EP0389229A2

EP0389229A2 - Image forming apparatus

Info

Publication number: EP0389229A2
Application number: EP90302946A
Authority: EP
Inventors: Satoshi Matsumoto; Shinobu Nakata; Hideshi Ishihara; Hiroshi Terada
Original assignee: Matsushita Electric Industrial Co Ltd
Current assignee: Panasonic Holdings Corp
Priority date: 1989-03-22
Filing date: 1990-03-19
Publication date: 1990-09-26
Also published as: KR900014945A; EP0389229A3

Abstract

An image forming apparatus includes a developer layer former (3) which charges developer to have a predetermined polarity in a developer unit (4) and forms a uniform thin layer of the developer on a developer carrier (1), an ion irradiator (6) which irradiates ions towards the developer on the developer carrier (1) partly so as to form a reversely charged latent image thereon in accordance with recording signals (5), and a transfer unit (8) which transfers the part of the developer on the developer carrier (1) corresponding to the latent image onto a recording medium (9) directly.

Description

BACKGROUND OF THE INVENTION

The present invention relates to an image forming apparatus such as a copying machine and a printer.
An image forming apparatus utilizing an electrophotographic system, which is disclosed, for example, in Japanese Patent Unexamined Publication No. 58-214175, is generally known as a conventional one for the copying machine, the printer and other like.
The conventional apparatus is constituted by, as shown in Figure 8, a photosensitive drum 101 serving as a latent image bearing medium, a charging unit 102 for establishing a uniform charge in the photosensitive drum 101, an optical system 103 such as a laser and an LED array for forming a latent image on the charged photosensitive drum 101 by applying a beam in accordance with recording signals, a developing unit 104 for making the latent image on the photosensitive drum 101 visible by means of a developer, a unit 105 for transferring the visible image on the photosensitive drum 101, which has been developed by the developing unit 104, to a recording medium 106, a cleaner 107 for removing the developer remaining on the photosensitive drum 101 after the transfer operation and disposing of the waste toner, and a fixing unit 108 for adhering the developer transferred onto the recording medium 106. Therefore, the apparatus has the advantage of higher speed operation and higher resolution owing to separation of function, in addition to the capability of recording to plain paper.
However, such electrophotographic system causes an enlargement of the apparatus because the image forming process is complicated. Further, since the developing portion where the developer is used and the cleaning process portion are separated from each other and, therefore, since the portions to be operated by the user for the supply of the developer, disposal of the waste toner and the like are dispersed, the electrophotographic system has been encountered the problems of the maintenability and the handling efficiency.
In recent years, an attempt has been made at integrating the photosensitive drum, the developing unit, the charging unit, the cleaner and the like together into a process cartridge so as to improve the maintenability. However, the process cartridge itself is large and the whole cartridge is replaced with a new one, resulting in an increase of the running cost.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an image forming apparatus which is made compact by adopting a simple image forming process and which exhibits excellent maintenability and handling efficiency for the user.
To this end, an image forming apparatus accord ing to the present invention is featured in that in a developer unit in which a developer, a developer carrier and layer forming means are integratedly unified, the developer charged to have a predetermined polarity is uniformly formed into a thin layer on the developer carrier by the layer forming means, a latent image charged to have the reversed polarity is formed on the developer carrier by applying ion onto the charged developer thereon in accordance with recording signals, and the developer only in the portion corresponding to the latent image is directly transferred from the developer carrier to a recording medium.
According to the present invention, therefore, the image forming process is very simplified since formation of latent image, developing and transfer are effected intensively on the developer carrier. This makes it possible to make compact the apparatus while disusing the photosensitive drum and the cleaner which are used in the conventional electrophotographic system.
Further, since the operation managed by the user is only the replacement of the developer unit including supply of the developer, the apparatus can be made very easy to maintain and handle.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a view showing an arrangement of an image forming apparatus according to one embodiment of the present invention;
Figure 2 is a perspective view of an ion generating portion and an ion controlling portion which constitute in combination a latent image forming means of the image forming apparatus shown in Figure 1;
Figure 3 is an illustration showing the manner of forming a latent image in the image forming apparatus shown in Figure 1;
Figure 4 is a view showing a modification of the image forming apparatus shown in Figure 1, which takes measures to prevent fog;
Figure 5 is a view showing another modification of the image forming apparatus shown in Figure 1, which takes measures to prevent unevenness of the density;
Figure 6 shows a further modification of the image forming apparatus shown in Figure 1, in which a bias voltage applying means is provided between a developer carrier and a brush roller as measures for preventing the developer from dropping and scattering;
Figure 7 is a view showing an arrangement of a color image forming apparatus according to the present invention; and
Figure 8 is a view showing an arrangement of a conventional image forming apparatus utilizing an electrophotographic system.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to Figures 1 and 2, a developer unit 4 comprises a developer carrier 1, casing containing developer 2 and a layer forming means 3 which are unified. The layer forming means 3 comprises a brush roller 3a, a supply member 3b and a blade 3c. The developer unit 4 is detachably mounted to an image forming apparatus likewise the process cartridge used in the electrophotographic system.
Ion irradiation means 5 comprises a generating portion 6 and a controlling portion 7. It is preferable to utilize the aerial discharge such as a corona discharge and a creepage discharge in order to generate ions. Figure 2 shows an example of the ion irradiation means 5 which utilizes the corona discharge. The generating portion 6 is formed by a so-called corona discharge device constructed such that a wire 6b of tungsten or the like extends inside a shield case 6a having a substantially U-shaped cross section. In the controlling portion 7, a sheet insulator 7a is provided with through holes 7d through which ions pass in accordance with dots to be recorded and which are arranged in a straight or zigzag manner. The sheet insulation 7 is also provided at one surface thereof with control electrodes 7b associated with the respective through holes 7d and at the other surface thereof with a common control electrode 7c to all the through holes 7d. The apparatus includes means 14 for applying a control voltage between the control electrodes 7b and 7c of the ion controlling portion 7. It is noted that the ion irradiation means 5 may be one which utilizes the creepage discharge as disclosed in Japanese Patent Unexamined Publication No. 57-501348.
The apparatus further includes transfer means 8, a recording medium 9, paper feeding means 10 constituted by a paper feeding roller and a cassette, and fixing means 11.
Next, a basic image forming process effected by means of this apparatus will be described.

(1) Uniform charge and uniform formation of thin layer of developer

In the developer unit 4, the developer 2 is supplied by the layer forming means 3 onto the developer carrier 1 to form a thin layer thereon. The supply member 3b serves to stir the developer 2 in the developer casing and supply it to the brush roller 3a. The brush roller 3a supplies a constant quantity of developer 2 to the developer carrier 1 while holding it in the brush. The developer 2 thus supplied is successively uniformed by the blade 3c into a thin layer on the developer carrier 1 rotating in the direction of an arrow shown in Figure 1. At this time, the developer 2 is charged to have an predetermined polarity due to the friction with the layer forming means 3. It is desirable, for assuring the uniform charge, that this thin layer is formed to have a thickness equal to that of a single layer of the developer 2.

(2) Formation of latent image on developer

The ion generating portion 6 of the ion irradiation means 5 generates ions having a polarity opposite to that of the charged developer 2. Irradiation of the ion from the generating portion 6 to the developer 2 is controlled by the ion controlling portion 7 in accordance with the recording signals so that only the developer in a portion corresponding to an image is charged to have the reversed polarity so as to form a charged latent image in order.

(3) Transferring and fixing

The portion of the developer 2 on the developer carrier 1 forming the charged latent image is transferred onto the recording medium 9 sent out from the paper feeding means 10 in the direction of an arrow shown in Figure 1 at a transferring position by means of the transfer means 8, such as a corona transfer device, which has a polarity opposite to that of the charged latent image, that is, the same polarity as the charge by the layer forming means 3.
The recording medium 9 is fed through the fixing means 11 in order to fix a transferred visible image of the developer 2 thereon, thereby completing a recorded image.
A part of the developer carrier 1 having passed through the transfer means 8 enters again the developer casing. The developer 2 is charged, and supplied onto and uniformed into a thin layer on the developer carrier 1, including the portion from which the developer has been removed by the transfer operation, by the layer forming means 3, thus preparing for the succeeding recording.
Next, more specified description will be given on an embodiment of the image forming apparatus according to the present invention.
The developer 2 used in this embodiment is a one-component non-magnetic toner with a particle size of about 10 µm, which is composed of styrene acrylic resin binder, pigment, charge controller such as silica, and the like. The developer carrier 1 used is a conductive metallic roller made of stainless steel, aluminum or the like. It is noted that the developer carrier 1 is provided in the surface thereof with fine concaves formed by means of sand-blasting or the like in order to facilitate the formation of the uniform thin layer of the developer 2.
The brush roller 3a of the layer forming means 3 has thin conductive filaments planted around a core, which filament has a specific resistance of 10⁵ to 10⁸ Ω·cm and is made of rayon containing carbon. The supply member 3b of the means 3 is made of a sheet of polyethylene terephthalate or the like. The blade 3c of the means 3 is made of an elastic material such as urethane rubber to press the developer carrier 1 at a line pressure of about 40 g/cm.
As the developer carrier 1 is rotated in this state, a uniform thin layer of the developer 2 is formed on the developer carrier 1, the thickness of which is substantially about 30 µm. The axial surface potential of the developer carrier 1 is about +25 V uniformly and the amount of charge of the developer 2 is about +1.5 µC/g.
The ion generating portion 6 is a corona discharge device in which the tungsten wire 6b (φ 60 µm) extends inside the shield case 6a having a substantially U-shaped cross section, as mentioned above. The ion controlling portion 7 includes the sheet insulator 7a having a thickness of 0.4 mm. A plurality of through holes 7d (each having a diameter of 0.4 mm) are formed in the insulator 7a in a gigzag manner, the number of which is in accordance with the recording dots. Ions pass through the holes 7d from the generating portion 6 to the developer carrier 1. The control electrodes 7b and 7c made of copper foil of 20 µm thickness are disposed opposite surfaces of the insulator 7a. The control electrodes 7b are fixed on one surface of the insulator 7a concentrically with the respective through holes 7d as shown in Figure 2, while the control electrode 7c extends on the other surface thereof common to all the through holes 7d.
Irradiation of the ions to the developer 2 on the developer carrier 1 is controlled by applying the voltage to the control electrodes 7b and 7c through the controlling voltage applying means 14. Namely, the developer carrier 1 is grounded, while the control electrode 7c is always applied with a voltage of -200 V. In case of no irradiation of negative ion onto the developer 2 (or when the irradiation of ions is inter rupted), the control electrode 7b is held at 0 V and an electric field is applied between the control electrodes 7b and 7c, which is inverse to the electric field between the wire 6b and the developer carrier 1, thereby preventing the negative ions from passing through the hole 7d. In case of irradiation of the negative ions onto the developer 2 to form a latent image pattern, the voltage of -300 V is applied to the control electrodes 7b to form an electric field of the same direction as the electric field between the wire 6b and the developer carrier 1, thereby allowing the flow of ions to pass through the hole 7d.
In this way, the latent image pattern is formed in accordance with the recording signals. As a result, as shown in Figure 3, the charge of the developer 2 in the portion corresponding to the latent image is changed from positive to negative, the surface potential of the portion of the developer carrier 1 corresponding to the latent image becomes about -70 V, and the amount of charge of the developer 2 becomes about -4.0 µC/g.
In order to transfer the developer 2 only in the portion corresponding to the latent image, which has been thus charged to have the negative polarity to the recording medium 9, a corona charger in used as the transfer means 8. A voltage of +6 kV is applied to a wire of the corona charger 8 to effect the transfer onto the recording medium 9. As a result, the developer in the portion corresponding to the latent image pattern is transferred. In this case, the recording medium 9 used is the plain paper for copying machine use.
Finally, a so-called thermal fixing device, which is used in the copying machine of the electrophotographic system and other like, is used as the fixing means 11 so as to melt and fix the developer 2 on the recording medium 9, thereby forming an image on the recording medium 9 in accordance with the recording signals.
The basic structure and the image forming process of the image forming apparatus according to the present invention have been described above with reference to the practical embodiment. Next, description will be given below of significant means for improving the picture quality and reliability of the recorded image based on the confirmation obtained through experiments.

(a) Preventing of fog

In the above image forming process, when the developer 2 in the portion corresponding to the latent image is transferred onto the recording medium 9, there is caused a so-called fog phenomenon that the developer 2 in the portion other than that applied with ions (or in the portion other than that corresponding to the latent image) is somewhat transferred as well.
In connection with the fog, even when the developer 2 is uniform into a thin layer on the developer carrier 1 and charged apparently to have the predetermined polarity with a surface potential of about +25 V by the layer forming means 3, if the developer 2 is laminated on the developer carrier 1 into multiple layers, for example, two to three layers, the developer 2 is not always charged entirely to have the positive polarity but partly has the negative polarity. The developer 2 thus charged to have the negative polarity, even if it is in the portion other than that corresponding to the latent image, is susceptible to be transferred to the recording medium 9 because of its polarity opposite to that of a charge for effecting the transfer, thereby causing the fog.
Therefore, in consideration of the uniform charge of the developer 2, it is preferred, as mentioned before, that the thin layer of the developer 2 is formed to have a thickness equal to that of a single layer of the developer 2, provided that the density of the recorded image is satisfactory.
To cope with the fog, as shown in Figure 4, a corona charger (using a wire of φ 60 µm) is provided at a position preceding to the ion irradiation means 5 as a first supplemental charging means 12 for the developer 2. A voltage of +4 kV is applied to the charging means 12 to generate and irradiate positive ions to the developer 2 on the developer carrier 1. In this case, the surface potential of the developer 2 is about +100 V. Therefore, the ion applying means 5 irradiate the ions to the developer 2 in the same manner as described before so as to form a latent image pattern. After this, a voltage of about +6 kV is applied to the corona transfer device 8 to effect the transfer to the recording medium 9. As a result, only the developer 2 in the portion corresponding to the latent image pattern can be transferred to the recording medium 9 without causing any fog.
According this, the developer 2 uniformly charged and formed into a thin layer on the developer carrier 1 by the layer forming means 3 is further charged with use of the charging means 12 in advance of the ion irradiation means 5, and therefore, the reversed polarity on the developer 2 can be canceled, resulting in the prevention of the fog. This contributes to the improvement of the picture quality of the recorded image.

(b) Prevention of unevenness of density

Further, in the case of repeating the above-described image forming process, the developer 2 cannot be supplied sufficiently to the portion on the developer carrier 1 from which the developer 2 has been removed by the transfer operation, thereby causing a little unevenness of the density on the recorded image.
Namely, in general, the developer 2 is uniformly charged to have the positive polarity and laminated into a thin layer by the layer forming means 3 in the developer unit 4, thus preparing for the succeeding recording. However, in case that the developer 2 having the negative polarity and remaining slightly on the developer carrier 1 still after having passed through the transfer means 8 enters the developer casing while sticking on the developer carrier 1, it is absorbed electrostatically to the developer 2 having the positive polarity and sufficiently sticking on the conductive brush roller 3a, so that the developer 2 cannot be satisfactorily supplied onto and formed into a thin layer on the developer carrier 1.
To cope with this, as shown in Figure 5, a corona charger (using a wire of φ 60 µm) is provided as a second supplemental charging means 13 at a position between the transfer means 8 and the developer unit 4 with respect to the direction of rotation of the developer carrier 1. A voltage of +4 kV is applied to the charging means 13 to generate and forcedly irradiate positive ions to the developer 2 on the developer carrier 1. Then, the developer 2 can be kept in the positive polarity and not absorbed by the developer 2 having the positive polarity on the brush roller 3a. Therefore, the developer 2 can be supplied sufficiently to the portion on the developer carrier 1 from which the developer 2 has been removed by the transfer operation without causing any unevenness of the density.
Namely, the developer 2 having the reversed polarity and remaining on the developer carrier 1 still after the transfer operation is charged forcedly to have the normal polarity that it has by the charging means 13. Therefore, in repeated forming a thin layer of the developer 2 on the developer carrier 1 by the layer forming means 3, the uneven supply of the developer 2, that is, the inequality of the layer thickness on the developer carrier 1, which is attributable to the developer 2 having the reversed polarity, can be eliminated and, hence, the unevenness of the density can be prevented, thereby improving the picture quality of the recorded image.
It is noted that the amount of charge of the developer 2 by the first and second charging means 12 and 13 varies depending upon the amount of ions to be irradiated. However, it is possible to control the amount of ions to be irradiated by changing the voltage to be supplied to the charging means and the time period during which the power is supplied thereto depending on the recording speed. Such control is conducted in accordance with the surface potential at the state free from the fog and with the state free from unevenness of the density attributable to the uneven supply of the developer. The corona charger is used as first and second charging means 12 and 13, however, it is also possible to use a solid-state discharge device in which a pair of strip electrodes are formed through a dielectric such as resin layer or ceramics. In the latter case, it is enable to make an apparatus compact and easy to handle.

(c) Prevention of drop and scatter of developer

In the above-described image forming apparatus, the developer carrier 1 carrying the developer 2 is partially exposed out of the developer casing. When an excessive impact or vibration is given to the apparatus, the developer 2 on the developer carrier 1 is caused to drop and scatter, resulting in the problem that the apparatus and recorded image are stained.
To cope with this, as shown in Figure 6, means 15 is provided to apply a bias voltage between the developer carrier 1 and the layer forming means 3, particularly, the brush roller 3a. After the image recording to a single or a series of sheets of paper has been finished, the developer carrier 1 is made to further rotate without recording operation. During such rotation, a negative voltage (ranging from -50 to -100 V preferably) is applied between the developer carrier 1 and the brush roller 3a by the bias voltage applying means 15, so that the developer 2 having the positive polarity on the developer carrier 1 is attracted into the developer unit 4 by the brush roller 3a, thereby removing almost all of the developer 2 from the developer carrier 1. On the other hand, on the occasion of the recording, the developer carrier 1 and the brush roller 3a are made to have the same potential, namely a switch of the bias voltage applying means 15 is turned as shown in Fig. 6. Thereafter, the developer carrier 1 is rotated in advance of the start of the recording, and then the developer 2 can be supplied sufficiently onto the developer carrier 1 and formed into a thin layer thereon.
Namely, since a bias voltage is applied between the developer carrier 1 and the brush roller 3a, there is developed an electric field which causes the developer 2 on the developer carrier 1 to be easily attracted to the brush roller 3a electrostatically. Accordingly, the developer 2 is returned from the developer carrier 1 to the brush roller 3a, that is, into the developer unit 4, and then the developer 2 can be removed from the portion of the developer carrier 1 exposed out of the developer casing.
Removal of the developer 2 from the developer carrier 1 is made by applying a voltage between the developer carrier 1 and the brush roller 3a, and therefore, it is possible to prevent the developer 2 from dropping and scattering from the developer carrier 1 even if it is subjected to an excessive vibration or impact when the apparatus is standing by or out of use. This contributes to the prevention of the stain of the apparatus and the recorded image and hence to the improvement of the reliability of the apparatus.
As has been described above, according to the present invention, since it is possible to effect formation of latent image, developing and transfer directly on the developer carrier 1 by forming a charged latent image on the developer 2 on the developer carrier 1 in accordance with the recording signals, and since the transferred portion on the developer 2 can be formed again into a thin layer on the developer carrier 1 can be recovered by the layer forming means 3 within the developer unit 4, the process can be simplified and the apparatus can be made compact and thin while disusing the photosensitive drum and the cleaner which are used in the conventional electrophotographic system.
Moreover, it is only the developer unit 4 that is replaced by the user on renewal of the apparatus, and therefore, the apparatus can be maintained and handled very easily.
Further, since the movable portion is only the developer carrier 1, it is possible to improve the stillness and the reliability of the apparatus as a whole.
In addition, the provisions of the first and second charging means 12 and 13, and the bias voltage applying means 15 assure the prevention of the fog, unevenness of the density, and drop and scatter of the developer.
It is noted that it is possible to realize a color image forming apparatus, as shown in Figure 7, by making use of the image forming process according to the present invention. The structure and the operation corresponding to each color are the same as the embodiment described in conjunction with Figures 1 and 2, and therefore, explanation thereof will be omitted.
Reference numerals 30Y, 30M and 30C denote developer carriers for yellow, magenta, and cyan, respectively; numerals 40Y, 40M and 40C denote developer units for yellow, magenta, and cyan, respectively; numerals 50Y, 50M and 50C denote latent image forming means for yellow, magenta, and cyan, respectively; and numerals 60Y, 60M and 60C denote transfer means for yellow, magenta, and cyan, respectively. An image forming process using a black developer may be provided before the yellow process or after the cyan process.
The respective color developer which form the respective latent images and reversely charged by irradiation of ions are transferred in order onto the recording medium 9 sent out from the paper feeding means 10 in superimposition manner at the positions corresponding to the respective transfer means 60. Then, after passing through the fixing means 11, a color recorded image is obtained.
In this case, like the monochromatic recording, first and second charging means may be provided around the respective developer carriers in order to prevent the recorded image from suffering the fog and the density unevenness, and bias voltage applying means may be provided between the developer carrier and the layer forming means, which is not shown, in order to prevent the drop and scatter of the developer.
Accordingly, by arranging a plurality of image forming processes used in the monochromatic recording in series correspondingly to the number of colors used, it is possible to provide a color image forming apparatus which has the effects of the basic structure, which can be made compact and thin, and which is easy to maintain and handle.
Moreover, although the one-component non-magnetic toner is used as the developer in the embodiment, the developer in the developer unit may be either of one- component and two-component toners so far as the developer is uniformly charged and laminated into a thin layer on the developer carrier at least after passing through the blade.
Furthermore, the brush roller and the blade of the layer forming means may be a roller of an elastic material such as sponge and a rigid blade which is in or out of contact with the developer carrier, respectively.
In addition, although the polarity of the first and second charging means and is the same as that of the developer charged in the developer unit in the embodiment, the polarity of the may be either positive or negative so far as the polarity of the developer in the portion corresponding to the portion other than the latent image and the polarity of the developer in the portion corresponding to the latent image charged by the ion irradiation means are differed from each other clearly.

Claims

1. An image forming apparatus comprising:
a unified developer unit including a developer casing containing developer therein, a developer carrier, and means for charging said developer at a predetermined polarity and for forming a uniform thin layer of said developer on said developer carrier;
means for irradiating ions towards said developer on said developer carrier to partly charge it so as to have the reversed polarity in accordance with recording signals, thereby forming a latent image on said developer; and
means for transferring a part of said developer on said developer carrier corresponding to said latent image onto recording medium.

2. An image forming apparatus according to Claim 1, wherein said layer forming means include a brush roller for supplying said developer onto said developer carrier, and a blade for forming a thin layer of said developer thereon.

3. An image forming apparatus according to Claim 1, wherein said irradiation means include an ion generating portion and an ion controlling portion for irradiating ions partially to said developer on said developer carrier in accordance with said recording signals.

4. An image forming apparatus according to Claim 1, wherein said apparatus further comprises additional means for charging said developer on said developer carrier uniformly at a predetermined polarity.

5. An image forming apparatus according to Claim 4, wherein said additional charging means charge said developer on said developer carrier at the same polarity said charging means do.

6. An image forming apparatus according to Claim 4, wherein said additional charging means are located upstream of said ion irradiation means in the direction of progress of said developer carrier.

7. An image forming apparatus according to Claim 4, wherein said additional charging means is located downstream of said transferring means in the direction of progress of said developer carrier.

8. An image forming apparatus according to Claim 1, wherein a voltage applying means is provided between said developer carrier and said layer forming means so as to collect said developer on said developer carrier into said developer casing after the recording by the action of an electric field developed by said voltage applying means.