Analysis of Blanket and Blanket in Offset Printing Process

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, which is capable of maximally absorbing ink during the printing process. Minimize moisture adsorption is best. 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, the cover layer is too thick and will make the oak. The skin layout department produces a large plastic deformation and breaks the uniformity of the blanket thickness. When the other products are changed, partial pressure insufficiency occurs.

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., and the non-polar solvent is linked. According to the principle of similar dissolving, the cleaning blanket should be mixed with organic solvent and water, and the organic solvent should be volatilized. The speed should be relatively fast, kerosene or other organic solvents with slow evaporation speed can easily make the blanket swell.

226. What is aging and what are the shortcomings of blanket aging 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; fight 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.

229. Why is there a slip on the drum surface of the embossing zone?

During imprinting, the radius of each point on the plate cylinder is equal because the main compression is blanket and liner, and the absolute value of its linear velocity is the same. However, the circumference of the blanket cylinder embossing area. The radius of each point is not equal, and its linear velocity is obviously not the same. In the circumferential direction of the embossing zone, the surface of the blanket cylinder and the surface of the printing cylinder can have at most two line speeds equal to each other. The improper cleaning agent used will cause the surface layer of the blanket to swell and gradually lose its elasticity and age. Therefore, we should choose a suitable blanket cleaning agent. The speed of each point must be different. There is a speed difference, and there is a slip in the speed difference. Sliding between the rollers is unavoidable. If the radius of the phase-rolling roller is not properly set, the slippage will be significantly increased. Therefore, in the offset printing production, it is necessary to minimize the slippage between the rollers.

230. What are the consequences of slippage between plate and blanket cylinders?

1 Because the rollers are rolled under a relatively large pressure, if there is slip in their embossed areas, there must also be friction. The degree of friction is proportional to the speed difference between the hand and the pressure of the printing. The friction on the printing plate will cause the alumina grains of the photosensitive layer and the blank portion of the plate on the plate to be worn, which will reduce the printing plate's resistance to printing.

2 The slippage between the rollers can cause circumferential deformation of the imprints or dots, making the imprints thicker in the circumferential direction, and the dots increase in an elliptical shape, so that the quality of the printed matter is affected by the larger shadow D direction. Too much friction on the blanket can cause the blanket to slip excessively and cannot be instantly reset, causing ghosting problems. Slip and friction can also cause "backhand" and "handslide" phenomena.

3 Sliding too much can cause liner under the blanket. The felt moved, even pleated, crushed the blanket.

231. What is the effect of slippage between the blanket cylinder and the impression cylinder?

Since the metal surface of the cylinder of the impression cylinder is much smoother than the surface of the blanket of the blanket cylinder, the friction coefficient between the front surface of the paper and the surface of the blanket at the time of imprinting is large, and the friction between the back surface of the paper and the surface of the cylinder surface of the impression cylinder is large. The coefficient is small, so the slippage between rollers during embossing mainly occurs between the back of the paper and the barrel of the impression cylinder.

When the radius of the blanket cylinder (including the rubber blanket lining) somewhere in the embossing zone is greater than the radius of the cylinder of the impression cylinder, the distance between the back of the page and the center of the blanket cylinder is greater than the radius of the cylinder of the impression cylinder. The simplified surface of the impression cylinder slides forward, which has no significant adverse effect on the imprinting and dot gain. This forward slip usually occurs when printing thicker paper.

When the radius of the blanket cylinder (including the rubber blanket lining) is greater than the radius of the cylinder of the impression cylinder, the distance between the back surface of the blanket and the center of the blanket cylinder is smaller than the radius of the cylinder of the impression cylinder, and the back of the sheet is relatively Slide back on the simplified surface of the impression cylinder. Because the paper in the imprint is pinched by the impression cylinder, the paper will stretch due to sliding. The paper that is sensitive to elastic deformation can be restored after the stamping is completed; and the paper that is plastically deformed will cause the paper to grow in the printing direction after being stretched. (Author: Yu Tian Peijuan savings)