Surface Printing Technology Essentials
Mr. Deng Zicong, the printing supervisor of Dongguan Meigao Containers Co., Ltd., Mr. Stephen Gilbertson, a United States-based Kammann Machinery Company, and Mr. Huang, Technical Service Manager of Dongguan Henghui Color Printing Machinery Co., Ltd., introduced us to the plastic bottle surface printing. Technical essentials. In the entire curved screen printing process, there are three important processes.
1. Separation
The first step in color separation is to ensure that the design elements on the original meet the factory's printing conditions. According to Mr. Deng Zicon, the thinnest line the company can print is 0.2mm (120 lines per inch). If the width of the thinnest line on the original is lower than this limit, it cannot be reproduced through printing. To advise the user to adjust the original, make the thickest lines thicker. The original image size is slightly smaller than the printable surface area of ​​the plastic bottle. In addition, since black is not conspicuous on various bottle surfaces, when the text on the user-provided original is black, it should be recommended to change it to white.
The second step is to use a reasonable separation method. Due to the special screen printing method, the transferred ink layer is thick, so the offset printing separation setting is not suitable for curved color screen printing. In order to obtain a satisfactory copying effect, various factors in the screen printing production must be taken into account in the color separation.
First of all, it is important to remember that the reproduction range of the curved screen printing is generally 15% to 85%. Secondly, the use of elliptical dots is more desirable for screen surface adjustment and screen printing. Because if a square dot is used, in the four-color printing, there will be four corners at the middle tone point, which will cause the dot to increase seriously, while the elliptical dot has only two edges at the same tone. Again, the optical density of the exposed part of the color separation sheet must be at least 4.00. For high quality plastic bottle screen printing, the resolution of the imagesetter should reach 2400 dpi. Finally, in order to facilitate print registration, registration marks should be placed on the color separation film.
2. Plate making
(1) The choice of wire mesh. The use of a yellow silkscreen for screen printing on plastic bottles can reduce the negative effect of light scattering on the sharpness of the image when exposed. Nylon yellow wire mesh 120-30Y is suitable for light-blocking large color block patterns and gold and silver printing; nylon yellow wire mesh 140-30Y is suitable for large color block text printing; nylon yellow wire mesh 165-30Y is the most widely used, printing text effect Especially good.
(2) Strictly control stretch net tension. The ideal screen tension after stretching is (1.8-2.0)×103 N/m, and it stabilizes at (1.5-1.6)×103 N/m after being placed for 24 hours. Since plastic bottles are printed as line contacts, the screen tension is slightly lower than flat screen printing. It is recommended to use a tension measuring instrument for normalization and data manipulation.
(3) The photoresist coating should be thin and smooth. The coating is too thick, the surface is smooth, the thickness of the transferred ink layer is thick, and the dots are enlarged so that the color is dirty; the coating is too thin and the flatness may be affected. If there is no instrument to measure and control the thickness of the photoresist coating, a 3-3 coating method is generally used, that is, three times of scraping, followed by drying followed by coating, for a total of three times.
(4) Correct exposure, development, and revision. The exposure time is determined according to the lamp power. When the power is 1 kW, the exposure time is generally 3 to 4 minutes; when the power is 2 kW, the exposure time is 1 minute and 50 seconds. Rinse the screen with high pressure water and check that the image is fully developed. On the light box, check whether there is light transmission in the non-graphic area, and repair the light-transmitting area of ​​the non-graphic area with a repairing slurry and dry it.
3. Printing
(1) Improve the adhesion of ink and plastic bottles. After the odor and safety meet the requirements of users, UV inks have become the main material for the surface printing of plastic bottles. The surface tension of UV ink is approximately (3.2-3.4)×10-2N/m. For PE and PP bottles with low surface energy, flame or corona treatment must be performed before printing to achieve a surface tension of 4.6×10. -2N/m or so. For PET bottles, direct printing is possible because their surface energy meets printing requirements.
(2) Test the ink adhesion. Many types of screen printing inks, improper selection, there will be due to ink adhesion and deinking phenomenon. Therefore, the ink adhesion test must be performed before printing. The specific method is to use 3M Company 610 tape to peel off the ink solidified on the bottle. If the ink does not come off, there is no problem with the adhesion of the ink.
(3) Ensure that the UV ink is completely cured. If the UV ink layer is not completely cured, its wear resistance, chemical resistance, weather resistance, and light resistance are poor. In general, yellow and magenta inks cure more easily, and white, blue, and black inks take longer to cure. If the curing time is sufficient, or ink adhesion is not fast, you must check whether the UV lamp aging and resulting in reduced power.
(4) Guarantee print registration. In the plastic bottle surface color screen printing, printing registration is a difficult point, it is closely related to the tolerances of screen printing machine and cylindrical bottle positioning holes. For a well-designed and well-manufactured curved screen printer, the positioning holes of the supporting bottles have a tolerance of ±0.01 cm. The precise dimension of the bottle positioning hole is the key to controlling the print registration. The dimensional tolerance must be strictly controlled in the design and manufacture of the mold.
In-mold label printing and processing technology
Although the printability of in-mold label materials is different from traditional paper and plastic, the printing of in-mold labels is no longer a difficult problem. Offset printing, gravure printing, letterpress printing, and flexographic printing can all be accomplished. So far no silk has been reported. Net printing in-mold labels.
For die cutting of in-mold labels, CNC automatic die-sinking machines, semi-automatic trademark die-moulding machines, and semi-automatic die-cutting machines can perform tasks. Wire-cutting and die-cutting methods are mainly applied to flexographic printing equipment. There are some post-processing steps after the in-mold labels are printed, such as hot stamping and then glazing, or first glazing and then hot stamping. Flexographic printing can even be used to print hot stamping stations online, but it has not been popularized in the country.
After in-mold labels are printed and processed, they are packaged and sent to plastic container processing plants where they are blow molded together with plastic bottles. Therefore, only qualified upstream and downstream companies can obtain qualified in-mold label plastic bottles. The special thing about labeling.
(to be continued)
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