JP6094801B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus using an electrophotographic system, and more particularly to an image forming apparatus that indirectly detects an electrical resistance of a transfer target member and adjusts a transfer voltage.

  In an electrophotographic image forming apparatus, a toner image formed on a photosensitive drum or an intermediate transfer belt is transferred onto a transfer member such as a sheet by electrostatic action due to a transfer voltage applied to a transfer member such as a transfer roller. Transfer to form an image. It has been conventionally known that the image quality of a toner image transferred to a transfer member is greatly influenced by the electric resistance of the transfer member.

  For this reason, an optimal transfer voltage is determined in advance for each type of member to be transferred, and a method for setting the transfer voltage according to information on the member to be transferred is performed. However, since the electrical resistance of the member to be transferred varies depending on the material and thickness, it is difficult to cope with all the members to be transferred on the market. Furthermore, the electric resistance of the member to be transferred varies greatly depending on the water content of the member to be transferred, and is easily influenced by the storage state of the member to be transferred.

  Therefore, in order to avoid such an influence on the image quality due to the environment, a method for controlling the transfer voltage by measuring the electric resistance of the member to be transferred has been proposed. There is also known a method in which a humidity sensor is provided and the transfer voltage is changed by its output. However, these methods must provide a dedicated resistance measuring means. In the case of laminated transfer members, the degree of influence of environmental humidity may be different between the upper layer and the lower layer, and even if the transfer voltage is appropriate for the upper transfer member, the upper layer and the lower layer may be different for the lower transfer member. Had the problem of an inappropriate transfer voltage.

  Patent Document 1 discloses a method for controlling transfer conditions by measuring a photoreceptor surface potential and humidity after transfer. That is, first, the detection value of the humidity sensor is compared with prestored data to perform primary correction of transfer conditions, and when image formation is performed continuously, the surface of the photoreceptor after transfer every time or every few sheets A method is shown in which the potential is measured and the measured value is compared with the potential data at the time of the first image formation to secondarily correct the transfer condition. This utilizes the fact that the surface potential of the photoconductor after transfer changes depending on the electric resistance of the transfer member that contacts the photoconductor during transfer. There is an effect of preventing deterioration in image quality due to fluctuations in electrical resistance.

JP-A-5-181335

  Since the method for estimating the electrical resistance of the member to be transferred from the humidity measured by the humidity sensor differs depending on the material and thickness of the member to be transferred, it is not realistic to cover all the members to be transferred. In addition, since the influence of the environment varies depending on the storage state of the member to be transferred, the method of estimating the electrical resistance from the humidity sensor has a large error.

  Further, the method described in Patent Document 1 requires a surface potential sensor to be provided in the vicinity of the outer periphery of the photoreceptor, which is disadvantageous for downsizing the apparatus.

  The present invention has been made in view of such conventional problems, and its purpose is to reliably eliminate the influence on the image quality due to the type of the member to be transferred and the use environment without installing a new member. An object of the present invention is to provide an image forming apparatus capable of stably obtaining high image quality.

  An image forming apparatus according to the present invention that achieves the object includes an image carrier, a charging device that applies a predetermined charging voltage to uniformly charge the surface of the image carrier, and the charged image carrier. An exposure device for forming an electrostatic latent image and a developing roller to which a predetermined developing bias voltage is applied, and the potential difference between the developing bias voltage applied to the developing roller and the surface potential of the electrostatic latent image A developing device for developing the electrostatic latent image with toner, a transfer device to which a transfer voltage is applied and transferring the developed toner image from the image bearing member to the transfer member, and a control unit for controlling the transfer voltage; An image forming apparatus comprising a density detection unit that measures the density of a toner image formed on the surface of the image carrier, and a portion of the image carrier that is in contact with a transfer target member after the transfer process, Toner image for transfer voltage adjustment While formed, the concentration of the formed toner image detected by said density detecting means, by said control means, and adjusts the transfer voltage based upon the detected concentration.

  Here, when the density of the toner image for transfer voltage adjustment is higher than the reference density stored in advance by the control means, the transfer voltage is set lower than the set voltage, and the density is higher than the reference density. When it is lower than a predetermined value, it is preferable to set the transfer voltage higher than the set voltage.

  The transfer voltage adjusting toner image is preferably formed by charging the surface of the image carrier with a charging voltage lower than the predetermined charging voltage in the charging device.

  The transfer voltage adjusting toner image is preferably formed by developing with a developing bias voltage lower than the predetermined developing bias voltage in the developing device.

  Further, it is preferable that the transfer voltage adjusting toner image is formed on a portion of the image carrier that is in contact with the non-image forming area at the rear end of the transfer member.

  In the image forming apparatus of the present invention, since the electrical resistance of the member to be transferred is estimated and the transfer voltage is adjusted without installing a new member, the size of the device to be transferred is not increased and the cost of the member to be transferred is not increased. The influence on the image quality by the type and usage environment can be surely eliminated, and high image quality can be stably obtained.

1 is a schematic diagram illustrating an example of an image forming apparatus according to the present invention. FIG. 2 is a control configuration diagram illustrating elements of a control system of the image forming apparatus in FIG. 1. FIG. 2 is a schematic diagram of an image forming unit of the image forming apparatus in FIG. 1. FIG. 3 is a diagram illustrating a surface potential of a photoreceptor in an image forming process of an image forming apparatus. The figure which shows the relationship between transfer efficiency and a transfer voltage. 6 is a graph showing a relationship between a transfer voltage applied to a transfer roller and a surface potential of the photoreceptor after contact with the transfer roller. FIG. 6 is a diagram for explaining that the photoreceptor surface potential in the non-image forming area after transfer varies depending on the electric resistance of the paper. FIG. 3 is a diagram illustrating a surface potential of a photoreceptor in an image forming process of an image forming apparatus. FIG. 4 is an explanatory diagram illustrating a reason why a toner density change is caused by electric resistance of a sheet. FIG. 3 is an explanatory diagram illustrating an example of a photoreceptor surface potential that is in contact with a sheet in a transfer process. 6 is a flowchart illustrating an example of control of a transfer voltage in the image forming apparatus according to the present invention. Explanatory drawing which shows the change of the photoreceptor surface potential in the other Example of this invention. Explanatory drawing in the case of changing a developing bias voltage in another Example of this invention.

  Hereinafter, the image forming apparatus according to the present invention will be described in more detail with reference to the drawings. However, the present invention is not limited to these embodiments.

  FIG. 1 is a schematic diagram showing an embodiment of an image forming apparatus according to the present invention. The image forming apparatus 1 shown in FIG. 1 includes an image forming unit 10, a sheet conveying unit 20 that conveys a sheet (a member to be transferred) P through a predetermined path, a fixing unit 30, and a control unit 40. The image forming unit 10 has a cylindrical photosensitive member (image carrier) D that carries a toner image and is driven to rotate counterclockwise, and a charging device that uniformly charges the surface of the photosensitive member D around the cylindrical photosensitive member D. 11, an exposure device 12 that irradiates the surface of the photosensitive member D with light to form an electrostatic latent image, and supplies toner to the photosensitive member D to develop the electrostatic latent image on the photosensitive member D to form a toner image. The developing device 13 to be formed, the transfer roller (transfer device) 14 for transferring the toner image on the photoconductor D formed by the developing device 13 to the paper P, and the photoconductor D remaining on the photoconductor D without being transferred to the paper P A cleaning device 15 that removes toner and a charge eliminating unit 16 that erases residual charges on the photoreceptor D are included.

  The charging device 11 is a scorotron charging method, and when a voltage of several kV is applied to the discharge electrode, corona discharge occurs, whereby the surface of the photoreceptor D is uniformly charged. The type of the charging device 11 is not particularly limited, and of course, a roller charging member, a blade charging member, a brush charging member, or the like may be used.

  The exposure device 12 performs exposure by selectively irradiating light on the surface of the photoconductor D uniformly charged by the charging device 11 based on image data input from an external device such as a personal computer, for example. A predetermined electrostatic latent image is formed on the surface of the photoreceptor D.

  The developing device 13 includes in the housing 131 a developing roller 132 that is rotatably provided facing the photoconductor D, and three transport rollers 133 that transport the developer toward the developing roller 132. In the housing 131, a developer composed of toner and a carrier (both not shown) is accommodated. Based on the potential difference between the voltage of the developing roller 132 and the electrostatic latent image on the photoconductor D, which is generated when a developing bias voltage is applied to the developing roller 132, the toner moves to the photoconductor D, and on the photoconductor D. The electrostatic latent image is visualized (toner image) with toner.

  The transfer roller 14 is rotatably provided by a drive motor (not shown) connected to the transfer roller 14 and is pressed against the photoreceptor D by a biasing member (not shown). The transfer roller 14 is applied with a transfer voltage having a polarity opposite to the charging polarity of the toner by a voltage application unit (not shown). When the sheet passes through the nip portion between the photoreceptor D and the transfer roller 14, a transfer voltage is applied to the transfer roller 14, and the toner image formed on the photoreceptor D is transferred to the sheet P.

  The cleaning device 15 includes a cleaning blade 151 that presses against the photoconductor D, and removes untransferred toner remaining on the surface of the photoconductor D from the photoconductor D.

  A paper feed cassette 21 containing paper P is detachably disposed below the image forming unit 10, and the paper P stored therein is a paper feed roller 22 disposed on the upper side of the paper feed cassette 21. , The sheets are fed one by one from the uppermost sheet to the transport path 23 in order. A plurality of transport roller pairs 24 are provided in the transport path 23, and transport the paper P to the nip portion between the transfer roller 14 and the photoconductor D in synchronization with the rotation of the photoconductor D. As described above, the toner image on the photoreceptor D is transferred to the paper P at the nip portion. The sheet P on which the toner image is transferred is heated and pressurized by the fixing unit 30, and the toner image is melted and fixed on the sheet P. The paper is discharged onto the tray 25 by the paper discharge roller pair 24. These sheet cassette 21, sheet feeding roller 22, conveyance path 23, and conveyance roller pair 24 constitute a sheet conveyance unit 20.

  The image forming apparatus 1 includes a control unit 40 that controls components. FIG. 2 shows a control configuration diagram. The control means 40 has a CPU 41 and controls the paper conveyance system / fixing load system 42, the development system load unit 43, and the like.

  The main body-attached nonvolatile memory 44 as a recording medium is connected to the CPU 41 and stores data measured by the CPU 41. Further, a unit-attached nonvolatile memory 45 is attached to a consumable such as toner, and consumable information such as a use history can be stored by connecting to the CPU 41.

  The CPU 41 controls the print head 46, the development system load 43, and the transfer voltage 48 when forming an image. Further, the exposure amount and exposure position of the print head 46 of the exposure apparatus 12 are controlled. Further, the development system control unit 47 is controlled. The developing system control unit 47 controls the charging bias 471 and the developing bias voltage, and the toner density sensor 17 transmits a detection signal of the toner image density formed on the surface of the photoreceptor D to the CPU 41. The CPU 41 also controls the transfer voltage 48. The control unit 40 and the controller unit 49 are connected to transmit / receive necessary information such as image data.

  When the CPU 41 forms an image, the print head 46, the charging bias 471, the developing bias voltage 472, and the transfer are set so as to obtain an optimum toner density based on the density of the toner image on the surface of the photoreceptor D detected by the toner density sensor 17. Each output of the voltage 48 is controlled.

  The surface potential of the photoconductor D varies depending on the developing bias voltage applied to the charging device 11, the exposure device 12, and the developing roller 132, the transfer voltage applied to the transfer roller 14, and the charge eliminating unit 16. A change in the surface potential of the photoreceptor D will be described with reference to FIGS. 3 and 4.

  FIG. 3 is a schematic diagram of the image forming unit. 4 shows the surface potential of the photoreceptor D at each position shown in FIG. 3, and the surface of the photoreceptor D uniformly charged to a minus potential is partially lowered by the exposure device 12 (zero). The state of potential) is shown as a downwardly convex view. In the following description, increasing the surface potential in the minus direction is referred to as increasing the surface potential or the surface potential is high, and decreasing in the zero direction is referred to as decreasing the surface potential or decreasing the surface potential.

  At the position (I) before transfer, the non-exposed portion Fw has a surface potential of −Vo, and the potential of the exposed portion Fr is lower than that of the non-exposed portion Fw. Then, at the position (II) after the transfer that has passed through the nip portion with the transfer roller 14, the surface potential of the photosensitive member D is generally lowered by the transfer voltage by the transfer roller 14. Next, at the neutralization position (III), the entire surface of the photoconductor D is neutralized in order to reduce variations in the surface potential of the photoconductor due to the presence or absence of a latent image. In general, static elimination is performed by light-exposing the surface of the photoconductor D. The surface potential of the photoconductor D is further lowered, and the potential difference between the non-exposed portion and the exposed portion is reduced. In addition, this static elimination process is not necessarily an essential process.

  At the next charging position (IV), the surface of the photoreceptor D is uniformly charged by the charging device 11 to form a new electrostatic latent image, and the surface potential becomes −Vo. At the exposure position (V), exposure according to the image is performed by the exposure device 12, and the surface potential of the exposure part Fr is lowered to -Vr. Next, at the development position (VI), the toner is transferred from the developing roller 132 to the exposed portion Fr of the photoreceptor D by the potential difference E between the developing bias voltage −Vdc applied to the developing roller 132 and the surface potential −Vr of the exposed portion Fr. Moves and development is performed. At this time, since the surface potential of the non-exposed portion Fw is higher than −Vo and the developing bias voltage −Vdc of the developing roller 132, the toner does not move to the non-exposed portion Fw. As described above, since the developing bias voltage −Vdc of the developing roller 132 is at an intermediate value between the surface potential −Vr of the non-exposed portion Fw and the surface potential −Vr of the exposed portion Fr, an electrostatic latent image on the surface of the photoreceptor D is formed. Developed with toner. Further, when the potential difference E is large, the force for moving the toner increases, and the toner image density increases. On the other hand, when the potential difference E is small, the force for moving the toner is small, and the toner image density is thin. The toner density sensor 17 is attached between the development position (VI) and the transfer position (I).

  Although it is desirable that all of the toner adhering to the exposed portion Fr of the photoconductor D moves to the paper P by the transfer voltage applied to the transfer roller 14, it does not actually move but remains on the surface of the photoconductor D. Toner is present. FIG. 5 shows the relationship between the transfer efficiency of toner onto the paper P and the transfer voltage. As is apparent from FIG. 5, the transfer voltage has an optimum transfer voltage at which the highest transfer efficiency can be obtained. This optimum transfer voltage varies depending on the electric resistance of the paper P. The optimal transfer voltage is higher for a paper having a higher electrical resistance than the standard plain paper, whereas the optimal transfer voltage is lower for a paper having a lower electrical resistance.

  FIG. 6 is a graph showing the relationship between the transfer voltage applied to the transfer roller 14 and the surface potential of the photoreceptor D after contacting the transfer roller 14. As understood from FIG. 6, when the transfer voltage is a predetermined voltage (broken line in FIG. 6), a difference occurs in the surface potential of the photoreceptor after transfer between a sheet with high electrical resistance and a sheet with low electrical resistance. That is, when the electric resistance of the paper is high, the surface potential is high, and when the electric resistance of the paper is low, the surface potential is low.

  FIG. 7 is a diagram for explaining that the degree to which the surface potential of the non-exposed portion Fw of the photoreceptor D decreases after transfer varies depending on the electrical resistance of the paper P. In the case of a reference sheet, the surface potential after transfer is reduced by Hs to −Vps. On the other hand, in the case of a sheet having high electrical resistance, the surface potential after transfer is reduced by Ha to −Vpa. On the other hand, in the case of a sheet with low electrical resistance, the surface potential after transfer is reduced by Hb to −Vpb. Therefore, in the present invention, the transfer voltage is adjusted by estimating the surface potential of the paper P using the difference in the surface potential of the photoconductor after transfer due to the difference in the electrical resistance of the paper.

  FIG. 8 is a diagram showing the surface potential of the non-exposed portion Fw at each position shown in FIG. At the pre-transfer position (I), the surface potential of the photoconductor D is −Vo, and at the post-transfer position (II), the surface potential is lowered to −Vps due to the influence of the transfer voltage. The surface potential at this time varies depending on the electric resistance of the paper P as described with reference to FIG. Then, static elimination and charging in the next process are not performed. As a result, the photoconductor D maintains a surface potential reflecting the electric resistance of the paper P.

  In the next exposure step (V), an electrostatic latent image of the transfer voltage adjusting toner image is created by exposure. The surface potential of the exposed portion Frr is lowered to -Vrs. The surface potential of the exposed portion Frr is affected by the surface potential before exposure -Vps, and if the surface potential before exposure is high, the surface potential after exposure becomes high, while if the surface potential before exposure is low, the surface potential after exposure is high. The potential is also lowered. Therefore, the surface potential of the exposed portion Frr is a surface potential that reflects the surface potential before exposure, that is, the electrical resistance of the paper P. In the developing step (VI), the toner adheres to the exposed portion Frr due to the potential difference Es between the surface potential -Vrs reflecting the electric resistance of the paper P and the developing bias voltage -Vdc. By detecting the toner density of the exposed portion Frr with the toner density sensor 17, the electrical resistance of the paper P is estimated.

  FIG. 9 is a diagram for explaining that the density of the transfer voltage adjusting toner image obtained in the developing step (VI) of FIG. In the case of a reference resistance sheet, the surface potential of the non-exposed portion Fw is −Vps, the surface potential of the exposed portion Frr is −Vrs, and the exposed portion Frr has a potential difference Es between the surface potential −Vrs and the developing bias voltage −Vdc. An amount of toner corresponding to the amount adheres. On the other hand, in the case of a sheet having a high electrical resistance, the surface potential of the non-exposed portion Fw is as high as −Vpa, and thus the surface potential of the exposed portion Frr is as high as −Vra. For this reason, the potential difference Ea between the surface potential −Vra of the exposed portion Frr and the developing bias voltage −Vdc is also reduced, and the amount of toner attached to the exposed portion Frr is reduced. On the contrary, in the case of a sheet having a low electric resistance, since the potential of the non-exposed portion Fw is as low as -Vpb, the surface potential of the exposed portion Frr is also low as -Vrb. For this reason, the potential difference Eb between the surface potential −Vrb of the exposed portion Frr and the developing bias voltage −Vdc increases, and the amount of toner attached to the exposed portion Frr increases. If the exposure of the electrostatic latent image of the transfer voltage adjusting toner image to be created is set to halftone, the difference in surface potential before exposure is easily reflected in the toner density difference.

  As described above, when the electric resistance of the paper P that has passed through the nip portion is high, the transfer voltage adjusting toner image becomes thin, while in the case of the paper P having a low electric resistance, the transfer voltage adjusting toner image becomes dark. Accordingly, when the density of the toner image for transfer voltage adjustment is detected by the toner density sensor 17, and the detected density is higher than the density stored in advance, it is determined that the sheet P having a low electrical resistance is used and the transfer voltage is used. Is lower than the set voltage, and when the detected density is lower than the density stored in advance, it is determined that the sheet P having a high electrical resistance is used, and the transfer voltage is set higher than the set voltage. As a result, the highest transfer efficiency can be obtained, and the optimum image density can be maintained.

  FIG. 10 shows an example of the position at which the transfer voltage adjusting toner image is formed. Since image information is exposed and developed in the image forming area, the surface potential of the photoreceptor changes variously and is not suitable for forming a transfer voltage adjusting toner image. Further, it is only affected by the transfer voltage of the transfer roller 14 between the next sheet after passing through the sheet and is not related to the electrical resistance of the sheet P. Therefore, it is preferable to form a transfer voltage adjusting toner image on the portion of the photoreceptor D that is in contact with the margin (non-image forming area) PW at the rear end of the paper P.

  FIG. 11 is a flowchart showing an example of transfer voltage control in the image forming apparatus according to the present invention. After the normal image forming operation, the control operation starts when the electrical resistance of the paper P is detected and the transfer voltage is corrected as a result. First, in step S1, it is determined whether or not a printing area that is an image forming area on the surface of the photoreceptor in normal printing has passed through an exposure unit by the exposure device 12. Then, when the printing area passes through the exposure part, in step 2, the static elimination means 16 and the charging device 11 are turned off. Next, in step S3, it is determined whether or not the trailing edge of the paper P has passed the transfer portion. When the trailing edge of the paper P passes the transfer portion, the transfer voltage is turned off in step S4. In step S <b> 5, it is determined whether or not the surface of the photoconductor D that has been in contact with the rear end of the paper P has passed through the charge eliminating unit 16 and the charging device 11. Next, when it is determined in step S6 that the exposure portion has been reached, the electrostatic latent image of the transfer voltage adjusting toner image is exposed in step S7, and then developed.

  In step S8, the toner density sensor 17 detects the density of the transfer voltage adjusting toner image. If the detected toner density is higher than the reference density upper limit GH stored in advance in step S9, the transfer voltage is lowered in step S10. On the other hand, if the detected toner density is lower than the reference density lower limit GL stored in advance in step S11, the transfer voltage is increased in step S12.

  This transfer voltage adjusting toner image is only formed on the surface of the photoconductor D, and is not required to be transferred to the paper P. When the transfer voltage is adjusted, the toner image for adjusting the transfer voltage of the photosensitive member may be removed by the cleaning device 15 after passing through the nip portion of the transfer roller 14 without passing the sheet.

  As an example of the transfer voltage correction method, there is a method of subtracting or adding a value obtained by multiplying the density difference from the reference density by a certain coefficient to the transfer voltage so far. In this case, for example, the reference density is obtained in advance through experiments for each type of paper P set by the user, such as general-purpose plain paper or photographic paper, and stored as data, and an allowable range of density change is determined. An upper limit GH and a lower limit GL for the reference density are determined. Then, the density change of the transfer voltage adjusting toner image caused by the change in the electrical resistance due to the environmental humidity and temperature, and the voltage to be added or subtracted at that time are obtained by experiment and stored as coefficients in the table. In addition, a method may be used in which all the relationships among the types of paper P, the reference density, the measured density, and the transfer voltage so far are collected as a table, and an optimum transfer voltage is selected according to the measured density. Such correction of the transfer voltage varies depending on the sensitivity of the photosensitive member D, and therefore, the correction coefficient and the transfer voltage may naturally vary depending on the model of the image forming apparatus.

  As shown in FIG. 9, when the surface potential of the non-exposed portion Fw is higher than the developing bias voltage −Vdc and the potential of the exposed portion Frr is lower than the developing bias voltage −Vdc, charging by the charging device 11 is performed. Without any problem, the toner image for adjusting the transfer voltage is developed without any problem. However, as shown in FIG. 12A, when the electric resistance of the paper P is extremely low, when the surface potential −Vpb of the non-exposed portion Fw is equal to or lower than the developing bias voltage −Vdc, the exposed portion Frr is developed. In addition, the toner adheres not only to the non-exposed portion Fw, but the toner adheres to the non-exposed portion Fw. Therefore, as shown in FIG. 12 (a), it is recommended that the charging device 11 be slightly charged at a voltage lower than usual and the surface potential of the photosensitive member be -Vpbb which is higher by Tb. Note that the charging voltage is appropriately selected from a range below a predetermined charging voltage, including zero (not charged), so that the surface potential of the photoreceptor reflecting the electric resistance of the paper P is not lost by charging. do it.

  Alternatively, as shown in FIG. 13, the development bias voltage is set to −Vdcc, which is lower than −Vdc by Td, so that the potential is lower than the surface potential −Vpb of the non-exposed portion Fw, thereby preventing wasteful consumption of toner. . The development bias voltage is lower than the surface potential of the non-exposed portion Fw of the photoreceptor D and the surface of the exposed portion Frr because the development bias voltage becomes lower than the surface potential of the exposed portion Frr. It must be higher than the potential. The degree of charging shown in FIG. 12 and the degree of developing bias voltage shown in FIG. 13 may be determined in advance through experiments or the like. Further, when changing the charging or developing bias voltage, the coefficient for adjusting the transfer voltage and the value of the table are also changed based on the detected density of the transfer voltage adjusting toner image described above.

  The embodiment described above is a so-called reversal development method in which toner is attached to an exposed portion and developed. However, the present invention can also be applied to a so-called regular development method in which toner is attached to a non-exposed portion and developed. it can.

  In the image forming apparatus of the present invention, it is possible to reliably eliminate the influence on the image quality due to the type of the member to be transferred and the use environment without increasing the size and cost of the apparatus, and it is possible to stably obtain high image quality and useful. is there.

1 Image forming apparatus D Photoconductor (image carrier)
P paper (transferred member)
DESCRIPTION OF SYMBOLS 11 Charging device 12 Exposure device 13 Development device 14 Transfer roller (transfer device)
17 Toner density sensor (density detection means)
40 control means PW non-image forming area 132 developing roller

Claims (5)

An image carrier, a charging device that applies a predetermined charging voltage to uniformly charge the surface of the image carrier, an exposure device that forms an electrostatic latent image on the charged image carrier, and a predetermined device. A developing device that includes a developing roller to which a developing bias voltage is applied, and that develops the electrostatic latent image with toner based on a potential difference between the developing bias voltage applied to the developing roller and the surface potential of the electrostatic latent image; A transfer device for transferring a developed toner image from the image carrier to a transfer member to which a voltage is applied; a control means for controlling the transfer voltage; and a density of the toner image formed on the surface of the image carrier. In an image forming apparatus provided with a density detection means for measuring
After the transfer process, a transfer voltage adjusting toner image is formed on the portion of the image carrier that is in contact with the transfer target member, and the density of the formed toner image is detected by the density detection unit. An image forming apparatus that adjusts the transfer voltage based on a detected density.   When the density of the transfer voltage adjusting toner image is higher than a preliminarily stored reference density by a predetermined value or more, the control means makes the transfer voltage lower than a set voltage and exceeds the reference density by a predetermined value or more. The image forming apparatus according to claim 1, wherein when the transfer voltage is low, the transfer voltage is set higher than a set voltage.   3. The image forming apparatus according to claim 1, wherein the transfer voltage adjusting toner image is formed by charging the surface of the image carrier with a charging voltage lower than the predetermined charging voltage in the charging device.   The image forming apparatus according to claim 1, wherein the transfer voltage adjusting toner image is formed by developing with a developing bias voltage lower than the predetermined developing bias voltage in the developing device.   5. The image forming apparatus according to claim 1, wherein the transfer voltage adjusting toner image is formed on a portion of the image carrier that is in contact with a non-image forming region at a rear end of a transfer member.