JPH05150577A - Image forming device - Google Patents

Abstract

PURPOSE:To prevent a rear end trim from being soiled with toner by preventing the rear end of a sheet material from turning up. CONSTITUTION:The sheet material P is held and carried between the photosensitive drum 1 and the transfer roller 6. A bias power source 26 is connected to the transfer roller 6 and, addition, an adjusting means for changing the time of switching at which a transfer bias voltage at the transfer time is switched to a weak bias voltage at the nontransfer time is connected thereto. When the sheet material P is supplied to the photosensitive drum 1, back torque differs depending on whether the sheet is fed from a sheet feeding roller 17 (at the time copying on the first side) or from a sheet refeeding roller (at the time of copying on the second side), and the first side and second side differ from each other in speed V1 at which the sheet material P is carried. Because of difference, when the time of switching is the same, the rear end turns up at the time of copying on the first side. When the first side of the sheet material is detected by a front/back detecting means, the time of switching is made early, thereby preventing the rear end from turning up. When it detects the second side of the sheet material, the time of switching is made late, thereby preventing the transfer defect of the rear end.

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 specifically, a transfer member to which a bias voltage is applied is pressed against an image carrier to sandwich a sheet material between them. The present invention relates to an image forming apparatus that transfers a toner image on an image carrier to a sheet material by conveying the image carrier.

[0002]

2. Description of the Related Art An outline of a conventional image forming apparatus using a contact type transfer roller as a transfer device is shown in FIG.

A photosensitive drum 1 as an image bearing member rotatably disposed in the substantial center of the apparatus main body M is formed by covering the surface of a conductive base material such as metal with a photosensitive layer. .. A primary charger 2 that uniformly charges the photosensitive drum 1 around the photosensitive drum 1 along the rotation direction R1 thereof,
An exposure unit 3 that forms an electrostatic latent image on the charged photosensitive drum 1, a developing device 5 that visualizes the electrostatic latent image, and a toner image visualized on the photosensitive drum 1 is transferred to a transfer material P. A transfer roller (transfer device) 6, a cleaner 7 for removing toner and paper dust remaining on the photosensitive drum 1, a charge eliminating lamp 9 for removing electric charges remaining on the photosensitive drum 1, and the like are provided. .. The sheet material P that is the transfer destination of the toner image is transferred to the photosensitive drum 1 via the transfer guide 10 on the upstream side of the transfer roller 6.
Is supplied to the transfer portion nip N between the transfer roller 6 and the transfer roller 6.
A registration roller 11 is arranged 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.

The sheet material P, which has been transferred, is conveyed by the conveyance guide 1.
The toner image is fixed by being sent to the fixing device 13 by 2 and heated under pressure. The fixing device 13 includes a fixing roller 15 and a pressure roller 16, and a fixing portion nip L is formed between them.

Here, the above-mentioned transfer roller 6 uses a roller which 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 harmful ozone generated is It can be significantly reduced as compared with the case of using corona discharge. In addition, the transport guide 12
In order to stably convey the unfixed sheet material P from the transfer portion to the fixing portion, the sheet material P is grounded to the apparatus main body M and is formed of a metal for electrostatically attracting the sheet material P.

Next, the paper feeding system will be described. In general,
Although double feeding of paper at the time of paper feeding becomes a problem, in the conventional example, the paper feeding roller 17 includes a feed roller 19 and a retard roller 20 for preventing double feeding of the sheet material P, and a pickup roller 21. The sheet material P sent from the paper feed cassette 22 into the apparatus main body M is sent to the registration roller 11 by the paper feed roller 17 and introduced into the transfer portion nip N. Here, the feed roller 1
9 is driven in the direction of 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 the feed roller 19) via a torque limiter (not shown).
Therefore, when the sheet material P is not fed or when only one sheet material P is conveyed, it rotates following the feed roller 19 (rotation in the direction opposite to the arrow R3). Retard roller 20 by action
Rotates in the direction of arrow R3 and sends the lower sheet material P back into the paper feed cassette 22 to prevent double feed. The back torque (back tension) of the retard roller 20 at this time is set to about 800 g, for example.

Further, in recent years, an image forming apparatus having an automatic double-sided printing function has been increasing, but in the case of the automatic double-sided machine of the conventional example, the sheet material P sent from the paper feed roller 17 is After the image formation on one side (front side) is completed through the fixing device 13 one by one, the image is fed to the lower sheet re-feeding conveyance path 23, inverted, and re-introduced into the registration roller 11 by the sheet re-feeding roller 25. Then, images are formed on the two sides (back side) this time. At this time, the refeed roller 25 does not need to consider the double feeding of the sheet material P, so that the back tension is 0.
It is g. Therefore, assuming that 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 sheet feeding roller 17 and the sheet refeeding roller 25 (one side: 800 g 2 sides: 0g)
When V ₁ ≦ V _{2 on} the first side, the sheet material P is conveyed in a straight posture, while on the second side, V ₁
> V _2, and the sheet material P sags between the transfer portion and the fixing portion.

Further, in the transfer device using the transfer roller 6, when the toner image on the side of the photosensitive drum 1 is transferred to the sheet material P, the transfer roller 6 presses the toner against the photosensitive drum 1, so that the central portion of the image is formed. Toner may not be transferred or the transfer rate may be low, resulting in a so-called "blank" transfer image. It has been found that, for example, the peripheral speed of the transfer roller 6 in the transfer portion nip N may be driven at a peripheral speed that is about 5 to 10% higher than the peripheral speed of the photosensitive drum 1 as a means for preventing the hollow portion.

The transfer roller 6 has a bias power source 2
A first bias voltage (transfer bias) required to transfer the toner image is applied by 6 to apply an electric charge 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 the second bias voltage (OFF or a weak bias lower than the transfer bias) by the constant voltage control to prevent the drum memory and the paper trace during the non-transfer. There is.

For this switching, as shown in FIG. 9, the image forming area S for the sheet material P is set to a front end P ₁ , a 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 It is performed outside the image forming area S 2 mm from the edge P ₂ .

[0011]

However, according to the prior art, when the trailing edge P ₂ of the sheet material P passes through the transfer portion nip N, trailing edge bounce occurs, and as shown in FIG. However, there is a risk that the inner edge of the sheet material P may come into contact with the internal structure of the apparatus and the edge dirt F may be generated at the rear end P ₂ of the sheet material P.

As shown in FIG. 12, after the trailing edge P ₂ of the sheet material P passes through the transfer portion nip N of the transfer roller 26,
Because the electrostatic attraction force of the photosensitive drum 1 causes the photosensitive drum 1 to be 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, the photosensitive drum 1 jumps up (arrow f). is there.

Further, the conditions under which the trailing edge bounce occurs will be described below. (1) Since the sheet material P having a light weight and a thin thickness is easily adsorbed to the photosensitive drum 1 and is difficult to be separated, the trailing edge bounce is likely to occur. (2) The occurrence of trailing edge bounce is influenced by the print pattern. When the toner is present 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.

That is, when the printing rate is small and the toner is not present in the vicinity of the rear end P ₂ of the sheet material P, the electric attraction force is strong and the rear end bounce is likely to occur. On the other hand, the printing rate is large and the trailing edge P
_When there is toner in the vicinity of ₂ , the trailing edge bounce is unlikely to occur. (3) The transfer bias V _T is switched at the trailing edge P of the sheet material 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
_With reference to ₂ , the transfer bias switching timing also varies, and for example, the transfer bias V is fully filled up to the rear end P _{2.
When T} is applied, the trailing edge P ₂ is attracted to the photosensitive drum 1 and trailing edge bounce is likely to occur. (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 Since it is stretched, it moves upward (to the side of the photosensitive drum 1) and the trailing edge bounce is likely to occur. On the other hand, in the case of V ₁ > V ₂ , the tension on the sheet material P does not work, so that the trailing edge bounce is less likely to occur. That is, as shown in FIG. 8, when double-sided printing is performed, 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 the back tension between the sheet feeding roller 17 and the sheet refeeding roller 25 as described above. ..

Therefore, according to the present invention, the switching timing from the first bias voltage (transfer bias) to the second bias voltage (weak bias) is appropriately set depending on, for example, the difference in the thickness of the sheet material or the front and back of the sheet material. It is an object of the present invention to provide an image forming apparatus in which the trailing edge of the sheet material is prevented from splashing and the trailing edge edge stain does not occur by changing the above.

[0016]

The present invention has been made in view of the above circumstances, and a sheet material is provided between an image carrier on which a toner image is formed and a transfer member to which a bias voltage is applied. In an image forming apparatus that transfers a toner image on the image carrier to the sheet material by nip-conveying, at least two types of first bias voltage at the time of toner transfer and toner, which are different in size, are applied to the transfer member. A bias power supply for selectively applying a second bias voltage during non-transfer, and an adjusting means for changing a switching timing of the bias power supply for switching from the first bias voltage to the second bias voltage. And

In this case, the adjusting means changes the switching timing according to the output of the thickness detecting means for detecting the thickness of the sheet material, or the rear end detecting means for detecting the rear end of the sheet material. Change the switching timing according to the output of
Alternatively, the switching timing is changed according to the output of the area detection unit that detects the image forming area of the sheet material, or the switching timing is changed according to the output of the front and back detection unit that detects the front and back of the sheet material, You may do it.

[0018]

According to the above structure, the adjusting means changes the switching timing of switching the bias power supply from the first bias voltage to the second bias voltage at the thickness detecting means, the rear end detecting means, the area detecting means,
Since it is changed according to the output of the front and back detection means, etc., the switching timing can be changed according to the thickness of the sheet material, the variation in the sheet material transportability, the relative position of the image forming area with respect to the sheet material, the front and back of the sheet material, etc. It can be suitably changed so that the rear end bounce does not occur.

[0019]

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

It should be noted that components having the same structure and operation as those of the conventional example described with reference to FIGS. 8 to 12 are designated by the same reference numerals, and the description thereof will be omitted.

The first embodiment shown in FIG. 1 differs from the conventional image forming apparatus M having the double-sided automatic printing function shown in FIG.
The bias power source 26 is connected to the adjusting means C.
The adjusting means C changes the switching time at which the bias voltage applied by the bias power source 26 to the transfer roller 6 is switched from the transfer bias (first bias voltage) V _T to the weak bias (second bias voltage) V _D. is there. Further, the adjusting means C changes the switching timing by the output of the front and back detecting means (not shown). The front and back detecting means may be, for example, a sensor or the like that detects that the sheet material P has been introduced into the re-feeding conveyance path 23 in preparation for copying the second surface (rear surface) after the completion of the first surface (front surface) copy. Enough The adjusting means C changes the switching timing according to the output of this sensor. When the sheet material P is conveyed to the photosensitive drum 1, the sheet feeding roller 1
As described above, the back torque varies depending on whether the sheet is conveyed through the No. 7 (one side) or the re-feed roller 25 (two sides). Therefore, the back torque is changed accordingly. The timing is adjusted.

For example, as shown in FIG. 2, the rear end P of the sheet material P is
_{On the} other hand, the transfer bias of the first side is switched to A ₁ mm, and the transfer bias of the second side is switched to A ₂ . However,
A ₂ <A ₁ . In the conventional example, A ₁ = A ₂ = 2 mm, etc., but at this time, the trailing edge bounce of the first surface occurs.

Further, if A ₁ = A ₂ = 7 mm, for example, the trailing edge bounce of the first surface is eliminated, but the transfer failure of the image trailing edge of the second surface occurs. This is because A ₁ = 7 because the sheet resistance value of the sheet material S on the first side is sufficiently low and transfer can be performed sufficiently even with a weak bias V _D.
Transfer defects do not occur even in mm. However, on the second side, the sheet is dehumidified through the fixing process (fixing device 13) and has a high resistance, so that a transfer failure is likely to occur. Therefore, a good image can be obtained by setting the switching timing of the transfer bias V _T to A ₂ <A ₁ in order to prevent both the trailing edge bouncing on the first surface and the transfer failure on the second surface.

The specific numerical values will be described below.

As the sheet material S, 64 g / m ² and a thickness of 9
The sheet was conveyed at a process speed of 100 mm / sec using a 0 μm, A4 size (paper grain is parallel to the conveying direction).

Transfer roller: Diameter: 20.8φ (conductive EPDM on 8φ core metal) Environment: At 23 ° C. and 60% humidity Resistance: 1.0 × 10 ⁸ (Ω) Transfer voltage: +3 KV, Paper-to-paper transfer voltage : + 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 it to form a blank, 15 ℃: 1
In three environments of 0%, 23 ° C .: 60%, 30 ° C .: 80%, 500 sheets (1000 images) of each side were printed with a character image having a printing rate of 4%.

Table 1 shows the occurrence of trailing edge bouncing and transfer failure at this time. In Table 1, A ₁ = A ₂ = 2 mm, A ₁ = A
Data for ₂ = 7 mm are also shown.

In Table 1, A ₁ is the first surface and A ₂ is the second surface. “Bounce” is a trailing edge bounce, and “rolling failure” is a transfer failure.

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

[0030]

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

Due to variations in transportability of the sheet material P,
The transfer bias V _T may be switched from 0.5 mm to ₂ mm from the trailing edge P ₂ of the sheet material P, no matter how many mm are specified.

[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
_By determining the timing for switching _T to the weak bias V _D in accordance with the sheet transport speed for each sheet, even if the transport speed V ₁ changes due to the print pattern or the difference in back tension between the one side and the two side. , 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 shows the adjustment when the printing ratio (image forming area) S is different.

When the printing rate is large, the trailing edge bounce is less likely to occur due to the interposition of toner. On the contrary, when the printing rate is small, for example, in the case of solid white, the trailing edge bounce is likely to occur. 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. In such a case, an effective method is to detect the rear end of the image forming area S and switch to the weak bias V _D. To detect the trailing edge of the image, for example, an image signal from a post computer is processed in the image forming apparatus to obtain the position of the trailing edge P ₂ of the image and switch the transfer bias V _T to the weak bias V _D. There is a way. In this way, by the method of switching to the weak bias by the image trailing edge detection, trailing edge bounce can be prevented without causing image defects such as transfer defects.

Adjustment when a host computer (not shown) is used will be described as a fourth embodiment.

The printing rate (for example, the average for the area of one sheet of paper) S is calculated from the data of the host computer, and the timing of switching the transfer bias V _T to the weak bias V _D is set to the later when the printing rate S is large. Since the end bounce is less likely to occur, the trailing edge P _{2 is set to the} limit ( ₂ mm from the trailing edge P 2), while when the printing rate S is small, the trailing edge bounce is likely to occur, so that the trailing edge P _{2 is} inside (the trailing edge P ₂ To about 7 mm), the trailing edge bounce can be prevented and the paper can be conveyed more stably.

As an example, the printing rate is 4% or more → switching at 2 mm from the trailing edge of the paper, and the printing rate is 4% or less → switching at 7 mm from the trailing edge of the paper.

FIG. 5 shows a fifth embodiment.

As described in the first embodiment, the trailing edge bounce can be prevented by making the timing of switching to the weak bias V _D for the first surface earlier than that for the second surface. 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 You can switch _T , without causing transfer defects,
It can solve the problem of trailing edge bounce.

As an example, on the first surface, 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 a bias of (transfer bias V _T + weak bias V _D ) / 2 and is switched to a weak bias V _D at the paper trailing edge of 7 mm, the effect of preventing trailing edge bounce without causing transfer defects Is improved.

Thin and light sheet material, ie, thickness t
The sheet material P having a small thickness is likely to cause a rear end bounce. Therefore, in the sixth embodiment shown in FIGS. 6 and 7, the thickness t of the sheet material P is detected, and the timing at which the transfer bias V _T is switched to the weak bias V _D is changed depending on the thickness t. It prevents the rear end from bouncing.

An example of a method of detecting the thickness t of the sheet material P is shown in FIGS. 6 and 7, and FIG. 6 is a vertical sectional view of the registration roller 11. By the sensor (thickness detecting means) 32 attached to the registration roller 11, the registration upper roller 11
The thickness t of the sheet material P is detected by measuring the distance between a and the lower resist roller 11b. When the distance between the center axis of the upper registration roller 11a and the center axis of the lower registration roller 11b when there is no sheet material P on the registration roller 11 is D ₁ (FIG. 6), the sheet material P is nipped and conveyed. Assuming that the center distance is D _{2 in} (FIG. 7), the thickness of the sheet material is represented by t = D ₂ −D ₁ .

Here, the relationship between the weight of the sheet material P and the thickness t is that the weight of 65 g / m ² paper is about 90 μm and the weight of 75 g / m ² paper is about 110 μm. In this embodiment, the trailing edge bounce occurs on the 65 g / m ² paper, but the trailing edge bounce does not occur on the 75 g / m ² paper.

Therefore, as described above, the sensor 32 detects the thickness t of the sheet material P, and when the thickness t is 100 μm or less, the transfer bias V _{T is} 7 mm from the rear end P ₂ of the sheet material P.
Is switched to a weak bias V _D, and when the thickness t is 100 μm or more, control is performed to switch from the trailing edge P ₂ to 2 mm, so that trailing edge bounce can be prevented and stable paper conveyance can be achieved. it can.

In the first to sixth embodiments described above, the adjusting means is used independently, but any two or more adjusting means, for example, the thickness detecting means and the front / back detecting means are used. It goes without saying that the conveyance of the sheet material P is further stabilized by finely adjusting the switching timing by combining and.

[0046]

As described above, the adjusting means is provided for changing the switching timing from the first bias voltage during toner transfer to the second bias voltage during non-transfer, and the adjusting means adjusts the thickness of the sheet material, By changing the switching timing depending on the trailing edge, image forming area, front and back, etc., even if the properties of the sheet material change, the trailing edge bounce of the sheet material is effectively prevented and the trailing edge of the sheet material becomes dirty. Can be prevented.

[Brief description of drawings]

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

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

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

FIG. 4 is a diagram showing a difference in bias voltage switching timing due to a difference in image forming area of a sheet material.

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

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

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

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

FIG. 9 is a diagram showing 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 dirt on the rear end.

FIG. 12 is a diagram showing a state where edge dirt is generated on the rear end.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Image carrier (photosensitive drum) 6 Transfer member (transfer roller) 26 Bias power source 31 Rear end detecting means (sensor) 32 Thickness detecting means (sensor) C Adjusting means P Sheet material S Image forming area t Sheet material thickness V _T first bias voltage (transfer bias) V _D second bias voltage (weak bias)

Front page continued (72) Inventor Hideo Nanaki 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Inventor Hideyuki Yano 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Inventor Yasumasa Otsuka 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) In-house Hiroto Hasegawa 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc.

Claims (5)

[Claims] 1. A toner image on the image carrier is transferred to the sheet material by sandwiching and conveying a sheet material between an image carrier having a toner image formed on its surface and a transfer member to which a bias voltage is applied. In the image forming apparatus, the transfer member includes at least two types of different sizes,
The switching timing of the bias power supply for selectively applying the first bias voltage at the time of toner transfer and the second bias voltage at the time of toner non-transfer and the switching time of the bias power supply for switching from the first bias voltage to the second bias voltage are changed. An image forming apparatus comprising: 2. The image forming apparatus according to claim 1, wherein the adjusting unit changes the switching timing according to an output of a thickness detecting unit that detects the thickness of the sheet material. 3. The image forming apparatus according to claim 1, wherein the adjusting unit changes the switching timing according to an output of a trailing edge detecting unit that detects a trailing edge of the sheet material. 4. The image forming apparatus according to claim 1, wherein the adjusting unit changes the switching timing according to an output of an area detecting unit that detects an image forming area of the sheet material. 5. The image forming apparatus according to claim 1, wherein the adjusting unit changes the switching timing according to an output of a front / back detecting unit that detects the front / back of the sheet material.