Stencil printer

Abstract

A stencil printer includes a plurality of ink drums spaced from each other in the direction of paper conveyance and each having a respective master wrapped therearound. An upstream ink drum and pressing means movable into and out of contact with the upstream ink drum define an upstream print position. A downstream ink drum and pressing means movable into and out of contact with the downstream ink drum define a downstream print position. An intermediate conveyor is located between the upstream print position and the downstream print position for conveying a paper carrying an image printed at the upstream print position toward the downstream print position. A distance which the paper moves from the upstream print position to the downstream print position is longer than a distance between the upstream print position and the downstream print position. Defective printing is reduced even when the peripheral speed of the upstream ink drum and that of the downstream ink drum are different from each other.

Description

BACKGROUND OF THE INVENTION

The present invention relates to a stencil printer of the type including a plurality of ink drums.

Japanese Patent Laid-Open Publication No. 7-17121, for example, teaches a stencil printer capable of printing a multicolor image on a paper by conveying the paper only once. The stencil printer taught in this document includes a plurality of ink drums spaced from each other in a direction of paper conveyance. A particular master is wrapped around each of the ink drums. An intermediate conveyor is positioned between the ink drums for conveying a paper carrying an image printed by an upstream ink drum toward a downstream ink drum. The intermediate conveyor conveys the paper at a constant speed while each ink drum rotate at a constant peripheral speed in synchronism with a paper feed timing. In this configuration, the paper coincides with an ink image formed on each drum at a print position where the ink drum is located.

A problem arises in the stencil printer of the type described when the size (overall length) of a paper is greater than -the distance between an upstream print position where the upstream ink drum is located and a downstream print position where the downstream ink drum is located. The drums are driven by a motor or similar drive source via a drive transmission mechanism including gears and belts. The peripheral speeds of the ink drums are not always equal to each other due to the expansion or contraction of the belts or various kinds of irregularities including the dimensional errors of the gears and those of the ink drums. When the peripheral speeds of the ink drums are different from each other, it is likely that the paper is pulled or slackened in the direction of paper conveyance. For example, assume that the peripheral speed of the downstream ink drum is higher than the peripheral speed of the upstream ink drum. Then, so long as the overall length of the paper is smaller than the distance between the two print positions, the leading edge of the paper successfully reaches the downstream print position after the trailing edge of the paper has moved away from the upstream print position. The paper is therefore conveyed by the peripheral speed of the downstream ink drum. However, if the overall length of the paper is greater than the above distance, then the paper extends over both the upstream print position and downstream print position. As a result, the paper is pulled in the direction of paper conveyance due to the difference between the peripheral speeds of the two ink drums while printing is under way at the upstream print position. This is apt to cause an image to be dislocated relative to the paper at the upstream print position in the direction of paper conveyance or relative to an image to be printed at the downstream print position, resulting in a defective printing.

On the other hand, when the peripheral speed of the downstream ink drum is lower than the peripheral speed of the upstream ink drum, the paper slackens on the intermediate conveyor. This also results in a defective printing although the deviation of the image printed at the upstream print position relative to the image printed at the downstream print position will be reduced, compared to the above condition.

Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 64-18682, 5-229243, 6-32038 and 10-315601 (corresponding to U.S. patent application Ser. No. 09/079,287 filed May 15, 1998), and Japanese Patent Application Nos. 9-321702 and 10-167322 (corresponding to the pending U.S. patent application Ser. No. 09/164,372 filed Oct. 1, 1998).

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a stencil printer of the type including a plurality of ink drums and capable of reducing defective printing even when the peripheral speeds of the ink drums are different from each other.

A stencil printer of the present invention includes a plurality of ink drums spaced from each other in a direction of paper conveyance, and each having a respective master wrapped therearound and fed with ink from an ink feeding device to an inner periphery thereof. A plurality of pressing devices each are movable toward and away from associated one of the ink drums. An intermediate conveyor is provided for conveying a paper on which an image is printed at an upstream print position where an upstream ink drum in the direction of paper conveyance and associated pressing device nip the paper to a downstream print position where a downstream ink drum and associated pressing device nip the paper. The intermediate conveyor is positioned between the upstream ink drum and the downstream ink drum. A paper conveyance distance which the paper moves while being conveyed from the upstream print position to the downstream print position is longer than a distance between the upstream print position and the downstream print position.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken with the accompanying drawings in which:

FIG. 1 shows the general construction of a stencil printer in accordance with the present invention;

FIG. 2 is a fragmentary plan view showing a specific configuration of an operation panel included in the stencil printer;

FIG. 3 is a block diagram schematically showing control means also included in the stencil printer;

FIG. 4 is a fragmentary enlarged front view showing an intermediate conveyor representative of an embodiment of the present invention;

FIGS. 5 and 6 are fragmentary enlarged front views each showing a particular modification of the embodiment of FIG. 4 ;

FIG. 7 is a flowchart demonstrating a specific operation of the embodiment of FIG. 4 ;

FIG. 8 is a fragmentary enlarged front view showing an alternative embodiment of the present invention including moving means;

FIG. 9 is a fragmentary enlarged plan view showing the moving means;

FIG. 10 is a fragmentary front view showing how the moving means extends a paper conveyance distance;

FIG. 11 is an enlarged perspective view of a guide member and a press roller included in the alternative embodiment;

FIG. 12 is a flowchart demonstrating a specific operation of the alternative embodiment;

FIG. 13 is an enlarged front view showing a modification of the moving means;

FIG. 14 is an enlarged front view showing how the moving means of FIG. 13 extends the paper conveyance distance;

FIG. 15 is a fragmentary perspective view showing the guide member;

FIG. 16 is a block diagram schematically showing control means representative of another alternative embodiment of the present invention; and

FIG. 17 is a flowchart representative of a specific operation of the embodiment shown in FIG. 16 .

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, the general construction and basic operation of the stencil printer in accordance with the present invention will be described. As shown in FIG. 1 , the stencil printer includes two ink drums 1 A and 1 B respectively located at the upstream side and downstream side in a direction X in which a paper or similar recording medium 22 is conveyed. With the two ink drums 1 A and 1 B, the printer is capable of producing multicolor printings (bicolor printings in this case). The ink drums 1 A and 1 B are substantially identical in configuration and function. Substantially identical ink feeding means, master making devices, master discharging devices and so forth are arranged around the ink drums 1 A and 1 B, as will be described specifically later. Such identical constituents around the drums 1 A and 1 B are designated by like reference numerals and simply distinguished from each other by suffixes A and B or a and b. When one of the identical constituents around the ink drums A and B is described in detail, a detailed description of the other constituent will not be made in order to avoid redundancy.

The printer is a conventional thermosensitive digital master making type printer. A master 33 a is wrapped around the outer periphery 1 A a of the ink drum 1 A. A master making device 41 a is positioned above and at the right-hand side of the ink drum 1 A for making the master 33 a . Paper feeding means 20 is positioned below the master making device 41 a for feeding papers 22 from a paper tray 21 one by one. A master discharging device 42 a is located above and at the left-hand side of the ink drum 1 A for peeling off a used master, not shown, existing on the ink drum 1 A and discharging it. A pressing device or pressing means 32 a is arranged below this ink drum 1 A for pressing the paper 22 being conveyed against the master 33 a wrapped around the ink drum 1 A. An air knife or separating means 7 a separates the paper or printing 22 on which an image has been printed at an upstream print position E 1 between the ink drum 1 A and the pressing device 32 a . The ink drum 1 A, master making device 41 a , paper feeding means 20 , master discharging device 42 a , pressing device 32 a and air knife 7 a constitute a first unit U 1 .

Likewise, a master 33 b is wrapped around the outer periphery 1 B a of the ink drum 1 B. A master making device 41 b is positioned above and at the right-hand side of the ink drum 1 B for making the master 33 b . A master discharging device 42 b is located above and at the left-hand side of the ink drum 1 B for peeling off a used master, not shown, existing on the ink drum 1 B and discharging it. A pressing device or pressing means 32 b is arranged below the ink drum 1 B for pressing the paper 22 being conveyed against the master 33 b wrapped around the ink drum 1 B. An air knife or separating means 7 b separates the paper or printing 22 on which an image has been printed at a downstream print position E 2 between the ink drum 1 B and the pressing device 32 b . The ink drum 1 B, master making device 41 b , master discharging device 42 b , pressing device 32 b and air knife 7 b constitute a second unit U 2 .

An intermediate conveyor (simply conveyor hereinafter) 17 conveys the printing 22 coming out of the upstream print position E 1 toward the downstream print position E 2 . A paper discharging device 35 is arranged below the master discharging device 42 b for driving the printing 22 out of the printer to a tray 37 .

Both the ink drums 1 A and 1 B may be used to print images in the same color as each other. Alternatively, one of the ink drums 1 A and 1 B may be fixedly assigned to a single document, i.e., a single master, in which case the other ink drum will be assigned to variable document information.

A document reading section or scanner, not shown, for reading a document and an operation panel 70 (see FIG. 2 ) are positioned above the master making devices 41 a and 41 b and master discharging device 42 a.

The ink drum 1 A has a conventional porous, hollow cylindrical configuration and is rotatably mounted on a shaft 2 a . A drum motor or drive source which will be described causes the ink drum 1 A to rotate in a direction indicated by an arrow. An openable damper 5 a is mounted on the outer periphery of the ink drum 1 A and extends in parallel to the axis of the drum 1 A. The camper 5 a clamps the leading edge of the master 33 a when closed. Specifically, opening and closing means, not shown, is located at a suitable position around the ink drum 1 A and causes the damper 5 a to open and close at a preselected position. Ink feeding means 10 a is arranged within the ink drum 1 A for feeding ink from the inner periphery 1 A b to the outer periphery 1 A a of the ink drum 1 A. In accordance with the present invention, the ink feeding means 10 a and ink feeding means 10 b arranged within the ink drums 1 A and 1 B feed magenta ink and black ink, respectively. Magenta and black will sometimes be referred to as a first color and a second color, respectively.

In accordance with the present invention, use is made of a stencil made up of a film of polyester or similar thermoplastic resin and a porous support adhered to the film and implemented by, e.g., Japanese paper. If desired, the stencil may consist only of an extremely thin, thermoplastic resin film.

The operator of the printer sets a desired document on a tray included in the document reading section and then presses a perforation start key 73 (see FIG. 2 ). In response, the printer starts a master making operation with both of the ink drums 1 A and 1 B. Specifically, the ink drum 1 A is rotated in the direction (counterclockwise) opposite to the direction indicated by the arrow. As a result, the used master existing on the ink drum 1 A is sequentially peeled off and conveyed to a waste master box, not shown, associated with the drum 1 A.

The document reading section reads the document in parallel with the above master discharging operation by use of a conventional reduction type reading system. An image optically read out of the document is incident to a CCD (Charge Coupled Device) image sensor or similar photoelectric transducer and transformed to an electric signal thereby. The electric signal output form the transducer is sent to an analog-to-digital (AD) conversion board, not shown, and converted to a digital image signal thereby.

The document reading section includes a construction having various functions for color separation essential with multicolor printing. Such a construction may be implemented by, e.g., a filter unit taught in Laid-Open Publication No. 64-18682 mentioned earlier and including a plurality of replaceable color filters. The filter unit is positioned on an optical path between a group of mirrors and a lens not shown.

While the document reading section reads the document, the master making devices 41 a and 41 b each perforate a respective stencil in accordance with the digital image signal output from the above section while conveying the perforated part of the stencil. Specifically, the master making device 41 a includes a flat thermal head, a platen roller pressed against the thermal head and a roller pair, although not shown specifically. The platen roller and roller pair cooperate to convey the perforated part of the stencil to the downstream side along a master transport path. The thermal head has a number of fine heating elements, not shown, arranged in an array in the main scanning direction of the head. The heating elements are selectively energized in accordance with the digital image data subjected to various kinds of processing by the A/D conversion board and a master making control board, not shown, following the A/D conversion board. Consequently, the thermoplastic resin film of the stencil contacting the thermal head is selectively perforated by heat in a pattern represented by the image signal.

The above roller pair conveys the leading edge of the perforated stencil or master 33 a toward the outer periphery of the ink drum 1 A. A guide plate, not shown, steers the leading edge of the master 33 a toward the damper 5 a held in its open position, i.e., causes it to hang down toward the damper 5 a . At this instant, the ink drum 1 A from which the used master has already been removed is held stationary in a stand-by position. A roller pair identical with the above roller pair conveys the leading edge of the master 33 b toward the outer periphery of the ink drum 1 B while a guide plate, not shown, guides the master 33 b in the substantially horizontal direction. At this time, the ink drum 1 B is held stationary at a stand-by position where a damper 5 b mounted on the drum 1 B is opened and located on substantially the top of the drum 1 B, as seen in FIG. 1 . The master 33 b is therefore inserted into the damper 5 b.

As soon as the damper 5 a clamps the leading edge of the master 33 a at a preselected timing, the ink drum 1 A is caused to rotate in the clockwise direction while sequentially wrapping the master 33 therearound. Cutting means, not shown, is included in the master making device 41 a and made up of, e.g., a movable edge and a fixed edge. The cutting means cuts the trailing edge of the master 33 a at a preselected length. When the master 33 a is fully wrapped around the ink drum 1 A, the master making and feeding operation ends.

After the masters 33 a and 33 b have been fully wrapped around the ink drums 1 A and 1 B, respectively, a trial printing and actual printing operation begins, as follows. First, the paper tray 21 is elevated to a level where the top paper 22 on the paper tray 21 contacts a pick-up roller 23 . In this condition, the pick-up roller 23 is rotated to pay out the top paper 21 . A pair of separator rollers 24 and 25 and a separator plate 26 cooperate to separate the top paper 22 from the other papers underlying it. The paper 22 is conveyed in the direction of paper conveyance X toward a pair of registration rollers 29 and 30 while being guided by an upper and a lower guide 28 and 27 , respectively. The leading edge of the paper 22 abuts against a portion of the registration rollers 29 and 30 just short of a nip and is caused to bend along the upper guide plate 28 .

On the start of the printing operation, the ink drum 1 A located at the upstream side in the direction of paper conveyance X starts rotating at a speed assigned to the printing operation. An ink roller 3 a and a doctor roller 4 a are disposed in the ink drum 1 A and form an ink well la therebetween. An ink distributor, not shown, feeds magenta ink to the ink well la. The ink roller 3 a and doctor roller 4 a in rotation knead and extend the magenta ink while causing it to uniformly deposit on the inner periphery 1 A b of the ink drum 1 A. Specifically, the ink roller 3 a rotates in contact with the inner periphery 1 A b of the ink drum 1 A in the same direction and at the same peripheral speed as the ink drum 1 A, feeding the ink to the inner periphery 1 A b . Ink sensing means shown in FIG. 2 of Laid-Open Publication No. 5-229243 mentioned earlier senses the amount of ink remaining in the ink well la. The ink distributor replenishes the magenta ink when it is short, as determined by the ink sensing means.

The pressing device 32 a includes the ink roller 3 a and a press roller 9 a , a bracket 11 a , a tension spring 13 a , and a cam 12 a . The press roller or pressing means 9 a presses the paper 22 being conveyed against the ink drum 1 A so as to form an image on the paper 22 . The press roller 9 a is rotatably supported by one end of the bracket 11 a and movable into and out of contact with the outer periphery of the ink drum 1 A. The tension spring 13 a is anchored to the other end of the bracket 11 a and holds it in contact with the contour of the cam 12 a . At the same time, the press roller 9 a tends to contact the ink drum 1 A due to the action of the tension spring 13 a .

A drum driveline 89 (see FIG. 3 ) rotates the cam 12 a in synchronism with the timing for feeding the paper 22 from the paper feeding means 20 and the rotation of the ink drum 1 A. When no papers 22 are fed from the paper feeding means 20 , a larger diameter portion forming a part of the cam 12 a faces the end of the bracket 11 a adjoining it. When the paper 22 is fed from the paper feeding means 20 , the cam 12 a is rotated such that a smaller diameter portion forming the other part of the cam 12 a faces the above end of the bracket 11 a , causing the press roller 9 a to angularly move clockwise, as viewed in FIG. 1 .

When the registration rollers 29 and 30 convey the paper 22 to the upstream print position E 1 in synchronism with the rotation of the ink drum 1 A, the press roller 9 a positioned below the ink drum 1 A is angularly moved upward so as to press the paper 22 against the master 33 a wrapped around the drum 1 A. The master 33 a is therefore brought into close contact with the outer periphery 1 A a of the ink drum 1 A due to the viscosity of ink penetrating through the porous portion of the drum 1 A. Further, the ink penetrates through the perforation pattern of the master 33 a and is transferred to the paper 22 . Consequently, an image is formed on the paper 22 in the first color.

When the leading edge of the paper 22 carrying the image of the first color approaches the edge of the air knife 7 a , the air knife 7 a is caused to pivot about a shaft 8 a toward the outer periphery of the ink drum 1 A in synchronism with the rotation of the drum 1 A. Then, compressed air is fed from an air pressure source, not shown, to the air knife 7 a and sent from the edge of the air knife 7 a . The compressed air separates the leading edge of the paper 22 from the ink drum 1 A. The conveyor 17 conveys the paper 22 separated from the ink drum 1 A to the downstream side in the direction of paper conveyance X.

The conveyor 17 includes a drive roller 15 , a driven roller 14 , a porous belt 16 passed over the two rollers 15 and 14 , and a suction fan 18 . Control means 34 (see FIG. 3 ) causes, via a conveyor driveline 88 (see FIG. 3 ), the conveying surface 16 a of the belt 16 to move at a peripheral speed or conveying speed V 3 substantially equal to peripheral speeds or conveying speeds V 1 and V 2 at which the ink drums 1 A and 1 B, respectively, rotate. The belt 16 turns in the counterclockwise direction in FIG. 1 . The suction fan 18 retains the paper 22 being conveyed by the belt 16 on the conveying surface 16 a of the belt 16 by suction. As a result, the paper 22 is conveyed by the belt 16 toward the downstream print position E 2 while being surely held on the belt 16 .

The ink drum 1 B is located downstream of the ink drum 1 A in the direction of paper conveyance X and assigned to the second color. The ink drum 1 B is caused to start rotating in synchronism with the ink drum 1 A at substantially the same peripheral speed as the ink drum 1 A in the direction indicated by the arrow in FIG. 1 (clockwise). An ink roller 3 b disposed in the ink drum 1 B rotates in contact with the inner periphery 1 B b of the drum 1 B at the same peripheral speed as the drum 1 B. The ink roller 3 b feeds ink of the second color to the inner periphery of the ink drum 1 B in exactly the same manner as the ink roller 3 a disposed in the ink drum 1 A.

When the paper 22 being conveyed by the belt 16 reaches the downstream print position E 2 , the press roller 9 b positioned below the ink drum 1 B is angularly moved upward so as to press the paper 22 against the master 33 b wrapped around the drum 1 B. The master 33 b is therefore brought into close contact with the outer periphery 1 B a of the ink drum 1 B due to the viscosity of ink penetrating through the porous portion of the drum 1 B. Further, the ink penetrates through the perforation pattern of the master 33 b and is transferred to the paper 22 . Consequently, an image is formed on the paper 22 in the second color in register with the image of the first color existing on the paper 22 .

When the leading edge of the paper 22 carrying the composite or bicolor image approaches the edge of the air knife 7 b , the air knife 7 b is caused to pivot about a shaft 8 b toward the outer periphery of the ink drum 1 B in synchronism with the rotation of the drum 1 B. Then, compressed air is fed from the air pressure source to the air knife 7 b and sent from the edge of the air knife 7 b . The compressed air separates the leading edge of the paper 22 from the ink drum 1 B. The paper discharging device 35 conveys the paper 22 separated from the ink drum 1 B further downward to the tray 37 in the direction of paper conveyance X.

The paper discharging device 35 includes a drive roller 38 , a driven roller 39 , a porous belt 40 passed over the two rollers 38 and 39 , and a suction fan 36 . The belt 40 is driven in synchronism with the rotation of the ink drum 1 B at a peripheral speed at least equal to the peripheral speed of the drum 1 B. The suction fan 36 retains the paper 22 on the belt 40 by suction. The belt 40 turns in the counterclockwise direction, as viewed in FIG. 1 , so as to convey the paper or trial printing 22 to the tray 37 . If the trial printing is acceptable, the operator inputs a desired number of printings on numeral keys 71 arranged on the operation panel 70 , FIG. 2 , and then presses a print start key 72 also positioned on the panel 70 . In response, the printer repeats the above paper feeding step, printing step and paper discharging step a number of times corresponding to the desired number of printings.

A preferred embodiment of the stencil printer in accordance with the present invention will be described hereinafter. As shown in FIG. 1 , the upstream print position E 1 and downstream print position E 2 are positioned substantially beneath the axes of rotation of the ink drums 1 A and 1 B, respectively. Therefore, in the illustrative embodiment, a distance L between the center of the print position E 1 and that of the print position E 2 is substantially equal to the distance between the axis of the ink drum 1 A and that of the ink drum 1 B.

The ink drums 1 A and 1 B each include a porous portion capable of accommodating the length of the paper 22 of size A3 (Japanese Industrial Standard) conveyed in a vertical position. In the illustrative embodiment, the distance L between the print positions E 1 and E 2 is smaller than the circumferential length of the porous portion of each of the drums 1 A and 1 B. That is, the distance L is smaller than the longitudinal dimension of the paper 22 of size A3.

Specifically, as shown in FIG. 4 , the conveying surface 16 a of the belt 16 has an upstream end 16 b and a downstream end 16 c adjoining the upstream print position E 1 and downstream print position E 2 , respectively. The upstream end 16 b and downstream end 16 c are positioned below an imaginary line 0 connecting the print positions E 1 and E 2 at the shortest distance. Stationary guide members 93 and 94 are respectively positioned between the print position E 1 and the upstream end 16 b of the belt 16 and between the downstream end 16 c of the belt 16 and the print position E 2 .

The intermediate conveyor 17 is arranged in the printer such that the conveying surface 16 a is parallel to and positioned below the line 0 . The guide member 93 adjoins the press roller 9 a and upstream end 16 b and has a flat top surface 93 a inclined downward in the direction of paper conveyance X. The guide member 94 adjoins the press roller 9 b and downstream end 16 c and has a flat top surface 94 a inclined upward in the direction X. In this configuration, the guide members 93 and 94 and belt 16 form a paper conveyance path R between the spaced print positions E 1 and E 2 . Because the paper conveyance path R includes the inclined flat surfaces 93 a and 94 a , the overall length of the path R, i.e., a paper conveyance distance W which the paper 22 moves from the print position E 1 to the print position E 2 is greater than the distance L by the sum of lengths P 1 and P 2 .

Assume that the paper 22 extends over both the print positions E 1 and E 2 , and that the peripheral speed V 2 of the ink drum 1 B is higher than the peripheral speed V 1 of the ink drum 1 A by one caused or another. Then, with the paper conveyance distance W greater than the distance L by the lengths P 1 and P 2 , it is possible to obviate an occurrence that the paper 22 is pulled by the downstream ink drum 1 B while printing is under way at the upstream print position E 1 . This insures expected printing at the print position E 1 and prevents the image of the first color from being dislocated relative to an image of the second color to be formed at the print position E 2 .

FIG. 5 shows a modified form of the intermediate conveyor 17 . As shown, the upstream end 16 b and downstream end 16 c of the belt 16 are positioned below the line 0 , but the downstream end 16 c is positioned above the upstream end 16 b . The conveying surface 16 a is therefore inclined upward in the direction of paper conveyance X. The guide members 93 and 94 are respectively positioned between the upstream print position E 1 and the upstream end 16 b and between the downstream end 16 c and the downstream print position E 2 , as in the above embodiment.

In the modification shown in FIG. 5 , the guide member 94 adjoining the press roller 9 b and downstream end 16 c has the top surface 94 a made up of an inclined portion contiguous with the conveying surface 16 a and a flat portion parallel to the line 0 . The inclined portion and flat portion merge into each other. In the modification, therefore, the paper conveyance path R between the print positions E 1 and E 2 is formed by the downwardly inclined surface 93 a , upwardly inclined conveying surface 16 a , and the surface 94 a made up of the inclined portion and flat portion. This is also successful to make the overall length of the path R, i.e., the paper conveyance distance W greater than the distance L by the sum of lengths P 1 and P 2 . The modification therefore also obviates the undesirable occurrence described in relation to the above embodiment.

FIG. 6 shows another modification of the conveyor 17 . As shown, the upstream end 16 b and downstream end 16 c of the belt 16 are positioned below the line 0 as in the illustrative embodiment. In this modification, the conveying surface 16 a has its intermediate portion 16 d curved downward in the direction perpendicular to the line 0 . It is to be noted that the downward curvature is an example of curvature perpendicular to the line 0 . A pair of rollers 95 (only one is visible) respectively contact opposite side edges of the intermediate portion 16 d which do not contact the paper 22 . Each roller 95 is constantly biased downward by a tension spring 96 . In this condition, the rollers 95 press the intermediate portion 16 a downward and thereby form the curved conveying surface 16 a . Again, the guide members 93 and 94 are respectively positioned between the upstream print position E 1 and the upstream end 16 b and between the downstream end 16 c and the downstream print position E 2 .

The top surfaces 93 a and 94 a of the guide members 93 and 94 , respectively, are so configured as to smoothly merge into the upstream end 16 b and downstream end 16 c , respectively. In this configuration, too, the overall length of the paper conveyance path R, i.e., the paper conveyance di stance W is greater than the distance L between the print positions E 1 and E 2 , also solving the problem discussed in relation to the illustrative embodiment.

Referring again to FIG. 1 , a paper size sensor or paper size sensing means 56 is mounted on the paper tray 21 in the vicinity of the outermost end 21 a of the tray 21 . A paper passage sensor or paper size recognizing means 46 is located on the paper conveyance path R in the vicinity of the registration rollers 29 and 30 and is responsive to the length of the paper 22 in the direction of paper conveyance X. The two sensors 46 and 56 constitute paper sensing means in combination.

The paper size sensor 56 is responsive to a reference length of the papers 22 stacked on the paper tray 21 and is spaced from the innermost end 21 b of the paper tray 21 by a preselected distance L 1 . The distance L 1 is slightly longer than the paper conveyance distance W. The paper size sensor 56 is implemented by a conventional reflection type photosensor made up of a light emitting device and a light-sensitive device. When the papers 22 are present on the tray 21 , a reflection from the papers 22 is incident to the sensor 56 and turns it on. The output of the sensor 56 therefore shows whether or not the papers 22 on the tray 21 has a preselected length (reference length) L 1 . The length L 1 should preferably be W+α, as will be described specifically later.

The paper passage sensor 46 is responsive to an interval between the time when the leading edge of the paper 22 coming out of the registration rollers 29 and 30 reaches the sensor 46 and the time when the trailing edge of the same paper 22 moves away from the sensor 46 . In the illustrative embodiment, the paper passage sensor 46 is used in combination with the paper size sensor 56 . When the paper size sensor 56 determines that the length of the papers 22 on the paper tray 21 is greater than the paper conveyance distance W, the paper passage sensor 46 translates the length into a period of time. The resulting time data allows how long the papers 22 actually is to be determined. The paper passage sensor 46 is also implemented by a reflection type photosensor.

The drum driveline 89 , FIG. 3 , is connected to the ink drums 1 A and 1 B via power transmitting means, not shown, and causes the drums 1 A and 1 B to rotate at identical peripheral speeds V 1 and V 2 , respectively. The conveyor driveline 88 , FIG. 3 , drives the belt 16 at the peripheral speed V 3 basically equal to the peripheral speeds V 1 and V 2 and drives the suction fan 18 .

As shown in FIGS. 2 and 3 , the numeral keys 71 , print start key 72 and perforation start key 73 are arranged on the operation panel 70 , and each is assigned to a particular function, as stated earlier. Further arranged on the operation panel 70 are a stop key 74 for interrupting the procedure ending with the printing step, a display 75 for displaying, e.g., the number of printings input on the numeral keys 71 and implemented by LEDs (Light Emitting Diodes), a monitor display 76 for showing the operator that the size of the papers 22 stacked on the tray 21 is inadequate, a clear key 77 for clearing, e.g., the number of printings input on the numeral keys 71 , and speed control keys 78 , i.e., an up key 78 b and a down key 78 a usable to vary the peripheral speeds of the ink drums 1 A and 1 B and belt 16 stepwise.

As shown in FIG. 3 , the control means 34 is implemented by a conventional microcomputer including a CPU (Central Processing Unit) 80 , an I/O (Input/Output) port, not shown, a ROM (Read Only Memory) 81 and a RAM (Random Access Memory 82 ) interconnected by a signal bus not shown. The control means 34 and the various keys 71 - 74 , 77 and 78 displays 75 and 76 of the operation panel 70 , paper passage sensor 46 and paper size sensor 56 are electrically connected to each other and interchange command signals, ON/OFF signals, and data signals. A power supply 47 is connected to the CPU 80 .

A master make and feed driveline 83 , a paper discharge driveline 84 , a paper feed driveline 85 and a pressure driveline 86 are additionally electrically connected to the control means 34 . The master make and feed driveline 83 drives the master making devices 41 a and 41 b and stencil feeding sections not shown. The paper feed driveline 85 drives the paper feeding means 20 . The pressure driveline 86 drives the pressing devices 32 a and 32 b . The paper discharge driveline 87 drives the air pressure source not shown. The control means 34 interchanges command signals, ON/OFF signals and data signals with drivelines 83 - 89 for controlling the operations of the entire system including the starts and stops and timings of the various devices and drive mechanisms.

When the length of the papers 22 is greater than the reference length, as determined by the paper size sensor 56 , the control means 34 inhibits the paper feeding means 20 from feeding the papers 22 . Also, when the length of the paper 22 determined on the basis of the output of the paper passage sensor 46 is greater than the paper conveyance distance W, the control means 34 interrupts the printing operation after the paper 22 sensed by the sensor 46 has been driven out to the tray 37 . For this purpose, the ROM 81 stores a table listing experimentally determined data representative of a relation between paper conveying times and the lengths of the papers 22 . The paper conveying times each correspond to a particular size of the papers 22 , a particular peripheral speed V 1 of the ink drum 1 A, and a particular peripheral speed V 3 of the belt 16 . The control means 34 reads such data out of the ROM 81 in accordance with the size of the papers 22 and peripheral speeds, as needed, and controls the various sections of the printer on the basis of the data. Further, the ROM 81 stores a program relating to the starts and stops of the various devices and drivelines as well as necessary fixed data.

Reference will be made to FIG. 7 for describing a specific operation of the illustrative embodiment. As shown, the control means 34 determines whether or not the operator has pressed the print start key 72 (step A 1 ). If the answer of the step A 1 is positive (Yes), then the control means 34 compares the length of the papers 22 in the direction of paper conveyance X determined by the paper size sensor 56 and the paper conveyance distance W (step A 2 ). If the length of the papers 22 is smaller than the distance W, i.e., if the paper size sensor 56 does not turn on (Yes, step A 2 ), then the control means 34 executes the previously stated printing operation with the papers 22 (step A 7 ).

Assume that the paper size sensor 56 has turned on, showing that the length of the papers 22 in the direction X is greater than the distance W (No, step A 2 ). Then, the control means 34 determines how long the paper 22 actually is on the basis of the output of the paper passage sensor 46 when the paper 22 is fed (step A 3 ). Subsequently, the control means 34 compares length data t output from the paper passage sensor 46 and the reference paper conveyance distance W+α (step A 4 ). If the length data t is greater than the distance W+α (Yes, step A 4 ), the control means 34 interrupts the printing operation after a trial printing has been driven out (step A 5 ) and informs the operator of the inadequate paper size via the monitor display 76 (step A 6 ). If the answer of the step A 4 is No, the controller 34 executes the printing operation (step A 7 ).

As for the reference transport distance W+α, α is the distance over which the ink drum 1 A conveys the paper 22 within the period of time in which the ink drum 1 B fully conveys the paper 22 by the sum of the distances P 1 and P 2 . The sum of the distances P 1 and P 2 is the difference, or margin, between the distances W and L.

As stated above, when the length of the papers 22 is greater than the reference paper conveyance distance W+α, the control means 34 interrupts the printing operation and informs the operator of the inadequate paper length. This prevents defective printing from continuing and clearly informs the operator of the reason why the printer has stopped operating. If desired, a drum speed sensor and a belt speed sensor may be arranged to sense the peripheral speed V 1 of the ink drum 1 A and the peripheral speed V 3 of the belt 16 , respectively. By counting the outputs of such additional sensors over the paper transporting time, it is possible to more surely determine a period of time needed for the passage of the paper 22 . Consequently, accurate recognition of the paper 22 being conveyed is promoted to further reduce defecting printing. While the illustrative embodiment compares the length data t with the reference paper conveyance distance W+α, the length data t may, of course, be compared with the paper conveyance distance W.

While the above embodiment includes the paper passage sensor 46 and paper size sensor 56 , the embodiment is operable only with the paper size sensor 56 if modified, as follows. In the modification, the paper size sensor 56 is spaced from the innermost edge 21 b of the paper tray 21 by a distance at least equal to the paper conveyance distance W. When the sensor 56 senses the papers 22 on the paper tray 21 and turns on, the control means 34 determines that the papers 22 are longer than a preselected length (reference length). The control means 34 then displays the inadequate paper length and inhibits the pick-up roller 23 from rotating. This modification therefore fully obviates defective printings including a trial printing and therefore the wasteful consumption of the papers 22 . The control means 34 may inhibit the registration rollers 29 and 30 from operating in place of the pick-up roller 23 , if desired.

An alternative embodiment of the stencil printer in accordance with the present invention will be described with reference to FIG. 8 . In this embodiment, structural elements identical with the structural elements of the previous embodiment are designated by like reference numerals and will not be described specifically in order to avoid redundancy. As shown, the printer includes moving means 53 for moving the intermediate conveyor 17 and a guide member 52 in accordance with the size (length) of the papers 22 such that the paper conveyance distance W increases. In the conveyor 17 , the driven roller 14 is mounted on a shaft 14 a . The shaft 14 a is movably received in an arcuate guide slot 54 formed in a frame 69 shown in FIG. 9 . The drive roller 14 is therefore angularly movable about a shaft 15 a supporting the drive roller 15 .

The moving means 53 selectively moves the conveyor 17 and guide member 52 to a first position shown in FIG. 8 or a second position shown in FIG. 10 . The conveyor 17 and guide member 52 define the reference paper conveyance distance W in the first position or extend the distance W in the second position. The moving means 53 includes a drive motor or drive source 49 having an output shaft 49 a . A drive gear 154 is mounted on the output shaft 49 a and held in mesh with gears 55 and 56 . The gear 55 is mounted on a shaft 58 journalled to the frame 69 . A pulley 65 is also mounted on the shaft 58 . A belt 66 is passed over the pulley 65 and a pulley 64 mounted on a shaft 63 rotatably supported by the frame 69 via a bearing 67 . A lever 68 is affixed to the shaft 63 at its one end and supports the shaft 14 a at its top 68 a . The lever 68 is usually positioned such that the conveying surface 16 a of the belt 16 remains substantially parallel to the line 0 .

The guide member 52 intervenes between the press roller 9 a and the upstream end 16 b of the belt 16 . One end 52 b of the guide member 52 adjoins the conveying surface 16 a at the upstream end 16 b side. The gear 56 is mounted on a shaft 90 mounted on the other end 52 a of the guide member 52 . The shaft 90 is journalled to the frame 69 so as to allow the guide member 52 to angularly move. The guide member 52 has a top surface 52 c made up of an inclined surface and a flat surface and guides the paper 22 moved away from the upstream print position E 1 to the belt 16 . As shown in FIG. 11 , the end 52 a of the guide member 52 is formed with a recess 52 d . The press roller 9 a is partly received in the recess 65 d , so that the paper 22 moved away form the print position E 1 can be smoothly handed over to the guide member 52 .

The top surface 52 c of the guide member 52 , conveying surface 16 a of the belt 16 and the top surface 94 a of the other guide member 94 form the paper conveyance path R. The overall length of the path R, i.e., the paper conveyance di stance W which the paper 22 moves from the upstream print position E 1 to the downstream print position E 2 is selected to be greater than the distance L between the positions E 1 and E 2 by the sum of the distances P 1 and P 2 . It is to be noted that the above distance W is a reference distance set up in the first position shown in FIG. 8 , i.e., when the conveyor 17 and guide member 52 are not moved.

The drive motor 49 is implemented by a stepping motor and connected to the CPU 80 via a driver 48 , as shown in FIG. 3. A displacement sensor 50 is mounted on the output shaft 49 a of the drive motor 49 for sensing the displacement of the guide member 52 and lever 68 . The displacement sensor 50 is a conventional rotary encoder responsive to the rotation angle of the output shaft 49 a . The output of the displacement sensor 50 is sent to the CPU 80 via a pulse detector 51 .

The ROM 81 stores a table listing experimentally determined data representative of a relation between the lengths of the papers 22 , the paper conveyance distances W, and the amounts of rotation of the drive motor 49 . The control means 34 reads such data in accordance with the outputs of the paper size sensor 56 and paper passage sensor 46 in order to control the direction and amount of rotation of the drive motor 49 .

FIG. 12 demonstrates a specific operation of the illustrative embodiment. As shown, the control means 34 determines whether or not the print start key 72 is pressed (step B 1 ). If the answer of the step B 1 is Yes, the control means 34 determines the length of the papers 22 stacked on the paper tray 21 on the basis of the output of the paper size sensor 56 (step B 2 ). When the paper size sensor 56 turns on (Yes, step B 2 ), the control means 34 determines that the length of the papers 22 is greater than the paper conveyance distance W, and advances to a step B 3 . If the answer of the step B 2 is No, meaning that the length of the papers 22 is smaller than the above distance W, then the control means 34 returns to the usual print control without controlling the paper conveyance distance.

In the step B 3 , the control means 34 determines the length of the paper 22 being conveyed on the basis of the output of the paper passage sensor 46 . The control means 34 compares length data t output from the sensor 46 and the paper conveyance distance W+α (step B 4 ). If the length data t is grater than the distance W+α (Yes, step B 4 ), the control means 34 executes a step B 5 ; if otherwise (No, step B 4 ), the control means 34 returns to the usual print control without controlling the paper conveyance distance. By so determining the actual length of the paper 22 , it is possible to see if the paper 22 will be pulled in the direction of conveyance when extending over both the print positions E 1 and E 2 or not.

In the step B 5 , the control means 34 calculates, based on the length data t and a set paper conveyance distance Wt, a correction value P for the paper 22 . Subsequently, the control means 34 corrects the distance W in the extending direction with the correction value P (step B 6 ) and then returns to the print control.

Specifically, the control means 34 calculated the correction value P causes the drive motor 49 to rotate by a preselected amount for extending the paper conveyance distance W. As shown in FIG. 8 , the rotation of the drive motor 49 is transferred to the gears 55 and 56 via the drive gear 154 . The gear 55 , in turn, causes the pulley 64 to rotate via the pulley 65 and belt 66 , thereby lowering the lever 68 via the shaft 63 . As a result, the shaft 14 a resting on the top surface 68 a of the lever 68 is moved along the arcuate guide slot 54 . The conveyor 17 is therefore lowered about the shaft 15 a to the second position.

On the other hand, the gear 56 causes the shaft 90 to rotate and thereby lowers the guide member 52 . Consequently, in the second position shown in FIG. 10 , the paper conveyance path W is extended by displacements P 3 and P 4 respectively implemented by the movement of the guide member 52 and that of the conveyor 17 .

As stated above, the paper conveyance path W increases with an increase in the length of the paper 22 . Therefore, even when the length or the conveyance speed of the paper 22 varies, the paper 22 is prevented from being pulled by the ink drum 1 B while an image is printed thereon at the upstream print position E 1 . This insures desirable printing at the upstream print position E 1 and prevents an image from being dislocated at the downstream print position E 2 .

FIG. 13 shows a modification of the above embodiment. As shown, the modification includes moving means 100 for moving the intermediate conveyor 17 up and down and renders the guide members 91 and 92 angularly movable and slidable. The modification is also successful to vary the paper conveyance distance W in accordance with the length of the paper 22 .

Specifically, the guide members 91 and 92 are respectively positioned between the upstream print position E 1 and the upstream end 16 b and between the downstream end 16 c and the downstream print position E 2 . The guide members 91 and 92 are angularly movable about shafts 193 and 194 , respectively. The guide member 91 is made up of a body 91 A and a member 91 B slidably mounted on the body 91 A and having a section in the form of a letter U. Likewise, the guide member 92 is made up of a body 92 A and a member 92 B slidably mounted on the body 91 B and having a generally U-shaped section. The guide members 91 and 92 have ends 101 A and 101 B adjoining the upstream end 16 a and downstream end 16 c , respectively. The body 91 A and slidable member 91 B respectively have flat tops 91 A a and 91 B a inclined downward from the upstream print position E 1 toward the upstream end 16 b . The body 92 A and 92 B respectively have flat tops 92 A a and 92 B a inclined upward from the downstream end 16 c toward the downstream print position E 2 .

The moving means 100 selectively moves the conveyor 17 to a first position shown in FIG. 13 or a second position shown in FIG. 14 . The conveyor 17 implements the reference paper conveyance distance W in the first position or extends the distance W in the second position. The moving means 100 includes a movable frame 195 connected at its bottom 195 b to a plunger 97 a extending out from an electromagnetic solenoid or drive source 97 . The shafts 14 a and 15 a of the driven roller 14 and drive roller 15 , respectively, are horizontally journal led to the movable frame 195 via bearings 99 A and 99 B, respectively. Guide slots 54 A and 54 B are formed in a frame 69 , and each extends over a preselected range in the up-and-down direction. The shafts 14 a and 15 a are respectively passed through the guide slots 54 A and 54 B and supported by the movable frame 195 . The solenoid 97 is affixed to the frame 69 by a mounting member 98 with the plunger 97 extending upward. A drive signal input to the solenoid 97 causes the solenoid 97 to pull the plunger 97 a thereinto.

Tension springs 96 A and 96 B are respectively anchored to the right portion and left portion of the upper part 195 a of the movable frame 195 at one end. The other ends of the tension springs 96 A and 96 B are anchored to the upper part 69 a of the frame 69 . The springs 96 A and 96 B therefore constantly bias the conveyor 17 toward the first position shown in FIG. 13 .

In the above configuration, the top surfaces 91 A a and 91 B a , top surfaces 92 A a and 92 B a and conveying surface 16 a form the paper conveyance path R. The overall length of the path R, e.g., the paper conveyance distance W which the paper 22 moves from the upstream print position E 1 to the downstream print position E 2 is greater than the distance L by the sum of P 1 and P 2 . It is to be noted that the above distance W is a reference distance set up in the first position shown in FIG. 13 ; the distance W in the first position is the reference paper transport distance.

In this modification, the solenoid 97 is connected to the CPU 80 , FIG. 3 , in place of the drive motor 49 via the driver 48 . The displacement sensor 50 and pulse detector 51 are therefore omitted. The modification is constructed such that when the paper size sensor 56 turns on, the length of the papers 22 is determined to be greater than the paper conveyance distance W, and a drive signal is input to the solenoid 97 . Therefore, when the paper size sensor 56 turns on, the solenoid 97 pulls the plunger 97 a downward with the result that the conveyor 17 is lowered from a position indicated by a dash and-dots line in FIG. 14 to a position indicated by a solid line in FIG. 14 .

As shown in FIG. 15 , the guide member 91 includes a rack 102 mounted on the inner side surface 91 B c of the slidable member 91 B. A drive motor 103 has an output shaft 103 a on which a pinion gear 104 is mounted. The pinion gear 104 is held in mesh with the rack 102 for causing it to move. The member 91 B is therefore slidable relative to the body 91 A independently of the conveyor 17 . This is also true with the other guide member 92 . The guide members 91 and 92 each are caused to move about the shaft 193 or 194 by the same mechanism as described in relation to the guide member 52 .

In the second position shown in FIG. 14 , the paper conveyance distance W is increased by the displacement or slide P 5 of the member 91 B and the displacement or slide P 6 of the member 92 B, compared to the first position shown in FIG. 13 . This is also successful to achieve the advantages discussed earlier.

FIG. 16 shows control means 340 representative of another alternative embodiment of the present invention. Briefly, when the length of the papers 22 in the direction of paper conveyance X determined by the paper passage sensor 46 is greater than the paper conveyance distance W, the control means 340 maintains the peripheral speed or conveying speed V 3 of the belt 16 higher than the peripheral speed or conveying speed V 1 of the ink drum 1 A while the paper 22 is conveyed from the print position E 1 to the print position E 2 . Let this mode be referred to as an acceleration mode. A key 105 for allowing the operator to select the acceleration mode is provided on the operation panel 70 and serves as conveyance control selecting means. Because the control means 340 is basically identical with the control means 34 shown in FIG. 3 , identical structural elements are designated by identical reference numerals.

As shown in FIG. 16 , drum motors 116 and 117 drive the ink drums 1 A and 1 B, respectively, and are connected to the CPU 80 via drivers 114 and 115 , respectively. Drum speed sensors 118 and 119 are respectively responsive to the peripheral speeds V 1 and V 2 of the ink drums 1 A and 1 B and implemented by rotary encoders mounted on the output shafts of the drum motors 116 and 117 , respectively. The outputs of the sensors 118 and 119 are sent to the CPU 80 via pulse detectors 120 and 121 , respectively.

A belt speed sensor 112 is responsive to the peripheral speed V 3 of the belt 16 and implemented by a rotary encoder. The rotary encoder is mounted on the output shaft of a belt motor 111 connected to the CPU 80 via a driver 110 . The output of the belt speed sensor 112 is sent to the CPU 80 via a pulse detector 113 . The key 105 mentioned earlier is ALSO connected to the CPU 80 .

The control means 340 includes a ROM 181 storing program relating to the starts and stops and timings of the various devices and drive sections. Further, the ROM 181 stores a table listing experimentally determined data representative of a relation between paper conveying times determined by the sizes of the papers 22 , the peripheral speeds V 1 and V 2 of the ink drums 1 A and 1 B and the peripheral speed of the belt 16 and the lengths of the papers 22 . In addition, the ROM 181 stores an equivelocity mode and the acceleration mode. In the equivelocity mode, the control means 340 controls the various drive sections such that the peripheral speeds V 1 and V 2 of the ink drums 1 A and 1 B and the peripheral speed V 3 of the belt 16 are equal to each other. In the acceleration mode, the control means 340 maintains the peripheral speed V 3 higher than the peripheral speed V 1 for a period of time necessary for the leading edge of the paper 22 to move from the print position E 1 to the print position E 2 . The fan 18 of the conveyor 17 is driven by a fan driver 188 .

A specific operation of the illustrative embodiment will be described with reference to FIG. 17 . As shown, the control means 340 determines the length of the papers 22 stacked on the paper tray 21 on the basis of whether or not the paper size sensor 56 has turned on (step C 1 ). If the answer of the step C 1 is No, then the control means 340 determines that the length of the papers 22 is smaller than the paper conveyance distance W, sets up the equivelocity mode (step C 4 ), and then returns.

If the answer of the step C 1 is Yes, meaning that the paper size sensor 56 has turned on, then the control means 340 determines whether or not the key 105 has been pressed (step C 2 ). If the answer of the step C 2 is Yes, the control means 340 sets up the acceleration mode (step C 3 ); if otherwise (No, step C 2 ), the control means 340 sets up the equivelocity mode.

Assume that the operator, looking at an image (deviation) printed on a trial printing, determines that the peripheral speed V 1 of the ink drum 1 A is higher than the peripheral speed V 2 of the ink drum 1 B. Then, the operator presses the key 105 . In response, the control means 340 maintains the peripheral speed V 3 of the belt 16 higher than the peripheral speed V 1 of the ink drum 1 A for a period of time necessary for the leading edge of the paper 22 to move from the print position E 1 to the print position E 2 . As a result, the paper 22 is preventing from bending on the belt 16 due to the peripheral speed V 2 lower than the peripheral speed V 1 . Should the paper 22 bend on the belt 16 , it would contact the ink drum 1 B and would be smeared thereby or would crease at the print position E 2 . Consequently, an image can be printed on the paper 22 at the print position E 2 in accurate register with the image printed at the print position E 1 .

While the above embodiment uses the paper size sensor 56 for determining the length of the papers 22 , it may, of course, use the paper passage sensor 46 for the same purpose. In the illustrative embodiment, the operator determines a difference between the peripheral speeds V 1 and V 2 by looking at a trial printing. Alternatively, the outputs of the drum speed sensors 118 and 119 may be compared in order to automatically determine a difference between the peripheral speeds V 1 and V 2 .

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

(1) A paper conveyance distance over which a paper is conveyed from an upstream print position to a downstream print position is longer than a distance between the two print positions. Therefore, even when the paper being conveyed extends over both the two print positions, it is prevented from being pulled in a direction of conveyance. It follows that defective printing is reduced even when the peripheral speeds of a plurality of ink drums are different from each other.

(2) An intermediate conveyor has a conveying surface for conveying the paper while retaining it thereon. At least one of the upstream and downstream end of the conveying surface adjoining the upstream print position and downstream print position, respectively, is positioned below an imaginary line connecting the two print positions. This also makes the paper conveyance distance longer than the distance between the two print positions and therefore successfully achieves the above advantage (1).

(3) A guide member is positioned between the upstream print position and the upstream end of the conveying surface and/or between the downstream end of the conveying surface and the downstream print position. The guide member guides the paper toward the conveying surface or the downstream print position. This is also successful to achieve the advantage (1). In addition, the leading edge of the paper is surely conveyed to the conveying surface or the downstream print position.

(4) When the size of papers stacked on paper feeding means has a reference length, the papers are not fed. This is also successful to achieve the advantages (1) and (3). In addition, no papers of inadequate size are fed, so that not a single paper is wasted.

(5) When the length of the paper recognized by paper size recognizing means positioned on a paper conveyance path is greater than the paper conveyance distance, the feed of a new paper and printing operation are interrupted. This also successful to achieve the advantages (1) and (3). In addition, only the papers recognized are fed and subjected to printing, so that no papers other than a trial printing are wasted.

(6) Moving means moves the intermediate conveyor and/or the guide member in accordance with the length of the paper in the direction of conveyance in a direction in which the paper conveyance distance increases. This is also successful to achieve the advantages (1) and (3). In addition, the paper conveyance distance is variable in accordance with length of the paper, so that the paper is prevented from being pulled or slackened in the direction of conveyance due to a difference in length.

(7) When the length of the paper in the direction of conveyance is greater than a reference paper conveyance distance, the intermediate conveyor and/or the guide member is brought to a second position for extending the paper conveyance distance. This is also successful to achieve the advantages (1), (3) and (6).

(8) When the ink drum located at the upstream side rotates at a higher peripheral speed than the ink drum located at the downstream side, an acceleration mode is selected on conveyance control selecting means. As a result, the paper conveying speed of the intermediate conveyor is made higher than the conveying speed of the upstream ink drum. This is also successful to achieve the advantage (1). In addition, the slackening of the paper on the intermediate conveyor is reduced more positively.

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

1. A stencil printer comprising:a plurality of ink drums spaced from each other in a direction of paper conveyance, and each having a respective master wrapped therearound and fed with ink from ink feeding means to an inner periphery thereof; a plurality of pressing devices each being configured to move toward and away from one of said plurality of ink drums; and an intermediate conveyor for conveying a paper on which an image is printed at an upstream print position where an upstream one of said plurality of ink drums in the direction of paper conveyance and one of said plurality of pressing devices associated with said upstream one nip said paper to a downstream print position where a downstream of said plurality of ink drums in said direction and the other of said plurality of pressing devices associated with said downstream one nip said paper, said intermediate conveyor being positioned between said upstream ink drum and said downstream ink drum, wherein a paper conveyance distance which the paper moves while being conveyed from said upstream print position to said downstream print position is made sufficiently longer than a distance between said upstream print position and said downstream print position such that the paper is not pulled by said downstream ink drum while being printed at said upstream ink drum.

2. A stencil printer as claimed in claim 1, wherein said intermediate conveyor has a conveying surface for conveying the paper while retaining said paper thereon and having an upstream end and a downstream end respectively adjoining said upstream print position and said downstream print position, at least one of said upstream end and said downstream end being positioned below an imaginary line connecting said upstream print position and said downstream print position.

3. A stencil printer as claimed in claim 2, further comprising a guide member positioned in at least one of a space between said upstream print position and said upstream end and a space between said downstream end and said downstream print position for guiding the paper being conveyed in the direction of paper conveyance to said conveying surface or said downstream print position.

4. A stencil printer as claimed in claim 3, further comprising:a paper feeding device configured to feed the paper toward said upstream print position; a paper size sensing device included in said paper feeding device and configured to sense a length of the paper relative to a reference length in the direction of paper conveyance; and a control device configured to interrupt paper feed from said paper feeding device in response to an output of said paper size sensing device representative of the length of the paper greater than the reference length.

5. A stencil printer as claimed in claim 3, further comprising:a paper size recognizing device positioned on a paper transport path and configured to recognize a length of the paper being conveyed in the direction of paper conveyance; and a control device configured to interrupt, when the length of the paper recognized by said paper size recognizing device is greater than the paper conveyance distance, at least one of paper feed from said paper feeding device and printing after the paper recognized by said paper size recognizing device has been driven out of said stencil printer.

6. A stencil printer as claimed in claim 3, further comprising:a paper sensing device configured to sense the length of the paper in the direction of paper conveyance; a moving device configured to move at least one of said intermediate conveyor and said guide member in a direction in which the paper conveyance distance increases; and a control device configured to operate said moving device in accordance with an output of said paper sensing device.

7. A stencil printer as claimed in claim 6, wherein said moving device is selectively movable to a first position where at least one of said intermediate conveyor and said guide member defines a reference paper conveyance distance and a second position where said reference paper conveyance distance increases, said control device causing, when the output of said paper sensing device shows that the length of the paper is greater than said reference paper conveyance distance, said moving device to move at least one of said intermediate conveyor and said guide member to said second position.

8. A stencil printer as claimed in claim 3, further comprising:a paper sensing device configured to sense the length of the paper in the direction of paper conveyance; and a conveyance control selecting device configured to select, when the length of the paper sensed by said paper sensing device is greater than the paper conveyance distance, an acceleration mode in which a conveying speed of said intermediate conveyor is maintained higher than a conveying speed of said upstream ink drum for a period of time necessary for said paper to move from said upstream print position to said downstream print position.

9. A stencil printer as claimed in claim 2, further comprising:a paper feeding device configured to feed the paper toward said upstream print position; a paper size sensing device included in said paper feeding device and configured to sense a length of the paper relative to a reference length in the direction of paper conveyance; and a control device configured to interrupt paper feed from said paper feeding device in response to an output of said paper size sensing device representative of the length of the paper greater than the reference length.

10. A stencil printer as claimed in claim 2, further comprising:a paper size recognizing device positioned on a paper transport path and configured to recognize a length of the paper being conveyed in the direction of paper conveyance; and a control device configured to interrupt, when the length of the paper recognized by said paper size recognizing device is greater than the paper conveyance distance, at least one of paper feed from said paper feeding device and printing after the paper recognized by said paper size recognizing device has been driven out of said stencil printer.

11. A stencil printer as claimed in claim 2, further comprising:a paper sensing device configured to sense the length of the paper in the direction of paper conveyance; a moving device configured to move at least one of said intermediate conveyor and said guide member in a direction in which the paper conveyance distance increases; and a control device configured to operate said moving device in accordance with an output of said paper sensing device.

12. A stencil printer as claimed in claim 11, wherein said moving device is selectively movable to a first position where at least one of said intermediate conveyor and said guide member defines a reference paper conveyance distance and a second position where said reference paper conveyance distance increases, said control device causing, when the output of said paper sensing device shows that the length of the paper is greater than said reference paper conveyance distance, said moving device to move at least one of said intermediate conveyor and said guide member to said second position.

13. A stencil printer as claimed in claim 2, further comprising:a paper sensing device configured to sense the length of the paper in the direction of paper conveyance; and a conveyance control selecting device configured to select, when the length of the paper sensed by said paper sensing device is greater than the paper conveyance distance, an acceleration mode in which a conveying speed of said intermediate conveyor is maintained higher than a conveying speed of said upstream ink drum for a period of time necessary for said paper to move from said upstream print position to said downstream print position.

14. A stencil printer as claimed in claim 1, further comprising:a paper feeding device configured to feed the paper toward said upstream print position; a paper size sensing device included in said paper feeding device and configured to sense a length of the paper relative to a reference length in the direction of paper conveyance; and a control device configured to interrupt paper feed from said paper feeding device in response to an output of said paper size sensing device representative of the length of the paper greater than the reference length.

15. A stencil printer as claimed in claim 1, further comprising:a paper size recognizing device positioned on a paper transport path and configured to recognize a length of the paper being conveyed in the direction of paper conveyance; and a control device configured to interrupt, when the length of the paper recognized by said paper size recognizing device is greater than the paper conveyance distance, at least one of paper feed from said paper feeding device and printing after the paper recognized by said paper size recognizing device has been driven out of said stencil printer.

16. A stencil printer as claimed in claim 1, further comprising:a paper sensing device configured to sense the length of the paper in the direction of paper conveyance; a moving device configured to move at least one of said intermediate conveyor and said guide member in a direction in which the paper conveyance distance increases; and a control device configured to operate said moving device in accordance with an output of said paper sensing device.

17. A stencil printer as claimed in claim 16, wherein said moving device is selectively movable to a first position where at least one of said intermediate conveyor and said guide member defines a reference paper conveyance distance and a second position where said reference paper conveyance distance increases, said control device causing, when the output of said paper sensing device shows that the length of the paper is greater than said reference paper conveyance distance, said moving device to move at least one of said intermediate conveyor and said guide member to said second position.

18. A stencil printer as claimed in claim 1, further comprising:a paper sensing device configured to sense the length of the paper in the direction of paper conveyance; and a conveyance control selecting device configured to select, when the length of the paper sensed by said paper sensing device is greater than the paper conveyance distance, an acceleration mode in which a conveying speed of said intermediate conveyor is maintained higher than a conveying speed of said upstream ink drum for a period of time necessary for said paper to move from said upstream print position to said downstream print position.