Sink Marks in Plastic Parts: Causes and Fixes

Sink marks are a common surface defect in injection molding, appearing as small depressions or dimples on thick sections, near ribs, bosses, or abrupt geometry transitions. While primarily cosmetic, they can also compromise dimensional accuracy, surface finish, and structural integrity, particularly in high-precision components used in automotive, electronics, and consumer products.

Causes of Sink Marks

Sink marks generally result from uneven cooling, material shrinkage, or inadequate packing. Some of the most common causes include:
  1. Uneven Wall Thickness: Thicker sections of a part cool and solidify more slowly than thinner areas. As the material cools and shrinks, the surface may depress inward, forming a sink mark. Abrupt transitions from thick to thin walls increase this risk.
  2. Inadequate Packing Pressure or Holding Time: During injection molding, molten plastic continues to shrink as it cools. If the packing pressure is too low or the holding time too short, the mold cavity is not sufficiently filled, leaving voids or surface depressions.
  3. Uneven Mold Cooling System: Non-uniform cooling causes certain areas of the mold to solidify faster than others. Hot spots, especially under thick sections, can exacerbate material shrinkage, leading to visible sink marks.
  4. Material Properties: Some plastics have higher shrinkage rates or are more prone to differential cooling, making them more susceptible to sink marks. For example, semicrystalline polymers like polypropylene (PP) and polyethylene (PE) often require careful design consideration.

Solutions to Sink Marks

Preventing or reducing sink marks involves a combination of design optimization, process control, and mold engineering:
  1. Optimize Part Design: Maintain as uniform a wall thickness as possible throughout the part. Gradual transitions between thick and thin sections, fillets, or ribs can help distribute material more evenly during cooling. Thick bosses or ribs should be supported with coring to reduce mass.
  2. Increase Packing Pressure and Holding Time: Adequate packing compensates for material shrinkage as it cools. Adjusting injection pressure and hold time ensures the cavity remains completely filled during solidification, minimizing depressions on the surface.
  3. Improve Mold Cooling Layout: A well-designed cooling system promotes uniform mold temperature, accelerating and equalizing the solidification process. Balanced cooling channels reduce hot spots and decrease the risk of surface depressions. Advanced techniques like conformal cooling using 3D-printed channels can further enhance heat removal efficiency.
  4. Select Low-Shrinkage Materials: Materials with lower shrinkage or high flowability help reduce sink mark formation. Some filled plastics, such as glass-reinforced polyamide (PA-GF), offer improved dimensional stability in thick sections.
  5. Use Simulation Tools: Computer-aided engineering (CAE) software can predict areas susceptible to sink marks before production. Moldflow or similar simulations help optimize wall thickness, gate location, cooling channels, and process parameters, reducing trial-and-error adjustments on the production floor.

Prevention Strategies

Proactive measures can further minimize the occurrence of sink marks:
  • Incorporate consistent wall thickness and smooth transitions during part design.
  • Balance packing pressure, injection speed, and holding time based on material properties.
  • Implement uniform mold cooling with adequate channels in thicker sections and around ribs.
  • Consider material selection, using plastics with lower shrinkage or adding fillers to reduce volumetric contraction.
  • Perform pre-production CAE simulations to identify potential sink mark areas and adjust design or process parameters.

Impact on Part Quality

Sink marks can affect both functional and aesthetic aspects of molded parts:
  • Visual Appearance: Surface depressions reduce the cosmetic quality of consumer products such as housings, handles, and decorative parts.
  • Dimensional Accuracy: Sink marks can distort critical surfaces, affecting assembly or mating with other components.
  • Structural Integrity: Concentrated sink marks near ribs or bosses may act as stress concentrators, reducing mechanical strength under load.
  • Cost Implications: Parts with sink marks may require secondary finishing, polishing, or even scrapping, increasing production costs.

Conclusion

Sink marks can be effectively minimized through careful part design, mold optimization, and process control. Effective management of sink marks not only reduces material waste and lowers production costs but also improves product reliability across automotive, electronics, consumer, and industrial applications.

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