GB2304643A - Stencil printer with an improved thermal head - Google Patents

Abstract

A stencil printer is disclosed capable of selectively perforating a thermoplastic stencil (61,Fig.7) with a thermal printhead (91,Fig.7) to form a pattern representative of a document image, the perforated stencil being moved downstream of the printhead (91,Fig.7) by roller means (93a,93b,94a,94b) to an outer periphery of a print drum (101,Fig.7) whereupon it is wrapped around the print drum (101,Fig.7) and secured by clamp means (102,Fig.7), ink is supplied from the inner periphery of the print drum (101,Fig.7) via the perforation pattern of the stencil (61,Fig.1) to a paper sheet (62,Fig.7) held against the print drum (101,Fig.7) by a press roller (103,Fig.7) to form an image corresponding to the document image on the paper sheet (62,Fig.7). The thermal print head (91,Fig.7) comprises a glaze layer of a thickness of 60um and has heating elements (10,Fig.4(a)) of a dimension (S10,Fig.4(a)) in a main scanning direction which is 30% to 95% of a pitch (p10,Fig.4(a)) between neighbouring heating elements (10,Fig.4(a)) and a dimension (F10,Fig.4(a)) in a subscanning direction which is 30% to 95% of the pitch (p10,Fig.4(a)) between neighbouring heating elements (10,Fig.4(a)).

Description

STENCIL PRINTER WITH AN IMPROVED THERMAL HEAD BACKGROUND OF THE INVENTION The present invention relates to a stencil printer having an improved thermal head.

It has been customary with a stencil printer to selectively perforate a thermoplastic stencil with a thermal head to form a pattern representative of a document image, to wrap the perforated stencil or master around a print drum, and to supply ink from the inner periphery of the drum to a paper via the perforation pattern to thereby form an ink image corresponding to the document image on the paper.

The thermal head is usually implemented by a thin film and provided with heating element portions each having a rectangular or a heat concentrating type of configuration. The head is made up of a base and a glaze layer, resistance layer, electrodes and protective layer sequentially formed on the base.

The prerequisite with the above stencil printer is that a desirable perforation pattern matching the resolution of the head be formed in the stencil in order to reproduce a document image faithfully without regard to the kind of the document image. Also, it is necessary to prevent ink from being transferred form the front of a paper to the rear of the next paper when they are stacked on a tray. To meet these requirements, each perforation formed in the stencil must be fine and discrete from adjoining perforations so as to reduce the amount of ink transfer to a paper.Japanese Patent Laid Open Publication No. 4-265759, for example, proposes to define particular ratios of the dimensions of the individual heating element portion to the pitch in the main scanning direction (distance between nearby heating element portions in the main scanning direction) and to the pitch in the sub scanning direction (distance in the subscanning direction).

However, the size of a perforation, of course, depends on the dimensions of the individual heating element. Hence, with the implementation taught in the above document, it i 5 necessary to further reduce the ratios in order to reduce the dimensions of each heating element portion. This brings about another problem that for a given peak temperature at each heating element portion, the energy required by the heating element portion for a unit area increases with a decrease i n the size of the portion due to, for example, the leakage of heat to the adjoining electrodes. As a result, the life of the heating element portions decreases with a decrease in the size of the same.

Assume that the glaze layer included in the head is relatively thick, e.g., more than 60 pm thick. Then, heat accumulates in the glaze layer more than in a thinner glaze layer during continuous print mode operation. Hence, the peak temperature of the heating element portions becomes far higher than when no heat accumulates in the glaze layer. As a result, the energy applied to the individual heating element portion is excessive for a unit area, causing a thermal stress to act on the portion. This, coupled with the fact that such a peak temperature accelerates, for example, the oxidation of the heating element portions and tends to change their electric resistance, further reduces the life of the portions.

Today, to reduce the master making time, a stencil printer operable at a printing period or line period of, for example, less than 2.5 msec/line is available. In this type of printer, more heat accumulates in the head than in the ordinary printer and further reduces the life of the head with respect to the resistivity to power. Again, this prevents discrete perforations from being formed in the stencil.

SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide a stencil printer having an improved thermal head and capable of extending the life of heating element portions without reducing their size to an excessive degree.

It is another object of the present invention to provide a stencil printer having an improved thermal head and capable of forming fine and discrete perforations in a thermoplastic stencil and reproducing a document image faithfully.

It is still another object of the present invention to provide a stencil printer having an improved thermal head and capable of preventing ink from being transferred from the front of an underlying paper to the rear of an overlying paper.

It is a further object of the present invention to provide a stencil printer having an improved thermal head and capable of reducing the master making time.

In accordance with the present invention, there is provided a stencil printer comprising: a thermal head comprising a number of minute heating element portions to be brought into contact with a stencil having at least a thermoplastic resin film, and caused to selectively generate heat in accordance with a document image to be printed to thereby form a perforation pattern representative of said document image in said thermoplastic resin film, said thermal head comprising a glaze layer having a thickness of less than 60 Hm; a print drum for causing said stencil perforated by said thermal head to wrap around said print drum; and ink supply means for supplying ink from an inner periphery of said print drum such that said ink oozes out via said perforation pattern, whereby an ink image corresponding to said document image is formed on a paper; wherein said heating element portions each have a dimension in a main scanning direction which is 30 % to 95 % of a pitch between neighbouring heating element portions, and a dimension in a subscanning direction which is 30 % to 95 % of said pitch.

Also, in accordance with the present invention, there is provided a thermal head for a stencil printer for selectively forming a perforation pattern representative of a document image to be printed in a thermoplastic stencil having at least a thermoplastic resin film, said thermal head comprising: a base; and a glaze layer formed on said substrate and having a thickness of less than 60 ym, and wherein a print drum for causing said stencil perforated by said thermal head to wrap around said print drum; and ink supply means for supplying ink from an inner periphery of said print drum such that said ink oozes out via said perforation pattern, whereby an ink image corresponding to said document image is formed on a paper; wherein said heating element portions each has a dimension in a main scanning direction which is 30 % to 95 % of a pitch between neighbouring heating element portions, and a dimension in a subscanning direction which is 30 % to 95 % of said pitch.

The above and other objects, features and advantages of the present invention will become more apparent from t h e following detailed description taken with the accompanying drawings, in which: FIG. 1A is a plan view of a rectangular heating element portion included in a conventional thermal head; FIG. 1B is a plan view of a heat concentrating type of heating element portion; FIG. 2A is an exaggerated section showing the structure of a glaze layer included in a conventional full-glaze type thermal head; FIG. 2B is a section similar to FIG. 2A, showing the structure of a glaze layer including in a conventional partial glaze type thermal head;; FIG. 3A shows a relation between the peak temperature of a thermal head and the number of pulses applied, using the thickness of the glaze layer as a parameter; FIG. 3B shows a relation between the peak temperature of a thermal head and the printing period, also using the thickness of the glaze layer as a parameter; FIG. 4, (a) is a plan view of heating element portions included in a thermal head; FIG. 4, (b) shows curves representative of the surface temperature distributions of heating element portions in the main scanning direction, using the thickness of the glaze layer as a parameter; FIG. 4, (c) shows curves representative of the surface temperature distributions of heating element portions in t h e subscanning direction, also using the thickness of the glaze layer as a parameter;; FIG. 4, (d) is a plan view showing perforations formed in stencils at AOC shown in Fig. 4, (b); FIGS. 5, (al) and (bl) are plan views of heating element portions for describing the size of the glaze layer; FIGS. 5, (a2) and (b2) are graphs respectively associated with FIGS. 5(al) and (b1), and each showing the surface temperature distributions of heating element portions set up in the main scanning direction by the first pulse and t h e 200th pulse, respectively; FIGS. 5, (a3) and (b3) are plan views of perforations formed in stencils at a point C of FIGS. 5, (a2) and (b2) after the application of the first pulse and the 200th pulse, respectively;; FIGS. 6, (al) and (a2) are graphs representative of the transition of the peak temperature of heating element portions and attributable to a difference in head temperature; FIGS. 6, (bl), (b2) and (b3) show perforations formed in the conditions shown in FIGS. 6, (a) and (b); FIG. 7 is a section of a stencil printer to which the present invention is applied; FIG. 8 is a block diagram schematically showing a control system included in the printer of FIG. 7; FIG. 9 is a side elevation showing a specific location of a thermistor responsive to head temperature; FIG. 10, (a) is a graph demonstrating an operation with thermal record control and an operation without it; and FIGS. 10, (bl) and (b2) are plan views of perforations respectively formed by the operation without thermal record control and the operation with it.

DESCRINION OF THE PREFERRED EMBODIMENTS To better understand the present invention, a brief reference will be made to conventional thin film type thermal heads. FIG. 1 shows a head having a rectangular heating element portion while FIG. 2 shows a head having a heat concentrating type of configuration. FIG. 2A shows a head having a full-glaze sectional configuration while FIG. 2B shows a partial-glaze sectional configuration. Each of the heads, generally 91, has a heating element portion 10, and electrodes 6. The head 91 shown in FIG. lA has a heat concentrating portion 15. Labeled S and F are respectively representative of the main scanning direction and the subscanning direction perpendicular to the direction S. The full-glaze type head 91, FIG. 2A, has a base 9 and a glaze layer 8 formed on the entire upper surface of the base 9.The partial-glaze type head 91 shown in FIG. 2B has the glaze layer 8 formed on the base 9 only beneath the heating element portion 10. The full-glaze type and partial-glaze type heads 91 both have a resistance layer 7, the electrodes 6 and a protective film 5 sequentially formed on the glaze layer 8 in this order. The base 9 is made of, for example, alumina. The glaze layer 8 is implemented by glass or similar electrically and thermally insulative material.

The resistance layer 7 and the electrodes 6 are respectively formed of tantalum silicon (TaSi), tantalum nitride (Tan2N) or similar substance having a high electric resistance, and an aluminum or similar metal. The protective film 5 is made up of an oxidation-resistive layer of, for example, silicon dioxide (SiQ), and a wear-resistive layer overlying the oxidationresistive layer and implemented by tantalum pentaoxide (tea205) or similar substance. The wear-resistive layer protects the electrodes 6 and resistance layer 7 from w e a r otherwise caused by their contact with, for example, a thermoplastic resin film which will be described.

The heads 91 described above each has a number of minute heating element portions 10 arranged an array in the main scanning direction at a predetermined pitch. A thermoplastic stencil, not shown, directly contacts the heating element portions 10 via the protective film 5 while being conveyed in the subscanning direction F, FIGS. lA and lB.

When a voltage is applied to between the electrodes 6, a current flows through the resistance layer 7 between the electrodes 6, i.e., through the heating element portion 10. As a result, the heating element portion 10 generates heat based o n the Joule's law. The heat melts the stencil contacting the heating element portion 10 and thereby forms a perforation therein.

However, a stencil printer having any one of the conventional heads has various problems left unsolved, as discussed earlier. Specifically, ink deposited on the front of one paper is transferred to the rear of the overlying paper.

The life of the heating element 10 is limited. In the perforated stencil or master, the perforations are prevented from being discrete from each other. Further, it is difficult to increase the master making speed. Particularly, as shown in FIGS. 2A and 2B, assume that the glaze layer 8 has a thickness tg greater than 60 llm, as measured at the heating element portion 10. Then, because heat accumulates more in the other heads during the course of continuous printing, the peak temperature of the portion 10 is far higher than when heat does not accumulate. In this condition, the energy applied to the portion 10 for a unit area is excessive and causes a thermal stress to act thereon. Moreover, the excessive energy accelerates the oxidation of the portion 10. These in combination reduce the life of the portion 10.

Thee present invention capable of eliminating the above problems will be described with reference to the accompanying drawings. In the figures to follow, the same or similar constituent parts as or to the parts shown in FIGS. 1A- 2B are designated by the same reference numerals.

First, a relation between the thickness tg of the graze layer 8 of the head 91 and the characteristics of the head 91 will be discussed. FIG. 3A shows a relation between the peak temperature of the head 91 and the number of pulses applied thereto with respect to three different thicknesses tg of the glaze layer 8. As shown, the accumulation of heat in the head 91 increases with an increase in the number of pulses applied to the head 91, but the former saturates as the latter reaches a certain value. Also, more heat accumulates in the head 91 as the thickness tg of the glaze layer 8 increases.FIG. 3 B shows a relation between the peak temperature of the head 91 and the printing period, also using the thickness tg of the glaze layer 8 as a parameter; the relation was determined when the 200th pulse was applied to the head 91. As FTG. 3B indicates, more heat accumulates in the head 91 as the printing period decreases, and less heat accumulates as the thickness tg decreases.

It will be seen from FIGS. 3A and 3B that the accumulation of heat decreases with a decrease in t h e thickness tg of the glaze layer 8, taking account of the number of pulses and printing period.

A reference will be made to FIG. 4 for describing the influence of the thickness tg of the glaze layer 8 on the surface temperature distribution of the head 91. As shown in FIG. 4, (a), the heating element portions 10 of the heads 91 are arranged in an array in the main scanning direction at a pitch of plO. Each heating element portion 10 is sandwiched between a pair of electrodes 6 and provided with a dimension F10 as measured in the subscanning direction F. FIG. 4, (b) shows the surface temperature distribution of each heating element portion 10 in the main scanning direction S, using the thickness tg of the glaze layer 8 as a parameter. FIG. 4, (c) shows the surface temperature distribution of each portion 10 in the subscanning direction F, also using the thickness tg as a parameter.Further, FIG. 4, (d) shows perforation patterns formed in a thermoplastic stencil 61 at a threshold temperature of AOC, FIG. 4, (b), at which a perforation is formed and spreads in the thermoplastic resin film of the stencil 61. The perforation patterns of FIG. 4, (d) respectively correspond to the surface temperature distributions of FIG. 4, (b).

As FIG. 4, (d) indicates, the thickness tg of the glaze layer 8 has noticeable influence on the surface temperature distribution or spread of the heating element portion 10 in the main scanning direction S. The difference between surface temperature distributions directly translates into a di fference in the size of a perforation h formed in the stencil 61. When the thickness tg is great, i.e., as great as 65 pm, nearby perforations h are communicated to each other. It will be readily understood from FIGS. 4, (a)-(d) that for given energy applied to each heating element portion 10, a finer perforation h can be formed in the stencil 61 as the thickness t g decreases. As shown in FIGS. 4, (b) and (c), the peak temperature of the heating element portion 10 increases with an increase in the thickness tg, as stated with reference to FIG.

3A. This is also true when the printing period is reduced to increase the printing speed, as stated in relation to FIG. 3B.

The increment of temperature due to heat accumulation translates into the excess temperature or overpower of t h e heating element portion 10, thereby reducing the life of the portion 10.

Assume that the glaze layer 8 has a relatively great thickness tg, as shown in FIG. 5, (al). Then, surface temperature distributions shown in FIG. 5, (a2) are set up on the heating element portion 10 in the main scanning direction in response to the first pulse and the 200th pulse, respectively. FIG 5, (a3) shows perforations h formed in the stencil 61 and respectively derived from the first pulse and the 200th pulse. FIGS. 5, (bl), (b2) and (b3) are similar to (al)-(a3) except that they are representative of a case wherein the thickness tg of the glaze layer 8 is relatively small. As shown, a difference in the thickness tg of the glaze layer 8 directly translates into a difference in the surface temperature of the heating element portion 10 and, therefore, in the size of the perforation h formed in the stencil 61.As for the glaze layer 8 having a great thickness tg, nearby perforations h in the stencil 61 are, in the worst case, communicated to each other when the 200th pulse is applied.

This is also true in the subscanning direction F.

In the stencil printer, ink coming out from the perforation pattern of the stencil or master 61 forms an ink image on a paper. The amount of ink transferred to the paper has influence on the smearing of the rear of the overlying paper; the greater the amount, the more the smearing is aggravated. Therefore, the smearing of the overlying paper cannot be obviated unless each perforation h of the pattern is discrete from the others, and unless the transfer of ink to the paper is controlled in amount. Hence, to eliminate t h e undesirable ink transfer, each perforation should be discrete from the others and, in addition, sufficiently fine.For example, in a facsimile apparatus using a thermal head, the glaze layer 8 is provided with a thickness as great as about 65 pm and used to prevent heat generating by the heating element portions of the head from being released downward, contrary to the glaze layer 8 of the present invention. Stated another way, the facsimile apparatus rather takes advantage of the heat accumulating function available with the glaze layer 8. It is, therefore, rather preferable for the apparatus that nearby heating dots be continuous in order to increase the density of a solid image portion having a substantial area.

By contrast, in the stencil printer, the perforations h in the stencil 61 should be sufficiently fine and be discrete from each other, as stated above. In this respect, thinning the glaze layer 8 is a very successful implementation.

Specifically, in accordance with the present invention, the thickness of the glaze layer 8 is selected to be less than 60 pm in order to reduce the diameter of perforations to b e formed in the stencil 61. In addition, because t h e accumulation of heat is adequately reduced, the thermal stress to act on the heating element portions of the head during printing is reduced. This successfully extends the life of the heating elements and provides the perforations with a moderate diameter.

The amount of ink to ooze out from the stencil 61 is proportional to the size of each perforation forming a perforation pattern. On the other hand, the size of each perforation is determined by the size of energy applied to each heating element portion. This is why the size of each perforation to be formed in the stencil 61 is controlled.

As shown in FIG. 6, for given energy applied to each heating element portion of the head, the peak temperature of the heating element portion increases with an increase in the head temperature sensed by head temperature sensing means. For example, FIG. 6, (al) compares a condition wherein the energy of a pulse current having a width tl is applied to a heating element portion under a head temperature of Tl, and a condition wherein it is applied under a head temperature of T2 higher than the temperature T1. As shown, the peak temperature of the heating element portion i S higher when the head temperature is T2 than when it is T1.

Each perforation h is greater in size when the head temperature is T2 than when it is T1, as seen from FIGS. 6, (bl) and (b2). In such a case, as shown in FIG. 6, (a2), the head is controlled such that the energy of a pulse current having a width t2 smaller than the width tí is applied to the heating element portion. As a result, the perforation h is successfully provided with the adequate size shown in FIG. 6, (bl).

More specifically, while heat generated by the heating element portions of the head is mostly used to perforate the stencil 61, part of the heat is transferred to the head body and elevates its temperature. Although the temperature elevation of the head body is generally not noticeable, it is not avoidable when the head is continuously operated for a long period of time. As a result, the heat sequentially accumulated in the head body is combined with the heat derived from the perforating energy, thereby increasing the diameter of the perforations. In addition, the size of each perforation increases with an increase in head temperature. Hence, when importance is attached to the influence of the temperature elevation of the head on the image density, it should be automatically compensated for.This can be done if the p u 1 se width is reduced when the head temperature, being sensed by head temperature sensing means, is high or is increased when it is low. Consequently, the peak temperature of the heating element portions and, therefore, the perforation diameter remains constant without regard to the head temperature.

Hence, the size of the perforation is determined by the head temperature and perforating energy. It follows that an optimal relation between the perforation diameter and the perforating energy exists which matches a head temperature.

This relation can be determined by experiments.

In accordance with the present invention, the energy to be applied to each heating element portion of the thermal head is controlled, based on the head temperature, to a value which forms an adequate perforation pattern matching the resolutions in the main and subscanning directions and faithful to any kind of document image. At the same time, the above control obviates the transfer of ink from the front of the underlying paper to the rear of the overlying paper.

Referring to FIG. 7, a stencil printer to which the present invention is applied is shown and includes a housing or cabinet 50. As shown, the cabinet 50 has a document reading section 80 in an upper portion thereof. A master making and feeding section 90 is positioned below the document reading section 80. A print drum section 100 is located at the lefthand side of the section 90 and includes a porous print d r u m 101. A master collecting section 70 is positioned at the lefthand side of the print drum section 100. A paper feed section 110 is disposed below the master making and feeding section 90. A pressing section 120 is disposed below the print drum 101. A paper outlet section 130 is located at the bottom left of the cabinet 50.

In operation, the operator lays a desired document on a table, not shown, provided on the top of the document reading section 80, and then presses a perforation start key. In response, a master discharge procedure begins. Specifically, the drum 101, carrying a used master 61b thereon, is rotated in the direction opposite to the direction indicted by an arrow A in FIG. 7. When the trailing edge of the master 61b approaches a pair of master separate rollers 71a and 71b which are in rotation, the roller 71b picks it up. A pair of discharge rollers 73a and 73b are positioned at the left-hand side of the roller pair 71a and 71b. A belt 72a is passed over the rollers 71a and 73a while a belt 72b is passed over the roller 71b and 73b. The roller pair 73a and 73b and belt pair 72a and 72b constitute a master separating and conveying section in combination.The used master 61b, having its trailing edge picked up by the roller 71b, is sequentially separated from the drum 101 by the above master separating and conveying section. Finally, the master 61b is fully separated from the drum 101 and collected in a waste master box 74. A compress plate 75 compresses the master 61b in the box 74.

In parallel with the master discharge procedure, the document 60 is read by the document reading section 80.

Specifically, the document 60 set on the table is sequentially conveyed by a separate roller 81, a first transport roller pair 82a and 82b, and a second transport roller pair 83a and 83b in directions indicated by arrow Y2 and Y3. When a plurality of documents 60 are stacked on the table, the lowermost document is fed first by a separate blade 84 cooperating with the separate roller 81. While the document 60 is conveyed on a contact glass 85, a fluorescent lamp 86 illuminates it. The resulting imagewise reflection from the document 60 is routed through a mirror 87 and a lens 88 to a CCD (Charge Coupled Device) array (photoelectric transducer) or similar image sensor 89. The image sensor 89, therefore, reads the document image by the conventional reduction type image reading scheme. The document 60 read is driven out to a tray 80A.An electric signal output from the image sensor 89 a n d representative of the document image is fed to an analog-to digital conversion (ADC) board, not shown, disposed in the cabinet 50, so that it is transformed to a digital image signal.

A master making and feeding procedure is effected in response to the digital image signal and in parallel with the image reading operation. The previously mentioned stencil 61 is implemented as a roll 61R supported by a core 61s. The core 61s is rotatably supported by a rotary support member, not shown, located at a predetermined position in the master making and feeding section 90. A platen roller 92 is pressed against a thermal head 91 with the intermediary of the stencil 61 paid out from the roll 61R. The platen roller 92 and a feed roller pair 93a and 93b are rotated to convey the stencil 61 to the downstream side in the direction of stencil transport. The head 91 has an array of minute heating element portions extending in the main scanning direction.The digital image signal from the ADC board is applied to the head 91 by way of a perforation control board, not shown, which executes various kinds of processing with the signal. The heating element portions of the head 91 selectively generate heat in accordance with the input image signal, thereby selectively perforating a thermoplastic resin film forming part of the stencil 61. As a result, a perforation pattern corresponding to the document image is formed in the stencil 61. The platen roller 92 is connected to a stencil feed motor or drive means, not shown, via a timing belt, not shown. The motor is implemented by a stepping motor and driven either intermittently or continuously. Consequently, the stencil 61 is moved, a predetermined pitch at a time, by the motor via the platen roller 92 in the subscanning direction perpendicular to the main scanning direction.

The leading edge of the perforated part of the stencil 61, i.e., a master 61a is driven toward the outer periphery of the print drum 101 by a pair of rollers 94a and 94b. At this instant, a guide member, not shown steers the leading edge of the master 61a downward. As a result, the master 61a hangs down toward a master damper 102 mounted on the drum 101. At this time, the master damper 102 is held in an open or unclamping position, as indicated by a phantom line in FIG.

7. The used master 61b has already been removed from the drum 101 by the previously stated procedure. The leading edge of the master 61a is clamped by the master damper 102 at a predetermined timing. Then, the drum 101 is rotated clockwise (arrow A) to cause the master 61a to sequentially wrap therearound. A cutter 95 cuts the master 61a at a predetermined length after the master 61a has been fully perforated. The master feeding procedure ends when the master 61a is fully wrapped around the drum 101.

As soon as the above master making and feeding procedure ends, a printing procedure begins. A paper cassette 51 is loaded with a stack of papers 62. The top sheet 62 is fed from the cassette 51 by a pick-up roller 111 and a separation roller pair 112a and 112b toward a registration roller pair 113a and 113b in the direction indicated by an arrow Y4. The registration roller pair 113a and 113b drives the paper 62 in synchronism with the rotation of the drum 101. When the paper 62 from the roller pair 113a and 113b arrives at a position between the drum 101 and the press roller 103 which is spaced from the drum 101, the press roller 103 is raised to press the paper 62 against the master 61a wrapped around the drum 101. As a result, ink is transferred to the paper 62 via the porous portion, not shown, of the drum 101 and the perforation pattern, not shown,.of the master 61a.As a result, an ink image corresponding to the document image is formed on the paper 62.

Specifically, in the drum 101, an ink supply tube 104 supplies ink to an ink well 107 formed between an ink roller 105 and a doctor roller 106. The ink roller 105 is held in contact with the inner periphery of the drum 101 and rotated in the same direction as and in synchronism with the drum 101. As a result, the ink is supplied from the ink roller 105 to the inner periphery of the drum 101. The ink is implemented by W/O type emulsion ink.

In the pressing section 120, the paper 62 carrying the ink image thereon is separated from the drum 101 by a separator 114. A belt 117 is passed over an inlet roller 115 and an outlet roller 116 and rotated counterclockwise. The paper 61 separated from the drum 101 is conveyed by the belt 117 in a direction Y5 while being sucked by a suction fan 118. Finally, the paper 61 is let fall onto a tray 52. This is the end of trial printing.

Subsequently, the operator enters a desired number of printings on numeral keys, not shown, arranged on the cabinet 50, and then presses a print start key, not shown. In response, the printer repeats the paper feeding step, printing step and paper discharging step a number of times corresponding to the desired number of printings, and then ends the entire printing operation.

Referring to FIGS. 8 and 9, there will be described an embodiment of the present invention pertaining to a control process for controlling the perforating energy to be applied to each heating element portion 10 of the head 91 to a predetermined value on the basis of the temperature of the head 91. As shown in FIG. 8, a microprocessor, or energy control means, 11 has a CPU (Central Processing Unit), an I/O (Input/Output) port, a ROM (Read Only Memory), a RAM (Random Access Memory), and other conventional constituents. The microprocessor 11 interchanges command signals and data signals with a thermistor 2 and the head 9 1, thereby controlling the entire energy control system. When the output of the thermistor 2 representative of the temperature of the head 91 is sent to the I/O port, the microprocessor 11 controls the perforating energy.For this purpose, the microprocessor 11 stores in its ROM a program defining a particular duration of a current supply to the heating element portions 10 for each head temperature and determined by experiments beforehand.

As shown in FIG. 9, the thermistor or head temperature sensing means 2 is located on a thermal head board 1 which is a circuit board carrying the head 91 thereon. In this condition, the thermistor 2 senses the temperature of the body of the head 91. Also shown in FIG. 9 are a portion 3 accommodating the heating element portions of the head 91, and a support/heat radiator 4 made of aluminum.

In FIG. 8, a power source 13 supplies the head 91 via a head driver, not shown, electric energy matching the energy necessary for the head 91 to perforate the stencil 61.

To control the perforating energy, the current or the voltage to be applied to the individual heating element 10 in accordance with the image signal may be controlled. However, in the illustrative embodiment, the perforating energy is controlled on the basis of the width of a pulse current to be applied to the individual heating element 10. Specifically, the microprocessor 11 receives the output of the thermistor 2, i.e., a head temperature data signal via the I/O port. In response, the microprocessor 11 sequentially compares the data signal with the data stored in the ROM, performs calculation by use of the CPU, and stores the result in the RAM. By such a procedure, the microprocessor selects a particular pulse width capable of forming perforations of adequate diameter in the stencil 61, and then drives the head 91.Hence, the head 91 causes its heating element portions 10 to selectively generate heat in response to the digital image signal on the basis of the pulse width selected by the microprocessor 11.

An alternative embodiment of the present invention will be described with reference to FIG. 10. This embodiment differs from the previous embodiment only in that it uses thermal record control means and a microprocessor 11 A (parenthesized in FIG. 8). The thermal record control means controls the individual heating element portion 10 of the head 91 on the basis of its thermal record. The microprocessor, or energy control means, 1 lA controls the individual heating element portion 10 such that when the portion 10 is controlled on the thermal record basis, a second pulse having a second width th for thermal record control is applied to the portion 10 with energy which is 40 % to 95 % of a first pulse having a first pulse width tp. The thermal record control means may be provided with a configuration taught in, for example, Japanese Patent Laid-Open Publication No. 5 780078. The thermal record control means, or record pulse control system, taught in the above document is applied to a thermoplastic recording apparatus and includes a first record pulse generator, a second record pulse generator, an N-bit serial-to-parallel (S/P) converter, and a circuit for comparing the data of the current line to be recorded and the data of the previous line recorded. The S/P converter converts the N-bit serial data to parallel data. If the data of the current line is black data and if the data of the previous data was also black data, a pulse from the first record pulse generator is applied to the heating element portion.However, if the data of the current line is black data and if the data of the previous line was white data, a pulse from the second record pulse generator is fed to the heating element portion. Further, if the data of the current line is white data, no pulses are applied to the heating element portion.

The microprocessor 11 A has the above functions in addition to the functions described in relation to the microprocessor 11. Hence, the ROM of the microprocessor 11 A stores, in addition to the previously stated program, a program for causing the second pulse having the pulse width th for thermal record control to be applied to the individual heating element portion 10 with energy which is 40 % to 95 % of the first pulse whose width is tp. This program is also determined by experiments beforehand.

As shown in FIGS. 10, (a) and (bl), assume that a certain heating element portion 10 to perforate the stencil 61 on the current line has perforated it on the previous line. Then, heat has accumulated in the glaze layer 8 beneath the heating element portion 10. Hence, unless the current or the voltage or the width of a pulse current to be applied to the portion 10 is changed to reduce the energy, the resulting perforation h will be excessively great and will fail to be discrete from the previous perforation h.In the illustrative embodiment, If the heating element portion 10 of interest has not generated heat on the previous line, the first pulse having the first pulse width tp is applied to the portion 10 for the current line; if it has generated heat, the second pulse having the second pulse width th, which is about 70 % of the first pulse in terms of energy, is applied for the current line. This is illustrated i n FIGS. 10, (a) and (b2). In FTG. 10, (a), the portion relating to the thermal record control is indicated by dashed lines, It is to be noted that the embodiment provides the second pulse and successive pulses with the second pulse width th.

The ratio of the second pulse width th to the first pulse width tp in terms of energy should preferably be 40 % to 95 % in respect of the discreteness of the perforations in the stencil 61. Ratios lower than 40 % reduce the size of the perforations excessively and prevent a solid image from being painted all over. Ratios higher than 95 % increase the size of the perforations excessively and cause the ink to be transferred to a paper in an excessive amount.

If desired, the second pulse width th may be followed b y a third pulse width, fourth pulse width and so forth in order to effect more delicate control.

When the head temperature does not change, or when heat does not accumulate in the head 91, i.e., when the printing period is long, or when the deterioration of image quality is acceptable, discrete perforations are achievable if the glaze layer 8 of the head 91 is 5 um to 60 pm thick, as stated earlier. In such a case, the arrangement for the pulse width control or the thermal record control and including the energy control means and microprocessor 1 lA are not necessary.

A specific configuration of the thermal head 91 and specific conditions for driving it will be described hereinafter.

The head 91 was implemented as a rectangular thin film type head having a full-glaze layer 8. While the glaze layer 8 may be formed of glass, epoxy resin or similar material, it was made of glass in the specific configuration. The head 91 had a resolution of 400 dots per inch (dpi). Each heating element portion 10 had a dimension S10 of 30 pm in the main scanning direction S and a dimension F10 of 40 pm in the subscanning direction F. The glaze layer 8 had a thickness t g of 40 um. The printing period was selected to be 2.25 msec/line. For the head temperature of 230C, the first pulse width tp and the second pulse width th were respectively selected to be 468 psec and 395 usec, and the power was selected to be 0.115 W.The stencil 61 consisted of a porous substrate implemented by Japanese paper, and a 2 um thick thermoplastic resin film adhered to the substrate. The stencil 61 had a total thickness of 40 llm .

In the above conditions, the head 91 was found to form discrete perforations each having an adequate size in the stencil 61. The stencil or master 61 with such perforations successfully formed a desired image on a paper, and prevented ink from being transferred from the paper to the rear of the next paper. In addition, the life of the heating element portions 10 was extended. These advantages were achieved even when the printing period was reduced to below 2.5 msec/line for a higher master making speed.

For comparison, use was made of a head 91 having a 65 pm glaze layer. When this head 91 was driven in the above conditions, it failed to form discrete perforations of adequate size in the stencil 61. With the resulting master, it was impossible to form a desired image on a paper or to prevent the undesirable ink transfer to the rear of the next paper.

Further, the life of the heating element portions 10 was shorter than that of the heating element portions 10 available with the 40 m thick glaze layer 8.

With a glaze layer 8 whose thickness tg was 60 um, the above advantages were achieved to a certain degree, but not to a satisfactory degree. Although even a glaze layer 8 thinner than 5 pm is expected to exhibit the effects described above, it is difficult to produce such a thin glaze layer with the state-of-the-art technologies. This, coupled with the fact t h a t the base 9 on which the resistance layer 7 should be formed with the intermediary of the glaze layer 8 must be provided with smoothness, results in a lower limit. Therefore, the thickness tg of the glaze layer 8 should preferably range from 5 um to 50 pm. The upper limit of this range is based on the results of experiments.

The stencil printer is operable with a thermoplastic stencil implemented substantially only by a thermoplastic resin film. For example, when a 2 um thick stencil of this kind was perforated by the head 91 with the width of the pulse current being changed, discrete perforations of adequate size were formed when the glaze layer 8 was 40 pm thick, as in the specific conditions described previously. The desirable perforations obviated the undesirable transfer of the ink from the front of a paper to the rear of the next paper. In addition, the life of the heating element portions 10 was extended. This was also true when the printing period was reduced to below 2.5 msec/line for a higher master making speed.

For comparison, the 2 um thick stencil stated above was used in combination with a 65 um thick glaze layer 8 and perforated under the same energy conditions (power and pulse widths). The resulting perforations were not discrete and failed to print a desirable image on a paper. The i n k transfer from the front of a paper to the rear of the next paper was noticeable. Further, the life of the heating element portions 10 was shorter than when the glaze layer 8 was 40 pm thick. When the glaze layer 8 was 60 Sum thick, the expected advantages were attained to a certain degree, but not to a satisfactory degree. Although a glaze layer 8 even thinner than 5 pm may be successful, the thickness tg of the glaze layer 8 should preferably range from 5 pm to 50 pm.

Regarding the dimensions of each heating element portion 10, the portion 10 should preferably have a dimension S10 in the main scanning direction S which is smaller than the distance or pitch plO between nearby heating element portions 20. In the illustrative embodiment, the pitch plO is selected to be 63.5 um. Also, the portion 10 should preferably have a dimension F10 in the subscanning direction which is also smaller than the pitch plO. With these dimensions, it is possible to form fine and discrete perforations in the stencil 61. More preferably, the dimensions S10 and F10 should be less than 95 % of the pitch plO each. This will further enhance the formation of fine and discrete perforations. Still more preferably, the dimensions S10 and F10 should be 30 % to 95 % of the pitch plO each.Should the dimension S10 or F10 be less than 30 Si of the pitch plO, the perforation h in the stencil 61 would be too small in size to form a smooth solid image. Should the dimension S10 or F10 be greater than 95 % of the pitch plO, the perforation h would be excessively great and would, in the worst case, prevent discrete perforations h from being formed. This would increase the amount of ink to be transferred to a paper and thereby aggravate the smearing of the rear of the next paper.

In summary, it will be seen that the present invention provides a stencil printer having various unprecedented advantages, as enumerated below.

(1) Heating element portions included in a thermal head achieve a long life without having their dimensions noticeably reduced.

(2) Fine and discrete perforations are formed in a thermoplastic stencil and form a desirable pattern matching resolutions in the main and subscanning directions. Hence, a faithful image can be formed on a paper without regard to the kind of a document image.

(3) The transfer of ink from the front of a paper to the rear of the next paper is obviated.

(4) A master can be formed in a short period of time.

(5) Energy for perforating the stencil can be delicately controlled.

(6) The printer is operable with a stencil implemented substantially only by a thermoplastic resin film in order to enhance image quality.

Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope of the invention. For example, the thermistor 2 may be disposed in the aluminum support/heat radiator 4. The head 91 may be of the kind having heating element portions arranged in a zigzag configuration or in two or more arrays in the main scanning direction.

Attention is directed to Application No. 9514326.9 (Publication No. 2294906) from which this application is divided and which claims a stencil printer in which the energy supplied to heating elements is controlled on the basis of the temperature of the head.

Claims (5)

1. A stencil printer comprising: a thermal head comprising a number of minute heating element portions to be brought into contact with a stencil having at least a thermoplastic resin film, and caused to selectively generate heat in accordance with a document image to be printed to thereby form a perforation pattern representative of said document image in said thermoplastic resin film, said thermal head comprising a glaze layer having a thickness of less than 60 ym; a print drum for causing said stencil perforated by said thermal head to wrap around said print drum; and ink supply means for supplying ink from an inner periphery of said print drum such that said ink oozes out via said perforation pattern, whereby an ink image corresponding to said document image is formed on a paper; wherein said heating element portions each have a dimension in a main scanning direction which is 30 % to 95 % of a pitch between neighbouring heating element portions, and a dimension in a subscanning direction which is 30 k to 95 % of said pitch.

2. A stencil printer according to claim 1, used in combination with a stencil which comprises substantially only said thermoplastic resin film.

3. A thermal head for a stencil printer for selectively forming a perforation pattern representative of a document image to be printed in a thermoplastic stencil having at least a thermoplastic resin film, said thermal head comprising: a base; and a glaze layer formed on said substrate and having a thickness of less than 60 ym, and wherein a print drum for causing said stencil perforated by said thermal head to wrap around said print drum; and ink supply means for supplying ink from an inner periphery of said print drum such that said ink oozes out via said perforation pattern, whereby an ink image corresponding to said document image is formed on a paper; wherein said heating element portions each has a dimension in a main scanning direction which is 30 g to 95 8 of a pitch between neighbouring heating element portions, and a dimension in a subscanning direction which is 30 % to 95 % of said pitch.

4. A thermal head according to claim 3, wherein the thickness of said glaze layer preferably lies in a range of from 5 ym to 50 ym.

5. A thermal head according to claim 3 or 4, wherein said glaze layer comprises one of glass and epoxy resin.