Dynamic Cavity Pressure Integral Correlation with Part Dimensions in Injection Moulding
Dynamic cavity pressure curve integration over hold time governs volumetric polymer density and predicts final part dimensions before tool ejection occurs.
Wall Thickness Variation
Meaning
Part geometry in injection moulding requires uniform thickness to ensure even cooling and consistent flow of the polymer melt. When a part has wall thickness variation, the different regions cool at different rates, which can lead to high residual stresses and dimensional defects. This variation is one of the most common causes of warpage and aesthetic flaws in molded plastic components.
Flow Behavior
Molten polymer always follows the path of least resistance, which can cause flow issues in parts with varying thicknesses. When wall thickness variation is present, the melt can hesitates in thinner sections while rushing through the thicker channels. This behavior, known as racetrack flow, can trap air and cause burns or weld lines in the finished product.
Structural Defect
Uneven cooling rates lead to differential shrinkage, which creates internal stresses and surface sink marks. In regions with wall thickness variation, the thicker sections remain hot longer than the thin walls, causing them to shrink more and pull the thinner walls inward. This unbalanced shrinkage results in severe warpage and can compromise the mechanical strength of the part under mechanical stress.
Over time, these molded-in stresses can also lead to environmental stress cracking or dimensional instability, especially when the parts are exposed to elevated temperatures.
Tooling Design
Tooling designers use core-outs and ribbing to maintain uniform wall thickness while keeping the required part stiffness. If wall thickness variation is unavoidable, the transition between different thicknesses must be gradual to prevent stress concentrations and flow disruptions. Mold-flow analysis is used to predict these transition zones and optimize gate locations to ensure even filling.