In addition to the fact that UV-gloss paints take some time to unroll before UV radiation is applied, the curing mechanism for UV inks and UV paints will remain substantially consistent. It should be noted that plastic substrates have different habits than paper or cardboard substrates. This article from paint supplier ACGIGATRA describes some special considerations.
The method of using UV radiation
Curing is one of the most innovative technologies for drying inks and coatings in the printing industry. With UV technology, printing can also be performed on a variety of non-absorbent substrates. The initial application in the early 1970s has taken full advantage of the same process advantages still valid today. In the meantime, UV curing has accounted for more than 90% of the market in narrow web printing. UV systems are also becoming increasingly popular among packaging printers. In this industry, sheet-fed offset printing is increasingly becoming the process of choice for printing and glazing.
The in-depth and meticulous research and development in raw materials, inks and coatings have further promoted the widespread application of this technology in many fields of modern printing industry and plastic film printing. However, when compared to paper and paperboard applications, there are various specialities that need to be considered if paint is to be applied on a plastic film.
Diaphragm heat-sensitive
UV lamps also emit a certain amount of infrared radiation (ie, heat). This heat of course will support the curing process, but if overheated it will easily lead to curling of the printed sheet. The problems that result are a decrease in register accuracy and an undesirable increase in stack temperature. It is therefore important to use only the actual amount of UV radiation required to cure the applied inks and coatings. Modifications to process parameters can help eliminate unnecessary heat input.
In general, UV curing will take place under the influence of ambient air. However, since the oxygen molecules in the air also work hard to react with the ink or paint composition, it is necessary to compensate for this competitive reaction by increasing the lamp power, which will naturally exert an unwanted heat load. On the substrate. One of the long-established techniques in roller feed applications, inert UV technology, can also provide a solution for sheet-fed offsets immediately. The method of replacing the parasitic oxygen with nitrogen in the gap between the UV lamp and the substrate makes this an inert atmosphere. A competing reaction with oxygen in the atmosphere is prevented so that the power of the lamp can be significantly reduced and thus the heat transferred to the substrate can be reduced. Another advantage of doing so is to reduce the amount of photoinitiator required for UV inks and coatings, which is a prerequisite for a small odor UV system.
Combination of paint and substrate
A variety of factors determine the combination with the plastic substrate. Since this combination is actually a process of mechanical adhesion, surface tension plays an important role. In order to achieve a good combination of inks or coatings, polyolefin substrates, such as polypropylene (PP) and polyethylene (PE) films, should have a surface tension of at least 38 millinewtons per meter, and preferably 40 millinewtons per meter. Meter. In most cases, the surface of the diaphragm has been corona treated by various suppliers.
However, we recommend checking the surface tension before use because the positive effects of the pretreatment will decrease over time and will fail after a maximum of six months, and this often happens much earlier (testing For details on the method, see the article on surface tension. If the measured surface tension is too low, the tension can be increased again by corona treatment again just prior to printing. Especially for PP and PE, this has shown to be a very convenient measure. On the other hand, corona treatment is not always necessary for PVC, PET, and PS substrates, but the usefulness of this process can still be demonstrated when adhesion problems are encountered. KBA will provide corona modules for on-line pretreatment in front of the first printing unit.
After printing and coating the substrate curl
The heat input is not the only factor that can cause the substrate to curl. The so-called polymerization shrinkage produces the same effect. The curing of UV inks and coatings reduces their volume by 2-10%, depending on the quality of the individual product. This also causes a reduction in the surface area of ​​the ink or paint film, causing the plastic film to curl.
A method of selecting a non-shrinking UV coating may be used to reduce this effect or at least have a significant effect on it. It is also important to ensure that the coating is not applied beyond the necessary thickness.
Static electricity generation and glass effect
All plastic films, whether in ribbon or sheet form, are often susceptible to static electricity. If it is in the form of a sheet, this will cause poor separation of the film in the feeder of the printing press and downstream processing. In addition to installing anti-static systems on presses and finishing machines (see Kersten's article on anti-static systems), proper storage can also reduce the generation of static electricity. To be able to adjust to the environment properly, the diaphragm should be maintained between 20°C and 22°C and the relative humidity maintained at 55% during the three days prior to printing.
Especially when processing very thin films, it is almost certain that poor separation of printed films will occur. For this reason, the use of a coating with a conductive agent composition may help avoid the so-called “glass plate effectâ€, that is, the mutual attraction between the single sheets due to the very smooth air and air between the sheets. The glazing surface is forced to discharge and enhance.
Coatings for plastic diaphragms in sheetfed offset printing
For the best results, the formulation of the raw materials for UV coatings for plastic films is different from the formulations for UV coatings for paper and board. As a result, UV film coatings have very good adhesive properties and also increase the flexibility of the final coating on a single sheet.
In addition to the standard high-gloss and lusterless paint types, the plastic film can also be provided with a variety of gold and silver effects, pearlescent finishes, and opaque whites of all grades. UV film coatings can also be customized to provide specific functions such as chemical resistance to most solvents, acids and alkalis. In addition, special formulations can affect the mechanical properties of the results. Excellent friction resistance and friction values ​​over the entire range from anti-slip to immediate release effects can be obtained. More options are heat resistant UV coatings for in-mold labels, or UV systems with low odor and low migration for food packaging.
The printing and coating of plastic substrates places special demands on all aspects of the process, including printer manufacturers, diaphragms, inks, and coatings suppliers, as well as printers at the chain terminal but not least. . Therefore, it is necessary for users to communicate with all partners regularly, because only then can you truly become a successful competitor in this interesting and innovative market segment.
The method of using UV radiation
Curing is one of the most innovative technologies for drying inks and coatings in the printing industry. With UV technology, printing can also be performed on a variety of non-absorbent substrates. The initial application in the early 1970s has taken full advantage of the same process advantages still valid today. In the meantime, UV curing has accounted for more than 90% of the market in narrow web printing. UV systems are also becoming increasingly popular among packaging printers. In this industry, sheet-fed offset printing is increasingly becoming the process of choice for printing and glazing.
The in-depth and meticulous research and development in raw materials, inks and coatings have further promoted the widespread application of this technology in many fields of modern printing industry and plastic film printing. However, when compared to paper and paperboard applications, there are various specialities that need to be considered if paint is to be applied on a plastic film.
Diaphragm heat-sensitive
UV lamps also emit a certain amount of infrared radiation (ie, heat). This heat of course will support the curing process, but if overheated it will easily lead to curling of the printed sheet. The problems that result are a decrease in register accuracy and an undesirable increase in stack temperature. It is therefore important to use only the actual amount of UV radiation required to cure the applied inks and coatings. Modifications to process parameters can help eliminate unnecessary heat input.
In general, UV curing will take place under the influence of ambient air. However, since the oxygen molecules in the air also work hard to react with the ink or paint composition, it is necessary to compensate for this competitive reaction by increasing the lamp power, which will naturally exert an unwanted heat load. On the substrate. One of the long-established techniques in roller feed applications, inert UV technology, can also provide a solution for sheet-fed offsets immediately. The method of replacing the parasitic oxygen with nitrogen in the gap between the UV lamp and the substrate makes this an inert atmosphere. A competing reaction with oxygen in the atmosphere is prevented so that the power of the lamp can be significantly reduced and thus the heat transferred to the substrate can be reduced. Another advantage of doing so is to reduce the amount of photoinitiator required for UV inks and coatings, which is a prerequisite for a small odor UV system.
Combination of paint and substrate
A variety of factors determine the combination with the plastic substrate. Since this combination is actually a process of mechanical adhesion, surface tension plays an important role. In order to achieve a good combination of inks or coatings, polyolefin substrates, such as polypropylene (PP) and polyethylene (PE) films, should have a surface tension of at least 38 millinewtons per meter, and preferably 40 millinewtons per meter. Meter. In most cases, the surface of the diaphragm has been corona treated by various suppliers.
However, we recommend checking the surface tension before use because the positive effects of the pretreatment will decrease over time and will fail after a maximum of six months, and this often happens much earlier (testing For details on the method, see the article on surface tension. If the measured surface tension is too low, the tension can be increased again by corona treatment again just prior to printing. Especially for PP and PE, this has shown to be a very convenient measure. On the other hand, corona treatment is not always necessary for PVC, PET, and PS substrates, but the usefulness of this process can still be demonstrated when adhesion problems are encountered. KBA will provide corona modules for on-line pretreatment in front of the first printing unit.
After printing and coating the substrate curl
The heat input is not the only factor that can cause the substrate to curl. The so-called polymerization shrinkage produces the same effect. The curing of UV inks and coatings reduces their volume by 2-10%, depending on the quality of the individual product. This also causes a reduction in the surface area of ​​the ink or paint film, causing the plastic film to curl.
A method of selecting a non-shrinking UV coating may be used to reduce this effect or at least have a significant effect on it. It is also important to ensure that the coating is not applied beyond the necessary thickness.
Static electricity generation and glass effect
All plastic films, whether in ribbon or sheet form, are often susceptible to static electricity. If it is in the form of a sheet, this will cause poor separation of the film in the feeder of the printing press and downstream processing. In addition to installing anti-static systems on presses and finishing machines (see Kersten's article on anti-static systems), proper storage can also reduce the generation of static electricity. To be able to adjust to the environment properly, the diaphragm should be maintained between 20°C and 22°C and the relative humidity maintained at 55% during the three days prior to printing.
Especially when processing very thin films, it is almost certain that poor separation of printed films will occur. For this reason, the use of a coating with a conductive agent composition may help avoid the so-called “glass plate effectâ€, that is, the mutual attraction between the single sheets due to the very smooth air and air between the sheets. The glazing surface is forced to discharge and enhance.
Coatings for plastic diaphragms in sheetfed offset printing
For the best results, the formulation of the raw materials for UV coatings for plastic films is different from the formulations for UV coatings for paper and board. As a result, UV film coatings have very good adhesive properties and also increase the flexibility of the final coating on a single sheet.
In addition to the standard high-gloss and lusterless paint types, the plastic film can also be provided with a variety of gold and silver effects, pearlescent finishes, and opaque whites of all grades. UV film coatings can also be customized to provide specific functions such as chemical resistance to most solvents, acids and alkalis. In addition, special formulations can affect the mechanical properties of the results. Excellent friction resistance and friction values ​​over the entire range from anti-slip to immediate release effects can be obtained. More options are heat resistant UV coatings for in-mold labels, or UV systems with low odor and low migration for food packaging.
The printing and coating of plastic substrates places special demands on all aspects of the process, including printer manufacturers, diaphragms, inks, and coatings suppliers, as well as printers at the chain terminal but not least. . Therefore, it is necessary for users to communicate with all partners regularly, because only then can you truly become a successful competitor in this interesting and innovative market segment.
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