The label is now a wonderful gift in the packaging decoration, and is increasingly favored by consumers. There are many types of label printing. From the outside, the plastic bottle surface screen printing and in-mold labeling methods all belong to the “no-tag†solution. The label corners are not visible on the bottle surface and cater to the packaging grades sought by consumers. From a technical point of view, although the processes are quite different, they do not require machine labeling or manual labeling processes. From the perspective of packaging style, bright color blocks and texts with strong readability make curved screen printing a representative of simple packaging styles; while in-mold labels, delicate screens, rich layers, and colorful colors. From an investment point of view, the investment threshold for curved screen printing is low, and the in-mold label belongs to the rich game. In short, two distinctive solutions have formed a unique landscape in the plastic bottle packaging printing market.
Surface Printing Technology Essentials
1. Color separation
The first step in color separation is to ensure that the design elements on the original meet the factory's printing conditions. The thinnest line that can be printed is 0.2mm (120 lines/inch screen). If the width of the thinnest line on the original is lower than this limit, it cannot be reproduced by printing. It is recommended that users adjust the original and change the thinnest line to the thickest line. some. The original image size is slightly smaller than the printable surface area of ​​the plastic bottle. In addition, since black is not noticeable on various bottle surfaces, when the text on the user-provided manuscript 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 method of screen printing, the transferred ink layer is thick, and offset printing separation is not suitable for curved color screen printing.
The surface tone copy of surface printing is generally 15% to 85%. Secondly, the use of elliptical dots is more desirable for screen surface adjustment and screen printing. If a square gauss dot is used, four corner borders will appear at the midtone dot in the four-color printing, resulting in a serious increase in dots, and the elliptical dots will have only two corners at the same gradation. 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. In order to facilitate print registration, registration marks should be placed on the color separation film.
2. Plate making
(1) The choice of screen. Plastic screen printing screen uses a yellow screen to reduce the negative effect of light scattering on the edge definition of the image when exposed. Nylon Yellow Wire Mesh 120-30Y is suitable for large blocks of light-blocking patterns and gold and silver printing: Nylon Yellow Wire Mesh 140-30Y is suitable for large color blocks Text Printing: Nylon Yellow Wire Mesh 165-30Y is the most widely used, printing text effect Special rape.
(2) Strictly control stretching force. 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 photosensitive adhesive 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 33 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 1kw, the exposure time is generally 3-4 minutes; when the power is 2kW, the exposure time is 1 minute and 50 seconds. With high-pressure water: wash the screen, check the picture 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. print
(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) x 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 x 10 -2N/m or so. For PET bottles, direct printing is possible because their surface energy meets printing requirements.
(2) Test 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.
Although the printability of in-mold labels is different from traditional paper and plastic, the printing of in-mold labels is no longer a difficult task. Offset printing, gravure printing, letterpress printing, and flexographic printing can all be accomplished. Wire-cutting and die-cutting methods are mainly applied to flexographic printing equipment. After the in-mold labels are printed there are some additional processes, such as first hot stamping and then glazing, or first coating and then hot stamping. 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.
(4) Correct exposure, development, and revision. The exposure time is determined according to the lamp power. When the power is 1kw, the exposure time is generally 3-4 minutes; when the power is 2kW, the exposure time is 1 minute and 50 seconds. With high-pressure water: wash the screen, check the picture 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) x 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 x 10 -2N/m or so. For PET bottles, direct printing is possible because their surface energy meets printing requirements.
(2) Test 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.
Although the printability of in-mold labels is different from traditional paper and plastic, the printing of in-mold labels is no longer a difficult task. Offset printing, gravure printing, letterpress printing, and flexographic printing can all be accomplished. Wire-cutting and die-cutting methods are mainly applied to flexographic printing equipment. After the in-mold labels are printed there are some additional processes, such as first hot stamping and then glazing, or first coating and then hot stamping. 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.