Electrophotography – Control of electrophotography process – Control of developing
Reexamination Certificate
2000-11-03
2003-03-18
Brase, Sandra (Department: 2852)
Electrophotography
Control of electrophotography process
Control of developing
C399S029000
Reexamination Certificate
active
06535700
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention is related to developing images using liquid toners.
2. Description of Related Art
An electrostatographic printing system uses a substantially uniformly charged photoreceptive member that is exposed to a light image of an original input document. The light image discharges selective areas of the photoreceptive member, forming an electrostatic latent image on the photoreceptive member. The electrostatic latent image is developed into a visible image by applying charged toner particles to the latent image. The developed image is transferred from the photoreceptive member to a copy substrate. The photoreceptive member is then cleaned to remove any residual charge or developing material.
In liquid electrophoretic image forming systems, the charged toner particles are part of a liquid developer material that is brought into contact with the latent image. The liquid developer material comprises charged toner particles dispersed in a liquid carrier material. Liquid toners have many advantages over powder toners. For example, images developed with some liquid toners adhere to the copy substrate without requiring fusing of the image. Other liquid toners require fusing, but require less fuser energy than dry powder toners. Also, liquid toner particles can be made significantly smaller than powder toner particles. This is particularly advantageous in multicolor processes where multiple layers of toner particles generate the final multicolor output image.
SUMMARY OF THE INVENTION
The developability of the liquid toner is the ability of the liquid toner to fully develop the electrostatic latent image on the photoreceptor to a desired image density. It is important to keep the developability constant to maintain print quality. The developability is kept constant by keeping constant the developed mass per unit area (DMA or M
D
). Conventionally, the developed mass per unit area is determined by developing solid area test patches of a known area on the photoreceptor. A scanning device, or other test device, is used to sense the toner mass density of the test patch. However, if the test patches are not finally transferred to an image media, the test patches place an undesirable load on the cleaning system of the print engine. If the test patches are finally transferred to an image media, their use increases paper waste and reduces printer availability to the customer.
In printing systems that provide custom colors by automating the mixing of primary colored toner components, the component concentrations in the mixed toner bath must be controlled to provide the correct custom color. Methods have been provided for continually sensing the color of the toner bath or the developed layer and calculating component concentrations. These methods are suitable for maintaining color during long runs in the presence of slow changes in developability. However, the use of these methods to obtain the proper initial color often takes several iterations of printing, measuring and proper adjusting of the developed toner color before the first print can be made.
Other conventional methods exist that measure properties of the liquid toner that are related to developability of the liquid toner. However, these methods of characterizing liquid toners are known to have disadvantages.
A first characterization method that measures toner conductivity, is provided by, for example, the Model 610 conductivity meter, sold by Scientifica, 340 Wall St., Princeton, N.J. The conductivity sensor measures the total conductivity of the liquid toner supply. The total conductivity of the liquid toner is a valuable characteristic for two reasons: (1) toners of very low conductivity usually develop very poorly, and (2) as conductivity increases above some optimum value, the developed mass per unit area (DMA) produced by a given voltage differential &Dgr;V
dev
between the voltage applied at the photoreceptor V
P/R
and the voltage applied at the development electrode V
dev electrode
usually decreases. The conductivity of liquid toner supply results from several conductive species and is calculated as:
&sgr;=&Sgr;
i=1
N
{Q
i
*N
i
*&mgr;
i
}=&Sgr;
i=1
N
{&sgr;
i
} (1)
where:
&Sgr;
i=1
N
{ . . . } denotes the sum from 1 to N of the values of the quantities inside the braces;
&sgr;
i
denotes the contribution to conductivity from the i
th
species;
Q
i
denotes the charge of the i
th
species;
N
i
denotes the number density of the i
th
species; and
M
i
denotes the mobility of the i
th
species.
Generally, toner particles represent only one of at least two species that contribute to the conductivity of the liquid toner. The particles in liquid toners generally get their charge from a chemical reaction that also produces dissolved molecular species of opposite sign charge. In many practical systems, there are three contributions to conductivity, &sgr;
particle
, &sgr;
minus
, and &sgr;
plus
, where:
&sgr;
particle
denotes the particle contribution to conductivity;
&sgr;
minus
denotes the negatively charged molecules' contribution to conductivity; and
&sgr;
plus
denotes the positively charged molecules' contribution to conductivity.
Conductivity alone is not able to identify systems in which dissolved molecules provide a large conductivity and particles are poorly charged. Nor can conductivity alone distinguish a small number of particles with high charge and mobility from a large number of particles with low charge and low mobility.
A second characterization method, laser velocimetry, usually called electrostatic light scattering (ELS), measures the velocities of toner particles. This has been described by Caruthers et al., “Liquid Toner Particle Charging and Charge Director Ionization,” pages 210-214, IS&T's 10
th
International Congress on Advances in Non
-
Impact Printing Technologies
(1994). Knowing the applied electrical field and the velocity, the toner particle mobility can be calculated. Liquid toners with high particle mobilities generally produce better print quality than toners with low particle mobilities. However, electrostatic light scattering requires very dilute solutions of the toner, such as 0.01% toner particles by weight, so that light is not multiply scattered in passing through the toner. Because this is much less than the 1-2% concentration of toner particles used in most printing engines, the mobilities measured by electrostatic light scattering may not predict the mobilities of toner particles in the printing engine's toner supply. Also, electrostatic light scattering requires that toner particles not move in and out of the laser beam during the measurement. For very mobile toner particles, this may require that the applied electric fields be much less than those used in a printing engine's development system. If the toner particle's mobility is field-dependent, then electrostatic light scattering may again fail to correctly predict behavior in the printing engine's development system.
A third method of characterization measures the intensity of an acoustic wave induced in a liquid toner by application of a high frequency electric field. Matec Applied Sciences' Electrokinetic Sonic Amplitude (ESA) system uses this principle. This characterization method has the great advantage of working well at high particle concentrations. However, as the density difference between the particle and the carrier liquid decreases, the signal becomes weaker. Since particles remain dispersed in the carrier liquid better as their density becomes more equal to the carrier liquid's density, this method may also fail for systems of practical importance.
All the above methods have the disadvantage that they measure properties of the liquid toner that are related to developability of the liquid toner, but they do not directly measure developability. Also, the conventional methods do not use the same development geometry used in the printing engine. Th
Brase Sandra
Oliff & Berridge PLC.
Xerox Corporation
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