Chapter 6. Imaging
Inkjet printing is a type of digital imaging where drops of ink are jetted onto the substrate in a very precise pattern from a nozzle. This nozzle, also called the print head, is required to be very precise and accurate, which is a challenge when you consider that the goal is to get many thousands of tiny drops of ink to land exactly where needed on the printed surface. Over time, inkjet technology has become more advanced, allowing greater resolution, more accurate colour, and overall, finer visual fidelity to the original. The most common method of inkjet printing for commercial purposes is called drop-on-demand (DOD). This type of inkjet print head only fires each individual droplet of ink when needed (on demand) and comes in two types, thermal or piezoelectric (see Figure 6.1). Accuracy in DOD inkjet printing is achieved by keeping the print head close to the surface being printed (substrate) as the velocity of the jetted ink is low.
In a thermal print head, each nozzle contains a special reservoir that is bounded by a heating element. When current is passed through the heating element, it causes the ink to expand rapidly, ejecting out of the nozzle to land on the substrate in a given position. The print head is made up of a matrix of many of these chambers, and each print head is connected to a different colour of ink. As the ejected ink leaves the chamber, fresh ink is drawn into the reservoir by surface tension and the vacuum created by the previous drop of ink leaving.
Thermal inkjet is most common in household and consumer grade inkjet printers. A major benefit to using thermal printhead technology is the relatively inexpensive print head. Since each colour printed requires a separate print head, and some print devices can contain eight or more colours of ink, thermal technology keeps the initial cost of the device low and reduces replacement costs when a print head fails, or is damaged.
Piezoelectric (piezo) print heads also use a tiny reservoir to hold a droplet of ink. However, unlike thermal printheads, piezo heads contain a small flexible membrane, or diaphragm, that moves up and down to squirt the ink out of the print nozzle. The pressure caused by the flexing of the piezo material is very precise, allowing a drop, or multiple drops, to strike the substrate accurately. Similar to thermal, the print head is made up of a matrix of a number of these individual nozzles. And by using multiple print heads, multiple colours are possible.
Piezoelectric is more common in commercial and large-format printing applications, although there are a few consumer grades of printers that use piezo as well. Piezo is more accurate, and because the ink in the chamber doesn’t have to be vaporized to form the droplets of ink, piezo can print with a wider variety of inks such as aqueous, ultraviolet, and latex.
Types of Ink
Inkjet printing has become more advanced not only in the mechanics of how the print heads work, but also in the variety and usage of different types of ink. Below are some common types of ink and a brief explanation of how they might be used.
Aqueous ink, as discussed earlier, is a water-based ink. This type of ink is used in consumer printers using thermal technology, but can also be used in commercial piezo printers as well. Aqueous is well suited to thermal technology because the formulation of the ink allows it to vaporize in the print head for expulsion onto the paper. The water component of the ink, however, also contributes to its greatest drawback: the susceptibility of the finished printed piece to run or smear if it gets wet. Many users of desktop printers in their homes have been disappointed when they take their printed pages outside in the rain. Even a few drops of water can cause the ink to run and bleed into the paper.
In commercial uses, aqueous inkjet is well known for colour fidelity and quality, but the finished piece has to be protected from moisture. These types of print products would most likely only be used indoors, mounted behind glass, or with a laminated plastic layer on top. There are water-resistant coatings that can be sprayed onto a finished product, but even then, you would not want to leave it outside for an extended period of time. Aqueous ink is a common choice for art prints.
Ultraviolet (UV) ink is a type of energy-cured ink that stays wet until it is bombarded with ultraviolet radiation. This UV radiation is commonly projected onto the freshly printed surface by means of a special high-intensity light bulb. Once the UV rays hit the ink, a special molecular process is triggered, causing the chains of molecules in the ink to bond and solidify instantly. UV ink does not dry from exposure to air, nor from heat. Once the UV ink has been cured, however, it is very solid and quite durable.
For commercial use, UV inks tend to be popular for outdoor uses such as banners and signage. Indoor signage is commonly printed using UV as well because of its durability and rub resistance. Since UV inks dry instantly, they can be removed from the printer and handled much sooner. UV inks sit mostly on top of the surface of the substrate, and because of their solid bond are more prone to cracking if bent or folded. UV is not a good choice of ink where flexibility of the substrate is required.
Latex ink is a newer formulation that has exploded onto the inkjet printing scene in the last few years. Latex inks are water based and cure primarily through heat, but more importantly, they are not subject to moisture damage once cured. This is because the pigment is carried by the latex molecules, and once the latex has bonded to the substrate, the pigment stays intact. Latex printed products dry immediately and are ready to use as soon as they come off the print device.
Latex inks are used in many commercial applications, particularly where outdoor durability and flexibility are needed. One of the many common uses of latex inkjet printing is in imaging car wraps. A car wrap is a flexible adhesive material that is printed flat, then stretched or wrapped around the contours of a vehicle, usually for marketing or advertising purposes. Figure I.1 in the introduction of this textbook shows an example of a car wrap. Because of this flexibility, latex printed signage can also be adhered to rougher surfaces such as wood or brick. The popularity of latex can be attributed to a previously popular inkjet formulation that is solvent based.
Because of the rise in popularity of latex ink over the last few years, there has been a great decline in the use of solvent inkjet inks. Formerly, this was the type of ink needed for flexible, durable printing. Solvent inks are formulated using solvent as a carrier for the pigment, and as the solvent dries, the pigment remains bonded to the substrate. A big concern for the use of solvent-based printing is the release of volatile organic compounds, or VOCs. These VOCs are released into the atmosphere during the printing and drying of solvent prints, and have to be vented outdoors so as not to pollute the workspace. Even newer eco-friendly inks still release VOCs, albeit at a lower level. Some areas have environmental laws that restrict the release of pollutants into the air (United States Environmental Protection Agency, 2000). Customers often complain about the smell of solvent prints, particularly when used indoors. Because of this, solvent inkjet is primarily chosen for outdoor uses such as large-format signage, banners, and car wraps. Solvent can be very economical, and while the quality isn’t as sharp as UV or aqueous, it is excellent for very large projects that will be viewed from even a moderate distance. Pressure on the solvent ink market comes because most of these uses can now be achieved with latex inks as well, and the industry has seen a divergence between companies that still use solvent or eco-solvent inks and those that are switching to latex.