Color difference control of solvent-based inks
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For ink producers and users, minimizing color difference is the direction of continuous efforts. In this paper, the effects of color difference and cell depth, scraper, viscosity and other factors are introduced in detail, and some objective solutions to control chromatic aberration are also proposed. Controlling chromatic aberration is the biggest problem in solvent-based ink control. The color of the printed product has two aspects: one is the same color as the same batch, and the other is that the colors of different batches are the same. For a print, it is very difficult to ensure that it is consistent with the same batch, and the consistency of different batches is more difficult.
Compared with printing processes such as lithography, flexo printing, and silk screen printing, gravure printing is a printing process in which the color difference of the product is small. The reason for the color difference may be mainly caused by the different ink preparation batches, which is relatively easy to compare and find. Therefore, in the process of transferring ink to the substrate, in addition to the influence of the amount of ink and the concentration of the ink, the influence of other factors on the color difference of the gravure can be said to be small.
First, the relationship between gravure cells and color
It can be known from the principle of gravure printing that the amount of ink in the gravure printing is mainly determined by the shape, size, depth and number of the printing cylinders, that is, the volume of the ink in the cell area per unit area, and the amount of ink in the plate cylinder is in accordance with the plate cylinder. The change of the change, so when the printing batch is large and need to use multiple sets of plate cylinders to continuously complete the task of the same product (such as smoked packaging), it is necessary to strictly control the various process parameters when making the same color plate cylinder. Such as the number of mesh lines, mesh angle, depth, trench, and even the hardness of the carved copper layer, so that they are consistent, to minimize the chromatic aberration caused by the replacement of the plate cylinder. Once this happens, it can only be adjusted in the ink. When using the new version, add a lighter amount of ink to reduce the tinting strength of the ink to make it close to the original color.
With the increase of the number of prints in the same plate cylinder, the surface chrome layer will gradually wear under the action of the ink scraper to make the mesh gradually change, and the ink amount of the plate cylinder will also become smaller, and the color of the printed matter will also be It gradually fades, although this is an inevitable process, but because the gravure cylinder is plated with a hard chrome layer of 0.08-0.10mm after engraving on the copper layer, this layer of chrome has good abrasion resistance, generally The normal service life (for example, the full version of the dark version) is between 1.5 and 2 million revolutions. Therefore, in the small batch printing, the color difference of the gravure printing product is not easy to be noticed, and only in the large-volume printing, the late product will have an appreciable color difference compared with the previous product. Some ink suppliers will have a special color supply to compensate for the tinting strength of some spot colors to delay the color difference caused by the shallower cells, which in turn extends the life of the plate cylinder. Fine adjustments are required during operation. For worn plate cylinders, confirm that the wall is not damaged, it can be re-chromed and then chrome-plated and reused. If the wall of the cell is damaged, the plate cylinder must be remade. The depth of the newly-made plate mesh is different from that of the old one. When using it, the ink should be adjusted according to the actual print.
The effect of ink concentration on chromatic aberration is mainly caused by changes in the ink pigment content per unit volume. The main factor that causes this change in the printing process comes from the continuous circulation of the gravure ink in the ink supply system, so that the solvent in the ink is continuously volatilized, causing the ink to become more and more concentrated, so the solvent is added to adjust the ink to the initial setting. The process of constant concentration is often short-lived, sometimes with excessive solvent addition, and repeated adjustments. This kind of frequent adjustment often reflects the color difference on the product. Therefore, when formulating the relevant operation regulations, the addition of solvent is more suitable and less suitable. The unit with conditions is better to use the automatic solvent adding device on the unit. At any time, the ink concentration in the ink tank is detected and adjusted, which is beneficial for reducing the chromatic aberration in batch printing.
Second, the main factors affecting the color difference
The first is the influence of ink, which is the main factor determining the uniform color of the printed matter. The color of the printing ink used should be stable. The same batch and different batches should be kept as small as possible; new ink should be added frequently during printing; for spot color ink ( In particular, some spot colors formulated with white ink should be stirred frequently to prevent separation of these composite colors; the ink is preferably kept constant during printing; the solvent added during printing cannot be contaminated by other colors.
In addition, the ink used in printing should be the same as that used in proofing (same manufacturer, same series, same model), in order to ensure consistent color.
The second is the effect of the scraper. This is because the material of the scraper, the size of the assembly, the position of the contact plate, the angle and the pressure have a great influence on the color of the print, especially the light of the shallow screen. The softer the blade (or when the assembly is long), the darker the print. The greater the distance between the contact point of the blade and the printing plate and the printing point, the lighter the color of the printed product. The greater the pressure of the blade, the lighter the color of the print.
The third is the influence of the plate roller. This is because the plate roller becomes shallow after a period of use, and the amount of ink is reduced to cause color deviation. Therefore, after a certain amount of printing, the re-plating roll should be considered. In addition, if the blockage occurs, the color will also be lighter.
The fourth refers to the influence of the light source. It is better to use the standard light source (D65) or daylight, and the light source should be located directly in front of the front. Because of the difference in the spectral band of the source, the hue of some colors is changed. The difference in the angle of the light source also affects the hue of the color.
Fifth, the influence of other printing conditions, such as the temperature and humidity of the printing workshop, the speed of printing and the drying speed of the solvent, the printing pressure, and the hardness of the printing roller will affect the color of the printing.
In the final analysis, when the color deviation of the printed product, it is necessary to consider the amount of ink in the ink, the amount of transfer, the color of the ink itself, and the light source conditions when viewing the color.
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Third, the color difference between ink manufacturers and printing plants
The first is the printing method. Most of the ink factories use small printing presses imported from the UK. The mesh of the machine is on a flat plate, and the printing is carried out by a circular embossing roller with a printing film. The printing machine is round. In the rounding mode, the mesh is on the rotating circumferential roller. The number of lines, angles, etc. of the two types of meshes are quite different, which makes the same ink, in the two printing modes, there is no small difference, sometimes not only the problem of dark color, but It is the effect that the hue, the brightness, etc. will bring a big difference. Some small factories use the ink stick to match the sample, which is even worse. Using the proofing machine of the plate making factory to match the color, the effect will be much better than that of the imported small printing machine, but the price is almost the same. This proofing machine can be made into the same version of the printing factory, and can design different levels and shades of printing according to the needs. The pattern, which makes the printing method basically the same as the printing house, and an important factor affecting the hue - the printing plate, is the same as the printing house.
Secondly, the plate is deep, different prints have different depths, and the ink factory's understanding or estimation of the plate depth used in the printing also affects the accuracy of the color correction. Obviously, if the ink factory prints with a 45μm deep plate for color, and the customer's version is much smaller than 45μm, the printed color will become lighter, and vice versa. Some people think that the ink is adjusted according to the standard ink provided by the user. The depth of the plate can be ignored. In fact, this is a theoretical view, but it is not the case in practice. From the theory, the same two inks (such as splitting a cup of ink into two), whether the plate is deep and shallow (other conditions are the same), the hue is consistent. However, in actual color grading, it is impossible to match the same ink, so there is often such a phenomenon; sometimes the printed version is closer to the color (can meet customer requirements), but if the version is printed, the color is It's a lot worse, so it's important to master the depth of the version. If the customer's version is deep, you must use a deeper version to print the color.
Again, the viscosity should be said. The printing viscosity of the ink factory to the color should be as close as possible to the printing viscosity when the printer prints the ink. The farther the difference between the two, the greater the difference in color. The ink factory uses 22s for coloring, while the customer uses 35s. At this time, the color is definitely much deeper, and vice versa. Some ink factories do not pay much attention to this problem. They do not consider the viscosity of the printing plant. They always use the uniform viscosity to compare the customer's standard samples (there are ink samples and printed samples), which causes a large color difference.
Another point is that the printing materials, the ink factory used in the color of the material and the printing plant (including other processes) will also cause a large color difference.
Some inks will be printed with a layer of white ink that will be closer to the customer's print, and some will be the opposite. Some ink customers do not change much after compounding, and some of them change very much, such as some transparent colors. Therefore, when the ink factory is toning, it is necessary to figure out the customer's process. The most basic ones include: whether to print the white ink tray, what compound Whether it is glazing oil.
Theoretically, the closer the printing conditions of the ink factory to the printing conditions when printing with the ink, the higher the accuracy of the inks that are recalled. However, due to the limitations of the conditions, there are still many differences between the two. For example, the printing speed, the environment for viewing the color, the pressure of the printing roller, etc. are impossible to be unified. As long as these four parts are controlled, it can definitely be greatly improved. The accuracy of the color correction of the ink factory.
In the printing process, how to keep the color of a batch of printed matter or batches of prints consistent is a basic requirement and an important problem. The hues of the hue can only be said to be relative. It is impossible to agree on the absolute hue. Especially when the number of times of printing is large, the color of this batch may be far worse than that of the other batch.
Fourth, how to control the hue to the maximum consistency
As the ink, it is required that the hue of the ink does not vary, and it is preferable to use the same batch of ink first. Because of the different batches of ink, although some hue has almost no change, some have changed, which requires a good supply of raw materials. The consistency of the ink hue is the most basic requirement and an important condition. If the ink hue deviation is too large, it is difficult to control the hue consistency by other means. When several colors are stacked or the ink layer is thick, the hue is The difference is obvious. Secondly, it is required to add less ink to the ink tray each time, so as to keep adding new ink frequently, especially some patterns with less ink loading. If too much ink is applied at one time, the ink is used for a long time without new ink added. Its hue will change. For example, a long black ink is gray, no new ink is so black, a long red ink will become dark red, no new ink so bright red. The third is to require the consistency of the ink to be consistent throughout, preferably with a viscosity controller.
In the same important position is the scraper. The position at which the blade contacts the plate has a large effect on the hue, especially the hue of the shallow mesh, so the scraper is required to be substantially at the same position from start to finish. The pressure of the scraper has little effect on the hue and can vary within a certain range, but it is also influential if the change is too large, especially for the printing of shallow netting.
The third aspect is the plate factor. The influence of the printing plate on the hue is mainly manifested in the following: the first is easy to block the plate, and the amount of ink transfer is reduced, thereby affecting the hue. Therefore, use a decontamination powder to clean the plate and print it under normal conditions. The second is the wear of the plate. When the printing plate has a large number of printing, as the degree of wear increases, the hue changes, and overall it becomes lighter. In order to solve this problem, if the printing batch is large and often printed, the ink can be diluted in the new version, and then as the printing volume increases, the degree of dilution is gradually reduced until it is no longer diluted.
Fifth, the normal printing status requirements
The change in hue is mainly related to the above problems, but it is also closely related to proofing and signing. It can be said that the problem of the proof is not solved. The control of the hue is simply not enough. Many printers print a sample first, and then let the customer confirm the effect and then formally print. Printing samples is critical, and it directly affects the ability to control the hue. It is summarized in one sentence; when printing a sample, it should be carried out under normal printing conditions, and it cannot be printed in an abnormal state in pursuit of a certain hue. The normal printing status includes the following aspects.
1. The viscosity of the ink should be suitable. The excessive viscosity is not conducive to the transfer of the ink, and it is easy to produce a paste, especially black ink. If it is a proof confirmed in a state of high viscosity, it is impossible to print for a long time. It will definitely paste the plate and make the printed products scrapped. When the viscosity is too small, it is easy to appear static electricity, and it is not conducive to the control of hue.
2, the ink is best mixed with old and new. If the printing time is too long, it is difficult to maintain the hue consistent with the ink proof that the hue has changed. The reason is simple, because without these same ink additions, only new inks are added, and the hue of the new ink is different from the hue of the original ink.
3. The position of the scraper to contact the plate should be appropriate. If the print is carried out at a distance between the position where the blade contacts the printing plate and the position where the rubber roller contacts the printing plate, in the case of official printing, it is difficult to transfer the film on the ink in some places with shallow texture. There will be flowers on the product. When the distance is shortened and the point is attempted to be removed, the hue will definitely change. Of course, if it is a solid printing, there is basically no such problem.
4. The control of hue is mostly to control the mixing effect of multiple colors. The above is for the change of one color. In the specific operation, it is necessary to accurately determine which color has changed so that the mixed hue has changed. This is the premise. If it is a judgment error, it is impossible to bring the hue to the correct direction. In combination with the above operation, the control of the hue is likely to be more satisfactory.
Sixth, the application of color measuring instruments
In the color measurement of printed matter, in order to improve the measurement accuracy and uniform test method, CIE (International Commission on Illumination, an international organization for color and color measurement) officially recommended the illumination and detection geometry used in four color measurement instruments in 1971. They are: 0/45°, 45°/0, 0/d, d/0. (In practical applications, considering the effects of including or eliminating specular reflection, is it more likely to use 8°/d and d/8°?).
We see that the basic principle of the 0/45° measurement geometry is that the light is incident perpendicularly and is received at 45° after being reflected by the surface of the object. (According to the reversibility of the optical path, we know that the measurement geometry of 45°/0 is similar in principle)? Single-angle incidence, single-angle reception, the principle of light reflection from the surface of the object, we know that the color measurement instrument using this kind of measurement geometry It is more suitable for the measurement of the surface color of general paper prints. X-Rite's 900 Series instruments use this type of measurement principle, while the X-Rite 500 Series Spectrodensitometer uses 45/0° measurement optics. These two series of instruments are well suited for the measurement and control of general paper print colors and achieve consistent and accurate results.
The basic principle of the d/8° integrating sphere measuring instrument is that the light is vertically irradiated on the measuring object, and the reflected light is uniformly reflected in the integrating sphere (the inner surface of the integrating sphere is completely reflected diffuse material, and the light can be reflected back and forth) Single angle reception. It is equivalent to a uniform sphere of light that illuminates the surface of the measuring object and measures its spectral reflectance. Generally, a gloss absorption trap is installed inside the sphere at a position opposite to the receiving angle. When the gloss absorption trap is opened, any specular reflection can be reduced to a minimum value. This measurement method is called "excluding specular reflection" (SPEX). The results are similar to those of the above 0/45° instrument. When we measure the color appearance of a specular surface, we need to consider the gloss effect of specular reflection, turn off the gloss absorption trap, and take the “including specular reflection” method (SPIN). There is no doubt that measuring and controlling the transfer of paper packets should be measured in this way.
There are several factors that should be considered when using SP instruments to control the color difference of printed paper packets. The following are the following:
1. When using color comparison, consider using a unified light source and on-site observation conditions to ensure consistency and comparability of measurement results.
2, due to the high-gloss reflection of the transfer paper, the measurement of the transfer paper and its printed products with SP series instruments should be measured by "including specular reflection" (SPIN).
3. For some special printing inks with high hiding rate printed on transfer paper, consider the “excluding specular reflection” (SPEX) method.
4. For white paper cigarette packs or general paper prints, when measuring with SP series instruments, they should be measured by “excluding specular reflection” (SPEX), and the color data is comparable with the data measured by other instruments.
5. It is important to choose a uniform color difference formula. According to my experience, the DEcmc color difference formula (LC=1.41) for measuring the transfer paper and its printed matter is in good agreement with the observation of the human eye.
6. The final problem with color measurement is to solve the problem of color difference evaluation between printed products and standard color samples and how to maintain the color consistency of large-volume prints. Due to the inevitable fluctuations in the printing production process, printing color fluctuations are inevitable. How to stabilize the production process and determine the color tolerance is a real problem that every printing company must face. According to my experience, some printing customers will directly specify a color tolerance standard, and even the same sheet will be handed over to the printing house, and the products delivered by the printing factory must meet the tolerance range. However, most printing customers currently do not have clear inspection standards. The printing companies themselves have not established internal color control standards according to their actual conditions, and finally cause quality disputes during delivery. If the printing factory usually uses the color measuring instrument to measure according to its actual situation, draw the color difference tolerance map to develop its own internal color difference control level, on the one hand, it can optimize internal management, improve product quality and production efficiency; on the other hand, it can be in advance with customers. Conduct color quality communication to reduce disputes.