Note: Descriptions are shown in the official language in which they were submitted.
~2359~3
The present invention relates to an image-forming
metl-lod in a laser color printer, colored image copying
machine, or the like and, more particularly, to a method
of forming images by superimposing a plurality of toner
images on an image retainer.
Copying machines have been known in which a
colored original picture is illuminated with light, and
the resultant light is decomposed into several components
by color filters. The light components are then caused to
fall on an electrically charged image retainer to form an
electrostatic image, which is then developed by applying
a powdered toner of a color, such as a yellow pigment,
magenta, Cyan, or a black pigment, which corresponds to
one of the color filters. The obtained toner image is
transferred to paper. Subsequently, a second electrostatic
image is formed in the same manner, and this image is then
developed by applying a powdered toner of another color.
The resultant toner image is transferred to the same paper.
~,
- 2 - ~35958
Similar operations are performed to superimpose toner
images on the paper. In this kind of copier, each time
one development in one color is completed, the toner
image is transferred to the paper, thus necessitating
a mechanism for rotating or reciprocating the paper.
This makes the machine bulky. Also, the time necessary
to form the whole image is lengthened. Another problem is
that it is difficult to make coincident positions at
which toner images are transferred to the paper.
In an attempt to overcome the foregoing problems
with the aforementioned copier, an electrophotographic
printer has been proposed in Japanese Patent Laid-open No.
144452/1981, where toner images are superimposed on an image
retainer and the superimposed images are subsequently
transferred to paper at once. In this apparatus, in order
to preventtonerimages previously formed on the retainer
from being disturbed by later developments, the powdered
toner is caused to make a flight from developer-feeding
carriers under an osci:Llating electric field during the
second and subsequent developments. Thus, the toner
adheres to the retainer, and this process is called
noncontact development. Although the toner is caused to
fly under the control of the oscillating field in a
noncontact manner as described above for permit
1~35958
superimposition of toner images on the retainer, it is
still likely that the powdered toner previously stuck to
the retainer is trans~erred back to the developer-feeding
carriers or the previously formed tonerimages are
disturbed by the action of the electric field. This makes
it impossible to vividly reproduce colored images. The
phase of the AC component of the oscillating electric
field produces an electric force that acts on the toner
existing between the retainer and each carrier in the
direction to move it from the carrier to the retainer, plus
a second electric force acting on the toner in the
opposite direction. The aforementioned undesired
phenomenor. is explained by this second force. If the DC
component of the oscilla-ting field is adjusted to prevent
the toner from being transferred back, fog will immediately
be produced. Hence, it is quite difficult to control the
travel of toner particles by the oscillating ~leld in such a
way that neither back-transfer nor fog takes place.
In view of the foregoing problems produced in
forming images by superimposing toner images on an image
retainer, it is the main object of the present invention to
provide an image-forming method which permits powdered toner to
- ~ - 1235958
travel while preventing the occurrence of fog and back-
transfer under the control of an oscillating electric
field, and which is thereLore capable of reproducing
color images stably a}lcl vividly without creating the
possibility that previously formed toner images are
disturned later or that toners of wrong colors are
adrnitted into developing devices.
The above object can be attained by a method of
forming an image comprising the steps of forming a
latent image on an image retainer, flying toner par-
ticles from a developer feeding carrier to attach on
said image retainer under an oscillating electric
field, repeating the steps of the above to superpose
a plurality of toner images on said image retainer by
using toner particles different in every steps, wherein
an electrical attracting force for attracting charged
toner particles on said developer feeding carrier to
said image retainer becomes larger according to the
increase of the number of the repetition of the above
steps.
The above Object can also be attained by a method
of forming an image comprising the steps of forming a
latent image on an image retainer, flying toner particles
from a developer feeding carrier to attach on said image
~235~315~3
retainer under an oscillating electric field, repeat-
ing the steps of the above to superpose a plurality of
toner images on said image re-tainer by using toner
particles different in every steps, wherein a flying
force of the toner particles flying from said developer
feeding carrier to said image retainer becomes smaller
according to the incre~se of the number of the repeti-
tion of the above steps.
The above object can also be attained by a method
of forming an image comprising the steps of forming a
laten-t image on an image retainer, flying toner par-
ticles from a developer feeding carrier to attach on
said image retainer under an oscillating electric
field, repeating the steps of the above to superpose
a plurality of toner images on said image retainer by
using toner particles different in every steps, wherein
such an image forming condition that the toner particles
formed on said image retainer is not separated from
said image retainer in sequential steps is set,
In summary, the image-forming method according
to the invention comprises the steps of: forming latent
images on an image retainer; causing a powdered toner
to fly from a developer-feeding carrier in the presence
of an oscillating electric field so that the toner may
~235958
adhere to the retainer; and carrying out similar steps
using differen-t -toners to superimpose a plurality of
toner images on the retainer. This process is charac-
terized in that the toners are subjected to weaker
magnetic forces as later steps are carried out.
Specifically, in each development done to form one image,
the developer-feeding carrier exerts a magnetic force
on the toner, and this force is made weaker as later
steps are performed. This allows powdered toners to
fly under the control of the oscillating field without
introducing fog or back-transfer. Accordingly, toner
images can be stably superimposed on the retainer.
An embodiment of the invention resides in an
image-forming method comprising the steps of: forming
a latent image on an image retainer; causing a powdered
toner to fly from a developer-feeding carrier to the
retainer at a developing station in the presence of an
oscillating electric field so that the toner may adhere
to the latent image; carrying out similar steps USillg
different powdered toners; whereby superimposing toner
images on the retainer. This embodiment is characterized
in that the quantity of toner conveyed from the carrier
to the developing station per unit time is made larger
as later steps are carried out. This permits the toner
12359~i8
to travel withou-t trans:Eerring back to the carrier and
without producing fog under the control of the oscillat-
ing field, therehy achieving the object described above.
Another embodiment of the invention resides in an
image-forming method comprising the steps of: forming a
laten-t image that has a potential oE a two-dimensional
distribution on the surface of an image retainer; causing
a powdered toner to fly from a developer-feeding carrier
in the presence of an oscillating electric field so that
the toner may adhere to the latent image; carrying out
similar steps using different powdered toners to super-
impose toner images on the retainer. This embodiment
is characterized in that the latent images are created
to form potential distributions having larger contrasts
as later steps are carried out, thereby accomplishing
the aforementioned object.
A further embodiment of the invention resides in
an image-forming methcd comprising the steps of:
developing latent images one after another in the pre-
sence of an electric field containing an AC component
to superimpose toner images on an image retainer, This
embodiment is characterized in that the waveform of the
AC component is made to have a larger amount of high
harmonic component as earlier developments are carried
1235~58
out, thereby attaining -the above-described object.
The AC componen-t of the electric field used for
the developing steps has a constant period. That is,
when i-t is represented in terms of ~ourier series, it
follows that the fundamental component has a constant
period, As earlier developing steps are conducted, the
waveform is rnade closer to a pulse waveform, i.e,, it
has many high harmonic components. As later developing
steps are carried out, the waveform is rendered closer
to the ~unclamental component, i.e., it has a less number
of high harmonic components.
In general, an electric field having a constant
period can be given by
CD
An sin ~n~t + ~n)
n=l
where ~ is the frequency of the fundamental component,
t is time, An and ~n are the amplitude and the frequency,
respectively, of the harmonic component of the n-th order.
When toner is placed in the electric field as given above,
a vibrational energy proportional to
= T r ~ ~ An sin ( n~t + ~n)~ dt
O n=l
1;~359~
is imparted to the toner, Accordingly, even when the
period and the amplitude are constant, if a development
is carried out in -the presence of an electric field
having many harmonic cornponen-ts, i.e., having a waveform
close to a rectangular wave, then a stronger vibrational
energy is given -to the toner. Thus, the toner can
readily move ~way from the developer-feeding carrier,
but after it has adhered to a latent image surface on
the retainer, it does not easily settle itself in that
location. Rather, it tends to return to the carrier.
On the other hand, if the AC component of the field has
a less number of harmonic components, i.e., it is close
to a sinusoidal wave, then the toner does not readily
move away from both the carrier and the retainer.
Also, when it is stuck to the retainer, a smaller impact
is created.
The method according to the invention utilizes
this phenomenon. ~ore specifically, when latent images
are developed to superimpose the images on an image
retainer in earlier steps, an electric field having
an AC component similar to a rectangular wave is set
up at a developing station. In later developing steps,
however, the waveform oi the AC component is made closer
to a sinusoidal wave to eliminate the possibility that
-- 10 --
~235958
previously formecl toner images are di sturbe~ in the
later developing steps or that toners of wrong colors
are introduced into developing clevices which are to be
used for later developing steps,
O-ther objects and features of the invention will
be seen by reIerence to the ensuring description, taken
in connection with the accompanying dr~wings.
Fig, 1 is a schematic diagram of a printer for
carrying out a method according to the present inven-
tion;
Fig, 2 is a partially sectional view showing the
structure of a developing device;
Figs, 3 and 5 are schematic diagramsof multi-
color image printer for carrying out other image-forming
methods according to the invention; and
Figs. 4(a) to 4(c) are waveform charts showing AC
components of an electric field for development.
Referring first to Fig, 1, there is shown a
printer for carrying out a method embodying the concept
of the present invention. This printer includes an image
1235958
retainer 1 in the form of a drum. The retainer 1 has
a photosensi-tive layer on its surface, and rotates in
the direc-tion indicated by the arrow. The surface of
the re-tainer is uniformly charged electrically by a
charging device 2 which consists of a scorotron in the
illustrated example. Light 3A associated with one
color is caused to strike the charged sur~ace to ~orm
an electrostatic image. Then, to develop it, a toner
of a color corresponding to the light 3A is applied to
the electrostatic image from a developing device 4A
constructed as shown in Fig. 2. The surface having
the toner image is caused to pass by developing devices
4B, 4C, 4D, a transfer device 5, and a separator 6
without being worked upon by these devices.
The developing devices 4B, 4C and 4D are similar in
structure to the developing device 4A, and receive
toners of different colors, respectively. Then, the
electric charge on the surface is dissipated by a
charge-eliminating device 7. Subsequently, the surface
from which the charge has been removed is uniformly
charged again by the charging device 2 without being
acted upon by a cleaning device 8. Thereafter, light
3B regarding another color is projected onto this
charged surface to form another electrostatic image.
- 12 ~ 58
This image i9 -then developeA by the developing device
4B. Similar steps are carried ou-t to form images until
the final developmen-t is done by the developing device
~D. Thus, a colored image consis-ting of the super-
imposition of color toner images is formed on the sur-
face oL the retainer 1. Paper P is moved into contact
with the surface of the retainer l in synchronism with
it, and the multi-colored image is transferred to the
paper by the transfer device 5. Then, -the paper is
separated from the surface of the retainer l by the
separator 6, and the coloredimage is fixed to the paper
by a fixing device (not shown). Subsequently, the
charge on the surface of the retainer from which the
colored image has been transferred is removed by the
charge-eliminating device 7, after which the remaining,
powdered toner is rernoved by the cleaning device 8.
Thus, the whole process of printing the colored image
is completed.
It is to be noted that all the charge-removing
operations carried out by the charge-eliminating device
7 can be omitted except for the final one. In this
case, when a powdered toner is stuck to areas on which
light falls for reversal development, if this light
does not cause a dot development in the same positions,
- 13 -
1235958
then the electrically charging operations per-formed by
the charging clev:ice 2 can be omitted except ror the
first charging~
During the image-forming method as described
above, a developer-feeding carrier 41 tha-t is incorpo-
rated in each of the developing devices 4A-4D and has
a magnet member 42 therein, as shown in Fig. 2, exerts
a magnetic force on the powdered toner on the developer
layer formed on the carrierO This magnetic force is
made weaker as later developments are made, and an
oscillating elec-tric field is applied between the
carrier 41 and the image retainer 1 by a power supply 9,
in order that the powdered toner fly from the carrier
41 to the retainer 1 in such a way that neither the
back-transfer of the toner nor fog is produced.
Consequently, toner images of different colors can be
stably superimposed on the retainer 1.
The magnetic force that the carrier 41 exerts
on the toner is made weaker in the manner described
below as later developments are carried out:
(1) the magnetic flux densities between the north and
south poles of the developing devices 4A-4D are made
smaller in turn in this order; (2) the ratios of the
magnetic material contained in the toners for the
1'~,359~;8
developing clevices gA-4~ are macle smaller in turn from
the devices4A -to 4l); or (3) the measures (1) and (2)
are taken a-t once. Tlle power supply 9 applies the same
or different bias voltages to tlle carriers 41 of the
developing devices 4A-4D to set up the oscillating
electric field between each carrier and the retainer 1
that has its body grounded.
The structures and the operations of the develop-
ing devices 4A-4D are now described in greater detail
by referring to Fig. 2, where the developer-feeding
carrier 41 is made from a nonmagnetic, electrically
conductive material such as aluminum or stainless steel,
and rotates in a counterclockwise direction. The magnet
member 42 within the carrier rotates in a clockwise
direction. Thus, a developer layer that moves in the
direction indicated by the arrow is formed on the
surface of the carrier 41. The magnet member 42 inside
the carrier 41 has the north and south poles which
exert a magnetic force on the ~dered toner on the
developer layer. Either the carrier 41 or the magnet
member 42 can make stationary,
The developer in a developer container 43 preferably
consists of a two-component developer that is a mixture
of a magnetic powdered carrier and a powdered toner,
12359~i8
because the quantity Oe -the magnetic material contained
in the toner can be reduced to a minimum necessary :eOr
the aforementioned controlled travel Oe the toner, but
it can also be a one-component developer that does not
contain magne-tic po~Ydered carrier, The developer held
in the container 43 is stirred by a stirring means 44,
and the toner is electrically charged by iriction.
The average amount of the charge on the toner preferably
ranges from 5 to 50 jUC/g. When a two-component developer
is used, it is easy to charge the toner within this
range, Therefore, the travel of the powdered toner can
be readily controlled by the oscillating electric field.
The above-described developer in the developer
container 43 is attracted to the surface oI the developer-
feeding carrier 41 by the magnetic force of the magnet
member 42 to form the developer layer that moves in the
direction indicated by the arrow as mentioned previously.
The thickness of the layer is limited by a blade 45.
The electrostatic image on the retainer 1 is developed
at a developing sta-tion where the carrier 41 is opposite
to the surface of the retainer 1.
The space between the carrier 41 and the retainer
1 at the developing station is set so as not to allow
the developer layer formed on the carrier 41 to make
- 16 -
1235958
contact with the surface of the retainer 1, and it
is desired tha-t the value lies in the range tends to
2,000 ,um, Accordingly, the thicklless o- the developer
layer that is limited by the blade 45 is made sma]ler
than this value. Ilowever, if the space a-t the develop-
ing station is narrowed excessively, the thickness of
the layer mus-t be made quite small, which will render
it impossible to make the thickness o~ the layer
uni~orm, Therefore, toner cannot be stably supplied
to the developing station, Further, electric discharge
tends to occur between the carrier 41 and the retainer
1, damaging the developer and increasing the possibility
that the toner is scattered, On the other hand, if the
space at the developing station is made too large, the
travel of toner cannot be controlled by the oscillating
iield.
When two-component developers are used, the
magnetic powdered carriers preferably exhibit insulat-
ing property, i,e " their resistivity is in excess of
108n cm, more preferably in excess of 1013n cm, to
prevent the occurrence of electric discharge between
the carrier 41 and the retainer 1 and to facilitate
controlling the travel of toner by the oscillating
electric field. The powdered carriers are coated with
- ~7
~23S958
a resinous .Cilm or consist of resinous par-ticles in
which magnetic part:icles are dispersed, I-t is to be
noted that the resistivity Or the insula-ting par-ticles
is measured by pu-t-ti.ng the particles into a container
having a cross-sectional area of 0,5 cm2, tapping it,
then applying a load of 1 1~g/cm2 to the stu~'fed par-
ticles, applying a voltage to produce an electric
field of 1,000 V/cm between the load and the bottom
electrode, and reading the value of the resultin~
current, At this time, the carrier particles are
tapped to a thickness of about 1 mm, Preferably, the
developer-feeding carrier 41 is coated with an insulat-
ing or somewhat insulating film such as a resinous or
oxide lilm to prevent the oecurrence of eleetrie
discharge.
By satisfying the above eonditions about the
developing devices 4A-4D, a bias voltage which is the
sum of appropriate AC and DC voltages ean be applied
to the developer-feeding carrier 41 by the power supply
9 without introducing any difficulty, This bias voltage
cooperates with the aforementioned magnetic force, which
is produced by the carrier 41 and exerts on the toner,
to ideally eontrol the travel of the flying toner,
To superimpose the -toner images with good resolution
- 18 -
123~9~3
and vividly, the aver~ge dialneter of the tc)ller par-ticles
of the developer is plc-fer<tbly less than 20 ,um, more
preferably from 1 to :L0 rlm. \~hen two-component devel-
opers are used, the average diameter of the c~rrier
particles preferably lies in the range 5 to 50 um. The
average diameter of the particles is a weight average
diameter of the particles, and is measured by an instru-
ment as manufactured by Calter Inc. under the product
name Coultex Coun-ter or by Bosch & Rohm Inc. under the
prvcluct name Omlli(on Alpha. ~'hen the average di~n-leter
of the toner particles is too small, the quantity of
electric charge given to each toner particle by friction
is small, and the van der Walls force assumes a large
value in inverse proportion to that quantity
The result is that the particles tend to collect together,
and therefore they cannot readily separate from each
other and fly. Inversely, when the average diameter
of the particles is too large, the quantity of electric
chalge per unit weight i5 small This makes it difficult
to control the travel of the particles. ~urther, a good
resolution cannot be obtained. When the average diameter
of the carrier particles is too small, the magnetic,
attracting force exerted by the magnetmember 42 issmall,
but the electrical Coulomb force and the van der Waals
*Trade Marks
.` '~ ~
9 --
1~35958
force are large. ThereEore, the carrier particles can
easily migrate to the surface of the image retainer 1
together with the toner particles. Inversely, when the
average diameter of the particles is too larye, the
developer layer formed on the feeding carrier 41 will
be coarse. This makes it difficult to form the developer
layer uniformly as a thin film. In addition, the condition
of adhesion of the toner particles on the developer layer
is not uniform. Further, a breakdown in the voltage applied
to the carrier 41 and electric discharge tend to occur.
Consequently, it is difficult to control the travel of
toner particles.
The electrostatic image on the image retainer 1
is developed by the developer layer on the carrier 41 at
the developing station as mentioned above. The remaining
developer layer is scraped off :Erom the surface of the
carrier 41 by a cleaning blade 46, and then it is returned
to the developer container 43. A toner hopper 47 supplies
toner to the container 43 via a toner replenishment roller
48 to make up the toner consumed by development. The
present invention is carried out as described thus far.
More detailed examples of the invention are next described~
EXAMPLE 1
- 20 -
~23~9S8
The printer shown in Figs. 1 and 2 was used.
The image retainer 1 had a photosensitive layer of Se on
its surface. The diameter oE the retainer was 120 mm,
and it was rotated in the direction indicated by the arrow
at a surface velocity of 120 mm/sec. The retainer was
electrically charged uniformly to ~00 v by the charging
device 2. The light 3A-3D for development was dot exposure
light produced by modula.ing a`He-Ne laser beam. Each
developer-feeding carrier 41 of the developing devices
4A-4D had a diameter of 30 mm. During development, the
carrier 41 was rotated in a counterclockwise direction at
a surface velocity of 120 mm/sec, and the magnet member 42
was rotated in a clockwise direction at 600 rpm. The
developing sleeve 41 and the magnet member 42 were at rest
except during development. The maximum magnetic flux
density at the surface of the carrier 41 was 800 G,
similarly to the developing devices 4A-4D.
Steps similarly done to form images resided in
forming negative latent images by means of light 3A-3D for
exposure and causing toners to adhere to the areas on
which the light 3A-3D fell by the developing devices
4A-4D. That is, these steps utilized reversal
development.
One-component developers consisting of toner
123S9S8
particles having an average diameter of 10 ~m were used
for the developing devices 4A-4D. The developers, or
toner particles, for the developing devices 4A-4D had the
following compositions:
Developer 4A: black powdered toner
polyester resin 70 wt. %
powdered ferrite 40 wt. %
carbon black 10 wt. %
electric charge-controlling agent 1 wt. %
eveloper 4B: powdered Cyan toner
polyester resin 80 wt. %
powdered ferrite 30 wt. %
phthalocyanine (blue) 10 wt. %
electric charge-controlling agent 1 wt. %
eveloper 4C: powdered magenta toner
polyester resin 80 wt. %
powdered ferrite 25 wt. %
rhodamine lake pigment 10 wt. %
electric charge-controlling agent 1 wt. %
ev~loper 4D: yellow powdered toner
polyester resin 80 wt. %
powdered ferrite 20 wt. %
Hansa Iyellow) 5 wt. %
electric charge-controlling agent 1 wt. %
- 2~ -
12359S8
For any of the developing devices 4A-4D, the
toner particles were electrically charged to +2 to 5
~C/g on the average. 'l'he amount of electric charge on
each toner particle ranged from negative to positive
values. The space between the developer-feeding carrier
41 of each of the developiny devices 4A-4D and the
retainer 1 was set to 0.8 mm, or 800 ~m. The thickness of
the developer layer was 0.5 mm. When developments were
made by the developing devices 4A-4D, a DC voltage of
500 V and an AC voltage as listed in Table 1 were
simultaneously applied to the carrier 41 by the power supply
9.
TABLE 1
Developing device
4A 4B 4C 4D
Ampl. Freg. Ampl. Freq. Ampl. Freq. Ampl. Freq.
(KV) (KHz) (KV) (KHz) (KV) (KHz) (KV) (KHz)
Run 1 2 3 2 3 2 3 2 3
Run 2 2 3 1.8 3 1.6 3 1.5 3
Run 3 1 1.5 1 L.7 1 2.0 1 2 5
(Note that each amplitude was measured from the central value.)
1235958
Colored lmages were printed under the above
conditions, and vivid colored images could be
reproduced stably at all times without disturbing any
color toner image for all the runs 1-3. Especially,
for runs 2 and 3, the obtained colored images were
good in color balance and excellent in vividness.
EXAMPLE 2
The same conditions as in Example 1 were adopted
except for the following conditions. The magnetizations
of the north and south poles of the magnet member 42 in
each of the developing devices 4A-4D were different from
each other to vary the magnetic flux density on the surface
of the carrier 41. Also, the space between each carrier 41
and the image retainer 1 and the thickness of the
developer layer were changed among the developing devices
4A-4D, as shown in Tabls 2.
TABLE 2
Developing Device
Condition
4A 4B 4C 4D
_
Maximum Magnetic1000 900 800 700
Flux DensitY (G)
Space (mm) 0.5 0.6 0.7 0.8
Thickness (mm) 0.3 0.4 O.S 0.6
- 24 - ~2~5958
Setting aside the kind of coloring component,
all of the developing devices 4A-4D used the one-
component developer of the same composition as the
powdered magenta toner in Example 1. The developers
used for the developing devices 4A-4D were yellow
powdered toner, powdered magenta toner, powdered Cyan
toner, and black powdered toner, respectively. A DC
voltage of 500 V and an AC voltage having an amplitude
of 1.5 KV and a frequency of 2 KHz were simultaneously
applied to all the carriers 41 of the developing devices
4A-4D.
Colored images were reproduced under the above
conditions, and vivid colored images where any of the
color toner images was not disturbed were obtained.
Further, after many copies were produced, no change was
observed in their image quality.
The examples described thus far rely on reversal
development, but the invention is not limited to this.
For example, ordinary development may also be utilized
to form images. Further, as already pointed out, two-
component developers may be used. Furthermore, the
invention may be applied to a system in which an image
retainer has a transparent, insulating layer on a
photosensitive layer to form an electrostatic latent
- 25 - ~235~S8
image. Addi-tionally, the invention may be applied -to
an elec-trostatic printer or magnetic printer. I-t is
also to he understood that the toner images super-
imposed on the image retainer can be transferred to
paper by u-tiliæing tackiness as well as by electro-
s ta t i cal process.
According to the invention, during developments
in which toner images are superimposed on the image
retainer, the developer-feeding carriers exert a
magnetic force on toner particles, and this force is
made weaker as later developments are performed. This
facilitates controlling the travel of toner particles
by an oscillating electric field. Accordingly, toner
images can be readily superimposed to form a complete
toner image without disturbing previously formed toner
images and without producing fog, Hence, colored
images can be reproduced stably and vividly.
As a yet further example of the invention, the
developing devices 4A-4D constructed as shown in Fig,
2 perform developing operations in turn in the printing
steps as mentioned previously, At this time, the
quantity of toner which is supplied to the narrow
developing station between the retainer 1 and each
carrier 41 of the devices 4A-4D by these carriers per
- 26 -
12~5958
unit time is increased as la-ter developmen-ts are con-
duc-ted. 'rherefore, as later developments are done,
it becomes easier to move toner particles from the
carrier 41 to tlle re-tainer 1 in the presence of the
oscill~ting electric field. This field serves to
prevent occurrence o~ fog and back-transfer of the
toner. Consequently, toner images of different colors
are superimposed on the retainer 1 stably without dis-
turbance.
This is described in greater detail by referring
to Fig. 2. The amount of toner supplied to the devel-
oping station is controlled by adjusting the thickness
of the developer layer by means of the blade 45, by
adjusting the velocity at which the developer layer
is moved, or, where the developer is a mixture of
toner particles and carrier particles, by varying the
ratio of the toner in the developer. 1`he velocity of
the movement of the layer can be adjusted by altering
one or both of the rotating frequencies of the carrier
41 and the magnet member 42, When the thickness of the
developer layer is changed, the amount of toner supplied
also relates to the gap between the develoer layer and
the surface of the retainer 1 or the gap between the
carrier 41 and the surface of the retainer 1 and so
- 27 -
123595~3
the result oL a change in the amount of toner supplied
cannot be iorecas-ted easily. Therefore, it is desired
tha-t the velocity oL the developer layer or the ratio
of the toner in a two-component developer be changed;
otherwise -these two methods may be used simultaneously.
In order to prevent unnecessary toner particles
from adhering to the image retainer 1 and to keep the
toner already forming a toner image from transferring
back to the developing device, the rotation of the
carrier 41 and the magnet member 42 in each developing
device not contributing to development is preferably
stopped so as not to m~e the developer layer,
The power supply 9 applies the same or different
bias voltages to the developer-feeding carriers 41 of
the developing devices 4A-4D to produce an oscillating
electric field between each carrier and the image
retainer 1 whose body is grounded. As stated above,
the amount of toner supplied to the developing station
by the carrier 41 is increased as later developments
are carried out. Thus, the oscillating field enables
every development to be stably done without disturbing
previously formed toner images and without producing
fog, Also in this case, it is desired that the AC
component of the bias voltage be not applied to those
- 28 -
1235958
developing devices not lnvolved in development, in
order to prevent both adhesion of unwanted -toner
particles to the retainer 1 and back-transfer of
toner particles already forming toner images to the
developing devices.
EXAMPLE 3
The printer shown in Figs. 1 and 2 was used under
the same conditions as in Example 1 except that the
distance between each carrier 41 of the developing
devices 4A-4D and the retainer 1 was set to 0.5 mm, or
500 um. Two-component developers were employed for the
developing devices 4A-4D. These developers consisted
of insulating powdered carriers which were coated with
styrene or acrylic resin. The average diameter of the
particles of the carriers was 20 jum, and silica was
appropriately added to them. The toner particles
consisted of polyester resin in which coloring agent
and electric charge-controlling agent were dispersed.
The average diameter of the toner particles was 12 ,um,
Yellow powdered toner, powdered magenta toner, powdered
Cyan toner, and black powdered toner were received in
the developing devices 4A-4D, respectively. The ratio
- 29 -
1235958
of the toners i.n their respective developers was 20% by
weight, The average amount of electric charge on the
toner particles in each developer container 43 for -the
developers was 20 ,uC/g.
During developmen-t, all the developing-feeding
carriers 41 of the developing devices 4A-4D were rotated
in their respective devices 4A-4D in a counterclockwise
direction, and -their surface speeds were 180 mm/sec,
200 mm/sec, 220 mm/sec and 250 mm/sec, respectively,
All the magnet members 42 were rotated in a clockvise
direction at 600 rpm. A developer layer which was
moved by each carrier 41 at a speed proportional to the
carrier was formed to a thickness of 0,3 mm on the
surface of the carrier 41 by the blade 45. The power
supply 9 applied bias voltages that were the combination
of a DC voltage of 500 V and the respective AC voltages
listed in Table 3 during the developing operations.
- 30 -
~23595;~3
TABLE 3
.
Develo in Device
. P ~
4A 4B 4C 4D
~npl. Fleq. Ampl. Freq. ~pl. Freq. A~npl. Freq.
(KV) (Kl~z) (KV) (K~z) (KV) (KHz) (KV) (KHz)
_ . _
Run 4 0.5 0.8 0.5 0.8 0.5 0.8 0.5 0.8
,
Run 5 2 3 1.8 3 1.5 3 1.2 3
.
Run 6 1 1,2 1 1,5 1 2 1 2.5
Toner images were formed by producing negative
latent images by the projection of light 3A-3D for devel-
opment and causing toner particles to adhere to areas on
which the light 3A-3D fell, using the developing devices
4A-4D. That is, reversal development was utilized.
Colored images were printed under the conditions
described above. For all the runs 4-6, vivid colored
images could be stably reproduced, i.e., the toner images
of the different colors were not disturbed. Especially
for runs 5 and 6, the obtained colored images were good
in color balance and excellent in vividness.
EXAMPLE 4
Colored images were reproduced under the same
~235958
conditions as in Example 3 except for the following
conditions. During every developments, the developer-
feeding carriers 41 of the developing devices 4~-4D were
rotated in a counterclockwise direction at the same surface
velocity of 220 rnrn/sec. The ratios of the toner particles
contained in the two-component developers received in the
developing devices 4A-4D were 15%, 18%, 20%, and 24% by
weight, respectively. Thus, the quantity of toner furnished
to the developing station was increased in turn. Colored
images could be ~eproduced stably and vividly for all the
runs 4-6 in the same manner as in Example 3. No disturbances
could be observed in the toner images of various colors.
EXAMPLE 5
The conditions adopted were the same as in Example 3
except for the following conditions. During developments,
the developer-feeding carriers 41 in the developing devices
4A-4D were rotated counterclockwise, and the magnet members 42
were turned clockwise. Their surface speeds were set as
listed in Table 4. The ratio of all the toners in the
developers was 15~ by weight.
- 32 -
~23~958
TABLE 'I
_
_ Developing Device
Conditlon _
4~ 4B 4C 4D
__ _
Surface Speed of 180 200 220 250
_ _
Magnet Member (rpm, _ 600 600 750
Colored images were reproduced under the above
conditions, and vivid colored images could be obtained for
all the runs 4-6 in Table 3, i.e., no disturbances occurred
in the toner images of different colors. Further, it was
found that no changes in color and in print quality could
be observed after many copies were created.
The above-mentioned Examples 3-5 all relied upon
reversal development. The invention is not limited to this,
and it is possible to form images by ordinary development.
Also, one-component developers consisting of magnetic
toner particles can be used.
In this example o~ the invention, the amount of toner
supplied to the developing station by each developer-feeding
carrier to superimpose toner images on the image retainer
is increased as later developing operations are performed.
This facilitates controlling the travel of toner particles by
the oscillating electric field during every development
without producing fog or back-transfer. Consequently, colored
- 33 -
35958
images can be stably and vividly reproduced.
As a still other example of the invention, the
potential difference between areas on which light falls
for development and areas on which no light falls is
increased, i.e., the contrast is increased, as later
developing operations are performed to derive toner images
from electrostatic images formed on the retainer 1. This
can be easily achieved by causing the charging device 2
to uniformly charge the retainer 1 at stronger intensities
as later toner image-forming steps are carried out.
Although the same result would be obtained by increasing
the contrast between the light 3A-3D for development as
later image-forming steps are carried out, the method using
the charging device 2 is most preferable in that it is
simple and can be used in wide applications. By forming
electrostatic images in this way, these images can be
readily developed and, accordingly, the travel of toner
particles can be easily controlled by the oscillating elec tric
field in such a way that the toner does not transfer back to
the developing device and that fog is not produced. Hence,
toner images can be superimposed on the retainer 1 without
disturbing them or mixture of colors, thus giving rise to a
vivid, superimposed toner image.
By adding the preferred conditions described above
- 34 -
1235958
to the image-forming process in which latent images are
successively formed with successively increased contrast,
further vivid color images can be reproduced.
EXA.~lPLE ~
The printer as shown in ~igs. 1 and 2 was used
under the same conditions as in Example 1 except for
the following. The gap between each developing sleeve 41 of
the developing devices 4A-4D and the image retainer 1 was
set to 0.7 mm, or 700 ~m. Two-component developers which
were mixtures of toner particles and carrier particles were
used for the developing devices 4A-4D. The toner particles
had an average diameter of 10 ~m, and consisted of polyester
resin in which coloring agent and electric charge-controlling
agent were dispersed. The carrier particles had an average
diameter of 30 ~m, and consisted of a styrene or acrylic
resin in which powdered ferrite was dispersed. Silica was
stuck to the surface of each carrier particle. The toner
particles for the developing devices 4A-4D were yellow
pigment, magenta, Cyan, and black pigment, respectively. The
ratio of the toner particles in the two-component developers
was 20~ by weight.
The developers in the developer containers 43 of
the developing devices 4A-4D were so electrically charged
~235958
that the amount oE electric charge on the toner particles
reached +20 to 30 ~C/g. A developer layer of 0.5 mm thick
was formed on each developing sleeve 41 by the blade 45.
The developing devices 4A-4~ caused the charging device 2
to charge the surEace Or the irnage retainer 1 to 500 v,
600 V, 700 V, and 800 V in turn, and then electrostatic
images were produced by exposing the surface to light
3A-3~. Tne bias power supply 9 applied bias voltages
which were combinations of the ~C components of 400 V,
500 V, 600 V, and 700 V, respectively, and the AC components
listed in Table 1 -to the developing sleeve 41 to develop
the images. The potential on the areas of electrostatic
images which were exposed to light was 20 V.
Colored images were reproduced under the
conditions shown in Table 1, and it was found that the toner
image of any color experienced no disturbance for all the
runs 1-3. Hence, all the colored images could be
reproduced stably and vividly. Especially for the runs
2 and 3, the obtained colored images were good in color
balance and excellent in vividness.
EXA~IPLE 7
Colored images were reproduced under tne same
conditions as in Example 6 except for the following
12~958
conditions. Sli-ts were provided so that the light
3A-3D for diEferent colors passes through their
respective slLts in the same way as in ordinary
elec-trostatic copiers to Eorm positive latent images.
Then, toner particles were caused to adhere to the
unexposed areas b~ the developing devices 4A-4D. One-
component developers consisting of toner particles were
used for the developing devices 4A-4D. The toner particles
consisted oE polyester resin containing powdered ferrite,
coloring agent, and electric charge-controlling agent.
The average diameter of the particles was 15 ~m. The amount
of electric charge on each toner particle in the developer
contai.ner 43 of the developlng devices 4A-4D ranged from
negative to positive values. The average value was -5 to
-2 ~C/g. Bias voltages which were combinations of a DC
voltage of 100 V and the AC components listed in Table 1
were applied to the developing sleeves 41 of the developing
devices 4A-4D. Colored images were reproduced under the
above conditions, and vivid colored images in which the
toner images of different colors were not disturbed were
derived. No change was seen in the print quality after a
number of copies were made.
In one aspect of the present invention, latent
images are produced with successively increased contrast
- 37 -
1235958
as later image-formillg steps are carried out. This
permits every developlnellt to be made in tne presence of -the
osci:Lla-ting electric Eield without producing back-
transEer of toner particles or fog. lience, vivid
colored images can be s~ably reproduced without incurring
disturbance of lmages and intrusion of wrong colors.
It is to be understood that the present
invention may be also applied to a printer in which an
image retainer has a transparent, insulating layer on a
photosensitive layer to forrn electros-tatic latent images.
Further, it may be applied to a system in whicn toner images
are transferred to paper under pressure making use of
viscous transfer. In addition, it may be applied to an
electrostatic printing system in which electric charges are
directly injected into an image retainer to form
electrostatic images. Furthermore, it may be applied to
printers in which non-electrostatic latent images such as
magnetic latent images are developed.
Referring next to Figs. 3-5, there is shown a
yet further example of -the invention. It is to be noted
that those components of the color printer shown in Fig. 3
WhiCIl are the same as those in Figs. 1 and 2 are
indicated by the same reference numerals as in Figs. 1 and 2.
This printer has multi-stylus electrodes 2A-2C for injecting
-- 3U --
~5~
electric charges into the surEace of the image retainer l
that turns in the direction indicated by the arrow, in
order -to form electrostatic latent images, wilich are
developed in-to -toner images by developing devices 13A-13C.
The structures of these devices 13A-13C are as sho~n in
Fig. 2. Bias power supplies 9A-9C apply bias voltages to
the developing sleeves 41 of -tne devices 13A-13C to set up
an electric field a-t the developing station between the
retainer 1 and each sleeve 41, each bias voltage being
ei,ther an AC voltage or t~le combination of an AC voltage
and a DC voltage. Charge-eliminating electrodes 15A and
15B ac-t to rernove electric charge from the surface of the
irnage retainer 1 on which a developrnent was previously maae~
in order that electrostatic latent images be smoothly formed
by the electrodes 2~ and 2C, respectively.
In this example, the bias voltages applied to
the developing sleeves 41 by the power supplies 9A-9C for
developl~ent contain at Eirs-t an AC component containlngmany
high harmonic components as shown in Fig. 4(a) to produce
an electric field containing an AC component a-t the
developing station Eor the first development that is done
by tne developing device 13A, for example. Then, the AC
component o~ the bias voltage is made to contain less high
- 39 -
5958
harmonic components as shown in Fig. 4(b) than the wave
of Fig, ~(a), for -the next development which is made by
the developing device 13~, for instance. Finally, the
AC component of the biasvoltage ismade eitherto contain
further less high harmonic componen-ts or to consist oL
only its fundamental component for the last development
that is carried out by the developing device 13C. This
eliminates the possibility that toner images previously
formed on the retainer 1 are disturbed by later develop-
ments or that the toner particles already adhering to
the retainer 1 transfer back to the developing sleeves
41 during later developments. As such, colored images
can be reproduced stably and vividly. As mentioned
previously, an appropriate DC voltage is added to the bias
voltage, as a ma-tter of course, in order to more stric-tly
prevent occurrence of fog or back-transfer. The waveform
of the AC component is shaped as shown in Fig. 4 -to help
prevent the occurrence of both fog and back-transfer,
which would usually have been a trade-off. The high
harmonic components can be controlled by providing an
RC integrator circuit and changing its time constant.
When one of the developing devices 13A-13C is
performing a developing operation, no bias voltage is,
of course, applied to the sleeves ~1 of the other devices.
1235958
Also, the sleeve 41 and -the magnet members 42 of the
other devices are not rotated.
Also in the above example of the invention,
developments are made by the developing devices 13A-13C
preferably under the condition that toner particles fly
from the developer layer formed on the developing sleeves
41 and adhere to the surface of tile retainer 1 sucn that
the developer layer does not make contact witn tne
surface of the retainer 1. This yields better advantage
than the process in which the developer layer makes a
sliding contact with the surface of the retainer 1.
Referring next to Fig. 5, there is shown a
two-color printer including an image retainer 1 that has a
photosensitive layer on its surface for employing the
principles of electrophotography. The surface of the
retainer 1 is uniformly charged by a charging eiectrode
(scorotron) 10. Light 11 is emitted by an exposure means
(not shown) so that it strikes the charged surface to form
an electrostatic latent image. This printer differs from
the printer shown in Fig. 1 in that toner images of two
colors are superimposed on the retainer 1 of Fig. 5 while
the retainer is rotated twice. The resultant image is
then transferred to paper P. More specifically, during
the first revolution of the retainer 1, an electrostatic
i23~958
image is formed on the retainer 1 and a development is
made by tne developing device 13A, for example. The
surface of the retainer L then passes by a transfer
device 5 and a cle~nillg device 8 without being acted
upon by these -two means. Subsequently, the retainer enters
into its second revolution, during which another
electrostatic latent image is formed by the action of both
the charging electrode 10 and light 11 for development.
This latent image is developed by the developing device 13B.
The superirnposed toner images are then transferred and
fixed to paper P in the same manner as in the printer of
Fig. 3. Those members whicl- are indicated by the same
reference numerals as in Fig. 3 function in the same way as
in Fig. 3. Where the exposure means which allows the
light ll to strike the retainer comprises a dot exposure
means such as a laser beam scanner, the uniform charging
by the charging electrode 10 during the second
revolution may be omittea, in which case the charge-
eliminating device 7 is deenergized and the retainer
passes by this electrode without being worked upon by it
during the first revolution. Where the uniform charging
is again made by the charging electrode 10 during the
second revolution, the charge-eliminating device 7 is
not required to operate.
- 42 -
12~5958
Also in the printer of Fig. 5, the first
development by the developing device 13A is carried out in
the presence of an electric field containing an AC
componen-t as shown in FigO 4(a) or ~ig. 4(b). The later
development by the developing device 13B is performed in
-the presence of an electric Eield containing an AC component
as shown in Fig. 4(b) or Fig. 4(c). Tnis exhausts the
possibility that toner images previously formea are
disturbed by later developments or that toners of wrong
colors are introduced due to back-transfer, in the same
fashion in the printer snown in Fig. 3. ~ence, two-color
images can be stably and vividly printed. It is also desired
for this printer that the toner particles travel from the
developer layer such that this ~ayer does not make contact
with the surface of the image retainer. Further, two-
component developers can preferably be used for this
printer.
In the case of a two-color printer, for example,
toner images of two colors can be superimposed during one
revolution of the image retainer l. In particular, an
electrostatic image consisting of three levels, i.e.,
positive, negative, and zero levels, are formed on the
surface of the retainer l using multi-stylus electrodes,
for example. l'oners wllich are oppositely charged are
- 43 -
~2359~B
received in two developing devices, respec-tively. One of
the developing devices acts to cause one toner to adhere
to the areas oE -the laten-t image which are at the
positive level. Tlle other serves to cause the other toner
to adhere to -the areas of the image which are at the
negative level. This method takes a reduced time to
print a two-color image, and facilitates putting the two
superimposed irmages into registry.
It is also possible to superimpose toner images of
two colors with a single developing device as described in
Japanese Pa-tent Laid-open No. 50548/1983. The present
invention is applicable to the printing of mul-ti-color
images as thus far described. Yet further example of the
invention is described below.
EXAMPLE 8
-
The apparatus shown in Figs. 2 and 3 was used.
The image retainer 1 had a diameter of 120 mm, and rotated
at a surface velocity of 120 mm/sec in the direction
indicated by the arrow. Multi-stylus electrodes 2A-2C
produced electrostatic latent images consisting of dots on
the surface of the image re-tainer 1 at 400 V, -500 V, and
600 V, respectively. Each of the developing devices
13A-13C had a developing sleeve 41 made of nonmagnetic
- 44 -
1235958
s-tainless s-teel. T}le sleeve 41 had a diameter of 30 mm,
and rota-ted at a surEace velocity of 12n mm/sec in a
counterclockwise direc-tion. The magnet member 42 had six
magnetic poles that gave a maynetic flux density to the
surface of the sleeve 4~, and the maximum value of the
magnetic ~lux density was 900 gauss. The sleeve -turne~ at
800 rmp in the direction indicated by the arrow. The
space between the retainer 1 and each sleeve 41 was 0.8 mm.
Two-component developers were used for the developing
devices 13A-13C. ~ commonly used carrier consisted of a
thermoplastic resin in which powdered ferrite was
dispersed and to which a fluldizer was added. The particle
sizes were selected so as to range from 20 to 40 ~m. The
used tonerseach consisted of a thermoplastic resin in which
coloring agent and electric charge-controlling agent were
dispersed. The particle sizes of the toners were so
selec-ted that they ranged from 10 to 20 ~m. The toners for
developing devices 13A, 13B, and 13C were magenta, Cyan, and
yellow pigment, respectively. The ratio of the carriers to
the toners was 4:1 by weight. 'l'lle average amounts of
electric charge on magenta, Cyan, and yellow pigment were
-15, 2U, and -~0 ~C/g, respectively. The thickness of the
developer layer on each sleeve 41 and the bias voltages for
developing operations were set as listed in Table 5 to
- 45 -
~2359~8
print colored images. The developments were made by the
developing devices 13i\, L3B, and 13C in this order. The
AC components of the bias voltages had the waveforms as
shown in Figs. 4(a) - 4(c) for the developing devices
13A - 13C, respectively.
TAsLF 5
__
Developing I~C Compo- AC Comyonent Thickness
Device nent (V) ~ pl (KV) Fre~ (K~z) of ~r(nm)
13A O 2 2 0.3
Run I 13B O 2 2 0.3
13C O 2 2 0.4
13A O 1.2 1 0.2
Run ~ 13B -100 1.2 1.5 0.2
_ 13C 100 1 1.5 0.3
13A O 2 3 0.4
Run II 13B O 2 3 O.5
13~ 100 2.5 3 0.5
~Note that an amplitude is half of a peak-to-peak value.)
The obtained colored images were vivid ones in
which no disturbances of images and no mixing of colors were
found for all the runs. l'ne color reproducibili-ty did not
change after many copies were made.
- 46-
359S8
In this example, the bias power supplies
9A-9C were used for -the developing devices 13A-13C,
respectively, but it is also possible to employ a single
bias power suppLy in comlno~ n this case, the waveform
of the AC component can be varied by controlling the time
constant of an inteyrator circuit.
EXAMPLE_9
The apparatus shown in Figs. 2 and 5 was used.
The image retainer 1 had a Se photosensitive layer and a
diameter of 120 mm. The retainer was turned at a
peripheral velocity of 180 mm/sec in the direction
indicated by the arrow. The surface was electrically
charged uniformly at 600 V by the charging elec~rode
10. Light 11 for exposure was emitted by a laser scanner
incorporating a Ele-Ne laser. The developing sleeves 41 oE
the developing devices 13A and 13B had a diameter of
30 mm. During developments, the sleeves rotated at a
peripheral velocity of 180 mm/sec in a counterclockwise
direction. Each magnet member 3 had six magnetic poles
that gave a magnetic flux density to the surfaces of the
corresponding sleeves 41. The maximum value assumed by
the magnetic flux density was 800 gauss. The members 3
rotated at 600 rpm in the direction indicated by the
- 47 -
~.~359S8
arrow. 'l`he space between the re-tainer 1 and each sleeve
41 was 0.5 mm. Electrostatic images formed by the light
11 in the two image-Eorming steps were negative latent
images. Tlle developing devices 13A and 13B made
reversal developments, i.e., toners were stuck to
exposed areas on the retainer 1. In this case, the
exposed areas were at a potential of 0 to 50 V with
respect to the corresponding image portions. The
unexposed areas were at a potential of 550 to 600 V with
respect to the corresponding nonimage areas. Two-
component developers were used for the developing devices
13A and 13B. The carrier used was the same as in Example 8.
Employed toners contained a red or reddish coloring agent
and a black or blackish coloring agent, respectively. In
other respects, the toners were the same as those in
Example 1. The red toner and black toner were used for
the developing devices 13A and 13B, respectively. The
ratio of the carriers to the toners was the same as that
in Example 3. The red and black toners each had electric
charge of 20 ~C/g. rl`he tllickness of the developer layer
on each sleeve 41 and the bias voltages were set as listed
in Table 6, and two-color images were printed. The
developments were made in red and then in black. The AC
components of the bias voltages had the waveforms as
- 4~ -
123~9~8
shown in Figs. 4(a) and 4(c) for -the developing devices
13A and 13B, respectively.
T~BL,E 6
¦Developing DC Compo- AC Component Thickness
Device nent (V) Ampl.(KV) ~req.(KHz) of Layer(r~)
~ . __ .
Run IV 13A 500 1.5 2 0.2
13B 500 1.5 0.2
Run V 13A 500 1.2 1.5 0.2
13B 550 1.2 2 0.2
13A 500 1.5 2 0.3
Run VI
13B 500 1.2 0.3
13A 450 2 3 0.3
Run V~ 13B 450 2 0.2
(No-te that an amplitude is a half of a peak--to-peak value.)
The obtained two-color images were vivid for all the
runs IV-VII in that no disturbances in images and no
mixture of the colors were observed. Mixture of the colors
or other undesired phenomena did not take place after
copies were successively made.
In the above example, the two-component developers
were employed, but it is also possible to use one-
component developers. Further, ordinary development may
- 49 -
~2359S~3
be utilized instead of reversal developmen-t.
For comparison, the rectangular wave shown in
Fig. 4(a) was used as the AC componen-t of -the bias
voltage also during the deve:lopment Inade ~y the
developiny device 13B. ~isturbances of images and
mixture oE colors were consL)icuous in the obtailled two-
color images. After many copies were created, a noticeable
quanti-ty of the red toner was rnixed in the developing
device 13B. This exarnple o:E the invention ensures that
vivid multi-colored images can be stably obtained since no
disturbance of images and no mixture of colors take place.
As a modified embodirment of the invention, toners
which are relatively electrically conductive are used to
prevent the toner once adhering to the image retainer
Erom transferring back to the developing device during the
next development. As an example, if the resistivity of the
toner particles is less than 1013 Qcm, preEerably in the
range from 106 -to 1012 ~2cm, the toner particles which have
been pulled to the surEace of the image retainer 1 by
electrostatic induction under the control of the vibrating
electric field will lose tlleir electric charge in a short
time because of the appropriately low resistance. ~ence,
it is unlikely that toner is Inoved back to the developing
- so -
~23~95~3
device by the action of -the oscillating ~ield or
electrostatic attraction during later developments.
Therefore, toner images can be superimposed without
resulting in fog or adciition of wrong colors. This
permits vivid colored images to be formed on the retainer 1.
The resistivity oE the toner particles was
measured as follows. First, -the particles were put into
a container having a cross-sectional area of 0.5 cn-2 and
then tapped. Subsequently, a load of 1 Kg/cm2 was
applied to the packed particles. A voltage was then
applied between the load and the bottom electrode to
produce an electric field of 1000 V/cm. The value of the
current flowing at this time was read, and then the
resistivity was calculated based on this value. Tlle
thickness of the packed toner particles was of the order of
1 mm.
The aforementioned low resistance of the toner
particles can also be attained by either adding conductive
powder to the resin besides the coloring agent or causing
conductive powder to adhere to the surface of the
retainer after granulation. Where the conductive powder
exhibits magnetic property or magnetic powder is added,
the toner particles will show magnetic property and so
the toner will be used as a developer by itself. However,
- 51 -
123595~3
developers for use in the developing devices 4A-4D are
not limited to this. A two-component developer which
is a mix-ture of magnetic carrier particles and toner
particles may also be employed.
The following embodiments could be considered in
order to superimposing good images by preventing -the
color images from being compounded:
The amplitude VAC of the AC component of the bias
voltage applied to the developing devices was 1.5 KV
and the potential of the DC component was 500 V. The
frequencies of the AC component were set to 1.8, 2.0
and 2.5 KHz in the order of developments performed.
These bias voltages were applied on the developing
devices which were then carrying out developing
operations. Those devices which were not involved
in developing operations were grounded. The space d
between the image retainer l and each developer-
feeding carrier 41 was 0.8 mm. The thickness of the
developer layer was 0.5 mm. Toners which were electric-
ally charged to 20, 22, and 25 ,uC/g in the order of
developments performed were received in their respec-
tive developing devices.
In the present example, the frequency of the AC
bias voltage was increased in turn for every subsequent
- 52 -
~23595~
development, and the amount of electric charge on the
toner used was also increased in turn, in order to
prevent the toner already adhering to the retainer l
from re-turning to the feeding carriers 41.
Multi-colored images were also ~ormed according
to the example described just above, and it was found
that visible images of sufficient density were derived
from the toner images without introducing such undesired
phenomenon that toner images previously formed on the
retainer were destroyed during subsequent developments
or that toners of other colors intruded into the
developing devices. Other examples using one-component
developers for the developing devices are described
below.
Toners which were electrically charged to O, 5,
and 10 luC/g in the order of developments carried out
were received in the developing devices. Then, colored
images were formed under the same conditions as the
example described just above except for the amounts of
electric charges. Although the average quantity of
electric charge on the toner used for the first devel-
opment was O yclg~ the quantity of charge on each toner
particle varies widely due to their mutual friction,
and the toner particles having the required charges
- 53 ~ 5958
were selected for development.
As a Lurther example of the invention, the ampli-
tudes VA~ of the AC components of -the bias voltages
applied during developments were set to 2.0, 1.~, and
1,5 KV in the orcler of developments done, and the
frequencies of the AC components Or the bias voltages
were set to 2.0, 2.2, and 2.5 KHz in the same order.
This gradual increase in frequency prevents the toners
from returning to the carriers. These bias voltages
were applied to only those developing devices which
were carrying out developing operations.
Those developing devices which were not involved in
development were grounded, The amount of electric
charge on each toner was 20 ,uC/g, The retainer 1 was
electrically charged to a potential of 600 V.
The space d between the retainer 1 and the carrier 41
was 0.5 mm. The thickness of the developer layer on
the periphery of the carrier 41 was 0.3 mm. A DC
voltage of 500 V was applied to the developing devices.
As a result, toner images of sufficient density were
formed without destroying the toner images previously
formed on the retainer 1 during subsequent developments
and without adding toners of wrong colors to the devel-
oping devices.
