Ways How to Resolve Silver Streak
What is the injection molding defect “Silver Streak”?
During injection molding, the gas (air) generated in the resin is stretched in the mold, resulting in streak-like “flow traces” that appear on the surface of the resin as silver streaks.
What are the causes and countermeasures for the silver streak defect in injection molding?
The causes of silver streaks include insufficient drying of the resin material, insufficient degassing in the cylinder of the Injection machine (moisture, volatile substance, etc.), Gas burn, thermal decomposition, and air entrainment in the mold.
Improvement measures by Injection Parameter Setting:
Overheated material is one of the most common causes of silver streaks.
When plastic resin overheats, it can degrade and off-gas.
However, there are several causes as below,
| Cause | Countermeasure |
| ① Part of the molding material thermally decomposes, producing volatile substances. | Lower cylinder temperature or shorten the molding cycle |
| ② Insufficient plasticization of the material | Improves plasticization |
| ③ Fast injection speed | Slow down the injection speed and suppress the occurrence of shear heat |
| ④ Insufficient injection pressure | Ensure sufficient injection pressure |
| ⑤Air gets caught in the cylinder | apply back pressure |
| ⑥Mold temperature is low | increase mold temperature |
In addition to the above,
• Check the pre-drying conditions for the resin and perform proper drying.
• Reduce the rotation speed of the screw cylinder. (avoid air entrapment)
• Shorten the residence time in the screw.
Improvement measures by Mold
Air trap is the one of most common causes of silver streak by mold.
The Air venting is the most important point during mold design to avoid silver streak or any gas related concern.
| Cause | Countermeasure |
| Poor gas venting (air venting) | Incorporate a gas venting structure at the appropriate location within the mold |
| The gate is not in the correct position | Determine the appropriate gate location and redesign the mold |
| ③Gate, runner, and sprue are smaller than the product. | Increase the size of gates, runners, and sprues |
In addition to the above,
• Addition and extension of cold slug well.
• Check to see if it is a threaded sprue and replace it.
• Check to see if there are any narrow spots on the way from the sprue to the gate.
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