Offset type and use of offset printing machine (1)

The essence of printing is the transfer of ink to paper or other substrates. Offsets are complicated by the presence of wetting fluid. On the surface, the water film of the printing plate is thinner than the ink film, but in fact, the wetting liquid will evaporate with the increase of the temperature, and the ink will slightly emulsify and transfer to the surface of the paper along with the ink. In the imprinting process, the wetting fluid also adheres to the paper through the blanket. Therefore, during the offset printing process, the consumption of water is much greater than the consumption of ink. The control of the amount of water is much more difficult than the control of the amount of ink. So far, there is no effective control method. The principle of general water supply is based on the fact that the blank part of the printing plate is not covered with ink, and the amount of water supplied by the plate is reduced as much as possible.

Kind of dampening fluid

In colloid chemistry, a property that lowers the surface tension of a solvent is called surface activity, and a substance that lowers the surface tension of a solvent is called a surface material. In offset printing, low-surface-active wetting fluids are currently used. In the wetting fluids, surface active substances or surfactants are added and they are configured as wetting fluids with low surface tension. They are widely used in offset printing machines. Alcohol wetting fluid and non-ionic surface wetting fluid.

1 Alcohol Wetting Fluid

It is usually prepared by adding ethanol or isopropanol to common wetting fluid. Ethanol is a surface active substance that can reduce the tension on the surface of the solution. In addition, the optimal range of ethanol concentration is 8 to 25%. Ethanol changes the spreading performance of the wetting fluid on the printing plate, making the amount of wetting fluid large. The reduction, therefore, also reduces the possibility of sticky water on the printed sheets and serious emulsification of the ink.

Since the volatilization of ethanol has a large latent heat of evaporation, a large amount of heat can be taken away during volatilization, so that the layout temperature is reduced, the dot gain value decreases, and the non-graphic part is not easily stained. Therefore, the use of alcohol wetting fluid can print high quality fine offset products. However, the volatilization rate of ethanol is faster. If it is not controlled, the wetting liquid will increase the surface tension due to the decrease of ethanol concentration, and the wettability will be weakened. Therefore, it is necessary to replenish the consumed ethanol in time, and at the same time, in order to reduce the ethanol Volatilization, we should try to control the temperature of the wetting fluid below 10 °C (preferably 4 ~ 9 °C). Therefore, offset presses that use alcohol wetting fluids mostly use Dacron wheel-type wetting devices to continuously supply the wetting fluid to the printing plate. And equipped with wetting fluid circulation cooling, auxiliary equipment to automatically add ethanol.

[Case 5-3]

A certain factory purchased an offset printing press with a cooling device and the cooling speed was very fast, causing the cooling roller to cool excessively. After inspection, the cooling valve was damaged and the alcohol valve could not be closed.

[Special note] In the actual production, we must pay attention to the alcohol control valve, whether the refrigeration valve fails.

2 Non-ionic surfactant wetting fluid

At present, all kinds of wetting powders sold on the market are such wetting agents. When used, as long as the powdered wetting agent is dissolved with a certain amount of water, it can be added to the water bucket for printing.

The non-ionic surfactant wetting fluid is cheaper than non-alcoholic wetting fluid, non-toxic, non-volatile, and does not require a dedicated wetting system for the offset press. A conventional swing-type water transfer device can be used.

Different offset printing inks have different emulsifying power due to their different hydrophilicity. Through experiments, it can be concluded that in the four-color offset printing ink, the blue ink (sky blue) has the highest water intake, the highest emulsifying power, followed by yellow and pink, and the black ink has the smallest water intake and the lowest emulsifying capacity.

If four-color ink printing is used, different amounts of water should be supplied in order to prevent the emulsification from increasing in accordance with the size of the ink's water intake. Therefore, the use of a low surface tension wetting fluid must strictly control the amount of water in the printing plate. In the case of a dirty plate, minimize the supply of low surface tension wetting fluid.