Hal Hinderliter

Gamut of toner vs ink?
November 22, 2007

Sponsored by:


Question: How can digital printing offer a larger color gamut than offset? One uses CMYK toner, the other CMYK ink. What's the difference?
– Dieter

For today’s answer, we turn to renowned color management consultant Lou Prestia, president of Prestia Color Consulting:

Digital printing in general does offer a broader gamut than conventional offset lithography. This is not always the case; digital color prints from a color copier using low grade paper may not have the gamut of certain types of offset and may not match the gamut of some other printing technologies such as flexography.

Toner versus ink
Digital printers use toner rather than ink, typically in powder form. The principle of laser printing is this: an image is marked into a charged photoreceptor drum in the printer with a laser, reversing the electrical charge of the drum in image areas so that the toner (which is charged with the opposite polarity) will adhere to the drum in these areas. The imaging drum and the substrate are brought in contact to transfer the toner, and page is passed through a “fuser” that melts the toner, adhering it to the particles in the substrate. This happens four times, once each for the four process colors.

Offset presses use paste inks that are suspensions of pigments in a carrier solution of oil. When the ink is transferred to the substrate in image areas it is partially dry but the printed sheet won’t be fully dry for some time. In process color printing, the sheets are imaged in all the process colors in rapid succession.

Caught in a trap
The method by which digital printing fuses each color to the substrate eliminates the issues faced by offset printing when it comes to “ink trapping.” Trapping is the term for how ink “sticks” to other colors that have already been printed. Commercial printing inks for multi-color presses must be carefully engineered so that they have the proper tack (stickiness) to adhere to the inks already on the paper that are not yet totally dry. Perfect trapping would provide 100 percent coverage of a wet ink onto a previously printed layer; offset printing typically displays from 80 to as little as 50 percent trap values.

Offset inks used for high-speed commercial printing are also limited in the amount of pigment they can contain, for both economic reasons and due to the complexity of the ink train. Toners by comparison can carry quite a lot of colorant, since they do not have to move through a complex system like the ink train of a printing press and are immune to concerns such as tack or viscosity.

The economics of toner
Pound for pound, toners are much more expensive than inks; the market supports this pricing structure because toners are purchased in smaller quantities. The high price point of toner allows manufacturers to include a greater “load” of colorant; in addition, these colorants are created from chemical compounds that offer greater purity than the organic materials mined for offset ink pigments. Greater purity for CMY toners means more effective absorption of red, green and blue light, a major contributor to digital printing’s larger color gamut.

Toner’s high density of pure colorant affects its price per pound, which in turn has a major impact on the economics of digital printing. To image 2,500 letter-sized pages at 50% coverage on one of today’s popular laser printers requires about $1,400 worth of toner. If offset printing inks were this expensive, a typical 10,00 impression run of a 16-page signature would require nearly $90,000 worth of ink! 

Posted by Hal Hinderliter on November 22, 2007 | Comments (0)