Meaning
Geometric constraints define the maximum allowable ratio between the length and diameter of a feature in a mold or a fiber. Engineers apply an aspect ratio limit to prevent the structural failure of core pins or the improper orientation of reinforcing glass fibers during the filling stage. When the melt flows past a slender pin, the resulting pressure can cause deflection or permanent bending if the limit is exceeded.
Design Constraint
Geometry determines the ease of ejection and the strength of the steel sections. A typical aspect ratio limit for a standing core pin in a high pressure mold is four to one.
Reinforcement Performance
Fiber orientation depends heavily on the geometry of the part and the flow front. If the aspect ratio limit of a carbon fiber is too high, the fiber may break during the high shear of the extrusion process or inside the screw. Internal breakage lowers the tensile strength of the finished part.
Short fibers provide isotropic properties while longer fibers offer directional strength.
Tooling Integrity
Tool life decreases when thin features are subjected to repeated thermal expansion and mechanical stress. An aspect ratio limit ensures that the heat transfer through a core is sufficient to prevent localized overheating of the resin. Hot spots lead to sink marks and degradation of the polymer chain.
Thickening the base of a feature can help maintain the ratio while improving the cooling rate across the tool.
