First, the definition of ink viscosity
Ink viscosity is usually divided into Newtonian viscosity, plastic viscosity, apparent viscosity and differential viscosity, etc., which are not comparable to each other, but as an important indicator of ink rheological properties, they are comparable in the same expression method. Large viscosity of ink means low fluidity, and low viscosity of ink has great popularity. In practical applications, the viscometer is usually used to detect the plastic viscosity of the ink. Ink viscosity is an important factor that shows the suitability of ink printing. It directly affects the entire printing process and the quality of the final print. As an important indicator of ink performance, ink viscosity reflects the printing suitability of ink from one side.
Types of printing inks for plastic films
1. Ink for gravure plastic printing Ink for plastic gravure is also called compound ink, which refers to the ink printed on the substrate. It is volatile and dry with low viscosity. It is mainly made of binder, polyamide resin, plus pigments and auxiliaries after grinding and filtering. The softening point of polyamide resin should be controlled at 121 °. Otherwise, the resin will have a low softening point, which will cause blocking of printed products during printing. The resin has a high softening point, and it will freeze when it is used in winter. You need to heat and mix to melt it.
The gravure plastic ink has good printability, strong adhesion to plastic films, fast drying speed, bright color and gloss of printed graphics. It belongs to volatile drying type, and the printing machine speed can be 40 ~ 160m / min according to different printing models.
2. Flexographic plastic printing inks The biggest difference between flexographic plastic inks and gravure plastic printing inks for general surface printing is that they cannot swell the plate. Its initial dryness, adhesion fastness and gloss are all similar to the surface printing gravure plastic ink. However, since the ink layer is thinner than the gravure after the transfer, compared with the surface printing gravure plastic ink, it has different concentrations and viscosity.
3, UV plastic ink UV plastic ink can be cured instantly, save space, short cycle; low temperature curing, suitable for a variety of substrates, including film materials; high solidification, no solvent (or minimal), can improve the working environment, Prevent air pollution. Abrasion resistance, solvent resistance, heat resistance, etc. meet the general requirements of current printing operations.
4. Water-based plastic ink Water-based plastic ink refers to the resin binder in the ink is dissolved with water (there may be a small amount of alcohol, ammonia, etc.). After the ink is transferred to the substrate, the water volatilizes into the environment or penetrates into the substrate. The ink dries as the water evaporates. The viscosity of this ink is easy to adjust, the printing process has no pollution to the environment, and the ink is non-toxic to the human body.
The influence of ink viscosity on printing effect
In printing production, it is necessary to rely on appropriate methods to keep the ink viscosity within a certain range to meet the needs of printing. In the printing process, the ink is continuously split and transferred between the ink roller and the ink roller, between the ink roller and the printing plate, between the printing plate and the blanket, between the blanket and the substrate, and the ink layer is split at the moment. Due to the cohesive effect of the ink system, a kind of resistance will be generated between the ink molecules, preventing the ink layer from splitting. If the viscosity of the ink is too large, the resistance between the ink molecules will also be very large, causing the ink layer to be difficult to split, and the substrate will be subject to burrs or even peeling, which greatly reduces the service life of consumables such as blankets and printing plates. For example, in general UV ink printing, the reason that ordinary printing plates and blankets are easily damaged is because the viscosity of UV ink is particularly large. In addition, high viscosity ink ink filaments are long and easily drawn into filaments. During the transfer process, the ink filaments often break into several knots and splash around, causing flying ink, and printing failures such as blockage and uneven ink transfer. . If the viscosity is too small, the cohesive force in the system is also small. When printing, the fountain solution can easily invade the inside of the ink, emulsifying the ink, causing floating stains and dot expansion. The ink is also not easy to adhere to the surface of the ink roller during transfer, which causes poor ink transfer, and causes printing failures such as stacking, stacking, and ink stacking rollers. Therefore, it is necessary to control the appropriate ink viscosity during production.
4. Problems caused by ink viscosity in plastic film printing
1. Dirty plates In plastic gravure printing, a thin layer of resin is attached to the non-printing surface of the cylinder (the resin is entrained with pigment), that is, a non-printing surface is often attached to the non-printing surface of the cylinder. , And transferred to the substrate to make the print appear flake or line-shaped dirt, this is the dirty version (also known as the fog version) failure. Generally speaking, the smaller the viscosity of the ink, the less likely it is to cause scumming problems, but special attention should be paid to the scumming phenomenon caused by high viscosity.
2. Flying ink When high-speed gravure printing (speed over 200m / min or more), if the gravure ink uses domestic chlorinated polypropylene resin with a wide molecular weight distribution, which causes the viscosity of the printing ink to be too low, it may cause flying ink. The problem of ink. The reason for this is that the viscosity of the printing ink is too low, and the cohesion of the ink is greatly reduced. When the ink is transferred to the printing film, the low molecular weight resin is easily separated, and small ink dots can be seen in the blank part of the printed matter. 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; the other is to appropriately increase the viscosity of the ink according to the printing situation.
3.Effect on the gloss of printed matter The lower the viscosity of the gravure ink (that is, the more solvent is added to the original ink), the lower the resin and pigment content in the printing ink. After the ink is transferred to the printing substrate and dried, it will not be easy to form. The smooth ink layer and the white flower phenomenon directly lead to the lack of gloss of the printed matter. Therefore, in order to obtain excellent gloss, it is necessary to ensure that the printing ink has the appropriate resin and pigment content.
4. Watermark The watermark phenomenon is mainly related to the ink viscosity and printing speed, but usually the printing speed is fixed, so in most cases, the problem of watermark must be solved by increasing the ink viscosity.
5. Effect on color, transfer rate and color saturation. Gravure ink is transferred to the printing substrate by the dot ink that is recessed by the gravure. Relatively speaking, the volume of printing dots is basically fixed. As the viscosity of the ink increases, the viscosity of the ink also increases. The more ink transferred from the dots, the higher the color density and saturation of the printed product. But there are limits. When the viscosity of the ink is continuously increased to a certain extent, the transfer rate of the ink has also increased simultaneously. If the viscosity of the ink continues to rise, the viscosity of the ink will become stronger and stronger, and it will become difficult for the ink to enter very small dots. The ink in the dots will become less and less, and the transfer rate of the ink will gradually decrease, and the color of the printed matter. The concentration also began to decrease. Therefore, in the actual printing process, in order to achieve a good print transfer effect, we must choose the viscosity very carefully. Generally speaking, the viscosity of gravure ink can be controlled in 9 ~ 13 seconds (No. 3 Zahn Cup).
The requirements for ink viscosity in printing mainly depend on factors such as the printing speed of the printing machine, the degree of softness of the substrate, and the changes in temperature and humidity of the surrounding environment. The factors that affect the viscosity of the ink mainly include the temperature and pressure of the environment, the volume ratio of the pigment to the binder in the ink composition and structure, the size and shape of the pigment particles, the presence of the surfactant, and the thixotropy of the ink. Generally speaking, the quality of plastic film composite printing is mainly controlled from two aspects: adjusting the composition and structure of the ink and controlling the thixotropy of the ink.