Hybrid scavengeless developer unit having a magnetic transport roller

An apparatus in which an electrostatic latent image recorded on a photoconductive member is developed with toner. A donor roll, spaced from the photoconductive member, transports toner to a development zone adjacent the photoconductive member. An electrode member is positioned in the development zone between the photoconductive member and the donor roll. The electrode member is electrically biased to detach toner from the donor roll so as to form a toner powder cloud in the space between the electrode member and the photoconductive member. A unitary magnetic roll is located in the housing to transport developer material to a reload zone where toner particles are attracted from magnetic carrier granules to the donor roll. The parameters are selected to optimize reload and suppress strobe lines.

This invention relates generally to an electrophotographic printing 
machine, and more particularly concerns a magnetic transport roller 
adapted to advance magnetic carrier granules having toner particles that 
adhere triboelectrically thereto a donor roller used in a scavengeless 
development system. Generally, the process of electrophotographic printing 
includes charging a photoconductive member to a substantially uniform 
potential so as to sensitize the photoconductive surface thereof. The 
charged portion of the photoconductive member is exposed to a light image 
of an original document being reproduced. This records an electrostatic 
latent image on the photoconductive member. After the electrostatic latent 
image is recorded on the photoconductive member, the latent image is 
developed by bringing a developer material into contact therewith. Two 
component and single component developer materials are commonly used. A 
typical two component developer material includes magnetic carrier 
granules having toner particles adhering triboelectrically thereto. A 
single component developer material typically comprises toner particles. 
Toner particles are attracted to the latent image forming a toner powder 
image on the photoconductive member. The toner powder image is 
subsequently transferred to a copy sheet. Finally, the toner powder image 
is heated to permanently fuse it to the copy sheet and image 
configuration. 
One type of single component development system is a scavengeless 
development system that uses a donor roll for transporting charged toner 
to the development zone. A plurality of electrode wires are closely spaced 
to the donor roll in the development zone. An AC voltage is applied to the 
wires forming a toner cloud in the development zone. The electrostatic 
fields generated by the latent image attract toner from the toner cloud to 
develop the latent image. A hybrid scavengeless development system employs 
a magnetic brush developer roller for transporting carrier having toner 
particles adhering triboelectrically thereto. The donor roll and magnetic 
roll are electrically biased relative to one another. Toner is attracted 
to the donor roll from the magnetic roll. The electrically biased 
electrode wires detach the toner from the donor roll forming a toner 
powder cloud in the development zone. The latent image attracts the toner 
particles from the toner powder cloud. In this way, the latent image 
recorded on the photoconductive member is developed with the toner 
particles. The hybrid scavengeless developer unit employs a magnetic 
roller. The magnetic roller heretofore used was a stationary magnet having 
a sleeve disposed thereabout. The sleeve is rotated and advances the 
developer material adjacent the donor roller. A magnetic roller of this 
type is relatively expensive to manufacture. Accordingly, it is highly 
desirable to reduce the manufacturing cost of the magnetic roller used in 
a hybrid scavengeless development system without decreasing the quality 
thereof. 
The following disclosures may be relevant to various aspects of the present 
invention: 
U.S. Pat. No. 3,392,432 Patentee: Naumann Issued: Dec. 16, 1964 
U.S. Pat. No. 4,868,600 Patentee: Hays et al. Issued: Sep. 19, 1989 
U.S. Pat. No. 4,984,019 Patentee: Folkins Issued: Jan. 8, 1991 
The relevant portions of the foregoing disclosures may be briefly 
summarized as follows: 
U.S. Pat. No. -A-3,392,432 discloses a magnetic roll which transports 
developer material to a development zone. In the development zone, toner 
particles are attracted from the magnetic carrier granules of the 
developer material to the latent image. In this way, the latent image is 
developed with the toner particles. 
U.S. Pat. No. -A-4,868,600 describes a scavengeless development system in 
which toner is detached from a donor roll and a powder cloud formed by AC 
electrically biased electrode wires. The donor roll is electrically biased 
by DC voltage. A magnetic roller transports two component developer 
material to a loading region with toner from the magnetic roller 
transferred to the donor roll. The magnetic roller includes a stationary 
magnet having a sleeve disposed thereabout. As the sleeve rotates, the 
developer material is advanced to the loading zone. 
U.S. Pat. No. -A-4,984,019 discloses a developer unit having a donor roll 
with electrode wires disposed adjacent thereto in a development zone. A 
magnetic roller transports developer material to the donor roll. Toner 
particles are attracted from the magnetic roller to the donor roller. The 
magnetic roller includes a stationary magnet having a rotating sleeve 
disposed thereabout. 
In accordance with one aspect of the present invention, there is provided 
an apparatus for developing a latent image recorded on a surface with 
toner. The apparatus includes a transport member, spaced from the surface, 
for moving toner to a development zone adjacent the surface to develop the 
latent image recorded thereon. A unitary magnetic member positioned 
adjacent the transport member, advances developer material to a position 
adjacent the transport member. The developer material is attracted from 
the magnetic member to the transport member. 
Pursuant to another aspect of the present invention, there is provided an 
electrophotographic machine of the type in which an electrostatic latent 
image recorded on a photoconductive member is developed with toner to form 
a visible image thereof. The improvement includes a transport member, 
spaced from the photoconductive member, for moving toner to a development 
zone adjacent the photoconductive member. The toner develops the latent 
image recorded on the photoconductive member. A unitary magnetic member, 
positioned adjacent the transport member, advances toner to a position 
adjacent the transport roller with toner being attracted to said transport 
member.

While the present invention will be described in connection with a 
preferred embodiment thereof, it will be understood that it is not 
intended to limit the invention to this embodiment. On the contrary, it is 
intended to cover all alternatives, modifications and equivalents as may 
be included within the spirit and scope of the invention as defined by the 
appended claims. 
Inasmuch as the art of electrophotographic printing is well known, the 
various processing stations employed in the FIG. 3 printing machine will 
be shown hereinafter schematically and their operation described briefly 
with reference thereto. 
Referring initially to FIG. 3, there is shown an illustrative 
electrophotographic printing machine incorporating the development 
apparatus of the present invention therein. The electrophotographic 
printing machine employs a belt 10 having a photoconductive surface 12 
deposited on a conductive substrate 14. Preferably, photoconductive 
surface 12 is made from a selenium alloy. Conductive substrate 14 is made 
preferably from an aluminum alloy that is electrically grounded. One 
skilled in the art will appreciate that any suitable photoconductive belt 
may be used. Belt 10 moves in the direction of arrow 16 to advance 
successive portions of photoconductive surface 12 sequentially through the 
various processing stations disposed about the path of movement thereof. 
Belt 10 is entrained about stripping roller 18, tensioning roller 20 and 
drive roller 22. Drive roller 22 is mounted rotatably in engagement with 
belt 10. Motor 24 rotates drive roller 22 by suitable means such as a belt 
drive. Belt 10 is maintained and tensioned by a pair of springs (not 
shown) resiliently urging tensioning roller 20 against belt 10 with the 
developed spring force. Stripping roller 18 and tensioning roller 20 are 
mounted to rotate freely. 
Initially, a portion of belt 10 passes through charging station A. At 
charging station A, a corona generating device, indicated generally by the 
reference numeral 26, charges photoconductive surface 12 to a relatively 
high, substantially uniform potential. High voltage power supply 28 is 
coupled to corona generating device 26 to charge photoconductive surface 
12 of belt 10. After photoconductive surface 12 of belt 10 is charged, the 
charged portion thereof is advanced through exposure station B. 
At exposure station B, an original document 30 is placed face down upon a 
transparent platen 32. Lamps 34 flash light rays onto original document 
30. The light rays reflected from original document 30 are transmitted 
through lens 36 to form a light image thereof. Lens 36 focuses the light 
image onto the charged portion of photoconductive surface 12 to 
selectively dissipate the charge thereon. This records an electrostatic 
latent image on photoconductive surface 12 which corresponds to the 
informational areas contained within original document 30. Alternatively, 
a raster output scanner may be used in lieu of the light lens system 
previously described to lay out an image in a series of horizontal scan 
lines with each line having a specified number of a pixels per inch. 
Typically, a raster output scanner includes a laser with a rotating 
polygon mirror block and a modulator. 
After the electrostatic latent image has been recorded on photoconductive 
surface 12, belt 10 advances the latent image to development station C. At 
development station C, a developer unit, indicated generally by the 
reference numeral 38, develops the latent image recorded on the 
photoconductive surface. Preferably, developer unit 38 includes donor roll 
40 and electrode wires 42. Electrode wires 42 are electrically biased 
relative to donor roll 40 to detach toner therefrom so as to form a toner 
powder cloud in the gap between the donor roll and the photoconductive 
surface. The latent image attracts toner particles from the toner powder 
cloud forming a toner powder image thereon. Donor roll 40 is mounted, at 
least partially, in the chamber of developer housing 66. The chamber of 
developer housing 66 stores a supply of developer material. The developer 
material may be a single component developer material of magnetic toner 
particles or, alternatively, a developer material including at least 
magnetic carrier granules having toner particles adhering 
triboelectrically thereto. In addition to an alternating voltage, 
electrode wires 42 are electrically biased with a constant voltage. 
Developer unit 38 will be described in further detail with reference to 
FIGS. 1 and 2. 
With continued reference to FIG. 3, after the electrostatic latent image is 
developed, belt 10 advances the toner powder image to transfer station D. 
A copy sheet 70 is advanced to transfer station D by sheet feeding 
apparatus 72. Preferably, sheet feeding apparatus 72 includes a feed roll 
74 contacting the uppermost sheet of stack 76. Sheet feeding apparatus 72 
advances sheet 70 into chute 78. Chute 78 directs the advancing sheet of 
support material into contact with photoconductive surface 12 of belt 10 
in a timed sequence so that the toner powder image developed thereon 
contacts the advancing sheet at transfer station D. Transfer station D 
includes a corona generating device 80 which sprays ions onto the back 
side of sheet 70. This attracts the toner powder image from 
photoconductive surface 12 to sheet 70. After transfer, sheet 70 continues 
to move in the direction of arrow 82 onto a conveyor (not shown) that 
advances sheet 70 to fusing station E. 
Fusing station E includes a fuser assembly, indicated generally by the 
reference numeral 84, which permanently affixes the transferred powder 
image to sheet 70. Fuser assembly 84 includes a heated fuser roller 86 and 
a back-up roller 88. Sheet 70 passes between fuser roller 86 and back-up 
roller 88 with the toner powder image contacting fuser roller 86. In this 
manner, the toner powder image is permanently affixed to sheet 70. After 
fusing, sheet 70 advances through chute 92 to catch tray 94 for subsequent 
removal from the printing machine by the operator. 
After the sheet is separated from photoconductive surface 12 of belt 10, 
the residual toner particles adhering to photoconductive surface 12 are 
removed therefrom at cleaning station F. Cleaning station F includes a 
rotatably mounted fibrous brush 96 in contact with photoconductive surface 
12. The particles are cleaned from photoconductive surface 12 by the 
rotation of brush 96 in contact therewith. Subsequent to cleaning, a 
discharge lamp (not shown) floods photoconductive surface 12 with light to 
dissipate any residual electrostatic charge remaining thereon prior to the 
charging thereof for the next successive imaging cycle. 
It is believed that the foregoing description is sufficient for purposes of 
the present application to illustrate the general operation of an 
electrophotographic printing machine incorporating the development 
apparatus of the present invention therein. 
Referring now to FIG. 1, therein is shown developer unit 38 in greater 
detail. As shown thereat, developer unit 38 includes a donor roll 40, 
electrode wires 42 and magnetic roll 44. Donor roll 40 conveys developer 
material comprising toner loaded thereon by magnetic roll 44. The donor 
roll can be rotated in either the "with" or "against" direction relative 
to the direction of motion of belt 10. The donor roll is shown rotating in 
the direction of arrow 41. 
Electrode wires 42 are located in the space between photoconductive surface 
12 and donor roll 40. The electrode wires include one or more thin 
tungsten wires which are lightly positioned against donor roll 40. The 
space between the electrode wires is preferably about 1.9 mm. The distance 
between the wires and the donor roll is approximately the thickness of the 
toner layer on the donor roll. The extremities of the wire are supported 
by the tops of end bearing blocks (not shown) which may also be used to 
support the donor roll for rotation. 
An AC electrical bias is applied to electrode wires by AC voltage source 
90. In addition, a DC power supply 50 electrically biases electrode wires 
42 and donor roll 40. DC power supply 50 establishes an electrostatic 
field between photoconductive surface 12 and donor roll 40 for attracting 
the detached toner particles from the cloud surrounding wires 42 to the 
latent image recorded on the photoconductive surface. Preferably, donor 
roll 40 has a diameter of about 16 mm. The donor roll has a tangential 
velocity preferably of about 145 mm per second at an angular velocity of 
about 172 revolutions per minute. The foregoing parameters are for a 
photoconductive belt having a velocity of about 63.5 mm per second. 
Magnetic roll 44, rotating in the direction of arrow 46, is a cylindrical 
magnet with an even number of alternating north and south magnetic poles 
equally spaced about the circumferential surface thereof. As shown in FIG. 
1, a magnetic roll 44 has six magnetic poles. The diameter of magnetic 
roll 44 is preferably about 18 min. Magnetic roll 44 rotates, in the 
direction of arrow 46, at a tangential velocity of about 560 mm per second 
at an angular velocity of about 594 revolutions per minute. The ratio of 
the tangential velocity of the magnetic roll to the donor roll is about 
3.85, with the ratio of the tangential velocity of the donor roll to the 
photoreceptor velocity being about 2.29. As magnetic roll 44 rotates in 
the direction of arrow 46, it advances the developer material to loading 
zone 48. The width of the loading zone is about 3.31 min. In loading zone 
48, toner particles are attracted from the carrier granules on magnetic 
roll 44 to donor roll 40. Scraper blade 58 removes the denuded carrier 
granules and extraneous developed material from the surface of magnetic 
roll 44. Metering blade 62 adjusts the quantity of developer material 
being advanced to the loading zone 48. 
As successive electrostatic latent images are developed, the toner 
particles within the developer material are depleted. Auger 54 is mounted 
rotatably in chamber 52 of housing 66 and mixes fresh toner particles with 
the remaining developer material so that the resultant developer material 
therein is substantially uniform with the concentration of toner particles 
being optimized. In order to translate the developer material in a 
longitudinal direction on magnetic roll 44, an arcuate plate 56 having 
threads or vanes therein is employed. Plate 56 extends substantially 
parallel to the longitudinal axis of magnetic roll 44 and along the length 
thereof. When such a plate is employed, it may perform two functions. One 
of these functions is to control the thickness of the developer material 
on magnetic roll 44 and, inasmuch as the threads are in a spiral, cause 
the developer material to translate as magnetic roll 44 rotates in the 
direction of arrow 46. The translation of the developer material in the 
longitudinal direction facilitates intermingling of the newly added toner 
particles with the denuded carrier granules and developer material 
remaining in chamber 52 of housing 66. Under these latter circumstances, 
metering blade 62 may be redundant. 
A DC power supply 60 establishes an electrostatic field between donor roll 
40 and magnetic roll 44. This electrostatic field attracts the toner 
particles adhering to the carrier granules on magnetic roll 44 to donor 
roll 40. 
Referring now to FIG. 2, therein is shown an alternate embodiment of 
magnetic roll 44. As shown thereat, magnetic roll 44 has a helical pattern 
of magnetic poles formed thereon. When a helically magnetized magnetic 
roll is employed, the angle of the vanes or threads of plate 56 and that 
of the magnetic poles on roll 44 should be substantially equal to one 
another to minimize the force required to move the developer material in 
the longitudinal direction. In addition to potentially eliminating the 
need for a metering blade, auger 54 may no longer be required and may be 
redundant as well in such a system. Thus, the cost of manufacturing a 
developer unit of this type may be significantly reduced by the 
elimination of the metering blade and auger. A developer unit of this type 
has a relatively high speed ratio between the magnetic roll and 
photoreceptor. This insures that the reloading of toner from the magnetic 
roll to the donor roll will be efficient while decreasing the amplitude of 
visible strobe lines on the copy sheet. By the appropriate selection of 
the reload speed, and spacing between electrode wires, strobe lines may be 
averaged out during the development process. Thus, it is seen that 
developer unit of the type described herein will significantly reduce the 
effects of strobe lines resulting in a higher quality copy at lower cost. 
In recapitulation, it is evident that the development unit of the present 
invention includes electrodes positioned closely adjacent to the exterior 
surface of a donor roll in a gap defining the development zone between the 
donor roll and the photoconductive belt. A unitary magnetic roll 
transports toner particles to a reload zone where they are transferred 
from the magnetic roll to the donor roll. The utilization of a unitary 
magnetic roll to achieve the foregoing enables higher speed reloading and 
significant reduction in strobe lines effects. 
It is, therefore, apparent that there has been provided in accordance with 
the present invention, a development system that fully satisfies the aims 
and advantages hereinbefore set forth. While this invention has been 
described in conjunction with a specific embodiment thereof, it is evident 
that many alternatives, modifications, and variations will be apparent to 
those skilled in the art. Accordingly, it is intended to embrace all such 
alternatives, modifications, and variations that fall within the spirit 
and broad scope of the appended claims.