About Blankets in Printing Presses

Offset printing blankets are often woven from four layers of linen that are bonded together with a special rubber adhesive and then their surface is coated with a rubber compound. The thickness of this rubber coating is 0.3 to 0.5 mm and the overall thickness of the offset printing blanket is about 1.5 mm. The thickness of offset printing blanket should be uniform throughout. Blanket thickness adjustments should not be more than 0.01 milliliters, otherwise the quality of the work will decrease.

In order for colors to overlap in multi-color printing, due to stretching of the paper during printing, minor changes in the underlaying of the offset printing blanket are necessary, but if the underlaying of the blanket is more than about 0.1 mm, the result is image vibration and dots will become bigger.

Normal offset printing blanket can be moved, but it does not completely return to its original shape. The structure of the pneumatic blanket is in a form that makes it resistant to pressure and destruction. Despite the fact that pneumatic blanket have a better performance than normal ones, the extra pressure and its high underlining cause the dots to be bigger.

The offset printing blanket is wrapped around the cylinder and placed under a certain tension to prevent it from sliding off the cylinder as a result of the printing pressure. Since blanket tightening is done by hand, it is not possible to accurately measure its tension. The tension of offset printing blanket around the cylinder varies between 0.2 and 0.35 newtons per centimeter, and this amount depends on the type of blanket and printing machine.

It is very important to use a graded wrench to tighten and evenly stretch the offset printing blanket in order to accurately adjust the tension. Especially in multi-color printing machines where all blankets must be closed uniformly and with equal pressure.

At the time of installing a new blanket, its stretching around the cylinder and the effect of being stretched by the printing pressure may cause the blanket to stretch and loosen, and the blanket must be tightened several times during printing

The pressure caused by printing in the first few thousand rotations reduce the thickness of the new blanket. If you measure the thickness of the offset printing blanket with an accurate micrometer, and the amount of under-blanket is also added to increase the printing pressure by 0.1 mm, you may notice the lack of printing pressure after a few hundred sheets of printing. Controlling the thickness of the blanket with a special micrometer may show that the thickness of the blanket has decreased by 0.05 mm and there is a need for additional underlayer for correct printing.

At first, offset printing blanket cover was made of natural rubber, but now this cover is made of synthetic rubber, which absorbs less oil and therefore shows more resistance to swelling, adhesion and glazzing.

The glazzing of the blanket surface usually occurs due to the drying of ink, gum, absorption of oil and other drying substances during work, which often causes the blanket coating to flatten and reduce its elasticity and, as a result, reduce the ability to receive the ink by blanket is offset printing. Strong chemicals can also destroy the surface layer of the blanket and make it hard. The transmission power of the ink in a dry and vitrified blanket is very weak. Rubbing the blanket with a piece of cloth should be soft, smooth and fluffy. Diluted blanket washing can prevent many problems. If the blanket cleaners are too strong, they will inflate on it, and if the chemicals used are weak, they will not be able to remove the ink and clean the blanket completely.


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