Sink marks are one of the most common and visible defects in injection-molded parts, especially in thick sections. They appear as localized depressions or dimples on the surface and can severely compromise both aesthetics and dimensional accuracy. Understanding the root causes and applying professional design strategies is essential for achieving high-quality, high-volume production.
1. Root Causes of Sink Marks in Thick Sections
Sink marks primarily result from uneven volumetric shrinkage during cooling. In thicker areas, the outer surface solidifies first while the molten core continues to cool and contract. This internal shrinkage pulls the still-soft surface inward, creating a visible depression. High material shrinkage rates, insufficient packing pressure, poor gate design, and inadequate cooling further aggravate the problem. Abrasive or high-shrinkage resins such as glass-filled materials intensify the issue by increasing internal stress and thermal gradients.
2. Uniform Wall Thickness Design
The most effective preventive measure is maintaining consistent wall thickness throughout the part. Sudden transitions from thin to thick sections should be avoided. Where thickness variation is unavoidable, gradual transitions or core-out features should be introduced to reduce material mass while preserving structural integrity. This approach minimizes differential shrinkage and significantly reduces the risk of sink marks.
3. Optimized Rib and Structural Design
Ribs and reinforcing features are frequent sources of sink marks if not properly proportioned. Industry best practice recommends limiting rib thickness to 50–60% of the nominal wall thickness and incorporating generous radii at the base. Strategic placement of ribs, combined with lightening pockets or core-outs, provides strength without creating excessive thermal mass that leads to surface depression.
4. Gate Location and Packing Optimization
Proper gate design ensures adequate packing pressure reaches thick sections before the gate freezes. Larger gates, valve gates, or hot-runner systems are preferred for thick-walled parts. Positioning the gate so that melt fills thicker areas first allows more effective compensation for volumetric shrinkage during the packing phase, dramatically reducing sink mark formation.
5. Advanced Cooling and Process Integration
Targeted cooling channels around thick sections help equalize temperature distribution and shorten the internal cooling time. When combined with mold-flow analysis and optimized packing profiles, this synergistic approach further suppresses sink marks. Selecting lower-shrinkage materials and fine-tuning process parameters completes a robust design-for-manufacturing solution.
By addressing wall thickness uniformity, rib geometry, gate strategy, and cooling design in a systematic manner, manufacturers can virtually eliminate sink marks in thick sections. Professional mold and part design focused on these principles delivers superior surface quality, tighter tolerances, and higher production efficiency — helping customers achieve reliable, cost-effective mass production with excellent cosmetic results.