1. Principle
UV varnish is mainly composed of photoinitiator, prepolymer (oligomer), monomer (reactive diluent) auxiliary (leveling agent, plasticizer, antioxidant) and so on. Under the irradiation of ultraviolet light with the appropriate wavelength and light intensity, the photoinitiator is arrogantly generated, generating free radicals, which can then initiate the unsaturated groups (usually acrylic phthalate groups) on the prepolymer and monomer. The polymerization reaction will eventually change it from liquid to solid quickly, generally only 0.1-0.3s.
2. Craft
There are two main processes in the UV glazing process for printed products:
(1) Apply UV oil. In fact, printing itself is a coating process, whether it is relief printing, gravure printing, offset printing, or missing printing or spray printing, all of which are enough to apply ink to the substrate. The UV varnish can also be applied to the substrate by the above method, but Jia Qiu is dense on the surface of the substrate and does not penetrate the varnish, because the viscosity of the general varnish is much lower than that of the ink. Coated paper, whiteboard paper, gold cardboard, plastic film, metal, Kimura, etc. can be coated with UV oil.
(2) Curing. UV varnish is applied to the substrate. After a period of time, the varnish flows into the ultraviolet radiation area and solidifies. The curing process is a chemical cross-linking process, that is, the unsaturated carbon-carbon double bond (liquid) in the prepolymer and monomer receives the energy transmitted by the photoinitiator and cross-links to form a network structure (homogeneous). But the curing changes with the thickness, time and UV radiation intensity of the UV oil. The relationship between radiant energy transfer and the thickness of the UV oil is exponentially decreasing. For example, 90? Of UV energy is absorbed in the uppermost layer of 1μm, and only 9? In the lower layer of 2μm. If you want to get the same energy in the lower layer of 2μm, the initial UV irradiation energy should be increased by 10 times. Generally, the thickness of the coating layer is only 3-5 μm, which is very thin, and the upper and lower varnishes are cured at the same time. However, when the coating is thick (such as glazing with a wire mesh machine or roller coater), the upper layer will be cured and the lower layer will not be cured. The assimilation changes with time. The initiation time of a large number of photoinitiators is generally 0.1-0.3s, but a part of the remaining (incompletely reacted) photoinitiators will continue to react for 6-24h, the so-called post-polymerization reaction. If the light is too much at the beginning, the UV oil will become more and more brittle due to the post-polymerization reaction, so it is not easy to overexposure when UV glazing. When using a UV curing machine, generally use a lamp to illuminate, and adjust the distance between the lamp and the object to be illuminated (generally 5-15cm). If a lamp cannot be cured, you need to increase the exposure. The curing changes with the UV radiation intensity. When the UV intensity per unit area increases, the curing rate will accelerate, but it is not a linear relationship but increases several times.
Second, how to choose the glazing method and glazing machine
At present, the UV coating methods used by the domestic printing industry are as follows:
(L) Three-roller type, namely ink roller, coating roller and pressure roller. The advantage of this method is that it is fast and adjustable; the disadvantage is that it is not suitable for coating on thin paper. Although some glazing machines have air knives, the glazing effect on thin paper is not good, and the oil layer is thick. Therefore, this method is more suitable for cardboard.
(2) The gripper type, that is, the impression cylinder has gripper teeth, generally adopts the pressure cylinder of the 01 type printing machine. The advantage of this method is that both thin and thick paper are suitable, fuel saving, the thickness of the oil layer is easy to adjust, the diameter of the impression cylinder is fixed, and the pressure is large; the disadvantage is that the speed is not easy to adjust.
(3) Embossing method, using a relief printing machine as the coating head, followed by a grouping device, such as letterpress Heidelberg, Wofei, etc. The advantage is that it can be partially polished, the thickness of the oil layer is very easy to adjust, and the smoothness after polishing is good.
In addition, there are coating methods such as gravure printing machine, offset printing machine and anilox roller. In short, it can be printed and glazed, as long as a set of curing device is installed, the printing machine can be transformed into a glazing machine. The process of UV varnishing machine is generally continued paper ~ powder removal ~ electric spark treatment ~ priming oil ~ UV oil coating ~ light curing ~ cooling ~ collecting paper. Which type of glazing machine to choose should be based on the characteristics of the live source.
The following aspects should be considered:
(1) Is partial or complete glazing required? It should be said that all varnishes can be partially varnished, but not the other way round. Partial glazing generally has a plate roller, and a resin plate is attached. It can also be combined with embossing, gravure, silk screen, flexo printing machine and curing device to achieve local glazing.
(2) According to the paper type. Use a gripper glazing machine or a combination of lithography, embossing and screen printing glazing machine for thin coated paper. Thick paper can use three-roll glazing machine. The roll paper can only be used with a combination glazing machine equipped with a curing device on a rotary machine.
(3) According to the number of live sources. Small amount of work and miscellaneous paper types, use gripper glazing machine, large amount of work with flexo printing machine or rotary machine.
(4) According to the thickness of the coated varnish. The varnish coating is required to be thick, and when you can feel a certain thickness by hand touch (such as local art glazing), you can only use a silk screen glazing machine. If the coating is required to be thin and uniform, it is best to use a flexo printing machine.
3. Common glazing faults and troubleshooting methods
1. UV varnish is not compatible with ink. The specific performance is that the UV oil is coated on a certain ink base to bead, and it is like orange peel after curing, or although it can be coated evenly without beads, but if scraped with a sharp tool, the UV oil layer is separated from the ink layer. The solution is to apply a layer of primer and then apply UV oil. Or apply UV oil after treatment with electric spark, mix with alcohol infrared varnish in an appropriate amount.
2. It does not light after UV coating. The reason is that the surface brightness of the paper is not enough, or the surface is rough and not dense. The solution is to prime the oil first and then apply the UV oil. In addition, expired UV oil or UV oil quality problems will also make it not bright after glazing, at this time, you can try another UV oil.
3. The UV oil layer is brittle, and the creases are brittle when folded. The solution is to use urethane oil, indentation first when folding, and thin paper folded by hand instead of bamboo. This fault will also occur when the UV oil layer is too thick, and the amount of oil can be adjusted.
4. Alcoholic UV oil is easily dissolved by ethanol after curing. For example, after the UV oil is applied to the wine box, the wine bottle will break during transportation and the wine will dissolve the UV oil layer. The solution is to use UV oil that is insoluble in alcohol.
5. Too thick UV oil coating is neither economical nor can cause knee cracking. Solve the problem of heating the UV oil to 50 ~ 55C or add a thinner to the UV oil.
6. The UV oil bursts automatically after curing. The reason is that the UV light is excessive and the surface temperature of the illuminated object is too high. The solution is to increase the distance between the lamp tube and the surface of the illuminated object, so that the illuminated object is far away from the focus of the lampshade. After solidification, it can be cooled rapidly. It can be cooled with cold air or cold roller, or with air-cooled or water-cooled light source.
7. Precautions for using UV lamp:
(L) Keep the high temperature above 800C around the lamp tube to facilitate the vaporization and conduction of mercury molecules, but the object needs to be cooled. Therefore, this must be considered when designing lamps and cooling devices.
(2) When using a focus lampshade, do not place the subject on the focus position to avoid overexposure.
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