Capillary Rheometry Data Corrections for Polymer Processing Accuracy
Correcting raw capillary rheometry data for entry losses, shear gradients, and wall slip prevents costly tooling rework and protects gross margins.
Nahme Griffith Number
Meaning
The ratio of viscous heat generation to thermal conduction across a flowing polymer melt measures the importance of shear heating in altering fluid viscosity. Polymer processing engineers utilize the Nahme Griffith Number to determine whether internal heat generation will destabilize the flow profile during extrusion or molding. This dimensionless parameter helps in identifying when temperature-induced viscosity drops will affect product quality.
A high value indicates that the temperature rise due to friction dominates the heat transfer within the channel.
Calculation Variable
Viscosity sensitivity to temperature, flow velocity, and thermal conductivity represent the primary inputs for this parameter. When calculating the Nahme Griffith Number, the activation energy of the polymer is compared against the rate of shear deformation. This relationship reveals how quickly the material’s flow resistance degrades under mechanical work.
Thermal Instability
Severe viscous dissipation can lead to localized thermal degradation and polymer chain scission. A high Nahme Griffith Number warns that the center of the flow stream is heating up faster than the die walls can conduct the energy away. This imbalance causes the flow to accelerate along the hot path, leading to uneven melt distribution and structural weaknesses in the finished profile.
To counteract this effect, operators must either reduce the screw speed or increase the cooling capacity of the die. Modifying the tooling to reduce the shear rate can also help lower the heat generation.
Engineering Margin
Design adjustments can keep the process within a safe operating window where thermal effects remain negligible. Under conditions where the dimensionless value is low, thermal conduction is sufficient to maintain a uniform temperature. This balance ensures that viscosity remains constant across the flow path, resulting in predictable extrusion dynamics.