Meaning
Velocity distribution across a flow channel describes the change in fluid speed from the center of the flow to the channel wall. This variation is the shear rate gradient, which is zero at the center of the stream and reaches its maximum at the mold steel boundary. It determines the viscosity of shear-thinning polymers during the filling phase.
It is a critical parameter in runner and gate design.
Frictional Heating
High shear rates at the flow boundary generate localized heat through viscous dissipation. Viscous heating, driven by the shear rate gradient, lowers the melt viscosity near the wall, allowing the polymer to flow easier. However, excessive shear can degrade the polymer chains and cause cosmetic surface defects.
Orientation Effects
Polymer molecules align along the direction of flow in response to high shear forces. High alignment creates a highly oriented skin layer at the part surface and a less oriented core region. Structural anisotropy is responsible for the unequal mechanical properties and directional shrinkage observed in molded parts.
Runner Optimization
Gate and runner dimensions must be designed to avoid extreme shear rates that could degrade the resin. Analyzing the shear rate gradient during flow simulation helps to determine the optimal gate size for a given material and flow rate. Optimal gate design ensures that the material is not degraded before it enters the cavity.
