Page 178 - Graphic Design and Print Production Fundamentals

166 Chapter 6. Imaging
Electrostatic Principles

Figure 6.2 Like charges repel each other while
opposite charges are attracted

To understand electrophotography, we must first understand some basic electrostatic principles. When
certain materials come in contact then separate from each other, these materials can become electrically
charged. Rubbing these materials together can increase this effect. This is called the triboelectric effect.
Static electricity buildup on your clothes in a dryer or from rubbing a balloon on your hair are examples
of the triboelectric effect. Charges can have either a positive or negative polarity. Like charges repel each
other while opposite charges are attracted, in much the same way as the polarities in magnets (see Figure
6.2).

These properties are at the core of the technology and are utilized in almost every stage of the digital
imaging process.

Toner Basics

Toner is a very fine, dry powder medium used in the electrophotographic or xerographic process. It is
composed primarily of a resin and includes pigment, wax, and process-enhancing additives. The term
xerography, in fact, is derived from the Greek words xeros, ‘dry’ and graphia, ‘writing,’ reflecting how
toner rather than ink is used in the imaging process. Toner particles become electrically charged when
stirred or agitated through a triboelectric effect. The composition of the toner not only contributes to
its imaging characteristics but to its ability to maintain and control its charge properties. The shape of
the toner also is a factor in its charging capability. This electrical charge is what allows the toner to be
precisely manipulated throughout the process.

There are two basic types of toner production, pulverized and chemical (Figure 6.3). Pulverized toner
was commonly used in earlier digital printers and is manufactured by successive compound mixing
and grinding steps until the desired consistency and size is achieved. The resulting toner particles are
irregular in size and shape and typically average around 6.2 to 10.2 microns in size. Pulverized toner
produces good results, up to 600 dpi resolution; however, a consistent size and shape along with a
smaller particle size is required to produce better clarity and detail at higher resolutions.