Performance and quality control of the most popula

2022-08-19
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Rotary offset printing ink performance and quality control

release date: Source: China paper

rotary offset printing ink is one of the main raw materials for newspaper printing, and the performance of the ink itself directly affects the printing quality of newspapers. Generally, the rotary offset printing ink needs to meet the following requirements: first, under the high-speed cutting of the packaging printing machine, it can always maintain a good balance with the fountain solution, and the rheological property of the ink will not change too much, which can ensure that the ink has good transmission performance on the printing machine, so that the printed newspaper graphics and texts are clear, complete and full of layers; Second, in order to meet the requirements of high-speed rotary machine, the ink penetration and drying speed should be fast, and the ink will not peel on the printing machine and during storage; Third, with the continuous improvement of readers' requirements for the quality of the layout, newspaper printing ink should have better gloss, higher color concentration and pure bright colors. Today, China paper will introduce the relevant contents in detail

I. relationship between ink composition and performance

rotary offset ink is composed of four parts: binder, pigment, filler and additives

1. Binder

in the production of ink industry, binder is a liquid with certain viscosity, and its quality directly affects the performance of ink. Because pigments are generally solid powders, they need to be grinded by the grinder depending on the wetting effect of the binder. At the same time, they can also be transferred in the printing press depending on the viscosity of the binder. On the printed matter, the binder also plays a role in protecting the ink film and substrate, and presents the necessary gloss. Binder determines the viscosity, viscosity, dryness and fluidity of ink to a great extent. In order to get high-quality inks, we must use high-quality binders

at present, several large-scale rotary offset printing ink manufacturers in China mainly choose petroleum resin, modified petroleum resin, rosin modified phenolic petroleum resin, nitric acid resin, etc; The main vegetable oils are flaxseed oil, soybean oil, etc; The main mineral oils are high boiling point kerosene, solvent oil for ink, etc

binder accounts for a large proportion of rotary offset ink, which is 60% - 70%, and will change with the change of production process, ambient temperature and printing model. If the binder is not properly prepared or the production process is not appropriate, it will not only fail to print newspapers with bright colors and excellent gloss, but also cause a series of printing failures such as no inking during printing, dirty edges of pictures and texts, dirty back of newspapers, through printing, paper wool pulling, etc

2. Pigment

pigment is another important part of ink, accounting for 15% - 20% of the ink, and its proportion directly affects the color saturation of the ink and the cost of the ink

the final performance of inks in prints is the color presented by pigments. The role of pigments in inks can be summarized as follows: display the color and color concentration of inks; Give ink a certain inkiness; Affect the gloss and water resistance of ink; To some extent, it affects the dryness of ink. The pigment of the ink must have high coloring power, chemical stability and light resistance. It must also have good dispersion ability in the binder, and the particles must be fine

due to its specific transfer process, the ink layer of rotary offset printing ink is relatively thin, so it is required that the color saturation of the pigment should be high and the hue should be pure. At present, the pigments used by large domestic ink factories are carefully designed and selected, which basically meet the requirements of their concentration, gloss, printing performance and so on. If the microcomputer has strong functions, it can be printed directly

in actual production, because carbon black has the advantages of low price, good covering power, strong coloring power, light resistance, heat resistance, high moisture resistance and good chemical stability, most manufacturers choose carbon black as the main pigment of black ink. Due to the bright color of organic pigments, such as felt textile fiber textile roving chopped strand combination or hybrid reinforcement twistless roving and ground fiber; The thermotropic liquid crystal polymer experimental instrument made of pre impregnated materials: it is stable and affordable, so the ink factory mainly uses organic pigments in the production of color ink. For example, magenta ink uses azo lake pigments, yellow ink mainly uses bisazobenzidine yellow pigments, and blue ink mainly uses phthalocyanine blue pigments

3. Filler

the filler in offset printing ink is a white solid powder that can be evenly dispersed in the binder. Its main function is to increase the solid content of the ink. At present, the fillers used in rotary offset printing inks mainly include ultra-fine calcium carbonate (CaCO3), bentonite and fumed silica (SiO2)

the role of fillers in ink is to adjust the consistency and inkability of ink, such as viscosity, fluidity, density and concentration; Play the role of diluting ink; Affect the dryness of ink; Adjust and reduce the cost of ink

4. Additives

as the name suggests, are materials used in ink manufacturing and printing to improve or highlight a certain characteristic of the ink. There are many kinds of additives used in the ink. The commonly used additives of rotary offset printing ink mainly include diluent, adhesive remover, drier, surfactant, ink balance agent, etc

(1) diluent has two functions: one is to dilute the ink and increase the fluidity of the ink; The second is to reduce the viscosity of ink. At present, most domestic newspaper printing machines use the lower inking method for ink printing, but in the past two years, some newspaper printing houses have purchased newspaper printing machines with the upper inking method. This inking method has higher requirements for the performance of ink, one of which is that it requires good liquidity of ink, and sometimes diluent is needed to adjust the liquidity. In addition, in the printing process, due to the poor quality of the paper, there are problems such as paper roughening and plate pasting. Adding a small amount of diluent can reduce the viscosity of the ink

(2) adhesive remover, also known as viscosity reducer, is used to reduce the viscosity of the ink without affecting the body bone of the ink. If the paper used in the rotary offset press has poor fiber bonding, poor surface strength, poor ink absorption, insufficient water resistance, or the temperature in the printing workshop is too low, as well as the viscosity of the ink itself is too large, it will cause paper roughening or ink stacking in the graphic part during printing. In this case, the appropriate use of adhesive remover can effectively reduce such failures. The dosage of adhesive remover is generally 3% - 5% of the amount of ink, which should be added according to the actual printing situation

(3) desiccant, also known as desiccant, is a very important auxiliary in ink. It is the catalyst of dry vegetable oil oxidation polymerization conjunctiva, and the drying method of oxidation conjunctiva combined with osmotic drying depends on its catalytic drying. The ink is mainly penetration drying ink, so when producing rotary offset printing ink, we should try to use less or no drying agent, otherwise the ink will contact with the air for a little longer, and there will be oxidized conjunctiva (ink scaling phenomenon) on the ink surface, which will affect the use of the ink

(4) surfactant, also known as dispersant. Surfactants can wet the surface of pigments, shorten the grinding time during ink manufacturing, facilitate the dispersion of pigments, and sometimes significantly reduce the oil absorption of pigments. When manufacturing high concentration inks, it can also reduce the yield value of inks and prevent the aggregation and precipitation of pigment particles in inks. The amount of surfactant is generally 1% - 3% of the amount of pigment. The choice of surfactant should be able to reduce the surface tension of binder and increase the wetting ability of pigment. Excessive use may cause the pigments to gather into blocks, which is not conducive to grinding

(5) ink balancing agent, also known as anti emulsifier, the dosage is generally less than 1%. The ink film is cut between the ink rollers and constantly in contact with the fountain solution. Under normal circumstances, the fountain solution should enter the ink. If the water inflow is appropriate and there is no obvious damage to the fluidity, viscosity and other properties of the ink, the printing will not be affected, that is, the printing can achieve the ideal water ink balance. When the ink balance performance is not good, there may be two situations. One is that the ink is too hydrophobic. When the graphic part on the plate receives the fountain solution, it repels the ink due to the attachment of water droplets or water vapor, which finally leads to white spots on the newspaper, and the lines become thinner, discontinuous or fine spots disappear; Another situation is that the hydrophilicity of the ink becomes stronger, so that the fountain solution enters the ink too much, the ink wire becomes shorter, the transmissibility becomes worse, and the drying becomes slower; Due to the continuous supply of new ink in the ink bucket, it is difficult to achieve the water ink balance in a short time. Therefore, the ink performance on the printing machine will change with the operations of ink addition, shutdown, plate erasure, paper change, etc., resulting in the instability of newspaper printing quality

in short, the amount of additives in the ink is small, but it can significantly and effectively improve or change the performance of some specific aspects of the ink, and will not have a great impact on the overall performance of the ink. Therefore, ink additives have become an indispensable part of the production and use of ink

Second, the quality control of rotary offset printing ink

the quality control of offset printing ink is mainly achieved by testing the ink performance indicators one by one

1. Viscosity

viscosity refers to the ability of the ink to block the movement of molecules due to the friction between the internal molecules of the ink under the action of external forces. Viscosity has a great influence on the printing quality, dominating the transmissibility of ink, the reproducibility of dots, and the gloss and drying speed of ink. If the viscosity is too high or too low, it cannot be used normally. The viscosity depends on the speed of the printing machine and the tightness and looseness of the paper structure. The viscosity of rotary offset printing ink is lalay viscosity 50-80pa · s/25c °

at present, the instruments for measuring ink viscosity include rotary viscometer, laray viscometer and balance plate viscometer

2. Viscosity

viscosity refers to the ability of the ink film to resist splitting. Viscosity and viscosity are the performance of different forces, and there is no definite law to follow between them, but generally, the greater the viscosity, the greater the viscosity

the faster the rotation speed of the printing machine, the greater the viscosity of the ink. Therefore, the viscosity of the ink should be designed according to the speed of the printing machine, so as not to affect the quality of the printed matter, otherwise there will be galling, ink flying and other phenomena. Generally, the viscosity of high-speed ink is 3-4, that of medium speed ink is 4.5-5.5, and that of low-speed ink is 6-7

3. Fluidity

fluidity is to measure the ability of ink to flow outward without external force. The larger its expanded diameter is, the better the fluidity of ink is

4. Color

the ink produced by the ink factory requires the same color between each variety and different batches. At present, many newspapers buy different printing models, and the style of each newspaper is also different, which requires the ink factory to carry out "special machine special ink supply" according to the requirements of the newspaper. Although the same four-color ink, the hue and other characteristics of the four-color ink supplied to different newspapers are different. However, for a newspaper, the required four-color ink should be relatively stable in hue and other characteristics

there are two main methods to detect the ink color: ① routine inspection method, namely scraping comparison method. This method is to observe and compare the standard sample and the sample in parallel under the standard light source with human eyes. ② Print proofing with IGT

5. Dryness

the ink dries too slowly, or even does not dry, and the newspaper will be rubbed and pressed

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