An Innovative Quality Mapping Technology for Photoreceptors

Two factors critical to print quality in electrophotography are coating uniformity and coating defects on the photoreceptor. In photoreceptor design, coating materials must therefore be optimized, and methods must be employed to ensure that the photoconductive coating is both free of defects and highly uniform in thickness and electrophotographic properties. A family of computerized photoconductor test systems now commercially available provides an innovative electrostatic mapping method for evaluating uniformity and defects. The systems combine conventional measurements such as charge acceptance and photosensitivity with the ability to detect and locate coating defects as small as 100 μm or less. Key to the success of the mapping method used is a measurement principle closely resembling the basic electrophotographic process. Strong correlations have been demonstrated between test results from these systems and the quality of prints from the photoreceptors tested. In this paper, the design methodology, operational characteristics, system performance, and practical applications of these systems are discussed. Importance of mapping Photoconductor mapping in quality measurement reveals coating uniformity — a great advantage, since inconsistencies in either the thickness or the photoelectric properties of the coating will affect print quality. As the demand for color printing increases, the need for detailed uniformity analyses of photoconductors becomes ever more acute. Mapping likewise reveals coating defects (pinholes, scratches, contamination, etc.), which also result in print defects. Besides exposing defects, mapping provides information for defect characterization. Further, mapping can be used to evaluate reliability, another important property of photoconductors. As photoconductors gain more widespread use in higher speed printers and digital presses, their physical wear behavior becomes increasingly important. Mapping technology for photoreceptors In pursuing significant improvements in photoconductor mapping methodologies for R&D and production applications, a substantial history of effort must be considered, though the results have been mixed. Scanning electron microscopy, for example, offers the benefit of high resolution, but is impractical for production use, major drawbacks being its inability to simulate the printing process and the fact that its operation requires a vacuum. Capacitive-coupled shielded probes have also been used, but have generally suffered from low signalto-noise ratios, and, like SEM systems, do not simulate printer operation. Popovic has offered a variation of the shielded probe technique, but it involves contamination of the photoconductor surface. Toner image methods also affect the photoconductor surface, and interpretation of results is complicated by uncontrolled variables introduced by the development system. Atomic force microscopy offers high resolution, but its practicality is limited, especially since, like the other systems mentioned, it does not simulate printer operation. Finally, the impressive ability of the human eye to detect and characterize detail, including defects in photoconductors, has inspired many proprietary efforts to develop optical mapping systems. In general, these efforts have been unsuccessful due to problems with speed, reliability, and flexibility. Furthermore, optical measurements are fundamentally indirect: a cosmetic defect may affect a photoconductor’s appearance without any degradation in its performance, while a serious functional defect may be optically invisible. A more reliable approach than any of these is electrostatic mapping based on direct measurement of the photoconductor’s electrophotographic properties. This approach, now commercially available, uses a proven electrostatic measurement technology — non-contact electrostatic voltage probes with vibrating electrodes. Unlike the other systems, this mapping system simulates the electrophotographic printing process and is designed to measure specifically those defects that affect print quality. It provides invaluable data in both R&D and production applications for materials development, process development, and quality control. Figure 1 illustrates an electrostatic mapping system. A brief discussion of each system component follows. Figure 1 Electrostatic mapping system measurement principle Design of an electrostatic mapping system –