EP0110164B1

EP0110164B1 - Device for charging electrophotographic apparatus

Info

Publication number: EP0110164B1
Application number: EP19830110835
Authority: EP
Inventors: Akira C/O Fuji Photo Film Co. Ltd. Yoda; Masamichi C/O Fuji Photo Film Co. Ltd. Satoh; Shuichi C/O Fuji Photo Film Co. Ltd. Ohtsuka
Original assignee: Fuji Photo Film Co Ltd
Current assignee: Fujifilm Holdings Corp
Priority date: 1982-10-30
Filing date: 1983-10-28
Publication date: 1987-05-20
Also published as: EP0110164A1; DE3371712D1

Description

The present invention relates to a charging device for electrophotographic apparatus.
As is well known, an electrophotographic apparatus records picture images by statically charging a photoconductive insulating layer, which is then exposed to form a static latent image. Charged toner is adhered to the static latent image for positive or inverse development before fixing the toner for recording the picture image. Such method has, therefore, an advantage over conventional films of silver salt in that the material obtains the photosensitivity only after it is charged. For this reason, the electrophotographic technique has found diversified applications. For example, the microfilm industry uses a film of a photoconductive transparent insulating layer formed over a support which is processed for conductivity as the photosensitive material, and records picture images by charging, exposing and developing a portion of the film to form latent images. Moreover, the picture images thus obtained together with the unrecorded portion of the film can be used for projection, etc. and also for additionally recording of fresh picture images. This was not possible in conventional silver halide photography and opened a new scope of uses for microfilm.
An electrophotographic apparatus comprises charging, exposure, developing and fixing devices. In order to obtain better image quality, it is essential that the entire area of the photosensitive material be uniformly charged. However, this has not been so easy because, for one reason, corona discharge is generally used in the charging device, and for another, it has been conventional in microfilm preparation to charge only the limited and desired region on the photosensitive material simultaneously without charging the relative positions of the electrophotographic material and the charging device. The charge efficiency was also unsatisfactory because only a very small portion of ions generated by corona discharge could be utilized.
A charging device used in general will be described referring to Fig. 1. In the drawing, the reference numeral 1 denotes a corona wire which is encased in a shield case 2 with a U-shaped cross section and which is made of metal plate as it is to be applied with a high electric field for generating corona ions. Said shielding case 2 is grounded as the opposing electrodes for the generating corona ion and said corona wire 1 is to be applied with high voltage. The high electric field causes the area near the corona wire 1 to be ionized so that discharge will occur between the corona wire 1 and the shielding case 2. A portion of the corona ions leaks from the opening 2a of the shielding case 2 and impinges on that portion of the film 3 which faces the opening 2a and charges the same. Since the electrode 2 extends close to the film 3, a large portion of the corona ions is absorbed by the electrode 2 before they reach the opening 2a, leaving a very small amount of ions to be discharged from the opening 2a, thus decreasing the charge efficiency. This requires a high voltage power source having a greater capacity. Moreover, the density of corona ions reaching the opening 2a would become uneven over the film 3 because the opening periphery is grounded as a part of the electrode and because the electricity will act in the direction of the periphery.
It should be noted, however, that such uneven distribution of the ions has posed no problem in the conventional copy machines because the photosensitive material and the corona discharge device have relative velocities respectively.
As one method for uniform charging, a charging device has been proposed to apply the conductive member provided on the periphery of the opening 2a with a bias potential which is substantially equivalent to the charged potential of the photosensitive material either by connecting said conductive member with an external pias power source or grounding the same via a high resistance, thereby correcting electric field distribution near the opening 2a. The device is effective in correcting uneven charging due to the turbulence of the electric field caused by the lower end of the shielding case on the periphery of the opening 2a and is particularly effective in correcting uneven charging of the edge portion in the longitudinal direction of the corona wire. This device, however, is not quite satisfactory as it causes uneven charging in the direction perpendicular to the corona wire when the device is used to uniformly charge a very limited narrow region under the condition in which the electrophoto- sensitive material and the charged member are relatively static to each other, the condition often required for forming picture images on microfilm. It also causes uneven charging as above when the distance between the corona wire and the electro- photosensitive material should be limited to be almost similar to the width of the opening due to the limitation in space. This device is further defective in that the charge in the periphery of a picture image region becomes lower than the charge at the center thereof because the distance from the corona wire differs between the center and the periphery. This device is also defective in that it is extremely difficult to apply a bias potential for complete charging under the condition in which the potential of the photosensitive material and that of the periphery of the opening become substantially the same because the potential of the opening periphery rises shortly after it is charged to cause a gap from the potential of the material or the potential of the photosensitive material becomes higher than that of the periphery after a given time.
A charging device has been disclosed in document US-A-3,991,311 which attempts to overcome the problems caused by the disagreement between the increase in surface potential of the material and the increase in periphery potential of the opening and to control potential in a satisfactory manner by connecting a capacitor so that the potential applied on the conductive member may vary chronologically along with the progress of the charging. But this device is also defective in that it presents difficulties in setting a capacitance of the capacitor suitable for a particular photosensitive material and that it increases cost.
Furthermore, prior art document DE-A--2 056 423 discloses a charging device for an electrophotographic apparatus comprising a corona wire and thin conductive members disposed parallel to a photosensitive material. The conductive members are positioned on each side of the centrally situated corona wire and act as grounded electrodes to generate corona ions.
In view of the circumstances described above, it is an object of the present invention to provide a charging device for electrophotographic apparatus which is simple in construction and is capable of efficiently and uniformly charging as well as uniformly imparting a charge over the entire area of said region on the photosensitive material where the picture image is formed.
The present invention provides a charging device for electrophotographic apparatus wherein a corona wire is provided in a charging chamber in parallel with a photosensitive material and opposing the center of the opening in the chamber to charge said photosensitive material, a conductive member is provided in parallel with said photosensitive material on each side of said corona wire, said conductive member acting as the grounded electrodes to generate corona ions, said charging device being characterized in that insulating film masking means are provided extending into said opening facing the photosensitive material so that the masking means can be brought nearly or completely to abut with said photosensitive material.
The inventors studied to find out an ideal form of an opposing electrode for generating ions to uniformly charge and found that in place of a shielding case, an opposing electrode made of thin conductive material in wire- or sheet-like form can be provided and grounded to provide a greater difference in potential as compared with that of a corona wire. In order to prevent fogging or increase in density at development caused by densely distributed charges or clearing or decrease in density at development due to low density charges around said region on the photosensitive material during charging and exposing by corona discharge, a masking means in the form of a film is provided at the opening to nearly or completely abut with the photosensitive material. It would be further effective if a conductive member is provided on the periphery of the opening, and a high resistance is connected to the conductive member to be grounded, thereby supplementarily applying bias potential to the conductive member.
Thus, the present invention enables a charging device in which a limited region of a photosensitive- material, where a picture image is formed, is efficiently and uniformly charged by a corona discharge. Fogging phenomenon and excessive increase in density at development are prevented, due to highly dense distribution of the electric charges around said region on the photosensitive material as well as clearing phenomenon or excessive decrease in density at development due to thin distribution of the electric charges, and furthermore, a uniform distribution of electric charges over the entire area of said region on the photosensitive material is achieved.
The charging device according to the present invention comprises conductive members provided on both sides of a corona wire in parallel with the surface of the photosensitive material, said corona wire being located in a chamber for charging the photosensitive material and the conductive members being provided as the opposing electrodes for generating corona ions.
Furthermore, film masking are projected toward the opening of the chamber where the corona wire is provided to charge the photosensitive material, the masking means being caused to nearly completely abut with the photosensitive material during charging.
The invention can be more fully understood from the following description of preferred embodiments when taken in conjunction with the drawings, in which:

Figure 1 is a sectional view of a conventional charging device,
Figure 2 is a sectional view of a charging device wherein Figure 2(a) shows the case where a non- picture image L is formed around the opening and
Figure 2(b) shows the case where portions to be processed in the subsequent step are integrally formed,
Figure 3 is a graph showing the relation between the relative positions of the corona wire and the opposing electrodes as against the uniformity in charge,
Figure 4 is a view to explain the method for evaluating uniform charge,
Figure 5(a) and (b) are sectional views of a modified embodiment of the device shown in Figure 2 wherein (a) shows the case in which opposing electrodes are of plate-like material and (b) the case in which opposing electrodes are of plate-like material and further provided with a conductive member,
Figure 6 is an explanatory view to indicate the discharge curves during charge,
Figure 7(a), 7(b) and (c) are sectional views to show embodiments of the present invention,
Figure 8(a) and (b) are to show advantages of the present invention wherein Figure 8(a) is a graph, and Figure 8(b) shows the voltage curve applied on the photosensitive material by corona discharge.

Figure 2(a) and (b) show a charging device in section wherein at least the inside of a chamber 4 is formed with an insulating material. A corona wire 1 is provided in parallel with the film surface and opposes the center of an opening 4a of the chamber 4, said opening 4a being located on the side where a film 3 for electrophotography passes. Two thin wires 5 and 6 are provided on both sides of the corona wire 1 horizontally in symmetry and also in parallel with the film 3. Adjacent portions of the opening 4a of the chamber 4 constitute projection 4b which blocks the charges to limit the charging region. Fig. 2(b) is a sectional view of a processing head which integrally houses various processing sections to meet the requirements in space that a charging section, an exposure section, a development section, a drying section and a fixing section are continuously provided in order to record a large number of picture images sequentially and continuously by each frame.
Corona ions are generated as the high voltage is applied between the corona wire 1 and the opposing electrodes 5 and 6. Because the electrodes 5 and 6 are made of thin wire, their respective surface is extremely small when viewed from the corona wire 1, and consequently the amount of corona ions absorbed by the electrodes become much less than in the case of Fig. 1. Instead, a large amount of corona ions will be discharged toward the film 3 through the opening 4a. The corona ion density becomes substantially uniform over the entire surface of the film 3 because the chamber 4 does not constitute an electrode and thus there is nothing in the area below the electrodes 5 and 6 and near the opening 4a to cause the electric force which acts in the direction to absorb the corona ions.
Efficient and uniform charging depends somewhat on the positional relation between the corona wire 1 and the electrodes 5 and 6. Qualitatively, the efficiency improves as the electrodes are located nearer to the film 3 because corona ions are attracted more in the direction of the film 3; on the other hand, the uniformity improves if the electrodes 5 and 6 are positioned farther from the film 3. The present inventors then studied the behavior of the charges on the film 3 while varying the positions of the electrodes 5 and 6 relative to the corona wire 1. The result is shown in Fig. 3. The distance d between the two electrodes 5 and 6 was set at as d = 12.3 mm and d = 24.7 mm respectively and the incident angle 8 formed by the lines a and b connecting the corona wire 1 and the centers of respective electrodes 5 and 6 against the horizontal plane H parallel to the film 3 is given as a variable. The length FWg_o of the region where the potential becomes greater than 90% of the maximum potential Vm was sought by measuring the potential distribution on the film 3 as shown in Fig. 4. The angle 8 is given as a positive value when the electrodes 5 and 6 are located below the horizontal plane H and as a negative value when they are above said plane. The distance between the corona wire 1 and film 3 was 14 mm.
It is understood from Fig. 3 that with θ = 0, or when the electrodes 5 and 6 are located very close to the plane H which includes the corona wire 1, the charges are most uniformly distributed. It is also noted, however, that uniform charge may be obtained over a considerably wider range.
As mentioned above, thin wires were used as electrodes 5 and 6, but electrodes made of a plate such as shown in Fig. 5 can also be used to obtain sufficiently uniform distribution of charges so long as the angle e formed by the horizontal plane H and the line connecting the bottom of the electrodes with the corona wire 1 can be maintained within the range of from 20° to 30°. It should be noted, however, that if the upper ends of the plate electrodes were to extend too far upward, a considerable amount of the corona ions would be absorbed in those regions. It would, therefore, be preferable if the thickness of the plate be as thin as possible and, the diameter of the electrodes be as small as possible, such that the angle a as determined with respect to the corona wire 1 become less than 10°. The angles 0 made between the plate material and the horizontal plane and between the wire and the horizontal plane are desirably ±30° or less and preferably ±10° or less.
As for the chamber 4 which encloses the corona wire 1 and the electrodes 5 and 6, the chamber 4 itself does not constitute an electrode; however, since the corona ions may also impinge on the chamber to impart it with a certain potential and somewhat affect the uniform distribution of the charges on the film 3, it is, therefore, necessary to design the chamber so that the wall thereof is spaced apart as much as possible from the electrodes 5 and 6.
If there are no projections 4b at the opening 4a of the chamber 4, the discharge would impinge on that region of the photosensitive material 3 where the picture image is to be formed to uniformly charge said region indicated by the solid lines in Fig. 6. On the other hand, presence of the projections 4b, if they are made of insulating material, would create an electric field F₁ in direction of the photosensitive surface and thereby cause fogging or excessive density at development around said region due to the highly dense charged region. Moreover, if the electric field F₁ is great, the charges would be concentrated at the central portion of the photosensitive surface leaving a very small area with charges. In case the projections 4b are made of conductive material and grounded, they would create an electric field F₂ in the direction of the projections and thereby cause clearing around said region of the photosensitive surface, which may lower the image quality. In order to overcome these defects, the present invention employs a masking means 7 made of flexible film projecting into the opening 4a from the projection 4b at the respective bottom facing the photosensitive material 3, as shown in Fig. 7(a). When the charges are distributed, the masking means 7 and the photosensitive material 3 are made to come into contact with each other. The thin masking means 7 would prevent the photosensitive material 3 from being affected by the potential inside the opening 4a, whereby a uniform charge can be obtained especially on the peripheries of picture image regions. If a large number of picture images must be recorded on a photosensitive material 3 sequentially and continuously, a high speed processing is required and space is limited, so that it is more effective to process charging and exposing at the same opening 4a. For such a case, a lens 8 for exposure is provided at the charging section.
Celluloid, vinyl chloride, PET, etc. can be used as insulating materials for the masking film 7. It is empirically proven that the thinner the masking film 7 is, the less the potential would rise. In other words, while the photosensitive surface of the photosensitive material 3 would be charged by the corona discharge assuming a voltage curve as shown by the solid line in Fig. 8(b), ideally it is necessary that the charging should occur by assuming a curve, as shown by the dot-and-chain line in Fig. 8(b), in order that the charges should be distributed uniformly over the entire area of said region on the photosensitive surface. If the distance between the masking films 7 is set as X, and the distance on the curve where the voltage assumes a value greater than 90% is given as a to obtain a/X(=a), then Fig. 8(a) indicates that the thickness of the masking means 7 made of celluloid should be less than 2.0 mm if the condition of a = 0.8 is to be satisfied. This leads us to conclude that a thin film masking means projecting at the opening 4a of the chamber 4 reduces the influence of the potential on the masking means 7 and the photosensitive material 3 and maintains the same substantially constant, approximately to the ideal state as shown by the dot-and-chain line in Fig. 8(b).
It was also found that the masking means 7 should preferably be projected as much as possible toward the opening 4a while maintaining the distance between the masking means 7 and the photosensitive material 3 as short as possible during charging; in this way, more uniform distribution of the charges can be obtained. The projections of the masking means 7 should preferably be long than 1.5 times the thickness of the means, and the masking means 7 and the photosensitive material 3 are preferably in complete abutment during charging. In case complete abutment is not obtainable because of various factors in the optical system, the masking means 7 should be so arranged that its surface facing the chamber 4 should be spaced 2.0 mm from the photosensitive material 3.
Fig. 7(b) shows a modified version of the embodiment shown in Fig. 7(a). The masking means 7 has an opening 7a which corresponds to one frame of the photosensitive material 3. The masking means 7 is bent toward the photosensitive material 3 with at least one end thereof affixed to the projection 4b. A stopper 9 is affixed to the projection 4b and a plate 10 is provided beneath the photosensitive material 3 which moves upward at the time of charging and exposing. As the plate 10 moves upward at the time of charging and exposing, the photosensitive material 3 is pressed upward to abut with the masking means 7. The stopper 9 restricts the upward movement of the plate 10 as it abuts with the stopper 9 via the photosensitive material 3. The masking means 7 becomes contracted as it abuts with the photosensitive material 3. The contraction is either absorbed by the flexibility of the masking means 7 itself or by a slight displacement thereof when one end is made free. Then, as the masking means 7 and the photosensitive material 3 come in close contact, the photosensitive material 3 is charged, and then exposed by means of the lens 8. During these processes, the photosensitive material 3 is pressed by the plate 10, which in turn is held in place by the stopper 9. Thus, the photosensitive material 3 is spaced from the lens 8 at a constant interval to achieve better exposure. It is noted that the interval between the photosensitive material 3 and the lens 8 must be positioned at a precision of several tens of pm to have an accurate focus. Voltage is applied to the wire 1 for corona discharge when the masking means 7 and the photosensitive material 3 come in close contact to charge the material 3. The masking means 7 and the photosensitive material 3 should be retained in such a way that no relative velocity exists therebetween. In the charging device which conducts charging and exposure at the same opening 4a, it is preferable to attach a lens 8 separated from the electric system so as to prevent charging from the corona wire 1 toward the lens 8.
In order to further enhance the effect of this invention, as shown in Figs. 5(b) and 7(c), it is possible that a conductive body 4c be provided on the periphery of the opening of the chamber 4 and a means be provided to apply the conductive body 4c with a potential which is substantially equivalent to the potential charged on the photosensitive material 3. This is especially effective in the case thatthe projecting length ofthe film-like masking 7 according to this invention cannot be made long. As the means which supplements the potential, the conductive body 4c provided on the periphery of the opening of the chamber 4 may be grounded via a high resistance 4d. The bias potential, therefore, may be determined by selecting the resistance so as to make the bias potential 80 to 120% of that on the photosensitive material 3. The practical value for such resistance may be several GO generally although it may vary depending on the potential of the photosensitive material, the property thereof orthe area of the conductive body 4c. The conductive body 4c may be made of such metals as copper, aluminum, stainless steel, etc. or of metal oxide. Alternatively, it may be coated with conductive paint.
As described in the foregoing, according to this invention a uniform charge may be obtained all over the whole area of a picture image region due to the effect achieved by the film-like masking to charge uniformly especially on the periphery of the picture image region and by supplying the conductive body 4c with a potential substantially equivalent to the surface potential on the photosensitive material 3.
In a charging device which is not provided with a film-like masking 7 as shown in Fig. 5(b), it is critical to arrange a conductive body 4c slightly apart from the photosensitive material 3 to separate them.
As has been described in the foregoing with respect to the preferred embodiments shown in the drawings, the charging device according to the present invention is capable of efficient and uniform charging because relatively thin electrodes are placed in opposition on both sides of the corona wire and because the shielding case does not constitute an electrode. Thin film masking means are projecting into the opening of the charge chamber, and masking means and the photosensitive material are arranged so as to come in close contact with each other at the time of charge. This simple construction allows the charges to be distributed evenly over the photosensitive surface of the photosensitive material.

Claims

1..A charging device for electrophotographic apparatus wherein a corona wire (1) is provided in a charging chamber (4) opposite the centre of an opening (4a) therein and parallel to a photosensitive material (3) to charge said photosensitive material (3); a conductive member (5, 6) is provided parallel to said photosensitive material (3) on each side of said corona wire (1), said conductive members (5, 6) acting as grounded electrodes to generate corona ions, characterized in that insulating film masking means (7) is provided extending inwardly from the periphery of said opening (4a) facing the photosensitive material (3) so that the masking means (7) can be brought nearly or completely into abutment with said photosensitive material (3).

2. A charging device for electrophotographic apparatus as claimed in claim 1, characterized in that said masking means (7) extends toward the photosensitive material by a distance greater than 1.5 times the film thickness and is spaced within 2.0 mm from the upper surface of said photosensitive material (3).

3. A charging device for electrophotographic apparatus as claimed in claim 1 or 2, characterized by a means (10) to cause said masking means (7) and said photosensitive material (3) to closely abut with each other by pressing said photosensitive material (3) toward said masking means (7) at the time of charging.

4. A charging device for electrophotographic apparatus as claimed in any one of claims 1 to 3, characterized in that said conductive member (5, 6) is made of a thin plate.

5. A charging device for electrophotographic apparatus as claimed in any one of claims 1 to 4, characterized in that said conductive member (5, 6) is so positioned that the line connecting its bottom end with said corona wire (1) and a horizontal plane (H) parallel to the photosensitive material (3) and crossing said corona wire (1) form an angle of +/-30° or less.

6. A charging device for electrophotographic apparatus as claimed in any one of claims 1 to 5, characterized in that the periphery of the opening (4a) of said chamber (4) is made of a conductive body (4c) and the conductive body (4c) is applied with a bias potential which is substantially equivalent to the surface potential of said photosensitive material.