Analysis of four issues to be considered in uv plastic printing

UV ink printing or UV coating on the surface of plastic substrates has become increasingly popular, and this is a challenging task. Plastic printing materials have been used in the printing industry for decades. With the prevalence of UV inks/sheaths, UV ink printing on plastic substrates has raised new issues for printers and ink manufacturers. The biggest problem is how The UV ink / varnish firmly adheres to the surface of the plastic substrate, that is, adhesion problems.

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, on-line processing is the best solution to ensure that the surface tension of the plastic substrate is maintained within an acceptable range when printing. 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 the PE (polyethylene) film is treated by corona discharge, unsaturated bonds such as carboxyl groups and carbonyl groups are formed on the PE molecular chains. 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/gloss, thereby increasing the imprinting strength. 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 can adhere 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. The study of a variety of ink formulations 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 ink/shade raw materials. Therefore, the use of this feature allows plastic substrates to be used in conjunction with certain UV inks to allow the ink/gloss 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, the temperature of the workshop can be increased 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 (Tg) of the ink

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 typically contain some of the smaller molecular weight materials. These materials, when cured, react to form a thicker, more cross-linked layer of ink. 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 layer in the finished product will not firmly adhere to the gold foil or the composite plastic film. UV inks/varnishes made using low Tg materials adhere well to gold foils and composite films in the postpress process. Sometimes in order to be able to make foiling or filming smooth, we have to reduce the operating temperature of the process.

4. Degree of curing

For any UV ink formulation, the proper photoinitiator must be selected to optimize the cured ink layer. Each UV ink/gloss formulation works with a specific, sufficient amount of UV energy. If the spectral 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/grease has been cured, but in reality, it is likely that the entire film layer has not completely cured inside. 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 the post-press processing process.

The plastic packaging market is an exciting market, and UV inks/varnishes 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 problems with UV inks and varnish in plastic packaging printing. The main reason is that printing customers continuously need new plastic substrate materials and new applications, so that the film adhesion 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.