JP2010072412A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus that suppresses both fog caused by deteriorated toner and accumulation of deteriorated toner in a developing device. <P>SOLUTION: One of two types of a development bias for a regular mode and a fog prevention mode used to suppress fog is selected according to environmental conditions etc. First, environmental information such as temperature and humidity is obtained. In this case, if the environment is likely to cause fog, a fog prevention bias is set, and if not, a regular bias is set. In the case that the fog prevention bias is set, background potential Vb and an amount of exposure light are adjusted along with this. <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 one-component toner. More specifically, the present invention relates to an image forming apparatus having an anti-fogging mode in which fog is suppressed by adjusting a developing bias.

  In a non-contact developing type image forming apparatus, there is a certain distance between the developing roller and the image carrier. For this reason, the toner moves from the developing roller to the image carrier by flying through this interval by the developing bias. Here, when the developing bias has an AC component, the toner reciprocates between the developing roller and the image carrier. Through this reciprocation process, the toner finally adheres to the image portion of the electrostatic latent image on the image carrier.

  Incidentally, the chargeability of the toner changes due to friction or the like. The toner has good chargeability when it is new. For this reason, the new toner is easily charged, and the variation in the charge amount is small. If the toner in the developing device is new, a large part of the toner on the developing roller is occupied by an appropriately charged toner (hereinafter referred to as “regularly charged toner”). However, as the toner deteriorates, the amount of toner with a small charge amount (hereinafter referred to as “low charge toner”) increases. This low-charged toner may adhere to the background portion of the electrostatic latent image. This becomes fogging and causes the quality of the printed matter to deteriorate.

  Further, when the toner further deteriorates, toner charged in the opposite direction to the normally charged toner (hereinafter referred to as “reversely charged toner”) is also generated. Once the reversely charged toner is separated from the developing roller, it receives a force opposite to that of the normally charged toner. For this reason, when the developing bias has an AC component, part of the reversely charged toner also reaches the image carrier. When the reversely charged toner adheres to the image carrier, it adheres more easily to the background portion than to the image portion. For this reason, the reversely charged toner also causes fogging. In particular, the reversely charged toner once adhering to the image carrier no longer leaves the image carrier even if it has the development side phase of the development bias. This is because the charge amount of the reversely charged toner is generally only small.

On the other hand, it is conceivable to suppress fogging by adjusting the developing bias. That is, the developing bias is adjusted to a value that does not contribute to the development of toner that has deteriorated and is no longer a normally charged toner. An example of such an image forming apparatus is Patent Document 1.
JP 2006-301479 A

  However, the conventional techniques described above have the following problems. That is, suppressing fogging by adjusting the developing bias has one aspect of suppressing the discharge of deteriorated toner from the developing device. For this reason, the deteriorated toner is accumulated in the developing device. As a result, fog may not be able to be suppressed, and toner may spill out of the developing device. That is, it is not preferable to always use a developing bias that suppresses fogging.

  The present invention has been made to solve the above-described problems of the prior art. That is, an object of the present invention is to provide an image forming apparatus that suppresses both the occurrence of fog caused by deteriorated toner and the accumulation of deteriorated toner in the developing device.

  An image forming apparatus designed to solve this problem includes an image carrier, an exposure device for forming an electrostatic latent image on the surface of the image carrier, and a non-magnetic 1 on the electrostatic latent image of the image carrier. In a non-contact developing type image forming apparatus having a developing roller for applying a component toner and a voltage applying unit for applying a developing bias between the image carrier and the developing roller, at least one of temperature and humidity The voltage application unit has a development voltage 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 the toner is caused to fly from the image carrier to the development roller. And a recovery voltage for forming an electric field in a direction to be applied alternately and, as the development voltage, the electric field strength in the image portion of the electrostatic latent image on the image carrier is the same as that of the toner. It is sufficient to fly from the roller to the image carrier, and the electric field strength at 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. The anti-fogging mode using the value and the electric field strength in the image portion and background portion of the electrostatic latent image of the image carrier are values that are sufficient for the toner to fly from the developing roller to the image carrier. The anti-fogging mode is selected as the developing bias when the measured value of the environmental sensor corresponds to a predetermined condition in which fogging is likely to occur. In other cases, the developing bias is selected as the developing bias. When the normal mode is selected and the anti-fogging mode is set, the background potential of the image carrier is set to a developing bias higher than the background potential in the normal mode. And a value close to the image voltage, the exposure amount of the exposure apparatus, is characterized in that greater than the value of the exposure light amount in the normal mode.

  Such an image forming apparatus can suppress the occurrence of fog due to deteriorated toner by applying the anti-fogging mode. Moreover, accumulation of deteriorated toner in the developing device can be prevented by applying the normal mode. Further, in both the normal mode and the anti-fogging mode, half unevenness that is a repeated stripe pattern does not occur in the image.

In the image forming apparatus described above, in the anti-fogging mode, the voltage application unit is expressed by the following expression: | Vmin−Vi |> Vpump
| Vmin−Vb | ≦ Vpump
Vmin Development side voltage of development bias
Vi Potential of image portion of image carrier
Vb Potential of background portion of image carrier
Vpump The critical flight voltage is satisfied, and in the normal mode, the following formula: | Vmin−Vb |> Vpump
A developing bias may be applied so as to satisfy the above condition. Occurrence of fog caused by deteriorated toner by using the normal mode in which the toner flies in both the image area and the background area, and the anti-fogging mode in which the toner flies in the image area and the toner does not fly in the background area. This is because it is possible to suppress accumulation of deteriorated toner in the developing device.

  In the image forming apparatus described above, the image forming apparatus includes a density sensor that measures the density of the developed toner image, and a control unit that controls the voltage application unit. The lower the humidity, the lower the development voltage at the background, and based on the set development voltage, set a background potential below the upper limit predetermined for each development voltage. The exposure light quantity is set so that the exposure light quantity increases as the distance increases. If the density is too high from the development voltage set above and the density value measured by the density sensor, the time occupancy rate of the recovered voltage Or increase the difference between the recovery voltage and the image part potential. If the density is too low, decrease the time occupancy of the recovery voltage or reduce the difference between the recovery voltage and the image part potential. Doo It should be noted that good. This is because a developing bias suitable for environmental conditions can be applied. Moreover, half unevenness can also be suppressed.

  In the image forming apparatus described above, the control unit may reset the developing bias, the background portion potential, and the exposure light amount when the temperature or humidity changes. This is because fog can be suppressed according to environmental changes.

  According to the present invention, both the occurrence of fog caused by deteriorated toner and the accumulation of deteriorated toner in the developing device are suppressed.

  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. In this embodiment, the present invention is embodied in a non-magnetic one-component non-contact developing image forming apparatus.

[First embodiment]
The first embodiment will be described below. The image forming apparatus according to the present embodiment uses two types of modes, a development bias for suppressing fogging and a normal development bias, depending on environmental values such as temperature and humidity measured by an environmental sensor. In other words, the image forming apparatus according to the present embodiment selects the anti-fogging mode for setting the developing bias for suppressing the fogging in an environment where the fogging is likely to occur, and sets the normal developing bias for the other cases. Select the mode.

[Constitution]
The image forming apparatus 100 of this embodiment is configured as shown in FIG. The image forming apparatus 100 includes a photoconductor 110, a charging device 120, an exposure device 130, a developing roller 140, a supply roller 141, a regulation blade 142, a charge eliminating sheet 143, a transfer device 150, a fixing device 160, and the like. , Cleaner 170, toner recovery container 171, buffer 180, upstream screw 181, downstream screw 182, voltage application unit 190, environment sensor 102, and general control unit 101.

  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 toner used in this embodiment is negatively charged.

  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 photoconductor 110. A developing bias is applied between the photoconductor 110 and the developing roller 140. The voltage application unit 190 applies the developing bias. The clearance between the photoconductor 110 and the developing roller 140 is a fixed value of about 130 [μm]. The toner on the surface of the developing roller 140 flies through this gap due to the developing bias, and finally adheres to the surface of the photoreceptor 110.

  The developing bias applied by the voltage application unit 190 includes an AC component as will be described later. That is, the voltage repeats the development voltage and the recovery voltage as shown in FIG. Further, there are two types, a normal mode shown on the left side of FIG. 2 and an anti-fogging mode shown on the right side of FIG. The general control unit 101 instructs the voltage application unit 190 to properly use the two types of development bias. The environment sensor 102 is a sensor for measuring temperature and humidity. The environmental values of temperature and humidity measured by the environmental sensor 102 are sent to the general control unit 101.

  The supply roller 141 is for supplying the toner stored in the buffer 180 to the developing roller 140. The supply roller 141 also serves to collect residual toner from the surface of the developing roller 140 as well as supply toner to the developing roller 140. For this reason, the supply roller 141 rotates with the developing roller 140 in the counter direction. The supply roller 141 is made of a foaming elastic member.

  The regulating blade 142 is for charging the toner supplied to the developing roller 140 and regulating the amount of toner transported on the developing roller 140. 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 eliminating sheet 143 is provided to improve the recoverability of the toner after development.

  The transfer device 150 is for transferring a toner image formed on the surface of the photoconductor 110 onto paper. The cleaner 170 collects the 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.

  As described above, the image forming apparatus 100 according to the present embodiment uses the anti-fogging mode and the normal mode developing bias in accordance with environmental values such as temperature and humidity. That is, the anti-fogging mode is applied when the temperature and humidity conditions are likely to cause fogging, and the normal mode is applied when not.

  Therefore, the relationship between environmental values such as temperature and humidity and the likelihood of fogging will be described. FIG. 3 is a graph showing the relationship between the temperature and humidity environment and fog. Vmin is a developing voltage of a developing bias described in detail later, and Vb is a potential of the background portion of the photoconductor 110 (see FIG. 2). The horizontal axis represents the effective developing voltage | Vmin−Vb | in the background portion, which forms the developing electric field in the background portion. The vertical axis represents the fogging ΔC *, which is a tint that is a substitute value for fogging. In FIG. 3, the low temperature and low humidity environment (10 ° C., 15%) is higher than the high temperature and high humidity environment (30 ° C., 85%) for any effective development voltage | Vmin−Vb | Fog is likely to occur.

  Therefore, the anti-fogging mode may be selected in a low temperature and low humidity environment where fog is likely to occur. In other cases, the normal mode may be selected. In other words, the image forming apparatus 100 according to this embodiment suppresses fogging by selecting the anti-fogging mode when fogging is likely to occur, and accumulates deteriorated toner in the developing device by selecting the normal mode in other cases. It can be avoided.

[Development bias]
Here, the developing bias having an AC component will be described in detail. FIG. 2 shows one cycle of the developing bias. An example of the development bias in the normal mode is shown on the left side of FIG. 2, and an example of the development bias in the anti-fogging mode is shown on the right side. In FIG. 2, the horizontal axis is time, and the vertical axis is potential. More specifically, the upper side of the vertical axis is the developing side (minus), and the lower side is the collecting side (plus). The meaning of each symbol in FIG. 2 is as follows. Note that Vmin in this embodiment corresponds to Vmax in Patent Document 1.
Vmin Development voltage Vmax Recovery voltage Vb Background part potential Vi Image part potential Vdc Development bias DC component Frq Development bias frequency Vave Development bias time average Duty Development voltage Vmin time share

Next, the force acting on the toner on the developing roller 140 when the developing voltage Vmin in FIG. 2 is applied will be described. The toner on the developing roller 140 receives a Coulomb force Fe and an adhesion force Fa. The Coulomb force Fe is a force that acts on the electric charge of the toner by the electric field by the developing voltage Vmin, that is, the developing electric field E. The magnitude of the Coulomb force Fe is expressed as follows:
Fe = q · E (1)
It is represented by That is, the Coulomb force Fe is proportional to the charge amount q of the toner. The direction of the Coulomb force Fe is a direction from the developing roller 140 toward the photoconductor 110 if the toner is negatively charged.

  The toner adhesion force Fa is the sum of the mirror image force Fi and the mechanical adhesion force Fv. These are all forces that prevent the toner from being detached from the developing roller 140. The main component of the mechanical adhesion force Fv is van der Waals force, and there are other liquid crosslinking forces. Hereinafter, the adhesion force Fa is referred to as a total adhesion force Fa.

The image force Fi when the toner adheres to the developing roller 140 is
Fi = (ε−1) · q ² / {4π · (ε + 1) · ε0 · D ² } (2)
ε Relative permittivity of developing roller
ε0 Air permittivity
D Expressed by the average particle size of the toner. Here, ε, ε0, and D are known constants. Therefore, the part of the coefficient of q ^{2 in} the right side of equation (2) (ε−1) / {4π · (ε + 1) · ε0 · D ² }
Is also a constant. When this is represented by “a”, the equation (2) is
Fi = a · q ² (3)
It becomes. That is, the mirror image force Fi is proportional to the square of the toner charge amount q. The mirror image force Fi decreases in proportion to the square of the distance between the toner and the developing roller 140. That is, once the toner is separated from the developing roller 140, this force is extremely weak.

  The mechanical adhesion force Fv can be measured experimentally. The present inventors used KONICA MINOLTA TECHNOLOGY REPORT VOL. 1 (2004) p15 “The particle size dependence of toner adhesion and electric field flight” was used. This is to obtain the vibration acceleration when the adhered toner is detached by vibrating the vibrator and applying vibration acceleration to the toner adhered by electrostatic force. From this value, Fv can be obtained.

  The measurement result Fv depends on the type and environment of the toner and the developing roller 140, but is found to be in the range of about 5 to several tens [nN] and does not depend on the toner charge amount q. Of course, Fv does not depend on the developing electric field E. Therefore, it can be regarded as a constant. The mechanical adhesion force Fv works only when the toner is substantially adhered to the developing roller 140. That is, once the toner is separated from the developing roller 140, this force does not work.

From the above, the resultant force F of the force applied to the toner attached to the developing roller 140 when receiving the developing electric field E is obtained by subtracting the mirror image force Fi and the mechanical adhesion force Fv from the Coulomb force Fe.
F = −a · q ² + E · q−Fv (4)
And a quadratic function of the toner charge amount q. In the equation (4), the first term is the mirror image force Fi, the second term is the Coulomb force Fe, and the third term is the mechanical adhesion force Fv. In the formula (4), the direction in which the toner moves from the developing roller 140 toward the photoconductor 110 is positive.

Here, for the parameters that appeared in equation (2),
ε> 1
ε0> 0
D ² > 0
So a is positive. Therefore, F is a quadratic function convex upward, and takes a maximum value at a certain charge amount q. The charge amount q at which F is maximized is the charge amount at which the toner is most easily detached from the developing roller 140.

  FIG. 4 shows the relationship between the toner charge amount q and each force acting on the toner. First, a graph of the resultant force F, Coulomb force Fe, image force Fi, mechanical adhesion force Fv, and total adhesion force Fa is shown in the upper part of FIG. The horizontal axis is the toner charge amount, and the vertical axis is the force acting on the toner. In the vertical axis, the upper side is the separation side and the lower side is the adhesion side.

Referring to FIG. 4, the following can be understood about the force that does not depend on the developing electric field E.
-The mechanical adhesion force Fv (broken line) is a negative constant value.
The mirror image force Fi (dashed line) is a parabola that opens downward with the origin where the charge amount and force are both zero as the apex.
-Therefore, the total adhesion force Fa (thin solid line), which is the sum of these, is a parabola obtained by translating the parabola of the mirror image force Fi downward. The amount of movement is equal to the value of the mechanical adhesion force Fv.

Regarding the force depending on the developing electric field E, the following can be understood.
-Coulomb force Fe (two-dot chain line) is a straight line going up to the left through the origin.
-For this reason, the resultant force F (thick solid line), which is the sum of all forces, must be a parabola that opens downward. The position of the vertex T is a position moved to the upper left as compared with the vertex of the total adhesion force Fa. The parabola of the resultant force F passes through the apex of the total adhesion force Fa.

  The Coulomb force Fe and the resultant force F in the upper part of FIG. 4 are obtained when the developing electric field E is set to a suitable value. The resultant force F here is a positive value in a certain range where the charge amount is on the negative side, and a negative value on both outer sides thereof. The range in which the resultant force F is positive is the range of the charge amount at which the toner detaches from the developing roller 140 and flies. The range in which the resultant force F is negative is the range of charge amount that the toner cannot detach from the developing roller 140.

  The lower part of FIG. 4 shows the toner charge distribution. The vertical axis represents the number of toner particles in the buffer 180. The horizontal axis represents the toner charge amount, which is the same as the horizontal axis on the upper stage. The toner in the range of charge amount (hatching range) guided by the toner from the upper stage separates from the developing roller 140 and flies by the developing electric field E. This range substantially coincides with the charge amount range in which the majority of toners are distributed. The range of the charge amount of the toner that is normally charged toner is almost the same as this. Even if the toner of the charge amount on both outer sides of this range is subjected to the developing electric field E, it cannot be detached from the developing roller 140 and continues to adhere as it is.

As described above, the Coulomb force Fe depends on the developing electric field E, and the resultant force F is also affected by the developing electric field E. FIG. 5 is a graph when the developing electric field E is made stronger than the developing electric field in FIG. When FIG. 5 is compared with FIG. 4, the following points are different, and the others are the same.
・ The slope of Coulomb force Fe (two-dot chain line) is steeper.
-The vertex T of the parabola of the resultant force F (thick solid line) is a position moved further to the upper left than the vertex T in FIG.
-As a result, the range in which the resultant force F is positive, that is, the flying range is wider than that in FIG. It spreads mainly to the left.

FIG. 6 is a graph when the developing electric field E is weaker than the developing electric field in FIG. When FIG. 6 is compared with FIG. 4, the following points are different, and the others are the same. In FIG. 6, the lower graphs in FIGS. 4 and 5 are omitted.
・ The inclination of Coulomb force Fe (two-dot chain line) is more gradual.
-The vertex T of the parabola of the resultant force F (bold solid line) is the position moved to the lower right from the vertex T in FIG. This position is on the horizontal line where the force is zero. That is, even at this vertex position, the resultant force F does not become positive.
-Thereby, the range where the resultant force F is positive, that is, the width of the flying range is zero. That is, since the developing electric field E is weakened, there is almost no toner that can fly.

FIG. 7 is a graph when the developing electric field E is further weaker than the developing electric field in FIG. When FIG. 7 is compared with FIG. 6, the following points are different, and the others are the same. Also in FIG. 7, the lower graphs in FIGS. 4 and 5 are omitted.
・ The inclination of Coulomb force Fe (two-dot chain line) is more gentle.
The vertex T of the parabola of the resultant force F (thick solid line) is a position moved further to the lower right than the vertex T in FIG. That is, even at this vertex position, the resultant force F is negative. Therefore, there is no toner that can fly.

Looking back at FIGS. 4 to 7, regarding the developing electric field E, FIGS. 4 and 5 are the conditions for the toner to fly, and FIGS. 6 and 7 are the conditions for the toner not to fly. In particular, the developing electric field E in the state shown in FIG. 6 is a critical developing electric field at the boundary between the condition for the toner to fly and the condition for the toner not to fly. Hereinafter, the developing electric field at this time is referred to as a critical flying electric field Epump, and the voltage between the photosensitive member 110 and the developing roller 140 at this time is referred to as a critical flying voltage Vpump. If the distance between the photoconductor 110 and the developing roller 140 is d, the critical flight voltage Vpump is
Vpump = Epump · d (5)
It is represented by

Here, the critical flight electric field Epump and the critical flight voltage Vpump are specifically calculated. In the state of FIG. 6, since the developing electric field E is the critical flying electric field Eump, from the equation (4),
F = −a · q ² + Epump · q−Fv (6)
It becomes. Then, the resultant force F received by the toner having the charge amount (referred to as qt) at the apex T in FIG. 6 is zero. Therefore, from equation (6)
0 = −a · (qt) ² + Epump · (qt) −Fv
Epump = a · (qt) + Fv / (qt) (7)
It becomes.

On the other hand, if the right side of the equation (6) is differentiated by q, it is zero at the vertex T (q = qt) in FIG.
2a ・ (qt) = Epump ............ (8)
It becomes. From equations (7) and (8),
(Qt) ² = Fv / a (9)
It becomes.

Here, in the image forming apparatus 100 of the present embodiment, the parameters appearing in the equation (2) are
ε = 3
ε0 = 8.85 × 10 ⁻¹² [F / m]
D = 6 [μm]
Met. Thereby, “a” in the equation (9) is determined. Furthermore, the mechanical adhesion Fv described in paragraph [0030] was 15 [nN]. This gives qt
qt = 1.1 × 10 ⁻¹⁴ [C]
Can be calculated.

Thus, using equation (8),
E pump = 2.8 [MV / m]
Is required. If the distance d is 130 [μm], from equation (5),
Vpump = 364 [V]
It becomes.

  Here, with respect to the image forming apparatus 100 of the present embodiment, the fog was measured by varying the value of the development voltage Vmin across the critical flight voltage Vpump. In this test, fog on the photoconductor 110 after development was measured at two levels of development voltage Vmin of 360 [V] (substantially close to the conditions of FIG. 6) and 650 [V] (which can be said to be the conditions of FIG. 5). did. The image was the entire background. The fog was evaluated by peeling the toner with a booker tape, attaching it to Konica Minolta J paper, and measuring C * with a color difference meter CR241 manufactured by Konica Minolta. As the toner, cyan toner, which has been printed to 2500 sheets at a printing rate of 5% and has deteriorated to some extent, was used.

As a result, the tint ΔC *, which is a substitute value for fog on the photoconductor 110, was as follows.
Development voltage Vmin Color ΔC *
360 [V] 0.52
650 [V] 2.36
The tint ΔC *, which is a substitute value for fog on the photoconductor 110, is 4 to 4 from the development voltage Vmin that does not exceed the critical flight voltage Vpump, that is, the development voltage Vmin that exceeds the critical flight voltage Vpump, that is, a condition that has been frequently used conventionally. It was confirmed that it improved about 5 times.

  The reason why fog occurs when the development voltage Vmin exceeds the critical flight voltage Vpump will be described. Since fog is a problem in the background portion, the surface potential of the photoconductor 110 is originally a potential at which normally charged toner is difficult to adhere. However, when the developing voltage Vmin exceeds the critical flight voltage Vpump, the regular charged toner on the developing roller 140 is detached from the developing roller 140 by the developing voltage Vmin. The separated regular charged toner hardly adheres to the photoconductor 110, but reciprocates between the developing roller 140 and the photoconductor 110 by the AC component of the developing bias. Therefore, a part of the reciprocating toner collides with the developing roller 140 and knocks out deteriorated toner such as weakly charged toner and reversely charged toner adhering to the developing roller 140.

  As described above, since the adhesion force Fa hardly acts on the deteriorated toner once knocked out, the toner is reciprocated by the Coulomb force Fe of the developing bias. Since such deteriorated toner hardly receives repulsive force due to the background portion potential of the photoconductor 110, a part of the deteriorated toner adheres to the photoconductor 110. Once the deteriorated toner adheres to the photoconductor 110, an adhesive force Fa acts between the deteriorated toner and the photoconductor 110. After that, it is almost impossible to expect the toner to be ejected from the photoconductor 110 by the normally charged toner. This is fogging.

  On the contrary, when the developing voltage Vmin does not exceed the critical flight voltage Vpump, there is no separation of the normally charged toner, so that the deteriorated toner does not knock out. For this reason, fog does not occur.

  A specific developing bias used in the image forming apparatus 100 of this embodiment will be described. As described above, there are two types of developing bias used in this embodiment, the anti-fogging mode and the normal mode. The normal mode is a mode generally used from the past. The anti-fogging mode is a mode in which fogging due to deteriorated toner is suppressed as compared with the normal mode. On the other hand, in the anti-fogging mode, the deteriorated toner tends to accumulate without being discharged from the buffer 180.

[Development bias in anti-fogging mode]
In the anti-fogging mode, the developing voltage Vmin on the right side of FIG. 2 is set so that the toner will fly in the image area and not in the background area. The toner is allowed to fly in the image area because otherwise image formation cannot be performed. On the other hand, in the background portion, in order to eliminate the cause of fog as much as possible, the toner does not fly.

That is, the effective developing voltage in the image area must exceed the critical flying voltage Vpump with respect to the above-mentioned critical flying voltage Vpump, and the effective developing voltage in the background part must not exceed the critical flying voltage Vpump. That is,
| Vmin−Vi |> Vpump (10)
| Vmin−Vb | ≦ Vpump (11)
Both formulas must be satisfied. The left side of equation (10) is the effective developing voltage for the image area, and the left side of equation (11) is the effective developing voltage for the background area.

  In other words, the relationship between the forces in the image portion must be in the state of FIG. 4 or FIG. 5, and the relationship of the forces in the background portion must be in the state of FIG. 6 or FIG. As a result, a situation where the toner flies in the image portion and does not fly in the background portion is realized.

[Development bias in normal mode]
In the normal mode, the development voltage Vmin is set so that the toner flies in both the image portion and the background portion. That is, with respect to the above-mentioned critical flight voltage Vpump, the effective development voltage is made to exceed the critical flight voltage Vpump in both the image portion and the background portion. In other words,
| Vmin−Vi |> Vpump (12)
| Vmin−Vb |> Vpump (13)
Is set to be Equation (12) is the same as (10). However, if equation (13) holds, equation (12) also holds. This is because the left side of equation (12) is necessarily larger than the left side of equation (13) (see the left side of FIG. 2). Therefore, in reality, only the equation (13) needs to be considered.

  That is, the normal mode is a mode in which the relationship between the forces is in the state shown in FIG. 4 or 5 in both the image portion and the background portion. As a result, a certain amount of fog occurs in the background. An example of normal mode setting is shown on the left side of FIG.

  In the image portion, the toner flies from the developing roller 140 toward the photoconductor 110 by the developing electric field. This is the same in both the anti-fogging mode and the normal mode. That is, equation (10) is satisfied regardless of the mode. In the background portion, the toner flies to the photoconductor 110 in the normal mode and does not fly in the anti-fogging mode. That is, in the anti-fogging mode, the expression (11) is satisfied, and in the normal mode, the expression (13) is satisfied. Therefore, whatever development bias is selected, it belongs to either the anti-fogging mode or the normal mode.

[Half unevenness and background potential]
If the anti-fogging mode and the normal mode described above are selectively used as the developing bias, accumulation of deteriorated toner can be avoided while suppressing fogging. However, when the anti-fogging mode is applied, there is a risk of causing half-uniformity in the image. Here, the half unevenness means unevenness that is a repeated stripe pattern as shown in the upper part of FIG. Needless to say, when half-uniformity occurs, the quality of the printed matter deteriorates.

  Here, the relationship between the background portion potential Vb and the half unevenness will be described. FIG. 9 shows the relationship between the background portion potential Vb and half unevenness. The horizontal axis is the background portion potential Vb, and the vertical axis is the rank of half unevenness. The rank of the half unevenness is evaluated in five stages of 1 to 5, rank 5 has the least unevenness, and rank 1 has the most unevenness.

  Under the conditions shown in FIG. 9, half unevenness is most likely to occur when the background portion potential Vb is −450 [V] while the effective developing voltage | Vmin−Vb | at the background portion is 350 [V]. That is, when the effective developing voltage | Vmin−Vb | in the background portion is set to a small value, that is, in the anti-fogging mode, half unevenness tends to occur if the background portion potential Vb is set to a value close to zero potential. For this reason, in the anti-fogging mode, the background portion potential Vb may be further changed to the negative side. This is to make negatively charged toner more difficult to adhere to the background. On the other hand, in the normal mode, half unevenness hardly occurs regardless of the background potential. Therefore, there is no need to change the background potential.

[Operation]
Here, the operation of the image forming apparatus 100 will be described. As described above, the image forming apparatus 100 according to this embodiment forms an image by selecting a development bias in the anti-fogging mode or the normal mode according to the environmental values such as temperature and humidity. Therefore, first, the anti-fogging mode or the normal mode is selected as the developing bias. The developing bias setting method performed at this time will be described with reference to the flowchart of FIG.

  First, environmental information such as temperature and humidity is acquired from the environmental sensor 102 (S1). Next, it is determined whether or not the acquired environmental values such as temperature and humidity are likely to cause fogging (S2). As described above, the anti-fogging mode is applied in a low-temperature and low-humidity environment, and the normal mode is applied in other environments. The temperature and humidity thresholds at this time may be determined in advance from the relationship with FIG. Therefore, if the temperature is low temperature and low humidity, the process proceeds to (S2: Yes), and if not, the process proceeds to (S2: No).

  If No in S2, the normal mode developing bias is set (S6). The developing bias in the normal mode refers to a developing bias that causes toner to fly toward the photoconductor 110 in both the image portion and the background portion of the electrostatic latent image. Next, the value of the background portion potential Vb in the normal mode is set (S7). The background portion potential Vb is the potential of the background portion of the surface of the photoconductor 110 and remains the potential charged by the charging device 120. As described above, the normal mode is less likely to cause half unevenness than the anti-fogging mode. Therefore, it is not necessary to increase the background potential to the negative side compared to the anti-fogging mode (see FIG. 2). Therefore, −450 [V] is set as the background portion potential Vb in the normal mode.

  Subsequently, the exposure light amount for the normal mode is set (S8). In this exposure step, the surface of the photoconductor 110 that is uniformly at the background portion potential Vb is selectively exposed to the portion that becomes the image portion, so that the potential at the exposed portion is set to the image portion potential Vi. is there. Therefore, 280 [μW] is set as the exposure light quantity in the normal mode. Due to the amount of exposure light, the potential of the image portion changes by an arrow A on the left side of FIG. Therefore, the image portion potential Vi at the exposed portion is obtained. The potential change amount | Vb−Vi | due to exposure is 400 [V]. Instead of performing the processing of S7 and S8, the background potential and the exposure light amount for the normal mode may be set at the initial stage of the flowchart.

  On the other hand, in the case of Yes in S2, a fog prevention mode for suppressing fogging is set (S3). Next, a value in the anti-fogging mode is set as the background portion potential Vb of the photoconductor 110 (S4). The background portion potential Vb of the photoconductor 110 charged by the charging device 120 is changed.

  The setting of the background portion potential Vb is necessary to avoid half unevenness that may be caused by applying the anti-fogging mode. Therefore, in order to prevent half unevenness from appearing, the value of the background portion potential Vb is set to the developing voltage Vmin side from the normal mode (see FIG. 2). Therefore, −650 [V] is set as the background portion potential Vb. Along with this change, the value of the effective developing voltage | Vmin−Vb | of the background portion is also changed.

  Next, the exposure light quantity is set to a value in the anti-fogging mode (S5). As described above, the background portion potential Vb is a potential uniformly applied to the surface of the photoreceptor 110 by the charging device 120. The portion selectively exposed on the surface of the photoconductor 110 having the uniform background portion potential Vb changes to the image portion potential Vi. For this reason, if the value of the background portion potential Vb is changed without adjusting the amount of exposure light, the value changes to the value of the image portion potential Vi accordingly. That is, it is difficult for toner to adhere not only to the background portion but also to the image portion. For this reason, there is a possibility that the fine line becomes thinner or disappears. Therefore, the exposure light amount is adjusted in S5.

  In this embodiment, 460 [μW] is set as the exposure light quantity. That is, the amount of exposure light in the anti-fogging mode is made larger than the value in the normal mode. Depending on the amount of exposure light, the potential of the image portion changes by an arrow B on the right side of FIG. Therefore, the image portion potential Vi at the exposed portion is obtained. The potential change amount | Vb−Vi | due to exposure is 600 [V]. This image portion potential Vi is substantially the same as the image portion potential Vi in the normal mode (see FIG. 2). Thus, even if the anti-fogging mode is applied, the toner density in the image portion can be prevented from being affected. This completes the setting of the development bias.

  Then, the surface of the photoreceptor 110 is uniformly charged by the charging device 120. At this time, the surface potential of the photoconductor 110 is the background portion potential Vb in the antifogging mode or the normal mode set above. Next, an electrostatic latent image is formed on the surface of the photoreceptor 110 by the exposure device 130. By this exposure, the potential of the portion corresponding to the image portion becomes the image portion potential Vi. Next, development is performed with the set development bias.

  Next, the toner on the surface of the photoconductor 110 is transferred onto the paper by the transfer device 150. Next, the transfer image to which the toner has been transferred is fixed by the fixing device 160. On the other hand, the toner remaining without being transferred to the surface of the photoreceptor 110 is collected by the cleaner 170. As described above, printing on paper is performed.

  The image forming apparatus 100 according to the present embodiment uses the developing bias in the antifogging mode and the normal mode properly according to environmental values such as temperature and humidity. In addition, the anti-fogging mode is selected in the case of low temperature and low humidity where fog is likely to occur, and the normal mode is selected in other cases. In addition, fogging can be suppressed while the anti-fogging mode is set, and accumulation of deteriorated toner can be avoided while the normal mode is set. As a result, an image forming apparatus capable of suppressing fogging and avoiding accumulation of low-charge toner and reverse-charge toner in the developing device is realized.

  Here, a modified example of the image forming apparatus 100 of the present embodiment will be described. In this embodiment, the anti-fogging mode developing bias is applied in a low temperature and low humidity environment, and the normal mode developing bias is applied in other environments. However, the development bias in the normal mode may be applied in a high-temperature and high-humidity environment, and the development bias in the anti-fogging mode may be applied in other environments. Further, the temperature / humidity region to which the anti-fogging mode developing bias is applied and the temperature / humidity region to which the normal mode developing bias is applied may be determined by two variables of temperature and humidity. The anti-fogging mode developing bias and the normal mode developing bias can be selectively used according to temperature alone or humidity alone.

  In addition to the temperature and humidity conditions, fog may occur. For example, when printing on cardboard or special paper. In addition, there are cases where it is desired to suppress fogging in terms of image quality. Therefore, the anti-fogging mode may be selected when fogging is likely to occur or when it is desired to suppress the fogging in terms of image quality. This is because if the anti-fogging mode is used in combination with the normal mode, the same effect can be obtained. Further, when the environmental value of temperature or humidity changes, the developing bias, the background portion potential, and the exposure light amount may be reset.

  As described above in detail, in the image forming apparatus 100 of the present embodiment, two types of development bias, that is, the anti-fogging mode and the normal mode are selectively used. This is because fogging and accumulation of deteriorated toner in the developing device can be suppressed. In the anti-fogging mode, low-charged toner or the like is likely to be accumulated in the developing device instead of suppressing the fog. On the other hand, in the normal mode, there is no effect of suppressing fogging, but since low-charged toner or the like adheres to the photoconductor 110 as appropriate, the toner is hardly accumulated in the developing device. When the anti-fogging mode is applied, the background potential Vb is adjusted and the exposure light quantity is also adjusted. As a result, the image forming apparatus 100 that can suppress the occurrence of fog caused by the deteriorated toner and the accumulation of the deteriorated toner in the developing device is 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, specific values such as the developing bias and the surface potential of the photoreceptor 110 used in this embodiment may be different values. Therefore, another value may be used for the background portion potential Vb for the anti-fogging mode. Further, it is preferable that the amount of exposure light is set to a value associated therewith. Further, the value of the critical flight voltage Vpump is an example, and becomes a different value under different conditions.

[Second form]
The second embodiment will be described. In the image forming apparatus 100 of the first embodiment, either the development bias in the anti-fogging mode or the development bias in the normal mode is set according to environmental values such as temperature and humidity. The image forming apparatus 200 according to this embodiment determines an optimum developing bias including environmental values such as temperature and humidity, and other information such as density. Therefore, the development bias belonging to the anti-fogging mode is not limited to one type. In other words, the image forming apparatus 200 according to the present embodiment uses several developing biases that belong to the anti-fogging mode.

  The mechanical configuration of the image forming apparatus 200 of the present embodiment is almost the same as that of the image forming apparatus 100 of the first embodiment shown in FIG. The difference from the image forming apparatus 100 of the first embodiment is that it has a density sensor 103 that measures the amount of toner on the photoconductor 110. The density sensor 103 is an IDC (Image Density Control) sensor that measures the adhesion amount of toner on the photoconductor 110. Therefore, the density sensor 103 measures the surface of the photoconductor 110 at a position downstream of the developing roller 140 and upstream of the transfer device 150 as viewed from the photoconductor 110. The density sensor 103 measures the density of the image portion of the photoconductor 110 and the fog of the background portion. Then, the test pattern is drawn and reflected in the next image formation. Other mechanical configurations are the same as those of the image forming apparatus 100.

  As shown in FIG. 11, the general control unit 101 of this embodiment has a control unit memory and a bias setting unit. The control unit memory includes a memory 1, a memory 2, a table 1, a table 2, and a table 3. The memory 1 stores information on the number of durable sheets. The durable number is a substitute value for the deterioration state of the toner that changes over time. The memory 2 stores the DS gap measured at the time of manufacture.

  Table 1 is for selecting the electric field strength of the developing electric field in the background portion from the temperature and humidity and the durable number. Table 1 is a table having 12 columns obtained by multiplying three temperature / humidity regions of low temperature and low humidity, medium temperature and medium humidity, and high temperature and high humidity, and four durability numbers of initial, first, middle, and second periods. . An example of Table 1 is shown in Table 1.

  In Table 1, “normal” characters are described in six of the twelve columns. This indicates that the value of the electric field strength of the developing electric field in the normal mode is stored. These six electric field strengths have the same value. That is, the electric field strength of one type of developing electric field belonging to the normal mode is commonly applied to six cases. On the other hand, in the remaining six columns of Table 1, the characters “antifogging” are described. This indicates that the value of the electric field strength of the developing electric field in the anti-fogging mode is stored. These six columns contain six different values. In other words, the electric field strengths of the six types of development electric fields belonging to the anti-fogging mode are properly used in six cases.

  In addition, as described above, fogging is more likely to occur as the temperature and humidity are lower. For this reason, the temperature and humidity are lowered, and the electric field strength of the developing electric field in the background portion is reduced. The effective developing voltage | Vmin−Vb | at the background portion also decreases accordingly. On the other hand, fog is likely to occur due to toner deterioration. This is because lowly charged toner and reversely charged toner that cause fogging increase. Therefore, the electric field strength of the developing electric field in the background portion is reduced as it approaches the later stage. As a result, the effective developing voltage | Vmin−Vb | in the background portion also decreases.

  Table 2 is for selecting the background portion potential Vb from the temperature and humidity and the effective developing voltage | Vmin−Vb | of the background portion. Table 3 is for selecting the exposure light quantity from the temperature and humidity and the background portion potential Vb.

Here, a setting method of the developing bias of the image forming apparatus 200 of this embodiment will be described. The parameters for determining the development bias are:
Development voltage Vmin
Recovery voltage Vmax
Time occupancy rate Duty
It is three. The frequency is fixed (see Fig. 2). Further, as described above, the background portion potential Vb is set as the potential of the photoconductor 110 so as not to cause half unevenness. The image portion potential Vi is fixed (see FIG. 2). Also, the exposure light quantity is set to an appropriate value.

Therefore, the developing bias of the image forming apparatus 200 of the present embodiment has the following effective developing voltage | Vmin−Vb |
Background potential Vb
Exposure light amount Recovery voltage Vmax or time occupancy Duty
It is determined by setting each value of.

  If the effective developing voltage | Vmin−Vb | and the background potential Vb in the background portion are determined, the developing voltage Vmin is determined. Then, as will be described later, the recovery voltage Vmax or the time occupation rate Duty is changed with the normal mode as a reference. Thereby, the density of the image can be controlled.

  Hereinafter, the procedure for setting the developing bias will be described with reference to FIG. First, environmental values of temperature and humidity are acquired from the environmental sensor 102. On the other hand, the durable number is read from the memory 1. Next, the electric field strength of the developing electric field in the background portion is selected from the environmental values of temperature and humidity and the durable number based on the table 1 prepared in advance. That is, one field intensity value of the developing electric field is selected from the 12 columns in Table 1.

  Next, the interval (DS gap) between the photoconductor 110 and the developing roller 140 is read from the memory 2. The DS gap varies by about ± 10% during manufacturing. Therefore, the DS gap is measured at the time of manufacture and stored in the memory 2. Subsequently, the value of the effective developing voltage | Vmin−Vb | of the background portion is calculated from the electric field strength of the developing electric field in the background portion selected from Table 1 and the DS gap. Note that the effective developing voltage | Vmin−Vb | of the background portion is the product of the electric field strength of the developing electric field in the background portion and DS (see equation (5)).

  Next, the value of the background portion potential Vb is selected according to Table 2 from the temperature and humidity, the durable number, and the effective developing voltage | Vmin−Vb | of the background portion. The background portion potential Vb selected at this time is a value that does not cause half unevenness. That is, in FIG. 9, the value is selected so that the half unevenness rank is 3 or more. This is a value less than or equal to the upper limit value determined for each development voltage. When the set electric field strength of the developing electric field corresponds to the anti-fogging mode, the background portion potential Vb is a value that also satisfies the expression (11). On the other hand, in the case of the normal mode, the background portion potential Vb is a value that also satisfies the expression (13).

  Next, the exposure light quantity is selected from the table 3 based on the temperature and humidity, the durable number, and the background portion potential Vb. The exposure light quantity set at this time is a value that makes the image portion potential Vi substantially constant regardless of the development bias to be set (see FIG. 2). That is, the amount of exposure light is set to increase as the background portion potential Vb is farther from the image portion potential Vb.

  Next, the value of the time occupation rate Duty or the recovery voltage Vmax is determined from the effective developing voltage | Vmin−Vb | of the background portion, the background portion potential Vb, the exposure light amount, and the toner adhesion amount acquired from the density sensor 103. To do. This is for adjusting the density of the image. Therefore, in order to maintain an appropriate image density, the image density is set in a range not less than a certain value and not more than a certain value. When the value is larger than the upper limit, the image density is too high. When the density is smaller than the lower limit value, the image density is too thin.

  If the concentration is high, increase the time occupancy of the recovered voltage. Alternatively, the potential difference | Vmax−Vi | between the recovery voltage Vmax and the image portion potential Vi may be increased. As a result, the image density can be kept within an appropriate range. If the concentration is low, reduce the time occupancy of the recovered voltage. Alternatively, the potential difference | Vmax−Vi | between the recovered voltage Vmax and the image portion potential Vi may be reduced. As a result, the image density can be kept within an appropriate range. The developing bias is determined by the above procedure.

  Thereafter, printing is performed using the set developing bias, as in the first embodiment. Further, it is possible to suppress the occurrence of fog due to the deteriorated toner and the accumulation of the deteriorated toner in the developing device. This is because the development bias is determined in accordance with the environmental conditions, so that the anti-fogging mode and the normal mode remain unchanged.

  Here, a modification of this embodiment will be described. In this embodiment, the density sensor 103 is used to determine the value of the development bias duty or the recovery voltage Vmax. However, the density sensor 103 can also detect half unevenness. That is, when half unevenness occurs on the photoconductor 110, the output voltage fluctuates at the unevenness period as shown in the lower part of FIG. For this reason, the fluctuation range ΔV of the output value may be set to a predetermined value or less.

  FIG. 12 shows the relationship between the fluctuation range ΔV of the output value of the density sensor 103 and the background portion potential Vb. The horizontal axis is the background potential Vb, and the vertical axis is the fluctuation range ΔV. In order to set the rank of half unevenness to 3 or more, the fluctuation width ΔV needs to be 0.5 [V] or less. From FIG. 12, the background portion potential Vb of the photoconductor 110 that satisfies the half unevenness rank of 3 or more is −550 [V] or less. That is, −550 [V] or less may be adopted as the background portion potential Vb. The background potential Vb may be changed every time the environmental value of temperature or humidity changes.

  In addition to the temperature and humidity conditions, fog may occur. For example, when printing on cardboard or special paper. In addition, there are cases where it is desired to suppress fogging in terms of image quality. Therefore, the anti-fogging mode may be selected when fogging is likely to occur or when it is desired to suppress the fogging in terms of image quality. This is because if the anti-fogging mode is used in combination with the normal mode, the same effect can be obtained.

  In this embodiment, the developing electric field is determined from the temperature, humidity, and the like and the number of durable sheets. However, the developing electric field may be determined without considering the durable number. Then, the effective development voltage | Vmin−Vb | may be determined only from the temperature and humidity environment. Further, the background portion potential Vb may be determined only from the effective developing voltage | Vmin−Vb |. Similarly, the exposure light quantity may be determined only from the background portion potential Vb.

  Further, when the environmental value of temperature or humidity changes, the developing bias, the background portion potential, and the amount of exposure light may be reset. In addition, although a table with two variables of temperature and humidity is used as the environmental value, a table with one variable of temperature or humidity may be used. In addition, if the development voltage, background portion potential, and exposure light quantity are determined based on the measured environmental values of temperature and humidity, means other than the table may be used. In this embodiment, one common type of development bias belonging to the normal mode is used. However, a plurality of development biases may be prepared and used depending on conditions.

  As described above in detail, the image forming apparatus 200 according to the present embodiment determines the development bias optimum for the usage environment from various information such as environmental values. Further, by setting the developing bias as described above, the developing bias belonging to the anti-fogging mode and the developing bias belonging to the normal mode are selectively used. Therefore, an image forming apparatus capable of suppressing the occurrence of fog due to deteriorated toner and the accumulation of deteriorated toner in the developing device is realized. Furthermore, it is possible to form an image with an optimum developing bias suitable for various conditions.

  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, specific values such as the developing bias and the surface potential of the photoreceptor 110 used in this embodiment may be different values. Therefore, another value may be used for the background portion potential Vb for the anti-fogging mode. However, it is preferable to set the exposure light amount to a value associated therewith.

  Further, in an image forming apparatus having an intermediate transfer belt, the density sensor 103 may be configured to measure the amount of toner attached on the intermediate transfer belt instead of on the photoreceptor 110. Further, the memory for storing the durable sheet number and DS gap value may be another memory. This is because it is sufficient that the total control unit 101 can read the data. Further, another value may be used as the threshold for setting the fluctuation range ΔV to a certain value or less.

Further, the DS gap can be measured after the image forming apparatus is manufactured. The DS gap can be obtained by a discharge between the photoconductor 110 and the developing roller 140. Here, Paschen's law shown below V = f (ρd)
ρ: Gas pressure (atmospheric pressure in this embodiment)
d: Distance between electrodes is used. The discharge starting voltage is a law expressed as a function of the product of the gas pressure and the distance between the electrodes. The interelectrode distance d corresponds to the DS gap. By performing this measurement at an appropriate frequency, the development bias can be further finely adjusted.

It is a figure explaining the image forming apparatus of a 1st form. It is a figure explaining normal mode (left side) and anti-fogging mode (right side) in the developing bias of the present invention. It is a graph for demonstrating the relationship between a temperature / humidity environment and fog. FIG. 6 is a diagram (part 1) for explaining a force acting on a toner receiving a developing voltage. FIG. 6 is a diagram (part 2) for explaining the force acting on the toner receiving a developing voltage. FIG. 6 is a third diagram illustrating the force acting on the toner receiving a developing voltage. FIG. 6 is a diagram (part 4) for explaining the force acting on the toner receiving the developing voltage. It is a figure for demonstrating the half unevenness and the output value of a density sensor. It is a graph for demonstrating the relationship between a background part electric potential and the rank of a half nonuniformity. It is a flowchart for demonstrating the setting method of fog prevention mode in 1st form, or normal mode. It is a block diagram for demonstrating the setting method of the developing bias in a 2nd form. It is a graph for demonstrating the relationship between a background part electric potential and the fluctuation range of the output value of a density sensor.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100,200 ... Image forming apparatus 101 ... General control part 102 ... Environmental sensor 103 ... Density sensor 110 ... Photoconductor 140 ... Developing roller 190 ... Voltage application part

Claims (4)

An image carrier, an exposure device that forms an electrostatic latent image on the surface of the image carrier, a developing roller that applies a non-magnetic one-component toner to the electrostatic latent image on the image carrier, and the image carrier And a non-contact development type image forming apparatus having a voltage application unit for applying a development bias between the development roller and the development roller.
An environmental sensor that measures at least one of temperature and humidity;
The voltage application unit is:
A developing voltage for forming an electric field in a direction in which toner is caused to fly from the developing roller to the image carrier;
And alternately applying a recovery voltage for forming an electric field in a direction in which toner is caused to fly from the image carrier to the developing roller,
As the development voltage,
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. An antifogging mode using a value at which the electric field strength in the toner does not cause the toner to fly from the developing roller to the image carrier;
The electric field strength in the image portion and background portion of the electrostatic latent image of the image carrier is different from the normal mode using a value that is sufficient for the toner to fly from the developing roller to the image carrier. Yes,
When the measured value of the environmental sensor corresponds to a predetermined condition in which fog is likely to occur, the anti-fogging mode is selected as the developing bias,
In other cases, select normal mode as the development bias,
When the anti-fogging mode is set,
The background portion potential of the image carrier is set to a value closer to the developing voltage of the developing bias than the background portion potential in the normal mode,
An image forming apparatus, wherein an exposure light amount of the exposure device is made larger than a value of an exposure light amount in a normal mode. The image forming apparatus according to claim 1,
The voltage application unit is:
In the anti-fogging mode, the following formula: | Vmin−Vi |> Vpump
| Vmin−Vb | ≦ Vpump
Vmin Development side voltage of development bias
Vi Potential of image portion of image carrier
Vb Potential of background portion of image carrier
Vpump meets the critical flight voltage,
In the normal mode, the following formula: | Vmin−Vb |> Vpump
An image forming apparatus, wherein a developing bias is applied so as to satisfy The image forming apparatus according to claim 1, wherein:
A density sensor for measuring the density of the developed toner image;
A control unit for controlling the voltage application unit,
The controller is
The lower the temperature or humidity acquired by the environmental sensor, the lower the development voltage at the background,
Based on the set development voltage, set the background potential below the predetermined upper limit for each development voltage,
Set the exposure light intensity so that the exposure light intensity increases as the set background potential is farther from the image potential.
From the development voltage set as described above and the density value measured by the density sensor,
If the density is too high, increase the time occupancy of the recovered voltage or increase the difference between the recovered voltage and the image area potential.
An image forming apparatus characterized in that when the density is too low, the time occupancy of the recovered voltage is reduced or the difference between the recovered voltage and the image portion potential is reduced. The image forming apparatus according to claim 1, wherein:
The controller is
When the temperature or humidity changes,
An image forming apparatus characterized by resetting a developing bias, a background portion potential, and an exposure light amount.

Images