PBT (Polybutylene Terephthalates) have high strength and rigidity for a wide range of applications. PBT is one of the toughest engineering thermoplastics.
This material is sensitive to hydrolysis at high temperatures. It is therefore important to dry the material before molding. Suggested drying conditions (in air) are 120 C [248 F] for 6- 8 hours (or 150 C [300 F] for 2 - 4 hours). Moisture levels must be below 0.03%. When using a desiccant dryer, drying at 120 C [248 F] for 2.5 hours is recommended.
>220 C-280 C [428 F - 536 F]; aim: 250 C [482 F]
40 C-60 C [104 F-140 F] for unreinforced grades. For other grades, a wide range of temperatures can be used, depending on the grade (15 C-80 C [59 F-176 F]). Heat removal must be fast and uniform. Cooling channels of 12-mm diameter are recommended.
Moderate (up to maximum of 150 MPa).
Fastest possible speeds (due to fast solidification of PBTs)
Full round runners are recommended to impart maximum pressure transmission. A guide line for the runner diameter is part thickness + 1.5 mm. You can use various gates. Use gate diameters between 0.8-1.0 times the part thickness. When using circular tapered gates, the minimum recommended diameter is 0.75 mm. If you carefully avoid drool and material degradation, you can also use Hot runners.
The poly condensation reaction of dimethyl terephthalate, a butanediol, produces PBT, a polyester. It is a semicrystalline material and has excellent chemical resistance, mechanical strength, electrical properties (high dielectric strength and insulation resistance), and heat resistance. These properties are all stable over a broad range of environmental conditions. It has 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 can cause warpage due to non-uniform cooling. In glass filled grades, shrinkage is reduced in the flow direction. However, in the cross-flow direction, it can 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 225C [437F]) and heat distortion temperatures are lower than for PET. The Vicat softening point is approximately 170C [338F]. The glass transition temperature ranges from 22C-43C [71F-109F].
The melt viscosity is fairly low and due to fast crystallization rates, cycle times are typically low.