Board recording apparatus with reduced smudge

A board recording apparatus in which a recording medium (1) in the form of an endless belt moves sequentially between an image formation station (6) and a charge elimination station (14). A charge elimination electrode (16) at the charge elimination station applied voltages via the residual electrically conductive toner (9) to eliminate charges which otherwise cause toner to adhere and produce smudges in subsequent displays.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a board recording apparatus capable of 
displaying an image such as a manuscript on a vertical plane like a 
blackboard. More particularly the invention concerns the elimination of 
smudges caused by previously displayed images on a board recording 
apparatus. 
2. Description of the Related Art 
The following is a description of a previously proposed board recording 
apparatus. As shown in FIG. 6, a board recording apparatus has a recording 
medium 102 in the form of an endless belt looped around a pair of 
vertically displaced rollers 100 and 101. An image formation apparatus 103 
is disposed ad]acent the outer surface of the belt near the lower roller 
101. This image formation apparatus is disclosed in Japanese Patent 
Publication No. 51-46707 (U.S. Pat. No. 3,914,771) and others; and it is 
constructed such that an image is produced by applying voltages 
corresponding to image signals from a recording control portion 105 to a 
recording electrode 104 and depositing toner onto the thus charged surface 
of the recording medium 102. The recording medium 102 on which the image 
is produced, is moved to a display area 106 and displayed. Thereafter, the 
deposited toner 107 is scraped and removed from the recording medium 102 
by a cleaning means 108, to enable a subsequent display on the recording 
medium. 
The above described board recording apparatus has certain problems. As can 
be seen from FIG. 7A, the image recording process carried out in the 
apparatus of FIG. 6 comprises the steps of applying predetermined positive 
voltages to the recording electrode 104, grounding a conductive layer on 
the back of the recording medium 102, and allowing the toner 107 to become 
charged to positive polarity so that it becomes deposited on the recording 
medium 102 as a result of electrostatic forces existing across the 
thickness of an insulating layer on the front of the recording medium. The 
toner 107 deposited on the surface of the insulating layer is thereafter 
scraped and removed by the cleaning means 108. However, if the insulating 
layer is a dielectric which has a high electrical resistivity or time 
constant, then the electrostatic charges from the toner 107 do not leak 
away fully from the insulating layer but instead remain as residual 
charges 111 (FIG. 7B). When the next image is developed in this 
circumstance, as shown in FIG. 7C, there arises a problem of smudge in the 
image on the recording medium 102 because minus charges are induced in the 
toner due to the residual plus charges in the insulating layer 110; and as 
a result, toner with a minus charge is deposited on the recording medium 
102 in addition to the toner deposited with a plus charge according to a 
new image. In order to solve this problem, it has previously been proposed 
to reduce the electrical resistivity of the insulating layer 110. However, 
this gives rise to another problem, namely an image of low density. This 
is because when the resistivity of the insulating layer 110 is low, the 
charges on the toner 107 leak away through the insulating layer 110 and it 
then becomes difficult to deposit toner on that layer. 
SUMMARY OF THE INVENTION 
The present invention is directed to overcoming the above described problem 
of the prior art. Thus, the present invention provides a board recording 
apparatus which can, without reduction of image density, enable a clear 
image to be produced. 
According to the invention there is provided a board recording apparatus 
which comprises a recording medium, an image recording station and a 
charge elimination station. The recording medium has a dielectric layer 
and an electrically conductive layer on one side of the dielectric layer; 
and it is mounted for successive movement between the image recording 
station and the charge elimination station. The image recording station 
comprises recording electrodes arranged to apply voltages between the 
conductive layer of the recording medium and electrically conductive toner 
at selected regions on the other side of the recording medium's dielectric 
layer to produce charges which cause the toner to adhere to the surface of 
the dielectric layer according to images to be recorded. The charge 
elimination station comprises a charge elimination electrode arranged to 
contact toner remaining on the recording medium. Means are also provided 
for applying voltages to the charge elimination electrode of a polarity 
opposite to that applied to the recording electrode. 
In one embodiment, the toner is magnetic and the charge elimination 
electrode is placed on the ends of a plurality of permanent magnets 
arranged in the form of a bar. In another embodiment, the toner is 
magnetic and the charge elimination electrode is in the form of a sleeve 
surrounding an assembly of radially extending permanent magnets. Also, 
either the sleeve or the assembly of magnets may rotate. 
The charge elimination electrode serves to eliminate the residual 
electrostatic charge which tends to cause the toner to adhere to the 
recording medium.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The board recording apparatus of FIG. 1 comprises a recording medium 1 
formed as an endless belt looped around a pair of rollers 2 and 3 which 
are disposed at the upper and lower ends, respectively, of an outer body 
30 of the apparatus. The recording medium 1 is supported by the rollers 2 
and 3 so as to move around them and pass through a flat vertical display 
plane at a display station 4. The recording medium 1 is driven in the 
direction of the arrows A (in FIG. 1) during image formation. At the 
lowest position of rotating path of the recording medium 1, that is the 
position adjacent the roller 3, there is provided an image recording 
station 6. This station records an image by applying voltages through 
electrically conductive toner 9 to the recording medium 1 at selected 
locations on the medium corresponding to the image. The toner 9, which is 
contained in a supply container 5 thus adheres electrostatically to the 
recording medium 1 in the image areas. 
The foregoing image production is carried out according to a method called 
"magnestylus" described in Japanese Patent Publication No. 51-46707 and 
others. The principle of this method will now be described. As shown in 
FIG. 2, a cylindrical magnet 8, having alternate north and south poles 
about its circumference, rotates in a non-magnetic hollow cylinder 7. 
Electrically conductive magnetic toner 9, pigmented in black or some other 
color, is carried on the non-magnetic cylinder 7 and then passes onto 
recording electrodes 10, made of electrically conductive material in the 
form of styli whch are arranged closely to one another along the 
non-magnetic cylinder 7 in the direction of its axis. Voltages are applied 
according to an image pattern between the recording electrodes 10 and a 
conductive layer 12 on the back side of the recording medium 1. The 
recording medium 1 also has an insulating layer 11 on its front side. The 
image is produced or developed by depositing the toner 9 onto the 
recording medium 1 so that adheres to the medium only in those regions 
that have an electrostatic charge, i.e. only on those regions of the 
recording medium 7 to which voltages from the recording electrodes 10 have 
been applied. The image information to be displayed is applied as electric 
signals from a recording control portion 13 (FIG. 1) to the recording 
electrodes 10. 
On the side of the recording medium 1 opposite the display station 4 there 
is provided a charge elimination station including removing means 14 for 
removing toner 9 deposited on the surface of the recording medium 1. This 
removing means 14 has a plurality of permanent magnets 15 in the form of a 
bar and arranged with their polar axes perpendicular to the plane of the 
recording medium 1 and with adjacent alternate north and south poles in 
the direction of movement of the medium. On the end of the magnets 15 
adjacent to and facing the recording medium, there is provided a plate 
like charge elimination electrode 16. The distance between the electrode 
16 and the recording medium 1 is 50 microns to 5 millimeters, preferably 
200 microns to 3 millimeters. This voltage applied to the electrode 16 has 
a polarity opposite to that applied to the recording electrodes 10. 
The above described board recording apparatus according to the present 
invention displays a first image and then may add and display a further 
image. Thereafter the apparatus cleans off the first image or images, and 
then adds a further image, and so on. While the recording medium 1 moves 
at a predetermined velocity, recording voltages V.sub.R of +30.about.+200 
V in accordance with image information are applied between the recording 
electrodes 10 and the conductive layer 12 of the recording medium 1, 
whereby minus charges are induced in the dielectric layer 11; and, as 
shown in FIG. 3A, toner 9 is deposited on the surface of the recording 
medium 1 to develop the image at the image producing station 6. The 
recording medium 1 then moves to the display station 4 where the image is 
displayed. After display, the image formed on the recording medium 1 moves 
to the charge elimination station where the image is eliminated by having 
the recording medium 1 on which toner 9 is deposited move past the 
eliminating means 14 and by scraping the toner 9 from the surface of the 
recording medium 1 by means of a toner brush 17 formed on the surface of 
the electrode 16 of the eliminating means 14. At the same time, a charge 
elimination voltage V.sub.E' of a polarity opposite to that of the 
recording electrodes 10, is applied to the electrode 16 of the eliminating 
means 14. This voltage applied to the electrode 16 is in the range of 
-100.ltoreq.V.ltoreq.0, preferably -30.ltoreq.V.ltoreq.-5. This causes the 
toner on the electrode 16 of the eliminating means 14 to become charged to 
minus polarity and also causes plus charges to be induced in the 
dielectric layer 11 as shown in FIG. 3B. Thus, in the dielectric layer 11 
of the recording medium 1, plus charges produced at the time of 
elimination neutralize the minus charges which were produced at the time 
of development. Consequently, after this charge elimination, no charge 
remains in the dielectric layer 11 as shown in FIG. 3C; and, at the next 
development stage, new toner 9 is applied to the recording medium 1 only 
according to the voltages applied to the recording electrodes 10 as shown 
in FIG. 3D. Therefore, smudges in the image caused by deposition of excess 
toner are prevented and a clear image can be obtained. 
In the graph of FIG. 4 the recording voltage V.sub.R applied to the 
recording electrodes 10 (shown on the horizontal axis) is plotted against 
the charge elimination voltage V.sub.E applied to the charge elimination 
electrode 16 (shown on the vertical axis). In this graph, areas A and B 
represent the ranges of recording and charge elimination voltages which 
permit smudge reduction; and area B represents the preferred ranges. Area 
C represents the condition where the reverse voltages applied to the 
charge elimination electrode 16 are high compared to the voltage applied 
to the recording electrodes 10. In this case the toner becomes adhered to 
the recording medium by a reverse electrostatic charge and the smudging 
problem is not overcome. The area D represents the other extreme where the 
elimination electrode 16 is insufficient to overcome the electrostatic 
attraction of the toner to the recording medium. In this case also the 
smudge problem is not overcome. By way of example when a recording voltage 
V.sub.R of plus 100 volts is applied to the recording electrodes 10, the 
charge elimination voltage V.sub.E on the elimination electrode 16 which 
will elimination smudge is -47=.ltoreq.V.sub.E .ltoreq.=0, more preferably 
-18.ltoreq.V.sub.E .ltoreq.2. Insufficient smudge elimination arises at 
the voltage of -2&lt;V.sub.E and toner deposition due to charge to opposite 
polarity appears at the voltage of V.sub.E &lt;-47. 
As can be seen from the graph of FIG. 4 the charge elimination voltage 
V.sub.E is of a polarity opposite to the recording voltage V.sub.R and has 
a value within a range whose first limit varies in a straight line from 
zero to five volts and whose other limit varies in a straight line from 
ten to one hundred volts as the recording voltage V.sub.R varies from 
thirty to two hundred volts (areas A and B). Also, it is preferred that 
the other limit of the range of the charge elimination voltage V.sub.E 
varies in a straight line from ten to thirty volts as the recording 
voltage V.sub.R varies from thirty to two hundreds volts (area B). 
FIG. 5 shows a further embodiment of the present invention. Portions of 
this embodiment which correspond to those of FIG. 1 are indicated by the 
same reference numerals. In the embodiment of FIG. 5, the charge 
eliminating means is constructed differently from the charge eliminating 
means in the previous embodiment. As shown in FIG. 5, a charge elimination 
electrode 16' is provided in the form of a cylindrical sleeve within which 
an assembly of radially extending permanent magnets 15 are disposed. When 
either the sleeve electrode 16' or the magnet assembly 15 is made to 
rotate, a toner brush produced on the surface of the sleeve elctrode 16' 
revolves. This improves efficiency of the charge elimination. A voltage 
having a polarity opposite to that of the recording electrodes 10 is 
applied to the sleeve electrode 16'. The remaining portions of the 
embodiment of FIG. 5 are the same as those in the embodiment of FIG. 1 and 
therefore will out be further described. 
The volume resistivity of the dielectric layer 1 of the recording medium 7 
used in the foregoing embodiments is preferably 10.sup.7 .about.10.sup.17 
ohm centimeters (.sup..OMEGA. cm). 
In order to protect the surface of the recording medium from mechanical 
scratches or stains the recording medium may have a protective layer 
covering its surface. 
In the above described embodiments, plus voltages are applied to the 
recording electrodes and minus voltages are applied to the electrode 16, 
16' of the eliminating means; however, reverse polarities may be used as 
well. For example, voltages of -30 to -200 volts may be applied to the 
recording electrodes and a voltage of 0&lt;V.sub.E .ltoreq.+100, preferably 
+5.ltoreq.V.sub.E .ltoreq.=30, may be applied to the elimination 
electrodes 16, 16'. In this case, the toner adhering to the recording 
medium becomes charged negatively. 
A description will be made hereinunder as to still another embodiment which 
employs a different type of the voltage applied to the charge elimination 
electrode 16 of the removing means. 
In the embodiments described before, the voltage applied to the charge 
elimination electrode has a polarity which is opposite to the polarity of 
the signal voltage applied to the recording electrode. This, however, is 
not exclusive and the invention may be carried out by applying an 
alternating voltage to the charge elimination electrode. 
Namely, in this embodiment, a voltage having an alternating component 
formed by superposing an A.C. voltage to a D.C. voltage is applied to the 
charge elimination electrode 16 of the removing means 14. It is even 
possible to apply an A.C. voltage having no D.C. component to the charge 
elimination electrode. In each case, the A.C. voltage may have a voltage 
V.sub.p-p of 10 to 100 V and a frequency f of 500 Hz to 5 KHz. The D.C. 
component, when employed, may be formed by applying a D.C. voltage having 
a voltage V.sub.Dc of +20 to -20 V. 
FIG. 9 shows the waveform of a voltage applied to the charge elimination 
electrode 16 of the removing means. This voltage has a sine waveform. Peak 
values of this voltage on the plus (+) side and on the minus (-) side are 
rspectively represented by V.sub.p + and V.sub.p -. Thus, the voltage is 
expressed by V.sub.p-p =V.sub.p +-V.sub.p -. When the recording voltage on 
the recording medium is a positive (plus) voltage, the control is 
conducted such that the voltage V.sub.p - (V.sub.p -&lt;0) is applied to the 
charge elimination electrode and such that the voltage is held in the 
negative level for a period which is not shorter than a quarter (1/4) of 
the period of the A.C. voltage. 
In this board recording apparatus, the charge elimination operation is 
conducted in a manner which will be explained hereinunder. As will be seen 
from FIG. 3a, the particles of toner 9 which have been charged in positive 
polarity during developing attach to the surface of the recording medium 1 
in such a manner as to induce negative charges in the conductive layer 12 
of the recording medium 1. During the charge elimination operation, the 
particles of the toner on the recording medium 1 are removed as they are 
rubbed by the brush of the toner particles formed on the surface of the 
electrode 16 of the removing means 16. During the charge elimination 
operation, the electrode 14 is supplied with a voltage of a polarity 
opposite to that of the recording voltage, so that negative charges are 
injected into the particles of the toner 9 on the charge elimination 
electrode 16, while. inducing positive charges in the conductive layer 12 
of the recording medium 1, as shown in FIG. 3B. In consequence, the 
positive charges produced during the development and the negative charges 
produced during the charge eliminating operation are negated by each other 
within the electrically conductive layer 12 of the recording medium 1, so 
that no residual charge is left in the conductive layer 12 of the 
recording medium 1 after the charge eliminating operation, as shown in 
FIG. 3C. Therefore, in the next image forming operation, the electrostatic 
charges attach only to the area which has been electrostatically charged 
in the new cycle of image forming operation, because there is no residual 
charge which would attract the toner particles onto the recording medium. 
Thus, a voltage V which ranges between 0 and V.sub.p -, having the polarity 
opposite to that of the recording voltage, is applied to the charge 
elimination electrode 16 of the removing means 14. Therefore, in the event 
of a change in the resistance values of the toner and the recording medium 
due to, for example, a change in the environmental condition, such a 
change in the resistance is absorbed by the value of the A.C. voltage, so 
that it is not necessary to change the voltage applied to the electrode 16 
of the removing means 14. 
It is assumed here that the recording medium exhibits a resistance value of 
10.sup.8 .OMEGA.-cm, while the toner exhibits a volumetric resistance 
value of 10.sup.6 .OMEGA.-cm, when the humidity is comparatively high, and 
that the resistance values are increased, respectively, to 10.sup.10 
.OMEGA.-cm and 10.sup.6 .OMEGA.-cm, as the humidity becomes low. In such a 
case, it is possible to eliminate charges satisfactorily without requiring 
any change in the voltage applied to the charge elimination electrode, if 
a voltage of a frequency f=1 KHz and having a D.C. component of V.sub.DC 
=-10 V and an A.C. component of V.sub.p-p =50 V is applied to the charge 
eliminatiOn electrode 16. 
The charge elimination electrode of the removing means is supplied with 
positive voltage for a period which is not longer than 3/4.times.T. When 
the frequency of the A.C. voltage is not lower than 500 Hz, the exchange 
of charges is effected rather than the application of the positive charges 
to the toner, so that the charges in the electrically conductive layer of 
the recording medium are eliminated, so that the recording medium becomes 
ready for the next recording operation without holding any residual 
charge. In contrast, when the frequency of the A.C. voltage is lower than 
500 Hz, the charge elimination electrode 16 of the removing means 14 may 
apply positive charges to the toner, depending on condition such as a 
large positive value of the voltage V.sub.p. In such a case, the charges 
in the electrically conductive layer in the recording medium cannot be 
eliminated completely, so that a substantially regular unevenness of toner 
density is caused in synchronization with the frequency of the A.C 
voltage. 
Although the recording voltage is assumed to be a positive voltage in the 
foregoing description of the embodiment, it will be clear to those skilled 
in the art that this embodiment can also be applied to the case where a 
negative voltage is used as the recording voltage. 
It will also be understood that this embodiment, which employs an 
alternating voltage with or without a D.C. component, can be used in the 
embodiment shown in FIG. 5 in which a brush of toner moves. 
In this embodiment, the charge elimination electrode is supplied with an 
alternating voltage so that, even when the factors such as resistance 
value of the recording medium is varied due to a change in the 
environmental condition, such a variation is effectively absorbed by the 
amplitude of the alternating current thereby enabling residual charges to 
be eliminated completely. 
A description will be made hereinunder as to the principle of charge 
elimination means in more detail, with specific reference to FIGS. 10A to 
10C. Referring to these Figures, a recording medium 1 is adapted to move 
in the direction shown by an arrow. The charge elimination means generally 
includes magnets 15, a non-magnetic backing plate 18, and a charge 
elimination electrode 16 is grounded and made of a non-magnetic metallic 
plate. In this embodiment, the charge elimination electrode 16, which is 
constituted by the grounded non-magnetic metallic plate, is disposed such 
that a gap of 0.3 mm to 1.2 mm is formed between itself and the recording 
medium 1. The magnets 15 are arranged such as to develop a magnetic flux 
of a density on the order of 500 to 1000 Gauss. 
Referring to FIG. 10A, the gap between the recording medium 1 and the 
electrode 16 is stuffed with particles of toner which are held by magnetic 
force. As the recording medium 1 moves from the upper side to the lower 
side as indicated by the arrow, the toner particles held on the recording 
medium and forming an image are rubbed by the toner particles with which 
the gap between the recording medium 1 and the charge elimination 
electrode 1 is stuffed so that they are scraped off the recording medium. 
Meanwhile, the electric charges on the recording medium 1 leak through the 
toner particles which have low electric resistance and then through the 
grounded charge elimination electrode 16 made of non-magnetic material, so 
that there is no residual charge on the recording medium coming out the 
charge elimination station. Although the gap between the recording medium 
and the non-magnetic electrode of the charge elimination means is stuffed 
with a large quantity of toner particles, the recording medium 1 is always 
kept at a constant distance from the charge elimination electrode 16 
because it is supported by the backing plate 18. 
The charge eliminating operation is conducted in the state shown in FIG. 
10B. As the recording medium further moves from the upper side to the 
lower side, the toner particles with which the gap between the recording 
medium and the charge elimination electrode is stuffed are progressively 
moved downward by the friction between these toner particles and the 
recording medium. In the region surrounded by a broken line in FIG. 10B, 
however, the toner particles stick to one another by the magnetic force 
and are arrested on the charge elimination electrode due to the magnetic 
force produced by the magnets 15. 
As the recording medium is further moved downward, the toner particles 
forming the image on the recording medium are successively separated from 
the recording medium as they are attracted by the magnetic force so that 
the amount of the toner particles exceed a predetermined capacity which is 
determined by the distance between the recording medium 1 and the charge 
elimination electrode 16. In consequence, the toner particles which have 
stayed in the lower region of the space between the recording medium 1 and 
the charge elimination electrode 16 in the state shown in FIG. 10A are 
forced out this region by the toner particles which have been separated 
from the recording medium and charged into the upper region of the space 
between the recording medium 1 and the charge elimination electrode, so 
that these toner particles are allowed to fall freely by the force of 
gravity. It will be understood that, in the region out of the broken line, 
the force of the gravity is more influential than the magnetic attracting 
force, so that the toner particles can drop freely. The thus freed toner 
particles fall onto the non-magnetic cylinder 7 (see FIG. 2) or onto the 
recording electrode 10 so as to be used again for the purpose of 
recording. 
It is thus possible to eliminate charges on the recording medium by a very 
simple arrangement. 
In the described arrangement for eliminating charges, it is possible to 
prevent the falling toner particles from scattering by designing the 
apparatus such that the distance between the charge elimination station 
and the recording electrode 10 or the non-magnetic cylinder 7 is 
sufficiently reduced. It is also possible to provide an anti-scattering 
member 19 in the path along which the toner particles fall, thereby 
preventing the toner particles from scattering (see FIG. 11). 
When the dielectric layer of the recording medium 1 exhibits a high 
resistance, there is a risk that a ghost image is formed undesirably due 
to insufficient leak of the residual charge. This problem, however, can be 
overcome by applying to the charge elimination electrode 16 a voltage of a 
polarity opposite to that of the recording voltage. 
FIG. 12 is a sectional view showing a different embodiment. In this 
embodiment, the magnets 15 are disposed behind the backing plate 21. The 
particles of the toner 9 rub the recording medium 1 while they are 
attracted by the magnetic force produced by the magnets 15, so that a high 
charge elimination effect is attained. In this embodiment, since the toner 
particles attracted by the magnet 15 tend to be moved together with the 
recording medium 1, the lower end portion of an opposing plate 20 is bent 
slightly towards the recording medium 1 so as to be able to retain the 
toner particles. The illustrated configuration of the opposing plate 20 is 
only illustrative and may be varied in various forms depending on various 
factors such as the nature of the surface of the recording medium 1 and 
arrangement of the magnets 15. The opposing plate 20 may be grounded or 
may be supplied with a bias voltage as required. The portion of the 
backing plate 21 which contacts the recording medium 1 is preferably 
treated so as to reduce the friction between the backing plate 21 and the 
recording medium 1, e.g., coating with teflon resin. 
In the foregoing illustrative embodiments the invention is used in a 
display apparatus. However, it will be apparent to those skilled in the 
art that the invention can also be applied to a device wherein the toner 
image produced on the recording medium is transferred to a transfer 
material with or without displaying. 
According to the foregoing structure and function of the present invention, 
since it is not necessary to reduce the resistivity of the dielectric 
layer of the recording medium, a high density image can be maintained and 
yet complete image elimination after display or transfer can be achieved 
so that subsequent images will be clear and free of smudges.