Developing apparatus which recycles used developer by effectively and thoroughly conveying the used developer from a developing sleeve to a developer conveying member

An apparatus for the development of electrostatic images includes a developer storing section, a developing sleeve, a first magnetic field generating member, a developer layer thickness regulator, a second magnetic field generating member and a developer conveying member. The second magnetic field generating member faces the developing sleeve and forms an attracting magnetic field to attract the carrier particles from the developing sleeve. The second magnetic field generating member, moreover, has a closer section located close to the developing sleeve and a farther section located far from the developing sleeve, the farther section being located downstream of the closer section with regard to the conveying direction. The strength of the attracting magnetic field at the farther section is stronger than that at the closer section so that the carrier particles are removed from the developing sleeve while the magnetic carrier particles is conveyed from the closer section to the farther section by the developing sleeve.

BACKGROUND OF THE INVENTION 
The present invention relates to a developing apparatus incorporated in an 
image forming device such as a copy machine, a printer, a facsimile, etc. 
Regarding developing methods utilized for electrophotographic devices, it 
is well known that digital printers, digital copiers, etc. employ a 
reversal developing method, while conventional copiers, etc. employ a 
standard developing method. In the reversal developing method, after a 
latent electrostatic image is formed on an image bearing element 
(hereinafter, referred as a photoreceptor drum) through processes of 
electrostatic charging and exposure of light emitted from a light source 
such as lasers, LED (Light Emitting Diode), etc., a visible toner image is 
developed, in which toner is charged at the same polarity as the charged 
photoreceptor drum. For instance, when the photoreceptor drum is charged 
negatively, a visible toner image is formed on the photoreceptor drum by 
developing with toner also charged negatively in accordance with the 
pattern of electrostatic potential differences generated from the exposure 
of light. After development, a visible toner image formed on the 
photoreceptor drum is transferred to a copy medium by charging the copy 
medium at a polarity opposite to the toner, using corona discharge of a 
transfer apparatus. After transfer, the attraction force of the copy 
medium with the photoreceptor drum is decreased by dropping the 
electrostatic potential of the copy medium with AC or DC corona discharge, 
which is then peeled from the photoreceptor drum by its own weight. 
In a conventional developing apparatus, a rotatable developing sleeve 
(developer bearing element) is located adjacent to a rotatable 
photoreceptor drum. Said developing sleeve, formed as a hollow cylinder, 
is mounted in a housing, wherein opening of the housing is located 
opposite the surface of said photoreceptor drum. A developing bias voltage 
of, for instance, -650V DC plus 2700V AC with a frequency of 8000 Hz is 
applied to said developing sleeve. A stationary magnet is mounted in the 
hollow interior of said developing sleeve, while a two component 
developer, composed of toner powder and magnetic particles (carrier), is 
attracted to the outer surface of said developing sleeve. 
A developing apparatus is comprised of: 
a housing means to store a two-component developer composed of toner powder 
and carrier particles, 
a developing sleeve (or a developing roll) as a transfer means of the 
developer, wherein a magnetic roll, serving as a magnetic field generating 
means, is mounted 
a developer feeding roller (hereinafter, termed a feed roller) 
a developer layer regulating member to regulate the thickness of the 
developer layer formed on the surface of the developing sleeve at a 
predetermined value, and 
a developer agitating screw (hereinafter, referred to as an agitating 
screw). 
The toner in said housing means, which is fed from a toner cartridge 
through a toner supply opening located at a top portion of said housing 
means, is mixed and agitated with the developer, stored in said housing 
means, by means of said agitating screw. As a result, toner of uniform 
density is supplied on the outer surface of said developing sleeve by 
means of said feed roller. 
In the configuration of the developing apparatus mentioned above, since the 
toner is attracted to the image bearing element at the developing 
position, it is required that the residual toner and magnetic carrier 
should be recycled to the developing sleeve after scrubbing the used 
developer off the developing sleeve and agitating it with toner at a 
predetermined ratio. 
For this purpose, there has been a technique that the developer is scraped 
off by physically contacting a scraper blade against the developing 
sleeve. 
According to the above technique, since the scraper blade directly contacts 
the developing sleeve, a material with high abrasion resistance, such as 
stainless steel (SUS), etc., should be employed for the developing sleeve. 
An employment of SUS, however, has required relatively high cost, due to 
its difficult workability, etc. 
Another technique is to remove the developer by means of a magnetic member 
arranged in a state of non-contacting with an aluminum alloy developing 
sleeve, which is relatively low in cost, set forth in Japanese Tokkai-sho 
No.60-91373, Tokkai-sho No.62-105174, Tokkai-hei No.6-194962, etc. 
In this technique, a magnetic brush is formed in a space between the 
developing sleeve and the magnetic member, which is inductively magnetized 
by a magnet mounted stationary in the hollow interior of the developing 
sleeve. This magnetic brush acts as a blocking means to scrape the 
developer off the developing sleeve. 
Although the above mentioned technique may be more or less effective to 
scrape the developer off the developing sleeve, some developer invariably 
passes through the magnetic brush due to the massive pressure of moving 
developer on the developing sleeve. Therefore, it may be virtually 
impossible for this technique to thoroughly remove said large amount of 
developer off the developing sleeve. 
In addition, since the induced magnetic force of the magnetic member is no 
stronger than that of the magnet mounted stationary inside the developing 
sleeve, no magnetic force oriented to scrubbing the developer off the 
developing sleeve is generated. Moreover, some quantity of the developer 
removed from the developing sleeve directly moves around the developing 
region of the image bearing element. Such developer would recombine with 
the surface of the developing sleeve without passing the agitating 
process, resulting in developing defects. 
As mentioned above, it is not effective for this purpose only to arrange 
the magnetic member in a state of non-contact with the developing sleeve. 
Especially, the conventional techniques are insufficient for a system 
where development is achieved by forming a thin layer of the developer on 
the developing sleeve. 
SUMMARY OF THE INVENTION 
To overcome the above mentioned drawbacks in the prior art, it is an 
objective of the present invention to provide a developing apparatus with 
which formation of high quality images is achieved, without occurrence of 
ghost images, developer deterioration nor abrasive scars on the developing 
sleeve. 
The present invention relates to a developing apparatus, which achieves the 
above mentioned objectives, comprising; 
a developer storing section for storing two-component developer including 
toner particles and magnetic carrier particles; 
a developing sleeve being a rotatable hollow cylinder; 
a first magnetic field generating member provided inside the developing 
sleeve and having plural magnetic poles to attract the magnetic carrier 
particles on the developing sleeve so that the developing sleeve conveys 
the two-component developer with its rotation; 
a developer layer thickness regulator to regulate the thickness of the 
two-component developer on the developing sleeve; 
a second magnetic field generating member provided so as to face the 
developing sleeve and for forming attracting magnetic field to attract the 
carrier particles from the developing sleeve, wherein the second magnetic 
field generating member has a closer section located close to the 
developing sleeve and a farther section located far from the developing 
sleeve, the farther section is located downstream of the closer section 
with regard to the conveying direction, and the strength of the attracting 
magnetic field at the farther section is stronger than that at the closer 
section so that the carrier particles are removed from the developing 
sleeve while the magnetic carrier particles is conveyed from the closer 
section to the farther section by the developing sleeve; and 
a developer conveying member provided in the vicinity of the farther 
section of the second magnetic member and to convey the removed carrier 
particles to the developer storing section. 
The present invention further relates to the above developing apparatus, 
wherein the first magnetic field generating means includes a first magnet 
and a second magnet both having the same polarity, the first magnet is 
located adjacent to the second magnet so that a repulsing magnetic field 
to remove the carrier particles from the developing sleeve is formed 
between the first magnet and the second magnet, the first magnet is 
located upstream of the second magnet in terms of the rotating direction 
of the developing sleeve, and the second magnetic field generating member 
is located adjacent to the first magnet.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Prior to describing the embodiments, the configuration and performance of 
the color printer which incorporates a plurality of the developing 
apparatus embodied in the present invention will be described in detail, 
referring to a lateral cross sectional view shown in FIG. 1. 
In this color printer, as a color image forming device, color image 
formation is achieved by overlapping each color image which is 
sequentially formed on the image bearing element and transferring the 
completed image on a sheet of recording paper, one at a time. 
Subsequently, the recording paper is peeled off the image bearing element 
by means of a separating means. 
In FIG. 1, grounded photoreceptor drum 10 as an image bearing element is 
rotationally driven in the direction shown by the arrow, whereon a coating 
layer of OPC (Organic Photo Conductor) is formed. Scorotron charger 11 
provides the surface of photoreceptor drum 10 with a uniform charge of 
high electrical potential V.sub.H, utilizing corona discharging action 
between a corona discharge wire and a screen grid, kept at grid voltage 
V.sub.G. Preceding the charging operation by scorotron charger 11, the 
surface of photoreceptor drum 10 is uniformly pre-charged by irradiating 
light with PCL (Pre-Charging Lamp) comprised of LED (Light Emitting 
Diode), etc., in order to eliminate an image hysteresis on the surface of 
photoreceptor drum 10, which was formed during the previous printing 
process. The image hysteresis mentioned above is referred as a residual 
image pattern on the surface of photoreceptor drum 10, or as a memory of 
photo sensitive element, which remained from the preceding image forming 
processes of charging and image exposure. 
After uniformly charging the surface of photoreceptor drum 10, image 
exposure is performed by image exposure means 13, based on image signals. 
Main scanning for the image formation is achieved by modulating a path of 
a laser beam emitted from laser diode (not shown) with a rotatable polygon 
mirror, a f.theta. lens and a cylindrical lens, which are mounted in image 
exposure means 13. The formation of a latent image is completed in 
association with the sub-scanning rotation of photoreceptor drum 10. In 
this embodiment, exposure of light is applied for the text portion of an 
original to form a reversal latent image so that the exposure potential 
V.sub.L is lower than the charge potential V.sub.H. 
Developing assembly 14 is mounted at the periphery of photoreceptor drum 
10, comprising developing units 14Y, 14M, 14C, 14K, each of which contains 
a two-component developer such as yellow (Y), magenta (M), cyan (C), black 
(B), etc. respectively. 
Initially, the first color image of yellow is developed by means of 
developing sleeve 141 which contains magnet roller 142 and rotates with 
the developer being absorbed on its surface. The developer comprises 
carriers of magnetite coated with insulating resin, toner made of 
polyester dispersed with pigments for each color, additives for charge 
controlling, silica, titanium oxide, etc. and is transferred to the 
developing region after regulating the thickness of the developer layer on 
developing sleeve 141 at a range of 100 to 600 .mu.m by means of developer 
layer regulating member 143, described later. 
In the developing region, the gap between developing sleeve 141 and 
photoreceptor drum 10 is set at a range of 0.5 to 1.0 mm which is larger 
than the thickness of the developer layer, wherein the overlapped bias 
voltage of DC voltage V.sub.DC and AC voltage V.sub.AC is applied. Since 
the DC bias voltage V.sub.DC, the charge potential V.sub.H of 
photoreceptor drum 10 and the charge potential of toner have the same 
polarity, the visual image formation with toner (the reversal development) 
is achieved in such a manner that the toner eventually left from the 
carrier by the AC bias voltage V.sub.AC is attracted to regions charged 
with exposed potential V.sub.L, which is lower than the DC bias voltage 
V.sub.DC, instead of being attracted to regions charged with non-exposed 
potential V.sub.H which is higher than the DC bias voltage V.sub.DC. 
After the formation of the first color image, photoreceptor drum 10 is 
uniformly re-charged by means of scorotron charger 11, as the initial 
process of the second color image formation and then, a latent image is 
formed by means of image exposure means 13, based on the second image 
signal for the magenta portion of the image. In the formation of the 
second color image, however, the pre-charging process with PLC 12 is 
excluded though it is applied in the formation of the first color image, 
for fear that the abrupt change of the potential around the toner would 
cause scattering of the first color toner image. 
The whole surface of photoreceptor drum 10 is uniformly re-charged with 
photoreceptor drum voltage V.sub.H. In the blank regions of the first 
color image, the formation of the latent image and the development of the 
visual toner image are achieved in the same manner as in the formation of 
the first color image. While, in the filled regions of the first color 
image, a latent image with slightly higher potential V.sub.M than the 
potential V.sub.L of the first color image is formed under the influence 
of the shading effect by the toner already attached in the formation of 
the first color image and the charge existing in the toner itself. 
Therefore, the second color image of toner is developed in response to the 
potential difference between the DC bias voltage V.sub.DC and the 
potential V.sub.M mentioned above. 
Regarding the third color image, cyan, and the fourth color image, black, 
the same image formation processes as those of the second color, magenta, 
are carried out. Finally, the visual image, superposed of the four colors 
of toner, is formed on the surface of photoreceptor drum 10. 
To control feeding of each of developing units 14Y, 14M, 14C and 14K, with 
new toner of each color, a toner supplying unit comprises a plurality of 
detachable toner cartridges 15 (Y, M, C, K), toner storage means 16 (Y, M, 
C, K) and toner transfer means 161 (Y, M, C, K). 
On the other hand, a copy medium (copy paper, etc.) drawn from paper 
supplying cassette 20 by means of crescent roller 21 is temporally stopped 
in the vicinity of the resist sensor after passing between intermediate 
conveyance rollers 22A and 22B. At the appropriate timing for transfer, a 
copy medium is fed to the transfer region by the rotating action of paired 
resist rollers 23. 
In the transfer region, synchronizing with the transfer timing, a 
multi-color image of toner on photoreceptor drum 10 is transferred onto 
copy medium P, one at a time, by pressing copy medium P against 
photoreceptor drum 10 with transfer means 17 which comprises a transfer 
roller, etc. to apply a transfer voltage onto the surface of photoreceptor 
drum 10. 
After the image transfer, copy medium P is discharged by means of 
separating means 18 such as a saw-toothed electrode, etc. to be peeled 
from photoreceptor drum 10. After separation, copy medium P is conveyed 
into fixing unit 24 to adhere the toner onto copy medium P with heat and 
pressure by means of heating roller (upper roller) 241 and pressing roller 
(lower roller) 242, and then, the finished copy medium is delivered to 
paper delivery tray 26 located outside of the device, passing between 
paired delivery rollers 25A, 25B and 25C. After the completion of the 
image transfer, transfer means 17 leaves the surface of photoreceptor drum 
10, to standby for formation of the next toner image. 
On the other hand, after the separation of copy medium P, the residual 
toner remaining on photoreceptor drum 10 is scraped off by pressing the 
surface with cleaning blade 191 mounted in cleaning unit 19. The next 
image formation will be started again through the pre-charging process by 
PLC 12 and the uniform charging process by scorotron charger 11, wherein 
immediately after cleaning the surface of photoreceptor drum 10, cleaning 
blade 191 retracts from the surface, and goes into the standby position. 
The residual toner, scraped into cleaning unit 19 by cleaning blade 191, 
is conveyed by conveyance screw 192 and stored in a residual toner storage 
(not shown). 
FIG. 2 shows a lateral cross sectional view of the developing assembly 
comprised of a plurality of developing units 14Y, 14M, 14C, 14K, embodied 
in the present invention, wherein developing units 14Y, 14M, 14C, 14K, are 
arranged vertically above each other so that each developing sleeve 141 
faces the surface of photoreceptor drum 10. Since developing units 14Y, 
14M, 14C, 14K, have substantially the same configuration, only the 
function of developing unit 14Y will be described below, referring to it 
as developing unit 14. 
FIG. 3 shows a lateral cross sectional view of developing unit 14 embodied 
in the present invention. FIG. 4 shows an enlarged lateral cross sectional 
view of developing unit 14 embodied in the present invention, illustrating 
the arrangement of developing sleeve 141 and the peripheral members. 
Specifically, FIG. 3 and FIG. 4 show: 
a developer housing (developer storage) 140 to store two-component 
developer composed of carrier and toner; 
a developing sleeve 141 which attracts the developer onto its surface, 
mounting a magnetic field generating means (magnetic roller) 142 comprised 
of stationary magnets in its hollow interior; 
a developer layer regulating member 143 to regulate the thickness of the 
developer layer formed on developing sleeve 141 at a predetermined value. 
A plurality of magnetic poles N1, N2, N3, S1, S2 are alternatively arranged 
in magnetic roller 142 which is mounted stationary inside of developing 
sleeve 141. In such arrangement of magnetic poles, adjacent magnetic poles 
of N2 and N3 are arranged at the same polarity as each other to form a 
repulsive magnetic field which acts as a scraper means for removing the 
developer from developing sleeve 141, wherein the adjacent magnetic poles 
of N2 and N3 are referred as scraping magnetic poles. In addition, 
magnetic pole N3 is located opposite to a developer layer regulating 
member 143. While, in FIG. 3, E1 and E2 designate AC and DC voltage 
sources, respectively. 
It is desirable that the outer diameter of developing sleeve 141 is in the 
range of .phi.8 to .phi.60 mm. If the diameter is less than .phi.8 mm, it 
is impractical to make magnetic roller 142 with, at least, five magnetic 
poles comprised of N1, S1, N2, S2, N3 which are necessary for high quality 
image formation. 
In case the diameter of developing sleeve 141 is larger than .phi.60 mm, 
the size of the developing unit would also increase. Especially in a color 
printer incorporating a plurality of such developing units (for instance, 
developing units 14Y, 14M, 14C, 14K), the larger the developing units, the 
bigger the outer diameter of photoreceptor drum 10 is required. Therefore, 
it becomes difficult to realize a separating action of copy medium 
utilizing a curvature of the drum after image transfer and discharge 
operations. In addition, the whole scale of the color printer would also 
become larger, associating with large-sizing of developing unit 14 and 
photoreceptor drum 10. 
As for the image formation device embodied in the present invention, the 
revised version of Konica KL-2010 Color Printer (manufactured by Konica 
Corp.) has been employed, wherein the outer diameter of developing sleeve 
141 is .phi.18 mm and that of photoreceptor drum 10 is .phi.100 mm. 
A developer layer regulating member 143 is comprised of magnetic members, 
such as stainless steel, iron, etc., and its surface located opposite 
developing sleeve 141 faces magnetic pole N3, while a minimum gap is set 
to a small predetermined value A (for instance, A=0.35 mm). Consequently, 
a wedge type space is created along a portion of the surface of developing 
sleeve 141, gradually widening the minimum gap to the upstream of the 
rotational direction. 
Developer feeding roller (hereinafter, referred to it as a paddle wheel) 
144 formed with a plurality of paddles 144A feeds developing sleeve 141 
with the recycled developer, as well as feeding agitating screw 146 with 
the used developer scraped off the surface of developing sleeve 141. The 
gap between the rotational tip locus of paddles 144A and the outer surface 
of developing sleeve 141 is set to a small predetermined value B (for 
instance, B=0.5 mm) as shown in FIG. 4. 
In FIG. 3, the rotational direction of each agitating screws (developer 
agitating screw) 145 and 146 is designated by the arrow and the flow 
direction of the developer is designated by the outlined arrow. Agitating 
screws 145 and 146, arranged in parallel, are mounted in first agitating 
chamber 140b and second agitating chamber 140c respectively and are 
rotated in directions opposite from each other, wherein the two chambers 
are divided by wall 140a protruding straight-up from the bottom of 
developer housing 140, while the tops of the first and the second 
agitating chambers 140b and 140c are closed with top cover 140A. 
The toner conveyed from toner cartridge 15 through toner storage means 16 
and toner conveyance means 161 is supplied to the first agitating chamber 
140b through a toner supply opening (not shown) located at top cover 140A. 
FIG. 5 shows the strength distribution of magnetic field generated by 
magnetic roller 142. When developing sleeve 141, which surrounds magnetic 
roller 142, made of a non-magnetic material, rotates in the direction 
shown by the arrow, it is actually observed that a region attracting a 
little developer was created on the surface of developing sleeve 141 by 
scraping the developer off the surface, ranging from the vicinity of 
magnetic pole N3 to the vicinity of magnetic pole N2 toward the upstream 
direction of its rotation, due to the repulsive magnetic field generated 
by magnetic poles N2 and N3. It is impossible, however, to completely 
remove the developer from the surface of developing sleeve 141 only with 
the repulsive magnetic field generated by scraping magnetic poles N2 and 
Nb 3. 
In the developing unit embodied in the present invention, magnetic field 
generating member 148, which generates a magnetic field with 
characteristics different from those of magnetic poles N2 and N3, is 
mounted at a fixed location adjacent to developing sleeve 141 near 
magnetic pole N2 upstream of said repulsive magnetic field. In other 
words, the pole of magnetic field generating member 148 is located 
opposite the vicinity of magnetic pole N2 to generate an attracting 
magnetic field. Magnetic member 147 firmly holds magnetic field generating 
member 148 at a fixed position near the inner side of top cover 140A, as 
well as guiding the used developer from developing sleeve 141 towards 
paddle wheel 144. V shaped magnetic member 147, made of magnetic stainless 
steel (SUS430, etc.) or iron, is comprised of developer guiding surface 
147A facing developing sleeve 141, attaching portion 147B which is fixed 
to top cover 140A and bent portion 147C. The angle of inclination .theta. 
(referred as a setting angle of magnetic member) at bent portion 147C is 
set to, for instance, .theta.=35.degree.. 
Developer guiding surface 147A faces the vicinity of magnetic pole N2, 
while a minimum gap, between developer guiding surface 147A and developing 
sleeve 141, is set to a predetermined value C (for instance, C=3 mm). 
Consequently, a wedge type space is created along the surface of 
developing sleeve 141, gradually widening the gap downstream in the 
rotational direction. The tip of developer guiding surface 147A is located 
near said rotational tip locus of paddles 144A, while the gap between them 
is set to a predetermined value D (for instance, D.apprxeq.2 mm). 
Magnetic field generating member 148, made of a permanent magnet, etc. is 
fixed on the back of developer guiding surface 147A by means of a 
double-sided adhesive tape. Since magnetic field generating member 148 
does not exist in the bent portion of magnetic member 147, as shown in 
FIG. 2 and FIG. 3, the magnetic field strength generated by magnetic field 
generating member 148 is attenuated upstream of developer guiding surface 
147A. Therefore, the used developer on developing sleeve 141 would 
smoothly flow along developer guiding surface 147A without being attracted 
by the attenuated magnetic field downstream of the rotational direction. 
Thus, a developer miss-flow, caused by a developer jamming in the vicinity 
of attaching portion 147B, is prevented. 
As for magnetic field generating member 148, employment of a rubberized 
magnet, BQC14, which belongs to a ferrite group and is manufactured by TDK 
Co., showed excellent effects in the above mentioned application. Said 
BQC14 is made of an NBR gum mulled with a strontium ferrite powder. Its 
characteristics as a material are shown in the following. 
Residual Flux Density (Br): 230-250 [mT] 
Coercive Force: H.sub.cB 163-195 [kA/m] 
Intrinsic Coercive Force: H.sub.cJ 279-358 [kA/m] 
Maximum Energy Product (BH)max: 10.3-11.9 [kJ/m.sup.3 ] 
In addition to the BQC14 mentioned above, magnetic materials, such as 
rubber magnets of ferrite groups (BQJ05, BQA14, BQE14, BQK12), cobalt 
magnets of neodymium/iron/boron rare earth metal group (NEOREC magnet 
series), cobalt magnets of rare earth metal group (REC magnet series), 
compound magnets of wet anisotropy ferrite magnetic material group (FB 
series), etc. are available, which produce substantially the same effect. 
Although all of the magnetic materials listed above are manufactured by 
TDK Co., the scope of the available magnetic materials is not limited 
within them as far as an employed material satisfies the required values 
of residual flux density, coercive force and maximum energy product. 
FIG. 6(a) shows an arrangement of developing sleeve 141, magnet roller 142, 
magnetic member 147 and magnetic field generating member 148, while FIG. 
6(b) shows an enlarged lateral cross sectional view of FIG. 6(a). 
The dimensions of magnetic field generating member 148, made of BQC14, are 
shown in the following: 
Thickness a (length between poles N and S)=1 mm 
Height b=4 mm 
Width c (length parallel to the axial direction of developing sleeve 
141)=300 mm 
The two-component developer, agitated by agitating screws 145 and 146, is 
conveyed into the gap between the slanted surface of developer layer 
regulating member 143 and the outer surface of developing sleeve 141, by 
means of paddles 144A of paddle wheel 144, which rotates clockwise as 
shown in FIG. 3. The developing sleeve 141 attracts the two-component 
developer in the vicinity of the gap by means of the magnetic force 
emitted from magnetic roller 142, and conveys it to the developing region 
where the surface of photoreceptor drum 10 is located opposite the surface 
of developing sleeve 141, while the thickness of the developer layer on 
developing sleeve 141 is regulated within a range of 100 to 600 .mu.m by 
means of minimum gap A. 
After development, the developer attracted on the surface of developing 
sleeve 141 passes through minimum gap C located between magnetic member 
147 and developing sleeve 141. Upstream of rotational direction of 
developing sleeve 141, the attractive force for the developer is 
relatively weak in the vicinity of magnetic pole N2 where magnetic field 
generating member 148 does not exist, while the strength of the magnetic 
field generated under magnetic field generating member 148 increases 
downstream of the rotational direction of developing sleeve 141. 
In gap C, after the developer is scraped off developing sleeve 141 by 
scraping magnetic poles N2 and N3, magnetic field generating member 148 
attracts the scraped developer towards the upper region of paddle wheel 
144 so as to prevent its scattering onto developing sleeve 141. 
Since the developer is gradually attracted by magnetic field generating 
member 148 according as the developer moves toward the downstream 
direction of rotational developing sleeve 141, it becomes possible to 
scrape the developer off developing sleeve 141. Therefore, it is not only 
possible to remove a sufficient amount of the developer from developing 
sleeve 141 but also possible to play an effective role even in a system in 
which the developing process is carried out by forming a thin layer of the 
developer on developing sleeve 141. 
The developer scraped off developing sleeve 141 through the above mentioned 
processes moves downward diagonally along developer guiding surface 147A 
being magnetically attracted by it and drops onto the rotational paddle 
wheel 144 in the vicinity of gap D located at the end tip of developer 
guiding surface 147A. 
Then, the developer scraped off developing sleeve 141 is engaged by 
rotating paddles 144A and conveyed to the rotating agitational screw 146 
to be agitated and recycled. The end tip of developer guiding surface 147A 
is located adjacent to and just above the rotational tip locus of paddles 
144A to form gap D. 
Since gap B between the tip locus of paddles 144A and the outer surface of 
developing sleeve 141 is set to an optimally small value and paddles 144A 
rotate at gap B in the direction so as to scoop up developer scraped off 
developing sleeve 141, almost all of the developer dropped from developer 
guiding surface 147A is engaged by paddles 144A and conveyed to agitating 
screw 146 without passing up through gap B. 
FIG. 7 shows a lateral cross sectional configuration of the color image 
forming device incorporating intermediate transfer drum 60 as an image 
bearing element. In this embodiment, a latent image formed on the surface 
of rotating photoreceptor drum 10 is developed by means of developing unit 
14Y, which incorporates developing sleeve 141, magnetic roller 142 
comprised of scraping magnetic poles, and magnetic member 148, to form a 
toner image, of color Y, to be transferred onto intermediate transfer drum 
60. As well as the above, a latent image formed on the surface of 
photoreceptor drum 10 is developed by means of developing unit 14M to form 
a toner image, of color M, to be transferred to intermediate transfer drum 
60. Through the same process, after development, a toner image of color C 
and a toner image of color K are sequentially transferred onto 
intermediate transfer drum 60 from photoreceptor drum 10. A multi-color 
toner image (superposed of Y, M, C and K images) is further transferred 
onto copy medium P at the point where transfer roller 17 contacts 
intermediate transfer drum 60 and said multi-color toner image on copy 
medium P is fixed by means of fixing unit 24 after a separating operation 
by means of separating means 18. 
FIG. 8 shows a lateral cross sectional configuration of another embodiment 
of the color image forming device incorporating developing units embodying 
the present invention. 
In FIG. 8, the same functional portions as illustrated in FIG. 1 will be 
given the same nomenclature and only the points which are different from 
the previous embodiments will be described in the following description. 
In this color image forming device, shown as well in FIG. 1, a plurality of 
image forming units (four units being shown) comprised of charging means 
11 (Y, M, C, K), image exposure means 13 (Y, M, C, K) as an external 
exposing type and developing means 14 (Y, M, C, K) are arranged at the 
periphery of an image bearing element, wherein a flexible and closed loop 
photo sensitive belt 100 (hereinafter referred to as photoreceptor belt 
100) and an optical unit of scanning laser beams are employed for the 
image bearing element, and image exposure means 13, respectively. 
Photoreceptor belt 100 is threaded on driving roller 101 and driven rollers 
102, 103, and is maintained in a taut state by means of tensioning roller 
104, being rotated clockwise as designated by the arrow, while partially 
contacting backup member 105. The purpose of backup member 105 is to 
position the surface for the developing regions of developing sleeves 141 
(Y, M, C, K) and focal planes of image exposure means 13 (Y, M, C, K). 
Four units of image forming means, comprised of charging means 11 (Y, M, C, 
K), image exposure means 13 (Y, M, C, K) and developing means 14 (Y, M, C, 
K), are arranged at the periphery of photoreceptor belt 100. 
At the initial step of image formation, photoreceptor belt 100 is rotated 
by driving roller 101 linked to a driving motor and is uniformly charged 
by means of scorotron charger 11Y in a charging operation. At the next 
step, a latent image of color Y is formed on the surface of photoreceptor 
belt 100 by means of image exposure means 13Y in an exposing operation, 
and by the belt rotation in a sub-scanning action, based on image signals 
of color Y as the first color. After said exposing operation, while 
photoreceptor belt 100 rotates, a reversal development of the latent image 
is carried out to form a toner image of yellow (Y) by means of developing 
unit 14Y in a state of non-contact with the developer adhered onto 
developing sleeve 141Y. 
Superposed on the yellow (Y) toner image, photoreceptor belt 100 is 
uniformly re-charged by means of scorotron charger 11M in a charging 
operation. A latent image of color M is formed on the surface of 
photoreceptor belt 100 by means of image exposure means 13M in an exposing 
operation, based on image signals of color M as the second color. After 
said exposing operation, reversal development of a latent image is carried 
out to form a toner image of magenta (M), superposed on the yellow (Y) 
toner image, by means of developing unit 14M in a state of non-contact 
with the developer adhered onto developing sleeve 141Y. 
Through the same process mentioned above, a toner image of cyan (C) is 
superposed on the toner images formed previously, by means of scorotron 
charger 11C, image exposure means 13C and developing unit 14C, as is also 
the case for the black (B) toner image, by means of scorotron charger 11B, 
image exposure means 13B and developing unit 14B. As a result, a 
full-color toner image is formed on the surface of photoreceptor belt 100 
within a period of its single revolution. 
During the developing operation utilizing developing units 14Y, 14M, 14C, 
14B, either a DC bias voltage or a DC plus AC bias voltage is applied to 
developing sleeves 141Y, 141M, 141C, 141B each of which bears a single- or 
a double-component developer on its surface, while the polarity of the DC 
bias voltage carries the same charging polarity of photoreceptor belt 100, 
the conductive layer of which is grounded. Under the above conditions, the 
non-contacting and reversal development is carried out by attracting the 
toner from the surface of developing sleeve 141 to the region of a latent 
image formed on photoreceptor belt 100. 
After normalizing the potential of attached toner by means of scorotron 
charger 11F, the discharge operation for the toner image of full-color, 
formed on the surface of photoreceptor belt 100, is carried out by means 
of a pre-transfer exposing apparatus. Then, the full-color toner image is 
transferred to a copy medium by means of transfer roller 17 located at the 
transfer position opposite the lower side of driving roller 101. The copy 
medium is fed to the transfer position by paired resist rollers 23, 
synchronized with the movement of the toner image to be transferred, after 
being drawn from paper supply cassettes 20(A) as a automatic paper supply 
apparatus, or 20(B) which serves as a cassette for manually loading of 
non-standard copy medium. 
The copy medium, carrying the transferred toner image, is peeled off the 
surface of photoreceptor belt 100 which is clung along the curvature of 
driving roller 101, and is conveyed into fixing unit 24 which fuses and 
fixes the toner on the copy medium with heat and pressure applied by 
fixing rollers in fixing unit 24. Passing between paired delivery rollers 
25A, 25B and 25C, the finished copy medium is delivered to delivery tray 
26 located at the upper area of the device in such a manner that the toner 
image faces down. 
After the separation of the copy medium, any residual toner remaining on 
photoreceptor belt 100 is scraped off by contact with cleaning blade 191 
mounted in cleaning unit 19. After said cleaning operation, the next image 
formation is started again with a process of pre-charging by PLC 12 to 
eliminate any hysteresis of the previous image on photoreceptor belt 100, 
or the device enters a standby mode. 
As shown in FIG. 2, magnetic poles N1, N2, N3, S1, S2 are mounted in each 
developing sleeve 141 of developing units 14 (Y, M, C, K). In this 
arrangement, adjacent magnetic poles are arranged in the same polarity 
next to each other to form a repulsive magnetic field for scraping the 
developer off developing sleeve 141. Meanwhile, magnetic field generating 
member 148 is mounted in the vicinity of scraping magnetic poles N2 and N3 
in a non-contact state to remove the developer from developing sleeve 141 
and to eliminate any hysteresis of the previous image on developing sleeve 
141 by generating an attracting magnetic field with a polarity different 
from that of scraping magnetic poles N2 and N3. The removed developer is 
conveyed to agitating screw 146, by rotation of paddle wheel 144. 
The scope of the present invention is not limited to only a full-color 
image forming device incorporating a plurality of the developing apparatus 
embodied in the present invention. It is also possible to embody the 
present invention in an application of a monochrome image forming device 
which requires only one developing apparatus. 
According to the developing apparatus embodied in the present invention, 
since a magnetic member, with a polarity different from those of the 
scraping magnetic poles, is located opposite them and in a state of 
non-contact with them to generate a strong attracting magnetic field 
directed outwards, the presence of ghost image is drastically reduced. 
According to the toner scraping technique embodied in the present 
invention, since the magnetic field for scraping toner is created by 
mounting the magnetic member in a state of non-contact with the developing 
sleeve, the drawbacks of the prior art due to the direct contact of the 
scraper blade with the developing sleeve, such as slippage of developer 
through the gap between the developing sleeve and the scraper blade, 
deterioration of developer caused by a scraping abrasion and damage to the 
developing sleeve, are successfully overcome. In addition, in an image 
forming device embodied in the present invention, a charge potential and 
an image intensity will not be decreased, even during continuous printing 
of many copies. 
Moreover, according to the present invention, since the magnetic field 
generating member for removing, attracting and conveying the developer 
from the developing sleeve is mounted in a state of non-contact with the 
developing sleeve, it becomes possible to change the material of a 
substantial member, which forms the outer surface of the developing 
sleeve, from stainless steel (SUS) to an aluminum alloy. This will result 
in reduction of manufacturing cost and reduction of its weight.