About Offset Printing (I)

Lithographic principle

In the principle of lithography, it has been said that lithographic printing is carried out using the law of repelling oil and water. Therefore, in addition to the ink supply device on the printing press, there is a water supply device. In the printing process, oil and water must be balanced to produce a good product. The balance of oil and water involves a series of factors, including the influence of air temperature and humidity. Therefore, the quality of lithography is greatly affected. Depends on operating skills and printing knowledge.

First, lithographic printing machine

Unlike lithographic and gravure printing machines, lithographic printing presses mainly differ in lithographic printing presses and water supply devices. In addition, lithographic printing mostly uses indirect printing. Therefore, in the printing section of lithographic printing presses, a blanket cylinder is also provided. The images and texts are first printed on the blanket cylinder and then transferred to the paper for printing.

In lithographic printing, direct printing is used for lithographic printing. Flat proofing press is used for round-press flat printing. The vast majority of other types use circular press-type indirect printing, that is, offset printing.

There are many kinds of offset presses. The printing device of the offset press has various models because of the different sizes of the cylinders, and the arrangement and the number of different types. As shown in Figure 1.

Second, printing preparation

1. Prepare with paper

Lithographic printing generally uses offset paper, coated paper, map paper, sea drawings, and the use of letterpress paper to print books.

The paper used for lithographic printing is required to be small in stretchability, tight in texture, non-skeined, and non-powdered, and has appropriate properties such as smoothness and whiteness.

The main component of paper is plant fiber, which absorbs moisture from the air. Therefore, the water content of the paper changes with changes in temperature and humidity. The moisture content of the paper changes, and the paper retracts. At the same time, the lithographic process Water is needed. After the paper comes into contact with water, the moisture content also changes. The size of the paper is difficult to maintain. For color printing, overprinting is not allowed. Experiments have shown that a general change in relative humidity of 10% in air causes the paper's moisture content to change by about 1%, and the paper size changes by about 0.1%, which can only control the temperature and humidity of the printing shop and reduce the change in the moisture content of the paper. To control its scaling changes.

Paper is also subject to deformation caused by climate change during shipment, transportation and storage from paper mills. If the water content in the air is less than the water content of the paper, the paper edge water will disperse and evaporate, and the phenomenon of “tight edges” will appear, as shown in Fig. 5-22a. When the water content in the air is greater than the moisture content of the paper, the paper edge absorbs moisture, and the paper If the water content in the middle is small, and the amount of water on the edges is large, the phenomenon of "foliage" appears, as shown in Figure 5-22b. When the water content on the front and back of the paper is not equal, the paper appears "curled", as shown in Figure 5-22c. . Deformation of the paper can cause misprints and wrinkles in the printing process.

In order to reduce paper failure during printing, the water content of the paper is adapted to the printing requirements, and it is compatible with the temperature and humidity of the printing environment. The plain printing paper generally needs to be subjected to suitable printing processing, and the processing method is preferably to use a paper-making method. The paper is in line with the temperature and humidity of the printing shop. The water content of the paper itself is even.

In the lithography, the fiber flow direction of the paper is also very important, and the flow direction of the fibers is inconsistent, affecting the paper feeding and overprinting. In order to ensure the accuracy of overprinting, it is preferable that the fiber flow direction of the paper used in the same printing plate is the same. The flow of the paper fibers is generally printed parallel to the print cylinder. In order to know the flow of fibers in the paper, it is necessary to make a judgment before printing. Commonly used methods include tearing, hanging law enforcement, wetting method, nail sliding method, or law enforcement.

After the printable paper is cut, it is stacked neatly on the transfer table of the offset press to ensure that printing can be performed smoothly.

2. Mix ink

The inks used for lithographic printing are required to have strong coloring power, sufficient fixation on the paper surface, acid resistance, insolubility in water, strong anti-emulsification, bright color, strong gloss, light resistance, and no fading. Ink factory ink is the original color ink, generally meet the above requirements, the printing plant in the specific use, due to different prints, require different hue, and according to paper and climatic conditions and other differences, the ink to be deployed.

Ink deployment includes: hue in accordance with the sample; viscosity of the appropriate flow; dry oil plus volume is correct, the amount of ink in accordance with the number of prints.

To determine the hue of an ink, first analyze the color of the print, analyze and synthesize the content of the original in the three primary colors, and determine the primary and secondary colors and the blending ratio. At the same time to consider overprinting the color sequence, printed paper, color ink and the nature of accessories, etc., such as the overprint of the ink to have a high degree of transparency, the paper's color will affect the color of the ink is equal. In the deployment of hue, a small amount of color ink must be gradually added until the hue meets the requirements.

The amount of ink is mainly estimated based on the area of ​​the layout graphic, the thickness of the printing ink layer, the strength of the ink absorption performance of the paper, the tinting strength and specific gravity of the ink, the size of the printed matter, and the number of prints. The thickness of the ink layer relates to the viscosity and fluidity of the ink. The viscosity of the ink is too large and the fluidity is small. Ink fountains are not easy to ink, and the ink on the ink roller is not easy to be uniform, so the ink on the ink is uneven. At the same time also caused the phenomenon of paper pulling hair; ink viscosity is too small, fluidity is large, convenient printing, but the layout of the ink absorption enlarged and paste, layout from the dirty phenomenon. The thickness of the ink is generally determined according to the temperature. If the temperature is high, the ink should be thicker and some should be thinner. If the surface of the paper is rough, the texture is loose, and the absorption is large, the ink should be thin, the printing is fine, and the ink should be thick. Printing in the field, the ink can be diluted; the speed of the machine should be faster than the slower speed. In short, all aspects must be comprehensively balanced.

The amount of dry oil depends on the temperature and humidity, the performance of the paper, the thickness of the ink layer, the amount of auxiliary materials to be added, and the performance of the ink jet. Excessive amounts of dry oil will generate ink stacks, blankets, and ink emulsions. , resulting in paste version and other drawbacks; the amount is too small, the ink can not be dried within the required time, the print will produce a scratch, the general principle of the amount of dry oil: coated paper, offset paper more than newsprint, winter more than summer, Diversify the ink more than the dark ink, color sequence in the back of the ink prior to the color sequence more than the amount of ink.

After the ink is dispensed, the ink is loaded into the ink fountain of the offset printing machine, and the inking device is adjusted so that the ink can be uniformly transferred.

3. Dampening potion adjustment

In the lithographic printing process, the layout must ensure that the blank part does not absorb ink after wetting with water, and the wetting water can use pure water. However, during the printing process, the layout is subjected to high-speed friction, so that the hydrophilic layer of the blank part is consumed to supplement. The hydrophilic layer of the layout is consumed, and the electrolyte is added to the water, but the hydrophilicity of the formed inorganic salt layer is not strong. Therefore, the hydrophilic colloid is added in the water to form and supplement the hydrophilic salt on the inorganic salt layer. Body membrane layer. Therefore, fountain solution contains: gum arabic, phosphoric acid, chromic acid, nitric acid and its acid salts.

The commonly used dampening potions are divided into red potion and white potion. The red potion contains chromic acid and its salts, so it is orange-red and the white potion does not contain chromic acid. Both have advantages and disadvantages. The red potion is easy to use when used, but the paper is prone to yellowing, toxic, and harmful to the health of workers; the white potion is free from the effects of chromium poisoning, but the ink is easily emulsified and can cause ink roller deinking.

The fountain solution is generally formulated as a stock solution, which is diluted when used. The dampening solution is acidic, and the acidity has a great influence on the printing plate, and the acidity is too weak. Inorganic salt layer cannot be formed on the plate, poor resistance to oil and water, the graphic part of the expansion, resulting in paste version; too strong acid, the resulting inorganic salt layer is dissolved, plateau grain is destroyed, reduce the resistance to force, accelerate the ink Emulsification; block blot drying, so the pH of the dampening solution must be controlled during printing.

The amount of fountain solution also depends on the nature of the ink, the thickness of the print ink layer, the amount of ink dry oil, the type of printing plate, the layout of the layout, the operating environment temperature, the nature of the paper and other factors.

A good dampening syrup was dispensed into the water bucket of the offset printing press, and the water supply device was adjusted so that the amount of water supplied was in compliance with the printing requirements to ensure the normal printing.