US3672326A - Method and apparatus for applying liquid developer to copy sheets - Google Patents

Abstract

Apparatus and method for developing electrostatically imaged copy sheets by the application of a liquid developer to the imaged surface thereof includes a cylinder mounted for rotation in a predetermined direction and at a predetermined speed while being dipped into a supply of developer liquid. A copy sheet is passed over the cylinder in the direction of rotation of the latter with the imaged surface of the copy sheet in contacting relation with the liquid coated surface of the cylinder. A sensor downstream of the cylinder along the copy sheet path detects the leading edge of the copy sheet and in response thereto reverses the direction of rotation of the cylinder until such time as the trailing edge of the sheet passes the sensor. In a preferred embodiment, the cylinder includes an internal auger which stirs the supply of developer liquid during rotation of the cylinder to maintain toner particles in the developer liquid in suspension. The outer surface of the cylinder includes a plurality of glass beads, each having a diameter in the range of 0.002 to 0.010 inches for carrying a predetermined quantity of developer liquid for application to the copy sheets.

Description

United States Patent Schulze 51 June 27, 1972 [72] Inventor: John J. Schulze, Prospect Heights, 111.-

[73] Assignee: Addressograph-Multigraph Corporation,

. Mt. Prospect, Ill.

[22] Filed: Oct. 13, 1970 [21] Appl.No.: 80,381

[56] References Cited UNITED STATES PATENTS 3,169,887 2/1965 York ..118/637 3,245,341 4/1966 Childress et al.. ....1 18/637 3,367,791 2/1968 L616 ..1 18/637 OTHER PUBLICATIONS IBM Tech. Disclosure Bulletin, Developing Electrostatic Charge Patterns, By T. M. Crawford, Vol. 8, No. 4, Sept. 65.

Primary ExaminerMervin Stein Assistant Examiner-Leo Millstein Attorney-Sol L. Goldstein 5 7] ABSTRACT Apparatus and method for developing electrostatically imaged copy sheets by the application of a liquid developer to the imaged surface thereof includes a cylinder mounted for rotation in a predetermined direction and at a predetermined speed while being dipped into a supply of developer liquid. A copy sheet is passed over the cylinder in the direction of rotation of the latter with the imaged surface of the copy sheet in contacting relation with the liquid coated surface of the cylinder. A sensor downstream of the cylinder along the copy sheet path detects the leading edge of the copy sheet and in response thereto reverses the direction of rotation of the cylinder until such time as the trailing edge of the sheet passes the sensor. In a preferred embodiment, the cylinder includes an internal auger which stirs the supply of developer liquid during rotation of the cylinder to maintain toner particles in the developer liquid in suspension. The outer surface of the cylinder includes a plurality of glass beads, each having a diameter in the range of 0.002 to 0.010 inches for carrying a predetermined quantity of developer liquid for application to the copy sheets.

9 Claims, 6 Drawing Figures EPMENTEDMN 1912 3, 6 72, 326

sum 1 or 2 (P Sal METHOD AND APPARATUS FOR APPLYING LIQUID DEVELOPER TO COPY SHEETS BACKGROUND OF THE INVENTION This invention relates generally to liquid developing apparatus used to deposit toner particles suspended in a liquid carrier onto the surface of moving copy sheet material in'accordance with a latent electrostatic image formed thereon, and more particularly to such an apparatus which applies the liquid developer only to one surface of the copy material.

Various arrangements for applying developing solution to one surface of copy sheet material in a copying machine are known in the art. One type in particular comprises a drum or cylinder mounted for rotation in a predetermined direction and at predetermined speed while being partially immersed or dipped in a supply reservoir of developer liquid. The rotational movement of the cylinder through the liquid causes the latter to be raised and flowed over the exposed surface of the cylinder. Copy sheets are fed in the direction of rotation of the cylinder with the imaged surfaces of the sheets substantially in tangential contact with the liquid coated cylinder surface, whereby liquid developer solution from the cylinder surface is applied to the imaged copy sheet surface to develop the latter.

In certain embodiments of developer apparatus of the type described heretofore, the cylinder is of a conductive material and a guide plate or the like element is mounted directly above the cylinder in spaced relation with respect to the exposed surface thereof. The guide plate slidably contacts the back surface of charged copy sheets passing between the guide plate and cylinder so as to maintain the imaged surface of the sheets in contacting relation with the developer liquid covering the cylinder surface and at a predetermined distance from the cylinder surface itself to provide a field control effect. Such a developer apparatus and an explanation of the field control effect are described in detail in -U.S. Pat. No. 3,367,791, issued Feb. 6, 1968.

While the above described liquid developing apparatus functions satisfactorily for the most part to effectively develop copy sheets, there are certain drawbacks in the design thereof. To explain, in the case wherein a guide plate of the type described is employed, often, due to the rotation of the cylinder, developer liquid is accumulated on the guide plate, and sheets subsequently fed through the developer apparatus emerge therefrom'with toner smeared on the back thereof. Also, as the trailing edge of a copy sheet passes over the cylinder surface, liquid developer solution, lifted by the cylinder which is rotating in the same direction as the copy sheet is moving, forms a beadof liquid between the cylinder surface and copy sheet which produces a darkened line along the trailing edge of the sheet and/or liquid is often splashed onto the back surface of the sheet itself.

In addition to the above, it has been difficult and expensive to provide a cylinder for a developing apparatus of the type discussed, which has a surface that applies a consistently uniform amount of liquid to copy sheets passing thereover. Also, developer liquid in an apparatus of the type described generally requires constant stirring or mixing in the reservoir to prevent the toner particles, which are suspended in the carrier liquid, from falling out of suspension. Normally such mixing is provided by a liquid circulating pump. The latter however, is costly and requires maintenance.

SUMMARY OF THE INVENTION Accordingly, it is a primary object of the present invention to provide new and improved liquid developing apparatus similar to the type described which overcomes the drawbacks of prior art apparatus.

It is another object of the present invention to provide a liquid developing apparatus of the last-mentioned type which is relatively simple in construction, inexpensive to fabricate and efficient in operation.

Briefly, a preferred embodiment of a liquid developer apparatus according to the invention includes a drum or cylinder mounted for rotation partially immersed in a reservoir of developer liquid. The cylinder is rotatable both in clockwise and counterclockwise directions. Copy sheet material is fed toward the cylinder surface and is directed by feed rollers below the point of tangency with respect to the cylinder. The rotation of the cylinder in the same direction as the movement of the copy sheet material and the stiffness of the copy sheet material itself carries the sheet over the cylinder surface in contacting relation with the layer of liquid developer covering the surface of the cylinder. Sensing means downstream of the cylinder spaced a predetermined distance therefrom along the copy sheet path, detects the leading edge of the copy sheet and in response thereto, reverses the direction of rotation of the cylinder. The latter causes liquid to be carried onto the cylinder surface in a direction opposite to the movement of the cylinder and thus no bead of liquid builds up at the trailing edge of the copy sheet, thereby eliminating the darkened line at the trailing edge of the sheet present on copy sheets developed with prior an apparatus described heretofore.

Y The need for a guide plate directly above the cylinder is likewise eliminated through the provision of a guide member taking the form of a single feed roller positioned downstream of the cylinder along the copy path which maintains the copy sheet, already directed toward the cylinder surface, in contact with the latter. The position of the single feed roller is well outside the area in which developer liquid from the cylinder surface might be splashed so that no developer liquid can be transferred from the roller to subsequently fed copy sheets.

The cylinder surface includes an outer layer of glass beads boned thereto. The beads range in size from 0.002 to 0.010 inches in diameter to provide an effective means to apply an amount of developer liquid to the imaged copy sheet which is sufiicient to uniformly develop the latter consistently while not wetting the sheet excessively. In addition, if the cylinder is of a conductive material, the beads serve to maintain the charged copy sheet a proper distance from the cylinder to produce a field control effect.

In a preferred embodiment of the developing apparatus, the internal core" of the cylinder takes the form of an auger so that the rotational movement of the cylinder in the developer liquid causes the auger to stir and agitate the developer liquid. Thus, the toner particles are maintained in suspension in the liquid carrier, and the need for a circulating pump is eliminated.

DESCRIPTION OF THE DRAWINGS A better understanding of the present invention and its organization and construction may be had by referring to the description below in conjunction with the accompanying drawings, wherein:

FIGS. 13 are end sectional views of a preferred embodiment of the liquid developer apparatus according to the invention, illustrating the operation thereof as copy sheet material is transported therethrough for development;

FIG. 4 is a side sectional view of the developer apparatus of FIGS. 1-3, illustrating the construction of the auger core of a preferred embodiment of the drum or cylinder employed therein;

FIG. 5 is a cut away perspective view of the drum of the developer apparatus according to the invention, illustrating in greater detail the liquid carrying surface thereof; and

FIG. 6 is an enlarged view of a section of the liquid carrying surface of the drum of the developer apparatus of the invention taken along line 6-6 illustrating the manner in which liquid developer is retained on the carrier surface.

DETAILED DESCRIPTION Referring now to the drawings in greater detail, FIGS. 1-3 thereof show end sectional views of a preferred embodiment of a liquid developing assembly 10 according to the invention. The first three figures (l3) illustrate the sequence of operation of the developing apparatus according to the invention as copy sheet material 12 is passed therethrough.

The assembly comprises developing apparatus 14 including a tank or reservoir 16 of liquid developing solution 18. A drum or cylinder 20 is rotatably mounted on a fixed axis 22 with respect to tank 16. The lower portion 24 of cylinder 20 dips into the quantity of developing liquid 18 and the rotational movement of the cylinder at a predetermined speed produces a washing of the developer liquid over the exposed surface 26 of the cylinder and maintains a layer of liquid thereon.

The layer 28 of the cylinder can be of a conductive material, such as for example, brass, if desired, to aid in moving toner particles in the liquid developing solution on the cylinder surface toward copy sheet material passing thereover due to the field control effect described in the above mentioned patent. It is not required however that layer 28 be conductive for the purposes of this invention.

A pair of copy sheet transport plus 30, 32 is provided upstream of the developing cylinder 20 along the copy sheet path designated by arrow 34 showing the direction of movement of sheet material being developed by the apparatus of the inventron.

The rollers 30, 32 are situated so that nip 36 thereof is at a level along a horizontal plane slightly nearer to the surface of cylinder 20 than the horizontal plane shown in dotted lines and designated by the numeral 37 extending tangent to the cylinder. Thus, copy sheets fed along copy path 34 by rollers 30, 32 are directed toward the cylinder surface.

A guide member taking the from herein of a rotatably mounted roller 38 is positioned at or slightly above the plane 37 of horizontal tangency with respect to the exposed cylinder surface 26, and the rotational axis thereof is aligned vertically, substantially along the plane 39 of vertical tangency (shown in dotted lines) with respect to the surface of cylinder 20. The direction of rotation of roller 38 as indicated by arrow 40 is the same as the direction of movement of copy sheet 12.

A second pair of feed rollers, 42, 44 is mounted along copy sheet path 34 downstream of the developing apparatus 14. The nip 46 of rollers 42, 44 lies along the same horizontal plane as the nip 36 of the roller pair 30, 32 at or slightly beneath the lower most peripheral surface 48 of guide roller 38.

A sensing device 50, here shown as a mechanical switching device including an armature arm 52 extending from a switch housing 54 into the copy sheet path 34, is mounted at a predetermined location downstream of the developing apparatus. The sensing device is spaced a distance from the applicator cylinder 20 sufficient to permit the leading edge of a copy sheet to reach the sensor while a portion of the surface of the sheet remains in contacting relation with the cylinder surface. The latter will be explained in greater detail, hereinafter when an operational description of the developing assembly according to the invention is given.

In response to the armature arm 52 of the sensing device 50 being engaged by the leading edge of the copy sheet material passing along path 34, sensing device 50 is actuated to reverse the drive mechanism (not shown) used to rotate cylinder 20 in the direction of movement of the copy sheet material, causing the cylinder to be rotated in the opposite direction. The reversed rotation of the cylinder continues until the trailing edge of the copy sheet leaves sensing arm 52. In this manner the washing of liquid onto the cylinder surface takes place at the downstream side of cylinder 20, thereby to eliminate any splashing of liquid developer upon the back surface of the sheet material as the trailing end of the sheet leaves the cylinder and to prevent the formation of a darkened line along the trailing edge of the sheet.

It should be noted that while a mechanical sensor device 50 including an actuator arm 52 engageable by the moving copy sheet material is illustrated in the drawings, a suitable substitute sensing means, such as a photoelectric device, could be used equally as well and still fall within the scope of the inventron.

For purposes of affording a more complete understanding of the invention, it is advantageous now to provide a functional description of the mode in which the component parts thus far described cooperate.

Subsequent to being electrostatically imaged in the usual manner by electrostatically charging and light exposing in accordance with an original document, a copy sheet, such as 12, having an electrostatic latent image thereon is transported into the vicinity of the developer assembly 10 with the imaged surface 13 facing the applicator cylinder 20. As the leading edge of the sheet enters the nip 36 of roller pair 30, 32, it is carried by the latter rollers along path 34 toward the surface 26 of liquid applicator cylinder 20 which at this time is also being rotated in the direction of movement of the copy sheet, (counterclockwise as viewed in FIGS. 1 and 2 of the drawings).

As seen Fig. l, the leading edge of the sheet is directed toward the liquid covered surface 26, slightly below the plane of horizontal tangency. The rotation of the cylinder causes the copy sheet to be carried over the surface 26 in intimate contact with the coating of developer solution so as to throughly apply a uniform quantity of developer solution to the imaged surface 13 of the copy sheet. The raising of liquid from the reservoir 18 onto the surface 26 by the rotational action of the cylinder 20 continues to supply sufficient amounts of developer solution to the surface 26.

As the leading edge of the copy sheet 12 passes beyond the cylinder 20 along path 34, it may have a tendency to extend in an upward direction away from path 34. To prevent the latter and to maintain the sheet on path 34 and in intimate contact with surface 26, guide roller 38, described heretofore, has been provided.

Roller 38 is positively rotated in the direction of movement of the copy sheet 12 and acts against the back surface 15 of the copy sheet 12 to maintain it on path 34. The roller 38 is well spaced from cylinder 20 so that no developer liquid is splashed thereon. Thus subsequently fed copy sheets are ensured of having dry, toner-free back surfaces.

The guide roller 38 aids in directing the leading edge of the copy sheet toward the nip 46 of the second pair of feed rollers 42, 44 downstream of applicator cylinder 20 and roller 38. Rollers 42, 44 transport the copy sheet away from the developer station and toward a dryer or exit (not shown) of the copying machine in which the developer assembly 10 is employed.

As the leading edge of the copy sheet passes beyond rollers 42, 44, it engages the amiature arm 52 of sensing device 50 (FIG. 3). The engagement of the leading edge of the copy sheet with the last-mentioned arm actuates sensing device 50. The sensing device 50 is connected by suitable means (not shown) to the positive drive (also not shown) for the applicator cylinder 20. In response to the actuation of sensing device 50, the positive drive mechanism rotating cylinder 20 in a counterclockwise direction as viewed in FIGS. 1 and 2, reverses so that cylinder 20 is driven in a clockwise direction, against or opposite to the direction of movement of the copy sheet 12 along path 34. The reversed rotation of cylinder 20 causes the washing or raising of developer liquid 18 from reservoir 16 to take place at the opposite, downstream side of cylinder 20 (FIG. 3).

The rotation of the cylinder remains opposite to the direction of movement of the copy sheet 12 until the trailing edge of the copy sheet moves past the cylinder 20 and releases actuator arm 52 of sensing device 50. Reversing the rotation of cylinder 20 to cause the washing of developer liquid as seen in FIG. 3 of the drawings, substantially eliminates the splashing of liquid over the trailing edge of the copy sheet and onto the back surface 15 thereof as the sheet passes beyond the cylinder and the formation of a bead of liquid along the trailing edge of the copy sheet as the latter leaves the cylinder surface. No darkened line at the trailing edge of the sheet is thus produced.

The location of sensing device 50 is, as explained heretofore, such that a portion of the copy sheet 12 being developed has moved past cylinder or drum 20, but so that a trailing portion remainsin contact with the liquid developer coated surface 26. The portion of the copy sheet length remaining to be developed upon reversing the direction of rotation of the cylinder is critical only in that a sufficient portion of the copy sheet must remain to be developed to permit the raising action of the liquid from the reservoir to begin-taking place on the downstream side of cylinder 20, thereby to prevent the splashing of developer liquid over the trailing edge of the copy sheet as well as to prevent a bead of liquid from forming thereat.

Referring now to FIG. 4 of the drawings, there is shown a side sectional view of the developing apparatus of FIGS. 1-3. It can be seen that the interior 56 of cylinder or drum is hollow and includes an auger 58 mounted on or formed integrally with the central axis 22 thereof for rotation therewith.

Auger 58, which as explained is rotated as cylinder 20 is rotated, stirs or mixes developer liquid 18 within reservoir 16, thereby to maintain the toner particles dispersed in the liquid carrier, in suspension therein. ,Movement of the developer solution is in the direction of the axis 22 of cylinder 20 either from left-to-right or from right-to-left (as indicated by the arrows), depending upon the direction of rotation of the cylinder 20.

Through the use of auger 58, the need for mixing or circulating pumps normally required to maintain toner particles in suspension in the developer solution, is eliminated.

The outer layer 28 of the applicator cylinder 20 of a preferred embodiment of the developer apparatus of the invention, includes a plurality of glass beads or spheres such as 60, shown in FIGS. 5 and 6 of the drawings. The beads are of a diameter in the range of from 0.002 to 0.010 inches and are bonded adjacent each other directly to the outer surface 66 of drum 20. The arrangement of the beads adjacent each other forms tiny openings or reservoirs 68 (FIG. 6) for holding developer liquid alongthe exposed surface of the cylinder or drum 20 and the smooth surfaces of the beads prevent scratching of the surface of the imaged copy sheet being transported across the beaded surface at a different speed than the rotational speed of cylinder 20.

The beaded outer surface 28 of the liquid applicator cylinder 20 according to the invention retains a sufficient quantity of developer liquid in the tiny reservoirs or openings 68 (FIG. 6) formed between the beads to effectiv'elydevelop copy sheets passing overthe cylinder in intimate contact therewith. The liquid developer solution likwise is applied uniformly, consistently to copy sheets being developed in the developer assembly.

As mentioned above, in the case wherein the cylinder 20 is constructed of a conductive material, the beads maintain the electrostatically imaged copy sheet a proper distance from the conductive cylinder, thus providing a field control effect as described in US. Pat. No. 3,367,791 without the need for a. guide plate.

While a particular embodiment of the invention has been shown and described, it should be understood that the invention is not limited thereto since many modifications may be made. It is therefore contemplated to cover by the present application any and all such modifications as fall within the true spirit and scope of the appended claims.

What is claimed is:

1. Apparatus for developing electrostatic images on sheets of copy material by the application of a liquid developer, comprising:

a reservoir of liquid developer containing toner particles dispersed throughout a liquid carrier;

a cylinder rotatably mounted to dip into said reservoir of developer liquid, said cylinder having an outer surface for retaining a quantity of developer liquid for application to said copy sheets;

means for transporting a copy sheet having an image to be developed on one surface thereof, along a predetermined path over said cylinder in a first direction with the imaged surface of said copy sheet in contacting relation with said cylinder; and means for rotating said cylinder to raise liquid developer from said reservoir onto the surface of said cylinder, said cylinder being rotatedin the same direction of movement as said copy sheetuntil a first portion of said copy sheet moves past said cylinder and thereafter reversing the direction of rotation of said cylinder while the remaining portion of said copy sheet is transported across said cylinder. 2. Apparatus as claimed in claim 1 further including sensing means located along the path of movement of said copy sheet, said sensing means detecting the passage of the leading edge of said copy sheet and in response thereto causing said means for rotating said cylinder to reverse the direction of rotation of said cylinder so that the latter is rotated in a direction opposite to the direction of movement of said copy sheet.

3. Apparatus as claimed in claim 1 wherein said copy sheet transport means includes a first pair of feed rollers positioned upstream of said liquid applicator cylinder for feeding copy sheets along said copy sheet path toward said rotating cylinder, a second pair of feed rollers downstream of said cylinder for feeding copy sheets along said copy sheet path away from said cylinder and a guide member between said second roller pair and said cylinder positioned adjacent said path for deflecting copy sheets tending to leave said path after passing over said cylinder, toward said copy sheet path.

4, Apparatus as claimed in claim 3 wherein said guide member includes a roller member mounted for positive rotation in the direction of movement of said copy sheets along said copy sheet path.

5. Apparatus as claimed in claim 1 wherein the outer surface of said cylinder includes a plurality of spherical members mounted thereon, the diameter of said spheres falling in the range of from 0.002 to 0.010 inches, said spheres being mounted closely adjacent to each other so that minute reservoirs are formed therebetween, each said reservoir carrying a quantity of developer liquid.

6. Apparatus as claimed in claim 5 wherein said spheres include glass beads, and wherein said glass beads are bonded to the surface of said rotating cylinder.

7. Apparatus as claimed in claim 6 wherein said cylinder is formed of a conductive material and wherein said glass beads maintain said electrostatically imaged copy sheet at a distance from said cylinder sufficient to produce a field control effect.

8. Apparatus for developing electrostatic images on sheets of copy material by the application of a liquid developer comprising:

a reservoir of liquid developer containing toner particles dispersed throughout a liquid carrier;

a hollow drum rotatably mounted along a central axis being partially immersed in said liquid developer in said reservoir; said drum having an outer surface for retaining a quantity of developer liquid for application to said copy sheets;

liquid ingress and egress means at the open ends of said drum;

means for rotating said drum to raise liquid developer from said developer on the surface of said drum;

means to rotate said drum alternately in opposite directions;

liquid propelling means inside said drum for moving liquid through said drum as it turns first in one direction and then in the opposite direction whereby said toner particles are maintained in suspension.

9. Apparatus as claimed in claim 8 wherein said liquid propelling means includes an auger member positioned concentrically with the central axis of said drum.

Claims (8)

1. Apparatus for developing electrostatic images on sheets of copy material by the application of a liquid developer, comprising: a reservoir of liquid developer containing toner particles dispersed throughout a liquid carrier; a cylinder rotatably mounted to dip into said reservoir of developer liquid, said cylinder having an outer surface for retaining a quantity of developer liquid for application to said copy sheets; means for transporting a copy sheet having an image to be developed on one surface thereof, along a predetermined path over said cylinder in a first direction with the imaged surface of said copy sheet in contacting relation with said cylinder; and means for rotating said cylinder to raise liquid developer from said reservoir onto the surface of said cylinder, said cylinder being rotated in the same direction of movement as said copy sheet until a first portion of said copy sheet moves past said cylinder and thereafter reversing the direction of rotation of said cylinder while the remaining portion of said copy sheet is transported across said cylinder.

2. Apparatus as claimed in claim 1 further including sensing means located along the path of movement of said copy sheet, said sensing means detecting the passage of the leading edge of said copy sheet and in response thereto causing said means for rotating said cylinder to reverse the direction of rotation of said cylinder so that the latter is rotated in a direction opposite to the direction of movement of said copy sheet.

3. Apparatus as claimed in claim 1 wherein said copy sheet transport means includes a first pair of feed rollers positioned upstream of said liquid applicator cylinder for feeding copy sheets along said copy sheet path toward said rotating cylinder, a second pair of feed rollers downstream of said cylinder for feeding copy sheets along said copy sheet path away from said cylinder and a guide member between said second roller pair and said cylinder positioned adjacent said path for deflecting copy sheets tending to leave said path after passing over said cylinder, toward said copy sheet path. 4, Apparatus as claimed in claim 3 wherein said guide member includes a roller member mounted for positive rotation in the direction of movement of said copy sheets along said copy sheet path.

5. Apparatus as claimed in claim 1 wherein the outer surface of said cylinder includes a plurality of spherical members mounted thereon, the diameter of said spheres falling in the range of from 0.002 to 0.010 inches, said spheres being mounted closely adjacent to each other so that minute reservoirs are formed therebetween, each said reservoir carrying a quantity of developer liquid.

6. Apparatus as claimed in claim 5 wherein said spheres include glass beads, and wherein said glass beads are bonded to the surface of said rotating cylinder.

7. Apparatus as claimed in claim 6 wherein said cylinder is formed of a conductive material and wherein said glass beads maintain said electrostatically imaged copy sheet at a distance from said cylinder sufficient to produce a field control effect.

8. Apparatus for developing electrostatic images on sheets of copy material by the application of a liquid developer comprising: a reservoir of liquid developer containing toner particles dispersed throughout a liquid carrier; a hollow drum rotatably mounted along a central axis being partially immersed in said liquid developer in said reservoir; said drum having an outer surface for retaining a quantity of developer liquid for application to said copy sheets; liquid ingress and egress means at the open ends of said drum; means for rotating said drum to raise liquid developer from said developer on the surface of said drum; means to rotate said drum alternately in opposite directions; liquid propelling means inside said drum for moving liquid through said drum as it turns first in one direction and then in the opposite direction whereby said toner particles are maintained in suspension.

9. Apparatus as claimed in claim 8 wherein said liquid propelling means includes an auger member positioned concentrically with the central axis of said drum.

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