Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to a device for the development of an
electrostatic charge image by means of a developing powder, comprising
magnetizable carrier particles and toner particles.
Such a device is known from United States Patent No. 3,639,051;
which issued on February 11, 1972 in the name of Savin Business Machines
Corporation.
This device is provided with a reservoir having a bottom, two side
walls and a front and rear wall, which reservoir can contain a quantity of
the developing powder mentioned, a cylinder rotatably mounted near the rear
wall between the side walls of the reservoir and a magnet cooperating with
this cylinder to a magnetic brush, means for transporting the support bearing
the charge image along this cylinder, one or more mixing members mounted in
bearings near the bottom of the reservoir between its side walls and means
for replenishing the amount of developing powder in the reservoir.
Such teveloping devices are for instance applied in electro-photo-
graphic copying devices, and thereby serve to bring the developing powder on
to a photoconductive material, provided with a latent charge image. For that
purpose the cylinder, situated with its lower side in the developing powder,
is causet to rotate, by which a certain amount of developing powder, which
has been mixed as homogeneously as possible by the rotating mixing members
in the reservoir, and thereby has also been brought to the desired tribo-
olectric charge height, is transported upwards through the space between the
cylinter and the rear wall of the reservoir as a result of the magnetic
attraction by the cylinder. When the developing powder has arrived near the
oagnet, the oagnetizable carrier particles of the developing powder are caused
to raise. In this way a kind of brush is formed on the cylinter wall. This
brush comes into contact with the photoconductive material which, preferably
in a direction opposite to the direction of rotation of the cylinder, is guid-
ed along the upper site of the cylinter. Thereby toner particles are attract-
ed from the developing powder by the latent charge image on the photoconduct-
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ive material, which charge image has a charge which is opposite to that
which is given to the toner particles by the triboelectric charging in the
reservoir by which the charge image is converted into a visible powder image.
The magnetizable particles of the developing powder and the toner particles
which were not attracted by the latent charge image, come loose from the
cylinder when they have arrived out of the magnetic field and are then again
fed back into the reservoir. Of course the developing powder in the reser-
voir must be replenished regularly with toner particles to replace the amount
of toner particles consumed. If necessary, also the iron particles are
refreshed regularly. When the device is in operation, the magnetizable
particles and the toner particles in the reservoir are continuously mixed
by the mixing members, in order to ensure that the developing powder taken
upwards from this reservoir by the cylinder has an optimally homogeneous
composition and the triboelectric charge height required for a good develop-
ment.
Conventional developing devices do not appear to be fully satis-
factory. When after a long period of non-use the copying device is again set
into operation, the triboelectric charge height of the developing powder
initially often is rather low, so that the copies made during that initial
period are not strong enough. In addition, for instance as a result of the
conditions of operation, which may highly vary, the composition of the develop-
ing powder in the reservoir often differs too much from place to place, and
therefore also the triboelectric charge height, as a consequence of which
the quality of the copies made with such a developing powder shows great
fluctuations or may sometimes even become insufficient.
The object of the invention is to provide a developing device in
which the above-mentioned disadvantages are obviated or mitigated, in that
upon leaving the reservoir the developing powder always has a very homogen-
eous composition and an amply sufficient triboelectric charge height. -
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According to the present invention, there is provided, in apparatus
for developing an electrostatic charge image on a support by applying thereto
a powder mixture of magnetizable carrier particles and toner particles,
comprising a tray for holding a quantity of said mixture, said tray having
a bottom wall, two side walls, a front wall and a rear wall, magnetic brush
forming means including a cylinder mounted for rotation between said side
walls in bearings thereon near said rear wall and a permanent magnet disposed
above said cylinder, means for transporting said support over said cylinder
through a magnetic field between said cylinder and said magnet, at least
two mixing elements mounted for rotation between said side walls in bearings
thereon near said bottom wall, and means for replenishing a deficiency of
toner particles in the powder mixture in the tray, the improvement which
comprises said rear wall having an overflow edge over which an excess of
powder approaching the apex of said cylinder will escape at a level between
said apex and the axis of the cylinder, and said replenishing means compris-
ing means for supplying a fresh m;~ture of magnetizable particles and toner
particles for admixture with the mixture returning to said tray from said
magnetic field and the mixture in said tray.
The invention will now be described in detail with the aid of the
accompanying drawings in which:-
Figure 1 i9 a cross-sectional view of a device according to the
invention; and ~:
Figure 2 is an exploded perspective view of the most important
parts of the device shown in Figure 1.
The device for the development of an electrostatic charge image,
consists of a reservoir 1 with a bottom 2, a front wall 3, a rear wall 4 and
two side plates 5 and 6 forming the side walls (Figure 2). Between the
plates 5 and 6 a cylinder 7 is rotatably installed in bearings, the cylinder
cooperating with a permanent magnet 8 to form a so-called magnetic brush.
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A belt 13 of photoconductive material guided along the upper side of the
cylinder 7 by means of rolls 9 and 10 and guides 11 and 12 is formed with a
charge image which is converted by the magnetic brush into a so-called
powder image. A channel-shaped trough 14 is installed between the plates 5
and 6 and a screwlike transporting or feed member lS is disposed along the
member 14 between bearings in the plates 5 and 6. The feed member 15 is
formed by a shaft 16, round which two coaxial helical coils 17 and 18 are
arranged which coils are fixed to the shaft 16 at their extremities. The
member 14 is provided with an opening 21 in one of the walls of the channel.
Mixing members 23, 24 and 25 are installed in bearings between the plates 5
and 6. A permanent magnet 53 is installed against the rear wall 4 of the
reservoir 1. The magnet is mounted in a holder 54 which has an elongate
side 55 which projects into engagement with the under surface of the
reservoir 1.
When the device described above is in operation, the cylinder
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rotates into the direction, indicated by the arrow A in Figure 1, as a result
of which a part of the developing powder in the reservoir 1, which powder
consists of a mixture of magnetizable carrier particles and toner particles,
is transported upwards. This developing powder thereby passes the permanent
magnet 53 and undergoes an additional and extra strong mixing in the magnetic
field formed between this magnet 53 and the cylinder 7. Subsequently the
developing powder arrives in the field of the permanent magnet 8, where there
is formed on the surface of the cylinder 7 a so-called magnetic brush, which
brushes against the belt of photoconductive material moving in the direction
of the arrows B. In this way the latent charge image on this belt is convert-
ed into a powder image. The opposite directions of movement of the cylinder
7 and the belt 13 produce an accumulation of the developing powder near 50
(Figure 1), but since the edge 51 of the rear wall 4 lies at a height of a
point intermediate the highest point and the centre of the cylinder 7, the
developing powder can freely escape outwards over this edge 51, by which
this accumulation is eliminated. The projecting side 55 of the magnet sup-
port 54 prevents this powder from reaching the magnet 53.
After the developing powder has left the magnetic field of the
permanent magnet 8, the magnetizable carrier particles and the remaining
toner particles fall from the cylinder 7 into the trough 14, after which
these particles are transported by the screwlike transporting member 15 to
the opening 21 and arrive into the reservoir 1 through this opening. In
addition, during the operation of the device a fresh mixture of magnetizable
carrier particles and toner particles is brought into the trough 14 by means
which are not shown but which lead into the trough 14 near the shortest part
of the trough adjacent the opening 21. When the device is in operation, the
developing powder is continuously mixed by the mixing members 23, 24 and 25.
The concentration of the toner particles in the developing powder is measured
optically via a window 33 in the bottom of the reservoir 1 by a device 52.
The window 33 is swept clean by a brush 32 which is fixed to the mixing
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member 24.
The details of the mixture replenishing device and the various
mixing members form no part of the present invention but for a fuller under-
standing of these details reference should be made to copending Canadian
application ser.no. 216,253, filed on December 17, 1974, in the name of
Mathias Jacobus Johannus Maria Vola et al.
The exact position of the magnet 53 outside the rear wall of the
reservoir can be chosen within very wide limits. However, it appears that
the best results are obtained, when the magnet is situated on or close to
the position where the distance between the rear wall and the rotatable
cylinder is a minimum. It is thought that the successful effect of the
magnet 53 mounted according to the invention must be ascribed to consequent
magnetic compression of the developing powder between the rear wall of the
reservoir and the rotating cylinder, by which a very intensive additional
mixing of the developing powder is effected, shortly before this powder
leaves the reservoir.
Surprisingly it has appeared, that the magnet 53 also effects the ~ -
formation of a higher magnetic brush having a very uniform meniscus between
the rotating cylinder and the other magnet 8 cooperating with it, which is
favourable for the quality of the copies made.
Although in principle the type of magnet is irrelevant and, for
example, an electromagnet coult be used, a permanent magnet will be preferred
for economical reasons. ~ ~
In general, the length of the magnet 53 will a~tcorrespond with -
the length of the rotatable cylinder. The width may amount to, for example,
one or re centimetres. The magnet may for instance have a rectangular
section or be shapet in accordance with the shape of the rear wall of the
reservoir at the place of its installation.
Particularly good results are obtained using the embotiment des-
cribet. However, the invention can be applied to similar embodiments which
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do not have the described means for replenishing the developer mixture or
the rear edge for preventing accumulation of mixture.
The presence of the magnet 53 in the developing device described
above not only appear to have the intended further improvement with regard
to homogeneity and triboelectric charge height of the developing powder
leaving the reservoir, but also appeared to have resulted in a considerable
improvement in the mixing in the trough 14. This is probably caused by the
fact that the additional magnet 53 eliminates or strongly reduces the lines
of force created by the magnet which forms the powder brush. -
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