Analysis of influencing factors of color differenc

2022-08-15
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Analysis of influencing factors of color difference in plastic flexible packaging printing (Part 2)

III. printing conditions and environment

in the printing process, the color of ink film mainly depends on the concentration and transfer rate of working ink. For a specific substrate and a specific working ink, the concentration of the working ink is relatively fixed, and the transfer rate depends on factors such as viscosity, dryness of the working ink and the condition of the plate face. At the same time, it is also affected by factors such as printing speed, printing pressure, scraper pressure, scraper position, scraper angle and environmental conditions (temperature and humidity). Here we need to point out that the influence of these conditions on the highlight, dark tone and middle tone of printed matter is different

1. the working viscosity and drying speed of the ink change

the change of the working viscosity of the ink can directly affect the color of the print. When the viscosity is high, the color concentration is relatively high, on the contrary, when the viscosity is low, the color concentration is also relatively low. This kind of influence is sometimes very obvious and must be paid great attention to. Due to the requirements of the printing process, the working viscosity of the ink should be moderate. When the working viscosity of the ink is too large, it is not conducive to the transfer of the ink, and it is easy to produce a paste phenomenon. When the working viscosity of ink is too small, the color of printed matter is prone to electrostatic phenomenon, which is not conducive to the control of ink color

the change of viscosity has a subtle effect on the flow area caused by the flow of ink or the flow mode of dots, especially in multicolor printing. Therefore, when the printing speed is constant, the viscosity of the ink is required to be consistent from beginning to end, and the highest strength and rigidity of the thermoplastic material can be maintained under the condition of up to 275 ° C. It is best to use the viscosity automatic controller

when the solvent proportion in the ink changes, the drying speed of the working ink will change, which will affect the ink transfer rate and printing color. For some less used inks, after being used in the ink tank for a period of time, the natural volatilization of the solvent will not only affect the working viscosity of the ink, but also affect the solvent proportion and volatilization speed, so it is necessary to supplement the solvent in time

2. temperature and humidity changes (environment)

if the room temperature and the liquid temperature of the ink change significantly, the flow shifting state will change and the hue will change accordingly. This is because temperature changes affect the viscosity of the ink. At the same time, the dry weather with high temperature and low humidity will have a significant impact on the ink transfer rate of the highlight part. Therefore, the temperature and humidity of the printing workshop should be controlled in any case when printing high-end products. In addition, when using ink in winter, it is necessary to preheat in advance to reduce the temperature change of the ink itself

3. Separation of synthetic colors

this phenomenon is easy to occur in light colors and mixtures of colors with widely different specific gravity, such as organic colors and inorganic pigments. Especially when the ink in the ink tray flows too slowly, the precipitation will promote the separation of synthetic colors, so pay attention to make the ink circulation smooth. In addition, if the mixing is not sufficient during color mixing, the concentration of a certain color tends to increase with the progress of printing. Therefore, before formal printing, a dissolving machine or other methods should be used for full mixing

4. the ink deteriorates and deteriorates the existing color.

due to long-time printing, the composition of ink solvent changes, or the water in the air mixes with the ink to cause the ink to deteriorate and deteriorate the existing color

in order to prevent the temperature drop caused by the heat of vaporization, slow drying solvents can be used together. In case of high temperature and humidity, a certain amount of water may be mixed into the ink. Therefore, in case of abnormalities, new ink should be added and finally, or all of them should be replaced. The remaining ink that has been used for many times should be filtered or discarded regularly because it is mixed with a lot of dust

5. Other printing conditions

printing discoloration will occur due to point coincidence, irregular wind exposure to the layout, or changes in printing speed and drying speed

IV. influence of post press processing and environment

1. change of post press processing technology

different post press processing technologies (such as extrusion compounding and dry compounding) also have a certain impact on the hue of prints. At the same time, the high temperature of extrusion composite sometimes causes the change of pigment. In addition, when hot porous polymer materials are stitched at a temperature above 200 ℃, due to the designability of their structure and function, the hue of the heating part may become darker, which is caused by the increase of coloring power due to the formation of micro crystals when the heating colorant is melted and cooled and recrystallized

2. Changes of post press processing substrate

changes of post press processing substrate, such as changes of aluminum layer of aluminized film, will directly affect the overall hue. Of course, the color deviation of printing substrate will also have a slight impact on the overall hue

3. photochemical reaction

when the light resistance and weather resistance of colorants are poor, the color may change due to photochemical reaction

4. After the contents affect the packaging contents, when the contents contain volatile and migratory active substances, the color of the packaging printing may also change due to corrosion. Therefore, the appropriate ink should be selected according to the end use of the packaging (or negotiate with the ink manufacturer)

v. color observation with the naked eye and color control instrument

customers hope that the ink color of packaging and printing materials is highly consistent, but everyone's color observation results are not exactly the same. What customers need is convincing data, that is, quantitative data of color. They don't want to say it's blue, but what are the L value, a value and b value. Although the naked eye is good at observing the color, it is not good at remembering the color (if the products are not placed together, the naked eye cannot accurately determine the color), so the naked eye is no longer considered as a reliable guarantee for judging the ink color, and the instrument shows its unique importance. From naked eye to densitometer, colorimeter and spectrophotometer. Later, it was developed to the current desktop spectrophotometer. Now various technologies can make up for the adverse effects of human eye fatigue, visual persistence, visual memory and so on on color observation, and the color measurement results are more accurate. Of course, it will take some time to use color control instruments in a large area of our country

as a part of the color management process, color detection is an indispensable step and the first step of color management. At the same time, because the color stability will be affected at any time during the operation of the printing machine, it will be an important development trend to use the color control device to measure and monitor the color stability

other

1. For plastic intaglio printing, there is no continuous and perfect color quantitative control system in the whole process from plate making to proofing. Therefore, when making plate and proofing, we should simulate printing conditions as much as possible to realize standardized proofing

2. The printing and proofing must be carried out under normal printing conditions, otherwise it will be difficult to follow up the samples. For example, if the scraper is too close to the imprint line during printing and proofing, it will be difficult to handle or adjust when there are scraper related faults in formal printing. If the printing sample is carried out when the scraper is far away from the imprint line, it is difficult to transfer the ink in some shallow places during formal printing, and there will be dots on the print. When the distance is shortened to try to eliminate these dots, the ink color will definitely change when the jaw splint is installed at last

3. The control of printing ink color is mainly to control the mixing effect of multiple colors. In the specific operation, we must first accurately determine which color has changed. Be careful not to patronize and adjust a certain local hue, and forget to patronize and their interrelated color changes

4. strengthen the color management of printing

sometimes the color looks the same under ordinary light, but it doesn't look the same under another light source. Therefore, a standard light source should be selected for color observation or comparison

the content of color management is very rich. It is very important for the plastic packaging and printing industry to formulate an effective color management system

5. Sometimes in the printing process, the situation is not always ideal. Some factors must be adjusted and a new balance must be achieved after this adjustment: the hue must change in the following occasions, so it is necessary to compare the color samples before starting the operation

(1) viscosity changes

(2) replace the scraper

(3) change the speed

(4) add supplementary ink

(5) add solvent

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