JP2004258534A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent shortness of previous rotation time, electrification inferiority, and transfer inferiority in an image forming apparatus for applying electrification bias selection and foreign matter removal bias during image formation in previous rotation by using voltage selection control between electrification alternating current peaks. <P>SOLUTION: An electrification bias selection process is previously performed, and a process for applying voltage between the maximum applicable alternating current peaks is performed thereafter for the electrification bias voltage in the previous rotation, and positive polarity bias of transfer is applied after completing the electrification bias selection process. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an image forming apparatus employing an electrophotographic method, an electrostatic recording method, or the like.
[0002]
[Prior art]
(1) Image forming process
The image forming apparatus generally includes a photosensitive drum 10 as a latent image carrier, a charging device 11 for uniformly charging the photosensitive drum, and a uniformly charged photosensitive drum as shown in FIG. Exposure device 12 for forming an electrostatic latent image by exposing the image to a developing device, developing device 13 for developing the electrostatic latent image by developing the electrostatic latent image with toner as a developer, and recording the toner image on the photosensitive drum on paper or the like The image forming apparatus includes a transfer device 15 for transferring the image to the medium 14, a fixing device 16 for fixing the toner image on the recording medium, and a cleaning device 17 for scraping off the toner remaining on the photosensitive drum 10. The photosensitive drum 10, the charging device 11, the developing device 13, and the cleaning device 17 are often of a process cartridge type that can be attached to and detached from the image forming apparatus main body.
[0003]
The image forming apparatus performs image formation by repeating the steps of charging, exposing, developing, transferring, fixing, and cleaning using the above-described means.
[0004]
(2) Image forming apparatus operation sequence
FIG. 9 shows an operation sequence of a general image forming apparatus.
[0005]
When the detachable process cartridge is inserted into the main body of the image forming apparatus and the power supply of the main body of the image forming apparatus is turned on, the main motor is driven, and the multi-rotation process starts. In this step, detection of the presence or absence of the process cartridge, cleaning of the transfer roller (discharging toner adhered to the transfer roller onto the photosensitive drum), and the like are performed.
[0006]
When the pre-multi-rotation step is completed, the image forming apparatus enters a standby state. When image information is sent from an output unit such as a host computer (not shown) to the image forming apparatus, the main motor drives the image forming apparatus main body to enter a pre-rotation step. In this step, a printing preparation operation of various process devices is performed, and preliminarily charging on a photosensitive drum, starting up of a laser scanner, determination of a transfer voltage at the time of image formation, temperature adjustment of a fixing device, and the like are mainly performed.
[0007]
When the pre-rotation step is completed, the image forming step is started. In the image forming process, feeding of a recording medium, image exposure on a photosensitive drum, development, transfer, fixing, and the like are performed at a predetermined timing.
[0008]
When the image forming process is completed, if there is a next print signal, the process enters the paper interval process and waits for the next printing operation until the next recording medium arrives. If there is no next print signal, the image forming apparatus enters a post-rotation step. In the post-rotation step, static elimination of the surface of the photosensitive drum, cleaning of the transfer roller, and the like are performed.
[0009]
When the post-rotation process is completed, the image forming apparatus returns to the standby state, and waits for the next print signal.
[0010]
(3) Charging device, charging bias voltage control method
As the charging device 11, a contact charging method in which a charging device is brought into contact with the surface of a photosensitive drum such as a roller type or a blade type and a voltage is applied to the charging device to charge the surface of the photosensitive drum is widely used. The roller type charging system can perform stable charging over a long period of time.
[0011]
The charging bias power supply applies a charging bias voltage to a charging roller as a charging device. As the charging bias voltage, the charging on the photosensitive drum may be performed by applying only a DC voltage. However, as shown in Patent Document 1, a DC voltage Vdc corresponding to a desired dark potential Vd on the drum is changed to a DC voltage Vdc. A bias voltage obtained by superimposing an AC voltage having a peak-to-peak voltage (Vpp) that is twice or more the discharge start voltage at the time of application is generally used (hereinafter, DC is DC, AC is AC, and main charging is AC + DC charging). Notation).
[0012]
This charging method is excellent in uniformly charging the photosensitive drum 10. When an AC voltage of a certain level or more is superimposed on the DC voltage, local unevenness of the potential (poor charging) on the photosensitive drum is eliminated by the smoothing effect of the AC component, and the charged potential Vd on the photosensitive drum surface is changed to DC. It converges uniformly to the voltage value Vdc.
[0013]
AC + DC charging is characterized in that the discharge current value to the photosensitive drum is larger than DC charging in which only a DC voltage is applied. When the discharge current value to the photosensitive drum increases, the molecules connecting the molecules on the surface of the photosensitive drum tend to be broken. As a result, the resin on the surface of the photosensitive drum has a low molecular weight and can be easily scraped off by the cleaning blade. As a result, the surface of the photosensitive drum is polished, and even when used repeatedly, for example, transfer residual toner does not contaminate the surface of the photosensitive drum, and is always kept in a refreshed state equivalent to that in the initial use. An image (charging step) can be entered.
[0014]
However, if an excessive discharge current is continuously applied to the surface of the photosensitive drum, the speed at which the surface layer of the photosensitive drum is abraded increases, so that the photosensitive layer of the photosensitive drum cannot perform its function at an early stage from the start of use. The film reaches its thickness and reaches its end of life. When the film thickness reaches the limit thickness, the function as the photosensitive layer is reduced, and minute charging unevenness occurs, and poor charging occurs along with a decrease in the surface charge holding ability. For this reason, in actual use, it is necessary to set the discharge amount to the photosensitive drum surface so as not to be too large.
[0015]
The relationship between the AC peak-to-peak voltage Vpp and the discharge current value is not always constant because it varies depending on the use environment (change in impedance of the charging roller), the thickness of the charge transport layer of the photosensitive drum, and the like. For example, even if the same AC peak-to-peak voltage Vpp is applied to the charging roller, the discharge amount is small because the impedance of the charging roller increases in a low-temperature and low-humidity environment, and conversely, the impedance of the charging roller decreases in a high-temperature and high-humidity environment. The amount increases. Further, even if the use environment is the same, if the surface of the photosensitive drum is scraped by the cleaning blade with use, the impedance is reduced as compared with the initial use, so that the discharge amount increases.
[0016]
In order to avoid this problem, Patent Document 2 proposes a method for controlling the AC component with a constant current. In this method, an AC current Iac flowing from the charging roller to the photosensitive drum is detected and controlled so as to be constant. When this method is used, an AC peak-to-peak voltage Vpp is applied to a change in impedance of the charging roller and the photosensitive drum. Changes freely, so that the discharge current value can be kept almost constant. This method has a very great effect in terms of ensuring good chargeability and preventing excessive discharge to the photosensitive drum.
[0017]
(4) Transfer device, transfer bias voltage control method
As the transfer device 15, a contact transfer system in which a transfer device is brought into contact with a latent image carrier to perform transfer to a transfer material is mainly used, and among them, a roller transfer system excellent in transfer material transferability in a transfer unit is used. It has become mainstream. In the roller transfer method, the transfer roller is pressed against the photosensitive drum with a total pressure of 4.9 to 19.6 N (0.5 to 2.0 kg) to form a transfer nip between the photosensitive drum and the transfer roller. The toner image on the photosensitive drum is transferred onto the transfer material by the action of the bias voltage applied to the transfer roller while nipping and transporting the transfer material at the transfer nip portion.
[0018]
In an image forming apparatus (for example, a copying machine or a laser beam printer) having a contact type transfer unit, a transfer bias applied to a contact transfer member is generally controlled by a constant voltage or a constant current.
[0019]
In the case of the constant voltage control, the resistance value of the transfer roller used as the contact transfer member changes over several digits depending on the environment, and it is difficult to always apply a stable transfer bias regardless of the environment.
[0020]
On the other hand, in the constant current control, the above-described drawback caused by the change in the resistance value of the transfer roller is eliminated, and the amount of charge required for transfer can be always secured. However, this type of image forming apparatus has various sizes. Normally, a transfer material can be used. Therefore, when a small-size transfer material is passed, a non-sheet-passing area where the photosensitive drum and the transfer member directly contact with each other is widened. Most of the current flows in the paper passing area, and particularly in a low-temperature and low-humidity environment, a transfer charge is insufficient and transfer failure occurs.
[0021]
In order to eliminate such a drawback, constant current control is performed during non-sheet passing when there is no transfer material at the transfer portion, and the voltage at this time is held, and constant voltage control is performed at this voltage during sheet passing. An active transfer voltage control (hereinafter simply referred to as “ATVC”) has been proposed.
[0022]
Specifically, a constant current is applied to the dark portion (Vd portion) of the photosensitive drum, which is a constant value, to monitor the generated voltage, and the voltage is increased by (1) equal times, (2) coefficients times, (▲). 3) The applied bias is controlled by performing a combination such as applying a constant voltage, and has a certain effect in preventing the occurrence of variations in transferability due to environmental fluctuations and differences in transfer material size.
[Patent Document 1]
JP-A-63-149669
[Patent Document 2]
Japanese Patent Publication No. 06-093150
[0023]
[Problems to be solved by the invention]
First, the background art of the problem to be solved by the present invention will be described.
[0024]
(1) Charge AC peak-to-peak voltage selection control
If a method of controlling the AC component with a constant current as the charging bias voltage is used as in Patent Document 2 described above, it is possible to ensure good charging properties and prevent excessive discharge to the photosensitive drum. However, in this method, in order to obtain a stable bias voltage, AC and DC power supplies to be superimposed are separated, and a boosting transformer as two voltage boosting means is required. A step-up transformer is relatively large and expensive in a power supply circuit. In particular, in a small-sized and low-cost image forming apparatus, a single voltage step-up means is independent of the use environment and the thickness of the photosensitive drum. It has been desired to output a stable charging bias voltage and supply a stable discharge current to the photosensitive drum.
[0025]
According to the study of the present inventors, it has been found that, with the following configuration, a stable discharge current can be supplied even with a charging bias power supply circuit using one voltage boosting unit regardless of the use environment.
[0026]
FIG. 10 shows a conceptual diagram of the charging bias power supply circuit. In this method, a plurality of AC oscillation outputs (Vpp-1, Vpp-2,..., Vpp-n: * Vpp-1>Vpp-2>...>Vpp-n>. The step-up transformer has only one step-up transformer that generates an AC component, and the step-up transformer generates the DC component by peak-charging the capacitor C10.
[0027]
The engine controller applies a plurality of AC peak-to-peak voltages Vpp from the AC oscillation output, and the AC current Iac flowing through the photosensitive drum 10 is equal to or higher than the required charging AC peak-to-peak voltage selection control threshold current Iac-0 for preventing occurrence of charging failure. And the minimum Vpp is selected as the charging AC peak-to-peak voltage during image formation.
[0028]
By performing such charging bias voltage control, it becomes possible to make a substantially constant current value transition regardless of the impedance change of the charging roller, the photosensitive drum, etc., as in the case of performing the constant current control. Was.
[0029]
Hereinafter, this charging voltage control method is referred to as charging AC peak-to-peak voltage selection control.
[0030]
(2) Foreign matter removal sequence on photosensitive drum
In FIG. 11, when the image forming process is completed and the image forming apparatus is stopped, foreign matters 19 such as transfer residual toner and shavings of the photosensitive drum are caught between the cleaning blade 17 and the photosensitive drum 19. Pressing against the photosensitive drum 10 makes it difficult for the photosensitive drum 10 to peel off. At the point X on the surface of the photosensitive drum to which the foreign matter 19 adheres, the coefficient of friction changes compared to the position on the photosensitive drum where the foreign matter 19 does not adhere.
[0031]
In this state, the next image forming process starts. When the point X goes around once and reaches the cleaning blade 17 again, only the point X has a different coefficient of friction as compared with the other points, so that the driving torque fluctuates. The rotation speed of the photosensitive drum changes. As a result, exposure blur occurs at the exposure position Y, and a white stripe image having a uniform length as shown in FIG. 12 is generated. When this position reaches the cleaning blade position again, the same phenomenon is repeated, and a white streak image is generated with a period R for one rotation of the photosensitive drum.
[0032]
This problem can be solved by extending the photosensitive drum rotation time during the previous rotation. If the rotation time of the photosensitive drum before image formation is extended, the chance of the foreign matter adhering portion passing through the cleaning blade is increased, so that the foreign matter can be completely removed finally. However, this method takes a long time to form an image and greatly increases the time of the entire printing process, which is not preferable from the viewpoint of usability.
[0033]
According to the study of the present inventors, the maximum AC peak-to-peak voltage that can be applied during the pre-rotation is set to at least one rotation of the photosensitive drum, more preferably 3 times, in order to remove the foreign matter attached to the photosensitive drum at the time of stop. It has been found that this problem can be solved by applying a voltage equal to or more than the circumference.
[0034]
By using this method, when the maximum applicable AC peak-to-peak voltage is applied, the foreign matter adhering to the photosensitive drum is efficiently peeled off by the cleaning blade, thereby rotating the photosensitive drum during the pre-rotation. Even if the time is not extended, the surface of the photosensitive drum is refreshed, and a good image can be always provided.
[0035]
Hereinafter, a high AC peak-to-peak voltage applied to the photosensitive drum during a part of the pre-rotation is referred to as a foreign matter removing bias.
[0036]
In an image forming apparatus which performs the above-described charging AC peak-to-peak voltage selection control and charging control for applying a foreign matter removing bias during the pre-rotation, various controls are performed during the pre-rotation. If it is applied, the pre-rotation process itself becomes longer, and the image formation itself takes time.
[0037]
Further, in the charging AC peak-to-peak voltage selection control, in a region where a low charging voltage during the previous rotation is applied, a charging voltage sufficient to charge the photosensitive drum is not applied. The potential Vd is in a state where the potential Vd is not sufficiently cut off. If a transfer voltage is applied for the purpose of the ATVC control or the like when the charging potential Vd on the photosensitive drum is unstable, the photosensitive drum surface is excessively positively charged due to the influence of the positive transfer voltage. If this occurs immediately before the image formation, in the worst case, due to the effect of the positive charging, a black dot-shaped charging failure may occur for one rotation of the photosensitive drum.
[0038]
Therefore, it has been desired to provide an image forming apparatus in which these controls performed during the pre-rotation are performed more speedily, and in which image harmfulness such as defective charging does not occur.
[0039]
The present invention has been made in view of the above-described problems, and its object is to speed control the charging AC peak-to-peak voltage selection control and the image forming apparatus performing the foreign matter removal bias application during the pre-rotation, and An object of the present invention is to provide an image forming apparatus which does not cause an adverse effect on an image such as poor charging.
[0040]
[Means for Solving the Problems]
The above problem can be solved by the following means. That is, a charging bias voltage control method of performing charging bias selection and foreign matter removal bias application of charging AC peak-to-peak voltage selection control during pre-rotation, and in an image forming apparatus that applies a positive transfer voltage during pre-rotation The application of the charging bias during the pre-rotation is performed before the application of the foreign matter removing bias, and the transfer voltage application is started after the completion of the selection of the charging bias; It is characterized by.
[0041]
BEST MODE FOR CARRYING OUT THE INVENTION
<First embodiment>
(1) Image formation process
The image forming apparatus generally includes an electrophotographic drum 2 serving as a latent image carrier as shown in FIG. 3, a charging device 3 for uniformly charging the photosensitive drum 2, and an image exposure L on the charged photoconductor. Exposure device 4 for forming an electrostatic latent image by developing the toner, developing device 5 for developing the electrostatic latent image by developing toner electrostatic toner with toner 5-a, and transferring the developed toner to paper or other transfer material The transfer device 6 includes a transfer device 6 that transfers the toner image to the transfer material 7, a fixing device 8 that fixes the toner image transferred to the transfer material 7, and a cleaning device 9 that scrapes off the toner remaining on the photosensitive drum 2.
[0042]
The image forming apparatus forms an image by repeating the steps of charging, exposing, developing, transferring, fixing and cleaning using the above-described means.
[0043]
A process cartridge C includes a photosensitive drum 2, a charging device 3, a developing device 5, and a cleaning device 9 and is detachable from the image forming apparatus main body 20. The mounting and dismounting of the process cartridge C is performed by opening and closing the open / close door 21 of the image forming apparatus main body 20.
[0044]
The charging device 3 is a charging roller, which is rotatably supported by a conductive bearing 3-a, and urges the conductive bearing 3-a in the direction of the photosensitive drum by a conductive spring member 3-b to cause the charging roller to rotate. 2 is pressed against the surface. The charging roller 3 rotates following the rotation of the photosensitive drum 2. Reference numeral 1 denotes a charging bias voltage power supply circuit for the charging device 3.
[0045]
Reference numeral a denotes an exposure window provided in the process cartridge, and the photosensitive drum exposure L by the exposure device 4 is performed through the exposure window a. In the developing device 5, 5-c is a developing sleeve, and 5-b is a developer coating blade. The transfer device 6 is a transfer roller that rotates in contact with the photosensitive drum 2. 6-a is a transfer bias voltage power supply circuit for the transfer roller 6.
[0046]
(2) Operation sequence of image forming apparatus
FIG. 4 is a flowchart illustrating the operation of the image forming apparatus used in the present embodiment.
[0047]
When image information is sent from an output unit such as a host computer to the image forming apparatus, the main motor drives the image forming apparatus and enters a pre-rotation step. In this step, a printing preparation operation of various process devices is performed, and preliminarily charging on the photosensitive drum, starting of a laser scanner, determination of a transfer voltage, temperature adjustment of a fixing device, and the like are mainly performed.
[0048]
When the pre-rotation step is completed, the image forming step (printing step) is continuously performed. In the image forming process, a transfer material is fed, an image is exposed on a photosensitive drum by an exposure device, a developing device is used to develop an electrostatic latent image on the photosensitive drum, a transfer device transfers the image to the transfer material, and a fixing device is used to transfer the image onto the transfer material. The unfixed developer is fixed to the permanent image.
[0049]
When the image forming process is completed, if there is a next print signal, the process enters a sheet interval process until the next transfer material arrives, and waits for the next printing operation.
[0050]
If there is no next print signal after the end of the image forming step, the image forming apparatus enters a post-rotation step. In the post-rotation step, steps such as charge elimination of the surface of the photosensitive drum and cleaning of the transfer roller, which discharges toner adhered to the transfer roller onto the photosensitive drum by applying a negative bias to the core of the transfer roller, are performed.
[0051]
(3) Charge AC peak-to-peak voltage selection control
The charging AC peak-to-peak voltage selection control used in this embodiment will be described.
[0052]
3-1) Flow from detection of AC current to selection of voltage between charging AC peaks
A method of selecting the charging AC peak-to-peak voltage in this example will be described. In FIG. 5, reference numeral 1 denotes a charging bias voltage power supply circuit capable of outputting a superimposed AC and DC voltage to a charging roller 3 serving as a charging unit by one voltage boosting unit and applying two or more types of AC peak-to-peak voltages in a stepwise manner. It is.
[0053]
In the circuit 1, a plurality of AC peak-to-peak voltages (Vpp-1, Vpp-2,..., Vpp-n,... * Vpp-1>Vpp-2>...>Vpp-n>. ) Can be applied to the charging roller 3 via a boosting means T1 as a voltage boosting means and the like, when the charging bias voltage is applied to the charging roller 3, the AC current Iac passes through the charging roller 3 and the photosensitive drum 2 and the high voltage power supply circuit GND Flows to At this time, the AC current detection circuit 9 serving as an AC current detection means samples this AC current only by an AC current having a frequency equal to the charging frequency by a filter circuit (not shown) including a resistor, a capacitor, and the like. Voltage conversion and input to the engine controller. The input voltage V sampled at a constant cycle is averaged in the engine controller.
[0054]
The average-processed input voltage Vave is compared with a charging AC peak-to-peak voltage selection control threshold V0 preset by comparison means in the engine controller. The charging AC peak-to-peak voltage selection control threshold V0 is an output voltage corresponding to a minimum AC peak-to-peak voltage at which charging unevenness does not occur, and its value is a minimum necessary current value Iac capable of performing uniform charging. It is determined based on −0 (charge AC peak-to-peak voltage selection control threshold current). Since the value of Iac-0 varies depending on the process speed and charging frequency of the device, and the constituent materials of the charging roller 3 and the photosensitive drum 2, the charging AC peak-to-peak voltage selection control threshold V0 may be set to an appropriate value each time. .
[0055]
3-2) Method of Selecting Charged AC Peak-to-Peak Voltage in Initial Rotation at Power-On (Multiple Rotations) When the power of the image forming apparatus main body is turned on, the main motor is driven to start the initial rotation (hereinafter, this is referred to as a multi-rotation). Called rotation). At this time, the engine controller of the image forming apparatus main body applies a part or all of the plurality of AC peak-to-peak voltages which can be applied stepwise to the charging roller, and at this time, the voltage flows from the charging roller via the photosensitive drum. The control is performed such that the AC peak-to-peak voltage at which the AC current value is equal to or higher than the charging AC peak-to-peak voltage selection control threshold V0 and which is the minimum is used as the charging bias voltage during image formation.
[0056]
Further, it is preferable that the application time of the applied charging bias voltage is a time during which the latent image bearing member makes one rotation or more. In the photosensitive drum, uneven film thickness in the circumferential direction may occur due to shaving unevenness due to eccentric rotation or the like. As a result, the alternating current Iac flowing varies with the rotation cycle of the photosensitive drum, so that current detection is performed with high accuracy. For this purpose, it is preferable to apply a bias voltage until the photosensitive drum makes one rotation or more. However, if the bias voltage application time is too long, the time of the entire process becomes long, so it should not be too long.
[0057]
3-3) Method of selecting voltage between charging AC peaks during pre-rotation
The selection of the charging bias at the time of image formation in the charging AC peak-to-peak voltage selection control used in this embodiment is also performed at the time of pre-rotation before image formation. The selection of the charging bias at the time of the pre-multi-rotation described in the section 3-2) is performed only when the pre-multi-rotation is performed when the power is turned on. If the charging bias is selected only at this time, an appropriate charging bias cannot be selected at all in an image forming apparatus in which the pre-multiple rotation is not performed (for example, when the power of the image forming apparatus main body is always on). Would. Therefore, by selecting the charging bias during the pre-rotation immediately before the image formation, an appropriate charging bias can be always applied during the image formation.
[0058]
The selection of the charging bias during the pre-rotation is performed according to the following procedure. In FIG. 6, when the charging bias at the time of image formation determined by the charging AC peak-to-peak voltage selection method at the previous multi-rotation described in the section 3-2) is Vpp-n, the Vpp-n at the pre-rotation Only the voltage Vpp- (n + 1) one step lower than that is applied.
[0059]
The charging bias used for image formation gradually decreases due to the influence of the photoreceptor drum surface scraping during use. Therefore, if the detection voltage Vn + 1 when applying Vpp- (n + 1), which is one step lower than the currently specified charging bias Vpp-n, is compared with the charging AC peak-to-peak voltage selection control threshold V0, the voltage switching can be performed. It can be performed at appropriate timing.
[0060]
The detection voltage Vn + 1 is averaged by a calculation means in the engine controller, and the averaged detection voltage Vn + 1-ave is compared with a charging AC peak-to-peak voltage selection threshold V0 by a comparison means. At this time, if Vn + 1-ave <V0, Vpp-n is selected as the charging AC peak-to-peak voltage during image formation, and if Vn + 1-ave ≧ V0, this is switched to Vpp- (n + 1). Perform formation.
[0061]
Since this method does not apply an unnecessary charging voltage and can be performed in a short time, the pre-rotation time is not extended.
[0062]
(4) Foreign matter removal sequence on photosensitive drum
When the main motor is stopped after the image formation is completed, it is possible to prevent a white stripe image having a uniform length from being generated mainly due to foreign matters such as untransferred toner adhered to the cleaning blade contact position on the photosensitive drum. During the pre-rotation, the maximum applicable AC peak-to-peak voltage (including a high AC peak-to-peak voltage close to the maximum AC peak-to-peak voltage) is set as the foreign matter removing bias at least for one rotation of the photosensitive drum, more preferably. Is applied for three or more rounds.
[0063]
Such a foreign matter removing bias has a great significance in being applied at the time of pre-rotation immediately before image formation. Foreign matter that has become difficult to adhere when the previous job is stopped is efficiently removed by immediately before the image formation by the foreign matter removal bias applied during the previous rotation, so that the surface of the photosensitive drum is refreshed immediately before the image formation. , And can always provide good images.
[0064]
(5) Charging / transfer sequence during pre-rotation
The feature of the present embodiment is that, prior to the selection of the charging bias in the charging AC peak-to-peak voltage selection control during the pre-rotation, the application of the foreign matter removing charging bias is performed, and the application of the positive voltage for the transfer is performed after the completion of the selection of the charging bias. To start with.
[0065]
In FIG. 1, in the pre-rotation, after the main motor starts rotating (time S1), first, a bias application for selecting a charging bias of the charging AC peak-to-peak voltage selection control is performed (time S2). The voltage applied between the charging AC peaks at this time is Vpp- (n + 1), which is one step lower than the voltage Vpp-n selected in the initial rotation when the power is turned on, and the application time T1 is one rotation of the photosensitive drum. is there.
[0066]
In order to accurately select the charging bias in the charging AC peak-to-peak voltage selection control, it is necessary to apply the AC peak-to-peak voltage for at least one rotation of the photosensitive drum as described above. In this embodiment, the time is longer and it takes time to complete the image formation, and the number of rotations of the photosensitive drum increases, so that the surface of the photosensitive drum is shaved and its life is shortened. One rotation of the photosensitive drum, which is the minimum necessary number of revolutions, was performed to select the charging bias in the charging AC peak-to-peak voltage selection control.
[0067]
Next, the charging bias power supply applies a foreign matter removing bias (time point S3). At this time, the maximum voltage Vpp-1 that can be applied, which is effective for removing foreign matter, is selected as the voltage between the charging AC peaks. In order to surely remove the foreign matter adhering to the photosensitive drum, the longer the foreign matter removing bias application time T2 is, the better, but at least one photosensitive drum or more, more preferably three photosensitive drums or more. Is applied. As a result, the foreign matter which has adhered to the photosensitive drum and has become difficult to peel off is efficiently removed, and the photosensitive drum surface can be subjected to image formation in a refreshed state. After the application of the foreign matter removal bias, the charging bias power supply applies an AC peak-to-peak voltage Vpp-n (or Vpp- (n + 1)) for image formation (time point S4).
[0068]
The transfer bias has a polarity opposite to the charging polarity of the toner. That is, if the charge polarity of the toner is negative (negative), the transfer bias is positive (positive). If the charge polarity of the toner is positive, the transfer bias is negative.
[0069]
In this embodiment, the charging polarity of the toner is negative, and the transfer bias is positive.
[0070]
Immediately after the application of the charging AC voltage selection bias is completed (time point S3), the transfer bias starts applying a positive bias (weak bias). During pre-rotation, ATVC control is performed to determine an appropriate transfer bias during image formation.
[0071]
Specifically, a constant current is applied to the dark portion (Vd portion) of the photosensitive drum, which is a constant value, to monitor the generated voltage, and the voltage is increased by (1) equal times, (2) coefficients times, (▲). 3) The applied bias is controlled by performing a combination such as applying a constant voltage, and has a certain effect in preventing the occurrence of variations in transferability due to environmental fluctuations and differences in transfer material size. When the pre-rotation step is completed, to start image formation, an appropriate transfer voltage (print bias) for image formation determined by the ATVC is applied (time S4), and the toner image on the photosensitive drum is transferred to the transfer material. .
[0072]
(6) Effect
When the charging AC voltage selection bias is applied, the potential Vd on the photosensitive drum is unstable because it does not exceed the desired drum potential Vd. When the foreign matter removing bias is applied, the potential Vd on the photosensitive drum is stable. Will come.
[0073]
The feature of this example is that the charging bias selection step at the time of image formation during the pre-rotation is performed first, and the application of the foreign matter removal bias is performed later.
1. Pre-rotation process time reduction
2. Prevention of charging failure due to positive charging on the photosensitive drum
It is.
[0074]
FIG. 2 shows the relationship between the charging bias application sequence and the potential on the photosensitive drum.
[0075]
According to FIG. 2A, which is a configuration of the present embodiment, during the charging bias selection step (time points S2 to S3), the potential on the photosensitive drum has not reached the desired value, but the foreign matter removing bias Vpp-1 has not increased. When the voltage is applied, the potential Vd on the photosensitive drum is stabilized to a desired value, so that the image formation can be started immediately after the application of the foreign matter removing bias Vpp-1 (time point S4). Further, during the charging bias selection step (time points S2 to S3), since the positive voltage for transfer is not applied, the photosensitive drum is charged positive by the transfer voltage, and a memory may be generated in the photosensitive layer on the surface of the photosensitive drum. Absent.
[0076]
Conversely, as shown in FIG. 2B, after applying the foreign matter removing bias first (time points S6 to S7), the charging bias selection step is performed (time points S7 to S8). During the application of the foreign matter removal bias (time points S6 to S7), the stable potential Vd on the photosensitive drum becomes unstable in the charging bias selection step (time points S7 to S8). In addition, during the charging bias selection step (time points S7 to S8), a positive transfer voltage (weak bias) is applied for the purpose of transfer control such as ATVC. In a region where the potential Vd on the photosensitive drum is unstable, when a positive voltage for transfer is applied, the photosensitive drum is positively charged under the influence of the transfer voltage. Therefore, when image formation starts immediately after the end of the charging bias selection step (time point S8), in this positively charged area, the photosensitive drum remains on the photosensitive drum during the first rotation of the photosensitive drum immediately after image formation (time point S8 to S9). A sufficient potential Vd cannot be applied, and in this portion (time points S8 to S9), charging failure occurs.
[0077]
In order to avoid this, before the image formation, a charging bias during image formation is applied by one rotation of the photosensitive drum (time points S8 to S9), and the photosensitive drum is pre-charged (time point S9). However, in this method, the pre-rotation step must be extended by the time T5, and it takes time for image formation.
[0078]
<Second embodiment>
In this embodiment, the configuration of the present invention is applied to a special mode in which the pre-rotation time is longer than usual in the same image forming apparatus main body.
[0079]
Note that the image forming process and the charging bias control method in this embodiment are the same as those in the first embodiment, and a description thereof will be omitted.
[0080]
When image information is sent from an output unit such as a host computer to the image forming apparatus, the main motor drives the image forming apparatus to enter a pre-rotation step. In this step, a printing preparation operation of various process devices is performed, and preliminarily charging on the photosensitive drum, starting of a laser scanner, determination of a transfer voltage, temperature adjustment of a fixing device, and the like are mainly performed.
[0081]
The pre-rotation step in normal use is determined in a fixed time to perform the above adjustment, but in a special case, the time of the pre-rotation step may be extended.
[0082]
For example, in order to fix a toner image satisfactorily regardless of the type of transfer material, the fixing device may change the fixing temperature depending on the type of transfer material. For example, when printing on thick paper having a thickness greater than that of plain paper as a transfer material, it is necessary to raise the fixing temperature by about 5 to 20 ° C. in order to perform good fixing compared to printing on plain paper. Since the temperature control is required, it takes time for the temperature to rise, so that the pre-rotation time is longer than usual.
[0083]
FIGS. 7A and 7B show an operation sequence chart and a flowchart of the main motor, charging, and transfer of the image forming apparatus used in the present embodiment. The case of the pre-rotation extension mode is shown.
[0084]
In FIG. 7B, a user using the image forming apparatus determines a print mode according to the type of transfer material and the like, and when a print signal is turned on, a fixing device according to the determined print mode is set inside the image forming apparatus main body. Is determined.
[0085]
Subsequently, it is determined whether the pre-rotation time of the fixing temperature can be handled in the normal mode. Here, when the pre-rotation time can be handled in the normal mode, the charging bias application time in the pre-rotation is set to T6 + T7 which is the charging application time in the pre-rotation normal mode shown in FIG. Is started.
[0086]
On the other hand, when it is necessary to extend the pre-rotation time, the image forming apparatus main body determines the time T8 required for the fixing device temperature adjustment shown in FIG. 7A, and sets the charging bias application time at the pre-rotation to T6 + T7 + T8. Set and start pre-rotation.
[0087]
When the pre-rotation is started, as for the charging bias, first, the bias for selecting the charging bias is first applied, and then the maximum applicable AC peak-to-peak voltage (foreign matter removing bias) is applied. The reason why the application is performed in this order is to prevent charging failure during image formation as described in the first embodiment.
[0088]
The voltage for selecting the charging bias is applied for the same time T6 (for one rotation of the photosensitive drum) as in the normal mode, and the maximum AC peak-to-peak voltage (foreign matter removing bias) that can be applied is T7 + T8, and the pre-rotation is performed. With the extension, the setting is made to be extended by T8 from the normal mode. As a result, the chance of polishing the surface of the photosensitive drum immediately before the image formation can be increased, so that the photosensitive drum is refreshed more at the time of image formation, and good image formation is performed.
[0089]
On the other hand, in the transfer bias, the application of the weak bias for controlling the ATVC or the like is extended by T8. If a strong positive voltage is applied directly to the photosensitive drum, there is a risk that a memory due to positive charging may be generated on the surface of the photosensitive drum. And the time during which a high positive voltage is applied is made as short as possible.
[0090]
【The invention's effect】
As described above, in the image forming apparatus using the charging AC peak-to-peak voltage selection control method, the pre-rotation time can be shortened and the image can be formed quickly by using the configuration of the present invention.
[0091]
Further, since the transfer voltage is applied in the region where the dark potential Vd on the latent image carrier is stable, the latent image carrier is not excessively charged by the transfer voltage, and image defects such as poor charging are caused. Does not occur.
[Brief description of the drawings]
FIG. 1 is a sequence chart of a first embodiment.
FIG. 2A is a diagram showing the relationship between the sequence chart of the first embodiment and the potential on the photosensitive drum.
FIG. 2B is a diagram showing the relationship between the sequence chart of the comparative example and the potential on the photosensitive drum.
FIG. 3 is a diagram illustrating an image forming apparatus according to a first embodiment.
FIG. 4 is a flowchart illustrating an operation sequence of the image forming apparatus according to the first embodiment.
FIG. 5 is a diagram showing a charging bias power supply circuit according to the first embodiment;
FIG. 6 is a flowchart illustrating a method of selecting a charging bias during pre-rotation according to the first embodiment.
FIG. 7A is a sequence chart of the second embodiment.
FIG. 7-b is a flowchart showing steps up to the start of pre-rotation in the second embodiment.
FIG. 8 is a diagram illustrating a conventional image forming apparatus.
FIG. 9 is a flowchart illustrating an operation sequence of a conventional image forming apparatus.
FIG. 10 is a diagram for explaining a charging bias power supply circuit of charging AC peak-to-peak voltage selection control;
FIG. 11 is a view for explaining a white streak image caused by a foreign substance attached to the surface of the photosensitive drum (part 1).
FIG. 12 illustrates a white streak image caused by a foreign substance attached to the photosensitive drum surface (part 2).
[Explanation of symbols]
1. photosensitive drum (latent image carrier), 2. charging device, 3. exposure device, 4. developing device, 5. transfer material, 6. transfer device, 7. fixing device, 8.・ Cleaning device

Claims (3)

At least, a rotatable latent image carrier,
Charging means for contacting the latent image carrier,
Cleaning means disposed in contact with the latent image carrier,
Transfer means for contacting the latent image carrier,
A charging bias voltage power supply circuit capable of outputting a superimposed voltage of AC and DC by one voltage boosting means to the charging means and applying two or more types of AC peak-to-peak voltages in a stepwise manner;
A transfer bias voltage power supply circuit capable of applying a positive bias voltage to the transfer unit,
Charging AC current detecting means capable of detecting a charging AC current flowing through the latent image carrier when a charging bias voltage is applied,
An image forming apparatus having a charging AC peak-to-peak voltage selection control,
In an image forming apparatus including a charging bias selection step during image formation and a step of applying a maximum AC peak-to-peak voltage that can be applied during pre-rotation,
The charging bias voltage during the pre-rotation is performed after the charging bias selection step is performed, and after the step of applying the maximum AC peak-to-peak voltage that can be applied, and the transfer bias is applied after the charging bias selection step is completed. An image forming apparatus. An image forming apparatus having a pre-rotation extension mode in which the pre-rotation time is longer than the normal mode depending on the image forming mode, wherein the charging bias selection step applied in the pre-rotation extension mode is equivalent to the normal mode, and is applicable. 2. The image forming apparatus according to claim 1, wherein the time of the step of applying the maximum AC peak-to-peak voltage is extended as the pre-rotation is extended. 3. The charging voltage application time for selecting the charging bias during the pre-rotation of the charging AC peak-to-peak voltage control is equal to or longer than the rotation time of the latent image carrier, and
3. The method according to claim 1, wherein the application time of the maximum AC peak-to-peak voltage that can be applied is a time during which the latent image carrier rotates one rotation or more, more preferably three rotations or more. 4. Image forming device.

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