Flexo printing-a new environmentally friendly printing process for hoses

At present, offset printing is more popular in China, and gravure printing has also occupied a large field in the printing and packaging industry. For the new technology of flexographic printing, people will inevitably compare it with offset printing and gravure printing. After decades of sustained and stable development, the flexographic printing industry in Western developed countries has grown to be almost equal to offset printing and gravure printing. As the call for environmental protection grows louder, the advantages of flexographic printing become more and more obvious.

The flexographic printing plate is soft. The reason why it is called flexo printing is that it uses soft, compressible and stretchable rubber. The engraving or exposure wash away makes the plate image higher than the blank part. This is different from the traditional The printing is the same, the difference is that you can use the computer to directly engrave or expose the image. The soft plate can make the printing plate and the surface of the substrate fully contact. He can print if he can print offset printing, and he can also print if he can't print offset printing. The printing material is very wide, fast, and environmentally friendly.

The basic principles and characteristics of flexographic printing

As a special relief printing method, flexographic printing was originally called "flexographic printing". By October 21, 1952, the printing method was formally named as "Flexography" at the 14th Symposium of the American Packaging Society: The flexible printing plate, the term "flexo printing" was also introduced to China shortly thereafter. Flexographic printing is similar to letterpress printing in printing principles, except that the composition of the printing plate and ink and the imprinting process are different from traditional letterpress printing. The image transmission in the printing process is through a flexible photosensitive resin plate obtained by photochemical reaction Or a rubber stick with an engraved pattern to pass the ink through the anilox roller.

The American Flexographic Printing Association formally defined flexographic printing in 1980 as: flexographic printing is a direct rotary printing method that uses a raised image printing plate with elasticity, and the printing plate can be fixed on a repeatable plate cylinder On the printing plate, a metal ink roller engraved with ink holes is used to apply ink. The metal ink roller (anilox roller) is controlled by another ink roller or doctor blade to transfer ink, which can transfer liquid and grease ink to all Kinds of materials.

Definition of Chinese printing technology standard terminology GB 9851.4-90: Flexographic printing is a printing method that uses flexographic printing plates to transfer ink through an anilox roller.

The flexographic printing machine is mainly composed of an inking unit, a printing plate and an impression cylinder. Compared with offset printing, the difference between flexographic printing:

(1) The ink is a low-viscosity volatile drying ink with alcohol as the main solvent. The drying speed is fast, and it is suitable for high-speed multi-color printing of flexo printing. The application of non-polluting, quick-drying water-based ink is very beneficial to environmental protection.

(2) The flexographic plate is a photosensitive rubber type printing plate, which has the characteristics of softness, flexibility and flexibility. Shore hardness is generally 25-60 degrees, good transfer performance for ink, especially for alcohol solvent ink. This is incomparable to the lead plastic version and photoconcentration version with a Shore hardness of 75 or more.

(3) Printing with light printing pressure, which is called KISS PRINT abroad (soft like kissing).

(4) The printing materials for flexographic printing are very wide.

(5) Has good printing quality. Due to the use of high-quality resin plates, ceramic anilox rollers and other materials, the printing accuracy has reached 175LPI, even 200LPI, and the level is also very full.

(6) High production efficiency. The flexographic printing equipment itself is fast and usually has the functions of multi-color groups and online multi-process operations.

(7) Easy operation and maintenance. Including the version, maintenance, ink color and other operations have become very simple.

There is no perfect printing method, and the flexible version is no exception:

(1) Compared with gravure printing and offset printing, the flexographic printing density curve is unstable, and the thickness of the printing plate has a great influence on the printing quality.

(2) Flexographic printing uses anilox rollers to transfer ink, and the screen line of the printing plate must match the screen line of the anilox roller.

(3) The dot deformation is large during the imprinting process.

(4) The pressure is small (only 2KG / CM, and the pressure of gravure printing is 50KG / CM, and the pressure of flat printing is 4-10KG / CM). A slight change in pressure will have an effect on the transfer of graphics.

(5) It has its own unique color separation principle, because the flexographic plate is a widely used method in the packaging and printing field. It often uses multicolor, 6 colors or even 8 colors. In addition, it is also considered to use multicolor to make up for the lack of dot performance .

(6) Due to the characteristics of on-line processing, flexo printing needs to comprehensively consider the post-printing link in the design process.

(7) The increase of the dots of flexo printing is non-linear, and the degree of increase of the dots of large dots and small dots during the printing process is different.

(8) When making a flexographic printing plate, it is necessary to consider the deformation of the flexographic printing plate in advance, so that after the printing plate is served on the drum, it will be deformed to the required length.

Types of flexographic printing machines: one is self-adhesive products, paper flexo printing machines; the other is flexo printing machines.

The flexographic printing process mainly includes two major steps of plate-making and printing. The plate-making process is generally as follows: manuscript—film (positive image) —back exposure—main exposure—developing and processing—drying—post-processing—post-exposure—plate-setting for machine print. The printing process is generally as follows: printing materials-printing (including anyway printing)-varnish or laminating-die cutting-cutting-inspection-storage.

Ink is currently the largest source of pollution in the printing industry. The world's annual output of ink has reached 3 million tons. Every year, the global VOC (organic volatile) pollution emissions caused by inks have reached hundreds of thousands of tons. These organic volatiles can form a more serious greenhouse effect than carbon dioxide, and under the irradiation of sunlight, oxides and photochemical smog will be formed, which seriously pollutes the atmospheric environment, which has aroused widespread concern in all aspects. Among the various printing methods, only flexographic printing has generally adopted environmentally friendly green printing materials-water-based ink, UV ink and alcohol-soluble ink.

RUV-650 flexible printing tube printing process

The RUV-650 flexographic hose printing machine developed by the well-known printing equipment manufacturer Hutong is a printing method that uses a flexographic plate to transfer ink through an anilox roller.

Working principle of flexible flexible tube printing machine

1) Process

Plate making process: manuscript → film → back exposure of printing plate → main exposure → plate washing (developing) → drying → post-processing → post-exposure → checking the cut plate → pasting plate for machine printing

Manuscript → Laser CTP → Printing plate → Check cutting plate → Paste plate for printing on machine

Printing process: printing materials → printing → receiving → inspection → finished products

2) Printing plate

The printing plate used by the RUV-650 flexographic hose printing machine is a photosensitive resin plate with a thickness of 1.7mm.

The flexible version is available in ordinary version and laser version:

Ordinary version: original → film → exposure → processing → printing plate;

Laser version: original → laser CTP → printing plate.

Using laser plate-making, the plate-making ability is greatly enhanced, and it is no longer a problem to produce plates with more than 175LPI.

Defects of traditional plate-making technology: The exposure time is determined by graphics and texts, and it relies too much on the skill of the operator. The film directly contacts the plate, which is easy to be invaded by dust, causing poor polymerization and depressions, and easy to produce vacuum leakage, which makes the negative film and plate poorly fit , Causing the dots in the highlight part of the picture and text to be higher than the solid part, which will cause a larger dot expansion in printing.

The advantages of laser plate-making technology: the exposure time has nothing to do with the type of graphics, the quality of the printing plate is stable, and the direct plate-making without film is realized, and there is no problem of poor polymerization and depression. No need to evacuate, there will be no dirty spots or rotten spots. Eliminate the phenomenon of dot expansion caused by UV light diffusion.

The selection basis of the flexographic platemaking line number: the pressure control of the printing equipment itself, the line number of the anilox roller, and the flatness of the printing material.

3) Types and selection principles of double-sided adhesive tape

The prepared printing plate is accurately adhered to the plate roller with double-sided adhesive tape for printing.

The thickness of the double-sided adhesive used for RUV-650 flexible printing machine is 0.38mm. The base material is closed-cell foam. There are three types: low density, medium density and high density. Low density is suitable for fine screen printing and fine line printing; medium density (comprehensive) is suitable for text, line and solid printing; high density is suitable for solid printing.

4) Anilox roller and doctor blade

The ink system of the RUV-650 flexible printing machine is composed of an anilox roller and a closed blade system.

This ink path system transfers ink uniformly and stably, and is little affected by external conditions. The amount of transferred ink is controlled by the number of lines of the anilox roller and the amount of ink carried. Anilox roller structure: metal roller base, ceramic layer, mesh hole. Composition of net hole: net hole depth, net cavity, net hole shape, net wall (regular hexagon), corner area, net angle (60 degrees).

How to choose the right anilox roller?

The anilox roller is selected according to the printing plate, the printing material and the ink used. The empirical reference: the number of anilox roller lines = the number of plate lines x 5-7.

First of all, according to the inked area of ​​the graphics and text of the printed products, based on daily experience, it is estimated how many lines / anilox rollers to choose.

Secondly, according to the amount of ink absorbed by the printing material, during the preliminary proofreading process, observe the hue of each color imprint and the saturation of the amount of ink, and then change the anilox rollers with different number of lines / load of ink.

The line number of the anilox roller can be slightly higher when using UV ink. Make data records during each printing process as an important basis for future production.

RUV-650 flexible printing machine uses UV ink to print. The main components of UV inks: pigments, photosensitive resins, reactive diluents, photoinitiators and additives, under the irradiation of ultraviolet light of a certain wavelength, chemical curing will instantaneously "cure".

Auxiliaries: UV active diluent, UV curing accelerator, UV ink cleaning agent, etc. Features: bright colors, good printability, fast drying speed, relatively low dosage, good stability, safety and reliability, environmental protection, wide range of materials, Cost-effective.

Storage: strictly avoid light and heat, storage temperature 5-30 degrees.

Safety protection: the harm of ultraviolet and UV ink to human body. UV flexo ink and technology will become the mainstream in the printing field in the future.

Comparison of flexographic hose printing and screen hose printing

Flexible version hose printing: using UV ink, safe and environmentally friendly, fast drying. There are many printing color groups, fast production speed and high efficiency. Color dot printing with high line count is good, and solid printing is thick. The ink path uses an anilox roller and a reverse scraper, the printing ink color is even and stable. Using light force printing, printing materials are very wide. Easy operation and maintenance. The flexible plate-making cycle is short, easy to transport, and low in cost. The flexographic version has a high printing durability. It is suitable for both large-scale printing and small-batch requirements. Multi-process combination printing, online glazing, online cold stamping, online coding, etc.

Screen hose printing: thick ink, bright color, strong three-dimensional sense. However, the printing speed is slow, the production volume is low, the performance of color printing is difficult, and mass printing is not suitable.