Process conditions of PBT injection molding

Process conditions of PBT injection molding

Processing properties of PBT

1) PBT has less hygroscopicity, but it is sensitive to water at high temperature. During the molding process, PBT molecules will be degraded, the color will be darker, and the surface will produce spots, so it should be dried.
2) PBT melt has excellent fluidity, so it is easy to form thin-walled and complex products, but pay attention to die overflow and nozzle salivation.
3) PBT has an obvious melting point. When the temperature rises above the melting point, the fluidity of PBT will increase sharply, especially attention should be paid.
4) PBT is suitable for rapid injection because of its narrow processing range, fast crystallization and good fluidity.
5) PBT has a large shrinkage and its shrinkage range, and the shrinkage difference in different directions is more obvious than other plastics.
6) PBT is very sensitive to the response of notches and sharp corners. In these locations, stress concentration is easy to occur, which makes the bearing capacity greatly reduced, and it is easy to crack when subjected to force or impact. Therefore, attention should be paid to this point in the design of plastic parts. Circular arc transition should be adopted at all corners, especially the internal corners.
7) The elongation of pure PBT can reach 200%, so the products with small depression can be forced to demould from the mold. However, after filling with glass fiber or filler, its elongation is greatly reduced. If there is a depression in the products, forced demoulding cannot be carried out.
8) When possible, the channel of PBT die is shorter and thicker, and the effect of circular channel is the best. General modified and unmodified PBT can be used in common flow channel, but glass fiber reinforced PBT can be used in hot flow channel molding.
9) Point gate and latent gate have great shear effect, which can reduce the apparent viscosity of PBT melt, and is beneficial to molding. They are often used gates, and the gate diameter should be larger.
10) The gate is preferably facing the core cavity or core, so as to avoid spraying and minimize the replenishment when the molten material flows in the cavity. Otherwise, the product is easy to produce surface defects, and performance degradation.
Main injection molding conditions of PBT
1) Barrel temperature. The selection of barrel temperature is very important for PBT molding, such as low temperature and poor plasticization, which will result in material shortage, depression, uneven shrinkage and lusterless. If the temperature is too high, the nozzle will salivate seriously, overflow the edge, darken the color and even degrade. Generally, the barrel temperature is controlled at 240-280 ℃, and the glass fiber-reinforced PBT is controlled at 230-260 ℃. The temperature of the nozzle should be controlled at 5 ~ 10 ℃ lower than that of the front section of the barrel.
2) Mold temperature. Mold temperature is directly related to dimensional stability, warpage, molding cycle and crystallinity of products. PBT is easy to crystallize, even at room temperature, it also crystallizes quickly, so the mold temperature does not need to be too high, usually 40 ~ 60 ℃, while the mold temperature of glass fiber reinforced PBT is slightly higher, usually 60 ~ 80 ℃.
3) Injection pressure. The melt viscosity of PBT is low and its fluidity is good. Medium injection pressure can be used, generally 60-90mpa, 80-100mpa for glass fiber reinforced PBT. Generally, the injection pressure increases with the increase of the thickness of the plastic part, but not more than 100MPa, otherwise it will make the demoulding difficult.