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.
Multiaxial Strain Gradient
Meaning
Three-dimensional variation in mechanical deformation across a localized region of a moulded part measures the intensity of localized stretching. This spatial change in strain, termed a multiaxial strain gradient, arises during complex forming and inflation processes like stretch blow moulding. It defines the transition zone where material thickness drops rapidly over a short distance, setting the boundary for part failure.
Stress Distribution
Differential stretching during part inflation creates uneven thinning across the transition regions of a container. A high multiaxial strain gradient develops when the resin is stretched rapidly in both axial and hoop directions over a small geometry change. In these regions, the polymer molecules experience differing stretch ratios across the wall thickness.
This variation leads to local stress concentration, affecting the load-bearing capacity of the final container.
Moulding Risk
Wall thickness variation and part failure represent the direct consequence of excessive strain changes in complex moulds. When moulding deep-drawn or highly contoured parts, the resin cannot distribute evenly, which forces some regions to thin out. A steep multiaxial strain gradient in these areas suggests that the wall thickness will drop below the structural specification.
Tool designers mitigate this risk by using generous radii and optimizing the temperature profile of the preform.
Mechanical Response
Yielding and puncture resistance are strongly influenced by the spatial distribution of strain. When a part with uneven strain profiles is subjected to impact, the region with the steepest gradient fails first. This localized failure occurs because the polymer chains in the highly strained region are already aligned and cannot absorb more energy.
In packaging applications, this limitation requires a conservative weight spec to ensure the thin sections do not rupture during palletization and transport. Consequently, minimizing these transition zones through better temperature control in the stretch rod and blow stages improves the drop performance of the finished bottle without increasing the overall resin weight.