Polybutylene Terephthalate (PBT) has high strength and rigidity for a wide range of applications. PBT is one of the toughest engineering thermoplastics.
Full round runners are recommended to impart maximum pressure transmission. A guide line for the runner diameter is part thickness + 1.5 mm. A wide variety of gates may be used. Gate diameters should preferably be between 0.8–1.0 times the part thickness. When using circular tapered gates, the minimum recommended diameter is 0.75 mm. Hot runners may also be used, taking care to avoid drool and material degradation.
PBT is a polyester that is produced by the polycondensation reaction of dimethyl terephthalate an butanediol. It is a semicrystalline material and has excellent chemical resistance, mechanical strength, electrical properties (high dielectric strength and insulation resistance), and heat resistance, all of which are stable over a broad range of environmental conditions. It has very low moisture absorption.
Tensile strength ranges from 50 MPa [7,250 psi] for unfilled grades to 170 MPa [24,650 psi] for glass reinforced grades. High levels of glass fillers make the material more brittle.
Crystallization is rapid and this could cause warpage due to non-uniform cooling. In the case of glass filled grades, shrinkage is reduced in the flow direction, but in the cross-flow direction it may be equal to that of the generic grade. Shrinkage is of the order of 0.015–0.028 mm/mm [1.5 -2.8%]. A 30% glass-filled material has a shrinkage range of 0.3–1.6%.
PBT melting point (approximately 225°C [437°F]) and heat distortion temperatures are lower than that of PET. The Vicat softening point is approximately 170°C [338°F]. The glass transition temperature ranges from 22°C–43°C [71°F–109°F].
The melt viscosity is fairly low and due to fast crystallization rates, cycle times are typically low.