Kunshan Xuan Yida machinery introduction: PVC material molding conditions are what? How is it processed?
PVC material is now more and more seen in life, modern life is obviously inseparable from his existence, so PVC material used in our raw materials of so many products, what is his molding condition? What is the processing method? Let's take a look.
A, PVC material forming conditions
1, hard PVC
Feed tube temperature: 160-190℃
Mold temperature: 40-60℃
Drying temperature: 80℃×2h
Injection pressure: 700-1500kg/cm2
Density: 1.4 g/cm3
Forming shrinkage: 0.1-0.5%
Thick meat: 2.0-50 mm
Water absorption (24h) : 0.1-0.4%
Melting degree softening point: 89℃
Thermal deformation temperature: 70℃
2, soft PVC
Feed tube temperature: 140-170℃
Mold temperature: 40-60℃
Drying temperature: 80℃×2h
Injection pressure: 600-1500 kg/cm2
Density: 1.4g /cm3
Forming shrinkage: 0.1-0.5%
Thick meat: 2.0-50 mm
Water absorption (24h) : 0.1-0.4%
Melting degree softening point: 85℃
Thermal deformation temperature: 55℃
Two, PVC material processing
1, PVC plastic forms are different, very different, processing methods are also diverse, can be pressed, extrusion, injection, coating, etc. Polyvinyl chloride resin particle size, fisheye, loose density, purity, foreign impurities, porosity have an impact on processing; The viscosity and gelatinizing properties of the paste should be considered.
2. Polyvinyl chloride is an amorphous polymer with small shrinkage. Powder should be preheated before processing to remove water, enhance the plasticizing effect, prevent bubbles. And PVC is easy to decompose, especially in high temperature and steel, copper contact easier decomposition (decomposition temperature 200 degrees). Forming temperature range is small, must strictly control the material temperature. When using screw injection machine and through nozzle, the aperture should be large to prevent the dead Angle stagnation. The mold pouring system should be thick, the gate section should be large, the mold should be cooled, the mold temperature is 30-60℃, the material temperature is 160-190℃.
3. Under the glass transition temperature (Tg, 80℃), polyvinyl chloride is glassy; In the Tg→ viscous flow temperature (Tf, about 160℃) is a high elastic rubber, plasticity; At Tf→ thermal decomposition temperature (Td), the flow is viscous. The higher the temperature, the easier the flow is. When the temperature exceeds Td, PVC dissolves a large amount of hydrogen chloride (HCl), and the material loses its chemical stability and physical properties, so Td is the upper limit temperature for processing and molding. Due to the high intermolecular force of polyethylene, Tf is very high, even close to the decomposition temperature, so it is necessary to add plasticizer to reduce Tf. On the other hand, it is also necessary to add stabilizer, so as to improve the Td of PVC, in order to be processed.
4. The glass transition temperature (Tg) is only related to the structure of the molecular chain segment, and has little relationship with the molecular weight, while the viscosity flow temperature (Tf) is the temperature at which the macromolecule starts to move, and is related to the molecular weight. The larger the molecular weight, the higher the Tf. For some processes, such as injection molding, it is necessary to reduce the molecular weight of the resin. According to the size of the different molecular weight, domestic suspended polyvinyl chloride resin is divided into 1-7 levels, the larger the serial number, the smaller the molecular weight. XJ-4 (XS-4) to XJ-7 (XS-7) resins are often used in the manufacture of hard tubes and hard plates, etc. Other resins with lower molecular weight need to add a large amount of plasticizer to reduce Tf due to high Tf, so they are often used in the manufacture of soft products. PVC with average polymerization degree below 1000 is called low polymerization degree PVC, which has better processing performance. In the processing process, less plasticizer can be added, so as not to accelerate the aging of products due to the migration of plasticizer. Polyvinyl chloride products with low degree of polymerization have good transparency, widely used in building materials, food and drug packaging materials and instead of plexiglass products.
5. Polyvinyl chloride melt is a non-Newtonian pseudo-fluid. The larger the shear velocity, the smaller the apparent viscosity, and the change is quite sensitive. Increase the temperature, viscosity is not reduced much, even if the plastic is below the decomposition temperature, but because of a long time in a higher temperature, it will also heat and oxidation degradation phenomenon, and affect its performance. Therefore, increasing shear rate (increasing pressure) should be considered to improve the fluidity of PVC melt. In fact, increasing the external force helps the movement of macromolecules, so that the Tf is reduced and the macromolecules can flow at a lower temperature.
