A printed matter must be completed in three stages: pre-press, mid-press, and post-press. Each process is related to the quality of the print. Therefore, the quality control of the print must start from the pre-press to the post-press. The quality control of printed matter should not only rely on experience, but on modern scientific methods. In particular, we must emphasize data, standardization, and standardization, and emphasize the entire process control from raw materials, production processes, and finished products.
Before printing, it includes design and plate making; after printing, it includes paper cutting, ink adjustment, and printing; after printing, it includes glazing, calendering, laminating, anodized aluminum hot stamping, embossing, stamping, mounting, die cutting, and paste box.
Printing is a very important part, it reflects the effect of design and plate making, and is the main production line of the product. Post-press processing can increase the added value of printed products and embellish products, which is the last link.
1 Design and plate making
Our packaging users expect that we can make their products new on the packaging box, so that consumers have a refreshing feeling, so as to achieve the purpose of promotion. Therefore, the design is a crucial part of the quality assurance of color cartons. It must achieve a comprehensive manifestation of patterns, colors, and text, highlight the main body, and at the same time consider the feasibility and actual effect of plate making and printing and post-processing. In the use of colors, reduce colors or overlapping colors as much as possible, with the exception of spot colors in special cases. The text must be arranged and combined properly in the text, as far as possible to avoid the use of too thin, too small Yin and Yang fonts. Avoid unnecessary complicated and complicated pattern structure, which will bring difficulties to printing.
Plate making is the intermediate link between packaging and decoration design and printing, and it is a bridge between design and printing. The task of plate making is not only to correctly reflect the requirements of the original design, but also to correct the parts that do not meet the requirements of the printing process. In the plate making work, we must first grasp the quality of the original manuscript, and if the quality of the manuscript does not meet the requirements, we will not use it. When making color separations, it is necessary to consider the equipment of the offset press. According to the traditional color separation or non-color color separation, it is more conducive to printing. There is also an understanding of the printing curve of each printing press. The quality of each process in plate making must be strictly controlled, and data monitoring must be carried out, such as printing, proofing, etc. [next]
2 Paper
1) General requirements for printing paper
The surface strength of the paper is better; the thickness is the same; the ink absorption is better; the stretchability is smaller; the pH value is preferably neutral or weakly acidic; it needs to be tough; the surface is smooth; the appearance of the paper is good.
2) Paper inspection by printing house
â‘ The moisture content of paper is very important for printing. Paper is made of plant fibers, fillers, rubber, etc., which easily absorb moisture. Changes in the moisture content of the paper during printing will cause changes in the size and shape of the paper, which will affect overprinting and paper feeding, so we must inspect the incoming paper to see if the moisture content reaches the standard. Generally, coated paper is about 5%, and whiteboard paper is about 7%.
â‘¡How does the whiteness of the paper determine the range of colors. The whiteness is high, the colors are bright, and the contrast of the color gradation is opened; the low whiteness reduces the color saturation of the ink, the color is dark, and the contrast of the color gradation cannot be opened. So we have to check the whiteness of the incoming paper to keep the whiteness of the whole batch of paper consistent.
â‘¢ The thickness of the thick and thin paper will affect the pressure during printing and make it difficult to distribute the printing pressure. It is reflected in whether the dots of the printed product are uniform, and whether the ground is strong, and the final printed product varies in depth, so we have to measure the thickness of the paper.
â‘£Whether the appearance of the paper surface is uneven, there are paper bumps, sand, stains, dust spots, aurora belts, etc., which will adversely affect the printing and cause quality problems in the printed matter.
⑤ The box for packaging the printed matter of gram weight must have stiffness, so the gram weight must be kept the same. We must test the basis weight of the paper according to the standard, and the deviation must not be too large.
â‘¥ Suitability Through printing suitability testing, you can know the printing performance of the paper, such as paper wool, ink absorption, drying, etc.
3) Paper storage
The quality of paper storage directly affects the quality and use of paper. In the storage process, the storage site should be selected reasonably, pay attention to the storage environment, and do a good job of cleaning, ventilating, avoiding light and preventing moisture.
â‘ It is not easy to stack the storage paper too high, and it should be stacked according to the variety and specifications.
â‘¡The moisture content of moisture-proof paper will change with the temperature and humidity of the environment, especially the paper pile after disassembly, must leave the floor mat shelf, and should not be piled against the wall to prevent poor ventilation and moisture, causing mildew. Generally the relative humidity is controlled at about 50% ~ 60%. The relationship between relative humidity and paper moisture content.
â‘¢The moisture content of the sunscreen paper pile will change sharply after being exposed to sunlight, which will cause deformation, paper brittleness, and yellowing.
④If the heat-proof environment temperature exceeds 38 ℃, the mechanical strength of the paper will decrease. Coated paper will cause paper-to-paper adhesion and even the coating to peel off.
⑤ Fireproof paper is flammable. Once a fire occurs, the consequences are unimaginable and must be strictly prevented.
â‘¥ When stacking anti-folding paper, each order should not be staggered and stacked with each other, otherwise the stacked edges will be staggered and stacked, causing the paper edge to wrinkle and deform due to moisture absorption or dehumidification. [next]
3 Ink
1) Performance of ink
Hue and brightness. The hue of the ink refers to the performance of the ink to reflect a certain part of the light wave, and it is also the place where the difference of the ink color is the largest. Various colors are determined by different wavelengths of light waves. It uses light absorption, reflection, and transmission to a certain wavelength to show their inherent colors. The brightness of the ink refers to the amount of light reflected by the ink. In the same color, the ink is bright when the reflection is large. Hue and brightness are expressed by Lab, L indicates light and dark, a indicates red-green direction, and b indicates yellow-blue direction, which can be measured with a spectrophotometer.
Coloring power. Indicate the density and saturation of the ink. Strong tinting strength, less ink consumption, it is a prerequisite for ink quality. The color strength of the ink is related to the chroma of the ink, that is, to the gray scale, color deviation, and color efficiency of the ink.
transparency. When the ink is coated into a uniform film, the background color of the material that bears the ink film can be displayed or not. This performance is called transparency or hiding rate. Transparency is very important when the ink is overprinted.
2) The relationship between ink color and printing
According to the calculation formula of the LTF Institute in the United States:
Grayscale = L / H × 100%;
Color deviation = ML / HL × 100%;
Color efficiency = 1-M + L / 2 × H × 100%.
3) Ink light resistance, acid resistance, alkali resistance, alcohol resistance and water resistance
Lightfastness. Under the effect of sunlight, the color change performance of the ink is called the light resistance of the ink.
Water resistance. The water resistance of the ink is of great significance to the principle of immiscibility of water and oil in offset printing. Water resistance is not good, "water" and "hydration" will occur during the printing process, so that the water solution and the entire printing plate will be dyed with a layer of colored water; emulsify the ink, reduce viscosity and adhesion, Make the color change. [next]
4) The relationship between ink viscosity and printing
The viscosity of the ink is the main means of conveying the paint.
The viscosity of the ink is the condition that determines the separation energy, and it is also the main condition for the ink to pass between each other. If the viscosity is too small, the ink is easy to stack plates, blankets, emulsification, floating dirt, and rubber rollers; the increased viscosity can make the dots clear, and the ink has a good gloss after drying. However, if the viscosity is too high, the ink layer will cut off the ink on the plate surface to cause the flower plate. The paper is brushed and de-powdered. The viscosity must be correctly grasped according to the paper performance, ink layer and dot level. The correct viscosity is easy to fix the pigment on the paper surface. After the ink connection material is completely dried, the pigment can also be fixed on the paper surface. During the drying process of ink without a certain viscosity, the binder can easily penetrate into the paper, making the printed ink lack gloss.
5) The relationship between ink fluidity and printing
Appropriate fluidity can ensure the normal transfer of ink, making the blot strong and even. Ink is too thick, because the ink is too thick, the ink supply is not normal, often making the same product printed before the dark and then light or light before the dark; too thin ink will cause lubrication, so that the ink roller can not be transmitted by friction, will Enlarge the imprint, loss of layers, and poor quality.
6) The relationship between ink drying and printing
Drying too slowly can make the printed matter sticky and dirty, and even overprinting work can not be carried out; too fast drying will cause the ink roller to bite, paste, pull, print too dry and vitrify, and the color printing is not good.
In the printing process, the drying of the ink is a complicated physical and chemical process, and there are many factors that affect the drying, and the changes are complicated. Correctly setting the dry oil is the key to solving this problem. But adding oil and drying oil is only one aspect of controlling the drying of the blot. The drying speed is also determined by various factors in the printing process. Because the emulsification of the ink, the thickness of the ink layer, the strength of the acidity of the fountain solution, the absorption performance of the paper, the temperature of the workshop and the storage and stacking of the paper are all closely related to the drying of the blot. [next]
3 Control of fountain solution
The role of offset fountain solution:
Form a uniform water film on the blank part of the printing plate to resist the ink from invading the blank part and prevent the dirty version.
Due to the continuous friction between the blanket cylinder, water roller, ink roller and the printing plate, the hydrophilic layer on the printing plate is destroyed. This requires the use of electrolyte in the fountain solution to chemically react with the aluminum plate metal exposed by wear and tear , To form a new hydrophilic layer, and constantly maintain the hydrophilicity of the printing plate.
Control the ink temperature of the printing plate, the viscosity of the general ink also changes with the temperature change, the experiment shows that the temperature increases from 25 ℃ to 35 ℃, the viscosity of the ink drops from 50Pa.s to 25Pa.s, the fluidity of ink The increase will cause the ink to spread seriously, which will seriously affect the quality of the printed matter.
1) Types of offset fountain solution
â‘ Common type fountain solution. Its main components are water, phosphoric acid, ammonium nitrate, ammonium dihydrogen phosphate, chromic acid, etc. This type of fountain solution has a high surface tension and poor spreading performance. The water film in the blank part is thicker and is available in red and white. The red fountain solution is no longer used because it is harmful to human skin, and the white fountain solution is generally used on single-color or two-color offset presses.
â‘¡Alcohol fountain solution. Alcohol fountain solution is generally prepared by adding ethanol or isopropanol to ordinary fountain solution. Ethanol is a surface-active substance, which can reduce the surface tension of the solution, which will change the spreading performance of the fountain solution on the printing plate and reduce the amount of fountain solution. And it's thin and even. Generally, the amount of ethanol is about 8% ~ 12%.
â‘¢ Non-ionic surfactant fountain solution. Various types of wetting powders currently on the market are wetting agents. In use, as long as the powdered wetting agent is dissolved with a certain amount of water, it can be added to the water bucket for printing. Non-ionic surfactant fountain solution is lower in cost than alcohol fountain solution, non-toxic and non-volatile. It can be used on the traditional water transfer device of offset press, without a special wetting system.
2) Use of offset fountain solution
When using offset fountain solution, pay special attention to the amount, that is, the ratio of fountain solution and water. There are many factors to be considered for the amount of the original solution: the consumption of the coating on the surface of the printing plate, the influence of the temperature and humidity of the workshop on the volatilization of the fountain solution, and so on. According to different situations, the ratio of fountain solution and water is different. Such as: different ink, its oiliness, viscosity, fluidity, etc. will be different, the amount of fountain solution will be different. The graphic structure of the printing plate, the size of the area, the size of the ink load, the environmental temperature and humidity, and the nature of the paper all affect the amount of fountain solution. [next]
3) Some problems that must be paid attention to in actual production
â‘ Dosage of alcohol. The addition of alcohol can reduce the surface tension of the fountain solution and make the printing plate more wettable. It can greatly reduce the amount of water used, thereby increasing the drying speed of the printing product, reducing the surface temperature of the printing plate and stabilizing the viscosity of the ink. However, if too much alcohol is added, due to volatilization and environmental pollution, it will cause fire or even explosion in severe cases. At the same time, if too much alcohol is added, it will also react with the gum of fountain solution to form a precipitate, which affects the work of the water delivery system and is detrimental to the balance of water and ink.
The surface tension of tap water is about 72 dynes, and the surface tension of ink is about 30 to 36 dynes. To make the surface tension of the fountain solution and the surface tension of the ink consistent, so that the two reach a balance, this can control the degree of emulsification of the ink. Therefore, the general alcohol consumption is controlled at about 8% to 15%.
â‘¡ Dosage of fountain solution. The amount of fountain solution is very important. If the amount of the fountain solution is small, the printing plate is easy to get dirty during printing; if the amount of fountain solution is too much, the ink is easy to emulsify and the printing plate is easy to flower. The amount of raw liquid added is generally controlled at about 2% to 3%.
③The setting of temperature and humidity. During printing, as the time increases, the speed increases, the surface temperature of the printing plate will increase. Increased temperature will make the surface water of the printing plate volatile, the ink viscosity will drop, the ink transfer will be poor, and the printing plate will become dirty. Therefore, reducing the surface temperature of the printing plate is very important for the balance of water and ink. Generally, the temperature of the workshop is about 25 ℃ ± 3, and the humidity is about 50% ~ 60%. The cooling box keeps the temperature of the fountain solution at about 10 ℃ ~ 12 ℃.
â‘£pH value and conductivity control. The pH and conductivity are used to reflect the control of the fountain solution. A lot of practice has proved that the pH value of fountain solution is relatively stable in the range of 5.0 ~ 6.0. The printing plate has a long service life, good ink and water balance, and good printing quality. The conductivity is easily affected by the hardness of the water, the amount of alcohol, dust, and ink inclusions, and it must be determined according to the water quality of the factory.
The water quality is divided into soft water and hard water. The general conductivity of soft water is around 0 ~ 225US, and the general conductivity of hard water is above 450US. (The hard water mainly contains more Ca2 +. Ca2 + will contact with the resin in the ink to form calcium saponification, which is lipophilic and will cause poor water transmission.) Therefore, the conductivity is generally controlled at about 800US ~ 1500US.
⑤Control of the printing plate surface wetting fluid. The amount of wetting fluid on the printing plate will affect the balance of water and ink. If it is too little, it will become dirty. Too much ink will emulsify, causing flying ink, dry blotting, etc. So we must control the amount of "water" on the printing plate. In actual production, we control the amount of water when the printed product is no longer dirty, that is, the minimum amount of printing plate water is required for printing.
In short, correct control and use of fountain solution can ensure the smooth progress of printing, improve the quality of printed matter, reduce the occurrence of printing accidents, and make production proceed in an orderly manner.
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