Thermal Boundary Layer Instabilities and Viscoelastic Melt Degradation during High Throughput Molding
High throughput molding requires balancing injection velocity against viscous dissipation to prevent polymer chain scission and wall slip defects.
Graetz Number
Meaning
Dimensionless parameter characterizes the ratio of heat convection to heat conduction during the flow of fluids through pipes or mold channels. Application of the graetz number allows engineers to determine if a thermal profile is fully developed inside a cooling or heating zone. The value relates the channel length and velocity to the thermal diffusivity of the material.
Thermal Diffusion
High values suggest that heat is moving mainly through the mass of the fluid rather than through the walls. When the graetz number is low, the temperature of the fluid becomes uniform with the surroundings more quickly. This transition point is vital for designing effective heat exchangers.
Flow Ratio
Length of the contact path changes how energy leaves the stream. Calculation of the graetz number uses the diameter and length of the pipe along with the thermal properties of the resin. Rapid transit through a short gate creates a different thermal result than slow travel through a long runner.
Scaling Analysis
Thermal designers use these calculations to size the cooling lines in complex injection tools. Because the graetz number links geometry to process speed, it predicts where the plastic will remain too hot for ejection. Proper scaling prevents regional overheating in the mold.