JPS6321657A - Image forming method - Google Patents

Abstract

PURPOSE:To form a spuriously inverted negative image, a normal positive image, and a positive contour images by using a positive original image by performing image formation through 1st and 2nd electrostatic charging processes, an exposing process, and a normal developing device. CONSTITUTION:In the 1st electrostatic charging process in inverted image formation mode, an electrostatic charger 2 gives constant-potential charges to the surface of a photosensitive drum 1 and then the surface is exposed to the positive original through a slit and a screen 31 to form an electrostatic latent image. Then, the surface of the drum 1 is given with charges having the opposite polarity from the electrostatic latent image by a scorotron charger 4 for reelectrification to form the spuriously inverted negative images, which is developed normally by a developing device 5. Consequently, the spuriously inverted negative image is formed from the positive original image, and image forming processes are changed properly to form the normal positive image and positive contour image.

Description

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

BACKGROUND OF THE INVENTION Normally, electrophotography uses a process to obtain a positive copy image, and for positive original images (electrostatic latent images), regular development is used to attach toner to high potential areas on the surface of the photoreceptor. , and for negative original images (electrostatic latent images), reversal development is performed in which toner is attached to low potential areas on the surface of the photoreceptor.

In this case, since the charge polarity of the toner is different between normal development and reversal development, in an image forming device that can selectively use the two development methods, two types of development devices are installed side by side along the surface of the photoreceptor. The operation is selectively changed over, and the two types of developing devices are replaced one by one for development.

On the other hand, it is conceivable to form a copy image by inverting a positive original image to a negative one, but if you want to perform reversal development for this purpose, two types of developing devices should be prepared: one for regular development and one for regular development to perform positive-positive copying. I have no choice but to do it.

By the way, the present applicant has proposed a new contour image forming method in electrophotography that can obtain a positive contour image through a series of image forming steps (Japanese Patent Application No. 37829/1983).
No. 61-37830, etc.). Subsequently, while developing and experimenting with this method, it was discovered that the halftone dot-like cumulative image was partially reproduced as an inverted image.

The present invention has been developed as an application of the contour image forming method, and is capable of forming a pseudo-inverted negative image from a positive original image, and by selectively changing the image forming process. It is an object of the present invention to provide an image forming method capable of forming a normal positive image and also forming a positive outline image.

The image forming method according to the present invention includes (i) a first charging step of applying a constant electric charge to the surface of the child photographic photoreceptor; and (i) electrons that have passed through the first charging step. an exposure step in which a positive original image is exposed onto the surface of the photographic photoreceptor through a screen having colored halftone dots, lines, etc. on a transparent substrate; A second charging T step in which the grid is re-charged while applying a voltage sufficiently lower than the surface potential of the electrostatic latent image portion formed in the exposure step to the grid and having the same polarity as the first charging step. and ('N) 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. shall be.

In other words, a positive original image is exposed through the screen onto the surface of the electrophotographic photoreceptor, which has been charged with a constant potential in the first charging step. As a result, a minute potential pattern is formed in the non-image area by the pattern-corresponding part of the screen and the light-transmitting part [see (c) in Figure 7], and then the first charging step is performed in the second charging step. A scorotron charger to which a DC voltage or an alternating voltage of opposite polarity is applied is used to charge the grid with a surface potential that is sufficiently lower than the surface potential of the electrostatic latent image area formed in the exposure process and the same as that of the first charging process. Recharging is carried out while applying a polar voltage. As a result, the surface potential of the electrostatic latent image area is reduced to approximately the grid voltage, leaving the contour area [see (d) in Figure 7]. In particular, when an alternating voltage is applied to the scorotron charger, the surface potential of the light-transmitting portion of the screen in the non-image area also rises slightly to approximately the grid voltage [see (a) in FIG. 13].

Next, the electrostatic latent image formed through the second charging step is regularly developed using a charged toner having a polarity opposite to that of the first charging step. As a result, charged toner adheres to the high-potential area, and the positive original image is developed as a pseudo-reversed negative image (see (e) in FIG. 7 and (b) in FIG. 13).

Further, the high potential portion (A') in FIG. 7(d) and FIG. 13(a) is the outline of the original image. Therefore,
If the image forming step is performed by exposing the image without passing through a screen, a contour image of the original image can be obtained as a positive image.

Furthermore, since regular development is performed in the present invention, if a positive image is exposed without passing through a screen and the second charging step is omitted, the image corresponds one-to-one to the original image. Ordinary standard copying is also possible.

[Refer to Embodiment 1, Figures 1 to 11] Figure 1 is from Book 1.
An outline of an electrophotographic copying apparatus for carrying out the embodiment is shown, and the electrophotographic photosensitive drum (1) is a well-known type having a photoconductive layer on the outer peripheral surface, and can be driven to rotate in the direction of arrow (a). The members and devices described below are installed around it.

The electrification charger (2) is for performing a first electrification that applies a constant electric charge to the surface of the photoreceptor drum (1), and a power source (21) is connected to the charge wire.

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 a well-known slit exposure method, and includes an exposure lamp (37).

It consists of mirrors, lenses, etc. Further, directly below the document table glass (33), a screen (31) having a fine colored line pattern on a transparent substrate in a direction perpendicular to the document scanning direction is placed in the exposure optical path with the support axis 32〉 as a fulcrum. It is set up so that it can be moved forward and backward. This screen (31) is movable together with the exposure lamp (37), and enters into the exposure optical path to apply partial pressure for image exposure during the inversion image forming mode described below, and during the contour image forming mode and standard copying. In the mode, it retreats from the exposure optical path. The screen (31) is attached to the photoreceptor drum (
1) is for forming a minute potential pattern in the non-image area of the electrostatic latent image formed on the surface. As shown in Fig. 1, in a format in which a screen (31) is placed in the exposure optical path and the screen (31) is moved relative to the original, line On the screen, the pattern direction must be perpendicular to the document scanning direction.

Even in the case of a line screen, when the pattern direction is made to match the document scanning direction, or when a halftone screen is used, the following two methods can be considered to prevent image defects caused by the pattern. The first method is to install a screen between the document and the document table glass (33). The second method, as shown in FIG.
, (36) and move it in synchronization with the document table glass (33). In this case, in addition to the screen (31) having a colored line pattern whose pattern direction is Wl and coincides with the document scanning direction in the recess shown in FIG.
As shown in the figure, a screen (31) having a transparent substrate with a fine colored halftone dot pattern can be used. That is,
When the inverted image formation mode is selected, as the original platen glass (33) moves in the direction of arrow (c) during exposure,
The screen (31) is wound up onto the drum (36), and the screen (31) is thus formed on the surface of the photoreceptor drum (1).
The original image is exposed together with the pattern 31). The screen (31) is rewound onto the drum (35) when the document table glass (33) returns, and is prepared for the next copying operation. Further, the end portion of the screen (31) may be a transparent portion without a pattern, and image exposure may be performed through the transparent end portion in the outline image forming mode or the standard copying mode.

In the following description, an example will be explained in which a line screen is provided such that its pattern direction is orthogonal to the document scanning direction. In this case, the line screen can be installed at any location in the exposure optical path.

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 a static iE latent image has been formed.The charge wire is connected to the power supply (41), and the grid (42) is connected to the power supply. (43) is connected. A DC voltage of opposite polarity to the charger (2) is applied from a power source (41) to the charge wire, and a power source (43) is applied to the grid (42).
Therefore, a voltage sufficiently lower than the surface potential of the electrostatic latent image area and having the same polarity as the charger (2) can be applied. In addition, the voltage that can be applied to the grid (42) is
It is necessary that the surface potential of the electrostatic latent image non-image area (low potential area) 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 of a well-known magnetic brush type and includes a developing sleeve (51) with a built-in magnetic roller (52) with north and south poles on its periphery, and also functions as a developing electrode. A power source for developing bias (<53) is connected to the developing sleeve (51). The developer is made of a mixture of a magnetic carrier and an insulating toner, and is charged to opposite polarities by the friction band 1, and the insulating toner is charged to an opposite polarity to the polarity of the charger (2). Here, if the insulating toner does not have magnetism, the developing sleeve (
51) is on the 1st floor! (53), a developing bias which is slightly higher than the surface potential of the electrostatic latent image area lowered in the second charging step and has the same polarity as the charger (2) is applied. The insulating toner may have magnetism, and a bias of a voltage slightly lower than the surface potential of the electrostatic latent image area lowered in the second charging step is applied to the developing sleeve (51). It is also possible to apply a developing bias in which an alternating voltage is superimposed, and if the toner has magnetism, it is possible to use only insulating toner.

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 copy paper (10), and a voltage of opposite polarity to that of the insulating toner is applied to the charge wire from a power source (61).

The luminescent charger (7) applies an alternating current electric field to the copy paper immediately after transfer, thereby eliminating electricity from the copy paper (10) and peeling it off from the surface of the photoreceptor drum (1). An alternating current voltage is applied to the charge wire from a power source (71).

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.

FIG. 3 shows the operation panel (100) of the present copying apparatus, (
101) is the standard copy mode selection key, (104) is its display lamp, (102) is the contour image formation mode selection key, (105) is its display lamp, (103) is the reverse image formation mode selection key, <106) is its indicator lamp. (LIO> is a print key, (111) is a numeric keypad, and fl12) is a clear/stop key.

(Li2) is a slide lever for adjusting image density.

The control is performed by a microcomputer (1
20) and each power supply (41).

(43) On/off control of the motor (34) etc. that drives the forward and backward movement of the screen (31) is performed. In addition, each mode selection key (101), (102), (103
) operations are SWIN f (SWI), (SW2), (
SW3) CI') is input to the microcomputer (120) as an on/off signal.

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

[I] When using non-magnetic insulating toner, charger [source (21)] Positive polarity +5.5kV Scorotron charger [1lj (4t)] Negative polarity -6, OkV Grid [Electrical! (43)] Positive polarity +200v Grid-photosensitive drum distance (dg) 1.5 nm Developing bias [ILE source (53)] Positive polarity +300■ Transfer charger [Power source (61) Positive polarity +5.5 kV Non-magnetic insulating toner Negative polarity [I[] When using magnetic insulating toner Charger [$E source (21)] Positive polarity +5.5kV Scorotron charger [source (at)] Negative polarity -
6, OkV Grid [Electric fi (43)] Positive polarity +200 ■ Grid-photosensitive drum distance (dg) 1.5 mm Developing bias [Power supply (53)] Positive polarity (DC) + 1,70 V (AC) 350 Vrms, 1 kHz Development start potential +250 Transfer charger [power supply (61) positive polarity +5.5 kV Magnetic insulating toner negative polarity Note that all of these polarities may be reversed,
Of course, the voltage value is only an example.

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

(Reverse Image Forming Mode) This reverse image forming mode is executed by operating the selection key <103) and turning on the switch (SW3). At this time, if the screen (31) is in the retracted position, the motor (34) is allowed to rotate and enter the exposure optical path. In addition, the scorotron charger (
Control the electric currents (41) and (43) so that 4) operates. At the same time, the display lamp (106) is turned on.

(i) First charging step A charger (2) applies a constant electric 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.

(i)! Light process A positive original image is slit-exposed to the surface of the photosensitive drum (1) charged to a potential of +600V through a screen (31) to form an electrostatic latent image. In this case, if the positive W, draft image is exposed without going through the screen (31), the image area (A) will be exposed as shown in (a) in Figure 7.
The charge of the part corresponding to remains as a potential of +600■,
The charge of the part corresponding to the non-image area is +100V when irradiated with light.
decreases to a certain degree. On the other hand, since the screen (31) is a transparent substrate with lines or halftone dot patterns that do not transmit light such as black, when exposing a white original through the screen (31), the screen (31) shown in FIG. As shown in b), a minute potential pattern corresponding to the pattern is completed.

Therefore, when the Bon original image is exposed through this screen (31), the image area (
The charge in the portion corresponding to A) is not affected by the pattern of the screen (31) and remains as it is at a potential of +600V. In addition, in the non-image area, the screen (31
) remains as a potential of +600V, and the charge of the transparent substrate portion, that is, the light-transmitting portion, decreases to about +100V upon irradiation with light.

(i) Second charging process The surface of the photoreceptor drum (1) on which the electrostatic latent image has been formed is re-charged by applying an electric charge of opposite polarity to that of the electrostatic latent image using a scorotron charger (4). do. At this time, the grid (
42) is applied with a voltage of +200V from the power supply (43). The voltage applied to the scorotron charger (4) is of opposite polarity to the first charging step, and the voltage applied to the grid (42) is applied to the electrostatic latent image area (A) and the pattern corresponding area (C). ) is sufficiently lower than the surface potential (+600V) and has the same polarity as the first charging step. Note that the voltage applied to the grid (42) is higher than the surface potential (+100 V) of the light-transmitting portion of the non-image portion of the electrostatic latent image.

Electric lines of force shown by arrows in FIG. 6 are formed between the surface of the photosensitive drum (1) and the grid (42).

Then, the negative polarity ions generated from the charge wire are subjected to a transport force along the lines of electric force. In this case, the electric lines of force that direct negative ions toward the surface of the photoreceptor drum (1) near the grid (42>) occur only in a portion other than the outline of the image area (A) (center portion). Therefore, the negative ions reach only the central part of the image area (A> as shown by the arrow →, and remove the charge in the area where they reach, increasing the grid voltage (Vg
).

That is, in terms of the surface potential of the photoreceptor drum (1), as shown in (cl) in FIG. is left as a high potential area with a constant width of approximately +600cm, which is the initial surface potential, and the light transmitting area of the non-image area is approximately +1
It remains as a low potential portion of 00V, and the central portion of the image portion (A) is reduced to approximately the grid voltage (Vg). Specifically, the surface potential of the central part of the image area (A) is
To scorotron charger power supply (41) (7)-6,
OkV, when a voltage of +200V is applied to the grid power supply (43), it decreases to around +200V 6 In other words, in this second charging process, the non-image area becomes screen <3
It has a minute potential pattern corresponding to the pattern 1), and the central part of the image area (A) is neutralized, that is, a positive original image is formed as a pseudo-inverted negative electrostatic latent image. It happens.

(iv) Developing step In the second charging step, the electrostatic latent image formed as a pseudo negative image is developed in a developing device (5). If the insulating toner does not have magnetism, the developing sleeve (51
) is applied with a developing bias of +300V. The voltage (vb) of this developing bias is such that the toner is attached to the central part of the electrostatic latent image area (A) where the surface potential has decreased in the second charging process (of course also to the light-transmitting part of the non-image area). In order to avoid overlapping, the central molecule position (+
200V) and has the same polarity as the first band T error.

On the other hand, when the insulating toner has magnetism, the developing sleeve (51) has AC350V, 1kHz and DC+17V.
A developing bias of 0V is applied. The developing bias voltage (Vb) is lower than the first position in the central part of the image area (A) where it has decreased to a value approximately equal to the grid voltage (Vg).
+200V). However, when an insulating toner having magnetism is used, since there is a threshold value due to magnetic binding force, development starts when the surface potential is around +250V. Therefore, toner does not adhere to and cover the central part of the electrostatic latent image area (A) (of course, the light-transmitting part of the non-image area), which has deteriorated in the second 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 contour area (A) of the image area (A).
゛) and adheres to the pattern-corresponding part (C) of the non-image area,
A so-called "pseudo-inversion" toner image can be formed by regular development.

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, the reason why the grid voltage (Vg) is made sufficiently lower than the surface potential (+600V) of the electrostatic latent image area is that the surface potential of the central part of the image area is set to be lower than the original surface potential in the second charging step. This is to reduce the temperature sufficiently.

Furthermore, as for the copied image, the non-image area does not become a complete high-density area, but becomes a minute density pattern corresponding to the pattern of the screen (31). However, the screen (31)
If a pattern with sufficiently fine spacing between lines and halftone dots is used, it can be visually recognized as a background with uniform density. However, if the pattern spacing is too small, a problem will occur as described below.

The first problem is that, as shown in FIG. 8, if the adjacent pattern-corresponding parts (C) are too close together, the lines of electric force cannot be directed toward the surface of the photoreceptor due to mutual repulsion in the second charging process. Since the non-image area is biased towards the grid, the charge is removed from the non-image area.

The second problem is that due to the optical performance and mechanical vibrations of the exposure device (3), if the pattern spacing is too small, it becomes difficult to form a clear latent image with a fine pattern on the surface of the photoreceptor. caused by. In this case, as shown in FIG. 9(a), the electrostatic latent image formed in the exposure process has almost no decrease in charge in the light-transmitting part in the non-image area, and in the pattern-corresponding area (C). Therefore, as shown in FIG. 9(b), not only the image area (A) but also the non-image area is neutralized in the second charging step.

According to experiments by the inventors, the spacing between lines and halftone dots is 50
Fineness on the order of μm was the limit at which a good inverted image could be obtained. Further, the limit of coarseness is about 300 μm, and if the spacing is larger than that, the pattern in the non-image area can be easily resolved visually, and some deterioration in image quality was observed. Further, the line width and dot diameter were preferably about 50 to 300 μm.

It should be noted that these numerical values depend on various setting conditions, various performances of the copying apparatus, etc., and are, of course, merely examples.

(Contour image forming mode) In this contour image forming mode, only the contour part of a positive original image is formed as a positive copy image, and the switch (Sen2) is turned on by operating the selection key (102). It can be executed by being given. At this time, the screen (31
) has entered the exposure optical path, the motor (34) is rotated to retreat. In addition, a power supply (41), (
43) is controlled in the same way as in the inverted image forming mode. At the same time, the display lamp <105) is turned on.

(i>first charging step This is the same as in the reverse image forming mode.

(i)! Light process A positive original image is slit-exposed to the surface of the photosensitive drum (1) charged to a potential of +600V without passing through the screen (31) to form a positive electrostatic latent image. In this case, as shown in FIG. 11(a), the image area (A
), (B) remains as a potential of +600■, and the charge of the part corresponding to the non-image area decreases to about +100■ upon irradiation with light.

(i) Second Charging Step The surface of the photosensitive drum (1) on which the positive electrostatic latent image is formed is recharged by a scorotron charger (4).

The operating conditions of the scorotron charger (4) in this step are the same as in the inversion image forming mode. However, the appearance of the electrostatic latent image formed is different. That is, lines of electric force shown by arrows in FIG. 10 are formed between the surface of the photosensitive drum (1) and the grid 42). Then, the negative 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 negative ions toward the surface of the photoreceptor drum (1) in the vicinity of the grid (42) are located in areas other than the outline of the surface image area (A) (
Therefore, the negative ions reach only the center part of the surface image area (A) as shown by the arrow →, eliminate the charge in the area they reach, and almost reach the grid voltage (Vg). The potential can be lowered to around the same potential.

To explain this as a surface potential of the photoreceptor drum (1), as shown in (b) in FIG. With a width of , the surface potential of the non-image area can remain as a high potential area of approximately +600■ which is the initial surface potential, and the surface potential of the non-image area can remain as a low potential area of approximately +100■, and the central part of the surface image area (A). decreases to approximately grid 1 pressure (Vg).

Note that although the surface potential of the line image portion (B″) hardly decreases, the width of the charge is slightly narrowed as a contour portion.

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

(iv) Developing step In the second charging step, the electrostatic latent image formed as a positive image of the contour portion is developed by a developing device (5).

The developing conditions in this step are the same as those in the reverse image forming mode. Therefore, the negatively charged insulating toner is applied to the high potential area of the photoreceptor drum (1), that is, the contour area (A'
), (B'), and a so-called "inner border" toner image is formed by regular development.

(Standard copy mode) In this standard copy mode, normal copying is performed in which a copy image is formed in one-to-one correspondence with the original image, and the switch (SWI) is turned on by operating the selection key (101). It is executed by At this time, if the screen (31) enters the exposure optical path, the motor (31)
4) Allow it to rotate and retreat. At the same time, the display lamp (1
04). Please note that the power supply (4t>,
(43) is different from the above two modes in that it is controlled.

(i) First charging step This is the same as in the above two modes.

(i) 1 light step The same as in the contour image forming mode [see FIG. 11 (
a) Reference.

(i) Second charger a The broken positive electrostatic latent image reaches the next development step as it is.

(■) Development process The same as in the above two modes. As a result, the negatively charged insulating toner adheres to the image areas (A) and (B) shown in (a) in FIG. 11, and a normal toner image corresponding one-to-one to the original image is formed. It can be formed by regular development.

[111] A et al. show an example in which the conditions of the scorotron charger (4) in the second charging step are changed when non-magnetic insulating toner is used in the first embodiment. The above [1] when using insulating toner
] Among the conditions, change the following items.

Scorotron charger [iJ source (41)] Negative polarity -7,0kV grid [Wataru! (43>Positive polarity +300V Grid-photoreceptor drum distance (dg) 1.0mm Developing bias [power supply (53)] Positive polarity +250V The image forming process was performed under the above CI], (:If) conditions in all three modes. This is the same as in .

However, in this variation, the electric! Increase the voltage of (41),
% between the grid (42) and the surface of the photoreceptor drum (1)
(dg) was made narrower, the scorotron charger (
4) The static elimination ability increases, and the potential at the center of the image area (A) is +230, which is slightly lower than the grid voltage (Vg).
It decreases to around v. Then, the developing bias voltage (v
b) is set to +250V, which is lower than the grid voltage (Vg) of +300V, but since the reduced surface potential is lower than the developing bias voltage (Vb), the image area (A
) There is no chance that toner will stick to the center of the image and cover it.

In the first embodiment described above, the photoreceptor drum (
The distance 1fi (dg) between the surface of 1) and the grid (42) (
Figure 6, 10r5! J) is 1.5mm or 1.0
It is cut to mm. This value is larger than the setting of the scorotron charger conventionally used in this type of electrophotographic copying machine. This is the surface image part (A)
This is to increase the extent to which electric lines of force are directed toward the surface of the photoreceptor in the edge portion (contour), the line image portion (B), or the pattern-corresponding portion (C) of the non-image portion, thereby obtaining a good contour image. The state of the electric lines of force changes depending on the disintegration (dg). That is, by changing the distance J (dg), it is possible to arbitrarily change the width dimension of the contour line obtained as an image, and the distance 11! As (dg) increases, the width dimension also tends to increase.

Note that this point also applies to the second embodiment described below.

[Second Embodiment, Refer to Figures 12 to 15] This second embodiment is different from the first embodiment in that the scorotron charger (4) for performing the second charging is connected to an alternating box from 'xi (41'). The difference is that voltage is applied.

The polarity and voltage of each charger, etc. in the second embodiment are the same as those shown in [I] and [I[] in the first embodiment.
The voltage of i (41') is AC±6.0 kV both when using an insulating toner that does not have magnetism and when using an insulating toner that has magnetism.

Next, the image forming method will be explained step by step for each mode.

(Reversal Image Forming Mode) (1) First Charging Step One charge of a certain amount is applied to the surface of the photosensitive drum (1) by the charging charger (2). In the second embodiment as well, the surface potential of the photosensitive drum (1) is +600V.

(i) Exposure step A positive original image is slit-exposed to the surface of the photosensitive drum (1) charged to +600V through a screen (31) to form an electrostatic latent image. In this case, the electrostatic latent image is in the same potential state as shown in (C) in FIG. 7 above.

(i) Second charging step The surface of the photoreceptor drum (1) on which the electrostatic latent image has been formed is recharged by the scorotron charger (4) to which an alternating voltage has been applied from the power source (41°). . At this time, a voltage of +200■ is applied to the grid (42) from the power supply (43), similar to [I, [■] in the first embodiment.

Electric lines of force shown by arrows in FIG. 12 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 negative ions toward the surface of the photoreceptor drum (1) near the grid (42) are generated only in the center of the image area (A), as in the first embodiment. Not yet. Therefore, the negative ions reach only the central portion of the image area (A) as shown by the arrow pass, eliminate the charge in the area they reach, and lower the potential to approximately the same as the grid voltage (Vg). Further, as shown by the arrow →, the positive ions reach only the light-transmitting part of the non-image area, and raise the potential of the reached part to a potential approximately equal to the grid voltage (Vg).

That is, in terms of the surface potential of the photoreceptor drum (1), as shown in FIG. is left as a high potential part with a constant width of approximately +600V, which is the initial surface potential, and the light transmitting part 7- of the non-image part rises to approximately the grid voltage (Vg), and the image part (A) The central portion drops to approximately the grid voltage (Vg). Specifically, the surface potential of the central part of the image area (A) and the surface potential of the light-transmitting part of the non-image area are ±6. okV, grid voltage (4
To 3) (7) + By applying voltage of 200V (7), +
It drops and rises to around 200V.

In other words, in the second embodiment, in the second charging step, the non-image area has a minute potential pattern corresponding to the pattern of the screen (31), and the image area (A)
The central portion of the image has been completely neutralized, that is, the positive original image has been formed as a pseudo-inverted negative electrostatic latent image.

(iv) Developing step In the second band T step, the electrostatic latent image formed as a pseudo negative image is developed by a developing device (5). The development conditions and development mechanism here are similar to m, Il:HJ in the first embodiment, but in the case of the second embodiment, the electrostatic latent image is removed by the second charging machine 3. Even if the surface potential of the light-transmitting part of the image area rises to approximately the same level as the grid voltage (Vg), toner will adhere and cover the image area (A) in the same way as in the center of the image area (A). do not have.

(contour image formation mode) (1) First charging step This is the same as in the reverse image forming mode.

(i) Exposure process A positive original image is exposed to slit light without passing through the screen (31) to form a positive electrostatic latent image [15th
See (a) in the figure.

(i) Second charging step The surface of the photosensitive drum (1) on which the positive electrostatic latent image has been formed is re-charged with the scorotron charger (4) to which an alternating voltage has been applied from the power source (41'). Become electrically charged.

The operating conditions of the scorotron charger (4) in this step are the same as in the inversion image forming mode. However, the appearance of the electrostatic latent image formed is different. That is, lines of electric force shown by arrows in FIG. 14 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 electric lines of force that direct negative ions toward the surface of the photoreceptor drum (1) near the grid (42) are directed toward the center of the surface image area (A), as in the case of FIG. only has occurred. Therefore, the negative ions reach only the central portion of the surface image area (A) as shown by the arrow →, and remove the charges in the area they reach, lowering the potential to approximately the same as the grid voltage (Vg). In addition, as shown by the arrow →, the positive ions
') reaches only the non-image area excluding the outside, and raises the potential of the reached area to a potential approximately equal to the grid voltage (Vg).

That is, when explaining the surface potential of the photoreceptor drum (1), as shown in FIG. 15(b), the image area (A).

The contour parts (A') and (B') of (B) remain as a high potential part with a constant width of approximately +600cm, which is the initial surface potential, and the outside of the contour parts (A') and (B') The non-image area partially remains as a low potential area of approximately +100■, and the non-image area excluding the outside of the contour areas (A') and (B') rises to approximately the grid voltage (Vg), And the surface image part (
The central part of A) drops to approximately the grid voltage EE (Vg).

In other words, in this second charging step, the image area (A), (
This means that the outline of B) has been completely formed as a positive electrostatic latent image.

(iv) Development process This is the same as in the reversal image forming mode, in which the negatively charged insulating toner is applied to the high potential areas of the photoreceptor drum (1>, that is, the contour areas (A') and (B'). It adheres and, so to speak, “
A toner image with "inner border" can be formed by regular development.

(Standard copy mode) This is the same as in the first embodiment, and the screen (3
1) is allowed to retreat from the exposure optical path, and the scorotron charger (4) will of course never become inoperable.

Effects of the Invention As is clear from the above explanation, according to the present invention, in the exposure step, a positive original image is exposed to light through a screen having colored halftone dots, lines, etc. on a transparent substrate, and the second charging is carried out. As a step, the surface of the electrophotographic photoreceptor that has undergone the exposure step is heated to a surface potential sufficiently lower than the surface potential of the electrostatic latent image image area formed in the exposure step on the grid using an O-tron charger. Since recharging is performed while applying a voltage of the same polarity as in the charging process, the outline of the original image and the corresponding part of the screen pattern remain as high potential areas on the surface of the electrophotographic photoreceptor, resulting in a pseudo-inverted state. After the electrostatic latent image has been formed, the electrostatic latent image is further developed in a developing step using a toner charged with a polarity opposite to that in the first charging step. In other words, the electrostatic latent image is exposed through a screen. By doing this, the non-image area was decomposed into a fine/J1 potential pattern, so
During regular development, toner adheres to the high potential portions of the electrostatic latent image, and a pseudo-inverted image without toner fog can be obtained. Moreover, by switching and controlling the screen and the second charging process, it is possible to form and separate positive and negative images by using only one developing device, and in addition, the above process can be performed without using a screen. If executed, a contour image can also be formed.

[Brief explanation of the drawing]

1 to 11 are for explaining the first embodiment of the present invention, in which FIG. 1 is a schematic configuration diagram of an electrophotographic copying apparatus, FIGS. 2a and 2b are a plan view of a screen, and FIG. Figure 3 is a plan view of the operation panel, Figure 4 is a block diagram of the control circuit,
Fig. 5 is an explanatory diagram showing another example of screen installation, Fig. 6 is a schematic diagram of electric lines of force in the second charging process in the reverse image forming mode, and Fig. 7 is a diagram showing static electricity in each process in the same mode. Graphs showing the potential of the electrostatic latent image, Figures 8 and 9 are schematic diagrams of lines of electric force and graphs showing the potential of the electrostatic latent image as a reference example in the same mode, Figure 10 is the contour image forming mode Schematic diagram of electric lines of force in the second charging step, 11th
The figure is a graph showing the potential of the electrostatic latent image in each process in the same mode. FIGS. 12 to 15 show the second embodiment of the present invention.
These are for explaining the embodiment, and FIG. 12 is a schematic diagram of the lines of electric force in the second charging step in the reverse image forming mode, and FIG. 13 is the potential of the electrostatic latent image in each step in the same mode. FIG. 14 is a schematic diagram of electric lines of force in the second charging step in the contour image forming mode, and FIG. 15 is a graph showing the potential of the electrostatic latent image in each step in the same mode. (1)... Electrophotographic photosensitive drum, (2)... Charger, (21)... Power supply, (3)... Exposure device, (31)... Screen, (4)... ... Scorotron charger, (41), (41') ... power supply,
(42)...Grid, (43)...Power supply, -(5
)...Developing device, (51)...Developing sleeve.

Claims (1)

[Scope of Claims] 1. A first charging step of imparting a constant electric charge to the surface of the electrophotographic photoreceptor, and a transparent image of a positive original on the surface of the electrophotographic photoreceptor that has undergone the first charging step. An exposure process in which the substrate is exposed to light through a screen having colored halftone dots, lines, etc., and an electrostatic latent image formed on a grid in the exposure process by using a scorotron charger on the surface of the electrophotographic photoreceptor that has undergone the exposure process. A second step of recharging while applying a voltage sufficiently lower than the surface potential of the image area and having the same polarity as the first charging step.
and 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. image forming method.