EP1864189B1

EP1864189B1 - Reverse flow binary image development

Info

Publication number: EP1864189B1
Application number: EP05709117A
Authority: EP
Inventors: Dror Kella; Zvi Cohen
Original assignee: Hewlett Packard Development Co LP
Current assignee: Hewlett Packard Development Co LP
Priority date: 2005-02-22
Filing date: 2005-02-22
Publication date: 2011-09-07
Anticipated expiration: 2025-02-22
Also published as: JP2008532063A; EP1864189A1; ATE523816T1; WO2006090352A1; JP4603051B2

Abstract

A binary image development printing system (100) using liquid toner, the system comprising: a) a developer member (104) configured so its surface moves in a first direction; b) at least one electrode (106, 108), separated from the developer surface by a gap (110) running part of the way around the developer surface; c) an inlet (111) for introducing the liquid toner into the gap (110); d) a voltage source (109) configured to apply a voltage difference between the electrode (106, 108) and the developer surface, sufficient to cause at least some toner particles in the liquid toner to be deposited in a toner layer (112) on the developer surface, when the liquid toner is introduced into the gap through the inlet (111); and e) a pump (138, 140) which causes a portion of the liquid toner introduced into the gap (110) through the inlet (111) to flow along the gap in a second direction opposite to the first direction, wherein the portion is more than 30% and/or wherein the portion is sufficiently great so that, when the voltage difference is applied, most of the toner particles deposited in the toner layer (112) come from said portion.

Description

FIELD OF THE INVENTION

The field of the invention is liquid toner printers and copiers.

BACKGROUND OF THE INVENTION

In a typical electrophoretic printer or copier, a photosensitive cylinder charged to a high voltage is exposed to light in certain regions, producing a latent image in which the voltage is reduced to a lower voltage depending on the exposure at each position. A toner, such as a liquid toner, with toner particles dispersed in a carrier liquid, is placed between the surface of the photosensitive cylinder and a development electrode, electrified to a voltage that is intermediate between the maximum and minimum voltage on the selectively exposed photosensitive layer. The development electrode thus produces an electric field normal to the surface of the photosensitive cylinder which is directed toward the photosensitive cylinder or away from it, depending on the potential at each position which in turn depends on how much light each position was exposed to.
Toner particles in the liquid toner migrate toward or away from the photosensitive cylinder, depending on the direction of the electric field at each position, and as a result, toner particles are selectively deposited on the surface of the photosensitive cylinder, converting the latent image into a developed toner image. For positions that were exposed to an intermediate amount of light, the density of toner particles may depend on the exposure at that position.
US patent 5,596,396 to Landa et al , and US patent 5,610,694, to Lior et al , describe a development method called binary image development (BID). In binary image development, instead of introducing a freely flowing liquid toner with charged particles against the surface of the photosensitive cylinder, a viscous concentrated layer of charged liquid toner particles, coating a developer cylinder, is placed against the surface of the photosensitive cylinder. The developer cylinder is at a voltage intermediate between the maximum and minimum voltage of the photosensitive cylinder. The two cylinders rotate, and different portions of the toner layer progressively come into contact with the photosensitive cylinder at a nip between the two cylinders. Depending on the direction of the electric field between the developer cylinder and the photosensitive cylinder at each point as it passes the nip, portions of the toner layer either are transferred from the developer cylinder to the photosensitive cylinder, or remain on the developer cylinder. This produces a developed toner image on the surface of the photosensitive cylinder, an image that, at each point, is either toned by the toner or left untoned.
Alternatively, as described in US patent 5,610,694 , less than the full thickness of the toner layer is transferred from the developer cylinder to the photosensitive cylinder, at those points where toner is transferred at all. This method may make the resulting developed image on the photosensitive cylinder less sensitive to possible non-uniformity of the toner layer on the developer cylinder.
To produce the layer of concentrated toner on the developer cylinder in the first place, liquid toner is run in a narrow gap between the rotating developer cylinder and an electrode, which produces an electric field which causes toner particles to adhere to the developer cylinder. As each portion of the surface of the development cylinder rotates beyond the end of the electrode, a squeegee removes excess liquid from that portion of the surface, leaving a uniform layer of concentrated toner coating the development cylinder. After each portion of the surface of the developer cylinder passes the nip and transfers part of the layer to the photosensitive member, a cleaning roller or scraper removes the remaining parts of the toner layer from that portion of the surface of the developer cylinder, providing a clean surface so that a uniform layer of toner can be coated on the developer cylinder for the next image as each portion of its surface passes the electrode again.
Japanese patent application number 09086192 (publication number 10282795 ) describes such an image development system in which a liquid toner flows into the gap between the electrode and the developer cylinder through an opening in the middle of the electrode. The electrode is adjacent to one side of the developer cylinder, whose surface is moving upward on that side. Some of the liquid toner is carried upward with the surface of the developer cylinder, while some of the liquid toner flows downward along the surface of the developer cylinder, moving in a direction opposite to the direction of motion of the surface. In both the upward and downward moving liquid toner, some toner particles migrate to the surface of the developer cylinder under the influence of the electric field produced by the electrode, and adhere to the developer cylinder.
A similar image development system is described as prior art in PCT publication WO 01/92962 , but with the electrode below the developer cylinder instead of to its side. Most of the liquid toner coming out of the opening in the middle of the electrode flows along the gap in the direction of motion of the developer cylinder, but some of it flows along the gap in the opposite direction.
Japanese patent publication 50-152741 describes an electrophoretic printer in which liquid toner emerges from an opening in the middle of an electrode, and flows in along a gap between the electrode and a rotating photosensitive cylinder. The toner flows in both directions from the opening, i.e., in the same direction as the rotating cylinder, and in the opposite direction.
US 6,108,513 describes a system for double-sided imaging on a continuous-web substrate. Pressurized liquid toner emerging from an inlet, which is formed between a charged electrode and an insulating material, enters a space between the charged electrode and a charged developer roller and is deposited thereon. Due to a large difference in voltage between the electrode and the development roller the toner particles adhere to the development roller resulting in a layer of liquid toner deposited on its surface.
Similarly in US 2004/0175206 liquid toner is forced by pressure via a passage in the middle of an electrode to enter a space between the electrode and a development member. The electrode and the development member are electrified to different voltages, so that the charged toner particles are plated onto the development member.
A wet type electrostatic development device is described in US 4,327,664 , wherein a development liquid is poured between a porous resilient developing roller and a trough. The development liquid is absorbed by the porous resilient roller when passing the trough, so that the developing roller has a thin layer of development liquid on its surface.

SUMMARY OF THE INVENTION

An aspect of an embodiment of the invention concerns a binary image development system as defined in claim 1. Liquid toner flows along the surface of the developer cylinder, in the gap between the electrode and the developer cylinder, in a direction opposite to the direction of rotation of the cylinder. Optionally, most, or almost all, of the liquid toner flows in this direction. Optionally, most (or almost all) of the toner particles coating the developer cylinder are deposited from liquid toner that flows in a direction opposite to the rotation of the developer cylinder. Optionally, a larger fraction of the toner particles adhere to the developer cylinder than in conventional binary image development systems, in which most of the liquid toner flows in the same direction as the developer cylinder.
This increased efficiency in coating the developer cylinder with toner means less toner has to be pumped, and less toner needs to be transported in the liquid transportation system which optionally recycles the toner after it goes through the gap. In addition, since most development takes place in the backflow position, the development takes place under strong shear conditions, which serves to break up agglomerates and helps spread a homogeneous uniform layer of toner on the developer. Since the general direction of the liquid toner is against the direction of the developer surface, less carrier liquid moves with the toner, and a mechanism which removes surplus liquid such as the squeegee has less work to do. Optionally there is no need for a squeegee at all.
In order to cause liquid toner to flow in a direction opposite to the motion of the developer surface, which acts as a pump drawing liquid toner to flow in the same direction as its motion, there is another pump which draws liquid toner in the opposite direction. Optionally, the pumping action in the opposite direction is done entirely or partly by a cleaning member, which cleans the old toner layer off the developer surface. Optionally, the toner enters the gap between the electrode and the developer surface near the end of the electrode at which the developer cylinder leaves the gap, so that the liquid toner that flows past the developer cylinder in the direction opposite to the rotation of the developer cylinder has a long path. Optionally at least 30% of the toner flows in the direction opposite to the rotation of the developer cylinder, or at least 50% of the toner flows in that direction, or at least 75% flows in that direction.
Another potential advantage of having much or most of the toner flow in the direction opposite to the rotation of the developer cylinder is that the remaining liquid from the toner used to develop the image is available to help clean the developer surface by the cleaning member. Because the toner particles have been largely deposited on the developer cylinder, the remaining liquid is relatively clear, and more suitable for cleaning than the toner with its original concentration of toner particles. In contrast, when almost all of the toner flows in the direction of rotation of the developer cylinder, then additional toner may have to be pumped to the cleaning member, in order to assist the cleaning. This additional toner is still full of toner particles, so is less suitable for cleaning, and the total amount of toner used is much greater than if the same toner is used first for developing and then for cleaning.
There is provided, in accordance with an embodiment of the invention, a binary image development printing system using liquid toner, the system comprising:

a) a developer member configured so its surface moves in a first direction;
b) at least one electrode, separated from the developer surface by a gap running part of the way around the developer cylinder;
c) an inlet for introducing the liquid toner into the gap;
d) a voltage source configured to apply a voltage difference between the electrode and the developer surface, sufficient to cause at least some toner particles in the liquid toner to be deposited in a toner layer on the developer surface, when the liquid toner is introduced into the gap through the inlet; and
e) a pump which causes a portion of the liquid toner introduced into the gap through the inlet to be drawn through the gap in a second direction opposite to the first direction;

Optionally, the pump causes at least 30% of the liquid toner introduced into the gap to flow in the second direction.
Optionally, the pump causes at least 50% or at least 75% of the liquid toner introduced into the gap to flow in the second direction.
In an embodiment of the invention, the developer surface is the surface of a cylinder and wherein said first direction is the direction of movement of the surface of the cylinder.
In various embodiments of the invention, at least 25%, 50%, 75% or 90% of the toner particles in the liquid toner which flows in the second direction are deposited in the tone layer.
In an embodiment of the invention, the inlet is situated at a position such that the liquid toner flowing in the second direction flows a greater distance along the gap than the liquid toner flowing in the first direction.
In various embodiments of the invention, the pump comprises one or more rotating surface elements forming a substantially sealed region at an end of the gap in the second direction from the inlet, that pump liquid away from the gap.
Optionally, the surface elements are surfaces of one or more rotating cylinders. Optionally, the surface elements are surfaces of a moving belt that rotates about one or more backing cylinders. Optionally, the one or more rotating surface elements comprise a surface of a cleaning element, which contacts the developer surface at a location past the end of the gap in the second direction from the inlet, thereby cleaning the developer surface. Optionally, the one or more rotating surface elements also comprise a surface of a sponge element, which contacts the surface of the cleaning element, thereby cleaning the surface of the cleaning element as the cleaning element and the sponge element rotate.
There is further provided, in accordance with an embodiment of the invention a method of binary image development printing, comprising:

a) introducing a liquid toner into a gap between an electrode and a rotating developer surface moving in a first direction;
b) drawing a portion of the introduced liquid toner along the gap in a second direction opposite to the first direction;
c) producing a toner layer on the developer surface by plating toner particles from the liquid toner in the gap onto the developer surface responsive to an electric field produced by a voltage difference between the electrode and the developer surface, and
d) using said toner layer to develop an image on a photosensitive surface, wherein more than half of the toner particles in the toner layer are plated onto the developer surface from toner flowing in the second direction.

Optionally, pumping comprises pumping at least 30% of the introduced toner, in the second direction.
Optionally, pumping comprises pumping at least 50% or 75% of the introduced toner, in the second direction.
Optionally, the electric field induces at least 25%, 50% or 75% of the toner particles in the liquid toner flowing in the second direction to adhere to the developer cylinder.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary non-limiting embodiments of the invention are described in the following sections with reference to the drawings. The drawings are generally not to scale and the same or similar reference numbers are used for the same or related features on different drawings.

Fig. 1 is a side cross-sectional view of a binary image development system, according to an exemplary embodiment of the invention; and
Fig. 2 is a side cross-sectional view of a binary image development system, according to a different exemplary embodiment of the invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Fig. 1 shows a binary image development system 100. A reservoir 102 contains liquid toner, for example Hewlett-Packard toner marketed under the trade name ElectroInk^®, or another liquid toner known in the art. A developer cylinder 104, rotating in a clockwise direction, has its surface in close proximity to a coating electrode having a forward electrode portion 106, and a back electrode portion 108. Developer cylinder 104 is separated from the electrode portions by a gap 110. Liquid toner is pumped through a channel 111 into gap 110 between the back and forward portions, such that some of it flows in a forward direction as shown by arrow 113, i.e. in the same direction as developer cylinder 104 is rotating, past electrode portion 106.
In accordance with an embodiment of the invention, most of the liquid toner, or a significant fraction of it, flows in a backward direction as shown by arrow 115, opposite to the direction of rotation of cylinder 104, past electrode portion 108. Optionally, electrode portions 106 and 108 are a single electrode, joined outside the plane of the drawing. Alternatively, they are optionally maintained at a same voltage. In any case, electrode portions 106 and 108 are each maintained, by a voltage source 109, at a voltage that is different than the voltage of the surface of developer cylinder 104, with the difference, for both electrodes, causing migration of the toner particles to and coating of the developer electrode.
As the liquid toner flows through gap 110, the electric field in the gap causes toner particles to migrate toward the surface of developer cylinder 104, and to adhere to (and coat) the surface. This happens for both the forward flowing toner, adjacent to electrode portion 106, and in the backward flowing toner, adjacent to electrode portion 108. The direction of the electric field needed depends on the charge of the toner particles, which can either be positive or negative, depending on the type of liquid toner used. The toner particles form a wet layer 112 of concentrated liquid toner on the surface of developer cylinder 104. Optionally, the fact that toner particles can adhere to the surface of developer cylinder 104 both adjacent to electrode 106 and adjacent to electrode 108 means that more of the toner particles adhere to the developer cylinder than if there were only an electrode adjacent to the forward flowing toner, as is in some other prior art printers.
As the developer cylinder rotates, the wet layer of toner passes an end 114 of electrode portion 106, and passes through a nip 116 between developer cylinder 104 and a preferably electrified squeegee roller 118. Squeegee roller 118 squeezes excess liquid out of the toner layer on the surface of developer cylinder 104, while the electric field formed between the squeegee roller and the developer cylinder urges the toner particles toward the developer cylinder. After passing squeegee roller 118, the concentrated toner layer is a viscous or semisolid layer 120 of concentrated toner particles, with a higher toner particle concentration than wet layer 112. For example, the liquid toner starts with a concentration of between 0.1% and 10% toner particles. In wet layer 112, the concentration of toner particles is between 2% and 20% (usually greater than 5% or 10%); while in layer 120 it is between 10% and about 50%. For non-tentacular toner and/or for toner particles that do not solvate the carrier liquid, higher concentrations may be possible.
As development cylinder 104 continues to rotate, concentrated layer 120 passes through a nip 122 between development cylinder 104 and a photosensitive cylinder 124. The surface of photosensitive cylinder 124 is electrified using a corotron or scorotron or other means known in the art and then selectively exposed by a laser 126, or other light source, prior to passing through nip 122, thereby producing a latent image on the surface of photosensitive cylinder 124. As in US patents 5,596,396 and 5,610,694 , the voltage of each point on the surface of photosensitive cylinder 124 is either greater than or less than the voltage of the surface of developer cylinder 104, depending on whether that point was exposed to light from the laser.
Where the electric field is pointing in one direction, at least a portion of the thickness of layer 120 breaks off from the surface of developer cylinder 104 and adheres to the surface of photosensitive cylinder 124 at that point Where the electric field is pointing in the other direction, layer 120 remains on developer cylinder 104, and the surface of photosensitive cylinder 124 remains free of toner at that point Similar to the situation with the electric field in gap 110, the sign of electric field needed in nip 122 to cause the toner layer to transfer to and adhere to the photosensitive cylinder depends on the charge of the toner particles, which may be positive or negative depending on the type of toner used.
As the surface of photosensitive cylinder 124 goes past nip 122, the latent image is thus developed and becomes a toner image, which is then transferred to a printing medium, by means well known in the art, but not shown in Fig. 1. Such transfer may be direct, in which case the image is transferred to the printing medium directly from the photoreceptor or may be made via an intermediate transfer member. Both methods and apparatus for their implementation are very well known in the art. The portions 128 of the toner layer which were not transferred to photosensitive cylinder 124 remain on developer cylinder 104.
As developer cylinder 104 continues to rotate, portions 128 of the toner layer are cleaned off the surface of developer cylinder 104, optionally by a cleaning cylinder 130. Optionally, cleaning cylinder 130 rotates with the same sense as developer cylinder 104, i.e. clockwise in Fig. 1, as shown by the arrow on cylinder 130 in Fig. 1, so that its surface rubs against developer cylinder 104, removing the toner layer at a contact point 132. Alternatively, cleaning cylinder rotates in the opposite direction from developer cylinder 104, i.e. counterclockwise. Optionally, cleaning cylinder 130 is maintained at a voltage such that the remaining toner layer 128 will be removed from the surface of developer cylinder 104 and adhere to cleaning cylinder 130, as it comes into contact with the remaining toner layer. The cleaning process is optionally helped by liquid 134 from the liquid toner, now at least partly (and generally mostly) depleted in toner particles, which has traveled through the gap past electrode portion 108 and reaches the surface of cleaning cylinder 130, wetting it. Optionally, the surface of cylinder 130 is resilient, so that it is in contact with developer cylinder 104 over an extended area.
Optionally, development system 100 includes a rotating cylinder 136, surrounded by a sponge 138, which rubs against the other side of cleaning cylinder 130, at a contact region 140, cleaning the toner particles off the surface of cleaning cylinder 130. Note that, although sponge 138 is optionally cylindrically symmetric, it is compressed where it comes into contact with cleaning cylinder 130, electrode 108, and another cylinder 137 which squeezes the excess toner liquid from sponge 138. Optionally, as shown by the arrows on cylinder 130 and 136 in Fig. 1, cylinder 136 rotates with the same sense as cylinder 130. Alternatively, cylinder 136 rotates with the opposite sense of cylinder 130, but optionally at a speed so that the surface of cylinder 130 rubs against the surface of sponge 138. Liquid 134 is also optionally absorbed by sponge 138, wetting the sponge so that it can more effectively clean off the cleaning cylinder. A scraper 142 optionally scrapes against the surface of cleaning cylinder 130 as it turns, removing toner, and/or loosening toner so that it can be more easily cleaned off by the sponge. Alternatively or additionally, other means are used to clean off cleaning cylinder 130.
The rotation of cylinder 136 (and in some measure cylinder 130) optionally draws liquid 134 away from the part of gap 110 that is adjacent to electrode portion 108. Particularly if liquid 134 fills the space between electrode portion 108, developer cylinder 104, sponge 138, and cleaning cylinder 130, with little or no air present, the rotation of cylinder 136 acts like a pump, drawing liquid toner from opening 111 through the gap between electrode portion 108 and developer cylinder 104, causing more than 30% of the liquid toner, or more than 50%, or more than 75%, or more than 90%, to flow in this backward direction, rather than in the forward direction past electrode portion 106. Liquid 134 then exits through one or more of the gap between cleaning cylinder 130 and developer cylinder 104, the gap between sponge 138 and cleaning cylinder 130, and the gap between sponge 138 and electrode portion 108, depending on the directions of rotation of cylinder 136 and cleaning cylinder 130.
Alternatively or additionally, there is a separate pump, not shown in Fig. 1, which causes most or much of the liquid toner to flow in the backward direction. It should be noted that in the absence of the pumping action, only a small fraction of the liquid toner, for example less than 30%, would flow past electrode portion 108, since, with the narrow gap used for development, the pumping action of cylinder 104 forces the liquid toner to flow past electrode portion 106. For example, if gap 110 is 0.5 mm wide, channel 111 is 5 mm wide, the velocity of the surface of the developer is 1m/s and the liquid toner is flowing at 0.062m/s through channel 111, and with sponge 138 not airtight so that the backward pumping is fairly weak, about 80% of the toner flowing from opening 111 flows in the forward direction past electrode portion 106.
Optionally, whether or not most of the liquid toner flows in the backward direction, most of the toner layer deposited on developer cylinder 104 comes from liquid toner that is flowing in the backward direction, past electrode portion 108. For example, if the liquid toner flowing backward past electrode portion 108 flows more slowly than the liquid toner flowing forward past electrode portion 106, then a larger fraction of the backward flowing toner particles may be deposited on the electrode, because they have more time to travel across the gap than the forward flowing toner particles.
Optionally, opening 111 is closer to the middle of the electrode pair 106/108 than shown in Fig. 1, so that electrode portion 108 is similar in width to electrode portion 106, or opening 111 is even closer to left end 114 of the electrode, so that electrode portion 108 is wider than electrode portion 106.
Fig. 2 shows a binary image development system 200 that is similar to system 100 in Fig. 1, but with only a single electrode 208, which extends over most of the area covered by electrodes 106 and 108 in system 100. The liquid toner flows into gap 110 through an opening 211 that is located near end 114 of gap 110. The pumping action produced by the rotation of cylinder 136, and/or by a separate pump if there is one, draws all, most or at least a large portion of the liquid toner through gap 110 along electrode 108, with relatively little toner, if any, flowing the other way, past end 114 of gap 110. Optionally, there is also a short electrode 206 on the side of the gap between opening 211 and end 114, which causes toner particles to adhere to developer cylinder 104, for the small fraction of the toner which flows in that direction. Even in that case, most of the toner particles which adhere to developer cylinder 104 do so in the part of gap 110 adjacent to electrode 208, and if electrode 208 is the only electrode, then all of the toner particles that adhere to developer cylinder 104 do so in the part of gap 110 that is adjacent to electrode 208.
Having most or all of the flow in the backward direction, and having most or all of the toner layer deposited from liquid toner flowing in the backward direction, offers several potential advantages, as noted previously. The high shear rate associated with back flow may tend to break up agglomerates in the liquid toner. The high shear rate may cause the developer cylinder to be coated more uniformly, possibly allowing a thinner layer of toner to be used. The high shear rate may also prevent much or almost all of the carrier liquid from following the toner particles and adhering to the developer cylinder. Squeegee roller 118 has less carrier liquid to remove, and optionally there is no squeegee roller. A higher fraction of the toner particles may be transferred to the developer cylinder when a large fraction of the liquid toner flows in the backward direction, than when it is flowing forward as in a prior art binary image development system since each particle has more time to cross the gap, as it flows first one way, then the other way, in the sheared flow. For all these reasons, systems 100 and 200 potentially use less toner per image than a conventional system.
In prior art systems with a cleaning cylinder and/or a sponge, a certain amount of toner liquid is needed in order to wet the cleaning cylinder and/or the sponge. If most of the liquid toner flows in the forward direction, then a large amount of liquid toner may have to be used, in order to ensure that the cleaning cylinder and/or sponge receives the minimum amount of liquid needed. In addition some of the liquid toner flowing in the forward direction may not be used for plating the developer roller. Both these reasons require that much more liquid flow in the system and flow of toner to the cleaning cylinder, which complicates the system.
In systems 100 and 200, the same liquid toner, flowing backwards, is used to coat the developer cylinder and to wet the cleaning cylinder and sponge, so little or no liquid toner is wasted. Furthermore, the liquid toner that reaches the cleaning cylinder, unlike in the prior art, may be largely depleted in toner particles, most of which may be deposited on the developer cylinder as it flows past electrode portion 108. The liquid reaching the cleaning cylinder and the sponge is thus cleaner, and more suitable for cleaning the cleaning cylinder and sponge.
The invention has been described in the context of the best mode for carrying it out. It should be understood that not all features shown in the drawings or described in the associated text may be present in an actual device, in accordance with some embodiments of the invention.
Furthermore, variations on the method and apparatus shown are included within the scope of the invention, which is limited only by the claims. For example, while the invention is described in an embodiment for which cylinders are used for many of the elements, most or all of these elements can be replaced by suitably backed belts or the like. As used herein, for example in the claims, the term "surface" is used to indicate this broader class of elements. Also, features of one embodiment may be provided in conjunction with features of a different embodiment of the invention. As used herein, the terms "have", "include" and "comprise" or their conjugates mean "including but not limited to."

Claims

A binary image development printing system (100, 200) using liquid toner, the system (100, 200) comprising:
a) a developer member (104) configured so its surface moves in a first direction;

b) at least one electrode (108, 208), separated from the developer surface by a gap (110) running part of the way around the developer cylinder;

c) an inlet (111, 211) for introducing the liquid toner into the gap (110);

d) a voltage source (109) configured to apply a voltage difference between the electrode (108, 208) and the developer surface, sufficient to cause at least some toner particles in the liquid toner to be deposited in a toner layer on the developer surface, when the liquid toner is introduced into the gap (110) through the inlet (111, 211);
characterized by

e) a pump configured to a portion of the liquid toner introduced into the gap (110) through the inlet (111, 211) to be drawn through the gap (110) in a second direction opposite to the first direction;
wherein said portion is sufficiently great so that, when the voltage difference is applied, most of the toner particles deposited in the toner layer come from said portion because the liquid toner drawn in the second direction past the electrode (108, 208) flows more slowly than the liquid toner flowing in the first direction.
A system according to claim 1, wherein the pump causes at least 30% of the liquid toner introduced into the gap to flow in the second direction.
A system (100, 200) according to any of the preceding claims, wherein the pump causes at least 50% of the liquid toner introduced into the gap (110) to flow in the second direction.
A system (100, 200) according to claim 3, wherein the pump causes at least 75% of the liquid toner introduced into the gap (110) to flow in the second direction
A system (100, 200) according to any of the preceding claims wherein the developer surface is the surface of a cylinder and wherein said first direction is the direction of movement of the surface of the cylinder.
A system (100, 200) according to any of the preceding claims, wherein at least 25% of the toner particles in the liquid toner which flows in the second direction are deposited in the toner layer.
A system (100, 200) according to claim 6, wherein at least 50% of the toner particles in the liquid toner which flows in the second direction are deposited in the toner layer.
A system (100, 200) according to claim 7, wherein at least 75% of the toner particles in the liquid toner which flows in the second direction are deposited in the toner layer.
A system (100, 200) according to claim 8, wherein at least 90% of the toner particles in the liquid toner which flows in the second direction are deposited in the toner layer.
A system (100, 200) according to any of the preceding claims, wherein the inlet (111, 211) is situated at a position such that the liquid toner flowing in the second direction flows a greater distance along the gap (110) than the liquid toner flowing in the first direction.
A system (100, 200) according to any of the preceding claims, wherein the pump comprises one or more rotating surface elements forming a substantially sealed region at an end of the gap (110) in the second direction from the inlet (111, 211), that pump liquid away from the gap (110).
A system (100, 200) according to claim 11, wherein the surface elements are surfaces of one or more rotating cylinders.
A system (100, 200) according to claim 11, wherein the surface elements are surfaces of a moving belt that rotates about one or more backing cylinders.
A system (100, 200) according to any of claims 11-13, wherein said one or more rotating surface elements comprise a surface of a cleaning element (130), which contacts the developer surface at a location (132) past the end of the gap (110) in the second direction from the inlet (111, 211), thereby cleaning the developer surface.
A system (100, 200) according to claim 14, wherein said one or more rotating surface elements also comprise a surface of a sponge element (138), which contacts the surface of the cleaning element (130), thereby cleaning the surface of the cleaning element (130) as the cleaning element (130) and the sponge element (138) rotate.
A method of binary image development printing, comprising:
a) introducing a liquid toner into a gap (110) between an electrode (108, 208) and a rotating developer surface moving in a first direction;

b) drawing a portion of the introduced liquid toner through the gap (110) in a second direction opposite to the first direction;

c) producing a toner layer on the developer surface by plating toner particles from the liquid toner in the gap (110) onto the developer surface responsive to an electric field produced by a voltage difference between the electrode (108, 208) and the developer surface; and

d) using said toner layer to develop an image on a photosensitive surface,
characterized in that the portion is sufficiently great so that most of the toner particles in the toner layer are plated onto the developer surface from toner flowing in the second direction because the liquid toner drawn in the second direction past the electrode (108, 208) flows more slowly than the liquid toner flowing in the first direction.
A method according to claim 16, wherein drawing comprises drawing at least 30% of the introduced toner, in the second direction.
A method according to any of claims 16-17, wherein drawing comprises drawing at least 50% of the introduced toner, in the second direction.
A method according to claim 18, wherein drawing comprises drawing at least 75% of the introduced toner, in the second direction.
A method according to any of claims 16-19, wherein the electric field induces at least 25% of the toner particles in the liquid toner flowing in the second direction to adhere to the developer cylinder (104).
A method according to any of claims 16-20, wherein the electric field induces at least 50% of the toner particles in the liquid toner flowing in the second direction to adhere to the developer cylinder (104).
A method according to any of claims 16-21 wherein the electric field induces at least 75% of the toner particles in the liquid toner flowing in the second direction to adhere to the developer cylinder (104).