UV ink, also known as ultraviolet drying ink, refers to ink that can change from a liquid state to a solid state under the irradiation of a certain wavelength of light. UV lithography refers to a printing method that applies UV inks and various auxiliary materials to lithographic offset printing presses, uses cross-linking polymerization under the irradiation of ultraviolet lamps, and instantly cures into a film.
UV printing started in 1946. It’s only developed in the last two or three decades. The world’s first UV lithography was used in the United States, followed by the United Kingdom in 1970.
Advantages of UV printing:
① High color saturation and good color effect.
② The printed pattern has high gloss and beautiful visual appearance.
③ It is suitable for many types of printed materials with completely different characteristics and chemical properties, including paper, cardboard, plastic, PVC, PE, PP, 3D, etc. paper and film.
④ It can be dried immediately when printing, which can greatly improve production efficiency and save production cycle.
⑤ Because it is dry immediately during printing, dusting is not needed, which can improve the quality problems caused by dusting and can be avoided.
⑥ Because it dries immediately during printing, no color change will occur after printing.
The resistance of the imprint is better.
Abrasion resistance is better.
⑨ belongs to environmental protection technology and is more friendly to environmental protection.
Disadvantages of UV printing:
① The cost of equipment and equipment for lithographic UV printing is high.
② The types of markets that can use lithographic UV printing are wider and more dispersed than the traditional lithographic brush market. Therefore, if manufacturers want to invest in the development of this technology, they should first carry out market analysis and research to avoid the difficulty of finding orders after investing in equipment.
③ The printing ink cost of lithographic UV printing is much higher than that of traditional printing, and it requires higher production technology, process technology and cost control.
④ The ink storage area needs to be controlled strictly, and it needs to be stored below 20 ℃, so the storage cost is high.
⑤ Difficulty in manpower supply familiar with operation technology
03 curing principle
UV inks selectively absorb UV light. Its drying is affected by the total energy radiated by the UV light source and the light energy distribution at different wavelengths. Under the irradiation of UV light, the photopolymerization initiator in UV ink absorbs photons of a certain wavelength, the excited state of the activator forms free radicals or ions, and then the polymerizable prepolymer and the photosensitive monomer are transferred through the intermolecular energy transfer. Polymers such as polymers become excited states, generating charge transfer conjugates. These conjugates are continuously cross-linked and polymerized and cured to form a film.
04 Related Materials
UV light source
① Structure of UV light source: It is usually composed of light boxes, tubes, reflectors, power supplies, controllers and cooling devices.
② UV light source is a device that emits UV light in a UV curing system.
③ According to the different contents of the tube, it can be divided into metal halogen lamps, high-pressure mercury lamps and odorless lamps.
④ Its performance parameters mainly include: arc length, characteristic spectrum, power, working voltage, working current and average life.
⑤ Although the UV light source emits mainly UV light, it is not light of a single wavelength, but light in a band. Different light sources have different wavelength ranges of emitted light. The distribution of spectral energy in the band is different. The UV light source radiates light from a band network, and the energy distribution of light at different wavelengths is different. Among them, the energy distribution of light having a wavelength of 300 to 310 nm and 360 to 390 nm is better.
① Special UV ink roller must be used for UV printing, otherwise the ink roller will be swollen, sticky, peeling and surface chipping.
② Generally made of printed EPDM material, but can not be refined with petroleum, should buy a special lotion.
③ If the UV ink and ordinary ink are switched, it must be cleaned very much each time to remove all the chemicals that may remain.
① When printing with UV, be sure to use a UV blanket, otherwise the surface of the blanket will become swollen, sticky, delaminated, and surface cracked.
② Generally, blankets printed with EPDM are used, but they may not be suitable for general ink printing. When changing to ordinary printing, you must replace the UV blankets.
③ lotion UV blankets can not be used in petroleum refining lotion, but only dedicated lotion.
④ Every time the leather cloth is removed, it must be cleaned to remove all chemicals that can be leftover to prevent the leather cloth from aging.
①Special PS plate for UV printing.
② If it is ordinary PS plate material, it must be processed through baking process to be resistant to printing.
05 Method for judging substrate adhesion
Dyne Pen Test
Dyne pen is a kind of colored water pen with felt tip, draw a wet mark on the surface of the substrate, and then observe whether the wet mark will show a spherical oil film. How long will it take to form a spherical oil film, and This method is used to judge the adhesion performance of UV ink on the substrate. The disadvantage of this method is that the test pen is easily soiled after using it for many times, which affects its accuracy. There are logos on the shell of the dyne pen, which are 38 dyne value, 39 dyne value, 40 dyne value and so on.
Test method for scratch resistance
Apply moderate pressure with the nails of your thumb and repeatedly scrape at the ink layer after printing and dried on the substrate. If the ink layer is scratched or the ink is peeling off, it indicates that the ink adhesion is not ideal.
Test method for resistance to tape peeling
Use adhesive tape to evenly adhere to the printed sample, and press it on the back of the tape with your fingers to make it stick firmly. Then, grasp the two ends of the tape with your hands and pull it at a medium speed to observe how the ink is stuck by the tape. It shows that the adhesion of the ink on the substrate is not good.
Test method for resistance to dry friction
Put the printed surfaces of the two printed samples on top of each other and rub them against each other with a pressure of 2-4Lb (1Lb = 0.4535920kg). If there are scratches or the ink film is peeled off, the ink adhesion is poor.
Test method for resistance to wet friction
The method is similar to the dry friction resistance test described above, except that a little water is required between the two printed samples.
Test method for detergents
Put a drop or a few drops of alkaline detergent on the printed sample. After 15C, suck it with a paper towel or a clean soft cloth, and then wipe it with a moderate pressure. If there is any change in the printed text, it indicates the adhesion of the ink. not good.
Dyne value Dyne value is derived from dyne. Dyne is a unit of force. Generally speaking, the surface tension and dyne value are commonly called, and the surface tension coefficient should be accurate. The definition is the force that pulls between two adjacent parts of a liquid surface within a unit length. The unit of surface tension is Newton / meter (N / m) in SI system, but dyne / cm (dyn / cm) is still commonly used, and 1dyn / cm = 1mN / m. The dyne value is actually the common name of dyne / cm, which expresses the size of the surface tension coefficient.
① Dyne value actually refers to surface tension, and surface tension and surface energy are two different concepts, but the values of the two are equal, so surface tension and surface energy can be expressed in dyne / cm, that is, dyne Factor value.
② Because surface capacity is one of the forms of free energy, according to the principle of minimum free energy of thermodynamics, the liquid surface energy has a tendency to automatically decrease, that is, by automatically reducing, its surface area S or from the surrounding medium through the liquid surface It is realized by automatically absorbing the adsorption of other substances. The relationship between the solid surface and its internal molecules is completely similar to that of the liquid, except that the shape of the solid surface is constant. The reduction of surface energy can only be achieved by adsorption of gas or liquid molecules on it.
③ The substrates we use all have a certain surface tension, that is, the dyne value, which is its own physical characteristics.
④ From the surface state, the ink is not easy to adhere, mainly because the free energy on the surface of the substrate is too low.
⑤ Theoretically, in order to obtain good printing suitability on the substrate, the surface tension of the printing ink must be lower than the surface tension of the substrate. The current printing ink must meet the corresponding requirements of printing, which is to pre-process the substrate, so that Its surface free energy can reach more than 36 dyne / cm2, thereby enhancing its affinity for printing ink.
⑥ The surface of the substrate with low ink transfer performance is treated to produce an oleophilic layer, which further improves the transferability of the ink.
⑦The substrate we use in practice, the substrate with a rough surface has a stronger adsorption capacity than a substrate with a smooth surface. The rougher the surface of the substrate, the larger its adsorption capacity, which increases the contact area between it and the printing ink. It also increases its surface tension in the unit. On the contrary, the smoother the surface, the smaller its adsorption capacity, and sometimes it will not be printed at all, and the critical zero value.
02How to determine the dyne value of the substrate
In actual production, we judge the dyne value of the printed materials by using a dyne pen to test. Generally, the dyne value should reach more than 38 dyne / cm. To be a reverse UV product, the Dyne value on the surface of gold and silver cards must reach 40 or more.
03 The dyne value of the substrate is fixed
If the dyne value is increased, the surface is treated to increase its dyne value, which is greater than the dyne value of the ink. At this time, it can meet the printing requirements. Dyne value (substrate)> dyne Value (printing ink), you can print, otherwise it does not adsorb, the adhesion is extremely poor, and there are adverse consequences such as pulling ink, not applying ink, peeling, and cracking.
The dyne value of a printing surface is related to its own roughness, hardness, surface strength, gloss, wetness and other properties.
Dyne pen Also known as surface tension test pen, Dyne pen, corona treatment pen, and plastic film surface tension detection pen. Dyne pen is a test tool for the corona degree (Dyne) of the film surface, which is specially used to determine the effect of the film after corona treatment. Dyne pen has 32, 34, 36, 38, 40, 42, 44, 46, 48, 50, 52, 54, 56, 58, 60 or more test pens with more than ten different tensions, which can accurately test plastic Whether the film surface tension reaches the value of the test pen. Let users know clearly whether this film is suitable for printing, laminating or vacuum aluminizing, etc., so as to effectively control the quality and reduce the loss caused by unqualified materials. Suitable for gravure printing, offset printing, UV printing. 01 How to use
With the dyne pen perpendicular to the plane of the film, apply a suitable pressure to draw a line on the surface of the film. Dyne pens with a smaller range are easier to draw on a straight line, so there is no need for too much pressure; while 40, 42, 44 Dyne pens need to apply more pressure when drawing lines. Under normal circumstances, to ensure the accuracy of the initial test, 6 different types of dyne pens are required; if it is determined that the surface tension of the film has a small change in number, at least 3 different types of dyne pens are required.
02 result analysis
1) If the corona-treated film is evenly distributed without drawing any beads, it means that the dyne on the surface of the film is higher than or equal to the index marked on the dyne pen
2) If the film without proper corona treatment draws a line slowly, it means that the surface of the film is dyne, which is lower than the index marked on the dyne pen.
3) If the film without corona treatment immediately shrinks and forms beads, it indicates that the surface dyne of the film is extremely lower than the index marked by the dyne pen.