Han Bangchun 1 Lu Xiangyang 2

(1. Xuzhou Golden Crown Industrial Co., Ltd. Xuzhou, Jiangsu 221009)

(2. Taicang Chang'e Industrial Co., Ltd. Taicang, Jiangsu 215437)

Abstract: Briefly describe the connotation of dimensional stability of BOM blanket, its impact on production, etc .; analyze the factors affecting the dimensional stability of blanket and the control approach.

Key words: BOM blanket, dimensional stability, influencing factors, control methods.

     BOM blanket (that is, high-wire bottom-net needle-punched paper blanket) is one of the valuable consumables on the paper machine. It is especially suitable for medium and high-grade paper grades made by medium and high speed paper machines. With the rapid development of the paper industry, the performance requirements of the papermaking blankets that are matched with it have become more refined and stricter. Therefore, in the design and production process of papermaking blankets, many factors that affect their performance must be strictly controlled. Among the many performances of papermaking felt, dimensional stability is one of its important basic performances, which cannot be ignored. This paper analyzes the factors that affect the dimensional stability of BOM blankets, and proposes a control method to improve their dimensional stability.

1. Definition of BOM blanket dimensional stability

      The stability of BOM blanket size usually refers to the change of its size before and after use, and is the main performance indicator to measure the amount of size change. When talking about the stability of BOM blanket size, we often refer to the change of its length and width, but it ignores the change of blanket thickness, but the change of thickness has a greater impact on the performance of the blanket. Therefore, the stability of BOM felt size should include changes in length, width and thickness.

The change process of BOM blanket size is divided into two stages: the first stage refers to the change of the length and width of the blanket after the machine is flattened and tensioned relative to the finished state under tension on the setting machine; the second stage refers to the blanket The change in length, width and thickness before the normal paper exit before flattening and tensioning with respect to the upper machine before lowering the machine.

     In the actual production and use process, people often only focus on the second stage size change of the blanket and ignore the first stage size change. The size change of the first stage is the basis of the second stage. If the size change of the first stage is small, the possibility of the size change of the second stage will be small; if the size change of the first stage is large, it will directly affect the second stage. The size of the stage changes.

     1. Length elongation

     The change in the length of the BOM blanket is generally expressed as an extension of the length, and there is rarely a phenomenon that the length of the blanket shrinks. Therefore, the change in the length is usually measured by the elongation rate.

     The elongation of BOM blanket length refers to the percentage of the difference in length between two different periods (state) and the length in the previous period (state) under a certain tension state. The different periods (states) mentioned here refer to the state in which the felt is tensioned on the setting machine, the state in which the felt is flattened on the machine, and the state before the felt is off the machine. It can be seen that the elongation of the BOM blanket length is divided into two stages. In general, we pay special attention to the elongation of the second stage, that is, during the use of the blanket, the length before the machine is flattened and tensioned at the beginning of the machine. The difference between the length and the percentage of the length of the blanket under the initial flattening and tensioning. Taking the second stage as an example, the elongation rate is expressed by the formula:

Among them: L1-represents the length of the blanket under the initial flattening tension (the tension of the normal paper production), unit: meter.

        L2－Indicates the length of the blanket before getting off the machine, unit: meter.

 2. Width shrinkage

     The width shrinkage rate refers to the percentage of the width shrinkage of the BOM blanket under a certain tension state in two different periods (states) and the width in the previous period (states). The content of the two different periods (states) mentioned here is the same as the content expressed by the elongation of the above length. The shrinkage of the width can be calculated by the following formula:

Among them: W1-represents the width of the blanket on the machine, unit: cm.

        W2-indicates the width of the blanket after getting off the machine, unit: cm.

     The change of the width of the papermaking blanket is shown in two aspects: on the one hand, the width of the blanket is widened, which is generally expressed in the first stage above. The blanket is affected by the redistribution of moisture, tension and internal stress. Trend; in the second stage, only partly due to improper processing of fully chemical fiber needle punched weftless papermaking felts will sometimes have a widening phenomenon, but BOM blankets rarely have a widening phenomenon, usually when the blanket is under high tension The lower width will shrink.

From another perspective, the width change is divided into the change of the overall width of the felt and the change of the local width. The change of the local width will also cause the overall size of the papermaking felt to be unstable.

     Different varieties of BOM blankets have different width shrinkage rates. Single-layer BOM blankets have the largest shrinkage rate during normal use, and the more the bottom mesh layer, the more stable the BOM blanket, the smaller the lateral shrinkage rate during normal use. .

     3. Compression rate of thickness

    The thickness compression rate refers to the percentage of the difference between the thickness of the BOM blanket before the machine is loaded and the thickness when the machine is off the machine, and is an important parameter to measure the elastic recovery and water filtration state of the BOM blanket. Formulated as follows:

    Among them: Z1-represents the thickness of the blanket before the machine, unit: mm.

            Z2-represents the thickness of the felt after getting off the machine, unit: mm.

     In the use of BOM blankets, we do not want the thickness compression rate to be too large, which means that the density change of the blanket during use is too large, which also means that the blanket's air permeability and water filtration performance change too much. This is something we do not want to see.

Second, the impact of BOM blanket size stability on production

      The stability of BOM blanket size is directly related to the quality of many use performances, and its main hazards are:

1. Elongation of length

     Excessive elongation of the length of the BOM blanket will lead to the reduction of the fracture strength of the blanket itself, the reduction of the pores of the water filter, the reduction of the efficiency of the water filter, the shrinkage of the width, etc. Tight, causing a lot of trouble in actual operation.

2. Width shrinkage

     Excessive shrinkage of BOM blanket width will result in insufficient effective width, vacuum box (roller) air leakage, carpet water filtration pores become smaller, water filtration efficiency decreases, carpet operation is unstable, paper feeding is difficult, wet paper sheet moisture fluctuations , Wet paper sheets frequently break the head and other problems. If the partial shrinkage of the blanket is too large, it will also cause the polarizer to work frequently and in a large range, and it is prone to deviations and other phenomena.

3. Compression rate of thickness

     Excessive compression ratio of BOM blanket thickness will lead to its loss of elasticity, blanket compaction, reduced air permeability, reduced water-containing porosity, reduced water filtration performance, difficult paper sheet dewatering, and uneven paper sheet dewatering.

     Figures 1 and 2: The change in air permeability and the corresponding thickness when using a 1200g / m2 double-layer BOM blanket on a paper machine with a speed of 400m / min are used as an example to illustrate the change in the thickness of the BOM blanket and the change in air permeability relationship:

      In our daily work, we must maintain the stability of the blanket size and reduce the occurrence of hazards. Maintaining the stability of the felt size is a necessary condition to ensure the stability and efficiency of paper machine production, and to produce high-quality paper; maintaining the stability of the size of the paper felt not only needs to be strictly controlled in the production process of the paper felt, but also needs to be used. Go to correct operation and maintenance.

3. The stability standard of BOM blanket size

      As we all know, the BOM blanket size change parameters described in the FZ / T 25002.5-2000 standard refer to the deviation between the length and width of the finished product in the natural state and the actual order size. However, it does not specify the change of the length and width of the BOM blanket after use compared to its value before it is not used, nor does it list in detail the parameters of the change of the length and width of different types of BOM blankets. The thickness of the blanket may be due to the large proportion of external factors that cause its change, and the thickness change can only occur after the blanket is used on the machine, so it is not specifically included in the standard. Analyzing and discussing the dimensional changes of BOM blankets during use is more conducive to correct judgment and analysis of their essential performance, and is more conducive to guiding the production of blankets. It is more intuitive and practical for blanket users.

The length and width requirements of the BOM blanket in the assessment indicators of the ministerial standards are shown in Table 1 below:

4. Factors affecting the dimensional stability of BOM blankets and their control methods

     There are many factors that affect the dimensional stability of BOM blankets. There are internal factors such as raw materials and manufacturing processes, as well as external factors such as storage, transportation, maintenance and use. The following are analyzed:

(1) Influence of raw materials

BOM blanket is composed of two parts: fiber layer and bottom net layer. The raw materials used in the blanket fiber layer are mainly polyester and nylon. The raw materials used in the bottom mesh layer are mainly nylon healds. The selection and use of these raw materials play a certain role in the stability of the BOM blanket size.

1. Fiber raw materials: Different fiber raw materials have different effects on the stability of the finished blanket. In comparison, the hygroscopicity of polyester is less than that of nylon. Therefore, the stability of the size of paper blankets produced with polyester is relatively better than that of nylon.

2. Fiber length: The longer the fiber length, the more unstable the felt size produced.

3. Fiber thickness: The larger the denier of the fiber, the thicker the fiber and the worse the dimensional stability of the felt produced.

4. Fiber curl: The smaller the fiber curl, the worse the dimensional stability of the felt produced.

3. Elongation of nylon heald: The greater the elongation of nylon heald, the more unstable the size of the felt produced.

(2) The influence of production process

1. The impact of the bottom net

    The bottom mesh layer of the BOM blanket plays an important skeleton structure in the blanket, which is the key to determining the performance of many blankets. The dimensional stability of the bottom mesh layer directly affects the dimensional stability of the finished blanket.

1) The organizational structure of the bottom net

    There are many organizational structures that constitute the bottom mesh layer of the BOM blanket, with or without node compound, 1/1 plain weave, 3/1 broken twill, 5/1 broken twill, 2/2 (broken) twill, double-layer structure, laminated composite Organizational structure, etc. Relatively speaking, the plain weave structure is more stable than the twill structure; the size of the bottom mesh layer with a larger warp density, thicker wire diameter, and more plied strands is smaller than the warp density, thinner wire diameter, and fewer plied strands; the plied weft is larger than the monofilament weft Stable; the latitude density is larger than the latitude density; the size is stable; the warp double layer is more stable than the same number of laminated composite layers; the larger the number of bottom net layers, the more stable the size.

2) Weaving of bottom net

    When weaving the bottom net, the greater the strength of warping, beating and winding the cloth, the denser and flatter the bottom net weaved and the more stable the size. BOM blankets for paper machines with high speed, high width, high linear pressure, and high-grade paper grades must have their bottom nets produced on a wide-width heavy loom.

3) The predetermined type of bottom net

    The bottom net of the BOM blanket must go through a predetermined type before laying the needles, so as to eliminate the stress of the bottom net, stabilize the position of the warp and weft of the bottom net, make the bottom net surface smoother, and the longitudinal elongation and lateral shrinkage rate are closer to and To meet the requirements of the finished blanket for this parameter. The pre-determined shape of the bottom net usually adopts low temperature and slow speed setting to avoid damage to the bottom net.

2. Influence of acupuncture process

    Whether the needle-punched flocking process is reasonable will directly affect the tightness (density) of the finished blanket, the smoothness of the surface and the degree of damage to the fiber and the bottom net, and will also affect the water permeability of the blanket, the hair removal and hair loss state, and the surface The properties of the silk state, the uniform density state and the strong loss state of the fiber bottom net also play a very important role in the stability of the blanket size. Therefore, the selection of the needle, the needle form of the needle board, the density of the needle, the depth of the needle, the number of needles, the speed of the cloth, the control of the needle breakage rate, and the state of the laying of the net need to pay close attention. , Strict control.

Practice has proved that in the process of acupuncture flocking, the smaller the damage to the bottom net, the deeper the acupuncture, the more the number of acupuncture passes, and the smaller the needle breakage rate, the greater the tightness of the felt produced. The more stable the size.

3. The influence of post-order finishing process

     The post-order finishing of BOM blankets includes washing, chemical treatment, heat setting, trimming, edge banding, marking spraying, dimensional inspection, quality inspection, roll wrapping and other processes. In these processes, the shaping process control of the felt is particularly important.

The shaping process control of the BOM blanket is to make the fibers in the fiber layer more stable and smoother on the surface, and at the same time make the bottom net shape and draw again, so that the internal stress in the blanket is more evenly distributed, thus To achieve a stable organizational structure. The control of the shaping process mainly includes the control of parameters such as temperature, speed, tension and pressure.

1) Control of setting temperature and speed: The heat setting temperature should not be too low. High is better under the possible conditions. The lower temperature will affect the setting effect. The relatively high temperature can make the fiber quickly meet the setting requirements. The far-infrared shaping has a poor heat penetration effect and needs to be adjusted in an appropriate manner; the hot-air shaping has a good heat penetration effect, but the blanket surface is poor; the oil heat setting has both good heat penetration effect and good Flatness of the blanket. The setting temperature and speed are related to each other, and it is necessary to determine a suitable data through measurement, which can not only make the felt get a good setting effect, but also ensure the water permeability of the felt.

2) Control of the setting tension: When setting the BOM blanket, the setting tension should be greater than the tension during normal use of the paper machine, otherwise the blanket will have a large elongation rate.

3) Control of setting pressure: BOM blankets apply a certain pressure during the setting process, which will help improve the setting efficiency, dimensional stability and initial adaptability of the blanket, and reduce the thickness of the blanket.

4) Others: In the post-sorting process of BOM blankets, the trimming edges should be level, without unevenness and "snake swing"; the edge banding should be even and firm; the roll cloth should be flat and tight on both sides; the packaging should be Waterproof and moisture proof. Changes in the above processes will have a certain impact on the stability of the blanket size.

4. Setting of structure ratio

     The structural ratio of the BOM felt refers to the ratio of the weight of the fiber layer to the weight of the bottom mesh layer. The setting of the structural ratio should be determined according to the speed of the paper machine, the line pressure, the use position of the felt, the washing state and the paper grade produced. The larger the structure ratio, the greater the proportion of the fiber layer, and the more the thickness of the felt decreases in use, otherwise, the smaller the thickness change.

(3) Impact of transportation and storage

     During the transportation and storage of BOM blankets, it is necessary to prevent the outer packaging from being damaged, and to be waterproof, moisture-proof, fire-proof, and chemical-proof. It is usually stored in a ventilated and cool place, so as to prevent the blanket from being damaged or other properties from changing and causing unnecessary trouble to the machine operation. The storage period of the blanket should not be too long, otherwise the performance of the blanket will change.

(4) Impact of use and maintenance

Figure 3: The standard line of papermaking blankets is tilted left and right

Figure 4: Papermaking blanket standard line before convex

      There are many factors in the use process that will affect the stability of the BOM blanket size. If used and maintained improperly, it will inevitably lead to excessive changes in the length, width and thickness of the blanket, thus greatly reducing the performance of the blanket. Therefore, the correct use and maintenance of papermaking blankets not only benefits the blanket itself

Dimensional stability, and is conducive to the performance of felt, in order to achieve the effect of extending the life of the felt.

1. The effect of tension

     The tension used by the BOM blanket is different depending on the location used on the paper machine. For BOM blankets, it is generally required to be tensioned once at the time of the machine. It does not need to be loosened at the beginning and gradually tensioned at the later stage like BOB blankets (that is, all chemical fiber needle punched paper blankets). The running tension of BOM blanket after machine is generally required to be between 2.5 ~ 4.5kgf / cm. If the tension is too large, the length elongation and width shrinkage of the felt will be too large, and the felt and rollers will easily break, and the running load will increase. Stable and easy to discount.

2. The impact of the standard line

Figure 5: After the standard line of the papermaking blanket is recessed

Figure 6: The standard line of papermaking felt is S-shaped

     In order to facilitate the observation and grasp of the change state of the water filter pores during the operation of the felt, and to effectively control the change of the width of the felt, each felt is provided with a standard line. The standard line is a striking wide line that is placed on the front (sticker side) of the blanket and is perpendicular to the warp center line of the blanket. During normal production, we require that the standard line be kept as straight as possible to prevent the standard line from deforming and causing the blanket Problems such as small pore size of water filtration, shrinkage of lateral width, and deterioration of water filtration performance occur. In severe cases, the operation of the blanket may become unstable and discounted, thereby reducing the life of the blanket and affecting product quality and normal production order.

1) There are many types of deformation of the standard line of blankets, which can be summarized as follows:

(1) The standard line is slanting from left to right, as shown in Figure 3.

(2) Before the standard line is convex, as shown in Figure 4.

(3) After the standard line is recessed, as shown in Figure 5.

(4) The standard line is S-shaped, as shown in Figure 6.

(5) Other irregular deformation.

The deformation of the above standard line can cause the width of the BOM felt to shrink. The larger the deformation range, the more the width shrinks.

For the shrinkage of the blanket caused by the deformation of the standard line, we take the convex and concave of the standard line as an example, and use a simple mathematical model to explain and calculate, as shown in the following figure:

2) The main cause of the deformation of the standard line of the blanket and its preventive measures:

(1) The tension on both sides of the blanket is inconsistent, one side is large and the other side is small. When the blanket is tensioned after being put on the machine, the tension on both sides of the blanket must be the same, and if it is found to be inconsistent, it should be adjusted in time.

(2) The pressure on the two sides of the press roll is inconsistent. During the pressing process of the press roll, both sides must be pressed at the same time, and the magnitude of the line pressure should be consistent. During the dry running of the new blanket, the linear pressure should be adjusted to the linear pressure during normal production as soon as possible to avoid long-term low-pressure dry running.

(3) The eccentricity of the two sides of the press roll, roll roll, and squeeze roll are inconsistent. During the production process, you should always check whether the eccentricity of each press roll is moderate and consistent, especially after replacing the blanket.

(4) The medium-high setting of the press roll is unreasonable, the medium-high is too large or too small, and the line pressure is too large or too small. The medium-high setting of the new press roll should be reasonable and accurate. During use, the medium-high wear change of the press roll and the consistency of the pressure in the nip should be regularly checked to prevent abnormal wear from causing trouble to the production. If the middle height of the press roll is too large or the middle height is correct and the pressure is not enough, the standard line of the felt will appear convex in the middle. The speed is faster, so that the standard line of the blanket is convex in the middle. The standard line of the felt appears concave in the middle, which is mainly caused by the insufficient pressure of the lower roller or the correct height and the excessive pressure. In other words, if the linear velocity in the middle of the roller surface is less than the linear velocity on both sides, the linear velocity of the felt in contact with the middle of the roller surface is slower, so that the standard line of the felt appears concave in the middle, as shown in Figure 5.

(5) The tangling slurry on the roller surface was not removed in time. Keeping the surfaces of the rollers clean at any time is good for reducing and eliminating blanket deviations, discounts and standard line deformation.

(6) The pressure roller has a displacement phenomenon, and the bearing is damaged and not replaced in time. In normal production, we must pay close attention to the positioning and lubrication status of each roller. If we find that the screws are loose and the bearings are damaged by heat, we should take measures to tighten, lubricate and replace them, especially the blanket rollers of the Yankee cylinder.

(7) The blanket rollers, press rollers and other rollers are partially unevenly worn. Rollers with uneven wear are first identified and replaced in time.

(8) The local tension of the bottom net (base net) of the papermaking felt is uneven. When producing paper-making blankets, it is necessary to ensure that the tension of the bottom net (base fabric) is uniform and consistent, and it is not allowed to use the bottom net (base mesh) with partial slack or elastic bands in the production of the blanket.

3. The impact of the blanket roller

     The function of the carpet splitting roller is to make the carpet felt by the spreading effect of the carpet splitting during each cycle of use, so that the carpet surface is smoother, the width is more stable, the operation is smoother, and the water filtering pores are smoother.

     Generally, blanket rollers include steel tube rollers, copper-clad or copper-plated rollers, anti-corrosion rollers, rubber-lined rollers, etc. There are several types of spiral strips for carpet rollers, such as copper, rubber, and anti-corrosion coating. The pitch of the spiral strip gradually increases from the center to both sides. The pitch of the spiral strip is generally 200 ~ 300mm; the height of the spiral strip is 5 ~ 15mm; the wrap angle of the felt on the split blanket roll is 90 ~ 1800. The smaller the pitch, the higher the height of the spiral strip, and the larger the blanket angle, the better the blanket roller splitting effect and the better the width recovery of the blanket, but the greater the wear on the blanket. The width recovery of the blanket is also related to the number of sub-carpet rollers and the placement position. Generally, a sub-carpet roller is provided for the blanket, and two sub-carpet rollers can be provided for the particularly long blanket.

4. The impact of the use of parts

     BOM blankets use different parts, and their length, width and thickness vary. For blankets of the same variety and weight, when used in the forming section (such as garden paper machine, super forming paper machine, etc.), the blanket has a larger elongation due to the larger load; when used in the press section, Its length and width change rate is smaller. As the line pressure increases, the thickness variation of the blanket will increase; when used in a Yankee cylinder to support the blanket, due to the effect of high temperature and high pressure, its width contraction and thickness reduction will be more prominent and easier Make the pores of the filter water filter become smaller and compact. These conditions require us to fully consider when designing and making BOM blankets.

5. Influence of washing system settings

    The quality of BOM blanket washing not only affects the water drainage performance of the blanket, but also affects the elastic recovery of the blanket, that is, the elastic recovery rate of the thickness. If the washing is not good, it will inevitably lead to the blanket being crushed, reduced in thickness, and increased in density. , Reduced permeability of filtered water, etc. One of the important functions of the high-pressure mobile spray pipe is to increase the loose thickness of the blanket. BOM blankets generally require high pressure water at 10 ~ 25kgf / cm2.

6. The impact of abnormal wear

     BOM blankets should reduce and eliminate abnormal wear during use. Unusual wear comes from vacuum box panels, blanket rolls, blanket rolls, squeezing rolls and other roller surfaces and the unreasonable setting of lubrication water. Abnormal wear and tear will inevitably lead to an abnormal decrease in the thickness of the felt and uneven water filtration.

    V. Conclusion:

    The dimensional stability of BOM felt should include length elongation, width shrinkage and thickness shrinkage. The stability of the blanket size directly affects the performance of the blanket, which plays an important role in extending its service life and ensuring the normal production of the paper machine. There are many factors that affect the performance of BOM blankets. We must make accurate analysis and correct judgments and strictly control them. Only in this way can the comprehensive performance of BOM blankets be fully exerted.

references:

[1] The People ’s Republic of China Textile Industry Standard, FZ / T 25002.5-2000

[2] Editor-in-chief of Guo Bingchen. "Performance and Testing of Nonwovens", Beijing, China Textile Press, 1998

[3] Long Yanquan editor. "Papermaking Principles and Engineering", Beijing, China Light Industry Press, 2002