Meaning
Dimensional reduction behavior measures the volumetric contraction of semi-crystalline polymers as they cool from melt to solid state. Managing PBT shrinkage requires precise tool sizing because polybutylene terephthalate exhibits significant density changes during crystallization. Mould designers scale core and cavity dimensions up to two percent above nominal part size to compensate for natural contraction.
Predictable shrinkage stops when cavity packing pressure drops prematurely, causing variable density distributions.
Anisotropic Contraction
Polymer molecules align along flow vectors during injection, causing higher longitudinal shrinkage than transverse shrinkage in unreinforced grades. Molecular orientation creates internal stress fields that twist flat box geometry during cooling outside the tool. Predicting PBT shrinkage along both primary axes prevents assembly fit issues in multi-component electronic connectors.
Process Sensitivity
Packing pressure and holding time dictate how much additional polymer enters the cavity during initial crystallization. Increasing packing pressure packs more mass into the cavity, directly reducing total PBT shrinkage across thick wall sections. Molders balance barrel temperature and mold wall chilling rates to hold tight dimensional tolerances across long production runs.
Filler Influence
Adding glass fibers suppresses longitudinal contraction while maintaining transverse shrinkage, altering the anisotropy ratio. Fiber reinforcement reduces overall PBT shrinkage down to one half percent in highly loaded engineering compounds.
