How much is the packaging offset? (220-228)

220. Why can't the blanket stretch too loosely?

A blanket that is too loose may be displaced when it is printed and cannot be reset in time, causing ghosting problems.

221. Why do the newly installed blankets need to be tightened several times?

The blanket stretched on the blanket cylinder was simultaneously subjected to the tension of the platen and the retraction force of the blanket itself. As time goes on, rubber molecules will undergo conformational changes, making their own retraction forces smaller. As a result, the blanket becomes slack and prone to ghosting failures. This phenomenon is called stress relaxation. Stress relaxation is more serious for newly-used blankets, so the new blankets begin printing after the first tightening, and need to be tightened again after a short period of time. After two or three repetitions, the relaxation of stress is slowed down and the tightness of the blanket can be kept stable for a longer period of time.

222. What is the torsional deformation of the blanket?

In the imprinting process, if the different positions of the blanket surface in the axial direction are subjected to forces in different directions or different sizes, the shape of the blanket may be torsionally deformed, resulting in deformation of the graphic shape, overprinting, and blankets. The service life is shortened and other adverse effects.

223. What causes the torsion deformation of the blanket?

1 drum axis is not parallel. When adjusting the center distance between the two ends of the drum, if the pressure regulator appears to be self-propelled, the center distance between the two ends of the drum will be unequal, which will cause the axis of the blanket cylinder to be non-parallel to the axis of the impression cylinder or plate cylinder. The end of the cloth is crushed by a large amount, and the speed of the forward hurry is faster. The squeezing at the other end is smaller and the speed of the forward hurry is slower, so that the blanket is torsionally deformed.
2 The operation method is not standardized when the blanket is cut or the blanket clamp is installed. When cutting the blanket, if the two sides of the clamping plate are not parallel, the tension in different axial positions will not be uniform when tightened, so that the blanket is tight on one side and the other side is loose. Tensile stress distribution is not uniform, resulting in torsional deformation. When the blanket is topped with a blanket, if the depth of the blanket is inserted into the plate or the individual screws are not tightened when clamping the platen, when the blanket is tightened, a small amount of the blanket where the screw has not been tightened will come off, resulting in the eraser. The tension of the cloth at different positions in the axial direction is different, resulting in local torsional deformation.

224. Why do you need to clean the blanket often during printing?

The surface of the blanket is composed of non-polar synthetic rubber. It is best to absorb the ink in the printing process with minimal adsorption of moisture. However, in the printing process, due to physical adsorption, a layer of cover layer is formed on the surface layer of the blanket. This layer of cover is mainly pigment particles in paper coating particles, plant fibers, and inks. Since most of them are polar Substances make the oleophobicity of the blanket down and the hydrophilicity rise, making the small dots on the printing plate not transfer to the substrate through the blanket, resulting in the loss of small dots and the appearance of powdery fine spots in the solid part, which seriously affects the printing quality. . On the other hand, increased hydrophilicity causes the blanket to absorb more wetting fluid from the non-graphical portion of the plate to the paper, causing the paper to stretch, resulting in misprints. In addition, if the cover layer is too thick, it will also cause a large plastic deformation in the rubber layout and damage the uniformity of the thickness of the blanket. When the other products are changed, partial pressure will be insufficient.

225. What kind of solvent should I use to clean the blanket?

Due to the covering layer on the blanket, there are polar paper powder, paper hair, etc., as well as non-polar binder solvent. According to the similar principle of dissolving, the cleaning blanket should be mixed with organic solvents and water, and organic solvents. The volatilization rate should be faster, kerosene or other organic solvents with slow evaporation speeds can easily make the blanket swell.

226. What is aging? What are the shortcomings of the aged blankets in printing?

The gradual deterioration of the properties of a polymer compound is called aging. In the printing process, the following drawbacks occur when the blanket is aged.
1 The surface is smooth, the wettability of the ink is reduced, the transfer of the dot is not complete, and the void in the field is not true.
2 The rubber is hardened and loses its balance elasticity.

227. What is the internal friction of the blanket?

During the imprinting process, the blanket is subjected to periodic alternating stress and deformed. However, because the development of blanket deformation always lags behind the change in stress, the elastic energy storage inside the rubber part is too late to be released, and it is converted into heat energy, which is called internal friction.

228. How to reduce blanket aging in production?

1 blanket in the printing process due to internal heat and heat, so that the temperature of the blanket to promote heat aging. Internal friction increases with the frequency of alternating stress. For high-speed printing presses, blankets with good elasticity and low internal friction should be used. In the hot season, controlling the temperature of the printing shop and the temperature of the wetting fluid can reduce the temperature of the blanket and reduce the heat aging of the blanket.
2 The blanket will produce photo-aging under light exposure, so the newly purchased blanket is wrapped in black paper before use.
3 When washing the blanket, be sure to wash the ink in order to prevent the residual traces from oxidizing and solidifying, so that the surface of the rubber blanket is aged.
4 improper use of cleaning agents will make the rubber surface of the blanket swelling and gradually lose elasticity and aging, so you should choose a suitable blanket cleaning agent.
5 Long-term tension of the blanket on the printing press will cause the internal stress to gradually decrease and lose its elasticity due to the change of the conformation of the rubber molecules. When the downtime is long, it is better to relax the blanket and make the internal stress recover. The blankets on high-speed printing presses age faster. If two blankets are used, they can be used in turn at regular intervals to extend the life of the blanket.

---"Printing Technology - Packaging and Printing"