Gravure printing ink transfer (3)

Fourth, gravure printing ink transfer

Gravure ink transfer and substrate surface properties, printing pressure and other factors.

1. Capillary action and gravure ink transfer. The mechanism of transfer of gravure printing ink is still not clear. The main reason is that the gravure printing ink volatilizes very quickly. The determination of the amount of ink transfer is very difficult, and it is not easy to find the resolution between the amount of ink x on the gravure and the amount of ink transfer y. relationship. Therefore, the research on the mechanism of transfer of gravure printing ink still remains in the qualitative analysis. There is a theory that the transfer of gravure inks first comes from capillary action. When the paper (or other substrate) and the gravure cylinder are separated, the ink in the cell is removed by capillary suction and adhered thereto. The smaller the gap between the printing plate and the substrate, the greater the capillary suction and the greater the amount of ink transfer.

Figures 6-9 are graphs of gravure ink transfer rates obtained with photocopy printing under the same printing pressure and ink viscosity conditions. The curves show that the substrates with high smoothness and soft texture can increase the capillary force between the substrate and the printing plate under the effect of larger printing pressure, thereby improving the transfer rate of the gravure printing ink.

2. Gravure printing pressure. Gravure printing is direct printing. The printing plate cylinder and the embossing cylinder coated with a blanket are in direct contact. After the two cylinders are pressed together, a greater pressure must be generated to transfer the ink in the plate base to the substrate. Because the pressure of gravure printing is greater than offset printing and letterpress printing, the requirements for technical quality indicators of blankets are even more stringent. Elastic blankets must have strong tensile strength, low elongation and proper hardness, and precise flatness. The surface of the blanket is generally made of nitrile rubber. The hardness varies slightly depending on the printing pressure required by different substrates. See Table 6-2. Underneath the blanket, commonly used liner materials are hard kraft paper, insulated hard paperboard, or multi-layered paper liners.

Many new gravure presses use sleeve-type gravure linings, and the blankets and liners are integrated.

In order to adjust the pressure quickly and accurately, the new gravure press is pressurized with compressed air. The operator can observe the change of pressure through the barometer. If you set one cylinder at each end of the impression cylinder and the pressure is equal, the pressure value displayed by the barometer has the following relationship with the pressure experienced by the plate cylinder:

PL= 2· π· R2· P/L

In the formula, PL is the linear pressure (N/m), R is the radius of the cylinder piston, P is the pressure value of the cylinder (N/m2), and L is the length of the plate cylinder (m).

Figure 6-9 Gravity ink transfer rate curve

Gravure pressure adjustment, generally after the plate cylinder is installed, first adjust the level between the impression roller and the plate cylinder so that the pressure at both ends of the roller is equal to ensure that the substrate is subjected to a balanced printing pressure, and then according to the substrate Apply different pressures.

Table 6-2 Gravure Printing Pressure of Different Substrates

Substrate Imprinting drum radius (mm) Rubber hardness (HS) Stamping width (mm) Printing pressure (kN/m)
Plastic film 120 to 150 60 to 70 10 0.98 to 4.9
Coated paper 150～200 70～80 13 7.8～14.7
Offset paper 150～200 85～90 15 24.5～39.2
Kraft paper 150～200 85～90 15 19.6～29.4