Developing device using two-component developer

A developing device has a vessel for holding a two-component developer composed of a toner component and a magnetic component. A magnetic roller is rotatably provided within the vessel to bring the developer to a developing zone for a development of an electrostatic latent image. An agitator also is provided within the vessel for agitating and circulating the developer to cause a triboelectrifiction between the toner component and the magnetic component and a uniform distribution of the toner component in the magnetic component. The agitator is arranged to present a uniform density mass of the developer to the magnetic roller for ensuring an even development of the latent image.

TECHNICAL BACKGROUND 
The present invention relates to a developing device used in an image 
formation apparatus such as a copying machine, a laser printer, a 
facsimile or the like, wherein an electrostatic latent image is 
electrostatically developed with a two-component developer. 
PRIOR ART 
Generally, in an image formation apparatus such as an electrophotographic 
recording apparatus, the following processes are typically carried out: 
a) a uniform distribution of electrical charges is produced on a surface of 
an electrostatic latent image carrying body; 
b) an electrostatic latent image is formed on a charged area of the body 
surface by an optical writing means such as a laser beam scanner, an LED 
(light emitting diode) array, a liquid crystal shutter array or the like; 
c) the latent image is developed as a visible image with a developer or 
toner, which is electrically charged to be electrostatically adhered to 
the latent image zone; 
d) the developed and charged toner image is electrostatically transferred 
from the body to a recording medium such as a cut sheet paper; and 
e) the transferred toner image is fixed and recorded on the cut sheet paper 
by a toner image fixing means such as a heat roller. 
Typically, the electrostatic latent image carrying body may be an 
electrophotographic photoreceptor, usually formed as a drum, called a 
photosensitive drum, having a cylindrical conductive substrate formed of a 
metal such as aluminum, and a photoconductive insulating film bonded to a 
cylindrical surface thereof and formed of an organic photoconductor (OPC), 
a selenium photoconductor or the like. 
As one type of developer, a two-component developer is well known, which is 
composed of a toner component (colored fine synthetic resin particles) and 
a magnetic component (fine magnetic carriers). Usually, a developing 
device using this type of developer includes a vessel for holding the 
two-component developer, wherein the developer is agitated by an agitator 
provided therein. This agitation causes the toner particles and the 
magnetic carriers to be subjected to triboelectrification, whereby the 
toner particles are electrostatically adhered to each of the magnetic 
carriers. The developing device also includes a magnetic roller provided 
in the vessel as a developing roller in such a manner that a portion of 
the magnetic roller is exposed therefrom and faces the surface of the 
photosensitive drum. The magnetic carriers with the toner particles are 
magnetically adhered to the surface of the magnetic roller to form a 
magnetic brush therearound, and by rotating the magnetic roller carrying 
the magnetic brush, the toner particles are brought to a nip zone or 
developing zone between the magnetic roller and the drum for development 
of an electrostatic latent image formed thereon. In the developing 
process, a developing bias voltage is applied to the magnetic roller so 
that the toner particles carried to the developing zone are 
electrostatically attracted only to the latent image, whereby the toner 
development of the latent image is carried out. The magnetic brush, from 
which the toner component is consumed for the development of the latent 
image, is removed from the magnetic roller and is then returned to the 
developer held in the vessel. For this reason, in the developer held in 
the vessel, the toner component cannot be uniformly distributed in the 
magnetic component. Of course, the non-uniform distribution of the toner 
component in the magnetic component causes an uneven development of a 
latent image. 
One type of developing device using the two-component developer, a 
developer circulation type, which is known, is provided with an agitator 
including a pair of screw members provided in the developer vessel and 
disposed in parallel with each other, and a partition member disposed 
between the screw members. The screw members are arranged and rotated in 
such a manner that a part of the developer held in the vessel is 
circulated between the screw members for the purpose of a uniform 
distribution of the toner component in the magnetic component. 
Nevertheless, the conventional developer-circulation type developing 
device is not satisfactory because an even development of a latent image 
cannot be sufficiently ensured, and that the developer can be prematurely 
deteriorated, as discussed in detail hereinafter. 
DISCLOSURE OF THE INVENTION 
Therefore, an object of the present invention is to provide an improved 
developer-circulation type developing device using a two-component 
developer, which is arranged so that an even development of a latent image 
can be sufficiently ensured. 
Another object of the present invention is to provide a 
developer-circulation type developing device as mentioned above, wherein a 
premature deterioration of a developer can be prevented. 
In accordance with one aspect of the present invention, there is provided a 
developing device using a two-component developer composed of a toner 
component and a magnetic component, which comprises: a vessel means for 
holding the developer; a magnetic roller means rotatably provided within 
the vessel means to bring the developer to a developing zone for a 
development of an electrostatic latent image; and an agitator means for 
agitating and circulating the developer to cause a triboelectrification 
between the toner component and the magnetic component and a uniform 
distribution of the toner component in the magnetic component, and in the 
present invention, the agitator means includes a presentation means for 
presenting a uniform density mass of the developer to the magnetic roller 
means. 
In the present invention, preferably, the presentation means includes a 
first screw means disposed in parallel with the magnetic roller means and 
rotated in such a manner that a developer entrapped by the screw mean is 
upwardly moved from a bottom side of the vessel means toward a top side 
thereof at a side of the first screw means adjacent to the magnetic roller 
means. Also, preferably, the magnetic roller means is rotated in the same 
direction as the first screw means so that a developer brought by the 
magnetic roller, passed through the developing zone, and removed from the 
magnetic roller is entrapped by the first screw means means. 
According to the present invention, the presentation means may further 
include a second screw member disposed in parallel with the first screw 
member at an opposite side thereof apart from the magnetic roller means, 
and the first and second screw means are arranged to define a developer 
circulating passage, and are cooperatively rotated in such a manner that a 
propellant force derived from the first and second screw means is 
prevented from being directed to a corner involved in the developer 
circulating passage. 
Preferably, the developing device further comprises a blade means engaged 
with the magnetic roller means for removing a developer brought by the 
magnetic roller and passed through the developing zone, from the magnetic 
roller, the blade means being at least extended to a location adjacent to 
a side of the agitator means next to the magnetic roller means. The blade 
means may have an acute edge formed thereat, and is arranged such that the 
acute edge thereof is close to the magnetic roller means at a transition 
at which a radial magnetic flux density is substantially zero. Also, the 
blade means may have a film edge element attached thereto, and is arranged 
such that the film edge element is in contact with the magnetic roller 
means at a transition at which a radial magnetic flux density is 
substantially zero.

BEST MODE OF CARRYING OUT THE INVENTION 
FIG. 1 schematically shows a laser printer as an example of an 
electrophotographic laser printer, in which the present invention is 
embodied. This printer comprises a printer housing 10, and a rotary 
photosensitive drum 12 formed as a latent image carrying body and housed 
in the printer housing 10. During an operation of the printer, the drum 12 
is rotated in a direction indicated by an arrow in FIG. 1. 
The printer also comprises an electric discharger 14 such as a corona 
discharger for producing a charged area on the photosensitive drum 12, and 
a laser beam scanner 16 is provided to write an electric latent image on 
the charged area of the drum 12. The laser beam scanner includes a laser 
source such as a semiconductor laser diode for emitting a laser light, an 
optical system for focusing the laser light into a laser beam LB, and an 
optical scanning system such as a polygon mirror for deflecting the laser 
beam LB along a direction of a central axis of the drum 12 so that the 
charged area of the drum 12 is scanned by the deflecting laser beam LB. 
During the scanning, the laser beam LB is switched on and off on the basis 
of binary image data obtained from, for example, a word processor, 
personal computer or the like, so that an electrostatic latent image is 
written as a dot image on the charged area of the drum 12. In particular, 
when a zone of the charged area is irradiated by the laser beam LB, the 
charges are released from the irradiated zone so that the latent image is 
formed as a potential difference between the irradiated zone and the 
remaining zone. 
The printer further comprises a developing device 18 for electrostatically 
developing the latent image with a two-component developer composed of a 
toner component (colored fine resin particles) and a magnetic component 
(magnetic fine carriers). In the developing device 18, the developer is 
agitated so that the toner particles are electrically charged with a given 
polarity by a triboelectrification with the magnetic carriers, and the 
development of the latent image is carried out by an electrostatic 
attraction of the charged toner particles to the latent image, as 
mentioned in the "Description of the Related Prior Art". Note, the 
developing device 18 is constructed as a developer-circulation type 
according to the invention, and an arrangement thereof is explained in 
detail hereinafter. 
Furthermore, the printer comprises a transfer charger assembly 20 for 
electrostatically transferring the developed toner image to a recording 
medium such as a cut sheet paper, which is introduced into a clearance 
between the photosensitive drum 12 and the transfer charger assembly 20. 
The transfer charger assembly 20 includes a transfer charger 20a, and an 
AC charge eliminator 20b disposed adjacent to the transfer charger 20a. 
The transfer charger, which may be a corona discharger, is subjected to an 
application of a DC electric energy to give the paper an electric charge 
having a polarity opposite to that of the electric charge of the developed 
toner image, whereby the toner image is electrostatically transferred from 
the drum 12 to the paper. The AC charge eliminator 20b, which also may be 
a corona discharger, is subjected to an application of an AC electric 
energy to partially eliminate the electric charge of the paper to which 
the toner image is transferred, whereby an electrostatic attraction acting 
between the paper and the drum can be weakened for an effective separation 
of the paper from the drum 12. 
The printer is provided with a paper cassette 22 in which a stack of paper 
is received, and a paper guide 24 extended from the paper cassette 22 
toward a pair of register roller 26, 26. During the printing operation, 
papers to be printed are fed one by one from the paper cassette 22 along 
the paper guide 24 by driving a paper feeding roller 28 incorporated in 
the paper cassette 22. The fed paper is stopped once at the register 
rollers 26, 26, and is then introduced, at a given timing, into the 
clearance between the drum 12 and the assembly 20 through a paper guide 30 
extended between the register rollers 26, 26 and the assembly 20, so that 
the developed toner image can be transferred to the paper in place. 
The paper discharged from the clearance between the drum 12 and the 
assembly 20, i.e., the paper carrying the transferred toner image, is then 
moved toward a toner image fixing device 32 along a paper guide 34 
extended between the assembly 20 and the fixing device 32 and having a 
paper guide roller 36 incorporated therein, and is passed through a nip 
between a heat roller 32a and a backup roller 32b of the fixing device 32, 
whereby the transferred toner image is thermally fused and fixed on the 
paper. The paper having the fixed toner image is fed to a pair of paper 
discharging rollers 38, 38 along a paper guide 40 extended between the 
fixing device 32 and the paper discharging rollers 38, 38, and is then 
discharged from the printer through the rollers 38, 38. 
The printer is also provided with a toner cleaner 42 having a charge 
eliminating lamp 42a attached thereon, and a fur brush 42b provided 
therein. The lamp 42a illuminates a surface of the drum 12 for eliminating 
the charges therefrom, and the fur brush 42b cleans the drum surface to 
remove residual toner particles not transferred to the paper in the 
transferring process. The toner cleaner 42 also has an outlet port 42c 
formed therein to discharge the removed toner particles therethrough, and 
the discharged toner particles are returned to the developing device 18 to 
be recycled. Note, in FIG. 1, reference numeral 44 indicates an main 
electric motor by which the drum 12, the developing device 18, the paper 
feeding roller 28, the register rollers 26, 26, the fixing device 32, 
etc., are driven. 
FIGS. 2 and 3 show an embodiment of the developer-circulation type 
developing device 18 constructed according to the present invention. The 
developing device includes a vessel 46 for holding the two-component 
developer, an existence of which is illustrated by a plurality of fine 
dots. Although the vessel 46 is covered by a cover plate member 46a, as 
shown in FIG. 3, this cover plate member 46a is omitted from FIG. 2 to 
illustrate an interior of the vessel 46. The developing device 18 also 
includes a developing roller or magnetic roller 48 rotatably provided in 
the vessel 46 in such a manner that a portion of the magnetic roller 48 is 
exposed therefrom and faces the photosensitive drum 12. The magnetic 
roller 48 has a rotatable sleeve 48a formed of a nonmagnetic material such 
as aluminum, and five bar-like permanent magnet elements elements M1, M2, 
M3, M4, and M5 immovably provided within the sleeve 48a and coextended 
therewith. During an operation of the developing device 18, only the 
sleeve 48a is rotated in a direction indicated by an arrow in FIG. 3 in 
such a manner that a rotating surface of the sleeve 48a ascends to a 
developing zone defined as a nip zone between the drum 12 and the magnetic 
roller 48. The magnet elements M1, M2, M3 and M4 are disposed at suitable 
intervals along a lower semicircular arc which is substantially defined by 
a horizontal plane including a rotational axis of the sleeve 48a of the 
magnetic roller 48, whereas the magnetic element M5 is disposed 
substantially at a top of the upper semicircular arc defined by said 
horizontal plane. 
The magnet elements M1, M3, and M5 are identically oriented with respect to 
the sleeve 48a, and also the elements M2 and M4 are identically oriented 
with respect to the sleeve 48a, but the orientation of the elements M1, 
M3, and M5 is opposite to that of the elements M2 and M4. Namely, for 
example, when each of the elements M1, M3, and M5 is oriented so that the 
S polarity thereof is directed to an inner surface of the sleeve 48a, each 
of the elements M2 and M4 is reversely oriented so that the N polarity 
thereof is directed to the inner surface of the sleeve 48a. Thus, a 
magnetic field is produced above an outer surface section of the sleeve 
48a defined by each of four sets of the two adjacent magnet elements M1 
and M2; M2 and M3; M3 and M4; and M4 and M5, but no magnetic field is 
produced above an Outer surface section of the sleeve 48a defined by the 
remaining set of the two adjacent magnet elements M5 and M1 because these 
magnetic elements are identically oriented with the respect to the sleeve 
48a. 
With the arrangement of the magnet elements M1 to M5, during the rotation 
of the sleeve 48a in the direction indicated by the arrow in FIG. 2, a 
part of the developer held in the vessel 48a can be magnetically adhered 
to and entrained by the sleeve 48a due to the production of the magnetic 
fields by the four sets of the two adjacent magnet elements M1 and M2; M2 
and M3; M3 and M4; and M4 and M5, so that the entrained developer is 
brought toward the developing zone for the development of the latent 
image. When the developer passed through the developing zone is moved into 
the non-magnetic field zone produced by the two adjacent magnet elements 
M5 and M1, the entrained developer is released from the magnetic adhesive 
force to the sleeve 48a, and is thus removed therefrom due to gravity. The 
removed developer is returned to the developer held in the vessel 46 so 
that a fresh part of the developer can be always entrained by the sleeve 
48a, whereby a proper development of the latent image can be ensured. 
Note, the developer passed through the developing zone has a smaller 
content of the toner component than that of the developer held in the 
vessel 46, because the development of the latent image is carried out by 
the toner component of the developer. 
The developing device 18 may be provided with a paddle roller 50 disposed 
beside the magnetic roller 48 and rotated in a direction indicated in FIG. 
3 for feeding a fresh part of the developer to the magnetic roller 48. 
Also, the developing device 18 may be provided with a doctor blade 52 for 
regulating an amount of the developer entrained by the magnetic roller 48. 
In particular, the developer is entrained by the magnetic roller 48 so as 
to form a magnetic brush having a plurality of fine spike-like elements, 
and a length of the fine spike-like elements should be uniformly regulated 
before an even development of the latent image can be ensured. Note, each 
of the spike-like elements is formed of a plurality of magnetic carriers 
magnetically connected to each other. 
The developing device 18 further includes a screw type agitator 54 provided 
in the vessel 46 to agitate the developer held therein, to cause the 
triboelectrification between the toner component and the magnetic 
component. The agitator 54 has at least two screw members 54a and 54b 
disposed in parallel with each other and rotatably supported at the ends 
thereof by end walls of the vessel 46, and a partition member 54c provided 
between the two screw members 54a and 54b and extended therealong. In this 
embodiment, each of the screw members 54a and 54b is provided with a 
right-hand flight as shown in FIG. 2. Accordingly, when the screw member 
54a is rotated as indicated by an arrow A (FIG. 3), a developer entrapped 
thereby is propelled in a direction indicated by an arrow A' (FIG. 2), 
and, when the screw member 54b is rotated as indicated by an arrow B (FIG. 
3), a developer entrapped thereby is propelled in the reverse direction 
indicated by an arrow B' (FIG. 2). Namely, each of the screw members 54a 
and 54b defines a passage for moving a part of the developer held in the 
vessel 46. As apparent from FIG. 2, the screw members 54a and 54b have the 
same length, but the partition member 54c has a length shorter than that 
of the screw members 54a and 54b, so that the developer passages defined 
by the screw members 54a and 54b are in communication with each other at 
the ends of the partition member 54c, whereby a part of the developer 
propelled by one of the screw members 54a, 54b is entrapped by the other 
screw member 54a, 54b. Thus, during an operation of the developing device 
18, a part of the developer held in the vessel 46 is always circulated 
along the developer passages defined by the screw members 54a and 54b, 
whereby not only can the toner component be sufficiently charged by a 
triboelectrification with the magnetic component, but also a uniform 
distribution of the toner component in the magnetic component can be 
ensured. Note, the screw member 54a, 54b may be integrally formed of a 
suitable resin material such as ANS resin, and have, for example, a shaft 
diameter of 8 mm, a screw flight diameter of 25 mm, and a screw flight 
thickness of 1 mm. 
As shown in FIG. 2, a gear train 56 is provided on one end wall of the 
vessel 46, and includes a gear 56a mounted on an end of the screw member 
54a, a gear 56b mounted on a corresponding end of the screw member 54b and 
engaged with the gear 56a, an idler gear 54c engaged with the gear 56a, 
and a gear 56d engaged with the idler gear 54c to rotate the sleeve 48a of 
the magnetic roller 48. With this arrangement of the gear train 56, when 
the gear 56d is subjected to a rotational drive force from the main motor 
44 (FIG. 1) through a drive mechanism (not shown) to rotate the sleeve 48a 
in the direction indicated by the arrow in FIG. 3, the screw members 54a 
and 54b can be rotated in the directions indicated by the arrows A and B, 
respectively. Note, of course, the gear train 56 may include a gear (not 
shown) for rotating the paddle roller 50 in the direction indicated by the 
arrow in FIG. 3. 
According to an aspect of the present invention, the developing device 18 
is characterized in that the screw member 54a disposed adjacent to the 
magnetic roller 46 is rotated in the direction indicated by the arrow A, 
so that the screw flight of the screw member 54a is upwardly moved from a 
bottom side of the vessel 46 toward a top side thereof at a side of the 
screw member 54a adjacent to the magnetic roller 48. Accordingly, the 
developer entrapped by the screw member 54a also is upwardly moved from 
the bottom side of the vessel 46 to the top side thereof at the side of 
the screw member 54a adjacent to the magnetic roller 48. On the other 
hand, the entangled developer is downwardly moved from the top side of the 
vessel 46 toward the bottom side thereof at the opposite side of the screw 
member 54a adjacent to the partition member 54c. As a result, the 
developer propelled by the screw member 54a in the direction indicated by 
the arrow A' has a uniform density at the side of the screw member 54a 
adjacent to the magnetic roller 48, and a nonuniform density at the 
opposite side of the screw member 54a adjacent to the partition member 
54c, as illustrated in FIG. 4(a). Namely, in this drawing, a part of the 
propelled developer having the uniform density is indicated as a hatching 
zone 58, and a part of the propelled developer having the non-uniform 
density is indicated as two kinds of hatching zones 60 and 62. Note, a 
density of developer of the hatching zones 60 is smaller than that of the 
hatching zones 62. The two kinds of hatching zones 60 and 62 alternately 
appear along a length of the screw member 54a at regular intervals, a 
pitch of which corresponds to a flight pitch of the screw member 54a. An 
appearance of the non-uniform density is derived from the fact that, 
during the rotation of the screw member 54a, the developer cannot be 
sufficiently fed to a trailing face of the screw flight (54a) at the side 
of the screw member 54a adjacent to the partition member 54c. In practice, 
a plurality of cavities discretely appears on an upper surface of the 
developer held in the vessel 46, at the locations corresponding to the 
hatching zones 60. Note, during the rotation of the screw member 54a, the 
developer can be sufficiently fed from the bottom side of the vessel 46 to 
the trailing face of the screw flight (54a) at the side of the screw 
member 54a adjacent to the magnetic roller 48, and thus the developer 
density can be uniformly maintained therein. 
If the screw member 54a is reversely rotated, as indicated by an arrow RA 
in FIG. 4(b), the developer is propelled in a reverse direction indicated 
by an arrow RA' in FIG. 4(b). In this case, the screw flight of the screw 
member 54a is downwardly moved from the top side of the vessel to the 
bottom side thereof at the side of the screw member 54a adjacent to the 
magnetic roller 48. Accordingly, the developer entrapped by the screw 
member 54a is downwardly moved from the top side of the vessel 46 toward 
the bottom side thereof at the side of the screw member 54a adjacent to 
the magnetic roller 48. On the other hand, the entrapped developer is 
upwardly moved from the bottom side of the vessel 46 toward the top side 
thereof at the opposite side of the screw member 54a adjacent to the 
partition member 54c. Thus, the developer propelled by the screw member 
54a in the direction indicated by the arrow RA' has a nonuniform density 
at the side of the screw member 54a adjacent to the magnetic roller 48, 
and a uniform density at the opposite side of the screw member 54a 
adjacent to the partition member 54c, as illustrated in FIG. 4(b). Namely, 
in this drawing, a part of the propelled developer having the uniform 
density is indicated as a hatching zone 58', and a part of the propelled 
developer having the non-uniform density is indicated as two kinds of 
hatching zones 60' and 62', Similar to FIG. 4(a), a density of developer 
of the hatching zones 60' is smaller than that of the hatching zones 62'. 
When the non-uniform density appears in the developer held in the vessel 
46, at the side of the screw member 54a adjacent to the magnetic roller 
48, as shown in FIG. 4(b), an even development of a latent image cannot be 
ensured because a non-uniform density correspondingly appears in the 
developer entrained by the magnetic roller 48. However, according to the 
present invention, as shown in FIG. 4(a), the appearance of the uniform 
density is assured at the side of the screw member 54a adjacent to the 
magnetic roller 48, and thus an even development of a latent image can be 
ensured. Namely, it is possible to present a uniform density mass of the 
developer to the magnetic roller 48 by the rotation of the screw member 
54a in the direction indicated by the arrow in FIG. 3. 
In practice, a printing of a toner-solid image on a sheet of paper was made 
on each of the two cases shown in FIGS. 4(a) and 4(b). The results are 
shown in a graph of FIG. 4(c), in which the abscissa indicates a distance 
measured from one side edge of the sheet of paper to the other side edge 
thereof, and the ordinate indicates a variation of an optical density (OD) 
of the printed toner-solid image determined along a width of the sheet of 
paper. In this graph, a curve represented by a plurality of small open 
circles ".smallcircle." indicates the case of FIG. 4(a), and a curve 
represented by a plurality of small open triangles ".DELTA." indicates the 
case of FIG. 4(b). As apparent from the graph, there is no variation of 
the optical density in the case of FIG. 4(a), whereas the optical density 
periodically varies in the case of FIG. 4(b). Note, a distance indicated 
by reference P in the graph of FIG. 4(c) corresponds to a screw flight 
pitch of the screw member 54a. 
Another advantage or merit can be obtained by the rotation of the screw 
member 54a in the direction indicated by the arrow A. In particular, the 
developer removed from the magnetic roller 48 at the non-magnetic field 
zone produced by the two adjacent magnet elements M5 and M1 has a small 
content of the toner component because the toner component of the removed 
developer is consumed for development of a latent image. For this reason, 
the removed developer should not be directly fed to the magnetic roller 48 
so that a proper development of a latent image can be maintained. 
According to the present invention, the direct feed of the removed 
developer to the magnetic roller 48 can be suppressed because the 
developer entrapped by the screw member 54a is upwardly moved from the 
bottom side of the vessel 46 to the top side thereof at the side of the 
screw member 54a adjacent to the magnetic roller 48. Note, if the screw 
member 54a is reversely rotated in the direction indicated by the arrow RA 
in FIG. 4(b), the direct feed of the removed developer to the magnetic 
roller 48 is facilitated because, of course, the developer entangled by 
the reversely-rotating screw member 54a is downwardly moved from the top 
side of the vessel 46 toward the bottom side thereof at the side of the 
screw member 54a adjacent to the magnetic roller 48. 
According to another aspect of the present invention, the developing device 
18 is arranged such that the developer propelled by the screw member 54a, 
54b cannot exert a pressure on a dead stock of developer which is produced 
at a corner involved in the passages for circulating the developer. In 
particular, as shown in FIG. 5(a), a section of the developer-circulating 
passage defined by the screw member 54b involves a corner indicated by 
reference numeral 64, and a dead stock of developer is produced at this 
corner 64, as indicated by hatching. The screw member 54b is rotated in 
the direction indicated by the arrow B in FIG. 5(b), so that the developer 
is propelled in the direction indicated by the arrow B' in FIG. 5(a). 
Nevertheless, the dead stock of developer cannot be subjected to a large 
pressure from the developer propelled by the screw member 54b because a 
propellant force which acts on the developer by the leading face of the 
screw flight (54b) is directed in a direction indicated by an arrow C in 
FIG. 5(a). 
If the screw member 54b is provided with a left-hand screw as shown in FIG. 
6(a), and if it is rotated in a reverse direction indicated by an arrow RB 
in FIG. 6(b), to propel the developer in the same direction indicated by 
the arrow B', the dead stock of developer produced at the corner 64 is 
subjected to a large pressure from developer propelled by the screw member 
54b because a propellant force which acts on the developer by the leading 
face of the left-hand screw flight (54b ) is directed in a direction 
indicated by an arrow C' in FIG. 5(a). Accordingly, the dead stock of 
developer is compacted to squeeze the toner particles included therein so 
that each of the magnetic carriers is coated with the squeezed toner 
material. Also, the dead stock of developer gradually grows into a large 
mass because a part of the propelled developer is added to the dead stock 
of developer. When the mass of the dead stock of developer becomes too 
large, a part of the dead stock of developer is separated therefrom, and 
is moved into the circulated developer. Thus, the developer held in the 
vessel 46 is prematurely deteriorated. 
In practice, with respect to the two cases of FIG. 5(a) and 6(a), a test 
was performed to learn how an upper surface of a developer held in a 
developer vessel is changed at a corner zone involved in a 
developer-circulating passage, due to a propellant force of a circulating 
developer. Note, in the test, a right-hand screw and a left-hand screw, 
each of which has a shaft diameter of 12 mm, a screw flight diameter of 30 
mm, and a screw flight pitch of 30 mm, were used and rotated at 120 rpm, 
and the vessel is filled with the developer up to a top level thereof. The 
results are shown in a graph of FIG. 6(c), in which the abscissa indicates 
a distance measured from an end wall of the vessel along a side wall 
thereof (note, these walls define the corner zone concerned), and the 
ordinate indicates a height of a raised developer surface measured from 
the top of the vessel. In this graph, a curve L indicates the case of FIG. 
6(a), and proves that a height of the raised developer surface is about 11 
mm when using the left-hand screw, and a curve R indicates the case of 
FIG. 5(a), and proves that a height of the raised developer surface is 
about 1 mm when using the right-hand screw. Accordingly, the graph proves 
that the arrangement of FIG. 5(a) according to the present invention is 
superior to that of FIG. 6(a). 
Also, in practice, with respect to the cases of FIG. 5(a) and 6(a), a 
printing test was performed to learn the relationship between a 
deterioration of developer and the number of printed sheets of paper. The 
results are as follows: 
In the case of FIG. 5(a), a proper quality of printed image could be 
maintained even after the number of printed sheets had exceeded 30,000, 
whereas, in the case of FIG. 6(a), a proper quality of printed image could 
not be maintained after the number of printed sheets had exceeded 20,000 
due to the deterioration of developer. 
As shown in FIG. 7(a), another section of the developer-circulating passage 
defined by the screw member 54b involves a corner indicated by reference 
numeral 66, and a dead stock of developer is produced at this corner 66, 
as indicated by hatching. However, this dead stock of developer cannot be 
subjected a pressure from the circulated developer because the screw 
member 54b is rotated in the direction indicated by the arrow B (FIG. 
7(b)), so that the developer is propelled in the direction indicated by 
the arrow B'. Also, as shown in FIG. 8(a), yet another section of the 
developer-circulating passage defined by the screw member 54a involves a 
corner indicated by reference numeral 68, and a dead stock of developer is 
produced at this corner 68, as indicated by hatching. However, this dead 
stock of developer also cannot be subjected a pressure from the circulated 
developer because the screw member 54a is rotated in the direction 
indicated by the arrow A (FIG. 8(b)), so that the developer is propelled 
in the direction indicated by the arrow A'. 
FIGS. 9 and 10 show a modification of the developing device 18 shown in 
FIGS. 2 and 3, and this modified embodiment is identical to that of FIGS. 
2 and 3 except that each of the screw members 54a and 54b is provided with 
a left-hand flight. Similar to the embodiment of FIGS. 2 and 3, the screw 
member 54a is rotated in the direction indicated by the arrow A in FIG. 
10, so that a developer entrapped thereby is propelled in a direction 
indicated by an arrow AA' in FIG. 9. Also, when the screw member 54b is 
rotated as indicated by the arrow B, a developer entrapped thereby is 
propelled in a reverse direction indicated by an arrow BB' in FIG. 2. 
Thus, a part of the developer held in the vessel 46 is always circulated 
along the developer passages defined by the screw members 54a and 54b. 
During the rotation of the screw member 54a, the screw flight thereof is 
upwardly moved from the bottom side of the vessel 46 toward the top side 
thereof at the side of the screw member 54a adjacent to the magnetic 
roller 48, so that the developer entrapped by the screw member 54a is 
upwardly moved from the bottom side of the vessel 46 to the top side 
thereof at the side of the screw member 54a adjacent to the magnetic 
roller 48, whereby the developer propelled by the screw member 54a in the 
direction indicated by the arrow AA' has a uniform density at the side of 
the screw member 54a adjacent to the magnetic roller 48, and thus an even 
development of a latent image can be ensured. Also, the 
developer-circulating passages defined by the screw members 54a and 54b 
involve two corners indicated by reference numerals 70 and 72, 
respectively, and a dead stocks of developer is produced at each of the 
corners 70 and 72. Nevertheless, the dead stock of developer cannot be 
subjected to a large pressure from the propelled developer because a 
propellant force which acts on the developer by the leading face of the 
left-hand screw flight is not directed to the dead stock of developer, as 
apparent from FIG. 9. 
FIG. 11(a) shows a multi-propeller member 74 which may be substituted for 
the screw member 54a, 54b in the above-mentioned embodiments. The 
multi-propeller member 74 includes an elongated shaft element 74a, and a 
plurality of propeller elements 74b fixed thereon at regular intervals. 
Each of the propeller elements 74b is provided with three blades, as shown 
in FIG. 11(b), and these blade are identically angled to define a given 
angle .theta. with a plane perpendicular to a central axis of the 
propeller element 74b, as shown in FIG. 11(c). Accordingly, In place of 
the screw member 54a, 54b, the multi-propeller member 74 can be used for 
propelling the developer. In this case, of course, the multi-propeller 
member 74 disposed adjacent to the magnetic roller 48 is rotated in such a 
manner that the blades of the propeller element 74b are upwardly moved 
from the bottom side of the vessel 48 toward the top side thereof. Note, 
although the illustrated multi-propeller member 74 is arranged as a 
left-hand flight type, it may be a right-hand flight type. 
FIGS. 12(a), 12(b), and 12(c) show a modification of the multi-propeller 
member 74 shown in FIGS. 11(a), 11(b), and 11(c). This modified 
multi-propeller member 74' has an elongated shaft element 74a', and a 
plurality of propeller elements 74b' fixed thereon at regular intervals. 
Each of the propeller elements 74b' is provided with four elongated 
rectangular blades, as shown in FIG. 12(b), and these blade are 
identically angled to define a given angle 8 with a plane perpendicular to 
a central axis of the propeller element 74b', as shown in FIG. 12(c). 
Similar to the multi-propeller member 74, the modified multi-propeller 
member 74' can be used in the place of the screw member 54a, 54b 
FIG. 13(a) shows a second embodiment of a developing device according to 
the present invention, which is identical to the first embodiment as shown 
in FIGS. 2 and 3 except that an endless belt type agitator 76 is 
substituted for the screw type agitator 54. The endless belt type agitator 
76 includes an endless belt 76a entrained with two pulleys 76b and 76c 
which are spaced from each other and displaced on a bottom of the vessel 
46 beside the end walls thereof. Each of the pulleys 76b and 76c has a 
shaft extended through the bottom of the vessel 46 and rotatably supported 
thereby, and a partition member 76d is fixed on the bottom of the vessel 
46 and is extended between the pulleys 76b and 76c inside of the endless 
belt 76a. The agitator 76 also includes a plurality of blade elements 76e 
attached to the endless belt 76a to define a given acute angle .theta. 
therewith, as shown in FIG. 13(b). During an operation of the developing 
device, the endless belt 76a is rotated in a direction indicated by an 
arrow D in FIG. 13(a), in such a manner that a part of the developer held 
in the vessel 46 is received in acute angle spaces between the endless 
belt 76a and the blade elements 76e. Thus, a part of the developer held in 
the vessel 46 is always circulated along the rotating endless belt 76a, 
whereby not only can the toner component be sufficiently charged by a 
triboelectrification with the magnetic component, but also a uniform 
distribution of the toner component in the magnetic component can be 
ensured. According to this second embodiment, a constant density can be 
given to a part of the developer fed from the endless belt type agitator 
76 to the magnetic roller 48. Namely, it is possible to present a uniform 
density mass of the developer to the magnetic roller 48. 
To rotate the endless belt 76a, a gear train 78 is provided on one end wall 
of the vessel 46, as shown in FIG. 13(a). The gear train 78 includes a 
gear 78a fixed on one end of a shaft 76f (FIG. 13(c)) extended through the 
end wall of the vessel 46, and the end of the shaft has a bevel gear 76f 
fixed thereon, which is engaged with a bevel gear 76g fixed on a free end 
of the shaft of the pulley 76b. The gear train 78 further includes a first 
idler gear 78b engaged with the gear 78a, a second idler gear 78c engaged 
with the first idler gear 78b, and a gear 78d engaged with the second 
idler gear 78c and provided to drive the magnetic roller 48. With this 
arrangement of the gear train 78, when the gear 78d is subjected to a 
rotational drive force from the main motor 44 (FIG. 1) through a drive 
mechanism (not shown) so that the magnetic roller 48 is driven in the 
manner as shown in FIG. 3, the endless belt 76a can be be rotated in the 
direction indicated by the arrow D. 
FIG. 14 shows a modification of the developing device as shown in FIGS. 
13(a), 13(b), and 13(c), and this modified embodiment is identical to that 
of FIGS. 13(a), 13(b), and 13(c) except that the blade elements 76e are 
reversely oriented with the endless belt 76a, and the endless belt 76a is 
rotated in a reverse direction indicated by an arrow D' in FIG. 14. Note, 
the gear train 78 includes only one idle gear 78b' provided between the 
gears 78a and 78d for the reverse rotation of the endless belt 76a. 
As mentioned hereinbefore, the developer entrained by the magnetic roller 
48 and passed through the developing zone, i.e., the developer used for a 
development of a latent image is removed at the non-magnetic field zone 
produced by the two adjacent magnet elements M5 and M1 (FIG. 3), but an 
amount of the removed developer is small. Accordingly, the removal of the 
used developer from the magnetic roller 48 should be carried out to as 
greater an extent as possible so that a proper development can be 
maintained. For this reason, the developing device 18 may include a 
scraper blade member 80 formed of a suitable metal material such as 
stainless steel, brass, aluminum or the like, as shown in FIG. 15(a), and 
the scraper blade member 80 is provided in the vessel 48 in such a manner 
that an acute edge of the scraper blade member 80 is tangentially oriented 
with respect to the magnetic roller 48 to remove the used developer 
therefrom. In this embodiment, the removed developer is returned to the 
developer held in the vessel 48, at a location between the magnetic roller 
48 and the screw member 54a, as indicated by an arrow E in FIG. 15(a), and 
thus a part of the returned developer can be directly fed to the magnetic 
roller 48. As discussed hereinbefore, since the returned developer has a 
small content of the toner component, the direct feed of the returned 
developer to the magnetic roller 48 should be effectively prevented. 
In an embodiment shown in FIG. 15(b), the scraper blade member 80 is 
extended to a location close to a side of the screw member 54a adjacent to 
the magnetic roller 48, and thus the main part of the removed developer 
can be entrapped by the scraper blade member 80, as indicated by an arrow 
F in FIG. 15(b). Also, in an embodiment shown in FIG. 15(c), the scraper 
blade member 80 is extended to a location close to a top edge of the screw 
member 54a adjacent to the magnetic roller 48, and thus all of the removed 
developer can be entrapped by the scraper blade member 80, as indicated by 
an arrow G in FIG. 15(c). Accordingly, in the embodiments shown in FIGS. 
15(b) and 15(c), a proper development of a latent image can be maintained 
over a long period of time. 
In practice, a printing of a solid-toner image on a sheet of paper was made 
with respect to each of the two cases shown in FIGS. 15(a) and 15(b). The 
results are shown in a graph of FIG. 15(d), in which the abscissa 
indicates a total length of printed sheet papers, and the ordinate 
indicates a variation of an optical density (OD) of the printed 
solid-toner image. In this graph, a curve represented by a plurality of 
small solid triangles ".tangle-solidup." indicates the case of FIG. 15(a), 
and proves that the optical density of the printed toner-solid image is 
gradually lowered as the total length of printed sheet papers is 
increased, whereas a curve represented by a plurality of small open 
squares ".quadrature." indicates the case of FIG. 15(b), and proves that 
there is no variation of the optical density. 
Also, after a printing of a solid-toner image on a sheet of paper was made 
with respect to each of the two cases shown in FIGS. 15(a) and 15(b) until 
a total length of printed sheet papers reaches 2 m, a part of the 
developer held in the vessel 46 was sampled at each of three locations 
where the magnetic roller 48, the screw member 54a, and the screw member 
54b are placed, and a content of the toner component in each of the 
samples was confirmed. The results are shown in the following table: 
______________________________________ 
Content of Toner Component (wt %) 
Difference 
at Sampling Location (Roller 48, 
between 
Screw 54a, and Screw 54b) 
Roller 48 & 
Case Roller 48 Screw 54a Screw 54b 
Screw 54 
______________________________________ 
FIG.15(a) 
2.27 2.99 5.02 2.72 
FIG.15(b) 
4.09 4.51 4.58 0.42 
______________________________________ 
Note: Initial Content of Toner Component = 5.5 wt % (No supplement of 
Toner Component) 
The removal of the used developer from the magnetic roller 48 cannot be 
completely carried out by using the scraper blade member 80, and an 
efficiency of removal of the used developer depends upon a positional 
relationship of the scraper blade member 80 with respect to the magnetic 
roller 48. In particular, as shown in FIG. 16, a distribution of vertical 
or radial magnetic flux density is established around the magnetic roller 
48, and a radial magnetic flux density is substantially zero in the 
non-magnetic field zone produced by the two adjacent magnetic elements M5 
and M1. Accordingly, as illustrated in FIG. 17, as soon as a spike-like 
element 82 of the magnetic brush formed around the magnetic roller 48 
passes through a transition TR at which a radial magnetic flux density is 
zero, the spike-like element 82 falls down on the surface of the magnetic 
roller 48. It is difficult to scrape the fallen-down spike-like elements 
82 without damaging the surface of the magnetic roller 48, and also 
scraping of the upright spike-like elements 82 is difficult due to the 
magnetic-adhesion thereof to the roller surface (48). 
In practice, a test for removing used developer was performed for three 
cases as follows 
Case I: 
An edge of a scraper blade is positioned on a first radial plan P.sub.1 
(FIG. 17) extended from a central axis of a magnetic roller through a 
transition TR as defined above, and a gap between the scraper blade edge 
and the magnetic roller surface is variously changed. 
Case II: 
The edge of the scraper blade is positioned on a second radial plan P.sub.2 
(FIG. 17) extended from the central axis of the magnetic roller through a 
magnetic field producing zone to define an angle of 5 degrees with the 
first radial plan P.sub.1, and a gap between the scraper blade edge and 
the magnetic roller surface is variously changed. 
Case III: 
The edge of the scraper blade is positioned on a third radial plan P.sub.3 
(FIG. 17) extended from the central axis of the magnetic roller through a 
non-magnetic field zone to define an angle of 5 degrees with the first 
radial plan P.sub.1, and a gap between the scraper blade edge and the 
magnetic roller surface is variously changed. 
The results are shown in a graph of FIG. 18, in which the abscissa 
indicates a width of the gap between the scraper blade edge and the 
magnetic roller surface, and the ordinate indicates an amount of developer 
not removed from the magnetic roller. In this graph, a curve represented 
by a plurality of small open squares ".quadrature." indicates Case I; a 
curve represented by a plurality of small open circles ".smallcircle." 
indicates Case II; and a curve represented by a plurality of small solid 
circles ".circle-solid." indicates Case III. As apparent from the graph, 
when the width of the gap is more than 0.5 mm, an amount of the 
non-removed developer is abruptly increased. An amount of the non-removed 
developer must be at most 15 g/cm.sup.2 before a proper development of a 
latent image can be maintained. 
Therefore, in the embodiments shown in FIG. 15(a), 15(b), and 15(c), the 
scraper blade member 80 should be arranged such that the acute edge 
thereof is positioned to be close on the transition TR to form a gap width 
of at most 0.5 mm with the surface of the magnetic roller 48, before an 
efficient removal of the used developer can be carried out. 
FIG. 19 shows another embodiment of scraper blade member, generally 
indicated by reference 84, which may be used in place of the scraper blade 
member 80. This type scraper blade member 84 has a rigid blade body 84, 
and a film edge element 84b attached to and extended along one side 
thereof, and is provided in the vessel 46 in such a manner that the 
film-like edge element 84a thereof is in contact with the surface of the 
magnetic roller 48 in vicinity of the transition TR. The film-like scraper 
member 84a has a thickness of at most 0.5 mm corresponding to the gap 
width with which the acute edge of the scraper blade member 84 is 
positioned with respect to the magnet roller surface, and thus an 
efficient removal of the used developer can be carried out. The film-like 
scraper member 84a may be formed of a suitable resin material such as 
polyurethane rubber, silicone rubber or the like or a suitable metal 
material such as stainless steel, phosphor bronze or the like. 
As apparent from the foregoing, since the toner component of the developer 
held in the vessel is consumed during the developing process, a toner 
component must to be supplemented to the developer held in the vessel, if 
necessary, so that a ratio of the toner component to the developer can 
fall within a given range to continuously maintain a proper and stable 
developing process. Accordingly, a content of the toner component in the 
developer must be properly detected before the supplement of toner 
component to the developer held in the vessel can be reasonably carried 
out. To detect the content of the toner component in the developer, a 
magnetic permeability is usually measured at a fixed location on a bottom 
of the vessel in which the developer is held. Namely, as the toner 
component formed of a non-magnetic material is consumed for the 
development of latent image, the magnetic permeability measured at the 
fixed location on the vessel bottom becomes larger. Nevertheless, it is 
difficult to carry out an accurate measurement of the magnetic 
permeability, especially in the developing device as mentioned above, 
because the developer is dynamically circulated in the vessel at all 
times. In particular, a level of the developer held in vessel is 
fluctuated due to the dynamic circulation thereof, so that a density of 
the developer is changed at the fixed or measurement location on the 
vessel bottom to thereby cause a variation of the magnetic permeability, 
and thus achievement of an accurate measurement is very difficult or 
impossible. Note, when a density of the developer is high, the magnetic 
permeability thereof is large due to a close congregation of the magnetic 
carriers and vice versa. 
In an embodiment shown in FIGS. 20(a) and 20(b), the developing device 18 
is arranged such that an accurate measurement of a magnetic permeability 
for detecting a content of the toner component in the developer held in 
vessel 46 can be ensured. In particular, the developing device 18 is 
provided with a permeameter 86 incorporated into the bottom of the vessel 
46 at a zone in which the screw member 54b is located, as shown in FIGS. 
20(a) and 20 (b), and the permeameter 86 measures a permeability of the 
developer at a fixed location on the vessel 46. The developing device 18 
is further provided with a barrage plate member 88 bridged between and 
supported by a side wall of the vessel 46 and the partition member 54c to 
uniformly regulate an upper level of the developer propelled by the screw 
member (having the left-hand screw flight) 54b in the direction indicated 
by the arrow BB'. As shown in FIG. 20(b), for example, the barrage plate 
member 88 is displaced at a distance of 100 mm from an end wall of the 
vessel 46, and the permeameter 86 is positioned at a location downstream 
with respect to the barrage in the propellant direction (BB') of the 
developer. With this arrangement, a level of the propelled developer is 
constant at the location at which the permeameter 86 is positioned, 
whereby a content of the toner component in the developer can be 
accurately detected and measured. 
In an embodiment shown in FIGS. 21(a) and 21(b), the developing device 18 
also is arranged such that an accurate measurement of a magnetic 
permeability for detecting a content of the toner component in the 
developer held in vessel 46 can be ensured. Note, in FIGS. 21(a) and 
21(b), the screw members 54a and 54b having the right-hand screw flight 
are rough illustrated. In this embodiment, the permeameter 86 also is 
incorporated into the vessel bottom at a zone in which the screw member 
54b is located, but a barrage block member 88' is used in place of the 
barrage block member 88 of the embodiment shown in FIGS. 20(a) and 20(b). 
As shown in FIG. 21b, the barrage block member 88' has a semi-cylindrical 
concave surface formed a lower side thereof to complementarily receive the 
screw member 54b. Accordingly, when the propelled developer is passed 
through a passage section defined by the barrage block member 88', the 
passed developer has a uniform cross section. Namely, a mass of the 
developer passed through the passage section is kept constant. Thus, a 
proper and accurate measurement of a magnetic permeability can be ensured 
by the permeameter 86 displaced below the barrage block member 88'. 
FIGS. 22(a) and 22(b) show a modification of the embodiment shown in FIGS. 
21(a) and 21(b). This modified developing device 20 is identical to that 
of FIGS. 21(a) and 21(b) except that a bypass passage 90 is provided 
beside the barrage block member 88' to bypass a part of the developer 
propelled by the screw member 54b. In particular, the bypass passage 90 is 
defined by a side wall section 46b extended from the vessel 46 and a short 
partition member 54c' disposed along a side of the barrage block member 
88' adjacent to the short partition member 54c', and a small screw member 
54b'is rotatably provided in the bypass passage 90. In this embodiment, 
the small screw member 54b'has a right-hand screw flight similar to the 
screw member 54b, and is rotated the same direction as indicated an arrow 
in FIG. 22(b). With this arrangement, the circulation of the developer can 
be smoothly carried out without any stagnation of the developer caused due 
to an existence of the barrage block member 88'. 
Note, in the embodiments shown in FIGS. 20(a) and 20(b), FIGS. 21(a) and 
21(b), and FIGS. 22(a) and 22(b), although the barrage member 88, 88' is 
associated with the screw member 54b, it may by provided in a section of 
the developer circulating passage defined by the screw member 54a, to 
uniformly regulate a mass of the developer at a suitable location of the 
section for a proper measurement of magnetic permeability of the developer 
mass, if necessary. 
In an embodiment shown in FIG. 23, a screw member 92 is used in place of 
the screw member 54b, and is suitable for an accurate measurement of a 
magnetic permeability for detecting a content of the toner component in 
the developer. In this embodiment, the screw member 92 has a left-hand 
screw flight and is rotated to propel the developer in a direction 
indicated by an arrow in FIG. 23. The screw flight of the screw member 92 
has a section 94 as a part thereof, a diameter of which is gradually 
reduced along the propellant direction of the developer. For example, when 
the screw member 92 has an outer flight diameter of 24 mm, a diameter of 
the reduced section 94 is gradually varied from 24 mm to 15 mm over a 
length of 60 mm, and is then kept constant over a length of 20 mm, as 
shown in FIG. 23. A barrage block member 96 is disposed above the screw 
member 92 in substantially the same manner as the barrage block member 88' 
shown in FIGS. 21(a) and 21(b), and has a concave surface formed on a 
lower side thereof to complementarily receive a part of the screw member 
92. Namely, the concave surface is formed of a tapered surface section for 
receiving the reduced section 94 of the screw member 92 and a 
semi-cylindrical surface section for receiving another section of the 
screw member 92. On the other hand, the vessel 46 has a partially raised 
portion on which a tapered concave surface section is formed to 
complementarily receive the reduced section 94 of the screw member 92. 
Thus, the tapered surface sections of the barrage member 96 and the vessel 
bottom define a tapered passage through which the developer is propelled 
by the reduced section 94 of the screw member 92. The permeameter 86 is 
incorporated into the raised portion of the vessel bottom in the vicinity 
of the smallest opening end of the tapered passage. With this arrangement, 
a magnetic permeability can be measured by the permeameter 86 on a small 
mass of the developer, and thus it is possible to eliminate uncertain 
factors which may disadvantageously affect the measurement of magnetic 
permeability. 
FIG. 24 shows a modification of the embodiment as shown in FIG. 23. In this 
modified embodiment, a semi-cylindrical plate member 98 is used in place 
of the barrage block member 96, and has a tapered section for 
complementarily receiving the reduced section 94 of the screw member 92. 
As apparent from FIG. 24, the semi-cylindrical plate member 98 is provided 
in the vessel 46 to be immersed in the developer held therein, so that the 
circulation of the developer can be smoothly carried out without any 
stagnation of the developer. 
FIGS. 25 and 26 show an office use type printer comprising a printer 
housing 100, a rotary photosensitive drum 102, an electric discharger 104, 
a laser beam scanner 106, a developing device 108, and a transfer charger 
assembly 110, a toner image fixing device 112, and a toner cleaner 114, 
and each of these elements corresponds to those of the printer as shown in 
FIG. 1. Note, the developing device 108 is constructed according to the 
present invention, and has at least one feature of the present invention 
as mentioned above. 
This office type printer is provided with a plurality of paper cassettes 
116, each of which receives a stack of cut sheet paper having a given 
paper size. A sheet of paper fed from one of the paper cassettes 116, is 
once moved to a pair of register rollers 118, 118 through a paper guide 
120, and is then introduced, at a given timing, into a clearance between 
the drum 102 and the transfer charger assembly 110, in which a toner image 
transferring process is carried out, as mentioned hereinbefore. 
Successively, the paper carrying the transferred toner image is moved to 
the fixing device 112, in which the transferred toner image is fixed on 
the paper, and then the paper having the fixed toner image is fed to a 
pair of paper discharging rollers 120, 120 along a paper guide 122, and is 
thus discharged from the paper discharging rollers 120, 120 to a paper 
receiver 124 provided at the top of the printer housing 100. 
The printer is constituted such that printing can be made on both side 
faces of a sheet of paper. To this end, the paper guide 122 includes a 
paper bypass guide 126 extended from a location adjacent to a 
paper-discharge side of the fixing device 112 to a location adjacent to a 
paper-entrance side of the register rollers 118, 118, and is provided with 
a paper switch 128 incorporated therein. On the other hand, the printer 
housing is provided with a provisional paper receiver 130 provided below 
the paper receiver 124. When double-sided printing is performed, a paper 
discharged from the fixing device 112, i.e., a paper having printing on 
one side face thereof is once introduced into the provisional paper 
receiver 130 by actuating the paper switch 128, and is then returned to 
the register rollers 118, 118 through the paper bypass guide 126 for 
printing on the other side face thereof. Thereafter, the paper having the 
printing on both side faces thereof is discharged from the paper 
discharging rollers 120, 120 to the paper receiver 124 through the paper 
guide 122. 
Note, in FIG. 26, reference 132 indicates a floppy disk driver for reading 
out code data from a floppy disk loaded therein, and reference 134 
indicates a controller for controlling an operation of the printer. The 
code data read out from a floppy disk is converted into image data, on the 
basis of which printing is carried out. 
In the office type printer as mentioned above, since a large amount of 
developer is consumed, the developing device 108 according to the present 
invention is provided with a cartridge type developer supplier 136, as 
best shown in FIG. 27. Note, the developing device 108 includes a vessel 
138 for holding the two-component developer, a developing roller or 
magnetic roller 140 for carrying the developer to the drum 102, a paddle 
roller 142 feeding a fresh part of the developer to the magnetic roller 
140, a doctor blade 144 for regulating an amount of the developer brought 
by the magnetic roller 140, and a screw type agitator 146 having two screw 
members 146a and 146b and a partition member 146c provided therebetween, 
and each of these elements corresponds to those of the developing device 
18 as shown in FIGS. 2 and 3. 
The cartridge type developer supplier 136 includes an outer cylindrical 
housing 136a, and an inner cylindrical container 136b rotatably housed 
within the outer container 136a. An interior of the container 136b is 
divided into two chambers by a partition 136c: one chamber is indicated by 
reference 136d in FIG. 27, but the other chamber is not visible in this 
drawing. The container has two outlet ports formed therein: one outlet 
port 136e is located on a wall section of the container 136b by which the 
chamber 136 is defined, and the other outlet port is located on another 
wall section of the container 136b by which the not visible chamber is 
defined. Also, the housing 136a has two outlet ports formed therein: one 
outlet port is indicated by reference 136f, but the other outlet port is 
not visible. As apparent from FIG. 27, the outlet ports 136e and 136f can 
be registered with each other by rotating the container 13b in the housing 
136a, and this is also true for the not visible outlet ports of the 
housing 136a and the container 136b. Note, the rotation of the container 
can be carried out by manually operating a pair of levers (not shown) 
provided at the outer end walls of the housing 136a, and the registration 
of the outlet ports with each other can not be performed until the 
developer supplier 136 is attached to the developing device 108. The 
developer supplier 36 also includes a rotatable paddle member 136g 
provided in the chamber 136d, and another rotatable paddle member provided 
in the not visible chamber. The housing has an extended portion integrally 
formed therewith, which defines an empty chamber 136h. 
When the developer supplier 136 is new, the chamber 136d holds a 
two-component developer composed of a toner component and a magnetic 
component, and the not visible chamber holds only a toner-component or 
supplemental toner. When a developer held in the vessel 138 is 
deteriorated, a new developer supplier is exchanged by the old developer 
supplier. In particular, first, a movable door 100a of the printer housing 
100 is opened to access the old developer supplier. After the old 
developer supplier is detached from the developing device 108, the new 
developer supplier is attached to the developing device 108. At this time, 
an outlet port 138a formed in a bottom of the vessel 138 is opened so that 
the vessel 138 is in communication with the empty chamber 136h. The 
deteriorated developer is discharged from the vessel 138 into the empty 
chamber 136h through outlet port 138a by driving the agitator 146, and 
this discharge of the developer is facilitated by a paddle roller 136i 
provided in the empty chamber 136h. 
After the discharge of the developer is completed, the outlet port 138a is 
closed, and the container 136b is rotated in the housing 136a for the 
registration of the outlet ports thereof. Then, the paddle member 136g is 
rotated to introduce the new developer from the chamber 136d into the 
vessel 138 of the developing device 108, and after the introduction of the 
new developer is complete, the developing device is ready. During an 
operation of the printer, the paddle member provided in the not visible 
chamber of the container 136b is rotated, if necessary, to supplement a 
part of the toner from the not visible chamber to the vessel 138 through a 
feeder pipe (not shown), one end of which is opened at a side wall of the 
vessel 138 as indicated by reference 148 in FIG. 27. This supplement of 
the toner can be reasonably controlled by the magnetic permeability 
measuring arrangement, as shown in FIGS. 20(a) and 20(b), FIGS. 21(a) and 
21(b), FIGS. 22(a) and 22(b), and FIGS. 23 and 24. Note, the toner cleaner 
144 is provided with a flexible toner feeder pipe 150, a free end of which 
is detachably connected to the developer supplier 136 so that a toner 
removed from the drum 102 by the toner cleaner 114 is fed to the chamber 
136 of the developer supplier 136. 
Finally, it will be understood by those skilled in the art that the 
foregoing description is of preferred embodiments of the present 
invention, and that various changes and modifications can be made without 
departing from the spirit and scope thereof.