Magnetic Printing Technology and Mechanical Exploration--Lithography (1)

Since the binder material of the magnetic ink is the same as the general ink, and the binder material mainly affects the printability of the ink, in principle, the letterpress, lithography, gravure, screen plate, and flexo plate can be printed with magnetic ink. . Therefore, magnetic printing may be relief printing, flat printing, concave printing, screen printing, and flexographic printing. Letterpress printing has been largely replaced by lithography, and there are fewer and fewer applications, so it is not discussed here.

First, lithography

(A) The basic characteristics of lithography

Compared with the techniques of letterpress printing and gravure printing, the production printing process has its unique features and is summarized as follows:

1. A lithographic printing plate, the blank part and the graphic part are almost on the same plane of the printing plate

The lithographic printing plate forms, the blank part and the graphic part are almost on the same plane of the plate without the aid of the instrument identification. Utilizing the law of oil and water incompatibility (that is, the ability of oil and water to selectively adsorb to different solid surfaces), technical processing enables the formation of hydrophilic and hydrophobic graphic portions and hydrophilicity on plates of the same plane. The oleophobic blank part. In the printing process, the wetting mechanism first applies a uniform amount of water to the blank portion of the printing plate so that the blank portion does not absorb the ink, and then the ink is used to make the graphic portion absorb an appropriate amount of ink, and the graphic is completed by the embossing mechanism. The transfer of copy, so as to achieve the purpose of printing.

2. The lithographic printing method is an indirect printing, the pattern on the printing plate is "positive"

The ink on the lithographic plate is not directly transmitted to the sheet, but passes through the middle drum of the elastomer. That is, during the printing process, the ink on the printing plate graphic is first transferred to the blanket cylinder, and the ink on the blanket cylinder is transferred to the printing substrate. Therefore, the graphic is "informal" and is indirect printing.

The blanket covered by the blanket cylinder has two characteristics: First, it has better hydrophobic, lipophilic characteristics, can fully transfer ink, and restrict the transmission of moisture; Second, the blanket is flexible, can be larger In the case of printing pressure and compression deformation, the prints are printed with a strong and low expansion rate.

3. Network fineness

The color of flat-printed products is ever-changing, and the formation of color is formed by nests and color inks, and dot color is the main one. It is guided by the theory of light color, using overlapping dots and juxtapositions, and using the theory of three primary colors plus black (four colors), through the photo separation or electronic color separation, the color of the picture is disputed into yellow with different reticulation angles. , magenta, cyan, black and other four color version, and then use four-color printing plate, overprinting to reproduce numerous layers and colors, to obtain a very rich color reproduction of the art.

4. Multi-color printing, wide range of printing

Lithography prints monochrome text, but mostly multi-color overlays. The printing format can be either large or small (4 sheets or less to double sheets) and can be printed with multi-colored products on the screen and in the field at the same time. The picture of the finished product is flat and colorful, which is not comparable to letterpress printing.

(II) Basic Principles of Lithography

The graphic and blank parts of the typographic plate are almost on the same plane, but the two parts have completely opposite properties for the intimate relationship between water and oil. The graphic part is lipophilic and hydrophobic, while the blank part is hydrophilic and oleophobic. Therefore, in the printing process, the graphic part absorbs the ink and repels the fountain solution, while the blank part of the printing plate absorbs the fountain solution and repels the ink. During the printing process, the blank portion is first wetted by the fountain solution, and then the graphic portion is coated with the ink, and then the graphic portion of the ink is transferred onto the paper. Therefore, lithography is based on the principle of oil, water immiscibility, the transfer of graphics on the plate above the substrate.

1. Water and oil insoluble principle

The main component of the fountain solution is water, which is composed of hydrogen atoms and oxygen atoms. The electronegativity of hydrogen is 2.1, and the electronegativity of oxygen is 3.5, so the polarity of the O-H bond in the water molecule is very strong.

Experiments show that the structural formula of water molecules is shown in Figure 2-23, and the angle between the two O-H bonds in the molecule is 104° 45. The dipole moment of the water molecule is not equal to zero. The "center of gravity" of the positive charge in the molecule and the "center of gravity" of the negative charge cannot be overlapped. As a result, one end of the hydrogen atom is positively charged and one end of the oxygen atom is negatively charged. Thus, it shows a strong Polarity, so water molecules are polar molecules that are very polar. Because of the electrostatic attraction between polar molecules, polar molecules have a certain affinity for other polar molecules, and substances composed of polar molecules have affinity for water. All substances that have affinity for water are called hydrophilic substances. Metal inorganic salts, metal oxides, and the like are all polar structures. They can have strong affinity with water and are hydrophilic substances. There are also substances where the molecular structure is symmetrical and the molecules lose their polarity. In the carbon dioxide molecule O=C=O, although the common electron pair biases oxygen, each C=O bond (the oxygen atom is partially negatively charged and the carbon atom is partially positively charged), but since the two C=O bonds are in On-line, ie, the geometry is symmetrical, the polarities of the two keys cancel each other out (see Figure 2-240.) In the entire CO2 molecule, the centers of gravity of the positive and negative charges are coincident, so it is a non-polar molecule. Organic compounds of saturated alkanes are all symmetric molecular structures.

Non-polar molecules have affinity for non-polar molecules, but have no affinity for polar molecules. This is based on the principle of mutual solubility between structurally similar substances. All substances made up of non-polar molecules have no affinity for water molecules. This substance is called a hydrophobic substance. In organic chemistry, "oil" is a general term for non-polar organic liquids. Therefore, hydrophobic substances must have lipophilic properties.

When some polar functional groups, such as hydroxyl (-OH), amino (-NH2), carboxyl (-COOH), carbonyl (-COH), nitro (NO2), etc., are introduced into the hydrophobic material, the hydrophobic material will have certain The polarity, and thus resulting in hydrophilicity. The size of this hydrophilicity depends on the length of the carbon chain in the molecule, the longer the carbon chain, the stronger the non-polarity of the molecule, the lower the hydrophilicity, and the higher the hydrophobicity.

2. Hydrophilicity and lipophilicity of printing plates

Wetting is the process of adsorbing a liquid on a solid surface. The degree to which the solid is wetted is measured by the size of the contact angle.

Droplets on the solid surface (Figure 2-25) form three phases: S-Solid, L-Liquid, G-Gas. The intersection of the three phases is E. The flat angle formed by the tangent of the droplet from the E point to the gas phase and the solid surface is called the contact angle.
The larger the angle is, the more the droplets bulge, the lower the degree of wetting of the solid surface; the smaller the angle, the flatter the droplets, and the lower the degree of wetting of the solid surface, and the smaller the angle, the flatter the droplets. The higher the degree of wetting of the solid surface. Therefore, the size of the contact angle can be used to measure the degree to which the solid surface is wetted. In general, θ = 90 is defined as the limit of whether the solid surface is wetted. θ>90 for no wetting; θ<90 for wetting. The smaller the wettability is, the better the liquid wets on the surface of the solid when θ=0.

A variety of common metals are used to make a smooth plate, the surface of which is cleaned, and water is dripping thereon to measure the contact angle of the water drop on the surface of these metals. According to the order of the measured contact angle, the following shall be discharged:

θCu>θFe>θZn>θAl>θNi>θCr

If oil is used for the above test, the contact angle of the oil on the surface of various metal plates is determined and the order of discharge according to its size is as follows:

θCu<θFe<θZn<θAl<θNi<θCr

The increasing tendency of the hydrophilicity and lipophilicity of the various metals thus obtained can be expressed as follows:

It can be seen from the above that copper has the lowest hydrophilicity and lipophilicity is the best; chromium has the best hydrophilicity and lipophilicity is the worst. There is a high degree of hydrophilicity and a high degree of lipophilicity, and such metals cannot be found. To make the surface of the same plate material, there are both graphic parts with good lipophilic properties and blank parts with good hydrophilic properties, which can only change the properties of the surface of the metal plate material. Metallic aluminum and zinc have a moderate degree of hydrophilicity and lipophilicity, and their wetting properties are easily changed. Therefore, aluminum or zinc plate is selected as the plate material in lithography. In order to improve the water wettability of the aluminum printing plate, a stable metal oxide hydrophilic film should be formed on the surface thereof. Due to the effect of affinity, the adhesion between the water and the printing plate is greater than the cohesion between the water molecules, so the water adheres to the surface of the metal inorganic salt or metal oxide, improving the affinity of the printing plate to the fountain solution. ability. The application of a layer of ink and fatty acid salts to the surface of the printing plate produces a lipophilic film, to which the ink can adhere.

The molecular structure of the fatty acid is R-COOH, and the molecular structure of the fatty acid salt is M(RCOO)n, where M is a metal ion and n is the valence of the metal ion. They have R at one end, R is a carbon chain of 17 or more carbon atoms, and they show strong non-polarity in the molecule; the other end of the molecule is -COOH, with weak polarity. When fatty acids or fatty acid salts are coated on aluminum plates or zinc plates, fatty acids or fatty acid salts attach to the surface of the metal at the polar end of the molecule, and the non-polar end of the molecule extends to the space. The outer layer of the seesaw or aluminum plate has adhesion. When the adhesion is greater than the cohesion of the ink itself, the ink is separated and a part is attached to the surface of the printing plate. The hardened photosensitive resin also has good lipophilic hydrophobicity and the ink adheres well to it.

In short, the formation of a stable film of fatty acid, fatty acid salt, or a photosensitive resin on the surface of an aluminum plate or a zinc plate can improve the wettability of the ink to the plate, that is, the pro-ink of the plate. Competence.