Flexo printing substrate - ink adhesion enhancement method

In order to obtain good ink adhesion on the substrate, the ink and substrate must have compatible surface tension and surface energy. Surface tension refers to the energy required for the liquid molecules to attach to the surface, and the surface energy is used to describe the energy required for attracting each other between solid surface molecules. Since both of these words can test the amount of energy molecules attract and attach to the surface of other molecules, many printers believe that the two terms are used interchangeably. The test units for surface tension and energy are both dyne/cm (dynes/cm).

When the surface tension of the liquid adsorbed onto the solid surface is higher than the surface energy of the solid substrate, the liquid molecules will aggregate together into larger droplets or droplets, which cannot be tiled on the surface of the substrate. This phenomenon is due to the Poor wetting caused. If the wettability between the ink supply system and the plastic surface is very poor, no ink can adhere to the surface of the plastic film.

When the surface tension of the liquid is lower than the surface energy of the solid substrate, the tendency of the liquid molecules to aggregate into droplets or droplets will be significantly reduced: the liquid can be well dispersed on the surface of the substrate and this can be called Good wetting. If the wettability between the ink supply system and the plastic film is the case, although it cannot be guaranteed that the ink adheres well to the solid surface, it is said that the surface of the printing material has the ability to absorb ink. The ink is attached to the substrate because of polar groups of molecules on the surface of the substrate and their attractiveness to the ink. This has a close relationship with the successful completion of printing on the film.

All plastic films have low surface blind energy, so printing on their surface is not easy. In addition, different types of films have different surface energies. PE films, polypropylenes, and copolymers have lower surface energies. They simply do not have enough attractive properties to attract ink. In addition, many packaging films contain additives that impart certain physical characteristics to the finished product such as resistance to blocking, increased slip, correction of shrink film tension, reduction of the surface tension of the anti-fog package, and adjustment of the air passage rate. These additives will greatly affect the surface energy of the plastic material thereby affecting the characteristics of the ink adsorbed by the material. Another widely used additive is the organic component of oils and waxes that usually contain large amounts of fatty acid amines. These ingredients can lubricate the surface of the material to reduce the coefficient of friction between the roll and the film on the packaging equipment. Usually they call it 'slip agent'. This additive would have resided in the interior of the plastic film. With time, the aging of the material will slowly transfer to the surface of the diaphragm. The rate of this change is a matter of course, even though the rate of change in different places of a roll of extruded film may be the same. Depending on the nature, these additives will reduce the surface energy of the film and may severely affect the ink adhesion properties on the press even within a few weeks after printing.

The most commonly used additives in high-speed printing films contain anti-adhesive ingredients. The most basic additive is an inorganic mixture of silicon, clay, and calcium, which are typically added to the base polymer of the film during the extrusion process to make a rougher film surface. By changing the smooth film surface to a pitted surface, the coefficient of friction between the film and the drum will be greatly reduced. This type of additive does not significantly affect ink adhesion.

Mixing additives is another area that makes flexo workers headaches. Usually these films have additives that resist blocking and enhance slipperiness. They may sometimes also carry other additives to give the polymer physical properties that are not obtainable by other methods.
The last additive is a surfactant, whose role is to reduce the surface energy to ensure that water droplets do not adhere to the final product or to fog. This film can be used to wrap meat or other foods that are transparent to the user. See how the quality of the product. Printing on this film so far is the least easy.

The best way to judge the required processing method and degree is to repeat the experiment. The first thing to know about the ink's dyne value is that ink suppliers generally provide this information. Also understand the change in the dyne value of the substrate for the degree of inactivity. To determine these recommendations it is best to measure the surface tension.

One of the methods is to measure the contact angle. This method uses a goniometer that can amplify the characteristics of the water droplets that have died from the printing process and reflect it on a pre-calibrated protractor.

One old way of measuring the contact angle is to measure static water droplets. In order to reach equilibrium, people always start reading several seconds after the water droplets have stabilized. However, a recent study showed that measuring the kind of dynamic droplets that swayed results in more accurate results. The advancing contact angle better reflects the wettability on the substrate and the receding contact angle better implies the adhesion properties of the liquid on the substrate.

The most common method of measuring surface energy is to use a series of different mixtures of the amines of formic acid and ethylamine. Each mixture represents a liquid with a different surface tension. Use a cotton cloth to coat this liquid on the surface of the substrate. If it is within a few seconds If no water droplets are formed within the species, the dyne level of the substrate is generally considered to be slightly higher than the dyne value of the liquid. This method can be repeated to gradually increase or decrease the dyne value of the liquid until the highest dyne value is found for wetting the substrate. The whole process is very subjective but from the point of view of the printer, this method is practical, low cost, and simple to operate. Because the degree of processing of plastic substrates continues to change over time, it must be able to withstand this change. As the film ages, some additives will gradually shift to the surface of the film, such as lubricants. This additive will offset the level of conventional processing, so it is necessary to increase the intensity of processing. Some factors must also be considered by the printer and the storage temperature. Even if the film supplier has processed the film before delivery, many printers reprocess it. This phenomenon has become more common.

More and more people in polypropylene film printing are using aqueous or UV inks. This is an important part of corona and paper products applications. Therefore, system manufacturers are also constantly accepting challenges to improve corona treatment to meet higher demand.

The so-called "universal process", these systems usually have the variable frequency power of IG dices: suitable for a variety of processing workstations. The processing stations are also becoming more common in design so that printers and packers can flexibly The special requirements of the prints change the configuration of the work station, such as the change from a bare cylinder to a cylinder with a liner to a double insulated cylinder.Although there are many ways to achieve this flexibility, all methods involve changing the electrode. With the drum configuration capabilities.

A common way to change the electrode configuration is to replace the electrode or rotate to change the position of the electrode. Replacing the treatment drum is not simple because the drum design depends on the size of the power and the type of insulation cover used. Therefore, paper product manufacturers must understand the types of insulators and their advantages and disadvantages.

Ceramic drums have a certain structural advantage. In order to maximize the voltage storage capacity of the insulator, a material with a higher dielectric constant should be selected. Insulation is the ability of a system to hold too much voltage. When the treated ear is high, excessive voltage will cause pinholes on the cover. The thickness of the electrode surface covering material depends on the size of the electrode. The smaller the insulation capacity, the thicker the thickness of the electrode. The thicker electrode of the cover layer requires higher power to produce an effective corona effect. There are a variety of insulation layers currently but it is not necessary for most applications and companies to change the insulation material often.

Source: Print Chemicals Network

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