JP2006178210A - Image forming apparatus and image forming method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prolong the life of a pre-transfer electrifier while restraining the separation of toner from recording material after transfer. <P>SOLUTION: The image forming apparatus is equipped with a photoreceptor drum 10 rotating in a direction shown by the arrow A, an electrifier 11 electrifying the photoreceptor drum 10, a laser exposure device 12 writing an electrostatic latent image on the photoreceptor drum 10, a developing device 13 developing the electrostatic latent image on the photoreceptor drum 10 with toner to be a visible image, the pre-transfer electrifier 14 electrifying the toner image on the photoreceptor drum 10 before electrostatic transfer in a transfer part 15, and a transfer unit 20 transferring the toner image formed on the photoreceptor drum 10 to recording paper (form) P being the transfer material in the transfer part 15. Control is performed so that pre-transfer electrifying operation by the pre-transfer electrifier 14 is not performed when the image density of the toner image formed on the photoreceptor drum 10 is under 30%, and the pre-transfer electrifying operation is performed when it is equal to or above 30%. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus such as a copying machine or a printer, and more particularly to an image forming apparatus using an electrophotographic system.

  In an image forming apparatus such as a copying machine or a printer using an electrophotographic system, for example, a photosensitive member (photosensitive drum) formed in a drum shape is uniformly charged, and the photosensitive drum is controlled based on image information. An electrostatic latent image is formed on the photosensitive drum by exposure with the exposed light. The electrostatic latent image is converted into a visible image (toner image) by toner charged to a predetermined polarity by a developing device, and the toner image is electrostatically transferred from the photosensitive drum to a recording material (paper) by a transfer device. After that, an unfixed toner image is fixed on the paper by heating and pressurizing with a fixing device (see Patent Document 1).

In such an image forming apparatus, the paper that has passed through the transfer device is conveyed to the fixing device in a state where toner is electrostatically adhered (unfixed state). At this time, among the toners adhering to the recording material, there is a possibility that toner with a low charge amount may be mixed because the developing device is insufficiently charged. Since the toner with a low charge amount has a weak electrostatic attraction force on the paper, the toner tends to be detached from the paper during conveyance and float in the image forming apparatus. If the toner floats in the apparatus in this manner, for example, the toner adheres to a fixing member (roll member or belt member) constituting the fixing device and causes a fixing defect or causes in-machine contamination.
In recent years, the image forming apparatus has been downsized, and there is a tendency that a sheet conveyance path is provided in a gap between each unit (apparatus) constituting the image forming apparatus. For this reason, it is difficult to ensure the sheet conveyance path in a straight line. Then, there is a problem that the sheet conveyed with electrostatically attracted toner collides with a guide provided in the sheet conveyance path or a gap between the sheets, and the toner is easily separated from the sheet due to the impact force received.

  Therefore, a pair of corona dischargers (post-transfer chargers) for charging the front and back sides of the paper with opposite polarities are mounted on the top and bottom of the paper conveyance path provided between the transfer device and the fixing device, A technique for suppressing toner scattering has been proposed (see Patent Document 2).

JP 2004-333587 A (page 5, FIG. 1) JP-A-61-100767 (page 3-4, FIG. 1)

  However, in Patent Document 2, unfixed toner on the sheet cannot be charged upstream of the pair of post-transfer chargers on the upstream side in the sheet conveyance direction. Here, since the toner with a low charge amount is detached from the paper immediately after being transferred to the paper, it is difficult to prevent the toner with such a low charge amount from floating.

In Patent Document 1, for the purpose of aligning the charging polarity of the toner before transfer, the toner adhering to the photosensitive drum after development is charged using a corona discharger (transfer pretreatment charger). Technology is disclosed. Therefore, by using this pre-transfer charger, the pre-transfer toner is sufficiently charged to strengthen the adhesion of the toner to the paper after transfer, and to prevent the toner from detaching from the paper. Conceivable.
However, since the life of the corotron charger, which is a transfer pretreatment charger, is determined by the product of the applied current value and the applied time, if the corotron charger is always operated during the image forming operation, its life is extremely short. turn into. If the service life of the transfer pretreatment charger is shortened, the replacement frequency of the transfer pretreatment charger increases, so that the period during which image formation cannot be performed increases, resulting in a decrease in productivity. Such costs also increase.

  The present invention has been made to solve such a technical problem, and an object of the present invention is to extend the life of the pre-transfer charger while suppressing the detachment of the toner from the recording material after the transfer. Is to plan.

For this purpose, the image forming apparatus to which the present invention is applied has an image carrier that carries toner, charges the toner on the image carrier by toner charging means, and is charged by the toner charging means. The toner on the body is electrostatically transferred onto the recording material by the transfer means, and the charging operation by the toner charging means is controlled by the charge control means based on the image density of the toner formed on the image carrier.
Here, the charging control means may be characterized in that the charging operation by the toner charging means is not performed when the image density is a predetermined value or less. Further, the humidity can be detected by the humidity detecting means, and the charging control means can control the charging operation by the toner charging means based on the image density and the humidity detected by the humidity detecting means. Here, the charging control means can be characterized in that the charging operation by the toner charging means is not performed when the humidity detected by the humidity detecting means exceeds a predetermined value.

Further, when grasping the present invention from the category of the method, the toner carried on the image carrier is charged by the pre-transfer charging unit, and the toner on the charged image carrier is transferred to the recording material by the transfer unit, An image forming method in an image forming apparatus in which a recording material to which toner has been transferred is conveyed through a non-linear conveying path and the toner on the conveyed recording material is fixed by a fixing unit. A first step of acquiring the image density of the toner to be formed and a second step of controlling the charging operation by the pre-transfer charging unit based on the acquired image density.
Here, when the image density acquired in the first step is less than or equal to a predetermined value, the charging operation by the pre-transfer charging unit is not performed in the second step. Further, the method may further include a step of acquiring humidity, and in the second step, the charging operation by the pre-transfer charging unit may be controlled based on the acquired humidity. Furthermore, in the second step, when the acquired humidity exceeds a predetermined value, the charging operation by the pre-transfer charging unit is not performed.

  According to the present invention, since the charging of the toner before transfer is controlled based on the image density of the toner and the like, the life of the pre-transfer charger is extended while suppressing the separation of the toner from the recording material after transfer. Can be achieved.

The best mode for carrying out the present invention (hereinafter referred to as an embodiment) will be described below in detail with reference to the accompanying drawings.
FIG. 1 is a schematic configuration diagram illustrating an image forming apparatus to which the exemplary embodiment is applied. The image forming apparatus shown in FIG. 1 includes a photosensitive drum 10 that rotates in the direction of arrow A, and a charger 11 that charges the photosensitive drum 10. Further, on the downstream side of the charging device 11 in the rotation direction of the photosensitive drum 10, a laser exposure device 12 for writing an electrostatic latent image on the photosensitive drum 10 (exposure beam is indicated by a symbol Bm in the figure), and toner is accommodated. Then, a developing device 13 is provided for visualizing the electrostatic latent image on the photosensitive drum 10 with toner. Further, around the photosensitive drum 10, a pre-transfer charger 14 that charges the toner image on the photosensitive drum 10 prior to electrostatic transfer in the transfer unit 15, and the toner image formed on the photosensitive drum 10 are transferred. A transfer unit 20 for transferring to a recording paper (paper) P, which is a transfer material, is attached to the section 15. Further, around the photosensitive drum 10, an image density sensor 18 for adjusting the image density, which is disposed in the vicinity of the photosensitive drum 10 on the downstream side of the transfer unit 15, and residual toner after electrostatic transfer. A pre-cleaning charger 16 for reducing the charge amount of the toner and a drum cleaner 17 for removing residual toner on the photosensitive drum 10 are provided.

Here, the photosensitive drum 10 as an image carrier is formed by forming an organic photosensitive layer on the surface of a metal thin cylindrical drum, and the organic photosensitive layer is made of a material that is negatively charged. . The development by the developing unit 13 is performed by a reversal development method. Therefore, the toner used in the developing device 13 is of a negative polarity charging type. Further, the pre-transfer charger 14 as a toner charging unit (pre-transfer charging unit) is composed of a corotron charger disposed in a non-contact manner with respect to the photosensitive drum 10, and performs negative discharge. It is configured as follows. The pre-transfer charger 14 is connected to a PTC (Pre Transfer Corotron) power supply 14a for supplying a current for generating corona discharge.
Further, the image forming apparatus includes a fixing device 60 as a fixing unit that fixes the unfixed toner image transferred onto the paper P, a control unit 30 that controls the operation of each device (each unit), and humidity detection that measures the humidity inside the device. A humidity sensor 40 is provided as a means.

The transfer unit 20 is disposed inside the transfer conveyance belt 21 stretched between the drive roll 22 and the idle roll 23, and is in pressure contact with the photosensitive drum 10 via the transfer conveyance belt 21. And a transfer roll 24 as transfer means. The transfer unit 20 has a function of transferring the toner image on the photosensitive drum 10 onto the paper P conveyed to the transfer unit 15 and conveys the paper P onto which the toner image has been transferred at the transfer unit 15 to the fixing device 60. It has the function to do. Further, the transfer unit 20 has a belt cleaner 25 for cleaning the transfer conveyance belt 21.
Here, the transfer conveyance belt 21 is constituted by a semiconductive rubber belt whose resistance value is adjusted to about 7 logΩ, for example.

  In addition, the image forming apparatus feeds out a paper tray 50 that accommodates the paper P, a pick-up roll 51 that picks up and transports the paper P accumulated in the paper tray 50 at a predetermined timing, and the pick-up roll 51 as a paper transport system. A transport roll 52 that transports the sheet P is provided. Further, the paper transport system sends out the transported paper P from the registration roll 54 that feeds the transported paper P to the transfer unit 15 at a predetermined timing, the transport chute 53 that guides the paper P transported by the transport roll 52 to the registration roll 54, and the resist roll 54. An inlet chute 55 is provided for guiding the sheet P to the transfer unit 15. Further, the paper transport system includes a fixing entrance guide 56 that guides the paper P to which the toner image is transferred by the transfer unit 20 and is transported to the fixing device 60.

  Next, the toner and developer used in the present embodiment will be described in detail. The toner used in the present embodiment has a polarity that is negatively charged as described above, and may be polyester, styrene acrylic, etc. by suspension polymerization, emulsion aggregation and coalescence, dissolution suspension, or the like. Fine particles obtained by internally adding a colorant and wax to the binder resin. The particle diameter was measured by a Coulter counter (manufactured by Coulter Co.). The volume average particle diameter was about 5 μm and the particle size distribution index (GSD) was 1.23. The toner shape (degree of sphere) is represented by a shape factor, and an enlarged photograph of the toner obtained with an optical microscope (Microphoto FXA: manufactured by Nikon Corporation) is subjected to image analysis by an image analyzer Luzex3 (manufactured by Nireco Corporation), and the following formula Calculated with

  This expression is expressed as a ratio of the projected area of the toner to the area of the circle circumscribing the toner, and is 100 for a true sphere, and the number increases as the shape moves away from the sphere. If the shape factor is small, the amount of toner (residual toner) that remains without being transferred during transfer decreases, so the shape factor of the toner is preferably about 100 to 140. In this embodiment, The shape factor of the toner was in the range of 129-134. Then, an appropriate amount of inorganic fine particles such as silica and titania having an average particle diameter of 5 to 200 nm are externally added to the toner as an external additive, and mixed with a carrier made of ferrite beads having an average particle diameter of 35 μm to form a two-component developer. did. The volume average particle diameter of the toner is preferably in the range of 3 to 10 μm from the viewpoint of forming a good image.

  Next, a basic image forming process of the image forming apparatus according to the present embodiment will be described. In the image forming apparatus as shown in FIG. 1, image data output from an image reading device (IIT) not shown or a personal computer (PC) not shown is subjected to predetermined image processing by an image processing device not shown. . In the image processing apparatus, the input reflectance data is subjected to predetermined image processing such as various kinds of image editing such as shading correction, position shift correction, gamma correction, frame deletion editing, and movement editing. The image data that has undergone image processing is output to the laser exposure unit 12.

  The laser exposure unit 12 irradiates the photosensitive drum 10 with an exposure beam Bm emitted from, for example, a semiconductor laser, according to the input image data. After the surface of the photosensitive drum 10 is charged to a predetermined charging potential by the charger 11, the surface is scanned and exposed by the laser exposure device 12 to form an electrostatic latent image. Next, the electrostatic latent image formed on the photosensitive drum 10 is developed by the developing device 13 to be a toner image.

Next, the toner image formed on the photosensitive drum 10 is charged by the pre-transfer charger 14 as necessary, and then conveyed to the transfer unit 15. Details of the operation of the pre-transfer charger 14 will be described later.
On the other hand, in the paper transport system, the pick-up roll 51 rotates in accordance with the timing at which the toner image on the photosensitive drum 10 is transported to the transfer unit 15, and the paper P of a predetermined size is supplied from the paper tray 50. The paper P supplied by the pickup roll 51 is transported by the transport roll 52, and reaches the registration roll 54 through the transport chute 53. In the registration roll 54, the paper P is temporarily stopped, and the registration roll 54 rotates in accordance with the movement timing of the photosensitive drum 10 carrying the toner image. Thereby, the position of the paper P and the position of the toner image are aligned, and the paper P is sent out from the inlet chute 55 to the transfer unit 15.

  In the transfer unit 15, the sheet P conveyed at the same timing is sandwiched between the photosensitive drum 10 and the transfer roll 24 via the transfer conveyance belt 21 of the transfer unit 20. At that time, a voltage (transfer bias) having a polarity opposite to the charging polarity (negative polarity in the present embodiment) of the toner is applied to the transfer roll 24, and the transfer roll 24 is transferred to the transfer conveyance belt 21 on the photosensitive drum 10. A charge having a polarity opposite to the toner charging polarity is applied. As a result, the toner image carried on the photosensitive drum 10 is electrostatically transferred onto the paper P in the transfer unit 15 pressed by the photosensitive drum 10 and the transfer roll 24. The residual toner adhering to the photosensitive drum 10 after the transfer is removed by the drum cleaner 17 after being neutralized by the pre-cleaning charger 16.

Thereafter, the sheet P on which the toner image has been electrostatically transferred is peeled off and conveyed from the photosensitive drum 10 while being electrostatically attracted to the transfer conveyance belt 21 of the transfer unit 20, and downstream of the transfer unit 20 in the sheet P conveyance direction. To the fixing device 60 provided on the side. A separation claw (not shown) is disposed in the vicinity of the surface of the photosensitive drum 10 on the downstream side of the transfer unit 15 to prevent the paper P from sticking to the photosensitive drum 10.
At the rear end portion of the transfer conveyance belt 21 on the fixing device 60 side, the sheet P is peeled from the transfer conveyance belt 21 due to the curvature when the transfer conveyance belt 21 is wound around the driving roll 22 and the stiffness of the sheet P. Then, the paper P is guided to the fixing inlet guide 56 and conveyed to the fixing device 60.
The unfixed toner image on the paper P conveyed to the fixing device 60 is fixed on the paper P by being subjected to fixing processing by heat and pressure in the fixing device 60. Then, the paper P on which the fixed image is formed is conveyed to a paper discharge tray (not shown) of the image forming apparatus, and a series of image forming operations is completed. Further, the transfer conveyance belt 21 after conveying the paper P is cleaned by a belt cleaner 25.

  In the image forming apparatus according to the present embodiment, a setup cycle is executed every predetermined cycle. That is, prior to the actual image forming operation, a predetermined patch area is developed under predetermined conditions. At that time, the transfer conveyance belt 21 of the transfer unit 20 is set in a state of being separated from the photosensitive drum 10. In this state, the developed patch area passes through the transfer section 15 as it is, and the density of the toner image in the patch area is measured by the image density sensor 18 disposed on the downstream side of the transfer section 15. The toner density signal measured by the image density sensor 18 is sent to the control unit 30. Then, in accordance with an instruction from the control unit 30 based on the signal from the image density sensor 18, the exposure amount by the laser exposure device 12 and the toner concentration in the developing device 13 are adjusted, and a predetermined image density is maintained. The Rukoto.

Next, the pre-transfer charging operation by the above-described pre-transfer charger 14 will be described.
FIG. 2 is a diagram illustrating functional blocks of the control unit 30 as charging control means. The control unit 30 controls the entire image forming apparatus according to the present embodiment. FIG. 2 shows only functional blocks related to the pre-transfer charging operation.
The CPU 31 of the control unit 30 executes processing while appropriately exchanging data with the RAM 33 according to the program stored in the ROM 32. Further, the control unit 30 receives image information from the image processing unit 70 (specifically, image density (image density) information of an image to be formed) and humidity from the humidity sensor 40 via the input / output interface 34. Information is entered. Further, the control unit 30 controls the power supply to the pre-transfer charger 14 by the PTC power supply 14 a via the input / output interface 34.

  Next, the pre-transfer charging operation in the image forming operation will be described. Here, the pre-transfer charging operation is preferably controlled based on both the image density information from the image processing unit 70 and the humidity information from the humidity sensor 40, but only on the image density information from the image processing unit 70. Control may be performed based on this. Hereinafter, each control example will be described.

◎ Control example 1
In this control example, the pre-transfer charging operation is controlled based only on the image density information input from the image processing unit 70. That is, the control unit 30 first acquires the toner image density from the image processing unit 70 (first step), and then determines whether or not the pre-transfer charger 14 is charged based on the acquired image density. (Second step). Table 1 shows the relationship between the image density D input from the image processing unit 70 and the PTC applied current value set as a result of the PTC power supply 14a.

  If the image density D exceeds 30%, if the absolute amount of unfixed toner present on the paper P after transfer is large, and the proportion of toner with a low charge amount contained in all toners is unchanged, It is estimated that the amount of toner leaving the paper P after transfer increases. In addition, when the image density D is high, there is a possibility that unfixed toner is attached to the paper P thickly (in a plurality of layers), and the adhesive force of some toner to the paper P is weak. It seems to be easy to become. For this reason, when the image density D exceeds 30%, the PTC applied current value is set to −55 μA, and the toner before transfer (charged to negative polarity) is further strongly charged to negative polarity, and then transferred to the paper P. To do.

  On the other hand, when the image density D is 30% or less, since the absolute amount of unfixed toner existing on the transferred paper P is small, the ratio of the low charge amount toner contained in all the toners is unchanged. In this case, it is estimated that the amount of toner that leaves the paper P after transfer is reduced. In addition, when the image density D is low, the unfixed toner adheres thinly to the paper P, so that it is considered that the adhesion force (electrostatic adsorption force) of the toner to the paper P is also strong. For this reason, when the image density D is 30% or less, the PTC applied current value is set to 0 μA, and the pre-transfer charging operation by the pre-transfer charger 14 is not performed, and the transfer to the paper P is performed as it is.

  As described above, when the image density D of the toner image to be formed is high (over 30%), the toner image formed on the photosensitive drum 10 is recharged by the pre-transfer charger 14, thereby The charge amount can be increased. Then, the adhesion force (electrostatic adsorption force) of the toner after transfer to the paper P is increased. As a result, it is possible to prevent a situation in which the toner is separated from the paper P and the image on the paper P is disturbed until the paper P is conveyed from the transfer unit 15 to the fixing device 60. Further, since the toner is not separated from the paper P, the amount of toner floating in the image forming apparatus can be remarkably reduced, and adhesion of the floating toner to the fixing device 60 and occurrence of contamination in the apparatus can be suppressed. become.

  Here, in the present embodiment, since a so-called spherical toner is used as the toner, the contact area between the paper P and the toner is small, and the adhesion force is particularly likely to be small. When the image density D is high, the toner may overlap and adhere to the paper P, further reducing the adhesion force. Further, since the particle size of the toner is small, the charge amount of each toner tends to be small as compared with, for example, conventional pulverized toner. In the present embodiment, recharging is performed by the pre-transfer charger 14 when the image density D is high. Therefore, even when spherical toner is used, scattering of toner from the paper P after transfer can be suppressed. it can.

  As shown in FIG. 1, in the present embodiment, for the sake of layout, the fixing nip of the fixing device 60 is set at a position higher than the sheet conveyance surface of the transfer conveyance belt 21. That is, in this image forming apparatus, the sheet conveyance path for conveying the sheet P from the transfer unit 15 to the fixing device 60 is non-linear. Therefore, in this image forming apparatus, a fixing entrance guide 56 is provided between the transfer conveyance belt 21 and the fixing device 60 so as to guide the paper P carrying an unfixed toner image. When such a configuration is adopted, the leading end of the sheet P carrying the unfixed toner image in the conveyance direction easily collides with the fixing inlet guide 56, and as a result, the unfixed toner on the sheet P is likely to scatter. On the other hand, in this embodiment, when the image density D is high, the toner before the transfer is charged to increase the adhesion of the toner to the paper P. Therefore, the conveyance path of the paper P after the transfer is linear. Even in the case where the toner cannot be formed in a shape, it is possible to suppress toner scattering.

  On the other hand, when the image density D of the toner image to be formed is low (30% or less), the toner image formed on the photosensitive drum 10 is transferred as it is without being recharged by the pre-transfer charger 14. Yes. When the image density D of the toner image is low, there is no possibility that toner scatters while the paper P is conveyed from the transfer unit 15 to the fixing device 60, so that no particular problem occurs.

  Here, in the corotron charger constituting the pre-transfer charger 14, a thin metal wire is generally used as the discharge electrode, and the discharge characteristics deteriorate as the energization time increases. Therefore, when the image density D of the toner image to be formed is low (30% or less), it is possible to extend the life of the pre-transfer charger 14 by not performing recharging by the pre-transfer charger 14. Is possible. In particular, in this embodiment, the use or non-use of the pre-transfer charger 14 is set with the image density D30% as a boundary, so that a general text image (image density D <10) that is frequently printed is used. %), It is not necessary to use the pre-transfer charger 14, so that the life can be further extended.

◎ Control example 2
In this control example, the pre-transfer charging operation is controlled based on the image density information input from the image processing unit 70 and the humidity information input from the humidity sensor 40. That is, the control unit 30 first acquires the toner image density from the image processing unit 70 (first step), acquires humidity from the humidity sensor 40 (step of acquiring humidity), and then acquires the acquired image. The presence or absence of charging by the pre-transfer charger 14 is determined based on the density and humidity (second step). Table 2 shows the relationship between the image density D input from the image processing unit 70 and the humidity H input from the humidity sensor 40, and the PTC applied current value set in the PTC power supply 14a as a result.

In general, when the humidity is low, a discharge phenomenon is likely to occur even at a low voltage, and as a result, scattering of unfixed toner on the paper P is likely to occur. On the other hand, when the humidity is high, the discharge phenomenon hardly occurs, and as a result, the unfixed toner on the paper P is hardly scattered.
Therefore, in this control example, in a low humidity environment where the humidity H is 30% or less, the PTC application current value is set to −55 μA regardless of the image density D. In a high humidity environment where the humidity H exceeds 70%, the PTC applied current value is set to 0 μA. Further, in a medium humidity environment where the humidity H is more than 30% and 70% or less, the PTC current value is varied according to the image density D. Specifically, when the image density D is 30% or less, the PTC application current value is set to 0 μA, and when the image density D exceeds 30% or more, the PTC application current value is set to −25 μA. .

  In a medium humidity environment, discharge is more likely to occur than in a high humidity environment, but discharge is less likely to occur than in a low humidity environment. Further, as described in the control example 1, when the image density D is low, scattering of unfixed toner on the paper P hardly occurs. Therefore, in a medium humidity environment, when the image density D is low, the pre-transfer charger 14 is not used, and when the image density is high, the pre-transfer charger 14 is used, but the PTC applied current value is small. is doing.

  In the present embodiment, whether or not to use the pre-transfer charger 14 according to the environment (humidity) in which the image forming apparatus is placed in addition to the image density D of the toner image to be formed (the magnitude of the PTC applied current value). ) Was decided. In a low humidity environment, a high (−55 μA) PTC applied current value is set regardless of the image density D, and in a high humidity environment, the PTC applied current value is set to 0 μA regardless of the image density D. The charger 14 was not used. In a medium humidity environment, the PTC application current value is set according to the image density D. When the image density D is high, a low (−25 μA) PTC application current value is set and the image density D is low. In this case, the pre-transfer charger 14 is not used by setting the PTC application current value to 0 μA. As a result, it is possible to set an appropriate PTC application current value according to the humidity, and to extend the life of the pre-transfer charger 14 while suppressing the scattering of toner from the paper P after transfer.

In this embodiment, the operation of the pre-transfer charger 14 is set with the image density D = 30% as a boundary. However, the present invention is not limited to this, and other image density D may be used as a reference. Of course. Also, for the humidity H, a reference value different from 30% or 70% may be adopted. Furthermore, in the present embodiment, the image density D is classified into two stages and the humidity is classified into three stages. However, the present invention is not limited to this, and it is needless to say that the image density D may be classified into finer classes. .
Further, in the present embodiment, the image forming apparatus configured to form a single color toner image has been described as an example. However, the present invention is not limited to this. For example, yellow, magenta, cyan, and black toner images are superimposed. It can also be applied to a color image forming apparatus that forms a color toner image.
Furthermore, in the present embodiment, an example of an image forming apparatus that directly transfers a toner image formed on the photosensitive drum 10 onto the paper P has been described. However, the present invention is not limited to this. An intermediate transfer member (for example, an intermediate transfer belt) is disposed to face the photosensitive drum, and the toner image formed on the photosensitive drum 10 is temporarily transferred to the intermediate transfer member and then temporarily transferred to the intermediate transfer member. It is also possible to apply to a so-called intermediate transfer type image forming apparatus that secondarily transfers the transferred toner image onto a sheet. In this case, the above-described method can be applied to primary transfer from the photosensitive member to the intermediate transfer member or secondary transfer from the intermediate transfer member to the sheet.

1 is a diagram for describing a schematic configuration of an image forming apparatus to which the exemplary embodiment is applied. It is a functional block diagram of a control part.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Photosensitive drum, 11 ... Charger, 12 ... Laser exposure device, 13 ... Developing device, 14 ... Pre-transfer charger, 14a ... PTC power supply, 15 ... Transfer part, 16 ... Pre-cleaning charger, 17 ... Drum cleaner , 18 ... Image density sensor, 20 ... Transfer unit, 21 ... Transfer conveyor belt, 22 ... Drive roll, 23 ... Idle roll, 24 ... Transfer roll, 25 ... Belt cleaner, 30 ... Control unit, 40 ... Humidity sensor, 50 ... Paper tray 56 ... Fixing entrance guide 60 ... Fixing unit 70 ... Image processing unit

Claims (8)

An image carrier on which toner is carried; and
Toner charging means for charging the toner on the image carrier;
Transfer means for electrostatically transferring the toner on the image carrier charged by the toner charging means to a recording material;
An image forming apparatus comprising: a charging control unit that controls a charging operation by the toner charging unit based on an image density of toner formed on the image carrier.   The image forming apparatus according to claim 1, wherein the charging control unit does not perform the charging operation by the toner charging unit when the image density is a predetermined value or less. It further includes humidity detecting means for detecting humidity,
The image forming apparatus according to claim 1, wherein the charging control unit controls a charging operation by the toner charging unit based on the image density and the humidity detected by the humidity detecting unit.   The image forming apparatus according to claim 3, wherein the charging control unit does not perform the charging operation by the toner charging unit when the humidity detected by the humidity detecting unit exceeds a predetermined value. . The toner carried on the image carrier is charged by the pre-transfer charging means, the charged toner on the image carrier is transferred to the recording material by the transfer means, and the recording material onto which the toner has been transferred is non-linear. An image forming method in an image forming apparatus which is conveyed through a conveyance path and fixes toner on the conveyed recording material by a fixing unit,
A first step of acquiring an image density of toner formed on the image carrier;
And a second step of controlling a charging operation by the pre-transfer charging unit based on the acquired image density.   6. The charging operation by the pre-transfer charging unit is not performed in the second step when the image density acquired in the first step is a predetermined value or less. Image forming method. Further comprising obtaining the humidity;
6. The image forming method according to claim 5, wherein in the second step, a charging operation by the pre-transfer charging unit is controlled based on the acquired humidity.   8. The image forming method according to claim 7, wherein, in the second step, the charging operation by the pre-transfer charging unit is not performed when the acquired humidity exceeds a predetermined value.

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