Flexographic printing belongs to letterpress printing, and its development has been a hundred years old. However, the application of UV ink in domestic flexographic printing has only been a few years. The green and non-polluting development direction is driving the rapid development of flexographic printing.
There are usually three types of flexographic inks: solvent-based inks, water-based inks, and UV inks. Water-based inks are the most widely used for flexo printing. Water-based ink has many advantages, but it is found in the actual application process, ink printing stability is very poor, difficult to control, the printed material also has a certain range of adaptation. The appearance of UV inks and their application on flexo presses have further broadened the development direction of flexo printing.
UV inks began in 1946 and have only developed rapidly in the last 20 years. The application time in domestic flexo printing is even shorter. UV inks are ultraviolet drying inks, and the curing principle is that the photoinitiator is dispersed into free radicals Râ†’R+R under the irradiation of ultraviolet light of the appropriate wavelength and intensity. These free radicals initiate the prepolymer and monomer. The unsaturated double bond polymerizes rapidly to form the monomer radical R+Mâ†’RM.
Then, the monomeric genes begin to undergo a chain reaction and rapidly increase the turnover of the network structure, RM+Mâ†’Rmmâ†’Rmmâ†’-------so that the ink is quickly solidified by liquefaction, forming an ink film on the surface of the substrate.
UV ink has many advantages. Compared with other inks, its biggest advantage is that it does not contain organic volatile solvents. It does not emit organic volatiles to the atmosphere during printing, which is beneficial to the working environment. The odor on the printed products is small, no adhesion pollution, and meets the requirements of environmental protection, especially in the fields of printing, medicine, beverages, foods and other hygienic requirements of more stringent application of more and more widely. UV ink needs to be equipped with an irradiation curing box, reducing the floor space.
UV inks are printed on the screen of flexographic printing products and maintain their absolute advantages. It cures immediately after printing, and the dot deformation rate is small, and it is not likely that the cable lining often appears like ink. The use process does not need to be adjusted as often as the ink, so that it is possible to maintain a small range of print color difference and uniform batch color.
Although the current price of UV inks is relatively high, there is no solvent volatilization during the printing process and 100% remains on the printed material. If the screen roller is used with ink and ink, UV inks show a thick and full ink layer. If printing 100m2 of prints requires 1kg of ink, then the water-based ink will need more than 2kg to print close to the effect of UV ink, and UV ink has obvious advantages in terms of luster and abrasion resistance.
Although UV ink has many advantages, due to its late appearance, the application time of flexo printing is relatively short, people still lack understanding of its knowledge and mastery. Now, several problems of UV ink in flexo printing and the printing industry are common. Explore and communicate.
One, UV ink curing problem
The ultraviolet wavelength and light intensity emitted by the UV lamp are the basic conditions for the formation of the UV ink. Dark ink, such as black ink, brown ink, etc., cures slowly, and is prone to smearing of the printed product, and the layout is blurred.
UV inks contain different wavelengths of pigments, and their UV absorption is not the same. Due to the poor reflection ability of dark pigments, most of the UV light is absorbed, and the UV light intensity is weakened, resulting in poor curing. Similar situations occur in daily operations, and sensitizers are often added to solve the problem.
However, adding an excessive amount of photosensitizer, UV ink will appear after the curing ink film performance decreases, the ink storage period is shortened, so the photosensitizer must be strictly proportional (usually not more than 3%) to add, not excessive.
The daily maintenance of the UV lamp is very necessary. The surface is covered with dust and dirt, and the generated UV light is weakened or obstructed, which will affect the curing and reduce the service life of the lamp.
When selecting an optional cable roller, you should use an anilox roller with a high number of screen lines to reduce the thickness of the ink layer so that the UV light can easily penetrate. In the ordering of the color sequence, under the premise of not affecting the registering pressure lines, the first printing of dark ink and the latter printing of light color inks; the first printing of solid portions and the latter of printing of the network lines. After repeated irradiation, the film is cured to the maximum extent.
In terms of the ink composition, the acrylic resin-based UV ink containing a cross-linking monomer cures faster than the epoxy resin-based UV ink, and an appropriate ink type is selected.
Under the premise of not affecting the effect of UV ink color in printing, transparent ink with high transparency is added to the ink force, so that the ultraviolet light can penetrate and enhance, and the purpose of reinforcement and reinforcement is achieved.
The above are several methods to solve the problem of curing the UV ink. In actual use, adjustments can be made according to specific conditions.
There are three ways to identify UV curing:
1. Scraping test method: Place the printed product on the flat surface and apply pressure to scrape the surface with a nail. If the scratched ink layer indicates that the curing is not complete, the normally formed ink layer is very hard. 2, chemical reagent testing method: In the print surface drops a few drops of acetone solvent, if the UV ink dissolved, indicating that it is not fully cured. 3, observation method: If the UV ink is not completely cured on the print, the pattern will produce a virtual shadow tail phenomenon. The above three methods can quickly and easily identify UV inks in a short time.
Second, the UV lamp overheating
This is a problem that arises with unreasonable equipment. Because the UV lamp generates heat while generating ultraviolet light, the longer the UV lamp is irradiated, the higher the amount of heat and the higher the amount of heat.
1. Overprinting of overprinted materials can easily lead to shrinkage, resulting in increased deviation of overprinting, which is not conducive to registering. 2. High temperature affects the printing environment, making it difficult to control the temperature and humidity. 3, equipment, electrical components at high temperatures, easy aging damage, easily lead to short-circuit fire, resulting in security risks. 4, UV ink sensitive to temperature, high temperature caused by ink viscosity decreases, thinning, so that the print product hue changes.
To sum up, it is necessary to reduce the temperature of the UV curing box. The most effective way to solve this problem is to increase the air intake volume and the air discharge volume of the curing box, and use the air flow to cool the UV lamp. If necessary, add the cooling equipment to the UV curing box, supplement the UV curing box with the cold air and cold water circulation system to force cooling. Considering the running cost, as long as the heat does not affect the operation of the equipment and the printing quality, the printing requirements can be satisfied by increasing the air volume.
Third, the viscosity of UV ink is higher
The high viscosity of UV inks in the field printing indicates that the prints are thick and have high color saturation, but the viscosity of the printed wire products is high and the leveling property is poor, which can easily cause the poor uniformity of the roller. The high viscosity of UV inks is mainly affected by their own binders. At present, experts in foreign countries have developed water-based UV inks for low-viscosity printing. However, many domestic manufacturers use the UV ink temperature control system on the printing equipment to use the characteristics of the viscosity of the UV ink, which is easily changed by the temperature, to achieve low viscosity printing.
IV. Daily savings of UV ink
Due to the influence of its raw materials, UV inks are currently relatively high in ink prices and are often wasted due to improper measures. Therefore, from the perspective of economic interests, the daily savings of UV ink is very necessary. Especially in the printing of large quantities of products and large field products, without affecting the color saturation, properly increase the number of line roller lines, saving ink from the ink supply. UV inks cure only when exposed to a certain intensity of UV light. According to this characteristic, UV inks do not â€œdryâ€ under UV light conditions on the press. The inks can be stored overnight in the ink fountains without ink. The same shutdown must be cleaned on the same day. Avoid waste caused by daily cleaning. UV ink in the application of flexo, as long as you can master it, and the rational use of it, its superior performance can be maximized.
With the development of flexo printing, narrow width and high speed are the current direction of flexographic presses. UV inks have a fast curing speed (300m/min), a wide range of applications (can be printed on a variety of substrates), and are healthy, environmentally friendly and pollution-free. They are also affecting and driving the development of flexo printing technology. Printing brings a new and broader field.
Source: Ink Information
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