PRINTING METHOD OF THERMAL PRINTER

Abstract: When a print line is printed on a printing paper by supplying a print current (C101 to C104) to a thermal head (4), heating elements (THa1 to THdn) within the thermal head and a transfer ribbon (13) are easily adhered together due to melted ink on surface of the transfer ribbon. Due to such adhering between the heating elements and the transfer ribbon, a white line is easily formed on a printing paper (14). In order to prevent such white line from being formed, the heating elements must be prevented from being cooled down after printing each print line. More specifically, in a period between a first time (t1) when each print line is completely printed and a second time (t4) when a printing paper is to be transported forward to a next print line, the heating elements are heated by relatively small heating value which is sufficient to melt a adhering portion formed between the heating elements and the transfer ribbon but which is insufficient to perform the printing. Thus, the heating elements are prevented from being adhered to the transfer ribbon, so that each print line can be printed with accuracy and without forming the white line.

Description: The present invention generally relates to a printing method of thermal printer, and more particularly to a printing method by which a printing quality of thermal printer can be improved. Fig. 1 is a block diagram showing an electric constitution of conventional thermal printer. In Fig. 1, 1 designates a line buffer for storing print data DB which have been subjected to a dot conversion, and 2 designates a control section having a micro processing unit (MPU), a working memory and a program memory. This control section 2 has a function for reading out the print data DB stored in the line buffer 1 and another function for inputting control signals and data into several kinds of circuits which will be described later. In addition, 3 designates an interface circuit which executes a communication of data between the control section and an external device (not shown; a micro computer, for example). Further, 4 designates a thermal head consisting of a shift register circuit 5, a latch circuit 6, a driver circuit 7 and a heating body 8. The shift register circuit 5 is constituted by a serial-in-parallel-out shift register, and the shift register circuit 5 reads the print data DB outputted from the control section 2 based on a clock signal CLK and then outputs the read print data DB to the latch circuit 6. The latch circuit 6 reads the output of shift register circuit 5 based on a latch signal DR outputted from the control section 2 and then outputs the read output of shift register circuit 5 to the driver circuit 7. This driver circuit 7 consists of four blocks, i.e., four drivers 7a to 7d. The driver 7a consists of NAND gates Ga1 to Gan, the driver 7c consists of NAND gates Gc1 to Gcn, and the driver 7d consists of NAND gates Gd1 to Gdn. Each of first input terminals of these NAND gates is connected to each output terminal of the latch circuit 6, while second input terminals of the NAND gates within each block (or each driver) are connected together in common. The heating body 8 consists of heating elements THa1 to THan, THb1 to THbn, THc1 to THcn and THd1 to THdn. Each of first terminals of these heating elements is connected to the output terminal of corresponding NAND gate within the drivers 7a to 7d, while second terminals of these heating elements are all connected in common to a positive power source +V. Next, 9 designates a timer circuit. When the control section 2 supplies common pulses CM1 to CM4 to the timer circuit 9, the timer circuit 9 sequentially generates current-on pulse signals C1 to C4 each having a pulse width W1 corresponding to current-on data TD supplied from the control section 2. These current-on pulse signals C1 to C4 are sequentially generated by predetermined intervals. Each of these pulse signals C1 to C4 is outputted to the common connection point between the second input terminals of the NAND gates within each driver. In Fig. 1, 10 designates a motor drive circuit which drives a pulse motor (or a step motor) 11 by one pulse based on a control signal MC supplied from the control section 2. This pulse motor 11 revolves a platen roller 12. In such thermal printer which is constituted as described heretofore, the control section 2 inputs the print data DB outputted from the external device via the interface circuit 3, and then the control section 2 stores the inputted print data DB in the line buffer 1. Next, the control section 2 supplies first print data DB(1) for printing a first print line to the shift register circuit 5 in synchronism with the clock signal CLK. In addition, the control section 2 supplies the current-on data TD to the timer circuit 9. When the first print data DB(1) have been stored in the shift register circuit 5, the control section 2 supplies the latch signal DR to the latch circuit 6 to thereby keep the first print data DB(1) in the shift register circuit 5. At the same time, the control section 2 supplies second print data DB(2) to the shift register circuit 5. Next, the control section 2 sequentially outputs the common pulses CM1 to CM4 to the timer circuit 9 by the predetermined intervals, so that the timer circuit 9 sequentially generates current-on pulses C11 to C41 show in Fig. 2. Each of these current-on pulses C11 to C41 is supplied to each common connection point of the NAND gates within each driver. Due to these current-on pulses C11 to C41, the output terminal of NAND gate whose first input terminal is at "1" level becomes "0" level. As a result, the current is flown through the heating element connected with the NAND gate whose output terminal is at "0" level. In this case, the area corresponding to the NAND gate whose output terminal is at "0" level is printed, but the area corresponding to the NAND gate whose output terminal is at "1" level is not printed. Thus, the printing of first print line will be executed. After this printing of first print line is completed, the control section 2 drives the pulse motor 11 so as to transport a printing paper forward by one step. In this case, a period T1 shown in Fig. 2 designates a period between a first time when the control section 2 supplies the control signal MC to the motor drive circuit 10 and a second time when the pulse motor 11 actually starts to revolve and then completes revolution of one step. Thereafter, the similar printing operation as described heretofore is repeatedly performed on the print data DB(2) to DB(N), so that printing of one page will be completed. Next, description will be given with respect to detailed printing process of thermal transfer type thermal printer in conjunction with Fig. 3. In Fig. 3, a transfer ribbon 13 and a printing paper 14 piled together are inserted between the thermal head 4 and the platen roller 12. In this case, the heating element THa1 arranged at a center portion of the edge end of thermal head 4 presses the transfer ribbon 13. This heating element THa1 is heated in a printing mode so that ink painted on the transfer ribbon 13 will be melted and then the melted ink will be adhered to the printing paper 14. Thus, the thermal transfer is performed. Meanwhile, in the case where the conventional thermal printer performs the printing when surrounding temperature is relatively low, white lines (or spaces) must be formed on the printed paper in a print line direction so that a phenomenon (so-called sticking phenomenon) in which the whole printed result must become whity will be occurred. Fig. 4 shows an example of printed result when the sticking phenomenon is occurred. In Fig. 4, a print line N+1 is shifted from a predetermined printing position and certain part thereof is printed over a print line N, so that interval portion between the print lines N+1 and N+2 must become large. Therefore, such interval portion can be seemed as the white line. Next, description will be given with respect to the cause for occurring the sticking phenomenon. As described above, this sticking phenomenon is occurred when the surrounding temperature is low. The cause of sticking phenomenon will be explained as follows. When the surrounding temperature is low, the control section 2 must widen the pulse widths of current-on pulse signals C1 to C4 in order to raise the heating temperature of each heating element to predetermined printing temperature. On the other hand, in the case where the pulse widths of these pulse signals C1 to C4 are widen, the heating elements which are supplied with the current-on pulses in initial orders must be cooled down. For this reason, after the ink on the surface of transfer ribbon is melted due to the heat of heating element, the melted ink is cooled and then adhered to the heating element. Therefore, when the printing paper 14 is driven by one step after the printing of one print line is completed, the printing paper can not be transported forward by a predetermined distance. As a result, the interval distances between the print lines will become irregular. As described above, in the case where the surrounding temperature is low, the transfer ribbon is adhered to the heating elements of thermal head so that the printing paper can not be transported forward in normal manner. Hence, the conventional thermal printer suffers a problem in that the white lines must be formed in the print direction of thermal head so that the whole printed result must become whity. In a device disclosed in US 4449136 an unevenness in recording density resulting from an influence of waste heat was proposed to be dealt with by providing a waste heat supplement signal of uniform pulse width during an idle time between adjacent line recording operations, the thermal head being excited periodically with impressed energy to a degree so as not to cause recording during the idle time between line recording operations. In EP 260917, published on 23.03.88 claiming priority from 19.09.86 and 09.10.86, desired printing density is obtained through one of two current on time characteristics being selected based upon the temperature of the thermal head which temperature is detected by a thermistor provided within said thermal head. Accordingly, it is a primary object of the present invention to provide a printing method of thermal printer by which the white lines are not formed on the printing paper in the print direction of thermal head even when the surrounding temperature is low. In a first aspect of the invention, there is provided a printing method of thermal printer in which a print current is flown into a thermal head to thereby print each print line so that a printing will be performed, comprising generating and supplying a current to the thermal head in a period between a first time when each print line is completely printed and a second time when a printing paper is to be transported forward to the next print line, said current being insufficient to perform the printing but wherein said current has a heating value which is sufficient to melt a adhering portion formed between heating elements of the thermal head and a transfer ribbon. In a second aspect of the invention, there is provided a printing method of thermal printer in which a print current is flown into a thermal head to thereby print each print line so that a printing will be performed, comprising generating a current which is insufficient to perform the printing, and supplying the current to the thermal head in a period between a first time when each print line is completely printed and a second time when the thermosensible paper is to be transported forward to a next print line, wherein said current has a heating value which is sufficient to melt a adhering portion formed between heating elements of the thermal head and a thermosensible paper. In a third aspect of the invention, there is provided a thermal printer which performs a printing by use of a thermal head including a plurality of heating elements with transporting a printing paper forward by each print line by driving a pulse motor which revolves a platen roller, the thermal printer comprising: (a) control means for generating a common pulse and current-on data; and (b) means for generating a current-on pulse signal in response to the common pulse and the current-on data, the current-on pulse signal consisting of a first pulse having a first pulse width corresponding to a first heating value for performing the printing and a second pulse having a second pulse width corresponding to a second heating value which is sufficient to melt a adhering portion formed between the heating elements and a transfer ribbon or a thermosensible paper but which is insufficient to perform the printing, the current-on pulse having the first pulse being supplied to a driver of the thermal head so that each print line will be printed when the printing is performed, and the current-on pulse having the second pulse being supplied to the driver of the thermal head so that the printing will be prevented from being performed in a period between a first time when each print line is completely printed and a second time when a printing paper or said thermosensible paper is to be transported forward to a next print line. Further objects and advantages of the present invention will be apparent from the following description, reference being had to the accompanying drawings wherein a preferred embodiment of the present invention is clearly shown. In the drawings: Fig. 1 is a block diagram showing an electric constitution of conventional thermal printer; Fig. 2 shows waveforms for explaining the printing operation of the conventional thermal printer; Fig. 3 is a side view showing a main part of conventional thermal printer for explaining the problem of conventional thermal printer; Fig. 4 shows an example of printed result for explaining the sticking phenomenon; Fig. 5 is a block diagram showing an electric constitution of thermal printer adopting the printing method according to the present invention; and Fig. 6 shows waveforms for explaining the present printing operation. Next, description will be given with respect to an embodiment of thermal printer adopting the printing method according to the present invention in conjunction with Figs. 5 and 6. Fig. 5 is a block diagram showing an embodiment of thermal transfer type thermal printer adopting the present invention. In this embodiment of Fig. 5, parts corresponding to those shown in Fig.1 are designated by the same numerals, and description thereof will be omitted. In Fig. 5, 100 designates a control section having the MPU, the working memory and the program memory. In the present embodiment, the timer circuit 9 outputs current-on pulse signals C101 to C104 as shown in Figs. 6(a) to 6(d) under control of this control section 100. These current-on pulse signals C101, C102, C103, C104 respectively include current-on pulses C11 and C111, C21 and C121, C31 and C131, C41 and C141, which will be described later. This control section 100 has the following function in addition to the functions of the control section 2 described in Fig. 1. More specifically, this control section 100 has the function for controlling the timer circuit 9 to generate current-on pulses C111 to C141 each having a pulse width W2 in a period between a time when the printing of one print line is completed and a next time when the printed paper is driven to be transported forward by one step. This pulse width W2 corresponds to the heating value which is sufficient to melt the ink on the surface of transfer ribbon but which is insufficient to perform the printing. Next, detailed description will be given with respect to timings for generating the current-on pulses C111 to C141 each having the above pulse width W2 in conjunction with Fig. 6. When the printing of one print line is completed, the control section 100 drives the pulse motor 11 to thereby revolve the platen roller 12 in order to transport the printing paper forward by one pitch distance. In this case, there must be a mechanical response delay between a time when the pulse motor 11 is started to be driven and a next time when the platen roller 12 is actually revolved. By considering such response delay period T3, the current-on pulse having the pulse width W2 is generated. More specifically, such current-on pulse must be generated at a time t2 just before a time t3 when the response delay period T3 has been passed and then the platen roller 12 is revolved as shown in Fig. 6(a). On the contrary, when the current-on pulse having the pulse width W2 is generated in an initial period of response delay period T3, the heating elements must be cooled so that the melted ink on the surface of transfer ribbon will become hard again and then adhered to the heating elements. This is why the current-on pulse must be generated at the time t2. Next, description will be given with respect to the printing operation of the present embodiment having the above-mentioned control section 100. At first, the control section 100 supplies the first print data DB(1) to the shift register circuit 5 in synchronism with the clock signal CLK. Next, the control section 100 supplies the current-on data TD1 to the timer circuit 9. Further, the control section 100 supplies the latch signal DR to the latch circuit 6 to thereby hold the first print data DB(1), and the control section 100 sequentially supplies the common pulses CM1 to CM4 to the timer circuit 9 by predetermined intervals. Then, the timer circuit 9 sequentially generates the current-on pulses C11 to C41 each having the pulse width W1 designated by the current-on data TD1 in accordance with the timings designated by the common pulses CM1 to CM4 as shown in Fig. 6, and these pulses are respectively supplied to the common connection points of the NAND gates within the driver circuit 7. Thus, the current is flown through the heating element connected to the NAND gate whose output terminal is at "0" level, and then this heating element will be heated. As a result, the printing of first print line will be completed. At this time, the control section 100 sequentially supplies the common pulses CM101 to CM104 to the timer circuit 9 by predetermined intervals at the time t2 just before the time t3 when the platen roller 12 is driven, so that the timer circuit 9 will sequentially generate the current-on pulses C111 to C141 each having the pulse width W2 designated by current-on data TD2 in accordance with the timings designated by the common pulses CM101 to CM104. Thus, the current is flown through the heating element connected to the NAND gate whose output terminal is at "0" level, and then this heating element will be heated. Next, the platen roller 12 is driven by one pitch distance in a period T4 between times t3 and t4 after the timer circuit 9 generates the current-on pulse C141. Thereafter, similar printing operation will be repeatedly performed with respect to the print data DB(2) to DB(N), so that the printing of one page will be completed. As described heretofore, at every time when the printing of each print line is completed, the current-on pulse having the pulse width by which the printing can not be performed is supplied to the heating elements just before the printing paper is driven by one step. Hence, the present invention can prevent the adhering state between the heating elements and the surface of transfer ribbon from being occurred. Above is a description of one embodiment of the present invention. It will be appreciated that this invention may be practiced or embodied in other ways without departing from the scope or essential character of the invention. In the present embodiment, the pulse width of each of current-on pulses C111 to C141 is set constant. However, cooling rate of each heating element must be increased in an order for sequentially supplying the current-on pulses C11 to C41. In order to compensate such cooling rate with accuracy, it is possible to set that the pulse width will become smaller in an order of current-on pulses C111, C121, C131 and C141. Instead of varying the pulse widths of current-on pulses C111 to C141, it is possible to vary amplitudes thereof. In addition, the present invention is applied to the thermal transfer type printer in the present embodiment. However, it is possible to similarly apply the present invention to a thermal printer using a thermosensible paper which consists of coloring layer and basic paper. In this coloring layer, a printing image is formed by applying the heat thereto by use of the thermal head. This coloring layer is formed on the basic paper. After all, the preferred embodiment described herein is illustrative and not restrictive, the scope of the invention being indicated by the appended claims and all variations which come within the meaning of the claims are intended to be embraced therein.

Claims: A printing method of thermal printer in which a print current (C11 to C41) is flown into a thermal head (4) to thereby print each print line so that a printing will be performed, comprising generating a current (C111 to C141) which is insufficient to perform the printing, supplying said current to said thermal head in a period between a first time (t1) when each print line is completely printed and a second time (t4) when a printing paper (14) is to be transported forward to a next print line, characterised in that said current has a heating value which is sufficient to melt an adhering portion formed between heating elements (THa1 to THdn) of said thermal head and a transfer ribbon (13). A printing method of thermal printer according to claim 1, wherein said current (C111 to C141) consists of a pulse signal having a pulse width (W2) which corresponds to said heating value which is sufficient to melt said adhering portion formed between said heating elements of said thermal head and said transfer ribbon but which is insufficient to perform the printing. A printing method of thermal head according to claim 1, wherein said heating elements within said thermal head are divided into some groups, each of which is sequentially supplied with said current so that said heating elements will be sequentially heated by said heating value by each group. A printing method of thermal printer in which a print current (C11 to C41) is flown into a thermal head (4) to thereby print each print line so that a printing will be performed, comprising generating a current (C111 to C141) which is insufficient to perform the printing, supplying said current to said thermal head in a period between a first time (t1) when each print line is completely printed and a second time (t4) when a thermosensible paper is to be transported forward to a next print line, characterised in that said current corresponds to a heating value which is sufficient to melt a adhering portion formed between heating elements of said thermal head and said thermosensible paper. A printing method of thermal printer according to claim 4, wherein said current (C111 to C141) consists of a pulse signal having a pulse width (W2) which corresponds to said heating value which is sufficient to melt said adhering portion formed between said heating elements of said thermal head and said thermosensible paper but which is insufficient to perform the printing. A printing method of thermal head according to claim 4, wherein said heating elements within said thermal head are divided into some groups, each of which is sequentially supplied with said current so that said heating elements will be sequentially heated by said heating value by each group. A printing method of thermal printer according to claim 1 or 4, wherein said heating value is controlled by changing an amplitude of said print current. A thermal printer which performs a printing by use of a thermal head (4) including a plurality of heating elements (THa1 to THdn) with transporting a printing paper (14) forward by each print line by driving a pulse motor (11) which revolves a platen roller (12), said thermal printer comprising: (a) control means (100) for generating a common pulse (CM1 to CM4; CM101 to CM104) and current-on data (TD1 and TD2); and (b) means (9) for generating a current-on pulse signal (C101 to C104) in response to said common pulse and said current-on data, said current-on pulse signal consisting of a first pulse (C11 to C41) having a first pulse width (W1) corresponding to a first heating value for performing the printing and a second pulse (C111 to C141) having a second pulse width (W2) corresponding to a second heating value which is sufficient to melt a adhering portion formed between said heating elements and a transfer ribbon (13) or a thermosensible paper but which is insufficient to perform the printing, said first pulse being supplied to a driver (7) of said thermal head so that each print line will be printed when the printing is performed, and said second pulse being supplied to said driver of said thermal head so that the printing will be prevented from being performed in a period between a first time (t1) when each print line is completely printed and a second time (t4) when a printing paper or said thermosensible paper is to be transported forward to a next print line. A thermal printer according to claim 8, wherein said heating elements are divided into n groups (where n denotes to as an arbitrary integral number larger than one) and said control means generates n common pulses and said means generates n current-on pulses each having said first or second pulse width, said heating elements being sequentially heated by said second heating value by each group in said period between said first time and said second time.

Inventors: ISHIYAMA, KIYOSHIGE C/O SHINKO ELECTRIC CO., LTD., IWATA, SATOSHI C/O SHINKO ELECTRIC CO., LTD.
Applicants: SHINKO ELECTRIC CO. LTD.
Application year: 1988
Main classification: B41J2
IPC codes: B41J35, B41J2