1, PBT process characteristics
PBT has a distinct melting point, melting point of 225 ~ 235 °C, is a crystalline material, crystallinity up to 40%.
The viscosity of PBT melt is not as great as the shear stress due to the influence of temperature. Therefore, in the injection molding, the influence of injection pressure on the melt flow of PBT is obvious.
PBT has good fluidity in the molten state and low viscosity, second only to nylon, and it is prone to "casting" during molding.
PBT molded article anisotropy. PBT is easily degraded when exposed to water at high temperatures.
2, injection molding machine
When using screw injection molding machine. Consider the following points.
1 The amount of the used product should be controlled at 30% to 80% of the maximum injection volume of the injection molding machine. Should not use large injection molding machines to produce small products.
2 Gradual three-stage screw should be used, length-to-diameter ratio is 15~20, compression ratio is 2.5~3.0.
3 Self-locking nozzles should be used, with heating temperature control device.
4 When molding flame-retardant PBT, the relevant parts of the injection molding machine should be treated with anti-corrosion.
3, product and mold design
1 The thickness of the product should not be too thick, PBT is very sensitive to the gap, therefore, the transition of the product such as right angle should be used arc connection.
2PBT molding shrinkage is relatively large, in 1.7% ~ 2.3%, the mold must have a certain degree of stripping.
3 mold need to set vent or exhaust slot.
4 The caliber of the gate should be large.
5 mold need to set the temperature control device. The maximum temperature of the mold cannot exceed 100°C.
6 flame-retardant PBT molding, the mold surface to be chrome-plated, to prevent corrosion.
4, raw material preparation
Dry before injection molding, to control the moisture content below 0.02%. When using hot air circulation drying, when the temperature is 105°C, 120°C or 140°C, the corresponding time does not exceed 8h, 5h, 3h. The thickness of the material layer is less than 30mm.
5, injection molding process parameters
1 The injection temperature PBT decomposition temperature of 280 °C, so the actual production is generally controlled at 245 ~ 260 °C.
2 injection pressure injection pressure is generally 50 ~ 100MPa.
3 Injection rate PBT has a fast cooling rate, so a faster injection rate is used.
4 screw speed and back pressure molding PBT screw speed should not exceed 80r/min, generally between 25 ~ 60r/min. The back pressure is generally 10% to 15% of the injection pressure.
5 mold temperature is generally controlled at 70 ~ 80 °C, the temperature difference between the parts does not exceed 10 °C.
6 molding cycle is generally 15~60 s.
6, matters needing attention
1 The use of recycled materials The proportion of recycled materials and new materials is generally 25% to 75%.
2 Use of release agents Normally, mold release agents are not used, and silicone release agents may be used when necessary.
3 Shutdown The stoppage time for processing PBT is within 30min. It can be stopped when the temperature drops to 200°C. When production is stopped after a long period of shutdown, the material in the barrel is to be drained, and new materials are added before normal production can be performed.
4 Post-treatment of the product does not require treatment in general, if necessary, treatment at 120 °C 1 ~ 2h.
