How to deal with insufficient toughness and cracking after stress relief in plastic products

Insufficient toughness and cracking after stress relief in plastic products is a very common issue in injection molding and plastic product applications. Insufficient toughness means the material cannot absorb impact energy, while stress cracking indicates significant residual stress inside the product. When exposed to environmental factors (like chemical solvents, temperature changes) or external forces, this stress is released, leading to the initiation and propagation of micro-cracks.

I. Optimize the Injection Molding Process (Most Direct, Lowest Cost)

• Increase Material and Mold Temperatures:
  • Principle: Low material temperature leads to poor plasticization and severe molecular orientation; low mold temperature causes the melt to cool too quickly, freezing the molecular chains in the surface layer without time to relax, dramatically increasing internal stress.
  • Action: Within the material's allowable range, appropriately increase the barrel temperature and mold temperature (e.g., for ABS, raise mold temp from 40-60°C to 60-80°C; for PC, aim for 80-100°C or higher). This helps molecular chains relax and reduces internal stress.
• Reduce Injection and Holding Pressures:
  • Principle: Excessively high pressures force molecules into extreme orientation, creating significant residual stress.
  • Action: Use multi-stage injection. Switch to a lower pressure for holding after filling most of the cavity. Use the lowest pressure possible that still produces a fully filled, non-short shot part.
• Ensure Smooth Ejection & Adequate Draft:
  • Principle: If ejection is unbalanced or draft angles are insufficient, the ejector pins will create immense mechanical stress on the part, leading to ejection stress marks or cracking.
  • Action: Check if all ejector pins are resetting evenly. Increase the ejection area or adjust ejection speed (slow-fast-slow) if necessary.
• Extend Cooling Time:
  • Ensure the part is fully cooled and solidified inside the mold to prevent stress concentration caused by post-ejection shrinkage.

II. Check and Adjust the Material (Root Cause Lies in Properties)

• Proper Drying:
  • Check: Verify the material is thoroughly dried. Hygroscopic materials like PC, PA, and PET will hydrolyze at high temperatures if not dried, reducing molecular weight and becoming brittle.
  • Requirement: Strictly follow the drying temperature and time specified by the material supplier.

• Consider Adding Impact Modifiers:
  • For modified materials, consider adding compatibilizers + elastomers (e.g., POE, SBS, high-rubber ABS powder). For instance, in glass-fiber reinforced materials (like PA6+GF30), high glass fiber content leads to poor flow, high anisotropy, and increased brittleness. Adding an appropriate impact modifier can help.

• Check Regrind Ratio:
  • Check if too much regrind (recycled sprue/runner material) is being used. Regrind undergoes multiple heat histories, breaking molecular chains and degrading performance. It's recommended to keep the regrind ratio below 20%-30%.

III. Review Part and Mold Design (Fundamental Solution Through Design)

• Add Radii (Fillets) at Corners:
  • Problem: Sharp corners and right angles are the most common sites for stress concentration.
  • Solution: Design transitions at corners, ribs, and sudden wall thickness changes with generous radii. The radius R should ideally be at least 0.5mm, and larger is better.

• Maintain Uniform Wall Thickness:
  • Problem: Significant variations in wall thickness (abrupt changes from thick to thin) cause uneven cooling rates and immense shrinkage stress.
  • Solution: Aim for uniform wall thickness. If variations are necessary, design gradual transitions.

• Optimize Gate Location:
  • A gate located in a stress-bearing or thin area can leave significant residual stress from the high-pressure holding phase. Use mold flow analysis (e.g., Moldflow) to position the gate in a non-stressed area or a thicker section.

IV. Post-Processing to Eliminate Residual Stress

V. Check for Solvent or Environmental Attack

• Check Contact Substances: Does the product contact oils, cleaning agents, rust preventatives, paints, etc.? Certain chemicals (e.g., alcohol on PC, motor oil on ABS) can significantly lower the material's critical stress for cracking.
• Suggestion: Avoid contact with organic solvents before stress relief, or switch to a material with better resistance to that specific chemical.

Summary: Recommended Troubleshooting Sequence

1. Start with Process: Ensure material is dried → Increase mold and material temperatures → Reduce injection and holding pressures.
2. Then Check Material: Check for excessive regrind usage → Confirm if the correct material grade was selected (e.g., using a filled compound instead of an impact-modified grade).
3. Post-Processing: If parts are already made and cracking, try annealing as a remedial measure.
4. Modify Mold/Tool: If the above methods fail, mold modifications may be necessary, such as adding radii or changing the gate location.