Offset ink and water balance discussion - wetting fluid

The type and use of wetting fluid

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 printing plate should be 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 liquid is usually prepared by adding ethanol or isopropyl alcohol to common dampening solution. 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.

3, high-quality dampening fluid

It is the newly developed wetting fluid. Generally, this wetting fluid has the following main components:
(1) Buffering agent: The purpose of adding the buffering agent to the wetting liquid is to stabilize the PH value of the printing water between 4.8 and 5.3. Equilibrium pH fluctuations are very beneficial to offset printing. The pH of the wetting fluid has a great influence on the transfer of offset inks. In addition, the plate base of the offset printing plate is aluminum or zinc, which is an active metal and is very unstable in a strong alkali and strong acid environment. Therefore, the pH of the wetting fluid must be strictly controlled. The buffer maintains the pH of the wetting fluid at a relatively stable level so as to maintain a stable ink balance.

(2) The printing plate protection agent, in the printing process, often occurs when the printing is restarted after the intermittent, but also need to re-adjust the ink balance, which is extremely unfavorable for the requirement of saving downtime. The addition of gum arabic to common wetting liquids serves the purpose of protecting the non-graphical portions of the PS plate, but because gum arabic is harmful to the graphic elements and repeat solubility is not good enough. Therefore, in the composition of high-quality dampening solution, some synthetic polymers are added as printing plate protection agents. Its role is:
a. Keep the new water of the non-graphic part.
b. Repeatedly slightly soluble.

(3) The wetting agent should enable water to be more fully wetted in printing. It is necessary to minimize the surface tension of the water, promote the cleaning and wetting of the printing plate, and allow the ink to be more quickly microemulsified. Therefore, wetting agents are generally added to the ingredients of the high-quality fountain solution.

(4) Preservatives:
The role of preservatives in high-quality wetting fluids is:
a Protects and protects the metal surface of the offset press.
b Protection of plate layers and blanket cylinders.

(5) Microbial killer Adding a microbicidal agent can prevent the growth of microorganisms in the water circulation system and thus play an important role in the stability of the printing process.

(6) Anti-foaming agent The anti-foaming agent works by dissipating the foam instead of dissolving it in the water circulation, which eliminates impurities.

Correct use of diuretic fluid

The correct use of wetting fluid is based on the different properties of raw materials and different printing conditions, strictly control the acid value of the water bucket solution, give full play to the role of the wetting fluid, and can not only generate inorganic salt layers stably, but also achieve the clean layout. The role of the graphics and blank parts remain relatively stable. From the requirements of the printing process, the correct use of the wetting fluid can:

(1) Make the inks bright and shiny, and meet the original requirements.
(2) The printed ink is dried rapidly by oxidizing the conjunctiva for a predetermined period of time.
(3) The ink is kept at a minimum emulsification value during printing, and the printability of the ink is not reduced due to ink emulsification.
(4) To make the graphic and blank base layers relatively stable, the graphic layers of the plates are clear and the dots are bright and clean.

1 amount of wetting fluid

The syrup in the bucket is a dilution of the stock solution (or powder), and its concentration is generally calculated as the weight or volume ratio of the stock solution (or powder) and fresh water. Adding stock solution (or powder) should consider the following factors:

(1) Types of inks Different types of inks have different requirements for the amount of raw liquid due to differences in the properties of pigments, oil content, oiliness, viscosity, fluidity, and acid resistance.
The general rule is: red, black, blue, and yellow in descending order, and darker colors than lighter ones.
(2) The viscosity and fluidity of the ink, the viscosity of the ink is large, the fluidity is large (ink thinning), the cohesion is small, and it is easy to spread on the layout, making the layout dirty. Therefore, the amount of stock solution must be properly increased.
(3) The ink-carrying capacity of graphic plate (graphic ink layer thickness) is large, and the amount of ink used is large, and the amount of raw liquid is large. The amount of ink contained in the graphic is small, and the amount of raw liquid is small.
(4) The structure and distribution of the layout and layout of the layout, the layout of the graphic is always composed of the field, texture, text, lines, etc., the amount of liquid used to take into account the use of both, can not make the field is not true, can not make the network to send Paste, but can not be due to the original volume is too large to spend.
(5) The higher the ambient temperature, the higher the amount of raw liquid. The lower the temperature, the amount of liquid used to reduce.
(6) The nature of the paper The nature of the paper refers mainly to the surface strength and pH of the paper. If the texture is loose, it is easy to print on the paper that is easy to lose powder and lint. Due to the viscosity of the ink, the paper and paper dust accumulate on the blanket, which increases the wear of the printing plate and makes the printing plate bright and dirty. Therefore, it is necessary to increase the amount of raw liquid to supplement inorganic salts and dirty oil. Particularly encountered acid paper, the amount of liquid can be reduced, if the alkaline paper, the amount of liquid can increase.

2 Problems that must be paid attention to when using the wetting liquid

(1) Minimize the amount of raw liquid without affecting the stability of the blank part of the printing plate.
(2) Prevent any blind increase or decrease in the amount of raw liquid in the bucket, because any increase or decrease in the volume of the original liquid will cause the acidity of the bucket wetted diluent to become large and small, which will inevitably affect the printing plate and blank parts. Relative stability.
(3) About 300 to 500 sheets of dilute or thickened water fountain wetting fluid should be replaced before each color change.
(4) Frequently measure the pH of the wetting fluid. For the alcohol wetting fluid, test its conductivity. These values ​​are the key factors for ink-water balance.
(5) When the stock solution is diluted with water, the gum arabic must be added in an appropriate amount to supplement the hydrophilic salt layer consumed by the layout.

Author/Li Yongqiang

Source: Ke Yin Network