PVC material processing

- May 11, 2018-

Polyvinyl chloride plastics vary in shape and vary greatly. Processing methods are also varied and can be suppressed, extruded, injected, and coated. Polyvinyl chloride resin particle size, fisheye, bulk density, purity, foreign impurities, porosity have an impact on processability; paste resin should consider the paste viscosity and gelation properties.

Polyvinyl chloride is an amorphous polymer and has a small shrinkage. Before the powder processing, it should be preheated to remove moisture, enhance the plasticizing effect and prevent air bubbles. Moreover, PVC is very easy to decompose, especially in contact with steel and copper at high temperatures, which is more easily decomposed (decomposition temperature 200 degrees). The molding temperature range is small and the material temperature must be strictly controlled. When using a screw-type injection machine and a straight-through nozzle, the hole diameter should be large to prevent dead material from stagnating. The mold casting system should be large, the gate section should be large, the mold should be cooled, the mold temperature is 30-60 degrees, and the material temperature is 160-190.

Below the glass transition temperature (Tg, 80°C), polyvinyl chloride is glassy; at Tg → viscous flow temperature (Tf, about 160°C), it is highly elastic rubbery and has plasticity; at Tf → thermal decomposition temperature (Td) For viscous flow, the higher the temperature, the easier it is to flow. When the temperature exceeds Td, PVC decomposes a large amount of hydrogen chloride (HCl) and the material loses its chemical stability and physical properties. Therefore, Td is the upper limit temperature for processing and forming. Due to the large intermolecular forces of polyethylene, Tf is very high, and even close to the decomposition temperature, it is necessary to add plasticizers to lower the Tf. On the other hand, it is also necessary to add a stabilizer so as to increase the Td of the PVC in order to carry out processing and shaping.