Electrophotographic printing is well known and has been widely refined. Using electrophotographic techniques, images are photoelectrically formed on a photoconductive layer mounted on a conductive base. Liquid or dry developer or toner mixtures may be used to develop a requisite image.
Liquid toner dispersions for use in the process are formed by dispersing dyes or pigments in natural or synthetic resin materials in a highly insulating high dielectric constant carrier liquid. Charge control agents are added to the liquid toner dispersions to aid in charging the pigment and dye particles to the requisite polarity for proper image formation on the desired substrate.
The photoconductive layer is sensitized by electrically charging whereby electrical charges are uniformly distributed over the surface. The photoconductive layer is then exposed by projecting or alternatively by writing an image over the surface with a laser, LED, or the like. The electrical charges on the photoconductive layer are conducted away from the areas exposed to the light with an electrostatic charge remaining in the imaged area. The charged pigment and or dye particles from the liquid toner dispersion contact and adhere to the image area of the plate. The image is then transferred to the desired substrate such as a sheet or paper.
With liquid electrophotography (LEP), the image development process requires that the toner be delivered to the developer at relatively constant concentration. However, usage of toner solids and toner carrier fluid in an LEP process are independent of each other. Toner solid consumption is proportional to the print coverage on the page and the number of pages printed. Whereas, toner carrier consumption is independent of print coverage and only a function of the number of pages printed. These characteristics result in toner concentration decreasing for above average page coverage and increasing for below average page coverage. Additionally, toner solids will settle out from the toner carrier over time. Thus, a toner concentration control is required for LEP processes.
Several active concentration systems are known in the art. Such active control systems generally measure one or more physical properties of the toner thereby determining an effective concentration of solids and carrier fluid. From this information the control system then enables the appropriate pumps to bring the concentration back into design specification. Such an arrangement requires the appropriate sensors, a metering pump for toner solids, a metering pump for the carrier fluid, a working reservoir in which the carrier fluid and solids are mixed and some means to mix within the working reservoir.
When an active toner concentration control system is applied to a color LEP printer the aforementioned items must be increased by a factor of four making such an arrangement prohibitively complex and costly. Another disadvantage of the active concentration control system is the difficulty and expense in incorporating all the elements into a single consumable. Some of the elements may actually become a permanent part of the printer. Such elements might include the working reservoir, the sensors, and metering pumps. The primary drawback of this approach is that they may need to be cleaned during toner replacement or addition due to contamination and sludge formation.