Injection Molding Sink Marks: Causes, Prevention, and Solutions

Sink marks are a common surface defect in injection molding, appearing as small depressions or indentations on the surface of the molded part. These marks are usually caused by the uneven cooling or insufficient packing of the molten material, leading to a localized shrinkage in certain areas of the part. While sink marks might not always affect the functionality of a part, they can be aesthetically displeasing and indicate potential issues in the Injection Molding Sink Marks that could affect overall quality.

Causes of Sink Marks

1. Thick Sections
   • Cause: Sink marks often occur in areas of the part where the wall thickness is greater. Thicker sections cool and solidify more slowly, leading to differential shrinkage that causes the surface to pull inward.
   • Example: Ribbed sections, bosses, or areas with varying wall thicknesses are particularly prone to sink marks.
2. Inadequate Packing Pressure
   • Cause: During the packing phase, insufficient pressure may be applied to the molten material, leading to incomplete filling of the mold cavity. As the material cools and shrinks, the surface can collapse inward, creating a sink mark.
   • Example: A part with intricate details or deep sections may require higher packing pressure to avoid sink marks.
3. Improper Cooling Time
   • Cause: If the cooling time is too short, the material may not fully solidify before the mold opens, causing the surface to contract and form a sink mark.
   • Example: Parts with thicker walls require longer cooling times, but if the cycle is rushed, sink marks may develop.
4. Gate Location
   • Cause: The location of the gate (where the molten plastic enters the mold) can influence how material flows and fills the mold. Poor gate placement can lead to uneven filling and localized shrinkage.
   • Example: A gate positioned far from a thick section may not apply sufficient packing pressure to prevent sink marks in that area.
5. Material Selection
   • Cause: Different materials have different shrinkage rates. Materials with high shrinkage tendencies are more likely to develop sink marks, especially in thick sections.
   • Example: Semi-crystalline materials like polypropylene (PP) and polyethylene (PE) are more prone to sink marks due to their higher shrinkage rates compared to amorphous materials like polystyrene (PS) or polycarbonate (PC).

Prevention and Solutions for Sink Marks

1. Optimize Part Design
   • Uniform Wall Thickness: Aim for consistent wall thickness throughout the part to minimize differential cooling and shrinkage. Gradual transitions between different thicknesses can help reduce the risk of sink marks.
   • Ribs and Bosses: Instead of increasing wall thickness for strength, use ribs or bosses. These features add strength without significantly increasing the likelihood of sink marks.
2. Increase Packing Pressure
   • Higher Packing Pressure: Apply sufficient packing pressure during the injection process to ensure that the mold cavity is completely filled and the material is compacted. This can help compensate for shrinkage as the material cools.
   • Extended Packing Time: Increase the duration of the packing phase to allow more material to enter the mold and fill in any potential voids.
3. Adjust Cooling Time
   • Extended Cooling Time: Allow enough time for the material to cool and solidify fully before ejecting the part. This is especially important for parts with thicker sections.
   • Controlled Cooling: Implement uniform cooling across the part to ensure even solidification. This can involve optimizing mold temperature and cooling channel design.
4. Optimize Gate Location and Design
   • Strategic Gate Placement: Position the gate closer to thicker sections of the part to ensure sufficient packing pressure and material flow. Multiple gates may be necessary for large or complex parts.
   • Gate Size and Type: Use larger gates or specific gate types (such as fan gates) to improve material flow and reduce the risk of sink marks.
5. Material Selection and Processing Adjustments
   • Material Selection: Choose materials with lower shrinkage rates for parts prone to sink marks. Amorphous materials typically have lower shrinkage and may be better suited for applications where sink marks are a concern.
   • Processing Parameters: Adjust the injection speed, temperature, and pressure to optimize material flow and reduce the likelihood of sink marks.
6. Mold Design Improvements
   • Cooling Channel Design: Design the cooling channels in the mold to provide uniform cooling across the part. This helps prevent hot spots and ensures even solidification.
   • Mold Venting: Proper venting of the mold can prevent air traps and ensure complete filling of the cavity, reducing the risk of sink marks.

Inspection and Quality Control

• Visual Inspection: Regularly inspect molded parts for surface defects, including sink marks. Early detection allows for timely adjustments to the process.
• Dimensional Analysis: Use tools like coordinate measuring machines (CMMs) to check for dimensional accuracy and detect any areas where sink marks may have caused deviations.
• Simulation Software: Use injection molding simulation software to predict potential issues, including sink marks, before production begins. This can help optimize design and process parameters to prevent defects.

Summary

Sink marks in injection molding are a common issue that can affect the appearance and quality of plastic parts. By understanding the causes and implementing preventive measures—such as optimizing part design, adjusting packing pressure, and improving cooling time—you can reduce or eliminate sink marks in your molded parts. Through careful attention to detail in both design and processing, manufacturers can ensure the production of high-quality parts with minimal defects.