JP3659302B2 - Image forming apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a so-called tandem type image forming apparatus in which a plurality of image carriers that carry each color component image are arranged in parallel, and in particular, a sheet carrier that is circulated and conveyed to a portion facing each image carrier. The present invention relates to an improvement in an image forming apparatus that sequentially transfers an image formed on each image carrier to a sheet held on the sheet conveyance body.
[0002]
[Prior art]
As a conventional tandem-type image forming apparatus, as shown in FIG. 11, for example, a photosensitive drum 201 (specifically, an image carrier on which a plurality of color toner images of yellow, magenta, cyan, and black are carried) The image forming units 200 (specifically, 200a to 200d; adopting, for example, an electrophotographic method) having 201 (1) to 201 (4)) are arranged in parallel, and a plurality of portions are provided at portions facing the respective photosensitive drums 201. A transfer belt 202 is provided that is circulated and conveyed over the tension rolls 203 and 204, and a transfer roll 205 to which a transfer bias is applied to the back side of the transfer belt 202 facing each photosensitive drum 201. (Specifically, 205 (1) to 205 (4)) are arranged, and each photosensitive drum is placed on a sheet (paper or OHP sheet) 207 held on the conveyor belt 202. A toner image in which a plurality of color toner images formed in 201 is sequentially transferred in a multiple manner is already known (see, for example, JP-A-5-53412).
In the figure, reference numeral 206 denotes a bias power source for applying a transfer bias to the transfer roll 205.
[0003]
[Problems to be solved by the invention]
By the way, in this kind of tandem type image forming apparatus (conveying belt + transfer roll method), for example, when an attempt is made to transfer to a high resistance sheet 207 such as an OHP sheet in a high temperature and high humidity environment, There has been a technical problem that image loss occurs at a distance corresponding to the pitch between the photosensitive drums 201 from the rear end.
It has been confirmed by experiments that this image loss becomes severe as the pitch between the photosensitive drums 201 becomes shorter when the same image forming apparatus is used, and this is a major obstacle to aiming for downsizing of the image forming apparatus in the future. It becomes.
[0004]
The present invention has been made to solve the above technical problem, and even when a high-resistance sheet is used, image omission at a distance corresponding to the pitch between the image carriers from the rear end of the sheet is effective. It is an object of the present invention to provide an image forming apparatus capable of preventing the above.
[0005]
[Means for Solving the Problems]
The inventor analyzed the cause of the above-described technical problem. As shown in FIG. 11, for example, immediately after the trailing edge of the sheet 207 passes through the first photosensitive drum 201 (1), the sheet 207 Since it does not exist, the combined resistance between the conveyance belt 202 and the photosensitive drum 201 (1) is rapidly reduced.
At this time, the transfer inflow current from the transfer roll 205 (2) corresponding to the second photosensitive drum 201 (2) is Itotal, the transfer current actually flowing to the second photosensitive drum 201 (2) is Itr, If the leakage current flowing to the second photosensitive drum 201 (1) side is Ileak, Itotal = Itr + Ileak, but the combined resistance between the conveyor belt 202 and the photosensitive drum 201 (1) is drastically reduced. In addition, it has been found that the leakage current Ileak increases as compared with the transfer current Itr, and a sufficient transfer current Itr cannot be supplied.
Therefore, the present inventor described above with the design philosophy of ensuring a sufficient transfer current Itr by suppressing the leakage current Ileak to the adjacent photosensitive drum 201 immediately before the trailing edge of the sheet 207 passes. The inventor obtained the knowledge that the technical problem can be solved, and came up with the present invention.
[0006]
That is, according to the present invention, as shown in FIG. 1, a plurality of image carriers 1 (for example, 1 (1) to 1 (4)) on which each color component image is carried are arranged in parallel, and each image carrier 1 is attached to each image carrier 1. A transfer unit 4 (for example, 4 (1)) is provided, in which a sheet transport body 2 that is circulated and transported is disposed on the opposite side, and a transfer bias is applied to the back side of the sheet transport body 2 facing each image carrier 1. -4 (4)), and an image carrier that sequentially transfers each color component image formed on each image carrier 1 to the sheet 3 held on the sheet carrier 2. The number of 1 is n (for example, n = 4), and the rear end of the sheet 3 passes through the j-1 (j: 2 to n) th image carrier, and the jth (j = 2 in FIG. 1) situations that passes between the image carrier 1 (j) and (j-1) -th image bearing member 1 (j-1) of example) in the corresponding to the j-th image bearing member 1 (j) transfer The transfer inflow current from the stage 4 (j) is Itotal (j), the transfer current that actually flows to the jth image carrier 1 (j) is Itr (j), and the j−1th image carrier 1 (j). The leakage current flowing to the side is Ileak (j-1), and the tolerance that the ratio of the leakage current Ileak (j-1) in the transfer inflow current Itotal (j) should be allowed is α (0 ≦ α ≦ 1). If
The present invention is characterized in that the leakage suppressing means 6 satisfying the condition of | α · Itotal (j) | ≧ | Ileak (j−1) |
[0007]
In such a technical means, an image forming apparatus to which the present invention is applied is a so-called tandem type in which a plurality of image carriers 1 on which color component images are carried are arranged in parallel, and each color component image is sequentially superimposed and transferred. The arrangement direction of each image carrier 1 may be a horizontal arrangement or a vertical arrangement.
Here, the image carrier 1 may be any material as long as it can carry at least a toner image, and an appropriate material such as a photoreceptor or a dielectric may be selected according to the type of the latent image writing unit. The form may be a drum shape or a belt shape.
The image forming method on the image carrier 1 may be selected as appropriate as long as it forms an image (toner image or the like) for each color component. For example, an electrophotographic method or an electrostatic transfer method is adopted. If it is an aspect, an electrostatic latent image may be written on the image carrier 1, and the electrostatic latent image may be visualized with each color component toner.
[0008]
In addition, as the sheet conveyance body 2, as long as the sheet 3 can be held and conveyed, a conveyance belt, a conveyance drum, or the like may be appropriately selected. A method or the like may be used.
Furthermore, in a mode in which the sheet transport body 2 is a transport belt, for example, a structure for stretching the transport belt is required. As this type of stretch structure, at least two stretch rolls are used, Can be made to function as a driven roll and the other as a driven roll, but other tension rolls can be added if necessary.
In this case, the added stretching roll may be a simple driven roll, a tension roll for applying tension, or a steering roll for meandering correction of the conveyor belt, You can choose.
[0009]
Furthermore, the transfer means 4 may be appropriately selected as long as it is placed in contact with the sheet transport body 2. However, from the viewpoint of stably transporting the sheet 3, the image carrier 1 is provided with a sheet. It is preferably a transfer roll that is arranged in pressure contact with the carrier 2 and rotates.
A predetermined transfer bias is applied to the transfer means 4 by a bias applying means 5. The bias applying means 5 may be individually provided for each transfer means 4, but may be configured to share all or a part.
[0010]
Further, the leak suppression means 6 can be selected as appropriate, and typical examples include the following three.
In the first embodiment, the system resistance between the jth image carrier 1 (j) contributing to the transfer current Itr (j) and the transfer means 4 (j) corresponding thereto is expressed as Ra (j), the leakage current. The system resistance between the transfer means 4 (j) corresponding to the jth image carrier 1 (j) contributing to Ileak (j-1) and the j-1th image carrier 1 (j-1) When Rb (j) is assumed,
Rb (j) / (Ra (j) + Rb (j)) ≧ (1−α) is satisfied.
[0011]
In the second embodiment, when the impedance of the j-th image carrier 1 (j) is Rpr (j),
Rpr (j) <Rpr (j-1) is satisfied.
[0012]
In the third mode, the bias applying means 5 for applying a transfer bias to each transfer means 4 is synchronized with the timing at which the rear end of the sheet 3 passes through the j-1st image carrier 1 (j-1). Thus, the transfer bias corresponding to the (j-1) th image carrier 1 (j-1) is turned off and the grounding state of the image carrier 1 (j-1) is floated.
[0013]
Next, the operation of the technical means described above will be described.
In FIG. 1, the leak suppression means 6 satisfies the condition of | α · Itotal (j) | ≧ | Ileak (j−1) | for the transfer portion of the j-th image carrier 1 (j).
At this time, since Itotal (j) = Itr (j) + Ileak (j-1),
Transfer current Itr (j) = Itotal (j) −Ileak (j−1) ≧ (1−α) Itotal (j) is set.
Therefore, the transfer current falls within a range of ± tolerance α with respect to the set current.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail based on embodiments shown in the accompanying drawings.
Embodiment 1
FIG. 2 shows a first embodiment of a color image forming apparatus to which the present invention is applied.
In the figure, the color image forming apparatus has four image forming units 22 (specifically, 22a to 22d) in a vertical direction in a main body housing 21 (in this embodiment, black, yellow, magenta, and cyan). A sheet supply cassette 23 that accommodates a sheet (not shown) such as a supply sheet is disposed below the sheet supply cassette 23 and disposed at a position corresponding to each image forming unit 22 from the sheet supply cassette 23. A sheet conveyance path 24 serving as a sheet conveyance path is arranged in a vertical direction.
[0015]
In the present embodiment, the image forming units 22 (22a to 22d) are for black, yellow, magenta, and cyan in order from the upstream side of the sheet conveyance path 24 (arrangement is not necessarily in this order). A toner image is formed, and includes a photosensitive cartridge 30 and an exposure unit 40.
Here, the photoconductor cartridge 30 includes, for example, a photoconductor drum 31, a charging roll 32 that precharges the photoconductor drum 31, and a static image formed by exposure on the charged photoconductor drum 31 by the exposure unit 40. A developing device 33 that develops the electrostatic latent image with a corresponding color toner (for example, negative polarity in the present embodiment) and a cleaner 34 that removes residual toner on the photosensitive drum 31 are integrally formed into a cartridge. .
On the other hand, the exposure unit 40 stores a semiconductor laser (not shown), a polygon mirror 42, an imaging lens 43, and a mirror 44 in the case 41, and deflects and scans light from the semiconductor laser (not shown) with the polygon mirror 42. An optical image is guided to an exposure point on the photosensitive drum 31 via the image lens 43 and the mirror 44.
[0016]
Further, in the present embodiment, as shown in FIGS. 2 and 3, a sheet conveyance path 24 is provided at a position corresponding to each photosensitive drum 31 (31 (1) to 31 (4)) of each image forming unit 22. A conveyor belt 80 that circulates and moves along is provided.
The conveying belt 80 is made of a belt material (such as a plastic film such as PET or PVdF or an elastic material such as urethane or urea) that can electrostatically adsorb the sheet 25, and in some cases, 10 ⁶ to 10 on the surface thereof. A high resistance coating layer (not shown) of about ¹² Ω · cm is formed.
The conveyor belt 80 is stretched between a pair of stretching rolls 81 and 82 (for example, a metal roll, an elastic body roll, etc.). In this embodiment, the upper stretching roll 82 is a drive roll, The tension roll 81 on the side is a driven roll.
[0017]
Furthermore, a transfer roll 50 (specifically 50 (1) to 50 (4), 10 ³ to ¹⁰ 10 in this example) is provided on the back side of the conveying belt 80 corresponding to the photosensitive drum 31 of each image forming unit 22. Ω · cm) is arranged, and the transfer roll 50 and the photosensitive drum 31 hold the sheet 25 on the conveyance belt 80 in a nip.
A transfer bias power source 51 is connected to the transfer roll 50, and a predetermined transfer bias (in this example, for example, positive polarity, +700 V) is applied through the switch 52 at an appropriate timing.
[0018]
A belt inlet sensor 101 and a belt outlet sensor 102 are disposed in the vicinity of the inlet side and the outlet side of the conveyor belt 80.
In the present embodiment, as each of the sensors 101 and 102, for example, an optical sensor that turns on when the leading edge of the sheet 25 passes and turns off when the trailing edge of the sheet 25 passes is used. For example, the switch 52 is turned on in synchronization with the on operation of the belt inlet sensor 101, and the switch 52 is turned off in synchronism with the off operation of the belt outlet sensor 102.
[0019]
Further, in the present embodiment, as shown in FIGS. 2 and 3, the sheet supply cassette 23 is provided with a takeaway roll 61 for picking up the sheet 25, and the takeaway roll 61 and the most upstream image forming unit 22a. A feed roll 62 for feeding the sheet 25 at a predetermined timing is provided in the sheet conveyance path 24 positioned between the transfer portion. An adsorbing roll 90 is disposed at the entrance portion of the conveyor belt 80.
Further, a fixing device 64 is provided in the sheet conveyance path 24 located on the downstream side of the most downstream image forming unit 22d, and a discharge roll 66 for discharging the sheet is provided on the downstream side of the fixing device 64. The discharge sheet is accommodated in an accommodation tray 67 formed on the upper portion of the housing 21. In FIG. 2, reference numeral 65 denotes a recording material guide for guiding the sheet 25 sent from the conveyance belt 80 to the fixing device 64.
[0020]
In particular, in the present embodiment, the following is performed at the transfer portion between each photosensitive drum 31 (31 (1) to 31 (4)) and the transfer roll 50 (50 (1) to 50 (4)). System resistance design is made.
Now, as shown in FIG. 4A, the sheet 25 is held and conveyed on the conveyance belt 80, and the state immediately after the rear end of the sheet 25 passes through the first photosensitive drum 31 (1). Suppose that
In this state, when attention is paid to the transfer portion of the second photosensitive drum 31 (2), as shown in FIG. 5, Itotal (2) the transfer current flowing from the transfer bias power source 51, the photosensitive drum 31 (2) If the transfer current that actually flows to Itr (2) and the leakage current that flows to the adjacent photosensitive drum 31 (1) side is Ileak (1),
Itotal (2) = Itr (2) + Ileak (1) …… ▲ 1 ▼
Is established.
[0021]
Further, Ra (2) represents the system resistance between the second photosensitive drum 31 (2) contributing to the transfer current Itr (2) and the corresponding transfer roll 50 (2), and the leakage current Ileak (1). Rb (2) is the system resistance between the transfer roll 50 (2) corresponding to the second photosensitive drum 31 (2) that contributes to the first photosensitive drum 31 (1).
Ra (2) = Rbelt (m) + Rsh + Rpr (2) (2)
Rb (2) = Rbelt (s) + Rpr (1) …… (3)
R belt (m): resistance in the thickness direction of the conveyor belt 80 (Ω)
Rbelt (s): resistance of the conveyor belt 80 in the “circumferential direction + thickness direction” (Ω)
Rsh: Resistance in the thickness direction of the sheet (Ω)
Rpr (1): resistance (impedance) in the thickness direction of the photosensitive drum 31 (1) (Ω)
Rpr (2): resistance (impedance) in the thickness direction of the photosensitive drum 31 (2) (Ω)
Is established.
[0022]
Here, a method for measuring each resistance will be described.
● Rbelt (m): Resistance in the thickness direction of the conveyor belt 80 (Ω)
This is because, as shown in FIG. 6 (a), while placing the conveyor belt 80 on the conductive base plate 120 that is grounded, and pressed by the pressing plate 121 of the conductive, due to the power supply 122 to the pressing plate 121 While applying a predetermined bias V, a plurality of earth electrodes 123 and 124 to be grounded (Gnd) are placed on the conveyor belt 80 to remove the resistance component in the circumferential direction of the conveyor belt 80, and the pressing plate This is obtained by measuring the current I flowing between 121 and the conductive base plate 120 and calculating V / I.
[0023]
Rbelt (s): Resistance in the circumferential direction + thickness direction of the conveyor belt 80 (Ω)
As shown in FIG. 6B, a pair of electrode rolls 125 and 126 arranged in contact with the front and back surfaces of the conveyor belt 80 are provided apart from each other by the photosensitive drum pitch. The bias V from 127 is applied, the other electrode roll 126 is grounded, the current I flowing through the electrode roll 126 is measured, and V / I is calculated.
[0024]
● Rsh: Resistance in the sheet thickness direction (Ω)
As shown in FIG. 7 (a), a measuring instrument having the same configuration as in FIG. 6 (a) is used, the sheet 25 is placed on the conductive base plate 120, the pressing plate 121, the conductive base plate. The current I flowing between 120 is measured, and this current and the bias V are used to calculate V / I.
[0025]
Rpr: resistance (impedance) in the thickness direction of the photosensitive drum 31 (Ω)
As shown in FIG. 7B, the conductive roll 128 is brought into contact with the photosensitive drum 31, and a bias V from a power source 129 is applied to the conductive roll 128, while the base of the photosensitive drum 31 is applied. The material portion is grounded, the current I flowing through the photosensitive drum 31 is measured, and V / I is calculated.
In general, since each photosensitive drum 31 has the same configuration, only one impedance is typically measured for impedance measurement.
[0026]
Furthermore, when Kirchhoff's law is applied to the system resistance circuit,
Itr (2) ・ Ra (2) ＝ Ileak (1) ・ Rb (2) …… ▲ 4 ▼
Is established.
From ▲ 1 ▼ ▲ 4 ▼,
Itr (2) = Itotal (2) · {Rb (2) / (Ra (2) + Rb (2))} …… (5)
Is guided.
[0027]
In the present embodiment, the latitude of the transfer current Itr (Itr (j)) during normal operation that is not affected by current leakage is:
1) For sheets with controlled resistance,
0.5 x Itr ≤ Itr ≤ 1.5 x Itr ... (6)
(Equivalent to ± 50% tolerance for the set current)
2) Including a commercially available general-purpose sheet
0.8 x Itr ≤ Itr ≤ 1.2 x Itr ...... (7)
(Equivalent to ± 20% tolerance for the set current)
It is.
[0028]
According to ▲ 5 ▼ ▲ 6 ▼,
Practical provisions for preventing leakage current to the adjacent photosensitive drum 31 (1) are as follows:
1) Rb (2) / (Ra (2) + Rb (2)) ≧ (1-50 / 100)
(Tolerance ± 50%).
Furthermore, for image forming devices that use versatile sheets,
▲ 5 ▼ ▲ 7 ▼
2) Rb (2) / (Ra (2) + Rb (2)) ≧ (1-20 / 100)
(Tolerance ± 20%).
More preferably, from the viewpoint of not causing an omission in actual use,
3) Rb (2) / (Ra (2) + Rb (2)) ≧ (1-1 / 100)
(Tolerance ± 1%).
[0029]
Further, as shown in FIG. 4B, the transfer current condition satisfies the tolerance according to the same calculation formula immediately after the trailing edge of the sheet 25 passes the third photosensitive drum 31 (3). Is grasped.
Therefore, in general, when considering the tolerance β% (α = β / 100) for the transfer current Itr (j) of the j-th photosensitive drum 31 (j),
Rb (j) / (Ra (j) + Rb (j)) ≧ (1-α) (8)
What is necessary is just to design so that.
With this design, the leakage current to the photosensitive drum immediately before the sheet passes can be effectively prevented.
[0030]
Examples and comparative examples relating to this will be described.
Example In the system shown in FIG. 2, the process speed is 56 mm / sec, the pitch between the photosensitive drums is 48 mm, the photosensitive drum diameter is φ10, the transfer roll diameter is φ10, and the developing unit has a nonmagnetic one-component development of φ10. The apparatus configuration is such that a developing roll is used.
In this example, a urea resin belt is used as the transport belt,
・ Rbelt (m) = 7.3 (Log Ω)
・ Rbelt (s) = 18.0 (Log Ω)
・ Rsh = 11.0 (Log Ω)
-Rpr = 8.0 (Log Ω).
Therefore, when calculating for Ra and Rb,
Ra = 26.3 (Log Ω),
Rb = 26.0 (Log Ω).
∴Rb / (Ra + Rb) = 0.50 ≧ 50/100
Meet.
In this example, for example, in the situation shown in FIG. 4A, the transfer current Itr (2) necessary for transfer on the second photosensitive drum 31 (2) is allowed, for example, 6 μA at a normal time with a tolerance of 50%. Then, since the leakage current Ileak (1) of the first photosensitive drum 31 (1) immediately before the sheet 25 passes is 3 μA or less, the transfer current is within the allowable range, and no image omission was observed. .
[0031]
○ Comparative Example Basically the same apparatus configuration as in the example, and in this example, a chloroprene belt is used as the conveyor belt,
・ Rbelt (m) = 6.3 (Log Ω)
・ Rbelt (s) = 9.0 (Log Ω)
・ Rsh = 11.0 (Log Ω)
-Rpr = 8.0 (Log Ω).
Therefore, when calculating for Ra and Rb,
Ra = 25.3 (Log Ω),
Rb = 17.0 (Log Ω).
∴Rb / (Ra + Rb) = 0.40 <50/100
It becomes.
In this example, for example, in the situation shown in FIG. 4A, the transfer current Itr (2) necessary for transfer on the second photosensitive drum 31 (2) is allowed, for example, 6 μA at a normal time with a tolerance of 50%. However, since the leakage current Ileak (1) of the first photosensitive drum 31 (1) immediately before the sheet 25 passes exceeds 3 μA, the transfer current Itr (2) <3 (= 6-3) μA. Image loss occurs.
[0032]
Embodiment 2
FIG. 8 shows an outline of the image forming apparatus according to the second embodiment.
The basic configuration of the image forming apparatus according to the present embodiment is substantially the same as that of the first embodiment, but unlike the first embodiment, the impedances of the photosensitive drums 31 (1) to 31 (4) are respectively set. In the case of Rpr (1) to Rpr (4),
Rpr (4) <Rpr (3) <Rpr (2) <Rpr (1)
It comes to satisfy.
[0033]
Therefore, according to the present embodiment, for example, in the situation shown in FIG. 4A, when the rear end of the sheet 25 is located immediately after passing the first photosensitive drum 31 (1), the second A leakage current tends to flow from the transfer portion of the photosensitive drum 31 (2) toward the photosensitive drum 31 (1). However, since the impedance of the first photosensitive drum 31 (1) is high, Leakage current hardly flows, and the transfer current in the transfer portion of the second photosensitive drum 31 (2) is sufficiently secured.
Furthermore, since the impedance of the photosensitive drum 31 located on the upstream side is always high at the transfer portion of the photosensitive drum 31 located on the downstream side, the leakage current is effectively suppressed.
Therefore, no image omission is observed at a distance corresponding to the pitch between the photosensitive drums from the rear end of the sheet due to the leakage current.
[0034]
Embodiment 3
FIG. 9 shows an outline of the image forming apparatus according to the third embodiment. Components similar to those in the first embodiment are denoted by the same reference numerals as those in the first embodiment, and detailed description thereof is omitted here.
The basic configuration of the image forming apparatus according to the present embodiment is substantially the same as that of the first embodiment, but unlike the first embodiment, a switch 52 (specifically, connected in series to each transfer bias power source 51). 52 (1) to 52 (4)) are individually controlled by the transfer bias controller 110, and the photosensitive drums 31 (31 (1) to 31 (4)) are interlocked with the switch 52. The switch 53 (specifically, 53 (1) to 53 (4)) is grounded.
[0035]
Therefore, according to the present embodiment, as shown in FIGS. 9 and 10, the transfer bias control device 110 turns on the switches 52 and 53 in synchronization with the on timing of the belt entrance sensor 101, and the transfer roll 50. A transfer bias is applied to (50 (1) to 50 (4)).
Thereafter, the sheet 25 is sequentially conveyed to the transfer portion of each photosensitive drum 31. The sheet 25 is turned off for a predetermined time t1 (corresponding to the timing at which the rear end of the sheet 25 passes) from the belt entrance sensor 101. The switch 52 (1), 53 (1) corresponding to the transfer portion of the photosensitive drum 31 (1) is turned off after the passage of the trailing end of the photosensitive drum 31 (1).
[0036]
In this state, since the photosensitive drum 31 (1) changes from the ground state to the floating state, the impedance of the photosensitive drum 31 (1) becomes infinite.
For this reason, there is almost no leakage current from the transfer portion of the second photosensitive drum 31 (2) toward the photosensitive drum 31 (1), and the transfer current is sufficiently secured.
Further, the sheet 25 corresponds to a predetermined time t2, t3, t4 (the timing at which the rear end of the sheet 25 passes the photosensitive drums 31 (2), 31 (3), 31 (4) from the off timing of the belt entrance sensor 101. ) After the lapse of time, the switches 52 (2) to 52 (4) and 53 (2) to 52 (4) corresponding to the transfer portions of the photosensitive drums 31 (2), 31 (3) and 31 (4) are turned off. .
As a result, the photosensitive drums 31 (2) to 31 (4) positioned immediately before the sheet 25 passes are sequentially changed from the grounded state to the floating state, and thus the impedances of the photosensitive drums 31 (2) to 31 (4) are Become infinite.
For this reason, the second and third photosensitive drums 31 (2) and 31 (3) located immediately downstream from the transfer portions of the third and fourth photosensitive drums 31 (3) and 31 (4) are directed. There is almost no leakage current, and a sufficient transfer current is secured.
Therefore, no image omission is observed at a distance corresponding to the pitch between the photosensitive drums from the rear end of the sheet due to the leakage current.
In the present embodiment, the belt exit sensor 102 functions as a jam sensor.
[0037]
In this embodiment, the present invention is not limited to the above-described mode. For example, the on timing of the switch 52 (transfer bias on timing) is synchronized with the passage state (nip state) of the sheet 25 with respect to each photosensitive drum 31. In addition, the photosensitive drum 31 may be grounded only in conjunction with the ON timing of the switch 52.
[0038]
【The invention's effect】
As described above, according to the present invention, since the leakage current to the adjacent image carrier immediately before the trailing edge of the sheet passes is suppressed and a sufficient transfer current is ensured, the high resistance sheet Can effectively prevent image omission at a distance corresponding to the pitch between the image carriers from the rear end of the sheet.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram showing an outline of an image forming apparatus according to the present invention.
FIG. 2 is an explanatory diagram showing an overall configuration of Embodiment 1 of an image forming apparatus to which the present invention is applied.
FIG. 3 is an explanatory diagram of a main part of the first embodiment.
FIGS. 4A and 4B are explanatory diagrams illustrating a sheet conveyance state process. FIGS.
FIG. 5 is an explanatory diagram showing a specific example of the system design of each transfer unit used in the first embodiment.
6A is an explanatory diagram showing an example of a method for measuring the resistance Rbelt (m) in the belt thickness direction, and FIG. 6B is an explanatory diagram showing an example of a method for measuring the resistance Rbelt (s) in the “circumferential direction + thickness direction” of the belt. It is.
7A is an explanatory diagram showing an example of a method for measuring the resistance Rsh in the thickness direction of the sheet, and FIG. 7B is an explanatory diagram showing an example of a method for measuring the resistance (impedance) Rpr in the thickness direction of the photosensitive drum.
FIG. 8 is an explanatory diagram illustrating a main part of an image forming apparatus according to a second embodiment.
FIG. 9 is an explanatory diagram illustrating a main part of an image forming apparatus according to a third embodiment.
FIG. 10 is a timing chart illustrating an example of a control operation of the transfer bias control apparatus according to the third embodiment.
FIG. 11 is an explanatory diagram illustrating an example of a conventional tandem type image forming apparatus.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 (1 (1) -1 (4)) ... Image carrier, 2 ... Sheet conveyance body, 3 ... Sheet, 4 (4 (1) -4 (4)) ... Transfer means, 5 ... Bias application means, 6 ... Leakage suppression means

Claims (4)

A plurality of image carriers that carry each color component image are arranged in parallel, and a sheet carrier that is circulated and conveyed to a portion that faces each image carrier, while a sheet carrier that faces each image carrier In the image forming apparatus, transfer means to which a transfer bias is applied is provided on the back side of each of the image forming apparatuses, and each color component image formed on each image carrier is sequentially transferred to a sheet held on the sheet conveying body. ,
The number of image carriers is n, and the rear end of the sheet passes through the j-1 (j: 2 to n) th image carrier, and the jth image carrier, the j- 1th image carrier, , The transfer inflow current from the transfer means corresponding to the jth image carrier is Itotal (j), and the transfer current that actually flows to the jth image carrier is Itr (j), j The leakage current flowing to the first image carrier side is Ileak (j-1), and the ratio of the leakage current Ileak (j-1) out of the transfer inflow current Itotal (j) is the allowable tolerance α (0 ≦ α ≦ 1)
An image forming apparatus comprising a leak suppression unit that satisfies a condition of | α · Itotal (j) | ≧ | Ileak (j−1) |. The image forming apparatus according to claim 1.
The leakage suppression means contributes to the system resistance Ra (j) and the leakage current Ileak (j-1) between the jth image carrier contributing to the transfer current Itr (j) and the transfer means corresponding thereto. When the system resistance between the transfer means corresponding to the j-th image carrier and the (j-1) -th image carrier is Rb (j),
An image forming apparatus characterized by satisfying Rb (j) / (Ra (j) + Rb (j)) ≧ (1-α). The image forming apparatus according to claim 1.
When the impedance of the jth image carrier is Rpr (j), the leak suppression means
An image forming apparatus characterized by satisfying Rpr (j) <Rpr (j-1). The image forming apparatus according to claim 1.
The leak suppression means is a bias application means for applying a transfer bias to each transfer means, and is synchronized with the timing at which the trailing edge of the sheet passes the j-1st image carrier. The image forming apparatus is characterized in that the transfer bias corresponding to is turned off and the ground state of the image carrier is floated.

Images