Meaning
Viscoelastic flow characterization of polymer melts is achieved using a specialized die that has an extremely short capillary length. A zero length orifice provides a way to isolate and measure the entrance pressure drop that occurs when a polymer melt experiences rapid acceleration at a channel restriction. By comparing this flow behavior with that of a long die, rheologists can separate the shear and elongational properties of the polymer melt.
It is used in laboratory capillary rheometers to analyze the processability of industrial resins.
Elongational Flow
Stretching flow dominates the behavior of polymer melts as they are forced through sudden contractions in runners and gates. Using a zero length orifice allows engineers to measure the resistance of the melt to this stretching action, which is a major factor in molding defects like melt fracture. The pressure drop across this orifice is almost entirely due to the elongational viscosity of the material.
This measurement is useful for optimizing the design of high-speed extrusion dies.
Flow Dynamics
Standard capillary dies measure a combination of shear and entry effects, which can make it difficult to determine the true material properties of the resin. Incorporating a zero length orifice in the testing setup allows the operator to subtract the entry pressure drop from the total pressure measured in longer dies. This technique, known as the Bagley correction, yields the true shear viscosity of the polymer.
This correction is a necessary step for creating accurate flow simulations.
Laboratory Setup
Practical testing involves a capillary rheometer equipped with two dies of equal diameter but different lengths, one being as close to zero length as possible. Data generated using the zero length orifice is plotted against flow rate to characterize the resin across the shear rates of interest. This profile allows compounding plants to compare resin batches for consistency before shipping.
It is a highly sensitive method for detecting changes in polymer structure that conventional shear rheometers might miss. By analyzing the flow characteristics in this manner, technicians can adjust the production line to accommodate small differences between material lots, which prevents issues such as uneven wall thickness in blow molding operations.
