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What are the process conditions for PBT injection molding?

Postview: 757 Date: July 20, 2019

[Introduction] PBT has less hygroscopicity, but it is sensitive to moisture at high temperatures. PBT molecules are degraded during molding, the color becomes darker, and the surface is stained, so it should be dried.

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PBT injection molding parts

PBT molding processabilities:

1) PBT has less hygroscopicity, but the moisture at high temperature is more sensitive. When the molding process is performed, the PBT molecules will be degraded, the color will become darker, and the surface will be stained, so it should be dried.

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

3) PBT has a distinct melting point. When the temperature rises above the melting point, the fluidity will suddenly increase, especially attention.

4) The PBT molding processing range is narrow, the crystallization is quick when cooling, and the fluidity is good, which is especially suitable for rapid injection.

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

6) PBT is very sensitive to the reaction of notches and sharp corners. Stress concentration is easy to occur at these locations, which greatly reduces the bearing capacity and is prone to cracking when subjected to stress or impact. Therefore, attention should be paid to the design of 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 articles with smaller depressions can be forced to release from the mold. However, after being filled with glass fiber or filler, the elongation is greatly lowered, and forced release may not be performed if there is a depression in the product.

8) The flow path of the PBT mold is preferably short and thick when possible, and the circular flow path is the best. Generally, both modified and unmodified PBT can be used in ordinary flow channels, but glass fiber reinforced PBT can be applied to hot runner molding to have good effects.

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

10) The gate is preferably facing the core cavity or core so as to avoid jetting and to minimize the replenishment of the melt as it flows through the cavity. Otherwise, the article is prone to surface defects and degrades performance.

PBT’s main injection molding conditions

1) Barrel temperature. The choice of barrel temperature is very important for the formation of PBT. For example, if the temperature is too low and the plasticization is poor, it will cause defects such as material shortage, dent, uneven shrinkage and dullness. If the temperature is too high, the nozzle will be severely flowed, overflowing, darkening and even degrading. Generally, the barrel temperature is controlled at 240~280°C, and the glass fiber reinforced PBT is controlled at 230~260°C. The nozzle temperature should be controlled below the temperature of the front section of the barrel by 5~10 °C.

2) Mold temperature. The mold temperature is directly related to the dimensional stability, warpage, molding cycle and crystallinity of the product. PBT is easy to crystallize, even if it crystallizes rapidly at normal temperature, so the mold temperature does not need to be too high, usually 40 to 60 ° C, while the glass fiber reinforced PBT has a slightly higher mold temperature, usually 60 to 80 ° C.

3) Injection pressure. PBT melt viscosity is low, fluidity is good, moderate injection pressure can be used, generally 60~90MPa, glass fiber reinforced PBT is 80~100MPa, usually the injection pressure increases with the increase of plastic part thickness, but not more than 100MPa Otherwise it will make demoulding difficult.