Optimizing Thermoforming Process Windows Using Strain Hardening Rheology Metrics
Strain hardening rheology metrics define thermoforming process windows by predicting extensional melt strength to eliminate corner thinning and sheet sag.
Linear Viscoelastic Envelope
Meaning
Rheological boundaries define the limits within which a polymer material exhibits a linear relationship between stress and strain. This linear viscoelastic envelope is established using dynamic mechanical analysis or rotational rheometry by performing strain sweep tests. Staying within this region is necessary to obtain reliable data on the material’s molecular structure.
Testing Method
Oscillatory tests apply a range of strain amplitudes at a constant frequency to locate where the storage modulus begins to decrease. Within the linear viscoelastic envelope, the polymer’s internal structure remains undisturbed by the measurement. Testing outside this region yields non-linear behavior that cannot be used to calculate molecular weight distribution.
Melt Stability
Extrusion and molding processes often subject the polymer to deformation rates that far exceed these linear limits. However, characterizing the polymer within the linear viscoelastic envelope provides the baseline values for zero-shear viscosity and relaxation time. These properties are critical for predicting how the resin will behave in the die or mold, particularly during the initial stages of flow.
If the resin exhibits a narrow linear region, it may be more sensitive to shear-induced degradation during high-speed processing. This sensitivity can lead to a loss of mechanical properties in the finished part.
Part Specification
Design engineers use rheological data to simulate the cooling and shrinkage of parts in the mold cavity. Inaccurate baseline data from outside the linear viscoelastic envelope leads to errors in mold flow simulations. These errors can result in incorrect gate placement or inadequate venting.