Injection Molding Defects: Internal Stress and Void Formation

In plastic molding and injection molding processes, cylindrical components are particularly prone to internal stress accumulation and internal void defects. These issues can significantly affect dimensional stability, mechanical strength, and long-term performance. This article analyzes the common problems, root causes, and practical corrective measures, and introduces a simple method to evaluate internal stress in molded parts.

Common Problems in Cylindrical Molded Parts

One of the most frequent issues observed in cylindrical plastic components is high internal stress. Due to structural constraints, the material is unable to shrink freely during cooling, which leads to residual stress locked inside the part.

Another common injection molding defect is the formation of voids or cavities in thicker sections. These internal holes are often invisible from the outside but can severely reduce strength, creep resistance, and service life.

Causes of Internal Stress and Voids

The primary reason for excessive internal stress is the restriction of shrinkage caused by the cylindrical geometry. As the material cools, radial and axial shrinkage are constrained, resulting in uneven stress distribution.

Void formation, on the other hand, is mainly caused by localized material accumulation. Thick sections cool more slowly than thin areas, leading to differential solidification. As the core material shrinks, it cannot be sufficiently compensated, resulting in internal cavities.

Solution for Internal Stress and Void

To minimize internal stress and void defects, several effective design and process optimizations can be applied:

• Reduce material accumulation by optimizing wall thickness and avoiding unnecessarily thick sections.
• Remove material at the bottom corner of the cylinder, especially at sharp transitions, to allow more uniform shrinkage and reduce stress concentration.
• Enhance mold core cooling, ensuring more balanced and controlled cooling throughout the part.
• Apply post-molding annealing when required. Depending on the application, annealing the molded part at approximately 140℃for 1 hour can significantly relieve internal stress. Excessive residual stress can greatly reduce the creep resistance and long-term mechanical performance of plastic components.

Evaluating Internal Stress Through Cutting Tests

To accurately assess internal stress levels and predict deformation trends, a practical experimental method can be used. The molded part is fully cut open, as shown in typical stress-release test examples.

Once the part is cut, the release of constrained deformation will allow internal stresses to partially relax. These stresses often cause visible deformation at the cut locations. By marking the force and reaction force directions at positions such as Cut 1 and Cut 2, engineers can observe that the deformation at the cuts theoretically occurs in opposite directions.

This simple yet effective test provides valuable insight into internal stress distribution and helps evaluate the overall molding quality of plastic parts before mass production.

Conclusion

Internal stress and void defects in plastic cylindrical molded parts are closely related to geometry, material distribution, and cooling conditions. Through thoughtful design optimization, improved mold cooling, and appropriate post-processing such as annealing, these issues can be significantly reduced. Combined with stress-release testing, manufacturers can achieve more reliable, stable, and high-quality molded components.

FAQ

What is the injection molding process?

Injection molding is a manufacturing process that injects molten plastic into a mold cavity to create parts. The process involves heating plastic pellets until liquefied, then forcing the material under high pressure into a precision mold where it cools and solidifies into the desired shape.

What are the 4 stages of injection molding?

The four stages are clamping, injection, cooling, and ejection. First, the mold closes and clamps shut. Second, molten plastic is injected into the cavity. Third, the part cools and solidifies inside the mold. Finally, the mold opens and ejects the finished component.

Is injection molding better than 3D printing?

Injection molding is better for high-volume production, while 3D printing excels at prototyping. Injection molding offers faster cycle times, lower per-unit costs, and superior strength for mass manufacturing. 3D printing requires no tooling, allows rapid design iterations, and is more cost-effective for small batches.

What products are made by injection molding?

Common injection molded products include plastic bottles, automotive parts, medical devices, and electronic housings.