EP2455224A1

EP2455224A1 - Method for printing on both sides of paper, paper feeding apparatus, and printer paper

Info

Publication number: EP2455224A1
Application number: EP10799551A
Authority: EP
Inventors: Iwao Matsumoto
Original assignee: Photo Craft Co Ltd
Current assignee: Photo Craft Co Ltd
Priority date: 2009-07-14
Filing date: 2010-05-10
Publication date: 2012-05-23
Anticipated expiration: 2030-05-10
Also published as: JP5349180B2; EP2455224A4; JP2011020292A; EP2455224B1; WO2011007487A1

Abstract

An object of the present invention is to perform printing an image onto both sides of a sheet with high accuracy, even if the sheet is large. A sheet and a feeder YH are used. The sheet has perforations formed at both ends of the sheet for sheet feed, and a feeder includes a drive roller 13 and two pin tractors 14, the drive roller rotating to drive the sheet in contact with the sheet, and the pin tractors including a plurality of pins to engage the perforations and being disposed in both ends of the drive roller to rotate to drive the sheet with a coaxial rotational driving force with the drive roller. The sheet is disposed in the feeder, and, in this state, the sheet is conveyed synchronously with movement of the pins to print an image onto one side of the sheet. The sheet is turned, onto one side of which printing has been performed, inside out and is disposed in the feeder, and, in this state, the sheet is conveyed synchronously with the movement of the pins to print an image onto another side of the sheet.

Description

TECHNICAL FIELD

The present invention relates to a method for performing printing on both sides of a sheet, a sheet feeder of a printer, and a print sheet used therein.

BACKGROUND ART

Silver-halide color films have been widely used in the past as image display media such as lit advertising signs and lit signboards. In recent years, however, an image display medium provided by printing an image onto a print sheet with an inkjet printer has often been used.
With such an image display medium, an image for transmitted light is required to have a density higher than that of an image for reflected light.
The silver-halide technology has a long history. The silver-halide technology involves using a pigment (coupler) having good transparency, so that improved color reproduction can be achieved even in a high density. In contrast, the inkjet technology has a short history. In the inkjet technology, quality is ignored in order to increase the density of an image. For example, the density of ink consisting of a pigment with poor transparency is increased simply, or, ink is applied repeatedly a few times.
Double-sided printing using the inkjet technology presents a problem that there is misregistration between an image on the obverse and an image on the reverse.
There is disclosed an ink jet printer in which a recording medium is sandwiched between a sheet feed roller and a pressure roller, carried thereby, a recording head is reciprocated on a platen in the right-angle direction to the medium carriage direction, and an image is printed (Patent Document 1: Japanese Laid-open Patent Publication No. 2001-335183 ). When such an ink jet printer is used to perform double-sided printing, the sheet is not in place on the sheet feed roller, which causes misregistration between images.
For this reason, such a method has been employed only when a large ink jet printer is used to perform printing onto a large sheet. To be specific, in the case of a large sheet, the distance between the sheet and a user is long, i.e., the user looks at the sheet from the position far away from the sheet. In such a case, misregistration between the image on the obverse and the image on the reverse is not so apparent, which is supposed to be acceptable.
In even such a case, however, it is true that there is large misregistration between the image on the obverse and the image on the reverse. When a user takes a closer look at the images, the misregistration therebetween is apparent, which is problematic in the light of image quality.
The following methods have been used to correct the misregistration: a method for automatically detecting image misregistration to correct an error manually; and a method for reading an error with a CCD camera to correct a print position automatically. Even when these methods are employed to correct the misregistration, however, it is still difficult to sufficiently correct the misregistration. The misregistration is not so apparent when a large sheet is used and the images are viewed from far away. However, in the case of a medium or small sheet, the misregistration is apparent, which still remains as an unsolved problem.
Examples of a recording sheet for transmitted light used in an inkjet printer are: recording material formed by providing a porous structured ink accepting layer containing titanium oxide therein (Patent Document 2: Japanese Laid-open Patent Publication No. 2004-167706 ); and a film material having an ink receiving layer containing therein water-containing silica particles and a water-retaining material (Patent Document 3: Japanese Laid-open Patent Publication No. 2001-135859 ).
Further, as a recording sheet for reflected light and transmitted light used in an ink jet printer, a sheet is proposed which has an ink receiving layer containing silica particles therein, and has an opacity of 70 % through 90 % on the whole sheet (Patent Document 4: Japanese Laid-open Patent Publication No. 2003-312131 ).
As described above, when the printer disclosed in Patent Document 1 is used to perform double-sided printing of images, the sheet positioning is difficult, which makes it impossible to register images with high accuracy.
When the sheet disclosed in Patent Document D2, D3, or D4 is used for double-sided printing of images, the sheet positioning is difficult, which makes it impossible to register images with high accuracy. In the case of double-sided printing of images on such a conventional sheet, after an image is printed onto the obverse of the sheet, the sheet is turned inside out and is placed in a correct position as much as possible, and an image is printed onto the other side (reverse) of the sheet. If misregistration occurs after printing is started, position correction is performed on an as-needed basis.
In such a method, however, a large amount of work is needed to place a sheet at the correct position in a sheet feed roller. In addition, a user will not find whether or not the sheet is placed at the correct position until printing is started. Even when the user finds that the sheet is not in place, it is not easy for him/her to set the sheet in place for a position correction. If such a position correction cannot be made, the user is required to place the sheet from the beginning again.
In the meantime, a continuous form has been conventionally used which is provided with perforations at both ends thereof for sheet feed. Since such a conventional continuous form is small like B5 size or A3 size and lightweight, it can be fed by a conventional pin tractor feed mechanism. In addition, the sheet positioning accuracy required is approximately 1 mm or less because the conventional continuous form is primarily used for letter printing.
In contrast, when images are printed on both sides of a sheet, accuracy incomparably higher than that in the conventional continuous form, e.g., accuracy in the micrometer range is required. In addition, a sheet used for lit advertising signs has a size of 1 meter or greater and is also heavy. In the conventional technologies, double-sided printing of images cannot be performed on a large sheet at high accuracy.
The ideal lit signboard has a structure in which a backlight is provided on the rear of an image, and the illumination on the obverse of the image by the backlight is higher than that on the obverse of the image by outside light. Such a condition is satisfied easily indoors while the condition is not satisfied easily outdoors.
The reason for this is as follows. Environment light within doors is artificial lighting, and the illumination on the obverse by the artificial lighting is not so high. Accordingly, if the light source of lighting from the reverse satisfies a predetermined condition, lightness on the reverse can be higher than the lightness on the obverse. In contrast, environment light outside is the sunlight, and the illumination on the obverse by the sunlight is very high. Accordingly, it is very difficult that lightness on the reverse is higher than the lightness on the obverse.
beauty of night view in a big city is created by geometric patterns by artificial 1 lighting. In the daytime, lighting of buildings is turned ON with some exceptions. However, building windows in the daytime seem to be totally different from those at night. This is attributed to the difference in brightness between artificial lighting and the sunlight. The illumination of surface light source of a lit advertising sign or lit signboard is approximately 5000-6000 lux in average, while the illumination of sunlight is as high as 100000 lux. The difference in illumination causes the difference in contrast, so that light from building windows within doors cannot be observed in the daytime. In view of the above, the ideal lit advertising sign has a structure in which images are created on both surfaces of a reflectable material, the images have appropriate brightness for viewing even if environment light on the obverse of the image is the sunlight or artificial light, and the images are visible with backlight at night. While salver-halide color films are not perfect as an image display medium for an image for the ideal lit advertising sign, there has been no material for lit advertising sign serving as substitutes for the color films.

DISCLOSURE OF THE INVENTION

The present invention is in view of the problems described above, and an object of the present invention is to provide a method for printing an image onto both sides of a large with high accuracy, a sheet feeder for carrying the large sheet with high accuracy of positioning, and a print sheet.
A method according one aspect of the present invention is a method for printing an image onto both sides of a sheet. The method includes using a sheet having perforations formed at both ends of the sheet for sheet feed; using a feeder including a drive roller and two pin tractors, the drive roller rotating to drive the sheet in contact with the sheet, and the pin tractors including a plurality of pins to engage the perforations and being disposed in both ends of the drive roller to rotate to drive the sheet with a coaxial rotational driving force with the drive roller; disposing the sheet in the feeder, and, in this state, conveying the sheet synchronously with movement of the pins to print an image onto one side of the sheet; and turning the sheet, onto one side of which printing has been performed, inside out and is disposed in the feeder, and, in this state, conveying the sheet synchronously with the movement of the pins to print an image onto another side of the sheet.
Preferably, the pin is formed to have a shape which makes a distance between a front base position of the pin at a time when the perforation of the sheet starts to disengage from the pin and a front position of the pin on a line along a movement direction of the sheet after the pin is moved equal to a distance of a movement of the front base position of the pin.

BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 is a diagram showing a schematic structure of a printer according to an embodiment of the present invention.
Fig. 2 is a front view of a sheet feeder of a printer.
Fig. 3 is a front view in which the main part of the sheet feeder shown in Fig. 2 is enlarged.
Fig. 4 is a cross-sectional view of the sheet feeder shown in Fig. 3.
Fig. 5 is a cross-sectional view cut along the A-A line of the sheet feeder shown in Fig. 2.
Fig. 6 is a partial enlarged view of Fig. 5.
Fig. 7 is a front view of a sheet.
Fig. 8 is an enlarged view of a part of a perforation of a sheet.
Fig. 9 shows diagrams depicting how a sheet is conveyed by a pin tractor.
Fig. 10 is a diagram depicting coordinate axes with respect to the shape of the front peripheral surface of a pin.
Fig. 11 is a diagram depicting a relationship of contact position between a sheet and the front peripheral surface of a pin.
Fig. 12 is a diagram depicting a method for determining the shape of the front peripheral 1 surface of a pin.
Fig. 13 is a diagram depicting another example of coordinate axes with respect to the shape of a pin.
Fig. 14 is a diagram depicting another example of a relationship of contact position between a pin and a sheet.
Fig. 15 is a diagram showing another example of a pin tractor.
Fig. 16 shows diagrams of images printed onto both sides of a sheet.
Fig. 17 is a diagram depicting a density of an image to be printed onto the reverse of a sheet.

DESCRIPTION OF EMBODIMENT(S)

Referring to Fug. 1, a printer 1 is provided with a body 11, a delivery roller 12, a drive roller 13, a pin tractor 14, a pressure roller 15, a recording head 16, a platen box 17, a fan 18, a sheet guide 19, a control circuit, and the like.
The printer 1 is connected to a computer via a non-illustrated interface. The printer 1 can be controlled through operation of a pointing device such as a keyboard, The control details, an image, and so on can be displayed on a non-illustrated display.
A roll sheet PY is rotatably placed in the body 11. The sheet PY is pulled out by a sheet feeder YH (described later). A recording head 16 serves to print an image GZ onto the obverse of the sheet PY. The sheet PY is a double-sided printable sheet, and is transparent or translucent. An image printed onto the sheet PY can be observed with reflected light or transmitted light.
Referring to Figs. 7 and 8, the sheet PY has perforations PA on both ends for sheet feed. The perforations PA are aligned at regular intervals along the length direction of the sheet PY. Each of the perforations PA has a circular shape. The rim of the perforation PA is cut out along the circumference thereof. Stated differently, the perforation PA has, on its rim, no jaggies which avoid interference with a pin. The outer surface of a pin comes in contact with the rim of the perforation PA, and thereby, the positional relationship therebetween is defined appropriately.
The sheet PY also has marks MKP indicating the correspondence relationship between the positions of perforations PA formed on one of the edges of the sheet PY and the positions of perforations PA formed on the other edge.
One mark MKP is provided for a predetermined number of perforations PA. In the illustrated example of Figs. 7 and 8, one mark MKP is provided for five perforations PA. The mark MKP may be a black dot, a color dot, an arrow, or the like. The mark MKP may be a hole.
The marks MKP indicate that perforations PA formed on the right or left of the marks MKP are located at the same position in the longitudinal direction. Accordingly, it is possible to optionally select the number, position, and shape of the marks MKP, and a method for adding the marks MKP as long as the marks MKP indicate which perforations PA are provided at the same position in the longitudinal direction.
The delivery roller 12 serves to deliver the sheet PY to the drive roller 13 and the pin tractors 14. The delivery roller 12 may be or may not be provided with pins to engage with the perforations PA. A guide plate may be provided instead of the delivery roller 12. Alternatively, a guide plate may be provided together with the delivery roller 12.
The drive roller 13 and the pin tractors 14 cooperate with the pressure roller 15 to correctly position the sheet PY With high accuracy, and to convey the sheet PY.
Referring also to Fig. 2, the drive roller 13 rotates to drive the sheet PY in contact with the lower surface of the sheet PY. The pin tractors 14 are provided on both ends of the drive roller 13. Each of the pin tractors 14 has a plurality of pins PN to engage the perforations PA, for sheet feed, formed on both ends of the sheet PY. The pin tractors 14 rotate to drive the sheet PY with a coaxial rotational driving force with the drive roller 13.
To be specific, the pin tractors 14 are driven to rotate by a drive shaft having the same axis as that of the drive roller 13. It is therefore easy to synchronize sheet delivery by the pin tractors 14 with sheet delivery by the drive roller 13.
In this embodiment, while the shaft of the pin tractors 14 the shaft of the drive roller 13 are axially connected together, they may be connected to each other through a power conveyance mechanism such as a coupling or a gear at an appropriate position in the axial direction.
Note that the pin tractor 14 on the left-land side of Fig. 2 is sometimes referred to as a "pin tractor 14a" and the pin tractor 14 on the right-hand side of Fig. 2 is sometimes referred to as a "pin tractor 14b".
The pressure rollers 15 ( pressure rollers 15a and 15b) are so provided as to face the drive roller 13 and the pin tractors 14, and serve to press the sheet PY against the drive roller 13 and the pin tractors 14.
To be specific, the pressure roller 15a is gently pressed against the drive roller 13. This prevents large friction from being produced on a contact surface between the drive roller 13 and the sheet PY. As a result, slight misalignment between the drive roller 13 and the sheet PY is tolerated.
In order to weaken the force pressing the pressure roller 15a against the drive roller 13, a force exerted by a spring that presses the roller 15a is preferably weakened. In the case of a structure in which the pressure roller 15a is pressed against the drive roller 13 due to the own weight of the pressure roller 15a, the weight of the pressure roller 15a may be reduced. Alternatively, it is possible to provide a spring for adjusting the pressing force of the pressure roller 15a. Alternatively, it is also possible to reduce the number of pressure rollers 15a or the length thereof.
Alternatively, it is possible to use, as a material of the surface of the pressure roller 15a, a material having a small coefficient of friction, instead of the reduction in pressing force of the pressure roller 15a, or, in addition to the reduction. Yet alternatively, it is also possible to use, as a material of the surface of the drive roller 13, a material having a small coefficient of friction.
The pressure roller 15b presses the sheet PY against the pin tractor 14. This allows the sheet PY to be pressed on the surface of a main roller 141 at the positions of the perforations PA, so that the bases TN of the pins PN engage the perforations PA. Thereby, the rotation of the pin tractors 14 moves the positions of the perforations PA precisely along a curve CV, discussed later, of the peripheral surface of a pin PN. Consequently, the sheet PY is conveyed properly with high accuracy.
If no pressure roller 15b is provided, or, if the pressure roller 15b does not press the sheet PY onto the surface of the main roller 141, the perforations PA reach not the bases TN of the pins PN but the other parts of the pins PN in some cases. In such a case, the sheet PY is not properly conveyed through the pins PN.
The sheet feeder YH is constituted by the drive roller 13, the pin tractors 14, and the pressure rollers 15a and 15b. The sheet feeder YH is driven to rotate at a controlled speed by a non-illustrated power conveyance mechanism such as a motor, a gear, or a belt.
The sheet feeder YH delivers the sheet PY synchronously with the movement of the pins PN. Then, the drive roller 13 rotates to send the sheet PY forward. Stated differently, both ends of the sheet PY are conveyed by the pin tractors 14, and the middle part of the sheet PY is conveyed by the drive roller 13. The conveyance speed by the pin tractors 14 is the same as that by the drive roller 13. Consequently, the sheet PY is linearly conveyed precisely at a controlled speed without being skewed or being loosened.
In practical cases, the conveyance by the pin tractors 14 is dominant compared to the conveyance by the drive roller 13, and the sheet PY is conveyed synchronously with the movement of the pins PN of the pin tractors 14. To be specific, if a change in temperature or humidity makes a small difference in conveyance speed between the pin tractors 14 and the drive roller 13, the conveyance speed by the pin tractors 14 is applied, and therefore, the drive roller 13 sends the sheet PY forward at substantially the same speed as the conveyance speed by the pin tractors 14.
The driver roller 13 may be structured to have a diameter slightly smaller than that of the main roller 141 in order that the conveyance by the pin tractors 14 is dominant.
In the meantime, the two pin tractors 14 have marks MKT showing the positional correspondence relationship in the conveyance direction between the pins PN provided in the pin tractor 14a and the pins PN provided in the pin tractor 14b.
The marks MKT have different colors or shapes in such a manner that the pins PN can be distinguished from one another. For example, the marks MKT may be provided by printing or engraving different color dots or arrows such as black, red, green, blue, yellow, and purple. Alternatively, it is possible to add a number or sign to each of the pins PN.
One mark MKT may be provided for a predetermined number of pins PN. For example, one mark MKT is provided for three pins PN.
The marks MKT indicate that pins PN provided on the right or left of the marks MKT are located at the same position in the longitudinal direction. Accordingly, it is possible to optionally select the number, position, and shape of the marks MKT, and a method for adding the marks MKT as long as the marks MKT indicate which pins PN are provided at the same position in the longitudinal direction.
Immediately after the sheet feeder YH delivers the sheet PY, the recording head 16 performs color printing on the obverse of the sheet PY by the inkjet method. The recording head 16 is attached to a non-illustrated carriage reciprocating along with a rail. The carriage reciprocates above the platen box 17 in a manner to cross over the sheet PY in the width direction thereof with a small gap kept between the carriage and the sheet PY. The recording head 16 has an ink nozzle facing the upper surface of the platen box 17. The ink nozzle blows ink onto the sheet PY adsorbed on the upper surface of the platen box 17, so that an image GZ is printed onto the sheet PY.
Assuming that the conveyance direction of the sheet PY is an X-direction, the movement direction of the recording head 16 is a Y-direction orthogonal to the X-direction. The sheet PY is moved in the X-direction and the recording head 16 moves in the Y-direction, and thereby, the image GZ is printed onto the XY-plane that is the obverse of the sheet PY.
In order to achieve a correct print position, it is necessary that the position of the image GZ generated by using ink blown from the recording head 16 and the position of the sheet PY in the XY-direction should have a predetermined proper relationship. Further, in order that the images GZ to be printed onto both sides of an identical sheet PY are registered at high accuracy with respect to each other, the sheet PY is required to be placed precisely at the correct position in the sheet feeder YH, and to be conveyed correctly without any misalignment.
In the sheet feeder YH of this embodiment, since the pin tractor 14 is used to convey the sheet PY, the position of the sheet PY is precisely defined thanks to the pins PN engaging the perforations PA of the sheet PY, and no misalignment occurs during the conveyance of the sheet PY.
In addition, as discussed later, the pin PN is formed to have a shape which makes the distance between the front base position of the pin PN at a time when the perforation PA of the sheet PY starts to disengage from the pin PN and the front position of the pin PN on a line along the movement direction of the sheet PY after the pin PN is moved equal to the distance of the movement of the front base position of the pin PN.
Stated differently, the pin PN is formed to have a shape which allows the pin PN to move, during a period from when the pin PN engages the perforation PA to when the pin PN is disengaged therefrom, in the direction which allows the front of the pin PN to be disengaged from the perforation PA in contact with the front rim of the perforation PA.
For this reason, the conveyance speed of the sheet PY by the pin tractors 14 is equal to the circumferential velocity of the drive roller 13, so that the sheet PY is delivered smoothly at a constant speed. The shape of the pin PN will be detailed later.
The platen box 17 has a fan 18 in the lower part thereof. The fan 18 exhausts air of the platen box 17, which reduces the pressure in the platen box 17. The platen box 17 has many pores in the upper surface thereof. The sheet PY is adsorbed due to the negative pressure, and is brought into intimate contact with the upper surface of the platen box 17. In short, the sheet PY is conveyed in intimate contact with the upper surface of the platen box 17.
The sheet PY onto which the image GZ is printed by the recording head 16 is outputted along the sheet guide 19.
As shown in Fig. 2 through Fig. 5, the drive roller 13 integrally has drive shafts 13a and 13b that extend linearly from both ends thereof. The drive shafts 13a and 13 are rotatably supported at ends thereof by bearings 21a and 21b attached to the body 11. The drive shafts 13a and 13b are rotationally driven by a non-illustrated power conveyance mechanism.
Since the surface of the drive roller 13 is knurled, adequate friction is produced between the drive roller 13 and the sheet PY. The drive roller 13 may be integrated by, for example, providing a screw in the inner peripheral surface of both ends of a hollow pipe, and by engaging threadedly, therein, a boss or threaded shaft to be screwed in the screw.
One of the drive shafts, i.e., the drive shaft 13a has, on its surface, a key groove provided in the axial direction. A key 22 is fitted into the key groove. The pin tractor 14a is provided with a key groove 23 to engage the key 22. The key groove 23 engages the key 22, and is axially movable along the key 22. The pin tractor 14a is also provided with a setscrew 24. The axial position of the pin tractor 14a is adjusted, and the setscrew 24 is fastened. Thereby, the position of the pin tractor 14a is fixed in the axial direction.
As with the case of the pin tractor 14a, the pin tractor 14b is also provided with a setscrew. After the position of the pin tractor 14b is adjusted in the axial direction, the setscrew is fastened. Thereby, the position of the pin tractor 14a is fixed.
Further, rollers 131 and 132 are interposed between the drive roller 13 and the pin tractor 14a to fill the space therebetween. The rollers 131 and 132 have the same outer diameter as that of the drive roller 13. Each of the rollers 131 and 132 is provided with a key groove to engage the key 22. The rollers 131 and 132 are axially movable along the key 22. Each of the rollers 131 and 132 is also provided with a setscrew. When the setscrew is fastened, the position of each of the rollers 131 and 132 is fixed in the axial direction.
It is possible to appropriately select the number of rollers 131 and 132, the length thereof in the axial direction, and the like. In a state where the drive roller 13 and the drive shafts 13a and 13b are detached from the body 11, the rollers 131 and 132 may be attached to the drive shaft 13a. Alternatively, it is possible to form the rollers 131 and 132 to have a double-split structure, to couple the split halves together with a screw or the like, so that the rollers 131 and 132 are attached to the drive shaft 13a.
Referring to Figs. 5 and 6, the main roller 141 of the pin tractor 14 has a plurality of equiangulary spaced pins PN. In short, the pin tractor 14 is a roller with pins. The main roller 141 has an outer diameter equal to that of the drive roller 13.
As clearly shown in Fig. 6, the outer peripheral surface of a pin PN is in the form of a circumferential surface, and the pin PN has a cannonball shape forming a smooth carve from the bottom position TN to the tip position TS of the pin PN.
The shape of the pin PN is detailed below.
Attention is focused on the center pin PN of Fig. 6. The pin PN is inserted through a perforation PA of a sheet PY, and the front bottom position TN of the pin PN contacts the front rim of the perforation PA of the sheet PY. Supposing that the outer diameter of the pin PN at the bottom position TN is denoted by DN, and that the inner diameter of the perforation PA is denoted by DA, the relationship therebetween is expressed as DA>DN. Accordingly, a gap is provided between the rear bottom position TN of the pin PN and the rear rim of the perforation PA.
To be specific, if the outer diameter of the main roller 141 is approximately 28 millimeters, the maximum value of the outer diameter of the pin PN is approximately 3 millimeters, the inner diameter of the perforation PA is approximately 3.2 millimeters, and therefore, the gap between the pin PN and the rim of the perforation PA is approximately 0.2 millimeters. The pin PN has a height of approximately 2.5 millimeters.
Under this state, the pin tractor 14 rotates in the direction of the right-hand side of Fig. 6, i.e., in the direction denoted by the arrow M1. Thereby, the sheet PY is conveyed in the direction of the right-hand side of Fig. 6, i.e., in the direction denoted by the arrow M2.
With respect to the center pin PN of Fig. 6, a pin PN provided ahead by one pitch is sometimes referred to as a pin PNZ. The pin PNZ is a state where a ridge formed by an imaginary surface MK extended from the outer peripheral surface of the pin PNZ and a plane MH passing through the tip position TS contacts the front rim of a perforation PA. In short, this state shows the moment at which the pin PNZ is disengaged from the perforation PA.
Note that, although the bottom position TN differs in height depending on the position of the pin PN in the peripheral direction, the difference is ignored in this description because of the very small value.
With respect to the center pin PN of Fig. 6, a pin PN provided behind by one pitch is sometimes referred to as a pin PNG.
Fig. 9 shows diagrams depicting how a sheet PY is conveyed with the rotation of the pin tractor 14.
Fig. 9A shows the same state as that of Fig. 6 described above. Under the state, the bottom position TN of the pin PN contacts the perforation PA at the part surrounded by a broken-line circle. Fig. 9B through Fig. 9F show the individual states in which the pin tractor 14 rotates by a predetermined angle in the direction denoted by the arrow M1. After the state shown in Fig. 9F, the state goes back to that of Fig. 9A again. In Fig. 9A through Fig. 9F, the bottom position TN of the pin PN contacts the perforation PA at the parts surrounded by broken-line circles.
Referring to Fig. 9A through Fig. 9D, only one pin PN contacts the perforation PA at the bottom position TN of the pin. Referring to Fig. 9E and Fig. 9F, two pins PN and PNG contact the perforations PA at the individual bottom positions TN of the pins PN. During the states shown in Figs. 9E and 9F, conveyance and positioning of the sheet PY is taken over from the pin PN to the pin PNG.
During the states shown in Figs. 9E and 9F, the pin PN moves in the direction which allows the front peripheral surface of the pin PN to be disengaged from the perforation PA in contact with the front rim of the perforation PA.
Referring to Fig. 10, in order to find the peripheral shape of the pin PN, the bottom position TN is set at the origin, the tangential direction of the surface of the main roller 141 is set as the U-axis, and a direction vertical to the U-axis is set as the V-axis. The peripheral shape of the pin PN is determined by obtaining the coordinates of the individual points on the curve CV in such a UV-plane.
As shown in Fig. 11, the U-axis and the V-axis move in accordance with the rotational angle of the main roller 141. It is assumed that the position of the origin is a position at which the bottom position TN of the pin PN is directly above the pin tractor 14, i.e., a position at which the pin PN is on the contact position of the main roller 141 and the sheet PY, the rotational angle on that position is set at zero degrees, and the rotational angle therefrom is set at θ degrees. With respect to the U-axis and the V-axis at various rotational angles θ degrees, coordinates of the contact position P to the sheet PY may be determined.
In Fig. 11, U is calculated as follows.
$U = cosθ \times D$
$\begin{array}{l} = cosθ \times (B - C) \\ = cosθ \times (r \times tanθ - 2 πrθ / 360) \end{array}$

Further, V is calculated as follows.
$\begin{array}{l} V = E - G \\ = (A - r) - (D \times sinθ) \\ = (r / cosθ - r) - (r \times tanθ - 2 πrθ / 360) \times sinθ \end{array}$

where r denotes the radius of the main roller 141. since it is assumed that the circumferential velocity due to the rotation of the main roller 141 is in synchronization with the conveyance speed of the sheet PY, the value "C" denoting the conveyance distance of the sheet PY is equal to the length of the circumference corresponding to the rotational angle θ.
In this way, coordinates of the contact position P at different rotational angles θ degrees are obtained, and the resultant is plotted on the UV-plane as shown in Fig. 12.
The curve CV shown in Fig. 12 is rotated with respect to the centerline TC of the pin PN, so that the outer peripheral surface of the pin PN is formed.
Referring to the curve CV of Fig. 12, a part of the curve CV far away from the origin corresponds to a line represented as the imaginary surface MK as discussed above.
Further, the peripheral shape of the pin PN can be determined in the following manner.
As shown in Fig. 13, the centerline TC of the pin PN is set as the V-axis, and a tangential line vertical to the V-axis on the surface of the main roller 141 is set as the U-axis. The peripheral shape of the pin PN is determined by obtaining the coordinates of the individual points on the curve CV in such a UV-plane.
As shown in Fig. 14 , the U-axis and the V-axis move in accordance with the rotational angle of the main roller 141. It is assumed that the position of the origin is a position at which the pin PN is directly above the pin tractor 14, i.e., a position at which the pin PN is on the contact position of the main roller 141 and the sheet PY, the rotational angle on that position is set at zero degrees, and the rotational angle therefrom is set at θ degrees. With respect to the U-axis and the V-axis at various rotational angles θ degrees, coordinates of the contact position P to the sheet PY may be determined.
In Fig. 14, U is calculated as follows.
$\begin{array}{l} U = cosθ \times D \\ = cosθ \times (C - B) \\ = cosθ \times (2 πrθʺ / 360 - r \times tanθ) \end{array}$

Further, V is calculated as follows. $\begin{array}{l} V = E + G \\ = (A - r) + (D \times sinθ) \\ = (r / cosθ - r) - (2 πrθ / 360 ʺ - r \times tanθ) \times sinθ \end{array}$

In the foregoing equation, $θʺ = θ + θʹ$

where θ' denotes a bias angle of the bottom position TN with respect to the centerline TC of the pin PN. The bias angle θ' is obtained as follows.
$θʹ = arc sin (rp / r)$

where rp denotes the maximum value of the radius of the pin PN. If the maximum value of the outer diameter of the pin PN is 3 millimeters, the value of the rp is 1.5 millimeters.
As described earlier, since it is assumed that the circumferential velocity due to the rotation of the main roller 141 is in synchronization with the conveyance speed of the sheet PY, the value "C" denoting the conveyance distance of the sheet PY is equal to the length of the circumference corresponding to the rotational angle θ. Note, however, that the value "C" in Equations (3) and (4) is the length of the circumference corresponding to the rotational angle θ plus the bias angle θ'.
The coordinates of the contact position P can be determined by a variety of methods other than the foregoing methods, The contact position P may be determined by using a plot. A mathematical method may be used to obtain an approximate expression of the curve CV. It is also possible for a computer to execute an appropriate program for the plot or the operation.
The pin PN having the shape discussed above is a rotor. Such a pin PN can be formed by cutting a metal material or a synthetic resin material. Alternatively, such a pin PN may be formed by metallic molding, and polishing may be performed after molding. Since the front half of the pin PN is important in order that the pin PN positions the sheet PY, only the front half of the pin PN may be finished precisely.
While the pin PN is formed to have a cannonball shape in this embodiment, the pin PN may be formed to have a cone shape. Further, although the pin PN has a circular shape as viewed from the tip position TS thereof, the pin PN may have a rectangular or polygonal shape as viewed from the tip position TS instead of the circular shape.
The pin tractor 14 discussed above is a roller with pins PN in which the main roller 141 has pins PN. Instead, however, the pin tractor 14 may be provided with a timing belt.
Descriptions are given below of a pin tractor 14B using a timing belt.
Referring to Fig. 15, the pin tractor 14B is provided with a casing 31, two rollers 32 and 33 rotatably attached to the casing 31 by bolts BT, a timing belt 34 running between the two rollers 32 and 33, a timing gear 35 that is rotatably provided in the casing 31 and is rotationally driven by a drive shaft 13a to drive the timing belt 34 rotationally, and so on.
The timing belt 34 has, on its outer peripheral surface, pins PN to engage perforations PA formed on a sheet PY.
The timing belt 34 is in parallel with the sheet PY between a position corresponding to the timing gear 35 and a position corresponding to the roller 33. The timing belt 34 gently slopes down toward the roller 32 from the timing gear 35 so as to be gradually away from the sheet PY.
Above the pin tractor 14B, a sheet keeping device S is provided which includes a hold-down roller 15B attached to a printer, a hold-down lever 43 rotatably provided with respect to a shaft 44, and a spring 44.
The rotation of the drive shaft 13 rotates the timing gear 35 integrally, and thereby, the timing belt 34 engaging the timing gear 35 runs. When the timing belt 34 runs, the pins PN engaging the perforations PA move the sheet PY.
The sheet PY is delivered accurately in the direction denoted by the arrow M2 in the parallel part between the position corresponding to the timing gear 35 and the position corresponding to the roller 33 because the plurality of perforations PA engage the pins PN sufficiently. The timing belt 34 gradually moves away from the sheet PY in the slope from the position corresponding to the timing gear 35 and the position corresponding to the roller 32. Along with this, the pins 36 are disengaged from the perforations PA smoothly.
As described earlier, the pin PN has a cannonball shape, and the shape of the pin PN can be determined as described earlier with reference to Fig. 9 through Fig. 14.
The pin tractor 14B may have a structure, shape, and so on different from the foregoing. For example, a structure is possible in which a roller similar to the rollers 32 and 33 is provided at the position of the timing gear 35, the timing gear 35 is provided between the roller and the roller 32, and thereby the timing belt 34 is driven.
A method is now described for printing an image GZ on both sides of a sheet PY with the printer 1.
The method uses a sheet PY having perforations PA on both ends thereof for sheet feed, and the sheet feeder YH that has a drive roller 13 rotating to drive the sheet PY in contact with the sheet PY, and two pin tractors 14a and 14b which have pins PN to engage the perforations PA and are disposed on both ends of the drive roller 13 to rotate to drive the sheet PY by a coaxial rotational driving force with the drive roller 13.
The positions of the two pin tractors 14a and 14b are adjusted to correspond to the positions of the perforations PA of the sheet PY to be used,
The sheet PY is placed on the sheet feeder YH, conveyed at a speed synchronously with the movement of the pins PN, and an image GZ1 is printed onto one surface HM1 of the sheet PY (see Fig. 16A).
After printing is performed on one surface UM1 of the sheet PY, the sheet is cut out from the rolled sheet at the corresponding position. The sheet PY thus cut out is turned inside out, placed on the sheet feeder YH. The sheet PY is, then, conveyed at a speed synchronously with the movement of the pins PN, and an image GZ2 is printed onto the other surface UM2 (reverse) of the sheet PY (see Fig. 16B).
At this time, the image GZ2 to be printed onto the reverse UM2 of the sheet PY is the reversed image of the image GZ1 printed onto the surface UM1, namely, a mirror image of the image GZ1. Stated differently, the image GZ2 has the same contents as those of the image GZ1, and is a reflected duplication of the image GZ1 that appears reversed. When the sheet PY is turned upside down, it is preferable to also turn the image GZ2 upside down.
The image GZ1 and the image GZ2 are printed in such a manner that they overlap each other at the same position. To be specific, in a state where the sheet PY is seen through, the origin GG2 of the image GZ2 printed on the reverse UM2 is located at the same position as the origin GG1 of the image GZ1 printed on the obverse UM1.
To be specific, if one perforation PAll on the sheet PY is made as a reference, it is preferable that the positional relationship between the perforation PAll and the origin GG1 is identical to the positional relationship between the perforation PA11 and the origin GG2 in the obverse and reverse of the sheet PY.
A pin PN11 inserted through the perforation PA11 during the print process on the obverse UM1 is to be inserted through a perforation PA21, which is horizontally opposed to the perforation PA11, during the print process on the reverse UM2. Accordingly, when the upper left position of the image CZ2 is defined as the origin GG21, the positional relationship is defined between the origin GG21 and the pin PN11. As for the image GZ2, since the positional relationship between the origin GG2 and the origin GG21 is already known, it is preferable that the positional relationship between the origin GG21 the pin PN11 may be defined based on the known positional relationship.
In the sheet feeder YH, the pin tractors 14 define the correct position of the sheet PY with high accuracy and convey the sheet PY. Accordingly, printing is performed after the print positions of the images GZ1 and GZ2 on the obverse and reverse of the sheet PY are determined as described above, so that the images GZ1 and GZ2 accurately correspond to each other without any misregistration.
In short, double-sided printing of images GZ can be performed at high accuracy even if a large sheet PY or a small sheet PY is used.
As discussed above, the sheet feeder YH enables double-sided printing of images at high accuracy even if a large sheet PY is used. Further, the sheet feeder YH enables conveyance of a sheet PY at high accuracy even if a large sheet PY is used.
As for the sheet PY onto which double-sided printing was performed, both ends of the sheet PY corresponding to the perforations PA may be cut out from the sheet PY.
It is preferable to adjust, in advance, the density of the image GZ2 to be printed onto the reverse UM2 of the sheet PY.
Fig. 17 shows a relationship between the density of the image GZ1 to be printed on the obverse (input image) and the density of the image GZ2 to be printed on the reverse (output image).
Referring to Fig. 17, as for the density of the output image, the dynamic range is smaller than that of the density of the input image. To be specific, the density of the output range is compressed to ND1-ND2 % as compared to the input image. In other words, the density distribution of the output image is so limited that it falls within the density ND1-ND2 of the intermediate portion in the grayscale or the scale of the brightness of the primary colors.
However, with respect to the density of approximately 0 to 2 % of the input image, i.e., with respect to the brightest density thereof, the brightest density is assumed to be obtained. This is because it is necessary to remain the brightest part unchanged as a bright part.
The specific range of the density ND1 is, for example, approximately 10-20 %. The specific range of the density ND2 is, for example, approximately 50-80 %. Accordingly, it is possible to limit the density of the output image to the range of approximately 20-70 % of the density of the input image. In this way, the density of the output image is limited, and the limited density is applied to the image GZ2 on the reverse. This enables the images printed on both sides of a sheet PY to be viewed clearly at natural density when the images are viewed with transmitted light.
It is possible to provide, on the end of the image GZ in the width direction, a marker for detecting image misregistration. In such a case, the marker may be provided by automatic printing in the outer periphery of the image size. If a print position error occurs after starting printing, it is preferable that the print position is set to be correctable through operation of a keyboard of a computer.
Alternatively, a camera with a CCD and so on may be used to detect misregistration of images GZ printed, and the print position may be corrected based on the detection result. In such a case, for example, an image GZ1 is printed onto the obverse of the sheet PY, the image GZ1 is taken by a camera, and the print position thereof is measured. Before an image GZ2 is printed onto the reverse of the sheet PY, the image GZ2 is taken by a camera, and the print position thereof is measured. Then, the amount of misregistration is so corrected that the print position of the image GZ2 corresponds to the print position of the image GZ1.
When the print position is corrected, as for the main-scan direction (X-direction), for example, the origin GG may be moved, and, as for the sub-scan direction (Y-direction), for example, a position correction signal may be outputted to a motor for driving the drive roller 13.
In this way, print positions of the images GZ1 and GZ2 can correspond to each other automatically, and the images GZ1 and GZ2 can be printed automatically on the obverse and reverse of a sheet PY.
In this embodiment, the images GZ1 and GZ2 identical to each other are printed on both sides of a sheet PY. Instead of this, however, images different from each other may be printed on both sides of a sheet PY. Alternatively, an image may be printed onto the same side of a sheet PY twice, three times or more, instead of being printed onto both sides of the sheet PY. For example, an image GZ1 is printed onto the obverse of a sheet PY, and then, an additional image may be printed on the obverse of the sheet PY. If such an additional image is printed, the color thereof may be changed to, for example, gold. Even when printing is performed on the identical side of a sheet PY a plurality of times, the position of the sheet PY can be maintained at high accuracy. As a result, images to be printed on the sheet PY can be in register.
the embodiment discussed above, intervals between the perforations PA formed on the sheet PY are so set as to be equal to intervals between the pins PN. instead of this, however, the intervals between the perforations PA may be smaller than the intervals between the pins PN.
In the embodiment discussed above, the structure, form, dimensions, quantity, material, composition, and the like of the entirety or individual portions of the drive roller 13, the pin tractor 14, the pressure roller 15, the pin PN, the sheet PY, the perforation PA, and the printer 1 may be altered as required in accordance with the subject matter of the present invention.

Claims

A method for printing an image onto both sides of a sheet, comprising:
using a sheet having perforations formed at both ends of the sheet for sheet feed;

using a feeder including a drive roller and two pin tractors, the drive roller rotating to drive the sheet in contact with the sheet, and the pin tractors including a plurality of pins to engage the perforations and being disposed in both ends of the drive roller to rotate to drive the sheet with a coaxial rotational driving force with the drive roller;

disposing the sheet in the feeder, and, in this state, conveying the sheet synchronously with movement of the pins to print an image onto one side of the sheet; and

turning the sheet, onto on side of which printing has been performed, inside out and is disposed in the feeder, and, in this state, conveying the sheet synchronously with the movement of the pins to print an image onto another side of the sheet.
The method according to claim 1, wherein the pin is formed to have a shape which makes a distance between a front base position of the pin at a time when the perforation of the sheet starts to disengage from the pin and a front position of the pin on a line along a movement direction of the sheet after the pin is moved equal to a distance of a movement of the front base position of the pin.
The method according to claim 1 or 2, wherein
the two pin tractors have marks showing a positional correspondence relationship in a conveyance direction between the pins provided in one of the pin tractors and the pins provided in the other pin tractor, and
the sheet has marks showing a correspondence relationship between positions of the perforations formed on one of edges of the sheet and positions of perforations formed on the other edge.
A sheet feeder of a printer, comprising:
a drive roller that rotates to drive a sheet in contact with the sheet;

two pin tractors that include a plurality of pins to engage perforations formed on both ends of the sheet for sheet feed, are disposed in both ends of the drive roller to rotate to drive the sheet with a coaxial rotational driving force with the drive roller; and

a press member that is so disposed as to face the drive roller and the pin tractors and presses the sheet against the drive roller and the pin tractors;

wherein the sheet is conveyed synchronously with movement of the pins, and the sheet is moved forward with rotation of the drive roller.
The sheet feeder according to claim 4, wherein the pin is formed to have a shape which makes a distance between a front base position of the pin at a time when the perforation of the sheet starts to disengage from the pin and a front position of the pin on a line along a movement direction of the sheet after the pin is moved equal to a distance of a movement of the front base position of the pin.
The sheet feeder according to claim 4, wherein the pin is formed to have a shape which allows the pin to move, during a period from when the pin engages the perforation to when the pin is is disengaged from the perforation, in a direction which allows the front of the pin to be disengaged from the perforation in contact with a front rim of the perforation.
The sheet feeder according to any one of claims 4 through 6, wherein the two pin tractors disposed in the both ends have marks showing a positional correspondence relationship in a conveyance direction between the pins provided in one of the pin tractors and the pins provided in the other pin tractor.
The sheet feeder according to any one of claims 4 through 7, wherein at least one of the two tractors is provided in such a manner that an axial position of said at least one of the two tractors is adjustable.
The sheet feeder according to claim 8, wherein, with respect to a drive shaft having a same axis as that of the drive roller, the pin tractors engage a key provided in the drive shaft in a rotational direction and the pin tractors are axially movable along the key.
The sheet feeder according to any one of claims 4 through 9, wherein each of the pin tractors has the pins on a peripheral surface of the pin tractor, is a roller with pins, and is provided on a same axis as that of the drive roller.
The sheet feeder according to any one of claims 4 through 9,
wherein
the pin tractor includes
a casing,

two rollers rotatably attached to the casing,

a timing belt that has the pins on an outer peripheral 1 surface of the timing belt, and is run between the two rollers, and

a timing gear that is rotationally driven by a rotational driving force of a same axis as that of the drive roller to run the timing belt, and
the timing belt has a parallel part that runs in parallel with the sheet and a slope part that gently slopes so as to be gradually away from the sheet.
A print sheet conveyed by a drive roller and a pin tractor, the print sheet comprising:
end portions that are provided along a width direction of the print sheet and are provided with perforations for sheet feed, the perforations being aligned along the length direction of the sheet to engage pins of the pin tractor; and

marks showing a correspondence relationship between positions of the perforations formed on one of edges of the sheet and positions of perforations formed on the other edge.