Standard Melt Flow Test Load Selection Criteria for Polyethylene Sourcing
Polyethylene sourcing requires specifying exact test loads, temperatures, and standards to isolate molecular structure and enforce contractual lot consistency.
Entry Pressure Loss
Meaning
Specific energy reductions occur at the transition from a wide reservoir to a narrow orifice. The term entry pressure loss describes the force consumed as the polymer melt rearranges its molecular chains to fit through a smaller opening. This phenomenon happens at the gate of a mould or the entry of a die.
Designers must account for this drop to ensure the machine has enough injection pressure.
Convergent Flow
The melt accelerates as it moves toward the restriction.
Molecular Elasticity
Long polymer chains resist the rapid deformation required to enter the die. This resistance contributes to the total entry pressure loss observed in highly branched materials like low density polyethylene. Linear polymers typically show a smaller drop because their chains slide past each other more easily.
Measuring this effect helps chemists understand the architectural complexity of the molecules.
Die Correction
Rheologists use the Bagley correction to separate the capillary resistance from the initial drop. Accurately identifying the entry pressure loss ensures that the viscosity data represents the material properties rather than the equipment geometry. Moulders use this information to size gates and runners for new tools.
Without this correction, the calculated viscosity would appear higher than the true value.