Ink ingredients fall into three main categories: pigment, vehicle, and modifiers/additives. Because there are so many different types of printing processes and print applications, the ingredients used in these three categories may vary widely. For example, for offset lithography, ink requires a higher degree of pigment than many other processes. This is because nearly half of the ink used in the offset process does not reach the printing surface. In direct printing methods, such as letterpress, a much higher percentage of the ink reaches the substrate.
The ingredients that comprise the color of the ink are referred to as pigment. The pigments are formulated from substances that create a desired color when the substances are blended together in specific proportions. Some of these substances may be ingredients found in nature or they may be produced synthetically. The substances can be in the form of dyes, powders, liquid dispersions, or concentrated pastes.
Pigments for printing inks are chosen for certain characteristics that make them suitable for use on printed products. For example, it is desirable for many print applications to be printed with inks that have a high degree fade resistance (light fastness), while other applications, such as food packaging, may be printed with nontoxic ink because of the possible contact with food items.
The vehicle is the substance in the ink mixture which carries the pigment and binds it to the printed surface. The vehicle can be formulated from petroleum or vegetable oils, solvents, or water. The vehicle must have the ability to be in a liquid form during the printing process, but dry rapidly when it reaches the printing surface. The illustration below shows how the vehicle carries the pigment through the roller train of a press and transfers it to the paper.
Modifiers and Additives
Modifiers and additives change the properties of the ink so that it can be used properly for different types of print processes and applications. Modifiers and additives may include the following:
- Driers: speed up the ink drying process.
- Waxes: reduce the possibility of ink from the front of one sheet transferring to the back of another sheet (setoff). The wax improves slip and scuff resistance of the ink. It is not possible to apply varnish to a printed sheet that has been printed with ink containing wax additives.
- Antiskinning agents: keep the ink from drying too rapidly and skinning over in the ink fountains of the printing press.
- Extenders: increase the coverage of the pigment in the ink.
- Distillates: improve the flow of ink.
The body refers to the consistency of the ink. The consistency of the ink may vary from the beginning of a press run to the end of a press run because of external factors, such as prolonged exposure to the air and the increase in heat produced by the roller train of the press.
The tack refers to the stickiness of the ink. The tack is determined by the resistance of the ink film to “split”. Splitting occurs when the ink from a printing plate is transferred to the printing blanket. During this process, only some of the ink is transferred, which splits the ink film. If the ink does not have the proper tack, the print quality will be poor because the improper quantity of ink is transferred to the printed surface.
Viscosity is the degree that ink resists flow when it is under force, such as in the roller train of a printing press. If ink does not flow easily, it has a high viscosity, and if it flows easily, its viscosity is low. As with body, the viscosity can change as it is used on a press, because of the friction and heat generated by the roller train. Viscosity is measured with an instrument called an inkometer.
Length is the ability of ink to flow and is sometimes confused with viscosity (the degree to which ink resists flow under force). It is necessary for the ink to move properly through the roller train of the press in order to achieve the best results. The friction and heat produced by the press may change the length of the ink.
Opacity refers to the covering power of ink. Different pigments have different degrees of opacity because of the substances used to produce the pigments. Opacity is sometimes referred to as color strength.
Tinting strength is the ability of an ink color to produce a tint with the addition of white pigment and is also referred to as coloring power.
The degree in which printed ink resists the fading power of light is known as permanency. Inks that have a greater proportion of their volume as pigment tend to fade more slowly. The rate that an ink fades is known as the degree of lightfastness and is especially important when printed items, such as an outdoor billboard, will be exposed to a high level of light.
Nearly all types of ink can be placed into one of two main categories:
- Standard Printing Inks: web offset ink (heatset and non-heatset), sheet-fed ink, soybean based ink, process ink for color printing, and others.
- Specialty Inks: metallic, fluorescent, security, phosphorescent, and others.
Standard Ink Types
Web Offset Non-Heatset Ink
The non-heatset variety of web offset printing ink is a common type of ink used on web presses for newspaper and business forms printing. Non-Heatset ink is printed on absorbent, uncoated paper stock. Coated stocks should not be used with this type of ink because the paper will not completely absorb the ink, resulting in excess smudging and smearing.
Web Offset Heatset Ink
The heatset variety of web offset ink contains special varnishes that help the ink dry when heat is applied. Heatset presses are equipped with drier units for this purpose. Due to the varnishes, the ink printed on the paper is highly flammable, so the drying units must be specially built and properly maintained to avoid potential hazards. The main advantage of heatset ink is a printed product with a higher degree of quality.
Quickset ink contains a special varnish to speed the drying process. Unlike heatset ink, quickset ink does not require a heat source for proper drying and curing. The ink will not dry out on the press, but will dry quickly after it has been printed onto the substrate.
The are four basic processes that allows quickset ink to dry depending on the formulation: evaporation, absorption, oxidation, and polymerization. Newer types of quickset ink have a greater proportion of antioxidants and higher boiling-point distillates, which evaporate more slowly, so the absorption process plays a greater role with the newer inks. All four processes share equally in the ink curing process with older ink types.
Uncoated paper stocks are best suited for quickset ink. This is because the low viscosity distillates and antioxidants are quickly absorbed by the substrate, which leaves the remaining pigment and vehicle to dry quickly on the surface.
Sheet-fed ink is manufactured specifically for sheet-fed presses and usually has a higher tack than web offset inks. The reason for this is that most sheet-fed presses run at slower speeds than web presses and a higher tack is necessary to provide the necessary quality.
Ink formulated with a rubber base is a good choice when flexibility in the printing process is important. Rubber-base ink can be printed on coated and uncoated paper and it dries quickly. It is most often used on small sheet-fed presses.
Soybean-base ink is becoming a popular alternative to petroleum-base ink because of the ease in which it is used and because it is environmentally friendly. Soy-base ink prints and handles similar to petroleum-base ink, but it is much less toxic because of the soybean oil. The soybean ink is biodegradable, meaning that it is eventually broken down and is much less hazardous to the environment. Some soy inks may contain petroleum additives, so if a client requires 100% soy-based ink for a print application, it is important to be fully informed on the type of soy ink that is used.