Must read: Tips for packaging and printing color difference control

The chromatic aberration problem of packaging printed products has always troubled printing workers. Whether the chromatic aberration problem can be effectively controlled is a measuring bar that reflects the company's product quality, reputation and skill level of the operator. Issues that all companies must pay attention to.

The chromatic aberration of printed matter is roughly divided into two phenomena. The first refers to the same color of the same batch of products, but there is a chromatic aberration with the color of the sample. The second refers to: there are chromatic aberrations in the same batch of colors, some are consistent with the sample, but some are inconsistent with the sample.
Now we will discuss and analyze with you from the following aspects.
1. The first phenomenon (the color of the same batch of products is the same, but there is a color difference with the color of 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 color of the same batch of products can be consistent, it means that the captain's skill level is still not low, but why dare to print it if it is inconsistent with the sample? Is the first sample signed? This is entirely a question of the captain's sense of responsibility. (It does not rule out mistakes in signing, if so, it is related to the responsibility of the signer).
2. Paper color:
Papers with different whiteness have different effects on the color of the printing ink layer. Because the different whiteness is equal to adding different black, red, blue or yellow to the ink, although the amount of ink and hue have not changed during printing, the actual ink has a certain transparency, and the color effect varies with the whiteness of the paper Shows differently, causing different color differences. When opening, it is necessary to use the same batch of paper. Although the basis weight, specification and size are the same, but the production batch date is different, the whiteness of the paper will have a certain difference, resulting in the color difference of the printed product. Therefore, paper with the same whiteness must be used as printing paper for the same product.
3. The gloss and smoothness of the paper:
The glossiness of the printed matter depends on the glossiness and smoothness of the paper. Offset color printing is when light is incident on the surface of the paper, the light is reflected to the retina of the human eye, and the color is seen through the photosynthesis received by the color sensor cells. If the gloss and smoothness of the paper are high, the color we observe is basically the color reflected through the ink layer, and the main color has high saturation. If the surface of the paper is rough and the gloss is low, diffuse reflection will occur, which will reduce the saturation of the main color light and make the color of the printed product feel lighter for the human eye. The density of the same ink volume is measured with a densitometer. Paper with high smoothness and glossiness has high density value. Paper with low smoothness and glossiness has low density value.
4. Surface treatment of printed sheets:
After the printed products are surface treated by laminating, glazing, calendering, oiling, and printing, there will be different degrees of hue changes. Some of these changes are physical changes and some are chemical changes. The physical change is mainly reflected in the increased specular reflection on the product surface, which has a certain effect on the color density. For example, compound film, UV varnish, calendering, etc., the color density will increase. The color density of the printed product decreases after the compound sub-film and the matte oil are applied. The chemical changes mainly come from laminating glue, varnish, UV oil and so on. These materials contain a variety of solvents, and they all cause the color of the printing ink layer to undergo a chemical reaction and cause a color change. Therefore, as the printed matter of packaging offset printing, if there is a post-press processing process during printing, the physical and chemical changes of the post-press processing must be taken into consideration to determine the density value and Lab value of the ink layer during printing.
5. Dry retreat density value:
Immediately after the offset printing product is 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. This is because the ink layer just printed has a certain leveling. It is mainly reflected by the specular reflection on the surface, which looks bright and shiny. When the ink layer is dry, there is diffuse reflection on the surface, and the natural luster appears dull and dull than when it was just printed. In order to ensure that the batch products reduce the color difference as much as possible during printing, we adopt the same wet density test method to control. That is, the first time the printed product is wet, the wet density value and L * a * b * value are measured immediately after the customer signs and agrees. When printing the product later (wet state), the color is controlled according to the density value and L * a * b * value measured for the first time. Of course, if a densitometer with a polarizer device is used, it can eliminate the light generated by the specular reflection on the surface of the ink layer. The measured wet density is relatively close to the dry density, and it can also help to control the reduction of color difference.
6. Printing pressure:
Printing pressure is one of the conditions for ink transfer. Since the surface of the printing plate, blanket, etc. cannot be absolutely flat, the surface of the paper is also inevitable with fine unevenness or uneven thickness. If the printing pressure is insufficient or uneven, the ink layer is prone to uneven density, so the process requires "three flats", that is, the printing plate surface, the blanket and the substrate and the liner surface must be relatively flat Conditions, in order to obtain a more uniform ink color on the printed matter with a thinner ink layer through a balanced printing pressure.
When the printing pressure is insufficient or uneven, and there is a poor contact between the backing body and the printing plate surface, it is necessary to increase the amount of ink infusion to meet the visual effect of the printing plate. But this will not only increase the consumption of ink, but also easily produce the color difference of the printing and the sticky phenomenon of the printed product, so do "three flat" (ink roller flat, water roller flat, roller flat) to use uniform and constant printing Pressure can prevent the color difference caused by inappropriate pressure.
7. The light source of the prototype:
Offset printing must have a light source when observing the color. If there is no light, the color cannot be seen, but if the characteristics of the light source are different, then the difference in color will be very large. Generally, our requirement is to observe the color under the condition of natural light (that is, standard light source). If you use ordinary light bulbs as the light source to observe the color, the color will appear yellow, and it is difficult to accurately identify the color. Finally, the printed product will have a serious color cast. In addition, the intensity of light and the angle of irradiation will also affect the color identification. 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. The closer is stronger, the farther is weak. Therefore, we usually pay attention to two points during the day and night at work: one is to use a standard light source (wavelength of about 400-700), and the other is to pay attention to the intensity of the light source and the angle of illumination. For products with high color requirements, the ink color samples of the printed sheets should be placed on the standard samples for comparison. In this way, due to the small angle of observation, the identification of colors is relatively accurate. Therefore, the process practice tells us that the difference between the characteristics of the light source and the viewing angle will directly affect the accurate discrimination of color (
Second, the second phenomenon (there is a color difference in the same batch of colors, some are consistent with the sample, but some are inconsistent with the sample)
1. Human factors:
I think that the first few colors are correct. When the ink and ink are not balanced, they start to open all the way. During the printing process, they constantly adjust the amount of water and ink, and adjust them while opening. This has a lot to do with the captain's skill level, experience, and personality. It also belongs to the color difference of the same batch of products caused by insufficient ability.
2. Fountain solution:
Offset printing is based on the principle of oil-water repulsion. This water refers to fountain solution. Its size directly affects the color of the ink layer. The water ink layer is light in color, and the water ink layer is dark. The greater the water, the more serious the emulsification of the ink. The emulsified ink is dull and the color is light. The printed products of emulsified ink not only have serious color difference, but also have quality problems such as stickiness, dryness, and poor imprinting. Therefore, in printing and operation, we must strictly control the size of the wetting liquid on the plate surface, and do a good job of water and ink balance. It is required that the water should be as small as possible without getting dirty.
3. Printing speed:
The speed of printing will affect the color depth of the printed matter. This is because the speed of the vehicle speed will affect the balance of ink and wash. The moisture content of the express plate and the slow-moving plate are different. The moisture content of the express plate is small, and the moisture content of the slow plate is large. Due to the change in the size of the water, the imbalance of the ink and water will inevitably cause the color change of the ink. Therefore, you must pay attention to constant speed when printing, and you must not drive fast after a while. After a while, I will drive slowly, so that the printed products will produce color difference. At the same time, it must be adjusted to a most stable speed according to the condition of the machine. For example, some imported offset printing machines indicate a maximum speed of 16,000 sheets / hour, and domestic machines have a maximum speed of 8,000 sheets / hour. These speeds are not stable. Generally speaking, whether it is an imported machine or a domestic machine, a stable speed of 70% of the maximum speed is the most ideal, that is: the imported machine is about 11,000 pieces / hour, and the domestic machine is about 5,600 pieces / hour. Such a speed can be easily controlled. Water and ink balance, reduce color difference.
4. Defective ink roller:
In the offset printing process, the quality of the rubber roller plays an important role in whether the printed product can obtain a uniform ink color. Generally there are three types of rubber rollers: ink transfer roller, ink leveling roller and inking roller, which have a great influence on the ink density. The elasticity, viscosity, center of circle, hardness and surface finish of the ink roller largely determine the quality of the ink color of the printed product. If the ink roller has poor elasticity, insufficient viscosity, the roller body is eccentric. When the colloid is too hard and the colloid is aging, it is easy to cause uneven or unstable ink in the printing process, which makes the ink color of the printed sheet inconsistent. Therefore, the quality of the rubber roller must be ensured during printing to ensure that the ink applied to the printing plate is basically the same every printing cycle. Therefore, the rubber roller should be cleaned in normal work, so that the paper dust and impurities remaining on the rubber roller can be removed, and the ink can be transferred evenly. At the same time, the contact pressure between the ink rollers and between the ink rollers and the printing plate should also be adjusted, otherwise it will cause uneven ink transfer and serious color difference. (
5. Maintenance of equipment:
Maintaining the good performance of the machine is a reliable guarantee for improving the quality of the printing ink color. If the machine is unstable, such as inconsistent pressure, unstable paper feeding, empty sheets or multiple sheets frequently occur during printing. It is bound to cause the color difference of printing. Therefore, we usually do well to maintain the equipment, pay attention to the lubrication of key parts of the machine, prevent the aggravation of wear, and keep the equipment in a stable state. As an offset printer, we must do "three good" (good management, good use, repair), "four meetings" (will use, maintenance, inspection, troubleshooting).
There are many reasons for the formation of printing chromatic aberration, and there are more than one control methods. This requires us to constantly explore and accumulate in the printing operation, and develop a good habit of taking notes, which is conducive to the improvement of our own ability. For books on printing theory, there is a theory to guide you, you can take a lot of detours, customers have higher and higher requirements for the quality of printed products. For us, pressure is more motivation, I believe as long as you can go with a positive attitude In the face, you will be more and more smooth on the road to success.

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Guangzhou Shiruoni Cosmetics Co., Ltd. ,