US3719165A

US3719165A - Tuner concentration control apparatus

Info

Publication number: US3719165A
Application number: US00177600A
Authority: US
Inventors: W Trachienberg; T Morse
Original assignee: Eastman Kodak Co
Current assignee: Eastman Kodak Co
Priority date: 1971-09-03
Filing date: 1971-09-03
Publication date: 1973-03-06
Anticipated expiration: 1990-03-06
Also published as: JPS4857638A; GB1398964A; JPS5432573B2

Abstract

An apparatus adapted for use in an electrographic magnetic brush development station for maintaining the concentration of toner in the developer mixture at a substantially constant level. Toner concentration is monitored by sampling the self-biasing potential generated by the brush as it periodically contacts uncharged areas of the electrographic recording element. Such potential has been found to be inversely proportional to the concentration of toner in the developer mixture.

Description

United States Patent Trachienberg et al.

TUNER CONCENTRATION CONTROL APPARATUS Inventors: William Trachienberg; Theodore H.

Morse, both of Rochester, NY.

Eastman Kodak Rochester, NY.

Filed: Sept. 3, 1971 Appl. No.: 177,600

Assignee: Company,

US. Cl. ..l18/7,1l7/17.5, 118/9, 118/637, 355/14 Int. Cl. ..B05c 11/00, 003g 13/00 Field of Search .....ll8/4, 7, 8, 9, 637; 117/17.5, 117/934 A; 355/3, 14; 346/74 References Cited UNITED STATES PATENTS Shelffo et a1. ..1l7/l7.5 Piper et al. ..1 18/2 1 March 6, 1973 3,599,605 8/1971 Ralston ..1l8/637 3,529,546 9/1970 Kollar 101/426 3,399,652 9/1968 Gawron.... ..l18/637 3,674,532 7/1972 Morse ..l l7/l7.5

Primary ExaminerMervin Stein Assistant Examiner--Leo Millstein Attorney-Robert W. Hampton et a1.

[57] ABSTRACT An apparatus adapted for use in an electrographic magnetic brush development station for maintaining the concentration of toner in the developer mixture at a substantially constant level. Toner concentration is monitored by sampling the self-biasing potential generated by the brush as it periodically contacts uncharged areas of the electrographic recording element. Such potential has been found to be inversely proportional to the concentration of toner in the developer mixture.

4 Claims, 4 Drawing Figures PAPER FEED/N6 CHARGING PATENTEUH R ems SHEET 10F 3 QERNQMK PATENTED 6|973 3,719,165 SHEET 2 BF 3 47 TONER REPLEA/(SHE? DELAY I 43 y FIG: 2 Q a THRESl-IOLD r L SAMPLE? SENSOR l L z 42 45 SHIFT REGISTER x t lb I 47 Reset ran/ER 3/ 40 NTER REPLE/V/SHER liese/ 42 b l a //v THRESHOLD g SAMPLE? AMP SENSOR SH/FT REG/5m? WILL/AM TRACHTE/VBERG 3 THEODORE H. MORSE INVENTORS AGENT M TUNER CONCENTRATION CONTROL APPARATUS BACKGROUND OF THE INVENTION This invention relates to electrographic development and more particularly to an apparatus for maintaining the concentration of toner particles in a magnetic brush development station at the substantially constant level required for producing copies of uniformly good density.

Electrographic developers commonly comprise a mixture of suitably pigmented or dyed resin-based particles, known as toner, and a granular carrier material which functions to carry toner by generating triboelecv tric charges thereon. In developing electrostatic charge patterns produced on the surface of an electrographic recording element, it is common to apply the developer to the charge pattem-bearing surface by contacting such surface with one or more rotating magnetic development brushes, the bristles of which comprise toner-coated metallic carrier particles. Upon contacting the surface of the recording element, the toner particles, which are charged to a polarity such as to be at tracted to the electrostatic charge pattern, are separated from the carrier particles by the stronger electrostatic forces and selectively. deposited on the surface in accordance with the charge pattern. Obviously, as successive electrostatic charge patterns are developed, there is a gradual depletion of toner from the development mixture, requiring subsequent toner replenishment to. avoid a gradual reduction in density of the developed or toned images.

To maintain the concentration of toner in the development brush at the level required for high density copies, it has been common heretofore to require the operator of an electrographic copier to periodically obtti saon serve the reproduction quality and to replenish the I development brush with additional toner particles when the legibility or density of the developed images dropped below a subjective acceptable level. While such a manual toner replenishment system is satisfactory for many applications, such a system has proven highly unsatisfactory when a high throughput application exists.

Although more sophisticated automatic toner replenishment systems have been proposed for maintaining the toner concentration at an acceptable level, no such system has proven entirely satisfactory 'to date. Generally, conventional automatic toner control systemshave proven unreliable over extended periods of use, insensitive to slight variations .in toner concentration of a magnitude adversely affecting image quality, costly to fabricate, anddifficult because of space limitations to incorporate in electrographic copiers. Moreover, most automaticsystems are adapted to sample the toner concentration of only a relativelysmall portion of the entire development mixture; thus, .when the toner concentration in such sampled portiondoes not accurately reflect the average toner concentration in the entire mixture, over or under-replenishment of toner results.

BRIEF SUMMARY OF THE INVENTION Still another object of the invention is to provide an improved automatic toner control apparatus which can be readily incorporated in conventional magnetic brush development stations.

Yet another Object of the invention is to monitor the average toner concentration in the entire electrographic developer rnix carried by a magnetic brush applicator and to replenish the mix with toner when the concentration thereof falls below an optimum level.

Briefly, such objects are achieved in accordance with the present invention by periodically sampling the elec tric potential of a magnetic development brush, connected to ground through a high resistance, when the brush is in contact with a uncharged portion (i.e.,a non-charge pattern-bearing portion) of the electrographic recording element. It has been found that the triboelectric interaction between the development mix carried by the magnetic development brush and they recording element causes charge to accumulate on the housing of the brush assembly. The intensity of such charge has been found to be inversely proportional to the concentration of toner in the development mix carried by the magnetic'development brush. By sampling the housing potential only when an uncharged portion of the recording element passes the developing station, the variable effect on the housing potential produced by the charge pattern is eliminated. The sampled potential is fed to a threshold sensing circuit which, when the input thereto exceeds a predetermined level indicative of a drop in toner concentration below an acceptable level, transmits anelectrical signal whereby a conventional toner replenisher can be activated.

In addition to the above objects, other objects of the invention and its various advantages will become apparent from the ensuing detailed description of preferred embodiments, reference being made to the accompanying drawings in which like characters denotelike parts and in which:

BRIEFDESCRIPTION OF THE DRAWINGS -toner concentration monitoring apparatus in accordance-with a preferred embodiment of the invention;

FIG. 3 is a schematic illustration of an alternative tonerconcentration monitoring apparatus embodied by the invention; and

FIG. 4 is a waveform diagram illustrating the output of the sampling and integrating components comprising the alternative embodiment of FIG. 3.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS For a general understanding of an electrographic reproduction apparatus wherein the invention has particular utility, reference is made to FIG. 1 wherein various components of an electrophotographic copier are schematically illustrated. As in most electrophotographic copies, an endless photoconductive recording element is advanced along a predetermined path adjacent to which are disposed the various electrophotographic processing stations which serve to form a toner image of the original document on the surface of such recording element. As shown in FIG. 1', the recording element may be in the form of a flexible belt 2 comprising a photoconductive. film 3 disposed on a conductive backing 4. The photoconductive film may comprise, for instance, a heterogeneous mixture of a bisphenol- A-polycarbonate binder, a triarylmethane organic photoconductor and pyrylium sensitizing dye. Belt 2 is driven and guided along a path in the direction indicated by the arrows past the processing stations by rollers 5, 6, 7, 8 and 9, roller being driven by motor 10. As belt 2 passes charging station 11, a portion of its photoconductive surface receives a uniform electrostatic charge'from a corona source, or the like, which charges the entire width of the belt. As the belt continues to advance, the uniformly charged portion passes exposure station 12 where it is imagewise exposed to actinic radiation in accordance with the indicia on the original document. Such imagewise exposure serves to selectively dissipate theuniform charge on the photoconductive surface to form'adevelopable electrostatic charge pattern thereon corresponding to the indicia on the original. Development of the electrostatic charge pattern is accomplished as the portion of belt 2 bearing such pattern is advanced past development station 13. The latter generally comprises a reservoir for containing an electrographic developer and a magnetic development brush for applying the developer to the electrostatic charge pattern to render it visible. As previously mentioned, the development mixture comprises, in general, a mixture of electroscopic toner particles charged to a polarity so as to be attracted to the electrostatic charge pattern, and granular carrier particles which carry the toner by generating triboelectric charges thereon. Upon contacting the charge pattern,.the triboelectric bonds between the toner and carrier particles are overcome by the stronger electrostatic attraction between the toner particles and the charge pattern, causing the toner particles to be separated from the carrier and deposited on the photoconductive surface of belt 2 in accordance with the charge pattern.

To reuse that portion of the photoconductive film bearing the developed or toner image, the toner image may be transferred to a receiving sheet 14 on which it can be subsequently permanently fused. Such a transfer is'commonly effectedby a sheet feeding device 15 which feeds receiving sheets, usually paper, film or other web material, from a, supply station 16 to a transfer station 17 simultaneously with the passage therepast of the toner bearing belt 2. A shift register R, disclosed in the commonly assigned copending U.S. Pat. application Ser. N01 19,999, serves to control the timing of the electrophotographic operations and to synchronize the feeding of the receiving sheets with the movement of the photoconductive belt. The shift register R includes a rotatable segmented and slotted cylinder 18 which is driven by suitable means, such as belt 19 extending from a pulley around roller 5 so that movement of cylinder 18 is in direct response to movement of the photoconductive belt 2.

Transfer station 17 commonly comprises means for electrostatically charging the receiving sheet so as to attract the toner particles from the belt thereto, After the toner image is transferred to the receiving sheet, the sheet-is peeled away from the belt as the latter passes over small roller 7. The toner-bearing receiving sheet is then attracted by an endless mesh belt transport 20, traveling about rollers 21 in a clockwise direction and at the same speed as belt 2, and is advanced thereby past a fusing station 22 where the toner image is permanentized by heat or the like. The receiving sheet with its toner-bearing surface facing downward is caused to adhere to transport 20 by a source of negative pressure on the rear surface of the lower leg of the transport. After fusing, the receiving sheet is dropped into a receptacle 23.

Referring now to FIG. 2, developing station 13 comprises a conventional magnetic development brush 31 which is journaled for rotation in a trough 34 containing the electrographic developer mix 35. Means are provided for rotatably driving the magnetic brush in a counterclockwise direction toward the approaching belt 2. The development brush generally comprises a magnetized metallic cylinder to which toner-coated magnetically attractible carrier particles, comprising the developer mix 35, are attracted in chainlike arrangements to simulate the bristles of a brush. Trough 34 is arranged adjacent the path along which belt 2 travels, being spaced therefrom such that the bristles of the development brush contact the photoconductive surface of belt 2 at all times.

As previously mentioned, only a portion of the photoconductive surface of belt 2 receives a uniform electrostatic charge as it passes charging station 11. Shift register R serves to activate the charging station only for a predetermined time period, sufficient to uniformly charge a sector of belt 2 long enough to receive an image of the longest original document which the copier is designed to handle. After each charging period, the charging station is inactivated and no charge is applied to the belt surface for a predetermined time period, usually a time sufficient for a few inches of the belt to pass by. Thus, as belt 2 approaches developing station 13, after being imagewise exposed at exposing station 12, its photoconductive surface bears equally spaced developable charge patterns 36, shown as a negative charge, between which uncharged areas 37, bearing virtually no charge whatsoever are present.

Now, in accordance with the present invention, it has been found that, as a magnetic development brush 31 which is connected to ground through some resistance R continuously rotates against, and thereby interacts with, the photoconductive surface of belt 2, an electrical potential is generated on the development brush. Moreover, it has been found that the two major factors which determine the magnitude of such potential are the concentration of toner in the developer and the charge on that portion of the belts surface contacting the brush. Toner concentration affects the potential of the brush due to the phenomenon of triboelectrification. As in the case of the electrographic developer, wherein two dissimilar materials are mixed together and become charged relative to one another, the surface of belt 2 and the development brush comprise such dissimilar materials which, when rubbed-together, as occurs when the rotating brush contacts the moving belt surface, generate triboelectric charges of opposite polarity on each other. Since the magnitude of the generated triboelectric charge is dependent on the composition of the materials, such composition determining the position of the materials in the triboelectric series, and since the composition of the development brush is a function of the toner concentration in the developer carried by such brush, the triboelectric charge generated on the brush is a function of the toner concentration in the brush. Actually, the brush potential has been found to be directly proportional to the carrier concentration, hence, inversely proportional to toner concentration. Since the composition of belt 2 is substantially constant, it has no variable effect on the brush potential. The charge pattern borne by the surface of belt 2 affects the brush potential due to induction. Since the charge pattern varies in intensity, it produces an undesirable variable effect on brush potential; To eliminate such effect, means are provided for sampling the brush potential only when the brush is contacting the uncharged portion of the belt between successive charge patterns. In this manner, there is no need to compensate for the variable effect on .brush potential produced by the charge pattern.

As shown in FIG. 2, an electrical conductor 40 is coupled to the magnetic brush 31 to sense the potential thereof. Actually, the brush potential can be sampled by connecting conductor 40 to trough 34, assuming it is electrically conductive, since the metallic carrier particles serve to electrically couple the brush and the trough. The development brush 3.1 is connected via conductor 40 to ground through a high resistance R,,, the value of which is selected so as to enhance the variability of brush potential. Typically, R is approximately 100 megohms, The brush potential sensed is amplified by a high input impedance operational amplifier 42 and passed through a sampling circuit 43 comprising a normally open switch which may be periodically closed by the shift register R for a short period during which the development brush is in contact with a non-charged portion 37 of the belt surface. The pulsed output a of the sampler is then fed to a conventional threshold sensor 45 which transmits a signal b when the amplitude of any pulse received thereby ex ceeds a predetermined threshold level. The output of the threshold sensor is then used to activate a conventional toner replenisher 47 which adds toner to the developing station when activated. To allow the added toner to blend in with the developer contained in trough 34 to form a substantially homogeneous mix, a simple delay circuit 48 is coupled with the output of the threshold sensor to override the sampling circuit for a predetermined time.

In FIG. 3, alternative signal processing circuitry is shown which includes an integrating operational amplifier 55. The integrating amplifier is used as a safeguard to prevent transients, noise or other electrical disturbances which might ocassionally affect the amplitude of the pulsed output a of sampler 43 from activating the toner replenisher when additional toner is not, in fact, required. As shown in FIG. 4, wherein the outputs of the sampler, integrating amplifier and threshold sensor are shown as a function of time, the integrating amplifier output 0 will gradually increase as positive-going pulses are applied to its input. Since the output of the integrating amplifier does not return to zero during the period sampling is not taking place, 2

means must be provided for periodically dumping the voltage accumulated by the amplifier. To accomplish this function, a conventional counting circuit 56 is provided which automatically dumps or resets the voltage accumulated by the integrating amplifier to zero volts after a predetermined number of pulses is counted. If the integrator output exceeds the threshold of sensor 45 before the counting circuit resets the integrator amplifier, the output of the threshold sensor can be used to reset the counter to zero.

EXAMPLE 1 A positively charged toner comprising carbon black, nigrosine base, docosanoic acid, poly(4,4'-isopropylidene diphenyl-alt-ethylene carbonate). was mixed with an electrographic carrier at a concentration of approximately 5.5 percent toner-to-carrier by weight to form a developer mixture. The carrier consisted of l-Ioeganaes EH sponge iron particles sieved +150, then nickeled to 1 percent by weight. The developer mixture was then used in a magnetic brush developing station to produce copies from various original documents. The level of potential of the magnetic brush was sampled before and after each copy. It was found during this period that the brush potential rose from -40 to -78 volts. The toner concentration was determined, after the copies were made, to have been reduced to approximately 4.8 percent.

EXAMPLE 2 The developer of Example 11 was manually replenished to about 6 percent toner-to-carrier by weight, and another process cycle of 29 copies was made from a single original. The brush potential during this period rose from -25 to -40 volts. The toner concentration was determined to have been reduced to 5.5 percent. The entire system was permitted to rest for 10 minutes. A series of seven copies was run and the brush potential initially registered -40 and then climbed to -45 volts.

EXAMPLE 3 A fresh developer and photoconductive belt combination was tested in the manner described in Example 1. Using a single original which had few solid areas, the potential rose -12 volts over a process cycle of 86 prints.

EXAMPLE 4 Example 3 was repeated, except that an original with an average amount of solid areas was used. In two separate process cycles, it was found that the potential rose -17 volts after 84 prints and -18 volts after 92 prints, respectively. Between the two cycles, 10 gm (approximately 0.4 percent) of toner was used to manually replenish the developer. It was noted that the potential dropped l 5 volts after replenishment.

From the foregoing examples, it is apparent that the concentration of toner in the developer carried by the brush is inversely proportional to the brush potential, the lower the toner concentration, the higher the brush potential.

This invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

We claim:

1. In an electrographic copier including means for advancing an electrographic recording element bearing spaced electrostatic charge patterns on a surface thereof past a rotatably mounted magnetic development brush carrying a developer mixture comprising toner and magnetically attractable carrier particles, such brush being electrically connected to ground potential through a resistor of substantial resistance and arranged to continuously contact the surface of the recording element as such element advances along the predetermined path, the improvement comprising:

apparatus for maintaining substantially constant concentration of toner particles in the developer mixture carried by the brush, said apparatus comprising means, electrically coupled with the magnetic development brush, for sampling the electrical potential of the brush as the developer mixture carried by the brush periodically contacts portions of the recording element between successive charge patterns, said potential resulting, at least in part, from the triboelectric interaction between the recording element and the developer mixture and being inversely proportional to the concentration of toner particles in the developer mixture.

2. The invention accordance to claim 1 wherein said apparatus further comprises means operatively coupled to said sampling means for activating a toner replenishing device when the sampled potential exceeds a predetermined threshold level.

3. The invention according to claim 1 further comprising an integrating means operatively coupled with said sampling means for accumulating the electrical potential sampled thereby, and means for activating a toner replenishing device when the potential accumulated by said integrating means exceeds a predefined threshold level.

4. The invention according to claim 3 further comprising counting means, operatively coupled with said sampling means, for counting the number of regions on the belt surface sampled by said sampling means and for resetting the potential accumulated by said integrating means to zero in the event the accumulated potential fails to exceed said predefined threshold level before said counting means counts a predetermined number of regions.

Claims

1. In an electrographic copier including means for advancing an electrographic recording element bearing spaced electrostatic charge patterns on a surface thereof past a rotatably mounted magnetic development brush carrying a developer mixture comprising toner and magnetically attractable carrier particles, such brush being electrically connected to ground potential through a resistor of substantial resistance and arranged to continuously contact the surface of the recording element as such element advances along the predetermined path, the improvement comprising: apparatus for maintaining substantially constant concentration of toner particles in the developer mixture carried by the brush, said apparatus comprising means, electrically coupled with the magnetic development brush, for sampling the electrical potential of the brush as the developer mixture carried by the brush periodically contacts portions of the recording element between successive charge patterns, said potential resulting, at least in part, from the triboelectric interaction between the recording element and the developer mixture and being inversely proportional to the concentration of toner particles in the developer mixture.