Meaning
Predictive analytical methods estimating the depth of surface depressions over internal ribs or bosses in molded parts prevent cosmetic blemishes before tooling fabrication. Performing a sink mark calculation allows design engineers to optimize the ratio between the nominal wall thickness and the rib thickness. This simulation identifies potential shrinkage problems that would otherwise require expensive tool modifications.
Shrinkage Kinematics
Thick intersections cool more slowly than the surrounding thin walls, leaving a pocket of molten material that remains hot while the skin solidifies. As this core eventually contracts, it pulls the rigid outer skin inward, creating a shallow depression on the outer cosmetic surface. A sink mark calculation simulates this thermal and volumetric shrinkage behavior by solving heat transfer and pressure equations over the local geometry.
This numerical analysis calculates the local density changes during the packing and cooling phases, identifying the exact millisecond when the gate freezes and stops further material from entering the cavity.
Geometric Ratio
Design rules generally dictate that the thickness of a reinforcing rib should be limited to sixty percent of the nominal wall thickness to minimize the risk of surface depressions. However, a detailed sink mark calculation can evaluate the exact impact of alternative configurations, such as adding gas channels or structural corrugations. This assessment helps to maintain the stiffness of the part while ensuring the surface remains flat.
Optimization Process
By running these calculations within a computer-aided engineering environment, designers can test different polymer grades with varying volumetric shrinkage rates. The software predicts the final sink depth in microns, allowing the team to compare the visual impact of each option. This pre-tooling analysis ensures that the mold is designed with optimized cooling and venting, which minimizes the need for high packing pressures to hide cosmetic defects.
