Meaning
Deformation behavior of thermoplastics at elevated temperatures combines time-dependent viscous flow with permanent plastic strain. This mechanical response, known as viscoplasticity, dictates how a polymer stretches when it is processed above its glass transition temperature. It is the dominant deformation mode during the stretch blow moulding of container preforms.
Rheological Description
Under this regime, the resistance of the polymer to stretching depends on both the rate of deformation and the current temperature of the melt. In the domain of viscoplasticity, the material displays a yield stress below which it behaves elastically and above which it undergoes permanent deformation. This transition is highly non-linear, with the rate of plastic flow increasing rapidly as the applied stress exceeds the yield point.
Understanding this behavior is essential for predicting the final wall thickness of the bottle.
Moulding Application
Moulding simulations must utilize constitutive models that incorporate these viscoplastic properties to generate accurate thickness profiles. If the simulation assumes a simple elastic or plastic model, the calculated deformation will not match physical trials. This modeling accuracy reduces the need for expensive tooling modifications.
Model Sensitivity
These material properties are highly sensitive to changes in polymer moisture content and molecular weight. Small variations in these raw material attributes can alter the stretching behavior during production. This sensitivity requires consistent pre-conditioning of the resin before extrusion or moulding.
