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Eliminating Sink Marks: Injection Molding' Hidden Challenge

Eliminating Sink Marks: Injection Molding’s Hidden Challenge

Pioneer Plastech

written by Joyce W.

edited by Joyce W.

Injection molding is a key process in manufacturing, used to make accurate and durable parts for products like cars, electronics, and everyday items. One common problem in this process is sink marks – small dents on the surface of molded parts that can make them look bad and even affect their performance.

What Are Sink Marks?

Sink marks are tiny dents or dips on the surface of a plastic part made by injection molding. They happen when the plastic cools unevenly, especially in areas where the part is thicker. Even though they might look small, sink marks can cause issues like product rejections and higher production costs.

Sink

Why Do Sink Marks Happen?

  1. Uneven Thickness: Thicker parts take longer to cool, leading to shrinkage and sink marks.
  2. Not Enough Packing Pressure: If the machine doesn’t press enough plastic into the mold, gaps can form as the material cools.
  3. Cooling Problems: Parts that cool too quickly or unevenly are more likely to have sink marks.
  4. Material Type: Some plastics shrink more than others, making sink marks more likely.
  5. Poor Mold Design: Molds with inconsistent walls or not enough venting can worsen the problem.

How Can We Fix Sink Marks?

1
Better Part Design:
o Use the same thickness across the part to avoid uneven cooling. o Smooth out transitions where thickness changes.
2
Adjust Machine Settings:
o Increase packing pressure and time to fill gaps as the plastic cools. o Control cooling rates to ensure even cooling.
3
Choose the Right Materials:
o Use plastics that shrink less to reduce the risk of sink marks.
4
Improve Mold Design:
o Add cooling channels to evenly distribute temperature. o Use ribs to strengthen the part without making it thicker.
5
Try Advanced Techniques:
o Gas-assisted molding: Helps reduce material in thicker areas. o Foam molding: Distributes pressure more evenly inside the part.

How to Prevent Sink Marks in the Future

  • Keep Molds in Good Condition: Regular maintenance helps ensure molds work as expected.
  • Train Your Team: Make sure everyone knows how to spot and fix sink marks early.
  • Use Simulation Tools: Software can predict sink marks before production starts, helping you fix designs ahead of time.

Conclusion

Sink marks might seem like a small issue, but they can have a big impact on product quality and costs. By understanding why they happen and how to fix them, you can create better parts and improve your manufacturing process. With careful design, smart material choices, and advanced tools, you can avoid sink marks and ensure your products meet the highest standards.

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