[China Packaging Network News] How to keep the consistency of the ink of a batch of printed products or batches of prints in printing is a basic requirement, and it is also an important problem. For manufacturers, the color of the packaging used by the product has become an important means and symbol for identifying the product (even as a corporate logo). No matter where, consumers can recognize Coca-Cola's red, Tide detergent orange and many products. If the appearance of the product packaging looks different, consumers may feel that the quality of the product is not stable and even suspect the authenticity of the product. Therefore, these well-known companies attach great importance to the color of the product packaging, and require its ink must be stable and consistent.
The consistency of color can only be said to be relative. Strict control of the chromatic aberration of the same batch is more difficult, and the strict control of the color difference between large batches or different batches is more difficult. Most of the plastic flexible packaging gravure printing is multi-batch printing and large-volume printing. Controlling the color difference is a very important issue for plastic flexible packaging printing enterprises.
First, ink
Ink is one of the main factors that determine the color consistency of printed products. The ink itself also has the same batch color difference and the color difference problem between different batches, which is an important indicator reflecting the quality of the ink.
To solve the problem of the color difference of the plastic gravure, first we must select the ink with stable quality and no hue deviation. It is best to use the same lot of ink (a best amount of spot ink is used when formulating a spot color ink) for a batch of printed products, because it is difficult to ensure that there is no difference in hue between batches of ink. If the hue deviation of the ink is too large, it is difficult to control the consistency of the printing ink by other methods, especially when printing a gradation version.
Second, it is required that the amount of ink added to the ink tank each time during the printing process is less, so as to keep adding new ink frequently. Especially for the pattern with less ink, if the amount of ink is too much at a time, the ink will become obsolete when exposed to the air for a long time. For example, the long black ink is gray and there is no new black; The long red ink will turn dark red, and there will be no bright red as new ink. In the printing process, the volatilization rate of the mixed solvent used in the ink must be strictly controlled. When the volatilization rate is too fast, the ink transfer rate may be reduced, and the color of the printed product may be affected.
In addition, when using a two-component ink, it should be fully stirred and less equipped. Otherwise it is difficult to maintain the stability of the ink viscosity, but also may cause other printing failures.
Second, scraper
The contact position, the angle and the pressure of the doctor blade and the plate cylinder have a great influence on the ink color of the print, especially the ink of the shallow layer. Therefore, the ideal state is that the doctor blade is in the same position from beginning to end. However, for various reasons, the position, angle, and pressure of the squeegee had to be adjusted during the printing process. Therefore, we must understand the effect of the change on the color of the printed product and take corresponding measures.
The contact position of the doctor blade and the plate cylinder determines the distance between the contact point of the knife and the plate and the imprinting point. The length of the distance should be adjusted according to the printing speed, ink viscosity, cell depth, layout quality, and the like. In general, the longer the distance, the greater the degree of drying of the ink in the cells and the lower the ink transfer rate. As far as its effect on the printing effect is concerned, it is equivalent to the partial withdrawal of the ink. At the same time, the long distance is also a test for ink adaptability. On the contrary, when the distance is short, the transfer rate of the ink is relatively high, and at the same time, some fine lines on the plate are transferred to the printing material.
The scraper angle is the angle between the tangent of the point of contact of the doctor blade and the plate cylinder and the doctor blade. Raising the angle of the scraper makes it easy to scrape off the ink on the layout to make the graphic clear, but it may also cause the ink layer to be too thin, the luster of the ink film to be poor, etc., and it is also easy to damage the plate cylinder. Conversely, when the angle of the blade is reduced, the printing ink layer becomes thicker and the gloss of the printed product is better, but the corners of the image are easily blurred.
Squeegee pressure also has a certain influence on the ink color, the ink becomes lighter when the pressure becomes greater; the ink becomes darker when the pressure becomes shallow.
Third, plate cylinder
The effect of the plate cylinder on the consistency of the ink color is mainly manifested in the following aspects:
1. Blocking: Blocking is the phenomenon that causes a blockage of the network (caused by the solidified material in the ink, insoluble matter of high-viscosity material or foreign matter, etc.) due to a certain cause, and causes a decrease in the transfer rate. The amount of ink transferred at this time is reduced, thereby affecting the hue. When a blockage occurs, use a solvent or a special cleaning agent (use common decontamination powders to cause other troubles) to clean the plate cylinder. At the same time, take corresponding measures against the reasons for plugging to make it normal.
There are many influencing factors in blocking the plate. In addition to factors that cause chemical changes when using inks that are likely to cause chemical reactions, such as two-component inks, which cause insolubility, other reasons can be attributed to the fact that the ink is solid on the plate, mixed with impurities, and the quality of the plate is low. factor. In addition, if water or other types of ink are mixed in the ink during the printing process, it will affect the fluidity of the ink, cause the reagglomeration of the pigment, and also cause blocking failure.
In addition, a detailed inspection of the plate cylinder is required before printing. If the plate cylinder adheres to oil stains or the like, a printing failure similar to the blocking plate will occur during printing.
2, the printing plate wear
When the print quantity of the print product is large, the degree of wear of the plate cylinder is large, and the printing ink color changes. On the whole, it tends to become lighter (because the cell network becomes lighter and the amount of ink is reduced). Therefore, after a certain number of printing, it is necessary to replate. Although the platen roller's cells can be lightened, other colors can be used to maintain the color stability. However, overuse will lose the value of replating plates.
The plate cylinder may also have various failures and damages during use. Since there are many influencing factors in platemaking, it is almost impossible to completely repeat the original platemaking process conditions when remaking, and thus it is difficult to obtain the same printing effect as the original even under the same printing conditions after remaking. In order to solve this problem, if the printing batch is large and it is a frequently printed job, it can be considered to make two sets or even multiple sets of solutions.
3, plate design and color management
In the design of the plate, the effects of the printing and post-printing processes must be fully taken into account. For example, the colors of some dot areas are prone to level and level, and they need to be processed during plate making. Many platemaking companies have begun to use color management software and have played a positive role in improving the quality of color reproduction.
Fourth, printing conditions and environment
In the printing process, the color of the ink film mainly depends on the ink concentration and transfer rate. For a specific substrate and a specific ink, the concentration of the ink is relatively fixed, the transfer rate depends on factors such as ink viscosity, dryness, and the condition of the screen, as well as the printing speed, the printing pressure, the squeegee pressure, and the doctor blade. Location, scraper angle and environmental conditions (temperature and humidity) and other factors. What needs to be pointed out here is that the influence of these conditions on the highlights, shadows, and midtones of prints is different.
1, the ink viscosity and drying speed change:
Changes in the viscosity of the ink can directly affect the color of the print. When the viscosity is high, the color concentration is relatively high, whereas, when the viscosity is low, the color concentration is also relatively low. This effect is sometimes very obvious and must be given high priority. According to the requirements of the printing process, the viscosity of the ink should be moderate, and the excessive viscosity is not conducive to the transfer of the ink, and it is easy to produce a paste-type phenomenon; the viscosity is too small to be prone to static electricity, and is also unfavorable to the control of the ink color.
The change in the viscosity of the ink has a subtle effect on the flow area created by the flow of the ink or the flow pattern of the dots, especially in multi-color printing. Therefore, when the printing speed is constant, it is required that the viscosity of the ink be consistent from beginning to end, and it is better to use an automatic viscosity controller.
When the proportion of the solvent in the ink changes, the drying speed of the ink changes, which in turn affects the ink transfer rate and the color of the printed product. For some inks with a small amount of ink, after being placed and used in the ink tank for a period of time, due to the volatilization of the solvent, not only will the viscosity of the ink be affected, but also the proportion of the solvent and the volatilization rate will be affected, so the solvent must be added in time.
2, the temperature and humidity changes
If the room temperature and the liquid temperature of the ink change drastically, the state of ink flow will change, and the hue will change accordingly.
This is because the change in temperature affects the viscosity of the ink. At the same time, high-temperature, low-temperature weather will have a significant effect on the ink transfer rate of highlights. Therefore, when printing high-end products, the temperature and humidity of the printing shop must be controlled anyway. In addition, when the ink is used in winter, it must be warmed up in advance to reduce the temperature change of the ink itself.
3, the synthesis of color separation:
This phenomenon is likely to occur in light colors and in color mixing materials having widely different specific gravities such as organic pigments and inorganic pigments. In particular, when the ink in the ink tank is not very fluid, precipitation will promote the separation of the synthesized colors, so care must be taken to make the ink circulate smoothly. In addition, if the stirring is insufficient, the concentration of a certain color tends to increase as the printing progresses. Therefore, the ink should be sufficiently stirred using a machine such as a dissolver or other method before the printing.
4, ink deterioration to make color deterioration
Due to long-term printing, the ink solvent composition changes, or the ink in the air mixed with the ink causes the ink to deteriorate, rendering the color development worse.
In order to prevent the temperature of the ink from falling due to the heat of vaporization, a slow-drying solvent may be used in combination. In high-temperature, high-humidity weather, there may be a certain amount of water mixed with ink, in the event of abnormalities should be added new ink, or all replaced, used more than a few times the remaining ink due to mixed with a lot of dust, should be regularly filtered or discarded .
5, other printing conditions
The discoloration of the printing may occur due to coincidence of the dots and the irregular wind blowing on the layout, or changes in printing speed and drying speed.
Fifth, the impact of post-press processing
1. Changes in post-press processing technology:
The use of different post-printing processes (such as extrusion compounding and dry compounding) also has a certain effect on the hue of the prints. At the same time, the extrusion of the composite high temperature sometimes causes the pigment to change. In addition, when the heat sealing is performed at a temperature of 200° C. or higher, the hue of the heated portion may be deeper because the heated colorant melts, and cooling causes recrystallization to form fine crystals, which causes an increase in coloring power.
2. Post-printing substrate changes:
Changes in the post-printing substrate, such as changes in the aluminum layer of the aluminized film, directly affect the overall hue. Of course, the color shift of the printed substrate will also have a slight effect on the overall hue.
3, photochemical reaction:
When the ink colorant has poor light resistance and weather resistance, the color of the print may change due to the photochemical reaction.
4, the impact of packaging content:
When the contents contain active substances that are volatile and migratory, the color of the packaged prints may also change due to corrosion. Therefore, the appropriate ink should be selected according to the end use of the packaged product (or negotiated with the ink manufacturer).
VI. Visual inspection and use of color control instruments
The customer wanted a high degree of consistency in the ink color of the packaged product, but the result of each person's observation of the color was not exactly the same. What clients need is persuasive data, which is the quantitative data of colors. Although the naked eye is good at color, it is not good at remembering colors (if you don't put the products together and you can't accurately measure the color with the naked eye), so the naked eye is no longer considered as a reliable guarantee for determining the color of ink. The instrument shows its uniqueness. importance. From naked eye to densitometer, colorimeter to spectrophotometer, to the current benchtop spectrophotometer, the color measurement results are more accurate enough to compensate for human eye fatigue effect, persistence of vision, visual memory, etc. The adverse effects. Of course, it will take time for the widespread use of color control instruments in our country.
As part of the color management process, color detection is an indispensable step and the first step in color management. At the same time, since the stability of the color during the operation of the printing press will be affected at any time, the use of an on-line color control device to monitor the color stability will be an important development trend.