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    2. HaiYi  Plastic

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      The basic parameters of screw
      Time:2009/5/4    The number of readers:2071    【Print
      Generally divided into three sections that screw feeding paragraph, compressed above, all of above.
          Feed above - at the end by the smaller, the main role is to transport raw materials to the latter, it is mainly the issue of transmission capacity, parameters (L1, h1), h1 = (0.12-0.14) D.
          Compression above - at the end by the change, the main role is to compaction, molten materials, the establishment of the pressure. Parameters of the compression ratio ε = h1/h3 and L2. Graded degree of accuracy should be A = (h1-h3) / L2.
          All of paragraph (measurement above) - will be compressed in paragraph quantitative melting temperature materials to screw onto the front of the parameters (L3, h3), h3 = (0.05-0.07) D.
          Screw on the whole, the aspect ratio parameters L/D-
          L / D the pros and cons of: L / D with speed n, is the ability and the effect of plasticizing screw an important factor, L / D is great material to stay in the barrel in a long time in favor of plastics, while the pressure of flow, leakage reduction improved plastics, while the temperature distribution of the materials require a higher benefit, but after the manufacture and assembly to use a negative impact on the general L / D for the (18 ~ 20), but there are increasing trends in .
          Other pitch S, helix angle φ = πDtgφ, the general D = S, then φ = 17 ° 40 '.
          φ affect the ability of plastics, in general φ is bigger faster transmission speed, so the shape of different materials, there are changes in the φ. Desirable powder around φ = 25 °, cylindrical material φ = 17 ° around the box material around φ = 15 °, but different φ, in terms of processing, but also more difficult, so check the general φ 17 ° 40 '.
          Ling W e, on the viscosity of the material in terms of small, e try to get bigger, too easy to drain flow, but too the General Assembly to increase power consumption, easy to overheat, e = (0.08 ~ 0.12) D.
          All in all, in the present circumstances, because of the lack of the necessary test instruments, the design of the screw and there is no means complete design. Most of all, depending on the nature of materials, to use their experience to develop parameters to meet the different needs, the plant or less the same.
      1. PC material (polycarbonate)
           Characteristics: ① non-crystalline plastic, no obvious melting point, glass transition temperature of 140 ° ~ 150 ℃, melting temperature 215 ℃ ~ 225 ℃, molding temperature 250 ℃ ~ 320 ℃.
           ② viscosity, temperature sensitive, in the normal processing temperature range of good heat stability, 300 ℃ basic non-stop long-term decomposition, more than the beginning of decomposition of 340 ℃, viscosity less affected by shear rate.
           ③ strong water absorption
           Selected parameters:
           a. L / D for its thermal stability, and viscosity characteristics, as far as possible to improve the plasticization effect of large aspect ratio select, factory from 26.
           Melting due to its wide temperature range, compression can be longer, so the use of gradient-type screw. L1 = 30% full-length, L2 = 46% full length.
           b. Compression ratio ε A degree from the gradual melting rate would need to adapt, but the melting rate is not calculated, according to PC from 225 ℃ to 320 ℃ between melt processability characteristics, and its gradient value relative degree A middle-check The value of bias in the case of the larger L2 ordinary screw-type gradient ε = 2 ~ 3, from 2.6 factory.
           c. Because of its high viscosity, water absorption, and all of it in before paragraph, after paragraph compression increase mixing in the screw structure, in order to enhance the dissolution of the solid bed at the same time, which allows water entrained into the gas escape.
           d. Other parameters such as e, s, φ, as well as the gap between the barrel and can be the same with other ordinary screw.
      二. PMMA (polymethyl methacrylate)
           Characteristics: ① glass transition temperature of 105 ℃, melting temperature greater than 160 ℃, decomposition temperature 270 ℃, forming a wide temperature range.
           ② viscosity, poor mobility, better thermal stability.
           ③ highly absorbent.
           Preferences
           a. L / D to select the aspect ratio of 20 to 22 of gradient-type screw, depending on the accuracy of its products generally forming L1 = 40%, L2 = 40%.
           b. Compression ratio ε, generally 2.3 to 2.6 selected.
           c. Some for its hydrophilicity, it is used in front-end mixing screw ring structure.
           d. Other parameters in general can design general-purpose screw, barrel and can not be too small space.
      三. PA (nylon)
           Characteristics: ① crystalline plastics, more and different types, and its not the same as the melting point and narrow melting point range generally used in PA66 melting point of 260 ℃ ~ 265 ℃.
           ② low viscosity, mobility, and there are more obvious melting point, poor thermal stability.
           ③ general absorbent.
           Preferences
           a. L / D to select the aspect ratio of 18 to 20 screw-type mutations.
           b. Compression ratio is generally selected from 3 to 3.5, which break down to prevent overheating h3 = 0.07 ~ 0.08D.
           c. Because of its low viscosity, it止逆barrel ring with a small gap should be about 0.05, screw and barrel gap of about 0.08 and, if necessary, depending on the material, the front-end can be止逆with Central, Nozzle should be self-locking.
           d. Other parameters may be universal screw design.
      四. PET (polyester)
           Characteristics: ① melting point of 250 ℃ ~ 260 ℃, blow molding grade PET is a wider point temperature about 255 ℃ ~ 290 ℃.
           ② PET blow molding grade high viscosity, the temperature influence on viscosity, poor thermal stability.
           Preferences
           ① L / D in general with 20, three sections of the distribution of L1 = 50% -55%, L2 = 20%.
           ② the use of low shear and low compression ratio of the screw, the compression ratio ε, general check of 1.8 ~ 2, at the same time lead to discoloration or overheating cut opaque h3 = 0.09D.
           ③ no mixing screw the front ring to prevent overheating, possession of materials.
           ④ because this material is more sensitive to temperature, and general manufacturers use recycled materials, in order to increase production, our plant is used in low shear screw, so there would be appropriate to improve motor speed in order to achieve the objectives. At the same time, the use of recycled materials (the majority of film materials), plant according to the actual situation, in order to increase the transmission capacity of the feed above, have also been taken to increase caliber blank cartridges in the slot machine, etc., and achieved better results.
      五. PVC (polyvinyl chloride)
           Heat-sensitive materials, generally divided into hard and soft, the difference between the raw materials added to the amount of plasticizer, 10 percent less than for the rigid, more than 30% for the soft.
           Characteristics: ① no obvious melting point, 60 ℃ soft, 100 ℃ ~ 150 ℃ viscoelastic state, 140 ℃ when the melting, decomposition at the same time, 170 ℃ decompose rapidly, softening point closer to the decomposition point, the release of decomposition gases in HC1.
           ② poor thermal stability, temperature, time will lead to decomposition of poor liquidity.
           A. Design Principles Strict temperature control, screw as much as possible the design of low shear to prevent overheating.
                   b. Screw, barrel to be anti-corrosion.
                   c. Injection process to be strictly controlled.
           Generally speaking, the screw parameters for L / D = 16 ~ 20, h3 = 0.07D, ε = 1.6 ~ 2, L1 = 40%, L2 = 40%.
           To prevent the possession of material, non-止逆ring, the head taper 20 ° ~ 30 °, than to adapt to the soft, such as the products require a higher, above the measurement can be no separation-type screw, which screw on the rigid PVC more appropriate and in line with temperature control, feeding increases above the internal screw cooling water or the hole, plus cold water barrel or tank, temperature control accuracy of about ± 2 ℃.
       
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