Molding performance of PBT
1) PBT has low hygroscopicity, but it is sensitive to moisture at high temperatures. During the molding process, the PBT molecules will be degraded, the color will become darker, and scars will appear on the surface. Therefore, it should usually be dried.
2) PBT melt has excellent fluidity, so it is easy to form thin-walled and complex-shaped products, but attention should be paid to mold overflow and nozzle drooling.
3) PBT has a distinct melting point. When the temperature rises above the melting point, its fluidity will increase suddenly, so special attention should be paid.
4) PBT has a narrow molding processing range, crystallizes quickly during cooling, and has good fluidity, making it particularly suitable for rapid injection.
5) PBT has a larger shrinkage rate and shrinkage range, and the difference in shrinkage rate in different directions is more obvious than other plastics.
6) PBT is very sensitive to notches and sharp corners. Stress concentration is likely to occur at these locations, which greatly reduces the bearing capacity and makes it easy to break when subjected to force or impact. Therefore, this should be taken into account when designing plastic parts. All corners, especially internal corners, should use arc transitions as much as possible.
7) Pure PBT has an elongation of up to 200%, so products with small dents can be forcibly demolded from the mold. However, after being filled with glass fiber or fillers, its elongation is greatly reduced, and if there are dents in the product, forced demolding cannot be implemented.
8) When possible, the runners of PBT molds should be short and thick, and round runners are the best. Generally, both modified and unmodified PBT can use ordinary runners, but glass fiber reinforced PBT can only achieve good results when hot runner molding is used.
9) Point gate and latent gate have great shearing effect, which can reduce the apparent viscosity of PBT melt and facilitate molding. They are commonly used gates, and the gate diameter should be larger.
10) It is best to have the gate facing the core cavity or core to avoid spraying and minimize the backfill of the melt when it flows in the mold cavity. Otherwise, the product is prone to surface defects and performance degradation.
Main injection molding conditions of PBT
1) Barrel temperature. The selection of barrel temperature is crucial for PBT molding. If the temperature is too low, poor plasticization can result in product defects such as missing material, dents, uneven shrinkage, and dullness. Excessive temperature can cause severe nozzle drooling, overflow, darkening of the color, and even degradation. Typically, the barrel temperature is controlled between 240ยฐC and 280ยฐC, and for glass-fiber-reinforced PBT, between 230ยฐC and 260ยฐC. The nozzle temperature should be 5ยฐC to 10ยฐC lower than the temperature at the front end of the barrel.
2) Mold temperature. Mold temperature is directly related to the product’s dimensional stability, warpage, molding cycle, and crystallinity. PBT crystallizes quickly even at room temperature, so mold temperatures do not need to be too high, typically between 40ยฐC and 60ยฐC. For glass fiber-reinforced PBT, mold temperatures are slightly higher, typically between 60ยฐC and 80ยฐC.
3) Injection pressure. PBT melt has low viscosity and good fluidity, so a moderate injection pressure can be used, generally 60~90MPa. For glass fiber reinforced PBT, it is 80~100MPa. Usually, the injection pressure increases with the thickness of the plastic part, but it should not exceed 100MPa, otherwise it will make demolding difficult.
