PBT (Polybutylene Terephthalate) is a typical crystalline engineering plastic widely used in automotive connectors, electronic and electrical housings, and other fields due to its excellent mechanical properties, electrical insulation, and chemical corrosion resistance. However, PBT has strong hygroscopicity, a large crystallization shrinkage rate (1.5%~2.5%), and is prone to hydrolytic degradation at high temperatures. Therefore, optimizing injection molding process parameters and adapting mold design are core to ensuring product quality.
Raw Material PretreatmentThe hygroscopic nature of PBT is the primary issue to address. Insufficiently dried PBT will hydrolyze in the high-temperature barrel, causing molecular chain breakage, reduced mechanical properties, and surface defects such as silver streaks and bubbles. Therefore, raw materials must be dried using hot air or vacuum drying before production to control the moisture content below 0.02%. The drying temperature is typically set at 120–140°C, with a drying time of 4–6 hours. For glass fiber-reinforced PBT, the drying time can be extended to 8 hours to ensure thorough drying.
Optimization of Injection Molding Process ParametersTemperature Control
Segmented barrel temperature setting is critical. The front section (hopper side) temperature is controlled at 220–230°C to prevent premature melting and adhesion; the middle section (plasticizing section) is raised to 240–250°C for complete plasticization; the rear section (nozzle section) is maintained at 240–250°C to prevent melt cooling and nozzle clogging. The overall temperature must be strictly controlled between 245–260°C to avoid exceeding PBT's decomposition temperature (280°C) and causing material degradation. Mold temperature directly affects PBT's crystallinity and dimensional stability, usually set at 60–80°C. Higher mold temperatures promote complete crystallization and reduce post-shrinkage deformation, suitable for precision parts; lower temperatures shorten the molding cycle, ideal for rapid production of appearance parts.
Pressure and Speed ControlInjection pressure is generally controlled at 50–100MPa for standard PBT, and 80–120MPa for glass fiber-reinforced PBT, to ensure sufficient filling without causing flash or internal stress. A segmented holding pressure strategy is recommended: initial holding pressure is 60%–70% of injection pressure, with duration adjusted according to product thickness (1–2 seconds per millimeter). In the later stage, holding pressure is gradually reduced to avoid warping and cracking. Injection speed should be moderate: too fast causes turbulence and air entrainment, leading to weld lines and bubbles; too slow results in premature melt cooling and insufficient filling. Medium-speed injection is preferred, with slightly lower speeds for glass fiber-reinforced PBT to reduce anisotropy caused by uneven fiber orientation.
Mold Design AdaptationGate Design
Large parts use side gates to minimize weld lines; small precision parts use point gates or submarine gates with a minimum diameter of 0.75mm to prevent flow obstruction.
Runner and Venting
Circular cross-sections are preferred for runners to reduce pressure loss. Hot runner systems should be avoided to prevent PBT degradation and leakage. Vent grooves must be 0.02–0.04mm deep and 5–10mm wide, located at the melt flow end to ensure smooth air exhaust without flash formation.
Ejection System
A draft angle of at least 1° is essential to prevent surface scratching during demolding. The ejection system should use uniformly distributed ejector pins or push plates to avoid product deformation due to uneven stress.
Practical Application GuidelinesProcess parameter optimization should be dynamically adjusted based on product characteristics and production requirements. For precision thin-walled parts like automotive connectors, increase mold temperature and holding time to ensure dimensional accuracy and mechanical strength. For electronic housings and other appearance parts, reduce mold temperature to shorten the cycle and improve efficiency. Through the coordinated adaptation of raw material pretreatment, process parameter optimization, and mold design, the performance advantages of PBT can be fully utilized to produce high-quality, stable products.
