Analysis of the Main Factors Affecting Printing Pressure

Offset presses are printed within the working pressure range to obtain quality prints. Excessive or too little printing pressure can affect the quality of printed products. Excessive printing pressure can shorten the service life of the printing press. Therefore, determining the pressure of printing has become a very important part of the printing process.

In the past ten years, China's offset printing technology has grown by leaps and bounds, and high-tech equipment and technologies have been emerging. However, at present, there are still quite a number of printing manufacturers still working to adjust the experience and determine the printing pressure, that is, to observe the restoration of the ink-colored dots of printed products. For clarity, etc., the printing pressure is adjusted until the printing pressure meets the requirements, and the printing pressure at this time is determined as the printing working pressure. In fact, the working pressure of the offset press, in addition to the performance of the offset press itself, is related to various factors such as paper and ink used in the printing process. In adjusting the working pressure of the offset press, various factors affecting the printing pressure should be fully understood.

First, the impact of roller lining on the printing pressure

The drum liner is divided into a printing drum liner and a rubber drum liner, and the impression cylinder is not covered. The blanket and the pad together form the blanket of the blanket cylinder of the offset printing press. The liner between the plate cylinder and the plate of the offset printing press is also lined with a liner. This liner is also sometimes called the liner of the plate cylinder.

The following mainly discusses the impact of the blanket lining on the printing pressure.

1. Packing function. The surface of the printing plates, blankets, and printed paper used in offset printing has microscopic irregularities. In order to increase the ink transfer rate, the surface of the printing plate should be brought into full contact with the surface of the blanket, the surface of the blanket, and the surface of the printing paper. In order to allow sufficient contact between the two pairs of surfaces, it is convenient to widen the deformation of the contact surface. Adding an elastic gasket can expand this deformation and overcome the poor surface contact caused by the rough printing surface. Therefore, there are two functions of the pad: 1 using a pad to use the elastic deformation of the pad to generate the contact pressure of the offset roller (if the pad is not covered by the metal cylinder, the pressure cannot be applied and the ink cannot be transferred, Printing cannot be done). 2 make up the printing plate, blanket, printed paper due to manufacturing accuracy

Inaccuracies such as unclear images and imprints due to errors or surface roughening.

2. Effect of Blanket, Liner Material Performance on Printing Pressure

The lining of the blanket cylinder includes a blanket and an elastic pad, both of which are very deformed and have a great influence on the printing pressure. There are two types of rubber blankets: ordinary rubber blankets and air-cushion blankets. The types of cushions are divided into three types: soft lining, neutral lining, and rigid lining. Their performance, differences and impact on printing pressure are as follows:

(1). Ordinary blanket is composed of a rubber layer with good oil resistance, a cotton cloth frame, and an elastic adhesive layer. The blanket is combined with various cushion materials and is classified into soft and neutral according to the elastic modulus. Hard lining. From the analysis of the lining performance, the soft elastic modulus of the soft lining is small, the compression deformation and the width of the embossing area are relatively large, the ink on the printing plate and the blanket dot is slightly squeezed, and the surrounding dots become less clear. The quality of the printing outlets was not ideal and the dot reproducibility was poor. However, the soft lining has the advantages of good elasticity, and the “ink bar” does not easily appear in the printing. Therefore, the soft lining is often used in offset printing where the printing accuracy is not high, but the printing surface wear is severe. For example, for silky rough and embossed paper, with a soft lining, the printing effect can also be improved to some extent; the elasticity of the hard lining is poor, the compressive deformation is small, and the nip line is also narrow. As a result, the printed dots are clean and the images are clear, which is suitable for printed materials with higher dot lines. However, hard liners used for printing are prone to “ink bars”, so they must be used on high-precision offset presses; the performance of neutral liners is between rigid liners and flexible liners, which often have both hard liners and The advantages of soft lining. Therefore, the utility model has the advantages of stronger adaptability, wider use, neutral liner lining and better dot reproducibility. The requirements for the accuracy of the offset press and the performance of the paper are not too harsh.

Experience shows that due to the elastic attenuation of the blanket, the printing pressure value stabilized by mass printing is lower than the printing pressure value at the beginning of printing, the soft wrapping lining is approximately 20-30% lower, and the neutral wrapping lining is approximately 10- 15%, the hard liner lining is about 5-10% lower. It can be seen that the pressure drop caused by the elastic damping of the rigid liner is much smaller than that of the soft and neutral liner. Therefore, the rigid liner is lined with the characteristics of maintaining the printing pressure.

(2). Effect of Air-cushion Blanket on Printing Pressure

The new air-cushioned blanket has been widely used in China. The biggest feature of air-cushioned blankets is compressibility. In addition, the liner latitude is greater than that of ordinary blankets. During printing, no convex bulges occur at the edges of the embossed area, and the general blanket pad has a low latitude and the bulge phenomenon becomes more serious as the pad thickens or the pressure increases. Therefore, the use of air-cushion blankets to reduce the network distortion, printing graphic blur, double printing and other ills.

Second, the impact of printing surface roughness on the printing pressure

Offset printing ink transfer is carried out on the surface of printing plates, blankets, paper (or other substrates). The thickness of printing plates, blankets, liners, and printing paper cannot be absolutely uniform due to manufacturing errors, that is, there is a certain degree of deviation, and all printing surfaces cannot be absolutely smooth, that is, have a certain degree of unevenness. If the surface of the printing plate has a grain of sand, the surface of the paper may have a certain roughness due to the interweaving of the fibers to form pores.

In order for the printing plate and blanket, blanket and paper to be in full contact, the printing pressure must also increase accordingly. Therefore, the use of low smoothness paper printing, it must be appropriate to increase the pressure of the printing, otherwise there will be printed network points are imaginary, lack of writing strokes and other defects, such as rough surface of the non-coated paper pressure is almost super smooth surface 3 times light coated paper.

Third, the impact of printing speed on the printing pressure

Printing pressure is also affected by the speed of printing, which is often overlooked by operators. The increase of printing speed will shorten the contact time between the printing surfaces. In order to ensure the full contact of the printing surface, this requires a greater printing pressure. In addition, the high-speed offset printing press uses a smaller diameter roller, in addition to a short contact time. Outside, and the nip is narrow. Hard liners can provide greater press pressure and narrower nip lines, so rigid liners are typically used on high-speed offset presses.

Fourth, the impact of other relevant factors on the printing pressure

In determining the working pressure of offset printing, in addition to considering the above factors, we must also consider factors such as the type of printing plate, the degree of grain size of the printing plate, and the characteristics of the graphic and text. For example, compared to the plano-gravure version, the surface of the PS plate is finely grained, and the picture is slightly higher than the publication surface. It is a plate with a plano-convex pattern. Compared to the peaceful gravure plate, it is necessary to print with a smaller printing pressure. In the printing plate, the number of outlets per unit area is large, and there is a slight over-expansion of outlets, and the phenomenon of loss of network level and level will occur. Compared with the printing of low-line graphics and texts, it is also required to print with relatively small printing pressure; Graphic printing plates and simple screen printing plates, as well as plates in the field and network lines, their printing pressure and eraser

Source: Industrial Control