Twenty Kinds of Plastics and Their Properties—(5) PBT
5.PBT polybutylene terephthalate
Typical application range:
Household appliances (food processing blade, vacuum cleaner element, electric fan, hair dryer shell, coffee containers etc.), electrical components (switches, motor shell, fuse box, computer keyboard, etc.) automobile industry (Radiator Grilles body panels, wheel covers, doors and windows components etc.).
Injection molding process conditions:
Drying process: this material is easy to hydrolyze at high temperature, so the drying process before processing is very important. It is suggested that the drying conditions in the air are 120 C, 6~8 hours, or 150 C, 2~4 hours. The humidity must be less than 0.03%. If drying with a moisture absorption dryer, the proposed condition is 150 C, 2.5 hours.
Melting temperature: 225~275 C, suggested temperature: 250 degrees C.
Mould temperature: 40~60 C for unreinforced material. The cooling cavity of the mold should be well designed to reduce the bending of the plastic parts. The loss of heat must be fast and uniform. It is suggested that the diameter of the die cooling chamber is 12mm.
Injection pressure: medium (maximum to 1500bar).
Injection speed: the speed of the injection should be as fast as possible (because the solidification of PBT is very fast).
Runner and gate: it is recommended to use a circular channel to increase the transfer of pressure (empirical formula: runner diameter = plastic part thickness+1.5mm). Various types of gates can be used. Heat flow can also be used, but attention should be paid to preventing material leakage and degradation. The gate diameter should be between 0.8~1.0*t, and here t is the thickness of the plastic parts. If it is a latent gate, it is recommended that the minimum diameter is 0.75mm.
Chemical and physical properties:
PBT is one of the most tough engineering thermoplastic materials. It is a semi crystalline material with excellent chemical stability, mechanical strength, electrical insulation and thermal stability.
The hygroscopic property of PBT is very weak. The tensile strength of the non enhanced PBT is 50MPa, and the tensile strength of the glass additive type PBT is 170MPa. Too much glass additives will cause the material to become brittle. The crystallization of PBT is very rapid, which will cause the bending deformation due to the inhomogeneous cooling.
For materials with glass additives, the shrinkage rate of flow direction can be reduced, but the shrinkage rate perpendicular to the process is basically the same as that of common materials. The shrinkage rate of the general material is between 1.5%~2.8%. The materials containing 30% glass additives shrink between 0.3%~1.6%. The melting point (225 C) and high temperature deformation temperature are lower than that of the PET material. The softening temperature of VEKA is about 170 degrees centigrade. The glass transition temperature (glass trasitio temperature) is between 22 and 43. Because of the high crystallization rate of PBT, its viscosity is very low, and the cycle time of the plastic parts is generally low.