Technical issues related to flexographic printing inks

Japan's gravure printing is mainly used in packaging printing, and its rate is close to 90%. In Europe and the United States, flexo printing dominates most of the market for packaging and printing, and the market is different.

Japan's gravure printing has entered a period of maturity today. Flexo printing has also gone from previous low-quality printing, or has changed considerably as a special-purpose image, and has made considerable improvements. Now it is the third largest printing method after offset printing and gravure printing.

One, gravure, flexo ink background

In 1995, Japan's total amount of printing inks was listed at more than 400,000 tons, of which packaging gravure ink accounted for 1/3. Since the late 1960s, gravure inks have rapidly become popular due to the replacement of cellophane with a variety of plastic film materials such as polyethylene, polypropylene, polystyrene, polyester, vinyl chloride, nylon, and so on. Big. However, the rate of growth has slowed since 1985, and its growth rate has been lower in the past five years. The number of flexo inks is still small, but its growth is extremely strong (Table 1).

Gravure inks are low-viscosity inks that are colored with a fast-drying organic solvent-soluble resin. In the past, its chromophores mainly used derivatives of natural resins such as nitrocellulose and rosin. However, with the widespread use of various film packaging materials, there has been an increase in the demand for inks that are more adherent and suitable for various applications and can maintain physical properties. All resins that are soluble in solvents can be applied to a variety of gravure inks. In recent years, water-based inks have been actively developed due to environmental requirements and hygiene issues.

The history of flexographic printing can be traced back to 1890 when flexographic printing was also referred to as flexographic printing and was limited to rubber flexographic printing. Therefore, it was used in the field of film packaging and was only used as a kraft paper for quite a long period of history. The printing method of the attachment of the bag machine only. In the 1970s, flexo printing was gradually developed in the field of corrugated paper printing, and this area now accounts for 70% of flexo printing.

Because flexo printing used rubber stamps at first, it was not possible to make high-quality images. Until 1973, when DuPont developed UV-cured resin plates, it was possible to obtain high-precision prints. The printing inks were also water-based inks. As well as alcoholic ink as the main body. The so-called alcoholicity means that 3 parts of a highly soluble ester-based solvent are mixed with alcohol (because there is very little resin that can be dissolved in a single alcohol solvent), and nitrocellulose, alkyd amine, polyamide, and other resins are used as the vehicle. . However, most resins used for gravure ink cannot be used for flexo printing.

Table 1. Change in the volume of major printing inks and its growth rate

Listed volume (kilotons) Growth rate (times)
85 years 90 years 95 years 90 years 95 years offset ink 73.7 108.4 119.7 1.47 1.10 1.62
Packaging Gravure Ink 83.2 115.9 121.3 1.39 1.05 1.46
Publication Gravure Ink 20.9 25.6 28.5 1.22 1.11 1.36
Metal plate ink 28.3 44.1 45.5 1.56 1.03 1.61
News printing ink 43.8 50.5 45.7 1.15 0.90 1.04
Rubber Letter flexo printing ink 16.5 21.2 25.6 1.28 1.21 1.56

Second, gravure printing ink

1, the development of gravure ink

In addition to lower cost hydrocarbons such as toluene, gravure inks use all fast-drying organic solvents such as esters, ketones, and alcohols. Therefore, gravure printing and ink production are subject to laws and regulations such as fire protection laws and labor safety, and organic solvents such as toluene tend to remain in the printed film. This type of printed matter will cause great harm if used in food packaging. So it is also subject to health laws.

2, the progress of water-based gravure ink

Water-based gravure inks were widely used in the printing of packaging paper and cardboard boxes in the 1970s. Due to the better evaporation drying and absorption drying in paper printing, the drying of water-based inks is better solved and popularized earlier. However, the ink in packaging film printing is water-based, and it has not yet reached a real practical stage.

Water-based printing in the field of flexible packaging, adherence to the film, improvement of humidity, and better maintenance of physical properties in various applications, although the requirements for the ink are higher, but the application of such high evaporation energy water-based ink In the middle (the evaporation energy of water is about 5 times that of organic solvents), the matching of desiccant and printing plate system is very important. In addition, due to the reduced lubricating performance caused by water, the frictional resistance increases, and the relative friction between the plate fog and the squeegee and the plate increases, so the improved squeegee system is not available for the entire printing system. Missing part.

Although the water-based gravure ink has entered the stage of use, it has yet to gain further popularity. This is because the promotion of the use of water-based inks not only enhances the performance of existing dryers, but also requires high-precision shallow holes. Printing plates, etc., and the improvement of the entire printing system requires considerable investment.

In the near future, the gravure printing will be completely separated from the organic solvent. Now a variety of printing tests are being carried out. It can be assured that the printing industry will soon usher in the era of full use of water-based inks.

Third, the flexographic printing ink

1, the directionality of flexo ink

Flexo inks are not completely different from gravure inks, but they are more similar. However, due to the different printing systems, there is a difference in printability with gravure printing. Basically, flexo printing uses water-based inks and alcohol-based inks.

Water-based flexo inks have long been used for the printing of corrugated paperboard and paper bags, while waterborne inks for flexible packaging film printing are still under development.

Flexographic printing of packaging films now uses alcohol solvent inks, but it is difficult to maintain its physical properties to suit multi-purpose film packaging materials, especially after various composite processing and cooking processes.

Another reason why flexo inks are rarely used in packaging film printing is because the print quality is far less than that of gravure printing. The quality of the two printed images is shown in Table 2. The films used in flexible packaging are basically oleophilic, hydrocarbons such as toluene are easy to adhere to, and the gravure ink is also good in transferability, and flexible printing inks mainly composed of polar solvents such as water and alcohols, It is difficult to ink the film, which has the disadvantages of poor transferability and poor leveling. In color printing, it is extremely difficult to handle large-format dot gains with high-precision image processing techniques. In addition to these deficiencies, coupled with many factors of flexo printing, it is necessary for the entire printing system to carry out precise management and transformation of operations.

The new choice for flexographic printing is the use of universal UV (ultraviolet curing) inks (the so-called universal inks, which are suitable for printing from paper to film), which have entered the practical stage in Europe and the United States, and some have reported good results.

The UV ink has good printability, does not change the physical properties during printing, and can be printed with higher viscosity (viscous as honey). Therefore, dot enlargement is greatly reduced in color printing, thereby improving print quality. In addition, after the use of UV inks, there is no problem of the discharge of solvents, which is conducive to environmental protection. In short, the trend of UV ink will cause constant concern.

2, flexo printing ink problems

Flexo is called Kiss touch. Light pressure printing, light press printing ink ink has a great impact on the quality of printed images. Water-based ink In the printing of hydrophilic paper, there is no major obstacle in the transfer of the ink, but it is difficult to apply ink to a strong, oil-repellent film, and it is prone to ink-moistening. The ink remaining on the printing plate is Become blocked version of the reasons.

In addition, the flexo printing ink has a smaller transfer amount than the gravure ink, and the ink path is long, and the ink scatters due to evaporation during the transfer, resulting in a change in the composition of the ink, resulting in blockage, poor transfer, and poor leveling. Therefore, the re-solubility is good, the change on the machine is small, the ink, adhesion is good, can maintain its physical properties in various uses, and good drying water-based ink is the current demand. However, the contradiction is that both the printability and the physical properties must be maintained. The opposite is true for the functional requirements of the ink. Therefore, the difficulty in designing water-based inks is conceivable.


Source: "Printing Intelligence"

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