Injection Molding Bubbles: Causes and Solutions for Different Defect Types
Bubbles in injection molded parts can appear in different forms depending on part thickness, molding conditions, material behavior, and mold design. Because the appearance of the defect varies, the root causes and corrective actions also differ. Understanding the type of bubble defect is essential for solving the problem efficiently and improving part quality.
1. Bubbles in Thick-Wall Products
In thicker plastic products, the surface may appear swollen or bulged, and bubbles may be visible when the part is cut open. This type of defect is often related to trapped air, insufficient cooling, improper feeding, or material degradation during molding.
Possible causes include:
- Injection speed is too fast, causing air to become trapped in the cavity and remain inside the part.
- Feed amount is too high or too low, and injection pressure is too high, which can worsen bubble formation.
- Cooling time is too short, especially for thick-wall products, so the inner material does not solidify properly before ejection.
- Material decomposition occurs because of excessive melt temperature, sometimes accompanied by black streaks or burn marks.
- The mold gate is too small, or venting is insufficient, which prevents gas from escaping effectively.
Recommended solutions include reducing injection speed, adjusting feed volume properly, lowering excessive injection pressure, extending cooling time, reducing melt temperature when necessary, and improving mold venting or gate size.
2. Voids and Blackened Internal Bubbles in Solid Products
Solid molded parts may sometimes develop internal holes or blackened voids. These defects are usually caused by air entrapment combined with overheating or burn caused by high-pressure friction.
When molding solid products, excessively high injection pressure and fast injection speed can trap air inside the part. The trapped air may then overheat under compression and create dark voids or burn-related internal defects.
In these cases, the following adjustments can help:
- Reduce injection pressure appropriately.
- Lower injection speed to reduce air entrapment and friction burning.
- Check whether feed amount is excessive and reduce it if necessary.
By lowering filling aggression and balancing shot size, internal void and blackening problems can often be reduced or eliminated.
3. Small Bubbles in Thick Sections of Uneven-Wall Products
When a plastic part has uneven wall thickness, small bubbles may appear in thicker areas, while extra-thick sections may even develop larger dark bubbles. This problem becomes more likely when multiple products with different shapes are molded in the same mold.
Common causes include:
- Uneven gate sizing relative to the weight and geometry of each molded part.
- Thick sections cooling more slowly than thin sections, which increases the risk of internal void formation.
- Insufficient mold venting, especially when parts tend to remain on the cavity side or front mold side, making gas evacuation more difficult.
- Excessive local wall thickness in product design.
To improve this type of defect, it is often necessary to modify mold design, improve venting, extend packing and cooling time, and ensure that gate dimensions are properly matched to each part. From a product design perspective, overly thick sections should be avoided whenever possible to reduce the risk of bubble defects.
Conclusion
Bubble defects in injection molding are not caused by a single factor. They may result from trapped air, poor venting, excessive injection speed, improper feed amount, insufficient cooling, high melt temperature, or poor product wall-thickness design. The defect pattern must be evaluated carefully so that the correct process adjustment or mold improvement can be made.
By optimizing injection speed, pressure, cooling time, gate design, venting, and part wall thickness, manufacturers can significantly reduce bubble defects and improve molded part consistency.