Precautions for PBT Plastic Injection Molding

PBT (Polybutylene Terephthalate) is a common engineering plastic with high strength, fatigue resistance, dimensional stability, and excellent electrical properties. It is widely used in connectors, coil bobbins, automotive parts, and electronic structural components, especially when reinforced with glass fiber. However, PBT has fast crystallization, high hygroscopicity, and temperature sensitivity. Improper parameter control during injection easily causes defects such as shrinkage, bubbles, warpage, weak weld lines, yellowing, and brittleness. This article details key precautions from material handling, mold/equipment requirements, process parameters, and defect control for on-site reference.

Material Drying and Handling

PBT is highly hygroscopic; even visually dry material contains moisture, which causes hydrolysis at high temperatures, reducing product strength and creating silver streaks/bubbles. Dry thoroughly before processing: 110–120°C for 3–4 hours.

For glass-fiber-reinforced PBT (PBT+GF), extend drying time to 4 hours to avoid delamination and fiber floating at the resin-fiber interface.

Use dried material within 2 hours; re-dry if exposed to air for too long. Equip the hopper with a hot air insulation device to prevent reabsorption.

Recycled material should not exceed 20% of the total, mixed uniformly with new material to avoid performance fluctuations.

Mold and Equipment Requirements

Design adequate mold venting, especially at flow ends, deep cavities, and insert peripheries. PBT melt flows fast; trapped gas causes burning and insufficient filling.

Ensure sufficient runner and gate size. PBT has good flowability, but reinforced grades have higher viscosity—small gates cause shear overheating and material degradation/yellowing.

Mold temperature significantly affects crystallization and warpage: 40–60°C for ordinary PBT; 60–80°C for reinforced PBT. Uniform mold temperature reduces warpage.

Use a balanced ejection system. PBT has high rigidity and slight brittleness; unbalanced ejection causes whitening and cracking.

A general three-stage screw is suitable: length-diameter ratio (L/D) = 18–22, compression ratio = 2.5–3.0. Avoid strong shear to prevent overheating and degradation.

Injection Process Parameter Settings

1. Barrel Temperature

Rear section: 210–220°C

Middle section: 230–240°C

Front section: 220–230°C

Nozzle: ~230°C

Excessively high temperature causes degradation, discoloration, and brittleness; low temperature causes uneven plastification, insufficient filling, and weak weld lines.

2. Injection Speed and Pressure

Use medium speed for pure PBT to avoid turbulence; medium-low speed for PBT+GF to reduce fiber floating and jetting.

Injection pressure: 60–90 MPa, adjusted by part thickness and flow length (higher for thin-walled, long-flow parts). Avoid excessive pressure to reduce internal stress.

Accurate V/P switchover: Too late causes flash; too early causes shrinkage and surface depression.

3. Packing and Cooling

Packing pressure: 50–70% of injection pressure; packing time: 5–10 seconds (determined by gate freeze-off) to prevent shrinkage.

Cooling time: 10–25 seconds, based on part thickness. Insufficient cooling causes warpage; excessive cooling reduces efficiency.

PBT crystallizes fast; uniform cooling ensures dimensional stability—design symmetric, unobstructed mold cooling channels.

4. Screw Speed and Back Pressure

Screw speed: 50–90 rpm (lower for PBT+GF) to prevent shear overheating and yellowing.

Back pressure: 3–8 MPa to ensure uniform plastification and reduce bubbles; slightly higher for reinforced grades to improve resin-fiber mixing.

Common Defects and Solutions

Warpage: Caused by uneven crystallization, cooling, or fiber orientation. Solution: Increase/uniformize mold temperature, extend cooling time, adjust packing, optimize gate position, and reduce injection speed if needed.

Fiber Floating/Rough Surface (PBT+GF): Caused by excessive shear, high speed, or low mold temperature. Solution: Reduce injection speed, increase mold/melt temperature, optimize gate shape, and increase back pressure.

Silver Streaks/Bubbles: Caused by insufficient drying, overheating, or poor venting. Solution: Extend drying time, lower barrel temperature, enhance venting, and check for nozzle leakage.

Weak Weld Lines: Caused by low mold/melt temperature or slow injection speed. Solution: Increase temperatures and speed, optimize venting, and avoid long multi-gate convergence.

Yellowing/Brittleness: Caused by overheating, long residence time, or strong shear. Solution: Lower barrel temperature, reduce residence time, decrease speed/back pressure, and clean material buildup.

On-Site Production Notes

Clean the barrel before startup, especially if previously used for PVC or POM (easily decomposed materials) to avoid contamination.

Empty the barrel or clean with PE/PP before shutdown to prevent residual PBT from decomposing and corroding the screw.

Post-treatment: Anneal high-precision parts at 120°C for 1–2 hours to eliminate internal stress and reduce deformation.

Ensure good ventilation—PBT emits minimal decomposition odors at high temperatures, but long-term production requires air circulation.

Conduct small-batch trial runs before mass production to confirm appearance, dimensions, and strength.

Conclusion

The core of PBT injection molding is: thorough material drying, moderate melt temperature (no decomposition), uniform mold temperature (reduce warpage), reasonable injection speed (avoid fiber floating), and adequate packing/cooling (control dimensions). Reinforced PBT requires extra attention to shear control and temperature matching. Mastering these key points ensures efficient, stable molding of high-quality PBT products with good appearance, strength, and dimensional stability.