Meaning
Structural gradient formed during the injection of a polymer melt into a cold mould results from the variation in shear stress and cooling rates across the thickness. Analysis of skin core shear deformation reveals why the surface of a part often has different physical properties than its center. The high shear at the wall aligns the polymer chains, while the slow-cooling core remains more isotropic.
Flow Dynamic
Friction between the flowing plastic and the tool surface creates a high-velocity gradient near the wall. This shear force stretches the molecules in the direction of the flow, creating a highly oriented skin layer. The thickness of this layer depends on the injection speed and the melt temperature.
Faster injection speeds generally increase the depth of the shear-oriented region. Thermal properties of the tool also influence how quickly this orientation is frozen into place.
Mechanical Anisotropy
Difference in molecular alignment leads to variations in strength and stiffness throughout the part. The oriented skin is typically stronger in the flow direction but weaker in the transverse direction compared to the unoriented core. This can lead to unexpected failures or warpage if the design does not account for the directional nature of the properties.
Visual Defect
Excessive shear can lead to surface issues like gate blush, silver streaks or delamination. If the skin layer is too thin or the transition to the core is too abrupt, the part may show visible flow lines or dull spots. Controlling the injection profile is the primary method for managing these deformations and ensuring a uniform appearance.
