Determine the theory and method of roller lining (2)

2. The characteristics of rubber.

Rubber is the main material for making blankets. The elasticity of the blanket, oil resistance, solvent resistance, and ink transferability are all related to the characteristics of the rubber.

The rubber used for making lithographic blankets is natural rubber and synthetic rubber. They are all high-molecular polymers that are highly elastically deformable and have the following characteristics compared to some solid materials:

First, the elastic modulus is particularly small, about 1/106 of steel and 1/104 of silk (see Table 5-12).

Second, the Poisson's ratio of rubber is almost higher than that of all other solid materials, close to 0.5 (see Table 5-12).

Third, the elongation is large, up to 100 to 1000% (generally less than 1% elastomer). Exothermic during stretching (ordinal when the elastomer stretches), the thermal effect increases with elongation (see Table 5-13). The elongated sample absorbs heat on its own retraction.

The above properties of rubber show that the high elasticity of rubber is extremely rare in all materials. This high elasticity, which is almost unique to rubber, is formed by the structural characteristics of rubber.

The molecular structure of natural rubber is shown in Figure 5-28. Under normal conditions, supple molecular chains are always spontaneously curled. Its end-to-end distance is 100 to 1000 times shorter than that of a fully-stretched molecular chain. Under the action of tensile force, the elongation of the rubber causes the spatial arrangement of molecular chains to have directionality, making the molecular chain oblige to follow. The direction of the external force field stretches. On the other hand, thermal motion attempts to disorder the molecular chain so as to return to the curled state, which results in the retraction force. This retraction force is very small compared to the force required to change the spacing between atoms in the metal lattice. Therefore, the rubber can generate a large deformation under a small external force, so that its elastic modulus is particularly small and it has a high elastic deformation capability.

The high elasticity of rubber shows that the stress and strain energy reach equilibrium instantaneously, that is, the speed of deformation can keep up with the speed of external force. If a tensile force is applied to the rubber material, it is immediately stretched, the tension disappears, and it instantly rebounds. If the target rubber material is subjected to a positive pressure, it is immediately compressed, the pressure disappears, and it immediately rebounds. This external force of the highly elastic rubber material disappears, and the phenomenon that the deformation is instantaneously restored is called "balanced high elasticity."

In the lithography process, although the blanket needs to withstand numerous rollings, the rubber has a balance of high elasticity, and the rollers can be in a state of complete contact and rolling with a small pressure, so that the dot definition is high and the level is high. Good reproduction of tone reproduction.

In aged rubber, the flexibility of the molecular chain deteriorates, and the movement of the molecular chain is difficult, and the speed at which the external force adjusts its conformation slows down. It is manifested as deformation during tension or rebound, and does not reach equilibrium with the momentary force. This feature is called "unbalanced high elasticity" or "delayed high elasticity."

Postponing high bullets can be seen as a combination of stickiness and elasticity. The viscoelastic properties of rubber are common in the offset printing process: creep, stress relaxation, internal friction, etc.

The blanket coated on the roller is simultaneously subjected to tension and printing pressure. As the number of rolling cycles increases, the blanket becomes stiff and loses the flexibility and high elasticity of the new blanket, even at the eraser. The surface of the cloth appears bright or cracked, which is the result of rubber creep.

The reason for the creep of rubber is that under the action of external forces for a long time, the rubber molecules have undergone conformational changes, which eventually led to irreversible plastic deformation. The stiffer the rubber's molecular chain, the worse the rubber's elasticity and the more serious the creep phenomenon. During the offset printing process, the greater the pressure on the blanket, the shorter the process of becoming stiff, ie, the earlier the creep time. In order to delay the occurrence of creep, a smaller pressure transfer ink should be used. In addition, the web of the printed paper is made to coincide with the web of the blanket as much as possible. Otherwise, the print pressure may be uneven due to creep of the blanket. For example, when printing on offset presses with 4 sheets of paper, the blanket creeps only on the web corresponding to the paper. If the change is changed to folio printing, the print pressure will be different due to the creep of the blanket. Uneven, so that the print quality.

The rubber blanket, which is fastened to the drum for a long period of time, gradually attenuates the internal stress over time, causing the degree of tightness to decrease, which is the result of stress relaxation of the rubber. Therefore, when the new blanket is installed, it must be tightened several times and the thickness of the liner should be appropriately increased.

The blanket cylinder is installed between the plate cylinder and the impression cylinder. Each tiny unit on the blanket must be contacted with the two cylinders once and each time it is subjected to pressure. Therefore, the force is cyclical. The rubber unit body enters into the embossing area, that is, it is compressed and deformed, and the rubber unit body leaves the embossing area, and its deformation recovers due to the disappearance of pressure. Because rubber has the property of delaying high elasticity, the development of high elastic deformation always lags behind stress. When the last deformation of the rubber unit body has not yet disappeared due to springback, it exerts the next stress, so that the continuous circulation continues, and the elastic energy stored in the rubber cannot be released into the heat. The more heat released, the greater the internal friction of rubber.

The internal friction of rubber increases with the frequency of alternating stress. The rubber blanket of high-speed offset press is subjected to high frequency of alternating stress. Compared with low-speed offset press, the internal friction of rubber is high. Therefore, it is easy to lose high elasticity and aging. This is the reason why high-speed offset press has to be used with good elasticity and low internal friction. The blankets and reasons for frequent replacement.

The glass transition temperature of rubber (the temperature at which the rubber transitions from a highly elastic state to a glassy state) is much lower than room temperature. Therefore, at room temperature, the disorder of the thermal motion of the rubber chains, the interaction between molecules, and the like are similar. In the case of molecular motion, Poisson is larger than other materials. This shows that when the rubber is deformed, its volume is nearly constant. If pressure is applied to the tiny unitary bodies of the blanket wrapped on the blanket cylinder, the deformation along the circumferential and radial directions of the cylinder is approximately equal. This is because rubber has incompressible properties.