Classification Of Offset Printing Inks

Offset printing is the most important printing method at present. Its characteristic is that a fountain solution is needed in the printing process. Therefore, offset printing inks require good water resistance. In the past, the binder of offset printing inks was mainly vegetable oils such as linseed oil, but now most of them have been replaced by various resins with better performance. Common resins include rosin modified phenolic resin, alkyd resin and polyurethane alkyd resin.

Offset printing inks are classified in different ways.

1. Classification by substrate

According to the chemical name of the substrate, it can be divided into: polyethylene, polypropylene (non-polar) ink. Polyvinyl chloride, polystyrene, ABS polycarbonate (polar) ink.

According to the form of the substrate, it can be divided into soft plastic ink and hard plastic ink.

2. Classification according to printing machinery

According to the classification of printing machinery, it can be divided into sheet-fed offset printing inks and rotary offset printing inks; the selection of offset printing inks for different printing machinery and different substrates.

1. Ink for web offset printing machine

The ink used in the web offset printing machine is usually called the offset printing web ink. This ink is based on the following two types:

a. Permeation and exudation drying type: Because the printing speed of newsprint and offset printing paper web offset printing presses is higher, and the drying speed of ink is fast, the commonly used inks are mostly permeation and exudation drying type.

b. Thermosetting ink: In modern commercial web offset printing presses, a drying device is added to the second half of the printing device. In order to adapt to this production premise, thermosetting ink is used. This ink is heated in the drying device. The volatilization of the solvent in the ink allows the ink film to dry quickly, and the content of dry oil (such as linseed oil) in the thermosetting ink is less. The content of petroleum agent is higher, and the other components are similar to those of sheetfed offset printing machine ink. The paper used with thermosetting ink is of better quality than the paper used with osmotic dry ink.

2. Sheet-fed offset printing ink

1), offset printing light

Offset bright ink is made of synthetic resin, dry vegetable oil, high boiling point alkane oil. Colloid ink composed of high-quality pigments and additives. It is used for single-color, two-color or multi-color offset presses to print picture albums, pictures and other advanced fine prints on coated paper. The luster of the ink is mainly produced by the conjunctivation after the binder is dried. The luster of the ink and the leveling of the ink film are good. It should be able to form a plane when attached to the surface of the substrate to reflect the incident light, that is, it has a mirror effect. The link material is required to stay on the surface of the substrate as much as possible.

2), offset printing resin ink

The offset printing resin ink is made of synthetic resin, dry vegetable oil, mineral oil, high-quality pigments and fillers, and is made by blending and grinding. Offset resin ink is characterized by low vegetable oil content and high solvent ratio, usually high-boiling alkane, which plays a role in reducing viscosity in the ink. After the ink is transferred to the paper, the solvent in this sector quickly penetrates into the paper fibers under the action of the capillary tube, which speeds up the drying of the ink and reduces the smear failure of the printed product.

3), UV offset printing ink

In offset printing, UV ink is used for printing, usually called UV offset printing. UV offset printing ink is a type of ink that uses ultraviolet (UV) radiation energy to quickly crosslink and cure liquid chemicals into an ink film. It requires a UV curing unit in the printer. As a kind of offset printing ink, UV offset printing ink must first be able to be printed in the way of offset printing, so UV offset printing ink should also have some characteristics of ordinary offset printing ink to meet the requirements of printing suitability.

4), offset printing synthetic paper ink

In the absence of a UV offset press, the use of ordinary four-color printers to print cardboard, synthetic paper and other special papers that are not easy to dry on the surface of the substrate requires the use of offset synthetic paper ink, which is suitable for printing Various synthetic paper (gold cardboard, silver cardboard, aluminum foil, special paper) and other non-absorbent printing substrates, the ink has the characteristics of fast drying and good adhesion on printed matter. Because the drying speed of this kind of ink is very fast, so when the printing process is stopped, the ink on the rubber roller should be cleaned in time to prevent the ink from drying out.

Three, according to the drying method classification

According to different drying methods, it can be divided into three types: osmotic type, oxidized conjunctival type, and volatile type. Let's learn more about volatile drying inks, UV curing inks, and oxidation drying inks.

1. Volatile dry ink: It is the most used ink in screen printing. The composition of the ink film is mainly high molecular material. After printing, the solvent volatilizes, and the high molecular material forms the ink film. This volatile drying process is reversible, that is, the dried ink film can be re-dissolved in a solvent.

After the ink is transferred to the substrate, there will be a solvent ink film, and the solvent volatilizes first. The solvent in the ink diffuses in the atmosphere due to the effect of vapor pressure, forms a liquid film on the surface of the ink film, and then volatilizes through the liquid film. In this drying process, the internal drying is generally slow, and sometimes it is necessary to blow air to accelerate the drying.

Volatile inks are easy to use and generally dry quickly, so they are widely used.

2. Ultraviolet curing ink (UV ink): UV ink can produce photochemical reaction, which can be completely cured in a few seconds, and it is used more and more in plastic printing. The main components of the ink are photopolymer resin, initiator, colorant and auxiliary agent. In principle, no organic solvent is used.

The main characteristics of UV ink are: no solvent volatilization, pollution-free, and environmental protection requirements, widely used in food packaging printing, good printing adaptability, suitable for printing paper jam, plastic products, metal, textiles and other printing materials, short curing time , Low energy consumption, good printing quality.

3. Oxidation drying ink: The ink contains polymer with a small molecular weight, which is oxidized in the air and forms a polymer film through the chemical reaction of heat, light or reactive substances. After this ink is printed on the surface of the substrate, it generally needs to be heated to promote hardening.

With the development of science and technology, people's requirements for environmental protection are increasing, so environmentally friendly inks have emerged. Environmentally friendly inks are also known as green inks. In offset printing, there are mainly soybean inks and ultraviolet curing inks.