TECHWATCH tm: Fusing Toner ... in a Flash!

Fusing the powdered “inks” used in most electronic printers means melting the toner onto the page. This inherently requires heat, which deforms and fixes those toner particles onto paper or other substrates. To meet increasing demands for speed, the emerging class of higher speed color digital printing systems must apply that heat energy very quickly, as well as do it for many colors in rapid succession.

This has lead to exciting new developments, as evidenced in Xerox's new 490 Color Continuous Feed, a 980-ppm tandem-engine printer. It wouldn't be far off the mark to think of this new device as a digital color web press—and its speeding web requires toner to fuse faster.

Heat roller fusing systems commonly used in digital printers present a host of taxing problems, not least of which is mechanical component wear, energy consumption and substrate limitations. But as speeds rise, heat quickly builds up in conventional fusing, setting a limit on speed for digital printing systems. Heat can scorch the substrate, damage the printed image and drive moisture from paper—posing other problems we will discuss shortly.

Typically, as toner particle temperatures rise, they tend to wet or melt and combine together to form a viscous liquid. Transferring from a powdered solid to melted, deformed toner particles provides the bonding to and penetration within the paper. As printing speed, numbers of colors along with register requirements increase in complexity, however, challenges arise. Process color, duotone and the ever-growing PMS toner color layers that must register or overprint are but a few examples.

Xerox's new CF490 engine, engineered to print four colors at 490 ppm within a single digital press unit (from a 20´´-wide roll) greatly increases fusing requirements. How this fusing problem was solved is a significant achievement is its own right.

The approach uses flashing or pulsing light energy of varying intensities and duration to quickly fuse toner without contact to substrates. The intensity rises and duration declines as the web moves at higher speeds. Flashing a page to fuse the toner is like snapping your fingers—“snap!” and the toner is fused. Flash fusing eliminates pressure and contact from fuser rollers and minimizes unnecessary heat energy that can damage printed pages.

Flash fusing has been practiced in electrophotographic printing systems for several decades. It is somewhat analogous to web offset UV ink curing, where photo-initiators in ink are hit by UV light and dry instantly. This instant drying/fusing is key. Flash fusing is a critical component to emerging higher speed, continuous-feed technologies.

Xerox first incorporated flash fusing in its continuous-feed monochrome model 495s, delivering very good text, line art and halftone images at 495 ppm (GAM April 2007, p.12). It's easy to conjecture that we will see ever-increasing speed and quality from a new generation of toner-based, color web presses.

Xerox uses non-contact, flashed light wave IR heat fusing incorporating specialized toners. Its 4-color CF490 uses Xenon lamps in a reflection housing. Pulsing many millions of times a month, this flash fuser housing has an advantage of bouncing light photon energy that may miss the substrate surface, returning it at light speed for a second try at fusing. A linear array of Xenon lamps span the paper path sequentially, exposing and fusing specially engineered color toners. The toners are prepared with special infrared, energy-absorption constituents.

Xenon lamps, which have fired scanners and litho cameras for years, have also historically been favored for photographic flash bulbs because of their way with color. Xenon lamp IR energy is ideal for flash fusing toner.

In flash fusing with the CF490, black toner is printed on the substrate first—and is easiest to fix, requiring slightly less energy. The cyan, yellow and magenta toners each have their own fuser behavior assignments. These colors fuse sequentially. As they are overlayed on the preceding toners, the energy builds with each lamp flash. Toners are laid down in order of their flash absorption—weakest is on top, where it gets the unattenuated light. Black absorbs the flash most easily and efficiently; that's why it is laid down first—and so it won't shield the color toners from fusing. All multi-layer colors, along with the black toner, have a liquid layer on the paper to assist adhesion.

In this process, traditional fuser technology components—such as heat and pressure rollers and belts to apply and fix the image—are eliminated, along with extra doses of electrostatic energy that are transferred to the paper with the toner. Since no pressure or heated contact is made by the engine with the paper, the substrate maintains more moisture, minimizing shrinkage and curl. This results in a wider range of substrates suitable for digital color printing at faster speeds.

The electrostatically charged papers that typically hit the delivery tray in electronic printing equipment have also proved to be a nightmare, at times, for binding and finishing operations. Leaving in more moisture also reduces static build-up. A byproduct of the process is that flash fusing heats the toner more effectively than it does white paper, so its temperature doesn't rise in the paper. And, unlike roll fusing, the energy that does get into the paper is concentrated on the thin surface layer and quickly dissipates into the bulk of the paper, lowering peak temperature (and reducing problems from electrostatic discharge in binding/finishing paper transport. Xerox also anticipates a payback in reducing customer paper waste and mail processing costs.