Sink marks during the T0 (first trial) stage of mold testing are common injection molding defects, typically caused by material shrinkage during cooling, resulting in insufficient volume. Below is a systematic analysis and step-by-step solution guide for troubleshooting.
1. Quick Diagnosis and Root Cause Analysis
First, identify the location, type, and severity of sink marks:
Common locations:
- Thick sections (ribs, boss columns, back side of reinforcing ribs).
- Areas far from the gate or at the end of flow.
- Sudden changes in part geometry.
Main causes:
- Insufficient packing pressure: Material not adequately replenished during cooling shrinkage.
- Too short packing time: Gate freezes too early, preventing continuous material compensation.
- Melt temperature too high or too low: Affects flowability and shrinkage rate.
- Uneven or insufficient cooling: Thick areas cool slower, leading to concentrated shrinkage.
- Poor gate design: Gate too small, poorly located, or insufficient in number.
- High material shrinkage rate: Crystalline materials like PP and PA are more prone to sink marks.
2. Process Parameter Optimization (Try First)
Recommended adjustment sequence:
- Increase packing pressure (gradually by 5%–10%): Ensure enough pressure to fill shrinkage voids, but avoid flash.
- Extend packing time:Determine via gate freeze test: Gradually extend packing time until part weight no longer increases, then add 1–2 seconds extra.
- Optimize packing switchover point: Switch from position-dependent to time-dependent control, or adjust timing so that packing starts when the cavity is 95%–98% filled.
- Adjust mold and melt temperatures: Raise mold temperature (by 10–20°C): Improves melt flow, delays cooling, and aids compensation.
- Appropriately lower melt temperature: Reduces overall shrinkage (especially for crystalline materials), but avoid short shots.
- Increase cooling time: Ensure complete part solidification to reduce post-ejection distortion that may appear as sink marks.
3. Mold Modification (If Process Adjustments Fail)
Optimize the feeding system:
- Enlarge gate size or switch to a fan gate to delay gate freezing.
- Increase number of gates or reposition them to shorten flow paths.
- Optimize runner design: Increase sprue/runner diameter to reduce pressure loss.
Improve mold cooling:
Add cooling channels near sink areas or use beryllium copper inserts for faster, more uniform cooling.
Enhance venting:
Check for trapped air in sink areas that may hinder packing; add vent slots if needed (depth: 0.02–0.04 mm).
Modify wall thickness (evaluate carefully):
Reduce local thickness (e.g., ribs to 50%–60% of nominal wall thickness), but confirm with the product designer.
4. Material and Design Collaboration
Change material:
Switch to a low-shrinkage grade (e.g., mineral-filled PP) or adjust material drying process.
Optimize product design:
Discuss with the client: Add fillets, apply fine texture (to hide sink marks), or allow slight wall thickness reduction in sink-prone areas.
5. Temporary or Emergency Measures
Reduce injection speed: Increase injection time for more effective packing pressure transfer.
Use multi-stage packing: Apply higher packing pressure to areas far from the gate.
Use mold temperature controllers: Rapidly raise local mold temperature (e.g., with hot oil circulation near sink areas).
6. Systematic Troubleshooting Process
Record current process parameters (packing pressure/time, mold temp, melt temp, etc.).
Perform short-shot tests: Observe fill pattern and weld line locations.
Weight analysis: Conduct packing time step tests to determine optimal packing time.
Use mold flow analysis software (e.g., Moldflow): Simulate sink marks and optimize gate/cooling design upfront.
Key Considerations
Avoid over-packing: May cause sticking, high internal stress, or dimensional issues.
Balance runners and gates: For multi-cavity molds, ensure uniform filling across all cavities.
Collaborate with material suppliers: Obtain accurate PVT data for process optimization.
Summary:
First, try to resolve ~70% of sink issues through process adjustments (packing, temperature, time). If ineffective, consider modifying the feeding or cooling systems. Finally, discuss with the product design team to optimize wall thickness distribution. Document each adjustment to build a systematic solution database.
