In the paper production process, it is inevitable to encounter some problems, resulting in a large number of “paper defects”. Only by analyzing the causes in a timely manner can effective measures be taken quickly to minimize losses. Here are some common press felt-related problems:
1.Paper page crushing
Paper crushing means that the wet paper sheet is subjected to pressure in the pressure zone to form fluid pressure. Under the action of the fluid pressure, the moisture in the paper sheet is discharged outward. If the drainage is not smooth, the fibers in the wet paper sheet will be destroyed. This phenomenon is called “crushing” of paper pages. It is the most common phenomenon among paper diseases and the most troublesome one.
Among the factors that cause paper sheets to collapse, paper making press felt have four factors:
(1) Improper selection of press felt causes paper page collapse
The selection of press felt must be based on various technical parameters of the paper machine and the production environment. The product name, weight, air permeability, thickness, and raw material origin of the supporting paper press felt must be selected. Communication and cooperation with the manufacturer’s technical personnel are required.
(2) The paper sheets were crushed due to dirt on the press felt
Strengthening the washing of paper making press felt and maintaining the cleanliness of paper making press felt are the top priorities to extend their service life and reduce the occurrence of paper defects.
(3) The wear of the press felt causes the paper page to collapse
There are two types of press felt wear: uniform wear and local wear.
Reasons for wear and tear: newly replaced vacuum box panel, insufficient needling tightness of the press felt, too light setting, too much vacuum, no lubricating water between the press felt and the vacuum panel, improper selection of vacuum panel material, excessive friction coefficientetc.
(4) Poor operation of the press felt causes paper sheets to collapse
The running states of the press felt include: tensioned state, transverse contraction state, press felt standard line straight state, press felt surface smooth state, etc.
The press felt tension is too small, the lateral shrinkage is too large, the vacuum suction box is leaking, the standard line of the press felt is deformed and bent, and the tension of the press felt base fabric is uneven, etc., which can easily lead to uneven dehydration of the wet paper and the collapse phenomenon.
2.drum broken
drum broken refers to the local accumulation of air brought in by the slurry or felt to form bubbles between the felt and the paper sheet, causing the paper sheet to burst or local delamination and blistering to occur between the layers of the wet paper sheet. It basically occurs on the cage of the multi-cylinder paper machine, at the entrance of the press nip and in the dryer section.
cause:
There is a large amount of air in the pores of the new press felt, and the new press felt cannot meet the needs of normal production due to factors such as uneven initial surface, excessive air permeability, insufficient vacuum, and high water content in the press felt. It is easy to cause the gap between the paper sheet and the press felt. Bubbles are generated at the entrance of the pressure zone.
When producing board paper, the degree of beating of each layer of slurry is too large, resulting in differences in bonding between layers.
Couch roll line is improperly adjusted; the slurry volume on each circular screen, the liquid level of the slurry on the online screen, and the water level difference between the inside and outside of the screen are improperly controlled; the vacuum in the vacuum suction box is too low, resulting in a huge disparity in the moisture content of the wet paper sheets formed between each slurry layer. This causes bubbling on the wet paper sheet.
Partial dirt on the net and press felt causes poor local dehydration and poor air permeability; the water-absorbing blade of the couch roll is partially jammed; when the water-retaining cloth is uneven or has holes, the pressed water will rewet, etc.
The angle before the wet paper sheet is pressed is incorrect, and the eccentricity and linear pressure of each press are incorrectly adjusted.
The drying temperature curve of the drying cylinder is not adjusted well, etc.
3.press felt Marks
press felt marks actually include two types of marks:
one is caused by the base fabric of the press felt, which is called “press felt base cloth marks”; the other is caused by the rough surface of the press felt, heavy needle punch marks, and too thick fibers on the paper surface. There are rough marks, which are called “press felt marks”.
The main causes of press felt base fabric marks are:
(1) The structure of the base fabric (bottom mesh) of the paper making felt is improperly selected.
(2) The structural proportion of the paper making press felt is unreasonable, that is, the fiber layer of the press felt, especially the fiber layer on the front of the press felt, is too small.
(3) The warp tension and beating force during bottom net weaving are too small, resulting in insufficient flatness of the woven bottom net.
(4) The first shaping effect of the bottom net is not good.
The main causes of wool marks on press felt are:
(1) On the side of the paper making felt that is in contact with the paper sheet (the working surface of the felt), the fiber layer on the surface is too thick.
(2) Due to the poor quality of the felting needles, improper use of needles, unreasonable needle layout on the needle plate, and unreasonable needle punching technology, the working surface of the press felt has heavy needle punch marks and dense, uneven pinhole marks.
(3) The wool net is uneven during the carding process, and “cloud flowers” of varying sizes appear.
(4) Traditional far-infrared shaping equipment is still used for press felt shaping. During the press felt shaping process, this shaping equipment uses high-temperature radiation to melt the fibers (fiber bundles) on the surface of the press felt into colloidal particles. These particles are very hard, thus As a result, the press felt cannot be pressed (grinded) flat for a long time during use.
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