PBT has a distinct melting point and a melting point of 225-235 ° C. It is a fast crystalline material with a crystallinity of 40%. The viscosity of PBT melt is not as much affected by temperature as the shear stress. Therefore, in injection molding, the injection pressure has a significant effect on the melt flow of PBT. PBT has good fluidity and low viscosity in the molten state, and only the secondary polyamide is prone to "casting" during molding.
PBT shaped articles are anisotropic. PBT is easily degraded when exposed to water at high temperatures. When selecting a screw type injection molding machine, the following points should be considered. 1 The amount of material used for the product should be controlled at 30%-80% of the rated injection volume of the injection molding machine. It is not advisable to use small injection molding machines to produce small products. When using a large barrel machine, a very small injection volume will cause the resin residence time to be unnecessarily prolonged, resulting in resin degradation;
2 The gradient type three-stage screw is selected, the aspect ratio is 15-20, and the compression ratio is 2.5-3.0;
3 should use self-locking nozzle with heating and temperature control device;
4 When molding flame retardant PBT, the relevant parts of the injection molding machine should be treated with anti-corrosion treatment.
Product and mold design should consider the following points:
1 The thickness of the product should not be too thick, and PBT is very sensitive to the gap. Therefore, the transition of the right angle of the product should be connected by a circular arc;
2 The unmodified PBT has a large molding shrinkage rate, generally between 1.7% and 2.3%, so the mold must have a certain degree of drafting; 3 molds need to have venting holes or exhaust grooves;
4 the size of the gate should be large;
5 mold needs to be equipped with temperature control device, the mold high temperature can not exceed 100 ° C;
6 flame retardant PBT molding, the surface of the mold should be chrome plated to prevent corrosion.
