JP3424382B2 - Image forming device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as an electrophotographic copying machine or an electrophotographic printer which develops with a developer such as toner.

[0002]

2. Description of the Related Art Conventionally, an image forming apparatus using an electrophotographic system has been widely spread. This electrophotographic image forming apparatus normally charges a drum-shaped image carrier (photosensitive drum) uniformly, and emits light carrying information of an image recorded on a document to the photosensitive drum. The electrostatic latent image is formed on the photosensitive drum by irradiating the surface of the photosensitive drum, and the electrostatic latent image is developed with a developer (toner) to form a developed image on the photosensitive drum. To print (copy) by fixing the developed image transferred to the paper
It is a device for obtaining images. As one of the transfer methods, an electric field (transfer electric field) is formed between the paper conveyed to the transfer area where the developed image on the photosensitive drum is transferred to the paper and the developed image on the photosensitive drum. An electrostatic transfer method is known in which a developed image on a photosensitive drum is transferred to a sheet by a transfer electric field.
In this electrostatic transfer method, when the paper leaves the photoconductor drum after transfer, discharge (peeling discharge) occurs between the rear end of the paper and the photoconductor drum, and the photoconductor drum becomes streaky in the direction of its rotation axis. May be charged. When the photoconductor drum is charged in a streak shape in this way, dark fine streaks may occur in the part of the paper that corresponds to the streak-charged part of the photoconductor drum after the next sheet. Transfer may not be possible.

Although a method of weakening the transfer electric field can be considered as a method of preventing peeling discharge, there is a disadvantage that good transfer may not be performed if the transfer electric field is weakened during the transfer (Japanese Patent Laid-Open No. 2-73278). See the bulletin). Therefore, a paper detection unit that detects the front end and the rear end of the paper is arranged upstream of the transfer area in the paper conveyance direction, and the transfer device is driven after a predetermined time has elapsed since the front end of the paper was detected.
A technique is proposed in which the transfer device is stopped after a lapse of a predetermined time after the trailing edge of the sheet is detected, and when the trailing edge of the sheet separates from the photosensitive drum after the transfer, the transfer electric field is weakened to prevent peeling discharge. (See Japanese Patent Laid-Open No. 2-115877).

[0004]

By the way, the above-mentioned image forming apparatus is usually provided with a transport mechanism for transporting a sheet along a transport path passing through the transfer area. As the transport mechanism, for example, a system in which a sheet is attracted to a transport belt by a sheet suction corotron arranged upstream of the transfer area in the sheet transport direction, and the sheet is transported to the transfer area in the suctioned state. It has been known. Further, in order to align the leading edge of the sheet with the leading edge of the developed image when the developed image on the photosensitive drum is transferred to the sheet that has been transported to the transfer area, it is generally upstream of the sheet suction corotron in the sheet transport direction. A registration roll (registration roll) which is disposed on the side and holds the front end of the sheet before transfer and keeps the sheet on standby is used.

The sheet before transfer is conveyed to the sheet suction corotron by the registration roll at the timing when the leading edge of the sheet and the leading edge of the developed image are aligned, is attracted to the conveyor belt, and moves integrally with the conveyor belt to transfer area. Be transported to. The photoconductor drum is rotating at a constant speed, and in the case of the electrostatic transfer method, a transfer electric field is formed by the transfer device between the rotating photoconductor drum and the sheet conveyed to the transfer area. The developed image on the photosensitive drum is transferred to the paper. If the speed of the transport belt fluctuates, the transport speed of the paper fluctuates, and good transfer cannot be performed. Therefore, the conveying speed of the conveyor belt is required to have high accuracy and stability. However, if the paper conveyed to the paper suction corotron by the registration roll is pulled between the registration roll and the conveyance belt, the conveyance speed of the conveyance belt may fluctuate. Will not be done. Therefore, normally, the paper is flexed between the registration roll and the paper suction corotron so that the paper is not pulled between the registration roll and the transport belt.
For this reason, the rotation speed of the registration roll is designed to be about 0.5% faster than the transfer speed of the transfer belt. Although the flexure of the paper gradually disappears when the trailing edge of the paper is separated from the registration roll, the maximum flexure of the paper increases as the length of the paper in the conveyance direction (length of the paper feed direction) increases. Here, if the length in the paper transport direction is L, the distance between the registration roll and the paper suction corotron is G, the rotational speed of the registration roll is Vr, and the transport speed of the transport belt is Vf, the maximum amount of deflection W of the paper is It is calculated by the following formula.

W = (L−G) × (1−Vf / Vr) As can be seen from the above formula, the maximum deflection amount W of the sheet changes when the length L in the sheet conveying direction changes. That is, the time from when the trailing edge of the sheet is detected at a predetermined position upstream of the transfer area in the sheet transport direction until the trailing edge of the sheet actually passes through the transfer area varies depending on the length L of the sheet transport direction. The maximum deflection amount W of the above is divided by the time V divided by the conveying speed Vf of the conveying belt. For example, the transport speed Vf of the transport belt is 16
0 mm / sec, the rotation speed Vr of the registration roll is 1
60.8 mm / sec, and when the distance G between the registration roll and the paper suction corotron is 96 mm, the length in the conveyance direction when the minimum size A6 size is LEF (convey in the direction orthogonal to the long side) The length is 105 mm, and the amount of deflection is 0.5 mm. Further, when the 17-inch paper is SEF (conveyed in the direction orthogonal to the short side), the length in the conveyance direction is 432 mm, and the amount of deflection is 2.1 mm. Therefore, the maximum deflection amount differs by 1.6 mm due to the difference in the paper size. This difference is 10 msec when converted to the time difference from when the trailing edge of the sheet is detected to when the trailing edge of the sheet passes through the transfer area.

Therefore, in the above-mentioned conventional technique in which the sheet detecting means is arranged upstream of the transfer area in the sheet conveying direction and the transfer device is stopped after a predetermined time has elapsed after the trailing edge of the sheet is detected, Since the length in the paper transport direction is not taken into consideration, the timing of stopping the transfer device and the timing of the trailing edge of the paper separating from the photoconductor drum are offset from each other, and the transferring device stops after the trailing edge of the paper leaves the photoconductor drum Or
On the contrary, the transfer device may be stopped while the developed image is being transferred to the sheet, and good transfer may not be performed.

In view of the above circumstances, it is an object of the present invention to provide an image forming apparatus capable of preventing occurrence of peeling discharge and performing good transfer.

[0009]

An image forming apparatus of the present invention for achieving the above object is to form an electrostatic latent image on an image carrier, develop the electrostatic latent image, and then develop the electrostatic latent image on the image carrier. An image forming apparatus that forms a developed image on the transfer material and transfers the developed image onto a transfer material that has been conveyed to a predetermined transfer area along a predetermined conveyance path to form an image on the transfer material. (1) A transfer device for applying a transfer electric field for transferring the developed image formed on the image carrier to the transfer material conveyed to the transfer area. (2) Above the transfer area. A detector (3) arranged on the upstream side of the conveyance path for detecting the rear end of the transfer material (3) at both the timing when the rear end of the transfer material is detected and the length of the transfer material in the conveyance direction. The application of the transfer electric field by the transfer device is stopped at a timing according to
Alternatively, it is characterized in that a controller for weakening the transfer electric field than the transfer electric field during transfer is provided.

Here, there is provided a length measuring means for measuring the length of the material to be transferred in the conveying direction before the material to be transferred is conveyed to the transfer area. At a timing corresponding to both the timing at which the rear end is detected and the length in the transport direction of the material to be transferred measured by the length measuring means, the transfer electric field is stopped by the transfer device, or the transfer electric field is changed. It is preferably weaker than the transfer electric field during transfer.

Further, the controller is switchably switchable at a first timing according to the timing at which the rear end of the transfer material is detected by the detector, or the rear end of the transfer material is detected by the detector. The transfer electric field is stopped from being applied by the transfer device or the transfer electric field is changed at the second timing corresponding to both the transfer timing and the transfer direction length of the transferred material.
It is preferable that the image forming apparatus is provided with a changeover switch for changing the first timing and the second timing, which is weaker than the transfer electric field during transfer.

[0012]

In the image forming apparatus of the present invention, the detector detects the trailing edge of the material to be transferred, and the transfer electric field by the transfer device is detected at both the timing detected and the length in the conveying direction of the material to be transferred. Is stopped or the transfer electric field is weakened as compared with the transfer electric field during transfer. The length of the transfer material in the conveyance direction is determined, for example, according to the paper tray in which the transfer material is stored, and the application of the transfer electric field by the transfer device is stopped, or the transfer electric field is transferred during transfer. The timing to weaken the electric field is determined according to both the timing at which the rear end of the transfer material is detected and the length of the transfer material in the transport direction, and thus the timing is adjusted with high accuracy. As a result, the application of the transfer electric field by the transfer device can be stopped or the transfer electric field can be made weaker than the transfer electric field during transfer in accordance with the timing when the rear end of the transfer material is separated from the image carrier. It is possible to prevent the application of the transfer electric field from being stopped while the developed image is being transferred, and to perform good transfer. Also, the occurrence of peeling discharge is prevented, and the peeling discharge causes the image carrier to streak. Is prevented from being charged in a line shape, and a line-shaped image is prevented from being generated on the next sheet.

Here, there is provided a length measuring means for measuring the length of the material to be transferred in the carrying direction before the material to be transferred is carried to the transfer area. At a timing corresponding to both the timing at which the rear end is detected and the length in the transport direction of the material to be transferred measured by the length measuring means, the transfer electric field is stopped by the transfer device, or the transfer electric field is changed. If the transfer electric field is weaker than the transfer electric field during transfer, the transfer direction length of the transfer material can be measured even if the transfer direction length of the transfer material changes due to, for example, manual feeding. Depending on the timing at which the trailing edge is detected, application of the transfer electric field by the transfer device can be stopped, or this transfer electric field can be made weaker than the transfer electric field during transfer, and peeling discharge can be prevented.

Further, the controller is switchably switchable at a first timing according to the timing at which the rear end of the transfer material is detected by the detector, or the rear end of the transfer material is detected by the detector. The transfer electric field is stopped from being applied by the transfer device or the transfer electric field is changed at the second timing corresponding to both the transfer timing and the transfer direction length of the transferred material.
In the case where the image forming apparatus is provided with a changeover switch for switching between the first timing and the second timing, which is weaker than the transfer electric field during transfer, when the image forming apparatus is manufactured, the registration roll and the sheet suction roller are used. Even if a manufacturing error occurs in the distance from the corotron or the rotational speed of the registration roll, the transfer switch stops applying the transfer electric field by the transfer device, or switches the timing to weaken the transfer electric field below the transfer electric field during transfer. Therefore, peeling discharge can be prevented.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the image forming apparatus of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic view showing an embodiment of the image forming apparatus of the present invention. The color copying machine 1 shown in FIG. 1 has four colors (black K, yellow Y, magenta M,
Cyan C) four monochromatic image forming apparatuses (engines) 10K, 10Y, 10M for developing with each toner
10C is incorporated. Each engine 10K, 10
Y, 10M and 10C respectively include photosensitive drums 11K and 11 that rotate in the direction of arrow A and serve as image bearing members.
Y, 11M, 11C are provided, and each charger 12 is provided in each photoconductor drum 11K, 11Y, 11M, 11C.
The electrostatic latent images are formed by the exposure lights 13K, 13Y, 13M, and 13C that carry image information and are primarily charged by K, 12Y, 12M, and 12C, and are emitted from an exposure device (not shown). The image is displayed on each developing device 14K, 14Y,
It is developed by 14M and 14C, and thereby, each developed image is formed on each photoconductor drum 11K, 11Y, 11M and 11C.

On the other hand, the color copying machine 1 is provided with a transfer belt 20 which rotates in the direction of arrow B along with four engines 10K, 10Y, 10M and 10C. In the vicinity of the transfer belt 20, in order to align the leading edge of the sheet 31 with the leading edge of the developed image, a registration roll 30 is arranged to hold the leading edge of the sheet and keep the sheet on standby. Further, a detector 32 that detects the presence or absence of the sheet 31 is arranged on the upstream side of the register roll 30 in the sheet conveying direction.
The detector 32 emits light toward the upper side of the detector 32, and when the reflected light from the paper is received, it is determined that the paper exists above the detector 32, and the reflected light from the paper is detected. When no light is received, it is a type that determines that there is no paper above the detector 32. Therefore, when the detection state in which the detector 32 receives the reflected light from the sheet shifts to the state in which the reflected light from the sheet is not received is detected when the trailing edge of the sheet passes above the detector 32. Therefore, the trailing edge of the sheet can be detected. The register roll 30 is controlled by a controller (not shown), and after the register roll 30 starts driving, the paper 3
Based on the time until the trailing edge of the sheet 1 passes the detector 32 and the rotation speed of the registration roll 30, the length of the sheet 31 in the conveyance direction is obtained in the controller. That is, the length measuring means referred to in the present invention is composed of the detector 32 and the controller here. In addition, the controller, in accordance with both the timing when the trailing edge of the material to be transferred is detected by the detector 32 and the length of the sheet in the sheet conveyance direction, is described later with a transfer corotron (an example of a transfer device according to the invention) 23K, The timing for stopping the application of the transfer electric field by 23Y, 23M, and 23C is determined, and the transfer corotrons 23K, 23Y, and 23 are determined according to this timing.
The application of the transfer electric field by M and 23C is stopped.

The sheet 31 conveyed by the registration roll 30 is adsorbed to the transfer belt 20 by the sheet adsorption corotron 21 and the sheet adsorption roll 22 and conveyed in the direction of arrow B as the transfer belt 20 rotates. The transfer belt 20 is made of, for example, a transparent body of PET (polyethylene terephthalate). The image formation in each of the engines 10K, 10Y, 10M, and 10C described above is performed in synchronization with the conveyance of the paper 31, and each developing device 14
K, 14Y, 14M, 14C for each photosensitive drum 11
The pre-transfer corotrons 15K, 15Y, 15M, 15 are included in the developed images formed on K, 11Y, 11M, 11C.
Charges suitable for transfer are given to the developed images by C, and the developed images are transferred on the sheet 31 by the transfer corotrons 23K, 23Y, 23M, and 23C so as to be superimposed on each other. After that, the paper 31 is peeled from the transfer belt 20 by the action of the peeling corotron 23, conveyed in the direction of arrow C, and fixed by a fixing device (not shown), whereby a color image is formed on the paper 31.

The transfer belt 20 after sheet separation is destaticized by the transfer belt destaticizing corotron 24, the surface thereof is cleaned by the fur brush 25a and the blade 25b provided on the transfer belt cleaner 25, and thereafter, the adsorbing corotron 21 and the adsorbing corotron 21. The paper 22 is used to electrostatically attract the next sheet. On the other hand, each engine 10K,
In 10Y, 10M, and 10C, after the development image is transferred, pretreatment for cleaning 16K, 16Y, 16M, 16
After the pretreatment by C, the cleaning blade 1
The surface is cleaned by 7K, 17Y, 17M and 17C, and the static elimination lamps 18K, 18Y, 18M and 18
The charge is removed by C, and the chargers 12K, 12Y, 12
Primary charging for the next image formation is performed by M and 12C.

The transfer timing will be described with reference to FIGS. Here, a case where the copying is restarted after the copying is not performed for a predetermined time will be described as an example. In addition, the operation of each device is performed in synchronization with the conveyance of the paper. When a copy start button (not shown) is pressed, the photosensitive drums 11K, 11Y, 11M and 11C start to rotate, and cleaning pretreatment corotrons 16K, 16Y and 1
6M and 16C are turned on at time t1, and the charger 12
K, 12Y, 12M, and 12C are turned on at time t2, and the primary charging is started on the photosensitive drums 11K, 11Y, 11M, and 11C. The leading edge of the sheet conveyed from the tray (not shown) passes the detector 32 at time t3 and is nipped by the registration roll 30. Therefore, at time t3, the detector 32 detects the sheet (the "on" state). At time t4, the exposure device is turned on, and electrostatic latent images start to be formed on the photoconductor drums 11K, 11Y, 11M, and 11C, and substantially at the same time, the development devices 14K, 14Y, and
14M and 14C also start driving and a developed image is formed. The formation of the developed image is performed in synchronization with the transportation of the paper 31 as described above. The exposure unit is turned off at time t7, and the first formation of the electrostatic latent image is completed. At time t5, the controller outputs a signal for rotating the registration roll 30, the registration roll 30 starts to rotate, and the paper 31 is conveyed toward the paper suction corotron 21 and the paper suction roll 22 and, as described above, the transfer is performed. It is attracted to the belt 20 and is conveyed in the direction of arrow B as the transfer belt 20 rotates. At time t6, the transfer corotron 23K, 23
Y, 23M, and 23C are turned “on”, a transfer electric field is applied, and transfer is started. At time t7, the detector 32 is turned “off”, whereby the timing when the trailing edge of the sheet 31 has passed the detector 32 can be known, and a signal indicating that the trailing edge of the sheet 31 has passed the detector 32 is output. Input to the controller. In the controller, the paper 31 is based on the time T1 from the time when the registration roll 30 starts driving (time t5) to the timing (time t7) when the trailing edge of the paper 31 passes the detector 32 and the rotation speed of the registration roll 30. The length in the transport direction of is required. Further, in the controller, the transfer corotrons 23K, 23Y, 23M, and the transfer corotrons 23K, 23Y, 23M, according to the timing when the trailing edge of the paper 31 passes the detector 32 and the length of the paper 31 in the transport direction.
The timing (time t8) to turn off 23C is determined. The time T2 from the time t7 to the time t8 is called a delay amount, and after the trailing edge of the sheet 31 passes the detector 32 (time t7), the blank portion within about 5 mm width of the trailing edge of the sheet 31 is transferred. The time required to reach the area
It changes according to the length in the transport direction of 1. For this reason, the rear end of the paper 31 has the photosensitive drums 11K, 11Y, 1
Transfer Corotron 23K, 2 before leaving 1M, 11C
3Y, 23M, and 23C will be turned off.
Thus, at time t8, the transfer corotron 23K,
23Y, 23M, and 23C are controlled to be “off”, and the first transfer is completed. As a result, the transfer corotrons 23K, 23Y, 23M, before the rear end of the paper 31 leaves the photosensitive drums 11K, 11Y, 11M, 11C.
Since the electric field due to 23C can be made zero, it is possible to prevent the transfer corotron from stopping while the developed image is being transferred to the paper, and to perform good transfer, and to prevent the occurrence of peeling discharge. This peeling discharge prevents the photosensitive drums 11K, 11Y, 11M, and 11C from being charged in a streak shape, and a good transfer can be performed without streaking on the next sheet. Here, at time t8, the transfer corotrons 23K, 23Y, 23M, and 23C are turned off to stop the application of the transfer electric field. However, the transfer electric field is controlled to be weaker than the transfer electric field during the transfer, and the peeling discharge is performed. May be prevented.

At time t9, the sheet is again conveyed from the tray to prepare for the second transfer, and the leading edge of the sheet passes the detector 32 and is nipped by the registration roll 30. After that, at time t10, the exposure device is turned on, at the time t11, a signal for rotating the registration roll 30 is output from the controller, and at time t12, the transfer corotrons 23K and 23K.
Y, 23M, and 23C are turned “on” to start the second transfer. At time t13, the detector 32 turns off, and the same operation as above is repeated.

Next, a case will be described in which the image forming apparatus 1 (see FIG. 1) is provided with a changeover switch for changing the delay amount T2 (see FIG. 2). The reason for providing the changeover switch will be described below. Variations in the manufacture of the image forming apparatus may include variations in the transport belt speed, errors in the transfer area position, and errors in the arrangement position of the detector 32 (see FIG. 1). Therefore, it is necessary to be able to adjust the above-described delay amount T2 (the time from the detection of the trailing edge of the sheet by the detector 32 to the turning off of the transfer corotron) for each device. The minimum unit for adjusting this delay amount is 10 msec (1.6 in terms of the length in the sheet conveyance direction).
(corresponding to mm), a maximum deviation of 3.2 mm occurred in terms of the length of the sheet in the transport direction due to an error in adjustment and a difference in the length of the sheet in the transport direction. In addition, the time from the detection of the trailing edge of the sheet to the turning off of the transfer corotron varies, and this variation causes a shift of 2.3 mm in terms of the length of the sheet in the transport direction. Therefore, the width (margin amount) of the margin portion at the trailing edge of the paper is required to be 5.5 mm. It is desirable that this margin amount be as small as possible,
Generally, it is considered that the allowable margin amount is a width of 5.0 mm or less. Therefore, if the minimum unit for adjusting the delay amount can be shortened sufficiently, for example, the time from the detection of the trailing edge of the sheet to the transfer corotron “OFF” is corrected for each length of the fixed-size sheet in the sheet conveyance direction, and the delay of the sheet may cause It is possible to reduce the difference in the delay amount T2 to the minimum unit. However, if the minimum unit of adjustment is reduced, the load on the CPU in the controller increases, and it is necessary to increase the processing speed of the CPU and use a plurality of CPUs, which increases the manufacturing cost of the device. Therefore, the manufacturing cost is suppressed and the delay amount T
A change-over switch is provided to adjust 2.

With this changeover switch, when the average value of the maximum length and the minimum length of the paper in the conveying direction is Wa, the papers larger than Wa are set to L group and Wa is set to Wa.
The smaller paper is divided into two groups as the S group. Further, the changeover switch is configured to change over to "A" or "B" in order to correct the error of the delay amount T2 due to the manufacturing variation of the image forming apparatus. Here, when "A" is selected,
In both the L group and the S group, the transfer corotron is turned "off" at the timing (first timing according to the invention) corresponding to the adjustment value stored in the controller that is adjusted in advance, and the discharge is stopped, and "B" is set. Is selected, the L group is set to the adjustment value and the S group is set to 1 (the 1
Is configured to stop applying the transfer electric field at a timing (second timing according to the invention) corresponding to the subtracted value (which is a time corresponding to 10 msec). This aspiration,
When the variation of the device is adjusted in the state of switching to “A”, the maximum margin is the proper amount of blank space, but when the margin is too small on the minimum size of the paper, the changeover switch is switched to “B”. It is possible to obtain an appropriate margin amount on paper.

Here, how to change the changeover switch will be described with an example. In order to surely turn off the transfer corotron at the margin of the trailing edge of the sheet to stop the discharge, the variation in the processing time from the detection of the trailing edge of the sheet to the stop of the discharging is 0 mm to 2 mm at the trailing edge of the sheet. It must have a margin of width 3 mm.
When making a copy using the image forming apparatus (manufacturing number S / N *** 1), the adjustment value is 4 (adjustment value 1 corresponds to 10 msec. The adjustment value 4 is set to correspond to the delay amount T2 of 40 msec), and a large number of copies are made with a 17-inch size paper, and the margin amount is measured and found to be in the range of 2.7 to 6.0 mm. LEF A6 size paper
Similarly, when the width of the margin is measured when (transporting in the direction orthogonal to the long side), it is 1.1 to 3.4 mm.
It varied in the range. That is, when "A" is selected by the changeover switch, the discharge of the transfer corotron can be stopped when the margin amount in the transfer area is within 5.0 mm, so the delay amount T2 is properly adjusted.
The adjustment value is set to 3 (the adjustment value 3 corresponds to the delay amount T2 of 30 msec) in the image forming apparatus of this manufacturing number S / N *** 1.
When set to 17, the margin amount for the 17-inch size is 6.6 mm, and the adjustment value is 5 (the adjustment value 5 is the delay amount T of 50 msec.
(Corresponding to 2), when A6 size paper is set to LEF, a copy with no margin appears. Therefore, it was found that both the adjustment value 3 and the adjustment value 5 are inappropriate.

Another image forming apparatus (serial number S / N ***)
When making a copy using 2), set the adjustment value to 4 while "A" is selected with the changeover switch,
When a large number of copies were made on a 17-inch size paper and the amount of blank space was measured, there was variation in the range from 1.2 mm to 3.5 mm. When the margin amount was similarly measured when the A6 size paper was LEF, the width of the margin portion varied from the one having no margin to the width of 1.9 mm. Since there are some with no margin, setting the adjustment value to 5 sometimes causes the 17-inch size margin to exceed 5.0 mm.
Adjustment value 5 was found to be inappropriate. That is, in the case of the image forming apparatus with the serial number S / N *** 2, it can be understood that the delay T2 cannot be properly adjusted while the changeover switch is switched to "A". When the changeover switch is switched to “B”, the L group of paper such as 17-inch size remains the adjusted value, and only the S group of paper when the A6 size paper is set to LEF is changed from the adjusted value. The value less one is applied. As a result,
When the adjustment value is 4, when A6 size paper is LEF, it is 1.2 mm to 3.5 m which is the same as 17 inch size.
The amount of blank space is up to m. The longest paper in the S group varies in the range of approximately 2.0 to 4.3 mm, and therefore the delay amount T2 can be appropriately adjusted for all the paper.

FIG. 3 is a flow chart showing the flow of determining the delay amount T2 when the changeover switch is provided in the image forming apparatus. When copying is started (S10), the length of the sheet in the sheet conveyance direction is detected by the detector 32 (see FIG. 1) and the controller described above (S12). The delay amount is once set to a predetermined delay amount T (S14), and then it is determined whether the changeover switch is "A" or "B" (S16). Changeover switch is "A"
If it is, it is determined whether the length of the sheet in the transport direction is longer or shorter than the average value Wa (S18). In either case, the delay amount is finally determined in advance. The delay amount T that has been set is set (S20, S22). When the changeover switch is set to "B", it is determined whether the length of the sheet in the conveying direction is longer or shorter than the average value Wa (S24), and the length of the sheet in the conveying direction is larger than the average value Wa. If it is too long, the delay amount is finally set to the predetermined delay amount T (S26). When the length of the sheet in the conveyance direction is shorter than the average value Wa, the delay amount is finally set to a value 10 msec later than the predetermined delay amount T (S28). By setting the delay amount in this way, it is possible to prevent the occurrence of peeling discharge and perform good transfer.

Although a color copying machine is taken as an example in the above-mentioned embodiment, the image forming apparatus of the present invention can be applied to a black and white copying machine. As shown in FIG. 4, the black-and-white copying machine 100 is provided with a photosensitive drum 102 that rotates in the direction of arrow D. The photosensitive drum 102 is primarily charged by a charger 104, and is exposed from an unillustrated exposing device. The emitted exposure light 106 carrying image information forms an electrostatic latent image, and the electrostatic latent image is developed by the developing device 108.
As a result, a developed image of the photosensitive drum 102 is formed.
Further, the sheet is guided and conveyed by the sheet guide 110. The same paper guide as the registration roll 30 shown in FIG. 1 is arranged. Further, a detector 3 for detecting the presence / absence of paper is provided on the upstream side of the register roll 30 in the paper conveyance direction.
2 are arranged. The detector 32 is the same as that shown in FIG. 1, and can detect the trailing edge of the paper. The registration roll 30 is controlled by a controller (not shown). The time from when the registration roll 30 starts driving until the trailing edge of the sheet passes the detector 32 and the registration roll 3
Based on the rotation speed of 0, the length of the sheet in the conveyance direction is obtained in the controller. Further, the controller controls the transfer electric field by the transfer corotron (an example of the transfer device according to the invention) 112 according to both the timing when the rear end of the transfer target material is detected by the detector 32 and the length of the sheet in the conveyance direction. Is determined, and the application of the transfer electric field by the transfer corotron 112 is controlled in accordance with this timing. As a result, the trailing edge of the paper is placed on the photosensitive drum
The application of the transfer electric field by the transfer corotron 112 can be stopped before it is separated from 02, and peeling discharge can be prevented from being generated, and good transfer can be performed without generating streaks on the next sheet.

[0027]

As described above, according to the image forming apparatus of the present invention, the transfer electric field by the transfer device is adjusted according to both the timing when the rear end of the transfer material is detected and the length of the transfer material in the conveying direction. The timing of stopping the application of the electric field or making the transfer electric field weaker than the transfer electric field during the transfer is prevented, so that the occurrence of peeling discharge is prevented and the image carrier is prevented from being charged in a stripe shape. Good transfer can be performed without generating streaks on the paper.

[Brief description of drawings]

FIG. 1 is a schematic view showing an embodiment of an image forming apparatus of the present invention.

FIG. 2 is a timing chart showing control timing of a transfer corotron.

FIG. 3 is a flowchart showing a flow in which a delay amount T2 is determined when a changeover switch is provided in the image forming apparatus.

FIG. 4 is a schematic view showing another embodiment of the image forming apparatus of the invention.

[Explanation of symbols]

1 color copier 11K, 11Y, 11M, 11C photoconductor drum 20 transfer belt 23K, 23Y, 23M, 23C Transfer Corotron 30 cash register rolls 31 sheets 32 detectors

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-6-208307 (JP, A) JP-A-7-92830 (JP, A) JP-A-5-188729 (JP, A) JP-A-7- 334018 (JP, A) JP 62-299371 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G03G 15/16 G03G 21/14

Claims (3)

(57) [Claims] 1. An electrostatic latent image is formed on an image carrier, the electrostatic latent image is developed to form a developed image on the image carrier, and the developed image is transferred along a predetermined transport path. In an image forming apparatus that forms an image on a transfer material transferred to a transfer material that has been conveyed to a predetermined transfer area, a developed image formed on the image carrier is transferred to the transfer area. A transfer device for applying a transfer electric field for transferring to the transferred material to be transferred, and a transfer device arranged on the upstream side of the transfer path with respect to the transfer area,
A detector that detects the trailing edge of the transfer material, and a transfer electric field generated by the transfer device at the timing when the trailing edge of the transfer material is detected by the detector and the timing in accordance with both the length of the transfer material in the conveyance direction. And a controller for making the transfer electric field weaker than the transfer electric field during transfer. 2. A length measuring device for measuring a length of a transfer material in a conveyance direction before the transfer material is conveyed to the transfer area, wherein the controller causes the detector to detect the rear end of the transfer material. The transfer device stops applying the transfer electric field or transfers the transfer electric field at a timing corresponding to both the edge detection timing and the transfer direction length of the transferred material measured by the length measuring means. The image forming apparatus according to claim 1, wherein the image forming apparatus is weaker than the internal transfer electric field. 3. The controller detects the trailing edge of the material to be transferred at a first timing according to the timing at which the trailing edge of the material to be transferred is detected by the detector. The application of the transfer electric field by the transfer device is stopped or the transfer electric field is made weaker than the transfer electric field during transfer at the second timing according to both the timing of the transfer and the length of the transfer material in the transport direction. 3. The image forming apparatus according to claim 1, further comprising a changeover switch that switches between the first timing and the second timing.