The relationship and influence of press felts and felt marks

Felt marks are fine streaks visible to the naked eye on the paper surface, which seriously affect the smoothness and appearance of the paper surface. The appearance of felt marks is directly related to the state of the paper making press felt. The causes and elimination of the marks are as follows:

The Influence of the Paper Making Felt Structure on the Felt Marks

Paper making felts are usually divided into three parts: base fabric, surface flocking layer, and bottom flocking layer. The selection of the base fabric, weaving method, and structural form will affect the felt marks. Generally, the base fabric of the commonly used paper making felt adopts a two-layer or three-layer structure. The base fabric layer constitutes the skeleton structure of the paper making felt. This skeleton structure has a certain hardness and incompressibility. When the wet paper sheet is attached to the felt and enters the pressing area, during the pressurized dehydration process, due to the insufficient pressure resistance of the felt and the hardness and incompressibility of the base fabric, the fibers in the wet paper sheet will be displaced under pressure. As a result, the fibers in the part squeezed by the skeleton have a high density and low water content, while the fibers in the other parts are relatively loose. Therefore, when observing the paper through light, it is obvious that the same and regular imprints as the arrangement of the paper making felt base fabric (bottom mesh) can be seen. In order to better play the performance of the paper making felt and avoid the occurrence of base fabric marks, two layers of separately woven base fabrics are composited together. The upper layer uses finer and denser silk to more evenly disperse the pressure in the pressing area, making the base fabric less likely to be compressed, thereby reducing the pressure and the probability of base fabric marks.

The Hair Marks on Paper Making Felt Lead to the Felt Mark

Felt marks caused by hair marks on paper making felts are mainly caused by the surface flocking layer fibers being too rough or the paper surface needle punching quality being poor, improper needle use, unreasonable needle board arrangement, resulting in heavy needle punch marks, dense and uneven needle hole marks, and uneven carding of the blanket, resulting in “clouds” of varying sizes. If this phenomenon occurs, the paper making felt needs to choose a finer surface layer of pile, with a surface layer weight of about 200gsm.

Impact of Different Paper Making Felt Positions and Replacement on Felt Marks

Paper making felts can be divided into forming felts, paper threading felts, and press felts. Their structures and the pressures they bear in the nip zones are different, and the degree of impact on felt marks is also different. Forming felts are mainly used in multi-circular paper machines. They bear very little pressure and generally do not leave felt marks on the paper sheets. Because paper feeding felts have vacuum suction rollers and the pressure in the nip zone is greater than that of forming felts, their influence on felt marks is greater than that of the former. Among them, press felts have the greatest impact on felt marks. When the press felts and paper sheets pass through the main nip zone, they are subjected to greater compression. The quality of their press felts directly affects whether felt marks will appear.
Through the analysis of the influence of press felt replacement in various parts of the paper machine on felt marks, it was found that the centralized replacement of press felts in various parts can improve the felt marks to a certain extent. Replacing a single press felt, especially the press felt in the downstream press, can improve the felt marks the most.

According to the above, the replacement coordination of press felts in various parts can be reasonably arranged to prevent the occurrence of felt marks most effectively. When the paper machine is shut down for maintenance, the press felt in various parts should be replaced as much as possible. During routine shutdowns for cleaning, when time and manpower permit, the blankets in various parts should be replaced together in a targeted manner according to the characteristics of the felt marks and the condition of the press felt themselves. At the same time, the entire set of press felt should be replaced separately in a timely manner. The total replacement interval should also be shortened as much as possible to keep the condition of the press felt in each part basically consistent.

Maintaining the Press Felt in Good Operating Condition is an Effective Measure to Felt Marks

During operation, acid and alkali cleaning should be performed alternately at irregular intervals depending on the contamination of the press felt. In particular, a thorough cleaning should be performed during downtime, and the surface condition of the press felt should be fully inspected, such as the integrity of the surface pile and whether there are dark lines on the surface. At the same time, an indentation test should be carried out to determine the impact of each pressing pass on the press felt mark and to effectively predict the operating cycle of each press felt.