Thermal transfer recording apparatus

Abstract: A thermal transfer recording apparatus for image recording by transferring ink contained in an ink sheet (14) onto a recording medium (11) comprises means (17, 18) for transporting ink sheet (14), means for transporting recording sheet (11), means for acting on the ink sheet by a thermal head (13) thereby recording an image on the recording sheet transported to the recording position by means for transporting recording sheet, and the center of the heat-generating resistor (132) of the thermal head is shifted by a predetermined distance (f) from the center of pressure exerted by the thermal head at the recording position to the upstream side in the transporting direction of the ink sheet, thus ensuring the transfer of ink onto the recording sheet to obtain a high-quality image for the recording.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a thermal transfer recording apparatus for recording an image by transferring ink contained in an ink sheet onto a recording medium of the kind defined by the pre-characterizing features of claim 1, and to a recording apparatus with a plurality of electrothermal converters arranged in series. Related Background Art In general, a thermal transfer printer uses an ink sheet with heat-fusible (or heat-sublimable) ink coated on its base film, and records an image by selectively heating the ink sheet with a thermal head in accordance with an image signal or image signals to transfer the fused (or sublimated) ink onto a recording sheet. In general, the ink contained in this ink sheet is completely transferred onto the recording sheet by one-time image recording (so-called one-time sheet). Therefore, it is necessary to transport the ink sheet by an amount equivalent to a length of the recorded letter or line when each recording is completed, so that an unused portion of the ink sheet is reliably brought to the position at which the next recording is performed. Consequently the consumption of the ink sheet increases, and the running cost of a thermal transfer printer tends to be higher than that of an ordinary thermal printer. In order to solve this problem, there has been proposed a thermal transfer printer wherein a recording sheet and an ink sheet are transported with speed difference as disclosed in Japanese Laid-Open Patent Application Nos. 57-83471 and 58-201686 or Japanese Patent Publication No. 62-58917. As described in the foregoing patent publications, there is known an ink sheet capable of recording image for plural times (n times) (so-called multi-print sheet) to be used in a thermal transfer printer. Therefore, in performing continuous recording for a record length L, it is possible to make recording by providing the length of transporting the ink sheet during or after the image recording shorter than the length L (L/n: n > 1) if this type of ink sheet is employed. Fig. 1 is an enlarged view showing the structure of a recording unit using a multi-print method such as this. Here recording is performed by transporting the ink sheet 203 and the recording sheet 204 in the opposite directions to each other. An ink layer 207 of the ink sheet 203 fused (or sublimated) by a heat-generating resistor 205 of this thermal head 201 is pressed to the recording sheet 204 between the thermal head 201 and a platen roller 202, and is transferred onto the recording sheet 204 while being rubbed with the recording sheet 204 at a relative speed. Here as clearly seen from Fig. 1, the center of the heat-generating resistor 205 of the thermal head 201 and the center of the platen roller 202 are arranged to be positioned at the normal line 206 of the platen roller 202 for exerting the pressure. Then the fused (or sublimated) portion of the ink layer 207 of the ink sheet 203 is transferred for recording while being transported or conveyed in the reverse direction a to the transporting direction (direction b) of the recording sheet 204. Therefore, the position where ink of the ink sheet 203 is actually transferred to the recording sheet 204 is the one slightly shifted to the upstream side in the transporting direction of the ink sheet 203, at which the pressure between the ink sheet 203 and the recording sheet 204 becomes slightly less than its maximum value (at the position where the normal line 206 runs through). As a result, there may occur the disadvantage that the recorded dots are blurred or their density becomes uneven because of the reduced pressure when the ink is actually transferred. In order to prevent such unevenness in the density it is necessary to intensify the energy applied to the thermal head 201 and maintain the density for the recording thicker. In this respect, there is known a structure in a thermal recording apparatus (Japanese Laid-Open Patent Application No. 57-157771, Laid-Open Date: September 29, 1982) in which a thermal head is mounted in such a manner that the center of a heat-generating element is arranged to be in a position shifted from the center of pressure exerted to a platen roller to the opposite side of the direction in which a recording sheet is transported, so that the thermal recording sheet is tightly in contact with the heat-generating element even if the platen roller is deformed by the rotation during the recording operation. There is also known a structure in a thermal recording apparatus of the kind defined by the precharacterizing features of claim 1 (Japanese Laid-Open Patent Application No. 57-103863, Laid-Open Date: June 28, 1982) in which the center of a heat-generating resistor is shifted from the center of the convex part thereof in order to transmit the heat energy generated by the heat-generating resistor effectively to a thermal recording sheet even if the portion of the thermal recording sheet on the side of the direction in which the head is relatively moved is raised or floated toward the head due to the pressure of the head or platen roller. Nevertheless, the inventions disclosed in the respective publications aforementioned have been given any attention to the lowering of the pressure when ink is actually transferred onto a recording medium. SUMMARY OF THE INVENTION An object of the present invention is to improve printing quality within a thermal transfer recording apparatus and a recording apparatus defined at the preamble, respectively. Another object of the present invention is to provide a thermal transfer recording apparatus defined at the preamble in which the pressure can be made the greatest when ink contained in an ink sheet is transferred onto a recording medium with a particular attention given to the lowering of the pressure at the time of the ink being actually transferred onto the recording medium. Another object of the present invention is to provide a thermal transfer recording apparatus defined at the preamble for recording a high-quality image by stabilizing the density for the recording. These objects are attained concerning the thermal transfer recording apparatus by the characterizing features of claim 1, and concerning the recording apparatus by the features of claim 4. The present invention, therefore, provides a thermal transfer recording apparatus for recording a high-quality image by stabilizing the density for the recording in such a manner that the central position of pressure exerted by a thermal head to a recording medium and an ink sheet is arranged to be shifted from the central position of a heat-generating member of the thermal head to the downstream side in the transporting direction of the recording medium so as to make the pressure the greatest when ink contained in the ink sheet is transferred. The present invention, further, provides a thermal transfer recording apparatus in which the center of the heat-generating resistor of a thermal head is shifted by a predetermined length from the center of pressure of the thermal head at the recording position to the upstream side in the transporting direction of an ink sheet. The present invention also provides a thermal transfer recording apparatus which comprises a recording head capable of exerting the greatest pressure for stabilizing the recording density at the time of performing a recording by transporting a recording medium relatively from the top of the convex part of the recording head toward the position of an electrothermal converter, so that a high-quality image can be recorded. Finally the present invention provides a thermal transfer recording apparatus in which a recording on a recording medium is performed by transporting a recording medium or recording head in such a manner that the recording medium is transported in the relative direction from the top of the convex part of the recording head toward an electrothermal converter. Advantageous developments of the invention are given in the subclaims. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a view showing the position of a heat-generating resistor in a conventional recording unit; Fig. 2 is an enlarged view showing the recording unit of a facsimile apparatus according to an embodiment suited for the present invention; Fig. 3 is a cross-sectional view showing the mechanical part of the facsimile apparatus of said embodiment; Fig. 4 is a block diagram showing the structure of the facsimile apparatus of said embodiment; Fig. 5 is a view showing the connection of the control unit and recording unit of the facsimile apparatus of said embodiment; Fig. 6 is a view further enlarged to show the recording unit shown in Fig. 1; Fig. 7 is a view showing the relationship between the shifting amount of the heat-generating resistor and the recording density; Fig. 8 is a flowchart showing a recording process in the facsimile apparatus of said embodiment; Fig. 9 is a view illustrating the recording principle in a case where a multi-ink sheet is used; Fig. 10 is a cross-sectional view showing a multi-ink sheet employed for the present embodiment; and Fig. 11 and Fig. 12 are enlarged views showing the recording unit of a facsimile apparatus according to a second embodiment suited for the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter with reference to the accompanying drawings an embodiment suited for the present invention will be described in detail. [Description of Facsimile Apparatus (Figs. 2-5)] Figs. 2-5 are views showing an example in which a thermal transfer printer using an embodiment of the present invention is applied to a facsimile apparatus. Fig. 2 is an enlarged view showing the structure of recording unit of the facsimile apparatus adopting the embodiment suited for the present invention; Fig. 3 is a lateral cross-sectional view illustrating the structure of mechanical part of the facsimile apparatus according to said embodiment; Fig. 4 is a schematic block diagram showing the structure of said facsimile apparatus; and Fig. 5 is a view showing the electrical connection between the control unit 101 and recording unit 102 of said facsimile apparatus. At first the structure of the apparatus will briefly be described with reference to Fig. 3. In Fig. 3 a numeral 10 denotes a roll of recording sheet 11 which is a plain paper wound around a core 10a. This rolled sheet 10 is rotatably housed in the apparatus so that the recording sheet 11 can be supplied to a thermal head 13 by the rotation of a platen roller 12 in the direction indicated by an arrow. In this respect, a numeral 10b denotes a rolled sheet loading unit in which the rolled sheet 10 can be detachably mounted. Further a numeral 12 denotes a platen roller which transports the recording sheet 11 in the direction indicated by an arrow b and at the same time, presses an ink sheet 14 and the recording sheet 11 against a heat-generating resistor 132 of the thermal head 13. The recording sheet 11 is transported toward discharge rollers 16 (16a and 16b) by further rotation of the platen roller 12 after an image has been recorded thereon by the thermal head 13, and is cut into the unit of one page with the engagement of cutter blades 15 (15a and 15b) when the image recording for one page is completed. A numeral 17 denotes an ink sheet feed roller with the ink sheet 14 wound thereon, and a numeral 18, an ink sheet takeup roller driven by a motor for conveying ink sheet, which will be described later, to take up the ink sheet in the direction indicated by an arrow a. In this respect, the ink sheet feed roller 17 and ink sheet takeup roller 18 are detachably mounted in an ink sheet loading unit 70 of the main body of the apparatus. Further a numeral 19 denotes a sensor for detecting the remaining amount of the ink sheet 14 and the carrying speed of the ink sheet 14. Also, a numeral 20 denotes an ink sheet sensor for detecting the presence of the ink sheet 14 and the remaining amount of the ink sheet 14 based on a mark provided on the ink sheet 14. A numeral 21 denotes a spring to press the thermal head 13 against said platen roller 12 through the recording sheet 11 and ink sheet 14. A numeral 22 denotes a recording sheet sensor for detecting the presence of the recording sheet. In this respect, the position of the thermal head 13 in this recording unit 102 will be described in detail later with reference to Fig. 2. Next, the structure of a reading unit 100 will be described. In Fig. 3, a numeral 30 denotes a light source for irradiating a manuscript 32. The reflected light from the manuscript 32 is inputted in a CCD sensor 31 through an optical system (mirrors 50 and 15, and a lens 52) for conversion into electrical signals. The manuscript 32 is transported by feed rollers 53, 54, 55, and 56 driven by a motor for conveying manuscript (not shown) corresponding to a speed at which the manuscript 32 is being read. In this respect, a numeral 57 denotes a manuscript stacker, and the plural sheets of manuscript 32 stacked on this stacker 57 is separated one by one and transported to a reading unit 100 by the feed roller 54 in cooperation with a pressurized separating piece 58. A numeral 41 denotes a control board comprising the principle portion of a control unit 101. This control board 41 outputs various control signals to each unit of the apparatus. A numeral 106 denotes a modem unit 107 and 107, an NCU board unit. Next, the structure of the facsimile apparatus according to the present embodiment will be described with reference to Fig. 4. In Fig. 4, a numeral 100 denotes the reading unit comprising a motor for conveying manuscripts and a CCD image sensor, etc. for reading manuscripts photoelectrically to output them to the control unit 101 as digital signals. Subsequently the structure of this control unit 101 is described. A numeral 110 denotes a line memory for storing the image data of each line of an image data. When a manuscript is transmitted or copied, the image data for a one-line portion from the reading unit 100 is stored, and when an image data is received, a one-line data of the decoded image data received is stored. Then an image formation on a recording medium, such as a recording sheet, etc., is performed by outputting the recorded data to a recording unit 102. A numeral 111 denotes an encoding/decoding unit for encoding an image information to be transmitted by an MH coding, etc. and at the same time, decoding the coded image data received to convert them into an image data. A numeral 112 denotes a buffer memory for storing the coded image data received or to be transmitted. Each of these units in the control unit 101 is controlled by a CPU 113, such as a microprocessor, etc. The control unit 101 comprises a control program for the CPU 113, ROM 114 for storing various data, RAM 115 for temporarily storing various data as a work area for the CPU 113, etc. in addition to the CPU 113. A numeral 102 is the recording unit comprising a thermal line head to cover the entire width of a recording area for performing an image recording on a recording sheet by a thermal transfer method. A numeral 103 denotes an operation unit including keys for various functional instructions and keys for inputting telephone numbers, etc., and 103a, a switch for an operation to instruct the kind of an ink sheet 14 to be used. When the switch 103a is on, it indicates that an ink sheet for multi-printing is currently mounted. When the switch is off, it indicates that an ordinary ink sheet 14 is currently mounted. A numeral 104 denotes an indicating unit which is usually provided in the operation unit 103 for displaying the state, etc. of each functional unit. A numeral 105 denotes a voltage source to supply electric power to the entire system. A numeral 106 denotes a modem (modulator-demodulator); 107, a network control unit (NCU); and 108, a telephone set. Fig. 5 is a view showing the electrical connection between the control unit 101 and the recording unit 102 in the facsimile apparatus of the present embodiment, and the components which are common to the other drawings are represented by the same numbers. The thermal head 13 is a line head. This thermal head 13 comprises a shift register 130 for receiving a one-line portion of the serial recording data 43 from the control unit 101, a latch circuit 131 for latching the data in the shift register 130 in response to latch signals 44, and a heat-generating register 132 comprising a heat-generating element for the one-line portion. The heat-generating register 132 is driven in separate blocks of m numbers identified by numerals 132-1 - 132-m. A numeral 133 denotes a temperature sensor mounted on the thermal head for detecting the temperature of the thermal head 13. The output signal 42 of this temperature sensor 133 is inputted into the CPU 113 after an A/D conversion process in the control unit 101. Thus the CPU 113 detects the temperature of the thermal head 13 to modify the pulse duration of a strobe signal 47 output from a driving circuit 46 in accordance with the temperature or the driving voltage of the thermal head 13, thus regulating the energy applicable to the thermal head 13 corresponding to the characteristic (kind) of the ink sheet 14. The characteristic (kind) of the ink sheet 14 is designated by the switch 103a of said control unit 103. In this respect, the kind, characteristic, etc. of an ink sheet 14 can be identified by a mark printed on the ink sheet 14 or a mark or a notch provided on a cassette or a cartridge of the ink sheet. A numeral 46 is the driving circuit for receiving a driving signal for the thermal head 13 from the control unit 101 to output the strobe signal 47 for driving each block of the thermal head 13. In this respect, the driving circuit 46 varies the duration of an applied voltage to a power source line 45 supplying current to the heat-generating resistor 132 of the thermal head 13 in accordance with an instruction from the control unit 101, so that an energy applicable to the thermal head 13 can be modified. A numeral 22 denotes said recording sheet sensor. A numeral 20 denotes an ink sheet sensor for detecting the presence of the ink sheet 14 and the transporting speed for the ink sheet 14; 31, 32, and 40, driving circuits for respectively driving a motor 24 for conveying recording sheet, a motor 25 for conveying ink sheet, and a motor 41 for winding ink sheet; also, 33, 34, and 35, driver circuits for respectively driving a pick up motor 37, a motor 38 for rewinding ink sheet, and a motor 39 for manuscript and copy. Although each of the motors employed in the present embodiment is a stepping motor, it is not limited thereto. A DC motor, etc. may also be used, for example. Fig. 2 is an enlarged view of the recording unit best showing the feature of the present embodiment. Here the center of the heat-generating resistor 132 of the thermal head 13 is arranged to be positioned in the upstream side in the transporting direction of the ink sheet 14 (downstream side in the transporting direction of the recording sheet 11) by a distance f identified by an arrow b from the central position 206 where the thermal head 13 is pressed by the platen roller 12. Hence, given a recording time for one line as Δt, and a recording length per line as Δℓ, the ink sheet 14 is transported in the direction indicated by an arrow a for a length Δℓ/n (n > l) during a time Δt while the recording sheet 11 is transported in the direction indicated by an arrow b for a length Δℓ. Thus an ink layer of the ink sheet 14 fused (or sublimated) by the heat-generating resistor 132 of the thermal head 13 is transferred onto the recording sheet 11 for recording. Here in Figs. 2 and 3, numerals 61 and 62 denote ink sheet guides to guide the transportation of the ink sheet 14. A numeral 63 denotes a recording sheet guide to guide the transportation of the recording sheet 11. In this respect, both of the guides 62 and 63 are not shown in Fig. 3. [Description of Features of Present Embodiment (Figs. 6 and 7)] Fig. 6 is a view enlarged further showing the vicinity of the heat-generating resistor 132 of the thermal head 13 to explain the present embodiment, and the components common to the other drawings are represented by the same numbers and marks. Ink 90 fused (or sublimated) by the heat-generating resistor 132 is being transported in the direction indicated by an arrow a and rubbed with the recording sheet 11 being transported in the direction indicated by an arrow b simultaneously for transfer recording. A position designated by a numeral 92 is the one where this transfer recording is performed. Here a numeral 206 denotes the center of pressure exerted by the platen roller 12 corresponding to the normal line direction as described earlier, and the position (92) at which ink of the ink sheet 14 is transferred is shifted from the position of the ink sheet 14 being heated to the position in the direction toward the center of pressure where the pressure becomes the greatest. Thus it is possible to perform the transfer accurately without irregularity in the density because ink is transferred to the recording sheet 11 while being pressed in the direction of the pressure becoming increasingly greater. Fig. 7 is a view showing the relationship between the recording density at each of the shifted positions of the heat-generating resistor 132 of the thermal head 13 and the shifting amount thereof when the recording is performed at the positions shifted from its central position. Here the relationship is shown for the case where the recording is performed by transporting the recording sheet 11 and the ink sheet 13 in the opposite direction to each other. As clearly seen in Fig. 7, a higher density is obtainable when the center of the heat-generating resistor 132 is shifted to the upstream side in the transporting direction of the ink sheet 14 (in the transporting direction of the recording sheet 11). Also, it is experimentally ascertained that the best quality of recorded image by transfer is obtainable when the central position of the heat-generating resistor 132 is shifted to the upstream side in the transporting direction of the ink sheet 14 from the center of the pressure 206 exerted between the thermal head 13 and the platen roller 12, most preferably by approximately 0.2 mm - 0.4 mm and preferably by approximately 0.1 mm - 0.6 mm. In this respect, the experiments have been conducted with the present embodiment on condition that the hardness of rubber for the platen roller is approximately 40°, the diameter of the platen roller is approximately 20 mm, and the pressure of the thermal head against the platen roller is approximately 4 kgf. Next, the operation of the facsimile apparatus of the present embodiment based on the above structure will be described with reference to a flowchart shown in Fig. 8. [Description of Operation (Figs. 2-8)] Fig. 8 is a flowchart showing the recording process for a one-page portion in the facsimile apparatus of the present embodiment. For the execution of the process thereof, the control program is stored in a ROM 14 in the control unit 101. This process is started when the recording operation is in a starting state subsequently to the storage of a one-line portion of the image data in the line memory 110 in the control unit 101. Here in this respect, the operation is described for a case where a multi-ink sheet is used as the ink sheet 14. At first, at a step S1 the recorded data for a one-line portion are output to the shift register 130 in serial. Then, when the recorded data for one line are completely transferred, a latch signal 44 is output at a step S2 to store the recorded data for the one-line portion in the latch circuit 131. Next, at a step S3 the motor 25 for conveying ink sheet is driven to transport the ink sheet 14 in the direction indicated by an arrow a in Fig. 2 for a (1/n) line portion. Then at a step S4 the motor 24 for conveying recording sheet is driven to transport the recording sheet 11 in the direction indicated by an arrow b for a one-line portion. In this respect, the length of this one-line portion corresponds to that of one dot to be recorded by the thermal head 13. Subsequently the process proceeds to a step S5 to energize each block of the heat-generating resistor 132 of the thermal head 13. Then, at a step S6 a detection is performed to determine whether or not the entire blocks of number m are energized. When the entire blocks of the heat-generating resistor 132 are energized to complete the recording of the one line, the process proceeds to a step S7 to examine whether or not the recording of one-page portion is completed. If the recording of the one-page portion is not completed, the process proceeds to a step S8 to transport the recorded data for the next line to the thermal head 13 and then returns to the step S2. At a step S7, when the recording of the one-page portion is completed, the process proceeds to a step S9 to transport the recording sheet 11 for a predetermined amount in the direction of the discharging rollers 16a and 16b. Then, at a step S10, cutters 15a and 15b are driven to engage their blades to cut the recording sheet 11 for the one page unit. Subsequently at a step S11, the recording sheet 11 is fed back for a distance corresponding to a space between the thermal head 13 and cutters 15 to terminate the recording process for one page. In this respect, said value n to determine the feeding of the ink sheet 14 can be modified not only by varying the rotational amounts of the motor 24 for conveying recording sheet and the motor 25 for conveying ink sheet but also by the moderating ratios of transmission gears in the driving system of the platen roller 12 and those in the driving system of the rewinding roller 18. Also, if the motor 24 for conveying recording sheet and the motor 25 for conveying ink sheet are both stepping motors, said value can be established by selecting their minimum step angles to set them to be different from each other. Thus it is possible to make the relative speed at which the recording sheet 11 and the ink sheet 14 are transported to be (1 + 1/n)V P . Besides as shown at the step S3 and step S4, it is desirable to actuate the motor 25 for conveying ink sheet for its transportation driving earlier than the motor 24 for conveying recording sheet for its transportation driving. This is because of the fact that there is a time delay before the transportation of ink sheet 14 is actually begun after the motor 25 for conveying ink sheet is driven for the characteristics of the motor and driving transmission system. In this respect, a similar effect can be obtained by actuating the motor for conveying recording sheet earlier, but if it takes a longer time before the actuation of the thermal head 13 (the recording operation shown at the step S4) after the transportation of the recording sheet 11 is begun, there may be brought about disadvantages that an unfavorable space is generated between recorded dots, etc. [Description of Recording Principle (Fig. 9)] Fig. 9 is a view showing the state of an image recording in the present embodiment when the image recording is performed by transporting the recording sheet 11 and the multi-ink sheet 14 in the reverse directions respectively. As shown in Fig. 9, the recording sheet 11 and ink sheet 14 are pinched between the platen roller 12 and the thermal head 13. The thermal head is pressed against the platen roller 12 by a spring 21 under a predetermined pressure. Here, however, the center of the heat-generating resistor 132 is slightly shifted to the upstream side in the transporting direction of the ink sheet 14 against the center of pressure to the platen roller 12 exerted by the spring 21. Then the recording sheet 11 is transported by the rotation of the platen roller 12 in the direction indicated by an arrow b at a speed V P while the ink sheet 14 is transported by the rotation of the motor 25 for conveying ink sheet in the direction indicated by an arrow a at a speed V I . Now, when the heat-generating resistor 132 of the thermal head 13 is energized by current from the voltage source (not shown) and is heated, a portion of the ink sheet 14 identified by slanting lines 81 is heated. Here a numeral 14a denotes a base film of the ink sheet 14 and a numeral 14b, an ink layer of the ink sheet 14. Energizing the heat-generating resistor 132 causes ink in the heated ink layer 81 to be molten, and its portion identified by a numeral 82 is transferred onto the recording sheet 11. This portion 82 of the ink layer to be transferred is equivalent to approximately a 1/n of the ink layer identified by the numeral 81. At this time of transfer, it is necessary to generate a shearing force against ink at the boundary line 83 of the ink layer 14b, so that the portion of the ink layer identified by the numeral 82 is transferred to the recording sheet 11. However, this shearing force tends to vary according to the temperature of the ink layer. The higher the temperature of the ink layer is, the smaller becomes the shearing force. Hence, the shorter the heating period of the ink sheet 14, the greater becomes the shearing force within the ink layer. Consequently, if the relative speed at which the ink sheet 14 and the recording sheet 11 are transported is made faster, it is possible to ensure the removal of the ink layer to be transferred from the ink sheet 14. According to the present embodiment, the heating period for the thermal head 13 of the facsimile apparatus is as short as 0.6 ms, and in order to make the relative speed at which to transfer the ink sheet 14 and the recording sheet 11 faster, the directions in which the ink sheet 14 and the recording sheet 11 are transported respectively are inversed (made opposite). [Description of Ink Sheet (Fig. 10)] Fig. 10 is a cross-sectional view of an ink sheet employed for a multi-printing in the present embodiment. Here it is formed in three layers. First, the second layer is a base film which functions as a supporting substrate for the ink sheet 14. In the case of a multi-printing, a high-thermal resistant aromatic polyamide film or condenser paper is preferably used because heat energy is repeatedly applied to the same position. A conventional polyester film, however, can also withstand this usage. From the viewpoint of a medium which the film must function to serve as its role, although the smaller the thickness is, the better for the printing quality, the thickness of the film should preferably be in a range of 3-8 µm in consideration of its strength. The third layer is an ink layer containing an amount of ink capable of performing transfers to a recording paper (recording sheet) n times. The principal composition of this layer is resin, such as EVA, etc. for adhesion, carbon black or nigrosine dye for coloring, and carnauba wax or paraffin wax as binder, which are mixed in such a manner that the layer is made to withstand the repeated use of n times at the same position. Also, there is a possibility that ink of the third layer is caused to smudge the surface of the recording sheet 11 when it is rubbed with the recording sheet. However, the multi-ink sheet used in the present embodiment is made with a precaution that it is not allowed to perform any transfer to the recording sheet 11 unless the temperature of its ink layer is caused to rise above a certain temperature under a pressure in a range of 3-5 kg. Although the coating amount of this layer should preferably be in a range of 4-8 g/m 2 , it can arbitrarily be selected because its sensitivity and density differ according to the coating amount. The first layer is a heat resistingly coated layer to protect the base film, which forms the second layer, from the heat generated by the thermal head 13. Although this is best suited for the multi-printing in which a heat energy is likely to be applied to a same position n times (when black information continues), its application can arbitrarily be selected. This layer is particularly effective for a base film of comparatively low heat resistivity, such as a polyester film. In this respect, the heating method is not limited to the one using the aforesaid thermal head, but a current supply method or laser transfer method can also be employed, for example. The recording medium is not limited to the recording sheet, either, but it can be any material if only capable of accepting an ink transfer, such as cloth, plastic sheet, etc. Also, the type of the ink sheet is not limited to the one having the roll formation as shown in the embodiment. It can be of a type, for example, using a housing with a built-in ink sheet, so-called ink sheet cassette, etc., which can detachably mounted in the main body of a recording unit. The thermal transfer material employed in the aforesaid embodiment has an ink layer as a supporting substrate made of a mixture of binding and coloring components. As a binder to be used in the ink layer, there can be employed, for example, carnauba wax, paraffin wax, sazol wax, microcrystalline wax, custard wax, and some others, high-class fatty acid such as stearic acid, palmitic acid, lauric acid, aluminum stearic acid, lead stearic acid, or their derivatives such as metallic acid, ester, etc., a kind or a mixture of two or more kinds of conventionally known resins, such as polyamide resin, polyester resin, epoxy resin, polyurethane resin, acrylate resin, etc. As a coloring material, a kind or a mixture of two or more kinds of conventionally known coloring materials are used. They are, for example, carbon black, nigrosine dye, lamp black, Sudan black SM, fast yellow G, benzine yellow, pigment yellow, Indian fast orange, ilgazine red, etc. Also, the thickness of the ink layer should preferably be in a range of 1-25 µm, and further, it should desirably be in a range of 3-15 µm. Here if the thickness of the ink layer is less than 1 µm, there is a possibility that a sufficient density of recording is not obtainable in a double density recording. If it exceeds 25 µm, there occurs an unfavorable possibility that the removal from the supporting substrate and the energy for recording must be increased. As described above, according to the present embodiment, the central position of the heat-generating resistor 132 of the thermal head 13 is arranged in the upstream side in the transporting direction of the ink sheet 14 from the center of pressure exerted to the platen roller 12 to intensify the pressurized contact between the ink sheet 14 and recording sheet 11. Thus it is possible to obtain an effect that the transfer of records is assured without any unevenness and blurring of recording dots. Further, with reference to Fig. 11 and Fig. 12, a second embodiment suited for the present invention will be described. Fig. 11 and Fig. 12 are enlarged views showing the recording unit of a facsimile apparatus using the second embodiment suited for the present invention. In the embodiment given below, the same reference numbers are designated to the same members appearing in the aforesaid embodiment to facilitate the description, and at the same time, the aforesaid Fig. 3-Fig. 5 and Fig. 8-Fig. 10 are occasionally referred to for the same purpose. Now, Fig. 11 is an enlarged view of the recording unit best showing the features of the present embodiment. Here the center of the heat-generating resistor 1132 of the thermal head 1113 is shifted for a distance f to the upstream side in the transporting direction of the ink sheet 14 indicated by an arrow b from the top 206 of the convex part 1113a of the thermal head 1113. As a result, the center of the heat-generating register 1132 of the thermal head 1113 is arranged in a position shifted for a distance f to the upstream side in the transporting direction of the ink sheet 14 indicated by an arrow a (to the downstream side of the transporting direction (indicated by an arrow b) of the recording sheet 11) from the central position 206 where the thermal head 1113 is pressed to the platen roller 12. Hence, given a recording time for one line as Δt, and a recording length per line as Δℓ, the ink sheet 14 is transported in the direction indicated by an arrow a for a length Δℓ/n (n > 1) during a time Δt while the recording sheet 11 is transported in the direction indicated by an arrow b for a length Δℓ. Thus an ink layer of the ink sheet 14 molten (or sublimated) by the heat-generating resistor 1132 of the thermal head 1113 is transferred onto the recording sheet for recording. Fig. 12 is a view further enlarged to show the vicinity of the heat-generating resistor 1132 of the thermal head 1113 to explain the present embodiment, and the components common to the other drawings are represented by the same numbers and marks. Ink 90 molten (or sublimated) by the heat-generating resistor 1132 is being transported in the direction indicated by an arrow a and rubbed with the recording sheet 11 transported in the direction indicated by an arrow b simultaneously for transfer recording. The position of this transfer recording is designated by a numeral 92. Here the numeral 206 denotes the normal line running through the top of the convex part 1113a of the thermal head 1113, and as aforementioned, shows the center of pressure exerted to the platen roller 12 corresponding to the normal line direction. Thus the position (92) at which ink of the ink sheet 14 is transferred is shifted from the position where the ink sheet 14 is heated to the position in the direction toward the center of the pressure where the pressure becomes the greatest. In this way it is possible to perform the transfer accurately without irregularity in its density because ink is transferred to the recording sheet 11 while being pressed in the direction where the pressure becomes greater between the ink sheet 14 and the recording sheet 11. As the above describes, according to the present embodiment, the central position of the heat-generating resistor 1132 of the thermal head 1113 is arranged in a position shifted to the upstream side in the transporting direction of the ink sheet 14 from the top of the convex part 1113a of the thermal head 1113 to intensify the pressure between the ink sheet 14 and the recording sheet 11, so that any unevenness and blurring of the recorded dots are eliminated, making it possible to obtain an effect that the transfer of records is assured. Also, it should be understood that although the description of the present embodiment has been made of a recording apparatus of full-line type, the present embodiment is not limited thereto, but it is also applicable to a recording apparatus of serial type. In each of the embodiments described, the description has been made of an application of the thermal transfer recording apparatus to a facsimile apparatus, but the present invention is not limited to this application. It is readily understandable that the thermal transfer recording apparatus of the present invention is also applicable to word processors and typewriters, for example. Also, in the present embodiment, the description has been made of a case where a multi-ink sheet is employed. The present invention is not limited thereto, but it is equally practicable when a usual one-time sheet is employed. As set forth above, according to the present invention, a recording of a high-quality image is possible without any unevenness and blurring thereof.

Claims: A thermal transfer recording apparatus for image recording by transferring ink contained in an ink sheet (14) onto a recording medium (11) in contact therewith, said ink sheet (14) and said recording medium (11) being supported by a curved surface of a platen (12), comprising: means (18) for transporting the ink sheet to a recording position, as well as means (12) for transporting the recording medium thereto, and recording means for acting, in the recording position, on the ink sheet by means of a thermal head, which comprises a heat-generating resistor (132), thereby recording an image on the recording medium, said heat generating resistor (132) being provided on the top of a convex portion of said thermal head, the recording medium is transported in the direction (b) opposite to that of the ink sheet, characterized in that the center of said resistor (132) is shifted by a predetermined length from the center (92) of pressure exerted by the curved surface of the platen (12) corresponding to the normal line direction to the upstream side in transporting direction (a) of the ink sheet. Apparatus as claimed in claim 1, wherein the thermal head is provided with a convex part faced to the ink sheet, with its heat-generating resistor being shifted by a predetermined distance from the top of said convex part to the upstream side in said ink sheet transportation direction (a). Apparatus according to claim 1 or 2, wherein the length of transportation of the ink sheet is made to be 1/n of the length of transportation of the recording medium, with n > 1, when using an ink sheet capable of performing n ink transfers from a same position. A recording apparatus with a plurality of electrothermal converters arranged in series, comprising: a recording head having said electrothermal converters arranged to be shifted by a predetermined distance from the top of a convex part of the recording head for pressing an ink sheet and a recording medium against a supporting means, means for transporting said recording medium or said recording head so that the relative transporting direction of said recording medium is from said top of the convex part of said recording head toward said electrothermal converters, means for transporting said ink sheet so that the relative transporting direction of said ink sheet is from the electrothermal converters toward the top of the convex part of the recording head. Apparatus according to claim 4, wherein the length of transportation of the ink sheet is made to be 1/n (n > 1) of the length of transportation of the recording medium, when using an ink sheet capable of performing n ink transfers from a same position. A recording apparatus according to claim 4 or 5, wherein the directions in which said ink sheet and said recording medium are transported are opposite to each other.

Inventors: AKIHIRO TOMODA, MAKOTO KOBAYASHI, MASAKATSU YAMADA, MASAYA KONDO, MINORU YOKOYAMA, SATOSHI WADA, TAKAHIRO KATO, TAKASHI AWAI, TAKEHIRO YOSHIDA, TAKESHI ONO, TOMOYUKI TAKEDA, YASUSHI ISHIDA, AKIHIRO, TOMODA, MAKOTO, KOBAYASHI, MASAKATSU, YAMADA, MASAYA, KONDO, MINORU, YOKOYAMA, SATOSHI, WADA, TAKAHIRO, KATO, TAKASHI, AWAI, TAKEHIRO, YOSHIDA, TAKESHI, ONO, TOMOYUKI, TAKEDA, YASUSHI, ISHIDA, AKIHIRO, TOMODA, TECHN. ENG., C/O CANON K. K., MAKOTO, KOBAYASHI, TECHN. ENG., C/O CANON K. K., MASAKATSU, YAMADA, TECHN. ENG., C/O CANON K. K., MASAYA, KONDO, TECHN. ENG., C/O CANON K. K., MINORU, YOKOYAMA, TECHN. ENG., C/O CANON K. K., SATOSHI, WADA, TECHN. ENG., C/O CANON K. K., TAKAHIRO, KATO, TECHN. ENG., C/O CANON K. K., TAKASHI, AWAI, TECHN. ENG., C/O CANON K. K., TAKEHIRO, YOSHIDA, TECHN. ENG., C/O CANON K. K., TAKESHI, ONO, TECHN. ENG., C/O CANON K. K., TOMOYUKI, TAKEDA, TECHN. ENG., C/O CANON K. K., YASUSHI, ISHIDA, TECHN. ENG., C/O CANON K. K.
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1990
Main classification: B41J2
IPC codes: B41J2