Precautions for the use of flexo printing inks

The special ink used in the flexographic printing machine is a water-based ink, which has the advantages of no pollution to the environment, no impact on human health, no combustion, good safety, stable ink performance, bright colors, no corrosion of the plate material, simple operation, and price Cheap, good adhesion after printing, strong water resistance, rapid drying, printing speed can reach 150 ~ 200m per minute. Due to the above characteristics, the flexo-printed water-based ink is particularly suitable for the printing and use of packaging and decorating printed products such as foods, medicines, and cosmetics that require strict sanitary conditions. What problems should it pay attention to when using it?

The main index of flexo ink

1, fineness

Fineness is a physical index that measures the size of pigments, fillers, and other particles in the ink and is directly controlled by the ink manufacturer. The user generally understands it and cannot change its size during use. The fineness of flexographic water-based ink is generally less than 20μm. The instrument for measuring fineness is a fineness scraper.

2, viscosity

Viscosity is one of the main indicators of water-based inks. Viscosity values ​​will directly affect the quality of prints. Therefore, the viscosity of water-based inks must be strictly controlled during flexographic printing. The viscosity of the water-based ink is generally controlled within the range of 30 to 60 seconds/25°C (with a No. 4 cup of paint), and the viscosity is generally controlled to be between 40 and 50 seconds. If the viscosity is too high and the leveling property is poor, the printability of the water-based ink may be affected, and the phenomenon of a dirty plate, a paste plate, etc. may easily occur. If the viscosity is too low, the ability of the carrier to drive the color material may be affected.

If aqueous inks are left for a long time, some poorly stabilized inks tend to precipitate and delaminate, and some may experience false thickening. At this time, sufficient stirring can be performed and the above problems can disappear naturally. When using fresh water-based ink, be sure to mix evenly beforehand and then make dilution adjustments. In normal printing, the ink fountain must be stirred regularly.

Water-based ink freezes easily below 0°C. If the water-based ink freezes, it can be placed in a room with high temperature, let it dissolve naturally, and after mixing, it can be used continuously. In the large temperature difference between winter and summer, the performance of viscosity is the most sensitive. When the temperature is high, the water evaporates quickly and the drying is fast. During the operation, it is necessary to pay attention to reducing the drying time or increasing the speed of the machine. When the temperature is low, the water evaporates slowly and the water-based ink will dry slowly. In the operation, the water-based ink may be dried faster. Speed ​​or add drying device. When using in winter, it is better to place the water-based ink stored in the warehouse into the workshop in advance to balance the viscosity.

3, drying

Drying is one of the most important indicators in water-based inks, because the speed of drying is the same as the viscosity and can be directly expressed in the quality of prints. The operator must understand the principle of drying in detail in order to properly set the drying time of the water-based ink according to the product or substrate. To ensure that the water-based ink dries well, it must also be considered that the viscosity is moderate or the pH is stable.

The drying of water-based ink includes three ways of volatilization, absorption and reactive solidification. After the water-based ink print is completely dried, it has a strong water resistance and rubbing resistance. Drying is too fast, the surface of the ink layer is easy to crust, the water-based ink is easy to dry onto the printing plate, causing the surrounding of the dirty plate and the pattern to be unclear, and the internal water cannot be excluded, resulting in inaccurate overprinting, which can be adjusted with a retarder. This can be solved by increasing printer speed or shutting down the drying system. When shutting down, it is best to clean the anilox roller or ink fountain. When it is stopped for a short time, it is necessary to stir the ink fountain frequently to prevent surface crusting. Drying is too slow, the paper is stretched, and the luster is reduced. This can affect the overlapping printing and may cause blocking. This brings a lot of trouble to the operator of the machine. At this time, it should first check the pH value is correct, according to the level of pH, add appropriate amount of stabilizer or ethanol adjustment, so that water-based ink drying faster.

4, pH

Water-based ink contains a certain amount of ammonium solution, which is used to increase its stability or enhance the water resistance after printing. Therefore, pH is one of the important indicators. The pH value of the water-based ink at the factory is generally controlled at about 9. The pH value of the machine table can be adjusted or controlled between 7.8 and 9.3. It should be flexibly controlled according to the different substrates and temperatures. At the same time, it is necessary to take into account the relationship between the drying and the viscosity, and add appropriate amounts of various additives to adjust. The pH of golden ink should not be too high to be controlled between 8 and 8.5, making it slightly alkaline. Usually 2 to 3 hours to test once, with the follow-up adjustment, try to control the pH value in the best printability range.

Water-based ink main additive types, uses, dosages

The main additives of water-based inks include diluting agents, stabilizers, retarders, cleaning agents, defoamers, gold ink thinners, and UV gold ink curing accelerators.

1, dilute agent

Diluent is a milky white paste liquid with a certain viscosity, high pH, ​​can be adjusted with various colors and dilute the four-color ink without lowering the viscosity and pH, and can be used directly on the machine to enhance the brightness of the print. The amount can be based on the user's color and color depth.

2. Stabilizer

The stabilizer is a colorless, transparent liquid with a pH value of up to 11, which increases the pH of the aqueous ink and enhances the curing and drying time of the aqueous ink. If the printing is normal, the pH value is relatively stable, and the viscosity is too large to adjust the viscosity of the water-based ink after dilution with 1 to 1 water, and the amount of use is generally controlled at 4% to 6%.

3, blockers

The retarder (slow-drying agent) is a slightly yellowish liquid with a pH of around 5.5. It inhibits and reduces the cure drying time of water-based inks, prevents surface crusting, is too slow for speed, and is caused by water-based inks. The phenomenon of dirty plate and stencil caused by drying too quickly has a certain inhibitory effect. When the viscosity of the water-based ink is too high, and it does not require the use of an excessive amount of retarder, it can be diluted with water, and the amount is generally controlled at 3% to 5%.

4, defoamer

The main performance of the defoamer is to eliminate air bubbles in the water-based ink and prevent problems such as white spots, blisters, and water lines arising from the appearance of air bubbles during printing. The amount is generally 0.5% ~ 1%, the amount of excessive, but affect the quality of the product, the best spray from the sprayer to use

5, gold and silver ink thinner

Gold and silver ink thinner is a kind of opaque turbid liquid, it has a certain viscosity, pH value is slightly alkaline, the main performance is to adjust the viscosity of gold ink, improve printing applicability, while not making gold and silver ink oxidation, darkening Keep the original luster of gold and silver ink. The dosage is generally controlled between 2% and 8%, and should be flexibly used according to the temperature, the substrate, and the speed of the vehicle.

6, UV gold ink curing accelerator

When UV inks are deployed on site, UV gold ink curing accelerators are mainly used to help increase the speed of photocuring and drying, increase vehicle speed, and increase production. The general amount of control can be 5% to 10%.

7, cleaning agent

The cleaning agent is mainly used to wash the printing plate and anilox roller on the machine table, and to process the dry ink and crusted water-based ink.

Recycling and utilization of remaining ink

Because ink fountains must maintain a certain amount of ink in order to ensure the normal printing of the printing presses, each machine can not run out of ink each time. At the end of each batch of product, there is always a certain amount of residual ink. Multi-type, multi-color printing units, each product will have a certain amount of remaining ink handling. If it is not adjusted in time, it will cause waste and increase costs when it is used continuously. This requires that the remaining ink should be kept to the lowest possible level during deployment. The remaining ink must be kept alone, sealed, and no ink or debris should be mixed in. If it is not clean, use a filter to filter it. When it is used for the next time, use the remaining ink first. The light color can be adjusted into the dark color. The rough paper or medium and low grade products can be used preferentially.

Water-based ink preservation

The shelf life of the flexographic water-based ink is generally one year, and it is longer than the shelf life or time, as long as the solidified colloidal state does not occur, but the surface is stratified or precipitated, and as long as it is sufficiently stirred, it can be used continuously. The storage temperature should be within the range of 5°C-30°C. It is best to store in the storehouse. Do not store it in the open air, do not expose it to sunlight, or store it in a storeroom below 0°C. Golden ink has a shelf life of six months or less.