Injection Molding Sink Marks: Causes, Solutions, and Process Optimization
Sink marks are a common molding defect that usually appear in areas where the part wall is too thick or where material shrinkage is not properly compensated during cooling. To solve this problem effectively, it is important to understand both the direct causes and the appropriate short-term and long-term solutions.
1. Causes of sink marks
(1) Uneven wall thickness in different areas of the product.
(2) Insufficient internal cavity pressure during molding.
(3) Inadequate mold cooling.
(4) Part deformation caused by insufficient cooling time.
2. Solutions for sink marks
(1) Immediate process adjustment
Increase injection pressure, extend holding time, reduce barrel temperature and mold temperature where appropriate, and apply forced cooling to the area where sink marks occur.
(2) Short-term mold adjustment
If sink marks appear in a specific area, the local flow path may be improved to help compensate for shrinkage. In some cases, the material distribution around the affected area may also need to be adjusted to improve pressure transmission and packing performance.
(3) Long-term design improvement
Large differences in wall thickness should be avoided as much as possible during product design. Ribs and other thick structural features that are likely to create sink marks should be kept as short and as thin as functionally possible.
When the molding temperature is reduced to a level where no sink marks appear, and sufficient cavity pressure is maintained during packing, the risk of sink formation can be minimized. However, pressure inside the mold cavity is not evenly distributed. Pressure is usually highest near the gate and decreases farther away from the gate. If material flows easily to all areas, the pressure difference throughout the cavity is smaller, which helps reduce sink marks and may also reduce internal stress in the product.
When part of the melt has difficulty flowing into a certain area, pressure may build up locally while pressure in other areas drops. This imbalance can lead to sink marks in low-pressure zones. In addition, areas with excessive local pressure may retain internal stress after molding. In an ideal molding condition, the melt temperature rises appropriately with mold temperature, the material flows well, and injection and packing occur under a more balanced cavity pressure state.
When adjusting molding conditions to reduce sink marks, combinations of temperature, pressure, and time should be planned in advance so the results can be evaluated more efficiently. In practice, it is often easier to understand small pressure changes after first extending the relevant molding time. It should also be noted that when temperature settings are changed, the final evaluation should be based on the stabilized molding result after several cycles, not immediately after the adjustment is made.