Bagley End Correction Methods for Polymer Capillary Rheometry Data Analysis
Bagley end corrections isolate true wall shear stress from entry pressure losses, preventing viscosity overestimation that distorts mold tooling and cash flow.
Die Swell
Meaning
Elastic recovery at a die exit results in a diameter increase when a compressed material is released from a constrained flow. The die swell ratio compares the final dimensions of an extrudate to the dimensions of the orifice it passed through. This behavior is a characteristic of viscoelastic fluids and does not occur in purely viscous Newtonian liquids like water or light oils.
Strain History
Molecular chains store energy as they are squeezed and stretched inside a tool. The die swell represents the release of this stored elastic energy once the wall constraints disappear.
Dimensional Tolerance
Extruded profiles must be sized to account for the predictable expansion of the melt. Designers use die swell data to cut the tool smaller than the desired final part size. A supplier’s forecast for part dimensions will fail if the processing temperature changes, because heat alters the elastic memory of the polymer.
Thermal Effect
Higher temperatures usually reduce the amount of expansion by allowing the molecules to relax more quickly. Managing die swell involves a balance between the speed of the line and the temperature of the die. If the cooling is applied too early, the expansion is frozen in place, leading to internal stresses.
Production yields drop when these stresses cause the part to crack or bow during storage.