The relationship between the printing plate and ink viscosity

1. Dirty plate In plastic gravure printing, a thin layer of resin (an entrained pigment in the resin) adheres to the non-printing surface of the platen, ie, a layer of ink that is often missing on the non-printing surface of the plate cylinder. , And transfer to the substrate, so that the printed material shows a sheet or line of dirt, which is dirty version (also known as fog) failure. The cause of the dirty plate problem is not only the scraper (including the hardness of the blade, the blade pressure, the angle of contact of the blade), the quality of the printing plate, and the ink formulation, but also mainly related to the viscosity of the ink. If the viscosity of the ink is too large, the viscosity of the ink becomes large, and the fluidity of the resin, the pigment, and the like is poor, and it cannot be uniformly dispersed, and is liable to be piled up together. The pigment in the ink is unevenly distributed in the ink system, or the resin in the ink fails to wrap the pigment particles well, so that the pigment particles will re-aggregate to form a larger pigment group (ie, a thickening phenomenon). The relatively high-speed movement of the squeegee and the stencil, the squeegee and the stencil are liable to be damaged. These large granules easily form a linear or stripe-like cutting line during printing, and a linear smear occurs. In addition, since the resin and the like are also in a supersaturated state, a large amount of the resins are agglomerated together and the resin has an affinity for the roller. Under the pressure of the squeegee, the resin adheres tightly to the drum (the pigment is entrained in the resin), so that the sheet becomes contaminated. In the high-speed movement of the printing plate, the ink will also have a great impact on the squeegee, so that it is difficult for the squeegee to smoothly scrape off the non-graphical ink to produce a dirty plate.

In general, the smaller the viscosity of the ink, the less likely it is that dirty print problems will occur. However, if the viscosity of the ink is too small, it indicates that the organic solvent content in the ink is high, and the resin, the pigment, and other components are relatively small, so that a smooth film cannot be formed when the coal is dry, and therefore, it is whitened and lacks luster. If the printed product needs to have a good gloss, it is generally considered to use a larger ink viscosity (13 to 19 seconds, Zahn No. 3 cup), that is, the resin and pigment components are relatively more to meet the needs of the printed product. . However, special attention should be paid to the dirty version caused by the large viscosity. In addition, when the ink is printed on a plastic and dried to become an ink film, the coated pigment should be firmly attached to the plastic surface, and the resin is the main material to which the plastic is firmly attached. Therefore, the firm adhesion of pigment is also related to the viscosity of the ink, which means that the proportion of the resin and the pigment in the ink should not be too large or too small, but should be within the normal range, and the color fastness of the pigment must meet the requirements. Under the condition of maintaining the normal environment humidity, the ink viscosity is more than 16 seconds (3 cups of Zahn), and the electrostatic phenomenon will not occur. The viscosity of the ink is less than 16 seconds (the Zahn No. 3 cup), and as the viscosity becomes smaller, electrostatic phenomena such as beard-like, streak-like, marginal repulsion, flying ink, poor transfer, and irregularly-formed water-stained patterns occur. And heavier. Therefore, in the actual printing process, special attention should be paid to the change of the viscosity of the printing ink, and the viscosity should be adjusted in time to improve the quality of the printed product.

2. Flying ink When high-speed gravure printing (speed exceeds 20m/min or more), if the gravure ink adopts a domestically produced chlorinated polyalkylene resin with a wide molecular weight distribution, the viscosity of the printing ink may be too low. The question of flying ink. The reason for this is that the viscosity of the printing ink is too low. The cohesion of the ink is greatly reduced. When the ink is transferred to the printing film, the low-molecular-weight resin can be easily separated and a small dot can be seen in the blank portion of the print. To solve the problem of flying ink, we mainly proceed from two aspects: one is to use a chlorinated polypropylene resin with a narrow molecular weight distribution, and the other is to appropriately increase the viscosity of the ink according to the printing conditions.

3. Effect on color, transfer rate, and color saturation The gravure ink is transferred to the printing substrate by means of gravure dot ink. Relatively speaking, the volume of printing dots is basically fixed. With the increase of the viscosity of the ink, the viscosity of the ink is also increased, and the more ink is transferred from the dot, the higher the color density and saturation of the printed article, but this is a certain limit. When the viscosity of the ink rises to a certain degree, the transfer rate of the ink also increases synchronously. If the viscosity of the ink continues to rise, the viscosity of the ink becomes stronger and the ink becomes difficult to enter into a small size. The number of inks in outlets and outlets is decreasing, and the ink transfer rate has gradually declined. The color density of prints also began to decline. Therefore, in the actual printing process, in order to achieve a good printing transfer effect, the viscosity should be selected very carefully. In general, the gravure ink viscosity can be considered in l3 ~ l9 seconds (3 Zahn Cup).

4. The impact on the printing speed According to the data, the current domestic gravure printing machine printing speed from 30 ~ 3O0 meters / min, the general printing press speed of 80 ~ 200 meters / min. In actual printing, it is selected according to the printing speed of the printing press. As the printing speed increases, the viscosity value will decrease. Of course, the most suitable viscosity is also affected by factors such as ink formulation, temperature and humidity, and printing plates. Even the same printing speed is due to the difference in ink formulation design. Its viscosity will also be different. In the actual printing process, it must be adjusted according to the effect of printing. Viscosity is also affected by the seasonal temperature. The viscosity of the same printing ink is different in the summer and winter. Although the ink manufacturers have been working hard to reduce the influence of temperature on the viscosity of the ink, this effect is still relatively common. In addition, the ink viscosity is also related to the printing plate, so the most suitable ink viscosity should be selected according to the actual conditions of the printing plate cylinder.

5. Effect on the glossiness of the printed matter The lower the viscosity of the gravure ink (that is, more solvent is added to the original ink), the lower the content of the resin and the pigment in the printing ink, the less the ink is transferred after being transferred to the printing substrate. Smooth ink layer, and the phenomenon of white flowers, leading directly to the lack of gloss print. Therefore, in order to obtain excellent gloss, it is necessary to ensure that the printing ink has a suitable resin and pigment content. In summary, the viscosity is an important parameter of the ink, and many factors affect this parameter. Ink viscosity has a great influence on the quality of printed matter. Different printing equipment, materials, substrates, and environmental factors all have different requirements for ink viscosity. Therefore, in the plastic film printing, according to the characteristics of the substrate, printing equipment and the environment is different, comprehensive consideration, and reasonable control of the viscosity of the printing ink, to ensure the printing of beautifully printed products.

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