Meaning
Localized surface depressions formed above internal ribs, bosses, or thick wall sections occur when volumetric shrinkage pulls outer polymer skins inward during cooling. The sink mark defect appears when localized core cooling is slower than surface skin solidification, creating internal vacuum voids or surface depressions. Increasing holding pressure and extending pack time forces additional polymer melt into the cavity to compensate for thermal contraction.
Mold design modifications, such as reducing rib thickness ratios to fifty percent of nominal wall thickness, mitigate surface depression risks.
Thermodynamic Drivers
Volumetric thermal contraction during polymer phase change from liquid to solid drives surface distortion in thick wall regions. Preventing sink mark defect formation requires maintaining adequate cavity packing pressure until gate seal occurs. Amorphous polymers exhibit less sharp volume changes during cooling than semi-crystalline polymers like polypropylene or nylon.
Insufficient holding pressure permits core resin to shrink away from cavity walls, leaving visible surface hollows.
Tooling Remediation
Mold cooling line placement directly influences temperature uniformity across thick section geometries. Adding conformal cooling channels accelerates heat extraction from heavy boss intersections, preventing localized thermal sinks. Gate positioning should prioritize feeding thick wall areas directly before thinner section flow paths freeze off.
Process Controls
Packing pressure profile adjustments pack additional polymer mass into shrinking sections during early cooling stages. Melt temperature reductions lower overall heat input, shortening the required holding time to achieve surface stability. Gas-assisted injection moulding eliminates sink marks by introducing pressurized gas cores into thick rib sections.
Surface texture application can cosmetically mask minor sink marks on visible cosmetic surfaces.
