The essence of printing is the transfer of ink to paper or other substrates. The offset printing is complicated by the participation 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 as the temperature rises, and the ink will emulsify slightly and transfer to the surface of the paper together with the ink. During the imprinting process, the dampening fluid will also adhere to the paper through the blanket. Therefore, during the offset printing process, the consumption of water is much greater than the consumption of ink, and the control of the amount of water is much more difficult than that of the ink. So far, there is no effective control method. The principle of the general water supply is based on the blank part of the printing plate not hanging ink, and the water supply of the printing plate should be reduced as much as possible.
A type of wetting fluid
In colloidal chemistry, the property that can reduce the surface tension of a solvent is called surface activity, and the substance that can reduce the surface tension of a solvent is called a surface substance. In offset printing, low-surface-active wetting fluids are currently used. Adding surfactants or surfactants to the wetting fluids configures low-surface-tension wetting fluids, which are widely used on offset printing machines. Alcohol wetting fluid and non-ionic surface wetting fluid.
1 Alcohol wetting fluid
It is generally prepared by adding ethanol or isopropanol to ordinary wetting fluid. Ethanol is a surface-active substance that can reduce the surface tension of the solution. In addition, the optimal range of ethanol concentration is 8-25%. Ethanol changes the spreading performance of the wetting fluid on the printing plate, which greatly increases the amount of wetting fluid. Reduction, therefore, also reduces the possibility of printed sheet sticking and serious emulsification of ink.
Because the evaporation of ethanol has a large latent heat of evaporation, a large amount of heat can be taken away during evaporation, so that the temperature of the layout is reduced, the dot increase value is reduced, and the non-graphic parts are not easily stained. Therefore, the use of alcohol wetting fluid can print high-quality fine offset products. However, the volatilization rate of ethanol is relatively fast. If it is not controlled, the wetting fluid will increase the surface tension due to the decrease of the ethanol concentration and weaken the wettability. Therefore, the consumed ethanol must be replenished in time. For volatilization, the temperature of the wetting fluid should be controlled as low as 10 ℃ or below (preferably 4-9 ℃). Therefore, most of the offset presses using alcohol dampening fluid use Dag wheel dampening device to continuously supply dampening fluid to the printing plate. It is also equipped with auxiliary equipment for circulating cooling of the wetting fluid and automatic supplement of ethanol.
[Case 5-3]
A factory purchased an offset printing machine with a squeegee roller with a cooling device. The cooling speed was very fast, which caused excessive cooling of the squeegee roller. After inspection, the refrigeration valve was damaged and the alcohol valve could not be closed.
[Special note] In actual production, it is necessary to pay attention to whether the alcohol control valve and the refrigeration valve are malfunctioning.
2 Non-ionic surfactant wetting fluid
At present, all kinds of wetting powders on the market are such wetting agents. In use, 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.
Non-ionic surfactant wetting fluid is lower in cost than alcohol wetting fluid, non-toxic, non-volatile, and does not require a special wetting system on the offset printing machine. Traditional swing water transfer devices can be used.
Different offset printing inks have different emulsifying ability due to different hydrophilicity of pigments. Through experiments, we can draw the following conclusions: Among the four-color offset printing inks, cyan ink (sky blue) has the highest water intake and the highest emulsification capacity, followed by medium yellow and pink, black ink has the smallest water intake and the lowest emulsification capacity.
If four-color ink is used for printing, different amounts of water should be supplied according to the amount of water absorbed by the ink to prevent the emulsification from intensifying. Therefore, the use of low surface tension wetting fluid must strictly control the amount of water in the printing plate. In the case of a dirty version, minimize the supply of low surface tension dampening fluid. [next]
3. High-quality wetting fluid
It is the latest developed wetting fluid. Generally, this kind of wetting fluid has the following main components:
(1) Buffering agent: The purpose of adding buffering agent in the wetting liquid is to stabilize the PH value of water in printing between 4.8 and 5.3. It is very favorable for offset printing to balance the fluctuation of PH value. The pH value of the wetting liquid has a great influence on the transfer of offset printing ink. In addition, the base of the offset printing plate material is aluminum or zinc, which is a lively metal and is very unstable in the environment of strong alkali and strong acid. Therefore, the PH value of the wetting fluid must be strictly controlled. The buffer keeps the PH value of the wetting fluid at a relatively stable level, so as to maintain a stable water-ink balance.
(2) The printing plate protection agent, in the printing process, when the printing is restarted after printing pauses, it is necessary to readjust the water and ink balance, which is extremely unfavorable for the requirement of saving downtime. Adding gum arabic to ordinary wetting fluid can achieve the purpose of protecting the non-graphic part of the PS version, but because gum arabic is harmful to the graphic part, and the repeated solubility is not good enough. Therefore, adding some synthetic polymers to the composition of high-quality wetting fluid as a plate protection agent, its role is:
a. Keep the new water-based non-graphical parts.
b. Slightly repetitive solubility.
(3) Wetting agent
To make the water more fully wetted in printing, it must be required to reduce the surface tension of the water as much as possible, promote the cleaning and wetting of the printing plate, and make the ink microemulsify faster. Therefore, wetting agents are generally added to the ingredients of high-quality fountain solutions.
(4) Preservative
The role of adding preservatives in high-quality wetting fluid is:
aProtect and protect the metal surface of the offset press.
b protects the printing plate layer and blanket cylinder.
(5) Microbicide
Adding a microbial killer can prevent the growth of microorganisms in the water circulation system, thereby playing an important role in the stability of the printing process.
(6) Anti-foaming agent
The anti-foaming agent works by dispersing the foam instead of dissolving it in the water circulation, which eliminates its impurities. [next]
Second, the correct use of wetting fluid
The correct use of wetting fluid is to strictly control the acid value of the hopper solution according to the different properties of the raw materials and different printing conditions. The effect is to keep the graphic part and the blank part relatively stable. Starting from the requirements of the printing process, the correct use of wetting fluid can:
(1) Make the ink color of the printed product bright, good gloss, and meet the requirements of the original.
(2) Make the printed graphic inks quickly oxidize the conjunctiva and dry within the prescribed time.
(3) Maintain the minimum emulsification value of the ink during printing, and not reduce the printability of the ink due to the emulsification of the ink.
(4) Make the graphics and text and the blank base layer relatively stable, the graphic level of the printing plate is clear, and the dots are clean.
1 Dosage of wetting fluid
The liquid medicine in the water hopper is a dilution of the original liquid (or powder), and its concentration is generally calculated based on the weight or volume ratio of the original liquid (or powder) and clean water. Add the stock solution (or powder) to consider the following factors:
(1) Types of inks Different types of inks have different requirements on the amount of the original liquid due to differences in pigments, oil content, oiliness, viscosity, fluidity, acid resistance and other properties.
The general rule is: red, black, blue, and yellow decrease in sequence, and the amount of dark color is more than that of light color.
(2) The viscosity and fluidity of the ink. If the viscosity of the ink is large, the fluidity is large (the ink is thin), the cohesion is small, and it is easy to spread on the plate, making the plate dirty. Therefore, the amount of stock solution must be appropriately increased.
(3) The amount of ink on the printing plate (the thickness of the layer of the graphic text) is large in the amount of graphic ink, and the amount of the original liquid is large;
(4) The layout and distribution of the graphics on the layout. The graphics on the layout are always composed of solid ground, net pattern, text, lines, etc. The amount of the original liquid is used for both purposes. It can't be printed because of too much original liquid.
(5) The higher the ambient temperature, the higher the amount of raw liquid. The lower the temperature, the lower the amount of stock solution.
(6) The nature of paper The nature of paper mainly refers to the surface strength and pH of the paper. If printing on paper with loose texture that is prone to dusting and lint, due to the viscosity of the ink, the paper wool and paper powder will accumulate on the blanket, increasing the wear on the printing plate and making the printing plate bright and dirty. Therefore, it is necessary to appropriately increase the amount of the original liquid to supplement the inorganic salts and clean the oil. Especially in the case of acid paper, the amount of raw liquid can be reduced. If it is alkaline paper, the amount of raw liquid can be increased.
2 Issues to be noted when using wetting fluid
(1) Under the premise of not affecting the stability of the blank part of the printing plate, try to reduce the amount of the original solution as much as possible.
(2) To prevent any increase or decrease of the amount of raw liquid in the water bucket blindly, because the arbitrary increase or decrease of the amount of raw liquid will cause the acidity of the water dilution of the water bucket to increase and decrease, which will affect the printing plate and blank Relative stability.
(3) Each time you change the color, you should dilute or thicken the water bucket wetting fluid about 300 to 500 sheets in advance.
(4) Frequently test the PH value of the wetting liquid. For the alcohol wetting liquid, the conductivity is to be tested. These values ​​are the key factors of the water-ink balance.
(5) When the stock solution is diluted with water, an appropriate amount of gum arabic must be added to supplement the hydrophilic salt layer consumed by the plate.
Soft Closing Elevator Basket,Kitchen Wall Cabinet Basket,Stainless Steel Pull Out Kitchen Faucet,Wall Mounted Dish Rack
ZHONGSHAN GRUWILL HARDWARE CO.,LTD , https://www.zsgruwill.com