Patent Publication Number: US-6708971-B1

Title: Paper cassette, printer for use with paper cassette, and paper supplying method

Description:
FIELD OF THE ART 
     The present invention relates a paper cassette and a printer for use with the paper cassette. The present invention relates also to a method of supplying recording sheets from the paper cassette to the printer. 
     BACKGROUND ARTS 
     Among printers, thermal printers are classified into thermosensitive printers and thermal transfer printers, each of these two types uses a specific kind of recording paper. Recording paper for the thermosensitive printer has a cyan thermosensitive coloring layer, a magenta thermosensitive coloring layer, and a yellow thermosensitive coloring layer formed sequentially atop another on a base material. The thermosensitive coloring layers are subjected to thermal recording sequentially from the obverse side, while being optically fixed before the next thermosensitive coloring layer is subjected to the thermal recording. The optical fixing is disabling coloring-capability of the individual thermosensitive coloring layer by exposing it to electromagnetic rays of a specific wavelength range, e.g. ultraviolet rays, that is determined for each thermosensitive coloring layer, so that already colored thermosensitive coloring layers will not be re-colored by the thermal recording on the next thermosensitive coloring layer. Because coloring characteristics of the recording paper for the thermosensitive printer are affected even by light from ordinary light sources if the recording paper is exposed for a long time, the recording paper is preserved in a light-tight bag, and the light-tight bags are encased in a paper box when the recording paper is shipped out of factory. 
     On the other hand, the thermal transfer printers are classified into a melting type where ink is melted or softened to be transferred from ink film onto the recording paper, and a sublimation type where dyes on an ink film are transferred onto the recording paper by sublimation or dispersion. For the melting type printer, a high-smooth coated paper is used as the recording paper. For the sublimation type printer, a specific paper coated with polyester resin is used as the recording paper. The recording paper for these thermal transfer printers is contained in a moisture-proof bag, and the bags are contained in a paper box at the shipment. 
     To load the recording sheets in a printer, first the bag is opened to take out. There is a recording sheet package that contain a stack of sheets of recording paper, and the recording sheet package is loaded in a paper cassette. Then, the paper cassette is placed in the printer, so the recording sheet is fed from the paper cassette into the printer. The paper cassette has an air-proof sponge or the like along a mating portion between a cassette body and a lid, for protecting the contained recording sheets from moisture, as disclosed for example in JPA 9-132330. 
     Although the above mentioned paper cassette is completely closed in its preservative condition where the lid is closed, but the lid must be opened for paper-feeding, then the moisture-shielding properties of the cassette decreases. Moreover, where the recording sheets are to be fed out from a stack, a trouble of feeding a plurality of recording sheets along with an uppermost one, called multi-feeding, is likely to occur. This trouble can occur at any kinds of recording sheets, including thermosensitive recording sheets. 
     As a result of multi-feeding, the recording sheets may jam up in a paper feeding path, or recording area on the recording sheet may be deviated. As a result of multi-feeding, leading ends of the multi-fed recording sheets may be left protruded from the paper cassette. Since recording surfaces of the recording sheets are protected from moisture and light by stacking the recording sheets in tight contact with each other, if the recording sheets are left protruded for a long time, coloring characteristics of the recording surfaces of the protruded recording sheets are affected by moisture and light. A change in the coloring characteristics results in undesirable variations in color or density of the recorded image. 
     The recording sheets can also protrude from a paper supply start position in the paper cassette or in the printer when the paper cassette or the printer with the paper cassette attached thereto is carried about. Also while the paper cassette is being attached to the printer, the recording sheets can protrude from the paper supply start position. If the recording sheets are in this condition at the start of paper feeding, it may cause multi-feeding, and jamming of the recording sheets are likely to occur. The same applies not only to the thermal printers but also to any other kinds of printers. 
     An object of the present invention is to provide a paper cassette, a printer and a paper feeding method, that prevent multi-feeding and keep protecting the recording surface of the recording sheet from moisture and light. 
     DISCLOSURE OF THE INVENTION 
     In a printer supplied with recording sheets one by one from a paper cassette by rotating paper supply rollers, the paper cassette being loaded with a stack of recording sheets, the present invention is characterized by comprising a paper supply roller control device that executes a paper supply mode and a returning mode, wherein the paper supply roller control device causes the paper supply rollers to rotate in a first direction in the paper supply mode, to feed an uppermost or a lowermost one of the recording sheets into the printer, or causes the paper supply rollers to rotate in a second direction in the returning mode, to return the recording sheet to a predetermined paper supply start position inside the paper cassette. 
     The returning mode is executed after an end of paper supply, when a power source of the printer is turned on, or when the paper cassette is loaded in the printer. 
     The paper supply mode is executed in response to a print start command. At that time, it is possible to execute the returning mode immediately before the paper supply mode. In that case, the paper supply rollers are rotated in the second direction to return the recording sheets to the predetermined paper supply start position inside the paper cassette before the paper supply rollers are rotated in the first direction to feed the uppermost or the lowermost recording sheet into the printer. 
     Even if the recording sheets are deviated from the predetermined paper supply start position because of multi-feeding or other reasons, these recording sheets are returned to the initial position, so that occurrence of multi-feeding and jamming of the recording sheets in the next paper supply mode is reduced. Beside that, the recording sheets are kept shielded from moisture and light. 
     A paper cassette according to the present invention comprises a cassette body loaded with a stack of recording sheets and having a parallelepiped shape with an open end that serves as a paper supply opening; paper supply rollers located in proximity to the paper supply opening of the cassette body in contact with a first one of the recording sheets that is placed at the uppermost or the lowermost position of the stack, the paper supply rollers being rotated to feed the recording sheet out of the cassette body; a recording sheet urging member that pushes the entire stack of the recording sheets to press the first recording sheet onto the paper supply rollers; and a recording sheet separator for separating the first recording sheet from a second and following recording sheets, the recording sheet separator being located in the paper supply opening and movable between a paper separating position where the recording sheet separator is in contact with leading ends of the second following recording sheets, and an open position away from the leading ends of the recording sheets to open the paper supply opening, wherein the first recording sheet is fed out from the cassette body through a paper passageway that is formed between the recording sheet separator and the paper supply opening while the recording sheet separator is in the paper separating position. 
     According to the present invention, the recording sheets are loaded in the cassette body that has only one end open as the paper supply opening, and the paper supply opening is also closed by the recording sheet separator such that the narrow paper passageway is left unclosed. Because the paper passageway is shielded when the paper cassette is placed in the printer, the paper cassette of the present invention is improved in light- and moisture-shielding properties. 
     Since the recording sheet separator is movable between the paper separating position where the recording sheet separator is in contact with the leading ends of the second and following recording sheets, and the open position retracted from the paper supply opening, the recording sheet separator in the paper separating position can separate the recording sheets from each other during the paper feeding, thereby preventing multi-feeding. On the other hand, in the retracted position, the recording sheet separator is retracted from the paper supply opening to open up the paper supply opening, so the recording sheets may be loaded through the paper supply opening into the cassette. 
     The light- and moisture-shielding properties are still more improved by providing the light-shielding member, which is made of a resilient plate bent into a wavy shape, in the paper passageway, such that the bent portions of the resilient plate are in contact with the recording sheet separator to close the paper passageway. The recording sheet passes through the paper passageway while being pressed gently by the bent portions against the recording sheet separator. 
     Incorporating the paper supply rollers into the paper cassette makes it unnecessary to provide roller openings that allows the paper supply rollers to enter the paper cassette, and thus provides improved light- and moisture-shielding properties. Where the paper supply rollers are not incorporated into the paper cassette, the sliding lid member is mounted to the roller opening, and is urged by a coiled spring to the closed position, so the roller opening is closed while the paper cassette is not attached to the printer. Thereby, the paper cassette is maintained light-tight and moisture-tight. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an appearance of a recording sheet package; 
     FIG. 2 is a top plan view illustrating the recording sheet package; 
     FIG. 3 is a sectional view taken along a line III—III of FIG. 2; 
     FIG. 4 is a perspective view illustrating the recording sheet package taken out from a bag; 
     FIG. 5 is a perspective view illustrating an unfolded casing of the recording sheet package; 
     FIG. 6 is a perspective view illustrating a paper cassette; 
     FIG. 7 is a perspective view illustrating the paper cassette with its lid open, and the recording sheet package; 
     FIG. 8 is a sectional view illustrating an interior of the paper cassette containing the recording sheet package; 
     FIG. 9 is a sectional view illustrating the interior of the paper cassette in which a few of the recording sheets remain; 
     FIG. 10 is a sectional view illustrating a paper remainder indicator of the paper cassette; 
     FIG. 11 is a schematic diagram illustrating essential parts of a thermosensitive printer according to a first embodiment of the present invention; 
     FIG. 12 is a perspective view illustrating an overall appearance of the thermosensitive printer of FIG. 11, with the paper cassette attached thereto; 
     FIG. 13 is a flow chart illustrating an operation of the thermosensitive printer of FIG. 11; 
     FIG. 14 is a schematic diagram illustrating essential parts of a thermosensitive printer with a paper sensor, according to a second embodiment of the present invention; 
     FIG. 15 is a flow chart illustrating an operation of a thermosensitive printer according a third embodiment of the present invention, wherein a returning mode for resetting the recording paper to a predetermined paper supply start position is carried out immediately after the printer is powered as well as immediately after the cassette is attached again; 
     FIG. 16 is a schematic diagram illustrating essential parts of a thermosensitive printer with a cassette sensor, according to a fourth embodiment of the present invention; 
     FIG. 17 is a flow chart illustrating an operation of a thermosensitive printer that carries out the resetting mode immediately before starting printing, according to a fifth embodiment of the present invention; 
     FIG. 18 is a perspective view illustrating an appearance of a paper cassette viewed from left, according to the present invention; 
     FIG. 19 is a perspective view illustrating an appearance of the paper cassette of FIG. 18, viewed from right; 
     FIG. 20 is a perspective view illustrating how to load a recording sheet package in the paper cassette of FIG. 18; 
     FIG. 21 is a vertical sectional view illustrating the paper cassette of FIG. 18; 
     FIG. 22 is a vertical sectional view illustrating the paper cassette of FIG. 18, in a condition where the recording sheet package is being loaded; 
     FIG. 23 is a perspective view illustrating the paper cassette of FIG. 18, in a condition attached to a thermosensitive printer; 
     FIG. 24 is a perspective view illustrating a paper cassette according to another embodiment, that is provided with an interconnection mechanism for interconnecting a recording sheet separator and a paper pressing plate; 
     FIG. 25 is a fragmentary sectional view illustrating an operation of the interconnection mechanism, wherein the recording sheet separator of the paper cassette of FIG. 24 is in an open position; 
     FIG. 26 is a sectional view illustrating essential parts of a paper cassette according to a further embodiment, attached to a thermosensitive printer; and 
     FIG. 27 is a sectional view illustrating the essential parts of the paper cassette of FIG. 26, detached from the thermosensitive printer. 
    
    
     PREFERRED EMBODIMENTS OF THE INVENTION 
     For detailed description, the present invention will be described with reference to the accompanied drawings. 
     As shown in FIGS. 1 to  3 , a recording sheet package  10  contains in a casing  11  twelve recording sheets  12  of thermosensitive type piled up in a stack with their base side upward and their thermosensitive coloring surfaces downward. However, as for recording sheets of a type that have an adhesive surface and a releasing paper for use as stickers and thus have a greater thickness, ten sheets are packed in a package. A protective sheet  12   a  is disposed under the bottom recording sheet  12 , in tight contact with the thermosensitive recording surface of the recording sheet  12 , so as to shield the thermosensitive recording surface from light and moisture. But the protective sheet  12   a  may be omitted. 
     As shown in FIG. 4, the recording sheet package  10  is enclosed in a light-tight and moisture resistant bag  13  in the stores. The bag  13  is opened by tearing off a margin  13   a  along a cutting line  14 , to use the recording sheet package  10 . The bag is provided with a plastic fastener  15  along an opening  13   b,  for permitting enclosing the recording sheet package  10  again after it is half-used. The fastener  15  consists of a pair of engaging ridges, of which male ridge is force-fitted in female ridge in a well-known manner to seal the bag. Designated by  16  is a hole through which the bag  13  may hang on a hook  17  for exhibition and sale. 
     As shown in FIG. 5, the casing  11  is shaped into a thin parallelepiped contour by folding a blade along folding lines  19 , the blade being formed by blanching a sheet of thick dust-proof paper made of long fibers. The dust-proof paper made of long fibers reduces generation of cutting dusts from the casing  11  at the blanching and at the paper feeding as well, and thus prevents bad influence of the cutting dust on the print. The casing  11  consists of rectangular top and bottom plates  11   a  and  11   b  that are slightly larger than the recording sheet  12 , an end plate  11   c  connecting the top plate  11   a  to the bottom plate  11   b,  side plates  11   d  and  11   e  on opposite sides of the top plate  11   a,  and supporting plates  11   f  and  11   g  that are respectively connected to the side plates  11   d  and  11   e  through the folding lines  19 . Another end of the casing  11  opposite to the end plate  11   c  is opened for use as a paper feed out opening  20 . To a short side of the top plate  11   a  that extends by the paper feed out opening  20 , a pressing plate  11   h  is connected through a folding line  30 . Between the top plate  11   a  and the pressing plate  11   f,  there is formed a roller entrance  22  for paper supply rollers  21  (see FIG. 8) that are provided in a thermosensitive printer. 
     Cutouts  23 ,  24 ,  25 ,  26  and  27  are formed through the side plates  11   d  and  11   e  and the end plate  11   c,  each of which extends from the top plate  11   a  through one of the plates  11   c  to  11   e  to the bottom plate  11   b.  As shown in FIG. 2, the cutouts  23  and  25  are located symmetrically about a center line CL 1  that extends in a paper supply direction of the recording sheet  12  across a center P of the casing  11 . The cutout  24  is not symmetrical with the cutouts  25  and  26  about the center line CL 1 . The cutouts  23  and  24  are not symmetrical with each other about a center line CL 2  that extends across the center P perpendicularly to the center line CL 1 . 
     As shown in FIG. 3, the pressing plate  11   h  is laid under the top plate  11   a  by folding at an angle of 180 degrees along the folding line  30 . Thereby, the roller entrance  22  is connected to the paper feed out opening  20 , and the pressing plate  11   h  urges the uppermost recording sheet  12  toward the bottom plate  11   b,  so the recording sheets  12  come into tight contact with each other. Accordingly, the thermosensitive recording surface is covered with the lower recording sheet  12 , such that air and light hardly enter the thermosensitive recording surface. That is, the pressing plate  11   h  provides moisture- and light-shielding effects, and prevents the coloring characteristics from being affected by moisture and light. 
     On those portions where the supporting plates  11   f  and  11   g  meet the bottom plate  11   b,  along opposite end portions from the paper feed out opening  20 , there are provided adhesive layers  31 . The adhesive layers  31  keep the casing  11  in the parallelepiped shape. Because the adhesive layers  31  are not provided along the entire meeting zones between the bottom plate  11   b  and the supporting plates  11   f  and  11   g,  but only along the opposite end portions from the paper feed out opening  20 , that is about one third to one half of the entire length (the long-side length) of the meeting zones, it is possible to push up almost a half of the bottom plate  11   b  on the paper feeding opening side, with decreasing number of the recording sheets  12 , as set forth later. 
     FIGS. 6 and 7 are perspective views of a paper cassette  40  for holding the recording sheet package  10 . For accepting the recording sheet package  10 , the paper cassette  40  consists of a cassette body  41  and a lid  42 , and has a thin parallelepiped shape. The lid  42  is hinged to the cassette body  41  through a mounting shaft  43  such that the lid  42  may be opened at an angle of 90 degrees. While the lid  42  is open and erect, the recording sheet package  10  is loaded in a package loading section  44 . When the lid  42  is closed into a horizontal position, as shown in FIG. 8, engaging claws  45  and  46  are engaged with each other, so the lid  42  is maintained in this closed position. To open the lid  42 , a disengaging blade  47  is pushed to disengage the engaging claws  45  and  46 , allowing the lid  42  to open. The disengaging plate  47  is coupled to a bottom plate  48  through mounting shafts  47   a,  such that the disengaging plate  47  is movable in the paper supply direction of the recording sheets  12 , and is urged by a coiled spring  47   b  to engage the engaging claw  46  with the engaging claw  45 . 
     As shown in FIG. 7, the package loading section  44  is made slightly larger than the recording sheet package  10 , so as to facilitate loading. On the bottom plate  48  of the loading section  44 , guide protrusions  50 ,  51 ,  52 ,  53  and  54  are formed in those locations corresponding to the cutouts  23  to  27 . The guide protrusions  50  to  54  are rectangular, and have an inclined guide surface  55  formed on their top faces. Through these guide surfaces  55 , the guide protrusions  50  to  54  are smoothly guided into the cutouts  23  to  27 . 
     When the recording sheet package  10  is placed properly in the loading section  44 , the guide protrusions  50  to  54  are inserted into the cutouts  23  to  27 , allowing loading the recording sheet package  10 . When the recording sheet package  10  is placed in the loading section  44  in a left-side-right posture or an inverted posture, the cutouts  23  to  27  do not fit in the guide protrusions  50  to  54 , so the recording sheet package  10  cannot be inserted, showing that the loading posture is wrong. In that case, the recording sheet package  10  is turned around to position the cutouts  23  to  27  in correspondence with the guide protrusions  50  to  54 , so the recording sheet package  10  may be reloaded correctly in the loading section  44 . 
     Among of the guide protrusions  50  to  54 , two disposed on the left side constitute a pair, and two disposed on the right side constitute another pair. As shown in FIG. 10, a distance W 1  from inner sides of one pair of the guide protrusions  50  and  52  to inner sides of the other pair of the guide protrusions  51  and  53  is slightly greater than the width of the recording sheet, so the recording sheets  12  are inserted in between these two pairs of the guide protrusions, and thereby aligned in the lateral direction (the direction perpendicular to the paper supply direction) inside the casing  11 . Therefore, even if the recording sheets  12  are loosely held in the casing  11 , the recording sheets  12  are automatically positioned in the perpendicular direction to the paper supply direction by the guide protrusions  50 ,  51 ,  52  and  53 , when the recording sheet package  10  is loaded in the paper cassette  40 . Accordingly, the recording sheet  12  would not be fed aslant, so that the image would not be recorded obliquely, or the recording sheet  12  would not get jammed. Designated by  12   b  is a cutout of the protective sheet  12   a.    
     As shown in FIG. 8, the guide protrusions  54  position the recording sheets  12  in the paper supply direction within the casing  11 . Therefore, even if the recording sheets  12  are loosely held in the casing  11 , the recording sheets  12  are automatically positioned in the paper supply direction by loading the recording sheet package  10  in the paper cassette  40 . A push-up plate  57  is provided on a portion of the bottom plate  48  of the package loading section  44 , and is coupled to the bottom plate  48  pivotally through a mounting shaft  58 . The push-up plate  57  is urged by a coiled spring  59  to move upward, so as to push up the bottom plate  11   b  of the casing  11  of the recording sheet package  10 . 
     A gap is formed between the cassette body  41  and the lid  42  in the vicinity of the mounting shaft  43 , to provide a paper passageway  60 . A recording sheet separator  62  is protruded into the paper passageway  60 , and a cork member  63  is cemented to a surface of the recording sheet separator  62 . A recording sheet pressing blade  61  is also provided in the paper passageway  60 , that is mounted to the lid  42  through a mounting shaft  61   a.  The recording sheet pressing blade  61  is formed by bending a leaf spring into a wavy shape, so bent portions  61   b  gently press the recording sheet  12  against the recording sheet separator  62 . Because of the cork member  63  and the recording sheet pressing blade  61 , if two or more of the recording sheets  12  are about to be fed out together, the lower recording sheet  12  is stopped by friction against the cork member  63 , so the upper recording sheet  12  alone is fed out. Although the cork member  63  covers substantially the whole surface of the upper portion of the recording sheet separator  62 , the cork member  63  may be provided only in a central portion that comes into contact with the recording sheet. The recording sheet separator  62  further has a pair of separating projections  64  for securer prevention against multi-feeding. The separating projections  64  are designed to confront leading ends of the lower ones of those recording sheets which are being fed together, and thus stop them from advancing. 
     The bent portions  61   b  are provided before and behind the mounting shaft  61   a  in the paper supply direction. Because the bent portions  61   b  are resiliently brought into contact with the recording sheet separator  62 , the paper passageway  60  is tightly closed. This blocks entrance of dusts or the like, and also increases light-tightness and moisture-tightness. Moreover, because the bent portions  61   b  close the passageway at two points, at least one of the bent portions  61   b  closes the passageway even while the leading end of the recording sheet passes through. 
     As shown in FIG. 9, roller openings  65  are formed through the lid  42  of the paper cassette  40  in positions corresponding to the roller entrance  22  of the recording sheet package  10 . Into these roller openings  65  are inserted paper supply rollers  21  when the paper cassette  40  is set in a thermosensitive printer  75 . Then, through the roller entrance  22 , the paper supply rollers  21  come to contact with the uppermost one of the recording sheets  12  in the recording sheet package  10 . The paper supply rollers  21  rotate in a paper supply direction at the start of printing, to withdraw the uppermost recording sheet  12  from the recording sheet package  10 , and supply it to a print stage  74  (see FIG. 11) of the thermosensitive printer  75 . 
     As shown in FIGS. 6 and 9, a top side of the lid  42  doubles as a paper ejection tray, so paper guides  66  and  67  and a stopper  68  are protruded from the lid  42 . The paper guides  66  and  67  are for guiding the recording sheet  12  at its lateral sides, and are formed along the length of the lid  42 . The stopper  68  is for stopping the recording sheet  12  from slipping off the lid  42 . 
     On an opposite side of the stopper  68  from the paper passageway  60 , there is provided a paper remainder counter  70 . As shown in FIG. 10, the paper remainder counter  70  has an indicator  73  whose pointer  73   a  indicates a remainder (the number of remaining ones) of the recording sheets  12  on scales  71  and  72 . The scales  71  and  72  are disposed on opposite lateral sides of the indicator  73 , wherein the right scale  71  is for ordinary sheets and has graduations ranging from “20” to “zero”. The left scale  72  is for thicker recording sheets, including the recording sheets for stickers, and has graduations ranging from “10” to “zero”. A lower end  73   c  of the indicator  73  is contacted with the uppermost one of the recording sheets  12 . 
     FIG. 12 is a perspective view showing the thermosensitive printer  75  loaded with the paper cassette  40 . The thermosensitive printer  75  has an inlet  77  on its front face  76 , into which the paper cassette  40  is inserted. When the paper cassette  40  is put in the inlet  77 , a click engagement member  83  is engaged in a recess  40   a  of the paper cassette  40 , as shown in FIG. 8, preventing the paper cassette  40  from dropping. Besides the inlet  77 , there are provided an operation panel  78 , an LCD  79 , an IC card slot  80 , a slot  81  for a smart media, and a power switch  82 . 
     As shown in FIG. 11, the paper supply rollers  21  are each pivoted on a roller holding arm  84 . The roller holding arms  84  are moved through an arm shifting mechanism  85  between a paper feeding position where the paper supply rollers  21  are in contact with the uppermost one of the recording sheets  12 , and a retracted position retracted upward from the paper feeding position, as shown in FIG.  8 . In the paper feeding position, as shown in FIG. 9, the paper supply rollers  21  are inserted in the openings  65  of the lid  42 . Since the recording sheets  12  are pushed upward by the push-up plate  57  inside the paper cassette  40 , the uppermost one of the recording sheets  12  is kept in contact with the paper supply rollers  21 . 
     The arm shifting mechanism  85  is constituted of a gear  86  secured to the roller holding arm  84 , and a motor  87  for rotating the gear  86  bi-directionally. But it is possible to use a cam mechanism or a link mechanism instead of the gear  86  in the arm shifting mechanism  85 . It is also possible to use a solenoid or the like instead of the motor  87 . 
     The paper supply rollers  21  are interconnected with a capstan roller  88   a  of a pair of feed rollers  88   a  and  88   b  through a not-shown timing belt. The capstan roller  88   a  is rotated by a pulse motor  89 . The feed roller pair  88   a  and  88   b  consists of the capstan roller  88   a  and a nip roller  88   b.  The driving power of the pulse motor  89  is also transmitted to a pair of ejection rollers  90  through the timing belt. The paper supply rollers  21 , the feed rollers  88   a  and  88   b  and the ejection rollers  90  are designed to have the same peripheral speed. 
     The feed rollers  88   a  and  88   b  feed the recording sheet  12  in the paper supply direction shown by an arrow A 1  upon forward rotation of the pulse motor  89 , and feed the recording sheet  12  in a printing direction shown by an arrow B 1  upon reverse rotation of the pulse motor  89 . On an upstream side of the feed rollers  88   a  and  88   b  in the paper supply direction A 1 , there are disposed a fixing device  91  and a paper sensor  92  in this order from the upstream side. On a downstream side of the feed rollers  88   a  and  88   b  in the paper supply direction A 1  are disposed a thermal head  93  and a platen roller  94 . The platen roller  94  is moved by a roller shift mechanism  95  between a recording position for pressing the recording sheet  12  onto a heating element array  93   a  of the thermal head  93 , and a retracted position away from the heating element array  93   a.    
     The respective motors  87  and  89  and the roller shift mechanism  95  are controlled by a system controller  96  through drivers  87   a,    89   a  and  95   a.  The heating elements of the thermal head  93  are controlled to heat in accordance with image data through a print section  97  and a head driver that is not shown but incorporated into the thermal head  93 . The system controller  96  is constituted of a well-known microcomputer, and controls the respective elements so as to record a full-color image on the recording sheet  12  in a three-color frame sequential fashion. The recording sheet  12  supplied through a paper supply path  98   a  of a paper guide  98  is fed to the printing stage  74  through a printing path  98   b.  In the three-color frame sequential printing, the feed rollers  88   a  and  88   b  convey the recording sheet  12  back and forth along the printing path  98   b  and a paper ejection path  98   c.  At the conclusion of printing, the recording sheet  12  is ejected by the ejection rollers  90  through the paper ejection path  98   c  out of an outlet  99  onto the top side of the paper cassette  40 . 
     Now the operation of the printer having the above described configurations will be described with reference to FIG.  13 . 
     A printing operation is designated by an operation on the operation panel  78 , the LCD  79  displays an image to print. After checking the displayed image, a print key on the operation panel  78  is operated to start printing. On printing, first the arm shifting mechanism  85  sets the paper supply rollers  21  to the paper feeding position. Thereafter, the pulse motor  89  rotates forward to rotate the paper supply roller  21  in the paper supplying direction. Upon this rotation of the paper supply rollers  21 , the uppermost recording sheet  12  as being in contact with the rollers  21  is fed out from the paper cassette  40  to the printing stage  74 . In two seconds after a leading end of the recording sheet  12  is detected by the paper sensor  92 , the paper supply rollers  21  are retracted from the paper supply position to the retracted position, assuming that the leading end of the recording sheet  12  reaches the feed rollers  88   a  and  88   b  at that timing. Thereafter, the recording sheet  12  is conveyed in the paper supply direction A 1  by the paper feed rollers  88   a  and  88   b.  When a trailing end of the recording sheet  12  moves past the paper sensor  92 , a trailing end passage signal causes the pulse motor  89  to stop the forward rotation, and the recording sheet  12  stops at a print start position. 
     Thereafter, the pulse motor  89  is reversely rotated to convey the recording sheet  12  in the printing direction B 1 . While being conveyed, the recording sheet  12  first has a yellow image recorded thermally in its recording area. During the yellow recording, a yellow fixing lamp  91   a  of the fixing device  91  is turned on to fix a yellow thermosensitive coloring layer. After the yellow image printing is accomplished, the pulse motor  89  rotates forward to move the recording sheet  12  back to the print start position through the feed rollers  88   a  and  88   b.  Thereafter a magenta image is thermally recorded in the recording area in the same way, and a magenta fixing lamp  91   b  is turned on to fix the magenta thermosensitive coloring layer during this thermal recording. After the magenta recording, the recording sheet  12  is returned to the print start position in the same way. Thereafter, a cyan image is thermally recorded in the recording area. During this thermal recording, the magenta fixing lamp  91   b  is turned on to bleach those areas having no image recorded thereon. After the cyan image is thermal recorded, the recording sheet  12  is ejected onto the top side of the paper cassette  40  through the ejection rollers  90 . 
     If the supply rollers  21  feed out not only the uppermost recording sheet  12  but also the second recording sheet  12  for some reasons, the lower recording sheet is caught on the recording sheet separator  62  by friction, and is stopped from moving out, so the upper recording sheet  12  alone is fed out. In that case, the lower recording sheet  12  stops in a half-drawn position. If the recording sheet  12  is left in this position, the leading end drawn out from the recording sheet package  10  does not get the moisture- and light-shielding effect that is provided by the tight contact between the neatly piled recording sheets  12 . So the leading end is affected by moisture and light to change its coloring properties. This results in unexpected density variations or color variations in the leading end. 
     To avoid this problem, according to the present invention, a returning mode is executed, where the supply rollers  21  are rotated reversely to the paper supply direction for a constant time, e.g. for two seconds. In the embodiment shown in FIG. 13, the returning mode is executed after the conclusion of printing on one recording sheet  12 . In this way, even if the following recording sheets  12  are partly drawn out along with the uppermost one, these sheets are fed back into the casing  11  without fail. Therefore, the entire surfaces of the recording sheets  12  are brought into tight contact with each other, and thus prevented from having their coloring properties affected by moisture and light. When the same image is to be printed on a plurality of recording sheets, the same processes, i.e., a paper supply mode and the three-color frame sequential recording and then the returning mode, are sequentially performed after the preceding recording sheet  12  is ejected. 
     In the above embodiment, the paper supply rollers  21  are rotated reversely for the constant time to feed the recording sheets  12  back into the package  10  even when the multi-feeding of the recording sheets  12  does not occur. However, as shown in FIG. 14, it is alternatively possible to provide a paper sensor  100  beside the recording sheet separator  62 , for detecting leading ends of the multi-fed recording sheets  12 , so as to rotate the paper supply rollers  21  reversely to the paper supply direction only when the paper sensor  100  detects the leading ends of the recording sheets  12 , thereby to feed the recording sheets  12  back into the package  10 . The paper sensor  100  may be of an interruption-type that detects the recording sheet  12  as the recording sheet  12  interrupts an optical path, or a reflection-type that detects the recording sheet  12  based on a change in light amount. It is to be noted that those elements which are equivalent to the above embodiment are designated by the same reference numbers. According to this configuration, the recording sheet protruded from a paper supply start position in the paper cassette is returned to the paper supply start position, reducing unexpected density- and color-variations. This configuration is more efficient, because the paper supply rollers are rotated reversely to the paper supply direction only when the recording sheet is partly drawn out. 
     FIG. 15 shows a flow chart of another embodiment, wherein the paper supply roller  21  are rotated reversely in a returning mode when a power source is turned on, and at each reloading of the paper cassette  40  after the start of power supply, for feeding the protruded recording sheets  12  from the paper supply start position back into the recording sheet package  10 . In this embodiment, when the thermosensitive printer is powered, its respective elements are initialized to be set in their initial position and, thereafter, the paper supply rollers are moved down and rotated reversely for two seconds. Thereby, those recording sheets, which are protruded from the recording sheet package for some reasons, are returned to the paper supply start position. Since the recording sheet does not start being supplied in the protruded position, multi-feeding and paper-jamming are prevented. 
     In order to start the returning mode at the reloading of the paper cassette  40 , a cassette sensor  101  is provided in the inlet  77  for the paper cassette  40 , so the cassette sensor  101  detects if the paper cassette  40  is loaded or not. The cassette sensor  101  may be a limit switch that is turned on upon being touched by the loaded paper cassette  40 , or may be an optical sensor or another kind of sensor. When the paper cassette  40  is removed after the printer is powered, for replacement of the recording sheet package  10  or for other reasons, the cassette sensor  101  detects the paper cassette  40  being reloaded. When the paper cassette  40  is reloaded, the paper supply rollers  21  are reversely rotated for a period in the paper feeding position to execute the returning mode, like when the printer starts being powered. There after when a print start signal is entered, the recording sheet  12  is supplied, and the three-color frame sequential printing is carried out, in the same way as in the above embodiment. 
     FIG. 17 shows a flow chart of another embodiment wherein the returning mode for returning the recording sheet is executed immediately before starting printing. This embodiment also prevents multi-feeding of the recording sheet by returning the recording sheet protruded from the paper cassette or the casing for some reasons to the original position immediately before starting printing. 
     Instead of executing the returning mode once, i.e., immediately after the completion of printing, or at the start of powering, or at the reloading of the paper cassette, or immediately before the start of printing, it is preferable to execute the returning mode several times on some of these occasions. It is also possible to execute the returning mode on each of these occasions. 
     Although the recording sheet package  10  is used for loading the recording sheets  12  in the paper cassette  40  in the above embodiment, the present invention is applicable to those cases where the recording sheets  12  are directly placed in the loading section  44  without the use of the casing  11 . 
     Although the paper supply rollers  21  are rotated reversely for a predetermined time, e.g. 2 seconds, to feed back the recording sheet  12  into the casing  11 , the time duration of this reverse rotation of the paper supply rollers  21  may be modified appropriately. To make sure to return the recording sheet  12 , the paper supply rollers  21  may be rotated reversely at several times. It is also possible to use the paper sensor  100 , as shown in FIG. 14, for checking if the recording sheet  12  is fed back into the casing  11  or not, so as to continue rotating the paper supply rollers  21  reversely till the recording sheet  12  is fed back into the casing  11 . 
     FIGS. 18 and 19 show the appearance of a paper cassette  110  with paper supply rollers  120 , according to another embodiment of the present invention. The paper cassette  110  consists of a cassette body  111  and a recording sheet separator  112  that functions also as a lid. The cassette body  11  has a thin parallelepiped shape with one end open for providing a paper supply opening  113 . 
     As shown in FIG. 20, a recording sheet package  114  is loaded in the cassette body  111 . A casing  115  of the recording sheet package  114  is formed from a thick dust-proof paper made of long fibers, and has a thin parallelepiped shape with one end open for providing a paper feed out opening  115   a.  The casing contains twenty recording sheets  12  neatly piled in a stack with their base side up and their thermosensitive coloring surfaces down. As for the thicker recording sheets that are used as stickers, ten sheets are contained in this instance. A protection sheet  12   a  is placed under the uppermost one of the recording sheets  12 . The protection sheet  12   a  is laid in tight contact with the thermosensitive recording surface of the recording sheet  12 , so as to protect the thermosensitive recording surface from light and moisture. The protection sheet  12   a  may be omitted. 
     As shown in FIG. 21, a pressing plate  115   c  is connected to a top plate  115   b  of the casing  115 . The pressing plate  115  is placed under the top plate  115   b  by being folded along a joint to the top plate  115   b.  Because of a resiliency of a bent portion  115   d,  the pressing plate  115   c  is urged downward, so the recording sheets  12  are clamped between a bottom plate  115   e  and the pressing plate  115   c,  and thus kept in tight contact with each other. 
     The recording sheet separator  112  has a parallelepiped shape with upper corners rounded. A cork member  123  is cemented on the recording sheet separator  112  from an upper portion of its front face through its top side to an upper portion of its rear face. The cork member  123  cooperates with a recording sheet pressing blade  124  as set forth later, such that a lower one of two recording sheets  12  which are fed out together is stopped by friction against the cork member  123 , so an upper one of the recording sheets  12  along is fed out. Although the cork member  123  covers substantially the whole surface of the upper portion of the recording sheet separator  112 , the cork member  123  may be provided only in a central portion that comes into contact with the recording sheet  12 . 
     The recording sheet separator  112  further has a pair of separating projections  125  for prevention against double-feeding. The separating projections  125  are formed to protrude vertically from the rounded corners  112   a,  so they stop a leading end of the lower one of the two recording sheets  12  which are being fed together, ensuring that the lower recording sheet is stopped from being fed out. 
     The recording sheet separator  112  is mounted pivotally about a mounting shaft  130  in the paper supply opening  113 . The recording sheet separator  112  is movable between an upright paper separating position shown in FIG. 21, and an open position shown in FIG. 22 where the recording sheet separator  112  is turned in a clockwise direction by an angle of 90 degrees from the paper separating position. In the paper separating position, the uppermost one of the piled recording sheets  12  is allowed to pass over the recording sheet separator  112 , whereas leading ends of the second and following recording sheets  12  come into contact with the recording sheet separator  112 , so these sheets cannot advance any further. In the open position, the recording sheet separator  112  is retracted from the paper supply opening  113 , allowing the recording sheet package  114  to be loaded through the paper supply opening  113 . 
     To permit switching the recording sheet separator  112  between the paper separating position and the open position, an operation lever  131  is secured to an end  130   a  of the mounting shaft  130 , as shown in FIG.  18 . For the sake of holding the recording sheet separator  112  at the paper separating position or the open position, a not-shown click stop mechanism is provided, so the recording sheet separator  112  would not easily be displaced from the paper separating position or the open position. 
     In the paper separating position, as shown in FIG. 21, a paper passageway  132  is provided by a gap between the recording sheet separator  112  and a top wall  111   b  of the cassette body  111 . In the paper passageway  132 , the recording sheet pressing blade  124  is disposed for urging the recording sheet  12  toward the recording sheet separator  112 . The recording sheet pressing blade  124  is mounted to the cassette body  111  through a mounting shaft  124   a.  The recording sheet pressing blade  124  is made of a leaf spring bent into a wavy shape, so bent portions  124   b  gently press the recording sheet  12  against the recording sheet separator  112 . 
     The bent portions  124   b  are provided before and behind the mounting shaft  124   b  in the paper supply direction. Because the bent portions  124   b  are resiliently pressed onto the recording sheet separator  112 , the paper passageway  132  is tightly closed. This blocks entrance of dusts or the like, and also increases light-tightness and moisture-tightness. Moreover, because the bent portions  124   b  close the passageway at two points, at least one of the bent portions  124   b  closes the passageway even while a leading end of the recording sheet  12  passes through. 
     The paper supply rollers  120  are disposed in the cassette body  111  in proximity to the paper supply opening  113 . The paper supply rollers  120  is rotated in a paper supply direction for drawing out the uppermost one of the recording sheets  12 , through a drive gear  135  (see FIG. 19) that is driven by a thermal printer as attached with the paper cassette  110 . 
     As shown in FIG. 21, a recording sheet urging plate  121  is pivotally mounted to a bottom wall  111   a  of the cassette body  111  through a mounting shaft  136 . Also a pressing spring  138  is mounted to the bottom wall  111   a  in a way to close a bearing portion  137  that holds the mounting shaft  136  such that the mounting shaft  136  may turn therein. A distal end  138   a  of the pressing spring  138  is in contact with the bottom side of the recording sheet urging plate  121 , to urge the recording sheet urging plate  121  upward. The casing  115  has a shorter length in the paper supply direction than the recording sheet  12 , so leading ends of the recording sheets  12  protrude from the casing  115 . Where the recording sheet package  114  is loaded in the cassette body  111 , the protruded leading ends of the recording sheets  12  are pushed from the top as well as from the bottom respectively by the paper supply rollers  120  and by an end of an urging ridge  121   a  of the recording sheet urging plate  121 . In this way, the recording sheets  12  are urged toward the paper supply rollers  120 , so the uppermost one of the recording sheets  12  is pressed onto the paper supply rollers  120 . The urging ridge  121   a  is formed on an distal end of the recording sheet urging plate  121 , to protrude upward. The end edge of the urging ridge  121   a  comes into contact with the bottom side of the leading end of the recording sheet  12 . 
     The mounting shaft  136  of the recording sheet urging plate  121  is urged by a coiled spring  139  to move in an opposite direction to the paper supply direction, (to the left in FIG.  21 ). The bearing portion  137  is elongated in the paper supply direction, so the mounting shaft  136  is movable in the paper supply direction inside the bearing portion  137 . Furthermore, the recording sheet urging plate  121  has a knob  121   b  that protrudes oppositely to the urging ridge  121   a.    
     The recording sheet urging plate  121  may be pulled down while pinching the knob  121   b.  When the end of the urging ridge  121   a  of the recording sheet urging plate  121  is placed under the bottom wall  111   a,  the recording sheet urging plate  121  moves to the left according to the force of the coiled spring  139 , so the end of the urging ridge  121   a  is brought into contact with the bottom wall  111   a.  Thus, the recording sheet urging plate  121  cannot return to an urging position, but is held in an open position where the urging ridge  121   a  is retracted from the paper supply opening  113 . 
     When the recording sheet separator  112  is switched to the open position, as shown in FIG. 20, the paper supply opening  113  is opened up, allowing loading the recording sheet package  114 . After the recording sheet package  114  is loaded in the paper cassette  110 , the operation lever  131  is erected to set the recording sheet separator  112  back to the paper separating position. The recording sheet urging plate  121  is then shifted in the paper supply direction by hand while pinching the knob  121   b.  Then the urging ridge  121   a  of the recording sheet urging plate  121  passes over an end of the bottom wall  111   a,  so the recording sheet urging plate  121  swings toward the paper supply opening  112 . Thus, the urging ridge  121   a  is brought into contact with the bottom of the leading end of the recording sheet, as shown in FIG. 21, urging the recording sheets  12  to the paper supply rollers  120 . 
     For loading the recording sheet package  114  in the paper cassette  110 , it is possible to set the recording sheet separator  112  from the paper separating position to the open position first, and then set the recording sheet urging plate  121  to the retracted position (see FIG.  22 ). 
     As shown in FIG. 18, a top wall  111   b  of the cassette body  111  doubles as a paper ejection tray, so paper guides  140  and  141  and a stopper  142  are protruded from the top wall  111   b.  The paper guides  140  and  141  are for guiding the recording sheet  12  at its lateral sides, and are formed along the length of the top wall  111   b.  The stopper  142  is for stopping the recording sheet  12  at its leading end, and preventing it from slipping off the top wall  111   b.    
     When the paper cassette  110  is inserted into an inlet  152  of a thermosensitive printer  150 , as shown in FIG. 21, a click engagement member  83  is engaged in an engaging recess  110   a  of the paper cassette  110 , preventing the paper cassette  110  from dropping off the thermosensitive printer  150 . On printing, the paper supply rollers  120  rotate in the paper supply direction (counterclockwise direction) to withdraw the uppermost one of the recording sheets  12  from the recording sheet package  112  in the paper cassette  110 , and feed it into the printer  150 , for subjecting it to the well-known three-color frame sequential printing. At the conclusion of printing, the recording sheet  12  is ejected onto the top wall  111   b  of the cassette body  111  through ejection rollers  90  that are provided in the thermosensitive printer. 
     In the above embodiment, the recording sheet urging plate  121  is moved from the urging position to the arrested position by pinching at the knob  121   b.  In an alternative shown in FIGS. 23 to  25 , a recording sheet urging plate  121  is moved in cooperation with a recording sheet separator  112  through an interconnection mechanism  160 . In this case, a gear train  162  is disposed at an opposite end of a mounting shaft  130  from an operation lever  131  of a paper cassette  161 , as shown in FIG. 23, so as to transmit rotational movement of the recording sheet separator  112  to an urging plate displacing lever  163  shown in FIG.  24 . It is to be noted the same elements as used in the above embodiment are designated by the same reference numbers, so as to avoid redundant descriptions of these elements. 
     Upon the recording sheet separator  112  being turned from a paper separating position shown in FIG. 24 to an open position shown in FIG. 25 by rotating an operation lever  131  in a clockwise direction, the rotational movement of the recording sheet separator  112  is transmitted through the gear train  162  to the displacing lever  163 . Then, the displacing lever  163  is turned counterclockwise, displacing the recording sheet urging plate  121  from an urging position shown in FIG. 24 to a retracted position shown in FIG.  25 . Because an urging ridge  121   a  is retracted along with the recording sheet separator  112  from a paper supply opening  113 , loading of the recording sheet package  114  into the paper cassette  160  is made easier. By operating the operation lever  131  to turn the recording sheet separator  112  from the open position to the paper separating position, the turning movement is transmitted to the displacing lever  163  through the gear train  162 , causing the displacing lever  163  to turn in the clockwise direction till the displacing lever  163  is put in an accommodation recess  164 . Since the displacing lever  163  does not push down the recording sheet urging plate  121 , the recording sheet urging plate  121  returns from the retracted position to the urging position according to a force of a pressing spring  138 . 
     Although the recording sheet separator  112  is cooperated with the recording sheet urging plate  121  through the gear train  162  in the above embodiment, they may be cooperated with each other through another interconnection device, such as a link mechanism. Although the operation lever  131  is used for displacing the recording sheet separator  112 , the operation lever  131  may be omitted. In that case, the recording sheet separator  112  is turned directly by hands. 
     In the above embodiment, the paper supply rollers  120  are incorporated into the paper cassette  110 , so the paper cassette does not need any roller opening for letting the paper supply rollers into it, and is therefore improved in light-tightness and moisture-tightness. However, it is possible to provide paper supply rollers  170  in a thermosensitive printer  171 , in a way as shown in FIG. 26, wherein the same elements as used in the above embodiment are designated by the same reference numbers. In this embodiment, a roller opening  174  is formed through a cassette body  173  of a paper cassette  172 . The paper supply rollers  170  are held on roller holding arms  175 . By moving the roller holding arms  175 , the paper supply rollers  170  are moved between a paper supply position (shown by solid lines) where the paper supply rollers  170  are inserted in the cassette body  173  through the roller opening  174 , and a retracted position (shown by phantom lines) retracted from the cassette body  173 . In the paper supply position, the paper supply rollers  170  are rotated in a paper supply direction to feed out the recording sheet. 
     To improve the light-tightness and the moisture-tightness, a sliding lid  176  is provided for opening and closing the roller opening  174 . The sliding lid  176  is mounted to a top wall  173   a  of the cassette body  173  such that the sliding lid  176  may slide in a paper supply direction. The sliding lid  176  is urged by a coiled spring  177  in the paper supply direction, so the sliding lid  176  closes the roller opening  174  when the paper cassette  172  is detached from the thermosensitive printer  171 . The sliding lid  176  has an engaging ridge  179  formed on upside of its leading end. The engaging ridge  179  is brought into contact with a frame  178  of the thermosensitive printer  178 . 
     Accordingly, as the cassette body  173  is inserted into a cassette inlet of the thermosensitive printer  171 , the sliding lid  176  is opened because the engaging ridge  179  is stopped against the frame  178 . When the cassette body  173  is completely inserted into the inlet, a click stopping member  83  holds the paper cassette  172  securely in a loaded position. In this position, the paper supply rollers  170  enter through the roller opening  174 , and come to contact with the uppermost one of the recording sheets  12 , for feeding it out. 
     According to this embodiment, while the paper cassette  172  is not loaded in the thermosensitive printer  171 , the sliding lid  176  is kept in the closed position by the urging force of the coiled spring  177 , so light and moisture would not enter the cassette body  173  through the roller opening  174 . By setting the paper cassette  172  in the thermosensitive printer  171 , the sliding lid  176  is automatically opened, so the paper supply rollers  170  may be placed in a paper supply position through the roller opening  174 . 
     It is preferable to use at least one of the paper feeding methods of the embodiments shown in FIGS. 13 to  17 , for feeding out the recording sheet from the paper cassettes shown in FIGS. 18 to  27 , because it prevents multi-feeding and provides better light- and moisture-tightness for the recording sheets. 
     Although the recording sheets loaded in the paper cassette are fed out one by one from the uppermost one in the above embodiments, the present invention is applicable to those cases where the recording sheets are fed out sequentially from the lowermost one. In that case, a paper passageway is provided on the side of the bottom wall of the paper cassette, and paper supply rollers are pressed onto the lowermost one of the recording sheets and rotated to feed it out. 
     Thus, the present invention is not to be limited to the above embodiments, but various modifications are possible without departing from the scope of claims. 
     INDUSTRIAL APPLICATION FIELD 
     The present invention is not only applicable to those printers and paper cassettes which use the thermosensitive recording sheets, but also to those printers and paper cassettes which use the sublimation type recording sheets or melting type recording sheets. Moreover, the present invention is applicable to printers of other printing types, such as an ink-jet type and a laser-printing type, and to paper cassettes for these printers.