Managing Hot Runner Shear Induced Thermal Imbalance across High Density Cavity Layouts
Managing shear imbalance across high-density cavities depends on rotating channel boundary layers at runner splits to equalize melt viscosity before gating.
Melt Temperature Asymmetry
Meaning
Uneven temperature distribution across the cross-section of a flowing polymer stream can arise from non-uniform shear or uneven heating along the flow channels. This melt temperature asymmetry creates regions of high and low viscosity within the same flow front, which leads to unbalanced filling when the stream is divided into multiple cavities. Because the hotter material flows more readily, it tends to fill certain areas of the mold faster, leading to part defects and weight variation.
Flow Asymmetry
Shear heating generates a thin band of hot polymer along the walls of the runner, while the center of the flow stream remains at a lower temperature. When the melt rounds a corner or splits at a junction, this temperature profile becomes asymmetrical, with one side of the channel carrying significantly hotter material than the other. This uneven distribution persists into the mold cavity, causing lopsided flow and differential cooling.
Downstream Deviation
Non-uniform cooling in the mold cavity occurs because the hotter regions of the part shrink more than the colder regions. This difference in shrinkage can cause molded parts to warp, twist, or fail to meet dimensional tolerances. The asymmetric flow also makes it difficult to pack the cavity evenly, leading to internal stresses that weaken the finished component.
Mold Quality
Minimizing this thermal variation involves optimizing the runner layout, using static mixers in the nozzle, or adjusting the melt rotation behavior at the runner junctions. These measures help to homogenize the melt before it enters the gates, ensuring a more uniform temperature distribution. The resulting balance improves the cosmetic and mechanical consistency of the molded parts.