Meaning
Flow profiles across a flow channel during melt processing represent the varying resistance to fluid movement caused by localized shear rates and temperature distributions. The melt viscosity gradient describes how flow resistance transitions from the high-shear region near the cool mold wall to the low-shear, hot region in the center of the flow channel. Controlling this behavior is essential for achieving balanced cavity filling and avoiding molded-in stresses.
Shear Thinning
Polymer melts exhibit non-Newtonian flow behavior where viscosity decreases as the shear rate rises. Near the stationary mold wall, the high velocity difference across a small gap creates extreme shear rates, which align polymer chains and reduce viscosity. This phenomenon establishes a steep melt viscosity gradient from the boundary layer to the slower, less sheared central flow stream.
Flow Behavior
Friction within the flowing polymer generates localized heat, which further alters the viscosity profile. This viscous dissipation raises the temperature in the high-shear zones, compounding the reduction in flow resistance near the channel walls. In contrast, the cold mold wall continuously extracts heat, creating a frozen skin layer that narrows the effective flow channel and increases the melt viscosity gradient in that boundary region.
Molding Defect
Non-uniform viscosity across the melt front causes unbalanced filling in multi-cavity molds when flow channels are not designed symmetrically. As the polymer flows through the runner system, the high-shear, low-viscosity material tends to separate from the higher-viscosity core, leading to uneven cavity packing and varying part weights. This viscosity variation can cause severe warping or surface blemishes when the material is forced into the cavities under inconsistent shear conditions.
Moulders must optimize melt temperature and injection speed to manage the melt viscosity gradient, ensuring that the material fills the cavity under stable, predictable rheological conditions.
