Quality Control of Printing Products in Offset (2)

Due to the nature of offset printing (need to print with the participation of water, the printing plate and the blank are almost on the same plane), the offset ink must have good water resistance, good adhesion, elasticity, and very good touch. transsexual.

1 water resistance. For smooth printing, it is necessary to select inks with good water resistance, but unlike other conditions, the water resistance of offset inks is not as strong as possible. Completely water-repellent offset inks cannot be printed on the machine. This is due to the fact that during the ink transfer process, water that has reached the ink path due to the high-speed shearing of the printing press will inevitably be encountered. When the oil and water are completely incapacitated, the normal ink transfer will be hindered.

· In printing, in order to ensure the ink's water resistance, we add additives (desiccant, remover, varnish, anti-fouling agent, etc.) to the ink to avoid reducing the cohesion of the ink, and in some cases According to the situation, a tackifier is added to the ink to restore the cohesion of the ink so as to restore the ink's water resistance. At the same time, the ink in the ink path and the ink fountain should be regularly cleaned to prevent the use of the residual ink on the machine and affect the water resistance.

2 sticky and elastic. Good adhesion is the premise to ensure that the ink can be properly transferred and transferred; and the ink's elasticity can ensure that the ink can quickly recover and level after transferring to the surface of the substrate. Good elasticity is the prerequisite for ensuring the smoothness of the surface of the printing dot.

• As mentioned above, except that good quality inks are selected, what we do in printing is to maintain the properties of inks. Many additives are added as mentioned above to minimize the ink properties due to human operations The change. Also pay attention to the control of the wetting fluid (discussed in detail in the process).

Thixotropic. The ink used in offset printing must have good thixotropy, that is, the fluidity increases under the action of the mechanical shear force when the ink passes out of the ink fountain, and the mechanical force when the ink is transferred from the surface of the blanket to the surface of the substrate. The disappearance of ink, the rapid decline in the flow of ink, began with the penetration of the binder in the ink on the surface of the substrate together to promote the initial curing of the ink. This process is very important for multi-color printing, can greatly affect the multi-color machine's overprint effect.

• Choosing a good thixotropic ink is very important for printing.

4 ink transparency. Offset inks require good transparency.

In fact, the transparency of printing inks in printing cannot be guaranteed, and the transparency of inks of different colors is not always the same. Therefore, in the printing process, the color sequence of the ink should be considered. A reasonable arrangement of the color sequence can effectively improve the color saturation of printed materials. degree.

5 ink particle size and coloring. Good printing inks require that the finer the particles of the ink pigment, the better the coloring properties. Select ink must not ignore these two points, the same is still the old saying, in the cost allowable conditions to select the finest particles of ink, high colorability of the ink, are conducive to print quality control.

6 ink stability. The stability of the ink includes the ability of the ink to resist light, heat, acid, alkali, and alcohol, and it requires that the ink does not change color and age under certain conditions, especially for inks with special uses (such as printing on billboards that need to be displayed in the sunlight, etc.) ), it is even more demanding that inks have this special resistance.

(3) Wetting fluid

The role of wetting fluid

The blank part of the printing plate forms a uniform water film to resist the infiltration of the graphic part of the ink to the blank part of the printing plate, preventing the dirty plate; forming a new hydrophilic layer, thereby maintaining the hydrophilicity of the blank part of the printing plate; Wet fluid to control the temperature of the plate surface. Because of the above effects of the wetting fluid, the requirements of the wetting fluid in the printing are: high surface activity (add surfactant or low surface energy substances in water); there is a certain degree of acidity (pH: 4.5- Between 6).

· The control of wetting fluid in the printing process is mainly considered from the pH value of the fountain solution. Here we briefly talk about the control of the PH value:

1 When the amount of desiccant in the ink is increased, although the drying speed is increased, the viscosity of the ink rises, the particles become thicker, and the adhesiveness to the blank part of the printing plate increases, and the dirty plate is prone to occur, so the amount of addition and discharge of the raw liquid is increased. The pH of the wetting fluid slightly decreased, which is beneficial for preventing the dirty plate.

2 If the non-coating paper with loose texture is used for printing, if the viscosity of the ink is large, the powder falling off is serious and the printing plate is dirty due to wear, it is necessary to properly reduce the pH of the wetting liquid. With advanced coated paper printing, the pH of the wetting fluid can be appropriately increased.

3 In the case of solid-printing, the pH of the wetting fluid can be lower. On the other hand, if dot printing is used, the pH of the wetting fluid can be higher.

4 When the temperature of the printing shop increases, the viscosity of the ink drops, and the fluidity increases. At the same time, the dry vegetable oil separates more free fatty acids and causes the layout to become dirty. Therefore, the pH of the wetting fluid should be lower.

(4) Blanket.

The ideal blanket should be completely non-absorbent, good ink, should have good elasticity, good compressibility, and can quickly restore the original shape after the pressure disappears, that is, all the elasticity in the printing performance Sensitive elasticity.

1 In printing, we want the blanket to deliver ink only to the surface of the substrate, and the moisture is that we do not want to transfer to the surface of the paper, otherwise it will cause the surface strength of the paper to be reduced or even deformed. Therefore, we hope to get a blanket that is water-friendly and ink-friendly.

But in practice, it is impossible to obtain blankets that are completely non-absorbent and have excellent ink-receptive properties. In addition, during the printing process, the rubber, fibers and other hydrophilic substances in the paper and the pigments in the ink are used on the blanket. Adsorption will result in a decrease in the ink-receptivity of the blanket during the printing process and an increase in hydrophilicity. Therefore, in addition to selecting a blanket with good surface properties before printing, care must be taken to clean the surface of the blanket during printing, so as to maintain a good ink-repellent hydrophobic property of the blanket.

2 Since blankets are periodically subjected to compression and elastic recovery during printing, this cycle is relatively short on current high-speed printing equipment. If the elasticity of the blanket shows a lag, it cannot be immediately after the pressure disappears. To restore, the compression range of the blanket will be affected during the next cycle of compression.

• The elasticity of the blanket can not be avoided, and even irreversible plastic deformation can occur. These are the factors that affect normal imprint during printing. Before printing, we must select the appropriate available blanket, and after the blanket has been pressed for a period of time, we must check the printing pressure (for example, it can be detected by pressing the bar marks). It also advocates the use of two blankets for rotation. Give the blanket a recovery time and reduce the blanket's aging cycle.

3 The ideal blanket should be compressible, that is, only a reduction in volume occurs in the compressed area, and the volume in the uncompressed area does not change. This can reduce the uniformity of the line speed ratio of the drum surface during the printing process within a considerable range, thereby reducing the distortion of the dot due to the speed difference.

• The blankets used in our daily production are generally incompressible. That is, in the vicinity of the embossed area, the blanket has a convex bulge and a convex bulge, which makes the linear velocity difference in the embossed area have a greater change. , which has a great influence on the correct transfer of outlets. Relatively speaking, the compressibility of the air-cushion blanket is quite good. For high-quality prints, using an air-cushioned blanket can get a good printing effect.