The binary ink developer (BID) in the HP-Indigo press converts low viscosity ElectroInk~? into a paste-like layer that is presented to the photoreceptor to form a real image. Liquid ink starts in the ink tank and is pumped into the BID where electrophoresis transfers electrically charged ink solids onto the developer roller. The adherent is subsequently compacted mechanically and electrically by a squeegee roller to produce the layer that interfaces with the photoreceptor that moves at 2.15 m/s in state-of-the-art HP Indigo presses. Nominal average solids concentration on the developer roller after exiting the squeegee is 25 wt %, as determined by removing the ink layer from the developer roller and comparing wet and dry weights. Ink transfer characteristics depend on ink solids content, which in turn varies with Electrolnk~? formulation and BID operating parameters. Quantification of ink solids content on the developer roller facilitates ink development and provides insight into BID operation. In this work we describe the use of a unique tool developed in-house for profiling ink density on the developer roller. This device controllably removes submicrometer strata of ink from the rotating developer roller by applying a step-wise variable force to a scraping blade in contact with the roller. We find that the solids distribution within the paste layer varies depending on the electrophoretic and the squeege roller voltages. A typical concentration profile starts at a minimum furthest from the roller, peaks within the paste layer and then drops again as the developer roller surface is approached. We discuss the origin of this profile.
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