns the micro switch  314  on. The micro switch  314  generates a detection signal, which is sent to the controller  322 . 
     In operation of the present embodiment, the recording sheet  304  is at first inserted in the entrance opening  305 . See FIG.  31 . The first margin  304   a  of the recording sheet  304  is contacted by the supply roller set  309 . During the insertion of the recording sheet  304 , the line of the first margin  304   a  is detected by the photo sensor  308 , of which a detection signal is sent to the controller  322 . 
     In response to a detection signal from the photo sensor  308 , the controller  322  causes the driver  323  to rotates the stepping motor  321  in the forward direction, so that the capstan rollers  315  and  318  are rotated in the clockwise direction. A pinch roller  316  is rotated by the contact with the capstan roller  315  in the counterclockwise direction, to nip the first margin  304   a  of the recording sheet  304 . 
     The recording sheet  304  conveyed by the supply roller set  309  is moved under the optical fixer  310 . In FIG. 36, the first margin  304   a  comes in contact with the plunger  314   a  of the micro switch  314 . The micro switch  314  being turned on, a signal from the micro switch  314  is sent to the controller  322 , which responsively stops the stepping motor  321 . Consequently the margin region L of the first margin  304   a  of the recording sheet  304  is stopped and confronted with a magenta fixer lamp  327 . See FIG.  34 . 
     At the same time as the stepping motor  321  stops, the controller  322  turns on the magenta fixer lamp  327  of the optical fixer  310 , to bleach a region which is defined between the line of the first margin  304   a  and a curved broken line  335  indicated in FIG.  34 . This bleached region is overlapped with a range L 1  which will be bleached in the course of printing operation. Consequently the entirety of the margin region L can be bleached. 
     This being so, the margin region of the recording sheet  304  is bleached upon supply of the recording sheet and before the printing. The recording sheet  304  after finishing the printing operation can be ejected immediately. The recording sheet  304  can be prevented from being forcibly pulled out of the printer body in the course of bleaching which would follow the thermal recording. It is possible to shorten the total time required for obtaining a full-color image, because the step of standby for printing has been a cause of increasing the total time of the printing, but utilized for the previous bleaching in the present embodiment. 
     After the magenta fixer lamp  327  is kept turned on for the predetermined duration of time by the controller  322 , the controller  322  turns off the magenta fixer lamp  327 . After the printer comes to stand by again for printing operation, the controller  322  causes the stepping motor  321  to rotate again. 
     The recording sheet  304  is conveyed by the supply roller set  309  toward the thermal head  312  while the first margin  304   a  is advanced. The recording sheet  304  is nipped by the feeder roller set  311 . The recording sheet  304  conveyed by the supply roller set  309  and the feeder roller set  311  passes between the thermal head  312  and the platen roller  313 . In FIG. 32, the recording sheet  304  is protruded from the inside of the thermal printer  302  through an auxiliary opening  306 . The controller  322  stops the stepping motor  321  while the second margin  304   b  of the recording sheet  304  remains nipped by the feeder roller set  311 . 
     In the present embodiment, a reference numeral  303  designates a printer body,  312   a  an array of heating elements,  319  a nip roller,  326  a yellow fixer lamp,  330  a shaft, and  331  a support lever. 
     In the above embodiment, the recording sheet  304  is conveyed horizontally. Alternatively the recording sheet  304  may be conveyed erectly with an inclination as depicted in FIG. 38. A color thermal printer  340  has an entrance opening  341  through which the recording sheet  304  is supplied. A thermal head  342  prints the three colors to the recording sheet  304 , which is fixed by an optical fixer  343 . A portion of the recording sheet  304  is caused to appear externally through an auxiliary opening  345 . After the printing operation the recording sheet  304  is ejected up through the entrance opening  341 . Of course the thermal printer  340  has a micro switch  346  for detecting one margin line of the recording sheet  304 . 
     In the above embodiments, the recording sheet is partially protruded from the inside of the printer in the printing operation. Of course the thermal printer of the present invention may be a type in which the entirety of the recording sheet is contained inside a printer body in the printing operation. 
     The embodiments of FIGS. 30 and 38 do not have feeder rollers between the fixer and the thermal head. Of course such an upright type of thermal printer in the present invention may include feeder rollers between the fixer and the thermal head. 
     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.