JP2009208346A - Method for cleaning thermal head - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for cleaning by which a deposit near a thermal head is effectively removed at low cost, and a load of the thermal head is reduced. <P>SOLUTION: The method for cleaning the thermal head is characterized in that heating elements 21a corresponding to printing pixels out of a plurality heating elements 21a are energized so as to attain a heating temperature for forming holes in a stencil printing base paper 23, and heating elements 23a not corresponding to the printing pixels are energized so as to attain a heating temperature for not forming holes in the stencil printing base paper 23, while printing the stencil printing base paper 23 by one printing plate. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a thermal head cleaning method, and more particularly to a method for cleaning deposits accumulated on a thermal head of a stencil printing press.

  In a stencil printing machine, before printing processing, plate making processing is performed in which image information read from a document is written as a punching pattern on a stencil sheet called a master. In recent years, heat-sensitive base paper in which a thermoplastic resin film and Japanese paper as a support are bonded together with an adhesive is widely used as the stencil base paper. A thermal head provided with a plurality of heating elements is used for writing a perforation pattern on a stencil sheet. In the plate making process, the stencil sheet is brought into contact with the surface of the thermal head, and a plurality of heating elements of the thermal head are selectively heated based on the image information, so that the thermoplastic resin film of the stencil sheet is melted and perforated. Will be formed.

  In the thermal head, the heating elements are arranged in a line along the main scanning direction, and the stencil sheet is conveyed in the sub-scanning direction while being pressed against the surface of the thermal head by a platen roller disposed opposite to the thermal head. The Here, the longitudinal direction of the thermal head is referred to as the main scanning direction, and the conveyance direction of the stencil sheet orthogonal to this is referred to as the sub-scanning direction.

By the way, when the heating perforation by the thermal head is repeated for a long time, the molten residue of the film, the adhesive or the like adheres and accumulates in the vicinity of the heat generating element to become a deposit. When such deposits become large, not only the adhesion between the film and the heating element is disturbed during plate making, but also the conveyance of the film is hindered because the frictional resistance between the film and the heating element increases. The perforation of the film is hindered, and the quality of the printed image gradually deteriorates. Therefore, in order to remove accumulated deposits, a special sheet such as a cleaning sheet is passed between the thermal head and the pressure roll (see Patent Document 1), or a cleaning area is provided in a part of the stencil sheet. The provided one has been proposed (see Patent Document 2).
Japanese Utility Model Publication No. 62-182756 JP-A-10-119330

  However, since special paper such as a cleaning sheet is disposable every time it is used, an increase in cost is inevitable when used for a long period of time. Further, dust generated due to poor conveyance of the stencil sheet or dust generated suddenly in the apparatus may be blocked by deposits and adhere to the surface of the heat generating element. In this case, in cleaning with a cleaning sheet, dust is not removed until cleaning is performed. Therefore, heat generated by the heating element may be hindered due to the adhesion of dust, and image degradation such as white stripes may occur.

  On the other hand, when a cleaning area is provided on a part of the stencil sheet, the cleaning area is smaller than that of a cleaning sheet in which the entire plate is used as a cleaning area, resulting in poor cleaning efficiency. In particular, when time has elapsed since the accumulation of the deposit and the deposit is fixed in the vicinity of the heating element, it is difficult to remove all the deposit.

  Further, when the stencil sheet provided with the cleaning sheet and the cleaning area as described above is passed through the thermal head in contact with the thermal head, all the heating elements are heated (all solid printing) and already accumulated. Although the removal of the deposits is facilitated, it is considered that the deposits are newly attached because the heat drilling is performed during the cleaning. In addition, every time the cleaning is performed, all the heating elements are heated to such an extent that perforations are formed. Therefore, wear and deterioration of the thermal head are inevitable in long-term use.

  The present invention has been made to solve the above-described problems of the prior art, and provides a thermal head cleaning method capable of efficiently removing deposits at low cost and reducing the burden on the thermal head. There is to do.

  In order to achieve the above object, according to the first aspect of the present invention, a plurality of heating elements are arranged in a line in a direction perpendicular to the conveyance direction of the stencil sheet, and the heating elements corresponding to the printing pixels are selected. A thermal head cleaning method for forming a perforation pattern on the stencil sheet conveyed while being in contact with the heat generating element by energizing and heating, and in a normal operation mode, a stencil sheet for one plate is formed. During printing, among the plurality of heating elements, the heating elements corresponding to the printing pixels are energized to a heating temperature at which perforations are formed in the stencil sheet, and the heating elements that do not correspond to the printing pixels The main point is to energize the stencil sheet so that the heating temperature is such that no perforations are formed.

  According to a second aspect of the present invention, in the confidential operation mode in which the stencil sheet for one plate not forming the perforation pattern is conveyed in contact with the heat generating elements of the thermal head according to the first aspect, The gist of the present invention is to energize the stencil sheet so as to have a heating temperature at which no perforations are formed.

  According to a third aspect of the present invention, in the second aspect, in the confidential operation mode, a perforation pattern is formed while energizing all the heating elements to a heating temperature at which the perforated stencil sheet is not perforated. The gist of the invention is to make the speed of conveying the stencil sheet not to be brought into contact with the heating element of the thermal head slower than usual.

  According to a fourth aspect of the present invention, in the second or third aspect, in the confidential operation mode, the heat generation is performed while energizing all the heat generating elements to a heating temperature at which no perforation is formed in the stencil sheet. The gist is to increase the interval until the element generates heat in the next line.

  According to a fifth aspect of the present invention, in any one of the first to fourth aspects, the temperature of the heat generating element that does not correspond to a print pixel or all of the heat generating elements is adjusted by adjusting the energization time to the heat generating element. The heating temperature is such that no perforation is formed on the stencil sheet.

  According to the first aspect of the present invention, when printing a stencil sheet for one plate in a normal operation mode, the perforation is not formed on the stencil sheet or perforations are formed on the stencil sheet for all the heating elements. Since energization is performed so that the heating temperature is reached, the deposit adhering to the vicinity of the heating element during plate making becomes warmed and softened, and when the stencil sheet is conveyed in contact with the heating element in this state, the deposit is The stencil sheet is pushed away from the vicinity of the heat generating element in the transport direction downstream. As described above, according to the present invention, since the thermal head is cleaned during plate making, the surface of the heating element can always be kept free of deposits. In particular, since special paper such as a cleaning sheet is not used in the present invention, the cost does not increase even when used for a long period of time, and the entire stencil sheet is used as a cleaning area, which is equivalent to the cleaning sheet. Therefore, it is possible to prevent a reduction in cleaning efficiency. In addition, since the deposit can be removed before the deposit adheres to the vicinity of the heating element, it is possible to prevent the deposit from adhering to the vicinity of the heating element and difficult to remove.

  Also, since cleaning is performed every time a stencil sheet for one plate is printed, dust generated due to poor conveyance of the stencil sheet or suddenly generated in the apparatus is blocked by deposits and generates heat. These dusts can be quickly removed without remaining in the vicinity of the element. For this reason, it is possible to prevent image deterioration such as white stripes caused by the heat generated in the heat generating element being hindered by the attached dust.

  According to the second aspect of the present invention, in the confidential operation mode, since all the heating elements can be energized so as to have the same heating temperature, the heating temperature for punching the stencil sheet for each heating element or Compared to the case where current is applied so that the heating temperature is not perforated, the temperature variation of each heating element is eliminated, and uniform cleaning can be performed.

  According to the third aspect of the present invention, in the energization of the second aspect, the speed of conveying the stencil sheet while being in contact with the heating element is made slower than that during normal plate making, so that the cleaning effect is further enhanced. Can do.

  According to the invention described in claim 4, by increasing the interval until the heating element generates heat in the next line, the temperature of the heating element is set to a heating temperature at which no piercing is formed on the stencil sheet. Further, the cleaning effect can be enhanced.

  According to the fifth aspect of the present invention, by adjusting the energization time to the heating element, the temperature of the heating element is set to a heating temperature at which no perforation is formed on the stencil sheet. The magnitude of the applied energy can be easily and accurately controlled.

  Hereinafter, as an embodiment of the present invention, a stencil printing machine for performing a thermal head cleaning method according to the present invention will be described.

  FIG. 1 is a block diagram showing the configuration of a control system of a stencil writing unit 20 of a stencil printing machine according to this embodiment, FIG. 2 is an overall configuration diagram of the stencil printing machine according to this embodiment, and FIG. FIG. 2 is a circuit configuration diagram of a thermal head.

  First, the overall configuration of the stencil printing machine 1 will be described. As shown in FIG. 2, the stencil printing machine 1 of the present embodiment is roughly composed of a document reading unit 10, a plate making / writing unit 20, a cutter unit 30, and a printing unit 40.

  The document reading unit 10 is a mechanism for performing a document reading process. The document reading unit 10 sets a document 7 that is a copy object, and a document sensor that detects the document 7 set on the document setting table 12. 17, a document conveying roller pair 14 that is rotationally driven by a detection signal of the document sensor 17, a contact image sensor 11 that optically reads an image of the conveyed document 7 and converts it into an analog electric signal, and an image A document discharge roller pair 15 for discharging the document 7 read by the sensor 11 to the document discharge tray 19 is configured.

  The document IN sensor 16 is a means for detecting the conveyed document 7 and determines the start of the plate making / writing unit 20 described later. Further, the document transport roller pair 14 and the document discharge roller pair 15 are rotationally driven by a stepping motor 18 as indicated by dotted lines in the figure.

  In the document reading unit 10, when a detection signal from the document sensor 17 is received by a system control unit (not shown), the document transport roller pair 14 is rotationally driven to transport the document 7 in a predetermined direction. The image of the document 7 is optically read. The original 7 from which the image has been read is discharged to the original discharge tray 19 by the original discharge roller pair 15. Further, an analog electrical signal for one plate read by the image sensor 11 is converted into 8-bit digital data by an A / D converter (not shown), and then sent to a stencil controller 60 described later.

  The plate-making writing unit 20 is a platen roller that conveys a thermal head 21 in which a plurality of heating elements 21 a are arranged in a line in the main scanning direction and a stencil sheet 23 fed from a stencil sheet roll 22 while pressing it against the thermal head 21. 24 and a base paper transport roller pair 26 for transporting the stencil paper 23 made by the thermal head 21 toward a clamp portion 32 of a drum 33 described later. Note that a writing motor 25 indicated by a dotted line in the drawing is a stepping motor, and rotationally drives the platen roller 24 and the base paper conveying roller pair 26.

  As shown in FIG. 3, the thermal head 21 has an AND circuit 27 connected to each of the plurality of heating elements 21a, and the other end is grounded. Each AND circuit 27 is connected to a latch circuit 28 and a shift register 29. A clock signal CLK and image data DAT output from a thermal head controller 80 described later are input to the shift register 29. The latch signal LAT is input to the latch circuit 28, and the strobe signal STB is input to each AND circuit 27.

  When printing (punching) on the stencil sheet 23, the image data DAT, which is serial data, is input to the shift register 29 at the timing of the clock signal CLK, replaced in parallel, and sequentially output to the latch circuit 28. The image data DAT latched by the latch circuit 28 is output from the latch circuit 28 when the latch signal LAT is input. At this time, the heating elements 21a are energized by the signal product of the image data DAT latched by the latch circuit 28 and the strobe signal STB, and the heating elements 21a are heated.

  Note that the image data DAT is composed of black-and-white data indicating presence / absence (on) / non-perforation (off), weak heating data, and heat history data indicating presence / absence (off) of heat history heating. Yes. The contents of each data will be described later.

  The cutter unit 30 is a mechanism for cutting the stencil sheet 23. When the stencil sheet 23 made by the thermal head 21 is wound around the drum 33 and has a predetermined length, the stencil sheet 23 is predetermined. A cutter 31 for cutting at a position is provided.

  The printing unit 40 includes a drum 33 having a built-in ink supply unit that supplies a predetermined amount of ink to an inner surface of an ink reservoir 58 formed between a doctor roller 56 and a squeegee roller 57, and a copy that is stacked on a paper feed table 44. Pick-up roller 46 that picks up and conveys the printing paper 43 one by one from the printing paper to become, a timing roller 42 that feeds the printing paper 43 conveyed from the pick-up roller 46 at a predetermined timing, and a timing roller 42 that feeds the printing paper 43 A press roller 35 that presses the printed printing paper 43 against the outer peripheral surface of the drum 33, a separation claw 55 for peeling the printed printing paper 43 from the drum 33, and a printing paper 43 that is peeled off from the drum 33 and discharged. The paper discharge tray 49 is configured to discharge and stack paper.

  On the outer peripheral surface of the drum 33, there is provided a clamp portion 32 for clamping the leading end portion of the stencil sheet 23 which has been made and conveyed by the thermal head 21. The stencil sheet 23 that has been pre-pressed and clamped by the clamp portion 32 is wound around the outer peripheral surface thereof by rotating the drum 33.

  A main motor 34 indicated by a dotted line in the drawing is a DC motor, and rotates the drum 33. Reference numeral 41 denotes a conveyance path.

  Next, a configuration related to the control system of the plate making / writing unit 20 will be described with reference to FIG. As shown in FIG. 1, the control system of the plate making / writing unit 20 includes a stencil control unit 60, a system control unit 70, and a thermal head control unit 80.

  The stencil controller 60 includes a motor drive circuit 61 and an image processing circuit 62. The motor drive circuit 61 controls the operation of the writing motor 25 that rotationally drives the platen roller 24 that conveys the stencil sheet 23 while pressing it against the thermal head 21. The image processing circuit 62 performs conversion processing for stencil printing on 8-bit (0 to 255) digital data sent from the document reading unit 10 in FIG. 2 and generates binary black and white data. Is. For example, in a character document, digital data is simply binarized (monochrome is determined with reference to a threshold value) to be 1-bit monochrome data. At this time, processing such as shading correction is performed. Also, in a photographic document, after converting the density of digital data, halftone processing (halftone dot / error diffusion) is performed to obtain monochrome data of 1 bit so that the density is expressed by the density of surrounding pixels. The black and white data represents the density (gradation) of the photographic document as a whole. The 1-bit monochrome data is, for example, a digital electric signal in which white data corresponding to white pixels is expressed as “0” and black data corresponding to black pixels as print pixels is expressed as “1”.

  The thermal head control unit 80 includes a weak heating control circuit 81, a thermal history control circuit 82, and a basic heating control circuit 83, and controls the operation of these control circuits to generate a clock signal CLK, image data DAT, and a latch signal. The LAT and strobe signal STB are output.

  In the normal operation mode, the weak heating control circuit 81 performs weak heating control so that all the heating elements 21a are energized so as to have a heating temperature at which the stencil sheet 23 is not perforated. Is generated. The weak heating data is output before the basic heating is performed by the black and white data, and each heating element 21a of one line is weakly heated by energization based on the weak heating data. Further, in the confidential operation mode described later, the weak heating control circuit 81 is such that no perforation is formed in the stencil sheet 23 for all the heating elements 21a while the stencil sheet 23 for one plate passes through the thermal head 21. The weak heating data for the confidential operation mode is generated so as to perform the weak heating control for energizing the heating temperature.

  The heat history control circuit 82 considers the heat history when the heat generating element 21a corresponding to the black pixel as the print pixel is perforated one or two lines before and the heat history of other adjacent heat generating elements. Thus, heat history data is generated so as to perform heat history control for supplying the optimum applied energy to the heating element 21a. The heat history data is generated as data indicating whether heat history heating is present (ON) / not present (OFF). Further, the heat history data is output after the basic heating by the black and white data is performed, and the heating element 21a corresponding to the black pixel to be heated by the heat history is heated following the heating by the black and white data.

  The basic heating control circuit 83 does not energize the heating element 21a corresponding to the white pixel that is not a printing pixel, and forms a perforation in the stencil sheet 23 for the heating element 21a corresponding to the black pixel that is the printing pixel. Monochrome data is generated so as to perform basic heating control in which power is supplied to achieve the heating temperature. This black and white data is output after the weak heating is performed by the weak heating data.

  In the black and white data of the present embodiment, the presence of perforation (on) is data indicating basic heating (heating in a normal operation mode), and is classified into short-time heating and long-time heating. That is, by performing the weak heating and the basic heating continuously, the heating is controlled for a short time with thermal history control. Moreover, it becomes long-time heating by which heat history control was carried out by continuing and performing the said weak heating, basic heating, and heat history heating. In black and white data, no perforation (off) is data indicating no heating.

  Due to the output of the black and white data, the heating element 21a corresponding to the black pixel is heated by energization, and the stencil sheet 23 is punched. On the other hand, since the heat generating element 21a corresponding to the white pixel is not energized and heated, the stencil sheet 23 is not perforated. In the following description, the presence / absence of punching of monochrome data (on) is referred to as monochrome data (on) and the absence of punching of monochrome data (off) is appropriately referred to as monochrome data (off).

  Here, the applied energy supplied to the heating element 21a of the thermal head 21 will be described. In the thermal head 21 of the plate-making writing unit 20, the applied energy supplied to the heating element 21a is applied power (current × voltage) × heating time (energization time to the heating element). In this embodiment, the magnitude of the applied energy is controlled by making the applied power constant and changing the length of the heat generation time. For this purpose, the heat generation time of the heat generating element 21a is adjusted by the pulse length (energization time) of the strobe signal applied to the thermal head 21. According to this, before transferring the monochrome data, the weak heating data is transferred, the latch signal for the weak heating is output, and then the strobe signal is turned on early so that the monochrome data is next transferred. Until the latch signal is output, the thermal head 21 can be weakly heated based on the weak heating data. However, the control of the applied energy can be realized not only by adjusting the heat generation time but also by adjusting the current and voltage that give the applied power.

  Each of the above control units can be constituted by a microcomputer including a CPU (Central Processing Unit), RAM, ROM, and an input / output interface (I / O interface). However, each of these units can be configured by a plurality of microcomputers, and may be configured as a device that executes a plurality of controls in addition to the control related to the stencil.

  Next, the processing procedure of the plate making operation by the thermal head control unit 80 will be described with reference to the flowcharts shown in FIGS.

  First (FIG. 4), the thermal head control unit 80 sets the uppermost line in the sub-scanning direction of the stencil sheet 23 as the target line from the black and white data sent from the stencil control unit 60 (step S101). Then, the weak heating data is transferred to the shift register 29 as the image data of the target line (step S102). Subsequently, the first latch signal LAT is output to the latch circuit 28 (step S103). In response to the latch signal LAT, the image data latched in the latch circuit 28 is output to the AND circuits 27 all at once.

  Next, the thermal head control unit 80 sets a pixel located at the left end in the main scanning direction as the target pixel in the target line (step S104), and determines whether the target pixel is a white pixel or a black pixel (step S104). S105). If it is a white pixel, monochrome data (off) is transferred to the shift register 29 as image data (step S106). If it is a black pixel, monochrome data (on) is transferred to the shift register 29 as image data. (Step S107).

  Subsequently, the thermal head control unit 80 determines whether or not there is a next pixel in the main scanning direction (step S108). If YES, the next pixel (right next to the previous target pixel in the main scanning direction) is determined. (Pixel) is reset as the target pixel (step S109), and the process returns to step S104. By going through the loop of step S104 to step S109, monochrome data for one line is transferred to the shift register 29.

  On the other hand, if NO in step S108, that is, if the transfer of monochrome data for all pixels in one line is completed, the thermal head controller 80 turns on the strobe signal STB to each AND circuit 27 of the thermal head 21. A state is set (step S110). As a result, each heating element 21a of the thermal head 21 has a heating temperature at which the heating elements 21a of all the lines are not perforated on the stencil sheet 23 based on the weak heating data output to the AND circuit 27. The weak heating which is energized to be performed is performed. The strobe signal STB remains on until it is turned off in step S121.

  Subsequently, the thermal head control unit 80 outputs the second latch signal LAT to the latch circuit 28 of the thermal head 21 when a preset time elapses after the strobe signal STB is output in step S110. (Step S111). By this latch signal LAT, the black and white data latched in the latch circuit 28 is output to the AND circuits 27 all at once. At this time, since the strobe signal STB is in the on state, each heating element 21 a of the thermal head 21 is energized / de-energized for each pixel based on the black and white data output to the AND circuit 27. Is done. That is, the heating element 21a corresponding to the black and white data (on) is energized so as to have a heating temperature at which perforations are formed in the stencil sheet 23, and the heating element 21a corresponding to the black and white data (off) is not energized. .

  Subsequently (FIG. 5), the thermal head control unit 80 sets a pixel located at the left end in the main scanning direction on the uppermost line in the sub-scanning direction of the stencil sheet 23 as a target pixel (step S112). Is a black pixel or a white pixel (step S113). If the pixel is a white pixel, the thermal history data (off) is transferred to the shift register 29 as image data (step S115). If the pixel is a black pixel, the pixel in front of the target pixel in the sub-scanning direction is black. It is determined whether or not it is a pixel (step S114). If YES, the thermal history data (OFF) is transferred as image data to the shift register 29 (step S115). If NO, the thermal history data (ON) is transferred to the shift register 29 as image data. Transfer (step S116).

  Next, the thermal head control unit 80 determines whether or not there is a next pixel in the main scanning direction (step S117). If YES, the next pixel (right next to the previous target pixel in the main scanning direction) is determined. Pixel) is reset as the target pixel (step S118), and the process returns to step S113. By going through the loop of step S113 to step S118, the thermal history data for one line is transferred to the shift register 29.

  On the other hand, if NO at step S117, the thermal head controller 80 outputs the third latch signal LAT to the latch circuit 28 of the thermal head 21 (step S119). In response to the latch signal LAT, the column history data latched by the latch circuit 28 is output to the AND circuits 27 all at once. At this time, since the strobe signal STB is in the ON state, each of the heating elements 21a of the thermal head 21 is energized / de-energized for each pixel based on the thermal history data output to the AND circuit 27. Is executed. That is, the heating element 21a corresponding to the thermal history data (ON) is energized so as to have a heating temperature at which perforations are formed in the stencil sheet 23, and the heating element 21a corresponding to the thermal history data (OFF) is not energized. become.

  Subsequently, the thermal head controller 80 turns off the strobe signal STB when a preset time has elapsed since the latch signal LAT was turned on in step S110 (step S120). Thereby, the heating based on the heat history data is stopped. Thereafter, the thermal head controller 80 determines whether or not the next line exists in the sub-scanning direction of the stencil sheet 23 (step S121). If YES, the next line is reset as the target line. (Step S122), the process returns to Step S102. If NO in step S122, the process ends.

  Next, image data and output timing of each signal in the plate making operation will be described with reference to a time chart of FIG. A specific example of the plate-making image is shown in FIG. FIG. 7A is an explanatory view showing a plate-making image for one plate made, and FIG. 7B is a schematic diagram of a perforation pattern showing a part of the plate-making image formed on the stencil sheet 23.

  In FIG. 6, when the weak heating data is transferred as the image data and subsequently the first latch signal LAT is output, the weak heating data latched in the latch circuit 28 is in an output waiting state in the AND circuit 27. Thereafter, the strobe signal STB is turned on, so that the weak heating based on the weak heating data is executed. The weak heating based on the weak heating data is performed regardless of whether the pixel is black or white, and the second latch signal LAT signal continues until the output after the strobe signal is turned on. In FIG. 7B, the white circle portion is a pixel that is slightly heated. The white circle portion is energized so that the stencil sheet 23 is heated so that the stencil sheet 23 is not perforated, so that perforation that allows ink to pass is not formed during printing.

  Further, when the monochrome data is transferred as the image data following the weak heating data, and the second latch signal LAT is output thereafter, the monochrome data latched in the latch circuit 28 is output to the AND circuit 27. . Here, since the strobe signal STB is in the ON state, the weak heating based on the weak heating data ends, and then the basic heating based on the black and white data is executed. In the basic heating based on the black and white data, the heating element 21a corresponding to the black and white data (on) is heated by energization following the weak heating described above, and the heating element 21a corresponding to the black and white data (off) is not energized. Therefore, it ends with only weak heating. Further, the basic heating with the black and white data continues until the third latch signal LAT signal is output.

  Further, when the thermal history data is transferred as image data following the black and white data, and then the third latch signal LAT is output, the thermal history data latched in the latch circuit 28 is output to the AND circuit 27. . Here, since the strobe signal STB is in the ON state, the basic heating based on the black and white data is completed, and then the heat history heating based on the heat history data is executed. In the heat history heating based on the heat history data, the heating element 21a corresponding to the heat history data (on) is heated by energization following the previous black and white data (on). On the other hand, in the heating element 21a corresponding to the heat history data (off), the energization is completed only with the black and white data (on).

  As shown in FIG. 7B, the heating element 21a corresponding to the black pixel is continuously subjected to the weak heating and the basic heating, so that the heating history is controlled for a short time, and the weak heating and the basic heating are performed. By continuously performing heating and heat history heating, the heat history is controlled for a long time. That is, weak heating is included in both short-time heating and long-time heating. Further, only weak heating is performed on the heating element 21a corresponding to the white pixel.

  As described above, in the stencil printing machine 1 according to the present embodiment, when the stencil sheet 23 is printed, the stencil sheet 23 is not perforated for all the heating elements 21a corresponding to the black pixels and the white pixels. Since the weak heating or the basic heating for forming the perforations in the stencil sheet 23 is executed, the deposit adhering to the vicinity of the heating element 21a during the plate making is warmed and softened, and in this state the stencil sheet 23 Is transported while being in contact with the heat generating element 21a, the deposit is moved away from the vicinity of the heat generating element 21a and is pushed downstream in the transport direction of the stencil sheet 23. As described above, in the stencil printing machine 1 according to the present embodiment, the thermal head 21 is cleaned during the plate making, so that the surface of the heating element 21a can always be kept free of deposits. In particular, the cleaning shown in this embodiment does not use a special sheet such as a cleaning sheet, so that the cost does not increase even when used for a long period of time, and the entire stencil sheet 23 is the cleaning area. Thus, since a wide cleaning area equivalent to the cleaning sheet can be secured, a reduction in cleaning efficiency can be prevented. Further, since the deposit can be removed before the deposit adheres to the vicinity of the heating element 21a, it is possible to prevent the deposit from adhering to the vicinity of the heating element and difficult to remove.

  Further, since cleaning is executed every time the stencil sheet 23 for one plate is printed, dust generated due to poor conveyance of the stencil sheet 23 or dust generated suddenly in the apparatus is blocked by the deposit. Even if it adheres to the surface of the heat generating element 21a, these dusts can be quickly removed. Therefore, it is possible to prevent image deterioration such as white stripes caused by the heat generation in the heat generating element 21a being hindered by the adhesion of dust.

  Further, since the heating element 21a that does not correspond to the printing pixel is weakly heated to such an extent that the stencil sheet 23 is not perforated, new deposits are attached as compared to the case where all the heating elements are heated (solid printing). It can be minimized. Further, as compared with the case of full solid printing, it is not necessary to heat all the heating elements 21a to such an extent that perforations are formed. Therefore, wear and deterioration of the thermal head can be minimized during long-term use. .

  In the present embodiment, the temperature of the heat generating element 21a is adjusted to a heating temperature at which no perforation is formed in the stencil sheet 23 by adjusting the energization time to the heat generating element 21a. The magnitude of the applied energy can be easily and accurately controlled.

  Thus, according to the thermal head cleaning method executed by the stencil printing machine 1 according to the present embodiment, deposits can be efficiently removed at low cost, and the burden on the thermal head can be reduced. it can.

  Next, the case where the cleaning shown in the present embodiment is executed in the confidential operation mode will be described.

  In general, in the stencil printing machine 1, even after printing is finished, the used stencil sheet 23 is still wound around the outer periphery of the drum 33, and when the next stencil start is instructed, the used stencil sheet 23 is used. Is discharged from the outer periphery of the drum 33. In this way, since the stencil sheet 23 used for the previous printing remains wound around the outer periphery of the drum 33 until the next plate making is started, the user who performs the next printing uses the stencil sheet 23 for the previous printing. By using it for printing, the contents of the original can be known. In the confidential operation mode, when it is not desired for the next person to print the used stencil sheet 23, the used stencil sheet 23 is peeled off from the outer periphery of the drum 33 after printing is completed, and nothing is made. It means that a new stencil sheet 23 is wound around a drum.

  In such a confidential operation mode, when a new stencil sheet 23 is conveyed while being in contact with the thermal head 21, the heating temperature is set so that no perforations are formed in the stencil sheet 23 with respect to all the heating elements 21a. The thermal head 21 can be cleaned by performing weak heating that is energized. In this case, since all the heating elements 21a are energized so as to have the same heating temperature, when weak heating, weak heating + basic heating, or weak heating + basic heating + heat history heating is performed during plate making Compared to the above, there is no temperature variation for each heating element 21a, and uniform cleaning becomes possible.

  Further, when the stencil sheet 23 is weakly heated in the confidential operation mode, the cleaning effect can be further enhanced by lowering the speed at which the stencil sheet 23 is conveyed while being in contact with the heat generating element 21a as compared with the normal plate making. Note that the speed at which the stencil sheet 23 is conveyed can be controlled by adjusting the rotational speed of the writing motor 25 by the motor drive circuit 61 shown in FIG. That is, when the execution of the confidential operation mode is instructed by the user, the stencil sheet control unit 60 controls the motor drive circuit 61 so that the rotation speed of the writing motor 25 is slower than that during normal plate making, thereby making the stencil sheet 23 The speed at which the sheet is conveyed can be reduced. Incidentally, since the confidential operation mode is performed after the printing is finished, even if the speed at which the stencil sheet 23 is conveyed is reduced, the printing operation is not affected, and the user's useless waiting time can be eliminated.

  In the confidential operation mode, in addition to slowing down the speed of transporting the stencil sheet 23 as described above, the printing cycle, that is, the interval until the heating element 21a generates heat in the next line is lengthened (slowed down). You may control as follows. Even in this case, the cleaning effect can be further enhanced. Further, the printing cycle may be controlled to be increased in accordance with the speed at which the stencil sheet 23 is conveyed. Also in this case, the cleaning effect can be further enhanced.

The block diagram which shows the structure of the control system of the plate-making writing part of the stencil printing machine concerning embodiment. 1 is an overall configuration diagram of a stencil printing machine according to an embodiment. The circuit block diagram of a thermal head. The flowchart which shows the process sequence of the plate making operation | movement by a thermal head control part. The flowchart which shows the process sequence of the plate making operation | movement by a thermal head control part. The time chart which shows the output timing of the image data and each signal in plate making operation | movement. (A) is explanatory drawing which shows the platemaking image for one plate made from platemaking. (B) is a schematic diagram of a perforation pattern showing a part of a plate-making image formed on a stencil sheet.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Stencil printing machine 10 ... Document reading part 11 ... Image sensor 20 ... Plate making writing part 21 ... Thermal head 21a ... Heating element 23 ... Stencil paper 24 ... Platen roller 25 ... Motor 27 ... AND circuit 28 ... Latch circuit 29 ... Shift Register 33 ... Drum 40 ... Printing unit 60 ... Stencil control unit 61 ... Motor drive circuit 62 ... Image processing circuit 70 ... System control unit 80 ... Thermal head control unit 81 ... Weak heating control circuit 82 ... Thermal history control circuit 83 ... Basic heating Control circuit

Claims (5)

A plurality of heating elements are arranged in a line in a direction perpendicular to the conveyance direction of the stencil sheet, and the heating elements corresponding to the print pixels are selectively energized and heated to contact the heating elements. A thermal head cleaning method for forming a perforation pattern on the stencil sheet to be conveyed,
In a normal operation mode, while printing one stencil sheet, a heating temperature at which perforations are formed in the stencil sheet for the heating elements corresponding to the printing pixels among the plurality of heating elements. A method of cleaning a thermal head, comprising: energizing the heat generating elements not corresponding to printing pixels so as to have a heating temperature at which perforations are not formed on the stencil sheet.   In the confidential operation mode in which a stencil sheet for one plate that does not form a piercing pattern is conveyed while being in contact with the heating element of the thermal head, the heating temperature is such that no piercing is formed on the stencil sheet for all the heating elements. The thermal head cleaning method according to claim 1, wherein a current is supplied to the head.   In the confidential operation mode, while energizing all the heating elements to a heating temperature at which no piercing is formed in the stencil sheet, while making the stencil sheet not forming a piercing pattern contact the heating element of the thermal head The thermal head cleaning method according to claim 2, wherein the conveying speed is made slower than usual.   In the confidential operation mode, while energizing all the heating elements to a heating temperature at which no perforation is formed in the stencil sheet, the interval until the heating elements generate heat in the next line is increased. 4. The thermal head cleaning method according to claim 2, wherein the thermal head is cleaned.   The temperature of the heating element that does not correspond to a printing pixel or all of the heating elements is adjusted to a heating temperature at which no perforation is formed on the stencil sheet by adjusting the energization time to the heating element. The method for cleaning a thermal head according to any one of claims 1 to 4.

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