Injection Molding Process Parameters for PBT Engineering Plastics

  Processing PBT requires precise control. The melt temperature range is narrow, typically between 250-260°C; temperatures below 240°C can lead to freezing, while exceeding 270°C risks thermal degradation. A barrel temperature profile maintaining around 210°C is recommended, with a mold temperature of 60-80°C. Injection pressure should be set between 100-140 MPa, with holding pressure at 50-60% of the injection pressure. A back pressure of 5-10 MPa helps avoid excessive frictional heat.

High injection speed is crucial due to PBT's rapid solidification and crystallization rates, preventing premature cooling in the flow path. Adequate mold venting is essential to avoid air traps and potential burning. Screw speed should correspond to a maximum linear speed of 0.5 m/s. The dosing stroke is recommended at (0.5-3.5)D, and the total residence time in the barrel should not exceed 5 minutes to prevent thermal degradation. Material must be pre-dried at 120°C for 4 hours.

For regrind usage, flame-retardant grades allow up to 10% recycled content if properly dried, while non-FR grades can incorporate up to 20%. Shrinkage largely depends on mold temperature, ranging from 1.4%-2.0% for unfilled PBT. Adding 30% glass fiber reduces shrinkage to 0.4%-0.6%. Gate design should favor direct or pinpoint gates for glass-filled grades, ensuring balanced filling. Hot runner systems require closed-loop temperature control. Standard screws with non-return valves and open nozzles are suitable. For machine downtime, the heater should be switched off and the barrel purged.