Patent Grant
7970304
This invention concerns a way of improving flat field uniformity in images produced by an image development system having a development roller interposed between a supply of developer and an imaging element.
In a two-component development system, the ability to apply sufficient developer (toner and carrier) to develop a latent image on a photoconductor is critical to the creation of images with high fidelity and quality. In general practice, developer “flow” is the common metric used to describe the amount of developer delivered to the toning zone per unit time. Flow measurement is accomplished by lowering a gate (2 inches wide) into the developer stream and collecting developer for a specified amount of time (0.5 seconds). This developer is then weighed, and developer flow is reported in units of grams/inch/second. Developer flow has been correlated against certain imaging properties of the developer, such as toning contrast, background, and so on. This measurement method, although useful, needs to be made with the developer station removed from the machine, requires a scale, and thus is not well suited for a real time application.
According to the present invention, therefore, a process of adjusting flat field uniformity in images produced by an image development system having a development roller interposed between a supply of developer and an imaging element is proposed. In this process, a raw feed of developer is supplied to the development roller, and both a metered feed and an overfeed of developer, which overfeed is returned to the supply, are produced from the raw feed. A plurality of mass densities of developer used in the system are determined, as are a plurality of developer velocities through the system. Respective product values of the developer mass densities and the developer velocities are then determined, and a maximum value of the respective product values is identified. The image development system is then operated so as to produce the maximum value.
In one preferred configuration, the metered feed is produced by way of a gap between a metering element and said development roller, and the metering element is a skive or gate. The metered feed can be made adjustable by modifying the gap mentioned.
Identification of the maximum value can be performed by adjusting the metered feed, the overfeed, or both, as well as by adjusting a rotational speed of the development roller.
Since the measurement of developer flow aggregates effects of developer mass density and developer velocity, this measurement is also proportional to the product of independently measured developer mass density, which is also referred to as nap density, and developer velocity, which is also referred to as nap velocity. Flat field uniformity can be improved by maximizing the product of the developer mass density and the developer velocity.
a is a plot of developer flow versus nap density.
b is a plot of developer flow versus nap velocity.
a-5c show parameters considered in determining a depletion metric.
Experiments were performed to understand the properties of the developer that influenced the development uniformity of flat fields. Flat field uniformity is a critical part of the quality of the output images of a printer. These experiments first concentrated on developing relationships between certain adjustments made to regulate the flow (grams/inch/second) of developer into the toning zone and the resulting changes in the physical properties of the developer.
The metering skive 14 allows only a prescribed amount of developer, referred to here as metered feed MF, to pass through a gap or spacing 16 defined between the metering skive 14 and the development roller 10. The raw feed RF generally exceeds the metered feed MF. Excess developer, resulting from the difference between the raw feed RF and the metered feed MF, is referred to here as overfeed OF. This excess developer is returned back to the sump for future metering.
The metered feed MF passing through the gap 16 is then transported by way of the development roller 10 to another gap or spacing 18 defined between the development roller 10 and an imaging cylinder 20.
The toner is selectively removed from the developer and deposited on charged areas of the imaging cylinder 20 or other appropriate photoreceptive element in conventional fashion to render an electrostatic latent image on that imaging cylinder or other element. The latent image can then be transferred by electric field application to a paper sheet or another desired substrate, again in conventional fashion, and then permanently affixed to the paper or other substrate through application of heat and pressure. The excess or remaining developer is removed from the development roller 10 at a strip area 22, and the removed developer can be replenished with more toner in the sump.
The metric used to evaluate flat field image quality in this experiment is that of “depletion.” The dimensionless depletion metric is obtained by way of the following relationship
where Dri is the reflection density at a particular crosstrack position i; and
As is apparent from
b shows RF and MF data on a graph of developer velocity, or nap velocity, as a function of developer flow. This confirms the anecdotal observation that images made without metering (images produced using raw feed only) looked much smoother and had better uniformity. The raw feed can run at higher flow rates before it reaches maximum density, since it does not densify as greatly as the metered feed does.
The data indicate that the measure of ND·NV is superior to the measurement of developer flow in different ways. First, because there is a limit on maximum developer density (over-compression can lead to catastrophic release of the developer from the toning station), measuring ND·NV reveals different aspects of the developer that can be varied (such as velocity) that improve developer ND·NV without the negative side effects of developer over-compression.
Second, the measurement of ND·NV is well suited for real time, non-contact measurements (such as conventional capacitive methods) that can be used for feedback control of feed parameters to optimize image quality.
a provides a schematic illustration of an imaged page and its associated crosstrack position,
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.
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