Vacuum press has the advantages of high dehydration efficiency, less paper breaks, long felt life, and is widely used. However, if the moisture content of the wet paper sheet entering the vacuum press zone is too high, the dewatering performance of the felt is poor, or the relevant process parameters are not adjusted properly, there may be a shadow corresponding to the press hole on the paper forming surface. Noodle kraft paper board has the characteristics of bottom layer and surface layer two kinds of slurry, large quantitative, and sometimes even the surface layer to be dyed, the problem is more complicated, and the vacuum press roll itself has a great impact. We will now organize our production practices in this area for reference by our peers.
1. Introduction of Raw Materials and Paper Machines Our factory produces A-grade kraft paperboard with imported natural wood pulp and American waste corrugated boxes as the main raw materials. The main equipments for slurry preparation are imported from Finland and Sweden and have advanced international level in gO era. Circular cylinder multi-cylinder paper machine width 1600mm, design speed 15Om/min, working speed 12Om/min. The net consists of a vacuum return roller and 5 Bristol rotary screen formers. The press section consists of a three-roll, two-compression compound press and a positive pressure. The recompression upper roll is cast iron and the outer coating rubber P&1O. The single-chamber vacuum press roll body ZQALg-4 has a hardness requirement of greater than Shore 90, a hole diameter of 3.2 mm on the surface of the roll body, and an opening rate of 16%. The re-compression roller material is made of cast iron, and the required hardness of the encapsulation is greater than that of the Xiao 90, surface groove pattern.
2. Characteristics of shadow marks From the pattern with shadow marks, the dots corresponding to the vacuum press holes are not only light in color, but also have exposed bottoms in the surrounding 4W, and these dots are spirally distributed. After the carton is formed, it is easy to visually cause problems with the carton itself or with incorrect printing and patterns.
3. Principles and solutions for generating shadow marks Since the surface of the roll body is provided with holes for sucking the moisture of the felt, the pressure and degree of dehydration of the paper sheet and the felt are inconsistent. In the nip, the pressure on the sheet and the corresponding part of the felt and eyelet is low, and the pressure on the unopened part is high, and there is a pressure gradient. Under the pressure gradient, the wet paper sheet is deformed, and the fine components are concentrated from the periphery of the hole to the hole, where the moisture content is also large. After the vacuum suction and drying process, the paper surface is left behind the surrounding paper surface. Lighter traces of color. Due to the shift of the fine components, the masking performance of certain parts of the surface of the paper sheet is reduced, thereby resulting in a bottoming phenomenon. If the moisture content of the wet paper sheet is too large or the external force exceeds a certain limit, the shadow marks will become more and more serious.
3. l The hardness of the rubber surface of the vacuum press roll I soon after the trial production of the vacuum roll, the rubber layer of the vacuum press roll fell off and needed to be re-wrapped. It was found that the hardness of the rubber surface is low, and only the Shore 82 is much lower than the requirement of Shore 90. After the feeding, the paper shadows are particularly serious. After that, the vacuum press roller was encapsulated according to international standards and the adhesive surface was over 90 Shore. After the feed was produced, combined with other solutions, the paper shadows disappeared. Verified our analysis results.
3.2 The rate of opening of the roll surface is small, the area of ​​opening is small, and the area of ​​dewatering is small; the rate of opening is large, the area of ​​force of the paper sheet in the nip area becomes smaller, and under the condition that the pressure on the wire is constant, the pressure received is increased. Hydraulic pressure will be higher and more likely to have shadows. The surface hardness of the rubber is low. Under the vacuum suction and the other two rollers, the edge of the hole wears, gradually becomes a tapered hole, the hole spacing becomes smaller, and the force area of ​​the paper sheet also decreases, so the paper sheet will appear obvious. Shadow marks. Therefore, in addition to ensuring that the hardness of the rubber roller is greater than gOO, there must be a suitable opening rate. The hole rate we finally selected was 16%.
3.3 Vacuum suction zone 1 According to the four-phase theory of the Wahstrom press, the vacuum suction zone should be located between the two nips and near the exit of the sheet. Otherwise, the water coming into the felt after the sheet has been squeezed and pumped will be sucked back by the sheet, causing a phenomenon of rewetting. After the nip of the upper press roller and the vacuum press roll, a shadow is formed. The angle of the vacuum zone can be adjusted according to the quantitative and bottom of the paper and the proportion of the dough. It is particularly pointed out that when replacing the roll body or the inner seal, attention must be paid to the installation location of the vacuum suction area in order to avoid production fluctuations.
3.4 Blanket ration Compared with the general online forming board machine, the board speed of our factory is higher, and the choice of blanket ration is very important. If the blanket is small or absorbent. Drainage performance is poor, so that the moisture content of the wet paper sheet is large, the buffering effect in the pressure area is small, the dewatering is uneven, and shadow marks are easily left. After our exploration, we chose to use a double-structure full-chemical fiber blanket made in Italy. The quantification of Baotou vacuum press roll blanket is 125Og/m2.
3.5 Vacuum degree and line pressure of the vacuum press roll To increase the speed and the basis weight of the paper machine, it is necessary to increase the vacuum degree of the vacuum press and the line pressure of the upper and lower press rolls correspondingly. However, if the degree of vacuum is too high, the dehydration effect is not only not obvious, but it causes uneven dehydration and rapid local dehydration. The fine substances or dyes accumulated in the pores are filtered through the blanket to form severe shadows; the press line pressure is too high, and the pressure is too high. The hydraulic pressure between the holes in the sheet increases, and it is also easy to form shadows. We control the degree of vacuum in the vacuum press. OS-O. O65MP. The upper pressure roller line pressure is controlled below 6OkN/m, and the lower roller line pressure is controlled below 4OkN/m. The press dehydration is compensated by the positive pressure, and the positive pressure coupling must be appropriately modified.
3.6 Face Assembly Compared to the production of high-grade kraft linerboards, we use imported wood pulp. Compared with the bottom slurry, the physical shape of the noodle pulp fiber is relatively uniform, the fiber is long, and the small and small fibers are small. Properly increasing the proportion of the noodle slurry can reduce the shadow. We have done experiments, when the batter ratio increased from ZO%h to 23%, the paper shadow completely disappeared. However, this will increase manufacturing costs.
3.7 The degree of beating of the slurry is well known, the degree of beating of the slurry is low, the strength of the sheet is affected; the degree of beating is high, the dewatering of the wet sheet is difficult, and a shadow is easily formed. As the bottom slurry and batter are two types of slurries with different dewatering properties, the degree of beating of controlling the two slurries is similar but different from the perspective of the combination of layers. We generally control the degree of beating pulp 34-36SR. About IOg wet weight, the difference between bottom pulp and batter pulping is controlled within 4SR.
3.8 The amount of water sprayed on the vacuum press roll is such that the holes of the vacuum press roll are not blocked by the slurry, and there is a spray water pipe in the interior. If there is too much water to be pumped away, it will flick from the inside of the roll body. Out, splashed on the felt that has been sucked dry wrapping the vacuum press roll, increased the vacuum suction load, it may also form a shadow. Therefore, we must strictly control the amount of spray water and water pressure.
3.9 Cleanliness of blankets Paper is made from waste paper. The content of fine fibers, fusible substances, and fillers in the slurry is high, which can easily contaminate the blankets and channels that block the water absorption and dehydration of the blankets, seriously affecting the ability of the blankets to absorb water and dewater. The sheet of paper entering the compound nip has large moisture and forms a shadow. In particular, forming a blanket is more important. Therefore, pay attention to the cleanliness of the blanket and clean it regularly. (Shandong Jining Paperboard Factory 272159)