UV wrinkle ink
Normally, when ink dries, wrinkling occurs if the surface layer shrinks differently from the underlying layer. This wrinkling would be considered a defect in printing, but if the wrinkling is deliberately accentuated and controlled to create a decorative effect with a unique pattern, it can give the printed matter a special charm.
Screen-printed UV wrinkle inks are usually used on substrates with a high gloss, such as gold and silver cardboard, but can also be printed on plexiglass, PVC sheets and plastic films. After printing, the ink is first exposed to 254 nm UV light from a low-pressure mercury lamp, which solidifies the surface of the ink and leaves the inside in a semi-solid state. The surface UV ink shrinks due to curing, creating a dimpled pattern. Then, the medium-pressure mercury lamp is used to irradiate the UV ink again, so that the UV ink is completely cured and a surface effect with a hammered metal or wrinkled appearance is achieved. Wrinkled paint has actually been around for a long time and is often used to coat the casings of electrical equipment or machines. However, it requires high-temperature baking to achieve a satisfactory wrinkled effect, so this process cannot be applied to packaging materials made from paper. UV wrinkle ink cleverly adopts the process of UV ink curing twice, and uses the volume shrinkage of the ink layer caused by UV ink curing to wrinkle the ink layer, creating a special decorative effect. Because UV wrinkle ink has the characteristics of a strong sense of three-dimensionality, luxury and elegance, a full ink film and a good visual effect, its application has been extended to the surface decoration printing of product packaging in fields such as high-end cigarette packs, wine boxes, gift packaging boxes, health product packaging boxes, cosmetic boxes, calendars, books, etc.
The size of the wrinkles produced using UV wrinkle ink is related to the mesh count and the thickness of the screen printing ink layer. The lower the mesh count, the thicker the ink layer and the larger the wrinkles; the higher the mesh count, the thinner the ink layer and the smaller the wrinkles. Generally, a mesh count of 100 to 200 is preferred.
The formation of wrinkles in UV wrinkle inks involves two stages: wrinkling and curing. Wrinkles are first formed on the surface by irradiation with a low-pressure mercury lamp, and then cured by irradiation with a medium-pressure mercury lamp. In addition to the thickness of the ink, the size of the wrinkles is also related to the speed at which the surface passes through the low-pressure mercury lamp. If the speed is too fast, no wrinkles will be produced; if it is too slow, the surface will have already cured and no wrinkles will be produced. By controlling the speed of the conveyor belt and changing the distance between the low-pressure mercury lamp and the high-pressure mercury lamp, the optimal process parameters can be determined.
The performance requirements for UV wrinkle ink are shown in the table.
UV wrinkle ink is not pigmented and is actually a kind of UV transparent varnish. For its preparation, see “3.17 UV varnish”.
The key points for using UV wrinkle ink are as follows.
① UV wrinkle ink should be thoroughly stirred before use, and generally can be printed directly without dilution. If dilution is necessary, it can be thinned with a special varnish (thinner) for UV wrinkle ink, and the ratio should generally not exceed 5%. If coloring is required, the corresponding color paste can be added and thoroughly mixed before printing, but the amount added should generally not exceed 3%. UV wrinkle inks of different series and brands generally should not be mixed with each other to avoid adverse reactions that affect the printing results.
② For some substrates with poor adhesion, the adhesion problem can be solved by first printing with the corresponding transparent ink and then printing with UV wrinkle ink.
③ Before using it in bulk, a small amount should be tried out and fully confirmed to meet the requirements of materials and equipment before it can be used in bulk. The ink should be stored in a cool, dry place, tightly sealed after use, and care should be taken to avoid contact with open flames and high temperatures. The shelf life is generally 1 year.
④UV wrinkle inks are usually used for screen printing, and different mesh counts of the screen are selected according to the desired wrinkle effect. The higher the accuracy requirements for the printed product in screen printing, the higher the mesh count of the screen selected, and vice versa. The higher the mesh count, the smaller the wrinkle effect, and vice versa. Therefore, in the screen printing of UV wrinkle ink, according to practical experience, a screen with a mesh count of 100 to 200 is generally used. If the printing equipment has high precision, a screen with a higher mesh count can be selected as appropriate.
⑤ Select an appropriate mesh distance. When the mesh distance is less than 5mm, the image can be completely displayed on the substrate. However, when the mesh distance exceeds 5mm, the image displayed on the substrate will be lost as the mesh distance increases. According to practical experience, a mesh distance setting range of 2 to 5mm is more appropriate.
⑥ Select an appropriate screen tensioning angle. According to practical experience with screen printing, the screen tensioning angle is 45°, the moiré effect is the most serious; the screen tensioning angle is 60° or 75°, the moiré effect is also relatively serious; the screen tensioning angle is 30°, , the moiré pattern is lighter, while at 0°, 10°, 20° and 25°, the printed matter basically does not produce moiré patterns. Therefore, the range of stretching angles can be selected to be less than 30°.
⑦ Pay attention to adjusting the printing speed and controlling the squeegee angle. Changes in the printing speed will not result in changes in the quality of the printed matter, but only in the size of the wrinkled pattern of the ink. A fast printing speed will result in a small pattern, while a slow printing speed will result in a large pattern. Similarly, a change in squeegee angle will not change the quality of the printed product, but only the size of the pattern. A small squeegee angle will result in a small pattern, while a large squeegee angle will result in a large pattern. Therefore, the optimal printing speed and squeegee angle should be adjusted to achieve the best pattern effect during UV curing.
⑧ Pay attention to controlling the wrinkling and drying speed of the ink. If the wrinkling speed is too slow, the printed matter cannot wrinkle. Experiments have shown that when the wrinkling speed is less than 14m/min, the ink basically cannot wrinkle; when the speed reaches 14m/min or more, the ink will wrinkle. Therefore, a wrinkling speed of 14 to 20m/min can be selected for general production.
⑨ Note the influence of the thickness of the ink layer and the color of the ink on UV curing. Because the ink layer thickness of screen printing is very large, when the ink is too thick, it will affect the curing effect. All factors that affect the thickness of the printed ink layer will affect the curing effect. Factors that affect the thickness of the ink layer include the mesh count, tension, thickness of the photosensitive adhesive, hardness of the squeegee, sharpness of the squeegee edge, printing angle, printing speed, pressure, etc., and these factors must be controlled during printing. In addition, pigments also have a significant effect on screen-printed UV inks. The UV curing effect of UV wrinkle inks of different colors and thicknesses also differs, mainly due to the absorption and reflection of light by various pigments and the different pigment contents in the inks. Generally speaking, white, black, blue and green are more difficult to cure, while red, yellow, varnish and transparent oil are easy to cure. By taking advantage of the characteristics of photoinitiators, the curing parameters of UV wrinkle inks of different colors can be made consistent under the same curing conditions by selecting the type of photoinitiator and reducing the color density. The thickness of the ink layer should be adjusted by changing the parameters of the screen, the hardness of the squeegee and the printing speed to ensure the rapid curing of the ink.
UV curing requires that the UV curing machine be equipped with a wrinkle-inducing device, the light source be a 20–40W low-pressure UV lamp, the power be 60–100W, and the distance between the UV wrinkle-inducing device and the UV curing lamp be no less than 1.2m. Otherwise, the printed product will be cured by the UV lamp without being wrinkled by the UV wrinkle-inducing device, and the pattern cannot be induced, and the the wrinkling effect. The power of the UV curing lamp should not be less than 3kW, and the speed of the conveyor belt depends on the printing and curing effect, generally 18-25m/min. The power of the UV wrinkling device and the distance from the conveyor belt directly affect the pattern of the product. The higher the power of the UV wrinkling lamp, the faster the speed of the conveyor belt, and vice versa.
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