16 Common Defects in the Injection Molding Production Process

1. Surface Sink marks: This is typically caused by uneven wall thickness of the product or insufficient holding pressure during the injection molding process.

2. Parting Line Flash: Excessive pressure during the molding process can cause flash along the parting line. If the flash occurs around the injection points, it is often due to excessive holding pressure. If it is not localized to the injection points, it may be a mold issue.

3. Splay mark around the injection gate: Insufficient melt time in the material heating process can cause Splay mark around the injectiongate area.

4. Weld Lines: The merging point of two melt flows can result in weld lines. To improve this, additional venting can be added at the merging point or mold temperature can be increased. If the weld line affects the product’s strength, reinforcement ribs can be added at the bottom of the weld line to resolve the issue.

5. Bubble Inside(Void): Air trapped in the mold cavity and mixed with the plastic material can cause bubbles. To address this, additional venting can be added to the mold or back pressure can be increased during the plastic material heating process. Thick wall sections of the product, especially in transparent materials, can also contribute to bubble formation.

6. Short shot/short molding : Adjusting the molding parameters can often improve short filling. If adjustments do not work, checking if the product thickness is above 0.8mm or adding additional injection points may help.

7. Burn Marks: Adding venting on the mold can help resolve localized burn marks.

8. Jetting Marks : Small gate size or excessive injection pressure can cause jetting patterns in the molded product. Adjusting the gate size or reducing injection pressure can help address this issue.

9. Glass emergence on the surface : Large temperature difference between the mold surface and the injection barrel can cause glass emergence on the surface. Increasing the mold temperature can help improve this.

10. Ejected mark: Insufficient cooling time on the mold or excessive clamping force can cause ejected mark on the product’s surface. Adjusting the molding parameters, adding ejector pins, or additional polishing can help improve this.

11. Drag mark: Surface scratches often occur on the parting line and can be caused by insufficient draft angles during the ejection process. Increasing the draft angle can help prevent surface scratches.

12. Surface Peel: Excessive regrind ratio in the plastic material or prolonged material heating time can cause the surface to peel. Small gate size and high-speed filling can also degrade the material’s physical properties and lead to surface peeling.

13. Warpage and Distortion: Warpage and distortion are common in molding processes and can be caused by various factors such as gate position, improper water flow design, and ineffective process setup.
Mold flow analysis and proper design can help minimize warping and distortion.

14. Color Variation: Color variation, or color deviation, is a common defect in injection molding and can be influenced by various factors such as resin material, color additives, mixing, molding process, and machine settings. Color difference meters and controlling the color difference value may be necessary for strict color requirements.

15. Dimensional Stability: For products with high precision requirements, automated production using a robotic arm and servo motor is recommended to ensure consistent molding cycles. Hydraulic and temperature systems should also be carefully controlled. Early stage parameter setting, fixed mold and machine, automated production, and comprehensive testing with customized gauges are necessary for achieving dimensional consistency.

16. Stress Marks: Stress marks on the product surface are caused by uneven shrinkage due to sudden changes in wall thickness. Properly designing the wall thickness and gate positions can help minimize stress marks.