Water-based printing ink, referred to as ink. Mainly used in flexo printing and gravure printing. Ink and ink have the ability to prevent air pollution, improve the working environment, eliminate harmful substances in the solvent on the human body and packaging, and prevent the risk of fire due to static electricity and flammable solvents. Therefore, ink painting is also called “green†printing material, and is particularly suitable for the packaging of instruments, medicines, beverages, alcoholic drinks and tobacco products that require strict hygienic conditions.
Flexo inks are liquid inks. It is much thinner than offset inks. This ink can be arbitrarily diluted with water and ethanol. Can be cleaned with tap water. However, after drying on the substrate, it is insoluble in water and has water resistance, the ink has high coagulability, low viscosity, clear marking, firm ink, and easy cleaning.
Ink is generally composed of insoluble pigments, water-soluble or water-dispersible resins, purified water and a small amount of ethanol, additives. The pigment in the ink is strong in alkali resistance. In the ink-manufacturing process, the pigments achieve a balance between mutual attraction and mutual repulsion of pigment particles through wetting, dispersion, and stabilization. Pigments not only determine hue and color saturation in ink but also have an important influence on many physical properties and storage stability.
The ink binder is mainly a water-soluble acrylic resin. The polymer resin contains a hydrophilic gene. After a certain process, it can be completely soluble in water, but it is insoluble in water after drying. The main properties of ink, viscosity, adhesion, gloss, drying and printability are mainly determined by the waterborne resin binder.
The solvent of ink is mainly pure water. In addition, a small amount of ethanol, cosolvents and various additives are added
The correct application of ink is an important factor in the normal printing and quality assurance. Because the characteristics of ink and the principle of ink transfer in flexographic printing are different from traditional printing, traditional ink concepts cannot be used to guide the application of ink. Must understand the characteristics and requirements of the flexographic printing process, must understand the choice and use of ink additives, must master the ink viscosity and PH value control and adjustment.
1, ink viscosity control and adjustment.
Flexographic inks should have relatively good fluidity and proper viscosity. The drying speed should be appropriate and the adhesion to substrates should be better. In addition, the materials in the ink cannot affect the flexographic plate and the squeegee, do not smell after drying, and have good storage stability. After a long period of storage, the ink is precipitated and stratified. After stirring, it should be easily dissolved and the hue cannot be changed.
Flexographic ink control indicators include: fineness, viscosity, drying speed and PH value. In addition to the gold ink, the PH value of ink is generally controlled within the range of 8.8 to 9.5. The pH of some special water-based inks is different from that of ordinary water-based inks. For example, the PH value of gold and silver inks is 7.5 to 8.0. The fluorescent ink is 7.5-8.3.
Viscosity is the most important control index in ink, and it directly affects the ink transfer transfer performance and product quality of flexographic printing. Viscosity is the degree to which a fluid resists flow, that is, the internal resistance to fluid flow. The viscosity of the ink is related to the viscosity of the binder, the amount of pigment added, and the particle size. It is related to the solvent and the compatibility. It is also related to the dispersion stability of the additive and the pigment. In addition, the temperature has a great influence on the viscosity and the temperature increases. The viscosity decreases.
The same kind of ink has different viscosity, and its performance is very different. If the viscosity is too low, the color will be light, the dot gain will be large, the dot will be false, the high-light dot will be deformed, and the ink will not evenly spread. Ink viscosity is too high, will affect the anilox roller on the ink transfer performance, resulting in uneven ink, the color is not printed deep, and prone to dirty version, paste version, blistering, quit, pull the paper and other ills.
The viscosity of the ink was measured using a "Tu-4" viscometer and a stopwatch. It is generally considered that the viscosity of the ink is better at 30 seconds/25°C—45 seconds/25°C. Of course, the viscosity needs to be determined according to the condition of the printing carrier (paper, aluminum foil, plastic, etc.), the type of the equipment, and the printing speed. In general, the faster the speed, the lower the viscosity should be. The ink viscosity indicators for the following set of papers are available for reference.
Yellow ink 40 seconds ± 2†25°C
Red ink 40 seconds ± 2†25°C
Blue ink 40 seconds ± 2†25°C
White ink 35 seconds ± 2†25°C
Black ink 32 seconds ± 2†25°C
Gold and silver ink 40 seconds ± 2†25°C
Waterborne Varnish 30 seconds ± 2†25°C
It should be pointed out that flexographic printing cannot use only the products of one production ink manufacturer, both domestically and abroad. When purchasing each brand of ink, it is absolutely necessary to ask the manufacturer for accurate technical parameters and usage methods, and classify them separately. When using it, be sure to notice if there is any difference between the products of each brand. And according to their respective viscosity and PH value, this will have certain benefits for product quality.
Viscosity adjustment before the machine can be used 1:1 mixture of ethanol and distilled water, the printing process viscosity adjustment requires the use of ink stabilizer.
2, PH value of ink
Another important indicator for aqueous inks used in flexographic printing is the pH. The previously described pH values ​​of commonly used inks are between 8.8 and 9.5, which are slightly alkaline. In this range, the printing performance of the ink is the best, and the quality of the printed matter is also the most stable. Determine the PH value is a solubilizer of the ink composition - amines. In the printing process, the amine is constantly evaporating, plus new inks and various additives are continuously added. As a result, the pH of the ink is constantly changing. When the pH is higher than 9.5, the alkali is too strong, the viscosity of the ink will decrease, the drying speed becomes slower, and the water resistance is too bad. When the PH value is lower than 8.8, the alkalinity is too weak, the viscosity of the ink will increase, and the drying will block the printing plate and the anilox roller too quickly, causing the plate to be dirty and foaming, so the pH value should come with the viscosity. monitor. Use a pH meter when monitoring. At the same time pay attention to grasp the PH value of various additives and strive to control the viscosity and PH value as accurately as possible.
According to relevant data, when a color is overprinted on another color, the viscosity of the ink should be slightly increased, and the PH value of the ink should be reduced. This helps the ink to dry and prevents the post-printing ink from causing the previously dried ink to dry again. Wet and affect the quality of the product.
3, the application of ink additives
The application of the ink additive can adjust and meet the different requirements of some printed products, improve the printability of the ink, and ensure the smooth progress of the printing process. In the printing process, the ink and the solvents such as amine, water, and ethanol continuously evaporate, and the viscosity and the pH thereof. The value will change, affecting the printability, and must be adjusted with the corresponding additives according to the specific situation.
Commonly used ink additives are:
(1) Ink stabilizers. Also known as PH stabilizer. It is the most commonly used additive in ink printing. It can replenish the amines and water evaporated from the ink transfer system during the printing process, helping to stabilize the viscosity of the ink and controlling the pH within the range of 8.8 to 9.5. At the same time, it is a very good diluent. When it is added, it can be added in an appropriate amount according to different conditions such as paper, product, and printing speed. Generally speaking, 1-2% of the total amount of ink added every 30-40 minutes is evenly mixed, and the effect is good. The ink stabilizer is divided into ordinary type and thin film type.
(2) The ink blocker can suppress and reduce the drying speed of the ink, prevent surface crusting, reduce the stencil, and improve the print sharpness, and can be used according to the required amount. Generally, the maximum amount of each added can not exceed 2-3%, commonly used propylene glycol, ethylene glycol monobutyl ether.
(3) Defoamer suppresses and eliminates ink bubbles. When the ink viscosity is too large and water is added too much, it is easy to foam. Defoamers are generally added to the ink prior to the start of printing. In the printing process, before adding the defoamer, the viscosity of the ink should be adjusted to the normal range before adding the defoamer, which is very effective. The amount of defoamer should not exceed 0.5-1%.
(4) Lighter can be used to lighten color. However, it does not reduce the viscosity and drying speed. Dilution agent is also a kind of water-based brightener that can improve the brightness of ink. It should be added according to the specific circumstances.
(5) Diluent Adding diluent to the ink can reduce the viscosity of the thickened ink without affecting the pH. The amount used should not exceed 5% of the total amount of ink. Water-based gold and silver ink should use a special rare release agent, water-based gold and silver ink is very easy to swell denaturation, not with amines, alkalis and polar solvents. Water-based gold and silver ink rare release agent is mainly used to adjust and reduce the viscosity of water-based gold and silver ink, in order to ensure the normal printing. Generally added at a certain interval.
(6) Ink cleaner (concentrate)
When using, use 8-10 times of water to release and mix evenly. It is mainly used for cleaning flexible plates, anilox rollers and ink transfer rollers. It is very effective for ink cleaning.
It should not be pointed out that when water is added to ink, pure water or steamed water must be added.
In the deployment of multicolor inks, not one brand of ink cannot be blended together. Since the ink additives and formulas of the respective brands are not the same, adverse reactions and precipitation will occur. Attention must be paid.
4, the storage of water-based ink
(1) The flexographic ink storage period is one year.
Storage conditions: The packaging window cover must be tightly capped; the storage temperature should be in the range of 5-30 °C; should be stirred again before use.
(2) Flexographic water-based gold and silver inks. Shelf life: Six months. Storage conditions: In addition to the general storage conditions of ink, the appropriate pH should be maintained.
(3) Flexible version of UV Varnish: Shelf life: six months;
Storage conditions: Tighten the cover of the packaging window; protect it from light; the storage temperature should be in the range of 5°C-30°C.
The shelf life and storage conditions of domestically produced flexographic ink and UV varnish are approximately the same as those of foreign products. The manufacturer is responsible for the quality of the promised storage. The actual product that does not exceed the storage period and is completely deteriorated can still be used normally after a slight adjustment. Of course, it is best to use it within the storage period.
The flexographic inks are basically pigments at home and abroad. Due to the low viscosity of the inks, the dispersed pigments and some additives easily sink (especially aqueous tipping paper inks), and they tend to produce delamination after storage for a time. Precipitate, forming rare above, thick below. This does not affect normal use. Just mix it well or stir it well before use. This is the preparation work that must be done before using ink.
5, inspection of ink
The inspection method of the flexographic ink is basically the same as that of the conventional liquid ink, and it can be performed with reference to relevant regulations and standards. Full-scale flexographic ink quality indicators include: viscosity, pH, fineness, color, dryness, gloss, adhesion, stability, etc.
Source: China Tobacco Packaging Ink Information Network