In plastic printing, the control of ink printing viscosity is very critical, especially in the high-speed rotary gravure, the ink printing viscosity control is appropriate, directly affect the transfer of pigments, the fastness of the paint's adhesion, the gloss of printed products, static electricity and other quality Problems affect the yield of printed products. In this paper, by analyzing the relationship between viscosity and print quality, some methods to control ink viscosity and improve printing quality are introduced.

Pigment transfer

The gravure printing ink is composed of organic solvents, resins, pigments, additives, additives, etc. The quality of the pigment transfer is related to the printing viscosity of the ink. Practice has proved that the ink printing viscosity is suitable between 11 ~ 24s (3 Zahn viscosity cup, the same below), the greater the ink printing viscosity, the poorer the effect of pigment transfer. Because the solvent is mainly dissolved resin, additives and additives, the pigment is easily dispersed and the ink has fluidity. When the viscosity of the ink is too high, the ink system is in an oversaturated state. The fluidity of the pigments and other substances is poor and cannot be uniformly dispersed. The inks cannot aggregate evenly, so that the pigments can not smoothly enter the cell site or form a blockage plate, let alone the substrate. Transferred. Therefore, it is necessary to ensure that the colloidal system composed of resin, pigment, and solvent is in a saturated or unsaturated state, so that pigments and other substances can be uniformly dispersed therein, and the pigment can enter the cell site smoothly.

Some ink manufacturers suggest that the ink printing viscosity control between 15 ~ 18s. However, in practical production, especially in the production of high-speed gravure printing machines (print speeds of 100 to 260 m/min), in order to ensure a good transfer effect, the viscosity of the ink should generally be controlled between 11 and 17 seconds.

In gravure printing, in the plastic gravure printing, the non-printing surface of the roller is often covered with a layer of resin (entrained with pigment in the resin), transferred to the substrate as a stained sheet, or a line of dirt. It is a dirty version. In addition to the hardness of the blade, the blade pressure, the blade contact angle, and the quality of the plate cylinder, the resulting dirty plate phenomenon is also related to the viscosity of the ink. When the printing viscosity is too high, resins, pigments, and other substances have poor fluidity and cannot be uniformly dispersed and accumulated, causing the blade and the plate cylinder to be damaged under high-speed operation, resulting in linear fouling. At the same time, since the resin and the like are also in a supersaturated state, a large amount of resin accumulates together and under the pressure of the doctor blade, the resin adheres to the drum (the pigment is entrained in the resin) due to the affinity of the resin to the drum, and the sheet becomes contaminated. In general, the smaller the ink printing viscosity, the less likely the dirty plate problem occurs.

Printing ink gloss is too low, indicating that the solvent content is high, while the resin, pigment and other components are relatively small, the ink transfer to the substrate is not easy to form a smooth film after drying, and may appear whitening phenomenon, so that the lack of imprinting luster. Therefore, to make the printed product has a good gloss, the ink viscosity should be controlled between 13 ~ 19s to maintain the appropriate ratio of resin and pigment.

Pigment Adhesion Fastness The ink is printed on the plastic and dried to form an ink film, which can hold the pigment and fix it. The resin is the main material for the pigment to adhere firmly. Therefore, the fastness between the pigment and the substrate is related to the content of the ink resin. The proportion of the resin and the pigment in the ink should be controlled within the normal range so that the fastness of the pigment on the substrate can be maximized.

Static electricity under normal humidity, ink viscosity above 16s, electrostatic phenomenon generally does not occur; ink viscosity below 16s, the lower the viscosity, beard-like, markings, edge exclusion, flying ink, poor transfer and irregular water stains The more prone to electrostatic faults such as electrical faults.

Based on experience, it is possible to find the ideal state of ink viscosity between 11 and 17 seconds. If static electricity occurs within this range, it is generally not required to increase the viscosity of the ink, but rather to change under the premise of maintaining normal ambient temperature and humidity. Diluent composition, installation of static brush, static electricity prevention agent and other methods to eliminate static electricity to ensure the quality of gravure, improve production efficiency and yield.

Source: PACK.CN

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