Developer unit utilizing a non-magnetic single component developer

A developer unit includes a porous conductive resilient member which rotates while partly contacting a developer holder to charge a non-magnetic single component developer and to supply it to the surface of the developer holder. A conductive constraining member forms a uniform thin layer of developer on the developer holder and also charges the developer to a given level. A developing bias voltage is applied to the developer holder via a rotatable fibrous charging member to cause the developer to fly to an image area of an electrostatic latent image formed on a latent image carrier which is disposed in opposing relationship with the developer holder with a gap therebetween. The bias voltage is also applied to the constraining member and to the charging member.

FIELD OF THE INVENTION AND RELATED ART STATEMENT 
The invention relates to a developer unit, and in particular, to a 
developer unit utilizing a non-magnetic single component developer. 
Prior art techniques to develop an electrostatic latent image which may be 
formed by an exposure of a uniformly charged photosensitive member in 
accordance with image information generally include a two component 
developer process which uses a toner and a carrier, in particular such 
process utilizing a magnetic brush, which will be hereafter referred to as 
a two component magnetic brush developing process. However this process 
suffers from practical difficulties including an increased size of a 
resulting developer unit, the difficulty to achieve a stable mixture ratio 
of toner and carrier and an associated difficulty to charge the toner. 
Recently, a magnetic brush developing process utilizing a single component 
developer in which the toner itself exhibits a magnetic property, 
hereafter referred to as a single component magnetic brush developing 
process, has been available on the market. However, while it achieves a 
reduction in the size of the resulting developer unit, the single 
component magnetic brush process presents difficulties in achieving a 
color image in view of the fact that a developing powder includes a 
magnetic powder. 
In view of the foregoing, there is proposed a developing process utilizing 
a non-magnetic, single component developer (hereafter referred to as a 
non-magnetic single component developing process), which is still under 
investigation. This process is again categorized into a process in which 
the development takes place by the contact between a developer and an 
electrostatic latent image holder such as a photosensitive member, for 
example, and another process in which the developer and the latent image 
holder are maintained out of contact from each other while the development 
takes place by flying the developer to the holder. 
The former or contact process results in excellent results in improving the 
image density and the ease of supplying a developer, but suffers from the 
susceptibility to the occurrence of a background fogging which is caused 
by the contact between the developer and the latent image holder. In 
addition, it exhibits a disadvantage that it cannot be adopted in a single 
drum, multiple color, single transfer process which is intended to achieve 
a simplification of an overall developer unit and a reduction in the cost 
in producing a color image, in view of the contacting nature of the 
process which gives rise to the problem of a color mixture. Accordingly, 
resort must be had to the latter or non-contact, flying developing process 
which utilizes the non-magnetic single component developer. 
In a conventional developer unit which utilizes the non-contact, flying 
developing process, the use of the non-magnetic single component developer 
may cause a poor image such as a thinning or breaking of an image unless a 
supply of a developer to the holder, its charging, the formation of a thin 
layer thereof, a conveyance to a developing zone and the flying capability 
are properly controlled together with a satisfactory achievement of the 
removal, stirring action and circulation of the developer. 
Considering, for example, a conventional developer unit in which a 
developer is charged and a thin layer is formed simultaneously by means of 
a constraining member such as a pressure blade, the degree to which the 
developer is charged cannot be stabilized in view of the triboelectric 
nature of charging, but undergoes a large variation subject to the 
material and a change in the surface condition of the constraining member, 
resulting in a poor reliability. In addition, a residual developer which 
remains on the holder after the developing step cannot be removed from the 
holder, and is allowed to be used again in the next following developing 
step as the holder rotates. In this manner, difficulties are experienced 
in achieving a stable charging of the developer and a satisfactory 
stirring action of the developer. 
In a conventional developer unit which utilizes the non-contact, flying, 
non-magnetic single component developing process, it has been known to 
apply a developing bias such as an electric pulse or an a.c. bias to the 
developer holder in order to prevent a non-image area in the latent image 
from being developed, to impart a proper amount of edge effect to the 
image or to improve the tone quality. However, a gap g between the 
developer holder and the electrostatic latent image holder must be 
maintained very small on the order of 0.5 to 0.02 mm. If a metal holder is 
used which is made of a commonly used metal such as aluminium, stainless 
steel or the like, the choice of a high developing bias which is applied 
to the holder from a high voltage source will cause the liability of the 
developing bias to discharge, which upon occurrence, reduces the potential 
of the developer holder to a point near the ground potential, with 
consequence that a resulting low bias phenomenon occurs across the entire 
developer holder to produce a black transversing pattern running across 
the background (non-image area) of a copy or the electric breakdown of the 
air will produce a white dot discharge pattern in the image area, thus 
causing a degradation in the image quality. On the other hand, the choice 
of a low developing bias cannot assure a satisfactory developing 
capability. These difficulties can be overcome by the use of an insulating 
developer holder, but this results in the loss of a developing electrode 
effect, degrading the reproducibility of a solid black image. 
To accommodate for this, there has been proposed a developer unit using a 
developer holder which comprises a cylindrical member formed by a 
conductive resin in which a conductive powder is dispersed and having 
openings at its opposite ends, to which a pair of end supports carrying 
stub shafts are coupled, with the resistivity of the conductive resin 
forming the cylindrical member chosen to be in a range from 10.sup.4 to 
10.sup.12 .OMEGA. cm with a wall thickness in a range from 0.5 to 3 mm 
(see, for example, Japanese Laid-Open Patent Application No. 80,875/1985). 
In the developer unit using such developer holder, the resistivity of the 
conductive resin suppresses the discharge of the developing bias, and thus 
the choice of a high developing bias applied cannot result in the 
appearance of a black transversing pattern in a background (or non-image 
area) of a copy which might have been otherwise caused by the discharge of 
the developing bias. In addition, the occurrence of a discharge pattern in 
the form of white dots in the image area is also avoided, and the 
reproducibility of a solid black image is not degraded. 
However, in this developer unit, the construction of the developer holder 
which is formed of a conductive resin and which is supported at its 
opposite ends by the pair of stub supports presents difficulty in securing 
the rigidity of the developer holder and the concentricity of the outer 
diameter thereof with respect to the axis. This in turn presents 
difficulty in maintaining a gap between the developer holder and the 
latent image holder to a high accuracy. Any variation in the gap is 
reflected in the non-uniformity of the image density. Where the developer 
holder has an outer diameter less than 30 mm or a length greater than 200 
mm, sufficient rigidity cannot be secured, causing a flexure therein. In 
addition, an error in the concentricity of the outer diameter with respect 
to the axis will exceed 10 .mu.m, causing a significant notability of the 
non-uniformity in the image density, which prevented its practical use. In 
addition, when a developing bias is applied to the cylindrical member, 
there will be produced a potential distribution lengthwise of the 
developer holder, which also contributes to increasing the non-uniformity 
in the image density. 
OBJECT AND SUMMARY OF THE INVENTION 
It is an object of the invention to provide a developer unit utilizing a 
non-magnetic single component developer in which the functions of 
controlling the supply, the charging, the formation of a thin layer of, 
the conveyance to a developing zone and the flying capability of a 
developer as well as the removal, stirring action and circulation of the 
developer are separated, and in which the charging of the developer which 
has been performed in the past only through the triboelectric charging 
operation is achieved by a positive charge injection operation by the use 
of a porous conductive resilient member or fibrous conductive member in 
combination with a constraining member to which a voltage is applied so 
that the developer is charged in a stable manner while improving the 
stirring action upon the developer to enable a stabilized developing 
operation which is free from any defect in the image quality. 
It is another object of the invention to provide a developer unit which 
permits a voltage applied to a developer holder, a porous conductive 
resilient member or fibrous conductive member and a constraining member to 
be controlled to enable a proper setting of developing conditions or 
parameters, thereby enabling a control to be exercised over the image 
density in the event of any variation in the environment, in the quality 
of the developer or the electrical resistance of component members or a 
variation from lot to lot. 
It is a further object of the invention to provide a developer unit which 
uses a developer holder carrying a conductive resin layer on its surface 
to prevent occurrence of a discharge of a developing bias to thereby 
enable a developing operation to be performed in a manner which avoids any 
defect in the image quality while allowing an image, free from a density 
non-uniformity, to be developed. 
It is an additional object of the invention to provide a process of 
manufacturing a developer holder in a facilitated manner and to a high 
accuracy, the holder preventing the occurrence of a discharge of a 
developing bias to enable an image, free from a density non-uniformity, to 
be developed. 
According to the invention, the control over the supply, the charging, the 
formation of a thin layer of, the conveyance to a developing zone and a 
flying capability of a developer as well as the removal, stirring action 
and circulation of the developer, which form essential steps in a 
developing process which utilizes a non-magnetic single component 
developer, are functionally separate from each other. This eliminates any 
instability in the developing conditions which may be caused by the 
triboelectric charging or a failure to take a flow condition of the 
developer into consideration. A developing electrode effect which is 
imparted to the developer holder can be advantageously established for a 
wide range of image varieties including a solid black image to a halftone 
image. Suitable developing conditions may be established which are adapted 
to the production of thin lines, in particular. In this manner, the 
reliability of the developing process can be improved by achieving a 
stabilized image quality. In addition, the invention exhibits a stabilized 
characteristic against environment by the use of a charge injection 
technique rather than the triboelectric charging technique which is 
greatly influenced by the environment factors or the surface condition of 
the material. 
The developer unit is internally constructed such that the porous 
conductive resilient member or fibrous conductive member is effective to 
feed the developer, so that in the event a foreign matter is present in 
admixture, it is only allowed to reach the top portion of such conductive 
member, but is prevented from proceeding into the following step, thus 
assuring an enhanced reliability in this respect. 
The developer holder comprises a metal carrier or core which is coated by a 
conductive resin layer. This reduces a change upon the image quality when 
a developing bias is applied to the developer holder and allows a 
fogging-free and sharply defined image to be obtained. The likelihood of a 
discharge is eliminated if a high voltage is applied as a developing bias. 
In addition, a high precision can be mechanically maintained for a 
developer holder of a reduced diameter and an increased length. 
A cylindrical member of conductive resin is fitted over and secured to the 
surface of the metal carrier, allowing a developer holder to be produced 
at a high accuracy and at a low cost through a mass production. The 
resulting developer holder is effective to prevent a discharge from the 
developing bias and to produce an image which is free from a 
non-uniformity in the image density.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 1 is a cross section of a developer unit according to a first 
embodiment of the invention. A developer unit according to this embodiment 
comprises a developer holder 1 which is rotatably supported in opposing 
relationship with a photosensitive member (electrostatic latent image 
carrier) 10 on which an electrostatic latent image is formed with a gap g 
therebetween, a porous conductive resilient member 2 which is rotatably 
mounted and partly maintained in contact with the developer holder 1, a 
conductive constraining member 3 for controlling the thickness of a layer 
of non-magnetic single component developer T to form a thin layer of 
developer T on the developer holder 1 and for charging the developer T to 
a given level, a stirring paddle 5 for stirring the developer T which is 
contained in a developer supply station, an anti-spill cover 6 for 
preventing the developer T from spilling over the top of the developer 
holder 1, a developer vessel 7 for defining a developer supply station and 
on which the members mentioned above are mounted, a high voltage source E1 
connected to the developer holder 1, and another high voltage source E2 
connected to the porous member 2 and the constraining member 3. 
The developer holder 1 is formed by a shaft of a metal such as aluminium or 
stainless steel. 
The porous conductive resilient member 2 comprises a roll of a material 
such as soft polyurethane foam having a three dimensional skelton 
structure and containing conductive carbon and which is formed on a metal 
shaft 2a which is supported in a rotatable manner by the sidewalls of the 
developer vessel 7. The porous member 2 is bonded to the metal shaft 2a by 
utilizing a conductive adhesive such as silver (Au) filler containing 
epoxy adhesive or carbon filler containing acrylic adhesive. The porous 
member 2 has a resistivity on the order of 10.sup.3 to 10.sup.6 .OMEGA. cm 
and hence there can be no leakage between the high voltage source E2 to 
which the porous member 2 is connected and the high voltage source E1 to 
which the developer holder 1 is connected, allowing high potentials to be 
independently maintained on the porous member 2 and the developer holder 
1. The developer T is charged to the same polarity as the polarity of the 
high voltage source E2. The porous member 2 has a porosity level, which 
may be from 15 to 45 pores or cells per 25 mm. It is found that the porous 
member 2 preferably has a contact depth (or depth of engagement) with 
respect to the developer holder 1 on the order of 0.5 to 1.0 mm in 
consideration of the efficiency of conveying the developer T and the 
removal of the developer T which may remain on the developer holder 1 
subsequent to the developing process. 
The constraining member 3 is formed of a silicone rubber sheet having a 
hardness from 60.degree. to 80.degree. and which is made electrically 
conductive by a dispersion or attachment of conductive material (for 
example, conductive carbon), the member having a thickness on the order of 
2 to 3 mm. The constraining member 3 abuts against the developer holder 1 
in its body portion or in both body and edge portion, and is effective to 
control the thickness of a layer of the developer T formed on the 
developer holder 1 so that the thickness may be on the order of 20 to 40 
.mu.m while charging the developer T to a given level. The constraining 
member 3 has a resistivity on the order of 10.sup.3 to 10.sup.10 .OMEGA. 
cm, and accordingly there occurs no leakage between the high voltage 
source E2 to which the constraining member 3 is connected and the high 
voltage source E1 to which the developer holder 1 is connected, allowing 
given high potentials to be maintained independently on the constraining 
member 3 and the developer holder 1. 
The stirring paddle 5 is not limited to any particular configuration, but 
preferably is shaped to achieve an effective stirring action and 
circulation of the developer T in the developer supply station defined 
within the developer vessel 7 without forming any stagnation or built-up 
of the developer T therein. 
Anti-spill cover 6 is suitably formed of a urethane rubber sheet having a 
thickness on the order of 0.02 mm. 
The developer holder 1, the porous member 2 and the stirring paddle 5 are 
connected together through gears, not shown, outside the developer vessel 
7, and are driven for simultaneous rotation in directions indicated by 
arrows as the developing process is started. 
In operation, as the developing process is started, the developer holder, 
the porous member 2 and the stirring paddle 5 begin to be driven to rotate 
in respective directions indicated. A quantity of the developer T which is 
contained in the developer supply station defined within the developer 
vessel 7 tends to be conveyed, as indicated by an arrow a in FIG. 2, by 
the rotation of the porous member 2 into an area of contact between the 
porous member 2 and the developer holder 1 where the developer T is 
charged by the porous member 2 which is connected to the high voltage 
source E2. 
The charged developer T moves in a manner indicated by arrows b shown in 
FIG. 2 as both the developer holder 1 and the porous member 2 rotate. 
Specifically, part of the charged developer T is conveyed to form a thin 
layer on the developer holder 1 while being controlled by the constraining 
member 3 to a thickness on the order of 20 to 40 .mu.m and is charged to a 
given level by the constraining member 3. The force which attracts the 
developer T to the developer holder 1 is a mirror image force acting 
between the charge of the developer T and the developer holder 1. 
A thin layer of the developer T which is formed on the developer holder 1 
is conveyed to a developing zone as the developer holder 1 rotates in 
order to develop an electrostatic latent image formed on the 
photosensitive member 10. When so conveyed, it will be located opposite to 
the photosensitive member 10 with a distance (which is equal to the gap g 
minus the thickness of the layer of developer T). 
The developer holder 1 is connected to a high voltage source E1. It will be 
seen that because in the developing zone, a surface charge density in an 
image area of the electrostatic latent image formed on the photosensitive 
member 10 is different from a corresponding density in a non-image area of 
the latent image, the electrostatic force of attraction F=qE (where q 
represents the charge of developer T and E represents the electric field 
in the developing zone) will be different between the image area and the 
non-image area. As a consequence, the developer T will fly beyond the 
developer holder 1 and toward the photosensitive member 10 for purpose of 
developing, only in the region of the image area. 
It is to be noted that a choice of a peripheral speed of the developer 
holder 1 which is greater than that of the photosensitive member 10 is an 
effective technique to assure an image density. 
An amount of developer T which remains on the developer holder 1 without 
being utilized in the developing process will be conveyed toward the 
anti-spill cover 6 as the developer holder 1 rotates so as to be received 
again within the developer supply station defined within the developer 
vessel 7. The anti-spill cover 6 is disposed in abutment against the 
developer holder 1, but such abutment takes place by a curve portion 
thereof which is held in gentle contact with the developer holder, and 
accordingly the developer T will be allowed to move into the vessel 7 
without being scraped off the developer holder 1 by the cover 6. 
The developer T remaining on the developer holder 1 and which is conveyed 
into the developer vessel 7 will be conveyed toward the porous conductive 
resilient member 2, as indicated by an arrow c, which member is effective 
to scrape it from the developer holder 1, allowing the scraped developer 
to be conveyed toward the stirring paddle 5 disposed within the vessel 7, 
in the manner indicated by an arrow b in FIG. 2, as the porous member 2 
rotates. The developer T will then be again stirred and circulated through 
the vessel 7 for repeated contribution to the developing process. 
The choice of a peripheral speed of the porous member 2 greater than that 
of the developer holder 1 is effective to improve the scraping effect upon 
the developer T which remains on the developer holder 1, and also 
contributes to the action of the porous member 2 which supply the 
developer T to the developer holder 1 and charges it in preparation to the 
next following developing cycle. It will be seen that the described 
operation is repeated to run a developing process. 
As the developer T is consumed, and a fresh quantity thereof is to be 
replenished to the developer supply station within the developer vessel 7. 
This may take place by opening a feed lid 7a or by utilizing a cartridge. 
It will be seen that in the developer supply station within the developer 
vessel 7, residual developer T and fresh developer T will be in admixture. 
However, since it is only that portion of the developer T subject to 
contact and a conveying action of the porous member 2 and the constraining 
member 3 which contributes to the deposition of the developer T upon the 
photosensitive member 10, the degree of charging can be controlled by such 
member, achieving a stabilized degree of charging irrespective of the 
history of the developer T. This in turn stabilizes the force F=qE with 
which the developer T flies to the image area during the developing 
process, thus achieving a stabilized image quality. 
While the high voltage source E2 is shown as a d.c. source, it is also 
effective to utilize a superimposed d.c. and a.c. source in order to 
prevent the agglomeration of the developer T while improving its conveying 
capability. However, if the a.c. is used in superimposition, the source 
still requires a d.c. component in order to prevent the polarity of the 
developer T from changing. 
FIG. 3 is a cross section of a developer unit according to the second 
embodiment of the invention. In this embodiment, the porous member 2 used 
in FIG. 1 is replaced by a fibrous conductive member 8. In other respects, 
the arrangement is similar to that of the first embodiment shown in FIG. 1 
and accordingly, corresponding parts are designated by reference numerals 
or characters, and the repeated description will be omitted. 
Specifically, the fibrous conductive member 8 is in the form of a brush 
comprising either a conductive resin fibre such as nylon or rayon in which 
a conductive carbon is dispersed or a conductive resin fibre such as nylon 
or rayon having a core of conductive material. The fibre may be made 
conductive by a post-processing step such as depositing fine particles of 
conductive carbon to the surface thereof. The thickness of the conductive 
resin fibre may be 100 to 2,000 denier/100 fibres, or each fibre may be on 
the order of 1 to 20 denier where one denier corresponds to the thickness 
of a fibre when one gram of the material is extended to a length of 9,000 
m. A suitable density will be on the order of 10 to 1,000.times.10.sup.3 
fibres per inch square. 
The fibrous conductive member 8 is formed as a brush mounted on a metal 
shaft 8a which is rotatably supported by the sidewalls of the developer 
vessel 7, in the similar manner as the porous member 2. The fibrous 
conductive member 8 may be bonded to the metal shaft 8a by utilizing a 
conductive adhesive such as silver (Au) filler containing epoxy adhesive 
or carbon filler containing acrylic adhesive as is the case with the 
porous member 2. 
The intended purpose of the fibrous conductive member 8 may be served by 
choosing a depth of contact between the fibrous conducting member 8 and 
the developer holder 1 on the order of 0.5 to 2.0 mm. 
The number of revolutions of the fibrous conductive member 8 depends on its 
diameter, but its peripheral speed is chosen to be equal to or greater 
than that of the developer holder 1 as in the case with the porous member 
2. 
The developer unit of the second embodiment operates in the similar manner 
as that shown in FIG. 1, and therefore will not be described. 
FIG. 4 is a cross section of a developer unit according to a third 
embodiment of the invention. In this embodiment, the developer holder 1 is 
provided by forming a dielectric layer 11 on the surface of a metal shaft 
which carries the developer holder 1, which remains the same as in FIG. 1. 
In other respects, parts shown in FIG. 4 are similar to those shown in 
FIG. 1, and accordingly are designed by like numerals and characters for 
purpose of avoiding a repeated description. 
The dielectric layer 11 may be formed of a polymer material such as 
polyester, polyethylene, polyvinylidene fluoride, polypropylene or the 
like, and desirably has a thickness on the order of 50 to 100 .mu.m. By 
providing a dielectric material in the form of electret, it may be 
rendered effective in preventing the developer T sputtering in addition to 
serving the conveyance of the developer T and development. 
The developer unit of the third embodiment operates substantially in the 
similar manner as the developer unit of the first embodiment shown in FIG. 
1, but the presence of the dielectric layer 11 results in a different 
nature of force acting upon the developer T. Specifically, the developer T 
which is conveyed by the porous member 2 upon initiation of the developing 
process will be held attracted to the developer holder 1 as a result of 
its charging the dielectric layer 11 on the developer holder 1 together 
with the porous member 2 connected to the high voltage source E2 and the 
constraining member 3 as the developer T itself is charged by the members 
2 and 3. It should be noted that the dielectric layer 11 is in effect 
charged by the charged developer T, and accordingly the force of 
attraction acting upon the developer T to the developer holder 1 will be 
an electrostatic force, rather than a mirror image force which was 
effective in the developer unit of the first embodiment. 
The electric resistance of the porous member 2, the constraining member 3 
and the dielectric layer 11 as well as the potential of the high voltage 
source E2 are chosen so that the potential of the porous member 2 and the 
constraining member 3 near their surface is greater in absolute magnitude 
than the surface potential of the dielectric layer 11. 
In the developer unit of the third embodiment, the relationship between the 
various potentials should be such that 
(1) For reversal development, 
.vertline.the potential of image area of electrostatic latent 
image.vertline.&lt;.vertline.the potential of developer 
holder.vertline.&lt;.vertline.the surface potentials of porous member and 
constraining member.vertline. 
(2) For normal development, 
.vertline.the potential of developer holder.vertline.&lt;.vertline.the surface 
potentials of porous member and the constraining 
member.vertline..ltoreq..vertline.the potential of image area of 
electrostatic latent image.vertline. 
While the residual developer T on the developer holder 1 is subject to the 
action of the porous member 2 to be removed therefrom, the surface 
potential of the dielectric layer 11 remains unchanged, which is effective 
to achieve a more stabilized deposition of the developer T upon the 
developer holder 1 in the next following developing process. 
It will be noted that the developer unit of the third embodiment may 
require at least one revolution of the developer holder 1 to charge the 
dielectric layer 11 before the developing step can take place, but this 
presents no problem whatsoever. 
In the developer unit of the third embodiment, it is also advantageous for 
the purpose of assuring a favorable supply of the developer T to choose a 
peripheral speed of the porous member 2 which is equal to or greater than 
that of the developer holder 1, as in the first embodiment shown in FIG. 
1. 
It is also to be noted that in the developer unit of the third embodiment, 
the porous conductive resilient member 2 may be replaced by the fibrous 
conductive member 8 shown in FIG. 3. 
FIG. 5 is a cross section of a developer unit according to a fourth 
embodiment of the invention. In the developer unit of this embodiment, the 
direction of rotation of the developer holder 1 is opposite from that 
shown for the first embodiment shown in FIG. 1. Accordingly, because 
corresponding parts are similar to those used in the first embodiment 
shown in FIG. 1, they are designated by like numerals and will not be 
described in detail. 
However, in the developer unit of the fourth embodiment, the constraining 
member 3 is disposed on top of the developer holder 1 while the anti-spill 
cover 6 is disposed adjacent to the bottom of the developer holder 1. 
The operation of the developer unit of the fourth embodiment remains 
substantially similar to that of the developer unit of the first 
embodiment shown in FIG. 1. 
The developer holder 1, the porous conductive resilient member 2 and the 
stirring paddle 5 rotate in directions indicated by arrows in FIG. 5, and 
the developer T is conveyed toward the developer holder 1 as the porous 
member 2 rotates. As it is being conveyed, it is charged by the porous 
conductive resilient member 2 which is connected to the high voltage 
source E2 and as it is charged, it is held attracted to the developer 
holder 1 by the mirror image force for its subsequent conveyance by the 
rotation of the developer holder 1. 
The constraining member 3 forms a thin layer of developer, on the order of 
20 to 40 .mu.m and also charges the developer T to a given stable level so 
that it is conveyed into the developing zone as the developer holder 1 
rotates. In the developing zone, the developer T is used to develop an 
electrostatic latent image formed on the photosensitive member 10 
according to the relative force relationship as mentioned above in 
connection with previous embodiments, and residual developer T which was 
not utilized in the developing step will move past the anti-spill cover 6 
as the developer holder 1 rotates to be removed therefrom by the porous 
member 2. Subsequently, it is subjected to a stirring and circulating 
action within the developer vessel 7 by the stirring paddle 5 located 
within the developer supply station for its use in subsequent development 
process. 
It is to be noted that the direction of rotation of the porous member 2 
shown in FIG. 5 is an example, but that the arrow may be oppositely 
directed so as to be capable of accommodating for a reduced amount of 
developer T within the developer supply station. 
It is also to provide a dielectric layer 11 on the surface of the developer 
holder 1 in the developer unit of the fourth embodiment, as shown 
previously in FIG. 4. 
Additionally, it is also possible to replace the porous member 2 by the 
fibrous conductive member 8 as shown in FIG. 3. 
FIG. 6 is a cross section of a developer unit according to a fifth 
embodiment of the invention. In this embodiment, the developer unit 
comprises a developer holder 1' which is rotatably supported and which is 
disposed in opposing relationship with a photosensitive member 10 with a 
gap g therebetween, a constraining member 3" for controlling the thickness 
of a thin layer of developer T which is formed on the developer holder 1' 
and for charging the developer T, a stirring paddle 5 for stirring 
developer T disposed within a developer supply station, an anti-spill 
cover 6 for preventing the developer T from spilling over the top of the 
developer holder 1', a developer vessel 7 for defining a developer supply 
station and on which the described parts are mounted, and a high voltage 
source E1 for applying a developing bias to the developer holder 1'. 
As shown in FIG. 7, the developer holder 1' comprises a cylindrical metal 
shaft or metal carrier 1a having a coating of conductive resin layer 1b 
thereon. The shaft 1a may be formed of aluminium, stainless steel or the 
like while the resin layer 1b may have a thickness on the order of 1.5 to 
5 mm and may be formed of a resin having conductive powder dispersed 
therein to exhibit a resistivity on the order of 10.sup.4 to 10.sup.12 
.OMEGA. cm. Conductive powder may comprise conductive carbon, aluminium 
powder or silver powder, and a resin may comprise a thermosetting resin 
such as phenol, urea or melamine resin or a thermoplastic resin such as 
polystyrene or acrylic resin. 
It is possible to achieve a resistivity in a range from 10.sup.4 to 
10.sup.12 .OMEGA. cm for the conductive resin layer 1b by using a 
dispersion of conductive powder in the resin on the order of 5 to 50 
percent by weight. A coating of the conductive resin layer 1b on the metal 
shaft 1a is formed by initially providing a hollow-cylindrical member of 
conductive resin, to which the metal shaft 1a is bonded by using a 
conductive adhesive or in which the metal shaft 1a is positioned as a 
press fit. 
More specifically, as shown in FIG. 8(a), a conductive resin which exhibits 
a resistivity on the order of 10.sup.4 to 10.sup.12 .OMEGA. cm and having 
a thickness on the order of 1.5 to 5 mm is initially formed into a hollow 
cylinder to provide a cylindrical member 1b' of conductive resin. 
Subsequently, as shown in FIG. 8(b), a metal shaft 1a carrying a pair of 
support stubs 1c at its opposite ends is polished to a high precision by a 
centered forced polishing operation. Then a conductive adhesive such as a 
silver filler containing epoxy adhesive or carbon filler containing 
acrylic adhesive which has a resistivity equal to or less than 10.sup.4 
.OMEGA. cm is applied to the surface of the metal shaft 1a as shown in 
FIG. 8(c), and then the cylindrical member 1b' is fitted over the metal 
shaft 1a. Finally, the centered forced polishing operation is again used 
to achieve a thickness of 1.5 to 5 mm for the conductive resin layer 1b 
and a tolerance of concentricity equal to or less than 10 .mu.m for the 
outer diameter of the developer holder 1' as referenced to the outer 
diameter of the support stubs 1c located on the opposite ends of the metal 
shaft 1a. In this manner, there is obtained a developer holder 1' having a 
resistivity on the order of 10.sup.4 to 10.sup.12 .OMEGA. cm, high 
rigidity and exhibiting a high dimensional accuracy. Since the metal shaft 
1a extends lengthwise through the developer holder 1', a non-uniformity of 
the potentials distributed lengthwise thereof can be avoided. 
It is noted that while the degree of the circularity of the external 
diameter of the developer holder 1' thus formed is slightly inferior to 
that of a developer holder 1 which is formed of a metal shaft alone, the 
presence of the conductive resin layer 1b thereon makes the resulting 
developer holder 1' permissible for practical purposes. Since no 
mechanical strength is required of the conductive resin layer 1b itself, 
accommodation for a reduced diameter or an increased length can be met by 
the configuration of the metal shaft 1a. 
To give an example, a developer holder 1' may be formed to an external 
diameter of 30 mm by utilizing a stainless steel shaft 1a having a 
diameter of 24 mm which is then coated by a conductive resin layer 1b 
having a thickness of 3 mm and having a resistivity of 10.sup.5 .OMEGA. cm 
which is attained by dispersion of conductive carbon in phenol resin. The 
resulting developer holder 1' is driven at the peripheral speed of 100 
mm/sec, for example, in the direction shown by the arrow. 
A conductive resin layer 1b having a resistivity in a range of from 
10.sup.4 to 10.sup.12 .OMEGA. cm may be formed on the metal shaft 1a by 
coating the metal shaft 1a with polyurethane or polyester resin having a 
dispersion of conductive powder such as conductive carbon, aluminium 
powder or silver powder so as to achieve a resistivity on the order of 
10.sup.4 to 10.sup.12 .OMEGA. cm. However, the application of the 
conductive resin layer 1b by the coating step may result in an 
insufficient adhesion of the conductive resin layer 1b to the metal shaft 
1a, causing an exfoliation after a prolonged period of use. In addition, 
the coating technique is not practical in view of the increased cost and 
the likelihood of producing pinholes. It is also contemplated that the 
metal shaft 1a be eliminated completely, and a cylindrical member 1b' 
formed of conductive resin having a resistivity on the order of 10.sup.4 
to 10.sup.12 .OMEGA. cm as a result of dispersion of conductive powder and 
which is free from flanges at its opposite sides may itself serve as a 
developer holder. However, in this instance, the application of a voltage 
from the source E1 will not be uniform lengthwise of the developer holder. 
In particular, for a developer holder having an external diameter equal to 
or less than 30 mm or having a length equal to or greater than 200 mm, the 
insufficient rigidity of the material may cause a flexure of the developer 
holder, and in addition, it becomes difficult to maintain the circularity 
of the developer holder which is directly related to an non-uniformity in 
the image density which is of paramount importance to the developer unit, 
thus presenting difficulties in their practical use. 
The developer holder 1' is located in the opening of the developer vessel 7 
which contains an amount of developer T as a developer supply station, and 
the developer holder 1' is coated with the developer T by an application 
roller, not shown. The constraining member 3" comprises a sheet of 
polyurethane rubber having a thickness of 3 mm and a rubber hardness of 
60.degree., for example, and is disposed in abutment against the developer 
holder 1'. 
In operation, as the developer holder 1' and the stirring paddle 5 rotate 
in directions indicated by arrows, the developer T in the developer supply 
station within the vessel 7 will be conveyed to form a thin layer under 
the control of the constraining member 3" to achieve a thickness on the 
order of 20 to 40 .mu.m, and will be triboelectrically charged to the 
positive polarity by a sliding contact with the developer holder 1' and 
the constraining member 3". The force which causes the adhesion of the 
developer T to the developer holder 1' will be electrostatic in nature in 
this instance. 
The thin layer of developer T which is formed on the developer holder 1' 
will be conveyed, as the developer holder 1' rotates, into the developing 
zone where it is in opposing relationship with the photosensitive member 
10 rotating at the peripheral speed of 50 mm/sec, for example, in a 
direction indicated by arrow, with a distance therebetween which is equal 
to gap g minus the thickness of layer of developer T. 
A developing bias of +500 V, d.c. for example, is applied to the developer 
holder 1' from the high voltage source E1, and because the surface charge 
density is different between an image area and a non-image area of the 
latent image on the photosensitive member 10 in the developing zone, the 
developer T will fly from the developer holder 1' toward the 
photosensitive member 10 and deposited thereon only in the region of the 
image area for purpose of development. A resistivity in a range from 
10.sup.4 to 10.sup.12 .OMEGA. cm, preferably around 10.sup.8 .OMEGA. cm, 
of the conductive resin layer 1b on the developer holder 1' yields a 
favorable development over a range of image varieties from a solid black 
to a halftone image while suppressing an excessive transfer of developer 
T. The effect of any fluctuation in the output from the high voltage 
source E1 is diminished by the resistance which the conductive resin layer 
1b on the developer holder 1' exhibits, reducing its influence upon the 
image quality. 
The developer T on the developer holder 1' which is left without being used 
in the developing process will be recovered into the developer supply 
station within the vessel 7 through the anti-spill cover 6 as the 
developer holder 1' rotates. The described cycle is repeated to proceed 
the developing process. 
It is to be noted that the principal force with which developer T on the 
developer holder 1' is transferred to an image area in the latent image on 
the photosensitive member 10 is an electrostatic force represented by F=qE 
where q represents the charge retained by the developer T as it is 
conveyed from within the vessel 7 to a position on the developer holder 1' 
where it is disposed in opposing relationship with the latent image, and E 
represents an electric field proportional to a difference between the 
potential Vs of an image area of the latent image and a bias potential Vb 
applied to the metal shaft 1a of the developer holder 1' or E=f.sub.0 
.times..vertline.Vs-Vb.vertline. where the f.sub.0 can be termed as a 
dielectric thickness. The dielectric thickness f.sub.0 can be determined 
from the following equation using a cross sectional arrangement of a 
dielectric layer model in the developing zone as shown in FIG. 9, which is 
formed by the photosensitive member 10 (photosensitive layer 10b and 
conductive substrate 10a), developer T, gap g and the developer holder 1': 
##EQU1## 
where r, d, g and h will be the thicknesses of the conductive resin layer 
1b, the thin layer of developer T, the gap g and the photosensitive layer 
10b, and .epsilon..sub.2, .epsilon..sub.1, .epsilon..sub.0 and 
.epsilon..sub.s represent the dielectric constants of the conductive resin 
layer 1b, the layer of developer T, the gap g and the photosensitive layer 
10b. By using relative dielectric constants, these dielectric constants 
can be rewritten as .epsilon..sub.1 =.epsilon..sub.0 .epsilon..sub.1', 
.epsilon..sub.s =.epsilon..sub.0 .epsilon..sub.s' and .epsilon..sub.2 
=.epsilon..sub.0 .epsilon..sub.2'. The equation (1) can then be rewritten 
as follows: 
##EQU2## 
For a developer holder 1 free of a conductive resin layer, the dielectric 
thickness f.sub.0 can be defined as follows: 
##EQU3## 
Substituting values of these parameters obtained in an actual developer 
unit into the equation (2), i.e., .epsilon..sub.1' =1.5, .epsilon..sub.s' 
=7, .epsilon..sub.2' =7, h=50 .mu.m, g - 100 .mu.m and 80 .mu.m, d=40 
.mu.m, r=5,000 .mu.m, 3,000 .mu.m, 1,000 .mu.m, 500 .mu.m and 0 .mu.m to 
derive a rate of change in f.sub.0 {f.sub.0 (80 .mu.m)-f.sub.0 (100 
.mu.m)}/ f.sub.0 (80 .mu.m) as the gap g changes from 100 .mu.m to 80 
.mu.m, there can be obtained a graph as shown in FIG. 10. The purpose of 
choosing a change of the gap g between 100 and 80 .mu.m in the graphical 
illustration in FIG. 10 is to consider a resulting change in the electric 
field E when the gap g varies due to mechanical accuracy of the developer 
holder 1'. The rate of change in f.sub.0 is plotted against the thickness 
r of the conductive resin layer 1b in this graph over 0 to 5,000 .mu.m in 
order to recognize the influence of the thickness (including the presence 
and absence) of the conductive resin layer 1b. 
Considering the graphical illustration of FIG. 10, which indicates the rate 
of change in the dielectric thickness f.sub.0 for a change in the gap g 
over varying thickness r of the conductive resin layer 1b, it will be seen 
that the rate decreases with an increase in the thickness of the 
conductive resin layer 1b. This means that the use of the conductive resin 
layer 1b on the developer holder 1' is effective to allow the accuracy 
which is required in machining the developer holder 1' to be alleviated 
than when no conductive resin layer is used. Accordingly, a developer 
holder 1' having a conductive resin layer 1b is seen to be more suitable 
for its mass production while reducing the manufacturing cost. 
A proper value of the thickness r of the conductive resin layer 1b will now 
be considered. Where no conductive resin layer is provided (r=0), it is 
necessary for the permissible development that a variation in the gap g 
remains within 8 .mu.m, which can be converted into the rate of change in 
the dielectric thickness f.sub.0 as follows: 
EQU {f.sub.0 (92 .mu.m)-f.sub.0 (100 .mu.m)}/f.sub.0 (92 .mu.m).div.0.06 (6%). 
Accordingly, if the gap g changes by 20 .mu.m, it is seen from FIG. 10 that 
the thickness r of the conductive resin layer 1b is equal to or greater 
than 1,500 .mu.m or 1.5 mm in order to achieve a satisfactory development 
for practical purposes. In addition, it is required that the developer 
holder 1' itself should achieve a tolerance of concentricity of its 
external diameter as referenced to the external diameter of the support 
stubs 1c located on the opposite end of the metal shaft 1a which is equal 
to or less than 10 .mu.m in consideration of the accuracies of related 
parts and assembly operation. 
A reduction in the absolute value of the dielectric thickness f.sub.0 which 
is caused by an increase in the thickness r of the conductive resin layer 
1b causes a reduction in the strength of the electric field E, which can 
be accommodated for by controlling the developing bias Vb. However, it 
will be noted from FIG. 10 that a decrease in the rate of change in the 
dielectric thickness f.sub.0 with an increase in the thickness of the 
conductive resin layer 1b is greatly reduced as the thickness further 
increases. In addition, an increased thickness of the conductive resin 
layer 1b causes a difficulty in achieving the accommodation by the 
adjustment of the developing bias Vb. Accordingly, it is preferable for 
practical purposes that the thickness r of the conductive resin layer 1b 
is limited to or less than 5 mm. In addition, it is undesirable to use an 
increased thickness for the conductive resin layer 1b in order to suppress 
a dimensional change during the operation and storage. 
If the conductive resin layer 1b is thin enough to be equal to or less than 
1 mm, this is likely to cause a non-uniformity in the image density due to 
a non-uniform dispersion of conductive powder within conductive resin 
layer 1b. In addition, the non-uniformity in the image density will also 
be caused by a non-uniformity in the thickness r of the conductive resin 
layer 1b, presenting practical problems. 
In an experimental development which is conducted by choosing a gap g 
between the developer holder 1' and the photosensitive member 10 which is 
less than 0.3 mm or to be equal to 0.1 mm, for example, with the 
resistivity of the conductive resin layer 1b on the developer holder 1' 
chosen to be 10.sup.7 .OMEGA. cm, it is found that if a discharge occurs 
in the presence of pinholes in the photosensitive layer 10b of the 
photosensitive member 10, the resulting discharge current is limited by 
the conductive resin layer 1b on the developer holder 1', preventing the 
potential of the developer holder 1' from being reduced to near the ground 
potential. In this manner, the occurrence of a black traversing pattern 
across a background of a copy, the occurrence of a discharge pattern in 
the form of white dots in an image area which would be otherwise produced 
as a result of the electric breakdown of the air or a nonuniformity in the 
image density of the copy is prevented. 
Thus by using the conductive resin layer 1b having a thickness of 1.5 to 5 
mm and exhibiting a resistivity on the order of 10.sup.4 to 10.sup.12 
.OMEGA. cm as a coating on the metal shaft 1a to provide the developer 
holder 1', any discharge which would be caused by a developing bias across 
the developer holder 1' and the photosensitive member 10 will be 
suppressed by the resistivity presented by the conductive resin layer 1b 
on the developer holder 1', with consequence that a high developing bias 
applied to the developer holder 1', if chosen, does not interfere with 
obtaining a sharp, fogging-free image exhibiting an enhanced edge effect, 
a low bias phenomenon caused by a discharge of the developing bias which 
would produce a black traversing pattern across a background of a copy and 
a discharge pattern in the form of white dots across an image area can be 
prevented and the developer holder 1' is enabled to act as a developing 
electrode to prevent any loss of the reproducibility of a solid black 
image. 
By maintaining a tolerance of the concentricity of the external diameter of 
the developer holder 1' as referenced to the external diameter of the 
support stubs 1c disposed at the opposite ends of the metal shaft 1a which 
is equal to or less than 10 .mu.m, the rigidity of the developer holder 1' 
is sufficient to maintain the gap g between the developer holder and the 
photosensitive member 10 to a high accuracy, enabling the development of 
an image which is free from a non-uniformity in the image density. Since 
the developer holder 1' comprises a coating of the conductive resin layer 
1b around the metal shaft 1a, there resulted no potential distribution 
lengthwise of the developer holder 1', which would have caused a 
nonuniformity in the image density. 
Since the cylindrical member 1b' of conductive resin is fitted over and 
secured to the peripheral surface of the metal shaft 1a, the developer 
holder 1', which is rendered incapable of producing a non-uniformity in 
the image density by preventing a discharge of the developing bias, can be 
provided at a reduced cost and at a high accuracy by means of a mass 
production. 
FIG. 11 is a cross section of a developer unit according to a sixth 
embodiment of the invention. The developer unit of this embodiment is a 
modification of that shown in FIG. 1 in that the developer holder 1' used 
in the developer unit of the fifth embodiment (shown in FIG. 7) is used as 
the developer holder 1 and separate high voltage sources are used, 
including a high voltage source E2 associated with the porous member 2 and 
a high voltage source E3 associated with the constraining member 3. It is 
to be noted that the polarity of the source E3 is chosen to be of the same 
polarity as the polarity to which the developer T is charged. In other 
respects, the arrangement is similar to those shown in FIG. 1, and 
accordingly corresponding parts are designated by like reference numerals 
or characters and will not be described in detail. 
The developer unit of the sixth embodiment operates substantially similarly 
as the developer unit of the first embodiment shown in FIG. 1. However, 
the force which attracts the developer T to the developer holder 1' is an 
electrostatic force acting between the charge of the developer T and the 
conductive resin layer 1b on the developer holder 1'. 
In FIG. 11, the constraining member 3 has been illustrated as a single 
member. However, the construction of the constraining member 3 is not 
limited thereto, and it may be constructed in different configurations as 
illustrated in FIGS. 12(a), (b) and (c). The only requirement is that a 
portion of the constraining member 3 including a surface which abuts 
against the developer holder 1' exhibits a given resistivity and is 
adapted to allow the application of a high voltage thereto. Any separate 
member may be used to support such portion so as to enable the mechanical 
abutment of such portion against the developer holder 1', and still the 
assembly can function as the constraining member 3. In FIG. 12(a), the 
constraining member 3 comprises a conductive material 32 on the surface of 
a resilient member 31 which may be formed of urethane rubber. The 
conductive material 32 may be coated on the resilient material 31, but a 
bonding by means of an adhesive or a mechanical attachment is preferred in 
view of the useful life and the stability. In FIG. 12(b), the constraining 
member 3 comprises a block of conductive material 34 secured to the free 
end of a resilient metal plate 33 which may be formed of phosphor bronze 
or spring steel. In FIG. 12(c), the constraining member 3 comprises a 
conductive material 37 applied to the surface of a resilient member 36 
which is in turn secured to a resilient metal plate 35 which may be formed 
of phosphor bronze or spring steel. 
FIG. 13 is a cross section of a developer unit according to a seventh 
embodiment of the invention. In this embodiment, the porous member 2 used 
in the sixth embodiment shown in FIG. 11 is replaced by the fibrous 
conductive member 8 used in the second embodiment shown in FIG. 3. In 
other respects, the arrangement is similar to that of the sixth 
embodiment, and accordingly, corresponding parts are designated by like 
reference numerals and characters and will not be described in detail. 
Again, this embodiment operates in the similar manner as the sixth 
embodiment shown in FIG. 11. 
FIG. 14 is a cross section of a developer unit according to an eighth 
embodiment of the invention where the direction of rotation of the 
developer holder 1' in the developer unit of the sixth embodiment shown in 
FIG. 11 is reversed. Accordingly, the arrangement of this embodiment is 
similar to that of the sixth embodiment shown in FIG. 11 unless otherwise 
specified, and accordingly corresponding parts are designated by like 
reference numerals or characters and will not be described. 
In the developer unit of the eighth embodiment, the constraining member 3 
is disposed on top of the developer holder 1' while the anti-spill cover 6 
is disposed alongside the bottom of the developer holder 1'. A partition 9 
is disposed on top of and above the porous conductive resilient member 2 
disposed within the developer vessel 7 for preventing the developer T 
distributed around the stirring paddle 5 from moving directly to the 
developer holder 1' without being previously engaged by the porous member 
2. In addition, the partition 9 is effective to introduce such portion of 
the developer T, which has been blocked from being conveyed into the 
developing zone as the constraining member 3 defines a thin layer as well 
as that portion of the developer T which is scraped off the developer 
holder 1' which remained after the development, into the developer vessel 
7 to the region of the stirring paddle 5. 
The partition 9 may be formed of a resin, for example, but is preferably 
formed of a metal which is then connected to the electrical ground in 
consideration of the charge of the developer T and the subsequent charged 
condition of the developer T. If placed in contact with the porous 
conductive resilient member 2, the partition 9 cannot cause a leakage of a 
high voltage from the source E2 because of the resistivity of the porous 
member 2 which is on the order of 10.sup.3 to 10.sup.6 .OMEGA. cm. 
The developer unit of the eighth embodiment operates in substantially the 
same manner as the developer unit of the sixth embodiment shown in FIG. 
11. 
In the developer unit of the eighth embodiment, it is possible to replace 
the porous member 2 by the fibrous conductive member 8 as used in the 
seventh embodiment shown in FIG. 13. In addition, the direction of 
rotation shown for the porous member 2 is exemplary only, and it may 
rotate in the opposite direction. The presence of the partition 9 is also 
preferred in this instance. 
For the developer units of the sixth to eighth embodiments, experiments 
have shown that images of a favorable quality has been obtained under the 
conditions indicated below: 
______________________________________ 
gap g 0.1 mm 
resistivity of 10.sup.10 .OMEGA. cm 
conductive resin layer 1b 
peripheral speed of 75 mm/sec 
developer holder 1 
resistivity of 10.sup.4 .OMEGA. cm 
porous conductive resilient member 2 
(or fibrous conductive member 8) 
peripheral speed of 125 mm/sec 
porous conductive resilient member 2 
(or fibrous conductive member 8) 
resistivity of constraining member 3 
10.sup.6 .OMEGA. cm 
voltage of source E1 
500 V 
voltage of source E2 
600 V 
voltage of source E3 
400 V 
peripheral speed of 50 mm/sec 
photosensitive member 10 
voltage of image area 
50 V 
on photosensitve member 10 
voltage of non-image area 
600 V 
on photosensitive member 10 
(reversal development) 
______________________________________ 
It is possible to exercise a control over the image density depending on 
the relative magnitude of the voltage of the sources E1, E2 and E3. For 
example, by choosing a relationship such that .vertline.voltage of source 
E2.vertline.&gt;.vertline.voltage of source E1.vertline., the image density 
can be increased. The image density can also be increased by choosing a 
relationship such that .vertline.voltage of source 
E3.vertline.&lt;.vertline.voltage of source E1.vertline.. 
FIG. 15 is a cross section of a developer unit according to a ninth 
embodiment of the invention. In the developer unit of this embodiment, the 
conductive constraining member 3 comprises a constraining member 3' formed 
as a lamination of a non-conductive portion 3a and a conductive portion 
3b, and a high voltage source E2 connected to the porous member 2 is 
separate from a high voltage source E3 which is connected to the 
conductive portion 3b of the constraining member 3. In other respects, the 
arrangement is similar to that of the first embodiment shown in FIG. 1 and 
accordingly, corresponding parts are designated by like reference numerals 
and characters and will not be described specifically. 
The purpose of replacing the conductive constraining member 3 by the 
laminate 3' is to improve the useful life and the reliability of the 
resulting developer unit. Specifically, if a constraining member 3 is 
formed by a dispersion of conductive material therein or containing a 
conductive material deposited on or coated to the surface thereof which is 
disposed for contact with the developer holder 1 which carries the 
developer thereon, a mechanical abrasion will be caused in the surface of 
the constraining member 3 which is placed in contact with the developer 
holder 1, in particular, when the surface of the developer holder 1 is 
roughened. Where the constraining member 3 is formed by dispersion, there 
results a differential abrasion between the resin which represents a 
dispersion medium and the conductive material which represents a dispersed 
phase. Where the constraining member 3 is imparted with the electrical 
conductivity by deposition or coating, the deposited or coated layer may 
be abraded or may become exfoliated. In either instance, the stability of 
the charging and the formation of the thin layer will both depend on the 
quality of the constraining member 3, resulting in a degraded reliability 
and a reduced life of the developer unit. 
The non-conductive portion 3a of the constraining member 3' is formed by a 
sheet of silicone rubber or urethane having a thickness on the order of 2 
to 3 mm and hardness on the order of 60.degree. to 80.degree., and the 
conductive portion 3b is applied to the opposite side thereof away from 
the side thereof which is disposed for abutment against the developer 
holder 1. The conductive portion 3b may be formed in a number of ways, 
including a coating of conductive material such as conductive carbon or 
metal filler on a resilient material which provides the nonconductive 
portion 3a, bonding a thin film of a metal such as copper, aluminium or 
stainless steel thereto by using a conductive adhesive such as a silver 
filler containing epoxy adhesive or carbon filler containing acrylic 
adhesive or evaporation of aluminium thereon. 
On the side disposed for abutment against the developer holder 1, the 
non-conductive portion 3a exhibits a resistivity equal to or greater than 
10.sup.13 .OMEGA. cm, and such insulating material is effective to prevent 
a leakage between the source E3 connected to the conductive portion 3b of 
the constraining member 3 and the source E1 connected to the developer 
holder 1, thus allowing the constraining member 3' and the developer 
holder 1 to be maintained at their respective high potentials. 
In operation, the developer T which is charged by the porous member 2 will 
be formed into a thin layer on the developer holder 1 under the control of 
the constraining member 3' so as to have a thickness on the order of 20 to 
40 .mu.m. Even though the non-conductive portion 3a of the constraining 
member 3' is insulating, it has a dielectric constant, so that when a high 
voltage is applied to the conductive portion 3b of the constraining member 
3' which is connected to the source E2, an induced charge will be 
developed on the side of the non-conductive portion 3a which is disposed 
for abutment against the developer holder 1, causing a charging by contact 
charging or triboelectric charging. 
In FIG. 15, the entire constraining member 3' is formed as a laminate 
construction, but the construction of the constraining member 3' is not 
limited thereto, but may assume different configurations as indicated in 
FIGS. 16(a), (b), (c) and (d). As far as the side of the constraining 
member 3' which is disposed for abutment against the developer holder 1 is 
formed as a non-conductive portion 3a while the opposite side is formed 
with the conductive portion 3b, the requirement for a mechanical abutment 
against the developer holder 1 is satisfied. Specifically, in FIG. 16(a), 
the constraining member 3' comprises an insulating resin layer 39 applied 
to a resilient plate 38 which may be formed of a metal such as phosphor 
bronze or spring steel. In FIG. 16(b), the constraining member 3' 
comprises a similar resilient metal plate 40, to the free end of which is 
secured a block of resilient material 41 having a conductive material 42 
formed on its surface. In FIG. 16(c), the constraining member 3' comprises 
a block of resilient conductive member 43 which may be formed by a sheet 
of silicone rubber or the like, having conductive material dispersed 
therein, and a portion of which that is disposed for abutment against the 
developer holder 1 is replaced by a block of an insulating material 44 
which may be formed of silicone rubber that do not have a dispersion of 
conductive material therein. In FIG. 16(d), the constraining member 3' 
comprises a so-called graded function material 45 which is formed by a 
metal sheet as may be formed by chromium dioxide (CrO.sub.2) on which a 
ceramic layer is grown as a crystal, with a high voltage being applied to 
the metal surface. 
FIG. 17 is a cross section of a developer unit according to a tenth 
embodiment of the invention in which the porous member 2 is replaced by 
the fibrous conductive member 8. In other respects, the arrangement is 
similar to that of the ninth embodiment shown in FIG. 15, and accordingly, 
the corresponding parts are designated by like reference numerals or 
characters and will not be described specifically. This developer unit 
operates in substantially the same manner as the developer unit of the 
ninth embodiment shown in FIG. 15. 
FIG. 18 is a cross section of a developer unit according to an eleventh 
embodiment of the invention in which the direction of rotation of the 
developer holder 1 is reversed from that used in the ninth embodiment 
shown in FIG. 15. In other respects, the arrangement is similar to that of 
the ninth embodiment shown in FIG. 15, and accordingly, corresponding 
parts are designated by like reference numerals or characters and will not 
be described specifically. 
In the developer unit of the eleventh embodiment, the constraining member 
3' is disposed on top of the developer holder 1 while the anti-spill cover 
6 is disposed on the bottom side thereof. In addition, a partition 9 
similar to that used in the eighth embodiment shown in FIG. 14 is disposed 
on top of and above the porous member 2 located within the developer 
vessel 7. The developer unit of the eleventh embodiment operates 
substantially similar as the developer unit of the ninth embodiment shown 
in FIG. 15. 
In the developer unit of the eleventh embodiment, it is possible to replace 
the porous member 2 by the fibrous conductive member 8 as in the tenth 
embodiment shown in FIG. 17. 
The direction of rotation shown in this Figure of the porous member 2 is 
exemplary, and it may rotate in the opposite direction. In tis instance, 
it is preferred that the partition 9 be provided. 
It is found by experiments that the developer units of the ninth to the 
eleventh embodiments produce favorable images under the same conditions as 
described for the developer units of the sixth to the eighth embodiments. 
FIG. 19 is a cross section of a developer unit according to an twelfth 
embodiment of the invention where the constraining member 3 is a composite 
of a resilient metal plate 33 and a conductive material 34 as shown in 
FIG. 12(b). In other respects, the arrangement is similar to that of the 
first embodiment shown in FIG. 1, and accordingly, corresponding parts are 
designated by like reference numerals or characters and will not be 
specifically described. 
It should be understood that the developer unit of this embodiment operates 
substantially similarly as the developer unit of the first embodiment 
shown in FIG. 1. 
The composite constraining member 3 may be replaced by a different 
composite constraining member 3 as shown in FIG. 12(a) or (c). The 
operation remains unchanged. 
As in FIG. 5, the composite constraining member 3 may be disposed on top of 
the developer holder 1 while the spill cover 6 may be disposed along the 
bottom thereof. In addition, as in the second embodiment shown in FIG. 3, 
the porous member 2 may be replaced by the fibrous conductive member 8. 
FIG. 20 is a cross section of a developer unit according to a thirteenth 
embodiment of the invention. In this embodiment, the developer holder 1 of 
the first embodiment shown in FIG. 1 is replaced by the developer holder 
1' (see FIG. 7) of the fifth embodiment shown in FIG. 6, and the 
conductive constraining member 3 of the first embodiment is replaced by a 
constraining member 3' comprising a laminate comprising a non-conductive 
portion 3a and a conductive portion 3b. In addition, a high voltage 
source E2 which applies a high voltage to the porous member 2 is separate 
from a high voltage source E3 which applies a high voltage to the 
conductive portion 3b of the constraining member 3. In other respects, the 
arrangement is similar to that of the first embodiment shown in FIG. 1, 
and accordingly, corresponding parts are designated by like reference 
numerals or characters as used in FIG. 1 and will not be specifically 
described. 
The developer unit of the thirteenth embodiment operates substantially 
similarly as the developer unit of the first embodiment shown in FIG. 1, 
but when the developer T is charged, it is supplied with charge from the 
porous member 2 to be electrostatically held attracted to the developer 
holder 1' and is charged in a stable manner by the induced charge which is 
developed at the non-conductive portion 3a of the constraining member 3 
before it is conveyed into the developing zone. 
In the developer unit of the thirteenth embodiment, the side of the 
constraining member 3' which is disposed for abutment against the 
developer holder 1' comprises the nonconductive portion 3a while the 
opposite side comprises the conductive portion 3b so that the charging and 
the formation of the thin layer of the developer T take place in a stable 
and reliable manner, assuring a stable and reliable image reproduction by 
the developer holder 1' which carries the conductive resin layer 1b. 
In the developer unit of the thirteenth embodiment, the constraining member 
3' may comprise a composite as shown in FIGS. 16(a) to (d), and in 
addition, the constraining member 3' may be disposed on top of the 
developer holder 1' while the anti-spill cover 6 may be disposed alongside 
the bottom thereof as shown in FIG. 5. 
FIG. 21 is a cross section of a developer unit according to a fourteenth 
embodiment of the invention, in which the porous member 2 in the 
thirteenth embodiment shown in FIG. 20 is replaced by the fibrous 
conductive member 8. In other respects, the arrangement is similar to that 
of the thirteenth embodiment shown in FIG. 20, and accordingly, 
corresponding parts are designated by like reference numerals or 
characters and will not be described in detail. 
The developer unit of the fourteenth embodiment operates in substantially 
the same manner as the developer unit of the thirteenth embodiment shown 
in FIG. 20. 
In various embodiments described above, it is assumed that the developer 
unit uses a developer T which is charged to the positive polarity, but it 
should be understood that the invention is equally applicable to developer 
units where a developer T is charged to the negative polarity.