Anisotropic Shrinkage Variance in Glass-Filled Polyamide Secondary Tooling Nests
Secondary tooling nests for glass-filled polyamide accommodate anisotropic shrinkage through floating datum pins aligned with transverse contraction vectors.
Post-Mould Crystallization
Meaning
Thermal transition phenomena occur when semi-crystalline polymers continue to develop their crystalline structure after the part has been ejected from the mold cavity. Post-mould crystallization occurs as the polymer chains slowly reorganize into denser spherulites over hours or days at room temperature. This ongoing structural change causes additional part shrinkage and can alter the mechanical properties of the molded part.
Crystalline Growth
Reorganization of polymer chains occurs because the cooling rate inside the mold was too fast to allow complete crystallization. As the material approaches its equilibrium state, the density increases and the part experiences dimensional contraction.
Processing Cause
Inadequate mold cooling and short cycle times are the primary process conditions that trigger this delayed structural development. If the mold temperature is set below the optimal glass transition temperature of the polymer, the material is quenched too quickly, freezing the molecules in an amorphous state. This unstable structure will reorganize as soon as the part is exposed to elevated operating temperatures or left to sit at room temperature for an extended period.
Dimensional Stability
Delayed contraction can lead to assembly failures and part warpage as the molded component changes size after inspection. Molders often use thermal annealing processes to force this crystallization to complete before the parts are shipped or assembled. This controlled heating step stabilizes the dimensions and ensures that the final product maintains its shape and performance over its service life.