PBT processing characteristics and successful injection molding conditions

2020/11/30

PBT characteristics:

1) PBT has low hygroscopicity, but it is more sensitive to moisture at high temperatures. It will degrade the PBT molecules during the molding process, darken the color and produce spots on the surface, so it should usually be dried.

2) PBT melt has excellent fluidity, so it is easy to form thin-walled, complex-shaped products, but pay attention to mold flashing and nozzle drooling.

3) PBT has an obvious melting point. When the temperature rises above the melting point, the fluidity will increase suddenly, so attention should be paid to it.

4) PBT has a narrow molding processing range, crystallizes quickly when cooling, and good fluidity, especially suitable for rapid injection.

5) PBT has a larger shrinkage rate and shrinkage range, and the difference in shrinkage rates in different directions is more obvious than other plastics.

6) PBT is very sensitive to the response of notches and sharp corners. Stress concentration is likely to occur at these positions, which greatly reduces the load-bearing capacity. It is prone to rupture when subjected to force or impact. Therefore, this should be paid attention to when designing plastic parts. All corners, especially the internal corners, should use arc transitions as much as possible.

7) The elongation of pure PBT can reach 200%, so products with smaller depressions can be forced to demold from the mold. However, after filling with glass fiber or filler, its elongation is greatly reduced, and if there are depressions in the product, forced demoulding cannot be implemented.

8) The runner of the PBT mold should be short and thick if possible, and the round runner will have the best effect. Generally, both modified and unmodified PBT can be used with ordinary runners, but glass fiber-reinforced PBT can only have good results when hot runner molding is used.

9) The point gate and the latent gate have a large shearing effect, which can reduce the apparent viscosity of the PBT melt, which is conducive to molding. It is a frequently used gate, and the gate diameter should be larger.

10) The gate is best to face the core cavity or core, so as to avoid spraying and minimize the filling of the melt when flowing in the cavity. Otherwise, the product is prone to surface defects and deteriorates performance.


Main injection molding conditions of PBT:

1) The temperature of the barrel. The selection of barrel temperature is very important for PBT molding. If the temperature is too low, poor plasticization will cause the product to lack material, dent, uneven shrinkage and dullness. If the temperature is too high, it will cause serious salivation of the nozzle, overflow, darken color, and even degrade. Generally, the temperature of the barrel 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 the temperature of the front part of the barrel.

2) Mold temperature. The mold temperature is directly related to the dimensional stability, warpage deformation, molding cycle and crystallinity of the product. PBT is easy to crystallize, and it crystallizes quickly even at room temperature, 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. PBT has low melt viscosity and good fluidity. Medium injection pressure can be used, generally 60~90MPa, and glass fiber reinforced PBT is 80~100MPa. Usually the injection pressure increases with the increase of the thickness of the plastic part, but should not exceed 100MPa , Otherwise it will make demoulding difficult.