JPS6341062B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electrophotographic developing method, more specifically, using a composite developer such as a two-component developer,
Varying the low frequency alternating electric field applied between the electrostatic image holder and the developing electrode supporting or in contact with the developer, depending on the original density or the electrostatic image potential,
The present invention relates to an electrophotographic developing method that provides visible images with excellent image clarity and no background fog.

2. Description of the Related Art Electrophotographic devices, such as copying machines, are required to produce copied images with high color density in image areas, no fog in non-image areas (ground areas), and rich tonality that is faithful to the original. Therefore, in general, in electrophotographic apparatuses, development is performed by applying a constant DC bias voltage that satisfies the above requirements to the developing device. However, this is inconvenient in the case of manuscripts in which the background area is colored (hereinafter referred to as colored manuscripts), manuscripts with low density, and manuscripts with thin text written in a specific color (hereinafter referred to as colored text manuscripts). tends to occur. For example, in the case of the former, background fog occurs due to the colored background,
In addition, the contrast between the image area and the background area becomes low, resulting in a reproduced image that is very difficult to see, and in the latter case, the image becomes blurred and has a low density. There were many things that happened. Conventionally, attempts have been made to increase the bias value to remove background fog, but this results in lower image density and contrast.Conversely, lowering the bias value to reproduce line images clearly causes background fog. The result was similar to that of the previous one.

The present invention improves the above-mentioned conventional drawbacks, and provides a developing method that can produce visible images with excellent image clarity, no background fog, and rich gradation for any type of original. The purpose is to provide the following.

The present invention that achieves this object uses a two-component composite developer having toner particles and carrier particles or a carrier liquid, and applies the composite developer to the image area (originally developed At the same time, the developer carrier and the electrostatic image holder are brought into contact with the developer carrier and the electrostatic image carrier. A low-frequency alternating electric field is applied between the image area and the image area, and the adhesion of developer to the non-image area is substantially removed. By changing the strength or alternating frequency of the applied alternating electric field accordingly, it is possible to always obtain an image with high image density and contrast, no background fog, or rich gradation, no matter what condition the document is in.

Preferred embodiments of the present invention are summarized as follows. A low-frequency alternating voltage is applied to the developing electrode to generate an alternating electric field between the electrostatic image holder and the developing electrode, and the intensity or frequency of the alternating electric field is changed by measuring the density of the non-image area of the document. . The higher the voltage applied to the developing electrode, the more effective it is in contributing to the above-mentioned improvement in image density and reduction in background fog density. It is necessary to take into account. Furthermore, the frequency of the alternating voltage applied to the developing electrode must be set to a frequency that does not cause density unevenness (so-called cycle unevenness) in the developed image. For this reason, the higher the frequency, the greater this effect.
On the other hand, the leakage current increases, so this factor must be taken into account and all the above factors must be set together.

To measure the condition of the document, you can measure the electrostatic image potential of the non-image area, or use a pushbutton or the like that has been set to an appropriate electric field strength and frequency based on the condition of the document. The selection means may be switched.

In order to prevent density unevenness from appearing in a microscopic image, the developing distance, that is, the distance in which the developer is in contact with the electrostatic image carrier, should be set as dmm, and the moving linear velocity of the electrostatic image carrier should be set as v.
mm/sec, and the frequency of the applied alternating voltage is Hz, it is necessary that ≧v/d (Hz)... (1). Of course, there is an upper limit to this from the viewpoint of improving gradation reproduction. The value suitable for the present invention will be explained in detail in the description of the following examples.

In addition, in order to prevent spark discharge from occurring due to dielectric breakdown at the shortest distance between the developing electrode and the electrostatic image holder, a protective resistor is installed between the developing electrode and the applied voltage source to limit the total current. It is preferable to insert it between. Then, even if dielectric breakdown occurs for some reason, the total current is limited, there is less risk of damage to the electrostatic image holding member due to spark discharge, etc., and the bias power source is It does not cause any damage. Although it is possible to limit the total current depending on the alternating current voltage generating device used, the former method is preferable because the device becomes complicated and it takes time to recover.

By applying a low-frequency alternating electric field between the electrostatic image holder and the developing electrode, background fog can be reduced to a greater extent than when no alternating electric field is applied. When the electrostatic image carrier is composed of a photoconductor such as Se, ZnO, or an organic semiconductor, depending on its properties, the potential of the non-image area of the electrostatic image carrier may be set to a potential that prevents toner from adhering to the electrostatic image carrier. In such a case, it is necessary to generate a slight DC electric field in addition to an alternating electric field to suppress the toner from adhering to the non-image area. In this case, in addition to the alternating voltage, a direct current voltage may be superimposed on the developing electrode, or an alternating voltage biased either positively or negatively may be applied. This further reduces fog due to the synergistic effect of the DC bias voltage and the alternating bias voltage application.
It is possible to substantially eliminate fog.

In this way, by applying an alternating bias voltage, image density can be increased and fog density can be reduced.
It is especially suitable for developing electrostatic images where the potential difference between bright and dark areas must be relatively low, such as the electrostatic image forming method (typically described in Publication No. 341, etc.). It is valid.

Embodiments to which the developing method and apparatus according to the present invention are applied will be described in detail below with reference to the drawings.

FIG. 1 is a sectional view of an embodiment of an electrophotographic apparatus having a developing device to which the developing method and device according to the present invention are applied, and FIG. 2 is an enlarged sectional view of the developing device.

In the figure, reference numeral 2 denotes an outer casing of the apparatus 1, and an original or the like is placed on an original mounting table 3 made of a transparent member such as glass on the upper part of the outer casing 2. That is, the mounting table 3 does not move, and the image is irradiated onto the photosensitive screen 4 by optical means such as a moving/fixed mirror and a lens system. This optical means belongs to conventionally well-known technology,
The first mirror 5 moves along the entire distance of the mounting table 3 at a speed v to the dotted line position at the right end together with the document illumination lamp 6. On the other hand, at the same time as the mirror 5 moves, the second
Mirror 7 moves at a speed of v/2 to the rightmost dotted line position. The original image guided by the first and second mirrors 5 and 7 is guided to the screen 4 via a lens system 8 having an aperture mechanism and a fixed mirror 9. Incidentally, the photosensitive screen 4 formed in an endless shape has the structure described in the above-mentioned Japanese Patent Laid-Open No. 51-341, and is made so that the side surface where the conductive member is exposed is on the inside. On the other hand, the formation of the primary electrostatic image by the photosensitive screen 4 is also carried out by the process clearly described in the above-mentioned Japanese Patent Laid-Open No. 51-341. (Of course, the present invention is not limited to this.) Among the constituent members arranged around the screen 4 in the figure,
Reference numeral 10 denotes a pre-exposure lamp, which is provided so that the photoconductive member constituting the screen 4 is always used in a stable light history state. A corona discharger 11 is a primary voltage applying means, and has a sufficient length in the circumferential direction of the screen 4 in order to charge the screen 4 to a sufficient voltage. Next, 12 is a corona discharger which is a secondary voltage application means, and the discharger 12
In order to irradiate an image onto the screen 4 through the screen, a part of the shield plate of the discharger 12 has an optically open structure. A lamp 18 is for illuminating the entire surface.

The corona discharger 19 generates a modulating corona ion stream for forming a secondary electrostatic image. A secondary electrostatic latent image is formed on the surface insulating layer 21 of the drum 20 facing the discharger 19 with the screen 4 interposed therebetween. Note that the insulating layer 21 of the drum 20 is arranged or attached on the conductive support 22,
The support 22 functions as a counter electrode in ion modulation. The drum 20 rotates in the rotation direction (arrow a) and speed (vmm/
sec) in the direction of arrow b. By the way, the secondary electrostatic latent image formed on the insulating layer 21 is
The toner image is developed by the developing device 23 using the developing method according to the present invention, and becomes a toner image. The toner image is transferred to copy paper using plain paper, which is a recording member, which has been conveyed at a transfer position 24. The residual toner on the insulating layer 21 that has passed through the transfer position 24 is removed by a cleaning means 25 using a blade or the like, and then the insulating layer 21 is made to have a uniform surface potential by a corona discharger 26, and if necessary, it is again subjected to secondary static. Forms an electrolatent image. On the other hand, the copy paper 27 to which the toner image is transferred is
The sheets are loaded in advance in a storage cassette 28, separated one by one by a delivery roller 29 and a separating claw 30, and transported to a transfer position. In the figure, 31 is a feed roller, and 32 is a corona discharger that applies a bias voltage to the copy paper 27 when transferring a toner image. After passing through the transfer position 24, the toner image on the surface of the copy paper 27 is fixed by the heater of the heat-fixing means 33, and the copy paper 27 is conveyed to an external finished copy paper storage tray 35 by the conveyance belt 34.

The developing device shown in FIG. 2 has two non-magnetic hollow cylinders (hereinafter referred to as scoops) facing a drum-shaped electrostatic latent image holder 21 and disposed within the downward rotation range of the holder. This figure shows an apparatus for developing an electrostatic image on the holder.

In the figure, the electrostatic image holder 21 is rotating in the direction of the arrow, and two non-magnetic rotating sleeves 36 are installed in the developing section so as to rotate in the same direction as the direction of the arrow.
A and 36B are arranged a short distance apart from each other. The rotational speed of this sleeve is
It is driven by a known driving means at a circumferential speed different from the circumferential speed of 1.

As an example, the peripheral speed of the electrostatic image holder is 500mm/
sec The peripheral speed of sleeve 36A in the front stage is 550mm/sec,
The peripheral speed of the downstream sleeve 36B was set to 550 mm/sec, and the diameter of the sleeve was set to 50 mm. Both sleeves have fixed magnetic means 37A, 37B inside them, and the magnetization poles of the respective magnetic means are different.

First, the magnetic pole arrangement of the first magnetic means 37A will be explained. The illustrated magnetic means 37A is formed as a magnetic roller, and has magnetized poles N ₂ , S ₂ , and N ₃ spaced apart from each other at equal angles near its surface, and the N ₃ pole is closest to the surface of the electrostatic image carrier 21 . are placed next to each other.

Next, a second magnetic means 37B is disposed downstream along the electrostatic latent image carrier of the first magnetic means,
As shown in the figure, it is formed as a four-pole magnetized roller. The N ₁ magnetic pole and the S ₁ magnetic pole are magnetic poles that have the function of drawing up and transporting the developer, and the S ₃ S ₄ magnetic poles are respectively located from the point where this magnetic roller is closest to the surface of the electrostatic image carrier. These are developing magnetic poles arranged in opposite directions and spaced apart from each other by a predetermined angle θ. These developing magnetic poles S ₃ and S ₄ of the same polarity are the developing magnetic poles of the above-mentioned first magnetic means.
It has been selected to be different from _N3 . 38 is
The developer in the developer housing 23a is comprised of a mixture of magnetic carrier particles and developer toner particles. Also, within this housing are two screws 39A and 39B for stirring the developer 38 in the direction of rotation of the sleeve, and a sleeve 37.
A blade 43 that regulates the thickness of the developer pumped onto the surface of the sleeve B to a predetermined value, a cleaning blade 44 that removes residual developer from the surface of the developer sleeve, and a replenishment developer storage chamber 40 and its lower part. It has a replenishment roller 41 provided in the opening. As the roller 41 rotates, the developer that has entered the recess provided on its surface is transferred to the roller 41.
With the rotation of the container housing 23a, the sleeve of the container housing 23a falls into the developing chamber 42, where the developer is replenished. In this developing device, the sleeve 36A and the magnetic means 37A constitute a first developing section, and the sleeve 36B and the magnetic means 37B constitute a second developing section.

In the figure, 45 is a power source that applies a low frequency AC bias voltage to both sleeves 36A and 36B, and 46 is the aforementioned protective resistor. The applied AC bias voltage was set to have an effective value of 200V. A rectangular wave was used as the voltage waveform. The frequency of the AC voltage is the same as that of the developing sleeves 36A, 3.
6B and the electrostatic image carrier in the developing section, the smaller one is d as shown in the figure, and this d
is approximately 5 mm, and the moving peripheral speed of the electrostatic image holder, that is, the process speed V, is set to 500 mm/sec, so the frequency of the AC voltage is ≧v/d=500/5=100 (Hz). It is good to have it, but from the viewpoint of effectiveness, the value should be 500 Hz or more, and the upper limit is to ensure sufficient fluidity of developer particles, maintain reproducibility of halftone images, and improve gradation. Good results were obtained by setting the frequency to 1KHz to avoid deterioration of the frequency. At this frequency, the effect gradually increased up to 10KHz, but above 10KHz, the leakage current only increased, and the effect did not increase at all. This is thought to be because at 10 KHz or higher, the toner cannot vibrate following the applied alternating voltage.

In addition, if the protective resistor 46 is set to 100KΩ,
Even with an applied voltage of 200 V, no excess current of 2 mA or more flowed through the sleeve, so the electrostatic image holder 21 was not damaged at all.

FIG. 3 shows a simplified configuration of the developing device shown in FIG. 2 by using only one sleeve. In the figure, 47 is
The electrostatic image carrier may be a photoreceptor on which an electrostatic image can be formed by directly performing processes necessary for electrostatic image formation such as photosensitive charging and image exposure.

For example, Special Publication No. 42-23910, No. 42
- Publication No. 19748, Publication No. 43-24748, Publication No. 44-
Photoreceptors and electrostatic image forming processes described in other publications such as No. 13437 can be applied.

48 is a housing of a developing device; 49 is a developer containing a mixture of carrier particles and toner particles;
50 is a non-magnetic sleeve supported to rotate;
51 is a magnet roll enclosed in the sleeve;
2 is a plate member that regulates the thickness of the developer replenished onto the sleeve; 53 is a scraping plate that removes residual developer from the sleeve surface after development; and 54 is a protection resistor 55 for applying an AC low frequency bias voltage to the sleeve. The power is applied through the The applied voltage, its frequency and protective resistance value are set in the same way as in the case of FIG.

Now, with respect to the developing apparatus illustrated in FIGS. 2 and 3, the phenomena and effects produced by generating an alternating current electric field between the electrostatic image holder and the sleeve acting as a developing electrode will be described.

The developer applied to the present invention has a complex component, a second component, and a second component.
The developer illustrated in FIGS. 3 and 3 is a mixture of toner and carrier particles. It is thought that in a developer made of such a mixture, the fluidity of the developer particles is substantially improved by applying an AC bias voltage between the electrostatic image carrier and the development electrode in the development section. This is because by creating an alternating electric field between the electrostatic image carrier and the developing electrode or between the electrostatic image carrier and the developer, the developer particles are subjected to a force along the alternating electric field. Therefore, not only the developer particles already in contact with the electrostatic image carrier, but also the developer particles present in close proximity to the electrostatic image carrier are urged to move alternately through the development gap, and the electrostatic It will repeatedly come into contact with and separate from the image carrier. Therefore, the amount of developer particles contributing to development becomes larger than when no AC bias voltage is applied.

The two-component developer is supplied by a sleeve so as to be in contact with the image area and the non-image area of the electrostatic image holder. When the electric field is to be attracted in the direction of the image area, it is strongly attracted over a wide range and adheres to the image area. In this way, once the toner adheres to the electrostatic image holder, physical adsorption force with the surface of the electrostatic image holder acts, so that it becomes substantially difficult to separate even in the case of a reverse electric field.

On the other hand, in the non-image area, even if toner adheres once during the electric field that should be attracted toward the image area, due to the alternation of the electric field, when the electric field of the other polarity is applied, the adhered toner or the vicinity The toner particles present in the toner are stripped off again by vibration,
The developer is returned to the developer carrying sleeve side. What should be noted here is that not all of the developer particles present between the electrostatic image carrier and the developer carrying member contribute to development; rather, in addition to the developer particles that come into contact with the electrostatic image carrier, , the developer particles attracted to the electrostatic image holder by the alternating electric field contribute to the development. At this time, other particles, for example, in the case of a two-component developer consisting of a toner and a carrier, the carrier behaves as a conductive particle, and the sleeve, which acts as a developing electrode, is farther away than the electrostatic image carrier. Even in this case, the carrier particles function as a developing electrode, and the gap between the electrostatic image carrier surface and the carrier is approximately the distance of several toner particles, approximately 10 to
Since the developing electric field between the electrostatic image holder and the developing electrode is considerably larger than the electric field applied to the developer carrying member, the effect becomes extremely large. The developer carrier particles that function as a developing electrode have a volume resistivity of 10 ¹² Ω·cm, preferably 10 ¹⁰ Ω·cm.
Must be below.

Furthermore, the following effects can be considered as effects of the alternating electric field. First, the developer particles attached to the electrostatic image carrier are also affected by the alternating electric field, and the developer particles repeatedly separate from and adhere to the electrostatic image carrier, but even in this case, the Because the developer particle carrier or conductive toner, which serves as a typical development electrode, is in contact with the surface of the electrostatic image holder or exists with a gap of 10 to 40 μm between only the toner particles, the strength of the alternating electric field becomes extremely large, and the development This serves to remove excess toner that has adhered to unwanted non-image areas, so-called background fog.

Here, it is also possible to mix a third component in addition to the toner and carrier. Examples include lubricants such as silica and Teflon, and abrasives such as potassium oxide talc.Since these are electrically charged, they are also affected by alternating current electric fields.

In the above description of the embodiment, a method of applying bias directly to the sleeves 36A, 36B, or 50 was shown as a method of applying bias to the developing electrode. A bias may be applied to this as an electrode.
Alternatively, a member that rubs the surface of the developer may be provided and a bias voltage may be applied to this member. As in the above example, an image with high image density and less fog can be obtained.

Next, the present invention is also effective in wet developing devices. This is because in a liquid developer in which toner particles are dispersed in a carrier liquid, the toner particles and the carrier liquid are charged differently. By applying an alternating current bias voltage to the developing plate or developing roller, an alternating electric field is generated between the developing plate or developing roller and the electrostatic image holder,
By forcing the toner particles suspended in the carrier liquid to move violently, the number of toner particles that come into contact with the electrostatic image carrier is increased, and the toner particles are charged to the opposite polarity to those of the toner particles. When the carrier liquid is also subjected to vibrations, toner particles adhering to non-image areas are peeled off, thereby reducing fog density and increasing image density. Therefore, the present invention also applies to wet development methods.
This method has the same effect as the dry development method. In the developing method using a developing plate, it is sufficient to apply an AC bias voltage to the developing plate, but in this case, since the developing plate functions as a developing electrode, due to its structure, it is necessary to apply an AC bias voltage to the developing plate. Since the interval is wide, it is desirable that the applied bias voltage is also higher than the others. An example applied to a wet development method will be described below.

FIG. 4 shows that the liquid developer 58 is pumped up onto the roller surface 57 by the rotation of the developing roller 56.
A method of developing by bringing it into contact with an electrostatic image holder 47,
This figure shows an outline of an apparatus applied to a so-called roller development method. A low frequency alternating bias voltage is applied to the developing roller 56 through the power source 54 and the protective resistor 55 in the same manner as shown in FIG.

Further, as illustrated in FIG. 5, a liquid developer 60 is drawn up by a liquid-absorbing developing roller 59 and brought into contact with the electrostatic image holder 47 to push out the liquid inside the developing roller 56. The present invention can be applied to a developing method in which the remaining liquid of the developer is absorbed by the liquid-absorbing developing roller and removed from the developing section, and similar effects can be obtained.

In this developing method, when the developing roller 59 has a mesh of, for example, 200 to 300 meshes as the outer circumferential member 61 and is electrically conductive by metal plating, conductive fibers, etc., the outer circumferential member 61 is In addition, when the liquid-absorbing member 62 of the developing roller 59 is conductive, with or without an outer peripheral member, for example, a conductive sponge, felt or web made of conductive fibers, etc. In this case, a bias voltage may be applied to the liquid-absorbing member 62 directly or via the conductive shaft 63. in this way,
Outer peripheral member 61 and liquid absorbing member 6 as a developing electrode
2 is close to the electrostatic image plane, the strength of the bias electric field obtained by applying the AC bias voltage from the power supply 54 becomes very large, and the above-mentioned effects of the present invention are great.

In addition, in the wet development method, image density can be reduced by applying an AC bias voltage to rollers such as a fog remover roller and a liquid squeezing roller that rotate in close proximity to the electrostatic image holder, as described above. The background fog can be removed without causing any damage, and the intended purpose can be achieved. This is because the antifogging roller, squeezing roller, etc. are also included in the liquid developer, and their construction is almost the same as the above-mentioned roller used for development. Therefore, by applying an alternating bias voltage to the roller means, an alternating electric field is generated between the electrostatic image bearing surface and the roller surface, thereby producing the effects described above. Furthermore, by bringing the conductive roller surface close to the surface of the electrostatic image carrier, both of these rollers can increase the strength of the applied bias electric field and attract the toner dispersed in the carrier liquid or vibrate the carrier liquid. As a result, more toner particles can be attached to the surface of the defogging roller to remove fog, or residual liquid developer can be squeezed out on the surface of the squeezing roller using the alternating current electric field.

The above examples are for explaining the present invention, and the present invention is not limited thereto. The waveform of the applied bias voltage may be any waveform as long as it is a low frequency alternating waveform, such as a sine wave, a rectangular wave, a triangular wave, or a sawtooth wave. Further, the waveform does not necessarily have to be symmetrical, and in relation to the electrostatic image potential, it may be preferable to have a waveform in which DC components are superimposed, or a waveform biased toward positive or negative polarity.

As explained above, by applying an alternating bias voltage between the electrostatic image holding member and the developing electrode, fog can be reduced, image density can be increased, and gradation can also be changed. be able to.

The strength of the applied alternating electric field, that is, the value of the output voltage of the bias power supply in this case, is a sine wave with an effective value of 50
~300V Preferably 100~300V, square wave, triangle wave etc. with amplitude 100~450V (peak to peak Vpp
=200~900V) is good. If it exceeds this, there is a risk of dielectric breakdown between the developing electrode and the electrostatic image holder, and leakage may occur to the developer thickness regulating member placed close to or in contact with the developer, resulting in the bias being substantially reduced. There is a risk that the result will be the same as not being applied.

Furthermore, as mentioned above, the period of the alternating electric field, that is, the frequency of the bias voltage, is preferably v/d 10KHz, preferably v/d1KHz, where the developing distance is dmm and the moving linear velocity of the electrostatic image holder is Vmm/sec. .

When applied to an actual device, the strength and frequency of the above-mentioned alternating electric field should be set so that a visible image with low fog, high image density, and rich gradation can be obtained within these conditions. Set.

Now, in the NP screen type copying machine shown in FIG. An electro-latent image was obtained. At this time, the dark surface potential on the drum is -150V,
The bright area potential was +50V. This latent image was developed using a two-component developer, and experiments were conducted by varying the strength or frequency of the alternating electric field applied.

FIGS. 6 and 7 are graphs obtained by changing the applied AC voltage value (effective value) V _AC and frequency _AC . This is the image reflection density D for the latent image potential V.
The curve below is called the V-D curve.

Figure 6 shows the V-D curves when the AC voltage (effective value) V _AC is 0V, 70V, 150V, and 200V when the frequency _AC = 1KHz, and as it is clear from the figure, the voltage value is increased. It can be seen that an image with high contrast can be obtained. For example, _{the dark} potential (-
The image density at 150 V) is high and an image with deep color can be obtained, and the image density at bright area potential (+50 V) is low and fog can be kept low.
This is because when the voltage value is increased to increase the electric field strength, a stronger force acts on the vibrating toner or carrier, causing more toner particles to adhere to the electrostatic latent image in the dark area, and the electrostatic latent image in the bright area. This is thought to be due to the fact that the toner adhering to the surface of the holder due to physical force is shaken off by the force of the strong electric field. Therefore, in the case of colored originals, low-density originals, and even originals with colored text, image density can be increased and fog can be reduced by changing the voltage value of the bias alternating current during development. A copy image with high contrast can be obtained.

Figure 7 shows the V-D curve measured by changing _{the frequency AC to} 0,500Hz, 1KHz, and 10KHz when the AC voltage value (effective value) VAC = 100V.As the frequency increases, Figure 6 As in the case of , it can be seen that the image density in dark areas is high and fog in bright areas is reduced, resulting in an image with extremely high contrast.
This is because by increasing the frequency of the applied alternating voltage, the toner that has adhered to the non-image area and the toner that exists in the vicinity vibrates more, and the amount of toner that ultimately adheres to the non-image area is significantly reduced. This is thought to be due to the fact that As is clear from the figure, when the frequency is around 10KHz, there is almost no difference in the effect obtained, and the leakage current increases and the power supply becomes larger, reducing the benefits. Therefore, the upper limit of frequency is preferably up to 10KHz.

However, as is clear from FIGS. 6 and 7, when the voltage value or frequency is increased, the so-called γ
The resulting image is not very desirable in terms of gradation (gamma=gradient of the characteristic curve of image density versus electrostatic image potential). Therefore, when copying an ordinary original, the voltage value or frequency is set to a low value, taking into account the gradation factor, and in the case of colored originals or color text originals, the gradation is set to a low value or frequency. It is better to increase the color density of characters (images) and eliminate background fog to obtain a high-contrast, easy-to-read copy image, even if this means damaging the image.

In this way, the intensity or frequency of the applied alternating electric field can be changed in accordance with the density of the original as described below. For example, in the case of a colored original, in order to detect the density of the background part of the original, a light receiving element is provided opposite to the original placed on the document placement glass, and this is moved to detect the density of the background part. . Alternatively, a plurality of light-receiving elements are randomly arranged facing the document placement surface, and the value with the lowest density among the detected values is taken as the reflection density of the background of the document. In the case of a color character original or an original with low image density, the strength and frequency of the alternating electric field may be changed in accordance with the value with the highest density among the values detected by the light receiving element.

It is also possible to measure the latent image potential on the electrostatic image holder using a surface electrometer or the like and change the alternating electric field in accordance with this value. Furthermore, a selection means may be provided in which several types of electric field strengths or frequencies are set in advance, and an appropriate value may be selected after confirming the density state of the original.

In general electrophotographic apparatuses, such adjustment of the alternating electric field must be performed before image exposure. However, in so-called NP screen type devices, the image is once formed on the screen drum and then the latent image is transferred to the insulating drum by corona discharge, so the operation of detecting the reflection density of the original is performed at the same time as image exposure. It's okay to be.

In explaining the embodiments of the present invention, a method of changing the effective value or amplitude (or V _PP ) of the applied alternating voltage was shown as a method of changing the strength of the alternating electric field. The strength of the alternating electric field may be changed by adjusting the distance between the body and the developing electrode.

As described above in detail, the present invention applies a low-frequency alternating electric field between an electrostatic image holder and a developing electrode when performing development using a composite developer containing chargeable composite components. Since the strength or alternating frequency of the alternating electric field is changed according to the density of the electrostatic image or the electrostatic image potential on the electrostatic image carrier, it is possible to change the original density by developing with emphasis on contrast or developing with emphasis on gradation. Development processing can be performed depending on the state of the image. Particularly in the case of originals with colored backgrounds, originals with low image density, or originals with colored characters, it is possible to increase the density of the image area, remove background fog, and obtain extremely clear copied images.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of an electrophotographic apparatus to which a developing method according to the present invention is applied, FIG. 2 is an enlarged sectional view of the developing device shown in FIG. 1, and FIG. 3 is a developing device shown in FIG. 2. Cross-sectional views showing modified examples of the device, FIGS. 4 and 5
The figures are cross-sectional views showing other embodiments of the developing method according to the present invention, and FIGS. 6 and 7 are graphs showing V-D curves measured by changing the value or frequency of the applied alternating voltage. In the figure, 21...electrostatic image carrier, 36A,
36B, 50...Nonmagnetic rotating sleeve, 37A,
37B, 51... Fixed magnetic means, 38, 49...
Developer, 43, 52... Thickness regulation blade, 4
4,53...Cleaning blade, 45,54
...Power source for applying low frequency alternating voltage, 46, 55
...represents protective resistance.

Claims (1)

[Scope of Claims] 1 In an electrophotographic development method in which an electrostatic image is visualized by bringing a developer into contact with an electrostatic image carrier, a composite developer containing a chargeable composite component is used, and the electrostatic carrier and the developer are brought into contact with each other. A low-frequency alternating electric field is formed between the electrode and the alternating electric field, and the strength or alternating frequency of the alternating electric field during development is varied depending on the type of original, its density, or the electrostatic image potential on the electrostatic image carrier. An electrophotographic developing method characterized by: 2. In the electrophotographic developing method according to claim 1, the distance between the contact portion between the electrostatic image holder and the developer in the developing section is dmm, and the linear velocity of the electrostatic image holder is vmm/sec. An electrophotographic developing method, wherein the frequency Hz of the alternating electric field is set to satisfy the following: v/d≦f≦10 (kHz). 3. An electrophotographic developing method according to claim 1 or 2, characterized in that a protective resistor is inserted between the developing electrode and a power source for forming an alternating electric field. Method. 4. The electrophotographic developing method according to any one of claims 1 to 3, wherein the voltage waveform of the alternating electric field is a sine waveform. 5. The electrophotographic developing method according to any one of claims 1 to 3, wherein the voltage waveform of the alternating electric field is a rectangular wave. 6. The electrophotographic developing method according to any one of claims 1 to 3, wherein the voltage waveform of the alternating electric field is a triangular wave or a sawtooth wave. Method. 7. The electrophotographic developing method according to any one of claims 4 to 6, wherein the voltage waveform is biased toward positive or negative polarity. Photographic development method. 8. The electrophotographic developing method according to any one of claims 1 to 7, wherein the developer includes chargeable toner particles and carrier particles. Method. 9. The electrophotographic developing method according to any one of claims 1 to 7, wherein the developer includes chargeable toner particles and a carrier liquid in which the toner particles are dispersed. Electrophotographic development method.