April 17, 2020

Practical experience in selection of resin for several commonly used plastic injection

PVC:

PVC is thermoplastic, generally divided into hard and soft, the difference is how much plasticizer is added to the raw material, less than 10% is hard and more than 30% is soft.

 

Features:

  • There is no obvious melting point, it becomes soft at 60 ℃, and it is viscoelastic at 100 ~ 150 ℃. It melts at 140 ℃ and decomposes at the same time. It decomposes rapidly at 170 ℃. The softening point is close to the decomposition point, and the decomposition releases HCl gas.
  • Poor thermal stability, temperature, time will cause decomposition, poor fluidity.

 

The following points should be noted for the screw of injection molded PVC:

  1. Strict temperature control, screw design should be as low as possible to prevent overheating
  2. Screws and barrels must be protected against corrosion
  3. The injection molding process needs to be strictly controlled

 

Generally, the screw parameters are L/D = 16~20, h3 = 0.07D, ε = 1.6~2, L1 = 40% (full length), L2 = 40% (full length). In order to prevent material storage, there is no non-return ring, and the head taper is 20 °~30 °, which is more suitable for soft resin. If the product requirements are higher, a non-metering zoon, separate screw can be used. In addition, in order to cooperate with temperature control, cooling water or oil holes are added inside the screw of the feeding section, and cold water or oil groove is added outside the barrel. The temperature control accuracy is about ± 2 ℃.

 

PC:

Features:

  • Amorphous plastic, no obvious melting point, glass transition temperature 140~150 ℃, melting temperature 215~225 ℃, forming temperature 250~320 ℃
  • High viscosity, sensitive to temperature, good thermal stability in the normal processing temperature range, stays at 300 ℃ for a long time, it will not decompose, and it will decompose when it exceeds 340 ℃
  • Strong water absorption

 

The following points should be noted for the screw of injection PC:

  1. L/D: In view of its good thermal stability and high viscosity characteristics, in order to improve the plasticizing effect, try to choose a large aspect ratio, generally 26. Due to its wide melting temperature range and long compression, a gradient screw is used. L1 = 30% (full length), L2 = 46% (full length)
  2. Compression ratio ε: The gradient A needs to be adapted to the melting rate, but the current melting rate cannot be accurately calculated by express delivery. According to the processing characteristics of PC melting from 225℃ to 320℃, the value of the gradient A take the upper-middle value. In the case where L2 is large, the ordinary gradient screw ε = 2~3, A generally takes 2.6.
  3. Due to its high viscosity and strong water absorption, a kneading structure is added to the screw before the homogenization section and after the compression section to strengthen the disintegration of the solid bed, and at the same time, the moisture contained therein can be turned into gas to escape
  4. Other parameters such as e, s, φ and the gap with the barrel can be the same as other ordinary screws

 

PMMA

Features:

  • PMMA temperature is 105℃, melting temperature greater than 160 ℃, decomposition temperature 270 ℃, wide forming temperature range
  • High viscosity, poor fluidity, good thermal stability
  • Strong water absorption

 

Pay attention to the following points when injection molding PMMA screw:

  1. L/D selects the gradient type screw with the length-diameter ratio of 20 to 22, according to the accuracy requirements of its product forming, generally L1 = 40%, L2 = 40%
  2. The compression ratio ε is generally selected from 2.3 to 2.6.
  3. It has a certain hydrophilicity, so the mixing ring structure is used at the front of the screw.
  4. Other parameters can generally be designed according to the general screw, and the gap with the barrel should not be too small.

 

PA:

Features:

  • There are many kinds of crystalline plastics, different types, different melting points, and narrow melting point range. Generally, the melting point of PA66 is 260~265 ℃.
  • Low viscosity, good fluidity, obvious melting point, poor thermal stability
  • General water absorption

 

Pay attention to the following points for the injection screw of PA:

  1. L/D selects mutant screw with length-diameter ratio 18~
  2. The compression ratio is generally selected from 3 to 3.5, which prevents overheating decomposition h3 = (0.07 to 0.08) D.
  3. Due to its low viscosity, the gap between the non-return valve and the barrel should be as small as possible, about 0.05, the gap between the screw and the barrel is about 0.08, if necessary, depending on the material, the front can be equipped with a non-return valve, and the nozzle should be self-locking
  4. Other parameters can be designed according to general screw

 

PET:

Features:

  • Melting point is 250~260 ℃, blow molding PET has a wider forming temperature, about 255℃~290℃
  • Blow molding PET has high viscosity, temperature has great influence on viscosity, and poor thermal stability

 

Pay attention to the following points when injecting PET screw:

  1. L/D is generally 20, three distribution L1 = 50%~55%, L2 = 20%.
  2. The screw with low shear and low compression ratio is adopted, and the compression ratio ε is generally 1.8 to 2, and the overheating of the shear causes discoloration or opacity, h3 = 0.09D
  3. There is no mixing ring at the front of the screw to prevent overheating and material storage
  4. Because this kind of material is sensitive to temperature, and general manufacturers use recycled materials, in order to increase production, low shear screws are generally used, so the motor speed can be appropriately increased to achieve the purpose. At the same time, in terms of the use of recycled materials (most of which are sheet materials), according to the actual situation, in order to increase the conveying capacity of the feeding section, methods such as increasing the diameter of the blanking and grooving in the barrel have been adopted to achieve better effect.
Injection Molding, Plastic Mold Case, Plastic Molding Design, Plastic Parts
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