Note: Descriptions are shown in the official language in which they were submitted.
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"Device for developing electrostatic charge images".
According to the present invention there is provided a device
for developing electrostatic charge images present on a record carrierJ com-
prising a developer chamber, for containing a developer powder consisting of
a mixture of toner and carrier material and accommodating two parallel adja-
cent transport rollers which are contra-rotatable and ~hich are provided
on their circumferential surfaces with respective blades for transporting
the developer powder to and fro respectively in the axial direction of the
rollersJ wherein each transport roller is prov;ded with a plurality of said
blades spacedly arranged in the form of a helix of varying pitch.
In electrostatic reproduction methods first a latent electro-
static image is formed on a record carrier~ the image being made visible in a
subsequent developing process. During this developing process electroscopic
particles are deposited on the charged areas of the record carrier, and are
subsequently fixed in a fixing station so that a non-erasable image is
obtained. -
The developer powder consists of fine electroscopic particles
and a carrier material, usually iron grains. The two types of material adhere ;;
because of friction-electrical charging. When this developer comes into
contact with the record carrier bearing the latent image, the electroscopic
particle - also referred to as toner - is transferred to the record carrier ~
due to the electrostatic attraction by the carrier material itself drops ;
bac~ into the developer chamber and is combined again with toner particles.
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Various devices are known in which toner
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partic~les cc~n bc dopo~itod in the de~cribe~ m~nner. The
cII~elop~r is constantly spread over the full width o~ th~
record carrier by two transport ~ollers (turbulence). A device
of this l~ d is known, ~or example, from German Paten-t
Specification 1,094,590, in which tl~o rollers are arranged
parallel ~ith respect to each other and with respect to the
recorcl carrier, and are rotated in opposite directions~
l`he blades pro~ided on the circwrlference thereof act as
sho~re~ 1~hi~h ~orm a wall o~ developer po1~der be~een the t~Yo
rollers, the upper side of the said 1Yall touching the record
carrier. Toner particles which are not electrostatically
attracted by the latent image drop back into the de~eloper
chamber by gravity and so does the carrier material. Howevsr,
this does not ensure uniform development of the latent image~
because in practice the images are not uniforml~ distributed
over the record carrier. Afte:r a ~ew hours o~ operation, a
difference in development ~Yill occur - vie-~ed over the fu1l
width o~ the record carrier - which may be so great that
parts o~ tha image remain unvisible. A oo~tinuous supply
20~ of fresh toner is then required over the full chamber width.
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This requires additional structural stepsO
These drawbacks can be avoided -to a given
ext-ent- by using instead o~ sho~el rollers transport rollerq
such as worm rollers. Thc deve~oper is then transported
parallel to the record carrier by the roller arranged near
the record carrier, and is returned by the second roller.
Howe~er, in such a device the toner must be supplied anew
over the full chamber width if oonstant intensity o~ tIIe
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developmont in the transport direction of the developer i~
to be ensur~d.
In addition, the use o* -transport rollers gi~es
rise to the probleltl o* building~up at the area o~ chanOe-
o~er from one transport direction to the other. It was found
-that developer polrder collects betwe~n the ends of the
rollers and the chalnber ~all. Because the developer powder
acts as a "viscous" liquid, at these locations in the
devetopiIlg chamber a comparativaly high thrust is built up
which sprcads in all directions. As a result, only a part
is taken up by the transpo-t roller rotating in the opposite
direc-tioll (as is desired), but a :~urth0r part forms a
superfluous wall,part of ~rhich slides to within reach ~P
the other roller by gravity. A third part presses on the
record carrier and thus causes unnecessa-~ colouring of -the :
edge o~ the record carrier~
The record carrier is Ped into the developer
chamber in one;way or~another, usually via a guide s].ot~
Ho1rever, bhe width of this slot does not correspond to the
~ridth of.the record carrier, so that an opening always remains
on the side. Because o* the thrustj the coloured developer
pol~der is *orced out through this opening, thus causing
contamination of the complete device as ~rell as oP the back
of the record carrierO ~his escape of po1rder also occurs
at comparatively high transpo~t speeds o~ the record carrier.
This problem could be mitigat~d by enlarging the developing
chamber so that enough ciearance ~rould be maintained for the
developer betlreen the ends of the rollers and the developer
chamber, The buildi~g-up o* developer is then elimi~ated,
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but the necessary level of the developer powder is no longer reached, so
that toner is no longer applied to the edges of the record carrier. ~oreover,
a substantial quantity of developer powder would be deposited in this space,
and this powder would be lost for the developing process. Therefore, at the
areas where the developer changes over from one direction of transport to
the other, a given thrust must be maintained whichJ however, should be so ~-~
large that the level of the developer powder is maintained at these areas on
the one hand, whilst on the other hand it should not force the developer
powder out of the chamber
The invention has for its object to provide a device for develop- `
ing in which the developer chamber is as small as possible on the one hand,
whilst on the other hand the level of the mixture required or develoying
and displaying the latent image remains constant over the full width of the
record carrier.
To this end, the device according to the invention is character-
ized in that the blades on the circumference of the transport rollers are
provided in the form of a helix of varying pitch so that they form a worm
having a pitch which varies in the axial transport direction. As a result, ~-
developer particles can pass between the blades over ~he full length of the
roller and be taken up by the counter roller. Thus, the developer is not
only fed along the record carrier, but is notably also mixed in an uninter-
rupted manner over the full width of the chamber, the relationship
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bet~een ~he quantities transported in the axial direction of the roller
and in the direction transverse thereto being determined by the pitch on
the one hand, and by the distance between adjoining blades on the other
hand. As a result~ a uniform developnlent of the latent image is always
achievèd The arrangement of the blades w;th respect to eacll other (pitch
o the worm) can be readily chosen over the length of the roller such that
the level of the develc~per remains constant over the full width9 for example,
the pitch may ~e smaller at one end of the roller than at the centre
thereof. If, moreover, the last blades or ~he first blades of each roller
are arranged parallel to the axis of the roller or in the opposite direction
~negative pitch~, no excessive thrust will be built up on the wall of the
chamber, so that no developer powder can escape from the guide slot of the
developer chamber even at low transport speeds o the record carrier.
The invention will be described in detail hereinafter with
reference to the drawing.
Fig. 1 diagrammatically shows an electrostatic printer, and
Fig. 2 shows the transport rollers according to the invention.
In the printer diagrammatically shown in Fig. 1, a record
carrier 1 (in this case a paper strip) is transported to a printing station
5 via a shaft 2 provided with a slide roller which comprises an indicator ;
wheel 4 and which cooperates with a pressure roller 3. At the printing
station, the latent image, for example, alphanumerical signs, is formed.
Subsequently, thP record carrier 1 reaches a developer chamber 6 and is
deflected upwards by a guide plate g. After the latent images of the record
carrier 1 have been in contact with toner 7, the superfluous developer ~;
particles are removed from the record carrier 1 by a sweeping magnet 24.
Subsequently, the record carrier 1 reaches a fixing station 25 in which the
coloured toner particles are melted by heating so that they are fixed to
the record carrier 1 in a non-erasable manner. The record carrier 1 is
discharged from the printer via drive rollers 26 and 27.
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The toner mate~ial 7 i~ fed to the deyeloper chamber 6 by way
o~ a t~ner cassette 23. The toner cassette 23 can be readily ~xchanged.
The supply of toner by means of the cassette 23 prevents contamination of
the apparatus. The toner 7 is introduced into the developer chamber ~, by a
toner feed roller 22, after which it is electrostatically combined with the
carrier material, for example, iron powder present in the chamber. In the
rest state, the developer chamber 6 is filled with this developer mixture
approximately up to the broken line 61.
Arranged on the bottom of the developer chamber 6 are two
parallel transport rollers 10 and 11 which can rotate in opposite directions
as indicated by the arrows. A plan view of these transport rollers is given
in Pig. 2. The rollers are provided on their circumference with the blades
12 which are arranged such that they constitute a worm. The developer
material is thus displaced in known malmer in ~he longitudinal direction
of the roller. Because openings are provided between these blades, during
the entire transport developer particles will drop out of reach of the one
roller and within reach of the other roller. In this manner, the developer
powder is not only transported along the record carrier 1, but is also
continuously mixed at the same time.
In order to prevent build-ups at the end of the rollers, the
embodiment shown in Fig. 2 has at least the first blade 14 and 16 of each
of the rollers 10 and 11, respectively, arranged parallel to the axis of
the roller. Because for the indicated direction of rotation of the rollers
the developer mixture is transported in the direction of the arrows 100,
110, the blades 14 and 16 cause a "wake" which draws the mixture supplied
by the rollers 10 and 11 away from these rollers. This effect can be
enhanced by arranging the last blade 28 and 29 opposite to the blade 16
and 14, respectively, in an opposite sense to the helix concerned. This
arrangement ensures in any case that the mixture cannot be pressed onto the
wall of the chamber. Moreover, it is alternatively possible to arrange the
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second bladcs 13 an(l 15 to he more incllned than the blades 12 constituting
the helix, or -to arrange them axially like the blades 1~ and 16.
The pitch of the blades 12 and the distance a ~Fig. 1) between
the end faces of the blades 12 determine the quantity transported by the
rollers in the aYlal direction (arrows 100, 110) on the one hand, and that
transported over the full length of the rollers in the direction transverse
thereto ~arrows 112) on the other hand. The pitch of the helix can thus be
adapted over the full length of each roller to provide a suitable transport
rate at eacll part of the roller. In order to achieve the same object, it is
alternatively feasible to arrange the blades 12 such that their position
~pitch) is variable or that the distance a from each other is variable,
for example, by rotation or shifting in the longitudinal direction. The
level of the developer mixture can thus be kept constant over the full width
of the record carrier l. The dimension of the arrows 112 indicates the
quantity of transversely transported developer mixture in this embodiment
with a regularly varying pitch of th0 helix.
In contrast with Fig. 2, instead of the first blade or blades
13 to 16, the last blades of the rollers 10 and 11 can alternatively be
arranged in the axial direction. In that case the build-up near the rele-
vant chamber wall is not eliminated by a "wake" but rather by pressure. ~
However, the same effect is thus achieved. ~;
The rollers 10 and ll are driven in opposite directions by an
electric motor 17 or other known manner by way of a drive system 18, 19J 20.
The rotation of the rollers lO and ll spins the developer mixture upwards
and against the record carrier. This effect is enhanced by the permanent
magnet 28 arranged behind the record carrier 1 and by the movement of the
record carrier 1. A shiel~ 21 arranged over the roller 11 prevents dropping
developer particles or carrier particles from being spun upwards again
before having been combined again with toner particles. This shield rather
directs the particles to the region between the two rollers 10 and 11.
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