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In recent years, UV (ultraviolet) glazing technology is gaining popularity in the printing field. 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 bring these questions to approach UV.
UV is the abbreviation of ultraviolet ultraviolet in English. Ultraviolet rays are a band in the electromagnetic spectrum. Waves with a wavelength of 100nm to 400nm are called ultraviolet rays. What we generally call UV in printing is actually the abbreviation of ultraviolet curing, that is, the process of drying the printed matter under ultraviolet irradiation after printing.
1. The advantages and disadvantages of UV coating:
Advantages â‘ There is no solvent in the formula, so there is no problem caused by solvent evaporation;
â‘¡ Can be cured quickly at room temperature;
â‘¢ Reduce energy consumption. Fast curing speed reduces the energy required per unit area;
â‘£ Coatable heat-sensitive substrate;
⑤ The coating process is simple and easy;
â‘¥ Save space, much shorter than infrared thermal drying oven;
⑦ Finished products can be stacked immediately;
⑧ Excellent coating performance.
Disadvantages â‘ If you do not use harmful monomers, it is often difficult to formulate coatings with very low viscosity;
â‘¡ The price is expensive;
â‘¢ Irritating to the skin;
â‘£ UV light source will produce ozone;
⑤ The coating has low adhesion and shrinkage.
2. The chemical composition of UV varnish.
The chemical components of UV varnish mainly include radiation prepolymer, diluent and photoinitiator:
1. Radiation prepolymer. Prepolymers are chemical systems with residual unsaturated molecules. This kind of molecule can cross-link with other unsaturated molecules when it is in a certain condition, changing from liquid to solid coating. It is required that these unsaturated molecules must be stable before cross-linking and do not react with each other.
Types of prepolymer:
1) Epoxy acrylate;
2) Acrylated oil;
3) Acrylic urethane;
4) Unsaturated polyester;
5) Polyester acrylate;
6) Polyether acrylate, etc.
2. Thinner. Diluent is also a chemical system containing unsaturated molecules. It can be used to adjust the viscosity while being a film-forming substance. In some cases, it can replace the prepolymer. In order to make the viscosity reach the required range, it is often necessary to add a small amount of volatile organic solvent to the mixture of prepolymer and diluent, but at most not more than 5% -10%.
Diluent is divided into active and inactive. There are two types of non-reactive diluents: solvents and plasticizers. Solvents are mainly volatiles. Plasticizers give the cured coating a certain degree of flexibility, which not only reduces viscosity but also facilitates coating and leveling. Adding reactive diluent to the prepolymer can change viscosity, tackiness, flexibility and hardness. Reactive diluents can be divided into two kinds of monofunctional groups and multifunctional groups.
Monofunctional reactive diluent, such as 2-ethyl ethyl acrylate, is flammable, volatile, and irritating to the skin. The multifunctional active diluent has low volatility, high flash point, and low skin irritation.
The role of reactive diluent is as follows:
â‘ Adjust viscosity and leveling;
â‘¡ Determine the curing speed with the photoinitiator system;
â‘¢Connect with high molecular weight prepolymer molecules to accelerate curing;
â‘£Promote and improve the performance of cured coatings.
3. Photoinitiator. The definition of a photoinitiator is a molecule that absorbs radiant energy and undergoes chemical changes to produce an active intermediate with the ability to initiate polymerization.
The photoinitiator does not participate in curing, so the concentration of the photoinitiator is low, and its dosage requirement: the initiator itself or its photochemical product must not adversely affect the chemical and physical properties of the polymer after curing. Usually benzophenones are used.
In the process of initiating polymerization by the photoinitiator, two other substances are indispensable. Photosensitive type refers to the chemical reaction that transfers energy to the photoinitiator molecules to initiate polymerization after absorbing light energy, and the photosensitizer returns to its original inactive state. The other is called photoactive agent or photo-synergist (usually amines), which mainly enhances the activation rate of the photoinitiator. It does not absorb radiation or initiate polymerization, but can increase the curing rate.
3. The chemical process of UV curing.
The basic composition of the photocurable material includes prepolymer, diluent, photoinitiator, auxiliary agent and pigment. The chemical process of light curing is that the photoinitiator absorbs the photon energy and is excited to form free radicals. These free radicals initiate the polymerization of the prepolymer and the diluent (initiate molecular chain growth).
Fourth, the process of UV varnish on the surface of the printed matter:
1. In addition to powder. Generally, there are powder removal devices on printing machines. When the printing ink color is large, powder spraying is often used to solve the problem of smudging. However, the powder spray 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 brush rolling direction is opposite to the paper moving direction; sweep up the powder, and then suck it to the outside through the suction duct. When the powder and ink are mixed into a granular shape and cannot be swept down, two smooth rollers can be used to press the surface of the paper to flatten the powder and ink particles.
2. Corona treatment. Some printing materials, such as PP or PET, have poor affinity for UV varnish, and some inks contain wax and are not compatible with UV oil. The high-voltage discharge treatment can be used to improve the affinity, and it can be evenly compatible with UV oil. Attach.
3. Priming oil. For oil-permeable paper, creamy white base oil should be applied first to seal the paper pores to improve the whiteness, and then the UV oil is best. For example, whiteboard paper with a thin surface coating layer, if directly coated with UV varnish. The oil will quickly penetrate into the bottom layer, no light will come on, but will reflect the gray background color of the bottom layer, so that the paper surface will become blue after glazing. This problem can be solved by applying a layer of primer and then applying UV oil. The base oil is generally heat-dried.
4. UV coating. Apply UV varnish.
5. Cool. Use a fan to blow the UV-cured printed matter to reduce its surface temperature. The first purpose is to avoid paper deformation, and the second is to prevent post-curing from continuing.
Fifth, we currently have the following four common UV glazing methods:
Three-roller coating principle.
The principle of three-roll coating is shown in the figure. UV varnish flows into the vinegar trough formed by rollers M and R, and is evenly squeezed by M-R. The varnish adheres to R. R and M squeeze to apply the oil on R to the paper. The scraper scrapes the excess varnish on the platen roller M into the oil receiving groove below and reuses it. The coating amount is determined by the pressure between M and R. This machine is characterized by fast glazing speed, simple machine structure, low price, and low technical operation. The disadvantage is that it is not suitable for thin paper, because the thin paper is easy to stick to the roller R and roll back. In some designs, the air knife can peel the paper from the roller R, so that the thin paper will not be taken away by the roller R, but it is not ideal for use. Another is that the coating amount is not easy to control, the coating is too thick, and the coating amount of varnish is not easy to adjust. The coating thickness can only be controlled by heating oil or adding a thinner.
The principle of gripping type glazing mechanism.
The principle of gripping 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 and the gripping paper platen L are pressed against each other to transfer the varnish to the paper to achieve coating. Advantages: Thin and thick paper is suitable, generally 80g /? Single-sided copper paper to 450g /? Whiteboard paper. The operation is simple, the structure is simple and easy to maintain. Due to the high pressure between rollers R and L, the ink leveling effect is better than the three roller type. Disadvantages: slow speed, generally 2,500 sheets / hour; many operators, at least 4 people; coating amount is not easy to control.
Four-roller anilox roller glazing machine structure.
The structure of the four-roller anilox glazing machine is shown in the figure. Comparing the three-roller coating machine, the four-roller type has an anilox roller (metering roller) and a corresponding scraper. The purpose of adding an anilox roller is to quantitatively control the amount of coating to ensure the consistency of the thickness of the film layer of the product. This type of model is generally used for long packaging, such as cigarettes, wine, soap, toothpaste, film and other product packaging boxes, which requires quantitative control of product quality to ensure the consistency of product quality. Another example is that a brand of soap box is produced in different periods, but placed on the same counter. If you use a three-roller glazing machine to glaze, the brightness of the surface of the box will be different. Batch production products.
Varnish with an offset press.
The methods and characteristics of coating with offset press are as follows:
1 The varnish is placed in a water tank and applied by a water roller.
2 Remove the water velvet cover of the water roller, and use the rubber roller to transmit the oil, the purpose is to reduce the amount of oil transmission.
3 Adjust the amount of water on the water roller, that is, adjust the amount of oil.
4 Widen the water tank so that all the ink rollers are on the water tank. Because the viscosity of the varnish is low, the oil on the water roller will drop down and fall on the paper below.
5 The cutting blanket is the same as the printed product width or slightly smaller than the printed product width.
6 UV oil and water-based varnish can be applied, but solvent-based thermal drying varnish cannot be applied, otherwise it will stick.
7 Use ordinary PS version and ordinary blanket.
8 The thickness of the coating oil layer should not be too large, otherwise pitting may occur.
9 Can not be partially polished, because the UV oil has no water ink separation characteristics for the PS version.
10 It is easy to partially polish by cutting the blanket.
11 When the coating needs to be thin, use the ink tank to polish.
If the workshop temperature is lower than 20 ℃, the coating thickness can be adjusted by adding thinner.
Sixth, the correct use of UV glazing oil UV oil is not a film, just like edible oil placed on the surface of the air will not form a dry film. UV oil sees ultraviolet light, such as sunlight, electric arc welding, printing light curing immediately. General fluorescent lamps, incandescent lamps, sodium lamps and other lighting sources will not cause curing. The UV oil contains photoinitiator and thinner, which has a certain stimulating effect on human skin.
In view of the above characteristics of UV oil, the following points should be noted when using:
①The normal use temperature of UV oil is 50 ℃ ~ 55 ℃. When using it, heat the oil in a constant temperature water bath to make the viscosity reach the design and use requirements, which is conducive to leveling and rapid curing.
② The UV oil passes through the area irradiated by the UV lamp. When the temperature is 50 ℃ ~ 60 ℃, the curing is fast and the adhesion is strong. But it is not that the lower the temperature under the UV lamp, the better.
â‘¢ The glazing machine should be placed in a position where direct sunlight cannot be applied, otherwise the UV varnish will be cured on the coating roller. If direct sunlight cannot be avoided, red and black curtains should be used to block the light.
â‘£ When glazing, the UV oil will disperse the ink on the glazing material into the UV oil to color the glazing oil, and it can be used after filtering and sedimentation, and does not need to be discarded.
⑤ When varnishing, if UV oil touches the skin, it should be washed away with soapy water immediately, otherwise the skin will be red and swollen and blistered.
â‘¥ The viscosity of UV oil varies with the type of glazing machine, and special glazing oil should be selected according to the model. If the viscosity required by the glazing machine is not achieved, thinner can be used to reduce the viscosity or tackifier to increase the viscosity. This method will lose the UV varnish curing speed, brightness, adhesion and so on.
⑦ UV oil flows to the rotating part of the drive shaft and will stick to the shaft at high temperature, affecting the drive, so pay attention to cleaning the shaft head.
According to the nature of the surface of the substrate, use special varnish, such as gold cardboard oil, PET special oil, etc.
Seven, UV curing time Theoretically, it can be strictly calculated how many photons a UV curing material needs to cure in a unit volume, and the photochemical reaction speed basically does not need much time. However, the addition of a polymerization inhibitor in UV oil or ink delays the molecular polymerization time (ie, curing time) after the photochemical reaction and is directly related to the photoinitiator and polymerization inhibitor.
In general UV varnish or ink formulation, the curing time is selected within 0.1 ~ 0.2s. But the specific time is determined by the following factors after the formula is determined:
â‘ Coating thickness. The surface of the coating cures quickly, but the surface layer strongly absorbs ultraviolet rays after curing, and the intensity of ultraviolet rays decays exponentially, reducing the curing speed. Therefore, the thin coating cures quickly, and vice versa.
â‘¡ UV intensity. The greater the strength, the shorter the curing time, and vice versa.
â‘¢ Add solvent. Adding thinner to UV varnish, such as ethanol, will reduce the curing speed.
④ Ambient temperature. The coating temperature is ideal ambient temperature when it is 50 ℃ ~ 70 ℃. The UV mercury lamp contains a lot of visible light and infrared components, which provides the ambient temperature for the coating surface. Therefore, when designing the curing machine, it is not that the lower the surface temperature of the material, the better.
⑤ The curing speed is fast with less oxygen. Oxygen has strong absorption of ultraviolet rays, and filling the surface with nitrogen will accelerate the curing speed.
The selection of specific curing time is related to the following aspects:
According to the characteristics of the coating machine, such as the screen printing machine, the printing speed is about 600 ~ 2000 sheets / hour, use a 350 mesh screen to apply UV varnish, and use a power density of 80W / cm lamp to meet the requirements. Use four-color offset printing machine to print UV offset printing ink, use three lamps with power density of 80W / cm, and the printing speed can reach 7000 sheets / hour. The flexo printing machine and gravure printing machine are coated with UV varnish. The number of screen lines is 165 lines of anilox roller. A 120W / cm lamp tube is used. The curing speed is 80W / min. Generally, the curing speed is faster than the coating speed.
According to the temperature resistance of the printed material. The temperature-resistant material should be cured quickly to avoid deformation of the material, such as plastic film, so fast that the coating just cures after the surface is exposed to light, but leaves the light before the temperature reaches the bottom, so that the material base does not deform. Whiteboard paper is easy to dehydrate and needs to be cured quickly.
8. Viscosity adjustment of UV varnish The UV varnish content is generally required to be 92% to 98%, of which there are few solvents. Solve the problem of improper viscosity from the formula, the viscosity is high and easy to achieve, just add the thickener, otherwise it is more difficult. The viscosity can also be adjusted by changing the ambient temperature. Generally, the roller and oil tank are heated to 55 degrees to reduce the viscosity. Another method is to add a thinner, but this method is not ideal. The thinner hinders curing, affects the brightness of the optical film, and the intermediate does not participate in curing, and ultimately wastes to cause environmental pollution.
9. Thickness of general UV coating:
The film thickness is related to the following aspects:
1. It is related to the gloss and density of the paper surface. The paper needs to be thin if it is bright, dense and not oily, but thick.
2. Related to customer's requirements. If it only needs to cover the ink, it needs to be thin, and if it needs three-dimensional effect, it needs to be thicker.
3. Related to glazing machine. Coating with offset presses and relief printing presses cannot be too thick because of the high pressure, and coating with a three-roll glazing machine cannot be thin because of low pressures. Coating with gravure and flexo printing machines can adjust the thickness quantitatively .
4. It is related to the operating speed of the coating machine. Such as flexo and gravure presses, when glazed, the speed is high, the oil is thin, and the anilox roll mesh number is high. The coating film layer is thin. If low mesh is used, the viscous oil solidifies before leveling, and the glaze layer is dot-like rather than a film layer.
Adjust the film thickness according to the above factors. Generally, the ink on the surface of coated paper is 1 ～ 4g / m², and the film thickness is 1 ～ 4μm; the ink used on the surface of white plate paper is 3 ～ 12g / m², and the thickness is 3 ～ 12μm.
Ten, the fuel consumption of UV varnishing UV varnish consumption can be determined according to the following factors:
1. Structure of glazing machine. Such as the simplest three-roll glazing machine, because the pressure of the roll is small and there is no uniform ink roller, the coating amount of varnish is relatively large, and the coating amount is often far beyond the required amount. The general coating amount is 6g /? The anilox roller is used for coating, and the coating amount is 2 g /? Or less. The coating amount of gripper glazing machine is 2 ～ 8g / ?. The coating amount of the screen machine can hit 50 g / ?. Generally, the local three-dimensional art is glazed, and the thickness of the glazing film is about 0.5 ?, which has a three-dimensional sense when touched by hand. Glue and embossing presses are coated with 1 to 3 g / ?.
2. The brightness of the paper surface. UV glazing should be said to be the icing on the cake, but it is only suitable for paper with high surface brightness or materials with poor varnish absorption on the surface. If the surface of the material is rough, it will seep into the lower layer immediately after the varnish is applied. There will not be a bright film on the surface, and there is no sense of glazing. Only the coated paper and whiteboard can be coated with UV oil. Copper card (glass card) oil coating amount of 2 ~ 3g /? The effect is ideal. Generally, the coated oil of coated paper is about 3 ~ 5 g / ?, and the amount of coated oil of white board is about 5 ~ 12 g / ?.
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