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Abstract: For ink producers and users, minimizing color difference values ​​has always been the direction of efforts. In this paper, the influence of factors such as chromatic aberration, cell depth, doctor blade, viscosity, etc. is introduced in detail, and some objective solutions to control the chromatic aberration are also proposed.
Keywords: Ink, color difference, hole depth, squeegee, viscosity, X-Rite SP instrument, control of color difference, is the biggest problem of solvent-based ink control. Consistent color of printed matter includes two aspects: on the one hand, it means the same color in the same batch, on the other hand, it means the same color in different batches. For a printed matter, it is very difficult to ensure that the same batch is consistent, but the consistency of different batches is even more difficult.
Compared with printing processes such as offset printing, flexo printing, and silk printing, gravure printing is a printing process in which the color difference of the product changes little. The reason for the color difference may be mainly caused by different batches of ink preparation, which is easier to find by comparison. Therefore, in the process of transferring ink to the substrate, in addition to the influence of the amount of ink and the concentration of ink, the influence of other factors on the color difference of gravure printing can be said to be very small.
1. The relationship between gravure mesh and color
It can be known from the principle of gravure printing that the amount of inking of gravure printing is mainly determined by the shape, size, depth and depth of the holes of the printing cylinder, that is, the amount of ink inked per unit area Change, so when the printing volume is large and multiple sets of plate cylinders are needed to continuously complete the tasks of the same product (such as cigarette packaging), it is necessary to strictly control various process parameters when making the same color plate cylinder Such as engraving the number of mesh lines, mesh angle, depth, trenches, and even the hardness of the engraved copper layer, etc., to make it consistent, and minimize the color difference caused by replacing the plate cylinder. Once this happens, the last resort is to adjust it in the ink. When using the new version, add appropriate amount of dilute ink to reduce the coloring power of the ink and make it close to the original hue.
With the increase of the number of prints in the same plate cylinder in use, the surface chromium layer will gradually wear out under the action of the ink scraper and the mesh hole will gradually change, the amount of ink on the plate cylinder will also become smaller, and the color of the printed product It gradually fades, although this is an inevitable process, because the gravure cylinder is coated with a hard chromium layer of 0.08-0.10mm after the copper layer is engraved, this chromium layer has good wear resistance, generally The normal service life (take the full version of the dark version as an example) is between 1.5 and 2 million revolutions. Therefore, the color difference of gravure printing products is not easy to be noticed when printing in small batches. Only when printing in large batches, there will be a noticeable color difference between later products and earlier products. Some ink suppliers will have a special supply of color enhancer, which can compensate the tinting power of certain spot colors to delay the process of color difference caused by the lightening of the mesh, which essentially extends the service life of the plate cylinder. Of course Fine adjustments are required during operation. For worn plate cylinders, make sure that the chrome wall can be re-plated and chrome plated again under the premise that the mesh wall is not damaged. It can also be used again. If the mesh wall is damaged, the plate cylinder must be remade. The depth of the newly-made printing plate mesh is different from the old one. When using it, the ink should be adjusted according to the actual printing.
The effect of ink concentration on color difference is mainly caused by the change of ink pigment content per unit volume. The main factor that will cause this change in the printing process is that the gravure ink continuously circulates in the ink supply system to continuously evaporate the solvent in the ink, 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 relatively short, sometimes excess solvent is added, and then repeated adjustments. This kind of frequent adjustment often reflects the color difference on the product, so when formulating the relevant operation regulations, it is more appropriate to add the solvent with diligent addition and less addition. It is best for qualified units to use automatic solvent addition devices on the unit , At any time to detect and adjust the ink density in the ink tank, which is very beneficial to reduce the color difference in batch printing.
Second, the main factors affecting color difference
One is the influence of ink, which is the main factor that determines the consistency of the color of the printed matter. The hue of the printing ink used should be stable, and the color difference between the same batch and the different batches should be kept as small as possible; new ink should be added frequently during printing; for spot color inks ( In particular, some spot colors formulated with white ink must be stirred frequently to prevent the separation of these composite colors; the ink should preferably maintain a constant viscosity during the printing process; the solvent added during printing should not 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), so as to ensure consistent colors.
The second is the influence of the scraper. This is because the material of the scraper, the size of the assembly, the position, angle and pressure of the contact plate have a great influence on the color of the printed product, especially the ink color of the light screen. The softer the blade (or the longer the assembly size), the darker the print. The greater the distance between the contact point of the doctor blade and the printing plate and the printing point, the lighter the color of the printed product. The greater the pressure of the scraper, the lighter the color of the printed product.
The third is the influence of the plate roller. This is because the plate roller will become lighter after a period of use, and the amount of ink is reduced to cause color deviation. Therefore, after a certain amount of printing, re-plating roller should be considered. In addition, if blocking occurs, the color will become lighter.
The fourth refers to the influence of the light source. It is best to use a standard light source (D65) or daylight for the light source, and the light source should be located directly above. Because the spectral band of the light source is different, it will change the hue of some colors. The difference in light source angle also affects the hue of the color.
Fifth, the impact 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 embossing rubber roller will all affect the color of the printed product.
In the final analysis, when the color of the printed product deviates, it needs to be considered in terms of the amount of ink on the ink, the amount of transfer, the color of the ink, and the light source conditions when viewing the color.
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3. Factors of color difference between ink manufacturers and printing houses
The first is the printing method. Most of the ink factories use small printing machines imported from the UK. The mesh of this machine is on a flat plate, and the printing is completed by moving the printing film with a round embossing roller. In the rounding method, the mesh is on the rotating circumferential roller. The line numbers and angles of the two meshes are quite different, which makes the same ink have a big difference between the two printing methods, sometimes not just the problem of light and dark colors, but It is the hue, lightness, etc. that will bring about a large difference in effect. Some small factories are even worse with the ink scraper. Use 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 about the same. The pattern, which makes the printing method basically the same as the printing factory, and an important factor that affects the hue-the printing plate, is also the same as the printing factory.
The second is the plate depth. Different prints have different plate depths. The ink factory's understanding or estimation of the plate depth used by the prints also affects the accuracy of color grading. Obviously, if the ink factory uses a 45μm deep plate for color printing, but the client's plate is much smaller than 45μm, the printed color will become lighter, otherwise, it will become darker. Some people think that the ink is adjusted according to the standard ink control provided by the user, and the depth of the plate can be ignored. In fact, this is a theoretical view, but it is not the case in practice. In theory, two identical inks (for example, a cup of ink is divided into two), whether the version is dark or light (other conditions are the same), the hue is the same. However, in actual color matching, it is impossible to formulate the same ink, so there is often such a phenomenon; sometimes the color of the light printed version is closer (can meet the customer's requirements), but if the dark version is printed, the color is There is a lot of difference, so it is very important to master the depth of the plate. The deeper the client ’s version, the darker the version must be used to print the color.
Again, let's talk about viscosity. The printing viscosity of the ink factory's color matching should be as close as possible to the printing viscosity of the printing factory when printing this ink. The farther the difference between the two, the greater the color difference. It takes 22s for color adjustment in ink factory and 35s for customers. At this time, the color must be much darker and vice versa. Some ink factories do not pay much attention to this problem, regardless of the viscosity of the printing factory, and uniformly compare the customer's standard samples (with ink samples and printed samples) with a uniform viscosity, which causes a large color difference.
Another point is the printing materials. The difference between the materials used by the ink factory and the printing factory (including other processes) will also cause a large color difference.
Some inks printed with a layer of white ink will be closer to the customer's prints, and some will be the opposite. Some ink customers have little change after compounding, while some have very big changes, such as some transparent colors, so when ink factories are toning, they must understand the customer's process conditions. The most basic ones include: whether to print white ink support, what materials are compound , Whether to varnish.
Theoretically, the closer the printing conditions of the ink factory during color adjustment and the conditions when the printing factory uses the ink for printing, the higher the accuracy of the transferred ink. However, due to the limitations of the conditions, there are still many differences between the two, such as: printing speed, color viewing environment, pressure of the printing press roller, etc., cannot be unified, as long as these four parts are controlled, it can be greatly improved Accuracy of color adjustment in ink factory.
In the printing process, how to make a batch of printed matter or several batches of printed matter maintain the consistency of hue is the most basic requirement and an important problem. The hue can only be said to be relative. Absolute hue consistency is impossible. Especially when there are many printing times, this batch and the other batch may be far from the possible hue.
Fourth, how to control the hue to the maximum consistency
In terms of ink, it is required that there is no deviation in the hue of the ink. First, it is best to use the same batch of ink. Because of different batches of ink, although some colors have little change, but some have changes, this requires a good check on the 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 consistency of the hue through other methods. When several colors are stacked together or the upper ink layer is thick, the hue The difference is obvious. Secondly, it requires that the amount of ink added to the ink tray be less every time to keep adding new ink frequently, especially some patterns with less ink. If too much ink is added at one time, the ink will be used for a long time without new ink being added. The hue will change. For example, black ink that has been used for a long time is gray, so black is not as pure as new ink, red ink that has been used for a long time will become dark red, and not so bright as new ink. The third is to require the viscosity of the ink to remain consistent from beginning to end, preferably with a viscosity controller.
At the same important position is the scraper. The position of the blade contacting the printing plate has a great influence on the hue, especially the hue of the light texture, so it is required that the blade is basically in the same position from beginning to end. The pressure of the doctor blade has little effect on the hue and can be changed within a certain range. However, if the change is too large, it also has an effect, especially for the printing of light texture.
The third aspect is the printing plate factor. The impact of the printing plate on the hue is mainly manifested in: the first is easy to block the plate, and the amount of ink transfer is reduced at this time, thereby affecting the hue. Therefore, it is necessary to clean the printing plate with decontamination powder to make it print in a normal state. The second is the wear of the printing plate. When the printing quantity of the printing plate is large, as the degree of wear increases, its hue will change, and overall it will become lighter. In order to solve this problem, if the printing volume is large, and the varieties that are often printed, you can dilute the ink in the new version, and then gradually reduce the degree of diluting with the increase of the printing volume until it is no longer dilute.
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V. Normal printing status requirements
The change of 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 signing samples cannot be solved, and the hue control is simply inadequate. Many printing plants print a sample first, and then let the customer confirm the effect and then officially print. It is very critical when printing samples, it directly affects whether the hue can be controlled well. To sum up in one sentence: when printing samples, it should be carried out in a normal printing state, and it cannot be printed in an abnormal state in order to pursue a certain hue. The normal printing state includes the following aspects.
1. The viscosity of the ink should be appropriate. Excessive viscosity is not conducive to the transfer of ink, and it is easy to produce paste plates, especially black ink. If the print is confirmed in the state of high viscosity, it is impossible to print for a long time. Of course, it will definitely paste the page and make the printed product scrapped. If the viscosity is too small, static phenomenon is easy to occur, which is not conducive to hue control.
2. The ink is best mixed with old and new. If the printing time is too long, it is difficult to keep the hue consistent with the ink proofing whose hue has changed. The reason is simple, because without these same inks, only new ink is added, and the hue of the new ink is different from the hue of the original ink.
3. The position of the blade contacting the printing plate should be appropriate. If the printing is performed at a distance between the position where the blade contacts the printing plate and the position where the rubber roller contacts the printing plate, during the official printing, it is difficult to transfer the ink on the film in some places with shallow texture. There will be flower spots on the product. When shortening the distance and trying to clear these flower spots, 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 hue of the mixture has changed. This is a premise. If it is a judgment error, it is impossible to bring the hue to the correct direction. Combined with the above operations, it is possible to obtain a more satisfactory degree of hue control.
6. Application of color measuring instruments
In the color measurement of printed matter, in order to improve the measurement accuracy and uniform test methods, CIE (International Commission on Illumination, an international organization for color and color measurement) officially recommended the lighting and detection geometric conditions for four color measuring instruments in 1971. They They are: 0/45 °, 45 ° / 0, 0 / d, d / 0. (In practical applications, considering the inclusion or exclusion of the effect of specular reflection, 8 ° / d and d / 8 ° are often used?).
We see that the basic principle of the 0/45 ° measurement geometry is: the light is incident vertically 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, and the principle of light reflection from the surface of the object. We know that this type of measurement geometry is used for color measuring instruments 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 X-Rite 500 series spectrodensitometers use 45/0 ° measuring optical geometry. The above two series of instruments can be well applied to the measurement and control of the color of general paper prints, and can achieve consistent and accurate results.
The basic principle of the d / 8 ° integrating sphere measuring instrument is: the light is irradiated vertically on the measuring object, and the reflected light is reflected uniformly in the integrating sphere (the inner surface of the integrating sphere is fully reflective and diffused, and the light can be reflected back and forth) Single angle reception. It is equivalent to a uniform sphere glowing, illuminating the surface of the measuring object and measuring its spectral reflectance. Generally, a gloss absorption trap is installed at a position opposite to the receiving angle inside the sphere. When this gloss absorption trap is turned on, any specular reflection can be reduced to a minimum. This measurement method is called "exclude specular reflection" (SPEX) The result is similar to the measurement result of the above 0/45 ° instrument. When we measure the color appearance of a high-gloss surface and need to consider the gloss effect of specular reflection, close the gloss absorption trap and take the "include specular reflection" measurement (SPIN). There is no doubt that measuring and controlling the transfer of paper cigarette packets should be measured in this way.
Nowadays, a considerable number of printing companies are equipped with X-Rite SP series instruments, which is also an important part of the measurement data processing. X-Rite's SP series instruments use d / 8 ° to measure optical geometric principles, and are particularly suitable for color control of transfer paper and cigarette packs printed on the transfer paper.
Generally, the design of cigarette packs mostly uses spot color printing. We know that using traditional density measurement to control spot colors often fails to achieve the desired results, and there is no way for density control to establish correlation with human eye observation. Therefore, spot color printing generally adopts the method of colorimetric measurement to control color difference to achieve color control.
X-Rite SP series spectrophotometer uses the principle of spectrometry, divides the visible light 400-700nm band into 31 measurement intervals, measures the spectral reflectance of the object color, and then obtains the CIELab value of the color, and directly calculates the excellent difference on the instrument .
X-Rite SP series instruments provide professional-grade color difference control accuracy. The measurement error of the same point can reach 0.05 DEab color difference unit, and the difference between instruments is 0.1 DEcmc color difference unit. It has a built-in pulsed tungsten filament lamp, which can avoid inconsistent visual effects caused by human eyes under different observation conditions. It also provides four levels of measurement management: Job, Project, Standard, and Sample. A measurement can provide 8 standard light sources and two fields of view (2 ° and 10 °) of XYZ, Lab, Lch values ​​and CIE color space at the same time. Color difference comparison value, and "color fingerprint"-spectral curve chart. The internal memory of the instrument can store 1024 standard and more than 2000 sample data. When you complete the quality inspection in the workshop, you can download the data on the instrument to the computer through X-Rite's special color quality control software after returning to the office. Back it up and use it as a reference standard until the next reprint.
There are several factors that should be considered when using the SP instrument to control the color difference of the transfer paper cigarette pack printing, there are the following points:
1. When doing color comparison, you should consider using a unified light source and on-site observation conditions to ensure the consistency and comparability of the measurement results.
2. Due to the high-gloss reflection of the transfer paper, the SP series instrument should be used to measure the transfer paper and its printed products using the "including specular reflection" method (SPIN).
3. For some special printing inks with high hiding rate printed on the transfer paper, the "exclude specular reflection" (SPEX) method can be considered for measurement.
4. For Whiteboard cigarette packs or general paper prints, when measuring with SP series instruments, it should be measured by "exclude specular reflection" (SPEX) method, and its color data is comparable to the data measured by other instruments.
5. It is of great significance to choose a unified color difference formula. According to my own experience, when the DEcmc color difference formula (LC = 1.41) is selected for the measurement of the transfer paper and its printed matter, it is in good agreement with the observation result of the human eye.
6. The final problem of color measurement is to solve the problem of evaluating the color difference between printed products and standard color samples and how to maintain the color consistency of mass prints. Due to the inevitable fluctuations in the printing production process, the printing color fluctuations are inevitable. How to stabilize the production process and determine the color tolerance is a practical problem that every printing company must face. According to my experience, some printing customers will directly specify a color tolerance standard, and even send the same sheet to the printing house, requiring that the products delivered by the printing house must meet the tolerance range. However, at present, most printing customers do not have clear inspection standards, and the printing companies themselves have not formulated internal color control standards according to their own actual conditions, which ultimately leads to quality disputes on delivery. If the printing factory usually uses color measuring instruments to measure according to its own actual situation, and draws a color difference tolerance chart to develop its own color difference control level, on the one hand, it can optimize internal management, improve product quality and production efficiency; Communicate color quality and reduce disputes.
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