Toner and the Two-Component Developer

Carlson's first developer was lycopodium powder - fern spores - dusted onto a charged plate. It proved the principle and nothing else. A machine that has to make ten thousand identical pages needs a powder whose charge you can specify, hold, and get back again. The answer, worked out at Battelle in the 1940s and refined ever since, is that the powder is never charged deliberately at all. Toner is mixed with much larger carrier beads and tumbled; the two materials sit at different places in the triboelectric series, so rubbing transfers charge from one to the other exactly as a balloon takes charge from a jumper. The beads then carry the toner to the drum on a magnetic brush, and the latent image's own field pulls the toner off. What makes this hard is that the numbers are close together. Work out the electrostatic force on a charged eight-micrometre particle in the development field and it comes to a few nanonewtons - the same order as the van der Waals and image forces gluing that particle to the bead. There is no comfortable margin. Under-charge the toner and it will not leave the carrier; over-charge it and it holds onto the drum through transfer and prints hollow letters. A developer is an engineered mixture held inside a narrow band, not simply a black powder. The particle size tells the rest of the story. Twelve micrometres was fine when a dot was eighty-five micrometres across. At 1200 dots per inch the dot is twenty-one micrometres and the particle IS the edge - which is what drove toner from ground-up resin to chemically grown particles of five or six micrometres, and eventually to suspending it in oil and giving up on powder altogether.