Generally, the process of electrophotographic printing includes charging a photoconductive member to a substantially uniform potential to sensitize its surface. The charged portion of the photoconductive surface is exposed to a light image from a scanning laser beam or an LED source that corresponds to an original document being reproduced. The effect of the light on the charged surface produces an electrostatic latent image on the photoconductive surface. After the electrostatic latent image is recorded on the photoconductive surface, the latent image is developed. Two-component and single-component developer materials are commonly used for development. A typical two-component developer comprises a mixture of magnetic carrier granules and toner particles. A single-component developer material is typically comprised of toner particles without carrier particles. Toner particles are attracted to the latent image, forming a toner powder image on the latent image of the photoconductive surface. The toner powder image is subsequently transferred to a copy sheet. Finally, the toner powder image is heated to permanently fuse it to the copy sheet to form the hard copy image.
The approach utilized for multicolor electrophotographic printing is substantially identical to the process described above. However, rather than forming a single latent image on the photoconductive surface in order to reproduce an original document, as in the case of black and white printing, multiple latent images corresponding to color separations are sequentially recorded on the photoconductive surface. Each single color electrostatic latent image is developed with toner of a color corresponding thereto and the process is repeated for differently colored images with the respective toner of corresponding color. Thereafter, each single color toner image can be transferred to the copy sheet in superimposed registration with the prior toner image, creating a multi-layered toner image on the copy sheet. Finally, this multi-layered toner image is permanently affixed to the copy sheet in substantially conventional manner to form a finished copy.
With the increase in use and flexibility of printing machines, especially color printing machines which print with two or more different colored toners, it has become increasingly important to monitor the toner development process so that increased print quality, stability and control requirements can be met and maintained. For example, it is very important for each component color of a multi-color image to be stably formed at the correct toner density because any deviation from the correct toner density may be visible in the final composite image. Additionally, deviations from desired toner densities may also cause visible defects in mono-color images, particularly when such images are half-tone images. Therefore, many methods have been developed to monitor the toner development process to detect present or prevent future image quality problems.
For example, it is known to monitor the developed mass per unit area (DMA) for a toner development process by using densitometers such as infrared densitometers (IRDs) to measure the mass of a toner process control patch formed on an imaging member. IRDs measure total developed mass (i.e., on the imaging member), which is a function of developablitiy and electrostatics. Electrostatic voltages are measured using a sensor such as an ElectroStatic Voltmeter (ESV). Developability is the rate at which development (toner mass/area) takes place. The rate is usually a function of the toner concentration in the developer housing. Toner concentration (TC) is measured by directly measuring the percentage of toner in the developer housing (which, as is well known, contains toner and carrier particles).
As indicated above, the development process is typically monitored (and thereby controlled) by measuring the mass of a toner process control patch and by measuring toner concentration (TC) in the developer housing. However, the relationship between TC and developability is affected by other variables, such as ambient temperature, humidity and the age of the toner. For example, a seven-percent TC results in different developabilities depending on the variables listed above.
One common type of development system uses one or more donor rolls to convey toner to the latent image on the photoconductive member. A donor roll is loaded with toner either from a two-component mixture of toner and carrier particles or from a single-component supply of toner. The toner is charged either from its triboelectric interaction with carrier beads or from suitable charging devices, such as frictional or biased blades or from other charging devices. As the donor roll rotates it carries toner from the loading zone to the latent image on the photoconductive member. There, suitable electric fields can be applied with a combination of DC and AC biases to the donor roll to cause the toner to develop to the latent image. Additional electrodes, such as those used in the Hybrid Scavengeless Development (HSD) technology may also be employed to excite the toner into a cloud from which it can be harvested more easily by the latent image. The process of conveying toner to the latent image on the photoreceptor is known as development.
A problem with donor roll developer systems is a defect known as ghosting or reload which appears as a lightened ghost image of a previously developed image in a halftone or solid on a print. The reload defect occurs when insufficient toner has been loaded onto the donor roll within one revolution of the donor roll after an image has been printed. In this situation, there will be a localized region of the donor roll that is not fully loaded with toner (it has been depleted of toner mass by the previous image). The donor roll thus retains the memory of the previous image, and a ghost of the previous image shows up if another image is printed at that time.
The susceptibility of the development system to a reload defect is dependent upon the image content of the print job (how much toner was removed from the donor roll by the image areas of the previous image) as well as the rate at which toner is reloaded onto the donor rolls (the maximum rate at which toner can be re-supplied to the donors). One way of improving the ability of the toner supply to provide an adequate amount of toner to reduce or prevent ghost images is to increase the peripheral speed of the magnetic brush or roll that transfers toner from the supply reservoir to the donor roll. However, as the relative difference in the speeds of the magnetic brush and donor rolls increases so do the collisions of the carrier or toner granules. The toner particles also impinge on the blade mounted proximate to the magnetic brush to regulate or trim the height of the magnetic brush so that a controlled amount of toner is transported to the developer roll. The collisions of the toner with the carrier and the trim blade tend to smooth the surface of the toner particles and cause the particles to exhibit increased adhesion. In general, the surface of the carrier particles can be affected by these collisions (with other carriers, trim bars, etc) as well. This general process is sometimes referred to as material abuse. The increased adhesion of the toner particles that have experienced a great deal of abuse causes less toner to be transferred to the photoreceptor to develop the latent image for a given development voltage. Thus, there is a tradeoff between increased speed of the magnetic brush to improve reload performance and the rate of material abuse. In most development systems, the tradeoff between increased toner supply and material abuse is made at design time. Typically the speed of the magnetic brush or roll is selected such that a solid patch can be developed within one donor revolution of another solid patch with minimal reload effects being observable in the developed mass image.
Material abuse is a problem for many development systems when printing low area cover (LAC) jobs. For LAC print jobs, there is little toner throughput and so the average age of the material in the developer sump can increase substantially. One potential problem as the age of the material in the sump increases is that the level of abuse that a given toner or carrier particle has experienced can actually become quite high. When this occurs, the developability of the toner particles generally tends to decrease, which then leads to a degradation in the performance of the development subsystem. In some circumstances, increased toner age and the associated increases in material abuse can also lead to problems in the transfer subsystem as well. Eventually these effects can lead to substantial print quality (PQ) problems that may require costly mitigation strategies.
One method for controlling the rate of material abuse in the developer housing is to maintain some constant level of abuse of the material independent of the image content that is being printed. This can be accomplished by adjusting how much energy is input to the developer housing based on the current image content of the customer's print job.
In another method, as disclosed in U.S. application Ser. No. 11/090,727 and is hereby incorporated by reference, employs the method of adjusting the speed of the magnetic roll on-the-fly based on image content to reduce material abuse. As discussed previously, reducing the speed of the magnetic roll can help to reduce the amount of material abuse that occurs within the developer housing. However, a major difficulty with reducing the speed of the magnetic roll is the occurrence of the reload defect. To minimize the occurrence of this defect, a reload sensitivity detection algorithm determines which pages within a customer's job are candidates for speed reduction without the possibility of inducing reload defects. Using this feed-forward information, the controller can then appropriately adjust the speed of the magnetic roll while attempting to minimize the chance for inducing reload defects in the output prints. Such an approach has enabled a performance improvement in terms of development stability and transfer performance over long LAC jobs. However, Applicants have found that there are significant undesirable Developed Mass per unit Area (DMA) shifts in the development performance when the speed of the magnetic roll is changed which then leads to undesirable color shifts and thus to poor output image quality.
Consequently, there is a need to provide a method and apparatus for maintaining stable DMA performance from the development subsystem independent of the speed profile applied to the magnetic brush or roll.