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Mobile: 13922171608 [WeChat]
Mobile: 13922338249 [WeChat]
Address: No. 47 Industrial Avenue, Xinlian Industrial Zone, Lishui Town (Navigation: Foshan Zhongli Printing)
The secret to controlling color difference in packaging box printing (Part 1)
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The color difference problem of printed products in packaging boxes has always plagued printing workers. Whether the color difference problem can be effectively controlled is a measuring stick that reflects the quality, reputation, and skill level of the company's products and operators. Therefore, it is a problem that every printing worker and their company must pay attention to.
The color difference of printed products can be roughly divided into two phenomena. One refers to the phenomenon where the color of the same batch of products is consistent, but there is a color difference from the sample. Secondly, it refers to color differences within the same batch, some of which are consistent with the sample, while others are inconsistent with the draft.
Now we will discuss and analyze the following aspects with everyone.
1、 The first phenomenon is that the color of the same batch of products is consistent, but there is a color difference from the sample
1. Human factors:
It has nothing to do with the captain's skill level, but it is related to the captain's sense of responsibility. Since the colors of the same batch of products can be consistent, it indicates that the captain's skill level is still not low. But why do they dare to print it if it is not consistent with the sample? Have you signed the first sample yet? This is entirely a matter of the captain's sense of responsibility. (It cannot be ruled out that there may have been a mistake in signing the sample, and if so, it is related to the responsibility of the signatory.).
2. Paper color:
Different whiteness of paper has different effects on the color rendering of the printing ink layer. Because different whiteness is equivalent to adding different black, red, blue, or yellow colors to the ink, although the ink volume and hue do not change in printing, the actual ink has a certain degree of transparency, and the color effect appears with different whiteness of the paper, causing different color differences. When cutting, the same batch of paper must be used. Because although the weight, specifications, and dimensions are the same, the batch number and date of production are different, resulting in differences in the whiteness of the paper and causing color differences in the printed products. So it is necessary to use paper with the same whiteness as the printing paper for the same product.
3. Glossiness and smoothness of paper:
The glossiness of printed materials depends on the glossiness and smoothness of the paper. Offset color printing is when light is incident on the surface of paper, and the light is reflected back to the retina of the human eye, where it is perceived through photosynthesis by color sensitive cells. If the glossiness and smoothness of the paper are high, the color we observe is basically the color reflected through the ink layer, and the saturation of the main color light is high. If the surface of the paper is rough and has low glossiness, diffuse reflection will occur, which will reduce the saturation of the main color light and make the color of the printed matter appear lighter to our eyes. Measure the density value using a densitometer for the same amount of ink. Paper with high smoothness and glossiness has a high density value. Paper with low smoothness and glossiness has a low density value.
4. Surface treatment of printed sheets:
After surface treatments such as lamination, glazing, calendering, oiling, and printing, printed materials will undergo varying degrees of color changes. Some of these changes are physical changes, while others are chemical changes. The physical changes are mainly reflected in the increased specular reflection on the surface of the product, which has a certain impact on the color density. For example, by applying complex light film, UV varnish, calendering, etc., the color density will increase. The color density of the printed product decreases after applying sub film and sub gloss oil. The chemical changes mainly come from laminating adhesive, varnish, UV oil, and so on. These materials contain various solvents that can cause chemical reactions in the color of the printing ink layer, resulting in color changes. Therefore, as a printed product of packaging offset printing, if there is a post press processing technology during printing, the physical and chemical changes of post press processing must be taken into account to determine the density value and Lab value of the ink layer during printing.
5. Dry density value:
After the offset printing product is just printed, the ink is still in a wet state, and the density value measured at this time is different from the density value measured after the printed product is dried. The density value is high when wet and low when dry, which is the phenomenon of drying out density value. This is because the newly printed ink layer has a certain degree of leveling. It is mainly manifested as mirror reflection on the surface, with bright colors and good luster. After the ink layer dries, there is diffuse reflection on the surface, and the natural luster appears dull and dull compared to when it was first printed. In order to minimize color difference as much as possible during regular printing, we adopt the same wet density testing method for control. The first printed product was immediately measured for wet density and L * a * b * value with the customer's signature and agreement in a wet state. In the future, when printing the product (in a wet state), the color control will be based on the density value and L * a * b * value obtained from the first measurement. Of course, if a density meter with a polarizing mirror device is used, it can eliminate the light generated by mirror reflection on the surface of the ink layer. The wet density measured by it is relatively close to the dry density, and it can also help control the reduction of color difference.
6. Printing pressure:
Printing pressure is one of the conditions for ink transfer. Due to the fact that the surface of the printing plate, rubber blanket, etc. cannot be absolutely flat, the surface of the paper is inevitably subject to subtle irregularities or uneven thickness. If the printing pressure is insufficient or uneven, the ink layer is prone to uneven concentration. Therefore, in the process, it is required to achieve "three flatness", that is, the surface of the printing plate, the rubber blanket, the substrate, and the lining surface must all reach a relatively flat condition, in order to obtain a relatively uniform ink color on the printed product through a thinner ink layer and balanced printing pressure.
When the printing pressure is insufficient or uneven, and there is poor contact between the backing body and the printing plate surface, it is necessary to increase the ink supply to meet the visual effect of ink on the plate. However, this not only increases the consumption of ink, but also easily leads to color difference and dirt in printing. Therefore, it is necessary to achieve "three levels" (ink roller level, water roller level, and drum level) by using uniform and constant printing pressure to prevent color difference caused by improper pressure.
7. Check the light source of the sample table:
When observing the color of offset printing products, there must be a light source. Without light, the color cannot be seen. However, if the characteristics of the light source are different, the difference in color will be significant. Generally, we require observing colors under natural light conditions (i.e. standard light sources). If a regular light bulb is used as a light source to observe color, the color will appear yellowish, making it difficult to accurately identify the color, and the final printed product will have severe color cast. In addition, the intensity of light and the angle of illumination can also affect color discrimination. Under the same light source, the intensity of light reflected on the illuminated sample is mainly determined by the distance between the sample and the light source. Those who are close are stronger, those who are far are weaker. So we need to pay attention to two things in our work, whether it's day or night: one is to use a standard light source (wavelength around 400-700), and the other is to ensure that the strength of the light source is equal to the angle of illumination. For products with high color requirements, the ink color sample of the printed sheet should be placed on top of the standard sample for comparison. This way, due to the small difference in observation angles, the identification of colors is relatively accurate. Therefore, process practice tells us that the characteristics of light sources and differences in observation angles will directly affect the accurate identification of colors
The color difference of printed products can be roughly divided into two phenomena. One refers to the phenomenon where the color of the same batch of products is consistent, but there is a color difference from the sample. Secondly, it refers to color differences within the same batch, some of which are consistent with the sample, while others are inconsistent with the draft.
Now we will discuss and analyze the following aspects with everyone.
1、 The first phenomenon is that the color of the same batch of products is consistent, but there is a color difference from the sample
1. Human factors:
It has nothing to do with the captain's skill level, but it is related to the captain's sense of responsibility. Since the colors of the same batch of products can be consistent, it indicates that the captain's skill level is still not low. But why do they dare to print it if it is not consistent with the sample? Have you signed the first sample yet? This is entirely a matter of the captain's sense of responsibility. (It cannot be ruled out that there may have been a mistake in signing the sample, and if so, it is related to the responsibility of the signatory.).
2. Paper color:
Different whiteness of paper has different effects on the color rendering of the printing ink layer. Because different whiteness is equivalent to adding different black, red, blue, or yellow colors to the ink, although the ink volume and hue do not change in printing, the actual ink has a certain degree of transparency, and the color effect appears with different whiteness of the paper, causing different color differences. When cutting, the same batch of paper must be used. Because although the weight, specifications, and dimensions are the same, the batch number and date of production are different, resulting in differences in the whiteness of the paper and causing color differences in the printed products. So it is necessary to use paper with the same whiteness as the printing paper for the same product.
3. Glossiness and smoothness of paper:
The glossiness of printed materials depends on the glossiness and smoothness of the paper. Offset color printing is when light is incident on the surface of paper, and the light is reflected back to the retina of the human eye, where it is perceived through photosynthesis by color sensitive cells. If the glossiness and smoothness of the paper are high, the color we observe is basically the color reflected through the ink layer, and the saturation of the main color light is high. If the surface of the paper is rough and has low glossiness, diffuse reflection will occur, which will reduce the saturation of the main color light and make the color of the printed matter appear lighter to our eyes. Measure the density value using a densitometer for the same amount of ink. Paper with high smoothness and glossiness has a high density value. Paper with low smoothness and glossiness has a low density value.
4. Surface treatment of printed sheets:
After surface treatments such as lamination, glazing, calendering, oiling, and printing, printed materials will undergo varying degrees of color changes. Some of these changes are physical changes, while others are chemical changes. The physical changes are mainly reflected in the increased specular reflection on the surface of the product, which has a certain impact on the color density. For example, by applying complex light film, UV varnish, calendering, etc., the color density will increase. The color density of the printed product decreases after applying sub film and sub gloss oil. The chemical changes mainly come from laminating adhesive, varnish, UV oil, and so on. These materials contain various solvents that can cause chemical reactions in the color of the printing ink layer, resulting in color changes. Therefore, as a printed product of packaging offset printing, if there is a post press processing technology during printing, the physical and chemical changes of post press processing must be taken into account to determine the density value and Lab value of the ink layer during printing.
5. Dry density value:
After the offset printing product is just printed, the ink is still in a wet state, and the density value measured at this time is different from the density value measured after the printed product is dried. The density value is high when wet and low when dry, which is the phenomenon of drying out density value. This is because the newly printed ink layer has a certain degree of leveling. It is mainly manifested as mirror reflection on the surface, with bright colors and good luster. After the ink layer dries, there is diffuse reflection on the surface, and the natural luster appears dull and dull compared to when it was first printed. In order to minimize color difference as much as possible during regular printing, we adopt the same wet density testing method for control. The first printed product was immediately measured for wet density and L * a * b * value with the customer's signature and agreement in a wet state. In the future, when printing the product (in a wet state), the color control will be based on the density value and L * a * b * value obtained from the first measurement. Of course, if a density meter with a polarizing mirror device is used, it can eliminate the light generated by mirror reflection on the surface of the ink layer. The wet density measured by it is relatively close to the dry density, and it can also help control the reduction of color difference.
6. Printing pressure:
Printing pressure is one of the conditions for ink transfer. Due to the fact that the surface of the printing plate, rubber blanket, etc. cannot be absolutely flat, the surface of the paper is inevitably subject to subtle irregularities or uneven thickness. If the printing pressure is insufficient or uneven, the ink layer is prone to uneven concentration. Therefore, in the process, it is required to achieve "three flatness", that is, the surface of the printing plate, the rubber blanket, the substrate, and the lining surface must all reach a relatively flat condition, in order to obtain a relatively uniform ink color on the printed product through a thinner ink layer and balanced printing pressure.
When the printing pressure is insufficient or uneven, and there is poor contact between the backing body and the printing plate surface, it is necessary to increase the ink supply to meet the visual effect of ink on the plate. However, this not only increases the consumption of ink, but also easily leads to color difference and dirt in printing. Therefore, it is necessary to achieve "three levels" (ink roller level, water roller level, and drum level) by using uniform and constant printing pressure to prevent color difference caused by improper pressure.
7. Check the light source of the sample table:
When observing the color of offset printing products, there must be a light source. Without light, the color cannot be seen. However, if the characteristics of the light source are different, the difference in color will be significant. Generally, we require observing colors under natural light conditions (i.e. standard light sources). If a regular light bulb is used as a light source to observe color, the color will appear yellowish, making it difficult to accurately identify the color, and the final printed product will have severe color cast. In addition, the intensity of light and the angle of illumination can also affect color discrimination. Under the same light source, the intensity of light reflected on the illuminated sample is mainly determined by the distance between the sample and the light source. Those who are close are stronger, those who are far are weaker. So we need to pay attention to two things in our work, whether it's day or night: one is to use a standard light source (wavelength around 400-700), and the other is to ensure that the strength of the light source is equal to the angle of illumination. For products with high color requirements, the ink color sample of the printed sheet should be placed on top of the standard sample for comparison. This way, due to the small difference in observation angles, the identification of colors is relatively accurate. Therefore, process practice tells us that the characteristics of light sources and differences in observation angles will directly affect the accurate identification of colors