Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
11 CKOSS REFERENCE
12
13 U.S. Patent No. 3,894,514 issued July lS, 1974 in the names of
14 A. H. Caudill, R. J. Hoekzema and C. C. Wilson, entitled
lS "Toner Recovery System" is directed to specific techniques
16 applying the filter of my invention.
17 BACKGROUND OF Tl~E INVENTION
18 Xerographic printers, such dS pl ain paper office copy
19 Illachines, produce images by use of a finely divided, colored
dielectric toner powder which is deposited in tne form of an
21 inldge in conformity with an electrostatic latent image
22 carried ~y an imaging surface. Once formed, the powder image
23 is transferred to a final support sheet and residual toner is
24 cleaned from tne imaging surface to prepare it for reuse.
Tne cleaning operation is accomplished by mechanical
26 dislodging the toner particles from tne imaging surface and
27 entraining the toner particles in an air stream.
28 Many cleaning systems in use resemble a household
29 vacuum cleaner employing a brush that dislodges the particles
and lifts them into a moving air stream where tiley are wisked
31 away from the surface being cleaned. The cleaning process
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1 mdy be enlldnced by d pre-clean chdrging treatlnen~ dnd light
2 exposure that provide optimum electrostatic conditions for
3 the cleaning operation. Like a household vacuum cleaner, the
4 toner laden air is ordinarily passed through a cloth-like fil-
ter where the toner particles are mechanically trapped to
6 clean ttle air prior to discharge from the machine. When loaded
7 with toner, the filter bag is usually replaced and discarded.
8 Electrostatic precipitators have been proposed for
9 cleaning the toner from the air. Such devices provide a strong
electrostatic field that tends to drive the toner to a moving
11 surface from which it can be scrapped and collected for re-use.
12 Electrostatic precipitators tend to add cost by requiring
13 both mechanical and electric additions to the copy machine
14 system.
DISCLOSURE OF THE INYENTION
16 I have discovered that the same carrier beads that inter-
17 act triboelectrically with the toner material in the develop-
18 ing station to charge the toner material will, if collected
19 in a body through which toner laden air can pass, thoroughly
filter tne toner particles from the air. The unusual
21 effectiveness of this carrier as a filter material can be
22 explained by the relatively large holding force that is
23 developed upon contact between a toner particle and the
24 carrier bead surface due to tne triboelectric relationship
between the materials of the particle and the surface. Tnis
26 holding force is substantially greater than the forces placed
27 on the toner particle to cause it to pass through the inter-
28 stices between adjacent carrier beads. Thus the force of the
29 alr stream that brings the toner particle into contact with a
carrier bead is substantially less than that necessary to
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1 dislodge the toner particle from a carrier bead which it has
2 contacted. This holding force is also large in comparison
3 with forces that might be developed due to the initial charge
4 on the toner and a carrier bead prior to actual contact
therebetween, since much of the field that would be created
6 by initial charge within a body of carrier Deads tends to be
7 negated by similar charge from adjacent carrier beads.
8 The ability of developer carrier beads to act as a
9 filter material greatly simplifies the reclamation of toner
particles since tne toner laden carrier of a saturated filter
11 body can simply be mixed into the developer ~ix from which
12 the toner originally came.
13 These and other objects, features and advantages of my
14 invention will be apparent to those skilled in the art from
the following description of a specific illustrative embodi-
16 ment thereof, wherein reference is made to the accompanying
17 drawing, of which:
18 The Figure shows a xerographic copy macnine having a
19 cleaning station including a toner filter operated in accord-
ance with my invention.
21 A copy machine is shown in the Figure and includes, a
22 drum 10 bearing a photoconductive imaging surface 11 that in
23 succession is charged by corona unit 12, image-wise discharge
24 by optical system 13 to form a differential electrostatic
charge latent image that is developed at developing unit 14
26 where a development mixture 20 comprising relatively large
,
27 carrier beads 21 and electroscopic resin toner particles 22
28 are presented in contact with imaging surface 11 to form a
29 physical powder image 23. Most of the powder image 23 formed
by developer unit 14 is transferred to a support sheet 15 by
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1 transfer corona unit 16. The sheet 15 is carried by vacuum transport
17 past image fixing means such as a radiant lamp 18 and is deposited
in an exit pocket 19. Residual toner particles 22 remaining on imaging
surface 11 are mechanically dislodged by brush 30 which wisks them
into an air stream in conduit 31. The structure thus far described is
conventional in the xerographic art.
To collect toner particles from the air-toner mixture in conduit
31 before ejecting the air from the machine, a container 32 having an
open mesh screen bottom 33 is supported in a filter box or housing 34
in alignment with the conduit 31. A body 35 of carrier beads 21 which
preferably are identical to carrier beads 21 in the developing station
14 is supported in container 32 in the path of air from conduit 31.
Filter box 34 is connected to the vacuum or inlet side of a fan 36 which
draws the air through conduit 31 and the body of carrier beads 21. If
desired, a fiberous filter 37 may be positioned in the outlet duct 38
of fan 36 to remove any toner particles that might remain.
In operation, toner laden air in conduit 31 is drawn through tor-
tious paths provided by interstices between carrier beads 21 in filter
body 35. Toner particles 22 are thereby caused to contact the external
surface of carrier beads 21 whereupon a triboelectric charge and re-
sultant holding force is created and the toner particle 22 is electro-
statically trapped. Copies are made until carrier body 35 becomes suf-
fici`ently laden with toner particles so that it no longer behaves as an
adequate filter. Container 32 is then removed from filter box 34 and
the toner laden body 35 is poured into the developer unit 14. A like
body of developer mixture 20 is removed from
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1 developiny unit 14 and placed in container 32 tllus providing
2 a rejuvenated filter.
3 The preferred implementation of my invention employs the
4 same carrier material for filtering as is employed in a
xerographic development system, an example of which is
6 described in United States Patent No. 3,947,271 issued
7 March 30, 1976 by C. A. Queener and H. E. Munzel and
8 entitled "Improved Coated Carrier Particles For Use In An
g Improved ElectrophotograPhic Process". Generally speaking,
however, the same triboelectric relationships that make a
11 particular carrier suitable for use as a developer with a
12 particular toner material will qualify that carrier as
13 suitable for use by my invention as a filter for toner
14 material.
The triboelectric behavior of pairs of materials is
16 known to those skilled in the art in a relative sense but is
17 difficult to express in absolute numbers. Under controlled
18 conditions, tne triboelectric charge development capability
19 of a pair of materials at least one of which is a dielectric
which can be measured in terms of the number of electronic
21 charges produced per linear unit of material diameter.
22 Materials producing higher triboelectric charging upon contact
23 (e.g. 4000 electronic cnarges per micron of diameter) will
24 exhibit satisfactory filter efficiency whereas those materials
producing lower triboelectric charging (e.g. less than 500
26 electronic charges per micron of diameter) will not.
27 The inherent filtering efficiency of a carrier material
28 with respect to a particular toner directly determines the
29 size of a body of carrier capable of providing a thorough
filter. The more efficient material will thoroughly filter
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1 with a shorter air flow path there-through. Th~ air flow path
2 in turn directly affects the pressure that must be applied
3 across the body of material to create the required air flow
4 through the filter. From these relationships, it can be seen
that carrier having low triboelectric charge development
6 capability with respect to a particular toner will be required
7 in larger quantity creating a longer flow path through the
8 filter with a resultant required higher pressure drop.
9 On the other hand, a carrier material having a high
triboelectric charge development capability for a particular
11 toner can thoroughly filter with a relatively short air flow
12 path reducing the required applied pressure drop to a minimum.
13 I have determined that carrier and toner materials
14 having triboelectric charge development capability of about
lS 4000 electronic charges per micron of diameter like that of
16 steel carrier with respect to powdered polyethylene will
17 produce a nighly satisfactory filter. Materials having a
18 tribolectric charge development capability of about 1000
19 electronic charges per micron of diameter such as steel
carrier and powdered sulphur will be marginally satisfactory.
21 Materials having a low triboelectric charge development
22 characteristic of less than 500 electronic charges per micron
23 of diameter such as sand carrier with respect to diatomaceous
24 earth will not function satisfactorily according to my
invention.
26 The triboelectric charge development characteristics of
27 any particular pair of materials relative to these exemplary
28 materials can be determined by a variety of known techniques.
29 One technique which I have found convenient and reliable is
as follows: A charge of carrier with about 1% by weight
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1 ~oner material thoroug/lly mixed therein is placed in d 30
2 Ml. ASTM 10-15 M Buchner funnel, the narrow end of which is
3 connected to a controllable source of metered air pressure.
4 A timed pulsing valve in the air line is arranged to period-
ically open and pass a puff of air upwardly through the charse
6 of material in the funnel. With materials whose behavior is
, 7 completely unknown I progressively increase the line pressure
8 and observe whether material is visibly blown out of the
9 sample cnarge. A good starting pressure is in the order of
magnitude of 10 pounds per square inch which I increase up to
11 about 40 pounds per square inch. The pressure attained when
' 12 material is first visibly blown from the charge is a rough
13 indication of the triboelectric charge characteristic of the
i 14 material. The higher the pressure, the greater the capability
of the materials for developing a triboelectric charge.
16 To more precisely compare the triboelectric charge devel-
17 opment characteristics of an unknown carrier and toner with a
18 known acceptable material, the mixed charge of each sample is
19 carefully weighed before it is loaded into the Buchner funnel.
The pressure line for both samples is set to a level previously
21 determined to be capable of noticeably blowing some material
22 from each of the two samples. An equal number of air puffs
23 at this pressure is delivered to each of the two samples,
24 after which the samples are carefully weighed to determine
~ 25 the weight loss of toner from each sample. The sample having
t 26 tne lower weight loss thus is demonstrated as having the
27 greater capability of triboelectric charge development.
28 In performing this measurement the following variables
29 should be controlled: Relative humidity should be constant
and preferably below 50%. The material of the samples should
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1 be of tne same respective size. Typical carrier bead size is
2 about 300 microns. Typical toner particle size is about 10
3 microns. The volume of both samples should be substantially
4 constant and the toner concentration should be constant and
in the neighborhood of about 1% by weight of the sample. The
6 weighing techniques should be capable of accuracy to at least
7 .0001 grams where a sample of nominal size of 50 grams is
8 employed.
9 Using this test method, I have determined that the charge
development capability of a preferred development mixture
11 comprising Teflon-*coated steel carrier as described in afore-
12 said United States Patent~io. 3,947,271 and a commercial toner
13 designated as IBM*Part Number 1162051 described in United
14 States Patent No. 3,778,262 issued December 11, 1973
by C. A. Queener, et al and entitled "Coated Carrier
16 Particles, Method of Making Same, and Improved Electrophoto-
17 graphic Process" is substantially in excess of the charge
18 development characteristic of uncoated S-70 steel shot meeting
19 SAE Specification J827 or SFSA Specification 20T-66 and
reagent grade powdered sulphur. In fact, the triboelectric
21 charge development capability of this developer mixture is
22 greater than that demonstrated by uncoated S-70 steel shot
23 meeting the above mentioned SAE or SFSA Specifications and
24 powdered polyethylene commercially available under the name
Microthene ~ M-711-939 from ~. S. I., a division of National
26 Distillers and Chemical Corporation.
27 Those skilled in this art will recognize that the
28 filtering techniques I have discovered is particularly useful
29 for removing xerographic toner particles from air. While
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preferred materials have been disclosed above it will be
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understood that various suitable materials can be identified by techniques
I have described. Accordingly, I intend my invention to be limited and
defined only by the following claims.
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