[0001] This invention relates to a developer mixture for developing a latent electrostatic
image. It also relates to apparatus and method for developing a latent electrostatic
image using the developer mixture.
[0002] In electrophotographic printing, a light image of an original document is projected
onto a charged photoconductive surface. This selectively dissipates the charge to
record an electrostatic latent image of the original document on the photoconductive
surface. The latent image is subsequently developed with toner particles. These toner
particles are transferred to the copy sheet and permanently affixed thereto by the
application of heat and/or pressure.
[0003] Generally, the developer material comprises toner particles adhering triboelectrically
to carrier granules. This two-component developer mixture contacts the electrostatic
latent image. The toner particles are attracted from the carrier granules to the latent
image rendering it visible. Generally, the carrier granules are made from a ferromagnetic
material the toner particles being made from a thermoplastic material.
[0004] Various methods have been devised for transporting the developer material to the
latent image. For example, the developer material may be cascaded over the latent
image and the toner particles attracted from the carrier granules thereto. Other structures
employed to develop the latent image include the use of magnetic field producing devices
which form brush- like tufts of developer material extending outwardly therefrom in
contact with the photoconductive surface.
[0005] In developing an electrostatic latent image, the toner particles are deposited thereon.
However, frequently the toner particles are not only deposited on the electrostatic
latent image but on the background areas. When the toner particles are transferred
from the photoconductive surface to the copy sheet, both the areas containing the
electrostatic latent image and the backgound are transferred- to the copy sheet. This
produces a gray background significantly deteriorating the resultant copy. Furthermore,
the toner particles frequently escape from the developer housing contaminating the
other systems of the printing machine. Various techniques have been devised to overcome
the foregoing problem.
[0006] U.S. Patent No 3 239 465 discloses a toner particle having magnetic particles held
in a binder. The magnetic material may be magnetite or hemitite with the binder being
an organic resin. The ratio of binder to magnetic particle can vary from 19 to 1 to
2 to 3 by weight. For best results, there should be at least 20% of the magnetic powder
but not over 70%. U.S. Patent No. 3 345 294 describes developer powder which reduces
the tendency of the toner to adhere to the background of the print. The toner comprises
a polymide resin mixed with a coloring agent and a magnetic substance. The magnetic
substance may be present in an amount as small as 1% by weight and preferably between
about 5% and about 25% by weight of the developer powder.. U.S. Patent No. 3 639 245
describes a dry ink made from magnetite blended with a resin and having carbon particles
embedded therein.
[0007] In accordance with the present invention, a developer mixture for developing an electrostatic
latent image is characterized by a magnetic carrier granule having a toner particle
adhering thereto, the toner particle comprising a non-magnetic portion with a magnetic
portion integral therewith. Preferably, the magnetic portion is from about 1% to about
10% of the volume of the toner particle, and is best about 10% of the volume of the
toner particle.
[0008] The invention also provides an apparatus for developing a latent image, including
a housing defining a chamber containing a supply of said developer mixture as described
above and means operatively associated with the developer mixture in the chamber of
said housing, for transporting the developer mixture closely adjacent to the latent
image.
[0009] Further in accordance with the. invention there is provided a method of developing
an electrostatic latent image recorded on a photoconductive surface, including the
steps of: storing a supply of developer mixture comprising a magnetic carrier granule,
and a toner particle adhering to the carrier granule, the toner particle having- a
non-magnetic portion and a magnetic portion integral therewith, and transporting the
developer mixture to a position closely adjacent to the latent image.
[0010] In order that the invention may be more readily understood, reference will now be
made to the accompanying drawings, in which:-
Figure 1 is a schematic elevational view depicting an electrophotographic printing
machine incorporating the features of the present invention therein;
Figure 2 is a fragmentary elevational view illustrating the developer unit employed
in the Figure 1 printing machine;
Figure 3 is a fragmentary elevational view showing the arrangement of the forces on
the developer mixture of the Figure 2 developer unit;
Figure 4 is a graph showing the force variation as a function of the percentage of
magnetic material in the toner particles for toner particles having a low charge;
and
Figure 5 is a graph depicting the force variation as a function of the percentage
of magnetic material in the toner particles for toner particles having a high charge.
[0011] For a general understanding of an electrophotographic printing machine incorporating
the features of the present invention therein, reference is made to Figure .1 which
depicts schematically the various components thereof. Hereinafter, like reference
numerals will be employed throughout to designate identical elements. Although the
developer mixture of the present invention is particularly well adapted for use in
the development system of an electrophotographic printing machine, it should become
evident from the following discussion that this mixture is equally well suited for
use in a wide variety of printing machines and is not necessarily limited to the particular
embodiment shown herein.
[0012] Inasmuch as the art of electrophotographic printing is well known, the various processing
stations for producing a copy of an original document are represented schematically
in Figure 1. Each of these processing stations will be described briefly hereinafter.
[0013] With continued reference to Figure 1, the electrophotographic printing machine employs
a drum 10 having a photoconductive surface 12 secured to the exterior circumferential
surface of a conductive substrate. 3y way of example, photoconductive surface 12 may
be made from a suitable selenium alloy with the conductive substrate being made from
aluminum. Drum 10 rotates in the direction of arrow 14 so that successive portions
of photoconductive surface 12 pass through the various processing stations disposed
about the periphery thereof.
[0014] Initially, drum 10 rotates a portion of photoconductive surface 12 through charging
station A. Charging station A includes a corona generating device, indicated generally
by the reference numeral 16. Corona generating device 16 is located closely adjacent
to photoconductive surface 12. When energized, corona generating device 16 charges
a portion of photoconductive surface 12 to a relatively high substantially uniform
potential.
[0015] The charged portion of photoconductive surface 12 is rotated to imaging station B.
Imaging station B includes an exposure system, indicated generally by the reference
numeral 18, comprising a transparent platen upon which the original document is disposed.
Lamps illuminate the original document. Scanning of the original document is achieved
by an oscillating mirror in a timed relationship with the movement of drum 10, or,
in lieu thereof by moving the lamps and lens system in synchronism to form a flowing
light image thereof. The light image of the original document is foeused onto the
charged portion of photoconductive surface 12. In this manner, photoconductive surface
12 is selectively discharged to record an electrostatic latent image thereon corresponding
the informational areas contained within the original document.
[0016] Next, drum 10 rotates the electrostatic latent image recorded on photoconductive
surface 12 to development station C. At development station C, a developer unit having
a housing with a supply of developer mixture therein renders the electrostatic latent
image visible. The developer mixture of the present invention includes magnetic carrier
granules having toner particles adhering triboelectrically thereto. The toner particles
comprise a non-magnetic portion and a magnetic portion. The magnetic portion is preferably
about 10% of the volume of the toner particle. The detailed structure of developer
unit 20 and the developer mixture employed therein will be described hereinafter with
reference to Figures 2 through 5, inclusive. In general, the electrostatic latent
image attracts the toner particles from the carrier granules to form a toner powder
image thereon.
[0017] With continued reference to Figure 1, a sheet of support material is advanced by
sheet feeding apparatus 22 to transfer station D. Sheet feeding apparatus 22 includes
a feed roll 24 contacting the uppermost sheet of stack 26. Feed roll 24 rotates in
the direction of arrow 23 to advance the uppermost sheet from stack 26. Registration
rollers 30, rotating in the direction of arrow 32, advance and align the sheet of
support material into chute 34. Chute 34 directs the advancing sheet of support material
into contact with drum 10 in a timed sequence so that the toner powder image developed
thereon contacts the advancing sheet of support material at transfer station D. Transfer
station D includes a corona generating device 36. Corona generating device 36 sprays
ions onto the side of the sheet of support material opposed from photoconductive surface
12. The toner powder image adhering to photoconductive surface 12 is then attracted
therefrom to the surface of the sheet of support material in contact therewith. After
transferring the toner powder image to the sheet of support material, endless belt
conveyor 38 advances the sheet of support material to fusing station E.
[0018] Fusing station E includes a fuser assembly, indicated generally by the reference
numeral 40. Fuser assembly 40 heats the transferred toner powder image to permanently
affix the toner particles to the sheet of support material. Preferably, fuser assembly
40 includes a heated fuser roller 42 and a backup roll 44. A sheet of support material,
with the powder image thereon, is interposed between fuser roll 42 and backup roll
44. The powder. image contacts fuser roll 42. Release material applicator 46 applies
release material to fuser roll 42. Blade 48, positioned closely adjacent to fuser
roll 42, regulates the thickness of release material coating fuser roll 42. After
the toner powder image is permanently affixed to the sheet of support material, stripper
blade 50 separates the sheet from fuser roller 42. Thereafter, the sheet of support
material is advanced by a series of rollers 52 to catch tray 54 for subsequeri- removal
from the printing machine by the operator.
[0019] In ariably, residual particles remain adhering to photoconductive surface 12 after
the transfer of the toner powder image to the sheet of support material. These residual
particles are removed from photoconductive surface 12 at cleaning station F. Cleaning
station F includes a cleaning mechanism, generally designated by the reference numeral
56, having a corona generating device and a brush contacting photoconductive surface
12. Initially, the particles are brought under the influence of the corona generating
device to neutralize the charge remaining on photoeonductive surface 12 and that of
the residual particles. Thereafter, the neutralized particles are removed from photoconductive
surface 12 by the rotatably mounted fibrous brush in contact therewith. After cleaning,
a discharge lamp floods photoconductive surface 12 to return it to the initial level
prior to the recharging thereof for the next successive imaging cycle.
[0020] Referring now to Figure 2 developer unit 20 is shown transporting the developer mixture
into contact with the electrostatic latent image recorded on photoconductive surface
12 of drum 10.
[0021] As illustrated in Figure 2, developer unit 20 includes a housing 58 defining a chamber
60 for storing a supply of developer mixture. 62 therein. Developer roller 64, mounted
in chamber 60 of housing 58, preferably includes a tubular member made from a non-magnetic
material such as aluminum. Tubular member 64 is interfit over an elongated magnetic
member 66. Preferably, magnetic member 66 is a bar magnetic made from barrium ferrite.
Magnetic member 66 is disposed interiorly of tubular member 64 and spaced therefrom.
Shaft 58 made preferably from steel, is mounted concentrically within tubular member
64 to support magnetic member 66. A motor (not shown), coupled to tubular member 64,
rotates tubular member 64 in the direction of arrow 70. Magnetic member 66 remains
substantially stationary. As tubular member 64 rotates relative to magnetic member
66, developer mixture 62 is advanced from chamber 60 to development zone 72. A power
supply (not shown) applies an electrical bias to tubular member 64. Preferably, the
voltage applied to tubular member 64 is about 500 volts. However, this voltage level
is adjustable and depends upon the background voltage level of photoconductive surface
12. In operation, developer mixture 62, in chamber 60 of housing 53, is attracted
by the magnetic fields produced by magnetic member 68 to tubular member 64. As tubular
member 64 rotates, the developer mixture moves into development zone 72. In development
zone 72, the toner particles are attracted from the carrier granules disposed on tubular
member 64 to the latent image recorded on photoconductive surface 12. In this way,
the toner particles are deposited on the electrostatic latent image forming a toner
powder image thereon. In order to minimize development of the background areas, the
toner particles include a magnetic portion. The carrier granules are magnetic and
are preferably made from ferromagnetic materials such as magnetite or hemitite. The
non-magnetic portion of the toner particle is made from fusable resin such as a polymide.
Thus, the toner particles comprise a fusable resin with magnetic particles such as
iron, magnetic iron oxide or magnetite; various magnetic metals and alloys, and the
like dispersed therein. The magnetic portion of the toner particle comprise preferably
10% of the volume of the toner particle with the resin or plastic material comprising
90% of the toner particle by volume. However, it has been found- that ranges of from
1 to 10% of magnetic material by volume improve development. Development is improved
when the toner particle charge ranges from about 1 to about 20 microcouloumbs per
gram. It is clear that the introduction of the magnetic portion in the toner particle
suppresses background development and facilitates development of the electrostatic
latent image. This produces sharper, darker copies. Furthermore, magnetic loading
causes the toner particles to better adhere to the carrier granules reducing toner
particle contamination and dirt throughout the printing machine.
[0022] Referring now to Figure 3, there is shown a schematic distribution of the forces
involved in development of the electrostatic latent image. As depicted thereat, during
development, developer mixture 62 is brought into the vicinity of photoconductive
surface 12. Carrier granule 74 acts as a transport for toner particle 76. Toner particle
76 acquires charge through triboeleetri- fication. This charge and the countercharges
on carrier granule 74 produces adhesion force F
R between toner particle 76 and carrier granule 74. The internal magnetic field also
induces a dipole in particle 76 which adds to F
R. As carrier granule 74 is transported into development zone 72 closely adjacent to
and in contact with photoconductive surface 12, toner particle 76 is transported therewith.
The electrostatic latent image recorded on photoconductive surface 12 produces an
electrostatic field which exerts a stripping force FD on toner particle 76. When stripping
force F
D exceeds retaining force F
R, the toner particles leave carrier granules 74 and move to photoconductive surface
12. In this way, the electrostatic latent image is developed. It has been found that
the magnetic portion of the toner particle causes the introduction of a magnetic dipole
which adds to F
R. In this way, F
R is increased and the background forces on photoconductive surface 12 are insufficient
to separate toner particles 76 from carrier granules 74. However, -the fields introduced
by the latent image produce a sufficient force to separate the toner particles from
the carrier granules resulting in development of the electrostatic latent image with
the background remaining substantially devoid of toner particles. In this way, background
development is suppressed and electrostatic latent image development enhanced.
[0023] Referring now to Figure 4, there is shown the force attracting the toner particles
produced by the background electrostatic field on the photoconductive surface as a
solid line, i.e. the stripping force, and the force retaining the toner particle to
the carrier granule as a dotted line for both 0% magnetic loading and 10% magnetic
loading. Figure 4 depicts the foregoing conditions for a low charge toner particle,
i.e. one having a 1 microcouloumb per gram charge thereon. As shown, when the magnetic
loading is 10% by volume of the toner particle, the electrostatic force produced by
the background area is never sufficient to strip the toner particle from the carrier
granule.
[0024] Referring now to Figure 5, the holding force is once again shown as a dotted line
for 0% and 10% magnetic loading with the stripping force being shown as a solid line.
The graph depicted in Figure 5 is for a highly charged toner particle, i.e. one having
a charge of 20 microcouloumbs per gram. Under these latter circumstances, there is
an increase in the required background force necessary to electrostatically strip
the toner particles from the carrier granules so as to develop the background areas.
Thus, it is clear that magnetic loading of the toner particles produces a significant
improvement when there are present toner particles with low charge and a minor improvement
when the toner particles are highly charged.
[0025] In recapitulation, the present invention is directed to a developer mixture which
may be employed to develop an electrostatic latent image. The developer mixture includes
a toner particle having a non-magnetic portion and a magnetic portion with the magnetic
portion being preferably about 10% by volume of the toner particle. A developer mixture
of this type improves development by reducing background development while maintaining
image development at a highly satisfactory level. In this way, contrast is improved.
Furthermore, machine contamination due to the escape of toner particles from the developer
housing is minimized.
1. A developer mixture for developing an electrostatic latent image, characterized
by a magnetic carrier granule (74); and a toner particle (76) adhering to said carrier
granule, said toner particle comprising a non-magnetic portion and a magnetic portion
integral therewith.
2. A developer mixture according to claim 1 wherein the magnetic portion is from about
1% to about 10%, preferably about 10%, of the volume of said toner particle (76).
3. A developer mixture according to claim 1 or 2,.wherein said toner particle (76)
has a charge ranging from about 1 microcouloumb per gram to about 20 microcouloumbs
per gram.
4. A developer mixture according to claim 3, wherein the non-magnetic portion of said
toner particle (76) is made from a plastic material.
5. A developer mixture according to claim 4, wherein the magnetic portion of said
toner particle (76) is made from a ferromagnetic material.
6. An apparatus for developing a latent image, including a housing (58) defining a
chamber (60) containing a supply of developer mixture (62) according to any of claims
1 to 5 and means (64, 66) operatively associated -with the developer mixture (62)
in the chamber of said housing, for transporting the developer mixture closely adjacent
to the latent image.
7. An apparatus according to claim 6, wherein said transporting means (64, 66) includes
a rotatably mounted tubular member (64) and a magnetic member (66) disposed interiorly
of said tubular member (64) for attracting the developer mixture (62) to the exterior
circumferential of said tubular member.
8. A method of developing an electrostatic latent image recorded on a photoconductive
surface, including the steps of: storing a supply of developer mixture comprising
a magnetic carrier granule, and a toner particle adhering to the carrier granule,
and a toner particle adhering to the carrier granule, the toner particle having a
non-magnetic portion and a magnetic portion integral therewith, and transporting the
developer mixture to a position closely adjacent to the latent image.
9. - A method according to claim 8, wherein the magnetic position of the toner particle
is from about 1% to about 10%, preferably about 10%, of the volume of the toner particle.
10. A method according to claim 8 or 9, wherein said step of transporting includes
the steps of attracting the developer mixture to the exterior circumferential surface
of a tubular member and rotating the tubular member to advance the developer mixture
to a position closely adjacent to the electrostatic latent image recorded on the photoconductive
surface.