Sink Marks / Depressions
Causes, Solutions & Prevention in Injection Molding
Sink marks occur when surface solidifies before the core, which then shrinks inward.
Below you will find general information about this defect. To get a pinpoint solution tailored to your specific machine, material, and process parameters using the world’s most comprehensive injection molding troubleshooting database — sign up and access AI-powered troubleshooting, 7 engineering calculators, and detailed project tracking.
🔍Symptoms & Visual Identification
⚠️Root Causes by Category
⚙️ Process
- Holding pressure too low
- Holding time too short gate not frozen
- Melt temperature too high
- Cushion too small less than 3mm
🔲 Mold
- Gate too small pressure drop
- Cooling channel far from thick section
- Insufficient venting near sink location
🧪 Material
- High shrinkage grade
- Rib thickness exceeds 50% of wall S=a/2 rule
- Boss diameter exceeds 60% of wall
- High regrind ratio
📋 General
- Wall thickness too thick
- No root radius at rib-wall intersection
✅Solutions & Corrective Actions
⚙️ Process
- Increase holding pressure in 10-15% steps target 50-80% of injection
- Extend holding time until gate freeze
- Increase cushion to 3-5mm
- WARNING Do NOT extend cooling time to fix sinks makes it worse
🔲 Mold
- Enlarge gate runner sprue to reduce pressure drop
- Move cooling channel closer to thick section
- Add vent near sink location
📋 General
- Apply S=a/2 rule rib max 50% wall boss max 60% wall
- Add root radius at all rib-wall intersections
- Chemical blowing agent if wall cannot be redesigned
🛠Step-by-Step Troubleshooting
Check Process Parameters
Increase holding pressure in 10-15% steps target 50-80% of injection
Inspect the Mold
Enlarge gate runner sprue to reduce pressure drop
Verify Material Condition
High shrinkage grade
❓Frequently Asked Questions
What causes sink marks / depressions in injection molding?
Sink Marks / Depressions is caused by several factors including: Holding pressure too low; Holding time too short gate not frozen; Gate too small pressure drop; Cooling channel far from thick section; High shrinkage grade; Rib thickness exceeds 50% of wall S=a/2 rule.
How do you fix sink marks / depressions in injection molding?
Start by checking key parameters: Holding pressure, Holding time, Gate size, Cooling time. Increase holding pressure in 10-15% steps target 50-80% of injection
Which plastics are most affected by sink marks / depressions?
Common materials include: ABS, PP, HIPS, PC, PA6. Pay special attention to drying, shrinkage and mold temperature for each grade.
How do I prevent sink marks / depressions in future production runs?
Prevention relies on three pillars: (1) process optimisation — verify Holding pressure, Holding time; (2) mold maintenance — check gates, vents and cooling channels; (3) material control — ensure correct drying and virgin/regrind ratio.
What is the difference between sink marks / depressions and similar defects?
Surface depressions on opposite side of thick features. Shiny dimples visible under raked light. This distinguishes it from related surface defects during visual inspection.
📐Related Calculators
Run these engineering calculators to verify your process parameters and prevent this defect.
🔗Related Defects
About MBPlastiPro
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