Electrophotography – Control of electrophotography process – Control of developing
Reissue Patent
2000-04-06
2002-09-24
Pendegrass, Joan (Department: 2852)
Electrophotography
Control of electrophotography process
Control of developing
C399S240000
Reissue Patent
active
RE037859
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to development control in elecrorostatographic imaging and, more particularly, to liquid toner development control.
BACKGROUND OF THE INVENTION
Generally, there are two types of development systems employed by electrostatographic imaging apparatus, namely, powder toner development systems and liquid toner development systems. Although powder toner is more conventional, liquid toner is often preferred for its higher intrinsic resolution. Considerable efforts have been made in the past to design more efficient and more convenient liquid toner development systems.
Liquid toner systems are sensitive to physical changes in the toner, such as changes in temperature, charge level, viscosity and liquid concentration, most of which are not relevant in powder toner systems. It is appreciated that these toner changes may affect the development level, thereby resulting in inconsistent imaging. Therefore, control of the liquid toner properties is generally considered to be crucial for maintaining a constant level of developed mass per unit area (DMA) on a photoreceptor of the imaging apparatus.
One current approach to maintaining image quality measures the optical density, volume and conductivity of the liquid toner used in the process. Based on these measurements, toner concentrate, carrier liquid a charge directs, respectively are added to the liquid toner. Such an approach is described in U.S. Pat. No. 4,860,924, the disclosure of which is incorporated herein by reference.
It is appreciated that construction and maintenance of a closed loop development system as described above is both complex and expensive. Therefore, liquid toner development systems have never been embodied in low-cost disposable cartridges, as normally is the case in powder toner systems.
In U.S. Pat. No. 4,341,461, the bias voltage of a development rolls in a powder development system la adjusted in accordance with a measurement of toner density on a developed patch on a photoreceptor. The toner density is measured by an infrared densitometer which apparently measures the optical density of the layer of toner developed on the photoreceptor.
U.S. Pat No. 4,678,317 describes a liquid toner system in which a sensor electrode is used to sense the potential of a charged photoreceptor and to adjust a development electrode voltage to compensate for variations in the sensed potential
WO 93/01531, the disclosure of which is incorporated herein by reference, describes a direct-transfer liquid toner development system. A layer of concentrated liquid toner coating a toning roller is brought into virtual contact with a photoreceptor, and portions of substantially even thickness are transferred from the toning roller onto attractive portions the photoreceptor.
Either the full thickness of the portions is transferred, is a binary mode of operation or, in a quasi-binary mode of operation, a partial yet even thickness is transferred. The voltage between the toning roller and the photoreceptor determines the thickness of the layer which is transferred. In the binary mode, the DMA on the photoreceptor is substantially equal to the DMA on the toning roller and, in the quasi-binary mode, the photoreceptor DMA is dependent in a well defined manner upon the toning roller DMA. For quasi-binary transfer the photoreceptor DMA is generally more uniform than the toning roller DMA.
The direct-transfer system described above normally employs a toner applicator and a squeegee associated with the toning roller.
SUMMARY OF THE INVENTION
It is so object of the present invention to provide an improved liquid toning system. In accordance with a preferred embodiment of the present invention, consistent toning of latent electrostatic images is maintained throughout numerous toning cycles without adding liquid toner or liquid toner components to the system and/or adjusting the material composition of the liquid toner, i.e. the ratio between toner particles and carrier liquid.
In general, liquid toner including charged toner particles and carrier liquid is contained in a sump of the toning system. The toner particles are selectively removed from the liquid toner during the toning process as they are transferred to a latent image bearing surface such as a photoreceptor. The carrier liquid is generally removed at a different rate, usually a lower rate. Thus, the percentage of toner particles in the liquid toner, hereinafter referred to as the solids concentration, rises or falls as a function of the total area toned by the toning system. For some colors, for which the proportion of printed surface to unprinted surface is small, the solids concentration may rise with time.
When either the solids concentration or the total quantity of liquid toner in the system is reduced below a pre-set value, either the sump or the entire toning system is replaced or refilled.
In accordance with a preferred embodiment of the present invention, there is thus provided a direct transfer toning system inducting an endless toning surface, preferably the surface of a toning roller charged to a predetermined voltage, coated with a lays of tone concentrate, a developed mass pet unit area (DMA) controller having an input for receiving an indication of the DMA on an imaging outface such as a photoreceptor, and adjusting the DMA on the toning surface in response to the received input, whereby the DMA on the toning roller is maintained substantially constant.
Preferably, the DMA controller controls at least one voltage which affects the DMA on the toning roller.
According to one aspect of the present invention, the input to the DMA controller is supplied by a DMA sensor which monitors the DMA on the imaging surface. Since, in direct-transfer toning systems, the DMA on the imaging surface is dependent upon the DMA on the toning rolls, by controlling the DMA on the toning roller, a consistent toning level is readily maintained.
In one embodiment of this aspect of the invention, the DMA sense includes an optical sensor which monitors the optical density (OD) on the surface of the photoreceptor or, alternatively, on the surface of the toning roller and supplies an indication of the OD to the input. In this case, the DMA controller includes a comparator which compares the signal to a value representative of a desired DMA and adjusts at least one voltage to product the desired DMA.
In accordance with another aspect of the present invention, the input to the DMA controller is generated by a solids concentration indicator responsive to the solids concentration of the liquid toner. In this aspect of the invention the development system preferably further includes apparatus for measuring the temperature of the toner. Based on the solids concentration indication and the measured toner temperature, the at least one voltage is adjusted according to a look-up table to provide the desired DMA.
According to one, preferred, embodiment of this aspect of the invention, the solids concentration indicator includes a concentration detector which measures the concentration of solids in the toner. The concentration detector may include a viscosity sensor an optical sensor, a permitivity sensor or a sensor of any other property of the tone which is related to the solids concentration.
According to another, preferred, embodiment of this aspect of the invention, the solids concentration indicator includes a concentration calculator which generates an output responsive to the total area toned by the toning system since the last refill/replacement of the toning system. Since the total toned area can be approximated by the number of toning cycles performed by the system, the concentration calculator may include a counter of the number of toning cycles performed since the last refill/replacement of the system it is appreciated that the concentration of solids is the liquid toner is substantially a function of the total area towed and, thus, only approximately, a function of the number of toning cycles performed by the system
Alternatively or additionally, the pr
Chatow Ehud
Lavon Amiran
Lior Ishaiu
Fenster & Company Patent Attorneys Ltd.
Indigo N.V.
Pendegrass Joan
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