1) Pressing of modern new papermaking machines:
Water removal during papermaking: 98.3% is removed in the wire section, 1.1% is removed in the press section, and 0.6% is removed in the drying cylinder section; cost comparison of removing the same weight of water in the wire section, press section, and drying cylinder section: 1:70:330;
2) Classification of press rolls:
According to the number of press rolls in each press, it can be divided into: double-roll press, multi-roll press
The function of press can be divided into: positive press, reverse press, gloss press, water squeeze press, paper feed press, high-strength press, composite press, hot press, etc.
3) Pressing felt
The role of felt:
Support and transport paper webs, cooperate with paper web dehydration, and drive other components of the press section
Requirements for felt:
Durability, dimensional stability, air permeability, cleanliness, softness, surface finishing
Development of felt:
Before the 1950s, it was mainly woven from fine yarn, The fleece blanket is made by mechanical method, which has low air permeability and short service life. In the 1950s and 1960s, the batt-on-base felt (also called base flocking blanket) was successfully developed, using a high proportion of synthetic fibers. In 1967, weftless base flocking blankets, multi-layer composite blankets, and non-woven blankets appeared.
4) Factors affecting the maximum pressure in the pressure zone:
Line pressure, pressure roller diameter, pressure zone structure, pressure roller hardness, blanket characteristics, compressibility of wet paper.
5) The main types of pressing rollers are:
Flat pressing roller: ordinary pressing and positive pressing, the upper roller is a stone roller, and the lower roller is a rubber roller.
Grooved roller: The upper roller is a stone roller, and the lower roller is a rubber-coated roller with spiral grooves (groove depth 2.5mm). Suitable speed: pulp board 90m/min (linear pressure 176kN/m), paperboard 245m/min (linear pressure 88kN/m), paper 850m/min (linear pressure 137kN/m).
Vacuum roller: used for medium and high speed paper machines, the upper roller is a stone roller, and the lower roller is a vacuum roller.
Blind hole roller: The blind hole depth is 12~15mm, the hole diameter is 2mm, and the opening rate is 25~30%, which can be applied to upper and lower rollers.
Controlled middle and high roller: In the 1970s, the winding degree of the roller can be controlled and adjusted according to production needs.
6) Take-of angle: The angle between the paper web and the tangent line of the web, blanket or roller at the peeling point when the paper web is peeled off from the web, blanket or roller.
The optimal peeling angle depends largely on the characteristics of the pulp. The peeling angle is closely related to the tension and strain when the wet paper sheet is peeled off. The Fourdrinier papermaking machine is generally 35~50°.
7) Factors affecting the dehydration of the wet paper sheet press section:
① The number of press sections, the form of the press rollers and their press combination, etc.;
② Line pressure, press roller diameter, press section structure, press roller hardness, felt characteristics, compressibility of wet paper;
③ Pressing dehydration temperature, etc.
8) Impact of pressing on paper properties:
① The bursting strength of paper increases with the increase of pressing force, because of the binding strength of paper sheets;
② The tensile strength changes with the change of the binding force between fibers, so increasing the pressing force can increase the tensile strength of paper;
③ The bulk decreases with the increase of pressing force;
④ The pressing force has a certain effect on the tearing degree, which mainly depends on the degree of pulping;
⑤ The porosity of paper increases linearly with the increase of pressing force;
The opacity of paper decreases with the increase of pressing force, because the reflectivity of paper decreases;
⑥ Improve the two-sidedness of paper sheets;
⑦ Pressing causes fiber keratinization, reduces swelling ability, and reduces water retention value, especially for sulfate wood pulp