Amorphous Thermoplastic Rheology and Extruded Sheet Stretch Dynamics
Amorphous sheet stretch dynamics depend on strain hardening and thermal saturation to prevent localized necking and corner blowout during deep thermoforming.
Fourier Number
Meaning
Dimensionless analysis characterizes the rate of heat conduction relative to the rate of thermal energy storage in a cooling polymer. In injection molding, the fourier number is used to calculate the time required for a molded part to cool to its ejection temperature. It governs the rate of heat transfer through the part thickness to the mold walls.
This calculation applies only to transient heat conduction during the cooling phase.
Cooling Calculation
Thermal diffusivity and cooling time together determine the value of this dimensionless parameter. A higher fourier number indicates that the polymer has reached thermal equilibrium with the mold, meaning it is cool enough to eject. Engineers use this relationship to estimate the necessary cooling time without relying solely on trial-and-error.
Thickness Influence
Part thickness has a quadratic relationship with the cooling time because heat must travel further to reach the mold surface. Double the wall thickness requires four times the cooling time to reach the same fourier number. This relationship means that minor increases in part thickness can extend the mold cycle.
Cycle Optimization
Controlling the cooling cycle prevents molders from ejecting parts that are too soft. A low value indicates that the center of the part is still molten. Ejecting too early causes part deformation and sink marks.