JP7722020B2 - Image forming device - Google Patents

Description

The present invention relates to an image forming apparatus.

Conventional image forming devices capable of double-sided printing on sheets have used 21 transport control, 2413 transport control, 2416 transport control, 2461 transport control, and other methods of sheet transport control. For example, with 2461 transport control, printing is performed in the following order: back side of the first sheet (page 2) → back side of the second sheet (page 4) → back side of the third sheet (page 6) → front side of the first sheet (page 1) (see Patent Document 1).

JP 2020-140114 A

According to the configuration of Patent Document 1, when a sheet is supplied from a sheet cassette while a preceding sheet is passing through the re-conveyance path, the preceding sheet is stopped on the re-conveyance path, and conveyance of the stopped sheet is resumed once the sheet has been completely supplied from the sheet cassette. This maintains a constant inter-sheet distance between the trailing edge of the sheet being supplied from the sheet cassette and the leading edge of the sheet passing through the re-conveyance path.

However, when the re-conveyance rollers are stopped to stop the sheet on the re-conveyance path, the sheet does not stop immediately but instead slips between itself and the re-conveyance rollers and stops. At this time, there is a risk that slip marks from the re-conveyance rollers may be left on the sheet.

The present invention aims to provide an image forming apparatus that can maintain a constant inter-sheet distance without stopping sheets on the re-conveyance path.

The image forming apparatus of the present invention comprises an apparatus main body having an image forming unit that forms an image on a sheet, a sheet tray that supports sheets to be supplied to the image forming unit, and a control unit. The apparatus main body has a feed path for feeding sheets from the sheet tray to the image forming unit, a re-conveyance path for re-conveying sheets with an image formed on one side by the image forming unit to the feed path, and a re-conveyance roller disposed on the re-conveyance path. The control unit is capable of performing normal control, in which the conveyance speed of the re-conveyance roller is a first speed, and deceleration control, in which the conveyance speed of the re-conveyance roller is a second speed slower than the first speed, during double-sided printing, in which the image forming unit forms images on the front and back sides of a preceding sheet and the front and back sides of a succeeding sheet fed from the sheet tray after the preceding sheet. During normal control, when the preceding sheet is positioned on the re-conveyance path and the succeeding sheet is fed to the feed path, the control unit performs the deceleration control so that the sheet-to-sheet distance between the leading edge of the preceding sheet conveyed by the re-conveyance roller and the trailing edge of the succeeding sheet becomes a predetermined distance.

With the above configuration, by slowing down the conveying speed of the re-conveying roller, the distance between sheets can be kept constant without stopping the sheets on the re-conveying path. As a result, slip marks from the re-conveying roller can be prevented from being left on the sheets.

In the above-described image forming apparatus, the apparatus main body may have a first sensor that detects a sheet upstream of the re-conveyance roller in the sheet conveyance direction on the re-conveyance path, and the control unit may perform the deceleration control a predetermined time after the first sensor detects the preceding sheet.

With the above configuration, by placing the first sensor immediately before the re-conveyance roller, the position of the sheet on the re-conveyance path can be detected with high accuracy.

In the above-described image forming device, the device main body may have a second sensor that detects a sheet being fed from the sheet tray to the feeding path, and the control unit may delay the timing of ending the deceleration control the longer the time from the start of the deceleration control until the second sensor detects the trailing edge of the subsequent sheet.

With the above configuration, by placing a second sensor in the feeding path, the timing for ending deceleration control can be controlled with high precision.

In the image forming apparatus described above, if the timing to end the deceleration control is Tm, the theoretical timing to end the deceleration control is Tt, the timing to start the deceleration control is T1, the timing when the second sensor detects the trailing edge of the subsequent sheet is T2, the theoretical time from the start of the deceleration control until the second sensor detects the trailing edge of the subsequent sheet is Tb, the first speed is V1, and the second speed is V2, then:
It can be calculated as Tm=Tt+{(T2-T1)-Tb}×V2/(V1-V2).

With the above configuration, the timing Tm for ending deceleration control can be calculated using a calculation formula.

In the above-described image forming apparatus, the device main body may have a drive motor and a drive force transmission unit that transmits the drive force of the drive motor to the re-conveying roller, and the drive force transmission unit may have a first transmission mechanism that drives the re-conveying roller at the first speed and a second transmission mechanism that drives the re-conveying roller at the second speed.

With the above configuration, the first transmission mechanism is used during normal control and the second transmission mechanism is used during deceleration control, eliminating the need to change the rotational speed of the drive motor.

In the above-mentioned image forming apparatus, the drive force transmission unit may have an electromagnetic clutch that switches between the first transmission mechanism and the second transmission mechanism.

With the above configuration, the drive force transmission unit can be made smaller by using an electromagnetic clutch.

In the above-mentioned image forming apparatus, the device main body may have a conveying roller that is arranged upstream of the re-conveying roller in the sheet conveying direction on the re-conveying path and driven at the same speed as the re-conveying roller.

With the above configuration, even short sheets can be transported.

In the above image forming device, the control unit may form images in the image forming unit in the double-sided printing process on the back side of the first sheet, the back side of the second sheet, the back side of the third sheet, the front side of the first sheet, and the back side of the fourth sheet in this order, and the preceding sheet may be the second sheet and the succeeding sheet may be the fourth sheet.

The above configuration can be applied to 2461 transport control.

In the above-described image forming apparatus, when the first sheet is positioned on the re-conveyance path and the third sheet is supplied to the feeding path during the normal control, the control unit may execute the deceleration control so that the inter-sheet distance between the leading edge of the first sheet and the trailing edge of the third sheet conveyed by the re-conveyance roller becomes the predetermined distance.

With the above configuration, the deceleration control can also be used to maintain a constant distance between the first and third seats.

According to the present invention, when performing double-sided printing in an image forming device, the distance between sheets can be maintained constant without stopping the sheets on the re-conveyance path.

1 is a central cross-sectional view showing an image forming apparatus to which a discharge unit according to an embodiment is attached; 1 is a central cross-sectional view showing an image forming apparatus with a cover attached thereto according to an embodiment; FIG. 2 is a block diagram illustrating a conveying unit of the image forming apparatus according to the embodiment. 10 is a flowchart of transport control. 10 is a central cross-sectional view showing the image forming apparatus in a state where the leading edge of the first sheet is at a position before reaching the flapper and the second sheet is being fed; FIG. FIG. 6 is a diagram showing a conveying state following FIG. 5 . FIG. 7 is a diagram showing a transport state subsequent to FIG. 6; FIG. 8 is a diagram showing a conveying state following FIG. 7; FIG. 9 is a diagram showing a conveying state following FIG. 8 . FIG. 10 is a diagram showing a conveying state following FIG. 9 . FIG. 11 is a diagram showing a conveying state following FIG. 10 . FIG. 12 is a diagram showing a conveying state following FIG. 11 . FIG. 13 is a diagram showing a transport state following FIG. 12 . FIG. 14 is a diagram showing a transport state following FIG. 13. FIG. 15 is a diagram showing a transport state following FIG. 14 .

In the following description, the left and right sides of Figure 1 are defined as the front and rear sides of the image forming apparatus 1, respectively, and the front and rear sides of the page are defined as the right and left sides of the image forming apparatus 1, respectively. Furthermore, the top and bottom sides of Figure 1 are defined as the top and bottom sides of the image forming apparatus 1, respectively.

[Overall Configuration of Image Forming Apparatus]
1 is a central cross-sectional view of an image forming apparatus equipped with a discharge unit according to an embodiment. The image forming apparatus 1 is a color laser printer that uses electrophotography to form an image on a sheet S, such as paper or an OHP sheet, by superimposing developer images of multiple colors. However, the image forming apparatus 1 can also be configured as a monochrome laser printer that forms an image on a sheet using a single-color developer image. The image forming apparatus 1 can also be configured as an inkjet printer.

The image forming apparatus 1 comprises a device main body 2, a feeding unit 3 that supplies sheets S, an image forming unit 5 that forms images on the conveyed sheets S, and a transport unit 7 that transports the sheets S conveyed from the image forming unit 5. The device main body 2 is a box formed in a substantially rectangular parallelepiped shape, and houses the feeding unit 3, image forming unit 5, and transport unit 7. An output tray 23a that is recessed and slopes downward from the front to the rear is formed on the top surface 23 of the device main body 2. The image forming unit 5 is included in the device main body 2.

The feeding unit 3 includes a sheet cassette 31, a first feeding roller 32, a pair of first conveying rollers 34, and a pair of registration rollers 35. A feeding path P0 is configured within the device main body 2 for feeding sheets S from the sheet cassette 31 to the image forming unit 5.

The sheet cassette 31 supports multiple sheets S in a stacked state. The sheet cassette 31 is an example of a sheet tray that supports sheets to be fed to the image forming unit 5. The sheets S supported by the sheet cassette 31 are fed one by one to the feeding path P0 by the first feeding roller 32. The sheets S fed to the feeding path P0 are transported toward the image forming unit 5 by the first conveying roller 34 and the registration roller 35.

The device main body 2 also has an opening 2A that opens to the front, and a front cover 29 that can open and close the opening 2A. The opening 2A is open when the front cover 29 is open, and is closed when the front cover 29 is closed.

The feeding unit 3 is equipped with a second feeding roller 38. When the front cover 29 is open, the sheet S placed on the front cover 29 is sent to the feeding path P0 by the second feeding roller 38. The sheet S sent to the feeding path P0 is transported toward the image forming unit 5 by the registration rollers 35. When open, the front cover 29 is an example of a sheet tray that supports the sheet to be fed to the image forming unit 5.

The image forming unit 5 has four drum units 51 arranged side by side in the front-to-rear direction. Each drum unit 51 corresponds to one of the colors: black, yellow, magenta, and cyan. Each drum unit 51 has a photosensitive drum 51a and a developing roller 51b.

The image forming unit 5 also includes a scanner unit 52 and a fixing unit 6. The scanner unit 52 is located at the top of the device main body 2, and laser light based on image data passes through a polygon mirror, lens, reflecting mirror, etc. and is scanned at high speed onto the surface of the photosensitive drum 51a corresponding to each color. The fixing unit 6 is located further downstream than the photosensitive drum 51a located furthest downstream in the transport direction of the sheet S.

A transfer belt 40 is disposed below the drum unit 51 in the image forming unit 5. The transfer belt 40 is stretched between a drive roller 41a and a driven roller 41b disposed behind the drive roller 41a. A transfer roller 42 is disposed opposite each photosensitive drum 51a across the transfer belt 40.

In the image forming unit 5, the photosensitive drums 51a, which are uniformly charged by a charger (not shown), are selectively exposed to light by the scanner unit 52. This exposure selectively removes charge from the surface of the photosensitive drum 51a, forming an electrostatic latent image on the surface of the photosensitive drum 51a.

A development bias is applied to the developing roller 51b, and when the electrostatic latent image formed on the photosensitive drum 51a faces the developing roller 51b, the potential difference between the electrostatic latent image and the developing roller 51b causes toner to be supplied from the developing roller 51b to the electrostatic latent image. This forms a toner image on the surface of the photosensitive drum 51a.

When the sheet S is transported toward the image forming unit 5 and is conveyed onto the transfer belt 40, it is transported by the transfer belt 40 and passes sequentially between the transfer belt 40 and each photosensitive drum 51a. Then, when the toner image on the surface of the photosensitive drum 51a faces the sheet S, it is transferred onto the sheet S by the transfer bias applied to the transfer roller 42.

The sheet S onto which the toner image has been transferred is transported to the fixing unit 6. The fixing unit 6 is equipped with a heating roller 61 that heats the sheet S, and a pressure roller 62 that is positioned opposite the heating roller 61. The sheet S transported to the fixing unit 6 passes between the heating roller 61 and the pressure roller 62, which are pressed against each other, thereby thermally fixing the toner image.

The sheet S with the thermally fixed toner image is transported downstream of the image forming unit 5 by the transport unit 7. The transport unit 7 has a transport path P1, a discharge path P2, and a re-transport path P3. The transport path P1 is the path along which the sheet S is transported from the image forming unit 5. The discharge path P2 branches off from the transport path P1 and is the path along which the sheet S is transported to the discharge tray 23a. The re-transport path P3 has a first path P31 that branches off from the transport path P1 and along which the sheet S is temporarily transported, and a second path P32 that branches off from the discharge path P2 and the first path P31 and along which the sheet S is transported to the image forming unit 5.

A pair of post-fixing rollers 71 that transport the sheet are provided downstream of the fixing unit 6 on the transport path P1 in the sheet transport direction. A pair of discharge rollers 73 that transport the sheet S are provided at the downstream end of the discharge path P2 in the sheet transport direction, and a pair of intermediate discharge rollers 72 that transport the sheet S are provided upstream of the discharge rollers 73 on the discharge path P2 in the sheet transport direction.

In the re-conveyance path P3, a pair of reversing rollers 74 that transport the sheet S is provided on the first path P31, and a pair of intermediate reversing rollers 75 that transport the sheet S is provided at the end of the second path P32 on the first path P31 side. A pair of first re-conveying rollers 76a and a pair of second re-conveying rollers 76b that transport the sheet S are provided downstream of the intermediate re-conveying rollers 75 on the second path P32 in the sheet transport direction. The first re-conveying roller 76a, an example of a transport roller, is positioned upstream of the second re-conveying roller 76b, an example of a re-conveying roller, in the sheet transport direction, and is driven at the same speed as the second re-conveying roller 76b. By providing the first re-conveying roller 76a, even short sheets can be transported.

A flapper 79 is provided at the branch point Ps between the conveying path P1 and the discharge path P2 and first path P31. The flapper 79 is configured to be switchable between a first position where it guides the sheet S conveyed along the conveying path P1 to the discharge path P2, and a second position where it guides the sheet S conveyed along the conveying path P1 to the first path P31. A solenoid 78 is provided in the device main body 2. The solenoid 78 is configured to be able to switch the flapper 79 between the first position and the second position.

The device main body 2 has a first motor 81, a second motor 82, and a third motor 83, which are examples of drive motors. The first motor 81 drives the first feed roller 32, the post-fusing roller 71, the first re-conveying roller 76a, and the second re-conveying roller 76b.

A first electromagnetic clutch 84 is interposed between the first motor 81 and the first feed roller 32, and the first electromagnetic clutch 84 allows for switching between transmitting and cutting off the driving force from the first motor 81 to the first feed roller 32. A second electromagnetic clutch 85 is interposed between the first motor 81 and the first re-conveyor roller 76a and the second re-conveyor roller 76b, and the second electromagnetic clutch 85 allows for switching between the transmission mechanism that transmits the driving force of the first motor 81 to the first re-conveyor roller 76a and the second re-conveyor roller 76b.

The second motor 82 drives the discharge rollers 73 and intermediate discharge rollers 72. The second motor 82 switches the rotation direction of the discharge rollers 73 and intermediate discharge rollers 72 between forward and reverse rotation. When the discharge rollers 73 and intermediate discharge rollers 72 rotate in the forward direction, they transport the sheet S toward the discharge tray 23a, and when they rotate in the reverse direction, they transport the sheet S toward the second path P32. This allows the transport direction in the discharge path P2 to be reversed. When the sheet S is reversed in the discharge path P2, the leading edge of the sheet S temporarily protrudes above the discharge tray 23a.

The third motor 83 drives the reversing roller 74 and the intermediate reversing roller 75. The reversing roller 74 and the intermediate reversing roller 75 are configured so that their rotation direction can be switched between forward and reverse by the third motor 83. The reversing roller 74 is configured to transport the sheet S away from the transport path P1 when rotating in the forward direction, and to transport the sheet S toward the second path P32 when rotating in the reverse direction.

This allows the conveying direction on the first path P31 to be reversed. The device main body 2 has an upper surface 23 with an upper surface opening 23b through which the sheet S conveyed on the first path P31 can pass. When the sheet S is reversed on the first path P31, the leading edge of the sheet S temporarily protrudes above the upper surface 23 through the upper surface opening 23b. The intermediate reversing roller 75 is configured to convey the sheet S on the second path P32 toward the image forming unit 5 when rotating in the reverse direction.

A first sensor 91 that detects the sheet S is provided between the first re-conveyance roller 76a and the second re-conveyance roller 76b on the second path P32 of the re-conveyance path P3. In other words, the first sensor 91 is located upstream of the second re-conveyance roller 76b in the sheet conveyance direction. By placing the first sensor 91 immediately before the second re-conveyance roller 76b, the position of the sheet S on the second path P32 can be detected with high accuracy.

A second sensor 92 is provided at the upstream end of feeding path P0 in the sheet transport direction to detect sheet S being fed from sheet cassette 31 to feeding path P0. A third sensor 93 is provided on feeding path P0 between registration roller 35 and image forming unit 5 to detect sheet S. A fourth sensor 94 is provided on second path P32 between intermediate reversal roller 75 and first re-conveyance roller 76a to detect sheet S. A fifth sensor 95 is provided on transport path P1 between fixing unit 6 and post-fixing roller 71 to detect sheet S. A sixth sensor 96 is provided on sheet cassette 31 to detect the presence or absence of sheet S supported by sheet cassette 31.

A discharge unit 100 having multiple discharge trays (not shown) is removably attached to the top surface 23 of the apparatus main body 2. The discharge unit 100 has a discharge unit path P4 along which sheets S are transported. The image forming apparatus 1 is configured so that sheets S can be transported to the discharge unit path P4 via the discharge path P2. The discharge unit 100 is an optional device that is detachable from the apparatus main body 2.

Furthermore, the sheet S being transported along the first path P31 in a direction away from the transport path P1 can protrude above the upper surface 23 through the upper surface opening 23b in the upper surface 23 of the device body 2. Once the sheet S has been transported until it protrudes above the upper surface 23, the transport direction is reversed when the rotation of the reversing roller 74 is switched from the forward rotation direction to the reverse rotation direction, and the sheet S is transported toward the second path P32.

On the top surface 23 of the device main body 2, the discharge unit 100 is disposed between the discharge tray 23a and the top opening 23b. The discharge tray 23a is disposed in front of the discharge unit 100, and the top opening 23b is disposed behind the discharge unit 100. Therefore, a sheet S transported along the first path P31 protrudes upward from the top surface 23 of the device main body 2 through the top opening 23b, behind the discharge tray 23a. In other words, the first path P31 is a path disposed on the opposite side of the discharge unit 100 from the discharge tray 23a.

In this way, sheets S that protrude above the upper surface 23 from the first path P31 will protrude upward behind the discharge unit 100. Therefore, sheets S that temporarily protrude from the first path P31 when the sheets S are reversed on the first path P31 are hidden by the discharge unit 100 and are difficult for users on the discharge tray 23a side, which is in front of the discharge unit 100, to see. This makes it possible to prevent users from removing the protruding sheets S.

The length of the first path P31 from the branch point Ps to the downstream end P31d is longer than the length of the discharge path P2 from the branch point Ps to the downstream end P2d. The downstream end P31d is located at the downstream end in the sheet conveyance direction when the reversing roller 74 on the first path P31 is rotating in the forward direction, and the downstream end P2d is located at the downstream end in the sheet conveyance direction when the discharge roller 73 on the discharge path P2 is rotating in the forward direction.

Therefore, the length of the sheet S protruding from the first path P31 when the sheet S is inverted on the first path P31 is shorter than the length of the sheet S protruding from the discharge path P2 when the sheet S is inverted on the discharge path P2, making it possible to prevent the sheet S protruding from the first path P31 from coming into contact with the sheet S discharged to the discharge tray 23a.

As shown in FIG. 2, when the discharge unit 100 is not attached to the image forming apparatus 1, a cover 101 that covers the top opening 23b can be attached to the top surface 23 of the apparatus main body 2. When the cover 101 is attached to the top surface 23 in this manner, the first path P31 is formed continuously inside the cover 101, and the downstream end P31d of the first path P31 is located above the front end of the cover 101 and the downstream end P2d of the discharge path P2.

In other words, the first path P31 is a path that guides the sheet S conveyed from the conveying path P1 above the discharge tray 23a, and the sheet S that protrudes from the downstream end P31d of the first path P31 to the outside of the device main body 2 is positioned higher than the sheet S that is discharged onto the discharge tray 23a from the discharge path P2.

Therefore, when the cover 101 is attached to the top surface 23 of the device main body 2, the sheet S protruding from the downstream end P31d of the first path P31 to the outside of the device main body 2 will be positioned higher than the sheet S discharged from the downstream end P2d of the discharge path P2 to the discharge tray 23a, making it possible to prevent the sheet S protruding from the first path P31 from coming into contact with the sheet S already discharged to the discharge tray 23a.

Furthermore, even when the cover 101 is attached to the top surface 23 of the device body 2, the length of the first path P31 from the branch point Ps to the downstream end P31d is longer than the length of the discharge path P2 from the branch point Ps to the downstream end P2d. This makes it possible to further prevent sheets S protruding from the first path P31 from coming into contact with sheets S discharged onto the discharge tray 23a.

As shown in FIG. 3, the image forming apparatus 1 has a control unit 9 consisting of a CPU (Central Processing Unit). Connected to the control unit 9 are a solenoid 78, a first motor 81, a second motor 82, a third motor 83, a first electromagnetic clutch 84, and a second electromagnetic clutch 85. Also connected to the control unit 9 are a first sensor 91, a second sensor 92, a third sensor 93, a fourth sensor 94, a fifth sensor 95, and a sixth sensor 96.

The control unit 9 controls the solenoid 78 to switch the flapper 79 between the first and second positions. By switching the flapper 79 between the first and second positions, the control unit 9 can set the flapper 79 to the first position when transporting the sheet S to the discharge path P2, and the flapper 79 to the second position when transporting the sheet S to the first path P31. In this way, by the control unit 9 switching the flapper 79 between the first and second positions, it is possible to switch the destination of the sheet S between the discharge path P2 and the first path P31 at the appropriate time.

The control unit 9 controls the first motor 81 to switch between rotating and stopping the post-fixing roller 71. The control unit 9 controls the first motor 81 and the first electromagnetic clutch 84 to switch between rotating and stopping the first feed roller 32. The control unit 9 controls the first motor 81 and the second electromagnetic clutch 85 to switch the conveying speed of the first re-conveying roller 76a and the second re-conveying roller 76b.

In this embodiment, the configuration for switching the conveying speed of the first re-conveying roller 76a and the second re-conveying roller 76b includes a driving force transmission unit 86 that transmits the driving force of the first motor 81 to the first re-conveying roller 76a and the second re-conveying roller 76b. The driving force transmission unit 86 includes a second electromagnetic clutch 85, and has a first transmission mechanism 86a that connects to the first motor 81 when the second electromagnetic clutch 85 is in the ON state, and a second transmission mechanism 86b that connects to the first motor 81 when the second electromagnetic clutch 85 is in the OFF state.

The first transmission mechanism 86a is a gear train that transmits the driving force of the first motor 81, which is driven at a predetermined rotational speed, so that the conveying speed of the first re-conveying roller 76a and the second re-conveying roller 76b becomes the normal first speed V1. The second transmission mechanism 86b is a gear train that transmits the driving force of the first motor 81, which is driven at a predetermined rotational speed, so that the conveying speed of the first re-conveying roller 76a and the second re-conveying roller 76b becomes the second speed V2, which is slower than the first speed V1.

The control unit 9 can perform normal control, which turns the second electromagnetic clutch 85 on, and deceleration control, which turns the second electromagnetic clutch 85 off. In this way, by providing the drive force transmission unit 86, the conveying speed of the first re-conveying roller 76a and the second re-conveying roller 76b can be switched without changing the rotational speed of the first motor 81. Furthermore, by using the second electromagnetic clutch 85, the drive force transmission unit 86 can be made smaller.

Note that the configuration for switching the conveying speed of the first re-conveying roller 76a and the second re-conveying roller 76b is not limited to the drive force transmission unit 86; for example, a separate drive motor may be provided to drive the first re-conveying roller 76a and the second re-conveying roller 76b at the second speed V2.

By controlling the second motor 82, the control unit 9 can change the rotation direction of the discharge roller 73 and intermediate discharge roller 72, and switch between rotating and stopping. By controlling the third motor 83, the control unit 9 can change the rotation direction of the reversing roller 74 and intermediate reversing roller 75, and switch between rotating and stopping.

In the image forming device 1 configured in this manner, when performing double-sided printing, in which images are formed on both sides of a sheet S, the control unit 9 executes transport control to print on the sheet S in a printing order that shortens the gap between conveyed sheets S, thereby improving printing speed. For example, the control unit 9 is configured to execute 2461 transport control, in which printing is performed in the following order: back side of the first sheet (page 2) → back side of the second sheet (page 4) → back side of the third sheet (page 6) → front side of the first sheet (page 1) → back side of the fourth sheet (page 8).

When the control unit 9 performs 2461 conveyance control, in order to print on multiple sheets S as quickly as possible, high-speed conveyance control is performed to invert the sheet S using discharge path P2 and re-convey it to the image forming unit 5 when the image forming unit 5 has finished printing on the back side of the first sheet (page 2) and the back side of the second sheet (page 4), and to invert the sheet S using first path P31 and re-convey it to the image forming unit 5 when the image forming unit 5 has finished printing on the back side of the third sheet (page 6) and the back side of the fourth sheet (page 8).

In addition, if the feeding of the second sheet, on which the back side (page 4) is to be printed, is delayed due to a feeding error or the like, and the first path P31 is empty when the second sheet, on which the back side (page 4) has been printed, reaches in front of the flapper 79, the system is configured to execute conveyance switching control to convey the second sheet to the first path P31, invert it, and re-convey it to the image forming unit 5.

Furthermore, during normal control, when the preceding second sheet is positioned on the second path P32 and the following fourth sheet is supplied to the feeding path P0, deceleration control is performed so that the inter-sheet distance between the leading edge of the second sheet conveyed by the second re-conveyance roller 76b and the trailing edge of the fourth sheet becomes a predetermined distance (design value).

[Normal control and deceleration control]
Next, the conveyance control of the sheet S during double-sided printing executed by the control unit 9, particularly the normal control and deceleration control when 2461 conveyance control is performed, will be described. Fig. 4 shows a flowchart of the conveyance control.

In the following description, the first sheet with page 2 printed on the back side will be referred to as first sheet S1-2, and the first sheet with page 1 printed on the front side will be referred to as first sheet S1-1. Furthermore, the second sheet with page 4 printed on the back side will be referred to as second sheet S2-2, and the second sheet with page 3 printed on the front side will be referred to as second sheet S2-1. Furthermore, the third sheet with page 6 printed on the back side will be referred to as third sheet S3-2, and the third sheet with page 5 printed on the front side will be referred to as third sheet S3-1. Furthermore, the fourth sheet with page 8 printed on the back side will be referred to as fourth sheet S4-2.

Note that each sheet that is printed on its back side is switched to a sheet that is printed on its front side when the conveying direction is reversed. The second sheet S2-1 is an example of a preceding sheet, and the fourth sheet S4-2 is an example of a succeeding sheet.

Conveyance control is initiated when a print command is input to the control unit 9. When conveyance control is initiated, the control unit 9 feeds the first sheet S1-2 supported by the sheet cassette 31 (S10). As shown in Figure 5, the first sheet S1-2 is conveyed to the conveyance path P1 after an image is formed on it by the image forming unit 5.

When the first sheet S1-2 is in the position shown in Figure 5, the control unit 9 feeds the second sheet S2-2 supported by the sheet cassette 31 (S11). In this case, the control unit 9 feeds the second sheet S2-2 when the sheet-to-sheet distance between the first sheet S1-1 and the second sheet S2-2 is wide enough that the leading edge of the second sheet S2-2 is nipped by the heating roller 61 and the pressure roller 62 when the trailing edge of the first sheet S1-1 passes the flapper 79.

As shown in FIG. 6, the first sheet S1-2 is transported to the discharge path P2 via the branching portion Ps. When the first sheet S1-2 transported along the discharge path P2 reaches the reversal position, the control unit 9 controls the drive of the second motor 82 to switch the rotation of the discharge rollers 73 and intermediate discharge rollers 72 from the forward direction to the reverse direction, thereby reversing the transport direction of the first sheet S1-2 (step S12). In this case, for example, the control unit 9 pre-sets the time from when the fifth sensor 95 turns off until the first sheet S1-2 reaches the reversal position, and reverses the transport direction of the first sheet S1-2 when that time has elapsed since the fifth sensor 95 turned off.

As shown in FIG. 7, the first sheet S1-1, whose transport direction has been reversed, is transported toward the second path P32, and when the leading edge of the first sheet S1-1 reaches the position of the intermediate reversal rollers 75, the first sheet S1-1 is transported along the second path P32 by the intermediate reversal rollers 75. As shown in FIG. 8, after the trailing edge of the first sheet S1-1 passes the flapper 79, the second sheet S2-2 is transported to the discharge path P2 via the branching portion Ps.

When the second sheet S2-2 is in the position shown in Figure 8, the control unit 9 feeds the third sheet S3-2 supported by the sheet cassette 31 (S13). In this case, the control unit 9 feeds the third sheet S3-2 at a timing when the inter-sheet distance between the second sheet S2-1 and the third sheet S3-2 is such that the leading edge of the third sheet S3-2 is nipped by the heating roller 61 and the pressure roller 62 when the trailing edge of the second sheet S2-1 passes the flapper 79.

As shown in FIG. 9, when the second sheet S2-2 being conveyed along the discharge path P2 reaches the reversal position, the control unit 9 reverses the conveying direction of the second sheet S2-2 (step S14). As shown in FIG. 10, the second sheet S2-1, whose conveying direction has been reversed, is conveyed toward the second path P32, and when the leading edge of the second sheet S2-1 reaches the position of the intermediate reversal rollers 75, the second sheet S2-1 is conveyed along the second path P32 by the intermediate reversal rollers 75.

When the first sheet S1-1 is in the position shown in Figure 10, the control unit 9 determines whether the feeding of the third sheet S3-2 is delayed (step S15). If the inter-sheet distance between the leading edge of the first sheet S1-1 and the trailing edge of the third sheet S3-2 is less than a predetermined distance, the control unit 9 determines that the feeding of the third sheet S3-2 is delayed (step S15: Y), terminates normal control, and executes deceleration control (step S16). The deceleration control decelerates the first sheet S1-1 and the second sheet S2-1. The deceleration of the second sheet S2-1 is achieved by decelerating the second motor 82 and the third motor 83.

The control unit 9 switches from deceleration control to normal control when the inter-sheet distance between the leading edge of the first sheet S1-1 and the trailing edge of the third sheet S3-2 reaches a predetermined distance (step S17), and proceeds to step S18. This timing is calculated using a method similar to steps S22 and S24, described below, and therefore will not be described here. In this way, by performing deceleration control when the feeding of the third sheet S3-2 is delayed, the inter-sheet distance between the first sheet S1-1 and the third sheet S3-2 can be maintained constant without stopping the first sheet S1-1 on the second path P32.

On the other hand, if the inter-sheet distance between the leading edge of the first sheet S1-1 and the trailing edge of the third sheet S3-2 is the predetermined distance in step S15, the control unit 9 determines that the feeding of the third sheet S3-2 is not delayed (step S15: N) and proceeds to step S18.

As shown in FIG. 11, when the leading edge of the second sheet S2-1 passes the first sensor 91, the control unit 9 determines whether a predetermined time has elapsed since the first sensor 91 detected the second sheet S2-1 (step S18). In other words, the control unit 9 determines whether a predetermined time has elapsed since the first sensor 91 was turned on by the second sheet S2-1. The predetermined time is the time it takes for the second sheet S2-1 to reach a position where it is nipped by the second re-conveyance roller 76b, and is set in advance.

When a predetermined time has elapsed since the first sensor 91 detected the second sheet S2-1 (step S18: Y), the control unit 9 terminates normal control and executes deceleration control (step S19). The deceleration control decelerates the second sheet S2-1. As shown in FIG. 12, while the second sheet S2-1 is decelerating, the control unit 9 feeds the fourth sheet S4-2, which is supported by the sheet cassette 31, between the first sheet S1-1 and the second sheet S2-1 (S20).

As shown in FIG. 12, the third sheet S3-2 is transported to the first path P31 via the branching section Ps. As shown in FIG. 13, when the third sheet S3-2 transported along the first path P31 reaches the reversal position, the control unit 9 controls the drive of the third motor 83 to switch the rotation of the reversal roller 74 from the forward direction to the reverse direction, thereby reversing the transport direction of the third sheet S3-2 (step S21). In this case, for example, the control unit 9 presets the time from when the fifth sensor 95 turns off until the third sheet S3-2 reaches the reversal position, and reverses the transport direction of the third sheet S3-2 when that time has elapsed since the fifth sensor 95 turned off.

As shown in FIG. 14, the third sheet S3-1, whose transport direction has been reversed, is transported toward the second path P32, and when the leading edge of the third sheet S3-1 reaches the position of the intermediate reversal rollers 75, the third sheet S3-1 is transported along the second path P32 by the intermediate reversal rollers 75. As shown in FIG. 14, while the third sheet S3-1 passes through the flapper 79, the first sheet S1-1 is transported to the discharge path P2 via the branching portion Ps.

Proceeding from step S21 to step S22, the control unit 9 determines whether the second sensor 92 has detected the trailing edge of the fourth sheet S4-2. As shown in FIG. 14, when the trailing edge of the fourth sheet S4-2 passes the second sensor 92, the second sensor 92 changes from on to off, and the control unit 9 determines that the second sensor 92 has detected the trailing edge of the fourth sheet S4-2 (step S22: Y). This means that the trailing edge of the fourth sheet S4-2 has been fed into the feeding path P0.

When the trailing edge of the fourth sheet S4-2 is fed to the feeding path P0, the control unit 9 calculates the timing Tm at which to end the deceleration control (step S23). Specifically, the calculation is performed using the following formula.
Tm=Tt+{(T2-T1)-Tb}×V2/(V1-V2)
Tt: Theoretical (design) timing to end deceleration control T1: Timing to start deceleration control T2: Timing to detect the trailing edge of the fourth sheet S4-2 by the second sensor 92 Tb: Theoretical (design) time from the start of deceleration control to the detection of the trailing edge of the fourth sheet S4-2 by the second sensor 92 V1: First speed V2: Second speed

As expressed by the above formula, the longer the time (T2 - T1) from the start of deceleration control until the second sensor 92 detects the trailing edge of the fourth sheet S4-2, the later the timing Tm at which deceleration control ends. Through steps S18, S19, and S22 to S25, when the second sheet S2-1 is positioned on the second path P32 and the fourth sheet S4-2 is supplied to the feeding path P0 during normal control, deceleration control is performed so that the inter-sheet distance between the leading edge of the second sheet S2-1 conveyed by the second re-conveyance roller 76b and the trailing edge of the fourth sheet S4-2 becomes a predetermined distance.

Therefore, by slowing down the conveying speed of the first re-conveyance roller 76a and the second re-conveyance roller 76b, the inter-sheet distance can be maintained constant without stopping the second sheet S2-1 on the second path P32. As a result, slip marks from the first re-conveyance roller 76a and the second re-conveyance roller 76b on the second sheet S2-1 can be prevented. Furthermore, by locating the second sensor 92 on the feeding path P0, the position of the trailing edge of the fourth sheet S4-2 can be detected with high accuracy, and therefore the timing Tm for ending the deceleration control can be calculated with high accuracy.

Proceeding from step S23 to step S24, the control unit 9 determines whether it is time Tm to end deceleration control. If it is time Tm to end deceleration control (step S24: Y), the control unit 9 returns from deceleration control to normal control (step S25). After returning to normal control, as shown in Figure 15, the second sheet S2-1 and the fourth sheet S4-2 are conveyed while maintaining a predetermined sheet-to-sheet distance.

In the above embodiment, there is no particular limitation on the path for reversing the sheet S; for example, the discharge path P2 and the first path P31 may be used alternately.

In the above embodiment, transport control when performing double-sided printing on sheets S supported in the sheet cassette 31 was described, but the transport control of sheets S according to this embodiment can also be applied to transport control when performing double-sided printing on sheets S supported in other sheet trays, such as the front cover 29.

In the above embodiment, an example was described in which deceleration control was applied to 2461 transport control, but there are no particular limitations on the form of transport control when performing double-sided printing where there are preceding and succeeding sheets, and it can also be applied to 2416 transport control, for example.

1 Image forming apparatus 2 Apparatus main body 5 Image forming unit 31 Sheet cassette (sheet tray)
76a First re-transport roller (transport roller)
76b Second re-conveying roller (re-conveying roller)
81 First motor (drive motor)
85 Second electromagnetic clutch (electromagnetic clutch)
86: Driving force transmission section 86a: First transmission mechanism 86b: Second transmission mechanism 91: First sensor 92: Second sensor P0: Feeding path P3: Re-conveying path S: Sheet

Claims (8)

an apparatus main body having an image forming unit that forms an image on a sheet;
a sheet tray for supporting sheets to be supplied to the image forming unit;
a control unit,
the apparatus body includes a feeding path for feeding a sheet from the sheet tray to the image forming unit, a re-conveying path for re-conveying a sheet having an image formed on one side thereof by the image forming unit to the feeding path, a re-conveying roller disposed on the re-conveying path, and a second sensor for detecting the sheet being fed from the sheet tray to the feeding path ;
The control unit, during double-sided printing in which the image forming unit forms images on the front and back surfaces of a preceding sheet and the following sheet fed from the sheet tray after the preceding sheet,
normal control in which the conveying speed of the re-conveying roller is set to a first speed;
a deceleration control for setting the conveying speed of the re-conveying roller to a second speed that is slower than the first speed,
When the preceding sheet is positioned on the re-conveyance path during the normal control and the succeeding sheet is supplied to the feeding path, the deceleration control is executed so that an inter-sheet distance between a leading end of the preceding sheet conveyed by the re-conveyance roller and a trailing end of the succeeding sheet becomes a predetermined distance;
The image forming apparatus is characterized in that the timing of ending the deceleration control is delayed as the time from the start of the deceleration control until the second sensor detects the trailing edge of the subsequent sheet becomes longer. the apparatus main body has a first sensor that detects a sheet, the first sensor being located upstream of the re-conveyance roller in the re-conveyance path in the sheet conveyance direction;
The image forming apparatus according to claim 1 , wherein the control unit executes the deceleration control after a predetermined time has elapsed since the first sensor detected the preceding sheet. The timing to end the deceleration control is Tm.
The theoretical timing for ending the deceleration control is Tt.
The timing at which the deceleration control is started is T1.
The timing when the second sensor detects the trailing edge of the succeeding sheet is T2.
Tb is a theoretical time from the start of the deceleration control until the second sensor detects the trailing edge of the succeeding sheet,
The first speed is V1,
If the second speed is V2,
3. The image forming apparatus according to claim 1, wherein Tm=Tt+{(T2-T1)-Tb}*V2/(V1-V2). The device body includes:
A drive motor;
a driving force transmission unit that transmits the driving force of the drive motor to the re-conveying roller,
The driving force transmission unit includes:
a first transmission mechanism that drives the re-feed roller at the first speed;
4. The image forming apparatus according to claim 1, further comprising: a second transmission mechanism that drives the re-feed roller at the second speed. 5. The image forming apparatus according to claim 4 , wherein the driving force transmission unit has an electromagnetic clutch for switching between the first transmission mechanism and the second transmission mechanism. 6. The image forming apparatus according to claim 1, wherein the device main body has a conveying roller arranged upstream of the re-conveying roller in the re-conveying path in the sheet conveying direction and driven at the same speed as the re-conveying roller. the control unit forms images in the image forming unit in the order of the back side of the first sheet, the back side of the second sheet, the back side of the third sheet, the front side of the first sheet, and the back side of the fourth sheet in the double-sided printing;
7. The image forming apparatus according to claim 1, wherein the preceding sheet is the second sheet, and the succeeding sheet is the fourth sheet. The image forming apparatus of claim 7, wherein the control unit executes the deceleration control so that when the first sheet is positioned on the re-conveying path and the third sheet is supplied to the feeding path during the normal control, the sheet-to-sheet distance between the leading end of the first sheet and the trailing end of the third sheet transported by the re-conveying roller becomes the predetermined distance.