Electrophotographic apparatus for developing latent electrostatic charge images

An apparatus for developing latent electrostatic images in an electrophotographic copier are disclosed, the developer including an image carrier having a conductive backing layer and a photoconductive layer, a magnetic brush developer, a voltage source connected with the magnetic brush developer, and a high-ohmic resistance connecting the backing layer with ground, the voltage source and high-ohmic resistance establishing a potential difference between the magnetic brush developer and the backing layer to prevent a voltage breakdown at the image carrier which may result in undesirable streaking and fogging of an electrostatic copy. The high-ohmic resistance may be adjusted in accordance with the amount of current generated in the developer mixture whereby the flow of current from the developer through the photoconductive layer to the backing layer is impeded to prevent a voltage breakdown. In a first embodiment, the image carrier is a planar printing plate. In a second embodiment, the image carrier is a cylindrical drum having the photoconductive layer arranged on its outer peripheral surface, with charge and transfer coronas arranged adjacent the photoconductive surface, and the embodiment includes a semiconductor unit connected in parallel with the high-ohmic resistance.

BRIEF DESCRIPTION OF THE PRIOR ART 
An electrophotographic process for developing electrostatic charge images 
is disclosed in German Pat. No. 1,024,988, wherein an image carrier 
bearing a latent electrostatic charge image is supported on a grounded 
base plate and a charged magnet is arranged above the image carrier, the 
magnet serving to loosely attract a developer mixture during development. 
The apparatus further includes a grounded magnetic roller which applies 
the developer mixture to the image carrier and a conductor plate arranged 
opposite the magnetic roller over which the image carrier passes. The 
image carrier may pass in direct contact with the conductor plate or in 
the immediate vicinity thereof. The conductor plate is connected with a 
voltage source through a potentiometer so that a bias voltage of the order 
of 700 volts is applied between the conductor plate and ground. 
In the electrophotographic method and apparatus disclosed in the German 
printed application No. 2,232,513, the apparatus for transferring charge 
images onto an image receiving material comprises an image carrier, 
including a metallic back connected directly to ground. 
While the prior electrophotographic copiers normally operate quite 
satisfactorily, the image carriers of the prior art developers are 
susceptible to voltage breadkowns resulting in streaking or fogging of the 
copy. The height of the residual voltage of a latent charge image, i.e, 
the voltage still present in the exposed areas after the image carrier has 
been charged under a corona and exposed, is determined by different 
background characteristics and the exposure time of the original. In order 
to avoid fogging of the copy, it is normally desirable for development to 
begin at a certain charge which is either equal to or slightly greater 
than the residual voltage. The residual voltage present on the image 
carrier is compensated by increasing the voltage applied to the developer. 
Application of the increased compensating voltage, whose magnitude is 
determined by the characteristics of the image carrier bearing the latent 
charge image and the carrier material of the developer mixture, involves 
the risk that the potential in the developer mixture may be displaced. 
If the surface of the image carrier allows a flow of electric current, 
i.e., if the surface is porous or if it contains damaged or uncoated 
areas, such as cut edges in the case of printing plates, and if the back 
of the image carrier is grounded, voltage breakdowns may occur in the 
developer mixture. These breakdowns occur above a certain voltage level, 
which is dependent upon the conductivity of the developer mixture. 
So-called conductive paths are thereby formed in the developer mixture 
along which voltage breakdowns may occur. If the conductive paths in the 
developer mixture come into contact with defective areas of the image 
carrier or with electrically conductive areas on the surface of the image 
carrier, the developer potential is displaced and flows off to the back of 
the image carrier. In this manner, annoying and undesirable black streaks 
and fogging may be produced in the copy. 
The formation of a voltage breakdown within the developer mixture depends 
upon the voltage difference between the developing voltage applied and the 
voltage at the back of the image carrier and also upon the conductivity of 
the developer mixture. Differences in the potential within the developer 
mixture result in high local field strengths between the carrier particles 
of the developer mixture so that individual discharges may occur. These 
discharges cause an increase in the voltage differences between the 
remaining carrier particles of the conductive chain, so that further 
discharges occur which may result in an avalanche of discharges within a 
short time and thus lead to a chain reaction. Through the conductive path 
formed in this manner, the developer voltage flows to the back of the 
image carrier. 
The present invention was developed to provide an improved 
electrophotographic method and apparatus for developing latent 
electrostatic charge images wherein the formation of streaks or fogging on 
the copy due to voltage breakdown between the developer mixture and the 
carrier for the charge images in defective or uncoated areas of the image 
carrier surface and at cut edges is avoided. 
SUMMARY OF THE INVENTION 
The primary object of the present invention is to provide a method and 
apparatus for developing latent electrostatic charge images comprising an 
image carrier including a conductive backing layer and an insulating 
photoconductive layer, a magnetic brush developer, and voltage breakdown 
protection means including a voltage source connected with the magnetic 
brush developer and a high-ohmic resistance connecting the backing layer 
with ground for maintaining the potential of the developer mixture and the 
charge exchange between the image carrier which is necessary for image 
development independent of changes in the conductivity of the developer 
mixture during application of the toner to the charge images. 
According to a more specific object of the invention, the high-ohmic 
resistance of the voltage breakdown protection means comprises a variable 
resistor adjustable between values of 10 M.OMEGA. and 200 M.OMEGA.. The 
ground connection applied to the backing layer of the image carrier by a 
high-valued resistor impedes the formation of discharge avalanches because 
the resistor limits the increased current flow which forms in the 
developer mixture and thus prevents the formation of low-ohmic paths and 
the resulting displacement of the developer potential. The high-ohmic 
ground connection of the backing layer of the image carrier must be 
adjusted so that the charge exchange necessary for image development is 
not impeded. Depending on the resistance value and the height of the 
developer voltage applied, the high-ohmic resistor is similar to a control 
element regulating the conductivity of the developer mixture. 
It is a further object of the subject invention that the high-ohmic ground 
connection of the image carrier increases the voltage at the backing layer 
of the carrier so that the voltage difference between the applied 
developer voltage and the voltage at the backing layer of the image 
carrier is reduced. The lower voltage thus applied to the developer 
mixture diminishes the avalanche effect and increases the ohmic resistance 
of the developer mixture, thus reducing the flow of current through the 
developer mixture and within the developer generator system comprising the 
developer mixture and the applicator element used for applying the 
developer mixture to the image carrier, such as a magnetic brush or 
roller. Thus, the voltage-dependent resistance behavior of the developer 
mixture, in combination with the ground connection of the backing layer of 
the image carrier, causes the controlling effect. 
It is still another object of the invention to provide an electrostatic 
copy free from fogging or streaking where the surface of the image carrier 
contains defective areas. 
An additional object of the present invention is to provide an 
electrophotographic copier in which additional voltages which may 
interfere with development and are formed at the backing layer of the 
image carrier as a result of simultaneous exposure, development, or 
transfer operations are grounded.

DETAILED DESCRIPTION 
Referring first more particularly to FIG. 1, the current, voltage, and 
resistance conditions in a fundamental developer apparatus are shown. The 
behavior of the system comprising the developer mixture and the image 
carrier for the charge images is not purely ohmic. The developing unit in 
the form of the magnetic brush 10 and a developer mixture 29, having a 
toner and carrier material, acts as an additional current and voltage 
generator 30, as is shown in the fundamental circuit diagram. The carrier 
material of the developer mixture may be magnetite, ferrite, or similar 
materials containing iron. The carrier material may be coated or uncoated, 
and the coating may consist of "Teflon" or an oxide layer. The toner 
materials for the developer mixture consist of polymers, for example 
styrene/methacrylate resins with carbon black or epoxy resins with carbon 
black. With the generator 30 short-circuited and an ohmic leakage 
resistance R.sub.1 of 100 M.OMEGA., the flow of current I.sub.3 depends 
upon two factors: the conductivity of the developer mixture 29 and thus on 
the voltage applied, and the load on the generator, i.e. the generator 
load resistance resulting from the system comprising the image carrier and 
developer mixture. The load resistance is influenced in different ways by 
the conductivity of the developer mixture 29 depending on the nature of 
the surface of the image carrier. An additional factor relating to the 
current and voltage generator 30 is the current produced during 
application of the toner which becomes more or less noticeable during 
development. If the surface of the image carrier is not completely 
insulating, this current is only of minor influence because the galvanic 
current flow prevails, but if the surface of the image carrier is 
definitely insulated, its influence must be considered. 
In the diagram of FIG. 1, the measured values I.sub.1, I.sub.2, and I.sub.3 
and the theoretical value I.sub.0 of the currents shown in the fundamental 
circuit diagram are within the range of from -1 .mu.A to 6 .mu.A, 
depending on the developer voltage U.sub.1 which is plotted along the 
abscissa. I.sub.1 is the current in the line supplying the developer 
voltage U.sub.1 to the magnetic brush 10, I.sub.2 is the current in the 
output of the generator 30 if the leakage resistance R.sub.1 is 100 
M.OMEGA., and I.sub.3 is the current when the generator 30 is 
short-circuited by the low-ohmic ammeter and the leakage resistance 
R.sub.1 has a value of 100 M.OMEGA.. It was determined that the value of 
I.sub.3 is higher than the expected theoretical value I.sub.0 calculated 
from the ratio of the developer voltage U.sub.1 to the leakage resistance 
R.sub.1. 
The displacement of the measured curves of I.sub.1 and I.sub.2 by 0.2 .mu.A 
from zero is caused by the generator 30 which, with a leakage resistance 
R.sub.1 of 100 M.OMEGA., generates a current of 0.2 .mu.A which 
counteracts current I.sub.1 of the developer voltage U.sub.1. If the 
generator 30 is short-circuited, a current of 1.5 .mu.A is measured which 
has the same polarity as current I.sub.1 and is added to the theoretical 
current I.sub.0. For example, if U.sub.1 has values of 100 volts, 200 
volts and 300 volts, respectively, and R.sub.1 =100 M.OMEGA., the current 
I.sub.0 =(U.sub.1 /R.sub.1)=1 .mu.A, 2 .mu.A and 3 .mu.A, respectively, as 
shown by the dotted line I.sub.0. The measured values of the current 
I.sub.3, however, are 2.5 .mu.A, 3.5 .mu.A, 4.5 .mu.A and so on at the 
voltages indicated, which indicates that the current generated by the 
generator increases the theoretical current flow I.sub.0 uniformly by the 
1.5 .mu.A mentioned, so that I.sub.3 =I.sub.0 +1.5 .mu.A. 
The difference between the currents I.sub.1 and I.sub.2 determines the flow 
of current within the generator 30 which is indicated in the diagram by 
the shading between the two measured curves I.sub.1 and I.sub.2 and 
plotted against the axis of the ordinate. 
The product of I.sub.2 and R.sub.1 indicates the voltage drop U.sub.2 at 
the leakage resistance R.sub.1. As already mentioned, FIG. 1 also 
illustrates the generator current I.sub.1 -I.sub.2 as a function of the 
calculated resistance R of the generator 30. The difference U.sub.1 
-U.sub.2 between the applied developer voltage U.sub.1 and the voltage 
drop U.sub.2 at the leakage resistance R.sub.1 yields the voltage drop at 
the generator 30, for which the following equation applies: 
EQU R=(U.sub.1 -U.sub.2)/(I.sub.1 -I.sub.2). 
Substituting values for points A (U.sub.1 =200 volts, U.sub.2 =140 volts, 
I.sub.1 -I.sub.2 =0.4 .mu.A) and B (U.sub.1 =300 volts, U.sub.2 =225 
volts, I.sub.1 -I.sub.2 =0.55 .mu.A) into the above equation, the 
resistance R at points A and B may be calculated equal to 150 M.OMEGA. and 
136 M.OMEGA., respectively. In this manner, the resistance curve of the 
load resistance R of the generator 30 may be plotted. 
FIG. 2 is a diagrammatic representation of the apparatus of a first 
embodiment of the present invention in which the magnetic brush 10 is 
connected with a voltage source 11 by lead 12 to provide the developer 
voltage U.sub.1. The image carrier 13 has a photoconductive layer 14 as 
described, for example, in U.S. Pat. No. 3,363,099, with a latent 
electrostatic charge image thereon, the photoconductive layer being 
positioned adjacent the magnetic brush 10. The backing layer 15 of the 
image carrier is grounded through lead 16 and a high-ohmic leakage 
resistor 17 which preferably has a value between 10 M.OMEGA. and 200 
M.OMEGA.. The potential difference between the magnetic brush 10 and the 
backing layer 15 lies within the range of from 30 to 100 volts. 
Application of the developer voltage U.sub.1 to the magnetic brush 10 
creates an electric field which causes the toner material in the developer 
mixture to migrate from the magnetic brush onto the photoconductive layer 
14 of the image carrier 13. 
If the image carrier has a completely insulating surface such as a 
photoconductive layer, voltage breakdowns within the developer mixture 29, 
which occur during development of the charge images, can not reliably be 
avoided by grounding the backing layer 15. This is due to the fact that an 
insulating photoconductive layer does not allow a galvanic flow of current 
to the backing layer so that the ohmic leakage resistor can not perform 
its control action. In the event of a voltage breakdown, the effective 
developing voltage U.sub.1 is applied directly onto the photoconductive 
layer 14 whereby the danger of puncturing the photoconductive layer is 
created which would result in a voltage drain at the photoconductor and 
ultimately in fogging on the copy. However, even though the high-ohmic 
leakage resistance 17 can not perform its control function, no breakdown 
of the developing voltage U.sub.1 to ground occurs if the photoconductive 
layer 14 is punctured, and, therefore, a black zone is prevented from 
being produced on the copy being developed. Hence, it is possible to 
provide a developed copy free from fogging in spite of a damaged 
photoconductive layer due to a voltage breakdown. 
If the development of the image is performed separately from the other 
process steps, such as charging of the image carrier 13 or image transfer, 
the backing layer 15 is preferably grounded solely by the high-ohmic 
resistor 17. 
FIG. 3 is a diagrammatic representation of the apparatus of a second 
embodiment of the present invention in which several of the aforementioned 
processing steps are performed simultaneously, each of these steps being 
possibly accompanied by a considerable flow of current which may influence 
each other in spite of the high-ohmic ground connection of the backing 
layer. As shown in FIG. 3, a magnetic brush 10 is again connected with a 
voltage source 11 through lead 12 to supply the necessary developer 
voltage of between 100 volts and 800 volts. The voltage source 11 supplies 
a voltage which is variable from 30 volts to 350 volts. An image carrier 
in the form of a cylindrical drum 19 has a photoconductive layer 20 
arranged on its outer peripheral surface and a conducting backing layer 
21. The photoconductive layer on the drum may be a photoconductive double 
layer of organic materials comprising a charge carrier-producing dyestuff 
layer of a compound corresponding to the general formula disclosed in U.S. 
Pat. No. 3,871,882, or it may be a layer containing a condensation product 
of 3-bromopyrene or 3-chloropyrene with formaldehyde or para-formaldehyde, 
as described in U.S. Pat. No. 3,842,038, or a polyvinyl 
carbazole/trinitrofluorenone layer as disclosed in U.S. Pat. No. 
3,484,237. Second and third voltage sources 24 and 25, each of which has a 
voltage range between 3.0 kilovolts and 7.0 kilovolts, are provided having 
first terminals of one polarity connected with the backing layer 21 at a 
terminal 26. The second terminal of opposite polarity of the voltage 
source 24 is connected with a charging corona 22 through lead 27 and the 
second terminal of opposite polarity of the voltage source 25 is connected 
with a transfer corona 23 through lead 28. The charge and transfer coronas 
are arranged adjacent the photoconductive layer 20. Terminal 26 is 
connected with ground through a leakage resistor 17 connected in parallel 
with a semiconductor 18. Preferably, the leakage resistor has a value 
adjustable between 10 M.OMEGA. and 200 M.OMEGA. and the semiconductor is a 
diode. 
Through application of charges to the photoconductive layer 20 by the 
charging corona 22 or the transfer corona 23, current is caused to leak 
from the backing layer 21 of the photoconductive layer 20, creating a 
voltage drop the magnitude of which depends, i.a., upon the intensity of 
the charging current and the value of the leakage resistor 17. The voltage 
created on the backing layer 21 disturbs the uniformity of the virtually 
simultaneous development of the image. The semiconductor unit 18 prevents 
the formation of such undesirable interfering voltages and provides an 
additional leak. The polarity of the diode depends upon the polarity of 
the counter-charge produced. 
The leakage resistor described in conjunction with the first and second 
embodiments is preferably a variable resistor which may be adjusted to the 
most favorable leakage resistance for the specific operating conditions in 
each instance. 
While in accordance with the provisions of the Patent Statutes the 
preferred form and embodiments of the invention have been illustrated and 
described, it will be apparent to those skilled in the art that other 
changes and modifications may be made without deviating from the inventive 
concepts set forth above.