The printability of paper usually means that the paper exhibits adaptability or reaction characteristics when printed, subjective to a large extent, and subject to different standards. For example, most papers can be printed with most printing processes from a technical point of view, but this will not work from a commercial or economic point of view. Similarly, when judging the quality of a printed matter, the characteristics of the printed matter must be considered. For example, the judging standard for a quality flexo-folding carton is different from the quality judgment standard for a student's exercise book.
Moreover, it must be clear that the quality of the printing depends on the combined effect of the paper, ink, plate, anilox rollers, and the printing press, and of course on the preparation and operation of the press. If there is something wrong with the other parts, even a paper with good printability will not achieve satisfactory print quality. The term printability is used to describe many physical and chemical properties of paper, which restrict the production of prints, the expected quality of commercial batch printing, and the speed of printing. This article will focus on the characteristics of paper (including paperboard, translator's note), ink, and the impact of printing plates on the printability of paper.
There are many types of paper used in flexible packaging , prepress and postpress, such as coated paper, semi-coated paper, uncoated paper and Kraft paperboard. Paper with good printability only absorbs the proper amount of ink at a predetermined location and does not affect its normal drying process. In addition, the paper should also have a certain degree of smoothness, ink penetration, suitable optical density and rheological properties (compressibility and elasticity). The paper should also have sufficient surface strength to prevent fuzzing; the shape and size should be stable and not subject to change in wetness. The surface of the paper should not have adhered dust particles and it is not easy to generate or maintain static electricity.
Speaking of the printing of the paper does not consider the situation when the ink is in contact with the paper. This involves the momentary printing pressure on the paper. After the paper is pressed, the smoothness is further improved, and at the same time, the pores on the surface of the paper are reduced. The compressibility of the paper plays a role, making the surface easily wet with ink, which is beneficial to ink absorption. In an ideal situation, the paper should be restored to its original shape after being pressed, especially those papers that have only been subjected to rough calendering.
Ink transfer ink transfer is characterized by ink absorption. But this statement is not precise enough, because it contains this meaning: paper inhales a lot of ink. Actually it does not need that, it will consume excess ink. Therefore, the term “inking performance†is now widely used and its meaning is relatively precise. Because the ink, paper, and smoothness are different, the pore size of the paper is also different. Therefore, under a certain printing pressure, the amount of ink seepage is not the same.
In fact, as long as the specified minimum ink transfer amount is sufficient, it can ensure the formation of the required ink film and quality, so that the ink is transferred to the surface of the paper as much as possible and firmly adheres to the surface of the paper. Does not cause stickiness. In order to obtain high ink hiding power, the binder (liquid phase) in the ink must penetrate the pores on the surface of the paper to allow the pigment to adhere to the surface of the paper. Smooth paper can achieve higher ink hiding power, while rough paper requires more ink to get the desired effect. Rough paper also tends to result in a greater rate of outliers than smooth paper. This is related to the type of paper, the roughness of the paper surface and the size of the pores. As long as the outlines on the printed sheets are compared with the printing plates or the outlines on the film, the degree of enlargement can be seen. The outliers are usually found using a densitometer, and then the outline values ​​are plotted on the graph. The plotted curve for each color embodies the printing performance of the printing press, paper, printing plates, and inks used, and presents us with important information about the enlarged tone reproduction. With specific inks, the data obtained after trials of printing plates and paper on a printing press has a very important guiding role for flexo printing and color separation.
Of course, the binder and the pigment should not be separated where the ink adheres to the surface of the paper, which cannot be ignored. In addition, overprinting has a great influence on printability. In order to obtain a good overprint effect, the pH value of the water-based ink should be valued. The pH of the ink used for the first ink should be between 9-9.3. The pH of the ink used for the first ink should not fall below 9. The ink used for the second ink should have a lower pH. The drying properties of various overprint inks also have a great influence on printability. In the flexo printing, the color of the ink is generally first light, then dark (yellow-magenta-cyan). When printing on coated or semi-coated paper, it is sometimes necessary to change the color to magenta-yellow-cyan in order to obtain a better overprint effect for the water ink.
Paper smoothness Paper smoothness affects the overall appearance of the ink film and the print sheet. There are two types of defects in smoothness. The former refers to defects that can be seen with the naked eye, such as uneven thickness or wrinkling of the weight. These defects are sometimes caused by the structure of the fibers. Small flaws are flaws that are invisible to the naked eye, such as pits and holes that can damage the dots and affect print quality.
Dust particles that adhere to the paper or web edge during printing can also affect printability. If no ink filter is installed on the flexographic printer, dust particles will collect in the ink and be carried on the plate. This phenomenon can be eliminated by installing an ink filter in front of the ink pump. In addition, dust particles may adhere to the underside of the squeegee, causing streaks on the printed sheets; sometimes, dust particles may adhere to the ink holes of the stencil roll, resulting in uneven ink transfer. Therefore, the ink filter should be installed as soon as possible to effectively control the dust particles.
Visual effects In the relationship between paper and printing effects, you should also pay attention to the effect of the paper's ink and coloring properties on the ink film. Although, the color of the paper has a certain influence on the effect of the printed sheet. Because of the difference in printing, the same ink produces a very different visual effect and can even change the color tone. The same kind of job is printed on different types of paper, and the quality of the print is different. It often makes people feel disappointed.
Chemical composition The chemical composition of paper has a great influence on printability. Excessive acid content in the paper slows down the drying of the ink. This is more evident in areas with high relative humidity and high moisture content. In general, non-coated papers with a pH of 5 or 5.5 (or slightly higher) and coated paper with a pH of 7-8.5 do not cause problems with ink drying unless the relative temperatures are high, such as In Southeast Asia, relative humidity is above 65%.
According to the specific circumstances, heating and rapid drying can be used. Understand the paper's moisture accountant and keep it at the required level, which is a basic guarantee for producing high-quality prints. Paper fibers, like sponges, absorb or release water molecules according to the humidity of the printing environment. The state in which paper fibers neither absorb nor release water molecules is called equilibrium. Different grades of substrates have different balance points. For example, at the same humidity, the absolute humidity of bleached paperboard can reach 6%, and the absolute humidity of unbleached kraft paper can reach 7.5%. Dry webs tend to break on the press because many flexo printers pull the web over the printing unit. The normal humidity of the paper is 5-7%. High humidity can affect the paper's ink performance. When the humidity is too low, the ink layer cracks when the printed sheet is folded. Paper fibers absorb moisture from the air, the volume becomes larger, and internal pressure is generated. Release of internal pressure can cause warpage of the paper edge.
(to be continued)
Moreover, it must be clear that the quality of the printing depends on the combined effect of the paper, ink, plate, anilox rollers, and the printing press, and of course on the preparation and operation of the press. If there is something wrong with the other parts, even a paper with good printability will not achieve satisfactory print quality. The term printability is used to describe many physical and chemical properties of paper, which restrict the production of prints, the expected quality of commercial batch printing, and the speed of printing. This article will focus on the characteristics of paper (including paperboard, translator's note), ink, and the impact of printing plates on the printability of paper.
There are many types of paper used in flexible packaging , prepress and postpress, such as coated paper, semi-coated paper, uncoated paper and Kraft paperboard. Paper with good printability only absorbs the proper amount of ink at a predetermined location and does not affect its normal drying process. In addition, the paper should also have a certain degree of smoothness, ink penetration, suitable optical density and rheological properties (compressibility and elasticity). The paper should also have sufficient surface strength to prevent fuzzing; the shape and size should be stable and not subject to change in wetness. The surface of the paper should not have adhered dust particles and it is not easy to generate or maintain static electricity.
Speaking of the printing of the paper does not consider the situation when the ink is in contact with the paper. This involves the momentary printing pressure on the paper. After the paper is pressed, the smoothness is further improved, and at the same time, the pores on the surface of the paper are reduced. The compressibility of the paper plays a role, making the surface easily wet with ink, which is beneficial to ink absorption. In an ideal situation, the paper should be restored to its original shape after being pressed, especially those papers that have only been subjected to rough calendering.
Ink transfer ink transfer is characterized by ink absorption. But this statement is not precise enough, because it contains this meaning: paper inhales a lot of ink. Actually it does not need that, it will consume excess ink. Therefore, the term “inking performance†is now widely used and its meaning is relatively precise. Because the ink, paper, and smoothness are different, the pore size of the paper is also different. Therefore, under a certain printing pressure, the amount of ink seepage is not the same.
In fact, as long as the specified minimum ink transfer amount is sufficient, it can ensure the formation of the required ink film and quality, so that the ink is transferred to the surface of the paper as much as possible and firmly adheres to the surface of the paper. Does not cause stickiness. In order to obtain high ink hiding power, the binder (liquid phase) in the ink must penetrate the pores on the surface of the paper to allow the pigment to adhere to the surface of the paper. Smooth paper can achieve higher ink hiding power, while rough paper requires more ink to get the desired effect. Rough paper also tends to result in a greater rate of outliers than smooth paper. This is related to the type of paper, the roughness of the paper surface and the size of the pores. As long as the outlines on the printed sheets are compared with the printing plates or the outlines on the film, the degree of enlargement can be seen. The outliers are usually found using a densitometer, and then the outline values ​​are plotted on the graph. The plotted curve for each color embodies the printing performance of the printing press, paper, printing plates, and inks used, and presents us with important information about the enlarged tone reproduction. With specific inks, the data obtained after trials of printing plates and paper on a printing press has a very important guiding role for flexo printing and color separation.
Of course, the binder and the pigment should not be separated where the ink adheres to the surface of the paper, which cannot be ignored. In addition, overprinting has a great influence on printability. In order to obtain a good overprint effect, the pH value of the water-based ink should be valued. The pH of the ink used for the first ink should be between 9-9.3. The pH of the ink used for the first ink should not fall below 9. The ink used for the second ink should have a lower pH. The drying properties of various overprint inks also have a great influence on printability. In the flexo printing, the color of the ink is generally first light, then dark (yellow-magenta-cyan). When printing on coated or semi-coated paper, it is sometimes necessary to change the color to magenta-yellow-cyan in order to obtain a better overprint effect for the water ink.
Paper smoothness Paper smoothness affects the overall appearance of the ink film and the print sheet. There are two types of defects in smoothness. The former refers to defects that can be seen with the naked eye, such as uneven thickness or wrinkling of the weight. These defects are sometimes caused by the structure of the fibers. Small flaws are flaws that are invisible to the naked eye, such as pits and holes that can damage the dots and affect print quality.
Dust particles that adhere to the paper or web edge during printing can also affect printability. If no ink filter is installed on the flexographic printer, dust particles will collect in the ink and be carried on the plate. This phenomenon can be eliminated by installing an ink filter in front of the ink pump. In addition, dust particles may adhere to the underside of the squeegee, causing streaks on the printed sheets; sometimes, dust particles may adhere to the ink holes of the stencil roll, resulting in uneven ink transfer. Therefore, the ink filter should be installed as soon as possible to effectively control the dust particles.
Visual effects In the relationship between paper and printing effects, you should also pay attention to the effect of the paper's ink and coloring properties on the ink film. Although, the color of the paper has a certain influence on the effect of the printed sheet. Because of the difference in printing, the same ink produces a very different visual effect and can even change the color tone. The same kind of job is printed on different types of paper, and the quality of the print is different. It often makes people feel disappointed.
Chemical composition The chemical composition of paper has a great influence on printability. Excessive acid content in the paper slows down the drying of the ink. This is more evident in areas with high relative humidity and high moisture content. In general, non-coated papers with a pH of 5 or 5.5 (or slightly higher) and coated paper with a pH of 7-8.5 do not cause problems with ink drying unless the relative temperatures are high, such as In Southeast Asia, relative humidity is above 65%.
According to the specific circumstances, heating and rapid drying can be used. Understand the paper's moisture accountant and keep it at the required level, which is a basic guarantee for producing high-quality prints. Paper fibers, like sponges, absorb or release water molecules according to the humidity of the printing environment. The state in which paper fibers neither absorb nor release water molecules is called equilibrium. Different grades of substrates have different balance points. For example, at the same humidity, the absolute humidity of bleached paperboard can reach 6%, and the absolute humidity of unbleached kraft paper can reach 7.5%. Dry webs tend to break on the press because many flexo printers pull the web over the printing unit. The normal humidity of the paper is 5-7%. High humidity can affect the paper's ink performance. When the humidity is too low, the ink layer cracks when the printed sheet is folded. Paper fibers absorb moisture from the air, the volume becomes larger, and internal pressure is generated. Release of internal pressure can cause warpage of the paper edge.
(to be continued)
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