Thermal Boundary Layer Instabilities and Viscoelastic Melt Degradation during High Throughput Molding
High throughput molding requires balancing injection velocity against viscous dissipation to prevent polymer chain scission and wall slip defects.
Shear Rate Gradients
Meaning
Mathematical distribution describes the variation in the velocity layers of a fluid moving between two boundaries, such as from the center of a runner to the mold wall. Understanding shear rate gradients allows designers to predict the alignment of fibers and molecules in the finished part. The gradient is sharpest where the velocity drop is most sudden.
Velocity Difference
Polymer at the wall is nearly stationary while the center moves at peak speed. Because shear rate gradients are high near the surface, this area experiences the most friction and orientation. These variations determine the local strength of the molded component.
Viscosity Change
Thixotropic materials become thinner in regions where the change in speed is most intense. Using the knowledge of shear rate gradients, engineers can simulate how a complex part fills by looking at the effective viscosity at every point in the flow. Flow becomes highly non-linear in narrow ribs.
Flow Profile
Parabolic or plug patterns describe how mass moves forward through the tool. If the shear rate gradients are too steep, the outer layers may cool too slowly or align so strongly that they become weak in the cross direction. Controlled filling profiles aim for uniformity in these shifts.