Toner is becoming ever more important as digital print continues to grow. What is it and where does it come from?
When discussing consumables in the printing industry, one must now start looking at the toner or ‘dry ink’ products that are used by digital print engines.
As digital devices become more pervasive in the industry, more knowledge of how the colour is put on the page will aid the buying decisions of new investors.
A bit of history
TO look at the toner market we must cast an eye at the history of xerography as a whole. The man credited with the invention of this technology was Chester Carlson. Back in 1938 in Astoria NY, Carlson and his assistant, Otto Kornei created the first image using the electrostatic technique. To accomplish this they used ‘lycopodium’ powder—the waxy spores from club moss.
From these inauspicious beginnings the technology was developed to use carbon particles mixed with a polymer. In the xerographic process electrostatic forces on the image plate attract the toner particles to a charged image on a drum. This image is then transferred to the output paper. The paper is ‘statically’ charged and therefore pulls the toner onto it rather than the traditional offset method that ‘applies’ the ink to the paper. The paper then has heat and pressure applied to a point where the polymer melts and fuses to the fibres of the paper. Once the toner is fused it binds very securely and it won’t bleed or smudge easily.
Originally the size of these toner particles was around twelve microns (thousandths of a millimetre) in diameter. As xerography moved from copying into digital imaging the demand on particle placement accuracy drove the manufacture of finer particles. By the time 600dpi printers were the standard, the toner size had reduced to around eight microns in diameter.
Conventional toner is manufactured by creating a mix of plastic, pigment, and other additives to improve the properties of the product. This block of composite plastic is pulverised to form a fine powder. This powder is processed and screened to remove chunks and ultra-fine particles. Those ‘wrong sized’ fragments are remelted and enter the process again.
Some desirable additives and pigments just would not withstand the melt mix step. Others would not distribute evenly to give each toner particle the right amount of added ingredients. Some formulas resulted in particles that wouldn’t flow. Lower melting plastics, which were desirable to produce toners that fused at lower temperatures, just wouldn’t pulverize efficiently. And waxes, that would have allowed oil-less fusers, disrupted the process at the melt mix stage. The result is a mixture of non-uniform, angular particles with sizes ranging from 6–8 microns depending on final usage. When particles get this small they behave in a similar fashion to a low viscous liquid—a bottle of toner tipped or shaken will flow quickly, not like the behaviour of printers ink.
To prevent toner particles adhering to the fuser roll, and to keep them on the paper a fuser oil is employed. This fuser oil can be an issue with newly printed sheets. On heavy toner coverage area the toner could appear as streaks. These tend to disappear as the fuser oil evaporates. In some cases the residue after evaporation can adversely affect laminating, or use of ‘Post-it®’ notes. In a move to lessen the reliance on fuser oils some manufacturers have started coating fuser rollers with Teflon, and have also pioneered toners with fusing agents such as wax contained within the toner particle.
Advances in requirement and colour accuracy have called for toners to ‘flow’ more like a liquid, to contain fusing agent within the particles, and also to fuse at a lower temperature.
The desirable qualities of modern toner govern how the latest generation digital presses operate. A low melt temperature means that the fuser in the print engine can get to temperature faster, allowing quicker ‘first print out time’, this allows the engines to cycle down and rest at lower power and therefore makes them more energy efficient.
In contrast to conventional powder-based toners, the more modern toner is formed through a chemical process by mixing, coalescing and then heating pigment and latex particles in a solution. These toners are known under different labels depending on printer manufacturer, EA (Emulsion Aggregation), Polymerised, and Simitri are common names.
The new techniques to manufacture toner can be likened to growing cultured pearls. A central ‘bead’ of material or wax is suspended in an emulsion where they are ‘grown’ to the required size, and indeed shape. This method ensures a consistent size of particle (from sizes as low as three microns) and smooth coating which can provide a smoother final printed document, with less gloss than with conventional toners. The modern toners are more efficient to manufacture (less CO2 emissions), more efficient in printing – with nearly 40% less toner used per print, and produce less waste toner in the xerographic process
These new techniques for manufacturing toner do not render conventional toners obsolete though. Currently the colour gamut of conventional toners exceeds that of the new toners, and so many of the current production range digital presses still use conventional toners. The heat required to fuse toner at higher speeds also limits the speed that polymer toners can operate. In addition the conventional toners can bond to more media types making them more suited to production environments where there is the need to respond to end user customer stock requests. These limitations will be overcome in time, but until then there is space for both families of toner in the marketplace.
