Ink development in HP Indigo digital presses

Abstract

In classical liquid electrophotography the latent image on the photoconductor is developed by electrophoresis of low viscosity low density ink dispersion onto the photoconductor followed by a metering step removing excess liquid. This technology was used in the first generation products that Indigo introduced to the market. As Indigo mastered the color switching architecture and strived for higher speeds, it became evident that simple electrophoresis was limited in its speed capability.In this paper we describe the novel ink development method employed in more recent Indigo products. A very thin electrically charged, high viscosity and highly concentrated ink paste is applied to an electrically biased elastomeric developer roller, which, when contacting the photoconductor, applies ink to the discharged areas of the photoconductor. The 2nd generation method can support a broad range of printing speeds, from desk top printer level to offset press level, and can accommodate a wide range of ink properties like viscosity and concentration. It also reduces carrier usage and relaxes tolerances around the photoconductor.With this method, an ink film is electrophoretically applied to the developer roller from low concentration ink dispersion, leveraging Indigo's technology for controlling ink characteristics. The next step compacts and meters the ink layer using a biased squeegee roller. Ink is then selectively transferred to the photoconductor by the electric fields. Finally, the residual ink is electrically transferred from the developer roller surface and subsequently dispersed back to low concentration for reuse.