Offset Rubber Blanket （4）
Correct measurement of the blanket
In offset printing production, the size of the printing pressure is usually expressed indirectly by the maximum compression of the lining on the cylinder. The correct measurement of the thickness of the lining is the key to calculating the printing pressure.
In offset printing, the blanket is stretched on the blanket cylinder, and the blanket stretches after being stretched in the warp direction, and the elongation is proportional to the stretch. However, in the circumferential direction of the roller, due to friction, the tension at different points of the blanket is different. The two ends close to the plate holder have the largest tension, and the middle position has the smallest tension, from the middle to the plate. The tensile force at both ends of the clamp increases sequentially. Since the rubber blanket is stretched and elongated, the adhesive layer will be thinned and the thickness will be reduced. Therefore, the thickness of the blanket measured in the free state between the blanket cylinders is not equal to the thickness after tensioning. To accurately measure the thickness of the lining should use a cylinder gauge to measure the thickness of the tensioned lining at different positions in the circumferential direction of the rubber cylinder, and calculate the printing pressure from this thickness. However, many small printing plants are not equipped with a tube diameter gauge, and usually use a micrometer to measure the thickness of the blanket and liner. Since the micrometer can only be measured in a free state before the liner is installed, the operator should take care of the tensioning of the blanket. The difference between the thickness and the thickness in the free state has a correct estimate, so that there will be no larger deviations in the calculation. For easy-to-use soft linings, this difference may have little effect. For hard linings, due to the large elastic modulus of rigid linings, a small amount of error in the calculation of compression will cause a larger brush pressure. Change, improper printing pressure will seriously affect the correct transfer of ink.
Method for identifying the position of the blanket in the warp and weft direction
The first method is to tear by hand. Because the blanket is composed of a surface adhesive layer and a base cloth, and the base cloth is composed of a strong long-staple cotton cloth, and there are warp and weft directions, so it is stretched. In the state, the one that stretches more is the weft direction, and the one that is not easy to stretch is the warp direction.
Method two, there will be a red or green line mark on the back of the blanket to indicate the warp direction of the blanket.
Maintenance of the blanket
Because the blanket has hot spots of light aging and heat aging, the blanket to be used after purchase should be wrapped in black paper and stored in a cool place.
When cleaning the blanket, the faster volatile organic solvent should be used as the detergent, and the slow volatile kerosene or its solvent will easily swell the blanket when washing deep rubber. Wash the blanket and dry it without leaving any residue. On the one hand, the residue is easy to oxidize and dry up, which will make the blanket aging prematurely. On the other hand, when the residue is printed on other products, the ink color is easy to appear uneven at the beginning.
After a certain product is printed, if the downtime is longer, the tensioning device of the blanket can be loosened, so that the blanket can relax and get an opportunity for internal stress recovery, which has the effect of actively preventing stress relaxation.
In the printing process, the ink roller must be cleaned when changing colors. After a period of printing, paper lint, paper dust, ink skin and other dirt will accumulate on the blanket, which reduces the quality of the printed matter, so the blanket should be timely Cleaning, especially when printing low-strength paper, the accumulation of paper lint and paper powder is more serious, so it is necessary to clean it frequently.
If the ink roller set is not cleaned during color change, it will affect the purity of the new ink. Pay special attention when changing from dark ink to light ink. For example, when changing from black ink to yellow ink, if the black ink is not cleaned, the yellow ink will turn black, which will affect the quality of the printed matter. Therefore, the ink roller set must be cleaned when changing colors.
Blanket roller
The blanket cylinder is on the outer surface of the cylinder, and is lined with a thicker, elastic blanket to transfer the print obtained from the printing plate.
The role of the blanket roller:
(1) Transfer ink;
(2) Make up for the comprehensive error of the machine;
(3) Buffer and absorb vibration.
During normal operation, the surface of the roller is subjected to a lot of forces, and the magnitude of the force is constantly changing, that is to say, the blanket roller is always in a complex vibration state, that is, the pressure of the lining at every contact on the surface of the blanket roller It is not consistent, so that the pressure changes caused by machine vibration and machine processing and installation errors are turned into the elastic energy of the rubber cloth. When in contact, elastic energy is produced; when in contact, the elastic energy is released; when in contact again, new elastic energy is produced and released again when in contact. This process is repeated continuously, but the damage caused by the impact and vibration of the machine is controlled to a minimum through the blanket roller. Therefore, in addition to transferring ink, the blanket cylinder also has the function of compensating for the overall error of the machine and buffering and absorbing vibration.
Disassembling and installing the blanket is a very delicate operation. The blanket and liner must be trimmed. Clean the blanket roller with pure alcohol before loading the blanket. If necessary, a non-corrosive oil can be applied to the surface.
If there is a small-size print with a large number of prints, the liner paper should be cut to a size slightly smaller than the printed sheet, so as to avoid piles of ink and paper lint on the blanket. If there is paper lint on the blanket cylinder or ink skin on the plate cylinder. Never use your hands when the machine is running at high speed to avoid personal accidents.
The neutral part of the blanket cylinder has clamping and tensioning mechanisms. One end of the blanket is fixed, and the other end is installed on a shaft that can be tensioned, and then it is tensioned with a ratchet mechanism or a worm mechanism
(1) Carefully remove the blanket and liner.
(2) Use a ballpoint pen or pencil to outline the crushed area on the back of the blanket.
(3) Use a ball-point pen or pencil to push up the damaged back surface, and apply lubricating oil on the front surface.
(4) Use a thin sulfuric acid paper for cushioning. If the crush is severe, use two layers of sulfuric acid paper.
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