JP2006091481A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus reducing a drum memory at the end part of a photoreceptor drum caused by transfer bias while suppressing an influence (such as poor transfer of high resistance paper) exerted on an image and preventing occurrence of edge fogging. <P>SOLUTION: In the image forming apparatus which forms a toner image on an image carrier, transfers the toner image on the surface of a recording material by the transfer bias to be applied to a transfer roller, and forms the image, it has a high voltage power source by which constant voltage control and constant current control are switched as a transfer bias application means and switches transfer bias control between the constant voltage control and the constant current control according to the predetermined number of prints. In addition, switching of the transfer bias control is changed according to a transfer roller resistance. Then, switching of the transfer bias control is changed according to use environmental conditions. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus using a printer such as a laser printer or an LED printer, an electrophotographic system such as a digital copying machine, or an electrostatic recording system, and in particular, a toner image on an image carrier is electrostatically applied on a recording material. The present invention relates to an image forming apparatus that uses a transfer roller as a transfer means.

  Conventionally, various image forming apparatuses having a so-called double-sided printing function capable of printing an image on both sides of a recording material have been developed (for example, Patent Document 1).

  FIG. 10 shows an example of a conventional electrophotographic laser printer that has been put into practical use as a representative example.

  As shown in the figure, this laser printer includes a primary charger 102 that uniformly charges the photosensitive drum 1 around the photosensitive drum (photosensitive member) 101 along the rotation direction, and a uniformly charged photosensitive drum 1. Exposure means 103 for forming an electrostatic latent image by exposing the toner, a developing device 104 for forming a toner image by attaching toner (developer) to the electrostatic latent image, and a toner image on the photosensitive drum 101 on the recording material P A transfer roller (transfer device) 105 for transferring and a cleaning device 107 for removing residual toner are provided. The recording material P as a transfer destination of the toner image is stacked and stored inside the cassette 111, fed by the pickup roller 112, and sent to the photosensitive drum 101 by the registration roller pair 113. The recording material P fed to the photosensitive drum 101 is transferred with a toner image by a transfer roller (transfer device) 105 and then conveyed to a fixing device 106, where the recording material P on which the toner image is fixed is moved out of the device. Discharged.

  When printing on both the front and back sides of the recording material P, the recording material P is sent to the paper discharge guide 120 side, and once (a front end portion) is discharged outside the printer main body, the paper discharge roller 121 is then discharged. Is switched back to be fed into the second sheet feeding roller 122. The recording material P that has been turned upside down in this way is sent again to the photosensitive drum 101 by the registration roller pair 113, and printing on the second surface of the recording material P is performed.

  Next, the transfer device 105 will be described.

  Conventionally, in this type of image forming apparatus, as means for electrostatically transferring the toner image on the image carrier 101, a corona transfer device using corona discharge, a conductive elastic roller (transfer roller) with toner and A roller transfer device that applies a transfer bias of reverse polarity and electrostatically transfers the recording material onto the recording material. The recording material is electrostatically adsorbed to the belt-like rotating member, and the toner is generated by the electrostatic force from the belt-like rotating member. A belt transfer device for transferring an image onto a recording material is widely used. Among these, the roller transfer device has been widely adopted in recent years because of the low generation of ozone and the simplification of the configuration of the image forming apparatus because the transfer roller can also serve as a recording material conveying roller.

  Hereinafter, bias control in the roller transfer device 105 will be described.

  FIG. 11 shows a transfer bias sequence. In the sequence shown in FIG. 9, first, in order to detect an appropriate voltage applied to the transfer roller, the amount of current flowing from the transfer roller to the photosensitive drum (image carrier) is controlled to be constant, and the bias required at that time The value Vt0 is obtained. At this time, the transfer bias voltage value Vt at the time of subsequent recording material transfer is determined based on the necessary bias voltage value Vt0.

  At this time, the transfer bias of the second side during automatic duplex printing is set to a bias setting higher than the transfer bias value of the first side. This is because the recording material P is subjected to a heat fixing process when the first image is formed, and is dried and has a high resistance, thereby preventing an image defect such as a transfer defect from being easily generated. is there. Vt0 is about 300V to + 4.5kV, and Vt is about + 500V to + 6.0kV.

  Next, the voltage applied to the transfer roller 105 is set to Vt0, and the transfer bias is set to Vt as the recording material P reaches the transfer nip.

  While the recording material P passes through the transfer portion, the transfer bias is set to Vt, and Vt → Vt0 is switched at the rear end portion of the paper. The transfer bias is set to Vt0 while the photosensitive drum is rotating even during post-rotation before the end of printing.

  By the above control, the toner image on the photosensitive drum can be transferred onto the recording material with an optimum transfer bias voltage value in accordance with the resistance value change due to the environmental fluctuation and durability fluctuation of the transfer roller.

JP 2001-75378 A

However, in the conventional transfer device, when forming images on both sides of the recording material, the transfer bias on the second surface is set stronger than the transfer bias on the first surface. When the drum memory is generated at the portion where the drum and the transfer roller are in direct contact, and the paper passing position of the succeeding paper is deviated from the paper passing position on the second surface, the problem is that fog occurs at the paper edge. Less than,"
Edge cover ”).

  The mechanism of occurrence of end fogging will be described below.

  FIG. 12 schematically shows the layout of sheets when continuous automatic duplex printing is performed. P1a is the first sheet for the first transfer, P1b is the first sheet for the second transfer, P2a represents the second sheet at the time of transferring the first side, and P2b represents the second sheet at the time of transferring the second side.

  When transferring to the second side of the first sheet, the image has already been transferred to the first side of the sheet and has been heated by the fixing device, so the sheet is dry and the electric resistance value of the sheet itself is high. It has become. For this reason, in the end region, a positive charge due to the bias applied to the transfer roller is not injected into the sheet P1a whose resistance value is increasing, and a large amount is applied to the photosensitive drum side negatively charged by the charging roller. Will flow into. Then, the photosensitive drum portion into which positive charge has flowed in the end region is neutralized and recharged to the negative side by the charging roller, but is not uniformly charged to the predetermined charging potential Vd, and is low in terms of absolute value. It becomes a state. For this reason, the toner adheres to the hatched portion on one side during the developing process on the second side of the first sheet P1b. Next, the toner adhering to the photosensitive drum portion corresponding to the end region is transferred as it is to the end portion of the first surface of the second sheet P2a conveyed at the regular position, which is the hatched portion in FIG. Appears as an end fogging.

  This edge fog is more likely to occur when the transfer roller resistance is lower because the amount of positive charge flowing into the drum in the non-sheet passing area increases. Therefore, in recent years, polymer conductive transfer rollers such as NBR, which have been adopted because there is little resistance unevenness and high image quality, have low resistance due to temperature rise in the machine during double-sided printing, and therefore edge fogging has occurred. It tends to be easy.

  This phenomenon is also closely related to the charging process, and tends to occur in a low temperature / low humidity environment where the charging roller and drum have a high resistance and the charging ability is reduced.

  The present invention has been made in view of the above problems, and the object of the present invention is to reduce the drum memory at the end of the photosensitive drum due to the transfer bias while suppressing the influence on the image (transfer failure of high resistance paper, etc.). An object of the present invention is to provide an image forming apparatus capable of preventing the occurrence of edge fogging.

  In order to achieve the above object, according to the first aspect of the present invention, a toner image is formed on an image carrier, and the toner image is transferred onto a recording material surface by a transfer bias applied to a transfer roller to form an image. In the image forming apparatus, the transfer bias applying means has a high voltage power source capable of switching between constant voltage control and constant current control, and the transfer bias control is switched between constant voltage control and constant current control for a predetermined number of prints. Features.

  A second aspect of the invention is characterized in that, in the first aspect of the invention, the switching of the transfer bias control is changed in accordance with a transfer roller resistance value.

  The invention described in claim 3 is the invention described in claim 1, characterized in that the switching of the transfer bias control is changed according to the use environment condition.

  According to the present invention, by changing the transfer bias control method for the second side at the time of automatic duplexing from constant voltage control to constant current control for a predetermined number of prints, the influence on the image (transfer failure of high resistance paper, etc.) is suppressed. However, the drum memory at the end of the photosensitive drum due to the transfer bias can be reduced, and the occurrence of end fogging can be prevented.

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

<Embodiment 1>
FIG. 1 shows an image forming apparatus according to the present invention, that is, an image forming apparatus provided with a fixing device according to the present invention. 1 is a longitudinal sectional view showing a schematic configuration of a laser printer as an example of an image forming apparatus according to the present invention. First, the configuration of a laser printer (hereinafter referred to as “image forming apparatus”) will be described with reference to FIG.

  The image forming apparatus shown in FIG. 1 includes a drum-type electrophotographic photosensitive member (hereinafter referred to as “photosensitive drum”) 1 as an image carrier. The photosensitive drum 1 is configured by providing a photosensitive material such as OPC (organic optical semiconductor), amorphous selenium, and amorphous silicon on a drum base on a cylinder formed of aluminum, nickel, or the like.

  The photosensitive drum 1 is rotationally driven at a predetermined process speed (circumferential speed) in the direction of arrow R1 by a driving means (not shown).

  The surface of the photosensitive drum 1 is uniformly charged to a predetermined polarity and potential by a charging roller (charging means) 2.

  On the photosensitive drum 1 after charging, an electrostatic latent image is formed by a laser scanner (exposure means) 3. The laser scanner 3 performs scanning exposure that is ON / OFF controlled in accordance with image information, removes the electric charge of the exposed portion, and forms an electrostatic latent image on the surface of the photosensitive drum 1.

  This electrostatic latent image is developed by a developing device (developing means) 4 and is visible. As a development method, a jumping development method, a two-component development method, or the like is often used, and a combination of image exposure and reversal development is often used. The above-described electrostatic latent image is developed as a toner image with toner attached thereto by the developing roller 4a.

  The toner image on the photosensitive drum 1 is transferred to the surface of the recording material P. The recording material P stored in the paper feed cassette 11 is fed one by one by the paper feed roller 12 and is transferred to the transfer nip between the photosensitive drum 1 and the transfer roller 5 via the registration roller 13 and the like. It is supplied to the part T. At this time, the leading edge of the recording material P is detected by the top sensor 9, and the leading edge of the recording material P is transferred to the transfer nip portion T from the position of the top sensor 9 and the transfer nip portion T and the conveyance speed of the recording material P. The timing to reach is detected. The toner image on the photosensitive drum 1 is transferred by applying a transfer bias to the transfer roller (transfer means) 5 on the recording material P that has been fed and conveyed at a predetermined timing as described above.

  The recording material P on which the toner image has been transferred is conveyed to a fixing device (fixing means) 6 and heated while being nipped and conveyed at a fixing nip portion between the fixing roller 6a and the pressure roller 6b in the fixing device 6. The pressure is applied to fix the toner image on the surface, and then the paper is discharged onto a paper discharge tray 10a formed on the upper surface of the image forming apparatus main body 10. On the other hand, the toner (transfer residual toner) that has not been transferred to the recording material P and remains on the surface of the photosensitive drum 1 after the transfer of the toner image is removed by the cleaning blade 7a of the cleaning device (cleaning means) 7, and the next image formation. Provided.

  By repeating the above operation, image formation can be performed one after another. Note that the image forming apparatus according to the present embodiment can perform printing at a printing speed of 600 dpi, 35 sheets / minute (LTR longitudinal feed: process speed of about 201 mm / sec). Only A4 and LTR support automatic duplex printing.

In the image forming apparatus configured as described above, as shown in FIG. 2, the transfer roller 5 is a sponge-like elastic material in which EPDM in which carbon is dispersed as a conductive material is foamed on a core metal 5a such as iron or stainless steel (SUS). A body 5b is provided. The hardness of the transfer roller 5 is preferably 20 to 70 degrees (when Asker C is loaded with 1 kg), and the resistance value is preferably in the range of 10 ⁶ to 10 ⁹ Ω. In this embodiment, the hardness is 29 degrees and the resistance value is 4 ×. A roller of 10 ⁷ to 1 × 10 ⁸ Ω (23 ° C., 50% environment) was used.

  The transfer roller 5 is pressed against the photosensitive drum 1 by a pressure spring 5 c and forms a transfer nip T with the photosensitive drum 1. The transfer roller 5 is driven to rotate in the direction of arrow R5 when a driving force is transmitted from a driving gear (not shown).

  At this time, a predetermined bias voltage value is applied to the transfer roller 5 by a high voltage power supply circuit (transfer bias application power supply) 5d. At this time, the current value flowing from the transfer roller 5 to the photosensitive drum 1 or the recording material P converts the transfer current flowing in the high-voltage power supply circuit 5d into a voltage and feeds it back to the CPU (control means) 5e via the A / D converter 5f. Is done.

  The CPU 5e converts the PWM signal for controlling the high-voltage power supply circuit 5d into a voltage value through the low-pass filter 5g in accordance with the amount of current, and obtains a predetermined transfer bias voltage value by driving the high-voltage power supply circuit 5d. be able to.

  The above-described control of the transfer bias voltage value will be described with reference to FIGS.

  When a print command is sent from the controller 8 to the engine control unit 14, the engine control unit 14 starts feeding the recording material P by the paper feed roller 12, and at the same time, the heating start-up operation of the fixing device 6 and the image forming process. Preparation rotation (pre-rotation) of the previous photosensitive drum 1 is started. During the pre-rotation, a predetermined bias voltage is applied to the charging roller 2 so as to keep the surface potential of the photosensitive drum 1 at the dark portion potential Vd by the charging roller 2. The transfer roller 5 gradually increases the PWM control value from the CPU 5e so that a predetermined current flows to the dark part of the photosensitive drum 1, and finely adjusts the PWM control value when the target current value is reached. Control is performed so that a constant current value flows to the photosensitive drum 1 while controlling the transfer bias voltage value.

  The PWM1 value (transfer to this PWM1) is output to hold the average value of PWM values during this control period as PWM0 (the transfer bias voltage value for this PWM0 is Vt0) and output the transfer bias voltage value during the image forming operation. The bias voltage value is Vt1). In the embodiment, PWM1 is determined based on a control expression expressed by a linear expression with respect to PWM0. More specifically, PWM1 = A × PWM0 + B, and A and B indicate constants. The relationship between the PWM value and the transfer bias voltage value Vt is linear as shown in FIG. 4, and the transfer bias voltage value Vt is determined by determining the PWM value.

  FIG. 5 shows an example of a specific transfer bias control expression. 5A shows a voltage format, and FIG. 5B shows a PWM format. The transfer bias on the first surface is represented by PWM1 (Vt1), and the transfer bias on the second surface is represented by PWM2 (Vt2). According to the control formula of FIG. 5, the transfer bias is controlled so that image defects such as transfer defects do not occur on not only plain paper but also high resistance paper.

  Next, a specific experimental example will be described.

  As described above, the edge fogging is a phenomenon that occurs remarkably in a low temperature / low humidity environment and a condition where the transfer roller resistance value is low.

When double-sided continuous printing is performed in the conditions shown in Table 1 where edge fogging is most likely to occur (low temperature / low humidity environment (room temperature 15 ° C., humidity 10%), transfer roller resistance lower limit (4 × 10 ⁷ Ω)) The state of occurrence of edge fogging is shown. The transfer bias control is constant voltage control by the control equation shown in FIG.

表１から連続プリントを続けると徐々に端部かぶりが悪化していくことが分かる。これは、２面目の転写バイアスによって感光ドラムの非通紙部でプラス電荷が蓄積されていくためである。尚、このときの２面目の転写バイアス値Ｖｔ２は１．５ｋＶ、転写電流は約１１μＡであった。

From Table 1, it can be seen that the edge fog gradually deteriorates when continuous printing is continued. This is because positive charges are accumulated in the non-sheet passing portion of the photosensitive drum by the transfer bias on the second surface. At this time, the transfer bias value Vt2 on the second surface was 1.5 kV, and the transfer current was about 11 μA.

  Next, Table 2 shows the relationship between the edge fog and the transfer bias.

表２より２面目の転写バイアスを低くすることによって端部かぶりが改善されることが分かる。尚、転写バイアス制御は定電圧制御である。

It can be seen from Table 2 that the edge fog is improved by lowering the transfer bias on the second surface. The transfer bias control is constant voltage control.

  Tables 3 and 4 show the relationship with transfer failure when the transfer bias is controlled at a constant voltage / constant current.

表３から分かるように端部かぶりが発生しない転写電圧値（１．２５ｋＶ）で定電圧制御した場合、高抵抗紙の転写不良マージンが十分でないために転写材の抵抗値のバラツキによって転写電流値がバラツキ転写不良が発生する場合がある。これに対して、端部かぶりが発生しない転写電流値（９μＡ）で定電流制御した場合、転写に必要な転写電流値で制御されるため、高抵抗紙でも軽微な転写不良が発生する程度である。

As can be seen from Table 3, when the constant voltage control is performed with the transfer voltage value (1.25 kV) at which the edge fog does not occur, the transfer failure value of the high resistance paper is not sufficient, and therefore the transfer current value due to the variation of the resistance value of the transfer material. However, there is a case where variation transfer failure occurs. On the other hand, when constant current control is performed with a transfer current value (9 μA) that does not cause edge fogging, control is performed with a transfer current value necessary for transfer. is there.

  From the above results, in the present embodiment, regarding the transfer bias control on the second side during double-sided continuous printing, constant voltage control is used for the fifth and lower sheets, and constant current control is used for the sixth and subsequent sheets.

  A specific example of the transfer bias control process will be described with reference to FIG.

  FIG. 5 is a flowchart showing the transfer bias control process.

First, after processing is started by a print command, PWM0 (Vto) is obtained (step S101). Next, it is determined whether the job is single-sided printing or double-sided printing (step S102). In the case of single-sided printing, the transfer bias value PWM1 (Vt1) for the first surface is determined from the previously determined PWM0 (Vt0) value (step S103), and PWM1 (in synchronization with the transfer material P reaching the transfer portion) Vt1) is applied to perform transfer (step S104). When the job is duplex printing, the transfer bias PWM1 (Vt1) for the first surface and the transfer bias PWM2 (Vt2) for the second surface are calculated from PWM0 (Vt0) (step S105). Thereafter, the number of continuous prints is counted (step S106). If the number of continuous prints is 5 or less, the transfer is performed by applying PWM1 (Vt1) at the time of transferring the first side and PWM2 (Vt2) at the time of transferring the second side (step S106). Step S107). When the number of continuous prints is 6 or more, the transfer is performed by PWM1 (Vt1) at the time of transfer of the first surface and 9 μA constant current control at the time of transfer of the second surface (step S108). Printing is performed until the designated number of prints is reached by the above transfer bias control (step S109).
Table 5 shows the effect of preventing edge fogging by the transfer bias control of this embodiment.

表５から本実施の形態の転写バイアス制御により連続両面プリントを継続しても端部かぶりの発生を防止することができることが分かる。

From Table 5, it can be seen that edge fogging can be prevented even if continuous double-sided printing is continued by the transfer bias control of the present embodiment.

  As described above, by changing the transfer bias control for the second automatic double-sided surface from constant voltage control to constant current control with a predetermined number of sheets, the influence on the image quality is minimized and the occurrence of edge fogging at the edge of the paper is generated. Can be prevented.

<Embodiment 2>
In the present embodiment, an example will be described in which NBR, which is a polymer conductive material, is used as the transfer roller 5 and transfer bias control switching is changed in accordance with the transfer roller resistance detection result (value of PWM0 (Vt0)). The other conditions are the same as those in the first embodiment, and the description thereof will be omitted.

  Compared to the electronic conductive material such as EPDM used in the first embodiment, the polymer conductive material such as NBR has less resistance unevenness in the circumferential direction and the longitudinal direction, and is excellent in dealing with high image quality and high speed. ing. On the other hand, NBR has a problem that resistance fluctuation is relatively large compared to EPDM depending on the use environment conditions such as temperature and humidity.

  FIG. 6 shows the relationship between the resistance value and temperature of the NBR transfer roller and the EPDM transfer roller. As can be seen from FIG. 6, the resistance value of the NBR transfer roller rapidly decreases as the temperature rises, but the resistance value of the EPDM transfer roller varies little even when the temperature changes.

  FIG. 7 shows changes in the transfer roller temperature during automatic duplex printing. As can be seen from FIG. 7, when automatic double-sided printing is continued, the transfer roller temperature gradually rises.

  6 and 7, it can be seen that the NBR transfer roller has a tendency that the resistance of the transfer roller gradually decreases and the edge fog tends to occur when automatic double-sided printing is performed.

  Table 5 shows the relationship between the transfer roller resistance value, that is, the value of PWM0 (Vt0) and the edge fogging.

表５よりＰＷＭ０（Ｖｔ０）の値が小さい、つまり、転写ローラの抵抗値が低いと端部かぶりが発生することが分かる。

From Table 5, it can be seen that edge fogging occurs when the value of PWM0 (Vt0) is small, that is, when the resistance value of the transfer roller is low.

  From the above results, in this embodiment, transfer bias control is switched according to the value of PWM (Vt0).

  The transfer bias control of this embodiment will be described in detail with reference to FIG.

  First, after processing is started by a print command, PWM0 (Vto) is obtained (step S201). Next, it is determined whether the job is single-sided printing or double-sided printing (step S202). In the case of single-sided printing, the transfer bias value PWM1 (Vt1) for the first surface is determined from the previously determined PWM0 (Vt0) value (step S203), and PWM1 (in synchronization with the transfer material P reaching the transfer portion) Vt1) is applied to perform transfer (step S204). If the job is duplex printing, the transfer bias PWM1 (Vt1) for the first surface and the transfer bias PWM2 (Vt2) for the second surface are calculated from PWM0 (Vt0) (step S205).

  Next, the value of PWM0 (Vt0) is determined (step S206). If the value of PWM0 (Vt0) is greater than 75 (1.4 kV), the number of continuous prints is determined, and if the value of PWM0 (Vt0) is 75 (1.4 kV) or less, the number of continuous prints is determined ( In step S207), if the number of continuous prints is 5 or less, transfer is performed by applying PWM1 (Vt1) at the time of transferring the first side and PWM2 (Vt2) at the time of transferring the second side (step S208). When the number of continuous prints is 6 or more, the transfer is performed by PWM1 (Vt1) at the time of transfer of the first surface and 9 μA constant current control at the time of transfer of the second surface (step S209). Printing is performed until the designated number of prints is reached by the above transfer bias control (step S210).

  Table 6 shows the result of comparison of the edge fogging between the case where the transfer bias control of the present embodiment is performed and the conventional transfer bias control.

  It can be seen from Table 6 that no end fogging occurs in the present embodiment. In this embodiment, the presence / absence of switching of the transfer bias control is changed according to the value of PWM0 (Vt0), but it is also possible to change the number of sheets to be switched.

  As described above, end fogging is likely to occur by changing whether or not the transfer bias control is switched from the constant voltage control to the constant current control according to the value of PWM0 (Vt0), that is, the transfer roller resistance value detection result. Since the transfer bias is changed only under conditions, it is possible to prevent edge fogging and at the same time obtain a good image even with high resistance paper.

<Embodiment 3>
In this embodiment, a sensor for detecting the temperature of the environment is provided in the apparatus main body, and whether or not to switch the transfer bias control on the second side during automatic duplex printing from constant voltage control to constant current control according to the detection result of the sensor. The control to change is demonstrated. The other conditions are the same as those in the above embodiment, and a repetitive description is omitted.

  As described above, the edge fogging is closely related to the charging process, and when the charging roller 2 has a capability of charging the photosensitive drum 1 (hereinafter referred to as charging capability), the drum memory due to the transfer bias may be erased. This is not possible and edge fogging is likely to occur.

  As a condition for reducing the charging ability, there is a case where it is used in a low temperature and low humidity environment. This is because the DC resistance value of the photosensitive drum 1 and the charging roller 2 is increased, so that a charging bias is supplied to the charging roller 2. This is because the high-voltage power supply cannot supply enough electric charge.

  Table 7 shows the relationship between room temperature and edge fogging.

表７から室温が低い方が端部かぶりのレベルが悪化し、発生しやすいことが分かる。

From Table 7, it can be seen that the lower the room temperature, the worse the edge fogging level and the more likely it occurs.

  From the above results, in this embodiment, the transfer bias control is switched according to the room temperature.

  The transfer bias control of this embodiment will be described in detail with reference to FIG.

  First, after processing is started by a print command, PWM0 (Vto) is obtained (step S301). Next, it is determined whether the job is single-sided printing or double-sided printing (step S302). In the case of single-sided printing, the transfer bias value PWM1 (Vt1) for the first surface is determined from the previously determined PWM0 (Vt0) value (step S303), and PWM1 (in synchronization with the transfer material P reaching the transfer portion) Vt1) is applied to perform transfer (step S304). If the job is duplex printing, the transfer bias PWM1 (Vt1) for the first surface and the transfer bias PWM2 (Vt2) for the second surface are calculated from PWM0 (Vt0) (step S305).

  Next, room temperature determination (step S306), PWM0 (Vt0) value determination (step S307), and continuous print sheet number determination (step S308) are performed. When the room temperature is 25 ° C. or less and the value of PWM0 (Vt0) is 75 (1.4 kV) or less, and the number of continuous prints is 6 or more, PWM1 (Vt1) is transferred at the time of transfer of the first surface, and transfer of the second surface The bias is set to 9 μA constant current control (step S309). In conditions other than the above, the transfer is performed by applying PWM1 (Vt1) at the time of transferring the first surface and PWM2 (Vt2) at the time of transferring the second surface (step S310). Printing is performed until the designated number of prints is reached by the above transfer bias control (step S311).

  Table 8 shows the result of comparison of the edge fogging level between the transfer bias control of the present embodiment and the conventional transfer bias control.

表８から分かるように温度検知結果に応じて転写バイアス制御の切り替えを行うことによって端部かぶりの発生が防止することができる。尚、本実施の形態では室温のみで転写バイアス制御の切り替えを行ったが、端部かぶりは低湿環境でも発生し易いため、湿度の条件に応じて切り替える方法も有効であった。

As can be seen from Table 8, the occurrence of edge fogging can be prevented by switching the transfer bias control according to the temperature detection result. In this embodiment, transfer bias control is switched only at room temperature. However, since edge fogging easily occurs even in a low humidity environment, a method of switching according to humidity conditions is also effective.

  As described above, by detecting the environment and changing the presence / absence of switching from the constant voltage control to the constant current control on the second surface of the automatic double-sided transfer from the detected environmental information (temperature / humidity information) At the same time, it is possible to obtain a good image even with high resistance paper in an environment where edge fog does not occur.

1 is a configuration diagram of an image forming apparatus according to the present invention. It is a block diagram of the transfer apparatus which concerns on this invention. It is a figure showing the relationship between a PWM value and a transfer voltage value. (A) is a figure showing the transfer bias value (voltage form) of this invention, (b) is a figure showing the transfer bias value (PWM form) of this invention. It is a figure showing the transfer bias control of Embodiment 1 of this invention. It is a figure which shows the relationship between transfer roller temperature and resistance value. It is a figure which shows the automatic double-sided printed sheet number and the transfer roller temperature transition. It is a figure which shows the transfer bias control of Embodiment 2 of this invention. It is a figure which shows the transfer bias control of Embodiment 3 of this invention. It is a block diagram of a conventional image forming apparatus. It is a chart showing a conventional transfer bias sequence. It is a figure explaining the subject which the present invention tends to solve.

Explanation of symbols

1 Photosensitive drum (image carrier)
2 Charging roller (charging means)
3 Laser scanner (exposure means)
4 Developing Device 5 Transfer Roller 5a Core Bar 5b Elastic Body 5c Pressure Spring 5d High Voltage Power Supply Circuit (Transfer Bias Application Power Supply)
5e CPU
5g Low-pass filter 6 Fixing device 6a Fixing roller 6b Pressure roller 7 Cleaning device 7a Cleaning blade 9 Top sensor 10 Image forming apparatus body 10a Paper discharge tray 11 Paper feed cassette 12 Paper feed roller 13 Registration roller 20 Paper discharge guide 21 Paper discharge roller 22 Second side paper feed roller P Recording material

Claims (3)

In an image forming apparatus for forming a toner image on an image carrier and transferring the toner image to a recording material surface by a transfer bias applied to a transfer roller to form an image.
The transfer bias applying means has a high voltage power source capable of switching between constant voltage control and constant current control, and the transfer bias control is switched between constant voltage control and constant current control for a predetermined number of prints. apparatus.   2. The image forming apparatus according to claim 1, wherein switching of the transfer bias control is changed according to a transfer roller resistance value.   2. The image forming apparatus according to claim 1, wherein switching of the transfer bias control is changed according to use environment conditions.

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