Fabric fiber screens, also known as screens (hereinafter referred to as screens), have always been important auxiliary materials for flour mills. Although they account for a small proportion of the entire mill's investment, they play a key role. Because the flour is finally screened through it, the milling process is also balanced by the grading and sieving. Choosing a good screen is actually one of the key technical indicators of the flour mill. Therefore, more and more milling workers believe that it is an important factor to ensure the balance and stability of flour quality. The author combines the practical use of many years to talk about the choice and use.
1. Classification and selection criteria of screens 1.1 The screens used in flour mills are divided into the following categories according to their materials:
1.1.1. Silk screen: It is made by weaving multiple strands of natural silk into silk, and then weaving it. It is characterized by good elasticity, not easy to relax, the pore size is not easy to be deformed, has good hygroscopicity and moisture release, and is not easy to produce. Static electricity, etc. However, it is not wear-resistant and easy to fluff.
1.1.2, nylon (nylon) sieve: woven from polyamide monofilament fiber. Wear-resistant, long service life, high strength, good overall effect.
1.1.3, polyester mesh, woven from polyester fiber monofilament. High strength, not easy to deform, but slightly less wear resistance and antistatic effect than nylon mesh.
In the past, most domestic flour mills chose silk screens, which is the use of imported nylon screens, the digestion and absorption of domestic screens, especially the extensive application of bandage machines, ensuring nylon screens for more than a decade. The tension of the time makes the nylon screen popularized and applied. But in some factories, it is still accustomed to use silk screens. As for the polyester screen, it is used less by the flour mill except for the use on the sieve of the powder cleaner. The physical properties of the three materials are shown in Table 1.
Table 1: Comparison of material performance parameters
| Performance / material | Nylon (polyamide) | Polyester (polyester) | silk |
| Tensile strength (Kg/mm2) | 41---67 | 45---75 | 36---45 |
| Stretching degree at break (%) | 20---35 | 15--30 | 15--25 |
| Resilience | 95% to 100% after 3% elongation | 90-95% after 3% elongation | 54-55% after 3% elongation |
| Temperature resistance | Below 100 ° C | Below 140 ° C | 100 ° C becomes brittle |
| Water absorption rate (65% relative humidity, 20 ° C) | 3.8% | 0.4% | 11% |
| Corrosion resistance | it is good | it is good | good |
| Mass specific resistance (Ω/cm.g) | 4.1X1010 | 2X1015 | good |
1.2. Standards and weaving methods of screens
1.2.1. From a historical perspective:
Silk mills have been widely used in foreign flour mills until the 1940s. At that time, the screens used for grading generally had special sieves (14-72) and three sieves (7-72). Species, roughly equivalent to the opening size of 1600-200um; for the screening of sieve powder, there are standard (code S, 3-25S), single material sieve (code X, 4X-17X), double material sieve (code XX , 3-17XX), three materials sieve (code XXX, 3XXX-18XXX) several, roughly equivalent to the mesh size of 300-60um.
From the domestic point of view, the flour mill also commonly uses silkworm mesh, the label is also quoted from foreign industry norms, "GG" (Grit Gauze) refers to the coarse mesh screen, meaning each Vienna inch (about 1.0375 inches) The number of sieve holes in the length, the weaving method is full twisting, and the "XX" refers to the fine mesh screen, which is mainly used for powder screening and semi-twisting.
1.2.2, entered the 1980s, GB2014-80 "Technical requirements for silk, synthetic fiber screen", GBn90∽93-80 "silk, synthetic fiber screen" national standard announcement (now GB/T 14014-92 "Silk, synthetic fiber screen" instead), the domestic screen marking (today) has been unified to the basis of the metric system, such as CB, JQ, JCQ, etc., the number of sieve holes per cm, the material has silk, The nylon and the silkworm are interwoven, and the weaving method has plain weave, semi-twisted weave, full twist weave and square weave.
1.2.3. Since the mid-1980s, with the introduction of imported nylon mesh materials for the production of powder production lines, the long life and good performance have attracted the attention of the industry, and the size of the mesh is directly marked. The opening size has been serialized, standardized, standardized, and the industry title and continuous inertia have been considered, respectively, corresponding to "GG, XXX, milling", where "GG" is plain weave; "XXX" is plain weave enhanced; "milling" It is used exclusively for flour mills and is also the most widely used flour mill. The weaving method is single and double warp weaving (that is, single and double filaments are used in the warp direction). In foreign countries, this weaving method is called “taffetaâ€, but the industry Insiders still follow the generic title: "XX".
It can be seen that the meaning of "GG, XX, XXX" in the name of the imported screen has changed a lot with the familiar domestic screen. The "XXX" has a thick wire diameter, long service life and stable "XX" size. The effective sieving area is slightly larger, and the surface roughening degree is high, and the automatic grading effect is good.
1.2.4. Domestic screen mills have also launched a number of digestion and absorption products, with the code name “JM†(single and double warp weaving) and “JMG†(plain weave), and now also launching a “PDM†( Single and double warp weaving), the specifications are different from the imported screen, but close.
In summary, there are many standards for circulating screens on the market, but it can be seen that the main difference is that the number of mesh holes in the previous length units is more, and the new numbering method is marked with the actual mesh size. It seems more reasonable and more practical, and the current industry awareness is gradually becoming the same as the latter, but due to decades of habits, it can be imagined that the above-mentioned multiple label names will still be mixed for quite some time.
1.3, the selection of flour screen:
1.3.1. Screen opening size: As can be seen from the above, due to the existence of various representation methods, it is preferable to select according to the actual size.
1.3.2, the degree of wear resistance: largely determines the service life of the screen, among all the materials currently used, nylon wear resistance is the best, polyester is only 70%, silk is the worst, the same material In the case of a screen, the thicker the wire diameter, the better the wear resistance.
1.3.3, elongation: From the value of Table 1, the silkworm mesh is obviously lower than polyester and nylon mesh, but now, most polyester and nylon meshes must be heat-set after being braided to prevent mesh deformation as much as possible. Therefore, it is basically close to the use effect of silkworm mesh.
1.3.4 Antistatic property: All flour screens are required to be antistatic. This is because the contact and friction between fibers and fibers, fibers and materials will cause the transfer of charge between the fibers to produce static electricity. Due to the finer grain size of the flour, it is easy to be attracted by the charge to cause sieving difficulties, even the paste net.
Generally, the fiber specific resistance can be used to indicate the antistatic characteristics of the fiber: the fiber specific resistance has three representation methods, the most common of which is the mass ratio resistance (code Rs, which means that a uniform sample is 1 cm long and has an ohm value of 1 g) It is generally considered that its value is less than 109 Ω/cm.g, and the antistatic effect is good. Therefore, it can be seen from Table 1 that polyester and nylon are subjected to antistatic treatment, and generally there are the following methods:
A. Surface treatment: generally treated with an antistatic agent;
B. Polymerization treatment: when the synthetic fiber is polymerized, a hydrophilic polymer or a conductive low molecular polymer is added;
C. Conductive fiber: such as adding carbon in the center of the monofilament or adding carbon in the whole monofilament;
D. The use of high-energy radiation, such as gamma radiation or electron beam radiation fabric, denatures the fiber, improves moisture absorption, and increases antistatic effect.
As can be seen from the above description, the latter three methods are actually anti-static treatment of materials, which is permanent, and the first method belongs to surface anti-static. However, the specific anti-static treatment process for the screens has not been described in detail as a technical secret. Therefore, this has become an important indicator for judging the quality of the screen.
1.3.5. Width: With the development of the milling process and the grain machine technology, especially the emergence of the eight-roll mill and the high-yield high-square sieve, it has indeed created conditions for the improvement of the milling efficiency and the increase of the production capacity. Therefore, the sieve grid is also accompanied by this development direction along the standard, expanded and super-large direction. The width of the imported screen is also 1.10 meters, 1.36 meters, 1.54 meters (the domestic screen width is 1.04-1.27 meters), it must be Choose the appropriate width according to the grid standard used by each factory.
1.3.6. Effective sifting area: refers to the percentage of the opening area of ​​the screen opening in the unit screen. It is worth noting that in the case where the size of the opening of the screen is determined, this value is actually related to the size of the wire diameter and the weaving method.
2, the tension of the screen:
2.1, classification of stretched and corresponding equipment: According to the actual use of the flour mill, from the stretch method and equipment can be divided into: manual method, self-tensioning method, mechanical stretching method, pneumatic stretching method.
2.1.1. Manual method: almost all the mills have used the method, which is completely stretched by the operator experience and work responsibility. Because of its obvious defects, it has been basically abandoned by large and medium-sized flour mills, but in many Small plants are still in use.
2.1.2 Self-tensioning method: It is mainly used for the sieve frame of the powder cleaning machine. With its special structure, the tensioning device of the peripherals is not used, and the meshing of the screen can be achieved only by special pliers. In foreign countries, high-strength screen cores also use this tightening method (designed with special tools and special screen core structure), but the ratio is not high.
2.1.3. Mechanical method: Actually, according to the structure, it can be divided into horizontal tension, vertical top tightness and a hybrid of two methods.
2.1.4. Pneumatic method: There are also two types of horizontal tension and vertical top tightening. The horizontal tension is generally a combined structure. It can be either single or double lattice at the same time. Vertical vertical tightening is mainly introduced. The Occle production line comes from a small number, so the following is an example of a horizontal pneumatic stretcher.
The latter two models have been introduced in professional publications and will not be described in detail here.
2.2. Stretching operation index, testing instrument and measuring method 2.2.1. Measuring the tension, there are special Newton tension meter and millimeter tension meter at home and abroad. Newton's unit is Newton/cm, and the price is high (there was an article in the professional journal that introduced its principle and structure); the millimeter is directly displayed in mm, the test principle is similar to Newton's tension meter, low precision, low price, but There are very few manufacturers in the domestic flour industry.
Of course, it is also possible to use a test method to control the degree of tension by using a relationship in which the elongation of the screen is positively correlated with the tension.
The flour mill using a pneumatic stretcher can also control the tension by adjusting the pressure. Of course, it must be prepared in advance or according to the data provided by the manufacturer of the stretcher. In addition, the air pressure must be adjusted when stretching the net. Only from small to large.
2.2.2. The purpose of the mesh stretcher is to ensure uniform and moderate screen tension. "The reason that the screen mesh is not tight will affect the screening effect" is often known to people, but the hazard of tension is not taken seriously, and there are even misconceptions that the tighter the tension is, so it is necessary to This highlights.
Being too tight will result in:
A. The mesh hole is deformed and the opening size is increased;
B. Because nylon mesh has its own extension limit, excessive stretch will shorten its service life;
C. Stretching too tight will reduce the elasticity of the screen, especially when sieving the powder, it will not play the role of the cleaning block, and it is easy to cause the screen hole to be blocked;
D. Excessive tightening may also cause deformation of the screen core (especially for screen cores with a size of 740mm × 740mm or more), causing leakage.
2.2.3. Summary: Although the process of stretching is seemingly simple, it requires actual and careful operation. Due to differences in screen materials, weaving equipment, technical level and thermal stability of different manufacturers, the recommended parameters shown in Table 2 apply only to SEFAR flour mesh, and the displayed tension values ​​are only applicable to SEFAR tension meters, and It is in an environment of 18-21 ° C and a relative humidity of 50-60%. For other brands of screens and for different seasonal and climatic conditions, please contact us.
Table 2: SEFAR Flour Net Bundling Parameters Recommendation Form
Screen opening (um) | Initial tension (N/cm) | Tension after three hours (N/cm) | Elongation rate (%) |
950---375 | 14---16 | 8--13 | 1 |
350---280 | 12---14 | 7---11 | 2 |
150---250 | 10---12 | 6---9 | 3 |
90---140 | 8--10 | 5---7 | 4 |
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