Rheological Determination of Ultra High Molecular Weight Tails in Bimodal Resins
Low-frequency storage modulus and creep recovery accurately quantify ultra-high molecular weight tails in bimodal resins, ensuring stress crack resistance.
Shear Rheology
Meaning
Characterization of material flow under the influence of parallel layers sliding past each other provides the data needed for most polymer processing simulations. Shear rheology focuses on the relationship between shear stress and shear rate in the molten state. This field of study is necessary for designing injection moulds and extrusion dies where the polymer is forced through narrow channels at high speeds.
Viscosity Measurement
Capillary and rotational rheometers are the primary instruments used to collect data across a wide range of flow conditions. In shear rheology, the viscosity is observed to drop as the shear rate increases, a phenomenon known as shear thinning.
Moulding Application
Filling patterns in an injection mould are predicted using the viscosity curves generated from these tests. By applying the principles of shear rheology, engineers can calculate the pressure required to fill a part and identify potential areas of high shear that could lead to material degradation. This analysis helps in selecting the correct gate size and runner layout to ensure balanced flow.
Data Interpretation
Results from these measurements allow for the comparison of different resin lots to ensure consistency in production. While simple melt flow index values give a single data point, shear rheology provides a complete picture of how the material will behave across the entire processing window. This depth of information is used to troubleshoot problems like short shots or flash.