Introduction to UV technology for printing

　　UV is the abbreviation of English UltravioletRays, that is, ultraviolet rays. Ultraviolet (UV) is invisible to the naked eye. It is a section of electromagnetic radiation other than visible violet light, with a wavelength in the range of 210-400nm.

　　 The technology of applying UV in the printing process is to use special ink to quickly form a three-dimensional network of macromolecules under the irradiation of ultraviolet light in a certain wavelength range, with stable physical and chemical properties and high surface brightness.

　　The status quo and development of printing application UV technology

　　 The first UV ink in the world was developed in the United States, probably in the early 1960s. In the late 1960s, UV inks had already begun industrial production in the United States, when they were mainly developed to meet the requirements of environmental pollution regulations. Japan started the industrial production of UV inks and manufactured UV ink printing equipment in the early 1970s.

　　 my country began to introduce UV glazing and UV ink printing technology and equipment in the 1980s. At the same time, Tianjin Ink Factory and other domestic ink companies also developed UV inks and glazing oils. So far, China has developed into the country with the largest installed capacity of UV printing presses and UV ink usage in the world.

The application of UV technology in the tin-printing can industry is mainly marked by the introduction of the world’s most advanced metalstar2 six-color UV iron printing machine from KBAMetalprint by Shanghai Baoyin Metal Color Coating Co., Ltd. in 2004, followed by the introduction of Hangzhou COFCO Packaging Co., Ltd. British Queli company's 5-color UV iron printing machine, UV iron printing technology has entered a new development period in China.

　　Traditional printing process and control points

　　1. Coating

　　 Coating is required before printing. Under normal circumstances, the front side of the tinplate needs to be coated and printed, and the inner side needs to be coated with paint to ensure that the printed iron products have good canning, canning and appearance decoration requirements. The coatings are classified into oily, phenolic, epoxy, polyester, vinyl, acrylic, amino, etc. according to the resin. According to the purpose, they are divided into internal coatings, external coatings, color coatings, varnish coatings, special coatings, etc.

　　 2. Plate making

　　 The plate making of iron printing is basically the same as that of ordinary offset plate making. However, since iron printing products must be made into cans in the end, the iron printing plate making must meet the requirements of can making, and the basic characteristics of metal packaging containers must be understood clearly when making plates. For example, the oil tin cans, the requirements of resistance welding should be considered when making plates, and there should be a certain distance between the patterns, so that the product can be easily cut, high-frequency welding and can making needs after iron printing processing, such as batteries shell. It is necessary to arrange as many numbers as possible within the specified tinplate size range, and to facilitate the cutting and punching of the battery casing. In addition, for candy boxes, it is necessary to understand the size of the blanking knife edge of the lid, and leave a certain amount of residual material accordingly, otherwise it will bring trouble to the punching of the lid

　　 3. print

　　The tinplate can be printed on the machine only after the inner surface is coated and the outer surface is coated (except for special processes). Iron printing machines are divided into three types: single color machine, double color machine and multi-color machine. At present, the imported machines used in China are mainly the two-color iron printing machine of British Queli Company and the two-color iron printing machine of Japan Fuji Company. The domestic machine mainly adopts the monochrome iron printing machine of Shanghai Goss Printing Equipment Co., Ltd. The structural principle of iron printing machine is similar to offset printing machine. When printing, water and ink balance adjustment, printing pressure adjustment and overprinting accuracy adjustment are required. Because the surface of tinplate is different from paper, it does not have the absorbency of paper pores, so large-area wet-pressure wet multi-color overprinting cannot be arranged when considering the color sequence , We can only implement wet pressure wet process for less than 50% of the outlets. When arranging the color sequence, it is generally necessary to comprehensively consider the main color of the layout, the accuracy of overprinting, and the pros and cons of the operation, and make specific analysis based on the characteristics of the product and the performance of the machine.

　　4. Glazing

　　After finishing all the color registration, a layer of bright oil should be applied to the printed iron surface to increase the gloss of the printed product surface, and at the same time, it can also increase the surface hardness of the product, so that the printed ink layer has a certain degree of flexibility and corrosion resistance.

　　5. dry

　　 Both paint and ink need to be dried in a tunnel oven. The drying condition of the iron printing ink is usually between 130°C and 160°C, and the time is controlled at about 10-12 minutes. The drying condition of the coating is between 170°C and 210°C. The tunnel oven is about 30 meters long. It consists of 3 stages, namely the heating zone, constant temperature zone and cooling zone. The temperature rise is the first stage after the tinplate enters the oven. The main function is to raise the tinplate from room temperature to the set temperature. The main function of the constant temperature zone is to maintain the temperature reached by the heating zone. The function of the cooling zone is to fully cool the dried tinplate prints. Once the cooling is not enough, the paint film is easily scratched when the tinplate is stacked, and the stacking fork is prone to sticking.

　　 Application of UV Technology in Printing Iron Cans

　　1. Application prospect

　　 Today, when environmental protection requirements are getting higher and higher, the environmental protection of UV technology is also fully understood. More and more iron-printing can making companies adopt UV printing technology to meet increasingly stringent environmental protection requirements. In addition, UV technology can also save considerable costs for companies. For example, after UV technology is adopted, the drying process is not required (traditional iron printing processes must have a drying room of more than 30 meters to keep the ink at 150 degrees. drying). Due to the non-absorption of tinplate and the coating on the tinplate, multi-color printing like the printing paper process cannot be used. Therefore, the traditional printing iron production line is mostly a single-color or two-color machine. The production of a product often requires printing several times. , After adopting UV technology, four colors or even six colors can be used for simultaneous printing, so that a product can be printed at one time, which can greatly improve efficiency.

　　 Due to the advanced nature of UV technology and its applicability to the iron-printing can industry, since 2004, COFCO Packaging transformed the original traditional iron-printing line into a UV line, and the development of UV technology in the iron-printing can industry has entered the fast lane.

　　 2. UV printing iron material

　　The materials currently used in UV printing technology are mainly inks, coatings and related auxiliary materials. Since UV inks and coatings are quite different from traditional inks and coatings, I will introduce them in detail here.

The main components of UV inks and varnishes are polymerizable prepolymers, photosensitive monomers, and photoinitiators, and auxiliary components are coloring pigments, fillers, and additives (leveling agents, defoamers, polymerization inhibitors, slip agents) Wait.

　　 UV ink for iron printing has high storage requirements. It needs to be stored in an environment of 5-25°C and the ink cannot be exposed to the air; heat sources and sunlight should be avoided.

The technical indicators of Printing Iron UV Varnish 971-0461 mainly include: Main ingredient: UV acrylic; solid content: 97.5±2.5% (ASTMD-5403; METHOD); VOC: 0.08Kg/L; viscosity: 65-95 seconds (25 ℃@Ford4 cup); curing conditions: 3300±75mj/cm2; film thickness: 5.4-7.0gsm; preheating varnish temperature: 32-38℃; flash point: 93℃ (closed cup); film performance: Good adhesion and mechanical processing performance, high gloss, high hardness, excellent surface slip properties.

　　 3. UV printing equipment

　　The main difference between UV printing equipment and traditional printing equipment is that there are UV ultraviolet drying equipment between each printing unit, which has less track-type drying room than traditional equipment.

　　UV printing machine

　　4. UV printing process

　　 Compared with the traditional printing process, the UV printing process has less thermal drying process. The UV ink is dried by ultraviolet light, while the traditional ink must be dried at a high temperature of about 150 degrees.

　　Because the ink of each unit of UV printing can be dried under ultraviolet light, it can be printed in multiple colors together. When arranging the color sequence, there is no need to consider the problem of wet pressing in the traditional iron printing process, which can be freely arranged according to the actual situation.

　　5. Common problems of UV printing technology

　　ア. Poor drying, incomplete light curing, sticky surface

　　 Causes: insufficient UV light intensity; UV lamp aging and weakening; UV ink storage time is too long; too much mixing oil is added; machine speed is too fast.

　　 Solution: When the curing speed is required to be less than 0.5s, it is necessary to ensure that the power of the high-pressure mercury lamp is generally not less than 120w/cm, and the lamp tube should be updated in time, and do not wait for it to be broken. If necessary, add a certain amount of UV varnish curing accelerator to accelerate drying.

　　B. The UV varnish on the surface of the printed matter cannot be applied, and it is blurred

　　 Causes: UV varnish has low viscosity and too thin coating; ink contains too much varnish or dry oil; ink surface has crystallized; ink surface anti-sticking materials (silicone oil, powder spray)

　　 Solution: When printing products that require UV glazing, corresponding measures must be taken to create conditions and apply a thicker UV varnish appropriately. If necessary, the problem can be solved by applying primer or replacing special varnish.

　　C. Big residual smell

　　 Causes: incomplete drying and curing; insufficient UV light source or aging of the lamp; UV varnish has poor anti-oxidant interference; excessive addition of non-reactive diluent in UV varnish.

　　Solution: UV ink and varnish must be dried and cured thoroughly. And to strengthen the ventilation, and replace the UV varnish when necessary.

　　 Comparison of traditional printing iron process and UV technology process

　　 UV printing iron technology has many advantages compared with traditional printing iron:

　　Reduce costs: UV metallic inks can reduce production costs and save energy. Using UV ink, the drying room part of the traditional printing machine is no longer needed. After drying with UV lamps, UV inks can dry quickly. This alone saves the company the cost of building a drying room, and also saves space in plant design.

　　 Speed ​​up: the use of UV metal ink can speed up the printing speed. Nowadays, ordinary printing machines usually have a speed of about 4000 sheets/hour, and after using UV technology, the speed can reach 6000 sheets/hour -7000 sheets/hour. In addition, because ordinary metal inks need to be dried, when printing 6-color finished products, if you use a two-color printing machine, you need to go through three drying rooms. If it is a monochrome printing machine, it will go through six drying rooms. After using UV technology, the time saved can speed up the production cycle and greatly improve production efficiency.

Green and environmental protection: The environmental performance of UV metal ink is its excellent quality. Because there are no volatile substances in UV ink, it solves the problem of residual solvents. Therefore, many companies choose this ink for products exported to Europe. . In particular, UV metallic ink is brighter and brighter than traditional ink after coating with varnish.

　　 At the same time, the use of UV printing technology also has its disadvantages, such as the need to adjust the machine. UV inks are more expensive than ordinary inks. In addition, because UV metallic inks are easily affected by ultraviolet rays, the shelf life is short. After opening and using, it should be used up as soon as possible, otherwise it can only be discarded.