Meaning
Velocity gradients across a flow channel define the variation in the rate of deformation experienced by a polymer melt as it is processed. Understanding the shear rate distribution is critical when designing gates and runners for high-performance injection moulds. The parameter dictates the local viscosity of the shear-thinning plastic, which affects the pressure drop and fill time of the cavity.
Velocity Gradient
Flowing polymer melt experiences zero shear at the center of the channel and maximum shear near the wall. This variation creates a stratified viscosity profile.
Defect Prevention
Extreme shear at the gate can cause polymer degradation, which appears as discolored streaks or weak spots in the part. By analyzing the distribution of shear rates, molders can adjust gate dimensions to keep peak shear below the resin’s critical degradation limit. This precaution reduces the risk of part rejection during high-volume production runs.
Proper gating also prevents fiber orientation issues that reduce the impact strength of reinforced plastic components.
Rheological Relevance
Inaccurate modeling of this distribution leads to incorrect predictions of injection pressure and clamp force. This parameter is used in process simulations to optimize the gating location and ensure that the melt fills the cavity uniformly. Maintaining a balanced distribution reduces molded-in stress and prevents part warpage after ejection.
