JP2009300483A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus that prevents the deposition of low charged toner and reversely charged toner to a background section of an electrostatic latent image of a photoreceptor and prevents the occurrence of fog in the background section of a printed matter, when high-definition mode or character mode is set or when printing is performed on special paper. <P>SOLUTION: When the high-definition mode or character mode is set or when printing is performed on special paper, setting of a developing bias is performed as follows. The value of developing voltage Vmin is set so that regular charged toner flies in an image section and the regular charged toner does not fly in the background section using the electric field intensity between a developing roller and the photoreceptor. Thus, the regular charged toner does not fly in the background section. Consequently, the flying regular charged toner does not tap out weakly charged toner and reversely charged toner. Accordingly, the weakly charged toner and reversely charged toner are not deposited on the background section, and the fog can be prevented. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an image forming apparatus of a non-contact development type using a non-magnetic single component toner. More specifically, the present invention relates to an image forming apparatus that suppresses fogging by adjusting a developing bias.

  The image forming apparatus forms an electrostatic latent image on the surface of a photoconductor, and applies toner to the electrostatic latent image for development. In an image forming apparatus 300 as shown in FIG. 1, the photosensitive member 310 is charged by the charging device 320, and the surface of the photosensitive member 310 is partially exposed by the exposure device 330 to form an electrostatic latent image.

  A specific image forming method is as follows. First, the charging device 320 uniformly charges the surface of the photoreceptor 310. Next, light corresponding to the image is applied to the uniformly charged surface of the photoreceptor 310 by the exposure device 330. Thereby, the electric potential of the part which received light changes. Here, the place where light on the surface of the photoconductor 310 is received and the place where no light is received correspond to an image portion to which toner is applied and a background portion to which toner is not applied. In this manner, an electrostatic latent image including an image portion and a background portion is formed on the photoreceptor 310. Next, charged toner is applied to the electrostatic latent image on the surface of the photoreceptor 310 by the developing roller 340. Thereby, a visible image is formed. Next, the toner on the surface of the photoconductor 310 is transferred to paper by the transfer device 350. Next, the fixing device 360 fixes the toner so that the toner does not peel from the paper. Note that the toner remaining on the surface of the photoreceptor 310 after the transfer is collected in the toner collection container 371 by the cleaner 370.

  The development in the non-contact development system means that the toner attached to the developing roller 340 is attached to the electrostatic latent image on the photoreceptor 310 by applying a development bias. For this reason, it is important for securing the quality of the printed matter to obtain a clear and moderately dark print by attaching a necessary amount of toner faithfully to the electrostatic latent image. In addition, it is necessary to prevent toner from adhering to the background portion.

  When this developing bias has an AC component, the toner reciprocates between the photosensitive member 310 and the developing roller 340. This reciprocation is shown in FIG. That is, the toner flies from the developing roller 340 toward the photosensitive member 310 by the developing voltage, and is pulled back from the photosensitive member 310 toward the developing roller 340 by the recovery voltage. By repeating this series of processes, the toner finally adheres to the electrostatic latent image on the photoreceptor 310 and is developed.

  An example of the developing bias to be applied is shown in FIG. The development voltage Vmin and the recovery voltage Vmax are opposite to each other and are applied alternately. For this reason, the charged toner receives a force in either direction. The toner tries to fly between the photoconductor 310 and the developing roller 340 according to this voltage.

  Incidentally, the charging characteristics of the toner change due to friction or the like. Initially, the charging characteristics of the toner are good. In addition, the variation in charge amount is small. Initially, the toner having good charging characteristics is stored inside the developing device. This is a toner (hereinafter referred to as “regularly charged toner”) having an appropriate charge amount and carried on the developing roller 140. However, when the toner deteriorates, a toner with a small charge amount (hereinafter referred to as “low-charged toner”) is generated. This low-charged toner may adhere to the background portion of the electrostatic latent image that should not adhere. This is a cause of fogging at the time of printing and degrading the quality of the printed matter.

  For this reason, a method for suppressing fogging due to low-charged toner has been taken. In Japanese Patent Application Laid-Open No. 2004-133260, development is performed by adjusting an absolute value | Vmax−Vd | of a difference between a peak voltage Vmax that forms an electric field in a direction in which toner is caused to fly to a photoreceptor and a background potential Vd of an electrostatic latent image. An apparatus and an image forming apparatus are disclosed. As a result, as the charge amount of the toner decreases, | Vmax−Vd | is adjusted to suppress fogging. The peak voltage Vmax described in Patent Document 1 corresponds to the developing voltage Vmin in FIG. 3, and the peak voltage Vmin described in Patent Document 1 corresponds to the recovered voltage Vmax in FIG. It is.

JP 2006-301479 A

  However, when the toner further deteriorates, toner having a charge opposite to that of the normal regular charged toner (hereinafter referred to as “reversely charged toner”) is generated. Once the reversely charged toner is detached from the developing roller 340, the reversely charged toner receives a force opposite to that of the normally charged toner. That is, a force from the photosensitive member 310 toward the developing roller 340 is received at the developing voltage Vmin, and a force from the developing roller 340 toward the photosensitive member 310 is received at the recovery voltage Vmax. For this reason, the reversely charged toner flies in the opposite direction to the regular charged toner.

  Since the reversely charged toner is more likely to adhere to the background portion than the image portion, it causes fogging. Once the reversely charged toner adheres to the photoconductor 310, it is difficult to fly the photoconductor 310 to the developing roller 340 again by the development voltage Vmin. This is because the reversely charged toner has a small absolute value of the amount of charge, and therefore the force received from the electric field is small. For this reason, it is difficult to fly. Therefore, the method of Patent Document 1 cannot prevent the reversely charged toner from adhering to the background portion of the electrostatic latent image.

  On the other hand, when the image forming apparatus is used in the high image quality mode, it is desirable to suppress the occurrence of fog as much as possible. Also, when there are many background parts, such as character mode, fog is likely to stand out. On the other hand, when printing on special paper such as thick paper, glossy paper, and OHP sheet, the transfer is performed under the condition that the developed toner is easily transferred. For example, the strength of the electric field formed by the voltage applied during transfer can be increased. For this reason, even the toner on the photoreceptor having a charge amount that cannot be transferred in normal use is transferred. As a result, fog is likely to occur on the printed paper.

  The present invention has been made to solve the above-described problems of the prior art. In other words, the problem is that when high-quality mode or character mode is set, or when printing on special paper, low-charge toner and reverse-charge toner adhere to the background of the electrostatic latent image on the photoreceptor. It is an object of the present invention to provide an image forming apparatus that suppresses the occurrence of fog and does not cause fogging in the background portion of the printed matter.

  An image forming apparatus of the present invention, which has been made for the purpose of solving this problem, includes an image carrier, a developing roller for applying a non-magnetic one-component toner to the image carrier, and the image carrier and the developing roller. A non-contact development type image forming apparatus having a voltage applying unit for applying a developing bias therebetween and an image quality mode adjusting unit for selecting an image quality mode, wherein the developing bias applied by the voltage applying unit includes: A development voltage Vmin for forming an electric field in a direction for causing toner to fly from the developing roller to the image bearing member and a recovery voltage Vmax for forming an electric field for causing the toner to fly from the image bearing member to the developing roller are alternately repeated. In addition, the voltage application unit includes an electric field strength in an image portion of the electrostatic latent image of the image carrier and an electric field strength in a background portion of the electrostatic latent image of the image carrier. The normal bias for setting the development voltage Vmin so that the toner is sufficient to fly from the developing roller to the image carrier, and the electric field strength in the image portion of the electrostatic latent image on the image carrier Is sufficient to fly from the developing roller to the image carrier, and the electric field strength at the background portion of the electrostatic latent image of the image carrier causes the toner to fly from the developing roller to the image carrier. Two development biases, i.e., an anti-fogging bias for setting the development voltage Vmin, are applied so that the image quality mode adjustment unit does not lead to this, and the image quality mode adjustment unit belongs to a mode group including at least a character mode and a high image quality mode An anti-fogging bias is applied when the image quality mode is selected, and a normal bias is applied except when the anti-fogging bias is applied. It is intended.

  Such an image forming apparatus can suppress the occurrence of fog when high image quality such as a high image quality mode is required. In addition, the occurrence of fog can be suppressed even when the fog is conspicuous in the background portion such as the character mode.

  The image forming apparatus of the present invention applies a developing bias between the image carrier, a developing roller for applying a non-magnetic single component toner to the image carrier, and the image carrier and the developing roller. A non-contact development type image forming apparatus having a voltage application unit and a paper identification sensor for detecting a type of paper to be printed, wherein the development bias applied by the voltage application unit is applied from the development roller to the development roller. A development voltage Vmin for forming an electric field in a direction for causing toner to fly to the image bearing member and a recovery voltage Vmax for forming an electric field for causing the toner to fly from the image bearing member to the developing roller are alternately repeated. The voltage application unit is configured such that both the electric field strength in the image portion of the electrostatic latent image on the image carrier and the electric field strength in the background portion of the electrostatic latent image on the image carrier, A normal bias for setting the developing voltage Vmin so as to be sufficient to fly from the roller to the image carrier, and an electric field strength in the image portion of the electrostatic latent image on the image carrier, so that the toner is the developing roller. And the electric field strength in the background portion of the electrostatic latent image of the image carrier does not cause the toner to fly from the developing roller to the image carrier. Thus, two developing biases, i.e., an antifogging bias for setting the developing voltage Vmin, are applied, and the paper group includes at least one of thick paper, an OHP sheet, and glossy paper by the paper identification sensor. The anti-fogging bias is applied when a sheet belonging to the above is detected, and the normal bias is applied except when the anti-fogging bias is applied.

  Such an image forming apparatus can suppress the occurrence of fog when printing on special paper such as thick paper, an OHP sheet, and glossy paper that are likely to cause fog.

  In the image forming apparatus described above, a mode group including a sheet identification sensor that detects a type of a sheet to be printed, wherein the voltage application unit includes at least a character mode and a high image quality mode in the image quality mode adjustment unit. And at least one of when the paper belonging to the paper group including at least one of thick paper, OHP sheet, and glossy paper is detected by the paper identification sensor. It is even better if the anti-fogging bias is applied where applicable.

  Such an image forming apparatus can suppress the occurrence of fog when high image quality such as a high image quality mode is required. In addition, the occurrence of fog can be suppressed even when the fog is conspicuous in the background portion such as the character mode. Further, even when printing on special paper such as thick paper, an OHP sheet, and glossy paper, occurrence of fog can be suppressed.

In the image forming apparatus described above, the anti-fogging bias satisfies the following expression in the image portion of the electrostatic latent image of the image carrier,
| Vmin-Vi |> Fa · d / q
Vi: Potential of the image area
Fa: Adhesive force acting on the toner adhering to the developing roller
d: Distance between the image carrier and the developing roller
q: average charge amount of toner so that the following equation is satisfied in the background portion of the electrostatic latent image of the image carrier:
| Vmin−Vb | ≦ Fa · d / q
Vb: It is more preferable that the background potential is set.

  This is because it is still possible to suppress the occurrence of fog. In addition, although toner flows in the image portion of the electrostatic latent image due to the developing bias, it does not fly in the background portion. For this reason, fog does not occur in the background.

In the image forming apparatus described above, the anti-fogging bias is in a range where ± Vmin−Vb | is within ± 20% of Fa · d / q in the background portion of the electrostatic latent image of the image carrier. like
Vb: background potential
Fa: Adhesive force acting on the toner adhering to the developing roller
d: Distance between the image carrier and the developing roller
q: The toner may have an average charge amount set.

  This is because it is still possible to suppress the occurrence of fog. In addition, although the toner flies in the image portion of the electrostatic latent image due to the developing bias, it hardly flies in the background portion. For this reason, there is almost no fogging in the background.

  According to the present invention, when the high image quality mode or the character mode is set, or when printing on special paper, the low charge toner and the reverse charge toner adhere to the background portion of the electrostatic latent image on the photosensitive member. There is provided an image forming apparatus that suppresses the occurrence of fog and does not cause fogging in the background portion of the printed matter.

[First embodiment]
DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the best mode for embodying the present invention will be described in detail with reference to the accompanying drawings. The present embodiment embodies the present invention with respect to a non-contact developing type image forming apparatus.

[Device configuration]
The mechanical configuration of the image forming apparatus of this embodiment will be described with reference to FIG. The image forming apparatus 100 includes a paper identification sensor 101, an image quality mode adjustment unit 102, a photoconductor 110, a charging device 120, an exposure device 130, a developing roller 140, a supply roller 141, a regulating blade 142, a static elimination device. The sheet 143, the transfer device 150, the fixing device 160, the cleaner 170, the toner recovery container 171, the buffer 180, the upstream screw 181, the downstream screw 182, and the voltage application unit 190 are provided.

  The photoconductor 110 has a cylindrical shape and is an image carrier on which an electrostatic latent image is written on the surface. The charging device 120 is for uniformly charging the surface of the photoconductor 110. The exposure device 130 is for applying light to the surface of the uniformly charged photoreceptor 110 to form an electrostatic latent image. The developing roller 140 is for applying toner to the electrostatic latent image on the photoconductor 110.

  The image forming apparatus 100 according to this embodiment performs development by a non-contact developing method in which the photoconductor 110 and the developing roller 140 are not in contact with each other. For this reason, the developing roller 140 is installed slightly offset so as not to contact the photoreceptor 110. Further, a developing bias can be applied between the photoconductor 110 and the developing roller 140. The voltage application unit 190 applies the developing bias. The voltage application unit 190 also controls the setting of the development bias according to the type of paper detected by the paper identification sensor 101 or the mode set by the image quality mode adjustment unit 102.

  The supply roller 141 is for supplying the toner stored in the buffer 180 to the developing roller 140. The supply roller 141 also supplies the toner to the surface of the developing roller 140 and simultaneously collects the development residual toner from the surface of the developing roller 140. For this reason, the supply roller 141 rotates in the counter direction with the developing roller 140. The supply roller 141 is made of a foaming elastic member.

  The regulating blade 142 is for further charging the toner supplied to the developing roller 140 and for regulating the toner conveyance amount. The buffer 180 is a container for storing toner therein. Further, the toner inside the buffer 180 is circulated by the upstream screw 181 and the downstream screw 182. In addition, a charge removal sheet 143 is provided to improve the recovery rate of the toner after development.

  The transfer device 150 is for transferring the toner on the electrostatic latent image formed on the surface of the photoconductor 110 to paper. The cleaner 170 collects toner remaining on the surface of the photoconductor 110 after being transferred to paper, and puts it in the toner collection container 171. The fixing device 160 is for fixing the toner on the paper so that the toner does not peel off from the paper. The paper identification sensor 101 is for determining the type of paper to be printed. The image quality mode adjustment unit 102 is for setting the image quality mode by a user input. Here, examples of the image quality mode include a high image quality mode and a character mode. The character mode is a mode in which, for example, a printing process suitable for an image with a large contrast such as a character placed on a blank paper background or an image in which the number of print dots in the image is a predetermined value or less. The high image quality mode is a mode that enables high image quality printing in the image forming apparatus, for example, in terms of high resolution or image reproducibility. The image quality mode adjustment unit 102 may automatically set the image quality mode.

  The gap between the photoconductor 110 and the developing roller 140 is about 130 μm. The toner adhering to the surface of the developing roller 140 flies through this gap by the developing bias, and finally adheres to the surface of the photoconductor 110.

[Device operation]
Here, a series of operations of the image forming apparatus 100 of this embodiment will be described. The bias set in the image forming apparatus 100 includes a normal bias and an anti-fogging bias. The normal bias is a developing bias that is set so that toner can fly in both the image portion and the background portion of the electrostatic latent image. On the other hand, the anti-fogging bias is a developing bias that is set so that the toner jumps in the image portion of the electrostatic latent image and does not fly in the background portion.

  The anti-fogging bias is applied when the high image quality mode or the character mode is selected as the image quality mode. This is because, in the high image quality mode, high quality is required, so it is necessary to suppress the fog. Also, in the character mode, fog is conspicuous because there are many background parts. On the other hand, even when printing on special paper such as thick paper, glossy paper, and OHP sheet, fog is easily noticeable. This is because the transfer is performed under conditions that make it easier to transfer the toner in the transfer process. For this reason, when printing on special paper, the anti-fogging bias is applied regardless of the image quality mode. In addition, it is better to apply anti-fogging bias when it is desired to suppress fogging.

  Therefore, the image quality mode is first checked to determine whether to apply the anti-fogging bias. If the image quality mode adjustment unit 102 selects a mode belonging to the mode group to which the high image quality mode and the character mode belong, the anti-fogging bias is applied. Next, the paper identification sensor 101 detects the type of paper to be printed. If the paper belongs to a paper group to which special paper such as thick paper, glossy paper, and OHP sheet belongs, anti-fogging bias is applied. So far, if the antifogging bias is not applied, the normal bias is applied. That is, the anti-fogging bias is applied when the high image quality mode or the character mode is selected, or when printing on special paper, and the normal bias is applied when neither of these conditions is satisfied.

  After these settings, the charging device 120 uniformly charges the surface of the photoconductor 110. Next, an electrostatic latent image is formed on the surface of the photoreceptor 110 by the exposure device 130. The surface potential of the photoconductor 110 on which the electrostatic latent image is formed is different between the image portion and the background portion. Next, the toner on the developing roller 140 is attached to the electrostatic latent image on the surface of the photoreceptor 110. Here, in the case of the anti-fogging bias, the toner adheres to the image portion of the electrostatic latent image, but does not adhere to the background portion of the electrostatic latent image. As a result, proper development is performed. Also, fog does not occur. On the other hand, in the case of normal bias, the toner can adhere not only to the image portion of the electrostatic latent image but also to the background portion.

  Next, the toner on the surface of the photoconductor 110 is transferred to paper by the transfer device 150. Next, the toner image on the paper to which the toner is transferred is fixed by the fixing device 160. For this reason, the printed toner does not peel from the paper. As a result, printing on paper is performed. On the other hand, toner remaining without being transferred to the surface of the photoreceptor 110 is collected by the cleaner 170.

[Cause of fog]
Here, the mechanism that causes fogging will be described with reference to FIG. FIG. 5 shows the movement of toner when a developing bias is applied between the photoconductor 110 and the developing roller 140. Here, the normally charged toner is negatively charged, and the reversely charged toner is positively charged.

  The normally charged toner flies from the surface of the developing roller 140 to the surface of the photoreceptor 110 by the developing voltage Vmin in the image portion. Thereafter, at least a part of the normally charged toner returns from the surface of the photoconductor 110 to the surface of the developing roller 140 by the recovery voltage Vmax. At this time, the returned normally charged toner may collide with the lowly charged toner and the reversely charged toner on the developing roller 140. As a result, low-charge toner and reverse-charge toner are knocked out from the surface of the developing roller 140.

  The subsequent movement of the knocked-out toner will be described. First, the low charge toner will be described. The lowly charged toner that has been knocked out is subjected to a force in the same direction as that of the normally charged toner by the developing bias. For this reason, the low-charge toner flies toward the photoconductor 110 by the development voltage Vmin. Since the low charge toner has a lower charge amount than the regular charge toner, it adheres to the background portion more easily than the regular charge toner. In addition, once the low-charged toner adheres to the background portion of the electrostatic latent image on the photoconductor 110, it is difficult to collect with the recovery voltage Vmax because the charge amount is small. In addition, there is a low possibility that the low-charge toner is knocked out to the regular charge toner again. That is, once the low-charged toner adheres to the surface of the photoconductor 110, it tends to remain as it is.

  Next, the reversely charged toner will be described. When the returned normally charged toner collides with the reversely charged toner on the developing roller 140, a recovery voltage Vmax is applied between the photoconductor 110 and the developing roller 140. Since the reversely charged toner receives a force in the direction opposite to that of the normally charged toner, the force toward the photoconductor 110 is received from the electric field by the recovery voltage Vmax. Therefore, the reversely charged toner that has been knocked out flies directly toward the photoconductor 110.

  The reversely charged toner easily adheres to the background portion and hardly adheres to the image portion. This is because the reversely charged toner has a charge opposite to that of the normally charged toner. It should be noted that the regular charged toner is likely to adhere to the image portion and hardly adhere to the background portion. That is, the reversely charged toner is more likely to adhere to the background portion than the normally charged toner. Furthermore, once the reversely charged toner has adhered to the background portion of the electrostatic latent image on the photoconductor 110, it is difficult to collect with the developing voltage Vmin because the charge amount is small. This is because there is hardly any regular charged toner that can knock out the reversely charged toner again.

  When the low-charged toner and the reverse-charged toner adhere to the background portion of the electrostatic latent image on the photoconductor 110, there is almost no means for removing it. And it appears as fog on the paper.

  In order to solve this problem, the development voltage Vmin may be adjusted so that the normally charged toner does not fly in the background portion. By doing so, the normally charged toner does not knock out the low-charged toner and the reverse-charged toner due to the recovery voltage Vmax. Thereby, it is possible to suppress flying of the low-charge toner and the reverse-charge toner in the background portion. Therefore, fogging in the background can be suppressed.

[Conditions where toner does not fly]
Here, conditions for preventing the normally charged toner from flying will be described. First, the force acting on the toner will be described. The force received by the toner will be described with reference to FIG. FIG. 6 shows a case where the toner adhering to the developing roller 140 receives an electric field by the developing voltage Vmin. At this time, the toner receives a Coulomb force Fe (hereinafter simply referred to as “Coulomb force”) from an electric field generated by the development voltage Vmin. Further, it receives a mirror image force Fi from the developing roller 140. Further, the developing roller 140 receives a mechanical adhesion force Fv (hereinafter simply referred to as “van der Waals force”) having a van der Waals force as a main component.

  Further, the Coulomb force Fe acts in a direction from the developing roller 140 toward the photoconductor 110. On the other hand, the mirror image force Fi works in the opposite direction to the Coulomb force Fe. Similarly, the van der Waals force Fv works in the opposite direction to the Coulomb force Fe. That is, the Coulomb force Fe works to release the toner from the developing roller 140, and the mirror image force Fi and the Van der Waals force Fv work so as not to release the toner from the developing roller 140.

Therefore, the resultant force F of the force acting on the toner when the toner adhering to the developing roller 140 receives the developing voltage Vmin is positive when the direction from the developing roller 140 toward the photoconductor 110 is positive.
F = Fe-Fi-Fv
It can be expressed. The toner flies when F> 0, and does not fly when F ≦ 0. That is, the resultant force F is a detachment force (hereinafter referred to as “detachment force F”) that determines whether or not the toner is detached from the developing roller 140. Note that gravity is ignored.

[Each power acting on toner]
Next, each force will be described. First, the Coulomb force Fe will be described. When the charge amount of the toner is q, the force that the toner receives in the electric field E is
Fe = q · E
It is. That is, Fe is proportional to the toner charge amount q.

Next, the mirror image force Fi will be described. If the charge amount of toner is q,
Fi = (ε−1) · q ² / {(ε + 1) 4π · ε0 · D ² } (1)
ε is the relative permittivity of the developing roller
ε0 is the dielectric constant of air
D is the average particle diameter of the toner. Here, ε, ε0, and D are known constants.

Here, if the coefficient of q ² in the equation (1) is a, that is,
a = (ε−1) / {(ε + 1) 4π · ε0 · D ² }
Then, the mirror image force Fi is expressed by the following equation.
Fi = a · q ²
Here, a is a known constant, and the mirror image force Fi is proportional to the square of the toner charge amount q. Note that the following value is calculated: ε = 3
ε0 = 8.85 × 10 ⁻¹² [F / m]
D = 6 [μm]
Will be used.

  An experimentally measured Van der Waals force Fv is used. For this measurement, KONICA MINOLTA TECHNOLOGY REPORT VOL. 1 (2004) p15 “The particle size dependence of toner adhesion and electric field flight” was used. In this method, the vibrator is vibrated, and Fv is measured by giving vibration acceleration to the toner adhered by electrostatic force. At that time, by gradually increasing the vibration acceleration, the vibration acceleration at which the adhered toner is detached is obtained. From this value, Fv can be obtained.

  The van der Waals force Fv works substantially only when toner adheres to the developing roller 140. In other words, once separated, this force hardly works until it adheres again. As a result of the measurement, it was found that Fv does not depend on the charge amount of the toner. Therefore, it will be regarded as a constant and expressed as “c” in the future.

From the above, the resultant force F of the force acting when the toner adhering to the developing roller 140 receives the developing voltage Vmin is
F = Fe-Fi-Fv
= -A · q ² + E · q-c
It can be expressed as This is a quadratic function of the toner charge amount q. here,
ε> 1
ε0> 0
D ² > 0
Therefore, a> 0. Therefore, F is a quadratic function convex upward, and takes a maximum value at a certain charge amount q.

Here, the upper part of FIG. 7 shows the relationship between the charge amount q of the toner and each force acting on the toner. 7 shows the separation force F, Coulomb force Fe, image force Fi, van der Waals force Fv, and adhesion force Fa. The horizontal axis is the toner charge amount, and the vertical axis is the force acting on the toner. Here, the adhesion force Fa is as follows:
Fa = Fi + Fv
It is the resultant of the mirror image force Fi and the van der Waals force Fv. Further, the adhesion force Fa is a force with which the toner adheres to the developing roller 140. Using this adhesion force Fa, the separation force F is
F = Fe-Fi-Fv
= Fe-Fa
It is expressed. As apparent from the above equation, when the Coulomb force Fe exceeds the adhesion force Fa, the separation force F becomes positive and the toner flies.

  The lower part of FIG. 7 shows the toner charge distribution. The horizontal axis represents the toner charge amount, and the vertical axis represents the number of toners having the charge amount in the buffer 180. In the upper part of FIG. 7, the toner flies in a range where the separation force F is positive. The toner within the range (hatched area) corresponding to this receives a force from the electric field generated by the developing voltage Vmin and flies from the developing roller 140 toward the photoconductor 110. That is, the area of the hatched area is proportional to the number of toners flying.

  As described above, the Coulomb force Fe is proportional to the charge amount q. On the other hand, the van der Waals force Fv is a constant that does not depend on the charge amount q. The mirror image force Fi, the adhesion force Fa, and the separation force F are quadratic functions of the charge amount q. Since the separation force F is a quadratic function of the charge amount q, the separation force F having a certain charge amount q in the toner charge distribution has a maximum value. The toner corresponding to the maximum value of the quadratic function, that is, the toner having the charge amount q that maximizes the separation force F is the toner that is most likely to fly.

  However, when the electric field strength E is changed, a toner other than the toner applied before the change of the electric field strength E corresponds to a toner that is most likely to fly, that is, a toner having the maximum F. For example, as shown in FIG. 8, when the electric field strength E is increased, toner having a larger absolute value of charge amount is most likely to fly. In FIG. 8, the extreme value of the separation force F is moving obliquely. That is, when the electric field strength is increased, a toner having a larger absolute value of the charge amount becomes “the toner that is most likely to fly”. This corresponds to the extreme value of the quadratic function changing with the linear coefficient. Further, by increasing the electric field strength E, the flying range in the toner charge distribution is expanded. A toner having a larger charge amount is likely to fly. On the contrary, when the electric field strength E is weakened, the toner with a smaller charge amount is most likely to fly. The toner that is most likely to fly has a charge amount q within the range of the charge distribution of the normally charged toner.

  As described above, if the toner having the charge amount q that is most likely to fly does not fly from the developing roller 140 toward the photoconductor 110, toner having another charge amount q should not fly. Therefore, it is only necessary to obtain a condition that the toner having the charge amount q that is most likely to fly does not fly from the developing roller 140.

  Here, the mirror image force Fi is a force determined by the toner particle diameter D and the charge amount q, and the van der Waals force is a force determined by the environment such as humidity. In other words, these are uncontrollable factors. On the other hand, the Coulomb force Fe can be controlled. Therefore, the toner can be prevented from flying from the developing roller 140 by setting the electric field strength to an appropriate value. That is, the detachment force F may be negative.

Here, the condition that the toner does not fly in the background portion will be described. By setting the above electric field strength, it is possible to prevent the toner from flying in the background portion. That is, the condition of F ≦ 0 may be imposed. Thereby, flying toner particles can be suppressed. That is, in the background portion, the following formula is Fe = q · E ≦ Fa
The developing voltage Vmin may be set so as to satisfy E that satisfies the above. If the voltage is too high, the toner will fly in the background. On the other hand, if the voltage is too low, the toner does not fly in the image area. In other words, development itself is not made.

Here, the boundary line when the toner flies and when it does not fluctuate is obtained. The limit of whether the toner is flying or not is when F = 0. Therefore, when the electric field intensity Epump when F = 0 is obtained,
Epump = a · q + c / q
It becomes. When this is differentiated by the toner charge amount q, the toner charge amount q that is most likely to fly is obtained. When q is obtained, q = 1.1 × 10 ⁻¹⁴ [C]. Ep at this time is Epump = 2.8 MV / m by substituting the value of q. Here, the shortest distance between the photoconductor 110 and the developing roller 140 is 130 μm. For this reason, the effective developing voltage | Vmin−Vb | for forming an electric field in the background is 360V. Here, Vb is the potential of the background portion.

  For this reason, if the effective developing voltage | Vmin−Vb | of the background portion is reduced, even the toner that is most likely to fly in the background portion can be prevented from flying. That is, as the developing voltage Vmin is reduced, the electric field strength is reduced accordingly. As a result, the toner does not fly. This is shown in the upper part of FIG. All toners do not fly at this field strength. 8 and 9 show the toner charge distribution as in the lower part of FIG.

[Development bias and force received by toner]
Here, the developing bias applied by the voltage application unit 190 in this embodiment will be described. The voltage application unit 190 not only applies a development bias, but also plays a role of controlling the setting value of the development bias. The developing bias in this embodiment is shown in FIG. The horizontal axis is time, and the vertical axis is potential. Vmin is a developing voltage, Vmax is a recovery voltage, Vb is a background portion potential, Vi is an image portion potential, and Vdc is a DC component of the developing bias. Frq is the frequency of the developing bias, Vave is the time average of the developing bias, and Duty is the ratio of the time for applying the developing voltage Vmin to the total time. Note that Vmin in this embodiment corresponds to Vmax in Patent Document 1.

  Here, the relationship between the developing bias and the force applied to the toner will be described. A case where the development voltage Vmin is applied will be described. Since the potential in the background portion and the image portion in the electrostatic latent image is different, the potential difference between the photoconductor 110 and the developing roller 140 is different in the background portion and the image portion. The effective developing voltage for the background portion is | Vmin−Vb |, and the effective developing voltage for the image portion is | Vmin−Vi |.

  An image portion when the development voltage Vmin is applied will be described. The toner receives a force from the developing roller 140 toward the photoconductor 110 by the electric field strength E generated by the effective developing voltage | Vmin−Vi | As a result, the toner on the developing roller 140 flies toward the photoconductor 110. Here, the effective developing voltage | Vmin−Vi | of the image portion is 760V.

  Next, the background portion when the development voltage Vmin is applied will be described. The toner receives a force from the developing roller 140 toward the photoconductor 110 by the electric field strength E generated by the effective developing voltage | Vmin−Vb | of the background portion. However, the toner on the developing roller 140 does not fly toward the photoconductor 110. This is because the electric field strength E formed by the effective developing voltage | Vmin−Vb | in the background portion is not large enough for the toner to fly as shown in the upper part of FIG. Here, the effective developing voltage | Vmin−Vb | of the background portion is 360V.

  Note that the developing bias of this embodiment has a smaller absolute value of the developing voltage Vmin compared to the conventional developing bias (FIG. 3). Other conditions have not changed. For this reason, the effective developing voltage | Vmin−Vb | of the background portion is correspondingly reduced from 650 V to 360 V. In addition, the absolute value of the average potential decreases as the developing voltage Vmin decreases.

  As described above, in the background portion, any of the normally charged toner, the low charged toner, and the reversely charged toner does not fly. In the image area, not only regular charged toner but also low charged toner and reverse charged toner fly. At this time, the number of flying toners is sufficient. This is because, as shown in the lower part of FIG. 8, even if the flying range changes, the number of toners in the flying range does not change greatly. For this reason, fog does not occur. The density in the image area is appropriate.

[Experimental results on development bias]
Here, the results of tests performed on the developing bias of the image forming apparatus 100 of this embodiment will be described. In this test, fog was measured using the value of the development voltage Vmin. To remove fog, the toner on the photoconductor 110 was peeled off with a booker tape, adhered to Konica Minolta J paper, and C * was measured with a color difference meter CR241 manufactured by Konica Minolta. In addition, cyan toner, which has been deteriorated to some extent after printing 2,500 sheets at a printing rate of 5%, was used.

As a result, the tint ΔC *, which is a substitute value for fog on the photoconductor 110, was as follows.
This form 0.52
Conventional condition 2.36
The tint ΔC *, which is a substitute value for fog on the photoconductor 110, is improved by about 4 to 5 times the conventional condition depending on the developing bias condition of this embodiment.

  FIG. 11 shows the relationship between the development voltage and the fog color ΔC *. The horizontal axis represents the effective developing voltage | Vmin−Vb | of the background portion. The vertical axis represents the tint ΔC *, which is a substitute value for fog. Here, a low temperature and low humidity environment is taken as an example. In FIG. 11, as the difference between the development voltage Vmin and the background portion potential Vb is reduced, the tint ΔC *, which is a substitute value for fog, is reduced. That is, as the effective developing voltage | Vmin−Vb | of the background portion decreases, fogging is less likely to occur.

  As described above, the experimental results support that the fogging of the background portion can be suppressed by adjusting the developing voltage Vmin so that the effective developing voltage | Vmin−Vb | of the background portion becomes small.

[Development bias to be set]
Here, the value of the development voltage Vmin to be set in the development bias will be described. The electric field intensity Epump at which the toner flies almost is F = Fe-Fa = 0,
q · Eump = Fa
Because
Epump = Fa / q
It is. The voltage Vpump that forms this electric field strength Epump is:
Vpump = Fa · d / q
d: A distance between the photosensitive member and the developing roller.

  Therefore, in order for the toner to fly in the image area, the value of the development voltage Vmin may be set so that the effective development voltage | Vmin−Vi | of the image area is larger than Vpump. At the same time, in order to prevent the toner from flying in the background portion, the value of the developing voltage Vmin may be set so that the effective developing voltage | Vmin−Vb | of the background portion is smaller than Vpump.

Therefore, the following expression is satisfied in the image portion of the electrostatic latent image of the photoconductor 110 by setting the developing voltage Vmin.
| Vmin-Vi |> Fa · d / q
Vi: Potential of the image area
Fa: Adhesive force acting on the toner adhering to the developing roller
d: Distance between the photoconductor and the developing roller
q: Average charge amount of toner At this time, toner flies in the image area.

Further, by setting the developing voltage Vmin, the following expression is satisfied in the background portion of the electrostatic latent image on the photoconductor 110.
| Vmin−Vb | ≦ Fa · d / q
Vb: Background portion potential At this time, toner does not fly in the background portion. The development voltage Vmin may be set so as to satisfy the above two formulas.

That is, the development voltage Vmin is
| Vmin−Vb | ≦ Fa · d / q <| Vmin−Vi |
This should be set within the range. As a result, the image portion can be developed with an appropriate density, and the background portion is not fogged. This is the condition of the development voltage Vmin of the development bias set in the anti-fogging bias. For the normal bias, for example, a developing voltage Vmin as shown in FIG. 3 may be set.

[Experimental results comparing fog generated on cardboard and fog generated on plain paper]
Here, a description will be given of test results obtained by comparing fog generated on cardboard and fog generated on plain paper. When printing on special paper such as thick paper, the toner on the developed photoreceptor is easily transferred onto the paper to be printed. This is because the transfer is performed under conditions that facilitate transfer, such as increasing the electric field strength. For this reason, when printing on special paper, fog on the photoconductor, which was not a problem with plain paper, may appear as fog on the paper. Therefore, in this test, the fog on the photoreceptor and the fog on the paper were compared between plain paper and cardboard.

  The fog on the photoconductor was measured by the following procedure. First, the toner adhering to the photoreceptor was peeled off with a booker tape. Next, a booker tape with toner adhered thereto was affixed to Konica Minolta J paper. Thereafter, C * of the toner adhering to the booker tape on the J paper was measured by a color difference meter CR241 manufactured by Konica Minolta. As the value of C * is larger, the fog becomes more conspicuous. When C * is about 3 or more, it is visible to the naked eye.

  On the other hand, fog generated on plain paper and cardboard was visually confirmed. The degree of fogging on the paper was evaluated in 9 grades (0.5 increments) from 1 to 5. Note that the lower the value, the more noticeable the fog, the higher the value, the less noticeable. In other words, the higher the numerical value of the rank on the paper, the better the image quality, and the lower the numerical value on the photosensitive member, the better the image quality. The results are shown in Table 1.

  Here, it is assumed that a rank of 3 or more satisfies the image quality. In other words, for plain paper, the fog C * on the photosensitive member is 0.5 to 3, and for thick paper and coated paper, the fog C * on the photosensitive member is 0.5 to 1.5. . Although not shown in the table, if the fog on the photoconductor is zero, no fog on the paper will occur. Therefore, in the case of plain paper, the fog C * allowed on the photosensitive member is 3 or less. In the case of thick paper / coated paper, the fog C * allowed on the photosensitive member is 1.5 or less. In other words, when printing on thick paper or coated paper, the fog on the photoreceptor tends to appear more remarkably, so it is necessary to suppress the occurrence of fog.

  Therefore, the anti-fogging bias is set when printing on thick paper or coated paper. The same applies when it is desired to suppress fogging in the high image quality mode. At this time, toner is allowed to adhere to the image portion of the electrostatic latent image, and not only low-charged toner and reversely-charged toner but also regular charged toner is not allowed to adhere to the background portion of the electrostatic latent image. It is. Thereby, even when the fog is easily noticeable, the fog can be suppressed. Further, there is no possibility that low-charge toner and reverse-charge toner are accumulated in the buffer 180. Similarly, the anti-fogging bias may be set when printing on a sheet on which the toner image is easily transferred, such as an OHP sheet or glossy paper.

The development voltage Vmin satisfies the following formula: | Vmin−Vb | ≦ Fa · d / q
When set, the low-charge toner and the reverse-charge toner do not fly to the photoconductor 110 and are accumulated in the buffer 180. As a result, the photoconductor 110 may cause filming. Therefore, it is not preferable to always set the anti-fogging bias. For this reason, in the image forming apparatus 100 of this embodiment, the anti-fogging bias is applied when it is desired to suppress the fogging, and the normal bias is applied in other cases.

  As described above in detail, the image forming apparatus according to the present embodiment sets the anti-fogging bias when the high image quality mode or the character mode is set, or when printing on special paper. . That is, toner is allowed to fly between the developing roller 140 and the photoconductor 110 in the image area, and toner is not allowed to fly between the developing roller 140 and the photoconductor 110 in the background area. For this reason, in the background portion, the normally charged toner does not knock out the lowly charged toner and the reversely charged toner. As a result, the low-charge toner and the reverse-charge toner can be prevented from adhering to the background portion of the electrostatic latent image formed on the photoconductor 110. In addition, an image forming apparatus capable of suppressing the fog even when the image quality mode in which the fog is likely to occur and special paper is printed has been realized.

  Note that this embodiment is merely an example, and does not limit the present invention. Therefore, the present invention can naturally be improved and modified in various ways without departing from the gist thereof. For example, the electric field strength may be obtained by differentiating the separation force F.

The separation force F acting on the toner on the developing roller 140 when the developing voltage Vmin is applied is
F = Fe-Fi-Fv
= -A · q ² + E · q-c
It is. When this is differentiated by the charge quantity q,
dF / dq = -2a · q + E = 0
It is. Therefore, when q = E / 2a, the maximum value,
(E ² / 4a) -c
Take. That is, the force F acting on the toner attached to the developing roller 140 under the electric field strength E is
F = −a (q−E / 2a) ² + (E ² / 4a) −c
It is.

Therefore, the electric field strength E can be set so that the toner does not fly from the developing roller 140 (F ≦ 0). That is, when the maximum value (E ² / 4a) -c is 0, the toner does not fly. Therefore, the electric field intensity E between the photoconductor 110 and the developing roller 140 is
(E ² / 4a) −c = 0
E = ± (4a · c) ^1/2
The development voltage Vmin may be set so that Note that a is a known value, and c is a value determined from experiments.

[Second form]
The second embodiment will be described. The image forming apparatus of this embodiment has the same mechanical configuration as that of the first embodiment. The anti-fogging bias is set in the same manner as in the first embodiment when the high-quality mode or character mode in which fogging is likely to occur is set, or when printing on special paper. A difference from the first embodiment is a method of setting the developing voltage Vmin of the developing bias. In this embodiment, an allowable range is provided for the developing voltage Vmin with reference to a voltage that forms an electric field intensity at which the toner that is most likely to fly in the background portion does not fly.

As in the first embodiment, the threshold value Epump of the electric field intensity at which the toner that is most likely to fly is used as a reference. That is, as in the first embodiment, the developing voltage Vmin is
| Vmin−Vb | = Fa · d / q
Vb: background potential
Fa: Adhesive force acting on the toner adhering to the developing roller
d: Distance between the photoconductor and the developing roller
q: When the toner is set to have an average charge amount, the toner does not fly at the background. At this time, as in the case of the first embodiment, the toner flies in the image portion.

  In the image forming apparatus of this embodiment, the developing bias is set according to the allowable range of the fogging ΔC * with the developing voltage Vmin as a reference. FIG. 12 is a diagram showing the relationship between the development voltage Vmin and the magnitude of the fogging ΔC *. The difference between high temperature and high humidity environment and low temperature and low humidity environment is shown. The low temperature and low humidity environment is the same data as in FIG. In both the high temperature and high humidity environment and the low temperature and low humidity environment, the fogging ΔC * is smaller when the effective developing voltage | Vmin−Vb | This result is also consistent with the fog mechanism described above.

  When the development voltage Vmin is set to a voltage Vpump (HH) at which the toner does not fly at high temperature and high humidity, the fogging ΔC * is about 0.5. Further, when the development voltage Vmin is set to a voltage Vpump (LL) at which the toner does not fly at low temperature and low humidity, the fogging ΔC * is about 0.5. Note that the value of fogging ΔC * that can be discriminated with the naked eye is about 3. Therefore, this level of fog is well within the allowable range.

  Due to the setting of the development voltage Vmin, the background is hardly fogged. However, in the above setting, the low-charge toner and the reverse-charge toner are not discharged from the buffer 180. That is, low-charge toner and reverse-charge toner are accumulated in the buffer 180. On the other hand, since the value of fog ΔC * that can be discriminated with the naked eye is about 3, fog ΔC * does not have to be as severe as 0.5. In other words, it may be a milder condition. For this reason, the setting of the development voltage Vmin is also provided with a wide range.

  Here, Vpump (LL) is a voltage at which the toner does not fly at low temperature and low humidity. Vpump (HH) is a voltage at which the toner does not fly at high temperature and high humidity. As a result of the experiment, when the set value of the developing voltage Vmin is 1.2 times the voltage Vpump (LL) at which the toner does not fly at low temperature and low humidity, the fogging ΔC * is 1.4. Further, when the set value of the developing voltage Vmin is 1.2 times the voltage Vpump (HH) at which the toner does not fly at high temperature and high humidity, the fogging ΔC * is 1.4. The condition that ΔC * is 1.4 or less is sufficient to ensure the quality of the printed matter.

On the other hand, if the set value of the development voltage Vmin is set to 0.8 times the voltage Vpump (LL) at which the toner does not fly at low temperature and low humidity, the fogging ΔC * should be sufficiently small although it is not an experimental value. The same applies to high temperatures and high humidity. For this reason, the anti-fogging bias according to the present embodiment sets the effective developing voltage | Vmin−Vb | at the background portion as Fa · d / q.
Vb: background potential
Fa: Adhesive force acting on the toner adhering to the developing roller
d: Distance between the image carrier and the developing roller
q: It is set so as to fall within a range of ± 20% of the toner charge amount. At this time, the fogging ΔC * is 1.4 or less. At this time, the toner flies in the image portion.

  In this test, the fog was measured by using the value of the development voltage Vmin as in the first embodiment. To remove fog, the toner on the photoconductor 110 was peeled off with a booker tape and attached to Konica Minolta J paper, and C * was measured with a color difference meter CR241 manufactured by Konica Minolta. In addition, cyan toner, which has been deteriorated to some extent after printing 2,500 sheets at a printing rate of 5%, was used.

  As described above in detail, the image forming apparatus according to the present embodiment sets the anti-fogging bias when the high image quality mode or the character mode is set, or when printing on special paper. . That is, the toner hardly fly between the developing roller 140 and the photoconductor 110. For this reason, the number of toner particles flying between the developing roller 140 and the photoreceptor 110 is reduced. As a result, it is possible to prevent the normally charged toner from hitting the reversely charged toner and the reversely charged toner from adhering to the surface of the photoconductor 110. In addition, an image forming apparatus capable of suppressing the fog even when the image quality mode in which the fog is likely to occur and special paper is printed has been realized.

  Note that this embodiment is merely an example, and does not limit the present invention. Therefore, the present invention can naturally be improved and modified in various ways without departing from the gist thereof. For example, an allowable range of the development voltage Vmin may be set according to a value for which the fogging ΔC * is desired to be guaranteed.

1 is a diagram illustrating an image forming apparatus of a non-contact type developing method using non-magnetic one-component toner. FIG. 6 is a diagram illustrating a state in which toner reciprocates between a photoconductor and a developing roller by a developing bias. It is a figure explaining the conventional developing bias. It is a figure explaining the image forming apparatus of this invention. FIG. 10 is a diagram illustrating a case where a normally charged toner knocks out a reversely charged toner by a developing bias. FIG. 4 is a diagram illustrating a force acting on toner attached to a developing roller when a developing voltage is applied. FIG. 6 is a diagram illustrating a relationship between a charge amount of toner attached to a developing roller and a force applied to the toner. FIG. 6 is a diagram for explaining changes in the force received by a toner when the electric field is strengthened and the charge amount of the flying toner. FIG. 6 is a diagram illustrating a relationship between a charge amount and a force when toner attached to a developing roller does not fly at the last minute. It is a figure explaining the developing bias of this invention. It is a figure which shows the relationship between the absolute value of the difference of a developing voltage and background part electric potential, and fog. It is a figure which shows the difference by the environment of the relationship between the absolute value of the difference of a developing voltage and background part electric potential, and fog.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 ... Image forming apparatus 101 ... Paper identification sensor 102 ... Image quality mode adjustment part 110 ... Photoconductor 140 ... Developing roller 190 ... Voltage application part

Claims (5)

An image carrier;
A developing roller for applying a non-magnetic one-component toner to the image carrier;
A voltage applying unit for applying a developing bias between the image carrier and the developing roller;
An image forming apparatus of a non-contact type development system having an image quality mode adjustment unit for selecting an image quality mode,
The development bias applied by the voltage application unit is:
A developing voltage Vmin for forming an electric field in a direction in which the toner is caused to fly from the developing roller to the image carrier;
And a collection voltage Vmax for forming an electric field in a direction in which toner is caused to fly from the image carrier to the developing roller, and alternately.
The voltage application unit is:
Both the electric field strength in the image portion of the electrostatic latent image of the image carrier and the electric field strength in the background portion of the electrostatic latent image of the image carrier cause the toner to fly from the developing roller to the image carrier. A normal bias for setting the development voltage Vmin to be sufficient for
The electric field strength in the image portion of the electrostatic latent image on the image carrier is sufficient for the toner to fly from the developing roller to the image carrier, and the background portion of the electrostatic latent image on the image carrier. The two developing biases, i.e., an antifogging bias for setting the developing voltage Vmin so that the toner does not fly from the developing roller to the image carrier, are applied.
An anti-fogging bias is applied when an image quality mode belonging to a mode group including at least a character mode and a high image quality mode is selected by the image quality mode adjustment unit;
An image forming apparatus characterized by applying a normal bias in addition to applying the anti-fogging bias. An image carrier;
A developing roller for applying a non-magnetic one-component toner to the image carrier;
A voltage applying unit for applying a developing bias between the image carrier and the developing roller;
A non-contact development type image forming apparatus having a paper identification sensor for detecting the type of paper to be printed,
The development bias applied by the voltage application unit is:
A developing voltage Vmin for forming an electric field in a direction in which the toner is caused to fly from the developing roller to the image carrier;
And a collection voltage Vmax for forming an electric field in a direction in which toner is caused to fly from the image carrier to the developing roller, and alternately.
The voltage application unit is:
Both the electric field strength in the image portion of the electrostatic latent image of the image carrier and the electric field strength in the background portion of the electrostatic latent image of the image carrier cause the toner to fly from the developing roller to the image carrier. A normal bias for setting the development voltage Vmin to be sufficient for
The electric field strength in the image portion of the electrostatic latent image on the image carrier is sufficient for the toner to fly from the developing roller to the image carrier, and the background portion of the electrostatic latent image on the image carrier. The two developing biases, i.e., an antifogging bias for setting the developing voltage Vmin so that the toner does not fly from the developing roller to the image carrier, are applied.
An anti-fogging bias is applied when a paper belonging to a paper group including at least one of thick paper, an OHP sheet, and glossy paper is detected by the paper identification sensor;
An image forming apparatus characterized by applying a normal bias in addition to applying the anti-fogging bias. The image forming apparatus according to claim 1,
It has a paper identification sensor that detects the type of paper to be printed,
The voltage application unit is:
When the image quality mode belonging to the mode group including at least the character mode and the high image quality mode is selected by the image quality mode adjustment unit, and any one of thick paper, an OHP sheet, and glossy paper is selected by the paper identification sensor An anti-fogging bias is applied in a case where at least one of the above-described cases where a sheet belonging to a sheet group is detected is applied. An image forming apparatus according to any one of claims 1 to 3, wherein
The anti-fogging bias is
The following expression is satisfied in the image portion of the electrostatic latent image of the image carrier:
| Vmin-Vi |> Fa · d / q
Vi: Potential of the image area
Fa: Adhesive force acting on the toner adhering to the developing roller
d: Distance between the image carrier and the developing roller
q: Average charge amount of toner To satisfy the following expression in the background portion of the electrostatic latent image of the image carrier:
| Vmin−Vb | ≦ Fa · d / q
Vb: an image forming apparatus in which the background portion potential is set. An image forming apparatus according to any one of claims 1 to 3, wherein
The anti-fogging bias is
In the background portion of the electrostatic latent image of the image carrier, | Vmin−Vb | is in the range of ± 20% of Fa · d / q.
Vb: background potential
Fa: Adhesive force acting on the toner adhering to the developing roller
d: Distance between the image carrier and the developing roller
q: An image forming apparatus in which an average charge amount of toner is set.

Images