UV (UV curable) ink refers to an imaging substance that uses different wavelengths and energies of UV light to polymerize monomers in the ink binder into a polymer under UV irradiation, and then solidifies into a film and dries. During the printing process, we often encounter adverse phenomena such as incomplete curing of UV ink, weak adhesion, stacking, drying, and flying ink, which affect the tone, color, clarity, and other aspects of the printed surface image. In this article, based on years of work experience, the author analyzes the common quality problems and solutions of UV ink in the printing process together for readers' reference.

Poor UV ink curing

Whether UV ink is completely cured determines the adhesion fastness of the printing ink layer, the reproduction of printing colors, and the glossiness of the printing ink layer. Therefore, it is important to make accurate judgments on whether UV ink is cured quickly on the production site.

Common problems with poor solidification

If UV ink is not cured thoroughly, the adhesion of the ink layer is relatively poor. During the printing process, it can cause the printing ink layer to stick to the back of adjacent prints, resulting in dirty prints; At the same time, the surface of the ink layer is not resistant to friction and is prone to scratches in subsequent processing and use of printed materials, seriously affecting printing quality.

In the process of large-scale on-site printing, due to the large amount of ink supply and the thick ink layer, the ink cannot be completely dried when the printed product is dried under UV light, resulting in the phenomenon of the surface layer of the ink drying and the bottom layer not completely drying. We call this the phenomenon of false drying of the ink. Especially when printing solid colors such as white and black (as shown in Figure 1), due to the slow curing speed, it is easy to produce false drying, leading to poor ink curing. On the one hand, this is because white can fully reflect all visible light, requiring higher energy during curing, making it difficult for white ink to cure; On the other hand, although black and dark colors can theoretically absorb all light, the blocking effect of pigment particles on light also hinders the deep curing of ink, which requires higher energy. Therefore, the curing speed of black and other dark color inks is slower.

Judgment method for curing defects

After years of production practice, we have summarized various judgment methods, which can be roughly summarized as appearance observation method, physical testing method, and chemical testing method.

(1) Appearance observation method. The appearance observation method does not require the use of any external tools, and is determined directly by observing whether the UV ink layer has a bright surface. Generally speaking, a fully cured UV ink layer has a shiny surface.

(2) Physical testing method. There are two commonly used physical testing methods. Method one is to lay a piece of paper on the UV ink layer on the surface of the printed product, press the paper with your hand, and rub it back and forth a few times on the UV ink layer. If damage is found on the surface of the UV ink layer on the printed product, then the UV ink layer is not fully cured. Method 2: Use a large needle to scratch the surface of the UV ink layer on the printed product, and then place it under a 200x magnifying glass to observe the scratches. If the scratches on the UB ink layer are found to be serrated under magnification, it indicates that the UV ink layer has fully cured.

(3) Chemical testing method. The chemical testing method requires the use of acetone (CH3COCH3) solvent for testing. The specific method is to drop acetone solvent onto the UV ink layer. If it is found that the UV ink layer can be dissolved, it indicates that the UV ink layer has not been fully cured.

Solution to poor solidification

Those with printing experience know that poor UV ink curing during the printing process mainly occurs when printing thin film products, while poor ink drying is relatively rare in printing paper products. This is mainly because the surface tension of thin film products is relatively low, and ink is not easy to firmly adhere to the surface of these products.

The most thorough way to solve the problem of poor curing of UV ink is to switch to UV ink with faster curing speed; You can also switch to ink with higher color concentration to achieve the expected color reproduction with a thinner ink layer, that is, by reducing the ink layer and improving the curing efficiency of the ink, the ink can be completely cured. In some cases, it is also possible to improve the curing efficiency of UV ink by adding an appropriate amount of surface smoothness aids or curing aids without replacing the ink, achieving complete curing of the ink layer. However, in general, the method of adding additives is not recommended to prevent chemical reactions from occurring.

Poor adhesion of UV ink

The weak adhesion of the printing ink layer is not always caused by the ink itself, but is also closely related to the surface performance of the printing material. When replacing or adjusting the ink cannot solve the problem and the printing material cannot be replaced, there are two solutions for printing enterprises to choose from.

Method 1: Before printing ink, apply a layer of transparent and adhesive medium through printing to improve the surface performance of the printing material, so that UV ink can adhere well to it. This plan is to print this ink hopper base oil in the first unit of the printing machine, using UV curing.

Method 2: Before printing, apply water-based varnish as a primer on the surface of the printing material to solve the problem of weak adhesion of ink layers such as explosive ink and edges. The water-based varnish is applied by roller coating method and dried using infrared radiation. This method is developed to address the unstable adhesion of printing materials such as gold and silver cardboard and laser cardboard to UV ink.

Poor UV ink transfer, stacking roller, dry roller

Stacking ink refers to a printing malfunction in which ink accumulates on the blanket, printing plate, and ink roller, resulting in poor ink transfer and blurring of the ink color of the printed product (as shown in Figure 3). There are many reasons for ink accumulation during the production process, such as poor ink fluidity, poor ink transfer, easy emulsification of ink and fountain solution, peeling of paper powder and mixing into the ink, and insufficient ink fineness, all of which can lead to ink accumulation problems.

Common methods to solve the problem of ink stacking include: (1) using ink with good water resistance; (2) Use inking oil or viscosity reducing agent to adjust the viscosity and viscosity of the ink; (3) Control the amount of water during the production process and avoid excessive amounts.

UV ink appears as flying ink

During the printing process, the phenomenon of ink flying occurs, which not only pollutes the environment but also damages the health of operators. It can also have a negative impact on product quality. For example, ink flying onto the surface of the printed product can cause a certain degree of dirt on the surface, affecting the clarity of the printed product, and in severe cases, it can also lead to the cancellation of the printed product.

Reasons for the occurrence of flying ink

The reason for the formation of flying ink is due to the fact that during the transfer of UV ink, the fibers are cut or elongated at two or more adjacent ink rollers, resulting in fragmented surfaces due to surface tension. At this point, the double layer of electricity at the interface between the fiber and the fragment is destroyed, with some remaining in the UV ink and the other escaping into the atmosphere with opposite charges. When the fibers shrink, the charges in the printing machine gradually accumulate on the surface of the ink roller and paper, and the fragments, due to the same charge as the ink, are repelled by the air when they are close to the ink roller or paper, forming flying ink.

Factors affecting flying ink

There are many factors that affect the occurrence of ink flying phenomenon in the printing production process, such as:

(1) The viscosity of UV ink. If the viscosity of UV ink is too high and it is stretched at the exit of the ink roller, it is easy to cause ink flying, and at the same time, it can also cause uneven ink transfer, paper shedding and powder removal, difficulty in peeling the paper from the rubber cloth, easy patterning, plate falling, and other problems; If the viscosity of UV ink is too small and the ink layer splits poorly, it is also easy to produce ink flying phenomenon, which can emulsify the ink, cause floating dirt, paste plate, and dot enlargement, thereby affecting the printing quality of the printed product.

(2) The thickness of UV ink. The thicker the ink layer of UV ink on the ink roller, the greater the probability of flying ink. If making samples on coated paper, the ink layer thickness is required to be 1.2 μ At m, the ink on the plate only needs to reach 2.8g/m2, while when printing with offset paper, the minimum ink on the plate should also reach 4g/m2. In addition, once encountering situations where the performance of the paper is not good for ink adhesion, it is easy to cause ink accumulation on the ink roller, resulting in ink flying.

(3) Ink roller. When the surface of the ink roller is uneven or there are dry knots and cracks on the surface, some ink layers become thicker, and the amount of ink mist increases, which is easy to cause ink flying.

(4) Printing speed. During the production process, if the printing speed increases, the probability of ink flying phenomenon will also increase.

(5) Air humidity. If the air humidity in the production workshop is relatively low, it will promote the occurrence of ink flying phenomenon; If the humidity increases relatively, the amount of charge in the air decreases, and the amount of ink flying will also decrease accordingly.

(6) Moisturizing solution. If there is too much fountain solution in UV ink, serious W/O emulsion ink will form, with a sharp decrease in viscosity and a shortened ink filament, resulting in serious ink flying. At the same time, the fountain solution immersed in ink can also corrode the metal ink roller, forming a hydrophilic layer on the surface of the ink roller and repelling the ink, causing deinking of the metal ink roller.

Flyink Solution

Once encountering ink flying problems during the production process, common solutions include: (1) adjusting the viscosity of UV ink, as different printing speeds of the printing machine adapt to different ink viscosities; (2) Regularly clean the rubber roller to ensure even distribution of the ink layer on the roller and improve ink transfer performance; (3) Control the printing speed. Excessive printing speed will increase the possibility of flying ink; (4) The thicker the ink layer on the ink roller, the greater the probability of flying ink generation; (5) Increase the air humidity in the production workshop appropriately, because when the air humidity is relatively high, the possibility of flying ink is lower; (6) During the offset printing process, attention should be paid to the balance control of ink and water.

In short, the phenomenon of ink flying may occur at any time during the printing process due to the contact and rotation of the two rollers, which requires operators not only to have high skills in operation, but also to be familiar with the data and specifications involved in printing, and to minimize the probability of ink flying as much as possible.

Generally speaking, the faster the curing speed of UV ink, the higher the surface hardness and flexibility of the printing ink layer, and the poorer the adhesion on the surface of the printing material, resulting in a lower glossiness of the printed product. In short, the various factors that affect the use of UV ink are closely connected and mutually influencing. Printing enterprises must consider and eliminate difficult problems from multiple perspectives in the process of applying UV ink, and fully leverage the advantages of UV ink.