JP3053282B2 - 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 a copying machine or a laser beam printer, and more particularly, to a transfer member to which a bias voltage is applied is pressed against an image carrier, and a sheet material is sandwiched between the two. The present invention relates to an image forming apparatus that transfers a toner image on an image carrier to a sheet material by being conveyed.

[0002]

2. Description of the Related Art FIG. 8 shows an outline of a conventional image forming apparatus using a contact type transfer roller as a transfer device.

A photosensitive drum 1 as an image carrier rotatably disposed substantially at the center of the apparatus main body M is formed so that the surface of a conductive substrate such as a metal is covered with a photosensitive layer. . Around the photosensitive drum 1, a primary charger 2, which uniformly charges the photosensitive drum 1 along the rotation direction R1,
Exposure means 3 for forming an electrostatic latent image on charged photosensitive drum 1, developing device 5 for developing this electrostatic latent image, and transferring the developed toner image on photosensitive drum 1 to transfer material P A transfer roller (transfer device) 6, a cleaner 7 for removing toner and paper dust remaining on the photosensitive drum 1, and a static elimination lamp 9 for removing charges remaining on the photosensitive drum 1. . The sheet material P to which the toner image is transferred is transferred to the photosensitive drum 1 via the transfer guide 10 on the upstream side of the transfer roller 6.
Is supplied to a transfer portion nip N between the transfer roller 6 and the transfer roller 6.
A registration roller 11 is provided upstream of the transfer guide 10, and supplies the sheet material P to the photosensitive drum 1 in synchronization with the rotation of the photosensitive drum 1.

[0006] The sheet material P after the transfer is transferred to a conveyance guide 1.
The toner image is fixed by being sent to the fixing device 13 and heated under pressure by 2. The fixing device 13 includes a fixing roller 15 and a pressure roller 16, and a fixing portion nip L is formed between the two.

Here, the above-mentioned transfer roller 6 uses a roller that comes into contact with the photosensitive drum 1 in a state where a voltage is applied to the surface layer. According to this method, the amount of ozone generated as harmful is reduced. This can be significantly reduced as compared with the case where corona discharge is used. In addition, the transport guide 12
In order to stably transport the unfixed sheet material P from the transfer unit to the fixing unit, the sheet material P is grounded to the apparatus main body M and is formed of metal for electrostatically attracting the sheet material P.

Next, the paper feeding system will be described. In general,
In the conventional example, the paper feed roller 17 includes a feed roller 19 and a retard roller 20 for preventing the sheet material P from being double fed. The sheet material P sent into the apparatus main body M from the sheet feeding cassette 22 by the feeding roller 17 is sent to the registration roller 11 by the sheet feeding roller 17 and introduced into the transfer portion nip N. Here, feed roller 1
9 is driven in the direction of the arrow R2. The retard roller 20 is pressed against the feed roller 19 and is driven in the direction of arrow R3 (counter direction with respect to the feed roller 19) via a torque limiter (not shown).
For this reason, when the sheet is not passed and when only one sheet P is conveyed, the sheet P is rotated by the feed roller 19 (rotation in the direction opposite to the arrow R3). The action of the retard roller 20
Is rotated in the direction of arrow R3, and the lower sheet material P is fed back into the sheet cassette 22 to prevent double feeding. At this time, the back torque (back tension) of the retard roller 20 is set to, for example, about 800 g.

Further, in recent years, many image forming apparatuses have an automatic double-sided printing function. However, in the case of the conventional automatic double-sided printing machine, the sheet material P sent from the paper feed roller 17 is After the image formation on one surface (front surface) is completed one by one through the fixing unit 13, the sheet is fed to the lower re-feeding conveyance path 23, reversed, and re-introduced into the registration roller 11 by the re-feeding roller 25. Then, an image is formed on two surfaces (back surface). At this time, since the re-feed roller 25 does not need to consider the double feeding of the sheet material P, the back tension is 0.
g. Accordingly, if the paper transport speed at the transfer portion nip N is V ₁ and the paper transport speed at the fixing portion nip L is V ₂ , the difference in back tension between the paper feed roller 17 and the re-feed roller 25 (one surface: 800 g) , Two sides: 0g)
When V ₁ ≤V _{2 on} the first side, the sheet material P is conveyed in a straight posture, whereas on the second side, V ₁
> V ₂ and turned to transfer part - made sheet P and sag system between the fixing unit.

In the transfer device using the transfer roller 6, the transfer roller 6 presses the toner against the photosensitive drum 1 when transferring the toner image on the photosensitive drum 1 to the sheet material P. May not be transferred, or a transfer rate may be poor, that is, a so-called “missing” transfer image may be generated. It has been found that, as a means for preventing this hollowing out, for example, the peripheral speed of the transfer roller 6 should be driven at the peripheral speed of the transfer portion nip N about 5 to 10% higher than the peripheral speed of the photosensitive drum 1.

The transfer roller 6 has a bias power source 2
6, a first bias voltage (transfer bias) necessary for transferring the toner image is applied, and a charge is applied to the sheet material P passing between the photosensitive drum 1 and the transfer roller 6,
The toner image on the photosensitive drum 1 is transferred to the sheet material P. After the transfer, the voltage applied to the transfer roller 6 is switched to a second bias voltage (OFF or a weak bias lower than the transfer bias) by constant voltage control, thereby preventing drum memory and paper marks during non-transfer. I have.

As for this switching, as shown in FIG. 9, the image forming area S for the sheet material P is set at the front end P ₁ , the rear end P ₂ ,
From right and left ends P ₃ each 5mm entered inner, also when the conveyance direction of arrow A direction, the switching from the transfer bias V _T to the weak bias V _D after the transfer as in FIG. 10, after the sheet material P is performed from the end P ₂ in the image forming area S outside of 2 mm.

[0011]

[SUMMARY OF THE INVENTION However, according to the prior art, the rear end bouncing occurs when the transfer unit nip N rear end P ₂ of the sheet P passes, as shown in FIG. 11, the unfixed image there was a possibility that edge stain F to the rear end P ₂ of the sheet P touches the device within the structure occurs.

This is because the rear end P ₂ of the sheet material P passes through the transfer portion nip N of the transfer roller 26 as shown in FIG.
Because the photosensitive drum 1 is attracted and moved (arrow e) in the rotation direction (arrow R1 direction) of the photosensitive drum 1 without being separated from the photosensitive drum 1 by the electrostatic attracting force, the photosensitive drum 1 jumps up (arrow f). is there.

Further, the conditions under which the rear end bounce occurs will be described below. (1) The light and thin sheet material P is easily adsorbed to the photosensitive drum 1 and is difficult to be separated, so that the rear end jumps easily. (2) The occurrence of trailing edge bounce depends on the print pattern. If toner is interposed between the sheet material P and the photosensitive drum 1, the electric attraction force is weakened, so that the sheet material and the photosensitive drum 1 are easily separated.

[0014] That low printing ratio, when the toner in the rear end P ₂ vicinity of the sheet material P is not present, easy electrical attraction force is strong rear splashing occurs. On the other hand, the printing rate is large,
_{When the} toner is present in the vicinity of ₂ , the rear end bounce is unlikely to occur. (3) the rear end P of the transfer bias V _T switching life sheet P
Be set from ₂ inner 2mm This is usually because due calculated back from the distal end P ₁ side, by registration roller 11 and variations in the paper transport (quality of conveyance accuracy), the rear end P
Looking to ₂ to the reference, the transfer bias switching stage also occur variations, such as transcription fills up the rear end P ₂ bias V
_{When T} is applied, the rear end P ₂ is attracted to the photosensitive drum 1, easily rear splash occurs. (4) the relationship between the sheet conveying speed V ₂ of the sheet conveyance speed V ₁ and the fixing portion of the transfer portion, in the case of V ₁ ≦ V _2, the sheet material P between the fixing device 13 and the transfer roller 6 Because of the tension, it moves upward (to the side of the photosensitive drum 1), and the rear end jumps easily. On the other hand, if V ₁ > V ₂ , no tension is applied to the sheet material P, so that rear end bouncing hardly occurs. That is, as shown in FIG. 8, when performing double-sided printing, the trailing edge bounce easily occurs on the first side and does not occur on the second side. This is because the first surface corresponds to V ₁ ≦ V ₂ and the second surface corresponds to V ₁ > V _{2 due} to the difference in back tension between the sheet feeding roller 17 and the re-feed roller 25 as described above. .

Accordingly, the present invention provides a sheet material on the rear end side of a sheet material.
Switching time of bias voltage to be applied in the sheet material area
Is changed between the first surface and the second surface of the sheet material to prevent the rear end of the sheet material from splashing, and to provide an image forming apparatus in which the rear end edge is not stained. Things.

[0016]

The present invention has been made in view of the above circumstances, and the present invention according to claim 1 has the following features.
An image carrier having a toner image formed on its surface, a transfer member to which a bias voltage is applied to transfer the toner image on the image carrier to a sheet material, and a sheet material having a toner image formed on a first surface. And a double-sided feeding means for feeding the sheet material again to a transfer portion having a transfer member for forming a toner image on the second surface. Switching means for switching the bias voltage applied to the transfer member from a high bias voltage to a low bias voltage in the sheet material area on the side of the sheet material, wherein the switching means is configured to perform the transfer on the first surface and the transfer on the second surface. Then, the switching timing of the bias voltage by the switching means is changed. The present invention according to claim 2 provides:
2. The image forming apparatus according to claim 1, wherein the switching unit includes:
The switching timing of the bias voltage of the second surface is set to a rear end side of the sheet material with respect to the switching timing of the bias voltage of the first surface. According to a third aspect of the present invention, in the image forming apparatus of the first or second aspect, the switching means switches a bias voltage applied in a sheet material area on a rear end side of the sheet material during transfer in a stepwise manner. It is characterized by the following. According to a fourth aspect of the present invention, in the image forming apparatus of the first, second, or third aspect, the transfer member is a transfer roller pressed against the image carrier.
It is characterized by the following.

[0017]

[0018]

[0019]

Embodiments of the present invention will be described below with reference to the drawings.

The same components and operations as those of the conventional example described with reference to FIGS. 8 to 12 are denoted by the same reference numerals, and description thereof will be omitted.

The first embodiment shown in FIG. 1 is different from the conventional image forming apparatus M having a double-sided automatic printing function shown in FIG.
An adjusting means (switching means) C is connected to the bias power supply 26. The adjustment means C, a bias voltage bias source 26 is applied to the transfer roller 6, the transfer bias (high bias voltage, the first bias voltage) weak bias (low bias voltage, the second bias voltage) from V _T to V _D This is to change the switching timing. Further, the adjusting means C changes the switching timing according to the output of the front and back detecting means (not shown). The front / back detection means is, for example, the first surface (front surface ; hereinafter, appropriately referred to as “one surface” ) of the sheet material P.
Is completed, the second side (the back side, hereinafter referred to as “two sides” as appropriate)
Say. ) Re-feeding conveyance path (double-sided feeder)
(Step) A sensor or the like for detecting that it is introduced into 23 is sufficient. The adjusting means C changes the switching timing according to the output of the sensor. When the sheet material P is conveyed to the photosensitive drum 1, the sheet material P is conveyed via the sheet feeding roller 17 (one surface).
Depending on whether the sheet is conveyed via the sheet re-feed roller 25 (two sides), as described above, the electrostatic
Since the suction force and the back tension vary, the switching timing is adjusted accordingly.

For example, as shown in FIG.
For _two transfer bias switching on the first side is performed in A ₁ mm, the transfer bias switching second surface and A _2. However,
A ₂ <A ₁ . In the conventional example, A ₁ = A ₂ = 2 mm, etc., but at this time, the rear end of the first surface bounces.

Further, for example, if A ₁ = A ₂ = 7 mm, the rear end jump of the first side is eliminated, while the transfer failure of the image rear end of the second side occurs. This is because, in the sheet material S on the first side, the resistance value of the paper is sufficiently low and the transfer can be sufficiently performed even with the weak bias V _D , so that A ₁ = 7
No transfer failure occurs even with mm. However, on the second side, the sheet is dehumidified through the fixing step (fixing device 13) and has a high resistance, and transfer failure is likely to occur. Therefore, in order to prevent both rear splashes first side and the second side of the transfer failure, the switching period of the transfer bias V _T With A ₂ <A _1, it is possible to obtain a good image.

Hereinafter, specific numerical values will be described.

As the sheet material S, 64 g / m ² , thickness 9
The paper was conveyed at a process speed of 100 mm / sec using an A4 size sheet (the abstract is parallel to the conveying direction).

Transfer roller: Diameter: 20.8φ (conductive EPDM on 8φ core) Environment: Temperature: 23 ° C., Humidity: 60% Resistance: 1.0 × 10 ⁸ (Ω) Transfer voltage: +3 KV, transfer voltage between sheets : + 2KV photosensitive drum: OPC drum image portion: -100 V non-image portion: an image forming apparatus having the automatic duplex printing function of FIG. 1 in -600V above conditions, an image from the rear end P ₂ of 5mm min of the sheet material P Mask to form margins, 15 ° C: 1
In three environments of 0%, 23 ° C .: 60%, and 30 ° C .: 80%, 500 double-sided prints (1000 images) of character images each having a print rate of 4% were performed.

Table 1 shows the state of occurrence of rear end jumping and transfer failure at this time. Table 1 shows that A ₁ = A ₂ = 2 mm and A ₁ = A
Data for ₂ = 7 mm is also shown.

In Table 1, A ₁ is the first side and A ₂ is the second side. "Bounce" means a trailing end bounce, and "rolling failure" means a transfer failure.

In the first embodiment, it is taken into consideration that the switching timing is changed depending on whether the sheet material P is conveyed by the sheet feeding roller 17 or the sheet re-feeding roller 25. Then, it goes without saying that the present embodiment may be applied not only to the copying on both sides of the sheet material P by the double-sided automatic printing function but also to the single-sided multiplex copy using the re-feed roller 25.

[0030]

[Table 1] Next, a second embodiment will be described with reference to FIG.

Due to variations in the transportability of the sheet material P, etc.,
Switching period of the transfer bias V _T is sometimes deviate more 0.5~2mm even specified from the rear end P ₂ of the sheet material P what mm.

[0032] To prevent this problem, adding a sensor (trailing edge detection means) 31 to the transfer guide 12 as shown in the figure, and detects the trailing end P ₂ of the sheet P, the transfer bias V
The timing for switching the _T to the weak bias V _D, by determining in accordance with the sheet conveying speed of each sheet, printing pattern and, also conveying speed V ₁ due to the difference of the back tension of the first surface and the second surface is changed , it is possible to switch the transfer bias V _T from the rear end P ₂ at a fixed distance to a weak bias V _D, it is possible to secure a stable sheet conveyance does not occur at the rear end bounce.

As a third embodiment, FIG. 4 illustrates adjustment when the printing ratio (image forming area) S is different.

When the printing rate is large, the trailing edge is less likely to occur due to the interposition of toner. Conversely, when the printing rate is small, for example, in solid white, the rear end jumps easily. Thus, the rear end bounce when printing to the rear end P ₂ barely is not easily generated, but if thus considerably picture ends in front of the rear end P ₂ are rear splashing tends to occur. Valid in this case is a method which detects the trailing end of the image forming area S'll switched to the weak bias V _D. The trailing edge detection of the image, for example, an image signal from the post computer in the image forming apparatus, by processing, switching the transfer bias V _T to the weak bias V _D to obtain the position of the rear end P ₂ of the image There is a way. In this manner, by switching to a weak bias by detecting the trailing edge of the image, it is possible to prevent the trailing edge from jumping without causing image defects such as transfer failure.

As a fourth embodiment, adjustment when a host computer (not shown) is used will be described.

The printing rate from the data of the host computer (e.g., the average of the area of one sheet of paper) to calculate the S, the timing for switching the transfer bias V _T to the weak bias V _D, when printing ratio S is large after while the since the end splashing hardly occurs rear P ₂ barely (2mm from the rear end P _2), since when printing ratio S is small tends rear splashing occurs inward from the rear end P ₂ (the rear end P ₂ (Approximately 7 mm from the front end) to prevent the trailing edge from jumping and to achieve more stable paper conveyance.

As an example, by switching the printing rate from 4% or more to 2 mm from the trailing edge of the paper, and switching the printing rate from 4% or less to 7 mm from the trailing edge of the paper, the trailing edge does not bounce and the paper transport is improved.

FIG. 5 shows a fifth embodiment.

[0039] As described in the first embodiment, it is possible to prevent the rear end bounce by give proper timing for switching to the weak bias V _D on the first side earlier than the second surface, where the first side of the transfer bias when switching the V _T weak bias V _D, as shown in the figure, when'll switched to stepwise weak bias V _D from a transfer bias V _T, the transfer bias V at an earlier timing than the first embodiment _T can be switched, without causing transfer failure,
The problem of rear end bouncing can be solved.

As an example, on the first side, the sheet material P
Transfer bias V _{T in} the range of 9 mm to 7 mm from the rear end P ₂
Is switched to the bias of (transfer bias V _T + weak bias V _D ) / 2, and the bias is switched to the weak bias V _D at the rear end of the paper 7 mm. Is improved.

A thin sheet material having a light weight, ie, a thickness t
However, a thin sheet material P is liable to cause rear end bouncing. Therefore, FIG. 6, in the sixth embodiment shown in FIG. 7 detects the thickness t of the sheet material P, by its thickness t, by changing the timing for switching the transfer bias V _T to the weak bias V _D This prevents rear end bouncing.

FIGS. 6 and 7 show an example of a method for detecting the thickness t of the sheet material P. FIG. 6 is a longitudinal sectional view of the registration roller 11. The sensor (thickness detecting means) 32 attached to the registration roller 11 detects the upper registration roller 11
Then, the distance between a and the registration lower roller 11b is measured, and the thickness t of the sheet material P is detected. When the distance between the center axis of the registration upper roller 11a and the center axis of the registration lower roller 11b when the registration roller 11 does not have the sheet material P is D ₁ (FIG. 6), and the sheet material P is nipped and conveyed. When the center distance D ₂ (FIG. 7), represented by the thickness t = D ₂ -D ₁ of the sheet material.

Here, the relation between the weight and the thickness t of the sheet material P is about 90 μm for a paper weighing 65 g / m ² , and about 110 μm for a paper weighing 75 g / m ² . In this embodiment, the trailing edge of 65 g / m ² paper does not cause the trailing edge of the 75 g / m ² paper.

[0044] Therefore, as described above, to detect the thickness t of the sheet material P by the sensor 32, when the thickness t is less 100μm is transcribed in 7mm from the rear end P ₂ of the sheet P biased V _T
Switching to a weak bias V _D, when the thickness t is more than 100μm, by performing control to switch 2mm from the rear end P _2, can prevent the occurrence of the trailing edge splashes, allowing stable sheet conveyance it can.

In the first to sixth embodiments described above, examples are shown in which the adjusting means is used independently. However, any two or more adjusting means, for example, a thickness detecting means and a front / back detecting means are used. Needless to say, if the switching time is finely adjusted by combining the above, the conveyance of the sheet material P is further stabilized.

[0046]

As described above , according to the present invention,
The bias applied in the sheet material area on the rear end side of the sheet material
The switching time of the ass voltage is different between the first surface and the second surface of the sheet material.
By making the change, it is possible to effectively prevent the rear end of the sheet material from splashing and prevent the rear end edge of the sheet material from being stained.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view showing an outline of an image forming apparatus according to the present invention.

FIG. 2 is a diagram illustrating a difference in switching timing of a bias voltage due to a difference in front and back of a sheet material.

FIG. 3 is a schematic diagram illustrating a mounted state of a sensor that detects a rear end of a sheet material.

FIG. 4 is a diagram illustrating a difference in bias voltage switching timing depending on a difference in an image forming area of a sheet material.

FIG. 5 is a diagram showing a state where a bias voltage is changed stepwise.

FIG. 6 is a schematic diagram showing sheet thickness detecting means.

FIG. 7 is a view showing the operation of a thickness detecting means.

FIG. 8 is a longitudinal sectional view showing an outline of a conventional image forming apparatus.

FIG. 9 is a diagram illustrating an image forming area of a sheet material.

FIG. 10 is a diagram showing a conventional bias voltage switching timing.

FIG. 11 is a diagram showing edge contamination at the rear end.

FIG. 12 is a diagram showing a state in which edge contamination at the rear end occurs.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Image carrier (photosensitive drum) 6 Transfer member (transfer roller) 23 Double-sided feeding means (re-feeding conveyance path) 26 Bias power supply 31 Rear end detecting means (sensor) 32 Thickness detecting means (sensor) C switching means ( Adjusting means) P sheet material S image forming area t sheet material thickness V _T high bias voltage (first bias voltage, transfer bias) V _D low bias voltage (second bias voltage, weak bias)

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Hideo Nanataki 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Inventor Hideyuki Yano 3-30-2 Shimomaruko, Ota-ku, Tokyo Within Canon Inc. (72) Inventor Yasumasa Otsuka 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Inventor Hiroto Hasegawa 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (56) References JP-A-61-185758 (JP, A) JP-A-2-39181 (JP, A) JP-A-3-91785 (JP, A) JP-A-62-56977 (JP, A) JP-A-59-206583 (JP, A) JP-A-1-157363 (JP, U) (58) Fields investigated (Int. Cl. ⁷ , DB name) G03G 15/00 303 G03G 15/36 G03G 21 / 00 370-540 G03G 21/02-21/04 G03G 21/14 G03G 21/20 G03G 1 3/14-13/16 G03G 15/14-15/16 103

Claims (4)

(57) [Claims] An image carrier on which a toner image is formed, a transfer member to which a bias voltage is applied to transfer the toner image on the image carrier to a sheet material, and a toner image formed on a first surface. And a double-sided feeding means for feeding the sheet material again to a transfer section having a transfer member for forming a toner image on the second surface of the sheet material. If the bias voltage applied to the transfer member in the sheet material area on the rear end side of the sheet material is a high bias voltage,
Switching means for switching to a lower bias voltage , wherein the switching means changes the switching timing of the bias voltage by the switching means between the time of transfer to the first surface and the time of transfer to the second surface. An image forming apparatus comprising: 2. The switching device according to claim 1, wherein the switching timing of the bias voltage of the second surface is set to a rear end side of the sheet material with respect to the switching timing of the bias voltage of the first surface. The image forming apparatus according to claim 1 . Wherein said switching means, the image formation according to claim 1 or 2 switches the bias voltage applied at the rear end side of the sheet region of the sheet material at the time of transfer in stages, it is characterized by apparatus. 4. The image forming apparatus according to claim 1, wherein the transfer member is a transfer roller pressed against the image carrier.