1. Printing process
At present, the more popular scratch-off printing process is: special paper for bills → printing anti-lighting ink on the back side → printing invoice graphic on the front side → printing tax stamp → printing invoice number and password → coating isolation oil → printing silver paste Cover layer → finished product.
Compared with plain paper, special invoice paper has high opacity and tensile strength, and has good adaptability to various printing methods. In order to prevent the information in the prize area from leaking during the observation of light transmission, a corresponding layer of anti-transparent ink is printed on the corresponding position of the scratch area on the back of the invoice, which is generally printed in black. Then, the device prints the front content through the automatic flip device, prints the shading and plain text on the front side, and prints the tax stamp with the spot color fluorescent ink, which is done by different color groups of the same printing machine. The number of the invoice and the content of the prize zone are completed by the digital printing system. After the content of the prize zone is sprayed, a layer of isolation oil is printed on the corresponding position. Finally, the silver paste coating is printed on the isolation oil. The 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
The scratch-off invoice is a kind of high-quality and difficult-to-copy printed matter. It uses a variety of printing methods such as ordinary offset printing, waterless offset printing, embossing, screen printing, flexo printing and digital printing, which fully exerts the powerful advantages of computer information technology. It embodies the perfect combination of modern printing methods and digital printing technology.
When printing scratch-off invoices, 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 ticket printing machine or a rotary printing machine with more than five colors; number and password. The variable information is completed by the digital printing system by inkjet method; the coating of the isolation oil usually uses the embossing method; the printing of the silver coating is the most technical and the most difficult part of the scratch-type invoice printing, so There is also some controversy about the way it is printed.
At present, the printing of the silver paste coating layer is often printed by screen printing, flexographic printing and letterpress printing. There are the largest number of 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 edge of the silver paste covering layer by screen printing is smooth, the layer is shiny and flat, and it is easy to scratch, and the printing effect is the best. However, due to the low printing durability of the screen version, difficulty in processing and cleaning, etc., the added value of the product is increased, which is bound to bring resistance to further development. With the further development and improvement of technology, flexographic and letterpress printing will gradually expand market share with its low cost and high efficiency, and its development prospects are promising.
3. Process requirements
(1) Printing speed
Since the printing of the scratch-off invoice is a combination of a plurality of printing methods, the printing speed is required to take into account the tolerance of various printing methods, so the printing speed is low, usually between 50 and 100 m/min. On the one hand, the scratch area should be printed with the number, isolation oil and silver paste cover respectively. The drying device installed on the equipment still can not meet the requirements of high-speed printing; the other is the high resolution of digital printing because The resolution of the coded content is 200-300dpi. It is necessary to distinguish the shape of the ink dot under the magnifying glass of 5 times. If the printing speed is too fast, the clear reproduction of the shape of the ink dot cannot be guaranteed. Finally, in order to ensure no leakage of information. The thickness of the ink of the silver paste coating layer is required to be not less than 5 μm, but the drying speed of the ink layer is slower, and the overall printing speed is limited.
(2) the ratio of silver paste
Silver paste (scratch layer ink) is made by uniformly mixing silver powder and silver oil in a certain ratio and stirring under an electric mixer. Among them, silver powder is a very fine aluminum powder, and silver-adjusting oil is a chemical raw material sensitive to ultraviolet light. After the silver paste is printed on the substrate, it is necessary to ensure that the transfer is transported with considerable fastness and that it is easy to scratch when the lottery is opened. The ratio of silver powder to silver silver oil in silver paste is generally between 1:9 and 1:6, but in actual production, the manufacturer will keep the specific distribution ratio confidential, the ratio 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 the silver paste mainly adopts the UV drying method. When the thickness of the silver paste is constant, if the sensitive wavelength of the silver-adjusted oil to the ultraviolet light is consistent 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 affect the drying speed, then the ratio will be larger. In addition, different silver powder and silver-tone oil have to print the same effect, and the ratio is different.
The ratio of silver paste is affected by a variety of variables. When the thickness of the silver paste layer is determined, the smaller the ratio, the more the liquid component, and the more difficult it is to dry. However, in actual production, the thickness of the silver paste coating is a variable factor, making the ratio of the silver paste more difficult to determine. Because the more liquid components, the silver paste coating can be printed thinner and the drying speed is faster; when the ratio is larger, although the liquid component is less, it is difficult to realize a thin ink layer, so the higher drying speed cannot be guaranteed. . In addition, the liquid component in the silver paste is a silver-adjusting silver-sensitive oil, and therefore, the increase in its content is also a factor contributing to drying, so that it has a different nature from the ordinary liquid 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 ticket printing, almost all printing units have corresponding drying devices. The offset printing part generally adopts the UV curing drying method, and the digital inkjet printing part generally adopts the hot air drying method, and the isolated oil printing part also adopts the UV curing drying method because of the component which is sensitive to ultraviolet light, and the scratching layer is not used. The printing method adopts UV curing drying method.
A UV lamp is used in the UV drying device. The power per inch of the lamp is generally between 10 and 20 kW, and the maximum is not more than 600 W. UV lamps generally have two types of mercury tubes and metal tubes. The UV ink should match the wavelength of the UV tube to improve drying efficiency. Manufacturers can also choose the appropriate drying method and drying device according to their own product characteristics.
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