GB2320702A - Structures of a drum and a stencil for a stencil printer - Google Patents

Abstract

In a stencil printer, a drum has the outer periphery thereof formed by a porous sheet having passages (14 a ) for ink therein. The pores of the sheet are sized smaller than perforations (50 f ) formed in a stencil.

Description

STRUCTURES OF A DRUM FOR A STENCIL PRINTER The present invention relates to a stencil printer for printing an image on a paper by supplying ink from the inner periphery of a drum to the paper via a stencil or master wrapped around the drum and perforated in accordance with image data. More particularly, the present invention relates to an improvement in the structure of the drum A stencil printer includes a thermal head having heating elements. The heating elements selectively generate heat to perforate a thermosensitive stencil in accordance with image data and thereby form an image in the stencil. The perforated stencil, or master, is wrapped around a drum made up of a porous support and a mesh screen of resin or metal. Ink is supplied from the inner periphery of the drum by a supply member while a paper is continuously pressed against the master by a press roller or similar pressing member. As a result, the ink oozes out via the pores of the drum and the perforations of the master, thereby printing an image on the paper The drum for the above application is disclosed in, for example, Japanese Utility Model Publication No. 59-229 and Japanese Patent Publication No. 63-59393. The drum taught in Publication No. 63-59393 has a porous support and a plurality of mesh screens wrapped around the support. The outermost mesh screen is implemented by members of about 250 mesh (pitch of about 100 from) and having a diameter of about 40 pm. Pores for passing ink therethrough are sized 60 pm to 70 pm square each.

The heating elements have customarily been sized about 40 pm square each and provided with a resolution of 400 dots per inch (dpi). The heating elements each forms a perforation of substantially the same size as itself in the stencil.

The conventional stencil has a laminate structure comprising a film of polyester or similar thermosensitive resin and as thin as about I pm to 2 llm, and a porous flexible support implemented as a layer of synthetic fibers or Japanese paper or a mixture layer of Japanese paper fibers and synthetic fibers. This kind of stencil has the following problems. In portions where the Japanese paper fibers are entangled together, ink is obstructed and prevented from being transferred to a paper. As a result, fiber marks appear in the resulting image. For example, a fiber pattern appears in a solid image portion having a substantial area, or thin lines become blurred. Another problem is that when the used master is discarded, the ink deposited thereon is also discarded. This is wasteful from the resource standpoint.

It is a common practice with the stencil printer to use sparingly volatile oil ink or emulsion ink in which oil wraps water. With this kind of ink, when the printer is operated after a long time of interruption, there can be obviated an occurrence that a number of papers are simply wasted due to the evaporation of ink from the support of the drum and mesh screen or the absorption of ink by the porous support of the stencil.

However, the problem with the ink of the kind described is that a long period of time is necessary for it to infiltrate into the paper and fully dry. In a continuous print mode, when a paper or printing is laid on the previous printing whose ink is still wet, the ink is transferred from latter to the rear of the former. This is particularly true with a solid image portion to which a great amount of ink is deposited.

In light of the above, it has been proposed to reduce the thickness of the support of the stencil or omit it in order to reduce the fiber marks and the amount of ink to be discarded.

However, the outermost mesh screen of the drum has openings of substantially the same size as perforations to be formed in the stencil. Hence, even the above proposed scheme scarcely obviates the transfer of the ink from the underlying printing to the rear of the overlying printing. Further, when the support of the stencil is omitted, the stencil or master contacts the protruding portions of the mesh screen. As a result, when the paper is pressed against the master during printing, the stencil suffers from wear and, therefore, holes due to friction. This causes the ink to smear the paper.

To eliminate the undesirable ink transfer, previously mentioned Publication No. 63-59393 teaches a drum having a hollow cylindrical support having a number of pores, an inner screen layer surrounding the support, and an outer screen ]ayer surrounding the inner layer. The mesh value i s sequentially increased from the support to the outer screen layer in order to reduce the size of ink passages, i.e., to reduce the amount of ink to be drawn out from the drum. This kind of structure is also disclosed in Japanese Utility Model Publication No. 5-41026 in which the mesh value is sequentially decreased from the inner screen layer to the outer screen layer. However, because the openings of the outer mesh screen have substantially the same size as the perforations of the stencil, the ink does not break off sharply and is transferred to a paper in a great amount. This also results in the transfer of the ink from the underlying printing to the rear of the overlying printing.

It is, therefore, an object to provide a drum and a stencil printer and capable of obviating the ink transfer to the rear of the overlying printing.

It is another object to provide a drum and a stencil for a stencil printer and capable of ensuring clear-cut images free from fiber marks.

It is a further object to provide a drum and a stencil for a stencil printer and capable of eliminating the waste of ink.

GB-A-2295166, from which the present case is divided, is directed to a porous sheet for forming an outer periphery of a drum of a stencil printer, and for supplying ink from an inner periphery of said drum to a paper via said porous sheet and a perforated stencil wrapped around said drum, said porous sheet comprising: a main body; inlet pores formed in said main body and for receiving the ink from the inner periphery of the drum; outlet pores formed in said main body and for discharging the ink; and passages formed in said main body and each for causing the ink entered any respective one of said inlet pores to be diverted from a single perpendicular to the drum at least once, and then flow out via at least one of said output pores.

It is also directed to a stencil printer including such a porous sheet, a stencil including the features of such a porous sheet and a stencil printer including such a stencil.

According to the present invention, there is provided a mesh screen forming an outer periphery of a drum of a stencil printer, and for supplying ink from an inner periphery of said drum to a paper via said mesh screen and a stencil having perforations and wrapped around said drum, said mesh screen being formed with openings having a smaller size than said perforations of said stencil.

The invention will be further described by way of non-limitative example with reference to the accompanying drawings, in which: FIG. 1 is a section of a stencil printer to which the present invention is applied; FIG. 2 demonstrates the transfer of ink in a conventional stencil printer; FIGS. 3 and 4 each shows an embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1 of the drawings, a stencil printer to which the present invention is applicable is shown and generally designated by the reference numeral 1.

As shown, the printer 1 is generally made up of a printing unit 10, an ink supply unit 20, a master making unit 30, and a paper feed unit, not shown.

The printing unit 10 has a drum 11 made up of three layers, i.e. a cylindrical porous support layer 12, an ink retaining layer 13 surrounding the layer 12, and a porous sheet 14 surrounding the layer 13 and forming the outer periphery of the drum 11. The layer 13 constitutes a mesh screen. The drum 11 is mounted on a hollow center shaft 15 which plays the role of an ink pipe at the same time, as will be described later specifically. The drum 11 is rotated around the shaft 15 by a motor, not shown.

The ink supply unit 20 is disposed in the drum 11 and has an ink roller 21 for supplying ink to the inner periphery of the support layer 12. A doctor roller 22 is positioned parallel to the ink roller 21 and spaced apart from the roller 21 by a small gap. The rollers 21 and 22 cooperate to form an ink well 23 therebetween. Ink is fed to the ink well 23 by the shaft 15. The rollers 21 and 22 are rotatably supported by axially opposite end walls, not shown, provided on the shaft 15 within the drum 11. The ink is fed to the shaft 15 under pressure by a pump, not shown, from an ink pack located at a suitable position of the printing unit 10. The ink is supplied from the shaft 15 to the well 23 via a hole l5a formed in the shaft 15. A paper P is fed from the paper feed unit toward the drum 11. A press roller 16 and a pair of registration rollers 17a and 17b are located in the vicinity of the lower part of the outer periphery of the drum 11. The press roller 16 presses the paper P against the drum 11. The registration rollers 17a and 17b cooperate to drive the paper P to between the drum 11 and the press roller 16 at a predetermined timing.

A clamping device 40 is mounted on the outer periphery of the drum 11. The device 40 is made up of a stage 41 and a damper 42. The stage 41 is made of a magnetic material and extends in the axial direction of the drum 11. The damper 42 is positioned to face the stage 41 and pivotably supported by a shaft 42a. A magnet is adhered to the surface of the damper 42 that faces the stage 41. The stage 41 may be made of a material such as a metal attracted by the magnet.

The damper 42 is opened and closed at a predetermined position by opening and closing means, not shown.

The master making unit 30 has stencil support means 50-1 supporting a stencil 50 rolled round a core 50-2. A rotatable platen roller 51 conveys the stencil 50 paid out from the roll. A thermal head, or perforating means, 52 is movable into and out of contact with the platen roller 51. A pair of rollers 53a and 53b are located downstream of the platen roller 51 in order to convey the stencil 50 perforated by the head 52. A pair of cutter members 54a and 54b are positioned downstream of the roller pair 53a and 53b and c u t s the stencil 50 at a predetermined length. A guide 55 guides the cut stencil, or master, 50 toward the clamping device 40.

The rotatable platen roller 51 is mounted on a shaft rotatably supported by end walls of the printer 1 and driven at a predetermined peripheral speed by a stepping motor, not shown. In this condition, the roller 51 conveys the stencil 50 while pressing it against the thermal head 52. The head 52 has an array of heating elements extending in the widthwise direction of the stencil 50 and is moved into and out of contact with the roller 51 by moving means, not shown. The heating elements are provided with a resolution of 400 dpi, and each is sized 40 pm square. Specifically, an analog image signal representative of a document image is converted to a digital image signal by an analog-to-digital converter included in an image reading section, not shown. The digital signal is processed by a perforation control section, not shown. The head 52 selectively perforates the stencil 50 in accordance with the processed digital signal, thereby forming an image in the stencil 50. The roller 53b is rotated by the stepping motor, which drives the platen roller 51, by way of a torque limiter. The roller 53a is drived by the roller 53b. The peripheral speed of the roller 53b is selected to be slightly higher than the peripheral speed of the roller 51. This difference in peripheral speed causes a predetermined degree of tension to act on the stencil 50 between the roller 51 and the rollers 53a and 53b, thereby preventing the stencil 5 0 from creasing.

It is to be noted that the platen roller 51, thermal head 52 and rollers 53a and 53b each has a width at least equal to the width of the stencil 50.

In operation, a document is set on the image reading section, and then a start button is pressed. In response, the drum 11 starts rotating. At this instant, a used master having been wrapped around the drum 11 is separated from the drum 11 and discarded by a discharging device, not shown.

The drum 11 is brought to a stop as soon as the clamping device 40 reaches substantially the uppermost position.

Then, the opening and closing means causes the clamper 42 to rotate about the shaft 42a away from the stage 41, i.e., to open. In this position, the damper 42 waits for a master.

Subsequently the stepping motor is driven to rotate the platen roller 51. The roller 51 pays out the stencil 50 from the core 50-2 and conveys it. At the same time, the heating elements of the head 52 selectively generate heat in response to the digital image signal fed from the perforation control section. As a result, the head 52 starts perforating the stencil 50 in accordance with the image signal.

The leading edge of the stencil 50 is sequentially conveyed toward the clamping device 40 by the rollers 51, 53a and 53b over the guide 55. When the number of steps of the stepping motor reaches a preselected value, it is determined that the leading edge of the stencil 50 has entered the clearance between the damper 42 and the stage 41. Then, the shaft 42a is rotated by the opening and closing means to close the damper 42. As a result, the leading edge of the stencil 50 is clamped by the stage 41 and damper 42. At the same time, the drum 11 is rotated clockwise, as viewed in the figure, at the same peripheral speed as the platen roller 51, wrapping the stencil 50 around it.

When the stencil 50 is wrapped around the drum 11 over a predetermined length, the rotation of the drum 11 and rollers 51, 53a and 53b is once stopped. At the same time, the cutter members 54a and 54b cut the stencil 50. Then, the drum 11 is again rotated clockwise to pull out the trailing edge. not shown, of the stencil or master 50 from the master making unit 30. Consequently, the master 50 is fully wrapped around the drum 11.

The drum 11 carrying the master 50 therewith i S rotated clockwise while a single paper P is fed from the paper feed unit to between the registration rollers 17a and 17b. The rollers 17a and 17b once stop the movement of the paper P and again drive it in synchronism with the rotation of the drum 11. Hence, the paper P is inserted between the master 50 on the drum 11 and the press roller 16 at an adequate timing. At this instant, the ink roller 21 is rotated in the same direction as the drum 11. The ink in the ink well 23 is deposited on the surface of the ink roller 21 being rotated. The doctor roller 22 regulates the amount of the ink being conveyed by the ink roller 21. The regulated amount of ink is supplied to the inner periphery of the support layer 12.

As the ink roller 21 is further rotated, the ink penetrates the porous support layer 12, ink retaining layer 13 and porous sheet 14 to reach the master 50. The sheet 14 has inlet pores and outlet pores, Subsequently, the press roller 16 is raised to press the paper P against the master 50 wrapped around the porous sheet 14 of the drum 11 being rotated. As a result, the ink is transferred to the paper P via the perforations of the master 50 which will also be described later specifically. The ink forms an image representative of the document image on the paper P. The paper P with the image, i.e., a trial printing is separated from the master 50 by a separator, not shown, and then driven out to a tray not shown. After the trial printing, the press roller 16 is moved away from the drum 11 to its original position. In this condition, the entire printer 1 waits for a print start command. Thereafter, the above procedure is repeated to produce a desired number of printings.

A reference will be made to FIG. 2 for describing the transfer of the ink from the front of one printing to the rear of the next printing. The transfer is attributable to the structure of a drum and that of a master included in a conventional stencil printer. It has been customary with a stencil printer to press a paper against a master wrapped around a drum by a press roller and thereby cause ink to ooze out from the inside of the drum, as stated above. After the transfer of the ink to a paper via the master, the paper is separated from the master and then driven out to a tray.

Specifically, as shown in FIG. 2, when a paper P is separated from a master 60, ink 80 is drawn out in a great amount via the pores of a support layer, not shown, the pores 70a of a mesh screen 70, the pores 61a formed in a porous substrate 61 forming part of the master 60, and pores 62a formed in a thermoplastic resin film 62 forming the other part of the master 60. At this instant, each pore 70a of the mesh screen 70 has centers C1 and C2 at the inlet side and outlet side, respectively, which are positioned on a single perpendicular S connecting the center and surface of the drum. As a result, the ink 80 is transferred to the paper P in a great amount, slipping on the inner walls of the pores 70a. Hence, a substantial period of time is necessary for the ink 80 to dry, resulting in the undesirable transfer of the ink to the rear of the next printing.

FIGS. 3 and 4 show an embodiment of the present invention. As shown in FIG. 3, a master 50A is implemented only by a thermoplastic resin film 50e and formed with perforations 50f. As shown in FIGS. 1 and 3, the drum is made up of three layers, i.e., the cylindrical porous support layer 12, the ink retaining layer 13 surrounding the layer 12, and a mesh screen 14J surrounding the layer 13 and forming the outer periphery of the drum 11.

The layer 12 is made of stainless steel or similar metal and formed with a number of pores 12a for passing the ink 80 therethrough. The layer 13 is implemented by a metal]ic mesh screen or foam resin in order to retain the ink 80 while passing it therethrough. A plurality of layers 13 may by provided, if desired.

As shown in FIG. 4, the mesh screen l4J is formed with square openings 14a for passing the ink 80 therethrough. The openings 14a are each sized 15 pm square which is smaller than the size B (about 40 llm) of a single substantially square perforation 50f formed in the stencil 50. The screen 14J has a line width C of 5 pm. The screen 14J is implemented by a thin sheet of, for example, copper or stainless steel and formed with the openings 14a by etching. The screen 14J has a uniform thickness and has at least one side thereof smoothed. The screen 14J is wrapped around the support layer 12 with the smoothed surface thereof facing outward.

The screen 14J may be produced by the electroforming of a thin sheet having a uniform thickness and formed with the openings 14a, at least one side thereof being smoothed. The screen should preferably be about several microns to 30 pm thick; it should be as thin as possible within a range which does not lower the strength to an excessive degree. The etching or electroforming reduces the production cost of the screen 141. Of course, the shape of each opening 14a is not limited to a square, but it may be a circle, hexagon, polygon or any other suitable shape.

The openings 14a of the screen 14J shown in FIGS. 3 and 4 is assumed to be 15 um long at each side A. However, when each side B of the perforation 50f is 40 pm long, the side A may range from 5 pm to 20 pm. Specifically, when the side A is 5 pm long, the ratio of the area of the opening 14a (5 x 5 pm2) to the area of the perforation 50f, i.e., j(5x5)/(40x40)]xl00 is 1.6 %. On the other hand, when the side A is 20 pm long. the above ratio is 25 %, i.e., ((20x20)/(40x40)1x100 = 25. If the radio is smaller than 1.6 %, the amount of the ink 80 to be transferred to the paper P is too small to provide a printing with sufficient density. If the ratio is equal to or smaller than 25 %, the ink 80 is transferred to the paper P in a desired small amount and, therefore, more effectively prevented from being transferred to the rear of the next printing. Hence, the above ratio should preferably lie in a range of from 1.6 % to 25 Se.

As stated above, in the embodiment shown in FIGS. 3 and 4, the openings 14a of the mesh screen 14J are each sized greater than the perforations 50f of the master or stencil 50A. Hence, the ink to ooze out from the perforations 50f is reduced in amount. It follows that the ink transferred to the paper P infiltrates and dries rapidly and is, therefore, scarcelv transferred to the rear of the overlying sheet. The master 50A, implemented only by the thermoplastic resin film 50e, reduces the amount of the ink 80 to be discarded therewith and obviates fiber marks. In addition, the master 50.t is wrapped around the mesh screen 14J which is smoothed on the side contacting the stencil 50A and provided with a uniform thickness. This allows the mesh screen 14J and master 50A to contact each other over a broad area, reduces the wear of the master 50A due to the paper P, and protects the master 50A from tearing.

Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof.

Claims (3)

1. l. A mesh screen forming an outer periphery of a drum of a stencil printer, and for supplying ink from an inner periphery of said drum to a paper via said mesh screen and a stencil having perforations and wrapped around said drum, said mesh screen being formed with openings having a smaller size than said perforations of said stencil.
2. 2. A mesh screen as claimed in claim 1, wherein said mesh screen is smoothed on the side contacting the stencil and provided with a uniform thickness.
3. 3. A mesh screen for use on the drum of a stencil printer, the mesh screen being constructed and arranged to operate substantially as hereinbefore described with reference to and as illustrated in Figures 3 and 4 of the accompanying drawings.