When the ink touches the paper, under a certain pressure between the rollers, the bonding material molecules are forced into the fiber gaps on the surface of the paper, and then the gap capillary phenomenon in the paper layer slowly penetrates freely. Therefore, pressure is an important condition for the fixing and drying of the binding material on the paper surface.

When the printing pressure exceeds a certain value, due to the excessive printing pressure, the amount of ink on the printing plate and text begins to spread to the blank part, the ink transfer rate begins to decrease, the printed dots of the printed matter expand seriously, the graphics and text are obviously distorted, The value of printing pressure is not suitable for printing.

The size of the printing pressure has a great influence on the degree of ink transfer from the printing plate to the paper. When the printing pressure is insufficient, the transfer of the ink is insufficient and it is not suitable for printing; when the printing pressure is excessive, due to the Ink spreads to blank areas other than pictures and text, and the ink transfer rate is not only not improved, but also brings other disadvantages, and is also not suitable for printing. Only within the proper printing pressure range can the printed matter with thick ink layer, clear image, good tone and color reproducibility be printed.

The effect of cylinder lining on printing pressure

The blanket cylinder of the offset press is named after the blanket is wrapped on the metal cylinder. Between the blanket and the cylinder is also lined with paper, wool and other materials. The blanket and the pad are together Formed the blanket of the blanket cylinder of the offset press. Similarly, the printing plate and pad on the plate cylinder also form the liner of the plate cylinder, and the liner of the impression cylinder is the printed paper. Roller lining is the basic source of printing pressure, and has a great influence on printing pressure.

The cushion materials used for offset printing mainly include cushion paper, plastic film, worsted wool fabric, blanket, etc.

The rubber cloth and various cushioning materials are combined together, and are divided into soft wrapping lining, neutral wrapping lining and hard wrapping lining according to their elastic modulus.

The printing pressure of the hard lining is large, the embossing line is narrow, the printing pressure of the soft lining is small, and the embossing line is wide. It can be seen that the hard lining has better dot reproducibility than the soft lining, but the soft lining It has the advantages of good elasticity and the difficulty of "ink marks" in printing. Therefore, the soft lining is often used in offset presses that have low printing accuracy requirements and serious wear on the printing surface. The hard liner has a small amount of compression deformation, and the dots on the printed matter are smooth and clear, so it is particularly suitable for printing high dots and lines. However, the hard liner is prone to "ink marks" and must be used in offset printing with higher accuracy on board.

The printing pressure is also affected by the printing speed. Increasing the printing speed will shorten the contact time between the printing surfaces, make the dots illegible, and reduce the printing quality. In order to ensure the full contact of the printing surface and improve the printing quality, it is required to control the printing machine speed under a certain printing pressure.

The performance of the neutral lining is between the hard lining and the soft lining, and it can be subdivided into neutral hard lining and neutral soft lining. Neutral linings often have the advantages of both hard linings and soft linings. Therefore, they are more adaptable and have a wider range of applications. They are the most commonly used lining forms on offset presses. The use of a neutral liner makes the dot reproducibility better. For the accuracy of offset presses, the paper performance requirements are not too harsh.

To sum up: different offset presses choose different liners, whether the liner choice is reasonable, not only affects the printing quality, but also affects the service life of the offset press.

[Case 6-5]

A printing factory used J2205 machine to print a batch of cartons with a thickness of 300g /? Whiteboard paper. The captain of the machine only adjusted the center distance between the rubber and the pressing roller. As a result, the machine was very noisy during the official printing.

[case study]

Judging from the treatment method, the case seems to have no major problems, but further analysis still feels a bit inappropriate. As we all know, the shaft end of the blanket roller of the J2205 machine is equipped with double sets of eccentric bearings. If the rubber to pressure center distance is adjusted to the limit, it will cause the imbalance of the variable load of the drum, thereby generating an inertial force, causing the drum gear to bear the impact load and causing noise.

[Expert Tips]

When increasing the center distance, the thickness of the lining can be considered at the same time.

The basic principle of reasonable lining is: under the condition of a certain printing surface deformation, the minimum pressure should be used evenly on the basis of sufficient printing and good reproduction of the dots to improve the printing plate durability and reduce the load of the roller gear.

The specific steps are:

(1) Accurately measure and correct the center distance of the drum.

(2) Accurately calculate and measure the thickness of roller pads.

(3) Choose the ideal liner and increase the minimum liner thickness within the limit to make the contact pressure ideal.

(4) Under the printing pressure that will be in contact but not in contact, correctly check the unevenness of the printing plate and the blanket.

(5) Effectively fill the unevenness of the blanket.

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