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The press section is the location where the impression is most likely to occur throughout the sheet making process. In general, the dryness of the paper exit portion is between 18% and 22%. The wetter the paper into the press section, the easier it is to change the arrangement of the fibers, causing the fibers to shift and the impression to appear. In addition, the high line pressure of the press is also a potential cause of the impression.
1, Maobujibu marks
During the use of the felt, the base fabric is exposed and the base fabric is rubbed on the sheet due to wear. Usually we will find obvious very straight pinstripes in the longitudinal direction of the paper, which are even and dense.
The base cloth marks generally appear in the middle and late stages of the use of the felt. In the process of paper sheet making, it is necessary to check the degree of oldness of the felt, and if it is acceptable, appropriately reduce the line pressure of the press section to reduce the base mark. If the line pressure is not improved, the cloth needs to be replaced at this time. Sometimes, because the pressure of the high-pressure spray water is too large, the nozzle wear causes the water column to disperse, or the vacuum of the vacuum box is too high, or the vacuum box panel is too rough, and the felt is excessively worn during use, thus making the felt The base cloth marks appeared in advance.
At this time, we need to check the pressure of the high-pressure spray water on the paper machine, the state of the nozzle, the configuration of the vacuum box, and check the condition of the press rolls in contact with the felt and the individual felt rolls. In addition, if the fine cloth can be found on the paper sheet in a short period of time after the machine is put on the machine, it may be related to the improper design of the felt cloth. For example, the felt is too light in weight to allow excess water to pass through the nip, thereby creating a dewatering mark on the sheet. This mark corresponds to the linear density of the base fabric.
In the initial stage of the felt, there are suggested solutions:
First, increase the weight of the velvet in the felt, increase the anti-wear time of the felt, and thus reduce the occurrence of the base cloth marks. On the other hand, increasing the grammage of the velvet, the water content of the felt is correspondingly increased, and the water flow resistance of the felt when passing through the nip is appropriately increased, thereby reducing the dehydration speed and reducing the base fabric trace which is excessively dehydrated and rubbed on the paper sheet.
Second, increase the weight of the felt base fabric, and use the elasticity of the fiber to reduce or cushion the pressure on the base fabric. Increasing the kibble weight can be achieved by increasing the number of layers of the base fabric, increasing the density of the warp and weft in the base fabric.
Third, according to the different positions on the paper machine and the requirements of the paper sheet, the base fabric of different weaves is selected. Generally speaking, the base cloth of the felt is more serious, the 2/2 broken twill is lighter, and the 1/3 broken twill is lighter.
2, traces of woolen layer
It is well known that the press section is the area where the sheet is subjected to the greatest line pressure throughout the paper machine. When the felt passes through the nip, the improper selection of the velvet and the improper operation of the paper machine will cause the crease marks on the paper. The feature is that by means of the emitted light, it is obliquely viewed from different angles or viewed from the light, and the paper sheet has a crease-like dent, a single fiber velvet mark, and a plurality of fiber ridge marks. The velvet marks usually appear at any time during the use of the felt. If there is a velvet mark at the beginning of use, the velvet of the felt is too rough. Appearing in the middle and late stages of the felt indicates that the fine velvet on the surface of the felt is worn and the velvet in the middle layer is exposed on the surface.
How to reduce the velvet marks?
In the wool fabric production plant, it is possible to consider a more delicate surface layer velvet to meet the requirements of the flatness of the paper sheet; increase the amount of surface layer velvet used in the felt fabric layer, and improve the abrasion resistance of the surface layer velvet. If necessary, it may be considered to add a part of the low-melting-point two-component velvet in the surface layer, and increase the temperature during the finishing of the felt to melt the surface layer of the low-melting velvet and firmly condense with the conventional velvet, thereby improving the abrasion resistance of the felt; Appropriate singeing treatment is used in the felt production process to reduce the surface fluff of the felt.
The felt user should pay attention to the new felt when it is installed on the machine, according to the indicated running direction (arrow). This is especially important for seaming felts, because the felts have already formed a velvet backing during puncturing and finishing. If the running direction of the paper machine is opposite to the reverse direction of the velvet, it is easy to cause abrasion of the felt and generation of velvet marks. After the machine is put on, fully wet the felt to reduce the pile marks caused by the dry grinding of the new felt. During the operation of the paper machine, pay attention to the configuration of high pressure water pressure and vacuum chamber vacuum.
For the vacuum box, we can check the vacuum box panel and the vacuum gap during the shutdown. At the same time, the water needle movement of the high-pressure water covers the entire width of the felt within one stroke, so that the felt is uniformly washed without missing or repeating. In addition, the low-pressure fan-shaped spray water in front of the vacuum suction box is also a critical part, and two adjacent low-pressure lubrication waters require cross-over coverage.
3, vacuum roller impression
When the press section of the paper machine is in the form of a three-roller two-pressure or a four-roller three-pressure composite press, the paper sheet often exhibits a vacuum roll impression consistent with the vacuum press roll pattern. This is because during the vacuum press dehydration process, the fine fibers, fillers and the like in the paper sheet form a higher ratio in the pore region than the non-porous region under the vacuum suction force, resulting in higher opacity. The paper machine solution has changed the pressing form (such as changing four rolls and three pressures into five rolls and three pressures), reducing line pressure, and reducing vacuum. In the design of the felt, a multi-layer base fabric can be considered, and the warp and weft of the base fabric of a larger diameter is used to improve the rigidity of the felt to obtain a bridge effect, and the void ratio and the compression resistance of the felt are improved.
4, spray water stripe imprint
Uneven high-pressure spray water makes the wool cloth banner uneven in water content, and the nip in the water is inconsistent, which will inevitably form a striped print on the paper surface. Such impressions can only be solved by enhancing on-site management of the paper machine.
5, the press dehydration marks on the paper sheet
When the sheet passes through the nip, a large amount of water is squeezed onto the felt and transferred to the roller or the boot to cause marks. The characteristics of the dewatering marks are also fine and straight regular marks in the longitudinal direction of the paper, or some regular hole-like marks. These marks may be felt base cloth, vacuum roll or blind hole roll print, grooved roll or grooved boot cover. When the pressure of the nip line is larger, the more moisture passes through the felt, the more likely it is to produce dewatering marks, especially in the middle and late stages of use of the felt.
The dewatering marks caused by the base fabric on the sheet are easily confused with the dewatering marks produced by the groove marks.
The method of distinguishing between the two: according to the number of impressions per centimeter on the paper, whether it matches the density of the base fabric or the density of the groove. In general, the linear density per centimeter in the base fabric will be higher.
There are many reasons for the paper mark on the wet end of the paper machine. Different solutions should be taken according to the actual situation. On-site management of paper machines is very important, and felt manufacturers should also strengthen on-site observation, gain in-depth understanding of the characteristics of paper machines, and provide felt products that are compatible with paper machines.
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