Abstract:
In thermal ink transfer printing, a layer of thermally fusable binder is deposited on a recording sheet prior to the transfer print of thermally fusable ink from a transfer sheet to provide a strong bond between the underlying recording sheet and the overlying ink deposits. In one embodiment, layers of thermally fusable ink and binder material are deposited on the base of a common transfer sheet in alternate fashion. The binder layer is initially positioned above a thermal head and the latter is heated so that the binder is fused and transferred to the recording sheet. The recording sheet and transfer sheet are advanced together with respect to the head so that the recording sheet is entirely covered with a layer of the binder material. Thereafter, the binder-coated recording sheet comes into contact with the ink layer and the thermal head is controlled to heat portions of the ink layer for transfer to the binder-coated surface of the recording sheet.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a thermal ink transfer printer. 
     Thermal printing employing a thermal ink transfer sheet is known in the art. In this type of printing, the transfer sheet is laminated with a recording sheet of paper and heated by a thermal head so that portions of ink are fused and transferred from the transfer sheet to the recording sheet. A shortcoming inherent in this type of printing resides in the fact that since the ink has no strong bond to paper, small dots of transferred ink tend to be easily displaced from the recording sheet. The disadvantage is particularly severe in the case of printing color images where layers of different color ink are printed one upon another since the upper layers tend to be more easily displaced than is the lowermost layer. 
     SUMMARY OF THE INVENTION 
     The invention contemplates to apply a coating of a binder material to the surface of a recording sheet prior to thermal printing operation. 
     According to the invention, there is provided a method for printing an image on a recording sheet in response to a video signal. The method comprises the steps of feeding a thermal ink transfer sheet with the recording sheet, the thermal ink transfer sheet having a base and a layer of thermally fusable ink deposited thereon, and simultaneously therewith heating successive portions of the ink transfer sheet in response to the video signal to fuse the portions of ink and transferring the fused ink portions from the base to the recording sheet. The invention is characterized by the step of applying a coating of a binder on the recording sheet prior to the step of heating the ink transfer sheet by heating successive portions of a layer of a thermally fusable binding material deposited on a base, fusing the portions of binding material and transferring the fused binding portions from the last-mentioned base to the recording sheet to allow the fused ink portions to be deposited on the coating. 
     In a preferred embodiment, a layer of thermally fusable binding material is deposited on a common transfer sheet on which a layer of thermally fusable ink is also deposited. The binding layer is positioned above the thermal head and the latter is first heated so that the binding material is fused and transferred to the recording sheet. The platen is rotated so that the start line of the recording sheet comes into alignment both with the start line of the ink layer and the thermal head. The latter is controlled so that it heats portions of the ink layer to effect the transfer of the heated portions to the binder-coated surface of the recording sheet. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described in further detail with reference to the accompanying drawings, in which: 
     FIG. 1 is an illustration of a printing apparatus according to a first embodiment of the invention; 
     FIG. 2 is an illustration of a portion of a transfer sheet on which layers of ink and binder are arranged in alternate fashion; 
     FIG. 3 is an illustration of a portion of a modified form of the transfer sheet of FIG. 2; 
     FIG. 4 is an illustration of a portion of a transfer sheet for color printing; 
     FIG. 5 is an illustration of a printing apparatus of a second embodiment employing the transfer sheets of FIGS. 2, 3 and 4, and FIG. 5a is an illustration of the printing operation of the apparatus of FIG. 5; and 
     FIG. 6 is a block diagram of a typical example of the control circuit for controlling the thermal head of FIG. 5. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, there is shown a thermal ink transfer printer according to an embodiment of the invention. The printer comprises a thermal head 1 located below a platen 2 on which is wound a recording sheet of paper 3. The platen 2 is driven by a motor, not shown, in a clockwise direction and a thermal-ink transfer sheet 7 is transported in a direction indicated by the arrow A from a supply reel 8a to a takeup reel 8b. 
     According to the invention, a binder applying device is provided which includes a supply reel 5a which supplies a thermal binder transfer sheet 4 to a takeup reel 5b which is driven by a motor, not shown. This binder sheet comprises a polyester base 4a having a thickness of 6 micrometers and a transparent layer of thermally fusable binder 4b deposited on the base 4a to a thickness of 6 micrometers. A suitable material for the thermally fusable binder 4b is a compound comprising 45 weight percent of carnauba wax and 55 weight percent of lubricating oil (copolymer such as polyvinyl acetate, polystylene and stylene-butadiyne). Such a binder material has a melting point of 60 degrees centigrade. 
     A thermal roller 6 is located between the reels 5a and 5b and is movable from a nonworking position to a working position in which it presses the binder sheet 4 into contact with the recording sheet 3 as illustrated. The thermal roller 6 is coated by a nonsticking material such as Teflon (trademark) and includes a resistance heater element which heats the surface of the roller 6 to allow the binding material 4b to fuse. The fused binder detaches from the base 4a and sticks to the recording sheet 3. 
     The thermal-ink transfer sheet 7 comprises a polyester base 7a, 6 micrometers thick and a layer of thermally fusable ink 7b deposited on the base 7a to a thickness of 3 micrometers. The fusable ink is a compound which comprises typically 25 weight percent of carnauba wax, 50 weight percent of pigment and 25 weight percent of lubricating oil of the kind mentioned above. The ink is fusable at a temperature of 70 degrees centigrade. 
     The ink transfer sheet 7 is brought into contact with the surface of the binder 4b which has been applied to the recording sheet 3 when the thermal head 1 moves into contact with the ink transfer sheet 7. The thermal head 1 extends across the width of the ink transfer sheet and includes an array of resistance elements. These resistance elements are heated by currents supplied from a control circuit which will be described later. 
     The heated portions of the ink transfer sheet 7 are fused and detach from the base 7a and stick to the binder-coated recording sheet as shown at 7b&#39;. The binder 4b provides a strong bond between the underlying sheet 3 and the overlying ink deposits 7b&#39;. 
     Preferably, the surface of the ink deposits 7b&#39; is covered with an additional binder layer to prevent them from detaching from the underlying binder layer 4b. This is accomplished by further rotating the platen 2 while continuing the operation of the binder applying device after the printing is completed. 
     In a modified embodiment, a transfer sheet as shown in FIG. 2 is employed instead of the binder applying device of FIG. 1. A transfer sheet 9 comprises a plurality of thermally fusable binder layers 9a and thermally fusable ink layers 9b which are deposited on the polyester base alternately along the length of the sheet. The transfer sheet 9 is supported on the supply and takeup reels 8a and 8b in an arrangement shown in FIG. 5. 
     The binder layer 9a is first positioned so that its start line is located above the thermal head 1 and aligned with the start line of the recording sheet 3. All the heating elements of the thermal head 1 are heated while the platen 2 rotates a first full turn to transfer the binder layer 9a to the recording sheet 3 wound on the platen 2. The start line of the ink layer 9b is again positioned on the thermal head 1 and the latter is activated in response to control signals to effect the transfer of ink to the binder-coated surface as shown in FIG. 5a. 
     Alternatively, a thermal roller may be provided to transfer the binder layer 9a. This thermal roller is heated and brought into contact with the recording sheet while the platen 2 turns a first full revolution and then moved to a retracted position to allow the thermal head 1 to be moved to the print position. 
     A transfer sheet shown in FIG. 3 is to provide a binder coat on the printed surface. Such coatings serve to prevent the ink deposits from detaching from the underlying binder layer. This transfer sheet differs from the sheet of FIG. 2 in that it includes an additional binder layer 9a&#39; between an ink layer 9b which is used to print an image on a given sheet 3 and a binder layer 9a which is used to coat the next sheet. The additional binder layer 9a&#39; is transferred to the printed surface of the given sheet by heating all the resistance elements of the head 1 at the end of the print operation. 
     FIG. 4 is an illustration of a further embodiment of the transfer sheet which is used for providing a color print. The transfer sheet 9&#39; includes a plurality of color ink layers 9c and a plurality of binder layers 9a each of which is arranged between different ones of the color ink layers so that a binder coat may be provided between the recording sheet 3 and a layer of given color deposits and between layers of different color deposits. 
     FIG. 6 is an illustration of a circuit arrangement for controlling the thermal head 1. For purposes of illustration, the thermal head 1 is provided with a linear array of 512 rectangular shaped resistance elements R 1  through R 512 . First terminals of these resistance elements are coupled together to a lower voltage source 31 or a higher voltage source 32 through a changeover switch 33 and their second terminals are coupled respectively to the collectors of transistors 21 1  to 21 512  whose emitters are connected to ground. The sheet edge detector 30 is located adjacent to the platen 2 to detect the start line of the recording sheet 3. A binder mode circuit 34 receives an output signal from the edge detector 30 to apply a binary 1 to all the bases of the transistors 21 1  through 21 512  for coupling all the second terminals of resistance elements to ground. At the same time the circuit 34 provides a switching signal to the switch 33 to connect the lower voltage source 31 to the first terminals of the resistance elements. All the resistance elements are thus heated to a temperature sufficient to fuse the binder, which is 60° C. as noted above. With the start line of a binder layer 9a being located above the thermal head 1, the rotation of the platen 2, FIG. 5, causes the binder layer 9 a to be fused and transferred to the recording sheet 3. When the binder layer 9a has been transferred to the sheet 3, the sheet edge detector 30 issues a signal to cause the binder mode circuit 34 to remove the binary 1&#39;s from the bases of transistors 21 and the switching signal from the switch 33 so that the higher voltage source 32 is coupled to the resistance elements. The apparatus is now ready to operate in a print mode in which the resistance elements are heated to a temperature sufficient to fuse the ink layer. 
     A signal source 10, which may be a television receiver or a video tape recorder or the like, supplies an analog television signal to an analog-digital converter 11 where the signal is sampled in response to clock pulses from source 15 and converted to a digital video sample of 6-bit data word, which is applied on parallel bit lines 12 to a data memory 13. The data memory 13 comprises a random access memory having a matrix of cell locations arranged in a pattern of 512 rows by 512 columns. The digital samples of each television line scan are sequentially written into the memory 13 in the direction of rows to store a complete picture frame. 
     After the complete picture frame has been stored in the memory 13, 512 digital samples are sequentially read from each row in response to a 9-bit address word generated by a 9-bit column address counter 14 in response to the clock pulse. As will be described, the digital samples of each row are repeatedly read out 64 times for comparison with a digital reference value to determine the tone value of the original picture element and thereupon the reading operation is shifted to the next row in response to a 9-bit row address code supplied from a 9-bit row address counter 16. 
     When 512 clock pulses have been counted the column address counter 24 supplies a carry or &#34;full count&#34; signal to a 6-bit reference counter 17 to increment its count from the initial value &#34;000000&#34; to the maximum of &#34;111111&#34; in response to each readout of 512 video samples. This reference counter can be considered as a digital sawtooth generator since its output stepwisely follows the waveform of a sawtooth. This time-varying 6-bit digital reference is supplied to a digital comparator 18 for comparison with the 6-bit data word read out of memory 13. The comparator 18 generates a logical one output if the digital sample has a tone value greater than the reference value and generates a logical zero output if the tone value is equal to or smaller than the reference value. When the maximum reference value &#34;111111&#34; is reached, the reference counter 17 applies a full-count signal to the row address counter 16 to shift the readout operation to the next row. The logical ones and zeros from the comparator 18 are clocked into the data input terminal of a shift register 19 having 512 bit positions. 
     It will be appreciated therefore that the shift register 19 is loaded with a random sequence of 512 binary l&#39;s and 0&#39;s depending on the relative values of the data words retrieved from the 512 cell locations of a given row to an instantaneous value of the digital reference. The binary data stored in the shift register 19 are transferred through a latch 20 to the bases of transistors 21 1  through 21 512 , respectively, in response to the carry signal from the column address counter 14 which is also supplied to the reference counter 17. The transistors 21 have their emitters coupled to ground and their collectors coupled to resistance elements R 1  to R 512  respectively. Those transistors which are supplied with binary 1&#39;s from the latch 20 are gated into conduction to generate currents in the associated resistance elements. 
     The reading operation begins with the application of a start signal applied to terminals 22 and 23 which may be derived from the binder mode circuit 34. This start signal resets the counters 14, 17 and shift register 19 to clear any stored contents. The column address counter 14 is incremented by the clock source 15 to address the data memory 13 to read out a series of 512 digital samples in succession out of the No. 1 row into the digital comparator 18. The same digital samples are read out 64 times in response to the clock pulse. In synchronism with these reading operations the reference counter 17 is incremented from the minimum value to the maximum value. The output of the digital comparator 18 is 64 series of 512 binary 1&#39;s and 0&#39;s, with each series being applied to the thermal head 1 in response to each count of 512 clock pulses. The ink layer 5 is fused in positions corresponding to the heated resistance elements, the fused portions of ink being transferred to the recording sheet 3 as described above. More specifically, a series of rectangles is produced in a direction perpendicular to the direction of advancement of sheet 3 for each of 64 iterative reading operations. With a continued advancement of recording sheet 3 and transfer sheet 4, 64 series of rectangles. 
     The length of each rectangle as measured in the longitudinal direction of the paper 3 varies depending on the density of the original picture element. 
     The foregoing description shows only preferred embodiments of the present invention. Various modifications are apparent to those skilled in the art without departing from the scope of the present invention which is only limited by the appended claims. Therefore, the embodiments shown and described are only illustrative, not restrictive.