11. Write down 10 sentences of your own using words with prefixes.

TEXT 2

Flexography

In the last several decades, a modified letterpress printing method, flexography, has become increasingly more prevalent, especially in the packaging industry, for printing on the most varied materials. The term “flexography” was introduced at the beginning of the 1950s (it was previously known as “aniline printing”). Unlike letterpress printing, flexography uses low viscosity inks and resilient or soft, flexible printing plates, and the pressure applied between plate cylinder and substrate is low (“kiss printing”). Because of the flexible printing plates, which were formerly made solely from rubber and are now made mostly from photopolymeric plastic, printing can be done on materials with very rough surfaces and even on fabrics. Flexography is the only printing technology by means of which printing can be done on very thin, flexible, and solid films, virtually all papers, thick cardboard and rough-surfaced packaging materials.

The print quality that can be achieved in flexography is lower compared to offset printing. The resolution is usually lower with flexography than with offset printing which has a standard of 60 up to 120 lines/cm (150 up to 300 lpi). However, modern printing plates, especially those produced using computer-to-plate imaging systems, have improved printing quality markedly. The use of new types of printing plates with adapted inks and technical printing press developments, especially with respect to inking (inking units), have tremendously improved the printing quality offered by flexography.

The resilience of the flexographic printing plate in conjunction with low viscosity ink makes it possible, in particular, to print on the non-absorbent and rough substrate surfaces that are often typical of packing. Moreover, flexography is particularly suitable for printing on flexible materials such as plastic film. The printing plates are made from rubber or photopolymers. Their hardness and thickness are adapted to the particular substrate and motif. Since, on the one hand, the types of substrate to be printed range from corrugated board via paper, plastic film, and metal foil to composite materials and on the other, extremely varied requirements (different requirements apply for the packaging of industrial products than those for the packaging of foodstuffs) have to be met, numerous types of ink are used for flexography – water-based inks, inks containing alcohol or benzene, ester inks, UV curing inks, and so forth. The printing plate material must be selected so that it will not be swollen, etched, or embrittled by the inks.

Printing plates are either flat and fastened onto the plate cylinder with adhesive or double-sided adhesive film, or they are produced in cylindrical form (e. g., sleeve technology).

Mounting the Printing Plate. The flat plates are fastened in accurate register onto the plate cylinder with double-sided adhesive film. When doing this, the plate must be brought into the shape of a cylinder shell. The image dimensions have to be reduced in prepress in the direction of printing to compensate for the longitudinal increase in size.

Sleeve Technology. The principle of sleeve technology consists of a thin-walled metal sleeve, the inside diameter of which is dimensioned so that the sleeve can be expanded under compressed air and pushed axially onto the plate cylinder. Once the compressed air has been turned off, the sleeve sits firmly on the plate cylinder by force fit. Before being pushed onto the plate cylinder, the entire outer surface of this sleeve is covered with plate base material. The cylindrical plate is directly imaged using lasers in a round imagesetter.

Printing units usually consist of an inking unit, a plate cylinder and an impression cylinder. There are two different types of inking unit in current use:

• Fountain roller system where ink metering is effected by squeezing off the ink supply in the roller nip between anilox roller and fountain roller.

• Doctor blade inking unit with anilox roller and blade, notably the chambered doctor blade.

Рис.4

Flexographic presses are predominantly designed as web presses in the three configurations:

• central impression cylinder system;

• in-line (unit) design;

• stack-type design.

Central impression cylinder presses (satellite printing units) have advantages over stack type presses in the precision of the longitudinal register, above all for printing flexible materials (plastic film), due to their large wrap angle.

In-line presses are flexible and expandable with respect to the number of printing units and the usable printing processes (combination of different processes).

The stack type and central cylinder system, on the other hand, are restricted to a specified choice of printing and inking units.

It is usual practice for the sheet-fed flexographic process to be applied in the coating units of sheet-fed offset presses. Multicolor sheet-fed flexographic presses are also used for special applications (e. g., high-quality printing of metallic inks on packaging material, or printing on plastic material).

The low-viscosity ink is transferred to the printing plate via a roller that is evenly screened with cells, the so-called anilox roller (screen width 200–600 lines/cm, ceramic or hardchromed metal surface). The rubber or plastic plate is attached to the printing plate cylinder. Ink is transferred to the printing substrate by the pressure of the impression cylinder. The use of a blade (together with the ink supply system) on the anilox roller has a stabilizing effect on the printing process resulting from even filling of the cells on the anilox roller.

The importance of flexographic printing has increased noticeably world-wide thanks to the many new developments and enhancements in this field and the good results achieved in the packaging industry. Depending on the substrate and the motif, the quality of products printed in flexography is improved and the gap to the high quality of offset and gravure printing is reduced. Flexography has turned out to be a qualitatively good printing technology suitable for industrial printing. At a rate of around 3% p. a., its market share has risen above average in recent years, especially in the packaging industry, and noteworthy growth rates are predicted in many markets for the coming years. On the other hand, flexography will play a subordinate role in the newspaper sector in the future.