Post-Cure Shrinkage Basics in Polymer Micro-Molding
Post-cure shrinkage in polymer micro-molding requires feature-specific tooling compensation and strict 48-hour environmental stabilization protocols.
Shear Orientation
Meaning
Alignment of polymer chain molecules in the direction of melt flow caused by the velocity gradients that develop during the cavity filling phase of injection molding. This shear orientation is strongest near the mold walls where the velocity gradient is highest, creating a distinct skin-core structure in the molded part. The degree of this alignment affects both the dimensional stability and the directional mechanical properties of the finished product.
Molecular Alignment
Flow of the polymer melt involves friction against the cooler mold surfaces, which forces the molecular chains to stretch and orient themselves along the flow path. This shear orientation occurs during the filling stage and remains frozen in the outer skin of the part if the cooling rate is high. In contrast, the slow-cooling core allows the molecules to relax and return to a randomized, unaligned state.
Understanding this skin-core differentiation helps molders design gate locations that minimize structural weaknesses in areas subject to bending or impact during product use.
Physical Anisotropy
Mechanical strength and thermal expansion coefficients differ along the flow direction compared to the transverse direction. When shear orientation is excessive, the part will exhibit high tensile strength parallel to the flow but will be prone to cracking and splitting along perpendicular axes. This directional difference also causes uneven part shrinkage, which leads to warping and twisting during the cooling cycle.
Process Control
Melt temperature and injection speed are the primary variables used to adjust the level of flow-induced alignment. When process engineers increase melt temperatures, the resulting lower viscosity decreases the shear orientation and encourages molecular relaxation before solidification. Maintaining a stable, balanced process window helps ensure that the structural properties of the molded parts remain within specified design limits.