Sink Marks: Tips For Solving The Problem

About the sink marks

1. Phenomenon of sink marks

The phenomenon that the surface of a molded product is dented due to shrinkage. It is often found in thick molded products.

2. Causes of sink marks

  • Small gates, sprues and runners
  • Poor gate position
  • Insufficient cooling time
  • Overly high or low mold temperature
  • Low injection/holding pressure
  • Short injection/holding time

3. Measures to avoid sink marks

(1) Measures related to mold

  • Lower the cavity surface temperature slightly.
  • Enlarge gates and runners
  • Make the sprue thicker.
  • Reexamine the mold cooling circuit to improve cooling efficiency.
  • Modify the cooling structure of the part that is hard to cool to a structure that is easy to cool.
    (Example: Baffle plate structure, cooling pipe structure, heat pipe, non-ferrous metal nest)
  • Increase the number of gates.
  • Change the gate position to the thick part.

(2) Measures regarding injection molding conditions

  • Increase the holding time.
  • Set a higher holding pressure.
  • Set the injection speed faster.
  • Lower the nozzle temperature.
  • Increase the measured value.
  • Increase the amount of cushion.
  • Try changing the injection molding machine.
  • Allow sufficient cooling time
  • Replace the backflow prevention ring of the injection unit.

(3) Measures related to molded product design

  • Remove the thick part of the molded product.

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