Transient Entanglement Disentanglement Kinetics in Deep Draw Thermoforming under Multiaxial Strain Gradients
Matching plug speed to polymer disentanglement kinetics prevents corner thinning and optimizes wall thickness in deep draw thermoforming.
Topological Constraints
Meaning
Physical boundaries imposed by the mutual impenetrability of long-chain molecules limit the free motion of polymer chains within a melt or solid. These physical barriers, termed topological constraints, prevent the chains from crossing each other, forcing them to move along a restricted path. They govern the viscoelastic behavior and elastic recovery of resins during extrusion and injection moulding.
Molecular Entanglement
Entanglement networks act like a system of temporary knots that resist rapid deformation under stress. In high molecular weight resins, these topological constraints are dense, resulting in a high viscosity and significant melt strength. The chains cannot slip past each other easily, which gives the polymer melt its elastic character during processing.
This elastic response is essential for processes like blow moulding and thermoforming, where the melt must support its own weight without tearing.
Melt Rheology
Deformation rates during processing determine how the entanglement network responds to applied forces. If the deformation is slow, the polymer chains have time to wiggle out of their restricted paths, and the melt behaves like a viscous liquid. At high deformation rates, the topological constraints act as fixed barriers, causing the melt to behave like an elastic solid.
This transition affects the pressure required to fill thin-wall moulds and determines the residual stresses in the finished part.
Processing Impact
Mould filling and part quality depend on managing these network structures through temperature and shear control. When a resin experiences high shear in the runner system, the topological constraints are partially bypassed as the chains align. This shear-thinning behavior reduces the melt viscosity, allowing the polymer to fill complex cavity details at lower pressure.
However, if the melt cools before the chains can re-entangle, the resulting part may have weak knit lines and poor impact strength, making it susceptible to failure under stress.