Ways How to Combat 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 trace” that appear on the surface of the resin as silver streaks.

What is the injection molding Causes and countermeasures for silver streaks defect “Silver Streak”?

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 Process flaws:

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,

① Part of the molding material thermally decomposes, producing volatile substances.Lower cylinder temperature or shorten molding cycle
② Insufficient plasticization of the materialImproves plasticization
③ Fast injection speedSlow down the injection speed and suppress the occurrence of shear heat
④ Insufficient injection pressureEnsure sufficient injection pressure
⑤Air gets caught in the cylinderapply back pressure
⑥Mold temperature is lowincrease 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.

 Poor gas venting (air venting)Incorporate a gas venting structure at the appropriate location within the mold
The gate is not in the correct positionDetermine 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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