Meaning
Time parameter describing how long it takes for a polymer melt to release internal stresses after a deformation is applied governs the orientation and warpage of molded parts. A relaxation time is directly related to the length and entanglement density of the polymer chains. This duration is a critical factor in determining how quickly a material will transition from a highly oriented shear flow state back to its random coil equilibrium state.
Viscoelastic Transition
Measuring this parameter involves subjecting the melt to a step strain and tracking the decay of the stress over time.
Polymer Orientation
High shear during the injection phase stretches and aligns the polymer chains along the flow direction. If the mold cools the resin faster than its relaxation time, this aligned orientation is frozen into the solid part. This frozen-in orientation causes anisotropic shrinkage and leads to part warpage.
Defect Prevention
Processors adjust the mold temperature and the injection speed to manage this thermal and structural behavior. Allowing sufficient time for the chains to relax before the part is ejected prevents the accumulation of residual stresses. This adjustment reduces the scrap rate, improves the dimensional stability of the finished product, and prevents failure under stress during service.
