Basic structure of paper machine （2）
The formed wet paper sheet is further dehydrated by means of pressure. It consists of two rollers with hard and soft surfaces, such as stone rollers and rubber rollers, to form a set of presses. The pressing section is composed of pressing groups of the same form or different forms, which conventionally also include a vacuum suction device. When squeezing, the paper sheet is supported by the appropriate specifications of felt, which can evenly distribute the pressure and take away part of the water squeezed out by the squeezing, and can increase the pressure and increase the dewatering capacity of the press. Low-speed paper machines mostly use 20-60kN/m line pressure, high-speed paper machines use up to 210kN/m line pressure, when high-impact presses are used, such as wet paper presses can be as high as 350kN/m line pressure; Vacuum pressure rollers, grooved pressure rollers, net-lined pressure rollers, blind hole pressure rollers, etc., to more effectively take the press water away from the press area, these are new technologies that have been adopted since the 1950s to improve the efficiency of press dehydration. After that, a compound press structure is used to form a closed thread. It is composed of two or three sets of presses and combined. The wet paper is peeled from the forming wire by a vacuum suction roller, and is fed into the composite press with a felt support. Later, the paper is transferred between the press rollers, thus eliminating the paper in the past. The squeezing is caused by the traction of factors such as its own weight, causing the phenomenon of broken ends. Since the 1980s, wide-nip pressing technology has emerged. The tough rubber belts or rubber rollers deform when subjected to strong pressure, resulting in a line pressure of up to 750kN/m and a wide contact surface, which has significantly extended dehydration time. The dewatering capacity of the press is greatly improved, and the effect is more prominent on the high-speed paper machine. The dryness of the pressed sheet can be increased from 30 to 37% in the past to 43 to 45%, and some even reach 50%. The number of times the paper sheet touches the smooth hard roller surface or the number of rough rubber surfaces and the amount of dehydration on each side will affect the surface quality of the paper sheet. Proper handling can significantly reduce the difference in smoothness on both sides of the paper, and vice versa, it can also increase the difference in smoothness on both sides.