1. Surface tension of plastic substrates
When printing plastics, the surface tension of the plastic substrate is the first factor to be considered first. Many plastic films have low surface tension before they are processed. Generally, they need to be surface-treated to increase the surface tension of the plastic substrate. After processing, they generally reach 40 dynes/cm or even higher. In general, plastic sheet suppliers have surface-treated plastics, but the effect is quickly lost.
For printers, connection processing is the best solution to ensure that the surface tension of the plastic substrate is maintained within an acceptable range. Corona discharge (oxidation) treatment is the most common treatment method. It is applied to the surface treatment of various plastic films without damaging temperature-sensitive plastic substrates. Corona discharge uses high-frequency high-pressure or medium-frequency high-voltage discharge, the plastic surface is treated to make its surface active, porous, in order to improve the adhesion of the plastic film surface to the ink and improve the printability of the film. For example, after a common pe film is subjected to corona discharge treatment, unsaturated bonds such as a carboxyl group and a carbonyl group are formed on the pe molecular chain. After the surface is oxidized, pe molecules are converted into polar molecules, increasing the surface energy. In general, the surface tension of pe increases from 31 dynes/cm to 39-40 dynes/cm, which increases the wettability and adhesion of pe film to uv ink/varnish, thus enhancing the robustness of the imprinting. degree.
The flame method is more used in the surface treatment of high temperature-resistant plastic products, so that the plastic under the effect of instantaneous high temperature, remove the surface oil and melt the surface layer to improve the ink ability; also use chemical treatment, but this method is usually Used in conjunction with corona discharge treatment, the chemical treatment uses an oxidizing agent to treat the surface of the polyolefin plastic to form polar groups on the surface so that the surface of the plastic substrate adheres well to uv ink/gloss. . In order to detect the effects of the plastic substrate surface treatment, plastic film printing manufacturers are better equipped with dyne test pens.
In the treated plastic film, the surface tension of the ink is lower than the surface tension of the plastic film. Similarly, in order that the optical oil wets well on the ink surface and firmly adheres to the surface of the ink, the surface tension of the optical oil is lower than the surface tension of the ink. Therefore, the selection of raw materials for uv ink formulations is very sophisticated, in order to obtain acceptable surface tension. Studies on a variety of ink formulations have found that solvent-based inks generally have lower surface tensions than other types of inks and can therefore be wetted on a variety of plastic film surfaces. Therefore, the surface treatment of plastic substrates, especially uv ink printing, is particularly critical.
2. Plastic substrate expansion and permeability properties
Unlike many commonly used printed papers and paperboards, the surface of the plastic substrate does not have micropores that allow ink/lubricants to penetrate into the interior. However, some plastic substrates may swell when exposed to some uv inks/varnishes. Therefore, the use of this feature allows the use of plastic substrates in conjunction with certain uv inks to allow the ink/gloss oil to penetrate the interior of the plastic substrate. At the time of curing, due to the internal penetration of the ink/lubricant, a strong and strong adhesion between the ink/varnish and the surface of the plastic substrate can be formed. In addition, it is also possible to increase the temperature of the shop floor to enhance the penetration of plastic substrates and inks/lacquers, which is also one of the ways to improve the adhesion between uv inks/varnishes and plastic films.
3. The glass transition temperature of the ink (tg)
In physics, glass is formed when certain substances are below or equal to the glass transition temperature of the substance. Compared with common inks/varnishes, uv inks/varnishes generally contain some of the smaller molecular weight materials. When these materials are cured, the ink layer formed by the reaction is thicker and has a higher degree of cross-linking. At the same time, the glass transition temperature (tg) of the film after curing with the uv ink/bright oil is higher and the film layer is harder, so that it has good abrasion resistance and chemical resistance.
If the tg of the film is higher than the working temperature in the bronzing or laminating process, the uv film in the finished product will not firmly adhere to the gold foil or the composite plastic film. The uv ink/varnish prepared using a low tg material adheres well to gold foil and composite film in the postpress process. Sometimes it is necessary to reduce the working temperature of the process in order to make the foiling or lamination smooth.
4. Degree of curing
For any uv ink formulation, the proper photoinitiator must be selected to optimize the ink's cured properties. Each uv ink/gloss formulation works with a specific, sufficient amount of uv energy. If the spectrum band and power of uv energy change, the performance of the ink layer after curing will be affected.
At the surface of the plastic substrate, it may feel that the uv ink/shade has been cured, but in fact, it is likely that the entire interior of the film has not completely cured. The degree of curing close to the underlying layer of the film is critical to obtaining good adhesion. If the interior and bottom layers of the film are not completely cured, then the penetration of the uv ink/gloss into the interior of the plastic substrate is meaningless. Once the curing energy changes, the surface tension of the film will also change, ultimately affecting the subsequent completion of postpress processing.
The plastic packaging market is an exciting market, and uv inks/glosses are commonly used materials for plastic packaging printing. Faster curing means higher production efficiency. Drying ink without heat means less impact on the environment and work shop.
Adhesion is one of the few issues in the uv ink/gloss in plastic packaging printing. The main reason is that the printing customers continuously need new plastic substrate materials and new applications, so that the adhesion of the film layer becomes uv plastic. Common problems in printing. As long as the above-mentioned four problems are fully taken into account, the problem of adhesion in plastic printing can be solved.
Source: Shenzhen Industry Information Network
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