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Description  |
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FIELD OF THE INVENTION
This invention relates to a corona discharge device, and more specifically
to a corona discharge device that can be conveniently used in various
types of electrophotographic process.
DESCRIPTION OF THE PRIOR ART
A corona discharge device having a discharge electrode, an opposing
electrode, a high voltage alternate current source for applying a high
alternate current voltage across the two electrodes is known in the prior
art. The prior art also teaches using a grid placed in the corona
discharge current flow path between the two electrodes and having applied
thereto a fixed bias voltage for use in an electrophotographic process
(for example, U.S. Pat. No. 2,777,957).
Japanese Patent Publication No. 5466/74(particularly FIG. 6-a), which
corresponds to U.S. Pat. No. 3,775,104 discloses a corona discharge device
including a discharge electrode, an opposing electrode and a grid disposed
between the two electrodes with a fixed DC bias voltage of a predetermined
magnitude. This device can be conveniently used for secondary charging in
an electrophotographic process which comprises primarily charging the
surface of a photosensitive laminate by applying a DC corona discharge
thereto, and simultaneously with light and dark imagewise exposure,
secondarily charging it by applying an asymmetric AC corona discharge. The
ratio of the discharge current, which has the same polarity as the primary
charge, to a discharge current having an opposite polarity thereto is
within a predetermined range.
Furthermore, Japanese Laid-Open Patent Publication No. 3747/73 discloses a
corona discharge device including a discharge electrode, an opposing
electrode, and a grid disposed therebetween and directly grounded for use
in secondary charging in an electrophotographic process. The process
comprises charging the surface of a photo-sensitive laminate to the
desired polarity by applying a DC corona discharge thereto, and
simultaneously with light and dark imagewise exposure, subjecting it to
secondary charging to remove charge from the surface of the photosensitive
laminate.
These known corona discharge devices including a discharge electrode, an
opposing electrode and a grid disposed in a corona discharge current flow
path between the two electrodes and either directly grounded or having
applied thereto a fixed bias voltage can be used conveniently in various
electrophotographic processes, as disclosed, for example, in Japanese
Patent Publication No. 5466/74 and Japanese Laid-Open Patent Publication
No. 3747/73. They however, suffer from various defects. For example, they
require a fixed bias voltage source for applying a fixed bias voltage to
the grid, and therefore, the initial manufacturing and operating costs are
relatively high. Since the grid is directly grounded or a fixed bias
voltage is applied to the grid, the action of the grid on the corona
discharge current and the characteristics of the corona discharge current
cannot be freely changed. Moreover, the charging and charge-eliminating
steps in various electrophotographic processes must be performed at higher
speeds in order to obtain latent images or visible images at higher
speeds. The known corona discharge devices described above cannot fully
meet this demand.
SUMMARY OF THE INVENTION
It is an object of the disclosed invention to provide a corona discharge
device which does not require a special voltage source for applying a bias
voltage to control the corona discharge current, and which has low initial
manufacturing and operating costs.
Another object of this invention is to provide a corona discharge device in
which the speeds of charging and charge-elimination are faster than in
conventional corona discharge devices.
Still another object of this invention is to provide a corona discharge
device in which the corona discharge current can be freely controlled.
The present invention provides a corona discharge device comprising a
corona discharge electrode, an opposing electrode disposed opposite to the
corona discharge electrode, a high voltage alternate current source
electrically connected between the two electrodes, and a grid disposed in
a corona discharge current flow path between the two electrodes and
grounded through a nonlinear bias element.
The corona discharge device of the invention does not require a special
bias voltage source, but the nonlinear bias element connected to the grid
forms a self-bias to cause the grid to act favorably on the corona
discharge current. In addition, the action of the grid on the corona
discharge current, and therefore the characteristics of the corona
discharge current itself, can be very easily controlled. This is
accomplished by adjusting the impedance of an AC impedance element, such
as by use of a variable resistance which constitutes part of the nonlinear
bias element. The corona discharge device of the invention can perform
charging and charge-elimination of chargeable surfaces in various
electrophotographic processes at higher speeds than the conventional
corona discharge devices.
Accordingly, the corona discharge device of the invention can be
conveniently used for charging and charge-elimination in various
electrophotographic processes. In particular, it can be conveniently used
as a corona discharge device for secondary charging in the
electrophotographic process disclosed in Japanese Patent Publication No.
5466/74 cited above. Therein charging the surface of a photosensitive
laminate by applying a DC corona discharge of a specified polarity
thereto, and then with simultaneous light and dark imagewise exposure,
performing secondary charging is taught. The secondary charging is
accomplished by applying a nonlinear AC corona discharge in which the
ratio of a discharge current of the same polarity as the primary charging
to a discharge current of the opposite polarity to the primary charging is
within a predetermined range. The corona discharge device of the invention
can also be used conveniently in a charge-eliminating step of various
types of electrophotographic processes which, for facilitating the
subsequent formation of a latent image, eliminates the remaining charge
which has been applied to the surface of a chargeable material by the
previous latent image-forming step. It can also be used conveniently in a
charging step of an electrophotographic process of the electrofax type in
place of the corona discharge devices disclosed in Japanese Patent
Publication No. 9791/65 and Japanese Utility Model Publication No.
20364/65.
The above and other objects of the invention will become apparent from the
following description.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a simplified view of the corona discharge device of this
invention;
FIGS. 2a and 2b are simplified views showing preferred forms of the
nonlinear bias element used in the corona discharge device of this
invention;
FIG. 3 is a simplified view of an electrophotographic apparatus in which
the corona discharge device of the invention is used for secondary
charging and charge elimination;
FIGS. 4a, 4b and 4c and FIGS. 5a and 5b are graphs showing changes in the
surface potential of a chargeable material which are caused by secondary
charging as compared with the position of the discharge electrode;
FIG. 6 is a simplified view of apparatus used in various experiments;
FIGS. 7a, 7b and 8 are diagrams showing changes in the surface potential of
a chargeable material which are caused by secondary charging various
corona discharge devices;
FIG. 9 is a diagram which shows the relation between the current flowing
through the grid of the corona discharge device of the invention and the
surface potential of a chargeable material;
FIGS. 10a, 12a, 13a and 14a are simplified views showing various
embodiments of a discharge electrode and a shield case;
FIGS. 10b, 11, 12b, 13b and 14b are diagrams showing the relation of the
distance X of the embodiments of the discharge electrode and shield case
to the discharge current;
FIG. 15 is a simplified view of a charging device made up of the corona
discharge device of this invention for charging the surface and back of a
chargeable material at the same time; and
FIG. 16 is a modified example of a nonlinear bias element that can be used
in the charging device shown in FIG. 15.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
By reference to FIG. 1, a preferred embodiment of the corona discharge
device of the invention is shown which is suitable for the charging or
charge-elimination of the surface of a chargeable material A. The material
A is composed of a single layer or a laminate comprising a conductive
layer, a photoconductive layer and an insulator layer.
The corona discharge device 2 comprises a discharge electrode 4 made of,
for example, a tungsten filament, a discharge needle, or a metal foil, an
opposing electrode 6 disposed opposite to the discharge electrode, and a
shield case 8. When the shield case 8 is used in the simultaneous step of
secondary charging and exposing, it may be opened at the top. Between the
discharge electrode 4 and the opposing electrode 6 is electrically
connected a high voltage AC source 10 for applying a high AC discharge
voltage across the two electrodes to generate a corona discharge current.
A grid 12 of a conventional type is disposed in a discharge current flow
path between the electrodes 4 and 6 (between the discharge electrode 4 and
the surface of chargeable material A in the drawing). The grid 12 is
connected to the opposing electrode 6 or is grounded through a nonlinear
bias element 14. In the embodiment illustrated, the grid 12 is connected
to a line connecting the opposing electrode 6 to the high voltage AC
source 10, and grounded.
The term "nonlinear bias element," as used in the present application,
denotes a bias element which gives different impedances according to
positive and negative currents, and, for example, can be built by
parallel-connecting an AC impedance element and a rectifier, or by
parallel-connecting an AC impedance element and a constant-voltage
rectifier.
The AC impedance element is an element that can be built of a resistor, a
condenser, and a coil, etc. taken alone or in combination, and is
preferably a variable AC impedance element.
FIGS. 2a and 2b show preferred form of the nonlinear bias element 14. The
nonlinear bias element 14 of FIG. 2a is made by connecting a rectifier 16
and a variable AC impedance element 18 in parallel. The nonlinear bias
element shown in FIG. 2b is constructed by series-connecting a rectifier
16a and a variable AC impedance element 18a, series-connecting a rectifier
16b and a variable AC impedance element 18b, and parallel-connecting the
two series connections. The rectifiers 16a and 16b have opposite
polarities to each other. In the nonlinear bias element 14 shown in FIG.
2a, the variable AC impedance element 18 acts as impedance to either a
positive or negative electric current. Accordingly, by adjusting the
impedance of the AC impedance element, either one of the positive and
negative currents, and therefore the bias voltage, can be selectively
adjusted with respect to to the other. In the nonlinear bias element 14
shown in FIG. 2b, the AC impedance element 18a acts as impedance to either
one of the positive and negative currents, and the AC impedance element
18b acts as impedance to the other current independant of element 18a.
Hence, the positive and negative currents passing through the nonlinear
bias element 14 and, therefore the bias voltage, can be separately and
independently controlled by adjusting the AC impedance elements 18a and
18b.
The corona discharge device 2 described hereinabove can be used as a corona
discharge device for secondary charging in the electrophotographic process
disclosed. For example, in Japanese Patent Publication No. 5466/74 which
basically comprises primarily charging the surface of a photosensitive
laminate by applying a DC corona discharge of a specified polarity
thereto, and then simultaneously with light and dark imagewise exposure,
secondarily charging it by applying an asymmetric AC corona discharge in
which the ratio of a discharge current having the same polarity as the
primary charging to a discharge current having an opposite polarity to the
primary charging is within a specified range. The device 2 can also be
conveniently used for eliminating charge on the surface of the
photosensitive laminate prior to the primary charging.
An electrophotographic apparatus for carrying out the aforementioned
electrophotographic process in which corona discharge devices in
accordance with this invention are used for secondary charging and charge
elimination is described briefly hereinbelow.
A photoconductive layer 22 and a light-transmitting surface insulator layer
24 are provided on a crylindrical electroconductive base plate 20 which is
grounded and acts as an opposing electrode for various corona discharge
devices to form a photosensitive drum. The photosensitive drum is rotated
in the direction of the arrow, and successively arrives at various
treating zones disposed on the periphery of the drum. First, a charge
eliminator 2a composed of the corona discharge device in accordance with
this invention reduces the surface potential of the surface 24 of the drum
to substantially a zero potential. The charge-eliminator 2a will be
described in detail later on. Then, a DC corona discharge device of a
conventional type having a discharge electrode 28 connected to a DC source
26 imparts a positive or negative DC corona discharge to the drum surface.
The drum surface is exposed imagewise by an exposing device 30 disposed
adjacent to the corona discharge device, and simulataneously charged by an
asymmetric AC corona discharge from a secondary charging device 2b
composed of the corona discharge device of this invention. The secondary
charging device 2b will be described in detail later on. Then, the drum
surface is uniformly exposed over the entire surface by a light source 32
whereby a light and dark electrostatic latent image having opposite charge
polarities and opposite potential polarities is formed on the drum
surface. The electrostatic latent image is developed with a toner charged
to the opposite polarity to the electrostatic latent image by means of a
magnetic brush 36 within a developing mechanism 34. The image developed in
this manner is transferred to a transfer sheet 40 in a transfer zone
equipped with a corona discharge device 38 of a known type. The transfer
sheet 40 having a toner image transferred thereto is conducted to a
fixation mechanism 42 where the toner image is fixed. In the meanwhile,
the photosensitive drum is cleaned by a cleaning device 46 equipped with a
toner-removing brush 44, and the next cycle of copying is performed.
The charge-eliminator 2a composed of the corona discharge device in
accordance with the present invention includes a discharge electrode 4a, a
shield case 8a which is grounded, a high voltage AC source 10a connected
to the discharge electrode 4a, and a grid 12a disposed between the
discharge electrode 4a and the surface 24 of the photosensitive drum and
grounded via a nonlinear bias element 14a. Since the charge on the border
between the photosensitive layer 22 and the surface insulator layer 24 is
generally difficult to remove by discharge current alone, it is preferred
that the top of the shield case of the eliminator 2a be opened so that
light from a suitable light source (not shown) may be irradiated onto the
surface of the photosensitive drum, preferably at the central portion of
the charge-eliminator.
Heretofore, an ordinary AC corona discharge device has been used as an
eliminator for eliminating charges on the surface of a photosensitive
drum. But as is well known to those skilled in the art, an AC corona
discharge current generated by an ordinary AC corona discharge device has
a somewhat greater negative current component than its positive current
component, and this accordingly tends to render the surface of the
photosensitive drum somewhat negative after elimination of the charge
thereon by the AC corona discharge. According to the corona discharge
device of this invention, however, the ratio of positive component to
negative component of the discharge current can be controlled as required
by prescribing the impedance of an AC impedance element of the nonlinear
bias element 14a connected to the grid 12a at a suitable value. For
example, the ratio can be adjusted exactly to 1:1, or the positive
component can be made larger than the negative component. The eliminator
2a composed of the corona discharge device of this invention can adjust
the surface potential of the photosensitive drum stably to substantially a
zero potential, or to any other desired value as a result of charge
elimination by the discharge current and the light irradiated onto the
surface of the drum through the top opening of the shield case.
The secondary charging device 2b composed of the corona discharge device of
this invention includes a discharge electrode 4b, a grounded shield case
8b having a top opening for simultaneous imagewise exposure by an exposing
mechanism 30, a high voltage AC source 10b connected to the discharge
electrode 4b, and a grid 12b disposed between the discharge electrode 4b
and the surface of the photosensitive drum and grounded through a
nonlinear bias element 14b.
In the secondary charging device 2b, the actions of the nonlinear bias
element 14b and the grid 12b, without requiring any bias voltage source,
convert the discharge current applied to the drum surface to an asymmetric
AC corona discharge current in which the ratio of a current component
having the same polarity as the primary charging to a current component
having an opposite polarity to the primary charging is within a specified
range. The asymmetry of the asymmetric AC corona discharge current can be
easily adjusted as desired by adjusting the impedance value of an AC
impedance element of the nonlinear bias element 14b.
As another important feature, it has been found that the secondary charging
device 2b can perform uniform charging at a faster charging speed than a
secondary charging device composed of a known corona discharge device
having a grid directly grounded or a grid having a fixed bias voltage
applied thereto by a bias voltage source.
Generally, in an electrostatic photographic process, higher secondary
charging speeds and shorter secondary charging periods result in a higher
contrast potential between the light areas and the dark areas of the
resulting electrostatic latent image, and clear copied images having
superior reproducibility of halftones and fine lines can be obtained. This
is probably because the charge induced by the primary charging in a
boundary layer between the surface insulator layer and the photoconductive
sensitive layer tends to disappear at the time of secondary charging. The
more induced charge which disappears results in the secondary charging
speed being lower and the secondary charging time being longer, which in
turn reduces the contrast potential between the light areas and the dark
areas.
This will be readily apparent from FIGS. 4a, 4b and 4c and FIGS. 5a and 5b.
FIG. 4a shows changes in surface potential versus secondary charging time
in a process which comprises charging the surface of a photosensitive
plate by an ordinary DC corona discharge device (primary charging; DC
source +5KV), then secondarily charging it by the corona discharge device
of this invention, and exposing it entirely at the times indicated by a, b
and c in FIG. 4a. FIG. 4b is a diagram similar to FIG. 4a which shows the
results obtained when the surface of the photosensitive plate is charged
primarily by a DC source of +6 KV; and FIG. 4c is a diagram similar to
FIG. 4a which shows the results obtained when the surface of the
photosensitive plate is charged primarily by a DC source of +7 KV. FIGS.
5a and 5b are similar diagrams obtained by using a DC source of +7 KV for
primary charging and an AC source of voltages, which differ between FIGS.
5a and FIG. 5b, for secondary charging.
It will be understood from FIGS. 4a to 4c that the contrast potential is
the greatest when the primary charging is about 6 KV (the contrast
potential can be considered to correspond to changes in potential after
exposure of the entire surface). If the primary charge is low, the amount
of the charge induced in a boundary between the photosensitive layer and
the surface insulator layer decreases, and therefore, the contrast
potential decreases. On the other hand, the contrast potential decreases
when the primary charge becomes higher. This is probably because the
charge induced at the boundary by the primary charging reaches saturation
when the primary charging voltage is about 7 KV. When the primary charge
becomes high, the rate of charge elimination by apparent secondary
charging decreases, and unless the charge-eliminating time is prolonged,
the surface potential cannot be sufficiently reduced. Furthermore, it will
be readily understood from FIGS. 4a to 4c and FIGS. 5a and 5b that the
contrast potential decreases when the secondary charging
(charge-eliminating) time increases.
Evidently, therefore, the use of the corona discharge device of this
invention having a higher charging (charge-eliminating) speed than the
known corona discharge devices can afford clear copied images having
superior reproducibility of halftones and fine lines.
The corona discharge device of this invention having a grid grounded via a
nonlinear bias element advantageously has a higher charging or
charge-eliminating speed than known corona discharge devices equipped with
a directly grounded grid, or a grid having a fixed bias voltage applied
thereto by a specified bias voltage source. Various experiments were
conducted using a corona discharge device illustrated in FIG. 6 in order
to compare the charging or charge-eliminating speeds of a known corona
discharge device having a directly grounded grid, a known corona discharge
device having a grid with a fixed bias voltage applied thereto by a bias
voltage source, and a corona discharge device in accordance with the
present invention having a grid grounded through a nonlinear bias element
or connected electrically to an opposing electrode. In the corona
discharge device shown in FIG. 6, 50a and 50b represent discharge
electrodes with a diameter of about 0.06 mm, and 52, a grid having a
diameter of about 0.1 mm. The discharge electrodes 50a and 50b and the
grid 52 are fixed to a shield case 54 through an insulator. A represents a
chargeable material, and 56, an opposing electrode. The opposing electrode
56 is grounded through an ammeter for measuring a corona discharge
current. On the other hand, the discharge electrodes 50a and 50b are
connected to a grounded high voltage source 60. The input voltage of the
electric source 60 is adapted to be changed by a slide regulator (not
shown). The grid 52 can be grounded through a nonlinear bias element 66
formed by parallel-connecting a variable resistance 62 having a maximum
value of 6 megohms and a high voltage rectifier 64 by a connection a, or
through a high voltage DC source by a connection b. By connecting the grid
52 to the nonlinear bias element 66 by the connection a, the corona
discharge device of FIG. 6 becomes a corona discharge device in accordance
with the present invention. When the grid 52 is grounded through the
electric source 68 by the connection b, the corona discharge device of
FIG. 6 becomes a conventional corona discharge device having a grid with a
fixed bias voltage applied thereto. When the voltage of the electric
source 68 is reduced to zero, the device becomes a known corona discharge
device having a directly grounded grid.
FIGS. 7a and 7b show changes in the surface potential of a chargeable
material which occurred when charging the material by a secondary charging
corona discharge device of FIG. 6. Grid 52 was grounded through DC bias
source 68 by connection b while maintaining constant the DC source voltage
of primary charging and the AC source voltage of secondary charging and
varying the voltage of the electric source 68 to 0 V (therefore, the grid
was directly grounded), -100V, -200 V, and -300 V, respectively. It will
be appreciated from FIGS. 7a and 7b that the charge-eliminating speed
increases as the fixed bias voltage becomes greater. For example, in FIG.
7a, when the fixed bias voltage is zero, a period of about 5 seconds is
required until the surface potential becomes 0 V. But when a fixed bias
voltage of -300 V is applied, this period becomes about 2.5 seconds, and
the charge-eliminating speed becomes approximately twofold.
FIG. 8 shows changes in surface potential which occurred when performing
primary charging by an ordirary DC corona discharge device having a DC
source of +7 KV. The secondary charging was subsequently performed
(charge-eleimination) by a corona discharge device shown in FIG. 6 in
which a self-bias by a nonlinear bias element 66 is connected to the grid
52 by connection a, and a device shown in FIG. 6 in which a fixed bias by
DC source 68 is connected to the grid 52 by connection b. The resistance
value of the nonlinear bias element was about 6 megohms. The self-bias
voltage has a much higher pulsation factor (50 or 60 Hz) than the fixed
bias, and exhibits a peak voltage of about 225 V when the effective
voltage is 160 V. From FIG. 8, it will be readily understood that the
self-bias gives a higher charge-eliminating speed than the fixed bias. It
will also be appreciated that even when the voltage of the self-bias is
pulse-like, the saturated value of the potential of the surface of the
chargeable material becomes equal to the peak value of that voltage.
As mentioned above, the use of the corona discharge device of this
invention in which the grid is grounded through the nonlinear bias element
gives a higher charging (charge-eliminating) speed than the known corona
discharge devices with the grid directly grounded, or grounded through a
fixed bias source. The reason for this can be ascribed to the following
FIG. 9 shows changes in the current flowing through the grid grounded via
the nonlinear bias element versus the surface potential of the chargeable
material. For example, when the surface potential of the chargeable
material is +1000 V, the positive component of the grid current is about
50 .mu.A and its negative component is about 14 .mu.A at an AC corona
source voltage of 5.6 KV and a self-bias resistance of 3 .times. 10.sup.6
ohms (3 megohms). When the corona discharge is continued, the charge on
the surface of the chargeable material is gradually eliminated, and its
potential is reduced. For example, when the surface potential of the
chargeable material is reduced to +200 V, the positive component of the
grid current becomes about 44 .mu.A, and its negative component increases
to about 30 .mu.A. This increase in the negative component and the
decrease in the positive component naturally mean an increase in the
negative component of the self-bias voltage and a decrease in its positive
component. Consequently, the self-bias voltage by the nonlinear bias
element becomes higher in the direction in which charging
(charge-elimination) proceeds with the advance of charging
(charge-elimination). This is considered to contribute to the increase of
the charging (charge-eliminating) speed.
Generally, the charging of the surface of a chargeable material is
determined by an arithmetic sum of the amount of charge from the discharge
electrode and the amount of charge leaked through the chargeable material.
In other words, the arithmetic sum of the amount of the charge which flies
to the surface of the chargeable material and the amount of the charge
that leaks through the bulk of the chargeable material is the amount of
the charge accumulated on the surface of the chargeable material per unit
time. The potential of the surface of the chargeable material is
determined by the amount of the charge accumulated. Accordingly, in order
to increase the speed of charging or charge-elimination of the surface of
a chargeable material, it is necessary to increase the amount of the
charge which flies to the surface of the chargeable material from the
discharge electrode per unit time. Generally, the current I is expressed
by the equation
I=dQ/dt
wherein Q is the charge, and t is the time. In order, therefore, to
increase the charging or charge-eliminating speed, the corona discharge
current must be increased.
The present inventors have experimentally ascertained that the discharge
current from the discharge electrode is considerably affected by the
relative positions of the discharge electrode and the shield case, the
shape and properties of the shield case, and the diameter of the discharge
electrode, etc. This experiment will be described below.
In Embodiment 10a, a device comprising a discharge electrode 4, a shield
case 8 and an opposing electrode 6 shown in FIG. 10a, changes in discharge
current according to changes in the distance x between the discharge
electrode and the side plate of the shield case 8 were examined. In FIG.
10a, the discharge electrode used was a tungsten filament having a
diameter of about 0.08 mm. The changes in discharge current versus the
changes in distance x are shown in FIG. 10b. In calculating the discharge
current per unit area, 2 x multiplied by l(l being the length of the
discharge electrode) was considered as a discharge area.
It will be understood from FIG. 10b that the maximum discharge current can
be obtained when the distance x is about 10 mm. If the distance x is less
than 10 mm, spark discharge is liable to occur between the discharge
electrode and the shield case. Hence, the distance x is preferably
adjusted to 10 to 15 mm. FIG. 10b refers only to the case of negative
corona discharge. It has been ascertained that in the case of positive
corona discharge, the current becomes maximum when the distance x is in
the vicinity of 10 mm.
In Embodiment 10a above, the distance between the discharge electrode 4 and
the opposing electrode 6 was adjusted to 10 mm. In order to examine the
relation of the distance between the discharge electrode 4 and the
opposing electrode 6 to the distance x, the distance between the discharge
electrode 4 and opposing electrode 6 was adjusted to 18 mm, and the
relation between the distance x and the discharge current was examined.
The results obtained are plotted in FIG. 11.
In FIG. 11, too, the discharge current per unit area is maximum when the
distance x is about 10 mm. Accordingly, there is no close relation between
the distance from the discharge electrode to the opposing electrode, and
the distance x from the discharge electrode to the shield case, and these
distances are considered to define the discharge current independently.
Based on the above information, the bottom ends of both side plates of the
shield case 8 were bent inwardly to form Embodiment 12a as shown in FIG.
12a so that the curved portions became substantially parallel to the
opposing electrode 6. The relation between the length l of the bent
portion and the discharge current was examined. The results obtained are
shown in FIG. 12b.
It is seen from FIG. 12b that the corona discharge current becomes maximum
when the length l is 3 mm. When l is 3 mm, the distance from the discharge
electrode to the end of the curved portion is calculated as follows:
.sqroot.8.sup.2 + 7.sup.2 = 10.6 mm.
Probably, the corona current will become maximum when this distance is 10
mm.
Accordingly, the suitable distance between the shield case and the
discharge electrode is 10 to 15 mm. Furthermore, it can be appreciated
that the discharge current is determined by a conductor which is closest
to the discharge electrode, and conductors situated farther from it do not
contribute to an increase in the discharge current.
Conductors situated in the vicinity of a circumference with a radius of 10
mm from the discharge electrode contribute to the increase of the
discharge current, and conductors located farther are useless. It appears
further that the conductors located outside the 10 mm-radius circumference
act as an absorbent for the discharge current, and reduce the discharge
current. In order to ascertain it, Embodiment 13a was tested wherein the
bottom portions of both side plates of the shield case to a height of x mm
from the bottom were covered with an insulating tape P as shown in FIG.
13a, and the relation between changes in length x and the discharge
current was examined. The result obtained are plotted in FIG. 13b.
It is presumed from FIG. 13b that the discharge current becomes maximum
when x is 16 mm; that is, the length of the insulating coating reaches the
same level as the discharge electrode. That the discharge current
increases as x increases from 0 to 16 mm coincides with the conclusion
obtained from Embodiment 12a above. This shows that the conductors
situated outside the 10 mm-radius circumference act as absorbers of the
discharge current. FIG. 13b gives the results obtained with a discharge
voltage of 6400 V, but the same phenomenon was observed when the discharge
voltage was otherwise.
In Embodiments 12a and 13a, a corona discharge device including one
tungsten filament as a discharge electrode was used. In Embodiment 14a two
tungsten filaments as discharge electrodes as shown in FIG. 14a were
tested. The relation between the distance y between the discharge
electrodes and the distance x between each discharge electrode and the
adjacent side plate of the shield case, to the discharge current, was
examined. The results obtained are shown in FIG. 14b.
In FIG. 14b, the discharge current did not show a peak when the distance x
between the side plate of the shield case and each discharge electrode was
about 10 mm. But as spark discharge tends to occur when this distance is
less than 10 mm, it is appropriate to adjust the distance x to 10 mm - 15
mm. Furthermore, FIG. 14b shows that the distance y between the discharge
electrodes does not exert a great influence on the discharge current.
Hence, the distance y may be suitably prescribed in consideration of other
requirements, for example, the uniformity of discharge.
Now, the discharge electrode itself will be described. Usually, a tungsten
filament having a diameter of not more than 0.1 mm is used as the
discharge electrode. However, since corona discharge is an electric field
emission, the mode of discharge is considered to be affected by the
surface condition of the discharge electrode. Therefore, the following
four types of tungsten filaments were examined.
(1) A drawn tungsten filament as drawn.
(2) A drawn tungsten filament as drawn chemically treated.
(3) A drawn tungsten filament electrolytically polished.
(4) A tungsten filament subjected to gold plating.
It is well known that generally in corona discharge, a positive corona has
good uniformity, but a negative corona has poor uniformity with discharge
points appearing at intervals of 3 to 5 mm. Experiments have shown that
filament (1) has extremely poor uniformity in negative corona; filament
(3) has somewhat good uniformity in negative corona; filament (2) showed
good uniformity in negative corona at a discharge voltage of 8000 to 7200
V; and filament (4) subjected to gold plating showed better uniformity in
negative corona than the other types of tungsten filament. Filament (4),
however, showed deterioration at the gold plated portion at points of even
slight spark discharge, and the current at these points is reduced. The
filament (4) subjected to gold plating, however, is considered as
preferred because it has resistance to oxidation by corona discharge as
long as spark discharge is prevented.
Generally, the discharge current decreases with increasing diameter of the
discharge electrode. For example, if the application of a discharge
voltage of 5600 V to a gold-plated discharge electrode having a diameter
of 0.1 mm produces a discharge current of 40 mA, the application of the
same discharge voltage to a gold-plated discharge electrode having a
diameter of 0.02 mm produces a discharge current of 100 mA. In order to
obtain a discharge current of 100 mA, a discharge voltage of 6400 V may be
applied to a gold-plated discharge electrode having a diameter of 0.06 mm.
Whether to apply a discharge voltage of 5600 V to a discharge electrode
with a diameter of 0.02 mm or a discharge voltage of 6400 V to a discharge
electrode with a diameter of 0.06 mm in order to obtain a discharge
current of 100 mA should be determined in consideration of, for example,
the service life of the discharge electrode, the workability of the
filament in setting up the discharge electrode, and the efficiency of the
voltage applied. In view of these factors, a discharge electrode with a
diameter of about 0.06 mm has been found to be generally preferred. For
example, a discharge electrode with a diameter of 0.02 mm has a surface
area about 1/9 of that with a diameter of 0.06 mm, and therefore, about 9
times as much current flows therethrough per unit area considerably
shortening its service life. On the other hand, with a discharge electrode
having a diameter of 0.1 mm, a considerably higher voltage (for example,
6800 V) must be applied that with a discharge electrode having a diameter
of 0.06 mm in order to obtain the same discharge current (for example, 100
mA), and therefore, the efficiency of the voltage applied becomes poor.
The above are summarized as follows:
(1) The suitable distance (shortest distance) between a discharge electrode
and the side plate of a shield case is 10 to 15 mm, for example, about 12
mm.
(2) Those parts within the shield case which are outside the range of the
above shortest distance are preferably insulation-coated.
(3) The number of discharge electrodes, and the distances between the
electrodes can be properly selected according to the optimum range of
charging.
(4) A gold-plated tungsten filament having a diameter of about 0.06 mm is
most preferred as the discharge electrode.
The corona discharge device of this invention has been described in detail
hereinabove in relation to the electrophotographic process which comprises
primarily charging the surface of a photosensitive laminate by applying a
DC corona discharge of a specified polarity thereto. This is followed by,
simultaneously with light and dark imagewise exposure, secondarily
charging it by applying an asymmetric AC corona discharge in which the
ratio of a discharge current component having the same polarity as the
primary charging to a discharge current component having an opposite
polarity to the primary charging is within a specified range. It should be
understood however that the corona discharge device of the invention can
be conveniently used also in an electrophotographic process of the
electrofax type. For example, the invention can be used as a charging
device for simultaneously charging the surface and back of a laminate
consisting of a back plate and a photoconductive surface layer as desired,
or in place of the charging devices disclosed in Japanese Patent
Publication No. 9791/65 and Japanese Utility Model Publication No.
20364/65.
FIG. 15 illustrates one embodiment of a charging device for simultaneously
charging the surface and back of a chargeable material which is
constructed of two corona discharge devices in accordance with the present
invention. This charging device includes two corona discharge devices 102a
and 102b which respectively have discharge electrodes 104a and 104b which
are electrically connected to each other via a grounded high voltage AC
source 110. In the device shown, therefore, the discharge electrode 104b
of the corona discharge device 102b acts as an opposing electrode of the
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