JPS6321660A - Contour image forming method - Google Patents

Abstract

PURPOSE:To form a negative reversal contour image whose contour part is white-omitted and to enable electrostatic transfer to normal paper, etc., by performing normal development with insulating charged toner which is electrified to the negative polarity in a developing process. CONSTITUTION:The surface of an electrophotographic drum 1 is given constant- potential charge and then exposed by an exposing device 3 to a positive image. Then, the surface of the drum 1 is applied with a specific voltage by a scorotron charger 4 to perform electrification again, thereby forming an electrostatic latent image. Then, while a specific bias is applied to a developing sleeve 51 from a power source 53, the normal development is carried out with the insulating toner which is electrified negatively in the developing process. Consequently, the insulating toner charged negatively sticks on an image part on the drum 1 which is a high-potential part to sufficient density and sticks on a no-image part as an intermediate-potential part thinly. Namely, the negative reversal contour image whose contour part is white-omitted is formed. Further, the insulating toner is used to improve the electrostatic transfer to normal paper.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a method of forming a negative inverted contour image corresponding to a positive original image, as one of the electrophotographic image forming methods.

BACKGROUND OF THE INVENTION Conventional techniques and their problems In general, contour lines are a part of an image that contains a lot of information, and at the same time sufficiently express the characteristics of the image, and thus play an important role in the image. Furthermore, an image that has been converted into a binary figure by drawing outlines is easier to identify than a normal grayscale image.

Extremely easy to handle in various processes such as determination and transmission,
Converting an image into a binary figure by drawing a contour line is extremely effective in terms of image pattern recognition, image correction and emphasis, bandwidth compression, and the like.

By the way, conventional methods for forming contour images of this type include:
The present applicant has already ranked No. 1 between the maximum and minimum surface potential of the member to be developed in a one-component toner development method in which an electrostatic latent image is developed with a conductive toner. A contour line of the electrostatic latent image on the developing member is extracted by applying a DC bias having a polarity opposite to that of the electrostatic latent image charge between the developing member and the conductive toner carrier. proposed a method (see Japanese Unexamined Patent Publication No. 134635/1983).

However, when a conductive toner is used as in this method, there is a problem in that electrostatic transfer to plain paper is difficult.

On the other hand, when copying images for office use, the entire paper is lightly colored in order to clearly distinguish the copied paper from other papers and to improve aesthetics and contrast. Therefore, as disclosed in Japanese Patent Application Laid-Open No. 58-37665, development is carried out by setting a developing bias for the selected color near the latent image formation level of the background area relative to the selected colored area. An electrophotographic recording method characterized by forming a thin background has been proposed.

However, according to this method, if there is a partial sensitivity unevenness in the photoreceptor, unevenness will occur in the light coloring of the background area, and if the sensitivity of the photoreceptor changes, the density of the light coloring in the background area will be uneven. There is a problem that the coloring varies, and in some cases, the coloring itself cannot be completely formed.

Therefore, the contour image forming method according to the present invention includes: (i) a first charging step of applying a constant electric charge to the surface of an electrophotographic photoreceptor; (i) An exposure step in which a positive image is exposed on the surface of the electrophotographic photoreceptor that has undergone the charging step; a second charging step of recharging while applying a voltage lower than the surface potential of the image area and having the same polarity as the first charging step;
) A developing step of regularly developing the electrostatic latent image formed through the second charging step with a charged toner having a polarity opposite to that of the first charging step.

In other words, a positive image is exposed on the surface of the electrophotographic photoreceptor which has been charged with a constant potential in the first charging step (see (a) in FIGS. 3 and 5), and then in the second charging step. A scorotron charger to which a DC voltage or an alternating voltage of the same polarity as that of the first charging step is applied, and the surface potential of the electrostatic latent image formed on the grid in the exposure step is lower than that of the first charging step. Recharging is performed while applying a voltage of the same polarity.

As a result, the surface potential of the non-image area of the electrostatic latent image rises to approximately the grid voltage, leaving the outline of the image area (see (b) in Figure 3, especially when an alternating voltage is applied to the scorotron charger). When the voltage is applied, the surface potential of the image area also decreases slightly to around the grid voltage, leaving the area near the contour area [see (b) in FIG. 5].

Next, when the electrostatic latent image formed through the second charging step is regularly developed with a charged toner having a polarity opposite to that of the first charging step, the charged toner adheres to the high potential area and the contour area is developed as a reversed contour image of the negative, which is marked as “white blank” [
See (c) in Figures 3 and 5.

On the other hand, the relationship between the voltage applied to the grid of the scorotron charger and the development bias voltage applied to the development electrode is determined by the surface potential of the non-image area of the electrostatic latent image recharged in the second charging step. If the setting is made to be slightly higher than the rising potential at the start of development in the process, the surface potential of the non-image area of the static 1 latent image in the second charging process will be approximately higher than the rising potential of the development characteristic, leaving the outline of the image area. also rises slightly higher [see (b) in Figure 6]. Next, when this electrostatic latent image is regularly developed, the charged toner adheres at a sufficient density to the electrostatic latent image image area, which is a high potential area, and the non-image area of the electrostatic latent image where the surface potential has increased slightly ( A negative inverted contour image with a thin layer of charged toner adhered to the background (background area) and a "blank white" outline can be developed [see (c) in FIG. 6].

Embodiment [First embodiment, see FIGS. 1 to 3] FIG. 1 schematically shows an electrophotographic copying apparatus for carrying out the first embodiment, in which the electrophotographic photosensitive drum (1) is It is a well-known device that has a light guide chamber 1 on its surface, and can be rotated in the direction of arrow (a), and around it are the following members,
The equipment has been installed.

The electrification charger (2) is for performing the first electrification that applies a constant electric charge to the surface of the photoreceptor drum (1). There is.

The exposure device (3) is for forming an electrostatic latent image corresponding to the original image on the surface of the photoreceptor drum (1) using the well-known slit exposure method, and is composed of an exposure lamp, a mirror, a lens, etc. It's broken.

The scorotron charger (4) is connected to the exposure device (3).
) to perform a second charge on the surface of the photoreceptor drum (1) on which an electrostatic latent image has been formed.The charge wire is connected to the power source (41), and the grid (42) is connected to the
．． The power source (43) is disconnected. A DC voltage of the same polarity as the charger (2) is applied from a power source (41) to the charge wire, and a voltage of /1! is applied to the grid (42). X(
43), a voltage lower than the surface potential of the electrostatic latent image area and having the same polarity as the charger (2) can be applied. Note that the voltage applied to the grid (42) is
It is necessary that the surface potential is higher than the surface potential of the non-image area of the electrostatic latent image whose potential has decreased in the exposure device (3).

The developing device (5) is a well-known magnetic brush type device that includes a developing sleeve (51) that includes a built-in magnetic roller (52) with N and S poles magnetized around its periphery, and also functions as a developing electrode. A power source (53) for developing bias is connected to the developing sleeve 51). The developer is made of a mixture of a magnetic carrier and an insulating toner, which are charged to opposite polarities by frictional charging, and the insulating toner is charged to a polarity opposite to that of the charger (2). Here, if the insulating toner does not have magnetism, the developing sleeve (51) is supplied with a power source (53) that has a surface potential that is lower than the surface potential of the non-image area of the electrostatic latent image that has risen in the second charging step. A developing bias having the same polarity as that of the charger (2) is applied. The insulating toner may have magnetism, and the developing bias may be one in which an alternating current voltage is superimposed. If the toner has magnetism, only the insulating toner can be used.

The transfer charger (6) applies an electric field from the back side to the copy paper (10) being conveyed in the direction of arrow (b), and the developing device (5) applies an electric field to the surface of the photoreceptor drum (1). It is used to transfer the formed toner image onto the copy paper (10), and its charge wire is electrically connected! From (61), a voltage having a polarity opposite to that of the insulating toner can be applied.

Separation Char Shaker 7) is used to remove electricity from the copy paper (10) and separate it from the surface of the photoreceptor drum (1) by applying an alternating current electric field to the copy paper immediately after transfer. The wire is marked with AC voltage from tmm).
I can finish JO.

The cleaning device (8) uses a blade method to remove toner remaining on the surface of the photoreceptor drum (1).

The eraser lamp (9) is used to remove charges remaining on the surface of the photoreceptor drum (1) by irradiating light in preparation for the next copying process.

Here, the polarity and voltage of each charger, etc. in the first embodiment will be shown.

Charger [i'FA(21)] Positive polarity +5.5
kV scorotron charger [source (4t)] positive polarity +
5.5kV Grid [power supply (43) positive polarity +500v Grid-photosensitive drum distance (dg) 1.5mm Development bias [power supply (53) positive polarity +300v Transfer charger [source (61)] positive polarity +5.5kV Non-magnetic insulating toner Negative polarity Note that all of these polarities may be reversed,
Of course, the voltage value is only an example.

The outline image forming method using the copying apparatus described above will be explained step by step.

b) First charging step: A charger (2) applies a certain amount of charge to the surface of the photoreceptor drum (1). As a result, in the first embodiment, the surface potential of the photoreceptor drum (1) is +600V.

<;>M light step A positive original image is slit/t exposed on the surface of the photosensitive drum (1) charged to a potential of +600V to form a positive electrostatic latent image. In this case, as shown in (a) in Figure 3,
The charge of the parts corresponding to image areas (A) and (B) is +60
The potential remains at 0V, and the charge in the portion corresponding to the non-image area decreases to about +100V by light irradiation.

(i) Second charging step The surface of the photosensitive drum (1) on which the positive electrostatic latent image is formed is charged with the same polarity as the electrostatic latent image using a scorotron charger (4). Recharge.

At this time, the grid (42) receives +5 from the power supply (43).
A voltage of 00■ can be applied. Scorotron Charge~
The voltage applied to (4) has the same polarity as the first charging step, and the voltage that can be applied to the grid 42) is slightly higher than the surface potential (+600V) of the electrostatic latent image areas (A) and (B). It is low and has the same polarity as the first charging step. In addition,
The tEE that can be applied to the grid <42> is sufficiently higher than the surface potential (+100 V) of the non-image area of the electrostatic latent image. This is because the surface potential of the non-image area of the electrostatic latent image is sufficiently increased in the second charging step relative to the original surface potential.

Electric lines of force shown by arrows in FIG. 2 are formed between the surface of the photosensitive drum (1) and the grid (42).

The positive polarity ions generated from the charge wire are subjected to a transport force along the lines of electric force. In this case, the lines of electric force that direct positive ions toward the surface of the photoreceptor drum (1) in the vicinity of the grid (42) are at the outside of the contour line (A'), ( This occurs only in non-image areas except for B'). Therefore, as shown by the arrows, positive ions are present in the image area (A), the area outside the contour line (A') of (B), and the area CB.
It reaches only the non-image area except for the area '), recharges the area it reaches, and allows it to rise to a potential nearly equal to the grid voltage (Vg).

That is, if we explain the surface potential of the photoreceptor drum (1), as shown in FIG. 3 (b), the image areas (A), (B)
The surface potential of the image area is maintained as a high potential area of approximately +600cm, which is the initial surface potential, and the outer areas (A') and (B') of the image areas (A) and (B) are of a constant width. Almost +1
It is left as a low potential part of 00V, and the part outside the contour line (
The non-image area excluding A') and (B') rises almost to the grid voltage (Vg). Specifically, by applying a voltage of +5.5kV to the scorotron charger power supply (41) and +500■ to the grid power supply (43), +5o
It rises to around ov.

In other words, in this second charging step, the image area (A), (
This means that the outline B) was formed as a negative electrostatic latent image.

(iv) Developing step In the second charging step, the electrostatic latent image formed as a negative image of the outline is developed by a developing device (5). At this time, a developing bias of +3 GOV is applied to the developing sleeve (51). This developing bias voltage (vb) is
In order to ensure that the toner adheres to the non-image area of the electrostatic latent image whose surface potential has increased in the second charging process (of course also to the image area), the voltage should be sufficiently higher than this non-image area (+500V) where the surface potential has increased to the highest level. It is low and has the same polarity as the first charging step.

In this way, the negatively charged insulating toner is applied to the high potential area of the photoreceptor drum (1), that is, the image area (A).

A negative inverted contour toner image, which is attached to the outside of the contour line in (B) <A゛) and other than (B'), is formed by regular development, so to speak, as an "outer edging".

This toner image is then transferred onto the copy paper (10) by positive discharge from the transfer charger (6), and is formed as a copy image via a fixing device (not shown).

By the way, since the insulating toner used in the first embodiment has a sufficiently high electrical resistance, it can be electrostatically transferred well even to plain paper in the transfer process. This point also applies to the second embodiment described below.

In addition, in the first embodiment, the distance (dg) between the surface of the photoreceptor drum (1) and the grid (42) (see FIG. 2)
is said to be 1.5 mm. This value is larger than the setting of the scorotron charger conventionally used in this type of electrophotographic copying machine.

This is to increase the extent to which the lines of electric force are directed toward the surface of the photoreceptor in the edge part (contour) of the area image area (A>) or the line image area (B), and to obtain a good contour image. The state changes depending on the distance II (dg).

That is, by changing the distance 11 (dg), it is possible to arbitrarily change the width of the contour line obtained as an image, and as the distance (dg) increases, the width tends to increase. This point will be explained in the second section below.
The same applies to the embodiments.

[See Second Embodiment, Figures 4 and 5] This second embodiment differs from the first embodiment in that an alternating voltage is applied from the power supply (41') to the scorotron charger (4) that performs the second charging. 4 corresponds to FIG. 2, and FIG. 5 corresponds to FIG. 3, respectively. Therefore,
The differences will be explained below.

The scorotron chart (4) is an exposure device (3
) An electrophotographic photoreceptor drum (1) on which an electrostatic latent image is formed
) is used to perform a second charging number on the surface of the grid (41'), and the electric fi (41') is connected to the charge wire, and
2) is connected to a power source (43). An alternating voltage is applied to the charge wire from the power supply (41°), and an electric current fi (41°) is applied to the grid (42) as in the first embodiment.
3), a voltage lower than the surface potential of the electrostatic latent image area and having the same polarity as the charger (2) can be applied. Note that the voltage applied to the grid (42) is
It is necessary that the surface potential is higher than the surface potential of the non-image area of the electrostatic latent image whose potential has decreased in the exposure device (3).

In addition, the polarity and voltage of each charger in the second embodiment are the same as those shown in the first embodiment, except for the scorotron charger (4).
) (7) Voltage is AC±6. OkV There is.

Next, the contour image forming method will be explained step by step.

(i) First charging step This is the same as in the first embodiment, and the surface potential of the photosensitive drum (1) is +600V.

(i) Exposure step The process is the same as that of the first embodiment, and as shown in FIG.
The potential remains at 600V, and the charge in the portion corresponding to the non-image area decreases to about +100V by light irradiation.

(i) Second charging step The surface of the photoreceptor drum (1) on which the positive electrostatic latent image has been formed is re-charged with a scorotron charger (4) to which an alternating voltage is applied from the power source (41'). Become electrically charged. At this time, the grid (42) receives +5oov from the power supply (43).
voltage can be applied. The voltage that can be applied to this grid (42) is the same as in the first embodiment.

Electric lines of force shown by arrows in FIG. 4 are formed between the surface of the photosensitive drum 1) and the grid (42).

Ions of negative and positive polarity generated from the charge wire to which an alternating voltage is applied are subjected to a transport force along the lines of electric force. In this case, the lines of electric force that direct positive ions toward the surface of the photoreceptor drum (1) in the vicinity of the grid (42) are located outside the contour lines (A') of the image areas (A) and (B).
, (B"). Therefore, as in the first embodiment, positive ions are generated only in the non-image areas except for the image areas (A) and (B") as shown by the arrows →. Outer part (A')
, (B') and reaches only the non-image area, and the area it reaches is recharged and raised to a potential approximately equal to the grid voltage (Vg). In addition, as shown by the arrow →, the negative ions reach only the part (center part) excluding the inner part of the outline of the image area (A), eliminate the charge in the reached part, and almost reach the grid voltage (Vg). It can be slightly lowered to near the same potential.

That is, if we explain the surface potential of the photoreceptor drum (1), as shown in FIG. 6 (b), the image areas (A), (B)
The inner part of the contour line remains as a high potential part of approximately +600cm, which is the initial surface potential, and the image parts (A) and (B
) outside the contour line (A'), (B''I are left as a low potential part of approximately +100V with a constant width, and the surface potential of the non-image area is outside the contour line of the image area (A), (B) Part (A'>,
(B') and rises to almost the grid voltage (Vg), and the surface potential at the center of the surface image area (A>) slightly reaches around the grid voltage (Vg) except for the inner part of the contour line. Specifically, ±6.OkV to the scorotron charger power supply (41'), grid power supply (
43> by applying a voltage of +5oov to the surface image portion (
The surface potential of the central portion of A) and the surface potential of the non-image area decrease and increase to around +5oov.

In other words, in the second embodiment as well, the contours of the image areas (A) and (B) are formed as negative electrostatic latent images in the second charging step.

(iv) Developing step In the second charging step, the electrostatic latent image formed as a negative image of the outline is developed by a developing device (5). The development conditions and development mechanism here are the same as those in the first embodiment, but in the case of the second embodiment, the center of the electrostatic latent image surface image area (A) is Even if the potential of the area slightly drops to approximately equal to the grid voltage (Vg), toner will not adhere to the area and cause fogging.

[Third embodiment, see FIG. 6] This third embodiment is a partial modification of the first embodiment, in which the electrostatic latent image non-image area recharged in the second charging step The voltage applied to the grid (42) of the scorotron charger (4) and the development bias voltage applied to the development sleeve (51) are adjusted so that the surface potential of The relationship was established. Specifically, the voltage of the grid power supply (43) was +33GV, and the voltage of the developing bias power supply (53) was +30GV.
0■, and the other conditions are the same as in the first embodiment.

Therefore, both the first charging step and the exposure step are the same as those in the first embodiment, and the second charging step is performed by connecting the scorotron charger to +5.5 kV to the source (41) and the grid power source (41).
3) under the application of a voltage of +330μ.

In this second charging step, the state of the electric lines of force formed between the surface of the photoreceptor drum (1) and the grid (42) is the same as that shown in FIG. 2, and positive ions generated from the charge wire. As shown by the arrow →, the image area (A),
It reaches only the non-image area excluding the outside parts (A゛) and (B') of the outline in (B), and recharges the reached part to raise the potential to approximately the same as the grid voltage (Vg: +330V). . That is, when explaining the surface potential of the photoreceptor drum (1), as shown in FIG. 6(b), the image area (A),
The surface potential of (B) is approximately +600■ which is the initial surface potential.
It remains as a high potential part of the image part (A), (B
) outside the contour line (A') and (B') are +1 with a constant width
00
'>, the non-image area excluding (B') is almost at the grid voltage (
The voltage rises to around Vg), which is considered to be a medium potential region of +330■.

In the next development step, the developing sleeve (51) is charged with +3+3 from the power source (53) using negatively charged insulating toner.
Regular development is performed under application of a development bias of 00V. The developing bias voltage (vb) is the grid voltage (
Vg: +330V) and has the same polarity as the first charging step.

As a result, the negatively charged insulating toner adheres in sufficient density to the image areas (A) and (B), which are high potential areas, of the photoreceptor drum (1), and the non-image areas, which are medium potential areas. It adheres thinly to some parts. That is, the contour line outer parts (A') and (B') are "
A negative inverted contour image is formed by regular development, in which the background part of the positive original image is lightly colored.

By the way, in the present invention, as the second charging step,
Since "processing" is added to the electrostatic latent image formed in the exposure process, even if the amount of exposure light varies, its adverse effects are eliminated. The present inventors conducted an experiment in which the amount of exposure light in the exposure process was varied, and found that, for example, the amount of exposure light was increased until the non-image area decreased to approximately +70V,
Even when the voltage was decreased to +150V (both image areas were +600V), the background area of the original image was colored uniformly and lightly, and its density did not change at all.

Here, the relationship between the grid voltage (Vg) and the developing bias voltage (vb) will be explained along with an explanation of such experimental results.

In this third embodiment, a scorotron charger (
In the second charging step (4), the surface potential of the non-image area is raised to a constant potential. The voltage will be approximately equal to the grid voltage (Vg). That is, as in the present third embodiment or the above-mentioned experimental example, the surface potential of the non-image area is +70 to +70.
Even if the voltage changes to about +150V, the surface voltage of the non-image area in the development process after the second charging process always rises to +330V, which is almost equal to the grid voltage (Vg), and as a result, the toner becomes thin. As a result, the density of the background area remains unchanged.

Next, the grid voltage (Vg) and the developing bias voltage (vb
). In the normal magnetic brush development method, the development characteristics start from the surface potential of the photoreceptor which is equal to the development bias voltage (vb).

When a normal scorotron charger is used for the second charging step, the surface potential of the photoreceptor is approximately the grid voltage (V
g), in order to lightly color the background portion of the original image, the grid voltage (Vg) may be set to a value slightly higher than the developing bias voltage (vb). By appropriately selecting the difference, it is possible to change the density of the background area, which is uniformly and lightly colored.

However, depending on the development system, the development characteristics may rise from a value different from the development bias voltage (Vb), and depending on the conditions of the scorotron charger, the surface potential may be recharged to a slightly higher or lower surface potential than the grid voltage (Vg). Sometimes.

Therefore, the grid voltage (Vg) and the developing bias voltage (
The value of vb) should be set in accordance with these conditions, and should be set so that the development characteristics start at a potential slightly lower than the surface potential of the background area recharged by the scorotron charger.

[Other Examples] By the way, since the development by the developing device (5) is based on the regular development method, the Suflotron Charge-r
If the second charging step (4) is not performed, the positive electrostatic latent image formed in the exposure step will be developed as normal regular development, and a normal positive image corresponding one-to-one to the original image will be obtained. be able to.

This is the scorotron charger (4) on.

It can be easily controlled by switching off.

As is clear from the above description, according to the present invention, as a second charging process, the surface of the electrophotographic photoreceptor that has undergone the positive image exposure process is subjected to the grid exposure process using a scorotron charger. By applying a voltage lower than the surface potential of the electrostatic latent image formed in the image area and having the same polarity as that of the first charging step, recharging is performed, so that the outline of the original image remains on the surface of the electrophotographic photoreceptor. A negative electrostatic latent image is formed in which the area remains as a low potential area, and in a further development process, the electrostatic latent image is regularly developed with a charged toner having a polarity opposite to that of the first charging process. In this regular development, toner adheres to the high potential portions of the electrostatic latent image, and a negative inverted contour image in which the contour portions are "blank" can be obtained. Moreover, since an insulating toner is used, electrostatic transfer to plain paper can be performed well.

Further, as in the third embodiment, the surface potential of the non-image area of the electrostatic latent image re-charged in the second charging step is made to be slightly higher than the rising potential of the development characteristics in the development step. By setting the relationship between the grid voltage and the developing bias voltage, it is possible to obtain a negative inverted contour image in which the background portion of the original image is lightly colored. Furthermore, this background portion can be reliably formed at a constant low density without being affected by local sensitivity unevenness of the photoreceptor or variations in the amount of exposure light.

[Brief explanation of the drawing]

1 to 3 are for explaining a first embodiment of the present invention, and FIG. 1 is a schematic configuration diagram of an electrophotographic copying apparatus;
FIG. 2 is a schematic diagram of the lines of electric force in the second charging step, and FIG. 3 is a graph showing the potential of the electrostatic latent image in each step. FIG. 4 is a schematic diagram of the lines of electric force in the second charging step in the second embodiment of the present invention, and FIG. 5 is a graph showing the potential of the electrostatic latent image in each step. FIG. 6 is a graph showing the potential of the electrostatic latent image in each step in the third embodiment of the present invention. (1)... Electrophotographic photosensitive drum, (2)... Charger, (21)... Power supply, (3)... Exposure device, (4>... Scorotron charger, (41)... )
, (41゛)...1! source, (42)...grid, (43)...power supply, (5)...developing device, (51
)...Developing sleeve, 53)...Power supply.

Claims (1)

[Scope of Claims] 1. A first charging step of applying a constant electric charge to the surface of the electrophotographic photoreceptor; and an exposure step of exposing a positive image to the surface of the electrophotographic photoreceptor that has undergone the first charging step. , Applying a voltage lower than the surface potential of the electrostatic latent image area formed in the exposure step to the grid using a scorotron charger to the surface of the electrophotographic photoreceptor that has undergone the exposure step, and having the same polarity as the first charging step. a second charging step of recharging, and a developing step of regularly developing the electrostatic latent image formed through the second charging step with charged toner having a polarity opposite to that of the first charging step. A contour image forming method comprising the following steps. 2. Apply to the grid of the scorotron charger so that the surface potential of the non-image area of the electrostatic latent image recharged in the second charging step is slightly higher than the rising potential of the development characteristics in the development step. 2. The contour image forming method according to claim 1, wherein a relationship between the voltage and a developing bias voltage applied to the developing electrode is set.