US20110233853A1

US20110233853A1 - Sheet stacking apparatus, sheet finishing apparatus, image forming apparatus and sheet stacking method

Info

Publication number: US20110233853A1
Application number: US13/027,200
Authority: US
Inventors: Tatsuhisa Naraoka
Original assignee: Toshiba Corp; Toshiba TEC Corp
Current assignee: Toshiba Corp; Toshiba TEC Corp
Priority date: 2010-03-26
Filing date: 2011-02-14
Publication date: 2011-09-29

Abstract

A sheet stacking apparatus according to an embodiment of the invention includes a movable tray having a sheet stack part whose center part is dented downward as the number of discharged and stacked sheets increases, and a movement control mechanism part to move and control the movable tray up and down.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority from U.S. provisional Application Ser. No. 61/318255 filed on Mar. 26, 2010. The entire contents of the provisional application are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a sheet stacking apparatus, a sheet finishing apparatus, an image forming apparatus and a sheet stacking method.

BACKGROUND

An image forming apparatus generally has functions to forma latent image by selectively exposing the surface of a photoconductive drum, to develop the latent image by a developer or the like to obtain a visible image, and to transfer it to a sheet to obtain a copy. Although no limitation is made to the image forming apparatus as stated above, a sheet copied by a copying machine is finally stacked on a discharge tray. The discharge tray may be provided as a sheet stacking apparatus separate from the image forming apparatus, or may be provided integrally with the image forming apparatus. When the number of sheets to be copied is small, a fixed tray may be used.
However, when the number of copied sheets is large, the amount of sheets stacked on the discharge tray increases. Then, in order to stably hold the stacked sheets, a movable tray capable of moving up and down is used in many cases, and the movable tray is lowered and sheets are stacked. The design can be made such that also when the number of sheets is small, they are discharged to the movable tray.
An apparatus is known in which when the stack amount increases, an angle of the movable tray is tilted more than usual to stably secure sheets.
However, even if the angle of the tray is changed, when the stack amount is large, the sheets can not be stably aligned and secured.
The invention provides a sheet stacking apparatus capable of aligning and stacking sheets even when the sheet stack amount increases.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing an outer appearance of a sheet finishing apparatus and a multi-function type color copying apparatus in an embodiment of the invention.

FIG. 2 is a main part explanatory view of the sheet finishing apparatus of the embodiment.

FIG. 3 is a perspective view of the sheet finishing apparatus of the embodiment.

FIG. 4 is a top view of a sheet stacking apparatus of a first embodiment.

FIG. 5 is a perspective view of a case where the number of stacked sheets is small in the first embodiment.

FIG. 6 is a perspective view of a case where the number of stacked sheets is large in the first embodiment.

FIG. 7 is a structural view of a second embodiment.

FIG. 8 is a perspective view of a case where the number of stacked sheets is large in the second embodiment.

FIG. 9 is a front view of a sheet finishing apparatus of a third embodiment.

DETAILED DESCRIPTION

According to an embodiment of the invention, a sheet stacking apparatus includes a movable tray having a sheet stack part whose center part is dented downward as the number of discharged and stacked sheets increases, and a movement control mechanism part to move and control the movable tray up and down.
Hereinafter, embodiments of the invention will be described with reference to the drawings. In the following, a description is made on the assumption that an image forming apparatus including the sheet stacking apparatus of the embodiments is a multi-function type color copying apparatus. FIG. 1 is a schematic view of the image forming apparatus of the embodiment. An image forming apparatus 1 includes an image reading part 2 to read an image of a read object, an image forming part 4 to form the image, and an operation panel 5. The operation panel 5 includes a touch-panel type display part 6 and various operation keys 7.
The operation keys 7 of the operation panel 5 include, for example, a numeric keypad, a reset key, a stop key, a start key and the like. The display part 6 gives various instructions of, for example, sheet size, the number of copies, print density setting and the like.
The image reading part 2 includes a document table 8, a carriage 9, an exposure lamp 10, a reflecting mirror 11, an imaging lens 12 and a CCD (Charge Coupled Device) 13. The CCD 13 is a photoelectric conversion element to capture reflected light and converts it into an electric signal. An auto document feeder 30 to convey a document to a read position is provided above the document table 8.
The image forming part 4 includes an intermediate transfer belt 14 as a transfer body, and four process units 16Y, 16M, 16C and 16K arranged side by side along the intermediate transfer belt 14 and corresponding to toners (recording materials) of respective colors of yellow (Y), magenta (M), cyan (C) and black (K). Since these process units have the same structures, the black process unit is typically selected here and this will be described.
The process unit 16K includes a photoreceptor 18K as an image carrier, a laser unit 20K to form an electrostatic latent image on the photoreceptor 18K, a charging device 22K disposed around the photoreceptor 18K, a developing device 24K, a primary transfer device 26K opposite to the photoreceptor 18K through the intermediate transfer belt 14, a cleaner 27K and a charge removing lamp 28K.
The operation of the process unit 16K will be described. First, a document is placed on the document table 8 or the auto document feeder 30 conveys a document to a read position. Next, the exposure lamp 10 supported by the carriage 9 irradiates light to the document from beneath the document table 8. An image of reflected light from the document is focused and is projected onto the CCD 13.
The CCD 13 captures the reflected light and outputs image information of the document as an analog signal. The electric signal transmitted by the CCD 13 is converted into a digital signal. The laser unit 20K receives the digital signal subjected to image processing.
When the image forming part 4 starts image formation, the charging device 22K charges the outer peripheral surface of the rotating photoreceptor 18K. The laser unit 20K irradiates a laser beam in accordance with the digital signal subjected to the image processing to the outer peripheral surface of the photoreceptor 18K charged to a uniform potential in the axial direction by the charging device 22K in order to form an electrostatic latent image.
The developing device 24K supplies a black developer (for example, toner) to the outer peripheral surface of the photoreceptor 18K, and develops the electrostatic latent image with the toner. The primary transfer device 26K electrostatically transfers the toner image to the intermediate transfer belt 14.
The cleaner 27K disposed downstream of the primary transfer device 26K in the rotation direction of the photoreceptor 18K removes the toner remaining on the photoreceptor 18K without being transferred. The charge removing lamp 28K disposed downstream of the cleaner 27 in the rotation direction of the photoreceptor 18K removes the remaining charge on the outer peripheral surface of the photoreceptor 18K. In the color image formation, the process units 16Y, 16M and 16C similarly perform the above operation.
A secondary transfer device 36 electrostatically transfers the toner image transferred on the intermediate transfer belt 14 to a sheet conveyed from a sheet feed device 32 through a conveyance path 34. A fixing device 38 fixes the toner image to the sheet. A conveyance roller 40 conveys the sheet on which the toner image is fixed to a branch member 42. The transfer device and the fixing device constitute a print part of the sheet.
The branch member 42 guides the sheet to a reverse roller 44 or a conveyance roller 54 based on the sheet processing. When the branch member 42 guides the sheet to the reverse roller 44, the reverse roller 44, a branch member 46 and a conveyance roller 48 convey the sheet to a sheet discharge roller 50, and the sheet discharge roller 50 discharges the sheet to an upper-stage fixed tray 107.
When two-sided printing is performed, the branch member 42 guides the sheet to the reverse roller 44, and the reverse roller 44 is reversely rotated and conveys the sheet to the branch member 42. The branch member 42 guides the sheet to a conveyance roller 56, and the conveyance roller 56 conveys the sheet to the secondary transfer device 36. As a result, the secondary transfer device 36 transfers a toner image to the opposite surface of the conveyed sheet. The fixing device 38 fixes the toner image to the sheet. Besides, for example, when the size of the sheet is, for example, A3 size and is large, the branch member 46 guides the A3 size sheet to a reverse paper path, and the reverse roller 44 is reversely rotated and conveys the A3 size sheet to the branch member 42.
When the branch member 42 guides the sheet to a conveyance roller 60, the conveyance roller 60 conveys the sheet to a carry-out roller 62, and the carry-out roller 62 discharges the sheet to a movable tray 140 of a sheet finishing apparatus 100 as indicated by a dotted line. The movable tray 140 is controlled by a movement control mechanism part 144. The movement control mechanism part 144 includes a tray position detection part 140 s to detect the position of the movable tray 140, a tray drive belt 140 b, and a tray drive motor 140 m to drive and control the rotation of the tray drive belt 140 b.
Incidentally, the sheet here is, for example, standard thick paper, thin paper, glossy paper, OHP sheet or the like.
FIG. 2 shows a schematic structure of the sheet finishing apparatus 100. The sheet finishing apparatus 100 functionally includes a finishing part 105 to perform finishing, such as stapling, after the sheet is conveyed, and a sheet stacking apparatus 130 for stacking the sheet subjected to the finishing. Although the sheet is discharged also to the fixed tray, the sheet stack part here indicates the portion of the movable tray 140 on which many discharged sheets are stacked.
The sheet finishing apparatus 100 processes the sheet discharged from the image forming apparatus 1 in accordance with input instructions from the operation panel 5 or instructions from a separately connected PC.
The sheet finishing apparatus 100 includes an inlet roller 102, a branch member 104, a sheet discharge roller 106, an exit roller 108, a standby tray 110, a standby roller 112, a processing tray 114, an alignment member 116, a stapler 118, a sheet bundle discharge member 120, the fixed tray 107, the movable tray 140 and a shutter 160.
The inlet roller 102 receives the sheet on which the image is formed by the image forming apparatus 1, and conveys it to the branch member 104. The branch member 104 guides the sheet to the sheet discharge roller 106 or the exit roller 108.
The branch member 104 discharges the sheet to the fixed tray 107. On the other hand, when the branch member 104 guides the sheet to the exit roller 108, the exit roller 108 conveys the sheet to the standby tray 110. The standby tray 110 temporarily holds plural conveyed sheets. After supporting a predetermined number of sheets, the standby tray 110 drops the supported sheets to the processing tray 114.
When after-mentioned lateral alignment or stapling is not performed using the processing tray 114, the standby tray 110 conveys the supported sheets directly in the direction of the movable tray 140, and discharges them. In this case, the standby tray 110 and the standby roller 112 do not stop the sheets at the standby tray 110, and discharges the sheets one by one to the movable tray 140.
When the lateral alignment or stapling is performed, the following operation is performed. The processing tray 114 receives the sheets dropped by the standby tray 110. While the sheets are aligned or stapled, the processing tray 114 supports the stacked sheets. The alignment member 116 aligns a sheet bundle on the processing tray 114 in the lateral direction crossing the conveyance direction. The stapler 118 staples the end of the aligned sheet bundle as the need arises. The sheet bundle discharge member 120 discharges the stapled sheet bundle to the movable tray 140.
The movable tray 140 is controlled by the movement control mechanism part 144 including the tray drive belt 140 b and the tray drive motor 140 m, and moves up and down. A detection member 132 detects the upper surface of the movable tray 140 or the uppermost surface of the sheets stacked on the movable tray 140. The detection member 132 is attached to the shutter 160.
For example, when one or plural sheets are discharged, the movable tray 140 is controlled by the movement control mechanism part 144, and moves downward. The stacked sheets are removed from the movable tray 140, and when a sheet on the movable tray 140 is not detected, the movable tray 140 is controlled by the movement control mechanism part 144, and moves upward. The movable tray 140 moves to the position where the detection member 132 detects the uppermost surface of sheets stacked on the movable tray 140, and stacks discharged sheets.

First Embodiment

FIG. 3 is a perspective view of the sheet finishing apparatus 100. As shown in FIG. 1, the sheet stacking apparatus 130 included in the sheet finishing apparatus 100 includes the movable tray 140 and the movement control mechanism part 144 to move and control the movable tray 140 up and down. The movement control mechanism part 144 includes the tray position detection part 140 s to detect the position of the tray, the tray drive belt 140 b, and the tray drive motor 140 m to drive and control the rotation of the tray drive belt 140 b.
The movable tray 140 includes a sheet stack part 141 for stacking a discharged sheet. FIG. 3 to FIG. 6 show a structure of the sheet stack part 141. FIG. 4 is a top view of the sheet stack part 141 which includes two plates 142 a and 142 b provided right and left in a discharge direction X of the discharged sheet. The two plates contact each other and are provided to be bent downward at the boundary surface thereof.
As shown in FIG. 5, a holding body 143 to hold the plates is provided under the two plates 142 a and 142 b. The plate 142 a includes, for example, four pressure springs 147 c, 147 d, 147 e and 147 f provided on the holding body 143. The plate 142 b includes, for example, four pressure springs 148 c, 148 d, 148 e and 148 f provided on the holding body 143. The elastic force of each of the pressure springs 147 e, 147 f, 148 e and 148 f farther from the boundary of the two plates 142 a and 142 b is higher than that of each of the pressure springs 147 c, 147 d, 148 c and 148 d nearer to the boundary.
Accordingly, when the number of sheets stacked on the sheet stack part 141 of the movable tray 140 is small, as shown in FIG. 5, the two plates 142 a and 142 b are positioned on almost the same surface and are flat. However, when a large number of sheets are stacked on the sheet stack part 141, the size of each of the pressure springs 147 c, 147 d, 148 c and 148 d nearer to the boundary of the two plates 142 a and 142 b becomes smaller than that of each of the pressure springs 147 e, 147 f, 148 e and 148 f farther from the boundary, the two plates 142 a and 142 b are dented downward near the boundary thereof, and the section becomes V-shaped.
Accordingly, as the number of discharged sheets increases, and the stack amount becomes high, a V-shaped recess becomes deep by the mechanical operation due to the difference in the elastic force of the pressure springs. Incidentally, since the position of the sheet discharge port is fixed, in general, when the sheet stack amount increases, in order to stack the sheets, the movable tray 140 is positioned downward as indicated by an arrow 149 in FIG. 6.
Although a sheet freely drops from the sheet discharge port to the sheet stack part, when the stack amount increases, the center part is dented and the sheet can be certainly stacked.

Second Embodiment

Next, a second embodiment of a sheet stacking apparatus will be described with reference to FIG. 7 and FIG. 8. A sheet stack part 141 of a movable tray 140 includes two plates 172 a and 172 b provided right and left in a sheet discharge direction X, a box body (holding body) 173 provided under the plates and integrally with the plates, three electromagnetic solenoids 174 a, 174 b and 174 c that are provided on the box body 173 and pull the connection position of the two plates 172 a and 172 b downward by electrical control, a sheet thickness sensor 176 to measure the thickness of sheets by using a thickness lever 175 which contacts the sheets stacked on the two plates 172 a and 172 b, a sheet weight detection part 177 to detect the weight of the stacked sheets, and a solenoid drive part 178 that applies power to the three electromagnetic solenoids 174 a, 174 b and 174 c to pull the vicinity of the boundary of the two plates downward according to the weight of the sheets detected by the sheet weight detection part 177.
Also in this embodiment, as shown in FIG. 1, the movement control mechanism part includes the tray position detection part 140 s to detect the position of the tray, the tray drive belt 140 b, and the tray drive motor 140 m to drive and control the rotation of the tray drive belt 140 b.
In the sheet stacking apparatus of this embodiment, as the weight of sheets stacked on the two plates increases, the force of pulling by the electromagnetic solenoids 174 a, 174 b and 174 c is made high.
Accordingly, the boundary portion of the two plates is pulled downward according to the weight of the sheets stacked on the sheet stack part 141, the V shape is formed in the direction vertical to the sheet discharge direction, and a large number of sheets can be certainly stacked on the sheet stack part 141. Since the height of the sheet discharge port can not be changed, also in this case, in general, as the number of stacked sheets increases, the movable tray 140 is positioned below as indicated by an arrow 179.
Incidentally, in the second embodiment, the sheet detection part 177 detects the weight of the sheets stacked on the sheet stack part 141, and the degree of pulling by the electromagnetic solenoids 174 a, 174 b and 174 c is changed according to the weight. However, information of the thickness of the stacked sheets detected by the sheet thickness sensor 176 is directly inputted to the solenoid drive part 178, and as the thickness of the sheets becomes large, the pulling force (attracting force) by the electromagnetic solenoids 174 a, 174 b and 174 c is made high, and the V shape of the two plates can be formed.
Besides, the weight of the stacked sheets is directly measured without measuring the thickness of the stacked sheets, and the solenoid drive part 178 may drive and control the electromagnetic solenoids 174 a, 174 b and 174 c.
In the second embodiment, as the thickness of the sheets stacked on the sheet stack part 141 increases, the degree of pulling by the electromagnetic solenoids is made high. However, the electromagnetic solenoids are not driven until the thickness of the stacked sheets becomes a predetermined thickness, and when the thickness becomes the predetermined thickness, the electromagnetic solenoids may be driven, so that the section of the two plates becomes V-shaped.
The predetermined thickness at which the electromagnetic solenoids are driven may be divided into two or more stages.

Third Embodiment

Since the height of the sheet discharge port through which the sheet enters the sheet stacking apparatus can not be changed, in general, as the number of stacked sheets increases, the movable tray is positioned downward. Accordingly, the height position of the movable tray is detected, and according to this, the V shape formed of two plates of the sheet stack part can be changed. A description will be made on a third embodiment in which the V shape of the sheet stack part of the movable tray is changed according to the position of the movable tray as stated above. In this embodiment, the shapes of two plates are deformed by a cam mechanism.
FIG. 9 is a front view of a sheet stacking apparatus of this embodiment. A sheet stacking apparatus 190 of this embodiment includes two plates 192 a and 192 b that contact each other, provided under a sheet discharge port 191 and are bendable, a holding body 193 that is provided integrally with the two plates 192 a and 192 b and supports these plates from below, a rotation shaft 194 provided to pass through the holding body 193, a pair of eccentric cams 195 a and 195 b that are provided on the rotation shaft and contacts the two plates 192 a and 192 b from below, a position detection part 196 to detect the position of the rotation shaft 194, fixing parts 197 a and 197 b to fix both ends of the rotation shaft 194, and rotation belts 198 a and 198 b to rotate in an up and down direction.
The position detection part 196 detects the position of the rotation shaft 194, and detects the positions of the two plates 192 a and 192 b and the holding body 193. When the number of sheets stacked on the two plates 192 a and 192 b increases, the rotation belts 198 a and 198 b are rotated and driven by a not-shown motor or the like, and the holding body 193 and the like are lowered as indicated by an arrow A. The position detection part 196 detects the lowering of the position, and the rotation shaft 194 is rotated. When the rotation shaft 194 is rotated, the eccentric cams 195 a and 195 b fixed to the rotation shaft 194 are rotated, and raise portions of the plates 192 a and 192 b farther from the boundary upward as indicated by arrows 199 a and 199 b.
Accordingly, the shape formed of the two plates 192 a and 192 b becomes V-shaped in the direction vertical to the sheet discharge direction, and as the sheet stack amount increases, the depth of the V shape becomes deep.
In the invention, the degree of denting of the center of the sheet stack part may be gradually changed to form the V shape according to the amount, such as thickness or weight, of stacked sheets, that is, may be changed in analog, or may be changed digitally to increase the dent amount stepwise when the sheet amount reaches a certain amount. The invention includes both the cases.
The sheet stacking apparatus of the embodiment of the invention may be included in the finishing apparatus, or may be provided integrally with the image forming apparatus.
In the embodiment, the description is made on the case where the invention is applied to the multi-function type color copying apparatus. However, the invention can be applied to not only the multi-function type color copying apparatus, but also a sheet stacking apparatus for stacking a sheet discharged by another image forming apparatus, such as a normal copying machine, a printer or a facsimile, which includes an image forming part to generate an image to be printed on a sheet and specifies the type of a print sheet.
While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and sprit of the inventions.

Claims

1. A sheet stacking apparatus comprising:

a movable tray having a sheet stack part whose center part is dented downward as the number of discharged and stacked sheets increases; and

a movement control mechanism part to move and control the movable tray up and down.

2. The apparatus of claim 1, wherein a dent of the sheet stack part is formed to be V-shaped in a direction vertical to a direction in which the sheets are discharged.

3. The apparatus of claim 2, wherein the sheet stack part includes pressure springs, and when the sheets are stacked, the V shape is formed by elastic forces of the pressure springs and by the sheets own weight.

4. The apparatus of claim 3, wherein

the sheet stack part includes:

two plates provided right and left in the discharge direction of the discharged sheets;

a holding body provided under the two plates and to the two plates;

a first pressure spring that is provided on the holding body and supports the two plates; and

a second pressure spring that is provided on the holding body, supports the two plates, is provided at a position farther from a boundary of the right and left plates than the first pressure spring, and has an elastic force higher than that of the first pressure spring.

5. The apparatus of claim 2, wherein the sheet stack part detects an amount of the stacked sheets, and forms the V shape by electrical control according to the amount.

6. The apparatus of claim 5, wherein

the sheet stack part includes:

a sheet amount detection part to detect a thickness or a weight of the stacked sheets;

two plates provided right and left in the discharge direction of the sheets;

a holding body provided under the two plates and to the two plates; and

an electromagnetic solenoid that is provided between the holding body and the two plates, and pulls a portion close to a boundary of the two plates downward as the thickness or the weight of the sheets detected by the sheet amount detection part increases.

7. The apparatus of claim 2, wherein the sheet stack part forms the V shape by a cam mechanism.

8. The apparatus of claim 7, wherein when the amount of the stacked sheets increases, the movable tray is lowered, and the sheet stack part detects the lowering of the movable tray and forms the V shape by the cam mechanism according to a detected position.

9. The apparatus of claim 8, wherein

the sheet stack part includes:

a position detection part to detect the position of the movable tray;

two plates provided right and left in the discharge direction of the sheets;

a holding body provided under the two plates and to the two plates;

a rotation shaft that is provided to the holding body to be rotatable in a direction vertical to a line of a boundary of the two plates, and is rotated as the position of the movable tray detected by the position detection part becomes low; and

a pair of eccentric cams that are provided on the rotation shaft, and contact lower surfaces of the two plates to raise ends of the two plates farther from the boundary as the position of the movable tray becomes low.

10. A sheet stacking apparatus comprising:

a movable tray including a sheet stack part that detects an amount of discharged and stacked sheets, and forms a V shape in a direction vertical to a discharge direction of the sheets by electrical control according to the amount; and

11. The apparatus of claim 10, wherein the sheet stack part includes:

a sheet thickness detection part to detect a thickness of the stacked sheets;

two plates provided right and left in the discharge direction of the sheets;

a holding body provided under the two plates and to the two plates; and

an electromagnetic solenoid that is provided between the holding body and the two plates, and pulls a portion close to a boundary of the two plates downward as the thickness of the sheets detected by the sheet thickness detection part increases.

12. The apparatus of claim 11, further comprising a weight detection part to obtain a weight of the sheets from the thickness of the sheets detected by the sheet thickness detection part, wherein the electromagnetic solenoid pulls the portion close to the boundary of the two plates downward as the weight detected by the weight detection part increases.

13. The apparatus of claim 10, wherein

the sheet stack part includes:

a sheet weight detection part to detect a weight of the stacked sheets;

two plates provided right and left in the discharge direction of the sheets;

a holding body provided under the two plates and to the two plates; and

an electromagnetic solenoid that is provided between the holding body and the two plates, and pulls a portion close to a boundary of the two plates downward as the weight of the sheets detected by the sheet weight detection part increases.

14. A sheet finishing apparatus for finishing a sheet, comprising a sheet stacking apparatus including:

a movable tray having a sheet stack part that detects an amount of discharged and stacked sheets and forms a V shape in a direction vertical to a discharge direction of the sheets according to the amount; and a movement control mechanism part to move and control the movable tray up and down.

15. The apparatus of claim 14, wherein the sheet stack part includes pressure springs, and when the sheets are stacked, the V shape is formed by elastic forces of the pressure springs and by the sheets own weight.

16. The apparatus of claim 14, wherein the sheet stack part detects the amount of the stacked sheets, and forms the V shape by the electrical control according to the amount.

17. The apparatus of claim 14, wherein the sheet stack part forms the V shape by a cam mechanism.

18. The apparatus of claim 17, wherein when the amount of the stacked sheets increases, the movable tray is lowered, and the sheet stack part detects the lowering of the movable tray and forms the V shape by the cam mechanism according to a detected position.

19. An image forming apparatus comprising:

an image forming part to form an image to be printed on a sheet;

a sheet print part to print the image formed by the image forming part onto the sheet;

a movable tray including a sheet stack part that stacks the sheet printed by the sheet print part, and has a center part which is dented downward as the number of the sheets increases; and

20. A sheet stacking method comprising:

forming a V shape vertically to a sheet discharge direction according to an amount of sheets stacked on a sheet stack part that stacks discharged sheets and is included in a movable tray; and

moving and controlling the movable tray up and down.