1. Printing process
The current popular scratch-off printing process is: special paper for bills → printing anti-light-transmitting ink on the back side → printing invoice graphics on the front side → printing tax stamps → printing invoice numbers and passwords → applying isolation oil → printing silver paste Covering layer → finished product.
Compared with ordinary paper, special invoice paper has higher opacity and tensile strength, and has good adaptability to various printing methods. In order to prevent information leakage in the award-winning area during light transmission observation, a layer of anti-light-transmitting ink should be printed on the corresponding position of the scratch area on the back of the invoice, generally printed in black. Then, the device prints the front content through the automatic reversing device. When printing on the front, the shading and ordinary text are printed first, and then the tax stamp is printed with spot color fluorescent ink. This is done by different color groups of the same printing machine. The number of the invoice and the content of the award area are completed by a digital printing system. After the content of the award area is sprayed, a layer of isolation oil is printed on the corresponding position. Finally, a silver paste overlay is printed on the isolation oil. The isolation oil is printed to ensure the silver paste. The cover layer can be easily scraped off. This completes the printing of a scratch-off invoice.
2. Printing method
Scratch-off invoice is a high-quality and difficult-to-forge printed matter. It uses various printing methods such as ordinary offset printing, waterless offset printing, relief printing, screen printing, flexo printing and digital printing. It fully exerts the powerful advantages of computer information technology. , Embodies the perfect combination of modern printing methods and digital printing technology.
When printing a scratch-off invoice, the ordinary graphic content is completed by offset printing, which also includes the printing method of waterless offset printing. The equipment can be a professional receipt printing machine or a rotary offset printing machine with more than five colors; number, password The variable information is completed by the digital printing system using inkjet method; the isolation oil is usually applied by the relief printing method; the printing of the silver paste cover layer is the highest technical content and the most difficult process in the scratch-off invoice printing, so There is also some controversy over its printing method.
At present, the printing method of the silver paste cover layer often uses screen printing, flexographic printing and letterpress printing. There are the most screen printing production lines in China, and the application of the other two methods is gradually increasing. Each of the three printing methods has its own characteristics. The silver paste cover layer with screen printing method has smooth edges, glossy and smooth surface, easy to scratch, and the best printing effect. However, due to the low printing durability of the screen version and the difficulty of processing and cleaning, the added value of the product has been increased, which will inevitably bring resistance to its further development. With the further development and improvement of technology, flexographic and letterpress printing will gradually expand their market share due to their low cost and high efficiency, and have a promising development prospect.
3. Process requirements
(1) Printing speed
Because the printing of scratch-off invoices is a combination of multiple printing methods, the printing speed must take into account the bearing capacity of various printing methods, so the printing speed is low, usually between 50-100 meters / minute. On the one hand, the scratch zone has to be printed with the number, isolation oil and silver paste cover layer. The drying device installed on the equipment still cannot meet the requirements of high-speed printing; the other aspect is the high resolution of digital printing, because The resolution of the printing content is 200 ～ 300dpi, and the shape of the ink dot can be distinguished under a magnifying glass of 5 times. If the printing speed is too fast, the clear reproduction of the ink dot shape cannot be guaranteed; the last point, in order to ensure that no information is leaked It is required that the ink thickness of the silver paste cover layer is not less than 5μm, but the drying speed of the ink layer will be slower, which limits the overall printing speed.
(2) The ratio of silver paste
Silver paste (scratch layer ink) is made of silver powder and silver blending oil in a certain ratio, evenly mixed under the stirring of an electric mixer. Among them, silver powder is a very fine aluminum powder, and silver blending oil is a chemical raw material sensitive to ultraviolet light. After the silver paste is printed on the substrate, it is necessary to ensure a considerable fastness during the transfer and transportation, and to ensure that it is easy to scratch when drawing. The ratio of silver powder and silver oil in the silver paste is generally between 1: 9 and 1: 6, but in actual production, the manufacturer will keep its specific composition ratio secret, the size of the ratio and the function of the drying device And the quality of printing paper. If the substrate is smooth and flat, the ratio can be smaller; if the substrate is rough, the amount of silver powder should be more and the ratio will be larger. The drying of silver paste mainly adopts UV drying method. Under the condition of a certain thickness of silver paste, if the sensitivity wavelength of the silver oil to ultraviolet light coincides with the main emission wavelength in the drying device, the drying speed is fast, and the ratio can be smaller ; If the wavelength does not match well, it will inevitably affect the drying speed, the ratio will be larger. In addition, different silver powder and silver blending oil should print the same effect, and the ratio is different.
The ratio of silver paste is affected by many variables. For example, when the thickness of the silver paste layer is determined, the smaller the ratio, the more the liquid component, and the harder it is to dry. However, in actual production, the thickness of the silver paste coating is a variable factor, which makes the ratio of the silver paste more difficult to determine. Because the more liquid components, the silver paste cover layer can be printed thinner, and the drying speed is faster; and when the ratio is larger, although the liquid component is less, because it is difficult to achieve a thin ink layer, it still cannot guarantee a higher drying speed . In addition, the liquid component of the silver paste is a silver-adjusting oil that is sensitive to ultraviolet light. Therefore, the increase in its content is also a factor that is conducive to drying, so it has a different nature from ordinary liquids in affecting the drying speed. Therefore, the ratio of silver paste is a difficult parameter to determine.
(3) Drying method In order to increase the speed of bill printing, almost every printing unit has a corresponding drying device. The offset printing part generally adopts the UV curing drying method, the digital inkjet printing part generally adopts the hot air drying method, and the isolating oil printing portion also uses the UV curing drying method because it contains components sensitive to ultraviolet light, and the scratch layer no matter what The printing method adopts UV curing and drying method.
A group of UV lamps are used in the UV drying device. The power per inch of lamps is generally between 10 and 20kW, and the maximum power cannot exceed 600W. There are generally two types of UV lamps: mercury lamps and metal lamps. UV ink should match the luminous wavelength of the UV lamps to improve the drying efficiency. Manufacturers can also choose the appropriate drying method and drying device according to their own product characteristics.
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