Announced at the 2016 DRUPA international print media fair, the HP Indigo 7900 Digital Press features a new, HP Labs-engineered charge roller that many experts in the field had believed to be impossible to create.
The charge roller initiates the printing process by laying down a uniform electrical charge onto a photosensitive drum, which is then selectively discharged to form the latent image. Unlike all other commercial charge rollers, which are made from a conductive rubber exterior, HP Labs’ new charge roller is made from metal, a counterintuitive choice given that the rollers need to be highly electrically stable while in operation.
“The secret lies in a unique, ceramic based coating that surrounds the roller and that gives us the electrical characteristics that we need,” reports Omer Gila, director of commercial printing research in HP Lab's Print and 3D Lab. “It has allowed us to replace a limited-lifespan consumable with something that is in effect permanent, offers significant improvements in print quality and reliability, and helps us reduce our environmental impact.”
Until now, all charge rollers have been made from rubber-based formulations. These formulations are designed to be ionically conductive with a great deal of precision. But no matter how well designed, they also possess inherent drawbacks and limitations. During operation, rubber-based rollers will slowly deplete their conductive agents until they are no longer conductive and need to be replaced. Moreover, the rollers’ dimensions are extremely hard to control in manufacturing and even harder to manage in the press due to humidity or oil absorption.
HP’s new metal based roller does not age and is extremely durable both electrically and mechanically. Controlling its dimensions is relatively simple, allowing the roller to operate floating above the photosensitive drum. This avoids cross contamination from contact, which is characteristic of rubber rollers, and makes the charging a clean and robust operation. The new metal-based roller is also no longer a consumable but now a permanent part of the press, further reducing down time, operating expenses, and waste.
Researchers have long aspired to make charge rollers from metals, but had always failed to achieve charging uniformity and stability until now. “The charging plasma always remained uncontrolled and jumped sporadically between “glow discharge” and “Townsend discharge” states,” explains Gila. “With the latter, the air becomes conductive and creates high intensity streamers, which locally overcharge the photosensitive drum. That was one of the main issues that we had to overcome.”
In response, researchers in HP Labs managed to develop a semiconducting ceramic-based overcoat that generates a controlled charging behavior with extreme uniformity while also blocking streamers from occurring. The research required that they make advances in fields such as materials science and plasma modeling, and the development of new tools including a highly sensitive electrical meter able to measure events only nanoseconds in duration.
The HP Labs team closely collaborated with HP Indigo’s research and development group throughout their work on topics ranging from materials and processes to print quality, as well as stress testing and manufacturing.
After successful field trials at HP Indigo customer sites over the past year, the permanent charge roller was integrated into the new 7900 Indigo sheetfed Digital Press ready for its DRUPA 2016 launch. In time, the permanent charge roller is expected to be implemented in other HP Indigo presses, including HP Indigo label and wide format products.