In recent years, UV (ultraviolet) coating technology has been gaining popularity in the printing industry. So what is UV? What is the use of UV for printing? What are the benefits of using UV technology for printing? and many more. Below we take these problems to get close to the UV.
UV is the abbreviation of Ultraviolet Ultraviolet in English. Ultraviolet light is a band in the electromagnetic spectrum. Waves with a wavelength of 100 nm to 400 nm are called ultraviolet rays. UV, which is generally referred to as printing, is actually an abbreviation of ultraviolet curing, that is, a process in which a printed product is dried under ultraviolet irradiation after printing.
First, the advantages and disadvantages of UV coating:
Advantage 1 There is no solvent in the formula, so there is no problem caused by solvent evaporation;
2 Fast curing at room temperature;
3 Reduce energy consumption. Curing speed, so that the required energy per unit area decreases;
4 can be coated with heat sensitive substrate;
5 The coating process is simple and easy;
6 Save space, much shorter than infrared heat drying oven;
7 Finished products can be stacked immediately;
8 Excellent coating performance.
Disadvantage 1 If you do not use harmful monomers, it is often difficult to formulate very low viscosity coatings;
2 expensive;
3 has skin irritation;
4 UV light source will produce ozone;
5 The coating has low adhesion and shrinkage.
Second, the chemical composition of UV coating.
The chemical composition of UV Varnish mainly includes radiation prepolymers, thinners and photoinitiators:
1. Radiation prepolymer. Prepolymers are chemical systems with residual unsaturation molecules. This molecule is capable of cross-linking with other unsaturated molecules when in certain conditions, from a liquid to a solid coating. It is required that these unsaturated molecules must be stable and not react with each other before crosslinking.
Prepolymer type:
1) Epoxy acrylate;
2) Acrylated oils;
3) Acrylic urethane;
4) unsaturated polyester;
5) Polyester acrylate;
6) Polyether acrylates, etc.
2. Thinner. Diluents are also chemical systems containing unsaturated molecules. Use it to adjust the viscosity, while it is a film-forming substance. Some situations can replace prepolymers. To make the viscosity reach the desired range, it is often necessary to add a small amount of volatile organic solvent to the mixture of prepolymer and diluent, but up to no more than 5%-10%.
Diluents are divided into active and inactive two. Inactive diluents include solvents and plasticizers. Solvents are mainly volatiles. Plasticizers impart certain flexibility to the cured coating, which not only reduces viscosity but also facilitates coating and leveling. The addition of a reactive diluent to the prepolymer can change the viscosity, tackiness, flexibility and hardness. Reactive diluents can be divided into monofunctional and polyfunctional groups.
Monofunctional reactive diluents, such as 2-ethylethyl acrylate, are flammable, volatile, and irritating to the skin. The polyfunctional reactive diluents have low volatility, high flash point, and low skin irritation.
The role of reactive diluents is as follows:
1Adjust the viscosity and leveling;
2 and photoinitiator system determine the curing speed;
3 with high molecular weight prepolymer molecules linked together to accelerate curing;
4 to improve and improve the performance of the cured coating.
3. Photoinitiator. Photoinitiators are defined as molecules that absorb radiant energy and undergo chemical changes to produce reactive intermediates that have the ability to initiate polymerization.
The photoinitiator does not participate in the curing, so the photoinitiator concentration is low, and the amount thereof requires that the initiator itself or its photochemical product must not adversely affect the chemical and physical properties of the polymer after curing. Benzophenones are usually used.
Photoinitiators In the process of initiating polymerization, two more substances are indispensable. Photosensitive type refers to a chemical reaction that transfers energy to a photoinitiator molecule to initiate polymerization after absorbing light energy, and the photosensitizer recovers to its initial inactive state. The other is called photoactive agent or photo-enhancer (usually amines). It mainly enhances the activation rate of photoinitiator. It does not absorb radiation by itself and does not initiate polymerization, but it can increase the curing rate.
Third, UV curing chemical process.
The basic composition of photo-curing materials are prepolymers, diluents, photoinitiators, auxiliaries, and pigments. The curing chemical process of light is excited by the absorption of photon energy by photoinitiators to form free radicals, which initiate polymerization of the prepolymer and the diluent (initiate molecular chain growth).
Fourth, in the print surface UV light oil process:
1. In addition to powder. In general, the printing machine has a powder removing device, and when the printing ink is large, dusting is commonly used to solve the dirt problem. However, the dusting glue sticks to the surface of the ink like sandpaper, and there is no gloss after applying UV varnish. Therefore need to remove powder. Removal method: First use a brush to sweep, the direction of the brush's rolling is opposite to the direction of the paper's movement; the powder is swept and then sucked into the outdoor through the suction duct. When the powder and the ink are mixed together into a granular form and cannot be cleaned, two light rollers can be used to press the surface of the paper to flatten the powder particles.
2. Corona treatment. Some printing materials, such as PP or PET, have poor affinity for UV varnish. Some inks contain waxes and are not compatible with UV oils. They can be used to improve the affinity of high voltage discharge and can be used with UV oil very evenly. Attached.
3. Priming oil. For oil-stained paper, first coat the white base oil, seal the pores of the paper, increase the whiteness, and then apply UV oil best. If the surface coating layer of thin white paper, if directly coated with UV varnish. The oil will quickly penetrate into the bottom layer and will not shine. Instead, it will reflect the bottom ash color and make the paper surface blue after glazing. If you first hit a layer of the bottom, then apply UV oil to solve this problem. Base oils are generally hot dry type.
4.UV polishing. Apply UV Varnish.
5. Cool. The UV-cured print was blown with a fan to reduce the surface temperature. One purpose is to avoid paper deformation and the other is to prevent post-curing from continuing.
Five, we are currently more common ways of UV polishing are the following four:
Three-roll coating principle.
The three-roll coating principle is shown in the figure. The UV varnish flows into the vinegar tank formed by the rollers M and R, and is uniformly pressed by the M-R, and the varnish adheres to the R. The R and M presses apply the oil on the R to the paper. The scraper scrapes the excess lacquer on the embossing roller M into the lower sump and reuses it. The amount of coating is determined by the pressure between M and R. This machine is characterized by fast glazing speed, simple machine structure, low price, and low operating technology. The disadvantage is that it is not suitable for thin paper because the thin paper tends to adhere to the roller R and rewind it. In some designs, the air knife can peel the paper from the roller R so that the thin paper is not taken away by the roller R, but it is not ideal for use. Then, the coating amount is not easy to control, the coating is too thick, the coating amount of varnish is not easy to adjust, and the coating thickness can only be controlled by heating oil or adding thinner.
The principle of fang-type polishing mechanism.
The principle of the dental glazing mechanism is shown in the figure. The ink stick M brings the oil from the oil tank to the platen roller R, and R presses against the pick-up platen L to transfer the varnish to the paper for coating. Advantages: Thick and thick paper are suitable, generally 80g/m2 single-sided copper paper to 450g/m2 whiteboard paper can be. Simple operation, simple structure and easy maintenance. Since the pressure between the rollers R and L is large, the ink distribution effect is better than the three-roller type. Disadvantages: slow speed, generally 2500 / hour; operators, at least 4 people; coating amount is not easy to control.
Four-roll anilox roller coater structure.
The structure of the four-roll anilox roller coater is shown in the figure. Comparison of three-roll coater, four-roller an anilox roller (metering roller), corresponding to a more scraper. The purpose of adding an anilox roller is to quantitatively control the amount of coating and to ensure the consistency of the thickness of the film on the product. General long version packaging with this type of machine, such as cigarettes, wine, soap, toothpaste, film and other product boxes need to quantitatively control the quality of products to ensure the consistency of product quality. Another example is a brand of soap box produced at different times, but placed in the same counter, if the use of three-roller polishing machine will appear different brightness of the surface of the box, and with anilox roller coating machine coating, it is not clear which Batch production of products.
Polished with an offset press.
The method of using the offset printing machine and its features are as follows:
1 Varnish is placed in a sink and coated with a water roller.
2 Remove the water fleece sleeve of the water roller and spread the oil with the rubber roller so as to reduce the oil transfer amount.
3 Adjust the amount of water on the roller, that is, adjust the amount of oil.
4 Widen the sink so that all ink rollers are above the sink. Because of the low viscosity of the varnish, the oil on the water roller drops downward on the underlying paper.
5 The cutting blanket is the same or slightly smaller than the width of the print.
6 can be coated with UV oil, water-based gloss oil, but can not be coated with solvent-based thermal drying Varnish, or sticky roll.
7 Use ordinary PS plate and ordinary blanket.
8 The thickness of the coating oil layer should not be too large, otherwise there will be pitting.
9 Can not be partially glazing because of the separation properties of UV inks on the PS plate.
10 It is easy to apply light locally by cutting the blanket.
11 When the coating is thin, the ink tank can be used for polishing.
If the shop temperature is below 20°C, the coating thickness can be adjusted by adding thinner.
Sixth, the correct use of UV Varnish UV oil is not conjunctiva, just like cooking oil on the surface of the air will not bind a layer of dry film. UV oil sees UV light, such as sunlight, arc welding, and curing light immediately. General fluorescent lamps, incandescent lamps, sodium lamps and other lighting sources do not cause curing. UV oil contains photoinitiators and thinners, which have a certain stimulating effect on human skin.
In view of the above characteristics of UV oil, the following points should be taken into account when using it:
1 Normal use temperature of UV oil is 50°C~55°C. When it is used, the oil is circulated and heated with a constant temperature water bath to make the viscosity meet the design requirements, which is beneficial to leveling and rapid solidification.
2 UV oil irradiation area through the UV lamp, the temperature is 50 °C ~ 60 °C, curing fast, strong adhesion. However, it is not the lower the UV lamp temperature, the better.
3 The polishing machine is placed in a location where the sun cannot reach directly, otherwise the UV varnish is solidified on the applicator roller. If it is not possible to avoid direct sunlight, red and black curtains are used to block the light.
4 When varnishing, UV oil will disperse the ink on the varnish material into UV oil to make the varnish colored. It can be used after filtration and precipitation, and it need not be discarded.
5 When applying the UV oil to the skin, immediately wash it off with soapy water, otherwise skin redness and blistering will occur.
6 The viscosity of UV oil varies depending on the type of glazing machine. Special varnishes should be selected according to the model. If you do not reach the required viscosity of the glazing machine, you can reduce the viscosity of the thinner or stick it with a tackifier. This method will lose UV light oil curing speed, brightness, adhesion and so on.
7 When UV oil flows to the rotating part of the drive shaft, it will stick to the shaft at high temperatures, affecting the transmission, so pay attention to cleaning the shaft head.
According to the nature of the surface of the substrate, choose a special light oil, such as gold cardboard oil, PET oil.
Seven, UV curing time can theoretically be calculated strictly within the unit volume, UV curing materials need to use a number of photons can be cured, and the photochemical reaction speed basically does not take much time. However, the addition of a polymerization inhibitor to the UV oil or ink delays the polymerization time (ie, the curing time) after the photochemical reaction has a direct relationship with the photoinitiator and the polymerization inhibitor.
For general UV varnish or ink formulations, the curing time is selected within 0.1~0.2s. However, the specific time after the formula is determined depends on the following factors:
1 coating thickness. The surface of the coating cures quickly, but the surface layer cures and absorbs ultraviolet light strongly. The intensity of the ultraviolet radiation decays exponentially, lowering the curing speed. Therefore, the coating is cured quickly and slowly.
2 UV intensity. When the intensity is high, the curing time is short, and vice versa.
3 Add solvent. Diluents, such as ethanol, in the UV varnish reduce the curing speed.
4 Ambient temperature. The coating temperature of 50 °C ~ 70 °C is the ideal ambient temperature. UV mercury lamps contain a large amount of visible and infrared components, which are the coating surface