Optimizing Dynamic Packing Profiles to Mitigate Glass Fiber Alignment Distortion
Stepped packing pressure profiles decay holding force to match gate solidification, preventing secondary shear flow and mitigating glass fiber alignment warpage.
Fountain Flow
Meaning
Fluid dynamics mechanism where the molten polymer at the center of a flow channel moves faster than the sides and rolls outward to the mold walls governs the filling of injection cavities. This fountain flow behavior determines the thermal and stress history of the polymer molecules that form the outermost skin of the part. The process continues until the flow front ceases to move or the polymer freezes completely.
Velocity Profile
Molten plastic behaves as a non-Newtonian fluid with a parabolic velocity profile that reaches its peak at the midpoint between the cavity walls. As the melt front advances, the fast-moving material from the center is pushed forward and then splits, stretching and depositing onto the cold metal surface of the mould. This action ensures that the polymer that touches the steel is always drawn from the hot, well-mixed core of the melt, creating a characteristic V-shaped or U-shaped velocity distribution.
The continuous deposition of material onto the cold walls means that the age of the polymer on the surface increases with distance from the flow front.
Surface Effect
Molecules deposited by this mechanism are subjected to rapid cooling and high shear stresses as they freeze against the mould surface. This rapid transition freezes the stretched conformation of the chains, creating a highly oriented skin layer with high internal stress. If the melt temperature is too low, this oriented layer can lead to surface defects such as gating blush or poor gloss.
Process Variable
Control over this flow dynamic is achieved by adjusting the injection velocity and the temperature of the mould steel. Faster injection speeds maintain a hot flow front, reducing the thickness of the frozen skin and allowing better replication of the mould surface texture. Conversely, slow filling increases the frozen layer thickness, which can restrict flow and raise the required injection pressure.
Controlling Anisotropic Shrinkage in Reinforced Polyamide Injection Moulding
Controlling anisotropic shrinkage in reinforced polyamides requires directional shrinkage tool scaling, steel-safe cavity design, and optimized hold pressure profiles.