PRINTER AND RIBBON CASSETTE THEREFOR

Abstract: A combination of a printer and a ribbon cassette (45) has a mechanism for preventing a portion of the inked ribbon extend­ ing between a supply core (51) and a take-up core (62) of the ribbon cassette (45) from shifting in the direction perpendicular to the longitudinal direction of the inked ribbon with respect to a printing head (40). The mechanism is constituted by a substantially planar surface of the platen (13), and the print­ ing head (40) is urged against the planar surface of the platen through the inked ribbon portion and a sheet (32) to which solid ink in the area on the inked ribbon portion is trans­ ferred by the printing head (40). The width of that area on the inked ribbon portion in the widthwise direction of the inked ribbon is at most equal to a half of the width of the inked ribbon. Alternatively, the mechanism (400) is constitued by a mechanism operative in response to the variation in dia­ meter of the inked ribbon wound on the supply core (51) for applying a variable back tension in the reverse direction to the tension acting on the inked ribbon portion by the take-up core (62).

Description: BACKGROUND OF THE INVENTION The present invention relates to a combination of a ribbon cassette and a printer for thermally transferring solid ink on the inked ribbon onto a sheet of the kind. A combination and a ribbon cassette are known from JP-A-56-120385. In a printer of the type described above, the inked ribbon cannot be used again, because the ink on the inked ribbon is thermally transferred onto the sheet during the printing operation thereby resulting in a problem that the running cost is raised. In order to solve the above problem, it has been proposed to divide the ink on the inked ribbon into at least a first and a second track extending longitudinally of the ribbon in parallel to each other and to use the second track after the first track has been used. Such a proposal is disclosed in Japanese Utility Model Laid-Open No. 58-194042 and Japanese Patent Laid-Open No. 57-47685, for example. However, in the case of the proposals of the above disclosures, the inked ribbon being fed from the supply core to the take-up core tends to be shifted perpendicularly to the longitudinal direction of the inked ribbon relative to a printing head facing against the inked ribbon. Due to the lateral shifting of the inked ribbon, a new problem occurs that the inked ribbon approaches an upper inner wall surface or a lower inner wall surface of a casing of the ribbon cassette as the inked ribbon is taken up onto the take-up core and, finally, the inked ribbon contacts with the upper or the lower inner wall surface. The contact of the inked ribbon being taken up onto the take-up core with the upper or lower inner wall surface makes it impossible to take up the inked ribbon onto the take-up core. Further, in case the inked ribbon has at least two tracks, the allowance of the lateral shifting of the ribbon relative to the printing head is smaller than the case in which the central area in the width of the inked ribbon is used in the printing operation. Thus, when the inked ribbon is shifted laterally with respect to the printing head even though a small amount, it will be shifted out of the printing area on the printing head or, when the second track on the inked ribbon is used in the printing operation, the printing area on the printing head will be positioned on the first track from which the ink has already been stripped out thereby causing fatal defects of incapability of printing characters. JP-A-56 120 385 discloses a combination of a ribbon cassette and a printer for thermally transferring solid ink on an inked ribbon within said ribbon cassette to a sheet, said combination comprising said ribbon cassette including a rotatable supply core having wound therearound the inked ribbon and a rotatable take-up core for taking up the inked ribbon; said printer comprising an elongated platen extending substantially perpendicularly to a direction of movement of the sheet upon new line starting, a carriage capable of being reciprocated longitudinally of said platen. Further, this document shows a ribbon cassette for a printer, comprising a casing; a supply core rotatably disposed within said casing, said supply core having wound therearound an inked ribbon; a take-up core rotatably disposed within said casing for taking up the inked ribbon with a taking- up tension (fo) upon operation of the printer. OBJECT AND SUMMARY OF THE INVENTION The object of the present invention is to improve the combination of a ribbon cassette and a printer so that the ink ribbon which has once been used in the printing operation can again be used. This object is accomplished with a combination and a ribbon cassette as claimed in claim 1. Dependent claims are directed on features of preferred embodiments of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic perspective view showing an entire printer; Fig. 2 is a fragmentary plan view showing a ribbon cassette loaded on a carriage of the printer, with a first casing half of the ribbon cassette being removed; Fig. 3 is a sectional view taken along the line III - III in Fig. 2; Fig. 4 is a perspective view in an enlarged scale showing the back tension applying mechanism shown in Fig. 2; Fig. 5 is a block diagram showing a circuit associated with a ribbon sensor; Fig. 6 is a schematic fragmentary side view of a prior art printer having a platen or a circular cross-section, illustrating experiments conducted by the inventors of the present application; Fig. 7 is an illustrative view when a thermally sensitive head shown in Fig. 6 is viewed from a rear surface thereof; Fig. 8 is a view showing the facts clarified by the results of the experiments conducted on the printer shown in Fig. 6; Fig. 9 is a view showing the state in which the inked ribbon is shifted downwardly with respect to the thermally sensitive head; Fig. 10 is a fragmentary sectional view of the ribbon cassette showing the irregularly taken-up inked ribbon resulting from the downward shifting of the inked ribbon shown in Fig. 9; Fig. 11 is a sectional view taken along line XI - XI in Fig. 1; Fig. 12 is a fragmentary side view partly in cross-section showing the relationship between the platen, the inked ribbon and the thermally sensitive head shown in Fig. 11; Fig. 13 is a view showing the tension acting on the inked ribbon shown in Fig. 12; Fig. 14 is a view similar to Fig. 1 but showing a printer according to the first embodiment of the present invention; Fig. 15 is a schematic fragmentary view showing the experiments carried out on the prior art printer by the inventors of the present application; Fig. 16 is a view showing the tension acting on the inked ribbon shown in Fig. 15; Fig. 17 is a view showing the distribution of the tension force acting on the ribbon in the combination of the printer and the ribbon cassette according to a printer known in the art as shown in Fig. 14, the view being taken from the rear side of the thermally sensitive head; Fig. 18 is a schematic view showing the path of feeding of the ribbon in the combination of a printer and a ribbon cassette of the prior art shown in Fig. 15; Fig. 19 is a graph showing the relationship between the tension acting on the ribbon and the diameter of the ribbon taken up onto the take-up core as the ribbon is moved along the path of feeding of the ribbon shown in Fig. 18; Fig. 20 is a graph similar to Fig. 19 but showing the relationship between the ratio of the tension and the diameter of the taken-up ribbon; Fig. 21 is a plan view of the ribbon cassette loaded on the printer shown in Fig. 14 with the first casing half being removed; Fig. 22 is a sectional view taken along the line XXII - XXII in Fig. 21; Fig. 23 is a perspective view in an enlarged scale of a variable back tension applying mechanism shown in Fig. 21; Fig. 24 is a schematic view showing the action of a plate spring in the variable back tension applying mechanism shown in Fig. 24, the figure showing the state of the plate spring in which the amount of flexure thereof is at the maximum; Fig. 25 is a view similar to Fig. 24 but showing the state of the leaf spring in which the amount of flexure is reduced to the minimum; Fig. 26 is a view similar to Fig. 25 but showing the state of the leaf spring in which the amount of flexure thereof is zero; Fig. 27 is a graph showing the relationship between the tension on the ribbon and the diameter of the taken-up ribbon and the relationship between the brake and the taking-up diameter of the ribbon as obtained in the combination of the printer and the ribbon cassette shown in Fig. 21; Fig. 28 is a graph similar to Fig. 27 but showing the relationship between the ratio of the tension and the diameter of the taken-up ribbon; Fig. 29 is a perspective view in an enlarged scale showing an alternative form of the variable back tension applying mechanism shown in detail in Fig. 23, with the mechanism being mounted on the carriage of the printer; Fig. 30 is a view similar to Fig. 1 but showing a combination of the printer and a ribbon cassette according to an embodiment of the present invention; Fig. 31 is a plan view of the ribbon cassette loaded on the carriage of the printer shown in Fig. 30; Fig. 32 is a view as seen in the direction of arrow XXXII - XXXII in Fig. 31; Fig. 33 is a fragmentary sectional view taken along the line XXXIII - XXXIII in Fig. 31; Fig. 34 is a plan view of the ribbon cassette shown in Fig. 31 which is inverted and again loaded in the carriage; and Fig. 35 is a view as seen in the direction of arrow XXXV - XXXV in Fig. 34. Referring to Fig. 1, a printer of the prior art has a pair of spaced, but opposed side wall members 11 and 12. An elongated platen 13 extends between the pair of side wall members 11 and 12 and mounted thereon. The platen 13 will be described in detail later. A shaft 14 has opposite ends fixedly supported by the pair of side wall members 11 and 12, respectively, and has an axis extending substantially in parallel to the platen 13. A carriage 15 is shiftably supported on the shaft 14 so as to be moved therealong. The carriage 15 is fixedly connected to a timing belt 16. The timing belt 16 is trained around a roller 17 rotatably mounted on the side wall member 11 and an output shaft 18 of a motor 19 supported on a bracket 21 which is secured to the side wall member 12. When the motor 19 is energized, the carriage 15 is reciprocally moved along the shaft 14 by means of the timing belt 16. A pinch roller 22 and a feed roller 23 forming a pair are rotatably supported by the side wall members 11 and 12. The feed roller 23 has an end 24 extending through and beyond the side wall member 12. A gear 25 is mounted on the protruding end 24 of the feed roller 23 for rotation therewith. A line feed motor 27 supported by the side wall member 12 has an output shaft (not shown) which is in mesh with the gear 26. A knob 28 is mounted on the protruding end 24 of the feed roller 23 thereby permitting the same to be rotated manually. A guide 31 supported by the side wall members 11 and 12 serves to direct the leading edge of a sheet 32 toward the nip between the rollers 22 and 23. The sheet 32 may be of any material such as paper, plastic, etc., insofar as ink can be transferred onto it. When the line field motor 27 is energized, the roller 23 is rotated to feed the sheet 32 onto the platen 13. The sheet 32 is pressed against the platen 13 by means of a plurality of press rollers 34. The press rollers 34 are rotatably supported on a rod 36. The rod 36 is swingably connected to the side wall members 11 and 12 through a pair of levers 37. A release lever 38 is connected to one of the levers 37 to move the rod 36 between a first position at which the rollers 34 press the sheet 32 against the platen 13 and a second position at which the rollers 34 are held spaced apart from the platen. A printing head 40 having provided thereon a heat generating resistance element 41 (Fig. 11) is mounted on the carriage 15 so as to be moved therewith thereby permitting the printing head 40 to move toward and away from the platen 13. A sensor 42 for detecting the home position of the carriage 15 is mounted on the side wall member 11. The carriage 15 is connected to a central processing unit (CPU) 43 of the control device through a flexible cable 44. In the similar manner, the motor 19, the motor 27 and the sensor 42 are also connected to CPU 43. A ribbon cassette generally designated by the reference number 45 can be loaded on the carriage 15. Referring to Figs. 2 and 3, the ribbon cassette 45 has a casing 46 constituted by a first casing half 47 and a second casing half 48. The first 47 and the second casing half 48 are joined together by screws (not shown) threadedly by engaging holes 47. A supply core 51, around which an inked ribbon 52 having thermally meltable solid ink applied thereon is wound, has a cylindrical body 53 and a circular rib 54 projecting radially inwardly from the inner peripheral surface of the cylindrical body 53 as shown in Fig. 3. The circular rib 54 is loosely fitted between a circular boss 56 formed in the first casing half 47 and a circular boss 57 formed in the second casing half 48 thereby permitting the supply core 51 to be rotatably supported by the casing 46. The circular rib 54 is provided with a plurality of projections 58 projecting radially inwardly therefrom. In the similar manner, a take-up core 62 for taking up the inked ribbon 52 therearound has a cylindrical body 63 and a circular rib 64. The circular rib 64 is loosely fitted between circular bosses 66 and 67 formed in the first and second casing halves 47 and 48, respectively, thereby permitting the take-up core 62 to be rotatably supported by the casing 46. The circular rib 64 has projections 68 similar to the projections 58 provided on the circular rib 54 and the projections 68 engage with projections 69 on a rotary driving shaft 71 provided in the carriage 15. A portion 72 of the inked ribbon 52 extending between the supply core 51 and the take-up core 62 passes through a back tension applying mechanism generally designated by the reference numeral 73. The back tension applying mechanism 73 has a friction material 74 made of felt which is sticked to a post 75 integrally formed in the second casing half 48 as shown in detail in Fig. 4. A plate spring 76 has an end rotatably supported on a post 77 which is integrally formed in the second casing half 48. The other end of the leaf spring 76 is provided with an engaging portion 78. The operation of the engaging portion 78 will be described later. A friction material 81 which is made of felt is sticked to a portion of the leaf spring 76 between one and the other ends thereof. A projection 48a integrally formed in the second casing half 48 abuts against the rear surface of the leaf spring 76 opposite to the side on which the friction material 81 is mounted. Due to the abutment of the projection 48a against the leaf spring 76, the friction material 81 on the leaf spring 76 is resiliently urged against the friction material 74 on the post 75. When the ribbon cassette 45 is loaded on the carriage 15 as shown in Fig. 2, and also when it is unloaded from the carriage 15, the friction material 81 is resiliently urged against the friction material 74, thereby applying a predetermined constant back tension to the portion of the inked ribbon 52 passing through between the friction materials 81 and 74. Referring again to Figs. 2 and 3, the casing 46 of the ribbon cassette 45 is formed with a pair of notches 83 and 84 positioned in symmetry to each other with respect to a center line C shown by the dot-and-dash line in Figs. 2 and 3 which center line C is at an equal distance from both the rotary axis of the supply core 51 and the rotary axis of the take-up core 62. When the ribbon cassette 45 is loaded on the carriage 15 as shown in Fig. 2, the thermally sensitive head 40 on the carriage 15 is positioned in one 83 of the pair of notches 83 and 84 and the portion 72 of the inked ribbon 52 extending between a pair of guides 86 and 87 provided in the second casing half 48 of the ribbon cassette 45 is located between the platen 13 and the thermally sensitive head 40. Each of the first and second casing halves 47 and 48 of the ribbon cassette 45 is formed with an opening 88, and the opening (not shown) formed in the first casing half 47 and the opening 88 formed in the second casing half 48 are positioned in symmetry to each other with respect to the center line C. When the ribbon cassette 45 is loaded on the carriage 15, a ribbon sensor 90 on the carriage 15 is introduced into the ribbon cassette 45 through the opening 88, and the portion 72 of the inked ribbon 52 is located between the ribbon sensor 90 and the platen 13. Fig. 5 is a block diagram showing an electric circuit associated with the ribbon sensor 90. The ribbon sensor 90 has a light emitting diode 91 for projecting light onto the portion 72 of the inked ribbon 52 extending between the supply core 51 and the take-up core 62 of the ribbon cassette 45 when the ribbon cassette 45 is loaded on the carriage 15 of the printer as well as a phototransistor 92 for receiving light from the diode 91 after reflected by a reflecting plate 93 on the carriage 15. The ribbon sensor 90 is adapted to issue a first signal when the portion 72 of the inked ribbon 52 is present between the ribbon sensor 90 and the platen 13, while it issues a second signal when the portion 72 is not present between the ribbon sensor 90 and the platen 13, and the signal thus issued is supplied to CPU 43 shown in Fig. 1 thereby permitting the second signal to be issued when the portion 72 of the inked ribbon 52 is deviated out of the thermally sensitive head 40 for some reason or when the ribbon cassette 45 is not loaded on the carriage 15 at the start of operation of the printer to render the printer to be inoperative. Further, the ribbon sensor 90 detects the terminal end of the inked ribbon 52 wound around the supply core 51 and supplies the second signal to CPU 43 to inform the operator of the fact that the inked ribbon 52 on the supply core 51 has been used up. Further, when the feeding of the ribbon 52 commences and the thermally sensitive head 40 moves toward the platen 13, the supply core 51 tends to overrun due to the inertia thereof because the thermally sensitive head 40 pulls out the inked ribbon 52 at a high speed, thereby feeding out the length of the ribbon from the supply core 51 greater than the length in the steady state of feeding of the ribbon. Therefore, the ribbon tends to be slackened because the amount of the ribbon taken up by the take-up core 62 can not corresponded to that fed out of the supply core 51. Such a slackening of the ribbon occurs between the supply core 51 and the back tension applying mechanism 73. Thus, the ribbon sensor 90 is arranged at a location where no such slackening of the ribbon 52 takes place, i.e. at a position between the back tension applying mechanism 73 and the thermally sensitive head 40 within the path of feeding of the ribbon. A guide 95 integrally formed in the second casing half 48 of the ribbon cassette 45 and extending between the first and second casing halves 47 and 48 is provided with a pair of projections 96 and 97 which are positioned substantially in symmetry to each other with respect to the center line C. The pair of projections 96 and 97 guide the portion 72 of the inked ribbon 52 with a minimum contact resistance so as to permit the portion 72 of the inked ribbon 52 to run along the predetermined path. A pair of guides 98 and 99 integrally formed on the second casing half 48 at either sides of the opening 88 serve to guide the inked ribbon 52 when the feeding of the inked ribbon 52 is commenced or stopped and when the head 40 is moved in the direction toward the platen 13 and in the direction apart from the platen 13 so that the portion 72 of the ribbon 52 is exactly located in front of the ribbon sensor 90 thereby preventing the false function of the ribbon sensor 90 due to the slackening of the ribbon 52. The ribbon cassette 45 shown in Figs. 1 to 3 can be used again in inverted position in like manner as the ribbon cassette disclosed in Japanese Patent Laid-Open No. 57-47685, for example. Specifically, the inked ribbon 52 in the ribbon cassette 45 is divided into first and second tracks extending lengthwise of the inked ribbon 52 in parallel to each other. In the loaded state as shown in Fig. 2, when the inked ribbon 52 is fed from the supply core 51 to the take-up core 62, printing operation is carried out by using the first track on the inked ribbon 52. After the inked ribbon 52 from the supply core 51 is completely taken up onto the take-up core 62, the ribbon cassette 45 is unloaded from the carriage 15. The unloaded ribbon cassette 45 is inverted in its position and is again loaded on the carriage 15 and the take-up core 62 then acts as a supply core, while the take-up core 51 acts as a take-up core. When the inked ribbon 52 from the core 62 is taken up onto the core 51, the second track on the inked ribbon 52 is used to carry out the printing. Since the ribbon cassette 45 is reusable in inverted position, it is formed in the configuration substantially in symmetry with respect to the center line C. Specifically, the cores 51 and 62 are formed in the same configuration and each of the cores 51 and 62 is placed between the corresponding bosses 56 and 66 on the first casing half 47 and the corresponding bosses 57 and 67 on the second casing half 48, respectively, with a minimum gap therebetween to thereby prevent the position of each of the cores 51 and 62 from being shifted when the ribbon cassette 45 is inverted. In order to apply a predetermined constant-back tension to the portion 72 of the inked ribbon 52 when the ribbon cassette 45 is inverted and the core 62 acts as the feed-out core, a back tension applying mechanism 100 is arranged adjacent the core 62. The back tension applying mechanism 100 is similar in construction to the back tension applying mechanism 73 arranged adjacent the core 51, and it has a friction material 101, a leaf spring 102 and a friction material 103 corresponding to the friction material 74, the leaf spring 76 and the friction material 81 of the back tensional force applying mechanism 73, respectively. Further, a projection 48b integrally formed on the second casing half 48 abuts against the rear surface of the leaf spring 102 opposite to the side thereof on which the friction material 103 is mounted. Due to the abutment of the projection 48b against the leaf spring 103, a pressing force is applied to the leaf spring 102 which tends to urge the friction material 103 to the friction material 101. When the ribbon cassette 45 is used under the condition shown in Fig. 2, a pin 104 on the carriage 15 engages with an engaging portion 105 of the leaf spring 102 to maintain the friction material 103 at a position spaced apart from the friction material 101. When the ribbon cassette 45 is loaded on the carriage 15 with the cassette being inverted so that the core 62 acts as the supply core, the engaging portion 105 of the leaf spring 102 does not engage with any pin, because no pin is provided at the right side of the carriage 15 corresponding to the pin 104 provided at the left side thereof. Therefore, the friction material 103 of the leaf spring 102 is urged resiliently against the friction material 101. Thus, the friction materials 101 and 103 cooperate with each other to apply the predetermined constant back tension to the portion 72 of the inked ribbon 52 passing through therebetween. When the ribbon cassette 45 is loaded on the carriage 15 with the cassette being inverted, the engaging portion 78 on the leaf spring 76 of the back tension applying mechanism 73 located adjacent the core 51 is engaged with the pin 103 on the carriage 15 passing through a hole 82 formed in the second casing half 48 of the ribbon cassette 45 to move the friction material 81 apart from the friction material 74. As shown in Fig. 3, a window 106 formed in the first casing half 47 serves to inform the operator of the amount of the inked ribbon 52 left wound around the core 51. A window formed in the second casing half 48 serves to inform the operator of the amount of the inked ribbon 52 left wound around the core 62 then acting as the supply core when the ribbon cassette 45 is loaded on the carriage 15 with the cassette being inverted. In operation of the prior art printer shown in Fig. 1, the take-up core 62 takes up the inked ribbon 52 thereon and the thermally sensitive head 40 is moved toward the platen 13 to urge the portion 72 of the inked ribbon 52 against the sheet 32 on the platen 13 thereby thermally transferring ink on the inked ribbon 52 onto the sheet 32 only when the carriage 15 is moved toward the side wall member 12. During the time the carriage 15 is moved toward the side wall member 11, the take-up core 62 does not take up the inked ribbon 52 and the thermally sensitive head 40 is maintained at a position spaced apart from the platen 13. The inventors of the present application have carried out experiments to find out the cause why the portion 72 of the inked ribbon 52 is shifted in the direction perpendicular to the longitudinal direction of the inked ribbon 52 relative to the thermally sensitive head 40 as mentioned in "BACKGROUND OF THE INVENTION". As to the results of the experiments, an explanation will be given below with reference to Figs. 6 to 10, inclusive. Fig. 6 is a fragmentary side view partly in cross-section of the prior art printer using a rubber platen 1 of the circular cross-section, the figure showing the positional relationship of the platen 1, an inked ribbon 2, a sheet 3 and a thermally sensitive head 4. The thermally sensitive head 4 is pressed against the platen 1 with the sheet 3 and the inked ribbon 2 interposed therebetween so as to apply a substantially horizontal force against the platen 1. Thus, the pressing force of the head 4 against the platen 1, the hardness of the rubber material forming the outer diameter portion of the platen 1 and the width of the head 4 are in general so set that the width P of the pressed area formed on the surface of the platen 1 is rendered to be slightly larger than the width H of the area of the heat generating resistance element 5. The inked ribbon 2 is set to have such a width R that the printing is carried out by the use of one of the divided two upper and lower tracks on the ribbon. In Figs. 6 and 7, an example is shown in which only the upper half track of the inked ribbon is utilized to transfer the ink on the upper half track to the sheet 3. After the printing operation using the upper half track of the ribbon 2, the ribbon cassette 6 is inverted in its position and loaded on the carriage to transfer ink on the remaining half track (the track not yet used up) to the sheet 3 by using the remaining half track. Thus, the center Ci of the width of the inked ribbon 2 and the center Ct of the width of the area of the heat generating resistance element 5 is offset by the distance Δℓ from each other as shown in Fig. 6. Fig. 7 is a view of the thermally sensitive head 4 as seen from the rear side thereof, and this figure typically shows the manner as to how the thermally sensitive head 4 transfers ink to the sheet by using the upper half track of the inked ribbon 2 while the thermally sensitive head 4 is moving in the direction indicated by the arrow 7 in Fig. 7. In Fig. 7, the shaded area shows the area of the inked ribbon 2 which is not yet used, while the white blank area shows the portion of the inked ribbon 2 from which the ink has been stripped off after the transfer of ink. The distribution of the tension acting on the inked ribbon 2 is shown in Fig. 8. The head 4 presses the hatched area of the upper half track of the inked ribbon 2 onto the platen 1. Under these conditions, the taking-up tension To acts onto the portion of the inked ribbon 2 extending between the supply core and the take-up core 8 in the ribbon cassette 6 in the direction indicated by the arrow A by means of the take-up core. The back tension Ti acts on the ribbon portion in the direction opposite to the tension To in order to insure the stable feeding of the ribbon by means of the back tension applying mechanism 9. While the tension forces To and Ti tend to act uniformly in equal distribution over the entire width R of the ribbon 2, the distribution of the tension of the ribbon 2 is made nonuniform as shown by the arrow B in Fig. 8 because the pressed area of the ribbon 2 by the head 4 is offset in the upper side, so that the tension force acting on the lower side of the ribbon 2 is rendered to be greater than the tension acting on the upper side thereof. In other words, a force W is generated at the position of the head 4 which tends to shift the ribbon 2 toward the lower side. As a result, the ribbon 2 is shifted downwardly at the head 4 as shown in Fig. 8. Since the inked ribbon 2 is in general made of a very thin base film on the order of about 4 - 8 micrometers, the inked ribbon 2 itself has substantially no rigidity. Thus, the inked ribbon 2, which has once been shifted to an incorrect position at the thermally sensitive head 4, has no ability in itself to correct its position to the proper position and, therefore, it is taken up onto the take-up core 8 under the condition that it is left shifted toward the lower side. As described above, when the ribbon is taken up onto the take-up core under the condition it is left shifted downwardly, the ribbon 2 taken up onto the take-up core 8 is rendered to be an irregularly wound form as shown in Fig. 10, so that the ribbon 2 will contact with the inner wall of casing CC in which it is received resulting in difficulty making the taking-up of the ribbon 2 impossible. The above-described example is to explain the phenomenon of shifting down of the ribbon 2 occurring at the head 4 in case the upper half track of the ribbon 2 is used in the printing. To the contrary, however, in case the lower half track of the ribbon 2 is used in the printing, a phenomenon of shifting-up of the ribbon 2 will take place, thereby resulting naturally in the similar difficulty described above. The phenomena of shifting-up and -down of the ribbon further cause another difficulty different from the incapability of the taking-up of the ribbon as described below. In case the printing is carried out by using one of at least two tracks into which the ribbon 2 is divided, allowance to the amount of shifting-up or -down of the position of the ribbon is smaller than the allowance in case in which the central area of the width of the ribbon is used in the printing. Therefore, when the shifting-up and -down of the ribbon occurs at the head 4, fatal defects will result such as the deviation of the ribbon 2 out of the heat generating resistance element 5 on the thermally sensitive head 4, or failure of printing characters due to positioning of the track from which ink has been removed by the previous printing operation at the heat generating resistance element 5. As described above, it is the indispensable requirements to prevent the shifting-up or -down of the ribbon at the thermally sensitive head in order to embody the construction in which the printing is carried out using one of at least two tracks into which the inked ribbon is divided, for the purposes of reduction in the running cost. The occurrence of the shifting-up or -down of the ribbon when the printing is carried out by using one of at least two tracks into which the ribbon is divided is due to the fact that the thermally sensitive head 4 presses the ribbon 2 against the platen 1 only at the area deviated to one side of the ribbon rather than the central area thereof. Therefore, the shifting-up or -down of the ribbon 2 can be prevented if the thermally sensitive head 4 presses the ribbon 2 over substantially the same width as the width R of the ribbon 2. However, in case the existing platen 1 of the circular cross-section mainly in use at present is used, more than twice of the width of the planar area on the platen 1 formed when the platen 1 is pressed by the head 4 will be required in comparison with the heretofore used width, and the pressing force of the head 4 and the diameter of the platen 1 and the like must be increased thereby resulting in disadvantages in the construction of the printer. Therefore, the thin printer according to the prior art does not use a platen of the circular cross-section but it is constructed from the beginning by the platen 13 having a planar surface 111 of substantially the same width P as the width R of the ribbon as shown in Figs. 11 and 12. By constructing the platen as described above, the distribution of the tension force acting on the surface of the portion 72 of the inked ribbon 52 is made the same value at the upper side and the lower side of the ribbon without adopting measures such as those increasing the pressing force of the head as shown in Fig. 13 whereby the shifting-up or -down of the ribbon will not occur. The thermally sensitive head 40, the platen 13 and the ribbon 52 of the first embodiment of the present invention will be described below with reference to Fig. 11. A ribbon cassette 45 reusable in the inverted position is loaded on the carriage 15 so that the take-up core 62 of the ribbon received in the cassette 45 is engaged by the take-up shaft 71 provided on the carriage 15. The transmission of the rotating force to the ribbon take-up shaft 71 is effected by the traversing movement of the carriage 15 through the timing belt 16, a pulley gear 112 engaging therewith, and a take-up power transmitting and cutting-off mechanism 113 engaging with the pulley gear 112. A solenoid 114 appropriately actuates and, deactuates the ribbon take-up power transmitting and cutting-off mechanism 113 correspondingly to the time requiring the taking-up of the ribbon and the time the taking-up of the ribbon is not required. The platen 13 is supported by a platen holder 115 and the feed roller 23 is arranged at the rear side of the platen holder 115. The sheet 32 is fed by urging the pinch roller 22 against the feed roller 23. On the other hand, the pressing operation of the thermally sensitive head 40 against the platen 13 is given by a solenoid 116 for pressing the head. In other words, a head supporting plate 117 is angularly tiltably supported on the shaft 14 with the outer periphery of a bearing 118 connected to the carriage 15 and supported on the shaft 14 serving as a guide. The shaft 14 supports the entire carriage 15 and is capable of traversing the carriage 15 in parallel to the platen 13. The thermally sensitive head 40 is mounted on the head supporting plate 117. The pressing operation of the thermally sensitive head 40 against the platen 13 is given by the attraction of the plunger 121 by the solenoid 116 which causes a solenoid lever 122 engaging with the plunger 121 to be tilted to urge the head 40 against the platen 13 by a head pressing pin 123 through the head supporting plate 117. The pressing force of the head 40, the width of the platen 13, the hardness of the rubber of the platen 13 and the like are so set that the width P of the planar surface 111 formed on the surface of the platen 13 when the thermally sensitive head 40 is urged against the platen 13 has the relationship with respect to the width H of the heat generating resistance element 41 such that P > nH (n=1, 2, 3 ...) where n designates the number of tracks for printing on the ribbon 52, n=2 indicating the upper and lower two tracks for printing. The position at which the heat generating resistance element 41 is mounted on the thermally sensitive head 40 is so set that, in the case of the upper and lower two tracks for printing, the mounting position is located at the upper or the lower side from the center of the width P of the planar surface 111 on the platen 13. The positional relationship between the inked ribbon 52 and the platen 13 is so set that their respective centers coincide substantially with other. The width R of the inked ribbon 52 is set to be R > P > nH to enable the printing by using one of the upper and lower tracks on the ribbon 52 for printing, and the center of the width R of the ribbon 52 is offset from the center of the width of the heat generating resistance element 41 by a distance Δℓ so that the printing is effected by using the upper track or the lower track on the ribbon 52. When constructed as described above, the destribution of the tension force acting on the portion 72 of the ribbon 52 is made uniform in the upper track and the lower track of the inked ribbon 52 thereby permitting no shifting-up or -down of the portion 72 of the ribbon 52 at the thermally sensitive head 40 to occur. Further, the width R of the ribbon 52 and the width P of the Planar surface 111 on the platen 13 are preferably made substantially equal to each other, but it is preferred to make R > P as described above in order to prevent the contamination of the sheet 32 due to the edges of the ribbon 52, and R ≃ P + (1 - 5) mm is preferred. As shown in Fig. 4, the width T of the friction materials 74, 78, 101 and 103 is set to be greater than the width R of the ribbon 52. With such arrangement, the shifting in position of the ribbon 52 at the back tension applying mechanism 73 and 100 can be prevented, even though the ribbon 52 tends to be shifted upwardly or downwardly with respect to the head 40, due to the fact that the friction materials 74 and 81 and 101 and 103 are brought into contact with each other at the both edges of the ribbon friction materials 74 and 81 and 101 and 103 to generate the resisting force against the shifting in position of the ribbon. With the arrangement of the prior art as described above, in the printing operation using one of two or more tracks on the ribbon for printing, the shifting-up or -down of the ribbon 52 can be prevented, not only enabling to prevent the difficulty of incapability of taking-up of the ribbon from occurring but also enabling to prevent failure of printing characters due to the shifting-up or -down of the ribbon from occurring. According to this printer of the prior art, since the shifting-up or -down of the ribbon 52 at the head 40 is prevented in the case where the center of the width R of the ribbon 52 is offset from the center of the width of the heat generating resistor 41 as in the case of the printing utilizing at least two tracks on the ribbon 52, the following effects are achieved: achieved: (1) Since there are provided upper and lower two tracks or more tracks on a single ribbon, the life of use of the inked ribbon is remarkably prolonged to permit the running cost of the ribbon to be greatly reduced. This fact affords the maximum merits to the user of the printer. (2) The life of use of the ribbon can be greatly increased with the construction of the printer substantially similar to that of the prior art printer using only one track on the ribbon. (3) Allowance to the width of the ribbon can be made small for the ribbon having a plurality of tracks, and still the defect of failure of printing characters can be prevented. (4) Since the regular form of the wound ribbon taken-up onto the take-up core is achieved, the load for taking-up the ribbon is reduced and the difficulty of incapability of taking-up of the ribbon can be prevented, while the tension for taking-up the ribbon can be set small thereby permitting the carriage per se to be made compact. FIRST EMBODIMENT Fig. 14 shows a first embodiment of the printer according to the present invention. The printer shown in Fig. 14 is substantially similar to the printer shown in Fig. 1 except that a platen 213 has a circular cross-section. In Fig. 14, parts and portions similar to those shown in Fig. 1 bear the same reference numerals as those in Fig. 1, and the description of these parts and portions will be omitted here for the purposes of avoiding duplication. The inventors of the present application have further conducted experiments in order to find out the cause why the portion of the inked ribbon is shifted in the direction perpendicular to the longitudinal direction of the inked ribbon with respect to the thermally sensitive head during the operation of the printer, as discussed in "BACKGROUND OF THE INVENTION". The results of the experiments will be described below with reference to Fig. 15 and 16. Fig. 15 is a view showing the tension acting on an inked ribbon 304 as the inked ribbon 304 is being fed while it is urged against a sheet 302 wound around a platen roller 301, with the printer being seen from the upper side. As shown in the figure, a ribbon take-up tension fo is given acting in the reverse direction to the direction of advance of the head while a back tension fi for stabilizing the feeding of the ribbon is given acting in the reverse direction to fo . In general, the back tension fi is set to be very small in comparison with the ribbon take-up tension fo . Fig. 16 is a view showing the phenomenon of shifting-down of the ribbon occurring when the printing is effected by using the upper track on the inked ribbon 304, wherein only the ribbon take-up tension fo is acting while the back tension fi is not acting. In the printing operation using the upper track on the inked ribbon 304, the area of the inked ribbon 304 pressed by the thermally sensitive head 303 is only the area A ○ in the figure, and it is offset to the upper side with respect to the center Cp of the inked ribbon. As described previously, since the inked ribbon 302 is in general constructed by a very thin base film on the order of 4 - 8 micrometers, it has substantially no rigidity. Thus, only the component of the tension exists, while substantially no compression component exists. Therefore, the area of the ribbon in which the tension is acting when the ribbon take-up tension fo is applied becomes as shown in B ○ in the figure to render the same to be nonuniform in the upper and the lower side with respect to the center Cp of the pressed area A ○ resulting in the nonuniform distribution of the tension on the ribbon. As a result, the downwardly directed component of the tension becomes greater than the upwardly directed component of the tension, thereby resulting in the deformation of the ribbon 304 as shown by the two-dot-and-dash line. In other words, the ribbon 304 is shifted downwardly at the head 303. For the similar reason, the ribbon 304 will be shifted upwardly when the lower track is used in the printing. The phenomenon of the shifting-up and -down of the ribbon when no back tension fi is applied has been described. However, it has been found that the shifting-up and -down of the ribbon can be prevented even though the position of the printing for the inked ribbon is deviated toward the upper track or the lower track by setting the back tension fi to be a value greater than a predetermined valve. The reason for the above will be described with reference to Fig. 17. Fig. 17 is a view of the distribution of the tension when the back tension fi is additionally acted in the condition shown in Fig. 16. As shown in this figure, the area in which the take-up tension fo is acting is only the area B ○, whereas the area in which the back tension fi is acting is the area C ○, whereby the tension is acted on the entire area of the ribbon. In other words, the back tension fi has the effectiveness reducing the nonuniformity of the tension given to the ribbon. According to the experiments, the amount of the back tension fi required for preventing the shifting-up or -down of the ribbon has an intimate relationship to the amount of the take-up tension fo and, further, it is in the intensive corelationship with the degree of deviation of the position of printing A ○ with respect to the ribbon. That is, greater the take-up tension fo and greater the degree of deviation of the position of printing A ○, the back tension fi required will become greater. However, the amount of the back tension fi is limited by the condition that the ribbon must be taken up until the last winding of the ribbon is wound up. The reason for the above will be described with reference to Figs. 18-20, inclusive. Fig. 18 is a view illustrating the feeding system of the ribbon and shows the printer as seen from the upper side thereof. The new ribbon prior to the ink-transfer which is wound around the ribbon supply core 305 is supplied to the front surface of the thermally sensitive head 303 through a back tension applying mechanism 306 for applying a constant back tension fi , a guide roller 307 and a post 308. At the front surface of the head 303, the transfer of ink is carried out. The ribbon after the transfer of ink is further taken up onto the take-up core 311 through a guide roller 309 and a post 310. The take-up core 311 is given a constant rotary torque T by the engagement thereof with a take-up shaft 312 on a carriage (not shown), thereby effecting the taking-up of the ribbon. In this case, the problem arises that the take-up tension fo varies as the taking-up of the ribbon proceeds because the diameter of the ribbon wound around the take-up core increases. In other words, assuming that the diameter of the ribbon at the beginning of the winding is D1 and the diameter of the ribbon at the termination of the winding of the ribbon is D2, the take-up tension fo on the ribbon varies from the beginning of winding f o1 = 2T/D1 to the termination of winding f o2 = 2T/D2. Fig. 19 shows the variation in the ribbon take-up tension fo with respect to the diameter D of the wound ribbon, and the back tension fi . As is clear from this Fig. 19, the ribbon take-up tension fo is rendered to be smaller as the diameter of the wound ribbon increases, whereas the back tension fi is kept constant. And, the requirement f o2 > K·fi must be satisfied where K is a coefficient of resistance to the feeding of the ribbon in the cassette casing in order to make it possible to take up the entire ribbon even when the diameter of the wound ribbon becomes the maximum (D=D2) and the take-up tension becomes the minimum (fo=f o2 ). K is in general to be K=2-3 although it varies depending upon the cassette casing used. As described above, the back tension fi is limited by the requirements that the entire ribbon must be taken up under the existence of the back tension fi . A description will be given below with reference to Fig. 20 describing in what range of the diameter of the wound ribbon the shifting-up or -down of the ribbon takes place when the above take-up tension fo and the back tension fi are applied. As described previously, whether or not the shifting-up or -down of the ribbon takes place is determined by the ratio between the take-up tension fo and the back tension fi , provided that the degree of the deviation of the printing position with respect to the ribbon is the same. Fig. 20 shows the relationship between fo and fi shown Fig 19 replaced by the term of fo/fi. As is clear from Fig. 20, if fo/fi > α, shifting-up or -down of the ribbon will take place, while shifting-up or -down of the ribbon will not occur if fo/fi < α. Here, α is a constant determined by the degree of deviation of the printing position with respect to the ribbon. That is, in the system of a constant back tension, shifting-up or -down of the ribbon takes place in the range from the diameter D1 of wound ribbon at the beginning of winding in which the take-up tension fo is great, to the diameter D3. The first embodiment of the present invention is so constructed that the value of the back tension fi is variable correspondingly to the diameter of the wound ribbon for the purposes of solving the above problem and it is directed to a construction preventing the shifting-up or -down of the ribbon wherein the ratio of tension fo/fi is kept less than the above described α. This construction will be described below with reference to Figs. 21 to 23, inclusive. As designated by the reference numeral 400 in Fig. 21, a mechanism is provided for applying a variable braking force, i.e., a variable back tension onto the outer periphery 404 of the ribbon 52 wound around the supply core 51. The variable back tension applying mechanism 400 will be described in detail below with reference to Fig. 23 which is a perspective view showing the detailed construction of the variable back tension applying mechanism 400. A post 438 for supporting a leaf spring is provided integrally with the second casing half 48 at a position spaced an equal distance L from the center of the supply core 51 and the center of the take-up core 62. A leaf spring 439 is leftwardly and rightwardly swingably mounted on the leaf spring supporting post 438. A friction material 440 and a friction material 441 each made of a material such as felt are sticked to the front surface and the rear surface of one end 439a of the leaf spring 439, respectively. An engaging or camming member 443 is connected to the leaf spring 439 which is engageable with a push pin 442 provided on the carriage 15. When the ribbon cassette is loaded on the carriage, the protruding pin 442 is inserted into the cassette casing 46 through a through-hole 444 formed in the second casing half 48 so that the tip 442a of the push pin 442 engages with the camming portion 443a of the engaging member 443 thereby urging the leaf spring 439 in the direction toward the supply core 52. With this construction, the friction material 440 provided on the leaf spring 439 is urged against the outer periphery 404 of the ribbon at the supply side thereby generating back tension against the ribbon being fed. When the cassette is inverted, since the supply core and the take-up core are reversely operated to act as a take-up core and a supply core, respectively, the push pin 442 (shown by the phantom line) at the inverted loading of the cassette engages with a camming portion 443b of the engaging member 443 so that it pushes at all times only the outer periphery of the ribbon at the supply side. The above described friction material 441 is provided for the purposes of applying a braking force onto the outer periphery of the ribbon at the supply side upon the inverted loading of the cassette. The reference numerals 450 and 451 designate stopper pins integrally formed in the second casing half 48 for limiting the respective positions of the leaf spring 439. The operation of the leaf spring 439 for applying the variable braking force to the outer periphery 404 of the ribbon at the supply side will be described with reference to Figs. 24-26, inclusive. Fig. 24 shows the state of the ribbon at the beginning of winding thereof, and Fig. 25 shows the state of the midway of taking up the ribbon, while Fig. 26 shows the state of the ribbon at the end of winding. As shown in Fig. 24 the take-up tension fo becomes the maximum value f o1 at the beginning of winding of the ribbon, but, since the diameter of the wound ribbon at the supply side is the maximum, the amount of flexure (amount of deformation) of the leaf spring 439 applying the braking force, i.e., the back tension f R to the outer periphery of the ribbon becomes also the maximum so that a large back tension f R acts on the ribbon. As the taking-up of the ribbon proceeds, the take-up tension f₀₁ decreases gradually, but, since the diameter of the ribbon at the supply side decreases also gradually, the flexure of the leaf spring 439 becomes zero at a certain point as shown in Fig. 25 so that the back tension f R will no more act. During the period from the time beginning at the state of Fig. 25 to the time ending at the end of winding of the ribbon shown in Fig. 26, f R does not act but only fi acts, since the leaf spring 439 does not contact with the outer periphery 404 of the ribbon at the supply side. The variation of the back tension (fi+f R ) with respect to the diameter D of the wound ribbon and the variation of the take-up tension fo on the ribbon are summarized in Fig. 27. Here, the area shown by the hatching is the back tension component f R acting on the outer periphery of the ribbon at the supply side. As shown in Fig. 27, the back tension (fi+f R ) becomes the maximum when the take-up tension fo is the maximum value f o1 , and both forces decrease as the taking-up of the ribbon proceeds. Fig. 28 shows the ratio of tension fo/(fi+f R ) which is substituted for the tension shown in Fig. 27. As is clear from Fig. 28, the ratio of tension fo/(fi+f R ) is held less than the constant α defining the limit of occurrence of the shifting-up or -down of the ribbon over the entire range from the beginning at the diameter D1 upon beginning of winding of the ribbon to ending at the diameter D2 upon the termination of winding of the ribbon. In the first embodiment of the present invention, the leaf spring 439 for applying the braking force to the outer periphery of the ribbon at the supply side is so shown as an example that only one such spring is used therefor. However, it may be so constructed that a leaf spring is provided at each of the outer periphery of the ribbon at the supply side and the outer periphery of the ribbon at the take-up side separately from each other in the similar manner of consideration to the back tension applying mechanisms 73 and 100, and the braking force of the leaf spring at the take-up side is released by means of the push pin from the carriage at the time of loading of the cassette 45 on the carriage. With the first embodiment of the present invention, since the shifting-up or -down of the ribbon at the head can be prevented for the construction in which the printing position with respect to the inked ribbon is offset from the center of the width of the ribbon as in the case of the inked ribbon having upper and lower two tracks for printing or more tracks thereon, the following effects are achieved. (1) Since printing is made possible by using each of at least two tracks for printing provided on a single inked ribbon, the period of interchange of the inked ribbon is prolonged, thereby permitting the running cost of the printer to be reduced greatly. This affords the maximum merits to the user of the printer. (2) The construction of the body of the printer may be the same as that of the prior art printer using a single track for printing on the ribbon. In other words, the user can utilize the existing printer by loading merely the ribbon cassette of the second embodiment of the present invention, thereby permitting the running cost to be reduced. (3) Since the shifting-up or -down of the ribbon can be prevented, the allowance to the width of the ribbon can be made small, thereby permitting the size of the ribbon which is the disposable article as well as the size of the ribbon cassette to be made compact. (4) Since the regularly wound form of the ribbon can be achieved, the load for taking up the ribbon is reduced and the failure as the incapability of taking-up of the ribbon can be avoided, while the take-up tension on the ribbon can be set small, thereby permitting the shape and the size of the carriage per se to be made compact. MODIFICATION OF THE FIRST EMBODIMENT The second embodiment shown in Figs. 21-23, inclusive, is so shown that the variable back tension applying mechanism 400 is arranged in the casing 46 of the ribbon cassette 45. However, a variable back tension applying mechanism 500 shown in Fig. 29 is provided on the carriage 15 and, therefore, each of the first casing half (not shown) and the second casing half 48 of the ribbon cassette 45 is formed with an opening 501 allowing the mechanism 500 to be introduced into the cassette 45 when the ribbon cassette 45 is loaded on the carriage 15. The variable back tension applying mechanism 500 shown in Fig. 29 itself is similar in construction to the mechanism 400 shown in Figs. 21-23, and it has a leaf spring 539 having one end rotatably supported by a post 538 on the carriage 15, a pair of friction materials 540 and 541 each made of felt and sticked to the respective surfaces of the other end 539a of the leaf spring 539, and a camming member 543a fixedly connected to the leaf spring 539. When the ribbon cassette 45 is loaded on the carriage 15, an edge 502 of the opening 501 formed in the second casing half 48 of the ribbon cassette 45 engages with the camming member 543a to resiliently urge the friction material 540 against the peripheral surface 404 of the inked ribbon 52 wound around the supply core 51. The movement and deformation of the leaf spring 539 are limited by a pair of pins 550 and 551. SECOND EMBODIMENT Fig. 30 shows a second embodiment of a printer according to the present invention. The printer shown in Fig. 30 is similar in construction to the printer shown in Fig. 14 and, therefore, detailed description of the printer is omitted here. Figs. 31-35, inclusive, show a ribbon cassette 645 used in the printer shown in Fig. 30. The ribbon cassette 645 is similar to the ribbon cassette 45 shown in Figs. 2 and 3 except that the inked ribbon 652 therein has a width about a half of the width of the inked ribbon 52 shown in Figs. 2 and 3, and that a shielding member 600 to be described later is provided. Therefore, the similar parts and portions of the ribbon cassette 645 to those of the ribbon cassette 45 are designated by the same reference numerals, respectively, and the description thereof will be omitted here. As shown in Figs. 31-33, the shielding member 600 is integrally formed on the second casing half 648 of the ribbon cassette 645 and is positioned adjacent the opening 88a formed in the first casing half 647. An inner surface 601 of the shielding member 600 formes the background having the same color as that of the inked ribbon 652. As is clear from Fig. 31, the shielding member 600 is associated with only the opening 88a, but is not associated with the opening 88 formed in the second casing half 648. "Side A" is printed on the outer surface of the first casing half 647, while "Side B" is printed on the outer surface of the second casing half 648 as shown in Fig. 34. As shown in Figs. 31 and 32, the ribbon cassette 645 is loaded on the carriage 15 with the mark "Side A" facing upwardly. In the operation of the printer, the inked ribbon 652 is fed out from the supply core 51 and taken up onto the take-up core 62, while the thermally sensitive head 40 thermally transfers ink on the inked ribbon 652 to the sheet 32 on the platen 213. As a result of the transfer, the inked ribbon 652 taken up onto the take-up core 62 has areas from which ink has been removed. When the inked ribbon 652 wound around the feedout core 51 has been completely taken up onto the take-up core 62, the ribbon sensor 90 detects the terminal end of the inked ribbon 652 and send a signal to CPU 43 as previously described in connection with Fig. 5 to render the printer to be inoperative and to inform the operator of the fact that the inked ribbon has been used up. An inked ribbon cassette used in the prior art thermally transferring printer is disposed each time it has once been taken up for printing. However, the area of the ribbon from which ink is actually removed during one printing operation is on the order of about 15% of the entire area of the inked ribbon and, therefore, this is very uneconomical. Although slight deficiency in printing characters might occur when the thus used inked ribbon is used again for printing, such a ribbon having been used once can be satisfactorily used in printing such documents as memorandums and the like other than important documents. However, when an inked ribbon having been used is used again, there might occur a problem that the ribbon sensor 90 detects the portions having removed therefrom the ink to judge that no inked ribbon is arranged at all times thereby stopping the printing operation and rendering the printer to be inoperative. With the second embodiment of the present invention, the ribbon cassette 645 is unloaded from the carriage 15 after the inked ribbon 652 from the supply core 51 has been completely taken up onto the take-up core 62. Then, the ribbon cassette 645 is inverted so that the mark "Side B" faces upwardly as shown in Figs. 34 and 35 and is loaded again on the carriage 15. At this time, the shielding member 600 having the inner surface 601 of the same color as that of the inked ribbon 652 is placed between the reflecting plate 93 and the inked ribbon 652. Although the inked ribbon 652 which is wound around the core 62 now acting as the supply core has areas from which ink has been removed by the previous printing operation, the light emitted from the light emitting diode 91 (Fig. 5) of the ribbon sensor 90 passes though the areas of the inked ribbon 652 without having ink and is absorbed in the inner surface 601 of the shielding member 600 so that no light is reflected from the shielding member 600. Thus, the phototransistor 92 (Fig. 5) of the ribbon sensor 90 does not receive any light thereby supplying to CPU 43 the first signal, i.e., a signal indicating that the inked ribbon 652 is present as previously described in connection with Fig. 5. Thus, CPU 43 supplies operating signals to the respective parts of the printer thereby permitting the head 40 thermally transfer ink remaining on the inked ribbon 652 to the sheet 32 on the platen 213. As described above, with the above construction of the printer, the ribbon cassette can be used twice in the reciprocating manner by merely inverting it for loading on the carriage, thereby achieving significant effectiveness in reducing the running cost. With the second embodiment of the present invention, since the mechanism of the thermally transferring printer per se may be the same as that of the prior art of the unidirectional printing can be effected by merely inverting the ribbon cassette, the effective length of the inked ribbon for the printing can be made twice longer in comparison with the prior art system, thereby affording to the user remarkable effectiveness in reducing the running cost. In the second embodiment described above, the shielding member 600 is provided on the ribbon cassette 645. However, it is possible to construct the printer in such a manner that the shielding member is provided on the carriage and the output signal of the inked ribbon sensor is switched to a signal indicating the existence of the inked ribbon regardless of the presence and nonpresence of ink on the inked ribbon. When the second embodiment of the present invention is used, since the inked ribbon already used once is used again to enable the reciprocal printing, so that a thermally transferring printer capable of lowering running cost can be provided affording to the user the maximum merits and, therefore, it is a superior invention having an effectiveness in practice. Although each of the above-described embodiments has been described and illustrated as having a thermally sensitive head provided with a heat generating resistance element, the head may be the one as disclosed in Japanese Patent Laid-Open No. 57-129763.

Claims: A combination of a ribbon cassette and a printer for thermally transferring solid ink on an inked ribbon within said ribbon cassette to a sheet, said combination comprising: said ribbon cassette (45) including a rotatable supply core (51) having wound therearound the inked ribbon (52) and a rotatable take-up core (62) for taking up the inked ribbon; said printer comprising an elongated platen (13) extending substantially perpendicularly to a direction of movement of the sheet (32) upon new line starting, a carriage (15) capable of being reciprocated longitudinally of said platen (13), whereby said ribbon cassette (45) is capable of being loaded on said carriage (15) for movement therewith, a printing head (40) is mounted on said carriage (15) for movement therewith, said printing head (40) being movable between a first position remote from said platen (13) and a second position where a portion of said inked ribbon (52) extending between said supply (51) and take-up cores (62) of said ribbon cassette (45) is urged by said printing head (40) against the sheet (32) on said platen (13) to thermally transfer the ink within an area on said inked ribbon portion to the sheet (32), said area on said inked ribbon portion having a width in the widthwise direction of said inked ribbon (52), the width of said area being at most equal to a half of a width of said inked ribbon, and means for rotatingly driving said take-up core of said ribbon cassette to impart a taking-up tension (fo) to said inked ribbon cassette, and means (400) are mounted on either one of said carriage of said printer and said ribbon cassette and operative in response to a change in diameter of the inked ribbon (52) wound around said supply core (51) for imparting a variable back tension (fi) opposite to said taking-up tension, to said inked ribbon portion so as to constantly satisfy the following inequality, said back tension being variable as to be reduced in accordance with the reduction in diameter of the inked ribbon (52) wound around said supply core (51), fo/fi < α where α: constant, said means (400) for imparting a variable back tension imparts braking force to the outermost turn of the inked ribbon wound around said supply core (51) and comprises elongated spring means (439) having one end thereof fixed to one of said carriage (15) of said printer and a casing of said ribbon cassette (45), and friction means (440, 441) mounted on the other end of said spring means (439) and resiliently urged against the outermost turn of the inked ribbon (52) wound around said supply core (51), said means for imparting a variable back tension being further located at substantially equal distances from a rotary axis of said supply core (51) and a rotary axis of said take-up core (62); and actuator means (442, 443) are mounted on the other of said carriage (15) of said printer and said ribbon cassette (45) for engaging said elongated spring means (439) when said ribbon cassette (45) is loaded on said carriage (15), to cause said elongated spring means (439) to be deflected, to thereby urge said friction means against the outermost turn of the inked ribbon (52) wound around said supply core (51) with a predetermined urging force. A combination as set forth in claim 1, wherein said means for imparting a variable back tension is disposed within the casing of said ribbon cassette (45), the one end of said elongated spring means is fixed to a wall of the casing, and said actuator means comprises a camming member connected to said spring means and a pin (442) fixed to said carriage (15) and protruding into said ribbon cassette (45), when the same is loaded on said carriage (15), to engage said camming member. A combination as set forth in claim 2, wherein said elongated spring means comprises a leaf spring (439). A combination as set forth in claim 3, wherein said friction means (440, 441) comprises felts respectively applied to opposite surfaces of the other end of said spring means (439). A combination as set forth in claim 1, wherein said means for imparting a variable back tension is mounted on said carriage (15) of said printer, the one end of said elongated spring means is fixed to said carriage (15), the casing of said ribbon cassette having an opening allowing said means for imparting a variable tension to be introduced therethrough into said casing when said ribbon cassette is loaded on said carriage (15), and said actuator means comprises a camming member connected to said spring means and an edge of said opening engaging said camming member when said ribbon cassette is loaded on said carriage. A combination as set forth in claim 5, wherein said elongated spring means comprises a leaf spring (439). A combination as set forth in claim 6, wherein said friction means comprises felts respectively applied to opposite surfaces of the other end of said spring means (439). A combination as set forth in claim 1, further comprising means for imparting a constant back tension opposite to said taking-up tension, to said inked ribbon portion. A combination as set forth in claim 7, wherein said means for imparting a constant back tension is disposed within a casing of said ribbon cassette (45) and comprises a first felt fixed to a wall of said casing, a leaf spring (102) having one end thereof fixed to the wall of said casing, a second felt attached to the other end of said leaf spring for resiliently urging a portion of the inked ribbon extending between said supply and take-up cores, against said first felt. A combination as set forth in claim 1, wherein said printing head (40) has a heat-generating resistance element (41) incorporated in an area of said printing head corresponding to said area on said inked ribbon portion.

Inventors: FURUKAWA, SHIGETAKA, KITAGISHI, TOMOJI, KOBAYASHI, RYOOICHI, MIKAMI, KATSUMASA, SASAKI, AKIRA, SUZAKI, MASAFUMI, TAKAHAGI, FUMIO
Applicants: HITACHI, LTD.
Application year: 1986
Main classification: B41J32
IPC codes: B41J32, B41J35