JP2009003262A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To charge a desired charge amount in an image forming apparatus provided with a charging means for charging a toner image on an image carrier before transfer, due to toner deterioration.
An output of a charging unit is adjusted based on the number of continuous image formations and an average value of image ratios at that time.
[Selection] Figure 2

Description

  The present invention relates to an electrophotographic image forming apparatus, and more particularly to an image forming apparatus having a pre-transfer charger that charges an image prior to transfer of an image on an image carrier to a transfer medium. .

  The transfer of the toner image (image) in such an image forming apparatus is to move the toner image to a transfer medium such as a recording material or an intermediate transfer member by an electrostatic force received from an electric field between the photosensitive drum and the transfer device. is there.

  Therefore, the charge amount of the transferred toner image greatly affects the transfer performance.

  When the toner charge amount on the photosensitive drum is large, the electrostatic adhesion force between the photosensitive drum and the toner image is also increased, so that the toner is not sufficiently transferred to the transfer medium, resulting in a transfer failure.

  On the other hand, when the toner charge amount on the photosensitive drum is small, the toner image is peeled off from the photosensitive drum even with a slight transfer electric field, so that the toner image moves to the transfer medium before reaching the transfer portion, The toner image is scattered.

  In order to avoid the image defects as described above, a pre-transfer charger is provided for adjusting the toner transferred before transfer to the transfer medium to a desired charge amount.

Patent Document 1 discloses an image forming apparatus that detects the environment in which the image forming apparatus is used and adjusts the output of the pre-transfer charger from the detection result.
JP-A-11-352793

  However, as will be described below, the charging ability of the toner decreases with long-term use of the image forming apparatus. However, the method of Patent Document 1 does not take into account fluctuations in the charging ability of the toner due to long-term use, and is controlled so as to always maintain the charging ability with respect to the output of the pre-transfer charger set initially. As a result, the output of the pre-transfer charger is not appropriate when considering fluctuations in the charging ability accompanying the deterioration of the toner characteristics, which may lead to transfer failure.

  Here, the fluctuation of the toner charging ability due to long-term use will be described.

  For example, when images with a small image ratio are output continuously, the toner consumption of the developing device for developing the electrostatic image on the image carrier is small. As a result, the same toner stays on the developing roller of the developing device for a long time, causing a deterioration in toner characteristics.

  Although there are many indexes representing toner deterioration, the deterioration of toner characteristics in the present specification refers to the degree of change in externally added particles due to embedding and releasing of the toner external additive.

  Actually, when the deterioration of the toner characteristics due to long-term use is examined, it can be seen that the toner deterioration is promoted as shown in FIG.

  The deterioration index shown in FIG. 1 is a numerical value of the degree of change of external additive particles that greatly contributes to the charging ability of the toner, and the amount of external additive of the initial toner is normalized to 1.

  That is, as the deterioration index becomes smaller, it means that the charging ability of the external additive is lost accordingly. In particular, it has been confirmed that deterioration is further promoted when images with a small image ratio are output continuously.

  As described above, when images with a small image ratio are continuously formed, even if the output control of the pre-transfer charger is performed by the method described in Patent Document 1, the output of the pre-transfer charger is changed according to the number of image formations. In some cases, image defects occurred at the primary transfer portion.

  By continuously forming images with a small image ratio as described above, the toner charging ability is lowered, and as a result, image defects are unavoidable. Therefore, it is necessary to grasp the change in the toner charging ability over time.

  The present invention is for solving the above-mentioned problems, and controls transfer failure due to toner charge amount by controlling the output of a pre-transfer charger against fluctuations in charging ability accompanying toner deterioration. The purpose is to prevent.

Representative means in the present invention for solving the above problems are an image carrier that carries an electrostatic image, a developing means that forms an image by developing the electrostatic image with toner, and a transfer of the image. Transfer means for electrostatically transferring to a medium;
In an image forming apparatus having a pre-transfer charging unit that applies a charge to the image prior to the transfer, depending on the number of images formed continuously and the image ratio of the images formed at that time, It has a control means for variably controlling the charging condition of the pre-transfer charger.

  According to the present invention, the charging condition of the pre-transfer charger in consideration of the charging capability of the toner can be controlled by variably controlling the charging condition of the pre-transfer charger according to the number of continuously formed images having a predetermined image ratio or less. Control can be performed.

  Hereinafter, the present invention will be described in detail based on embodiments shown in the accompanying drawings.

[Example 1]
Hereinafter, embodiments of the present invention will be described with reference to the drawings.

<Embodiment 1>
Apparatus configuration of the present embodiment An embodiment of an image forming apparatus according to the present invention will be described with reference to FIG. FIG. 2 is a schematic side view illustrating a configuration example of the image forming apparatus according to the first embodiment.

  A photosensitive drum 1 is provided as an image carrier.

  The photosensitive drum 1 is an amorphous silicon drum having a positive charging polarity and has a diameter of φ84 mm. The photosensitive drum 1 is rotated by an electric motor (not shown) in the direction of arrow R1 at an image forming speed of 300 mm / s.

  A primary charger 2, an exposure device 3, a developing device 4, a density detection sensor 5, a pre-transfer charger 6, an intermediate transfer belt 7, a primary transfer roller 8, and the like around the photosensitive drum 1 almost in the rotation direction. A cleaning device 9 is provided.

  As the primary charger 2, for example, a scorotron type charger is used.

  The primary charger 2 is disposed opposite to the surface of the photosensitive drum 1 and is biased by a charging bias application power source (not shown). In this embodiment, the surface of the photosensitive drum 1 is uniformly and uniformly charged at about + 500V.

  For example, a laser scanner is used as the exposure apparatus 3. A laser beam is oscillated from the exposure device 3 based on the image information, and the charge in the exposed area is removed to form an electrostatic image.

  The developing device 4 as a developing unit is disposed downstream of the exposure unit 3 and upstream of the primary transfer transfer roller 8.

  The developing sleeve 4b is rotatably installed in an opening portion of the container 4a facing the photosensitive drum 1 and containing a one-component developer (black). Further, in the developing sleeve 4b, a magnet roller 4c for supporting the developer on the developing sleeve is fixedly disposed so as not to rotate with respect to the rotation of the developing sleeve 4b. The container 4a can be attached to and detached from the developing device 4, and when the one-component developer is consumed, it can be replaced with a new container 4a filled with the one-component developer. Further, the developing device 4 itself can be attached to and detached from the image forming apparatus main body, and can be replaced when the function is deteriorated due to wear of components.

  The negatively charged developer in the developing device 4 rises by the magnetic force of the developing main pole located in the developing region of the magnet roller 4 c and comes into contact with the surface of the photosensitive drum 1. By applying a developing bias to the developing sleeve 4b by a power source (not shown), the developer adheres to the image portion which is a non-exposed area of the electrostatic image, and a toner image t (image) is formed on the photosensitive drum 1. The

  In the present embodiment, development is performed with a developing bias that has an AC component of 1.2 kVpp (3 kHz) and a DC component that is variable from +100 to + 450V.

  The density detection sensor (density detection means) 5 detects the density of the toner image t on the photosensitive drum 1. The density detection sensor 5 includes an infrared light emitting unit and a light receiving unit. The toner amount is detected by irradiating the photosensitive drum 1 with a certain amount of light and detecting the amount of light reflected from the toner image t. Measure.

  The density information is sent to the CPU 30 and fed back to the developing bias voltage to adjust the density.

  A pre-transfer charger 6 as a pre-transfer charging means is a corona charger composed of a discharge wire 61 and a shield 62. The pre-transfer charger 6 applies a charge of the same polarity to the toner on the photosensitive drum 1, and determines the toner charge amount after development. adjust. The pre-transfer charger 6 is applied with a bias in which a DC voltage is superimposed on a rectangular AC voltage having a frequency of 1000 Hz from a power source (not shown).

  In this embodiment, the amplitude Vpp of the AC voltage is fixed at 8 kV, and the output of the pre-transfer charger 6 (the amount of current flowing from the discharge wire 61) is adjusted by adjusting the DC voltage.

  Note that the polarity of the DC voltage applied to the pre-transfer charger 6 is the same polarity as the toner polarity, and constant current control is performed by the control unit 14 while detecting the value of the current flowing through the discharge wire 61 by the wire current detection sensor 63. Is done.

  Further, an endless intermediate transfer belt (intermediate transfer member) 7 as a transfer medium is stretched below the photosensitive drum 1. The intermediate transfer belt 7 is supported by a driving roller 16, a tension roller 15, and a backup roller 10, and travels at 300 mm / s, which is the same as that of the photosensitive drum 1.

The intermediate transfer belt 7 is formed so as to have a volume resistivity of 10 ⁶ to 10 ¹⁰ Ω · cm by using a material containing an appropriate amount of an antistatic agent such as carbon black in a resin such as polyimide or polycarbonate. The thickness is about 0.1 mm.

  Further, on the traveling path of the intermediate transfer belt 7, there are a primary transfer portion T1 and a secondary transfer portion T2.

  In the primary transfer portion T1, an intermediate transfer belt 7 is passed between the photosensitive drum 1 and the primary transfer roller (primary transfer means) 8.

The primary transfer roller 8 is made of a φ8 mm, SUS shaft and a conductive urethane sponge layer having a thickness of 4 mm, and the length of the sponge portion is 300 mm. The resistance value is a relationship of current measured by rotating the transfer roller 8 at a peripheral speed of 300 mm / s with respect to the ground under a load of 500 g and applying a voltage of 1500 V to the surface of the primary transfer roller 8. And the value was about 1 × 10 ⁷ Ω (23 ° C., 50% RH).

  Both ends of the primary transfer roller 8 are pressed against the surface of the photosensitive drum 1 by a pressing member such as a spring (not shown), and are driven to rotate in the direction of the arrow R2 as the photosensitive drum 1 rotates in the direction of the arrow R1.

  In this embodiment, constant current control is performed on the bias applied to the primary transfer roller 8.

  Actually, a desired voltage is applied to the primary transfer roller 8 from the primary transfer power supply 100 of FIG. 2 so as to secure a transfer current +70 μA. As a result, the transfer charge determined by the surface potential of the primary transfer roller 8 is charged, and the toner image t on the photosensitive drum 1 is electrostatically transferred onto the surface of the intermediate transfer belt 7.

  The photosensitive drum 1 after the primary transfer is subjected to removal of deposits such as residual toner by the cleaning device 9.

  The cleaning device 9 includes a cleaner blade 9a. The cleaner blade 9a is in contact with the photosensitive drum 1 at a predetermined angle and pressure by a pressing unit (not shown) and remains on the surface of the photosensitive drum 1. Collect toner etc.

  On the other hand, at the secondary transfer position T2, a secondary transfer outer roller 11 as a secondary transfer means is disposed at a position facing the backup roller 10, and an intermediate transfer is performed between the secondary transfer outer roller 11 and the backup roller 10. A belt 7 is passed.

The secondary transfer outer roller 11 is composed of a 12 mm diameter SUS shaft and a conductive urethane sponge layer having a thickness of 6 mm, and the length of the sponge portion is 330 mm. The resistance value is a current measured by rotating the secondary transfer roller 11 at a peripheral speed of 300 mm / s with respect to the ground under a load of 500 g and applying a voltage of 3000 V to the surface of the secondary transfer roller 11. The value was about 6 × 10 ⁷ Ω (23 ° C., 50% RH).

  In this embodiment, the bias applied to the secondary transfer outer roller 11 employs constant voltage control.

  This is because the secondary transfer portion T2 must perform sufficient transfer on recording materials of various sizes and types, and the bias to be applied must take into account the shared voltage of the paper. Because of that.

  Therefore, the secondary transfer portion T2 needs to perform ATVC (Active Transfer Voltage Control) control in order to determine the applied bias.

  The ATVC control executed in this embodiment is performed at the time of image pre-rotation, and three kinds of biases having different voltages are applied, and a current value flowing at that time is detected. And the voltage value with respect to the target current is calculated from the result.

  The bias actually applied to the secondary transfer portion T2 is determined from the result of the previous ATVC control and a paper sharing voltage table prepared in advance in the apparatus.

  In this way, a positive voltage is applied to the secondary transfer outer roller 11 from the secondary transfer power source 200 shown in FIG. As a result, the transfer charge determined by the surface potential of the transfer roller is charged, and the toner image t on the intermediate transfer belt 7 is transferred to the surface of the recording material P.

  The belt cleaning device 12 includes a cleaner blade 12a, and the cleaner blade 12a is in contact with the intermediate transfer belt 7 by a pressing means (not shown) at a predetermined angle and pressure, and remains on the surface of the intermediate transfer belt 7. Collected toner and the like.

  The recording material P to which the toner image t has been transferred is introduced into the fixing device 13 and heated and pressurized. As a result, the toner image t is fixed on the surface of the recording material P.

  Further, a video counter 40 is installed in the reader unit of this embodiment, and an image signal is sent to the video counter 40. The video counter 40 detects the number of pixels of the document and integrates the number of pixels. That is, the pixel data of the document is accumulated for each pixel, and for example, the number of pixels per document is counted. Then, the CPU 30 calculates an image ratio that is a value obtained by dividing the number of pixels of the document by the total number of pixels that can be formed on the document.

  Further, during continuous image formation, the CPU 30 integrates the image ratios of the respective images, counts the number of formed images, and calculates an average value of the image ratios per document.

  These processes are performed by the CPU 30 of the main device, and the processing results are stored in the memory 35.

  In this embodiment, when the developing device 4 is replaced or when toner is replenished by replacing the toner container 4a, the integrated value of the video count and the continuous output number are reset.

<Control action>
Next, the control operation of the present embodiment will be described with reference to FIG. First, after the power is turned on, a toner patch is formed on the photosensitive drum 1 (S101). The density of the toner patch on the photosensitive drum 1 is detected by the image density detection sensor 5 (S102).

  The density detection result is sent to the CPU 30, and it is determined whether or not the density of the toner patch is within the reference range (S103). When the value is outside the reference range, the voltage value of the developing bias, the voltage value applied to the primary charging device 2, the light amount of the exposure device 3 and the like are adjusted, and the image forming conditions are changed. This process is repeated until the image density is within the reference range.

  Next, the output of the pre-transfer charger 6 is controlled so that the toner charge amount Q / M before the primary transfer becomes a desired value. For control, first, the CPU 30 calculates the charge amount Q / M of the developed toner image (S105).

  In this embodiment, the toner amount M and the toner charge amount Q are calculated to determine the charge amount per unit amount of the toner image.

  That is, the toner charge amount Q / M can be determined by measuring the surface potential of the toner image with respect to the image portion potential of the electrostatic image formed on the photosensitive drum 1. Then, the toner charge amount Q / M is calculated by knowing the weight M of the toner image from the detection result of the density detection sensor 5.

  In this embodiment, Q is the charge amount (μC), and M is the weight (g).

  Actually, first, the toner loading amount on the photosensitive drum 1 is grasped from the density detection sensor 5. The relationship between the density detected by the density detection sensor 5 and the applied amount of toner on the photosensitive drum 1 is obtained in advance, and the optimum applied amount in this embodiment is about 0.65 mg / cm 2.

  In the present embodiment, a potential sensor 20 that measures the potential on the photosensitive drum 1 is provided in association with the density detection sensor 5. This potential sensor 20 measures the potential of the toner image.

  The result is sent to the CPU 30, and the toner charge amount Q / M is calculated from the potential of the image portion (in this embodiment, +500 V) and the potential of the developing bias (developing contrast).

  Then, it is determined whether the calculated toner charge amount Q / M is within a desired range (in this embodiment, the reason is -20 μC / g to -30 μC / g will be described later) (S106). If the charge amount is within this range, the toner image is not charged by the pre-transfer charger 6 (S107), and image formation is started immediately.

  However, when the toner charge amount is out of the range, it is necessary to adjust the toner charge amount by the pre-transfer charger 6, and the output value determination of the pre-transfer charger 6 is controlled.

<Target charge amount of toner before primary transfer>
In the present embodiment, a spot phenomenon is an example of the primary transfer failure caused by the toner charge amount. This phenomenon is a phenomenon in which the toner image is disturbed in a spotted pattern due to a discharge phenomenon in which the toner image primarily transferred onto the intermediate transfer belt 7 is generated in the primary transfer portion T1.

  The reason for this is that if the toner charge amount is small, the electrostatic adhesive force with the intermediate transfer belt 7 becomes weak. Of course, if the toner charge amount is large, this spot phenomenon does not occur.

  Here, the relationship between the toner charge amount before the primary transfer, the primary transfer efficiency of the solid image, and the speckle phenomenon was examined. The results are shown in Table 1.

  In the column of primary transfer efficiency, “x” indicates that the efficiency is insufficient, and “◯” indicates that it is sufficient. In the spot phenomenon column, “x” indicates that the spot phenomenon occurs, and “◯” indicates that the spot phenomenon does not occur. As can be seen from Table 1, it can be seen that in this example, the lower limit of the charge amount before transfer must be about −20 μC / g.

  On the other hand, if the toner charge amount before the primary transfer is set to −30 μC / g or more, the electrostatic adhesion force between the photosensitive drum 1 and the toner image increases, and the transfer current set in this embodiment is sufficient. The toner image cannot be moved. As described above, when the toner charge amount is set to −30 μC / g or more, the transfer property of the solid image in the primary transfer portion is deteriorated (strong loss). Therefore, in this embodiment, the target value of the toner charge amount is set to −20 μC / g. g to −30 μC / g.

<Bias setting of pre-transfer charger>
In general, the toner charge amount and charging ability greatly vary depending on the resistance value of the toner particles and the amount of the external additive attached to the toner particles. When the resistance of the toner particles is high and when the amount of the external additive attached is large, both the toner charge amount and the charging ability are high.

  On the other hand, when the toner particles and the external additive absorb water and the resistance value decreases, or when the adhesion amount to the toner particle is small due to embedding or releasing of the external additive, the toner charge amount and charging ability are lowered.

  Therefore, in the present invention, attention is paid to the use environment (water content) and toner deterioration in order to grasp the toner charging ability.

  First, as shown in Table 2, the usage environment was mainly defined by humidity (water content), and in this example, it was divided into seven environments and managed. In addition, when the moisture content is 21.6 g / m ^ 3 or more, it is recognized as environment No7.

  Next, toner deterioration will be described. The toner deterioration referred to in the present invention indicates a decrease in the amount of the external additive attached to the toner particles. The main cause of the toner deterioration is that the external additive is embedded and released from the toner particles when the toner is used for a long time. The amount of adhering external additive varies depending on the number of continuous images formed and the average value of the image ratio of images formed at that time. Table 3 shows the results of examination of toner deterioration with respect to the average value of the continuous image formation number and the image ratio.

In Table 3, toner degradation is indicated by a degradation index. This deterioration index is an index indicating the amount of the external additive adhering to the toner particles. The amount of the external additive adhering to the toner particles in the initial state at the time of developing device replacement or toner replacement is 1.0. Yes.
In this embodiment, the output of the pre-transfer charger 6 is adjusted in consideration of the charging ability that changes due to toner deterioration.

  From FIG. 1 and Table 3, it can be seen that the toner deterioration is promoted under the condition of a low image ratio.

  In particular, when the image is continuously formed at an image ratio of 1%, the deterioration index is reduced to about 0.8, but is subsequently saturated.

  On the other hand, in the case of an image with an image ratio of 20% or more, there is almost no toner deterioration, and it is saturated at a deterioration index of about 0.98 as shown in FIG.

  Next, in this embodiment, in order to prevent transfer failure, it is necessary to set the toner charge amount before the primary transfer to -20 μC / g or more, and the output of the pre-transfer charger 6 required at that time is examined. It was.

  In this embodiment, the output of the pre-transfer charger 6 that is necessary to increase the toner charge amount after development to about −5 to −10 μC / g or more to −20 μC / g was examined. The result is shown in FIG.

  FIG. 4 shows the output of the pre-transfer charger 6 required for the deterioration index in each environment. In FIG. 4, the absolute value of the output of the pre-transfer charger 6 is shown, and the polarity of the current actually flowing from the discharge wire 61 is negative.

  As can be seen from FIG. 4, the smaller the deterioration index value, the higher the output is required. Also, the required output becomes higher under the high humidity environment.

  Based on the above results, the output control method of the pre-transfer charger 6 of this embodiment will be specifically described below with reference to FIG. In the present invention, the toner charging capability and the usage environment are detected, and the output (charging condition) of the pre-transfer charger 6 is adjusted based on the result.

  First, the CPU 30 reads the number of continuously formed images and the average image ratio at that time from the memory 35, and calculates a deterioration index based on the relationship shown in Table 3 (S108).

  Subsequently, the use environment is detected by the environment sensor 50 (S109).

  Then, based on the deterioration index and the use environment, the CPU 30 determines the output of the pre-transfer charger 6, and the control unit 14 adjusts the amount of current that flows from the pre-transfer charger power supply 300 to the pre-transfer charger 6. S110). In this embodiment, the relationship between the deterioration index and the output of the pre-transfer charger 6 is stored in the memory 35 for each use environment. The CPU 30 determines the output of the pre-transfer charger 6 based on this relationship. This relationship is shown in Table 4.

  In Table 4, Y indicates the absolute value of the output of the pre-transfer charger 6. The polarity of the current actually flowing through the discharge wire 61 is negative. X is a deterioration index.

<Operation check in this embodiment>
In the configuration as described above, in this example, an image was created using toner in an initial state in an environment of 23 ° C. and 50% RH. In the environment of 23 ° C. and 50% RH, the amount of water is 1.0 g, and the classification shown in Table 2 is environment No. 2.

  First, after the power is turned on, the density detection sensor 5 detects the density of the toner image on the photosensitive drum 1, and the bias conditions of the primary charging device 2, the exposure device 3, and the developing device 4 are determined based on the detection result. It was done. In this embodiment, on the photosensitive drum 1, the surface potential of the image portion is + 500V, the surface potential of the non-image portion is + 200V, and the DC voltage of the developing bias is + 300V.

  Next, the toner charge amount after development was calculated from the previous development bias condition and the density of the detected toner image. As a result, the toner charge amount after development was −10 μC / g.

  Since the toner is in an initial state now, it is determined that the toner deterioration index is 1.0 and the continuous output number is 0, and the output setting of the pre-transfer charger 6 is set to −100 μA together with the use environment detection result. It was.

  When image formation was performed under these conditions, no spot phenomenon, which was a primary transfer defect, was observed.

  When the charge amount of the toner image before the primary transfer was confirmed, it was -24 μC / g, and it was confirmed that the pre-transfer charger 6 was optimally set.

  Next, 20000 sheets (predetermined number) of images having an image ratio of 1% were continuously formed under these conditions. The transition of the output of the pre-transfer charger 6 at that time is as shown in FIG.

  As can be seen from FIG. 5, the output of the pre-transfer charger 6 is controlled in accordance with toner deterioration. In the image formed in this example, no transfer defect such as a spot phenomenon or strong omission was observed.

  On the other hand, when 20000 images with an image ratio of 20% or more are output continuously in the same manner, the transition of the output of the pre-transfer charger 6 is as shown in FIG. 5B, and the output of the pre-transfer charger 6 is The toner is controlled to be almost the same value as in the initial state. Of course, no transfer failure was confirmed.

  As described above, in this embodiment, since the output of the pre-transfer charger 6 is controlled according to the degree of toner deterioration, it is possible to suppress the transfer failure occurring in the primary transfer portion T1.

<Example 2>
In the first embodiment, the current value flowing through the discharge wire 61 of the pre-transfer charger 6 is controlled.

  Here, as shown in FIG. 6, the current flowing through the discharge wire 61 is classified into a charging current flowing in the direction of the photosensitive drum 1 and a shield current flowing through the shield 62 of the pre-transfer charger 6.

  What is important for charging charge control of the toner image is the charging current flowing through the photosensitive drum 1. In this embodiment, the charging current is directly controlled so as to make the adjustment of the charge amount of the toner image more accurate.

  Due to long-term use of the image forming apparatus, the shield 62 of the pre-transfer charger 6 may be contaminated by toner scattering or the like, and current may not easily flow from the discharge wire 61 to the shield. In this case, the ratio of the shield current flowing from the discharge wire 61 to the shield 62 becomes low, and the charging current flowing to the photosensitive drum 1 becomes excessive. As a result, a transfer failure occurs at the primary transfer portion T1.

  The configuration of the image forming apparatus in this embodiment will be described below.

  Since the basic configuration is the same as that of the image forming apparatus according to the first exemplary embodiment, the description thereof will be omitted, and the configuration of the pre-transfer charger 6 that is characteristic of the present exemplary embodiment will be described.

  In this embodiment, as shown in FIG. 6, a shield current detection sensor 64 for detecting a shield current flowing in the shield 62 of the pre-transfer charger 6 is newly provided to detect the charging current.

  Next, an actual control method will be described with reference to FIG. The difference from the first embodiment is that the shield current changes when the total current flowing from the pre-transfer charger power supply 300 to the discharge wire 61 of the pre-transfer charger 6 is determined based on the calculation result of the deterioration index and the use environment. Is to be considered.

  In FIG. 11, in S201-205, the same control as S11-15 of FIG. Subsequently, it is determined whether or not the toner charge amount is within a desired range (S206). If the charge amount is within this desired range, the toner image is not charged by the pre-transfer charger 6 (S207), and image formation is started immediately.

  However, when the toner charge amount is out of the range, it is necessary to adjust the toner charge amount by the pre-transfer charger 6, and the output (charging condition) of the pre-transfer charger 6 is controlled.

  As in the first embodiment, first, the CPU 30 reads the number of continuous images formed and the average image ratio at that time from the memory 35, and calculates a deterioration index based on the relationship shown in Table 3 (S208). Subsequently, the use environment is detected by the environment sensor 50 (S209).

  Then, based on the deterioration index and the usage environment, the CPU 30 determines the amount of charging current (S210).

  Here, the amount of charging current necessary to increase the toner charge amount after development to about −5 to −10 μC / g or more to −20 μC / g was examined. The result is shown in FIG.

  In this embodiment, the relationship between the degradation index and the amount of charging current required for desired charging is stored in the memory 35 for each use environment. This relationship is shown in Table 5.

  The CPU 30 determines the charging current amount based on the relationship shown in Table 5. The relationships shown in Table 5 are obtained from the results shown in FIG. In FIG. 8, Y indicates the absolute value of the charging current amount. Actually, the polarity of the current that the pre-transfer charger 6 flows to the photosensitive drum 1 is negative. X is a deterioration index.

  Subsequently, the control unit 14 adjusts the total current supplied by the pre-transfer charger power supply 300 so that the charging current amount becomes the amount determined in (S210).

  The control unit 14 calculates the charging current from the difference between the total current amount passed through the discharge wire 61 of the pre-transfer charger 6 and the shield current amount, and adjusts the current that the pre-transfer charger power supply 300 flows.

<Operation check in this embodiment>
First, the amount of charging current flowing from the pre-transfer charger 6 to the photosensitive drum 1 was confirmed in the initial state immediately after replacement under the same conditions as in Example 1. When the total current flowing from the pre-transfer charger power supply 300 was −100 μA, the charging current amount was −20 μA.

  From this result, it is recognized that the ratio of the charging current amount flowing to the photosensitive drum 1 to the total current amount in the pre-transfer charger 6 in the initial state is about 20%.

  Next, as in Example 1 above, 20000 images having an image ratio of 1% were continuously formed.

  FIG. 9 shows the charging current flowing through the photosensitive drum 1 and the total current flowing through the pre-transfer charger 6 at that time. In FIG. 9, the charging current amount and the total current amount are shown as absolute values. The charging current and the total current are negative.

  As shown in FIG. 9, it can be seen that the total current amount determined by the control of the second embodiment is slightly smaller than the total current amount determined by the control of the first embodiment. However, a sufficient amount of charging current flowing through the photosensitive drum 1 is secured for charging to a desired amount of charge. Of course, no transfer failure of the primary transfer portion T1 was found in this embodiment.

  From this result, it is predicted that the pre-transfer charger 6 is contaminated according to the number of continuously formed sheets, the shield current amount is reduced, and the ratio of the charging current flowing through the photosensitive drum 1 is increased.

In this embodiment, 2000 continuous images were formed using the pre-transfer charger 6 in the initial state, and the ratio of the charging current to the total current was examined at that time.
As a result, the initial state was about 20%, but it was about 23% when 20000 sheets were formed, and the charging current flowing through the photosensitive drum 1 increased.

  In this embodiment, the charging current actually flowing through the photosensitive drum 1 is controlled. Therefore, if the shield 62 is contaminated and it becomes difficult for current to flow through the shield, the total current flowing through the pre-transfer voltage precharger 6 becomes smaller than when control is performed using the method of the first embodiment.

  Changes in the charging current flowing through the photosensitive drum 1 and the charge amount of toner after transfer were examined when continuous image formation was performed while controlling by the methods of Examples 1 and 2. The result is shown in FIG.

  As can be seen from FIG. 10, in the control of Example 1, as the number of continuously formed sheets increases, the charge amount of the toner charged on the pre-transfer charger 6 tends to increase. This is because the shield 62 of the pre-transfer charger 6 is contaminated with long-term use, and as a result, the ratio of the charging current increases.

  On the other hand, in the control of the second embodiment, since the charging current largely related to the charge amount of the toner charged by the pre-transfer charger 6 is detected and controlled, the pre-transfer charger even if image formation is continuously performed. 6 is prevented from being excessively charged.

  Therefore, it becomes possible to maintain a desired charge amount.

  It is desirable to use the control method of this embodiment in an image forming apparatus in which a transfer failure is particularly likely to occur in the primary transfer portion T1 due to an increase in toner charge amount.

  As described above, by controlling the total current flowing through the pre-transfer charger 6 by the method of this embodiment, the charge amount of the toner can be controlled with high accuracy.

FIG. 6 is a diagram illustrating a relationship between the number of continuously formed images and toner deterioration at each image ratio in the present invention. 1 is a schematic view of an image forming apparatus according to the present invention. FIG. 3 is a flowchart from the start of control operation to the start of image formation in Embodiment 1. 6 is a diagram illustrating a relationship between a deterioration index and an output of a pre-transfer charger in Embodiment 1. FIG. 4 is a transition of the output value of the pre-transfer charger with respect to the continuously formed number in Example 1. 6 is a schematic diagram of a pre-transfer charger configuration in Embodiment 2. FIG. FIG. 10 is a flowchart from the start of control operation to the start of image formation in Embodiment 2. It is a figure showing the relationship between the degradation parameter | index and charging current in Example 2. FIG. It is a figure which compares the total current of Example 1 and 2. FIG. FIG. 6 is a diagram illustrating a change in toner charge amount of Examples 1 and 2.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Photosensitive drum 2 Primary charger 3 Exposure apparatus 4 Developing apparatus 5 Density detection sensor 6 Pre-transfer charging apparatus 7 Intermediate transfer belt 8 Primary transfer roller 9 Photoconductor cleaning apparatus 10 Backup roller 11 Secondary transfer roller 12 Intermediate transfer belt cleaning apparatus 13 Fixing Device 14 Pre-Transfer Charger Control Unit 15 Steering Roller 16 Drive Roller 20 Potential Sensor T1 Primary Transfer Unit T2 Secondary Transfer Unit P Recording Material

Claims (3)

An image carrier for carrying an electrostatic image;
Developing means for forming an image by developing the electrostatic image with toner;
Transfer means for electrostatically transferring the image to a transfer medium;
In an image forming apparatus having a pre-transfer charging unit for applying a charge to the image prior to the transfer,
An image forming apparatus comprising: control means for variably controlling the charging condition of the pre-transfer charger according to the number of images formed continuously and the image ratio of the images formed at that time. .   The absolute value of the amount of current flowing from the charger to the image when the average value of the image ratio of the image formed continuously is small is the image of the image formed continuously. 2. The image forming apparatus according to claim 1, wherein when the average value of the ratio is large, the absolute value of the amount of current flowing from the charger to the image from the charger is larger.   The image forming apparatus according to claim 2, wherein the voltage applied to the pre-transfer charger is a voltage obtained by superimposing an AC voltage on a DC voltage.

Images