Electrostatic latent image developing device with developing agent-limiting means

A device for developing an electrostatic latent image includes a developing roller disposed in a developing housing and a developing agent-limiting device for limiting the amount of the developing agent held on the peripheral surface of the developing roller. The developing agent-limiting device includes a flexible blade brought into pressed contact with the peripheral surface of the developing roller, a flexible support plate that is mounted on the blade and supports the blade in a predetermined positional relationship with respect to the developing roller, and a resilient urging device that is constituted by a plurality of spring members arranged on the back surface of the support plate at a distance in the direction of width thereof and pushes the blade in a direction in which it comes into pressed contact with the peripheral surface of the developing roller via the support plate.

FIELD OF THE INVENTION 
The present invention relates to a device for developing electrostatic 
latent image used for developing electrostatic latent image into toner 
image in an image-forming machine such as electrostatic copying machine or 
laser printer. More specifically, the invention relates to a device for 
developing electrostatic latent image of the type equipped with a 
developing roller which holds the developing agent on the peripheral 
surface thereof to convey it to a developing zone and a developing 
agent-limiting means which acts on the peripheral surface of the 
developing roller to limit the amount of the developing agent held on the 
peripheral surface thereof. 
DESCRIPTION OF THE PRIOR ART 
As is well known, a device for developing electrostatic latent image of the 
type equipped with a developing roller which is rotated in a predetermined 
direction has been put into wide practical use in the image-forming 
machines in order to develop electrostatic latent image into toner image. 
The developing roller is rotated in a predetermined direction so as to 
travel through a developing agent-holding zone, a developing 
agent-limiting zone and a developing zone, successively. In the developing 
agent-holding zone, the toner which is the developing agent fed by a 
suitable system is held on the peripheral surface of the developing 
roller. In the developing agent-limiting zone, the developing 
agent-limiting means acts on the toner which is the developing agent held 
on the peripheral surface of the developing roller to limit the amount of 
the toner held on the peripheral surface of the developing roller to a 
required amount. In the developing zone, the toner which is the developing 
agent is applied to the peripheral surface of an electrostatic latent 
image carrier such as a rotary drum, which has an electrostatic 
photosensitive layer on the peripheral surface thereof, and electrostatic 
latent image formed on the surface of the electrostatic latent image 
carrier is developed into toner image. To accomplish good developing, it 
is important to suitably limit the amount of the toner held on the 
peripheral surface of the developing roller by the developing 
agent-limiting means and to form a toner layer on the peripheral surface 
of the developing roller in a predetermined thickness and sufficiently 
uniformly in the axial direction. 
Japanese Patent Publication No. 16736/1988 discloses a device for 
developing electrostatic latent image equipped with a limiting means 
having a blade which is made of a rubbery elastic material composed of a 
synthetic rubber such as urethane rubber, silicone rubber or the like. The 
rubbery elastic material constituting the developing agent-limiting means 
is brought, at its one surface or front edge, into pressed contact with 
the peripheral surface of the developing roller, limits the amount of the 
toner held on the peripheral surface of the developing roller to a 
considerably small amount thereby to form a thin toner layer on the 
peripheral surface of the developing roller. Furthermore, the device 
having the developing agent-limiting means constituted by the blade made 
of a thin spring stainless steel plate having a thickness of 0.1 to 0.2 mm 
has been put into practical use. In the developing agent-limiting means 
using an elastic blade made of a rubbery elastic material or a thin 
stainless steel plate, however, the rigidity becomes low at both ends of 
the elastic blade and the limiting force at the both ends becomes smaller 
than that at the central portion. Therefore, the toner layer formed on the 
peripheral surface of the developing roller becomes thicker toward both 
ends than at the central portion. Experiment conducted by the present 
inventors teaches that when a urethane blade is used, the toner layer at 
both ends becomes thicker than the central portion by 3.5 to 7.0 .mu.m. 
When the elastic blade is used, as described above, it is difficult to 
form the toner layer on the peripheral surface of the developing roller in 
a uniform thickness over the whole width in the axial direction and in 
consequence, a uniform image is not obtained. Besides, the elastic blade 
has an abrasion resistance which is not so high, and hence, is not 
necessarily satisfactory from the standpoint of life. 
In order to solve the above-mentioned problems of the elastic blade, there 
has been proposed an invention in which the blade constituting the 
developing agent-limiting means is made of a rigid member such as glass or 
the like (Japanese Laid-Open Patent Publication No. 36277/1995). By 
forming the blade using a sheet glass which is a rigid material, it is 
made possible to uniformly limit the thickness of the toner layer formed 
on the peripheral surface of the developing roller in the direction of 
width thereby to improve abrasion resistance and to lengthen the life to a 
satisfactory degree. 
In the case where the blade made of a rigid member is used to constitute 
the developing agent-limiting means that limits the amount of the 
developing agent held on the surface of the developing roller, however, if 
the developing roller and the blade become out of parallel due to 
deviation of the shaft of the developing roller caused by production error 
or due to warping of the developing housing, the contacting force of the 
blade does not become uniform in the direction of the width, and it 
becomes no longer possible to uniformalize the thickness of the toner 
layer formed on the peripheral surface of the developing roller over the 
whole width in the axial direction. 
Further, in the case where the blade made of a rigid member such as a sheet 
glass is used to constitute the developing agent-limiting means that 
limits the amount of the developing agent held on the peripheral surface 
of the developing roller, if the developing roller and the blade become 
out of parallel due to deviation of the shaft of the developing roller 
caused by production error or due to warping of the developing housing, 
the contacting force of the blade does not become uniform in the direction 
of the width, and it becomes no longer possible to uniformalize the 
thickness of the toner layer formed on the peripheral surface of the 
developing roller over the whole width in the axial direction. 
It was also learned through experiments conducted by the present inventors 
that the thickness and stability of the toner layer formed on the 
peripheral surface of the developing roller are greatly affected by the 
amount of projection of the blade constituting the developing 
agent-limiting means from the center of contact with the peripheral 
surface of the developing roller to the lower end of the blade. That is, 
when the amount of projection is smaller than a predetermined range, the 
thickness of the developing agent layer formed on the peripheral surface 
of the developing roller tends to become extremely small. When the amount 
of projection is larger than the predetermined range, on the other hand, 
the thickness of the developing agent layer formed on the peripheral 
surface of the developing roller becomes extremely large. 
It was also learned through experiments conducted by the present inventors 
that the thickness and stability of the toner layer formed on the 
peripheral surface of the developing roller are greatly affected by the 
contacting force of the blade constituting the developing agent-limiting 
means, exerted on the peripheral surface of the developing roller. That 
is, when the contacting force is larger than a predetermined range, the 
thickness of the developing agent layer formed on the peripheral surface 
of the developing roller tends to become very small. When the contacting 
force is smaller than the predetermined range, on the other hand, the 
thickness of the developing agent layer formed on the peripheral surface 
of the developing roller becomes very large. 
Furthermore, it was learned through experiments conducted by the present 
inventors that the thickness and stability of the toner layer formed on 
the peripheral surface of the developing roller are greatly affected by 
the roughness of the peripheral surface of the developing roller. That is, 
when the roughness of the peripheral surface of the developing roller is 
smaller than a predetermined range, the thickness of the developing agent 
layer formed on the peripheral surface of the developing roller tends to 
become very small. When the surface roughness is larger than the 
predetermined range, on the other hand, the thickness of the developing 
agent layer formed on the peripheral surface of the developing roller 
becomes very large. 
It was also learned through experiments conducted by the present inventors 
that setting of the developing roller having a suitable hardness plays an 
important role for forming the toner layer in a predetermined thickness on 
the peripheral surface of the developing roller. Experiment teaches that 
the thickness of the toner layer formed on the peripheral surface of the 
developing roller decreases with a decrease in the hardness of the 
developing roller, and the thickness of the toner layer formed on the 
peripheral surface of the roller increases with an increase in the 
hardness of the developing roller. Moreover, the thickness of the toner 
layer formed on the peripheral surface of the developing roller varies 
depending upon the contacting force of the blade constituting the 
developing agent-limiting means, which acts on the peripheral surface of 
the developing roller; i.e., the thickness of the toner layer formed on 
the peripheral surface of the developing roller increases with a decrease 
in the contacting force, and the thickness of the toner layer formed on 
the peripheral surface of the developing roller decreases with an increase 
in the contacting force. It was further learned that the uniform toner 
layer can not be obtained stably and variation in thickness of the toner 
layer frequently occurs when the contacting force is too small or is too 
large. It is known that the quality of image greatly varies depending upon 
the thickness of the toner layer formed on the surface of the developing 
roller. When the thickness of the toner layer is smaller than, for 
example, 20 .mu.m, the image density becomes so low that a copied image 
loses vividness. When the thickness of the toner layer becomes larger 
than, for example, 30 .mu.m, a so-called image base fogging occurs in 
which toner adheres to the areas other than the image areas. The 
developing roller is generally constituted by a rotary shaft made of a 
metal material and a rubber roller fitted onto the rotary shaft. 
Therefore, it was learned through experiments conducted by the present 
inventors that the substantial hardness of the developing roller as an 
assembly varies depending upon the thickness of the rubber roller fitted 
onto the rotary shaft and the thickness and stability of the toner layer 
formed on the peripheral surface of the developing roller are greatly 
affected by the substantial hardness of the developing roller. 
It was further learned through experiment conducted by the present 
inventors that even the toner layer formed in a predetermined thickness on 
the peripheral surface of the developing roller through limitation made by 
the blade undergoes a change in the density, i.e., undergoes a change in 
the amount of the developing agent conveyed to the developing zone 
depending upon the amount of the developing agent fed onto the peripheral 
surface of the developing roller by the replenishing roller in the 
developing agent-holding zone. It was further learned that the density of 
the developing agent layer formed on the peripheral surface of the 
developing roller varies even depending upon the thickness of the blade 
itself. The density of the image decreases with a decrease in the amount 
of the developing agent that is conveyed to the developing zone being held 
on the peripheral surface of the developing roller, whereas the so-called 
fogging phenomenon takes place when the developing agent is conveyed in 
too large amounts. 
SUMMARY OF THE INVENTION 
A first object of the present invention is to provide a device for 
developing electrostatic latent image in which the contacting force of the 
blade on the developing roller is uniformalized in the direction of the 
whole width, and the thickness of the toner layer formed on the peripheral 
surface of the developing roller is uniformalized over the whole width in 
the axial direction even though the developing roller and the blade become 
out of parallel due to deviation of the shaft of the developing roller. 
A second object of the present invention is to provide a device for 
developing electrostatic latent image in which the contacting force of the 
glass blade on the developing roller is uniformalized in the direction of 
the whole width, and the thickness of the toner layer formed on the 
surface of the developing roller is uniformalized over the whole width in 
the axial direction even though the developing roller and the glass blade 
become out of parallel due to deviation of the shaft of the developing 
roller. 
A third object of the present invention is to provide a device for 
developing electrostatic latent image which is capable of stably forming a 
developing agent layer in a thickness within a predetermined range on the 
peripheral surface of the developing roller by setting the amount of 
projection of the blade constituting the developing agent-limiting means 
from the center of contact with the peripheral surface of the developing 
roller to the lower end of the blade within a predetermined range. 
A fourth object of the present invention is to provide a device for 
developing electrostatic latent image which is capable of stably forming a 
developing agent layer in a thickness within a predetermined range on the 
peripheral surface of the developing roller by setting the contacting 
force of the blade constituting the developing agent-limiting means 
exerted on the peripheral surface of the developing roller within a 
predetermined range. 
A fifth object of the present invention is to provide a device for 
developing electrostatic latent image which is capable of stably forming a 
developing agent layer in a thickness within a predetermined range on the 
peripheral surface of the developing roller by setting the roughness of 
the peripheral surface of the developing roller within a predetermined 
range. 
A sixth object of the present invention is to provide a device for 
developing electrostatic latent image which is capable of stably forming a 
developing agent layer in a thickness within a predetermined range on the 
peripheral surface of the developing roller by setting the substantial 
hardness of the developing roller within a predetermined range, the 
developing roller being constituted by a rotary shaft formed of a metal 
material and a solid synthetic rubber roller fitted onto the rotary shaft. 
A seventh object of the present invention is to provide a device for 
developing electrostatic latent image which is capable of setting the 
conveyance amount of the developing agent held on the peripheral surface 
of the developing roller within a predetermined range by setting the 
thickness of the blade constituting the developing agent-limiting means 
and the nipping width in which the developing roller and the replenishing 
roller are in contact with each other within predetermined ranges. 
In order to accomplish the above-mentioned first object, the present 
invention provides a device for developing electrostatic latent image 
comprising a developing housing, a developing roller disposed in the 
developing housing to hold, on the peripheral surface thereof, a 
developing agent in a developing agent-holding zone and convey the thus 
held developing agent to a developing zone to apply it to the 
electrostatic latent image, and a developing agent-limiting means for 
limiting the amount of the developing agent held on the peripheral surface 
of the developing roller in a developing agent-limiting zone positioned 
between the developing agent-holding zone and the developing zone, wherein 
the developing agent-limiting means includes a flexible blade of which the 
one surface is brought into pressed contact with the peripheral surface of 
the developing roller, a flexible support plate which is mounted on the 
other surface of the blade and supports the blade maintaining a 
predetermined positional relationship with respect to the developing 
roller, and a resilient urging means which is constituted by a plurality 
of spring members arranged on the back surface of the support plate at a 
distance in the direction of width thereof and pushes the blade in a 
direction in which it comes into pressed contact with the peripheral 
surface of the developing roller via the support plate. 
Furthermore, in order to accomplish the above-mentioned first object, the 
present invention provides a device for developing electrostatic latent 
image comprising a developing housing, a developing roller disposed in the 
developing housing to hold, on the peripheral surface thereof, a 
developing agent in a developing agent-holding zone and convey the thus 
held developing agent to a developing zone to apply it to the 
electrostatic latent image, and a developing agent-limiting means for 
limiting the amount of the developing agent held on the peripheral surface 
of the developing roller in a developing agent-limiting zone positioned 
between the developing agent-holding zone and the developing zone, wherein 
the developing agent-limiting means includes a flexible blade of which the 
one surface is brought into pressed contact with the peripheral surface of 
the developing roller, a flexible support plate which is mounted on the 
other surface of the blade and supports the blade maintaining a 
predetermined positional relationship with respect to the developing 
roller, a flexible pushing member which is disposed by the side of the 
back surface of the support plate and has a protrusion formed along the 
direction of width to come into contact with the back surface of the 
support plate, and a resilient urging means which is constituted by a 
plurality of spring members arranged on the back surface of the support 
plate at a distance in the direction of width thereof and pushes the blade 
in a direction in which it comes into pressed contact with the peripheral 
surface of the developing roller via the pushing member and the support 
plate. 
In order to accomplish the above-mentioned second object, the present 
invention provides a device for developing electrostatic latent image 
comprising a developing housing, a developing roller disposed in the 
developing housing to hold, on the peripheral surface thereof, a 
developing agent in a developing agent-holding zone and convey the thus 
held developing agent to a developing zone to apply it to the 
electrostatic latent image, and a developing agent-limiting means for 
limiting the amount of the developing agent held on the peripheral surface 
of the developing roller in a developing agent-limiting zone positioned 
between the developing agent-holding zone and the developing zone, wherein 
the developing agent-limiting means includes a blade made of a flexible 
sheet glass of which the one surface is brought into pressed contact with 
the peripheral surface of the developing roller, a flexible support plate 
which is mounted on the other surface of the blade and supports the blade 
maintaining a predetermined positional relationship with respect to the 
developing roller, and a resilient urging means which is constituted by a 
plurality of spring members arranged on the back surface of the support 
plate at a distance in the direction of width thereof and pushes the blade 
in a direction in which it comes into pressed contact with the peripheral 
surface of the developing roller via the support plate, and wherein the 
blade made of the sheet glass has a thickness of from 0.5 to 2.0 mm. 
In order to accomplish the above-mentioned third object, the present 
invention provides a device for developing electrostatic latent image 
comprising a developing housing, a developing roller disposed in the 
developing housing to hold, on the peripheral surface thereof, a 
developing agent in a developing agent-holding zone and convey the thus 
held developing agent to a developing zone to apply it to the 
electrostatic latent image, and a developing agent-limiting means for 
limiting the amount of the developing agent held on the peripheral surface 
of the developing roller in a developing agent-limiting zone positioned 
between the developing agent-holding zone and the developing zone, wherein 
the developing agent-limiting means includes a blade made of a flexible 
sheet glass of which the one surface is brought into pressed contact with 
the peripheral surface of the developing roller, a flexible support plate 
which is mounted on the other surface of the blade and supports the blade 
maintaining a predetermined positional relationship with respect to the 
developing roller, and a resilient urging means which pushes the blade in 
a direction in which it comes into pressed contact with the peripheral 
surface of the developing roller via the support plate, and wherein the 
amount of projection of the blade from the center of contact with the 
peripheral surface of the developing roller to the lower end of the blade 
is set to be from 0.5 to 2.0 mm. 
In order to accomplish the above-mentioned fourth object, the present 
invention provides a device for developing electrostatic latent image 
comprising a developing housing, a developing roller disposed in the 
developing housing to hold, on the peripheral surface thereof, a 
developing agent in a developing agent-holding zone and convey the thus 
held developing agent to a developing zone to apply it to the 
electrostatic latent image, and a developing agent-limiting means for 
limiting the amount of the developing agent held on the peripheral surface 
of the developing roller in a developing agent-limiting zone positioned 
between the developing agent-holding zone and the developing zone, wherein 
the developing agent-limiting means includes a blade made of a flexible 
sheet glass of which the one surface is brought into pressed contact with 
the peripheral surface of the developing roller, a flexible support plate 
which is mounted on the other surface of the blade and supports the blade 
maintaining a predetermined positional relationship with respect to the 
developing roller, and a resilient urging means which pushes the blade in 
a direction in which it comes into pressed contact with the peripheral 
surface of the developing roller via the support plate, and wherein the 
contacting force of the blade exerted on the peripheral surface of the 
developing roller by the resilient urging means is from 4.0 to 12.0 g/mm 
as the line pressure. 
In order to accomplish the above-mentioned fifth object, the present 
invention provides a device for developing electrostatic latent image 
comprising a developing housing, a developing roller disposed in the 
developing housing to hold, on the peripheral surface thereof, a 
developing agent in a developing agent-holding zone and convey the thus 
held developing agent to a developing zone to apply it to the 
electrostatic latent image, and a developing agent-limiting means for 
limiting the amount of the developing agent held on the peripheral surface 
of the developing roller in a developing agent-limiting zone positioned 
between the developing agent-holding zone and the developing zone, wherein 
the roughness Rz (ten-point average roughness specified under JIS B 0601) 
of the peripheral surface of the developing roller is set to be from 5.0 
to 12.0 .mu.m. 
In order to accomplish the above-mentioned sixth object, the present 
invention provides a device for developing electrostatic latent image 
comprising a developing housing, a developing roller disposed in the 
developing housing to hold, on the peripheral surface thereof, a 
developing agent in a developing agent-holding zone and convey the thus 
held developing agent to a developing zone to apply it to the 
electrostatic latent image, and a developing agent-limiting means for 
limiting the amount of the developing agent held on the peripheral surface 
of the developing roller in a developing agent-limiting zone positioned 
between the developing agent-holding zone and the developing zone, wherein 
the developing agent-limiting means includes a blade made of a flexible 
sheet glass of which the one surface is brought into pressed contact with 
the peripheral surface of the developing roller, a flexible support plate 
which is mounted on the other surface of the blade and supports the blade 
maintaining a predetermined positional relationship with respect to the 
developing roller, and a resilient urging means which pushes the blade in 
a direction in which it comes into pressed contact with the peripheral 
surface of the developing roller via the support plate, and wherein the 
developing roller is constituted by a rotary shaft formed of a metal 
material and a solid synthetic rubber roller fitted onto the rotary shaft, 
and the developing roller has an Asker's C hardness of from 60 to 80. 
In order to accomplish the above-mentioned seventh object, the present 
invention provides a device for developing electrostatic latent image 
comprising a developing housing, a developing roller disposed in the 
developing housing to hold, on the peripheral surface thereof, a 
developing agent in a developing agent-holding zone and convey the thus 
held developing agent to a developing zone to apply it to the 
electrostatic latent image, a replenishing roller which is disposed in 
parallel with the developing roller and feeds the developing agent onto 
the peripheral surface of the developing roller in the developing 
agent-holding zone, and a developing agent-limiting means for limiting the 
amount of the developing agent held on the peripheral surface of the 
developing roller in a developing agent-limiting zone positioned between 
the developing agent-holding zone and the developing zone, wherein the 
developing agent-limiting means includes a flexible blade having a 
thickness of from 0.1 to 2.0 mm, of which the one surface is brought into 
pressed contact with the peripheral surface of the developing roller, a 
flexible support plate which is mounted on the other surface of the blade 
and supports the blade maintaining a predetermined positional relationship 
with respect to the developing roller, and a resilient urging means which 
pushes the blade in a direction in which it comes into pressed contact 
with the peripheral surface of the developing roller via the support 
plate, and wherein the nipping width in which the developing roller and 
the replenishing roller come into contact with each other is set to be 
from 0.1 to 0.6 mm. 
Other objects and features of the present invention will become apparent 
from the following description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Preferred embodiments of the device for developing electrostatic latent 
image constituted according to the present invention will now be described 
in further detail with reference to the accompanying drawings. 
FIG. 1 illustrates an image-forming machine equipped with the device for 
developing electrostatic latent image constituted according to the present 
invention. The illustrated image-forming machine is equipped with a rotary 
drum 2 having, arranged on the peripheral surface thereof, an 
electrostatic photosensitive material that serves as an image carrier. The 
rotary drum 2 is mounted by means of a rotary shaft 21 in a machine 
housing that is not shown, and is allowed to freely rotate. Around the 
rotary drum 2 that rotates in a direction of an arrow 22, there are 
arranged a corona discharger 4 for charging the photosensitive layer of 
the rotary drum 2 to a particular polarity, a laser optical device 6 which 
is an exposure means for forming an electrostatic latent image on the 
photosensitive layer of the rotary drum 2 charged to the particular 
polarity by the corona discharger, a device 8 for developing electrostatic 
latent image formed by a laser beam irradiated from the optical device 6 
into toner image, a transfer roller 10, a cleaning device 14 and a 
charge-removing lamp 16, as viewed in a direction of rotation. 
The device 8 for developing electrostatic latent image is equipped with a 
developing housing 30 that can be formed of a synthetic resin. In the 
developing housing 30 are arranged a developing roller 40, a replenishing 
roller 50, a stirring means 60 and a developing agent-limiting means 70. 
On the developing housing 30 is mounted a toner cartridge 100 as a 
developing agent container. 
The developing housing 30 is formed of a synthetic resin, and has a 
developing chamber 31 and a stirrer chamber 32. The developing housing 30 
is provided at an upper portion thereof with a toner cartridge-mounting 
portion 34 having an opening 33 in the upper part thereof, and a toner 
cartridge 100 is mounted on the mounting portion 34. 
The developing roller 40 is disposed in the developing chamber 31 in the 
developing housing 30, and includes a rotary shaft 41 that is supported 
between both side walls (not shown) of the developing housing 30 so as to 
rotate and a solid synthetic rubber roller 42 fitted to the outer 
peripheral surface of the rotary shaft 41. The rotary shaft 41 can be made 
of a suitable metal material such as stainless steel. The solid synthetic 
rubber roller 42 is constituted by a relatively soft material having 
electrically conducting property or an electrically conducting solid 
synthetic rubber such as urethane rubber. In the illustrated embodiment, 
the peripheral surface of the solid synthetic rubber roller 42 has a 
roughness, i.e., a ten-point average roughness Rz as stipulated under JIS 
B 0601, of 5.0 to 12.0. The solid synthetic rubber roller 42 has a volume 
resistivity of from about 10.sup.6 to about 10.sup.9 .OMEGA..cm. In the 
developing roller 40 constituted by the rotary shaft 41 made of a metal 
material and the solid synthetic rubber roller 42 fitted to the outer 
peripheral surface of the rotary shaft 41, the rotary shaft 41 has an 
outer diameter of, for example, 8 to 10 mm and the solid synthetic rubber 
roller 42 has an outer diameter of, for example, 16 mm. When the rotary 
shaft 41 has an outer diameter of 8 mm, therefore, the solid synthetic 
rubber roller 42 has a thickness of 4 mm in the radial direction and when 
the rotary shaft 41 has an outer diameter of 10 mm, the solid synthetic 
rubber roller 42 has a thickness of 3 mm in the radial direction. When a 
rubber that constitutes the solid synthetic rubber roller 42 has the same 
hardness, therefore, the hardness of the developing roller as an assembly 
varies. That is, the hardness of the developing roller using the rotary 
shaft of an outer diameter of 8 mm becomes smaller than that of the 
developing roller using the rotary shaft of an outer diameter of 10 mm. In 
the illustrated embodiment, the developing roller, as an assembly, 
constituted by the rotary shaft 41 and the solid synthetic rubber roller 
42 fitted to the outer peripheral surface of the rotary shaft 41 has a 
hardness of from 60 to 80 in terms of Asker's C hardness. The roller 42 of 
the thus constituted developing roller 40 is exposed through the opening 
formed in the developing housing 30 and is positioned being opposed to the 
rotary drum 2. The peripheral surface of the roller 42 constituting the 
developing roller 40 is brought into pressed contact with the peripheral 
surface of the rotary drum 2, and is elastically compressed to some extent 
in the press-contacted region. The rotary shaft 41 of the developing 
roller 40 is continuously rotated by a drive means that is not shown, in a 
direction indicated by an arrow 401 in FIG. 1. With the rotation of the 
rotary shaft 41, the roller 42 is also continuously rotated in the 
direction indicated by the arrow 401, and the peripheral surface of the 
roller 42 passes through a developing agent-holding zone 402, developing 
agent-limiting zone 403 and developing zone 404, successively. In the 
illustrated embodiment, a constant voltage of 300 V is applied to the 
rotary shaft 41 of the developing roller 40. 
The replenishing roller 50 is disposed in the developing chamber 32 in the 
developing housing 30 in parallel with the developing roller 40, and 
includes a rotary shaft 51 rotatably supported between both side walls 
(not shown) of the developing housing 30 and a roller 52 fitted to the 
outer peripheral surface of the rotary shaft 51. Like the rotary shaft 41 
of the developing roller 40, the rotary shaft 51 can be made of a suitable 
metal material such as stainless steel. The roller 52 is constituted by a 
foamed material such as foamed silicon or foamed urethane. The foamed 
material constituting the roller 52 has a hardness which is considerably 
smaller than the hardness of the roller 42 of the developing roller 40. 
For example, a material having an Asker's C hardness of about 35 is used 
as the foamed material. The roller 42 of the developing roller 40 and the 
roller 52 of the replenishing roller 50 are brought into pressed contact 
on their peripheral surfaces as shown in FIG. 2, and a nipping width (L) 
in which they come in contact with each other is set to be from 0.1 to 0.6 
mm. That is, the distance between the axis of the developing roller 40 and 
that of the replenishing roller 50 is so set that the nipping width (L) 
lies from 0.1 to 0.6 mm. The roller 52, too, has electrically conducting 
property, and has a volume resistivity of from about 10.sup.6 to about 
10.sup.9 .OMEGA..cm. The roller is continuously rotated by a drive means 
that is not shown, in a direction indicated by an arrow 501 in FIG. 1. In 
the illustrated embodiment, the rotary shaft 51 of the developing roller 
50 is impressed with a constant voltage of 450 V which is higher than the 
voltage applied to the developing roller 40. 
The stirring means 60 is disposed in the stirrer chamber 32 of the 
developing housing 30 in parallel with the replenishing roller 52, and 
includes a rotary shaft 61 supported rotatably between both side walls 
(not shown) of the developing housing 30, a stirrer member 62 fitted to 
the rotary shaft 61, and a resilient stirrer sheet member 63 mounted on 
the stirrer member 62. The stirrer member 62 is made of a synthetic resin 
and has a plurality of openings formed in the direction of width thereof. 
The stirrer sheet member 63 is made of, for example, a polyethylene 
terephthalate (PETP) resin and has, at its end portion, openings that 
correspond to the openings formed in the stirrer member 62, and is 
fastened to the stirrer member 62 using an adhesive or the like. The other 
end of the stirrer sheet member 63 protrudes beyond the stirrer member 62. 
The thus constituted stirring means 60 is continuously rotated by a drive 
means that is not shown, in a direction indicated by an arrow 601 in FIG. 
1. 
The developing agent-limiting means 70 comprises a flexible blade 71 
brought into pressed contact with the peripheral surface of the roller 42 
which constitutes the developing roller 40, a flexible support plate 72 
constituting a support means of the blade 71, a resilient urging means 73 
that pushes one surface of the blade 71 toward a direction to come into 
contact with the peripheral surface of the roller 42, and a support holder 
76 for supporting the blade 71, support plate 72 and resilient urging 
means 73. 
The support holder 76 is formed by, for example, extrusion-molding an 
aluminum alloy. The support holder 76 has a length in the direction of 
width which corresponds to a distance between both side walls of the 
developing housing 30, and comprises an upper wall 761 for mounting the 
upper end of the support plate 72, a lower wall 762 formed at a 
predetermined distance from the upper wall 761, and a rear wall 763 that 
connects the upper wall 761 to the rear end of the lower wall 762. The 
thus constituted support holder 76 is disposed at a predetermined position 
between both side walls of the developing housing 30, and is mounted on 
the developing housing 30 by a fastening means such as tightening bolts 
(not shown) that is fitted penetrating through both side walls of the 
developing housing 30. 
The flexible blade 71 has a length in the direction of width which 
corresponds to the length of the roller 42 that constitutes the developing 
roller 40, and has at least one surface thereof (surface brought into 
pressed contact with the peripheral surface of the roller 42 constituting 
the developing roller 40) constituted by a flat plate-like member that 
extends in the direction of width (direction perpendicular to the surface 
of the paper in FIG. 1) along the peripheral surface of the roller 42. It 
is desired that at least the region of one surface of the blade 71 brought 
into pressed contact with the peripheral surface of the roller 42 has a 
sufficiently small surface roughness and a ten-point average roughness Rz 
specified under JIS B 0601 of from 5.0 to 12.0. As the surface roughness 
on one surface of the blade 71 becomes too great, the surface of the toner 
layer formed on the peripheral surface of the roller 42 that constitutes 
the developing roller 40 is not flattened to a sufficient degree and is 
liable to be nonuniform. A sheet glass placed in the market can be 
exemplified as a suitable material that can be used as the blade 71 
relatively cheaply yet exhibiting a sufficiently small roughness, a high 
hardness and a large abrasion resistance. The sheet glass having a 
thickness of about 0.5 to 2.0 mm can be used. By using three kinds of 
sheet glasses having a length of 218 m and widths of 3.0 mm, 7.0 mm and 
10.0 mm, the present inventors have tested the amount of deflection and 
have obtained the results as shown in FIG. 9. The amounts of deflection of 
FIG. 9 are those when the amount of deflection of a sheet glass having a 
length of 218 mm, a width of 7.0 mm and a thickness of 3.0 mm is regarded 
to be 1. The sheet glass having a length of 218 mm, a width of 7.0 mm and 
a thickness of 3.0 mm is a nearly rigid material and, according to 
experiments conducted by the present inventors, poorly follows the 
peripheral surface of the developing roller. If the developing roller and 
the blade become out of parallel, the contacting force of the blade no 
longer becomes uniform in the direction of the width, and the thickness of 
the toner layer formed on the peripheral surface of the developing roller 
varies in the axial direction. According to experiments conducted by the 
present inventors, on the other hand, it was found that when the amount of 
deflection of the blade 72 is not smaller than 2.4, the blade follows the 
deflection of the shaft of the developing roller and exerts a uniform 
contacting force over the whole width in the lengthwise direction. The 
blade 71 deflects by an amount which is not smaller than 2.4 when it is 
made of a sheet glass having a width of 10.0 mm and a thickness of not 
larger than 2.0 mm. It was further learned that the sheet glass tends to 
be cracked during the operation when its thickness is not larger than 0.5 
mm. It is therefore desired that the sheet glass constituting the blade 71 
has a thickness of from 0.5 to 2.0 mm. When it is desired to apply a 
required voltage to the blade 71 to control the charging property of the 
toner, an electrically conducting film may be applied to one surface of 
the sheet glass. The blade 71 can also be constituted by using a suitable 
flexible metal plate such as of a stainless steel in place of using the 
sheet glass. To sufficiently decrease the surface roughness on one surface 
of the metal plate constituting the blade 71, a suitable surface treatment 
may be effected for one surface of the metal plate, as required. 
In the illustrated embodiment, the support plate 72 is constituted by a 
plate spring member such as thin spring steel plate or thin stainless 
steel plate, and has a length in the direction of width nearly the same as 
that of the blade 71. The support plate 72 must have flexibility and, 
hence, it is desired to use a steel plate having a thickness of about 0.1 
mm. As shown in FIG. 3, the support plate 72 has a plurality of oval holes 
721 (five holes in the illustrated embodiment) formed in the upper end 
portion thereof at a predetermined distance in the direction of width. To 
the back surface at the lower portion of the support plate 72 are attached 
a plurality of spring support projections 722 (six spring support 
projections in the illustrated embodiment) at an equal distance in the 
direction of the width. The thus constituted support plate 72 is fastened 
at the surface of the lower end thereof to the other surface of the blade 
71 by a fastening means such as adhesive agent. When it is desired to 
apply a required voltage to the blade 71 to control the charging property 
of the toner, an electrically conducting adhesive is used as the fastening 
means thereby to allow to apply the required voltage to the blade 71 
through the support plate 72. As described above, the support plate 72 
fastened to the other surface of the blade 71 is fastened and supported by 
using screws 78 that engage, through a patch 77 and the holes 721, with 
the end surface of the upper wall 761 that constitutes the support holder 
76 at the upper end thereof. 
The resilient urging means 73 comprises a plurality of compression coil 
springs 731 (six compression coil springs in the illustrated embodiment), 
is disposed on the spring support projections 722 attached to the back 
surface of the support plate 72, and is arranged between the spring 
support projections and the rear wall 763 that constitutes the support 
holder 76. When the back surface of the support plate 72 is pushed by the 
resilient urging means 73 constituted by the plurality of compression coil 
springs 731 that are arranged at an equal distance in the direction of 
width, the one surface of the blade 71 is uniformly brought into pressed 
contact with the surface of the roller 42 constituting the developing 
roller 40 over the whole width even though the shaft of the developing 
roller 40 is deflected to some extent since the support plate 72 and the 
blade 71 have flexibility. The contacting force of the blade 71 acting on 
the peripheral surface of the roller 42 constituting the developing roller 
40 can be suitably set depending upon the thickness of the developing 
agent layer that is to be formed on the peripheral surface of the roller 
42. The thickness of the developing agent layer formed on the peripheral 
surface of the roller 42 decreases with an increase in the contacting 
force. As the pressing force becomes excessive, on the other hand, smooth 
rotation of the roller 42 is likely to be impaired. In the developing 
system of the illustrated embodiment, in general, the toner layer formed 
on the peripheral surface of the roller 42 is from about 20 to about 35 
.mu.m. To suitably form the developing agent layer having such a 
thickness, the blade 71 should be brought into pressed contact with the 
peripheral surface of the roller 42 with a line pressure (pressure per a 
unit length in the direction of width) of from 4.0 to 12.0 g/mm. 
It is desired that, as shown in FIG. 8, the lower end of the blade 71 is 
slightly protruded toward the upstream side, as viewed in a direction in 
which the roller 42 moves, beyond a portion where the blade 71 is 
contacted to the roller 42 constituting the developing roller 40. In the 
illustrated embodiment, the amount (S) of projection of the blade 71 from 
the center of contact with the roller 42 to the lower end of the blade 71 
is set to be from 0.5 to 2.0 mm. When the amount (S) of projection is 
smaller than 0.5 mm, the limiting action by the blade 71 becomes so 
excessive that it becomes difficult to form a favorable developing agent 
layer. When the amount (S) of projection becomes larger than 2.0 mm, on 
the other hand, the thickness of the developing agent layer formed becomes 
too great and loses stability. 
Another embodiment of the limiting means 70 will now be described with 
reference to FIGS. 4, 5 and 6. The image-forming machine equipped with the 
device for developing electrostatic latent image constituted according to 
the present invention shown in FIG. 4 is substantially the same as the 
embodiment shown in FIGS. 1 to 3 except the developing agent-limiting 
means 70. Therefore, the same portions are denoted by the same reference 
numerals but are not described again. 
The support holder 76 in this embodiment has a guide rail 764 that inwardly 
protrudes at an intermediate portion of the upper wall 761 and extends in 
the direction of the whole width and, further, has a plurality of 
pin-insertion holes 765 (three pin-insertion holes in the illustrated 
embodiment) formed between the guide rail 764 of the upper wall 761 and 
the rear wall 763. In the lower wall 762 is formed a guide groove 766 
extending in the direction of the whole width from the rear wall 763 up to 
a position corresponding to the guide rail 764. 
In the embodiment shown in FIGS. 4 to 6, a pushing member 74 is disposed 
between the support plate 72 and the resilient urging means 73. The 
pushing member 74 has a length in the direction of the width which is 
nearly the same as that of the blade 71 and has the shape of a hat in 
cross section of which the rear end is opened, and is constituted by a 
front wall 741 that is opposed to the support plate 72, an upper wall 742, 
a lower wall 743, and guide support portions 744 and 745 that extend 
upwards and downwards from the rear ends of the upper wall 742 and the 
lower wall 743, respectively. A protrusion (elongated protrusion) 746 is 
formed on the front surface of the front wall 741 to constitute a pushing 
portion at an intermediate portion in the up-and-down direction over the 
whole width. Between the upper wall 742 and the lower wall 743, 
furthermore, there are formed a plurality of spring-fitting chambers 749 
(six chambers in the illustrated embodiment) by partitioning walls 747 and 
748 formed at an equal distance in the direction of width, and a 
compression coil spring 731 is fitted in each of the chambers. The thus 
constituted pushing member 74 is molded as a unitary structure using, for 
example, a synthetic resin. It is important that the pushing member 74 has 
flexibility. The end on one side of the compression coil spring 751 is 
fitted to each of the plurality of spring-fitting chambers 749 formed in 
the pushing member 74, and the end on the other end thereof is brought 
into contact with the front surface of the intermediate wall 763 
constituting the support holder 76. By being constituted as described 
above, the pushing forces of the plurality of compression coil springs 751 
disposed in the direction of the width of the pushing member 74 at an 
equal distance, act on the blade 71 via the pushing member 74 and the 
support plate 72 causing one surface of the blade 71 to be brought into 
pressed contact with the surface of the roller 42. In this case, the one 
surface of the blade 71 is brought into pressed contact with the surface 
of the roller 42 constituting the developing roller 40 uniformly over the 
whole width even though the shaft of the developing roller 40 is deviated 
to some extent since the pushing member 74, support plate 72 and blade 71 
have flexibility. The support plate 72 mounted on the blade 71 is pushed 
by the protrusion 746 formed on the front surface of the pushing member 
74. Since the contact area is small, therefore, a stable and uniform 
pushing force is obtained over the whole width. 
The developing agent-limiting means 70 has a pressure-canceling means 79 
for canceling the pressure exerted on the support plate 72 and on the 
blade 71 by the resilient urging means 73. In the illustrated embodiment, 
the pressure-canceling means 79 has three stopper pins 791. At the time of 
assembling the developing agent-limiting means 70, the three stopper pins 
791 are inserted, as shown in FIG. 6, in the three pin-insertion holes 765 
formed between the rear wall 763 and the guide rail 764 of the upper wall 
761 that constitutes the support holder 76, and act on the front surface 
of the guide support portion 764 that constitutes the pushing member 74 to 
cancel the pressing force of the resilient urging means 73 acting on the 
back surface of the support plate 72. After the developing agent-limiting 
means 70 is assembled on a predetermined portion of the developing device 
8, the three stopper pins 791 are removed from the pin-insertion holes 
765, so that the pushing forces of the compression coil springs 751 act on 
the back surface of the support plate 72 as shown in FIG. 4. 
Described below is the procedure for assembling the developing 
agent-limiting means 70 and for mounting the developing agent-limiting 
means 70 in the developing housing 30. In assembling the developing 
agent-limiting means 70, an end of the compression coil spring 731 is, 
first, fitted to the spring-fitting chamber 749 of the pushing member 74, 
as shown in FIG. 5, and while being compressed, the compression coil 
spring 731 is inserted from an end of the support holder 76 toward the 
other end of the holder 76. At this time, the guide support portion 744 of 
the upper side of the pushing member 74 is located between the rear wall 
463 and the guide rail 764, and the guide support portion 745 of the lower 
side is located in the guide groove 766, so that the pushing member 74 is 
inserted in a predetermined position of the support holder 76 to form the 
resilient urging means 73. When the resilient urging means 73 is mounted 
on the support holder 76, as described above, the front wall 741 of the 
pushing member 74 is pushed toward the rear wall 763 against the force of 
the compression coil springs 731, so that the guide support portion 744 is 
moved toward the side of the rear wall 763 beyond the pin-insertion holes 
765. In this state, stopper pins 79 are inserted in the pin-insertion 
holes 765 as shown in FIG. 6, whereby the stopper pins 79 act on the front 
surface of the guide support portion 764 that constitutes the pushing 
member 74 to limit the forward motion of the pushing member 74, so that 
the pushing forces of the compression coil springs 731 no longer act on 
the back surface of the support plate 72. In the state where the action of 
the pushing forces of the compression coil springs 731 is canceled, the 
upper end of the support plate 72 fastened to the other surface of the 
blade 71 is fitted by screws 78 to the end surface of the upper wall 761 
that constitutes the holder 76. The developing agent-limiting means 70 
that is assembled in a state where the pushing force acting on the back 
surface of the support plate 72 is canceled, is disposed at a 
predetermined position shown in FIG. 3 between the two walls of the 
developing housing 30, and is mounted using a fastening means such as 
tightening bolts (not shown) that is fitted penetrating through both walls 
of the developing housing 30. After the developing agent-limiting means 70 
is mounted at the predetermined position in the developing housing 30, the 
stopper pins 79 are removed from the pin-insertion holes 765, whereby the 
pushing member 74 is permitted to move forward, i.e., toward the support 
plate 72, and the protrusion 746 formed on the front surface of the 
pushing member 74 is pushed onto the back surface of the support plate 72 
by the compression coil springs 731. As described above, since the 
developing agent-limiting means 70 according to the illustrated embodiment 
is assembled in a state where the pushing force acting on the back surface 
of the support plate 72 on which the blade 71 is mounted is canceled, the 
peripheral surface of the developing roller 40 does not suffer from by the 
blade 71 at the time of mounting the developing agent-limiting means 70 in 
the developing housing 30. 
Described below with reference to FIG. 7 is another embodiment of the 
developing agent-limiting means 70. In the developing agent-limiting means 
70 according to this embodiment, the angle .theta. formed between the 
front wall at the rear part of the support holder 76 and the upper surface 
of the lower wall 762 is set to be smaller than 90 degrees (80 degrees in 
the illustrated embodiment), and the front lower edge of the pushing 
member 74 is brought into contact with the upper surface of the lower wall 
762. That is, the pushing direction of the compression coil spring 731 or 
the axial line of the compression coil spring 731 is not in parallel with 
the upper surface of the lower wall 762 constituting the upport holder 76, 
and the upper surface of the lower all 762 approaches the axial line of 
the compression coil spring 731 as it goes forward. Being constituted as 
described above, the front lower edge of the pushing member 74 is brought 
into intimate contact with the upper surface of the lower wall 762 
constituting the support holder 76 as the pushing member 74 is moved 
forward by the compression coil spring 731, and the pushing member 74 is 
urged by the compression coil spring 731 under the thus intimately 
contacted state. Therefore, the toner is reliably prevented from 
infiltrating into the developing agent-limiting means 70 from between the 
pushing member 74 and the upper surface of the lower wall 762 constituting 
the support holder 76. In the illustrated embodiment, furthermore, the 
front upper part of the front wall 741 of the pushing member 74 is tilted 
rearwardly to form an escape 741a. According to this constitution, the 
front upper part of the front wall 741 of the pushing member 74 is 
prevented from coming into contact with the back surface of the support 
plate 72. 
The device for developing electrostatic latent image according to the 
illustrated embodiments is constituted as described above. Mentioned below 
is the operation. Upon starting the operation of the device for developing 
electrostatic latent image, the roller 42 of the developing roller 40, 
roller 52 of the replenishing roller 50 and stirring means 60 are rotated 
in the directions indicated by arrows by a drive means that are not shown. 
As the stirring means 60 is rotated in the direction indicated by an arrow 
601, the developing agent contained in the stirrer chamber 32 is stirred 
and is fed into the developing chamber 31. Meanwhile, the used developing 
agent after held on the peripheral surface of the roller constituting the 
developing roller 40 and passed through the developing zone 404, is 
transferred onto the surface of the replenishing roller 50 at a portion 
where the developing roller 40 and the replenishing roller 50 are in 
contact with each other, and is mixed in the developing chamber 31 
together with the developing agent that is fed by the stirring means 60. 
The developing agent mixed in the developing chamber 31 is held on the 
peripheral surface of the roller 52 that is made of a foamed material and 
constitutes the replenishing roller 50, and is conveyed toward the 
developing roller 40. 
The developing agent held on the peripheral surface of the replenishing 
roller 50 and is conveyed toward the developing roller 40, is supplied to, 
and is held by, the peripheral surface of the roller 42 constituting the 
developing roller 40 in the developing agent-holding zone 402, and is 
conveyed toward the developing agent-limiting zone 403. In the developing 
agent-limiting zone 403, the blade 71 of the developing agent-limiting 
means 70 acts on the developing agent held on the peripheral surface of 
the roller 42 of the developing roller 40, and limits the developing agent 
held on the peripheral surface of the roller 42 into a predetermined 
amount to form a thin layer. Here, since the blade 71, support plate 72 
and pushing member 74 have flexibility, the one surface of the blade 71 is 
brought into pressed contact with the surface of the roller 42 of the 
developing roller 40 uniformly over the whole width even though the shaft 
of the developing roller 40 may be deflected to some extent. 
In the developing zone 404, the developing agent is applied to the 
electrostatic latent image on electrostatic photosensitive material 
disposed on the peripheral surface of the rotary drum 2, so that the 
electrostatic latent image is developed into a toner image. For instance, 
the electrostatic latent image has a non-imaged region electrically 
charged to about +600 V and an imaged region electrically charged to about 
+120 V, and the toner which is the developing agent is adhered to the 
imaged region (so-called reversal development). The rotary drum 2 is 
continuously rotated in a direction indicated by an arrow 22 in FIG. 1 
and, hence, the peripheral surface of the rotary drum 2 and the peripheral 
surface of the roller 42 constituting the developing roller 40 move in the 
same direction in the developing zone 404. The moving speed V2 of the 
peripheral surface of the roller 42 is set to be slightly larger than the 
moving speed V1 of the peripheral surface of the rotary drum 2 and it is 
desired that a relationship between V1 and V2 is 1.2 
V1.ltoreq.V2.ltoreq.2.2 V1. In this case, the developing agent is conveyed 
in sufficient amounts into the developing zone 404 by the roller 42 of the 
developing roller 40, the developing agent adhered to the non-imaged 
portion of the electrostatic latent image is suitably peeled off by the 
rubbing action of the peripheral surface of the roller 42 with respect to 
the peripheral surface of the rotary drum 2 and, thus, a good toner image 
having a suitable developing density and being free from fogging can be 
obtained. Desirably, the developing agent comprises only a toner having a 
volume average particle diameter (Vol. 50%: the volume of the toner having 
sizes smaller than the volume average particle diameter is equal to the 
volume of the toner having sizes larger than the volume average particle 
diameter) of from about 8.0 to 12.0 .mu.m and a volume resistivity of not 
smaller than 10.sup.8 .OMEGA..cm. 
Described below is how to set the amount (S) of projection of the blade 71. 
FIG. 10 is a diagram of experimental results showing a relationship 
between the thickness of the developing agent layer formed on the 
peripheral surface of the roller 42 and the amount (S) of projection of 
the blade 71 constituting the developing agent-limiting means 70 from the 
center of contact with the roller 42 constituting the developing roller 40 
to the lower end of the blade 71 in the device for developing 
electrostatic latent image shown in FIG. 4. In FIG. 10, a solid line 
represents values of when the developing roller 40 has a roughness Rz 
(ten-point average roughness specified under JIS B 0601) of 12.0 .mu.m on 
the peripheral surface of the roller 42 and the pushing force of the 
resilient urging means 73, i.e., the contacting force of the blade 71 
exerted on the peripheral surface of the roller 42 is 4.0 g/mm as the line 
pressure (pressure per a unit length in the direction of width). A broken 
line represents values of when the developing roller 40 has a roughness Rz 
(ten-point average roughness specified under JIS B 0601) of 5.0 .mu.m on 
the peripheral surface of the roller 42 and the contacting force of the 
blade 71 exerted on the peripheral surface of the roller 42 is 12.0 g/mm 
as the line pressure (pressure per a unit length in the direction of 
width). As will be understood from the experimental results, when the 
amount (S) of projection is smaller than 0.5 mm, the thickness of the 
developing agent layer formed on the peripheral surface of the roller 42 
becomes very small. When the amount (S) of projection is not smaller than 
2.0 mm, on the other hand, the rate of increase in the thickness of the 
developing agent layer on the peripheral surface of the roller 42 becomes 
great. When the amount (S) of projection lies within a range of from 0.5 
to 2.0 mm, the thickness of the developing agent layer formed on the 
peripheral surface of the roller 42 remains stable lying within a range of 
from 20 to 35 .mu.m even if the roughness Rz (ten-point average roughness 
specified under JIS B 0601) of the peripheral surface of the roller 42 
varies over a range of from 5.0 to 12.0 .mu.m, and the contacting force of 
the blade 71 upon the peripheral surface of the roller 42 varies over a 
range of from 4.0 to 12.0 g/mm as the line pressure (pressure per a unit 
length in the direction of width). Therefore, the amount (S) of projection 
should be set to lie from 0.5 to 2.0 mm. 
Described below is how to set the contacting force of the blade 71. FIG. 11 
is a diagram of experimental results showing a relationship between the 
thickness of the developing agent layer formed on the peripheral surface 
of the roller 42 and the contacting force (line pressure) of the blade 71 
constituting the developing agent-limiting means 70 upon the peripheral 
surface of the roller 42 constituting the developing roller 40 in the 
device for developing electrostatic latent image shown in FIG. 4. In FIG. 
11, a solid line represents values of when the developing roller 40 has a 
roughness Rz (ten-point average roughness specified under JIS B 0601) of 
12.0 .mu.m on the peripheral surface of the roller 42 and the amount (S) 
of projection of the blade 71 is 2.0 mm. A broken line represents values 
of when the developing roller 40 has a roughness Rz (ten-point average 
roughness specified under JIS B 0601) of 5.0 .mu.m on the peripheral 
surface of the roller 42 and the amount (S) of projection of the blade 71 
is 0.5 mm. As will be understood from the experimental results, when the 
contacting force of the blade 71 lies within a range of from 4.0 to 12.0 
g/mm as the line pressure, the thickness of the developing agent layer 
formed on the peripheral surface of the roller 42 remains stable lying 
within a range of from 20 to 35 .mu.m even if the roughness Rz (ten-point 
average roughness specified under JIS B 0601) of the peripheral surface of 
the roller 42 constituting the developing roller 40 varies over a range of 
from 5.0 to 12.0 .mu.m, and the amount (S) of projection of the blade 71 
varies over a range of from 0.5 to 2.0 mm. Therefore, the contacting force 
(line pressure) should be set to lie from 4.0 to 12.0 g/mm. 
Described below is how to set the roughness of the peripheral surface of 
the roller 40 constituting the developing roller 40. FIG. 12 is a diagram 
of experimental results showing a relationship between the thickness of 
the developing agent layer formed on the peripheral surface of the roller 
42 and the roughness Rz (ten-point average roughness specified under JIS B 
0601) of the peripheral surface of the roller 42 constituting the 
developing roller 40 in the device for developing electrostatic latent 
image shown in FIG. 4. In FIG. 12, a solid line represents values of when 
the amount of projection of the blade 71 from the center of contact with 
the roller 42 to the lower end of the blade 71 is 2.0 mm, and the pushing 
force of the resilient urging means 73, i.e., the contacting force of the 
blade 71 exerted on the peripheral surface of the roller 42 is 4.0 g/mm. A 
broken line represents values of when the amount of projection of the 
blade 71 is 0.5 mm and the contacting force of the blade 71 is 12.0 g/mm. 
As will be understood from the experimental results, when the roughness Rz 
(ten-point average roughness specified under JIS B 0601) of the peripheral 
surface of the roller 42 constituting the developing roller 40 lies within 
a range of from 5.0 to 12.0, the thickness of the developing agent layer 
formed on the peripheral surface of the roller 42 remains stable lying 
within a range of from 20 to 35 .mu.m even if the amount of projection of 
the blade 71 varies over a range of from 0.5 to 2.0 mm and the contacting 
force of the blade 71 varies over a range of from 4.0 to 12.0 g/mm. 
Therefore, the roughness Rz (ten-point average roughness specified under 
JIS B 0601) of the peripheral surface of the developing roller should be 
set to lie from 5.0 to 12.0 .mu.m. 
Described below is how to set the hardness of the developing roller 40. 
FIG. 13 is a diagram of experimental results showing a relationship 
between the thickness of the developing agent layer formed on the 
peripheral surface of the solid synthetic rubber roller 42 and the 
hardness of the developing roller 40 which is an assembly constituted by 
the rotary shaft 41 and the solid synthetic rubber roller 42 fitted to the 
outer peripheral surface of the rotary shaft 41 in the device for 
developing electrostatic latent image shown in FIG. 4. Here, the 
developing roller is constituted by the rotary shaft having an outer 
diameter of 10 mm made of a stainless steel and the solid synthetic rubber 
roller having an outer diameter of 16 mm made of urethane rubbers having 
various degrees of hardness. In FIG. 13, the abscissa represents the 
hardness of the developing roller which is an assembly constituted by the 
rotary shaft and the solid synthetic rubber roller fitted thereon. In FIG. 
13, a solid line represents values of when the amount (S) of projection of 
the blade 71 is 2.0 mm and the contacting force of the blade 71 is 4.0 
g/mm as the line pressure. A broken line represents values of when the 
amount (S) of projection of the blade 71 is 0.5 mm and the contacting 
force of the blade 71 is 12.0 g/mm as the line pressure. According to the 
experimental results, when the amount (S) of projection of the blade 71 is 
2.0 mm and the contacting force of the blade 71 is 4.0 g/mm as the line 
pressure, the thickness of the toner layer formed on the surface of the 
developing roller increases with a decrease in the hardness of the 
developing roller, and the thickness of the toner layer formed on the 
surface of the developing roller decreases with an increase in the 
hardness of the developing roller, as shown by a solid line. On the other 
hand, when the amount (S) of projection of the blade 71 is 0.5 mm and the 
contacting force of the blade 71 is 12.0 g/mm as the line pressure, the 
thickness of the toner layer formed on the surface of the developing 
roller decreases with a decrease in the hardness of the developing roller, 
and the thickness of the toner layer formed on the surface of the 
developing roller increases with an increase in the hardness of the 
developing roller, as shown by a broken line. When the hardness of the 
developing roller lies within a range of from 60 to 80 in terms of the 
Asker's C hardness, the thickness of the developing agent layer formed on 
the peripheral surface of the solid synthetic rubber roller 42 remains 
stable within a range of from 20 .mu.m to 35 .mu.m even if the amount (S) 
of projection of the blade 71 varies over a range of from 0.5 to 2.0 mm 
and the contacting force of the blade 71 varies over a range of from 4.0 
to 12.0 g/mm as the line pressure. 
It was also learned that variation in the thickness of the toner layer 
formed on the surface of the developing roller increases as the hardness 
of the developing roller becomes smaller than 60 in terms of the Asker's C 
hardness. Desirably, therefore, the lower limit of the hardness of the 
developing roller 40 should be set to be 60 in terms of the Asker's C 
hardness, the developing roller 40 being the assembly constituted by the 
rotary shaft 41 and the solid synthetic rubber roller 42 fitted onto the 
outer peripheral surface of the rotary shaft 41. 
Discussed below is the upper limit of the hardness of the developing roller 
which is the assembly constituted by the rotary shaft and the solid 
synthetic rubber roller fitted to the outer peripheral surface of the 
rotary shaft. As the hardness of the developing roller increases, the 
thickness of the developing agent formed on the peripheral surface of the 
developing roller tends to be stabilized within the above-mentioned 
predetermined range. As the hardness of the developing roller 40 
increases, however, the contacting force of the developing roller 40 
relative to the rotary drum 2 increases and the mechanical torque 
increases since the developing roller 40 is brought into contact with the 
rotary drum 2. Consequently, as the hardness of the developing roller 
exceeds 80 in terms of the Asker's C hardness, furthermore, it becomes 
necessary to increase the capacity of the electric motor to drive the 
rotary drum 2 and the developing roller 40. It is therefore desired to set 
the upper limit of the hardness of the developing roller 40 which is the 
assembly constituted by the rotary shaft 41 and the solid synthetic rubber 
roller 42 fitted to the outer peripheral surface of the rotary shaft 41, 
to be 80 in terms of the Asker's C hardness. 
Next, described below are how to set the thickness of the blade 
constituting the developing agent-limiting means and how to set the 
nipping width (L) in which the peripheral surfaces of the developing 
roller and the replenishing roller come into contact with each other. FIG. 
14 is a diagram illustrating a relationship between the nipping width (L) 
in which the peripheral surface of the roller 42 constituting the 
developing roller 40 comes into contact with the peripheral surface of the 
roller 52 constituting the replenishing roller 50 and the conveyance 
amount of the developing agent which is held on the peripheral surface of 
the developing roller 42 constituting the developing roller 40 in the 
device for developing electrostatic latent image shown in FIG. 4. 
The conveyance amount of the developing agent is expressed by a value that 
is obtained by sucking the developing agent held on the peripheral surface 
of the roller 42 constituting the developing roller 40 over a 
predetermined area and by dividing the weight of the sucked developing 
agent by the area. The amount suited for forming the image will be from 
0.2 mg/cm.sup.2 to 1.0 mg/cm.sup.2. For the experiment there are used four 
kinds of blades 71 constituting the developing agent-limiting means, i.e., 
a glass blade having a thickness of 1.1 mm, a glass blade having a 
thickness of 2.0 mm, a glass blade having a thickness of 3.0 mm, and a 
stainless steel blade having a thickness of 0.1 mm. The pushing force of 
the resilient urging means 73, i.e., the contacting force of the blade 71 
upon the peripheral surface of the roller 42 constituting the developing 
roller 40, was set to be 7.3 g/mm as the line pressure (pressure per a 
unit length in the direction of width). As will be understood from the 
experimental results, in the case where the glass blade having the 
thickness of 3.0 mm is used, the conveyance amount of the developing agent 
is about 0.2 mg/cm.sup.2 when the nipping width (L) is 0.05 mm, while the 
conveyance amount of the developing agent is about 1.0 mg/cm.sup.2 when 
the nipping width (L) is 0.15 mm, as represented by a dot-dash chain line 
in FIG. 14. It will be understood that the developing agent becomes short 
in supply when the nipping width (L) is smaller than 0.05 mm, while the 
developing agent becomes excessive in supply when the nipping width (L) is 
larger than 0.15 mm. The amount of supplying the developing agent exceeds 
1.0 mg/cm.sup.2 as the nipping width (L) becomes larger than 0.15 mm. This 
is because, when the glass blade has a large thickness, the developing 
agent-limiting means fails to perform good developing agent-scraping and 
the developing agent layer is formed highly densely on the peripheral 
surface of the developing roller. When the glass blade having the 
thickness of 3.0 mm is used, therefore, the nipping width (L) must be set 
to be from 0.05 to 0.15 mm. By taking into consideration the error in 
manufacturing the developing roller and the replenishing roller and the 
error in assembling them together, however, a considerably high degree of 
manufacturing precision is required to produce products confining the 
nipping width (L) within the range of from 0.05 to 0.15 mm. 
In the case where the glass blade having the thickness of 2.0 mm is used, 
on the other hand, the conveyance amount of the developing agent is about 
0.5 mg/cm.sup.2 when the nipping width (L) is 0.1 mm and the conveyance 
amount of the developing agent is about 0.8 mg/cm.sup.2 when the nipping 
width (L) is 0.6 mm, as represented by a broken line in FIG. 14. In the 
case where the glass blade having the thickness of 1.1 mm is used, 
furthermore, the conveyance amount of the developing agent is about 0.4 
mg/cm.sup.2 when the nipping width (L) is 0.1 mm and the conveyance amount 
of the developing agent is about 0.6 mg/cm.sup.2. When the nipping width 
(L) is 0.6 mm. In the case where the stainless steel blade having the 
thickness of 0.1 mm is used, furthermore, the conveyance amount of the 
developing agent is about 0.2 mg/cm.sup.2 when the nipping width (L) is 
0.1 mm and the conveyance amount of the developing agent is about 0.3 
mg/cm.sup.2 when the nipping width (L) is 0.6 mm, as represented by a 
two-dotted chain line in FIG. 14. 
Thus, even when the nipping width (L) is 0.6 mm, the conveyance amount of 
the developing agent can be suppressed to be not larger than 1.0 
mg/cm.sup.2 on account of the reasons that the blade is thin, flexible and 
is brought into pressed contact uniformly on the peripheral surface of the 
developing roller over the whole width, enabling the developing 
agent-limiting means to exhibit the developing agent-scraping action 
smoothly and favorably, and permitting the developing agent layer to be 
formed not so highly densely on the peripheral surface of the developing 
roller. It is therefore desired that the thickness of the blade which 
constitutes the developing agent-limiting means is set to be in the range 
of from 0.1 mm which is the lower limit of when the blade is made of a 
metal material and with which thickness the blade can be produced 
substantially easily maintaining flexibility to exhibit suitable 
developing agent-scraping function to 2.0 mm which is the upper limit of 
when the blade is made of the glass blade and with which thickness the 
blade exhibits flexibility and suitable developing agent-scraping 
function. It is further desired that the nipping width (L) is set to be 
from 0.1 to 0.6 mm. When the sheet glass constituting the blade has a 
thickness which is smaller than 0.5 mm, the sheet glass may be cracked 
during the operation. When the glass blade is used, therefore, it is 
desired that the thickness is set to be from 0.5 to 2.0 mm. With the 
nipping width (L) being set in a wide range of from 0.1 to 0.6 mm, the 
developing roller and the replenishing roller can be produced without 
maintaining so high precision, and these two rollers can be assembled 
together without maintaining so high precision, either, making it possible 
to produce at a lower cost a device for developing electrostatic latent 
image.