Meaning
Volumetric depression appearing on the exterior surface of an injection moulded polymer component results from localized volumetric shrinkage during cooling. Sink mark depth quantifies the physical height differential between the nominal wall plane and the lowest point of the surface indentation. This geometric deviation governs aesthetic acceptance in class A automotive trims and visible consumer electronics housings.
The measurement applies strictly to thick ribs, bosses and wall intersections where cooling rates diverge from adjacent thin sections.
Tooling Thermal Balance
Cavity design requires careful management of localized heat accumulation to suppress the formation of surface depressions. Unbalanced cooling circuits create hot spots where molten polymer remains liquid longer than the surrounding material. Excessive wall thickness at structural attachment ribs exacerbates this thermal differential during the holding phase.
Toolmakers mitigate these risks by thinning rib bases to fifty or sixty percent of the adjoining nominal wall thickness.
Resin Shrinkage Kinetics
Semicrystalline polymers exhibit significantly higher volumetric contraction during phase change than amorphous counterparts. Polypropylene and polybutylene terephthalate demand precise packing pressure profiles to force additional melt into the cavity during solidification. Regrind material blends alter melt flow rates and modify this crystallization behavior, which changes the shrinkage magnitude across identical tooling geometries.
Datasheet values for volumetric shrinkage provide a baseline, but actual cavity conditions dictate the final dimensional deviation on the pressed part.
Structural Compensation
Process technicians adjust barrel temperatures and injection velocities to counteract surface depression anomalies during production runs. Extended holding pressure durations drive extra material into the solidifying skin before gate freeze off occurs. Part specifications dictate allowable dimensional tolerances, which separates cosmetic rejects from functional components on the assembly line.
Optimizing cycle times prevents incomplete packing while avoiding residual internal stress buildup within the moulded structure.
