Image forming apparatus

An image forming apparatus includes means for supplying a developing agent between electrically independent recording electrodes and a recording medium which is movable relative to the recording electrodes while a signal voltage is applied to the recording electrodes so as to form images. In order to prevent electric charges from being induced in the recording medium due to the electric charges accumulated in the vicinity of the recording medium, and thereby prevent unnecessary toner from being attached to the recording medium, the surface of a resin member which opposes the recording medium fulfills a charging preventing function.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an image forming apparatus, and more 
particularly, to an image forming apparatus, such as a printer or a 
display apparatus, in which a developing agent, such as a toner, is 
supplied between a plurality of recording electrodes and a recording 
medium while a signal voltage is applied to the recording electrodes so as 
to attach the toner to the recording medium. 
2. Description of the Related Art 
The aforementioned type of conventional image forming apparatus employs the 
image forming method disclosed in Japanese Patent Publication No. Sho 
51-46707 (which corresponds to U.S. Pat. No. 3,914,771), and in U.S. Pat. 
Nos. 4,831,394, 4,910,538, 4,943,819 and 5,001,501. FIG. 2 shows an image 
forming apparatus which employs the above method and which is available on 
the market. 
In FIG. 2, reference numeral 1 denotes recording electrodes; 2, a 
conductive magnetic toner; 3, a recording medium consisting of a surface 
layer 3a, a resistance layer 3b, a conductive layer 3c and a support layer 
3d; 4, a plastic member; 5, a control circuit substrate; 6, a cleaning 
brush; 7, a rotary magnet; 8, a non-magnetic cylinder; and 9, a recording 
medium conveying roller. In the apparatus shown in FIG. 2, the conductive 
magnetic toner 2, which is a coloring substance, is conveyed over the 
non-magnetic cylinder 8 by the rotation of the rotary magnet 7, and is 
thereby supplied to a position overlying the recording electrodes 1. An 
electric charge (10.sup.-8 to 10.sup.-7 coulomb) is induced in the toner 2 
by the application of a voltage corresponding to an image signal generated 
by the circuit substrate 5 between the conductive layer 3c of the 
recording medium 3 and the recording electrodes 1, by which the toner 2 is 
electrostatically attached to the recording medium 3 (1000 to 10000 
pF/cm.sup.2) and an image is thereby formed. The endless belt-like 
recording medium on which the image has been formed makes one revolution, 
during which the cleaning brush 6, which utilizes volume resistance, 
scrapes the toner on the recording medium over the recording electrodes 1 
located just below the brush 6 so that the toner can be used again. 
In the aforementioned conventional image forming apparatus, a plastic 
forming resin is used to reduce the weight of the apparatus and production 
cost, and the member made of this plastic forming resin is provided near 
the surface of the recording medium (at a distance of 2 mm to 19 mm) in 
order to achieve reduction in the size of the apparatus. This approach 
causes the following problems. 
Generally, plastic members are readily charged unless a special measure is 
taken to prevent charging. Normally, the plastic member has a surface 
potential between .+-. several tens of volts and .+-. several hundreds of 
volts. External influences, such as rubbing, can increase this potential 
to .+-. several kilo-volts. 
The conventional image forming apparatus shown in FIG. 2 is characterized 
by requiring a recording voltage which is an order of magnitude less than 
that required by another recording means which employs a toner, such as an 
electrophotographic recording apparatus, i.e., the apparatus of FIG. 2 
requires a voltage of several tens of volts. Therefore, the surface 
potential of the recording member must be more strictly controlled so that 
the recording medium has a desired potential over the entire surface 
thereof uniformly. 
However, the use of the charged plastic member near the surface of the 
recording medium causes distribution of the potential on the surface of 
the recording medium to vary with time. This change in the distribution of 
the potential affects images adversely. The mechanism of generating 
changes in the potential distribution is estimated as follows. 
FIG. 3 schematically illustrates the effect of an electric field generated 
between the recording medium 3 and the plastic member 4. The negative 
charge in the plastic member 4 generates an electric field between the 
plastic member 4 and the grounded conductive layer 3c in the recording 
medium in the direction indicated by the longer arrows. The generated 
electric field induces positive charges in the conductive layer 3c, and 
the induced positive charges move through the resistance layer toward the 
surface layer over a finite time in the manner indicated by the shorter 
arrows and are trapped in the interface between the resistance layer and 
the surface layer 3a. The trapped positive charges are not easily 
cancelled: they induce negative charges in the electrodes when 0 volt 
(representative of a white image signal) is applied to the recording 
electrodes during a subsequent recording, by which Coulomb's attractive 
force is generated and toner, which would not be otherwise attached, is 
attached to the recording medium. Consequently, unnecessary toner is 
attached to the white area of the recording medium where no toner image 
should be formed, and a phenomenon which deteriorates the image quality 
(hereinafter referred to as fog), thus occurs. 
SUMMARY OF THE INVENTION 
A primary object of the present invention is to provide an image forming 
apparatus which is capable of preventing application of an unnecessary 
electric field to a recording medium from a plastic member close to the 
recording medium, so as to ensure recording of images of high quality. 
To achieve this object, the present invention provides an image forming 
apparatus which comprises electrically independent recording electrodes, a 
recording medium which can be moved relative to the recording electrodes, 
developing agent supply means for supplying a conductive developing agent 
between the recording electrodes and the recording medium, means for 
applying a signal voltage to the recording electrodes, and a resin member 
which opposes the recording medium. The resin member has a charging 
preventing function. 
These and other objects, features and advantages of the present invention 
will be more fully understood from a consideration of the following 
detailed description of the preferred embodiments and the accompanying 
drawings, in which like reference characters indicate like elements 
throughout.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The preferred embodiments of the present invention will be described below 
with reference to the accompanying drawings. 
FIG. 1 illustrates a first embodiment of an image forming apparatus 
according to the present invention. As mentioned, in FIG. 1, the members 
which are the same as those shown in FIG. 2 are denoted by the same 
reference numerals. In the recording medium 3, the surface layer 3a is 
made of a 1 to 20 .mu.m thick plastic forming resin having a volume 
resistance of 10.sup.7 to 10.sup.16 .OMEGA.cm, such as a butyral resin, an 
acrylic resin, a nylon resin, a polyester resin, urethane resin or phenol 
resin or a combination thereof. The resistance layer 3b has a volume 
resistance of 10.sup.0 to 10.sup.7 .OMEGA.cm and a thickness of 5 to 30 
.mu.m, and is made of a plastic forming resin in which titanium oxide, 
aluminum oxide, tin oxide or indium oxide (or a combination thereof) is 
disperse. The conductive layer 3c is a 5 to 30 .mu.m thick deposited 
conductive film having a volume resistance of 10.sup.0 to 10.sup.2 
.OMEGA.cm and made of, for example, aluminum. The support layer 3d is made 
of a 70 to 300 .mu.m thick plastic forming resin, such as polyethylene 
terephthalate resin or polyimide resin or both. 
The conductive magnetic toner 2 is manufactured first by internally adding 
magnetic powder to a plastic forming resin and then by externally adding 
carbon to the plastic forming resin. The electric resistance of the 
conductive magnetic toner 2 in the direction in which toner particles are 
coupled to each other in a chain by the magnetic brush method is 10.sup.3 
to 10.sup.8 .OMEGA.cm. 
In this embodiment, a conductive member 10 which is a metal plate or a 
conductive tape is fixed to the surface of the plastic member 4 using, for 
example, an adhesive. The conductive member 10 is grounded through a lead 
using, for example, soldering. 
Consequently, no matter how the plastic member 4 is charged, the surface 
potential of the plastic member 4 which opposes the surface of the 
recording medium 3 is maintained at 0 volt, and application of an 
undesired electric field to the recording medium 3 is thus avoided. As a 
result, generation of fog is prevented. 
FIG. 4 illustrates a second embodiment of the image forming apparatus 
according to the present invention. In this embodiment, the recording 
medium 3 and the toner 2 are the same as those used in the first 
embodiment. 
In this embodiment, the surface of the plastic member 4 is coated with a 
coating 10' which may be a conductive coating material capable of reducing 
the surface resistance to 10.sup.8 .OMEGA.cm or less, or a known 
antistatic agent capable of suppressing generation of charging. 
Consequently, no matter how the plastic member 4 is charged, the surface of 
the plastic member 4 which opposes the surface of the recording medium 3 
is not readily charged and the surface potential thereof is thus 
maintained substantially at 0 volt. As a result, application of an 
undesirable electric field to the recording medium 3 can be avoided, and 
generation of fog can be prevented. 
FIG. 5 illustrates a third embodiment of the image forming apparatus 
according to the present invention. The recording medium 3 and the toner 2 
which are used in this embodiment are the same as those used in the first 
embodiment. 
In this embodiment, a conductive plastic member 4' having a surface 
resistance of 10.sup.8 .OMEGA.cm or less replaces the conventional plastic 
member 4 in order to suppress generation of charging. 
Consequently, the surface of the plastic member 4' is not readily charged 
and the surface potential thereof is thus maintained substantially at 0 
volt. Therefore, application of an undesirable electric field to the 
recording medium 3 can be avoided, and generation of fog can be prevented. 
FIGS. 6 through 9 illustrate a fourth embodiment of an image display 
apparatus according to the present invention. This image display apparatus 
employs a rigid substrate in place of the flexible substrate used in the 
aforementioned embodiments as the substrate on which the recording 
electrodes are formed. That is, the image display apparatus employs a 
recording head 15 shown in FIG. 6 which includes a rigid substrate 11 made 
of an insulating resin, the recording electrodes 1 formed on the substrate 
11 in the same manner as that of the first embodiment, voltage application 
drive elements 13 fabricated on the substrates, bonding wires 12 for 
performing electric connection of the voltage application drive elements 
13, and a sealing resin 14 for protecting the drive elements 13 and the 
bonding wires 12. Such a recording head 15 is fixed to the non-magnetic 
cylinder 8 which is the same as that used in the first embodiment by means 
of a fixing base 16. 
In the image display apparatus arranged in the manner described above, 
since the recording head is located close to the recording medium 3, when 
the sealing resin 14 is charged, fog may be generated for the reason 
mentioned in connection with the conventional apparatus. 
Hence, a conductive member 10" may be adhered to the surface of the sealing 
resin 14 and be grounded through a lead in the same manner as that in the 
first embodiment, as shown in FIG. 8. Alternatively, a coating material 
may be coated on the surface of the sealing resin 14 to prevent charging, 
as in the case of the second embodiment, or the resin itself is made 
conductive, as in the case of the third embodiment. In the last case, 
since an excessive drop in the resistance can create a short circuit 
between the electrodes of the drive elements 13, the resistance of the 
conductive resin must be set between 10.sup.3 and 10.sup.8 .OMEGA.cm. 
In order to prevent the effect of a charged sealing resin, a protecting 
cover 17 may be provided over the sealing resin 14, as shown in FIG. 9. 
This protective cover 17 may be made of a metal or conductive resin or may 
be processed in the same manner as in the first and second embodiments. 
FIG. 10 illustrates a fifth embodiment of an image forming apparatus 
according to the present invention. In this apparatus, a body frame 19 
which surrounds the recording medium 3 is made of a resin. In order to 
prevent charging of the inner wall of the body frame 19, the inner wall is 
processed in the same manner as that of the first, second or third 
embodiment. A panel 20 for covering an opening 21 of a display portion is 
also made of a resin. The panel 20 may be processed in the same manner as 
in the first embodiment using a transparent conductive film, such as an 
ITO film. Alternatively, the panel 20 may be processed in the same manner 
as that of the second embodiment, using a transparent coating material. 
The recording member and toner used in the fourth and fifth embodiments are 
the same as those used in the first embodiment. 
The device shown in FIG. 10 can replace the recording electrodes shown in 
FIG. 1, the necessary electric field being provided by means of electrodes 
provided in unit 18. 
As will be understood from the foregoing description, in the image forming 
apparatus of the present invention, production of electric charges in the 
surface of the plastic member located close to the recording medium can be 
suppressed by processing the plastic member in any of the aforementioned 
manners, and the potential of the surface of the plastic member which 
opposes the recording medium can thus be reduced. Consequently, 
application of an unnecessary electric field to the recording member can 
be prevented, and generation of fog due to the effect of such an electric 
field can be eliminated. 
While the present invention has been explained in detail by reference to 
the preferred embodiments thereof, those of ordinary skill in the art will 
appreciate that many modifications and variations of those illustrative 
embodiments are possible within the scope of the present invention, which 
is accordingly not to be limited by the particular details described 
herein.