The effect of the performance of the main raw materials on the printing quality
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The selection of raw materials in the printing process will have an effect on the artistic effect and technical performance of the printed image, and also affect the stability of the printed image quality. In addition, the performance of raw materials also affects production efficiency. Printing production inevitably requires the use of a variety of materials, and the quality of the materials used is an indispensable condition for ensuring the establishment of high-quality work processes.
There are many kinds of materials used in the printing process, including paper, ink, blanket, adhesive, varnish, etc. Among them, paper and ink are the main influences on printing quality.
First, the impact of paper performance on printing quality
The printing performance of the paper determines whether the printing process can proceed smoothly and whether high quality prints can be obtained. The quality of paper is related to the quality of printed products. Therefore, correctly understanding and understanding certain properties of paper is of great significance for accurately grasping the suitability of printing technology and ensuring the quality of printed products.
1. Surface strength
Surface strength refers to the bond strength between the surface fibers of the paper, the size of the compound, between the fillers, or between the coating particles on the surface of the paper and between the coating and the paper base. It has a decisive influence on the abrasion resistance, powder drop resistance and lint resistance of the paper surface during printing. In order to obtain clearer dots during printing, some inks with higher viscosity can be used. If the surface strength of the paper is insufficient, it is prone to powder falling and lint, and adhere to the surface of the plate.
During the printing process, when the ink is split between the paper and the blanket, peeling tension is generated due to the adhesive force of the ink and the adhesion between the blanket and the paper. When the bonding strength is insufficient, a paper stick phenomenon occurs during the printing process to form a powder or a lint, which may cause the paper to rupture in severe cases. On the surface to which the paper is stuck, white spots are formed where ink should be present. The loose coating, filler particles and fiber can cause adverse consequences: First, the pollution of the graphic part, the more paper and paper powder accumulated on the printing plate, will lead to paste, make the printed matter The writing is blurred, the picture produces spots, which seriously affects the quality of the printed product. Secondly, the number of cleaning of the blanket and the ink roller in the offset printing increases, which reduces the printing efficiency. Increased the labor intensity of the printer.
If the surface bonding strength of the paper is insufficient, the viscosity of the printing ink must be appropriately reduced before printing to reduce the adhesion of the ink to the paper, and also to prevent surface sticking during printing.
2. Ink absorption
The so-called paper absorbency refers to the ability of paper to absorb ink. The ink absorption performance of paper is an important printability index that affects the quality of printed matter. The ink absorbency of the paper mainly depends on the tightness of the paper fibers (the size of the gap). Printing paper requires a proper porosity of the paper fiber structure.
If the paper is too slack, the fiber gap is too large, and the ink absorption of the paper is too strong. On the one hand, the paper is easily pulled, and on the other hand, the binder is excessively absorbed into the paper layer or coating, causing the ink layer on the paper surface. The lack of binder allows the pigment particles to be suspended on the paper surface. After the conjunctiva is dried, the printed matter is old or tarnished. Even if the imprint is rubbed, the pigment will peel off in a granular form, causing pulverization. Paper that is too absorbing to the ink may also cause offset.
If the paper absorbs ink too little, it will slow down the drying speed of the ink on the surface of the printed product, the printing speed will not be lifted, and the back of the printed matter will be dirty, especially for the letter-drying and high-speed printing. In addition, in the printing, paper with poor ink absorption performance is used, and the ink adsorption performance after inking is also poor.
In addition, the paper should also have ink-absorbing uniformity, otherwise the ink color of the writing is different.
3. Smoothness
The smoothness of the paper refers to the smoothness, uniformity and smoothness of the paper surface, which is mainly determined by the manufacturing process and the nature of the raw materials, and has a direct impact on the printing quality. It is an important printing performance of paper, and printing smoothness is the primary condition for obtaining a print that is faithful to the original on paper.
Paper with a high degree of smoothness, using a small printing pressure during printing, allows the plate to be in closer contact with the paper. In this way, the layout image on the printed product can be faithfully reproduced, and the printed matter with clear writing, deep and uniform color, and ink consumption can be obtained.
Paper with low smoothness, due to uneven surface during printing, uneven contact between the paper surface and the printing plate during imprinting, so the pressure generated everywhere is not uniform, and the amount of ink transferred is affected, which is reflected on the printed sheet. The thickness of the ink film is inconsistent, and the fine dots on the printing plate cause copying failure due to the depression of the paper, that is, the brightness level of the image is lost. If it is the solid part of the image, the uniformity of the ink color is lowered, and the expected printing density is not achieved. It can be observed that the print on the printed sheet is faint, and the ink color is not uniform, which affects the image copying effect.
In addition, the smoothness of the paper also affects the gloss of the printed matter, and the high smoothness of printing facilitates the formation of a uniform and smooth ink film on the paper surface, thereby improving the gloss of the printed matter, and the printed matter is bright and beautiful. Therefore, only the paper that is very smooth and smooth can reflect the level and dot shape of the original tone more completely, so that the reproduction of the reproduction is ideal and the level is clear.
To ensure the smoothness of the paper, mainly by adding and calendering. Thus, the tightness of the paper is improved, and the elasticity and ink absorbing property are relatively lowered, which tends to cause back smearing. In addition, when the printing pressure is too large, the ink layer is easily spread, the print is enlarged and deformed, and the defect of the print is falsified. Based on these conditions, when printing a paper with poor smoothness, a clear imprint should be obtained by appropriately increasing the printing pressure and the amount of ink according to the actual situation, thereby improving the printing quality.
4. Glossiness
The quality of the gloss of the paper itself also determines the gloss of the print. For paper with high gloss, the more vivid the ink of the printed product, the better the visual effect of the ink, and the higher the gloss of the printed color. For texts and texts whose layout is mainly text, paper of general gloss should be used to avoid reflection and fatigue of the eyes. For the printing of the plate-based layout, it is advisable to use paper with high gloss to make the printed ink uniform, thick and bright.
5. Water content
The weight of the water contained in the paper as a percentage of the weight of the paper is called the water content of the paper. The amount of moisture in the paper is also one of the important factors affecting the quality of the printing process and printed matter.
5.1 When the water content of the paper is too large, the bonding force between the fibers, the filler and the rubber in the paper is low, which may affect the surface strength of the paper, so that powder removal and paper dust may occur during the printing process. Under the action of external force, the fiber will be pulled out, the plasticity will be enhanced, and the drying speed of the blot will be affected. If the water content of the paper is too small, the paper will be brittle, which will easily cause damage, and will also generate static electricity. At the same time, the printing dot enlargement rate is large.
5.2 Poor control of water content will cause wrinkles. Wrinkled paper is a common phenomenon in light printing. It is essentially a paper problem. Paper is a highly polar hydrophilic substance that has a strong adsorption effect on water. If the paper is wet in the air, it will form a ruffle; in dry air, a tight edge will occur. In both cases, even though the paper feeding device is well delivered, it still wrinkles after being imprinted by the blanket cylinder and the impression cylinder, causing headaches for the worker.
5.3 Poor water content control will result in inaccurate overprinting. Overprinting is the key to ensuring the correct reproduction of the color tone. The change in the moisture content of the paper causes the registration of the front gauge and the side to be inaccurate, which affects the registration accuracy. This is especially true when the printing of single and double color machines (the semi-finished product is placed for too long) and the double-sided printing is required.
5.4 In the dry winter, the paper is prone to static electricity. Static electricity not only makes the paper surface absorb dust and paper hair, but also makes the papers stick to each other, and there are double or multiple paper feeding failures, which will aggravate the flying phenomenon and cause fire in serious cases. Therefore, it is necessary to increase the water content of the paper to eliminate static electricity on the paper.
The fiber of the paper is hygroscopic. The temperature and humidity of the surrounding environment determine the water content of the paper. The water content of the paper is balanced with the temperature and humidity of the environment, and the humidity changes with temperature. Therefore, to control the water content in the paper, it is necessary to control the temperature and humidity of the workshop. The general temperature is controlled at 18 to 24 ° C, and the relative humidity is controlled at 60% to 65% to maintain the balance of water content.
6. Acid alkaline
Generally, the pH of the printing paper is neutral or weakly alkaline, and the acidity and alkalinity of the paper are uncomfortable, which can discolor the ink which is not resistant to acid and alkali, and affect the drying property of the print.
If the paper is too alkaline, the alkaline substance in the paper neutralizes the dampening solution, which directly affects the pH of the dampening solution. At the same time, the alkaline substance also reduces the interfacial tension on the interface of the printing ink, thereby emulsification of the ink, resulting in a portion of the printed product being dirty and affecting the quality of the product. Conversely, if the paper pH is too low, it will cause corrosion and sticking of the printing plate, and inhibit the drying of the print. In addition, the acidity and alkalinity of the paper also has an effect on the durability of the printed matter and the service life of the printing plate.
7. Two-sidedness
Due to the different formation processes of the paper sheets, the properties of the front and back sides are different, which is called the two-sidedness of the paper. The influence of the two sides of the paper on printing is mainly manifested in two aspects: First, the curl of the paper, which is mainly caused by the poor expansion and contraction of the front and back sides. It is not allowed to use severely curled paper for printing. This will cause paper feeding difficulties and malfunctions. The paper should be fully conditioned before printing to achieve a water balance between the front and back sides of the paper. Second, due to the difference in performance such as smoothness, gloss, and absorbency, there is a significant difference in the reproducibility of the printed paper between the front and the back. Therefore, in the printing production, it is necessary to distinguish the front and back of the paper.
8. Anisotropy
Paper anisotropy refers to some of the performance inconsistencies that paper exhibits in all directions.
The paper is longitudinally fed, and the suitability on the printing machine is that the paper feeding stiffness is good, which is convenient for paper feeding and paper feeding; the tensile strength of the paper feeding direction is high, the paper is not easy to be broken, but the moisture absorbing deformation direction of the paper during printing is generated. It is inconsistent with the printing rolling direction, but perpendicular to each other, so the paper is easy to wrinkle during printing, and the offset printing process cannot solve the overprinting error of the axial direction of the roller caused by the deformation of the paper, thereby causing the overprinting to be inaccurate.
The paper is fed laterally, and the direction of moisture absorbing deformation during printing is the same as the direction of printing rolling, in which case paper wrinkles can be avoided or reduced. In addition, the amount of deformation of the graphic can be consistent with the amount of deformation of the paper by the method of drawing, thereby ensuring accurate overprinting.
9. Whiteness
Paper whiteness refers to the ability of paper to be totally reflected after being exposed to light, and is also known as the brightness of paper, expressed as a percentage. Whiteness is an important primary color for the correct color of paper. When printing a color image, the whiteness of the paper is closely related to the vividness of the reproduced color. The whiteness of the paper is high, and the contrast of the color gradation is pulled open, and the gradation of the reproduction is more abundant. Paper with low whiteness, because it only absorbs part of the color light, can not faithfully express the contrast between the light and dark parts, and easily cause color cast. When the paper itself is color cast, the color printed on the paper is the effect of the combination of ink and paper, which will inevitably lead to color cast.
According to the characteristics of the paper color, the ink should be correctly selected to eliminate the color cast. On the other hand, the color shift can be corrected by the ink adjustment process. For fine color prints, high whiteness paper should be used for printing. For books and magazines, because the whiteness is high, it will cause visual fatigue, so the whiteness of such prints need not be too high.
10. opacity
The opacity means that when the printed characters and pictures are on one side of the paper and the other side is also printed with words or pictures, they will not appear to be transparent. Only paper with a certain degree of opacity can prevent the paper from penetrating to ensure the quality of printing. For writing paper, a certain degree of opacity is also required to facilitate the writing on both sides of the paper. For some special papers (such as opaque glossy paper), opacity is also very important.
Second, the impact of ink performance on printing quality
Ink is one of the important raw materials for printing production, and its performance directly affects the printing quality and production efficiency of the product. Therefore, to correctly understand and understand the performance of the ink, in the actual production process, according to the printing conditions and characteristics, appropriate adjustment and improvement of certain properties of the ink, will play a better role in improving production efficiency and product quality.
1. Thixotropy
The thixotropy of the ink means that the ink becomes thin as soon as it is stirred and rubbed, and its fluidity increases, and after standing for a while, the ink returns to its original thicker state. During the printing transfer process, the ink is heated by the rotation friction of the ink fountain and the rubber roller, so that the fluidity and the ductility are also increased until the ink is transferred to the printed sheet, and the fluidity is lost due to the external force disappearing. The ductility is reduced, and the thickness is thickened to ensure the clarity of the printed ink. If the ink is too thixotropy, it is easy to cause ink jam, paste and other defects, and as the printing process progresses, the ink becomes more and more dilute, and the ink color of the printed product becomes lighter and lighter, resulting in unclear graphics. problem. Ink thixotropy is too large and too small to have a bad effect on printing. Printing requires the ink to have proper thixotropy.
2. Viscosity
Viscosity is a property that prevents the flow of fluid material. It is a measure of the interaction between fluid molecules that produces the ability to block relative movement between molecules, that is, the resistance of fluid flow. If the viscosity of the ink is too large, the transfer of the ink during the printing process is not uniform, and the printed sheets are sticky, the ink is transferred, the ink is unevenly distributed, the paper powder is pulled off, and the layout is creased or embossed; if the viscosity of the ink is too small, It is prone to slipping, causing printing bar marks, as well as poor prints, ink emulsification, and dirt.
3. Liquidity
Fluidity means that the ink will flow like a liquid under its own gravity, determined by the viscosity, yield value and thixotropy of the ink, and also closely related to temperature. The fluidity of the ink affects the transfer of ink between the ink tank, the ink fountain, the ink roller, and the substrate, thereby affecting the printing effect. The ink with high viscosity has poor fluidity; the ink with low viscosity has good fluidity.
4. Dryness
The ink should have good drying properties, and the ink drying too fast or too slow will affect the control of the printing process and the printing quality. If the ink is too dry, the printed matter will easily become dirty on the back side during the process of accumulation. In severe cases, adhesion will occur; the print will be dull or even powdered. If the ink is dried too fast, the fluidity of the ink is difficult to control during the printing process, which causes the ink roller to pile up and convey ink, and the ink color is inconsistent, the paper is detached, the ink film is crystallized, and the ink is destroyed. Roll surface performance.
5. Fineness
The fineness of the ink refers to the size of the particles of the pigment (including the filler) in the ink and the uniformity of the distribution of the pigment particles in the binder. Inadequate fineness of the ink in the printing process is prone to quality problems such as ink transfer and uneven printing, ink paste, wear plates, and poor color rendering. The fineness of the ink is relatively large, and the printing is also relatively clear. If the ink fineness is poor, the print dot layout is easy to hair, and the printing plate has a low printing durability.
6. Tinting strength
The amount of ink tinting force determines the depth of the ink hue. When the ink coloring power is large, the printing hue is deep; on the contrary, the printing hue is shallow. The ink with high tinting strength consumes relatively little ink in printing, and the ink color quality is also good. Conversely, the ink with small tinting strength uses relatively more ink during printing, and the ink color is relatively light.
7. Yield value
The yield value is the minimum shear stress required to cause the liquid to begin to flow. Ink with excessive yield value, poor flowability; ink with too small yield value, printed dots are easy to faint and unclear.
8. Lightfastness
The performance of the ink under the action of light, the relative change of the color light is called the light resistance of the ink. In fact, there is no ink that does not change color. Under the action of light, the color of any ink will change more or less. The ink with good light resistance, the printed product has bright color, and the dots on the plate are full and solid, full of three-dimensional sense, and can be preserved for a long time. In the case of an ink having poor light resistance, the printed ink color is liable to cause fading and discoloration.
9. Acid and alkali resistance and alcohol resistance
The ink is resistant to acid, alkali and alcohol. The printed products are bright in color, clear in layout and clear in dots. Printing inks with acid, alkali and alcohol resistance are the basic requirements of packaging products, such as those used for packaging soaps, sodas or foods. The inks used must have good acid and alkali resistance. In addition, the glazing oil contains an alcohol solvent, so products that require glazing must be considered to use alcohol-resistant inks to avoid undesirable phenomena.
The properties of paper and ink are varied, and they all affect the quality of printed products to some extent. Understanding and understanding the properties of paper and ink helps to avoid the blindness of printing process operations, prevent troubles in the printing process, and improve printing efficiency and quality.