Injection Molding Sink Marks: Causes, Solutions, and Process Optimization
Sink marks are one of the most common defects in injection molding. They usually appear in thick-wall areas, around ribs, bosses, or other heavy sections where plastic shrinkage is not fully compensated during cooling and packing. To reduce sink marks effectively, manufacturers need to evaluate product design, mold structure, cooling performance, and molding conditions together.
1. Common Causes of Sink Marks
Sink marks are typically caused by one or more of the following conditions:
- Uneven wall thickness across different areas of the molded part
- Insufficient cavity pressure during packing and holding
- Inadequate mold cooling efficiency
- Part deformation caused by insufficient cooling time before ejection
In most cases, sink marks occur because the outer surface solidifies first while the inner material continues to shrink. If the cavity pressure is too low during this stage, there is not enough additional material packed into the part to compensate for volume shrinkage.
2. Immediate Process Adjustments for Sink Marks
When sink marks appear during production, the first step is usually to optimize the molding process. Common short-term process adjustments include increasing injection pressure, raising holding pressure, extending holding time, reducing melt temperature where appropriate, lowering mold temperature in selected cases, and improving local cooling efficiency. Additional forced cooling may also help in areas where sink marks are concentrated.
These adjustments are intended to improve packing efficiency, reduce material shrinkage, and allow the part to maintain better dimensional stability during cooling.
3. Short-Term Mold Improvements
If sink marks continue to appear in a specific location, local mold modifications may be required. In some cases, the material flow path should be improved so pressure can be transmitted more effectively to the affected area. This may involve adjusting gate position, runner balance, or local flow distribution. Improving pressure transmission to the problem area often helps compensate for shrinkage more effectively during the packing stage.
4. Long-Term Product Design Improvements
From a product engineering perspective, large wall thickness differences should be avoided whenever possible. Thick sections tend to cool more slowly and create a higher risk of visible sink marks. Structural features such as ribs, bosses, and reinforcement sections should be designed to be as thin and as short as functionally possible. Good wall thickness control is one of the most effective long-term methods for reducing sink mark risk in plastic parts.
5. The Relationship Between Pressure Distribution and Sink Marks
Even when molding conditions seem acceptable, pressure inside the mold cavity is never completely uniform. Cavity pressure is usually highest near the gate and gradually decreases in areas farther from the gate. If the plastic melt flows smoothly throughout the cavity, the pressure difference is smaller and the risk of sink marks is reduced. Better flow balance can also help reduce internal stress inside the finished part.
However, when the melt has difficulty flowing into certain areas, pressure may remain high in one zone while dropping in another. This pressure imbalance often causes sink marks in low-pressure areas. At the same time, high-pressure areas may retain excessive internal stress after molding, which can affect part quality and long-term performance.
6. Why Balanced Temperature and Flow Matter
Under ideal molding conditions, melt temperature, mold temperature, filling behavior, and packing pressure work together to maintain a more balanced cavity pressure state. When the melt flows well and packing pressure reaches all critical areas effectively, shrinkage can be compensated more evenly and the surface of the molded part remains more stable.
7. How to Optimize the Process More Efficiently
When adjusting molding conditions to reduce sink marks, changes in temperature, pressure, and time should be planned systematically. This makes it easier to evaluate which factor is actually improving the result. In practice, it is often easier to understand the effect of small pressure changes after extending the relevant molding time first. Once the holding stage becomes more stable, pressure changes can be evaluated more clearly.
It is also important to remember that temperature adjustments should not be judged immediately after a single cycle. Final evaluation should always be based on stable molding results after several cycles, once the machine, mold, and material condition have fully stabilized.
8. Reducing Sink Marks for Stable Production
To minimize sink marks in injection molding, manufacturers should combine process optimization, local mold improvement, and proper part design. Lower molding temperatures, sufficient packing pressure, balanced cavity flow, effective cooling, and reasonable wall thickness design all play important roles. A stable process with controlled shrinkage behavior is the foundation for producing plastic parts with good appearance and consistent quality.