PBT Injection Molding: Key Properties and Important Considerations

Polybutylene terephthalate (PBT) is a versatile engineering plastic widely used in electronics, automotive, and industrial applications. Its excellent electrical insulation and good flow properties make it a popular choice — especially when reinforced with glass fiber.

Below is a practical guide to PBT’s key properties and important considerations for injection molding.

1. Key Properties of PBT

①PBT appears as a milky white to light yellow material with a glossy surface. Its density ranges from 1.31 to 1.55 g/cm³, and its typical molecular weight is between 30,000 and 40,000.

②Neat PBT resin has relatively low mechanical properties. However, when modified with glass fiber reinforcement, PBT exhibits high tensile strength, high flexural strength, and an elevated heat deflection temperature — reaching up to 203°C with 30% glass fiber reinforcement. It also has a low coefficient of friction and good wear resistance. On the other hand, it is relatively notch‑sensitive and has low notched impact strength.

③The most outstanding property of PBT is its excellent electrical insulation. It maintains good electrical insulating performance even in humid, high‑temperature, high‑frequency, or harsh environments. This makes PBT highly valuable in the electrical and electronics industries.

④Glass‑fiber‑reinforced PBT has a melting point (Tm) of approximately 225°C. The melt crystallizes relatively easily and at a fast rate.

⑤PBT melt has low viscosity and excellent flowability, providing sufficient flow length for molding various parts — including thin‑wall products, complex shapes, and parts with long flow paths.

2. Important Considerations for PBT Injection Molding

① Drying is essential

Although PBT absorbs very little moisture, even trace amounts can cause hydrolysis at high temperatures, making parts brittle. Therefore, PBT must be dried before molding to a moisture content below 0.03%. A hot air oven set to about 130°C for 3–4 hours is typically sufficient.

② Use fast injection speed

PBT crystallizes and solidifies quickly. Even at room temperature, the mold can produce parts with high crystallinity. To prevent short shots, weld lines, or rough surfaces, use fast injection speeds. This also shortens cycle time and improves production efficiency.

③ Avoid overheating

If PBT melt stays above 270°C for too long, thermal degradation can occur, causing a drop in mechanical properties.

④ Watch for shrinkage

Unfilled PBT has a molding shrinkage of about 0.9%, while 30% glass‑fiber‑reinforced PBT shrinks about 0.4%. This should be considered during mold design.

⑤ Temperature guidelines

A screw‑type injection molding machine is recommended. The barrel temperature should be controlled between 230°C and 270°C. Higher temperatures may reduce tensile strength. For mold temperature:

• Unreinforced PBT: around 60°C

• Glass‑fiber‑reinforced PBT: around 80°C

These mold temperatures help produce parts with high surface gloss and also facilitate demolding.

⑥ Injection pressure

Injection pressure should not be too high — otherwise, flash or demolding difficulties may occur. A typical range is 50–100 MPa.