UV glazing - materials and process indicators
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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 take these questions to approach the UV.
UV is an abbreviation for ultraviolet ultraviolet in English. Ultraviolet light is a band in the electromagnetic spectrum. Waves having a wavelength of 100 nm to 400 nm are referred to as ultraviolet rays. What we generally call UV is actually the abbreviation of UV curing in printing, that is, the process of drying the printed matter under ultraviolet irradiation after printing.
First, the advantages and disadvantages of UV glazing:
advantage
1 There is no solvent in the formula, so there is no problem caused by solvent evaporation;
2 can be cured quickly at room temperature;
3 Reduce energy consumption. The curing speed is fast, so that the energy required per unit area is reduced;
4 can be coated with a heat sensitive substrate;
5 The coating process is simple and easy;
6 save space, much shorter than infrared hot drying oven;
7 The finished product can be stacked immediately;
8 Excellent coating performance.
Disadvantage
1 It is often difficult to formulate very low viscosity coatings without the use of hazardous monomers;
2 expensive;
3 Irritating to the skin;
4 UV light source will produce ozone;
5 The coating has low adhesion and shrinkage.
Second, the chemical composition of UV coating oil.
The chemical composition of UV coatings mainly includes radiation prepolymers, diluents and photoinitiators:
1. Radiation prepolymer. Prepolymers are chemical systems with residual unsaturated molecules. Such molecules, when under certain conditions, crosslink with other unsaturated molecules and change from a liquid to a solid coating. These unsaturated molecules are required to be stable before crosslinking and do not react with each other.
Prepolymer type:
1) epoxy acrylate;
2) an acrylated oil;
3) Acrylic urethane;
4) unsaturated polyester;
5) polyester acrylate;
6) Polyether acrylate and the like.
2. Thinner. Diluents are also chemical systems containing unsaturated molecules. It can be used to adjust the viscosity while being a film-forming substance. In some cases, the prepolymer may be substituted. In order to achieve a viscosity range, it is common to add a small amount of a volatile organic solvent to the mixture of the prepolymer and the diluent, but not more than 5% to 10%.
Diluents are active and inactive. The non-reactive diluent has two types of solvents and plasticizers. Solvents are primarily volatiles. Plasticizers impart a certain degree of flexibility to the cured coating, which not only reduces viscosity but also facilitates coating and leveling. The addition of reactive diluent to the prepolymer can change viscosity, tack, flexibility and hardness. The reactive diluent can be classified into a monofunctional group and a polyfunctional group.
Monofunctional reactive diluents, such as 2-ethylethyl acrylate, are flammable, volatile, and irritating to the skin. The polyfunctional reactive diluent has low volatility, high flash point and low irritation to the skin.
The role of reactive diluents is as follows:
1 adjust viscosity and leveling;
2 and the photoinitiator system determines the curing speed;
3 is linked with high molecular weight prepolymer molecules to accelerate curing;
4 Improve and improve the performance of the cured coating.
3. Photoinitiator. A photoinitiator is defined as a molecule that absorbs radiant energy and undergoes chemical changes to produce an active intermediate that initiates polymerization.
The photoinitiator does not participate in the curing, so the concentration of the photoinitiator is low, and the amount required: 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.
Photoinitiators are also indispensable in the initiation of polymerization. Photosensitive type refers to a chemical reaction that transfers energy to a photoinitiator molecule to initiate polymerization after absorption of light energy, and the photosensitizer returns to its original inactive state. The other, known as photoactive or photo-enhancing agents (usually amines), primarily enhances the rate of activation of the photoinitiator, which itself does not absorb radiation and does not initiate polymerization, but increases the rate of cure.
Third, the chemical process of UV glazing.
The basic composition of the photocurable material is a prepolymer, a diluent, a photoinitiator, an auxiliary, and a pigment. The curing chemistry of light is that the photoinitiator absorbs photon energy to excite, forming free radicals that initiate polymerization of the prepolymer and diluent (initiating molecular chain growth).
Fourth, the process of UV varnish on the surface of printed matter:
1. Remove powder. Generally, there are powder removing devices on the printing machine. When the printing ink is large, the powder is usually used to solve the problem of smudging. However, the powder adhered to the surface of the ink like sandpaper, and after applying UV varnish, it has no luster. Therefore, it is necessary to remove the powder. Removal method: first use the brush to sweep, the brush rolling direction is opposite to the paper moving direction; sweep the powder and then suck it out through the suction duct. When the powder and the ink are mixed into a granular form and cannot be swept down, the surface of the paper can be pressed with two light rollers to flatten the toner particles.
2. Corona treatment. Some printing materials, such as PP or PET, have poor affinity for UV varnish. Some inks contain wax and are not compatible with UV oil. They can be treated with high pressure discharge to improve affinity and can be evenly mixed with UV oil. Attached.
3. Base oil. For oil-bleed paper, the milky white base oil should be applied first to seal the pores of the paper to improve the whiteness, and then the UV oil is best. For example, if the surface coating layer is thinner, apply UV varnish directly. The oil will quickly penetrate into the bottom layer, and no light will come out. Instead, the bottom gray color will be reflected, so that the paper will be blue after glazing. If you first apply a layer of bottom, then apply UV oil to solve this problem. The base oil is generally of a hot drying type.
4. UV glazing. Apply UV varnish.
5. Cool down. The UV-cured print is blown with a fan to lower the surface temperature. The purpose is to avoid paper deformation and to prevent post-cure from continuing.
Fifth,There are four main ways in which we currently have UV glazing:
The principle of three-roll coating.
The principle of three-roll coating is shown in the figure. The UV varnish flows into the vinegar tank formed by the rolls M and R, is uniformly extruded by M-R, the varnish is attached to R, and the R and M presses apply the oil on R to the paper. The scraper scrapes the excess varnish on the platen roller M into the lower oil sump and reuses it. The amount of coating is determined by the pressure between M and R. This machine features 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 tends to stick to the roll R and roll back. There are also some designs. The air knife can peel the paper from the roller R, so that the tissue is not taken up by the roller R, but it is not ideal for use. Then there is the coating amount is not easy to control, the coating is too thick, the amount of varnish coating is not easy to adjust, and the thickness of the glazing can only be controlled by heating oil or adding a thinner.
The principle of the enamel glazing mechanism.
The principle of the enamel 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 crepe paper cylinder L to transfer the varnish to the paper to achieve coating. Advantages: thin and thick paper is suitable, generally 80g/m2 single-sided copper paper to 450g/m2 white paper. Easy to operate, simple and easy to maintain. Since the pressure between the rolls R and L is large, the leveling effect is better than that of the three rolls. Disadvantages: slow speed, generally 2500 sheets / hour; more operators, at least 4 people; coating amount is not easy to control.
Four-roll anilox roller glazing structure.
The structure of the four-roll anilox roller glazing machine is shown in the figure. Comparing the three-roll coater, the four-roller type has an anilox roll (metering roll), and a corresponding scraper is added. The purpose of adding an anilox roll is to quantitatively control the amount of coating to ensure uniformity of the thickness of the coating layer on the product. Generally, long-term packaging uses such models, such as cigarettes, wine, soap, toothpaste, film and other product packaging boxes to quantitatively control product quality and ensure product quality consistency. Another example is that a certain brand of soap box is produced in different periods, but it is placed on the same counter. If a three-roller glazing machine is used, the brightness of the surface of the box will be different, and with the anilox roller glazing, it is impossible to tell which Batch produced products.
Gloss with an offset press.
The methods and features of varnishing with an offset press are as follows:
1 The varnish is placed in a water tank and coated by a water roller.
2 Remove the water velvet cover of the water roller and transfer the oil with a rubber roller to reduce the oil transfer.
3 Adjust the amount of water on the water roller, that is, adjust the amount of oil.
4 Widen the sink so that all the rollers are on top of the sink. Because the viscosity of the varnish is low, the oil on the water roller will drop down on the paper below.
5 The cut blanket is the same width as the print or slightly smaller than the print width.
6 Can apply UV oil, water-based varnish, but can not apply solvent-based hot-drying varnish, otherwise it sticks.
7 Use ordinary PS version, ordinary blanket.
8 The thickness of the coating oil layer should not be too large, otherwise pitting will occur.
9 Cannot be partially glazed because of the separation characteristics of UV oil on the PS version of waterless ink.
10 Simple partial glazing by cutting the blanket.
11 When the coating is required to be thin, the ink tank can be used for glazing.
If the shop temperature is below 20 ° C, the coating thickness can be adjusted by adding a diluent.
Sixth, the correct use of UV coating oil
UV oil is not a conjunctival, just like an edible oil placed on the surface of the air does not form a dry film. UV oil sees ultraviolet light, such as sunlight, electric arc welding, and printing light. Generally, illumination sources such as fluorescent lamps, incandescent lamps, and sodium lamps do not cause curing. UV oil contains photoinitiators and thinners, which have certain stimulating effects on human skin.
In view of the above characteristics of UV oil, the following points should be noted when using:
1 UV oil normal use temperature is 50 ° C ~ 55 ° C, use the constant temperature water bath to heat the oil cycle, so that the viscosity meets the design requirements, which is conducive to leveling and rapid curing.
2 UV oil passes through the UV lamp irradiation area. When the temperature is 50°C~60°C, the curing is fast and the adhesion is strong. But not the lower the temperature of the UV lamp, the better.
3 The glazing machine is placed in a position where the sunlight cannot be directly exposed. Otherwise, the UV varnish is solidified on the coating roller. If it is not possible to avoid direct sunlight, the red and black curtains are used to block the light.
4 When glazing, the UV oil will disperse the ink on the glazing material into the UV oil, so that the glazing oil is colored, and it can be used after being filtered and precipitated, and does not have to be discarded.
5 When glazing is applied, the UV oil should be washed off with soapy water immediately, otherwise the skin will be red and swollen.
6 UV oil viscosity varies with the type of glazing machine, special varnish should be selected according to the model. If the viscosity required by the glazing machine is not reached, it can be viscous with a thinner or tackified with a tackifier. This method loses UV varnish cure speed, brightness, adhesion, and the like.
7 UV oil flows to the rotating part of the drive shaft and sticks to the shaft at high temperatures, affecting the drive, so pay attention to cleaning the shaft head.
According to the nature of the surface of the substrate, special varnish, such as gold cardboard oil, PET special oil, etc., are selected.
Seventh, UV curing time
Theoretically, it is possible to strictly calculate how many photons need to be cured by the UV curable material per unit volume, and the photochemical reaction rate does not require much time. However, the addition of a polymerization inhibitor to the UV oil or ink retards the molecular polymerization time (ie, curing time) after the photochemical reaction is directly related to the photoinitiator and the polymerization inhibitor.
In general UV varnish or ink formulation, the curing time is selected within 0.1~0.2s. However, the specific time is determined by the following factors after the formulation is determined:
1 Coating thickness. The surface of the coating cures quickly, but the surface layer strongly absorbs ultraviolet light after curing, and the ultraviolet intensity is exponentially attenuated, reducing the curing speed. Therefore, the coating is thin and fast, and vice versa.
2 UV intensity. If the strength is large, the curing time is short, and vice versa.
3 Add solvent. The addition of a diluent such as ethanol to the UV varnish reduces the rate of cure.
4 Ambient temperature. The coating temperature is ideal for ambient temperatures from 50 ° C to 70 ° C. UV mercury lamps contain a large amount of visible and infrared components that provide an ambient temperature for the surface of the coating. Therefore, when designing the curing machine, the lower the surface temperature of the material, the better.
5 Oxygen cures less quickly. Oxygen has a strong absorption of ultraviolet light, and the surface of the coating is filled with nitrogen to accelerate the curing speed.
The choice of specific curing time is related to the following aspects:
According to the characteristics of the coating machine, such as screen printing machine, the printing speed is about 600~2000 sheets/hour, and the UV varnish is coated with 350 mesh net, and one lamp with a power density of 80W/cm can meet the requirements. The UV offset printing ink was printed by a four-color offset printing machine with three lamps of power density of 80 W/cm and a printing speed of 7,000 sheets/hour. The flexo printing machine and the gravure printing machine are coated with UV varnish. The number of the wire mesh is 165 wire anilox roller, and a 120W/cm lamp tube is used, and the curing speed is 80 W/min. The curing speed is generally chosen to be faster than the coating speed.
According to the temperature resistance of printed materials. The temperature-resistant material does not cure quickly, so as to avoid deformation of the material, such as plastic film, the coating is just as fast as the surface is exposed to light, but the temperature is not transmitted to the bottom to leave the light, so that the material base layer is not deformed. White paper is easy to dehydrate and needs to be solidified quickly.
Eighth, the adjustment of the viscosity of UV coating oil
The UV varnish content is generally required to be between 92% and 98%, with little solvent. Solve the problem of improper viscosity from the formula, the viscosity is high and easy to achieve, just add the tackifier, and vice versa. The viscosity can also be adjusted by changing the ambient temperature. Generally, the roller and the storage tank are heated to 55 degrees to lower the viscosity. Another method is to add a diluent, but this method is not ideal, the diluent hinders the curing, affects the brightness of the film, and the intermediate does not participate in the curing, and finally wastes environmental pollution.
Ninth, general UV coating thickness:
The film thickness is related to the following aspects:
1. It is related to the gloss and density of the paper surface. If the paper is bright, dense, and does not seep, the film layer needs to be thin and vice versa.
2. Relevant to the customer's request. If you only need to cover the ink, it should be thin. If you want to make a stereo effect, you need to be thicker.
3. Related to the glazing machine. Coating with offset press and embossing machine may not be too thick, because the pressure is large, and it is impossible to coat with a three-roll glazing machine because the pressure is small, and the thickness can be adjusted by coating with a gravure press or a flexo press. .
4. It is related to the operating speed of the coater. For example, when the flexo printing and gravure printing machines are glazed, the speed is high, the oil is thin, and the number of mesh rolls is high, and the coating film layer is thin. If a low mesh is used, the viscous oil will solidify without leveling, and the glazing layer is a dot-like layer.
The film thickness is adjusted according to the above factors. Generally, the ink surface of the copper paper is 1 to 4 g/m 2 and the film thickness is 1 to 4 μm; the surface of the white paper is 3 to 12 g/m 2 and the thickness is 3 to 12 μm.
Tenth, Fuel consumption of UV glazing
The amount of UV varnish can be determined according to the following factors:
1. The structure of the glazing machine. For example, the simplest three-roller glazing machine has a small roller pressure and no uniform ink roller. Therefore, the varnish coating amount is relatively large, and the coating amount is often far exceed the required amount, and the general coating amount is 6 g/m 2 or more. The glazing is applied by an anilox roll, and the coating amount is 2 g/m2 or less. The coating amount of the enamel coating machine is 2 to 8 g/m2. The screen machine can be applied in an amount of 50 g/m2. Generally, the partial three-dimensional art glazing, the thickness of the glazing film layer is about 0.5 mm, and the three-dimensional feeling is touched by hand. The glue and convex printing machine are coated with glazing 1~3 g/m2.
2. The brightness of the surface of the paper. UV glazing should be said to be the icing on the cake, but it is only suitable for papers with high surface brightness or materials with poor surface absorbing properties. If the surface of the material is rough, the varnish will penetrate into the lower layer immediately after it is applied, and there will be no bright film on the surface, and there is no sense of glazing. The paper material is only coated paper and white paper can be coated with UV oil. Copper card (glass card) oil coating 2 ~ 3g / m2 effect is very good. Generally, coated paper coated oil is about 3 to 5 g/m2, and white paperboard is oiled at about 5 to 12 g/m2.