Flexographic printing on packaging films is still a very complicated issue. This is mainly due to the fact that there are too many types of packaging applications, films, processing technologies, and ink supply systems. This article will briefly introduce the application of several common packaging films and their related treatment methods. In order to obtain good ink adhesion on the substrate, the ink and the substrate must have compatible surface tension and surface energy. Surface tension refers to the energy required for the liquid molecules to attach on the surface, and the surface energy is used to describe the solid The energy needed to attract each other between surface molecules. Because these two words can test the amount of energy molecules attract and attach to the surface of other molecules, many printers believe that the two terms can be used interchangeably. The test units for surface tension and energy are both dyne/cm (dynes/cm). When the surface tension of the liquid absorbed onto the solid surface is higher than the surface energy of the solid substrate, the liquid molecules will gather together. Become larger droplets or beads, not tiled on the surface of the substrate, this phenomenon is due to poor wetting. 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 roughness of the solid substrate. The tendency of liquid molecules to agglomerate into water droplets or droplets will be significantly reduced, and the liquid can be well dispersed on the surface of the substrate. This can be referred to as good wettability. This is the case if wetting between the ink supply system and the plastic film. Although there is no guarantee that the ink will adhere well to the surface of the solid, after all, the surface of the substrate has the ability to absorb ink. The ink is attached to the substrate because of the polar molecular groups on the surface of the substrate and their attractiveness to the ink, which is closely related to the successful completion of printing on the film. All plastic films have a low surface energy, so printing on their surface is not easy. In addition different types of film have different surface energies. Polyethylene films, polypropylenes, and copolymers have a low surface energy portion and they do not have enough attraction to adsorb ink molecules at all. In addition, many packaging film parts contain additives that make the final product have certain physical characteristics, such as: anti-blocking, enhanced slip, correct shrink film tension, reduce the surface tension of the anti-fog package, and adjust the air passing rate. . These additives will greatly affect the surface energy of the plastic material, thereby affecting the characteristics of the material to adsorb ink. A currently widely used additive is oil and wax. Passage. These ingredients can lubricate the surface of the material and reduce the coefficient of friction between the drum and the film on the packaging equipment. They are often called "slip agents." This type of admixture originally resides in the interior of the plastic film. Over time, the aging of the material will slowly transfer to the surface of the film. The rate of this change process is random, even if it is a roll. The rate of change in different parts of the extruded film may be different. Depending on the nature, these additives will reduce the surface energy of the film, which may severely affect the ink adhesion properties on the press even within a few weeks after printing. In the idle printing film. The most commonly used additives contain anti-adhesive ingredients. The most basic additive is an inorganic mixture of silicon, clay, and calcium. Generally, these additives are added to the basic polymer of the film during the extrusion process to produce 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 anti-blocking and slipperiness additives. They sometimes carry other additives. In order to let the polymer have physical properties that cannot be obtained by other methods. The last additive is a surfactant, whose role is to reduce surface energy to ensure that water droplets do not adhere to the final product or fog. This kind of film can be used to wrap meat or other foods. It is a transparent film. Users can see the quality of the product through packaging. However, printing on such a film is not easy. The best way to judge the required processing method and degree is through repeated experiments. The first thing to know is the dyne value of the ink. In general, the ink supplier will provide this information. Also understand how the different treatment levels change the dyne value of the substrate. In order to judge these. It is recommended to measure the surface tension. One of the methods is to measure the contact angle. This method involves the use of a goniometer, which can amplify the characteristics of the water droplets on the substrate 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 after a few drops of water have stabilized. However, a recent study shows that measuring the dynamic droplets that are still shaking is more accurate. The advanced contact angle reflects the wettability on the substrate better, 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 formamide and ethylamine. Each mixture represents a liquid with a different surface tension. A liquid is applied to the surface of the substrate using a cotton cloth. If no bead is generated within a few seconds, it is generally considered that the dyne level of the substrate is comparable to or slightly higher than the dyne value of the liquid. This method can be repeated to gradually increase or decrease the liquid selection value 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 feasible, low cost, and simple to operate. Because the degree of processing of plastic substrates continues to change over time, this change must be sustained. With the aging of the film. Some additives will gradually shift to the surface of the film. For example: antimony slip agent. This additive will offset the conventional treatment, so it is necessary to increase the intensity of the treatment. Some factors that need to be considered by U-printers include physical processing and storage temperatures. Even though the film supplier has processed the film before shipping, many printers will reprocess it on the press. This phenomenon has become very common. In the printing of polyethylene and polypropylene films, more and more people are starting to use aqueous or uV inks, which makes corona treatment an important part of printing and paper product applications. Therefore, system manufacturers are also constantly accepting challenges to improve corona treatment technologies to meet higher demand. There have been many recent introductions to the so-called "universal processing technology." These systems typically have variable frequency power based on IGBTs and are suitable for a variety of processing station configurations. Workstations have also become more versatile in design, allowing printers and packers the flexibility to change the configuration of workstations according to the specific needs of the print, such as from bare drums to lining cylinders to double insulated cylinders. change. Although there are many ways to get this flexibility. However, all methodologies involve changing the electrode and drum configuration. 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 advantages and disadvantages of insulators from the m category and their performance. A material that can concentrate electron charges is interpreted as a dielectric constant. Defect processing can be performed by using a material with a higher dielectric constant. Materials with higher dielectric constants (such as ceramics) can make the power supply more easily compatible with existing systems. Ceramic drums have a certain structural advantage, which is lacking in bare drums using ceramic tubes. 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. At higher levels of processing, excessive voltages can cause pinholes in the covering. The thickness of the electrode surface covering material depends on the capacity of the electrode, and the smaller the insulation capacity, the thicker the electrode thickness. Thicker layers of electrodes require higher power to create an effective corona effect. There are a variety of insulation layers currently, but for most applications and companies, there is no need to change insulation materials often.
Source: China Printing Chemicals Network