With the progress of our country's printing industry, the development momentum of China's ink manufacturing industry is very rapid. The technical indicators of the main products of domestic large-scale ink manufacturing enterprises have reached or are close to the advanced level of similar foreign products. Besides satisfying the needs of the domestic printing industry and the packaging and decoration industry, they are also exported to many countries and regions in the world and have achieved foreign exchange through exports. However, we must realize that compared with the international advanced level, China's ink manufacturing industry still has a certain gap, mainly reflected in the lag of production technology and packaging level, the poor consistency of the quality of domestic raw materials, research and development capabilities and analytical methods to be further improved. Poor environmental protection awareness. The key targets for the development of the ink industry in the future are commercial web offset inks, high-grade offset inks, environment-friendly inks and special inks. Offset UV ink is one of the special inks.
1. Characteristics of offset UV ink
Compared with ordinary inks, UV inks have good printability, color stability, overprinting effects, and lightness.
1 It has a fast curing speed and can be cured in a few seconds under ultraviolet light irradiation. Especially in packaging printing, it often needs to print a large area on the spot and requires a higher density, so it has to increase the amount of ink. When the drying problem becomes very important, the ink is not easy to dry and often affects the printing production efficiency. With the UV light-curable ink, this problem is solved.
2 UV ink can also prevent ink crusting, which solves the worries of offset presses when they stop.
3 Its conjunctival property is good, and it has good adhesion to all types of substrates. Since the UV ink contains no solvent, the thickness of the cured ink layer is basically the same as the thickness of the printing ink layer, and the unit weight of the UV ink is compared with the ordinary ink. , Printing area 30% -50%. No penetration when cured, can be printed on non-absorbent printing materials, such as card printing, etc. This is an area where ordinary offset printing ink can not be involved.
4 Because the UV ink does not need to be powdered during printing, the printing environment has been improved, avoiding the troubles caused by powder spraying to the post-press processing, such as the effect on coating and film coating, and Wired processing can be performed.
5 UV inks are brightly colored, and their scratch resistance, abrasion resistance, and acid and alkali resistance are better than ordinary inks. Therefore, although their price is much more expensive than ordinary inks, they are still widely used.
2. Composition of offset UV ink (UV)
UV inks are composed of binders, pigments, photosensitizers, and adjuvants.
1 The binder is an adhesive fluid that plays a linking role. It is composed of photo-curable resin and prepolymer, cross-linker (monomer cross-linker and pre-polymer cross-linker) and so on. The binder increases the fluidity of the ink, allowing the ink to pass from the ink fountain to the printing plate to the substrate, completing good ink transfer, and producing a chain polymerization under the action of a certain wavelength of light, cross-linking and curing to form a film, so that the pigment is firmly attached On the printed surface. The binder is the main flow part of the ink and the heart of the ink.
2 As a diluent, the cross-linking agent can reduce the viscosity of the ink, improve the printability, and play a role in crosslinking and curing in the photopolymerization reaction.
3Because there are some materials in the connecting material can not directly absorb the photon energy, it needs to add photosensitizer, also called photoinitiator, in the ink. Under the action of certain wavelength light, the photosensitizer molecule breaks down into two free radicals. The electrons in the molecule jump from the low energy level to the high energy level, and generate vibrations that split the unsaturated double bonds in the photo-curable resin and the photo-viscosity agent to produce a chain polymerization and further cross-link curing.
4 The auxiliary agent is an additional part of the ink except the main part, mainly wax materials such as microcrystalline wax, polyethylene wax, and the like. The auxiliary agent can reduce the viscosity of the ink and enhance the abrasion resistance of the ink film.
3. Precautions when using offset UV inks
1 When blending UV inks, the photocurable inks can be blended with each other, but cannot be mixed with ordinary inks. When printing, when ordinary ink is changed into ultraviolet ink, it is necessary to use a special cleaning agent to thoroughly clean the printing part of the machine, otherwise there will be poor ink drying and bad ink.
2 UV ink auxiliary materials are mainly remover, driers, hardeners, thinners and so on. The main function of the dissolving agent is to reduce the viscosity of the ink. The amount of the dissolving agent is generally 5% of the ink. Too much will cause the ink adhesion to decrease; the role of the driers is to accelerate the curing speed of the ink. The amount of the ink is generally ink. 2%, the role of hardener is to enhance the wear resistance of the ink, the amount of ink is generally 5% to 10%; the role of the diluent is to increase the flow of the ink, the amount of ink is generally 3% to 5%, the amount of Too much will cause poor drying of the ink film.
3 ink is full of the test method: the back of the print should be clean, no dirty, hand touch ink film is not sticky; with the thumb pressure under the pressure of 50N pressure, ink film no indentation, not broken; with nails hard strokes Ink film, no scratches.
4. Offset UV (UV) Ink Printing Process Requirements
1 color sequence arrangement: UV offset color arrangement problem is more complex, due to different colors of ultraviolet photon absorption is different, so the drying rate of different inks are not the same. Magenta ink has the best UV transmittance, followed by yellow ink, blue ink, and black ink. Since the black ink is a physics full absorption spectrum, the white ink is a total reflection spectrum, all affect the absorption of the UV light by the photosensitizer, thus affecting the drying speed, so its color arrangement should be white, black, cyan, yellow, product The red ink absorbs as many photons as possible in the light with poor light transmission and enhances its curing effect. Such color arrangement is particularly effective in monochrome or two-color printing. Because this is equivalent to adding a photo-curing opportunity to white ink and black ink, the drying effect is naturally good; in multi-color printing presses with multiple sets of UV lamp drying devices, this color sequence arrangement is also reasonable; In the case of color printing, the line or text version should be printed first. The dark inked, small-size field version should also be printed first. The large-area field version should be printed afterwards. This arrangement can achieve the highest ink volume and fuller color, and it can also avoid large prints. The ink film of the area was smeared and scratched.
2 fountain solution: multi-color offset printing machine with alcohol dampening device, to change the bucket fluid to a special UV bucket fluid, alcohol does not change. In monochrome offset printing, alcohol can be added to the water to reduce the amount of water, reduce the emulsification of the ink, reduce the surface tension of the ink, and increase the drying speed of the ink. The cleaning should be cleaned with a special UV car wash water. If there is no special UV car wash water, the effect will be good with absolute ethanol or xylene.
3 printing plate: As the UV ink and UV car wash water corrosive, ordinary PS version of the print force is very low, generally about 1000 to 2000 India, so use a special UVPS version or baked ordinary PS version. In the case of a temperature of 250°C, a conventional PS plate dried according to a conventional method needs to be baked for 10 to 15 minutes, and a CTP plate also needs to be baked for 10 to 15 minutes.
Selection of 4UV lamps: experiments proved that when the printing speed is 300 rpm, three 8KW UV lamps are used; when the printing speed is 6000 to 8000 rpm, six 8KW UV lamps can be used to complete the print dry. When the drying effect is not ideal, a photosensitizer may be added as appropriate, and the amount added depends on the degree of drying. The effective wavelength range of UV curing is 200-400nm. The wavelength of some light sources in the printing room is also within this range, which will cause the surface of the ink to slightly cure. Therefore, the effect of ambient light on the drying of UV ink should be taken into account.
Due to the special drying method of UV ink, it is suitable for printing absorptive and non-absorbing materials and other special materials, and the printing effect is excellent, and the printing performance is stable. With the resolution of UV printing technology, the future UV offset printing will be Have more room for development.