We live in a colorful world, and the sky, grass, ocean and grassland have their own colors. In printing production, color is not only an element of design, but also an important standard during product acceptance. A printed product with poor color quality may not be patronized next time even if the client barely accepts it. Many products lose important customers due to poor control of color quality issues. It can be seen that mastering the law of color presentation and controlling the color quality is a technology that must be mastered in production and production. It only has advanced equipment and does not have good process technology cooperation. In the fierce industry competition, it is inevitable to be eliminated.
1 The role of color quality control
Color control is the core of printing quality control. In the entire production management of printing enterprises, color management is at the center. With the development of economy and the improvement of material and cultural living standards, customers' requirements for print quality are constantly increasing. In the face of the intensifying competition between enterprises in the industry, printing companies must continuously improve product quality and improve the tone and color of printed products. Reproduce quality. To control the quality of product images, the key is to control the color. In the printing process, effective methods must be adopted to control various parameters to ensure the repeatability and quality stability of product replication.
2 Color quality requirements for printed products
Color quality control is a wide-ranging management process, involving a series of processes such as manuscript, pre-press design, plate making, printing, proofing, printing, etc. It includes pre-press color management and color and tone control during printing From the perspective of printing image reproduction technology, the quality of a printed product has four main requirements (image quality characteristic parameters): tone and color reproduction, image resolution, moire and other fault patterns and surface characteristics. For color dot prints, tone and color reproduction are required to be faithful to the original, no color cast, uniform ink color, good gloss, dots are not deformed, accurate overprinting, no ghosting, through-printing, various bars, backside stickiness and mechanical marks.
There are many factors in the tone of the printed image and color reproduction, such as: ink film thickness, dot expansion, printing color sequence and overprint, ink hue and solid density, ink temperature and viscosity, ink supply (water-ink balance), paper properties, plate layout Depth, printing pressure, etc.
Now we mainly analyze the control methods that affect the color tone of the printed product and the quality of color reproduction from the aspect of water-ink balance in the printing process. [next]
3 Analysis of the effect of ink-water balance on color
Offset printing is based on the principle of water and oil intolerance. The quality control of water-ink balance directly affects the quality of printing quality. Ink control is not only static, but also dynamic control throughout the entire production process. Sometimes, when the same product is produced on the same equipment, in several batches, or on different machines at the same time, the hue is often inconsistent, and even the parameters of the equipment are adjusted correctly. After entering mass production, the front and back ink colors will also appear Inconsistent situations, at this time, it is particularly important to control the balance of water and ink. In actual printing, many operators do not pay enough attention to the balance of water and ink. They realized it was necessary to control when the ink was seriously emulsified after a period of printing, which caused the ink roller to deink, the printing product was dirty, the color was dull, the image was white, and the paper was damp. Ink balance.
1) Relatively incompatible ink and wash
Ink and water are not absolutely incompatible. In the actual production process, there is pressure between each water roller and between the water roller and the plate cylinder. The ink and water will be fused together after being squeezed by the rubber roller and the rubber roller to emulsify the ink, so the water-ink balance is relative of.
In the transfer process of water ink, it is impossible to maintain a strict boundary between water and oil. Some additives in the binder of offset resin ink are hydrophilic, and some additives are both hydrophilic and lipophilic, so emulsification of ink is inevitable of. There are two emulsified states of the ink: when the water is large in ink, it forms "oil-in-water", and when the water is small in ink, it forms "water-in-oil".
Once the ink is emulsified, its hue, brightness and saturation will be damaged due to the penetration of water molecules. Products printed under such ink conditions will definitely have a problem of color shift, and because the saturation of the ink is reduced, When printing, you have to increase the amount of ink, and it will bring about the problem of increasing the density value on the ground.
Generally, the water ambassador ink is emulsified, the ink color is light, and the small water makes the blank part of the layout adhere to the ink, resulting in dirty marks. Therefore, the key to printing is to control the water-ink balance of offset printing.
2) The effect of water and ink imbalance on the color quality of printing
â‘ Large layout
Under the mechanical pressure between the ink roller and the printing plate and the rolling effect between the ink roller, the ink contains fine water droplets, which is easy to cause the dots to be slurred, unclean, opaque, and dark in color. 1. The vividness is reduced, the ink color is light, and water traces are generated on the ground. Sometimes too much water will be thrown onto the printed matter, causing the paper to stick and damage the quality of the graphics.
â‘¡Water and ink
When excessive water and ink accumulate on the surface of the water roller and the ink roller, a series of obvious shallow streaks in the axial direction will appear on the printing plate surface and the printing piece, resulting in "water bar" and "ink bar". Excessive moisture on the plate surface will be transferred to the paper after the printing plate and blanket are transferred, causing the paper to be uneven, the four corners are raised, or even curled, which affects the next color overprint and the back printing, which is easy to cause single-sided four-color overprint Irregular or front and back overprint is not correct. When the printed sheets with incorrect front and back overprint flow into the binding process, it will have a great impact on the binding folding, collating and binding quality.
â‘¢Ink emulsification
When the ink on the ink roller is emulsified, the nature and structure of the ink will be changed and destroyed. The ink roller is easy to flick the ink and splashes on the equipment and printed matter, resulting in flying ink dots. The emulsified ink layer on the printed matter will prolong the time of ink oxidation and film formation, and slow down the drying speed, causing the printed matter to appear sticky and dirty on the back, causing the printed image to become false and white, resulting in color distortion.
â‘£The water volume is too small
When the amount of water is too small, ink stains will appear on the blank part of the printed product first, and burrs appear on the text and lines. The printing plate will start to become dirty. With the extension of time, the ink gradually blocks the non-inked part, and will appear " Paste version ".
The small layout water is reflected in the printed matter more intuitively than the layout water conference. It is easy to correct and avoid as long as the sample is carefully checked in time. [next]
4 How to balance water and ink in daily operation
Ink and ink balance can be well controlled in production. Ink emulsification should be controlled to the minimum range to ensure that the printed product has distinct layers and bright colors. It is a reflection of the operator's ability to grasp the ink and ink balance and the comprehensive ability of operation skills.
1) Reasonably adjust the pressure between the water transfer roller and the plate water roller, the ink roller and the plate cylinder, and the ink rollers
When adjusting the pressure, it is necessary to achieve "Sanping" (the mutual pressure between the water roller, the ink roller and the cylinder should be flat), "Sanheng" (printing pressure, rotation speed, PH value must be constant), and correctly master "Sanqin" ( (Look at the proofs, wash the rubber, and stir the ink fountain) are the prerequisites to ensure the balance of water and ink. When replacing a new water rubber fleece cover, the force should be even, the surface should be uniform, and the tension should be consistent.
2) The ink in the ink fountain cannot be adjusted too thin.
Too thin ink, large fluidity, reduced viscosity, reduced water resistance, easy to emulsify. The amount of ink adjustment oil should be controlled within 10%, offset printing can be slightly higher. Dry oil is both a drier and an emulsifier. Excessive use will also cause emulsification or burning of the ink. Generally, the amount of red dry oil should be controlled at 1%, and the white dry oil should be controlled at less than 0.5%.
3) The operator should pre-adjust the ink and wash according to the graphic area of ​​the layout
When reprinting new products, it should be noted that after the plate is changed, the pre-opening ink volume of the layout should not be too large, otherwise the amount of water will have to be increased when the machine is turned on, which may cause large water and ink. In the process of pattern registration and the correction of the color of the layout, it is necessary to repeatedly start and stop. At this time, the ink and ink control is a difficult point. It should be highly valued. The ink and ink control after normal printing is relatively easier.
The pre-opening water consumption should also be controlled, not too large, otherwise it will cause large water and small ink, resulting in deinking of the ink transfer system and accelerated ink emulsification. In the normal printing process, as long as the normal water supply and the layout are not dirty, try to use a small amount of water.
4) Keep the machine running at a constant speed to avoid changes in water speed and ink volume caused by changes in the machine speed, which can easily cause water and ink imbalance and make the ink color of the printed product inconsistent. [next]
5) Reasonably control the ratio of fountain solution
Reasonable configuration of fountain solution is an important condition for water-ink balance. The ink volume of each color group and the proportion of fountain solution can be determined according to the color requirements of different sheets. The fountain solution in the fountain solution is controlled at about 5%, and the addition amount of the dampening powder is controlled at about 4%.
The pH value of fountain solution should be set between 5 ~ 6. If the pH value is too low, the acidity will be strong, and the blank part of the printing plate will be deeply corroded, with coarse trachoma, and it is not easy to form a strong hydrophilic layer, which causes the oil-in-water phenomenon in the blank part due to the presence of more fountain solution. The printing plate is dirty, weakening the printing durability of the printing plate, causing dot damage, and "plate-off". The ink contains acidic water beads, which will cause the decomposition of the pigment, the powdering of the ink, the dull color, the decrease of the saturation, and the speed of the oxidative film formation of the ink layer. If the pH value is too high, the diazo compounds in the graphics and text will be dissolved, causing the graphics and text to be incomplete, which will also damage the printing plate and reduce the printing resistance.
The acidity and alkalinity of the paper will also affect the pH control of the fountain solution. In general, the alkaline fountain solution for printing acid paper and the acid fountain solution for alkaline paper are relatively comprehensive and neutral as much as possible.
The amount of alcohol added to the fountain solution should be controlled at about 15%. Adding a proper concentration of alcohol to the fountain solution can form a water film on the surface of the water transfer roller, and the water transfer is uniform, which helps the fountain solution to fully spread on the surface of the printing plate. The ink color is bright and easy to dry. If the alcohol concentration is not enough, the fountain solution on the surface of the water transfer roller is transferred in a linear form, which is easy to cause uneven water transfer and the printed product pulls ink. Excessive alcohol will cause waste and increase printing costs.
The water temperature of the alcohol dampening system should be controlled at about 8 ° C, to avoid the heat generated by the high-speed running of the ink roller to reduce the moisture of the plate surface, and to increase the amount of water on the plate surface, resulting in oil emulsification.
6) Production environment control
The ideal printing workshop should have constant temperature and humidity, keep doors and windows as little as possible, avoid wind, keep the temperature between 22 and 25 ° C, and keep the relative humidity between 55% and 65%. This will help reduce the surface temperature of the printing plate, reduce the alcohol volatilization and the amount of fountain solution, and help control the water-ink balance. When the temperature is higher than 25 ℃, the water content on the layout is easy to evaporate, and the layout is easy to get dirty. Relative humidity is higher than 65%, it is easy to cause ink emulsification, the surface strength of the paper is reduced, and the drying of the ink layer is slower than 45%. The paper is easily charged with static electricity, which increases the difficulty of printing and affects the quality of printing. [next]
5 Ink and ink imbalance manifestations and identification
For research or experiment products, colorimeters, densitometers and other equipment can be used to monitor the whole process of the printing process. After a few prints, use the instrument to measure, find the places to be adjusted, make adjustments, try printing, and adjust again. … But for the production of some enterprises, due to the limitation of production cycle and production process control, there is a certain gap between the actual control method of color in large production and the method used in research work, and because of the time in the production process Tight and heavy tasks, it is impossible to accurately measure with a density meter and other instruments every time, usually only for the sample sheet, after the machine, the captain will adjust, and frequently adjust the printing pressure, wrapping or frequent Downtime has a great impact on the production efficiency of printing, so the captain is required to have a strong ability to control the color and equipment, so as to ensure both production efficiency and product quality during the printing process.
In the production process of printing, color realization does not exist independently, but is integrated into equipment and operation methods, and is closely connected with equipment, operation methods and production processes, and is highly unified. After the production prints are determined, in order to ensure the accuracy and consistency of the printed colors and make the products bright, saturated, and without color difference, the operator should also listen, see, and sample more during the production process to discover the impact of the product The problem of quality.
For example, if the captain can hear the sound of "silk" from the ink roller during the transportation of the machine, and check the printed sheet at this time, the print is dull and dull. At this time, the captain should judge that the ink is unbalanced , Serious problem of ink emulsification.
Sometimes the ink color on the printed matter is light, which does not mean that just increasing the amount of ink can immediately saturate the ink color. If it is found that after increasing the ink volume, the ink color is not deepened in time, it may be determined that the water volume is too large, and the water-ink balance needs to be adjusted. If the ink color of the printed product is dark and the paper is soft, the surface of the printing plate is shiny when checking the printing plate, which also means that the amount of water is too large.
Quality problems such as inaccurate overprinting, overprinting faults, paper surface roughening or peeling are very common in printing. Sometimes it is a paper problem, sometimes a printer failure, but if the paper quality and equipment failures are ruled out, there are still these If there is a problem, then it may be determined that the situation is big and big.
During the printing process, the operator must not only ensure the normal operation of the equipment and to continuously check the quality of the product, but also check the ink supply and water supply systems of the equipment. When water droplets are found in the ink fountain, the amount of water must be considered It is too large because the excess water will pass back to the ink fountain through the ink roller, forming small water droplets. Similarly, if water droplets are found outside the trailing edge of the blanket and the printing plate when cleaning the rubber, it can also be determined that the amount of water on the plate is too large. If the amount of water is too large, the performance of the paper collection part is that the printed products are not well received and the paper is curled. [next]
6 Summary
In order to obtain high-quality prints for lithography, it is very important to master the water supply of the printing plate. Therefore, it is particularly important to control the water-ink balance and standardize the operation. The amount of water on the plate is closely related to the transfer of ink, precision of overprinting, emulsification and drying of the ink, printing durability of the plate, and gloss of the print. The water supply of the plate should be determined according to the actual printing conditions. However, because there are too many variable factors, it is necessary to pay close attention to the changes in the water volume of the plate during the printing process and adjust the water supply of the plate in time. In short, according to the principle of water-ink balance, the amount of water supplied to the plate should be controlled to the minimum at which the blank part of the printing plate starts to get dirty.
Today, with the continuous development of printing technology, the continuous pursuit of high-quality and high-quality printed products in the market has made printing companies pay more attention to improving the color quality of products in addition to greatly improving their production efficiency. Careful, in order to obtain the ideal print, the printing operator and quality management link should carefully monitor and control many influencing factors in the printing process, water-ink balance, appropriate printing pressure, good registration of each color version and appropriate ink drying Speed, etc., in order to ensure the stability of the color of the printed product, reduce the scrap rate, improve the quality of the printed product, and also extend the service life of the printing equipment and save consumables. This is a problem for printing companies that are currently in fierce competition. Said, has very good practical significance.
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