Sheet processing apparatus and image forming apparatus having the same

Abstract

An adsorption standby unit is provided upstream in the sheet conveying direction of a sheet stacking portion, which stacks sheets to be processed thereon. During processing of sheets on the sheet stacking portion, the next sheet to be processed is adsorbed and put in a standby state by the adsorption standby unit, so that the sheet can be retained and put in a standby state without switching the sheet back.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to sheet processing apparatuses, in which the next sheet is put in a standby state while a sheet is processed, and an image forming apparatus having the sheet processing apparatus, and in particular relates to a sheet processing apparatus capable of holding a sheet in a standby state without switching back the sheet and an image forming apparatus having the sheet processing apparatus.

2. Description of the Related Art

Recently, an image forming apparatus, such as an electrophotographic copy machine and a laser beam printer, has been provided with a sheet processing apparatus as an option, such as a sorter for sorting sheets discharged from an image forming apparatus body to have images formed thereon. By such a sheet processing apparatus, not only the sorting, but also various kinds of processing have been carried out, such as stacking of a plurality of sheets, alignment, or binding sheets together with staples by a stapler.

In the sheet processing apparatus having the stapler, when binding sheets, sheets conveyed to the sheet processing apparatus are stacked on a processing tray after passing them through a conveying path within the body, and then bound. When a sheet bundle is bound, by moving the stapler, the bundle is bound at one point or plural points (two points generally).

During binding operation (binding processing), sheets for the next job cannot be stacked on the processing tray, so that the sheets have been generally spaced for the next job during the binding. This results in reduced productivity.

Then, in order to prevent the deterioration of the probability, during processing the sheets on the processing tray, the next sheets to be processed are put in a standby state in the upstream side of the processing tray without conveying them to the processing tray (see Japanese Patent Laid-Open No. 2003-081517).

FIGS. 10A and 10B are drawings showing such a conventional sheet processing apparatus, wherein FIG. 10A shows a sheet bundle 52 placed on a processing tray 51 going to be bound by a stapler (not shown). In such a state, when a first sheet 53 for the next job is conveyed to the inside of a finisher by an inlet roller 54, the sheet 53 is first conveyed to the downstream of a buffer guide 55 by the rotation in arrow direction of a buffer roller 50 rotatable in normal and reverse directions.

Then, when the sheet 53 is conveyed downstream by a predetermined distance X (switchback point) from the buffer guide 55, the buffer roller 50 is reversely rotated in arrow direction shown in FIG. 10B, so that the sheet 53 is switched back (reversely conveyed) so as to abut the buffer guide 55. Thereafter, the buffer guide 55 is rotated in arrow direction by a solenoid (not shown) so as to retain the trailing end of the sheet 53, thereby putting the sheet 53 in a standby state at this position.

However, in the conventional sheet processing apparatus structured as above, the conveyed sheet 53 is switched back after being once stopped every time, so that the sheet 53 is liable to be jammed as well as productivity is reduced because the number of processed sheets per unit time decreases due to switching back operation time.

Also, images on the sheet may deteriorate during switching back by the buffer roller 50 because the sheet is rubbed with a guide plate (not shown) arranged below the buffer roller 50 at point A. Moreover, the switched back sheet may be damaged at an end by being abutted by the buffer guide 55.

In addition, since the sheet 53 directly after having images formed thereon generally is at high temperature, guides retaining the sheet 53 are increased in temperature during putting the sheet 53 in a standby state. As a result, operability in jamming treatment and durability of the buffer guide 55 may be reduced.

SUMMARY OF THE INVENTION

The present invention is directed to a sheet processing apparatus capable of reserving sheets without switching them back and an image forming apparatus having the sheet processing apparatus.

In one aspect of the present invention, a sheet processing apparatus operable to process sheets includes a sheet stacking portion configured to stack the sheets to be processed thereon; a sheet conveying path facilitating conveying a sheet to be processed to the sheet stacking portion; a sheet conveying portion disposed in the sheet conveying path; an adsorption standby unit disposed in the sheet conveying path, and configured to adsorb a next sheet to be processed so as to be put in a standby state during processing of the sheets on the sheet stacking portion, wherein the adsorption standby unit adsorbs an end of the sheet conveyed to the sheet conveying path by the sheet conveying portion.

According to the present invention, during processing of the sheets on the sheet stacking portion, the next sheet to be processed is adsorbed so as to put it in a standby state by the adsorption standby unit which is disposed upstream in the sheet conveying direction of the sheet stacking portion, so that the sheet can be retained and put in a standby state without switching the sheet back.

Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an image forming apparatus having a sheet processing apparatus (finisher) according to an embodiment of the present invention.

FIG. 2 is a first drawing illustrating the schematic structure and buffer operation of the sheet processing apparatus.

FIG. 3 is a second drawing illustrating the buffer operation of the sheet processing apparatus.

FIG. 4 is a third drawing illustrating the buffer operation of the sheet processing apparatus.

FIG. 5 is a fourth drawing illustrating the buffer operation of the sheet processing apparatus.

FIG. 6 is a fifth drawing illustrating the buffer operation of the sheet processing apparatus.

FIG. 7 is a sixth drawing illustrating the buffer operation of the sheet processing apparatus.

FIG. 8 is a drawing showing the situation in that a plurality of sheets are stacked like brick work in the sheet processing apparatus.

FIG. 9 is a drawing illustrating another structure of the sheet processing apparatus.

FIGS. 10A-B are drawings illustrating the buffer operation of a conventional sheet processing apparatus.

DESCRIPTION OF THE EMBODIMENTS

Best mode for carrying out the invention will be described below in detail with reference to the drawings.

FIG. 1 is a schematic view of an image forming apparatus having a sheet processing apparatus according to an embodiment of the present invention. Referring to FIG. 1, reference numeral 100 denotes an image forming apparatus; numeral 100A denotes an image forming apparatus body (referred to as an apparatus body below); and numeral 19 denotes a sheet processing apparatus for processing sheets having images formed thereon and discharged from the apparatus body 100A. According to the embodiment, the sheet processing apparatus (referred to as a finisher below) 19 is detachably mounted on the apparatus body 100A as an independent optional apparatus. Alternatively, the sheet processing apparatus 19 of the present invention can be integrally incorporated to the image forming apparatus 100 so as to be provided in the apparatus body 100A.

The apparatus body 10A is provided with an image forming portion 100B and a sheet feed portion 100C for supplying a sheet, such as a plastic sheet accommodated in a cassette 115A, towards the image forming portion 100B. On the apparatus body 100A, an automatic document feeder 102 is provided.

In the image forming apparatus 100 structured as above, for forming an image on a sheet, first, a document G placed on a document tray 103 is corrected in skewing by a pair of register rollers 105 after being sequentially separated one sheet at a time by a feed portion 104. Then, the document G is passed through a reading position 107 via a lead path 106. The reflection light obtained by irradiating the document G passing through the reading position 107 with light by an illuminating system 111 is led to an optical element 113 (a CCD or other elements) with a mirror 112 so that images formed on the surface of the document are read so as to obtain image data.

The document with images being read out during passing through the reading position 107 is discharged onto a discharge tray 109 after passing through a discharge path 108. When both sides of the document are read, after the top surface is read in such a manner, the document G proceeds through the discharge path 108 so as to be reversed by a pair of reversing rollers 110 and to be fed to the pair of register rollers 105 again in the inverted state.

Furthermore, after being corrected in skewing by the pair of register rollers 105 in the same way as that of the top surface reading, the document G passes through the lead path 106 so that images formed on the surface (bottom surface at this time) are read at the reading position 107. Thereafter, the document G is discharged to the discharge tray 109 via the discharge path 108.

In the image forming portion 100B, a photoconductor drum 114 is irradiated with laser light from an exposing unit (not shown) based on the obtained image data so as to form latent images on the surface of the photoconductor drum 114. In addition, the photoconductor drum 114 may also be directly irradiated with reflection light by the mirror 112.

Then, the latent images formed on the photoconductor drum 114 in such a manner are developed by toner supplied from a toner supplying device (not shown), thereby forming toner images on the photoconductor drum 114.

Along with such image forming operation, in the sheet feed portion 100C, a sheet accommodated in the cassette 115A is fed to a position opposing the photoconductor drum 114 in accordance with a recording signal, and then, the toner images formed on the photoconductor drum 114 are transferred onto the sheet by a transfer device 116. Then, the sheet having the toner images transferred thereon is fed to a fixing device 117 so that the images are permanently fixed on the sheet by pressurizing and heating them in the fixing device 117.

When images are formed on each side of a sheet, the sheet having images fixed on one surface by the fixing device 117 in such a manner is again conveyed to between the photoconductor drum 114 and the transfer device 116 via an each-side path 118 arranged downstream to the fixing device 117. Thereby, toner images are also formed on the bottom surface of the sheet. The sheet having images on its one side or each side in such a manner is discharged outside (to the finisher 19).

FIG. 2 is a drawing of the structure of the finisher 19. Referring to FIG. 2, reference characters 1A and 1B denote a pair of inlet rollers. A sensor 15 detects the leading end of a sheet guided in a conveying path 2 arranged in the finisher 19. A buffer roller 4 is rotated about a shaft B as a fulcrum in the horizontal direction.

Reference character 5A denotes an upper discharge roller; character 5B denotes a lower discharge roller; character 7A denotes an upper bundle discharge roller; character 7B denotes a lower bundle discharge roller; and numeral 6 denotes a holder movable in the vertical direction for rotatably holding the upper discharge roller 5A and the upper bundle discharge roller 7A. Reference character 6A denotes a push-up member, and the holder 6 is moved upward by the push-up member 6A. When the holder 6 is moved upward in such a manner, the upper discharge roller 5A and the upper bundle discharge roller 7A are separated from the lower discharge roller 5B and the lower bundle discharge roller 7B, respectively.

Reference numeral 13 denotes a step portion arranged upstream of the conveying path 2 (downstream of the pair of inlet rollers 1A and 1B). Under the step portion 13, an adsorption standby unit such as an adsorption fan 3 is provided. As will be described later, when the adsorption fan 3 is driven after the trailing end of a sheet S1 arrives at above the step portion 13 so as to stop, the trailing end of the sheet S1 is adsorbed while being curved along an inclined surface 13a and the bottom surface 13b of the step portion 13.

Referring to FIG. 2, reference numeral 10 denotes a stack tray rotatable in the normal and reverse directions for stacking sheets discharged by the upper and lower pair of bundle discharge rollers 7A and 7B; numeral 14 denotes a processing tray downward inclined from the pair of bundle discharge rollers for stacking sheets to be processed; and numeral 9 denotes a stapler for binding the sheet bundle placed on the processing tray 14.

Reference numeral 12 denotes a rear end stopper disposed adjacent to the inclined end of the processing tray 14 for abutting a sheet dropping along the processing tray 14. The rear end stopper 12 is fixed to a belt 12a. The belt 12a is rotated by a driving motor 11. The rear end stopper 12 is moved towards the stack tray with the belt 12a rotated by the driving motor 11 after the binding process of the sheet bundle placed on the processing tray 14 is finished. By moving the rear end stopper 12 in such a manner, the bound sheet bundle is discharged to the stack tray 10.

In addition, by the rotation of the pair of bundle discharge rollers 7A and 7B in a direction reverse to arrow direction, a sheet placed on the processing tray 14 is returned with a return roller 8 towards the rear end stopper 12, so that the position of the trailing end during stapling is restricted by the rear end stopper 12.

After a predetermined number of sheets is abutted to the rear end stopper 12 so as to form a sheet bundle with such normal and reverse rotations of the pair of bundle discharge rollers 7A and 7B and the return roller 8, the sheet bundle is bound by driving the stapler 9. After completion of the binding operation, by rotating the pair of bundle discharge rollers 7A and 7B in arrow direction and by moving the rear end stopper 12, the bound sheet bundle is discharged to the stack tray 10.

During the sheet processing in such a manner, when the first sheet for the next job is discharged from the apparatus body 1, the sheet S1, as shown in FIG. 2, is led to the conveying path 2 within the finisher with the pair of inlet rollers 1A and 1B, and then the end of the sheet S1 is detected by the sensor 15. Based on the detection signal from the sensor 15, a controller (not shown) drives the buffer roller 4 and the pair of discharge rollers 5A and 5B so as to convey the sheet S1 to a predetermined position thereafter.

Then the sheet S1, as shown in FIG. 3, is conveyed to the pair of bundle discharge rollers 7A and 7B, thereby being placed on the stack tray 10 at its end. When the trailing end of the sheet S1 reaches on the step portion 13, the controller stops the buffer roller 4, the pair of discharge rollers 5A and 5B, and the pair of bundle discharge rollers 7A and 7B, and drives the adsorption fan 3 as well. The sheet S1, as shown in FIG. 4, is thereby adsorbed along the inclined surface 13a and the bottom surface 13b of the step portion 13 in a curved state.

According to the embodiment, the step portion 13 has a depth so deep that the next sheet S2 proceeding toward the conveying path 2 cannot abut the trailing end of the adsorbed sheet S1. Accordingly, when the sheet S1 is held at its rear end to such step portion 13, the leading end of the sheet S2 entering freshly cannot catch on the trailing end of the temporarily retained sheet S1, as shown in FIG. 5. The sheet S1 discharged from the apparatus body 1 is generally in a high temperature because it has passed through the fixing device 17. However, a conveying guide 2a of the conveying path 2 cannot be in a high temperature since it is effectively cooled by the adsorption fan 3.

Then, after the sheet S1 is adsorbed in such a manner, the push-up member 6A, as shown in FIG. 5, is upward rotated about a shaft C as a fulcrum with a driving unit (not shown). When the push-up member 6A is rotated upward, the holder 6 is rotated upward about a shaft D as a fulcrum so that the upper discharge roller 5A and the upper bundle discharge roller 7A are separated from the lower discharge roller 5B and the lower bundle discharge roller 7B, respectively. As a result, the pair of discharge rollers 5A and 5B and the pair of bundle discharge rollers 7A and 7B do not apply a conveying force to the temporarily retained sheet S1.

Then, in this state, when a second sheet S2 is conveyed with the pair of inlet rollers 1A and 1B from the apparatus body 1 as shown in FIG. 5, the sensor 15 detects the leading end of the sheet S2, and then the sheet S2 is conveyed with the buffer roller 4 after its trailing end has passed between the pair of inlet rollers 1A and 1B.

At this time, to the temporarily retained first sheet S1, the conveying force obtained by multiplying the pressurizing force of the buffer roller 4 by the friction coefficient between the two sheets S1 and S2 is applied. However, since the adsorption force of the adsorption fan 3 is established to be larger than this conveying force, the first sheet S1 cannot be brought by the second sheet S2. Only the second sheet S2 is thereby conveyed with the buffer roller 4. Then, the sheet S2 is controlled by the controller (not shown) to move until the trailing end of the sheet S2 reaches above the trailing end of the adsorbed sheet S1.

Thus, when the trailing end of the second sheet S2 arrives at the upper position of the trailing end of the first sheet S1, the second sheet S2 is stopped by upwardly rotating the buffer roller 4 about the fulcrum B with a driving unit such as a solenoid (not shown). Thereafter, the push-up member 6A is downwardly rotated about the shaft C and the holder 6 is downwardly rotated about the shaft D.

Thereby, the two sheets S1 and S2 are pinched between the upper and lower discharge rollers 5A and 5B and between the upper and lower bundle discharge rollers 7A and 7B. Then, when the lower discharge roller 5B and the lower bundle discharge roller 7B are rotated, the temporarily retained first sheet S1 and the second sheet S2 are conveyed. At this time, the first sheet bundle bound on the processing tray 14 is already discharged on the stack tray 10.

When the two sheets S1 and S2 pass through between the pair of discharge rollers 5A and 5B, and the pair of bundle discharge rollers 7A and 7B are reversely rotated thereafter, the two sheets S1 and S2 drop on the processing tray 14 so as to be abutted to the rear end stopper 12 by the return roller 8, as shown in FIG. 7. Thereafter, sheets are sequentially conveyed one by one from the apparatus body 1 to the finisher 19. When the number of sheets placed on the processing tray 14 becomes a predetermined number of sheets, the stapler 9 binds sheet bundles within the processing tray 14.

After the binding processing is finished in such a manner, the rear end stopper 12 is moved toward the pair of bundle discharge rollers with the belt 12a driven by the driving motor 11. With the rear end stopper 12 and the pair of bundle discharge rollers 7A and 7B, the bound sheet bundle is discharged on the stack tray 10. In addition, while the second sheet bundle is bound, the first sheet of the next job (third sheet bundle) is conveyed in the conveying path 2, and is temporarily retained as shown in FIGS. 2 to 4.

While sheets on the stack tray are processed with the adsorption fan 3 in such a manner, by putting at least one sheet to be processed in the next in a standby state, the sheet can be retained without switching it back, thereby achieving high productivity and reducing jamming.

The rubbing and damage in images produced during switchback operation are eliminated, thereby providing high quality sheets. Using the adsorption fan 3 enables the buffer portion including the step portion 13 for putting a high-temperature sheet in a standby state to be cooled by air, preventing deterioration in durability of the buffer portion and in jamming operability.

In the above description about controlling, after the leading end of a sheet is detected by the sensor 15, the sheet is stopped at a predetermined position. Alternatively, after the trailing end of a sheet is detected by the sensor 15, the sheet may be conveyed by a predetermined distance so as to have the same effect.

In the above description, the number of sheets to be temporarily retained is one. However, the number is not limited to this. As shown in FIG. 8, by alternately shifting the first sheet S1 and the second sheet S2 so as to have a phase difference of E between the first and second sheets S1 and S2 at trailing ends, a plurality of sheets can be temporarily retained even by adsorbing with air. When a plurality of sheets are superimposed so as to shift the trailing ends of sheets in the sheet conveying direction, the second sheet S2 may be controlled to stop at a position upstream remote from the first sheet S1 in the sheet conveying direction. That is, since the trailing end of the sheet S2 protrudes from the trailing end of the sheet S1 by the phase difference E upstream in the sheet conveying direction, the trailing end portion of the sheet S2 can be directly and securely adsorbed with the adsorption fan 3.

Furthermore, in the above description, the trailing end portion of the sheet has been adsorbed with the adsorption fan 3 (adsorbing unit). However, the invention is by no means limited to this, so that the leading end portion of a sheet or the entire sheet may be adsorbed. In other words, the same effect can be obtained as long as an adsorbing force is secured sufficiently enough not to be conveyed by a frictional force of the other sheet therebetween.

The adsorption method using air due to the adsorption fan 3 has been described. However, the invention is not limited to this, and as shown in FIG. 9, an electrostatic adsorption unit 20 may be used as an electrostatic adsorption standby unit so as to adsorb a sheet with static electricity.

In the case of such electrostatic adsorption, an adsorption force due to static electricity is exerted through the sheet, the sheets S1 need not to be placed like brick work as shown FIG. 8, and a plurality of sheets can be temporarily retained in a state that sheet trailing ends are aligned. Since the electrostatic adsorption may not adsorb the sheet at high speed unlike in the air adsorption, when the finisher 19 is connected to a high productive image forming apparatus with a short time interval between sheets, the buffer operation should be assisted by ejecting air toward the electrostatic adsorption unit 20 so as to urge the sheet toward the electrostatic adsorption unit 20, as shown in FIG. 9.

In the adsorption methods using air and static electricity described above, it is easily assumed that the required adsorption force be different corresponding to the size, kind, and weight of the sheet. Thus, the adsorption force may be established so as to satisfy all the temporarily retained sheets. Alternatively, in order to reduce electric power consumption and noise, the adsorption force is controlled by the controller corresponding to at least one of the size, kind, and weight of the sheet, for example.

The adsorption unit is not limited to the air adsorption unit and the electrostatic adsorption unit described above, and any unit that can control the sheet adsorption of course has the same effect. Moreover, in the above-description, the step portion 13 is provided below the upstream of the conveying path 2. However, the invention is not limited to this, so that the step portion 13 may also be provided above the upstream of the conveying path 2 as long as it does not prevent the next sheet from being conveyed while the adsorption standby unit may be provided above the step portion 13.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all modifications, equivalent structures and functions.

This application claims the benefit of Japanese Application No. 2004-269018 filed Sep. 15th, 2004, which is hereby incorporated by reference herein in its entirety.

Claims

1. A sheet processing apparatus operable to process sheets, comprising:

a sheet stacking portion configured to stack the sheets to be processed thereon;

a sheet conveying path facilitating conveying a sheet to be processed to the sheet stacking portion;

a sheet conveying portion disposed in the sheet conveying path; and

an adsorption standby unit disposed in the sheet conveying path, the adsorption standby unit configured to adsorb a next sheet to be processed so as to be put in a standby state during processing of the sheets on the sheet stacking portion,

wherein the adsorption standby unit adsorbs an end of the sheet conveyed to the sheet conveying path by the sheet conveying portion.

2. The sheet processing apparatus according to claim 1, further comprising:

a step portion disposed upstream of the sheet conveying path,

wherein the adsorption standby unit is provided in the step portion.

3. The sheet processing apparatus according to claim 2, wherein the adsorption standby unit adsorbs the trailing end portion in the sheet conveying direction of the next sheet to be processed along a sheet stacking surface of the step portion.

4. The sheet processing apparatus according to claim 2, wherein the step portion has a depth which prevents a sheet next to the adsorbed sheet adsorbed by the adsorption standby unit from abutting the trailing end in the sheet conveying direction of the adsorbed sheet during proceeding towards the sheet conveying path.

5. The sheet processing apparatus according to claim 1, further comprising:

a detection sensor configured to detect a sheet conveyed to the sheet conveying path; and

a controller controlling driving of the sheet conveying portion so as to stop the trailing end in the sheet conveying direction of the next sheet to be processed at a position opposing the adsorption standby unit in accordance with a signal from the detection sensor.

6. The sheet processing apparatus according to claim 1, wherein the adsorption standby unit adsorbs the sheet by air adsorption.

7. The sheet processing apparatus according to claim 1, wherein the adsorption standby unit adsorbs the sheet by electrostatic adsorption.

8. The sheet processing apparatus according to claim 6, wherein the adsorption standby unit adsorbs a plurality of sheets by shifting the trailing ends in the sheet conveying direction of the sheets.

9. The sheet processing apparatus according to claim 1, further comprising a controller which controls a sheet adsorbing force of the adsorption standby unit to correspond to at least one of the material, size, and weight of the sheet.

10. An image forming apparatus comprising:

an image forming portion configured to form images on a sheet;

a sheet processing apparatus processing the sheet having images formed thereon by the image forming portion, the sheet processing apparatus including: a sheet stacking portion configured to stack the sheets to be processed thereon; a sheet conveying path facilitating conveying a sheet to be processed to the sheet stacking portion;

a sheet conveying portion disposed in the sheet conveying path; and an adsorption standby unit disposed in the sheet conveying path, the adsorption standby unit configured to adsorb a next sheet to be processed so as to be put in a standby state during processing of the sheets on the sheet stacking portion, wherein the adsorption standby unit adsorbs an end of the sheet conveyed to the sheet conveying path by the sheet conveying portion.

11. The image forming apparatus according to claim 10, wherein the sheet processing apparatus further includes:

a step portion disposed upstream of the sheet conveying path,

wherein the adsorption standby unit is provided in the step portion.

12. The image forming apparatus according to claim 11, wherein the adsorption standby unit adsorbs the trailing end portion in the sheet conveying direction of the next sheet to be processed along a sheet stacking surface of the step portion.

13. The image forming apparatus according to claim 11, wherein the step portion has a depth which prevents a sheet next to the adsorbed sheet adsorbed by the adsorption standby unit from abutting the trailing end in the sheet conveying direction of the adsorbed sheet during proceeding towards the sheet conveying path.

14. The image forming apparatus according to claim 10, further comprising:

a detection sensor configured to detect a sheet conveyed to the sheet conveying path; and

a controller controlling driving of the sheet conveying portion so as to stop the trailing end in the sheet conveying direction of the next sheet to be processed at a position opposing the adsorption standby unit in accordance with a signal from the detection sensor.

15. The image forming apparatus according to claim 10, wherein the adsorption standby unit adsorbs the sheet by air adsorption.

16. The image forming apparatus according to claim 10, wherein the adsorption standby unit adsorbs the sheet by electrostatic adsorption.

17. The image forming apparatus according to claim 15, wherein when the adsorption standby unit adsorbs a plurality of sheets by shifting the trailing ends in the sheet conveying direction of the sheets.

18. The image forming apparatus according to claim 10, further comprising a controller which controls a sheet adsorbing force of the adsorption standby unit to correspond to at least one of the material, size, and weight of the sheet.