Abstract:
A thermal head avoids a drop in print quality caused by adhesion of printing chaff. A thermal head  39  that presses against thermal paper that moves from one side to the other side and prints by melting dye contained in the thermal paper has a glazed layer  150  that is formed in an area on one part of a ceramic substrate  43  and stores in-flowing heat, and a heating resistor  140  that is located offset to one side from the center of the glazed layer  150,  selectively heats the thermal paper S pressed in contact therewith, and melts a dye material contained in the thermal paper. A smooth surface P against which the thermal paper S heated by the heating resistor  140  slides is formed to the other side of the glazed layer  150  from the heating resistor  140.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of Invention 
         [0002]    The present invention relates to a thermal head and to a printer that prints by selectively heating thermal paper that reacts to heat energy. 
         [0003]    2. Description of Related Art 
         [0004]    Thermal printers are a common type of printer. Thermal printers use a thermal head having small heating elements that selectively produce heat when energized arrayed in a line on a ceramic substrate. Thermal printers use a thermochromic printing technique to print on thermo-sensitive paper (thermal paper) by selectively melting dyes contained in the coating on the thermal paper. The thermal head used in this thermochromic printing method has a glazed layer that functions as a heat storage layer below the small heating elements as taught in Japanese Unexamined Patent Appl. Pub. JP-A-H07-137317. There are two types of glazed layers, a flat glazed type in which the glazed layer covers substantially the entire surface of the ceramic substrate, and a partially glazed type in which the glazed layer is formed only around the heating elements. The partially glazed type is widely used today because its high speed heat dissipation performance avoids the ghosting effect produced by residual heat in the heating elements during high speed printing when the heating elements are caused to respond rapidly. 
         [0005]    The thermal paper has an undercoating applied to the base paper fiber matrix, and a thermochromic coating is then applied over the undercoating. In addition to improving the smoothness of the thermal paper, the undercoating also functions to cool the melted dye and fix the color. 
         [0006]    However, thermal paper that does not have this undercoating is also widely used as a low cost thermal paper. With this type of paper the melted dye may separate before it can cool and be fixed, and printing chaff such as the melted dye then clings to and accumulates on the shoulder of the glazed layer adjacent to the heating elements. As this chaff accumulates, optical density, which is a measure of print density, drops as shown in  FIG. 8 , printing therefore becomes lighter, and print quality drops. Such printers are also preferably compact and as inexpensive as possible. 
       SUMMARY OF THE INVENTION 
       [0007]    A printer according to a preferred aspect of the invention has a thermal head that prints on thermal paper, and a roller that is disposed opposite the thermal head with the thermal paper therebetween and conveys the thermal paper. The thermal head includes a substrate; a heat storage layer formed on a part of the substrate for storing input heat; and a heating element formed on the heat storage layer for melting a dye agent contained in the thermal paper; and the heating element is disposed on the upstream side in the thermal paper transportation direction from the center of the heat storage layer. 
         [0008]    Because the heating element is disposed on the upstream side in the thermal paper transportation direction from the center of the heat storage layer, the thermal paper heated by the heating element cools gradually while in contact with the heat storage layer. The printing chaff that results while the dye agent contained in the thermal paper cools from the melted state is thus produced gradually, the chaff does not adhere and accumulate in one location, and a drop in print quality caused by printing chaff can therefore be avoided. Furthermore, because the size of the heat storage layer itself does not change, the conventional size of the printer can be maintained and the cost can be kept down. 
         [0009]    Preferably, a surface is formed parallel to the substrate contacting the thermal paper on the heat storage layer downstream in the thermal paper transportation direction from the heating element. 
         [0010]    This aspect of the invention enables the thermal paper to separate more smoothly so that the printing chaff does not adhere and accumulate in one location, and a drop in print quality caused by the printing chaff can be avoided. 
         [0011]    Further preferably, the width of the heat storage layer is 1.5 mm or greater. 
         [0012]    By rendering the heat storage layer in this range, a suitable heat storage effect can be achieved, adhesion of printing chaff can be reduced, and printing is possible without a drop in quality. 
         [0013]    Yet further preferably, the thickness of the heat storage layer is uniform. This enables a printer that can maintain print quality while printing at a high speed because the color coating layer of the thermal paper that is heated by the heating element cools gradually while held reliably in contact with the heat storage layer. 
         [0014]    Yet further preferably, the heating element is aligned in a direction intersecting the transportation direction of the thermal paper. This enables a printer that can maintain print quality while printing at a high speed. 
         [0015]    Yet further preferably, the roller is disposed so that the position where a perpendicular to the substrate of the thermal head passing through the axial center of the roller intersects the thermal head is located on the downstream side in the thermal paper transportation direction from the position where the heating element is formed. This improves the printing chaff removal effect, and avoids a drop in print quality caused by printing chaff. 
         [0016]    Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is an oblique view showing the appearance of a printer according to a preferred embodiment of the invention. 
           [0018]      FIG. 2  is an oblique view of the print mechanism unit when the cover frame is open. 
           [0019]      FIG. 3  is an oblique view of the print mechanism unit when the cover frame is closed. 
           [0020]      FIG. 4  is a side section view of the print mechanism unit. 
           [0021]      FIG. 5  is a view of the thermal head from the side of the print mechanism. 
           [0022]      FIG. 6A  is a plan view from the platen side of the heating unit of the thermal head,  FIG. 6B  is a section view of the thermal head, and  FIG. 6C  is another section view of the thermal head. 
           [0023]      FIG. 7A  is a graph describing the relationship between the thickness of the glazed layer and the tendency for ghosting of the printed image,  FIG. 7B  shows the relationship between the volume of the glazed layer and the tendency for adhesion of printing chaff, and  FIG. 7C  shows the tendency for printing chaff adhesion relative to the width of the glazed layer. 
           [0024]      FIG. 8  shows the relationship between print density and printing chaff adhesion. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0025]      FIG. 1  is an oblique view showing the appearance of a printer  1  using a thermal head according to a preferred embodiment of the invention. The printer  1  is a thermal printer that is used as a receipt printer in a POS system, for example. The printer  1  uses thermal paper S that is wound in a roll (see  FIG. 4 ), and has a print mechanism unit ( FIG. 2 ) for printing information on the thermal paper S, a paper cutting unit for cutting the printed thermal paper S, and a roll paper compartment for storing the thermal paper S. 
         [0026]    The print mechanism unit  8  ( FIG. 2 ) is attached to a bottom case  4  made of plastic, the side and back portions are covered by a top case  3 , and the front portion is covered by a panel  2 . A paper cutter unit is disposed at the top of the panel  2 . The paper cutter unit is covered by a cutter cover  6 , and the cutter cover  6  can be slid out in the direction of arrow A. 
         [0027]    An open button  7  that drives an internal cover opening lever  9  to rotate an internal cover frame  10  ( FIG. 2 ) in order to remove the thermal paper S is disposed at one side of the top case  3 . This cover frame  10  ( FIG. 2 ) is connected to a top cover  5 . When the open button  7  is pressed in the direction of arrow B, the cover opening lever  9  rotates clockwise and a lock mechanism disengages so that the top cover  5  can open in the direction of arrow C and the roll paper compartment  17  ( FIG. 2 ) is exposed. 
         [0028]      FIG. 2  and  FIG. 3  are oblique views of the print mechanism unit  8 ,  FIG. 2  being an oblique view of the print mechanism unit  8  when the cover frame  10  is open, and  FIG. 3  being an oblique view of the print mechanism unit  8  when the cover frame  10  is closed. 
         [0029]    The print mechanism unit  8  has a cover frame  10  that opens and closes freely to the top of a main frame  13  that is typically metal, and an automatic paper cutter unit  11  that houses a movable knife  32  and a drive means for the movable knife. When the thermal paper S is not cut, the movable knife  32  is stored inside the automatic paper cutter unit  11  and the movable knife  32  is not exposed. When thus positioned, the movable knife  32  is said to be in the standby position. 
         [0030]    A fixed knife  33  that crosses the movable knife  32  with a scissor action is disposed to the cover frame  10  opposite the automatic paper cutter unit  11 . A blade shutter  34  is disposed above the fixed knife  33 . The blade shutter  34  is urged by a shutter spring  35  in the direction covering the cutting edge of the fixed knife  33 , but when the cover frame  10  is closed as shown in  FIG. 3 , part of the blade shutter  34  contacts an engaging part disposed to the main frame  13  so that the blade shutter  34  is lifted slightly open. The cutting edge of the fixed knife  33  is thus exposed so that the movable knife  32  can move across the fixed knife  33  with a scissor action to cut the paper. 
         [0031]    The cover frame  10  is attached to pivot, that is, open and close freely, on support pins  14  provided at the top part on both sides of the main frame  13 . A cover part  15  disposed to the cover frame  10  is curved so that the cover part  15  does not contact the thermal paper S when the cover frame  10  is closed. When the orientation of the printer installation is changed, this cover part  15  also functions as a holding member that receives the thermal paper S. 
         [0032]    A cover detector  44  that detects if the cover frame  10  is closed is disposed on the right side of the main frame  13 . This cover detector  44  is a transmission type photodetector, and detects whether or not the cover frame  10  is closed correctly based on whether the light beam from the detector is interrupted by a part of the cover frame  10 . 
         [0033]    A near-end detector  24  and a paper detector  30  described below are also provided in addition to this cover detector  44 . Leads  12  from these detectors, the automatic paper cutter unit  11 , and a paper transportation motor  23  described below are connected to a relay board attached to the right side of the main frame  13 . The relay board and a main circuit board (not shown in the figure) that controls the printer  1  are connected by a flat flexible cable, for example. 
         [0034]    A plastic roll paper compartment  17  is disposed inside the open cover frame  10 . The paper detector  30  for detecting if paper is present is disposed to the roll paper compartment  17 . The paper detector  30  is a reflection type photodetector, and a group of holes  31  is disposed on the upstream side of the paper detector  30 . The holes  31  allow foreign matter and chaff clinging to the thermal paper S to drop out so that the paper dust or other foreign matter does not interfere with detector operation. Slots  27  for engaging the right and left side panels of the main frame  13  are also rendered in the roll paper compartment  17 . When these slots  27  engage the right and left side panels of the main frame  13 , the widthwise position of the roll paper compartment  17  is fixed and the inside of the roll paper compartment is held at a width suitable to the thermal paper S. 
         [0035]    A platen  18 , which is a cylindrical rubber roller, is supported rotatably on the cover frame  10  by a platen shaft  20 . A platen gear  19  is press fit to one end of the platen  18 . A groove part  21  is rendered to the main frame  13  so that when the cover frame  10  closes, the platen shaft  20  is guided by a guide incline  45  disposed to the end part of the heat radiation plate  47  ( FIG. 4 ) and then contacts the groove part  21  so that the platen  18  is positioned in a prescribed position. Pressure from the thermal head  39  ( FIG. 4 ) on the platen  18  works to push down on the cover frame  10  and determine the position of the platen  18 . The platen gear  19  and paper transportation transfer gear  22  also mesh and power is transmitted from the paper transportation motor  23  to the platen  18 , causing the platen  18  to rotate in a predetermined direction. 
         [0036]    The near-end detector  24  for detecting if the thermal paper S is near the end of the roll is disposed freely rotatably on a support pin  25  on the left side of the main frame  13 . This arrangement enables the near-end detector  24  to be optimally positioned according to the orientation of the printer. For example, when the printer is used with the bottom  28  of the cover frame  10  down as shown in  FIG. 2 , the actuator  26  of the near-end detector  24  is fixed inside a hole  32   a  rendered in the cover frame  10 . When the printer is used with the back  29  of the cover frame  10  down, however, the actuator  26  is fixed in position in a separate hole  32   b.  A support channel unit  50  that supports the thermal head  39  ( FIG. 4 ) and the head pressure plate  41  ( FIG. 4 ) is rendered at the left and right sides of the main frame  13 . 
         [0037]      FIG. 4  is a side section view of the print mechanism unit  8 , and shows the thermal paper S roll paper compartment  17  held in the roll paper compartment  17  with the leading end delivered in the discharge direction (D).  FIG. 4  shows the thermal paper S when the diameter is large. As the paper is advanced and the diameter of the thermal paper S becomes small enough, the thermal paper S drops into the recess  38  and the near-end detector  24  thus detects that the roll diameter of the thermal paper S has become a certain small size. 
         [0038]      FIG. 5  is a view showing the thermal head  39  from the side of the print mechanism unit  8 . As shown in this figure, the thermal head  39  is based on a heat radiation plate  47 . A head support pin  40  is disposed on both sides of the thermal head  39 , and the head support pins  40  are supported on a part of the support channel unit  50  disposed to the main frame  13 . A ceramic substrate  43  made from an alumina ceramic is disposed on one side of the heat radiation plate  47 . This ceramic substrate  43  is located at a position opposite the platen  18 , and is urged by a spring  42  disposed on the other side of the heat radiation plate  47  toward the platen  18 . The other end of the spring  42  is affixed to the head pressure plate  41 , and the head pressure plate  41  is supported by the support channel unit  50 b disposed to the main frame  13 . 
         [0039]    With this configuration the thermal paper S is held between the platen  18  and the ceramic substrate  43  with the platen  18  pressing the thermal paper S from the surface S 2  on one side of the thermal paper S to the thermal head  39 , and the ceramic substrate  43  opposite the platen  18  pressed against the other surface S 1  of the thermal paper S. 
         [0040]      FIG. 6A  is a plan view from the platen  18  side of the of the thermal head  39 , and  FIG. 6B  is a section view of the thermal head  39  through line A-A in  FIG. 6A . In  FIG. 6  the discharge direction (D) of the thermal paper S is from right to left. 
         [0041]    As shown in  FIG. 6A  and  FIG. 6B , a glazed layer  150  of a substantially constant thickness (h) is formed from glass, for example, on the side of the ceramic substrate  43  facing the platen  18  in one area near the end in the direction that the thermal paper S is conveyed (to the left in  FIG. 6 ). A smooth surface P with a predetermined width (W) is formed on the glazed layer  150  substantially parallel to the ceramic substrate  43 . A linear heating resistor  140  that converts an applied current to heat is disposed perpendicularly to the discharge direction (D) of the thermal paper on the smooth surface P on the side from which the thermal paper S is conveyed (the right side in  FIG. 6 ). 
         [0042]    This heating resistor  140  is approximately 200 μm wide and approximately 6 μm high. The heating resistor  140  has hundreds of fine heating elements arrayed in a line, and by selectively energizing the heating elements, only the energized heating elements instantaneously emit heat. The heating resistor  140  is located on the upstream side of the approximate point of tangency between the platen  18  and the thermal head  39 . In other words, the center axis of the platen  18  is located downstream from the heating resistor  140  of the thermal head  39 . 
         [0043]    The glazed layer  150  functions as a heat storage layer for storing heat coming from the energized heating resistor  140 , and also functions to quickly dissipate heat to the heat radiation plate  47  when energizing the heating resistor  140  stops. The glazed layer  150  also functions to smoothen the surface roughness of the ceramic substrate  43 , and facilitate forming a fine pattern coated onto the glazed layer  150 . Segment electrodes and a common electrode not shown are thus formed on the glazed layer  150  near the heating resistor  140 . A protective film  160  made of lead glass, for example, for protecting the parts disposed on the ceramic substrate  43  is also coated over substantially the entire surface over the topmost layer of the glazed layer  150 . 
         [0044]    An epoxy molding  130  containing a sealed driver chip for selectively energizing the heating resistor  140  is disposed on the ceramic substrate  43  near the end on the side from which the thermal paper S is conveyed (the right side in  FIG. 6 ), and a glass epoxy circuit board  135  wired to the epoxy molding  130  is suspended therefrom. A connector  46  ( FIG. 5 ) connected by a flat flexible cable, for example, to the main circuit board (not shown in the figure) that controls the printer  1  is disposed to other end part of the glass epoxy circuit board  135 . 
         [0045]    As the platen  18  in this configuration turns, the thermal paper S is conveyed in the discharge direction (D) with pressure applied thereto from the one surface S 2  so that the other surface S 1  sequentially contacts the heating resistor  140  on the ceramic substrate  43 . As the paper advances, the heating resistor  140  emits heat from the heating elements in response to signals sent from the main circuit board (not shown in the figure) through the connector  46 . The thermal paper S is thus selectively heated across the width. A color coating with a plurality of dyes held separated by binder is formed on the other surface S 1  of the thermal paper S, and the color coating that contacts the energized heating elements changes to a melted state. As the thermal paper S moves, the melted color coating separates from the heating resistor  140  that it touched, pressure is released, and the other surface S 1  of the thermal paper S slides over the smooth surface P of the glazed layer  150 . Because this other surface Si is hotter than the smooth surface P, the heat energy of the thermal paper S moves through the smooth surface P of the glazed layer  150  into the glazed layer  150 . 
         [0046]    However, in addition to the glazed layer  150  already storing heat from the heating resistor  140 , the heating resistor  140  is located offset to the right side of the glazed layer  150 . Because the smooth surface P is sufficiently long, the melted color coating is not suddenly cooled and instead cools and solidifies gradually. As a result, printing chaff does not occur on one concentrated area of the glazed layer  150 , and instead is desirably dispersed. In addition, because the thickness (h) of the glazed layer  150  is substantially constant and the surface is not rough, and the surface roughness of the smooth surface P is smooth, the conveyed thermal paper S can separate smoothly and gradually from the smooth surface P without the printing chaff and dust produced on the other surface S 1  adhering to the smooth surface P. As the heated portion of the thermal paper S sequentially separates from the glazed layer  150 , the color coating cools further and solidifies. The color of the thermal paper S is therefore fixed and the color coating changes to a stable state. Information corresponding to the print signals is thus sequentially printed on the thermal paper S. 
         [0047]    After passing the glazed layer  150 , the thermal paper S is conveyed in the discharge direction (D) and the other surface S 1  of the thermal paper S contacts the guide incline  45  so that the paper is conveyed upward without curling and is guided into the paper cutter by the guide portions  48  ( FIG. 4 ) disposed to the cover frame  10 . The thermal paper S guided to the paper cutter unit passes between the movable knife  32  and fixed knife  33 , and is discharged from the printer  1 . 
         [0048]    The perpendicular Pc to the center axis of the platen  18  is preferably on the discharge side of the center Ph of the heating elements in the thermal head. The platen is elastic and deforms when it turns. This increases the load on the heating elements, and improves contact between the thermal paper S and the heating elements. Thermal conduction from the heating elements to the thermal paper therefore improves and sharp printing is possible. After printing, contact is held on the discharge side of the heating element center Ph, and any printing chaff and dust that sticks to the print head surface can be wiped off to the discharge side. The heat stored in the glazed layer also works to prevent the printing chaff from fixing on the head. Even if some printing chaff does adhere, it adheres at a place separated from the heating elements, and the effect on print quality is therefore reduced. 
         [0049]    Tests showed that the distance between the platen center Pc and the heating element center Ph is optimally 0.2 mm to 0.5 mm. If greater than 0.5 mm, contact between the heating elements and the platen becomes weaker and print tends to be lighter. 
         [0050]    The dimensions of the glazed layer  150  are described next with reference to  FIG. 7A  to  FIG. 7C .  FIG. 7A  shows the relationship between the thickness (h) of the glazed layer  150  and the tendency for ghosting of the printed image. High speed printing here indicates a paper speed near the thermal head  39  of approximately 170-200 mm/s. Low speed printing indicates a paper speed near the thermal head  39  of less than approximately 150 mm/s. 
         [0051]    As shown in  FIG. 7A , when the thickness (h) of the glazed layer is thin, the heat storage capacity of the glazed layer  150  drops and thermal response improves, and ghosting is therefore reduced and print quality improved, whether during high speed printing or low speed printing. The thickness (h) of the glazed layer  150  is therefore as thin as possible, but when the thickness (h) of the glazed layer  150  was less than 20 μm in this embodiment of the invention, the heat storage effect was minimal and the print density was light.  FIG. 7B  shows the relationship between the volume of the glazed layer  150  and the tendency for adhesion of printing chaff. As shown in  FIG. 7B , printing chaff adhesion declines as the glazed layer  150  volume increases. However, if the thickness (h) of the glazed layer  150  exceeds 50 μm, heat storage increases, ghosting increases, and print quality decreases. As a result, the thickness (h) of the glazed layer  150  in this embodiment of the invention is preferably 20-50 μm. 
         [0052]      FIG. 7C  shows the tendency for printing chaff adhesion relative to the width (W) of the glazed layer  150  when the thickness (h) of the glazed layer  150  was 20-50 μm. As shown in  FIG. 7C , when the width (W) of the glazed layer  150  exceeds 1.5 mm, there is a rapid drop in the adhesion of printing chaff. The width (W) of the glazed layer  150  in this embodiment of the invention is therefore preferably 1.5 mm or greater. By using a glazed layer  150  of these dimensions, the thermal head  39  significantly reduces ghosting and printing chaff adhesion compared with the related art and print quality is not degraded. 
         [0053]    A preferred embodiment of the invention is described above with reference to the accompanying figures, but the actual configuration of the invention is not limited to the foregoing embodiment and can be varied in many ways without departing from the scope of the accompanying claims. For example, as shown in  FIG. 6C , the heating resistor  140  could be disposed in the middle of the glazed layer  150 . 
         [0054]    The invention being thus described, it will be obvious that it may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.