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 several common packaging film applications and related treatment methods.
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 dyne/cm (dyne/cm).
When the surface tension of the liquid adsorbed on the solid surface is higher than the surface energy of the solid substrate, the liquid molecules will gather together to become larger drops or droplets, which cannot be tiled on the surface of the substrate. It 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 energy of the solid substrate, the tendency of the liquid molecules to aggregate into droplets or droplets will be significantly reduced, and the liquid can be well dispersed on the surface of the substrate. It is called wettability. If the wettability between the ink supply system and the plastic film belongs to this kind of situation, although it can not be guaranteed that the ink can adhere well to the solid surface, it means that the surface of the printing material has the ability to adsorb 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 low surface energy, so printing on their surface is not easy. In addition, different types of films have different surface energies, and polyethylene films, polypropylenes, and copolymers have low surface energy, and they do not have enough attraction to adsorb ink molecules.
In addition, many packaging films contain additives that make the final product have certain physical characteristics 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 material to adsorb ink.
Another widely used additive is oils and waxes, which usually contain large amounts of organic components of fatty acid amines. These ingredients can lubricate the surface of the material and reduce the coefficient of friction between the roll and the film on the packaging equipment. They are usually called "slip agents." This additive would have resided in the interior of the plastic film, and over time, the material's aging will slowly transfer to the film surface. The rate of this change is random, and the rate of change in different places of a roll of 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 high-speed printing films, the most commonly used additives 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 anti-blocking and slipperiness additives. They may also sometimes have other additives to allow the polymer to 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. 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 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. To judge these, it is recommended 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 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 several seconds after the water droplets have stabilized. However, a recent study shows that measuring the dynamic droplets that are still shaking is more accurate. The advancing contact angle better reflects the wettability on the substrate, while the receding contact angle better implies the adhesion properties of the liquid on the substrate.
The most common method for 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 droplets are 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 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 feasible, 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 migrate to the surface of the film, such as lubricants. This additive will offset the conventional treatment, so it is necessary to increase the intensity of the treatment. Some factors that printers need to consider include physical processing and storage temperatures. Even though film suppliers have already processed films before shipment, many printers have reprocessed them on the press. This phenomenon has become more 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 IGBT-based variable frequency power 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 achieve this flexibility, all methods 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 types of insulation and their advantages and disadvantages.
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 a higher dielectric constant, such as ceramics, can make power supply more easily compatible with existing systems.
The ceramic drum has a certain structural advantage, which is lacking in the use of a ceramic tube. 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 on the cover. 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 effective corona effects. There are a variety of insulation layers currently, but for most applications and companies, there is no need to change insulation materials often.
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 dyne/cm (dyne/cm).
When the surface tension of the liquid adsorbed on the solid surface is higher than the surface energy of the solid substrate, the liquid molecules will gather together to become larger drops or droplets, which cannot be tiled on the surface of the substrate. It 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 energy of the solid substrate, the tendency of the liquid molecules to aggregate into droplets or droplets will be significantly reduced, and the liquid can be well dispersed on the surface of the substrate. It is called wettability. If the wettability between the ink supply system and the plastic film belongs to this kind of situation, although it can not be guaranteed that the ink can adhere well to the solid surface, it means that the surface of the printing material has the ability to adsorb 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 low surface energy, so printing on their surface is not easy. In addition, different types of films have different surface energies, and polyethylene films, polypropylenes, and copolymers have low surface energy, and they do not have enough attraction to adsorb ink molecules.
In addition, many packaging films contain additives that make the final product have certain physical characteristics 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 material to adsorb ink.
Another widely used additive is oils and waxes, which usually contain large amounts of organic components of fatty acid amines. These ingredients can lubricate the surface of the material and reduce the coefficient of friction between the roll and the film on the packaging equipment. They are usually called "slip agents." This additive would have resided in the interior of the plastic film, and over time, the material's aging will slowly transfer to the film surface. The rate of this change is random, and the rate of change in different places of a roll of 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 high-speed printing films, the most commonly used additives 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 anti-blocking and slipperiness additives. They may also sometimes have other additives to allow the polymer to 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. 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 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. To judge these, it is recommended 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 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 several seconds after the water droplets have stabilized. However, a recent study shows that measuring the dynamic droplets that are still shaking is more accurate. The advancing contact angle better reflects the wettability on the substrate, while the receding contact angle better implies the adhesion properties of the liquid on the substrate.
The most common method for 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 droplets are 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 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 feasible, 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 migrate to the surface of the film, such as lubricants. This additive will offset the conventional treatment, so it is necessary to increase the intensity of the treatment. Some factors that printers need to consider include physical processing and storage temperatures. Even though film suppliers have already processed films before shipment, many printers have reprocessed them on the press. This phenomenon has become more 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 IGBT-based variable frequency power 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 achieve this flexibility, all methods 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 types of insulation and their advantages and disadvantages.
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 a higher dielectric constant, such as ceramics, can make power supply more easily compatible with existing systems.
The ceramic drum has a certain structural advantage, which is lacking in the use of a ceramic tube. 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 on the cover. 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 effective corona effects. There are a variety of insulation layers currently, but for most applications and companies, there is no need to change insulation materials often.
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