Meaning
Applied shear stresses in a flowing polymer melt can align molecular chains and drastically accelerate the rate of crystal formation. This phenomenon, known as shear induced crystallization, occurs primarily in high-shear regions such as gates and thin wall sections during the filling phase. It causes the aligned chains to crystallize at much higher temperatures than they would under static conditions.
This can create a highly oriented skin layer with high tensile strength but low impact resistance. It also alters the flow behavior of the polymer by increasing the viscosity rapidly.
Molecular Orientation
Flow velocity gradients across the mold cavity force the long polymer chains to stretch and orient in the direction of flow. In shear induced crystallization, this orientation reduces the entropic barrier to crystal nucleation. This results in a fast crystallization rate that can freeze the polymer before it reaches the end of the flow path.
It creates highly anisotropic properties in the molded part.
Viscosity Shift
Solidification during the filling phase can cause premature freeze-off and incomplete parts. Since shear induced crystallization increases the local viscosity, it can block the flow of material if the injection speed is too low. Proper management of injection speed is required to balance the heating effect of shear against the accelerated crystallization.
High mold temperatures can help keep the polymer flowing.
Defect Reduction
Warp and dimensional instability are often linked to the uneven crystal distribution caused by shear forces. Moulders use wider gates and thicker walls to reduce the shear rates and minimize the effect.
