Liquid Toner Conductivity Effects on Laser Exposed Color Halftone Dots

Abstract

Graphic Arts imaging requires high contrast materials to generate halftone dots for controlled tonal reproduction in lithographic printing. Off-press color proofing systems must accurately reproduce the halftone patterns with very controlled maximum reflection density. Direct digital color proofing (DDCP) writing technologies are currently being developed to produce full color halftone proofs directly from digital data with electrophotographic processes having advantages in use of cost effective lasers and high spot addressability. Colored liquid toners are known to produce high resolution and contrast dependent on the electrophoretic mobility and conductivity of the toner dispersions. However, the interactions of the materials in the electrophotographic machine with a specific writing light source are critical to the precise reproduction of the full halftone dot range in the DDCP. The effects of colored liquid toner electrophoretic properties on the reproduction of halftone dots developed onto laser exposed photoreceptor are described in this paper. Several liquid toners with different particle mobility, conductivity, and charge per mass are used to relate process conditions such as deposition field and development time to optical density of the image and area coverage or halftone dot gain of the toner deposit. Microdensitometer traces and scanning electron micrographs of liquid toner developed laser scan lines are used to evaluate edge sharpness and density uniformity. The electrostatic field generated by the latent charge pattern in a uniform geometry is calculated and an electric field dependent deposition model is described to determine the size and optical density of the laser scan line for various liquid toner formulations. This model may be used to optimize the process conditions of a specific laser writer to obtain the required halftone reproduction for direct digital color proofing.