JPH11272090A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device which is inexpensive, a small and simple structure, reduces image irregularities, and ensures stable transferability. SOLUTION: A pre-transfer guide 40, with which a transfer material is brought into contact further upstream than a transfer position T in a transfer- material carrying direction, is made of a conductive material. A pre-fixing guide plate 63, with which the recording sheet P is brought into contact further downstream than the transfer position T in the carrying direction before the front end of the recording sheet P in the carrying direction passes through a fixing position F and before its rear end in the carrying direction moves apart from the pre-transfer guide 40, is also made of a conductive material. Further, a casing 64 is also made of a conductive material. A casing 65 and the pre-transfer guide plate 40 are conductively connected via leakage-current feedback lines 71 and 72, and a resistance 81 A is interposed between the leakage-current confluence point 73 of the leakage-current feedback lines 71 and 72 and the ground.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an image forming apparatus such as a laser printer and a copying machine, and more particularly to an improvement in a technique for electrostatically transferring a toner image onto a transfer material.

[0002]

2. Description of the Related Art For example, in a laser printer, as shown in FIG. 11, an image is reproduced by an electrophotographic method based on image data received from a personal computer or the like. The light beam emitted from the diode is light-modulated with the received image data, and the peripheral surface of a photosensitive drum (image carrier) 210 driven and rotated by the light beam is exposed and scanned to form an electrostatic latent image. You. The electrostatic latent image is developed by receiving a supply of toner from the developing device 240, thereby forming a toner image on the peripheral surface of the photosensitive drum.

On the other hand, in synchronization with this image forming operation, copy paper or O
A recording sheet (transfer material) P of a predetermined size such as an HP sheet
Is transported from a paper feed cassette (not shown) to a transfer position T directly below the photosensitive drum 210 via a registration roller 300 and a pre-transfer guide plate 400. In this transfer position T, by an electrostatic force of a corona transfer device 25F which transfer voltage V _T is applied from the transfer DC constant voltage power supply 250, the photosensitive drum 2
The toner image formed on the recording sheet P is drawn onto the recording sheet P and is directly transferred. Thereafter, the recording sheet P on which the toner image has been transferred is conveyed to the fixing device 600 via the transfer / fixing guide plate 500, and heat at a transfer position F between the upper fixing roller 610 and the lower fixing roller 620 of the fixing device 600. The toner particles on the surface of the recording sheet are fused and fixed to the surface of the recording sheet by the pressure and pressure.

[0004] In order to ensure that the recording sheet P can pass through the transfer position T even when the recording sheet P is weak, such as thin paper or wet paper,
Guide plate 400 before transfer, guide plate 500 between transfer and fixing
Are disposed immediately upstream and downstream of the transfer position T, respectively. In order to reduce the size of the apparatus, the lower fixing roller 620 of the fixing device 600 is
The recording sheet P is disposed close to the position where the distance along the transfer material transport direction from the recording sheet P is shorter than the length of the recording sheet P in the transport direction.

In a low-humidity environment, that is, in a dry environment, electric charges are accumulated on the recording sheet P due to friction during conveyance of the recording sheet P to the transfer position T. If the recording sheet P is transported to the transfer position T in a state where the electric charge is accumulated, the accumulated electric charge may be sparked on the photosensitive drum 21 and adversely affect the transfer of the toner image. In addition, electric charge that has been dielectrically polarized during transfer of the toner image remains accumulated on the recording sheet P, or friction between the transfer sheet and the guide plate 500 during transfer of the recording sheet P to the fixing device 600. Charge is also accumulated on the recording sheet P. When fixing is performed in a state where the electric charges are accumulated, the recording sheet P is wound around the upper fixing roller 610 and the lower roller 620, and the toner to be fixed adheres to the fixing roller. (Referred to as “electric offset”).

Accordingly, the charge accumulated on the recording sheet P is released to the ground before the transfer, and
Before passing through the fixing position F, it is necessary to discharge the electric charge accumulated therein to the ground. As a countermeasure, the pre-transfer guide plate 400 is formed of a conductive material, and a sheet contact member in the fixing device 600, for example, the lower fixing roller 62
It is conceivable that 0 is formed of a conductive material and these are directly connected to the ground.

On the other hand, a recording sheet whose surface has a low resistance value due to moisture absorption in a high humidity environment or because of a surface coat layer (a recording sheet whose surface has a low resistance value is hereinafter referred to as
It is called "low resistance paper". ) May be printed through paper. In this case, if the pre-transfer guide plate 400 and the lower fixing roller 620 are configured to be directly connected to the ground, the transfer current to be applied to the transfer of the toner is reduced by the low-resistance paper and the pre-transfer guide plate 400 on both sides. Lower fixing roller 6
A large amount leaks to the ground via each of them 20 (the current leaking to the ground without contributing to the toner transfer in this manner is hereinafter referred to as “leakage current”). FIG. 12 shows the relationship between the transfer surface T between the transfer position T and the pre-transfer guide plate 400 when the pre-transfer guide plate 400 is directly connected to the ground, and the leakage current (sequence 1) to the pre-transfer guide plate 400 side. Is shown. This leakage current varies depending on the internal resistance of the corona transfer unit 25F, the transfer current flowing from the corona transfer unit 25F, and the like. However, when the surface resistance of the recording sheet between the transfer position T and the pre-transfer guide plate 400 decreases, the leakage current increases. Become. Such a situation is related to the relationship between the paper surface resistance between the transfer position T and the lower fixing roller 620 when the lower fixing roller 620 is directly connected to the ground, and the leakage current to the lower fixing roller 620 (series 2). The same is true for As a result, the transfer voltage V _T is reduced, the photosensitive drum 2
Insufficient transfer current to flow to the 10 side causes transfer failure.

Therefore, in the conventional laser printer, the resistance element 100 is interposed between the pre-transfer guide plate 400 and the ground, and the resistance element 200 is interposed between the pre-transfer guide plate 400 and the lower fixing roller 620 of the fixing device 60. The leakage current is reduced by the resistance elements 100 and 200, so that insufficient transfer is eliminated. Such a technique is disclosed in Japanese Patent Publication No. 5-29910.

In the specification of the present application, in addition to a resistor, a member exhibiting a blocking effect against a DC current, such as a Zener diode or a paristor, is called a "resistor element". It is used as a concept including the internal resistance of the wire rod and the internal resistance of the member to which the resistance element is connected.

[0010]

However, in the conventional printer, when an image is reproduced on low-resistance paper, before and after the leading end of the recording sheet in the conveying direction enters the lower fixing roller 620, or at the rear end of the recording sheet in the conveying direction. Is the guide plate 40 before transfer.
The amount of leakage current changes significantly before and after the value of 0 is separated.
For this reason, there was a great difference in the transfer efficiency of the toner image within one recording sheet, and noticeable density unevenness occurred in the reproduced image.

In order to solve this problem, the internal resistance of the DC constant voltage power supply for transfer 250 is reduced so that the leading end of the low-resistance paper enters the lower fixing roller 620 before and after, and the rear end of the low-resistance paper comes before the transfer. Even if the leakage current changes significantly before and after the guide plate 400 is separated, it is conceivable to make the transfer current to the photoconductor drum side substantially constant. However, in this case, the transfer DC constant voltage power supply 250 becomes expensive. Further, even if the internal resistance of the transfer DC constant voltage power supply 250 is reduced,
Since the large internal resistance of the corona transfer device 25F is added, the transfer current to the photoconductor drum side cannot be made substantially constant, and the transfer current to the photoconductor drum side decreases in inverse proportion to the leakage current. Would.

Also, before the leading end of the recording sheet in the conveying direction enters the lower fixing roller 620, the fixing device is disposed further downstream so that the trailing end of the recording sheet in the conveying direction is separated from the pre-transfer guide plate 400. Although it is conceivable to provide a recording sheet, the transport path of the recording sheet must be lengthened accordingly.
The device becomes larger.

Such a problem also occurs when the toner image is once transferred to the intermediate transfer member and then the toner image is re-transferred from the intermediate transfer member to the recording sheet by an electrostatic transfer method.

The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide an image forming apparatus capable of reducing image unevenness and securing stable transferability with a low-cost, small-sized and simple configuration. Aim.

[0015]

In order to solve the above-mentioned problems, an image forming apparatus according to the present invention provides an image forming apparatus that electrostatically transfers a toner image formed on an image carrier onto a transfer material conveyed at a predetermined transfer position. An image forming apparatus for forming an image by transferring the image and fixing the image at a predetermined fixing position, wherein the first conductive member contacts a transfer material upstream of the transfer position in a transfer material transport direction, and the recording device includes: Before the leading end of the sheet in the conveying direction passes through the fixing position, and before the trailing end of the sheet in the conveying direction separates from the first conductive member, the second contacting the transfer material downstream from the transfer position in the conveying direction. And a conductive member, wherein the first conductive member and the second conductive member are connected to ground via a common first resistance element.

Further, in the image forming apparatus according to the present invention, a second resistance element interposed between the first conductive member and the first resistance element; A third resistive element interposed between the resistive element and the third resistive element.

Further, according to the image forming apparatus of the present invention,
Each of the resistance values of the first to third resistance elements is R₁ , R
_Two , R_Three And the combined resistance value of the first to third resistance elements
To R ₀ During transfer, the resistance value becomes
(1) so that equation (2) and equation (3) are satisfied at the same time.
It is characterized by being set. R_Two ≧ R₁ ... (1) R_Three ≧ R₁ … (2) 1GΩ ≧ R₀ ≧ 100MΩ (3)

Further, in the image forming apparatus according to the present invention, the fixing device for fixing the toner image electrostatically transferred onto the transfer material comprises: a pair of upper fixing roller and lower fixing roller forming the fixing position; And a front guide plate for guiding the transfer material to pass through the fixing position, wherein at least one of the upper fixing roller, the lower fixing roller, and the pre-fixing guide plate is used as the second conductive member. .

[0019]

(Embodiment 1) Hereinafter, an example in which an embodiment of an image forming apparatus according to the present invention is applied to a laser printer will be described.

(Embodiment 1) FIG. 1 is a diagram showing the overall configuration of a laser printer according to Embodiment 1. As shown in FIG. 1, the laser printer 1 reproduces an image by electrophotography based on image data received from a personal computer or the like, and includes an exposure scanning unit 10, an image forming unit 20, and a fixing unit. Unit 60 and control unit 90
Etc.

The exposure scanning section 10 includes a laser diode (not shown), a polygon mirror, a scanning lens, and the like. The laser diode receives a drive signal from the control unit 90 and emits light modulated laser light. The modulated laser light is reflected and deflected by a mirror surface of a polygon mirror that is driven to rotate at a constant speed, passes through a scanning lens, and exposes and scans the surface of the photosensitive drum 21 of the image forming unit 20.

The image forming section 20 moves at a predetermined system speed determined by a driving device (not shown) at a predetermined system speed.
The photosensitive drum 21 is rotated around the photosensitive drum 21. The eraser lamp 22 is provided around the photosensitive drum 21. The eraser lamp 22 is rotated in the direction of arrow b in synchronization with the rotation of the photosensitive drum 21. The charging charger 23, the developing device 24, and the photosensitive drum 21 are pressed upward from directly below, and are driven to rotate in the direction of arrow c in synchronization with the rotation of the photosensitive drum 21, so that the transfer position between the charging charger 23 and the photosensitive drum 21. A transfer charger 25 for forming T is formed.

The charging charger 23 is composed of a semiconductive brush, and has a DC constant voltage power supply 23 for the charging charger.
At the predetermined timing from 0, a predetermined transfer voltage VG is supplied and the surface of the photosensitive drum 21 is uniformly charged while the transfer residual toner on the surface of the photosensitive drum 21 is wiped off, and the toner is absorbed and cleaned. It also has a cleaning function. For this reason, a so-called cleaner-less system is provided.

The transfer charger 25 is made of EPDM (Ethyl) in which a conductive material such as carbon black is dispersed.
ne-Propylene-Diene Methyl
ene linkage; ethylene-propylene-diene terpolymer), Si rubber, NBR (Nitril)
e-Butadiene Rubber (nitrile-butadiene-rubber) is formed of a semiconductive urethane foam roll formed by firing a resin material. It is designed to be transported at a speed. The transfer charger 25 has a DC constant voltage power supply 25 for transfer.
0 A predetermined transfer voltage V _T is applied at a predetermined timing from this transfer voltage V _T, is adapted to transfer the toner image on the photosensitive drum 21 to the recording sheet P.
In the transfer charger 25, the transfer current is not diffused to a position other than the transfer position T which occurs in the case of the corona discharge transfer charger, and the transfer current can be concentrated at the transfer position T without waste. since it is also possible to make transfer time, with a low transfer voltage V _T of the transfer charger of a corona discharge type, so that significantly high transfer efficiency can be obtained.

A pre-transfer guide plate 40, a paper feed unit (not shown), and the like are provided upstream of the transfer position T in the recording sheet conveyance direction. The paper feed unit includes a paper feed cassette for stacking and storing recording sheets P of a predetermined size,
And a timing roller 30 for setting the timing for feeding the recording sheet P onto the pre-transfer guide plate 40. The pre-transfer guide plate 40 is formed of a good conductive material such as SECC (electroplated steel plate).
The recording sheet P is guided to a transfer position while being in contact with the recording sheet P fed out from the printer.

A transfer / fixing guide plate 50, a fixing device 60, and the like are provided downstream of the transfer position T in the recording sheet conveyance direction. The transfer / fixing guide plate 50 is formed of a good conductive material such as SECC, is floating from the ground, and contacts the recording sheet P conveyed from the transfer position T. The fixing device 60 is guided.

The fixing device 60 is driven to rotate in accordance with the conveyance speed of the recording sheet, and is conveyed via a pair of upper fixing roller 61 and lower fixing roller 62 forming the fixing position F, and a transfer / fixing guide plate 50. Pre-fixing guide plate 6 for guiding the recording sheet P coming into contact with the fixing position F while contacting it.
3, a thermistor 64 for detecting the surface temperature of the upper fixing roller 61 heated by a heater (not shown), a casing 65 surrounding these, and the like.

The upper fixing roller 61 has a structure in which a surface of an aluminum core is coated with a fluorine resin and further subjected to a conductive treatment, and a heater is arranged at a position of an axis. The lower fixing roller 62 is formed by attaching a fluororesin tube to silicone rubber,
The recording sheet P is pressed from directly below and nipped at the fixing position F while being conveyed. For this reason, the fixing time can be increased by this nip, the thermal conductivity can be increased, and the recording sheet can be transported at the system speed while reliably fixing the toner on the recording sheet. The pre-fixing guide plate 63 and the casing 65 are formed of a good conductive material such as SECC, and are arranged to be conductive.

The casing 65 and the pre-transfer guide plate 40 are electrically connected to each other via leakage current feedback lines 71 and 72, and between the leakage current junction 73 of the leakage current feedback lines 71 and 72 and the ground. , And a resistor 81A. The resistance value of the resistor 81A is 10 ⁸ Ω or more in order to secure the transferability of the toner image even in a high-humidity environment low-resistance paper.
And the occurrence of discharge noise of triboelectric charge in a low humidity environment,
In order to prevent winding of the recording sheet and occurrence of fixing electrostatic offset, the range is set to 10 ⁹ Ω or less, and 400 MΩ in the first embodiment, and further, image unevenness does not occur. The details will be described later.

The photosensitive drum 21 serving as an image bearing member is irradiated with an eraser lamp 22 before being subjected to the above-described exposure, and is neutralized. Thereafter, the photosensitive drum 21 is uniformly charged by a charging charger 23 and removes residual toner on the surface of the photosensitive member. When the photosensitive drum 21 is exposed in such a charged state, an electrostatic latent image is formed on the photosensitive member on the surface of the photosensitive drum 21 and is developed by the developing device 24 to form a toner image. You.

On the other hand, the recording sheet P accommodated in the sheet cassette is guided to the timing roller 64, and the timing roller 64 synchronizes with the image forming operation on the photosensitive drum 21 on the guide plate 40 before the guide transfer. To the transfer position T while contacting the guide plate 40 before the guide transfer.
Transported to

When the recording sheet P reaches the transfer position T,
DC constant voltage power supply 2 for transfer in accordance with an instruction from control unit 90
From 50, an appropriate transfer voltage is applied to the transfer charger 25, and the toner image on the photosensitive drum 21 is electrostatically transferred onto the recording sheet P by the appropriate transfer voltage VT between the transfer charger 25 and the photosensitive drum 21. The recording sheet P to which the toner image has been transferred is transferred from the transfer position T to the transfer / fixing guide plate 5.
The toner is heat-fixed at a transfer position F between the fixing roller 61 and the fixing roller 62 while contacting the guide 63 while being in contact with the transfer / fixing guide plate 50. Thereafter, the image data is discharged onto a discharge tray (not shown), whereby the image formation based on the received image data is completed.

The control unit 90 internally stores a CPU, a ROM in which programs for controlling the various units of the printer and initial values of control variables are stored, and temporarily stores received image data and current values of control variables. And a RAM for providing a work area. The CPU reads a necessary program from the ROM, and controls each operation of the image forming unit 20, the paper feeding unit, and the like in a unified manner while taking timing. Further, the CPU constantly monitors the surface temperature of the upper fixing roller 61 detected by the thermistor 64 of the fixing device 60, and controls power supply to the heater so that the surface temperature becomes a temperature suitable for toner fixing. It has become.

The inventor of the present application examined the laser printer 1 and the leakage current feedback lines 71 and 72 and the resistor 81 from the laser printer 1 in order to examine the effect of the configuration of the first embodiment.
A is removed, and a resistor 100 (resistance = 400 MΩ) is provided between the pre-transfer guide plate 40 and the ground, and the casing 65
Resistor 200 (resistance value = 400 MΩ) between ground
Were connected to each other, and a comparison test with a conventional one was performed.

FIG. 2 is a diagram showing an equivalent circuit relating to the transfer current in the first embodiment, and FIG. 3 is a diagram showing an equivalent circuit relating to the transfer current in the conventional case. FIG.
4 (a) to 4 (d) are diagrams showing a comparison between the transfer and fixing operations of the two. FIG. 4 (a) to FIG.
(H) shows the case of the first embodiment, respectively.

In FIGS. 2 and 3, R _T represents the internal resistance of the transfer DC constant voltage power supply 250 and the transfer device 25, and R _T represents the internal resistance of the transfer device 25.
_{Sheet 0} shows the internal resistance of the recording sheet on the photosensitive drum side, and R _{sheet 1}
Is the surface resistance of the recording sheet before the transfer of the guide plate, R _{sheet 2} is the surface resistance of the recording sheet before the fixing of the recording sheet, R _{P / C} is the internal resistance of the photosensitive drum and toner, and R ₀ is the resistance 81A.
, R ₁₀₀ indicates the resistance 100, and R ₂₀₀ indicates the resistance 200. Further, I _T denotes a transfer current flowing from the transfer charger 25, I _{P / C} denotes a transfer current flowing to the photoconductor drum side, I ₁ and I ₁₀₀ denote leakage currents flowing to the pre-transfer guide plate 40 side, and I ₂ and I ₂ : I ₂₀₀ indicates a leakage current flowing to the casing 65 side, and I ₀ indicates a combined leakage current flowing to the resistor 81A.

[0037] In the case of image formation on the recording sheet P in a low humidity environment, since the surface resistance of the recording sheet is high, the first embodiment and both of the conventional, continuously during imaging R _{sheet 1,} R
_{Sheet 2} , R _{sheet 0} > R _{P / C} , no leakage current (I ₁ ,
I ₁₀₀ , I ₂ , I ₂₀₀ , I ₀ = 0) and I _T = I _{P / C} , and all of the transfer current was used for toner transfer, so that a reproduced image without unevenness could be obtained. Further, the triboelectric charge and the dielectrically polarized electric charge charged on the recording sheet are released from the pre-transfer guide plate 40 side before the transfer, and are released from the pre-fixation guide plate 63 and the casing 65 side before the fixing, so that they are accumulated. No spark of the charged electric charge to the photosensitive drum 21 occurs,
Neither winding of the recording sheet nor fixing electrostatic offset occurred.

In the case of forming an image on a recording sheet P in a high-humidity environment, that is, on a low-resistance paper, the surface resistance of the recording sheet is low. R _{sheet 1} , R _{sheet 2} , R _{sheet 0} <R _{P / C} , and a leakage current occurs (I _T = I _{P / C} + I ₁ + I ₂ , I _T = I _{P / C}
+ I ₁₀₀ + I ₂₀₀ ).

In synchronization with the operation of forming the toner image on the surface of the photosensitive drum, the leading end of the recording sheet P in the conveying direction contacts the pre-fixing guide plate 40 and the transfer of the toner image is started. Before the leading end comes into contact with the pre-fixing guide plate 63, both of the leakage currents are the pre-transfer guide plate 4.
4 leaks to the ground only from the 0 side (FIG. 4 (a), FIG.
(E)). Transfer position T relating to leakage current at this time
The combined resistance value between the ground and the ground is 400 MΩ. More precisely, the transfer position T and the pre-transfer guide plate 40
However, the resistance of the recording sheet P between the transfer position and the pre-fixing guide plate 63 and the R _{sheet 1} are also added, but this resistance is about 5 MΩ, which is smaller than the resistance 81A. Small enough. Therefore, even if this resistance value is added, there is almost no difference in the value of the leakage current. Therefore, the description will be made with the R _{sheet 1} omitted.

Next, after the leading edge of the recording sheet P in the transport direction contacts the pre-fixing guide plate 63 and before the trailing edge of the recording sheet P in the transport direction separates from the pre-transfer guide plate 40, the leakage current of both of them is It leaks to the ground from both sides of the pre-transfer guide plate 40 side and the pre-fixing guide plate 63 / casing 65 side (see FIGS. 4B and 4E). At this time, the combined resistance value between the transfer position T and the ground with respect to the leakage current is 200 MΩ in the conventional case, and 400 MΩ in the first embodiment. The position on the recording sheet where the transfer charger 25 nips when the leading end of the recording sheet P in the transport direction contacts the pre-fixing guide plate 63 is shown in FIG.
(D), and is indicated by the symbol α in FIG. Furthermore, accurately, but also applied resistance component R _{sheet 2} of the recording sheet between the transfer position and the fixing front guide plate 63, R _{sheet 1} in this case
Therefore, the description will be made with the R _{sheet 2} omitted.

For this reason, in the conventional case, the combined resistance value is equal to the parallel value of the resistors 100 and 200 at the nip position α.
The leakage current is doubled (I ₁₀₀ + I ₂₀₀ ) because it is halved to 0 MΩ (see FIG. 4D). As a result, the transfer current for transferring the toner is reduced by that amount (I _{P / C} = I _T − (I ₁₀₀
+ I ₂₀₀ )), and the image density decreases. On the other hand, in the first embodiment, the combined resistance value is maintained at 400 MΩ only by the resistance 81A (see FIG. 4H), and the leakage current is kept constant (I ₀ = I _1). + I ₂ ). As a result,
Transfer current for transferring the toner does not change _{(I P / C = I T} -
(I ₁ + I ₂ )), and the image density is kept constant.

Next, after the trailing edge of the recording sheet P in the transport direction separates from the pre-transfer guide plate 40, the leakage current leaks to the ground only from the pre-fixing guide plate 63 and the casing 65 side. At this time, the combined resistance value between the transfer position T and the ground with respect to the leakage current is both 400 MΩ. The position on the recording sheet where the transfer charger 25 nips when the trailing end of the recording sheet P in the transport direction separates from the pre-transfer guide plate 40 is indicated by the symbol β in FIGS. 4D and 4H. .

Therefore, in the conventional case, the nip position β is
At the boundary, the combined resistance doubles to 400 MΩ
(See FIG. 4D), the leakage current decreases (I
₂₀₀). As a result, the transfer current for transferring the toner
Increase (I_{P / C} = I_T − (I ₁₀₀ + I₂₀₀ )), Image density
The degree rises. On the other hand, in the case of Embodiment 1,
Has a combined resistance value of only 81A and maintains 400MΩ
(See FIG. 4 (h)), and the leakage current is kept constant.
(I₀ = I_Two ). As a result, the transfer current for transferring the toner
Does not change (I_{P / C} = I_T -I_Two ), Constant image density
Will be kept.

Therefore, during printing on a single recording sheet, remarkable image unevenness was generated because the resistance value doubled in the conventional case, but in the first embodiment, such a resistance change was observed. A value change does not occur at all, can be kept constant, and image unevenness can be completely eliminated with a simple configuration.

(Embodiment 2) FIG. 5 is a diagram showing a configuration of a laser printer 2 according to Embodiment 2 of the present invention, in which parts corresponding to those in Embodiment 1 are assigned the same reference numerals and description thereof is omitted. I do.

Here, the laser printer 1 of the first embodiment
In, although image unevenness could be eliminated during printing of one sheet on low-resistance paper in a high-humidity environment, the image density decreases due to the effect of leakage current as compared with a reproduced image in a low-humidity environment. Therefore, in the laser printer 2,
The transfer voltage _VT is controlled by feeding back the leakage current to the transfer DC constant voltage power supply 251 without directly escaping to the ground.

FIG. 6 is a circuit diagram showing a specific configuration of the transfer DC constant voltage power supply 251 shown in FIG. The transfer DC constant-voltage power supply 251 shown in the figure includes an AD / DC converter 253 for converting AC from a commercial AC power supply 252 to DC of a desired voltage, and a transistor 254 for DC current control.
When the capacitor 255 for smoothing the transfer voltage V _T
A resistor 256 having a low resistance value for converting a leak current into a leak voltage and detecting the leak current; a constant reference voltage;
Comparator 2 which compares the detected voltage with the detected voltage
57, and a control unit 258 that controls the output current of the transistor 254 based on the comparison result of the comparator 257.

When the surface resistance of the recording sheet P increases due to the low humidity environment, the leakage current decreases. As a result, the detection voltage decreases, becomes lower than the reference voltage, and the comparison result of the comparator 257 decreases. Therefore, the control unit 258 decreases the output current of the transistor 254. Therefore,
The transfer voltage _VT becomes low, and an appropriate image density can be obtained.

On the other hand, when the surface resistance value of the recording sheet P decreases due to the high humidity environment, the leakage current increases. As a result, the detection voltage increases, becomes higher than the reference voltage, and the comparison result of the comparator 257 increases. Therefore, the control unit 258
Increases the output current of transistor 254. Accordingly, the transfer voltage V _T is increased, it is possible to obtain the same image density as low-humidity environment.

(Embodiment 3) FIG. 7 is a diagram showing a configuration of a laser printer 3 according to Embodiment 3 of the present invention. Is omitted.

In the laser printers 1 and 2 of the first and second embodiments, the pre-transfer guide plate 40 and the pre-fixation guide plate 63 and the casing 65 are grounded via a common resistor 81A. . This is the guide plate 4 before transfer.
0 and the guide plate 63 and the casing 65 before fixing have resistance 8
Floating at 1A means that it is easier to pick up disturbance noise. Therefore, in a case where the recording sheet is removed due to a paper jam, if the operator's hand approaches or touches the pre-fixing guide plate 63 or the casing 65, the charged charges accumulated in the operator are removed. Leakage easily occurs in the thermistor 64, and the CPU of the control unit 90 monitoring the temperature detection result of the thermistor 64 may malfunction or break. From this viewpoint, it is preferable to reduce the combined resistance as much as possible within a range in which image unevenness is not conspicuous.

Therefore, in the third embodiment, the pre-transfer guide plate 40, the pre-fixing guide plate 63 and the casing 6
5 are grounded via a common resistor 81B, a resistor 82 is interposed between the pre-transfer guide plate 40 and the resistor 81B, and a resistor 83 is interposed between the casing 65 and the resistor 81B. It is supposed to be. Also,
The resistance values of the above 81B, 82, and 83 are R ₁ , R
₂ , R _3, and the combined resistance value of 81 B, 82, 83 is R ₀
Is set such that the resistance value simultaneously satisfies the following equations (1), (2) and (3) during transfer.

R_Two ≧ R₁ ... (1) R_Three ≧ R₁ … (2) 1GΩ ≧ R₀ ≧ 100 MΩ (3) where R_Two≒ R_Three Is preferable, and particularly, R_Two = R_Three Is preferred
New In this embodiment, R_Two = R_Three = 250
MΩ, R₁ = 150MΩ, R ₀ = 400MΩ, 275M
And the combined resistance is within the range where image unevenness is not
It is designed to be as low as possible.

The inventor of the present application examined the laser printer 3 and the first embodiment to check the effect of the configuration of the third embodiment.
A comparative test with the conventional example used in the comparative test was performed.

FIG. 8 is a diagram showing an equivalent circuit relating to the transfer current in the third embodiment, and FIG.
FIG. 10 is a diagram showing a comparison of the fixing operation, in particular, FIGS.
FIG. 9D shows the case of the related art, and FIGS. 9E to 9H show the case of the third embodiment. In the case of forming an image on the recording sheet P in a low humidity environment, the data was the same as that in FIG.
That is, the case of forming an image on low-resistance paper will be described in detail.

In synchronization with the toner image forming operation on the surface of the photosensitive drum, the leading end of the recording sheet P in the transport direction comes into contact with the pre-fixing guide plate 40 and the transfer of the toner image is started. Before the leading end comes into contact with the pre-fixing guide plate 63, both of the leakage currents are the pre-transfer guide plate 4.
The leakage to the ground only from the 0 side (FIG. 9A, FIG.
(E)). Transfer position T relating to leakage current at this time
The combined resistance value between the ground and the ground is 400 MΩ.

Next, after the leading end of the recording sheet P in the transport direction comes into contact with the pre-fixing guide plate 63 and before the trailing end of the recording sheet P in the transport direction separates from the pre-transfer guide plate 40, both of the leakage currents are reduced. It leaks to the ground from both sides of the pre-transfer guide plate 40 side and the pre-fixing guide plate 63 / casing 65 side (see FIGS. 9B and 9E). At this time, the combined resistance value between the transfer position T and the ground with respect to the leakage current is 200 MΩ in the conventional case, and 275 MΩ in the third embodiment.

For this reason, in the conventional case, the combined resistance value is equal to the parallel value 20 of the resistors 100 and 200 at the nip position α.
Since the resistance is halved to 0 MΩ (see FIG. 9D), the leakage current is doubled. As a result, the transfer current for transferring the toner decreases correspondingly, and the image density decreases. On the other hand, in the third embodiment, since the resistors 81 and 83 are simply added in parallel and the resistor 81B is added in series, the combined resistance value is 2
It is reduced to only 75 MΩ (see FIG. 9 (h)), and the leakage current is only increased 1.45 times. As a result, the change in the transfer current for transferring the toner becomes small, and the image unevenness is less noticeable.

Next, after the trailing edge of the recording sheet P in the transport direction separates from the pre-transfer guide plate 40, the leakage current leaks to the ground only from the pre-fixing guide plate 63 and the casing 65 side. At this time, the combined resistance value between the transfer position T and the ground with respect to the leakage current is both 400 MΩ.

For this reason, in the conventional case, the combined resistance doubles to the value of 400 MΩ of the resistor 200 at the nip position β (see FIG. 9D), so that the leakage current is reduced. As a result, the transfer current for transferring the toner increases accordingly, and the image density increases. In contrast, in the case of the third embodiment, the combined resistance value is slightly increased to 400 MΩ (see FIG. 9H), and the leakage current is only slightly reduced. As a result, the change in the transfer current for transferring the toner becomes small, and the image unevenness is less noticeable.

On the other hand, from the viewpoint of preventing the charge accumulated in the operator from leaking to the thermistor 64, the conventional device has a lower combined resistance value between the nip positions α and β, and is desirable. Prevention cannot be compatible.

Therefore, during printing on a single recording sheet, remarkable image unevenness occurred because the resistance value doubled in the conventional case, but in the first embodiment, such a resistance change was observed. It is possible to make the image unevenness inconspicuous with a simple configuration with a small change in value, and also to prevent leakage to the thermistor 64.

(Modification) Although the image forming apparatus according to the present invention has been described based on the embodiment, it goes without saying that the content of the present invention is not limited to the above-described embodiment. Such a modified example is conceivable.

In the first to third embodiments, the toner image is directly transferred to the recording sheet. However, after the toner image is once transferred to a transfer belt or an intermediate transfer member, the toner image is re-transferred to the recording sheet by the electrostatic transfer method. You may make it transfer.

In the first to third embodiments, a resistor is used as a resistance element. However, another resistance element such as a Zener diode can be used, and the guide plate before transfer, the guide plate before fixing, the casing, and the like can be used. May be formed of a semiconductive material, and the leakage current may be reduced by the internal resistance.

In Embodiments 1 to 3, the transfer / fixing guide plate 50 is floated, but this may be connected to the leakage current feedback line 72. In addition, at least the peripheral surface of the upper fixing roller 61 and the lower fixing roller 62 is formed of a conductive material,
5 may be conducted.

In the first to third embodiments, a semi-conductive urethane roll is used. However, PFA (Poly (Fluoro-Al
koxy-ethylen); a tube coating such as an alkoxyethylene fluoride resin), a coating or the like, and lamination.

Further, in the first to third embodiments, the roller type transfer charger 25 is used, but instead of this, the rotating brush 25 shown in FIG.
A, the semiconductive blade 25B shown in FIG. 10B, the elastic blade 514 and the elastic film shown in FIG.
The transfer may be performed by a transfer charger such as a combination of a corona charger 25D and a push-up member 25E shown in FIG. 10D, and a non-contact transfer charger such as a corona discharger 25F can be used instead of the contact transfer. .

Although the charging charger 23 is provided with a cleaning function, the charging function may be narrowed down to a single charging function by a fixed brush, blade, film, roller, corona charger, or the like. In this case, if a cleaner is separately provided, Good.

Further, in the above embodiment, the present invention is applied to a laser printer. However, in addition to a digital or analog type color / monochrome copying machine, a tandem type color copying machine, a facsimile machine, a micro reader print, and a combination thereof. The present invention can also be applied to an image forming apparatus such as a printer.

[0071]

According to the image forming apparatus of the present invention, the toner image formed on the image carrier is electrostatically transferred onto a transfer material conveyed at a predetermined transfer position. An image forming apparatus for forming an image by fixing the recording sheet at a predetermined fixing position, wherein the first conductive member contacts the transfer material on the upstream side in the transfer material transport direction from the transfer position; A second conductive member that contacts a transfer material downstream of the transfer position in the transport direction before the leading end in the transport direction passes through the fixing position and before the trailing end in the transport direction separates from the first conductive member. And the first conductive member and the second conductive member are connected to the ground via a common first resistance element. Image forming equipment that reduces and ensures stable transferability It is possible to provide a.

Further, according to the image forming apparatus of the present invention, the second resistive element interposed between the first conductive member and the first resistive element, and the second conductive member and the second resistive element, And a third resistance element interposed between the first resistance element and the third resistance element, so that the combined resistance can be reduced as much as possible within a range in which image unevenness is not conspicuous, and paper jams occur and the recording sheet is printed. In the case of removal, even if the operator's hand approaches or touches the pre-fixing guide plate or the casing, the charged charges accumulated in the operator are prevented from leaking to the thermistor, and the thermistor is prevented from being charged. There is no possibility that the CPU of the control unit monitoring the temperature detection result malfunctions or breaks.

According to the image forming apparatus of the present invention,
Each of the resistance values of the first to third resistance elements is R
₁ , R ₂ , and R ₃ , and when the combined resistance value of the first to third resistance elements is R ₀ , the resistance values during transfer are expressed by the following equations (1), (2), and (3). Are set at the same time. R ₂ ≧ R ₁ (1) R ₃ ≧ R ₁ (2) ₁ GΩ ≧ R ₀ ≧ 100 MΩ (3) For this reason, the combined resistance can be reliably reduced within a range where image unevenness is not conspicuous.

Further, according to the image forming apparatus of the present invention, the fixing device for fixing the toner image electrostatically transferred on the transfer material comprises a pair of a fixing upper roller and a fixing lower roller forming the fixing position. And a front guide plate that guides the transfer material so as to pass through the fixing position. Since at least one of the upper fixing roller, the lower fixing roller, and the pre-fixing guide plate is used as the second conductive member, The charge on the recording sheet can be easily released to the ground, and the winding of the recording sheet around the roller and the fixing electrostatic offset can be reliably prevented.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an overall configuration of a laser printer according to a first embodiment of the present invention.

FIG. 2 is a diagram showing an equivalent circuit relating to a transfer current in the first embodiment.

FIG. 3 is a diagram showing an equivalent circuit relating to a transfer current in a conventional case.

FIG. 4 is a diagram illustrating a comparison between a conventional transfer and fixing operation.

FIG. 5 is a diagram showing a configuration of a laser printer 2 according to Embodiment 2 of the present invention.

FIG. 6 is a circuit diagram showing a specific configuration of the transfer DC constant voltage power supply 251 shown in FIG.

FIG. 7 is a diagram illustrating a configuration of a laser printer 3 according to Embodiment 3 of the present invention.

FIG. 8 is a diagram showing an equivalent circuit relating to a transfer current in the third embodiment.

FIG. 9 is a diagram showing a comparison of a transfer and fixing operation with a conventional one.

FIG. 10 is a view showing a modification of the transfer charger.

FIG. 11 is a diagram showing the overall configuration of a conventional laser printer.

FIG. 12 is a diagram illustrating a relationship between the surface resistance of a recording sheet and a leakage current.

[Explanation of symbols]

1, 2, 3 Laser printer 10 Exposure scanning unit 20 Image forming unit 21 Photoconductor drum 24 Developing device 25, 25A to 25E Transfer charger 40 Guide plate before transfer 50 Guide plate between transfer and fixing 60 Fixing unit 61 Upper fixing roller 62 Fixing Lower roller 63 Pre-fixing guide plate 64 Thermistor 65 Casing 71, 72 Leakage current feedback line 73 Leakage current junction 81A, 81B, 82, 83 Resistance 90 Control unit 230 DC constant voltage power supply for charging charger 250, 251 DC constant voltage for transfer Power supply T Transfer position F Fixing position P Recording sheet

Claims (4)

[Claims] An image forming apparatus for electrostatically transferring a toner image formed on an image carrier onto a transfer material conveyed at a predetermined transfer position, and fixing the toner image at a predetermined fixing position to form an image. A first conductive member that comes into contact with the transfer material upstream of the transfer position in the transfer material conveyance direction, and a front end of the recording sheet in the conveyance direction before passing through the fixing position, and a rear end of the conveyance direction in the conveyance direction. A second conductive member that contacts a transfer material downstream of the transfer position in a transport direction before separating from the first conductive member, wherein the first conductive member and the second conductive member are , The common first
An image forming apparatus, which is connected to the ground through the resistance element. 2. A second resistive element interposed between the first conductive member and the first resistive element, and a second resistive element interposed between the second conductive member and the first resistive element. An image forming apparatus, further comprising: a third resistance element. 3. The resistance value of the first to third resistance elements
To R₁ , R _Two , R_Three And the first to third resistors
The combined resistance value of the resistance element is R₀ During transfer, the resistance value is calculated by the following formulas (1), (2) and
(3) is set to satisfy at the same time
The image forming apparatus according to claim 2. R_Two ≧ R₁ ... (1) R_Three ≧ R₁ … (2) 1GΩ ≧ R₀ ≧ 100MΩ (3) 4. A fixing device for fixing a toner image electrostatically transferred on the transfer material, wherein a pair of a fixing upper roller and a fixing lower roller forming the fixing position and a transfer material pass through the fixing position. And a front guide plate that guides the fixing member, and at least one of the upper fixing roller, the lower fixing roller, and the pre-fixing guide plate is used as the second conductive member. The image forming apparatus as described in the above.