Gravure printing process quality control (below)

(3) tension control of receiving traction

Between the roller and the receiving traction, usually the distance is increased for drying. Due to the dry heating, the extension car changes, causing the material to expand and contract. These are the reasons for overprinting. Therefore, as with the feeding traction part, a better and accurate tension control is required. In addition, materials with extremely low tension and low tension are also susceptible to electrostatic adhesion due to the wind blown by the dryer, which causes minor changes. These must also be considered.

(4) tension control of take-up section

The tension control at the winding section (between the take-up pulling roller and the receiving shaft) is also an important issue. This part is to send the printed materials as final products to the next process of compounding, slitting, and bag making.

At the same time, care should be taken to ensure that evenly coiled products are free from scratches and wrinkles. It is worth noting that in plastic films and aluminum foils, care must be taken to prevent thickness variations. This is not just a problem of how much tension is taken up, but it is important to adjust the tension while the diameter of the coil is increasing, whether it is adjusted with a constant tension or with a dimensional tension.

There are generally two types of drive in the take-up section: surface drive and center drive. Although the center-driven type generally uses more, the surface winding type also has good properties, and the condition is good when the roller peripheral speed is constant. Therefore, it is used in some industries. Compared with the receiving department, the receiving department should pay more attention to taper tension and inertia compensation error. The center drive is to wind the material directly on the paper tube. In the past, the friction control method was used, but now the height, automatic, and high-performance electrical control methods are used. The take-up coiling methods are as follows: 1 using a magnetic powder clutch; 2 using a torque motor ;3 Use a DC motor.

The magnetic powder clutch can obtain excellent performance at a low price; the torque motor cannot be used when the coil diameter changes greatly; the DC motor is often used for large capacity and automatic paper feed. Table 1 lists the tension values ​​of several web printing materials in different sections of the press.

Taper tension

Under normal circumstances, the use of constant tension to take up the material, with the increase of the coil relative to the material inside the torque becomes larger, resulting in slipping that is the coiling shrinkage. Further, due to the shrinkage of the material and the release of air toward the center, the material is squeezed or laterally extruded, resulting in a so-called bamboo shoot phenomenon. Wrinkles occur near the core, making the surface uneven. To solve these problems, the tension should gradually decrease when the roll diameter gradually becomes large, that is, taper tension is used. The initial tension at the winding determines the tension at the end of the coiling. The degree of reduction is called taper. A taper of 10% to 50% is generally used. For example, with a 30% taper; that is, starting with a tension of 10kg, the tension from 7kg ends, and the rate of change at this time is different depending on the properties of the material and the mechanism. Manufacturers should test to make various types of test curves so that the control can be used in production. Adjustment.

3. Inertia compensation

The so-called inertia compensation is to compensate for the change in tension caused by the inertia of the coil when the machine is started or when the speed is changed during the operation or when the speed reaches a predetermined speed. In practice, the non-linear relationship between the moment of inertia and the diameter is proportional to the density of the material and the width of the material. The printing manufacturer applies the formula to calculate the average value and accurately calculates the compensation curve to guide the production.

Third, the printing pressure control

The force of interaction between embossed bodies during the printing process is called printing pressure. Gravure printing is the same as offset printing and embossing. It is necessary to transfer the graphic on the printing plate surface to the printing material by means of printing pressure. Printing pressure is not only the fundamental guarantee for realizing the printing process, but also determines the quality of the printing to a large extent. To improve the quality of gravure printing, the printing pressure must be controlled, so what factors should be taken into account when determining the printing pressure:

The hardness of the blanket

Gravitational pressure is generated by the deformation of the blanket. As the printing pressure increases, the deformation of the blanket increases, but they are nonlinear.

Generally, when the pressure is less than 10 kg/cm, the relative deformation of the blanket changes in proportion to the pressure. Afterwards, as the pressure increases, the relative deformation increases slowly. When the deformation increases to 25%, the pressure increases and the deformation does not increase significantly. It can also be seen from FIG. 1 that in the case where the blanket deformation is the same, the pressure of the blanket I having a higher hardness is higher than the pressure of the blanket II having a lower hardness.
2. Substrate properties

With different thicknesses and substrates with different surface smoothness, the amount of pressure needed for printing is different. For papers with poor smoothness, it is necessary to overcome the rough surface conditions to obtain a solid print. For this reason, only increasing the printing pressure can make the substrate and the plate surface fully in contact. Table 2 lists the range of printing pressure required for various substrates.

3. Printing speed

Practice has proved that when the printing speed increases, the printing pressure also needs to increase accordingly. This is because the speed is high, and the contact time of the printing surface is reduced. In order not to reduce the degree of full contact of the printing surface, the ink is facilitated from the printing plate. When the surface is transferred to the surface of the paper, the printing pressure must be increased.

4. The number of prints

In the printing process, as the number of printed products increases, the printing pressure will decrease, as shown in Figure 2. This is because the blanket and the liner undergo plastic deformation during the printing process, which causes the thickness to become thinner and the pressure to decrease, which in turn reduces the transfer rate of the ink. Therefore, in order to obtain a stable quality print, a certain number of imprints are printed. After the product, check the change in printing pressure. Make appropriate adjustments to maintain pressure stability.

5. Print quality requirements

Different prints have different copying requirements. For example, a solid version can use slightly more pressure to achieve the requirement of imprinting, while for fine mesh graphics, it is very important to use as little stress as possible under the condition of clear prints. .