Sink Marks on Molded Parts: Practical Troubleshooting Steps

Sink marks are local depressions that appear when one area shrinks more than the surrounding surface. In vertical molding, they may appear near thick sections, bosses, ribs, insert backs, or material-rich areas that cool or cure differently from the rest of the part.

Quick Answer

Sink marks are rarely solved by the machine alone. Review product geometry, wall thickness, insert heat, gate location, packing behavior, cooling or curing time, and material condition before assuming a single setting is responsible.

What You May See

• A shallow depression appears on the surface opposite a thick feature.
• Sink marks appear near a metal insert, boss, rib, or material-rich area.
• The mark becomes visible after the part cools, cures, or leaves the machine.
• The same area sinks in every cycle, while the rest of the surface is stable.
• Changing cycle time or part removal timing changes the final surface.

Likely Cause Areas

• Wall thickness or rib design may create uneven shrinkage.
• Packing behavior may not compensate for local material shrinkage before gate freeze or cure progression.
• Cooling or curing may be unbalanced between thick and thin sections.
• Metal inserts may hold or transfer heat differently from the surrounding material.
• Material moisture, filler content, or batch variation may change shrinkage behavior.
• Part removal or handling may deform the part before the shape is stable.

Safe Checks Before Adjustment

Stop automatic operation if movement, alarm, guarding, or operator safety is uncertain. Do not place hands in the mold, table, injection, or guarded area while movement is possible. Use photos, controller screenshots, cycle logs, and safe observation before changing several settings.

Diagnosis Path

1. Mark the sink location relative to ribs, bosses, insert backs, wall thickness, and gate position.
2. Compare the defect immediately after demolding and after the part reaches room condition.
3. Check whether the defect changes with cooling or curing time, part removal timing, or fixture support.
4. Review material batch, drying condition, and any change in filler or colorant.
5. Inspect whether insert temperature, insert mass, or insert position creates a local heat concentration.
6. Confirm whether part removal force or handling pressure is deforming the surface.
7. Record the original process before testing one controlled change.
8. If geometry appears dominant, involve mold or product engineering instead of relying only on machine settings.

Evidence to Collect

Check	Why It Matters	Evidence to Collect
Sink location	Location shows whether wall thickness, ribs, bosses, or inserts are involved	Marked photos and simple section notes
Time after demolding	Some sink marks develop after cooling or curing outside the mold	Photos at demolding and after cooling
Cycle condition	Cooling or curing balance affects shrinkage	Cycle time, hold or cure notes, removal timing
Material record	Material behavior changes shrinkage and surface appearance	Material name, batch, drying and changeover notes
Insert condition	Metal inserts may change local heat and shrinkage behavior	Insert material, size, position, and temperature notes if available

What Not to Change Too Quickly

• Do not promise that a machine setting can remove every sink mark.
• Do not change packing, temperature, and cooling or curing time together without a record.
• Do not ignore product geometry when the mark follows a thick section every cycle.
• Do not remove parts too early if they are still deforming during handling.

When to Contact Deboge

Contact Deboge when sink marks remain after process checks or when the mark is linked to insert heat, table workflow, or part removal timing. Send product photos, cross-section or drawing if available, material details, cycle notes, and machine model.

Related Reading

• Molding Defects
• Cycle Time and Loading Time
• Materials
• Troubleshooting Center
• Machine Support