Inkjet

Technology

The first printing device that used drops of ink emitted from a nozzle was developed by Lord Kelvin in 1867 to record telegraph transmissions. It created a modulating single line that represented the dots and dashes of Morse code. It wasn’t until over a hundred years later that inkjet printing devices for printing text became commercially available. During the 1970’s several devices were marketed but were not commercially successful. In 1984, Hewlett Packard introduced the first desktop inkjet printing device for computer hardcopy. From then, rapid developments in technology led to improvements in print quality, the inclusion of color, and the printing of photographic images.

All inkjet printers use drops of ink emitted from a nozzle to create colored dot patterns on paper (or other printing substrates). There are two main types of inkjet printers – continuous and drop-on-demand.

Continuous Inkjet

Continuous inkjet printers were the first to be used for images. Their original purpose was for creating printer’s proof in commercial printing facilities. Continuous inkjet printers emit a constant stream of charged ink drops. The stream of charged drops can be electronically deflected into a recycling system or allowed to pass and hit the paper (or the reverse where the deflected drops hit the paper and the undeflected drops enter the recycling system). The printing paper is attached to a large cylinder that rotates under the print head. A variety of papers can be used as long as they are flexible enough to fit around the printing drum. These printers are large and have only been used in commercial printing environments. The use of these devices for printing individual images has dramatically declined, but the technology is currently being repurposed into large-scale production printers to compete with offset lithography.

Drop-on-Demand Inkjet

Drop-on-demand printers only eject drops of ink as required. There is no need for a deflection or recycling system. There are two different ways in which ink is forced from the printing nozzle. The first, called thermal inkjet printing (also known as bubble jet printing), utilizes a heating element inside the print head to temporarily vaporize some of the ink and create a bubble. The formation of the bubble forces a drop of ink out the nozzle. In piezo printing a ceramic piezo electric tile in the nozzle flexes into the ink reservoir in the print head forcing the drop from the nozzle. In both systems ink is only forced out of the nozzle when the digital signal asks for it. All desktop inkjet printers, and most wide format printers use drop-on-demand technology.

Inkjet printers can be used to print both images and documents. Inkjet printers can print on a greater variety of papers than dye sublimation or electrophotographic, though care must be taken to match the right papers with the right inkjet printers.

Image Structure

Like electrophotographic images, most inkjet printers can only produce a single, dark tone of color for each of its colors (whether monotone black or multi-color), they must create shades of color by utilizing the white of the paper. Dark colors are created by placing dots of ink close together, while light colors are created by placing dots further apart. This process is similar to the screening technique used in offset printing and is known as halftoning. However, as the dots get farther from each other, the image quality decreases. To address this some inkjet printer manufacturers have added lighter colored inks, so the dots can be printed closer together and maintain image quality. The lighter colored inks are only used in photo printers and are often referred to as light cyan and light magenta. There are some printers that also have various levels of gray to improve black-and-white images. Light yellow ink is not needed as that color is already very light in tone.

The dots in inkjet images can range from highly circular with smooth edges to ragged with feather edges. Smooth edges indicate that the print was made on a paper with a special coating to improve image quality. Feathered edges indicate the print was made on plain office paper.

Inkjet on plain office paper	Inkjet on coated photo paper

Colorants

Early continuous inkjet printers such as those produced by Iris Graphics, used dye as their colorants, while current continuous inkjet digital presses uses pigmant colorants. Drop-on-demand inkjet printers use either dye-based systems or pigment. In general the pigment colorants have tended to be more resistant to environmental deterioration (moisture, pollution, light), but pigment prints are often more sensitive to physical damage during handling. Dye technology has advanced though, and many dyes are significantly longer lasting than earlier formulations.

There are three classes of inkjet inks –aqueous, solvent, and UV-curable. The solvent and UV curable inks are primarily used for industrial and commercial applications and not for document and photograph printing. They will not be discussed here as they are outside the scope of this project.

The aqueous inkjet inks may be either dye or pigment. The dyes are organic compounds that are soluble in water. Pigments are usually inorganic but are sometimes organic or a combination of both.

Inkjet printers are often labeled as dye or pigments systems, but many dye printers actually use pigment for their black inks. This can have a dramatic effect on the permanence of documents and photos produced. If a printer uses black pigment to create text and color dyes to create photos, the documents could be long lasting and the images prone to rapid fade.

Inkjet color sets can be larger than the other printing methods. While most printing systems just use cyan, magenta, yellow, and black colorants, some high-end inkjet photo printers include inks of other hues such as orange, red, green, blue, or violet to improve the range of colors and tones that can be printed.

Papers

More types of papers are available for inkjet printing than any other digital printing technology. The various papers are used for different purposes and to create different effects.

Plain Papers

Plain paper primarily is used for printing documents. Water-based inkjet inks on plain paper can be problematic when printing images. This is because the large amount of ink needed to print images can cause the paper to wave and cockle. Another problem for images is that ink will spread slightly when absorbed into the plain paper’s surface. This can make colors appear both lighter and duller and the image details less sharp. There are some special plain papers available for inkjet printers that have been chemically treated to reduce the amount of ink spread and these associated problems. The label on the package will state that the paper is intended especially for inkjet printers. However, pictures will be only slightly better in terms of color and sharpness. To print high-quality photos, polymer or porous papers are needed.

Inkjet-sized Paper

Inkjet-sized paper is either plain paper or artist paper that has been coated with a special ink-receiver layer (IRL) that can absorb the ink but hold it close to the surface so that the color remains vibrant. It also keeps the ink from spreading so that the lines remain sharp. Both dye and pigment inkjet can be used on these papers. Images and documents printed on these papers are more like to fade or become physically damaged than those on plain papers.

Photo-Coated Paper

In addition to plain and cast papers, there are also papers made especially for imaging. Some use the same paper base used in traditional color photography so that they look and feel like “real” photographs. These papers are coated on both sides with a thin plastic layer and are called RC papers (resin-coated papers). There are two types of photo-coated RC papers – polymer and porous. There are also fine art papers which look and feel more like traditional artist papers, such as watercolor paper. Below are descriptions of the various photo-coated papers.

Polymer Inkjet Photo-Coated Paper

Polymer photo-coating on inkjet paper swells and absorbs the liquid inks as they are sprayed onto the paper by the printer. These papers are typically used with dye ink sets, because many pigment particles are too large to be absorbed into the coating.

There are several advantages to these papers. One is that they can be manufactured to be much glossier than the porous-coated papers. Also, because the ink is absorbed into the polymer layer, the prints are more resistant to abrasion or fade from airborne pollutants. These papers are becoming less popular though because they can take from several minutes to several hours to fully dry. Many manufacturers have been switching to porous-coated papers, because they dry instantly. Handling polymer-coated prints while they are still wet can lead to smudging of the colorants. The figure below shows a cross-section of a dye inkjet print on polymer-coated paper. The dark inks can be seen absorbed into the thin layer of polymer on the print’s surface.

Porous Inkjet Photo-Coated Paper

The term porous inkjet paper really refers to three sub-types: porous, micro-porous and nanoporous papers. They are similar in that they all have small pores that absorb the ink; they differ in the types of chemicals used to make the coatings and in the size of the pores. The main advantage of the smaller nano-sized pores is that the paper will appear glossier than porous or microporous papers, but nanoporous papers are more difficult and expensive to produce. The porous coating is a thin layer of mineral particles held to the paper by a polymer binder. The spaces between the mineral particles are the pores. When the inks are sprayed onto the paper they are absorbed into the pores and form the image. The water from the ink is quickly drawn down further into a second, deeper layer of pores resulting in an “instant dry” print. The figure below shows a cross-section of a dye inkjet print on porous paper.

Porous photo papers can be used with both dye and pigment inkjet printers. One disadvantage is that while nanoporous papers are often advertised as glossy, they are not as glossy as polymer inkjet, dye-sub, or traditional glossy photos. Also because the pores remain open, even after drying, the colorants are not protected from the environment they are more susceptible to pollutant-induced fade.

Unfortunately, the labels on most photo-coated inkjet paper packages do not state whether the products are of the polymer or porous type. Sometimes the information can be found on their websites. Here are a few hints, though. Most, if not all, papers labeled as “instant dry” will be porous. Polymer papers will be listed as fast-drying, and they may be labeled “high gloss” rather than just “gloss.” because they can be made glossier than porous papers.

Fine Art Inkjet Paper

In the early days of inkjet printing actual artist papers were used, but, as with plain office papers, the colors appeared dull and the images less sharp. For this reason, some paper companies have created special fine-art inkjet papers with porous-type ink receiver coatings or chemical treatments to improve the image quality. These papers are usually capable of accepting either dye or pigment inks. The image below shows a cross section of a typical fine art photo paper for inkjet. Notice the ink within a porous coating and the absence of RC layers. The absence of the RC layer allows the print to maintain the color and texture of the paper.