Abstract:
The present invention is directed to a thermal transfer recording apparatus, and an object is to prevent causes of print failure, such as wrinkling and transport failure of an ink sheet, resulting from the partial shrinking of the ink sheet due to the heating of the thermal head during printing. The thermal transfer recording apparatus of the present invention comprises a thermal head and a platen roller opposed to the thermal head, and prints on a sheet by causing a wax, a sublimation dye, or the like applied on the ink sheet to melt or sublime by the heat of the thermal head. The thermal transfer recording apparatus is constructed so that the take-up tensile force applied to the ink sheet is made greater at both edges of the ink sheet than at the center thereof.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a thermal transfer recording apparatus equipped with an ink sheet, for transferring wax or the like of the ink sheet onto paper. 
     2. Description of the Related Art 
     Printing apparatuses of the type that transfers wax or the like from an ink sheet onto paper by utilizing the heat of a thermal head are ubiquitously used as word processor printers, facsimile printing apparatuses, and the like. Of such printers, specifically, the type of printer that prints one line at a time widthwise on an A4, B4, or like-size sheet requires the use of a wide ink sheet which is prone to partially shrink due to heating during printing, thus tending to cause ink sheet transport failure and print failure. Various methods of correction using a sheet correcting plate, etc. have been tried to address the problem, but none of them have been successful in providing a perfect preventive measure. 
     Japanese Unexamined Patent Publication JP-A 4-148971 (1992) discloses a method in which ribbon guide rollers so arranged as to be able to align an ink ribbon (ink sheet) at the center are provided in the transport path of the ink ribbon to prevent wrinkling from occurring during recording and thereby to obtain high quality recording results. Further, Japanese Unexamined Patent Publication JP-A 4-25486 (1992) discloses a method in which an electrically conductive elastic member is provided in the transport path of the ink ribbon to ensure stable transport of the ink ribbon, thereby preventing image degradation from occurring due to wrinkling or slacking of the ink ribbon. On the other hand, Japanese Unexamined Patent Publication JP-A 7-266649 (1995) discloses a method in which the portion of a separation member which contacts the center portion of a thermal transfer ribbon is curved downward relative to both edges thereof, thereby absorbing relative stretching at the edges of the thermal transfer ribbon. 
     Printing techniques for transferring wax from an ink sheet onto paper by the heating of a thermal head are widely used. Specifically, when printing one line at a time along the width of a commonly used sheet such as an A4- or B4-size sheet, the width of the ink sheet is made equal to the length of the shorter side of the A4 or B4 size, for example. 
     During printing, the ink sheet partially shrinks due to the heat of the thermal head, as a result of which wrinkles are formed in the ink sheet and transport failure occurs, causing print failure. In particular, in the-case of a wide ink sheet, it is difficult to eliminate such print failure, and a measure that can perfectly prevent it has yet to be devised. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to prevent the causes of print failure, such as wrinkling and transport failure of the ink sheet, resulting from the partial shrinking of the ink sheet due to the heating of the thermal head during printing. 
     In a first aspect, the invention provides a thermal transfer recording apparatus comprising: 
     a thermal head ( 31 ); and 
     a platen roller ( 110 ) opposed to the thermal head ( 31 ), 
     the thermal transfer recording apparatus carrying out printing on a sheet (P) by causing a wax, a sublimation dye, or the like applied on an ink sheet ( 42 ) to melt or sublime by the heat of the thermal head ( 31 ), the ink sheet ( 42 ) being transported between the thermal head ( 31 ) and the platen roller ( 110 ), 
     wherein a tensile force applied to a portion of the ink sheet ( 42 ) which lies downstream of the thermal head ( 31 ) as viewed along a direction of ink sheet travel (d) is greater at both edges of the ink sheet ( 42 ) than at the center thereof. 
     According to the invention, the thermal transfer recording apparatus, comprising the thermal head and the platen roller opposed to the thermal head, prints on a sheet by causing a wax, a sublimation dye, or the like applied on the ink sheet to melt or sublime by the heat of the thermal head. In this thermal transfer recording apparatus, the take-up tensile force applied to the ink sheet is greater at both edges of the ink sheet than at the center thereof. With this arrangement, since vector components directed from the center toward both edges occur in the ink sheet tensile force, the ink sheet can be prevented from wrinkling due to the shrinking of the ink sheet caused by printing heat. Accordingly, the thermal transfer recording apparatus can prevent print failure and ink sheet transport failure. 
     In a second aspect of the invention, it is preferable that the thermal transfer recording apparatus further comprises a friction member for tensioning the ink sheet by applying friction to the ink sheet, wherein 
     the friction member is disposed downstream of the thermal head as viewed along the direction of travel, 
     a contact face of the friction member that is brought into contact with the ink sheet extends along full width of the ink sheet, and 
     a coefficient of friction at a center portion of the contact face of the friction member is higher than a coefficient of friction at both end portions of the contact face. 
     According to the invention, the friction member for frictionally tensioning the ink sheet is disposed downstream of the thermal head as viewed in the direction of ink sheet travel and in such a manner as to extend along the full width of the ink sheet. Further, the coefficient of friction of the contact face of the friction member is higher at the center portion than at both end portions thereof. The thus structured friction member can make the tensile force applied to both widthwise edges of the ink sheet greater than the tensile force applied to the center thereof. Accordingly, the thermal transfer recording apparatus can prevent print failure and ink sheet transport failure. 
     In a third aspect of the invention, it is preferable that a length of contact along which the center portion of the contact face of the friction member contacts the ink sheet is greater than a length of contact along which each of the end portions of the contact face of the friction member contacts the ink sheet. 
     According to the invention, the thermal transfer recording apparatus is constructed so that the length of contact along which the friction member for frictionally tensioning the ink sheet contacts the ink sheet is greater at the center than at both end portions. With this structure, the tensile force applied to both edges of the ink sheet can be made greater than that applied to the center thereof. 
     In a forth aspect of the invention, it is preferable that the length of contact along which each of the end portions of the contact face of the friction member contacts the ink sheet increases with increasing distance from each end of the contact face toward the center thereof. 
     According to the invention, the friction member can gradually reduce the ink sheet tensile force along the width thereof from both edges toward the center. Accordingly, the thermal transfer recording apparatus can prevent print failure and ink sheet transport failure more effectively. 
     In a fifth aspect of the invention, it is preferable that both end portions of the contact face of the friction member are each covered with a substance whose coefficient of friction is lower than the coefficient of friction of the center portion of the contact face. 
     According to the invention, in the thermal transfer recording apparatus, as a method of varying the coefficient of friction of the friction member for frictionally tensioning the ink sheet, a film such as a fluororesin sheet with a low coefficient of friction is glued to each end portion of the friction member, thus coating the end portion with fluororesin or the like. With this structure, the tensile force applied to center of the ink sheet can be made smaller than that applied to both edges thereof. 
     In a sixth aspect of the invention, it is preferable that the center portion of the contact face of the friction member is covered with a substance whose coefficient of friction is higher than the coefficient of friction of both end portions of the contact face. 
     In a seventh aspect of the invention, it is preferable that a film made of a substance whose coefficient of friction is higher than the coefficient of friction of both end portions of the contact face of the friction member is glued to the center portion of the contact face. 
     In an eighth aspect of the invention, it is preferable that the center portion of the contact face of the friction member is treated with surface roughening. 
     According to the sixth to eighth aspects of the invention, the center portion of the friction member for frictionally tensioning the ink sheet is coated with a substance or a film having a high coefficient of friction or is treated with surface roughening by sand blasting. Since this serves to increase the coefficient of friction of the center portion, the tensile force applied to the center portion of the ink sheet can be reduced compared to that applied to both end portions thereof. 
     As described above, according to the first to eighth aspects of the invention, by constructing the thermal transfer recording apparatus so that a greater take-up force is applied to both edges of the ink sheet than to the center thereof, vector components directed from the center toward both edges occur in the ink sheet pulling tensile force; this serves to prevent the ink sheet from wrinkling due to the shrinking of the ink sheet caused by printing heat. Accordingly, the thermal transfer recording apparatus can prevent print failure and ink sheet transport failure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other and further objects, features, and advantages of the invention will be more explicit from the following detailed description taken with reference to the drawings wherein: 
     FIG. 1 is a diagram showing the external view of a thermal transfer facsimile apparatus equipped with a thermal transfer recording apparatus  3  according to the present invention, and the arrangement of important units thereof; 
     FIG. 2 is a diagram showing the thermal transfer facsimile apparatus of FIG. 1 in a condition in which a printer cover and a head unit are opened; 
     FIG. 3 is a diagram showing in detail a thermal head and its adjacent parts in the thermal transfer facsimile apparatus of FIG. 1; 
     FIG. 4 is an enlarged view of the head unit in the thermal transfer facsimile apparatus of FIG. 1; 
     FIG. 5 is a perspective view showing in detail the basic structure of a friction member  41  contained in the thermal transfer facsimile apparatus of FIG. 1; 
     FIG. 6 is a perspective view showing a friction member of a structure in which the area of friction at both end portions is reduced; 
     FIG. 7 is a perspective view showing a friction member of a structure in which a fluorine-based film is glued to each end portion; 
     FIG. 8 is a perspective view showing a friction member of a structure in which the coefficient of friction at a center portion is increased; and 
     FIG. 9 is a block diagram showing the electrical configuration of the thermal transfer facsimile apparatus according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now referring to the drawings, preferred embodiments of the invention are described below. 
     FIG. 1 is an external view of a thermal transfer facsimile apparatus equipped with a thermal transfer recording apparatus  3  according to the present invention. FIG. 1 also shows the arrangement of important units within the thermal transfer facsimile apparatus. In this specification, those units which are located within the thermal transfer facsimile apparatus but are irrelevant to the present invention are omitted to avoid complexity. 
     The thermal transfer facsimile apparatus includes, in addition to the thermal transfer recording apparatus  3 , an operation panel  12 , a recording paper cassette  14 , a communications device  4 , and a document reading device  19 . The thermal transfer recording apparatus  3 , the operation panel  12 , the communications device  4 , and the document reading device  19  are arranged inside the thermal transfer facsimile apparatus. The recording paper cassette  14  is mounted detachably to the facsimile apparatus body. The thermal transfer recording apparatus  3  includes a take-up roller  15 , a head unit  16 , an ink sheet roll  17 , a pick-up roller  18 , and a platen roller  110 . The ink sheet roll  17  has an unused portion of an ink sheet  42  wound around it. Used portion of the ink sheet  42  is wound around the take-up roller  15 . The head unit  16  comprises a thermal head (hereinafter simply referred to as “head”)  31  and a head frame  32 . 
     The exterior of the facsimile apparatus body comprises a lower cabinet  13 , the operation panel  12 , and a printer cover of  11 . The recording paper cassette  14 , which is capable of holding a plurality of cut sheets of recording paper, is inserted in the apparatus body from the front thereof. The operation panel  12  has a group of dialing buttons, a group of facsimile operation buttons, a display device, etc. and provides a man-machine interface between the facsimile apparatus and the user. 
     The ink sheet  42  is fed out of the ink sheet roll  17 , is passed between the platen roller  110  and the head unit  16 , and is wound around the take-up roller  15 . In FIG. 1, the feed direction of the ink sheet  42  is indicated by arrow d. The recording paper P is fed from the recording paper cassette  14  one sheet at a time by means of the pick-up roller  18 ; the sheet is then passed between the platen roller  110  and the head unit  16 , and exits from a paper exit port  111 . The head  31  in the head unit  16  prints an image on the recording paper using the ink sheet  42  based on image data. The image data here refers to data transmitted from another facsimile apparatus or communications device and received by the communications device  4  or data created by reading a document by the document reading device  19 . 
     Though not shown, documents to be read are stacked on the printer cover, loaded into the apparatus one sheet at a time through the back of the operation panel, read by the document reading device  19 , and stacked on a top face of the recording paper cassette  14  inserted in the front. 
     FIG. 2 is a cross sectional view showing the thermal transfer facsimile apparatus in a condition in which the printer cover  11  is opened to remove jammed recording paper or to replace the ink sheet. As shown in FIG. 2, a printer cover  21  is opened upwardly in the rearward direction, and a head unit  22  is also moved upwardly in the rearward direction by being supported on a head unit supporting arm  23  shown in FIG. 2 but not shown in FIG.  1 . In this condition, the ink sheet  42  can be inserted or removed together with the ink sheet roll  17  and the take-up roller  15 . This facilitates the removal of jammed paper and the replacement of the ink sheet. 
     FIG. 3 is a diagram showing in detail the thermal head  31  and its adjacent parts, providing a detailed illustration of how the ink sheet and the recording paper are transported within the facsimile apparatus. FIG. 4 is an enlarged view of the head unit  16 . The following description is given with reference to FIG. 3 in conjunction with FIG. 4. A plurality of recording paper sheets are loaded into the recording paper cassette  14  and are urged upward by means of a rotation plate  35 . Both sides of the leading edge of the recording paper are held by pawls not shown. When the pick-up roller  18  rotates, the top sheet of recording paper is pushed forward and fed with the leading edge riding over the pawls. This mechanism ensures that the recording paper is always fed one sheet at a time. The recording paper thus fed is passed through a pair of first paper-feed rollers  36  by being guided on a lower paper guide  34  and a first upper paper guide  3 S, and fed to the position between the platen roller  110  and the head  31 . 
     The head  31  and head frame  32 , forming part of the head unit  16 , also act to guide the transportation of the recording paper and the ink sheet. The recording paper printed at the position between the head  31  and the platen roller  110  is guided by the lower paper guide  34  and a second paper guide  37  and is transported by a pair of second paper-feed rollers  38  into the paper exit port  111  for discharge outside. 
     The ink sheet  42 , fed out of the ink sheet roll  17 , is transported by being guided by a front head frame  32   a , the head  31 , and a rear head frame  32   b , and is wound around the take-up roller  15 . When the recording paper is fed to the position between the head  31  and the platen roller  110 , positional relationship at the portion of the head  31  is such that the head  31 , the ink sheet  42 , the recording paper, and platen roller  110  are stacked from top to bottom in this order. The head  31  has a heater with a heater dot density of eight dots per millimeter, extending along the full width of the recording paper in the depth direction in the plane of FIG. 3, and is pressed toward the platen roller  110  by a spring (not shown) exerting a constant pressure. 
     By applying a current to the appropriate dots of the heater in accordance with the image data to be printed, only the dots necessary to melt the wax on the ink sheet  42  are heated and the wax is transferred onto the recording paper, thus printing one line at a time. In this way, when the recording paper is fed past the head  31 , the printing is completed. 
     A member  41  for frictionally tensioning the ink sheet  42  (hereinafter called “friction member”) may be provided between the rear head frame  32   b  and the take-up roller  15 . The simplest construction of the friction member is to use the rear head frame  32   b  itself, at least a portion thereof, as the friction member  41 . The description hereinafter given assumes that the rear head frame  32   b  is constructed so that one end thereof located near the head  31  serves as the friction member  41 . 
     The head  21  in FIG. 4 is pressed by the spring against the platen roller  110 , as earlier described. The ink sheet  42  is fed by relying solely on the rotating action of the platen roller  110 . The ink sheet  42  is sandwiched between the head  31  and the platen roller  110 . This is equivalent to holding the ink sheet  42  stationary under tension at the head  31 . The take-up roller  15  takes up the printed portion of the ink sheet  42  with a strong rotational force. With this take-up tension, the friction member  41  can serve the function of frictionally tensioning the ink sheet when, as shown in FIG. 4, the rear head frame  32   b  is constructed so that the friction member  41  protrudes beyond a reference line L 1  toward the platen roller side, i.e., below the reference line L 1  in the plane of FIG. 4, the reference line L 1  being the line joining the head  31  to the take-up position of the take-up roller  15 . 
     In the present invention, the take-up tensional force applied to the ink sheet  42  at the print position of the head  31  is made greater at the widthwise edges of the ink sheet  42  than at the center thereof in order to prevent the ink sheet  42  from wrinkling with the center portion thereof shrinking due to the heating of the head  31  and thereby to prevent print failure. This also serves to prevent transport failure of the ink sheet  42 . 
     The structure of the friction member  41  will be described in detail below. In the thermal transfer recording apparatus  3 , the friction member  41 , which forcefully applies a frictional force to a portion of the ink sheet  42  lying in the section between the head  31  and the take-up roller  15 , is disposed between the head  31  and the take-up roller  15  and along the full width of the ink sheet. Further, the frictional force of the friction member  41  is made grater at the center than at both end portions thereof. With this structure, the tensile force applied to both edges of the portion of the ink sheet  42  which faces the head  31  can be made greater than that applied to the center of that portion. 
     There are several structures in which the frictional force of the friction member  41  can be made greater at the center than at both end portions thereof, for example, a first structure in which the length of contact between the ink sheet  42  and the friction member  41  is varied, a second structure in which the coefficient of friction of the friction member  41  is made lower at both end portions than at the center portion, and a third structure in which the coefficient of friction of the friction member  41  is made higher at the center portion than at both end portions. Specific methods of mounting will be presented below using several examples. 
     FIG. 5 is a detailed perspective view showing the friction member  41  of the basic structure. In this structure, the rear head frame  32   b  is used as the friction member  41 . In FIG. 5, the friction member  41  is shown upside down from that shown in FIG. 4, and the ink sheet  42  is fed in the direction of ink sheet travel  56  shown by arrow  56 . The rear head frame  32   b  is a member whose cross section cut along an imaginary plane perpendicular to the width direction of the ink sheet  42  is substantially formed in the shape of the letter L. Of all the surfaces of the rear head frame  32   b , the surface most protruding beyond the reference line L 1  toward the platen roller side is called a “friction portion”. The friction portion is the surface that applies friction to the ink sheet  42 . A friction member  51 A of FIG. 5 is formed from uniform material, and the surface of the friction portion  52 A is flat. The coefficient of friction at the center portion of a friction portion  52 A in FIG. 5 is equal to that at both end portions thereof. The first to third structures forming friction members  41 B to  41 D in the present invention are identical to the structure of a friction member  41 A shown in FIG. 5, the only difference being in the structure of the respective friction portions. 
     FIG. 6 is a perspective view showing the friction member  41 B of the first structure. In this structure, both end portions  54 B and  55 B of a friction portion  52 B are recessed to provide lower faces than a center portion  53 B; that is, a first recess  71  is formed in one end portion  54 B of the friction portion  52 B, and a second recess  72  is formed in the other end portion  55 B thereof. In this way, the length along which the center portion  53 B of the friction portion  52 B of FIG. 6 contacts the ink sheet  42  is made greater than the length along which each of the end portions S 4 B and  55 B of the friction portion  52 B of FIG. 6 contacts the ink sheet  42 , so that the center portion  53 B of the friction member  41 B of FIG. 6 provides a greater frictional force than that defined by the coefficient of friction of each of the end portions  54 B and  55 B of the friction member  41 B. Accordingly, in the structure of the friction member  41 B in FIG. 6, the tensile force applied to both widthwise edges of the ink sheet  42  can be made greater than that applied to the widthwise center thereof. 
     In FIG. 6, the recesses  71  and  72  in the respective end portions are formed widest at the respective ends, the width gradually decreasing toward the center of the friction portion  52 B. With this gradually narrowing recess structure, the friction member  41 B of FIG. 6 can gradually reduce the tensile force with increasing distance from each widthwise end toward the center. 
     FIG. 7 is a perspective view showing the friction member  41 C of the second structure. In this structure, a first fluorine-based film  61  is glued to one end portion  54 C of the friction portion and a second fluorine-based film  62  to the other end portion  55 C. Generally, a fluorine-based film has a lower coefficient of friction than members made of other materials. Thus, the friction portion  52 C of the friction member  41 C in FIG. 7 has a lower coefficient of friction at both end portions  54 C and  55 C than at a center portion  53 C, relatively. Accordingly, in the structure of the friction member  41 C in FIG. 7, the tensile force applied to both widthwise edges of the ink sheet  42  can be made greater than that applied to the widthwise center thereof. 
     FIG. 8 is a perspective view showing a friction member  41 D of the third structure. In the friction member  41 D of FIG. 8, a center portion  53 D of a friction portion  52 D is covered with a substance having a high coefficient of friction. 
     More specifically, a substance  81  with a high coefficient of friction is applied only on the center portion  53 D of the friction portion. This serves to reduce surface smoothness of the center portion of the friction portion  52 D; as a result, the friction portion  52 D of the friction member  41 D in FIG. 8 has a lower coefficient of friction at both end portions  54 D and  55 D than at the center portion  53 D, relatively. Accordingly, in the structure of the friction member  41 D in FIG. 8, the tensile force applied to both widthwise edges of the ink sheet  42  can be made greater than that applied to the widthwise center thereof. When adjusting the frictional force by treating only the center portion  53 D of the friction portion, a film with a high coefficient of friction may be glued to the center portion  53 D of the friction portion, or the surface of the center portion  53 D of the friction portion may be roughened using a sand blasting or like method. With such treatment, in the structure of the friction member  41 D in FIG. 8, the tensile force applied to both widthwise edges of the ink sheet  42  can be made greater than that applied to the widthwise center thereof. 
     In the usual printing process, the widthwise center portion of the ink sheet  42  is frequently used for printing, increasing the tendency of the center portion of the ink sheet to shrink due to subjection to the heat of the thermal head  31 . Both edges of the ink sheet include portions not used for printing and, therefore, are less likely to be subjected to the heat of the head  31 . By designing the thermal recording apparatus  3  so that both widthwise edges of the ink sheet  42  is pulled with a greater tensile force than the center portion thereof is pulled and so that the direction vector of pulling is made to direct outward, transport failure of the ink sheet due to shrinking can be prevented, ensuring proper transportation of the ink sheet  42 ; furthermore, it becomes possible to prevent the ink sheet from wrinkling due to shrinking. 
     A realistic approach to achieving a structure that can apply a greater tensile force to both edges of the ink sheet than the center thereof is to reduce the ink sheet take-up force by friction and vary the rate of reduction between the widthwise center and the widthwise edges of the ink sheet. Accordingly, in the thermal transfer recording apparatus of the embodiment, the friction member for tensioning the ink sheet by forcefully applying friction to it is constructed so as to contact the full width of the ink sheet, and the frictional force of the friction member is made smaller at both end portions than at the center portion so that a greater tensile force is applied to both edge of the ink sheet. To achieve the purpose, in the specific structure shown in FIG. 6, the width of the friction member for providing friction is varied in such a manner as to increase the length of contact along which the ink sheet contacts the center portion. In FIGS. 7 and 8, the coefficient of friction of the friction member is varied between the center portion and the end portions to achieve the above purpose. 
     FIG. 9 is a block diagram showing the electrical configuration of the above-described thermal transfer facsimile apparatus according to the present invention. A brief description of the facsimile apparatus will be given with reference to FIG.  9 . The illustrated configuration concerns a conventional facsimile, and various other embodiments are also possible. The thermal transfer facsimile apparatus comprises a CPU  100 , a document transport section  102 , a document transmitting section  103 , and a memory  104 , in addition to the communications device  4  and a recording section  101  which includes the thermal transfer recording apparatus  3 . The CPU  100  controls the functions of the devices  4  and  101  to  104  in the facsimile apparatus. The recording section  101  records images received by the communications device  4  onto the recording paper. The recording section  101  includes a thermal head driver. The document transport section  102  transports a document holding thereon an image to be transmitted. The document transmitting section  103  transmits the document via the communications device  4 . The document transmitting section includes the document reading device. The memory  104  is used to store received images as well as abbreviated dialing and speed dialing numbers. 
     The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and the range of equivalency of the claims are therefore intended to be embraced therein.