Why Does Inadequate PC Drying Cause Silver Streaks?

The silver streak problem in PC (polycarbonate) is arguably one of the most classic—and most easily misdiagnosed—defects on the injection molding floor. Radial silvery streaks appear on the product surface, looking as if the material has "burned." The operator's first reaction is often to lower the melt temperature and reduce injection speed. But many times, the real cause is hidden in the hopper—PC pellets were not dried adequately, and moisture was carried into the barrel.

Why is PC so sensitive to moisture? Why does inadequate drying cause silver streaks on the surface? The answer must be understood from two levels: PC's molecular structure and the physical behavior of moisture during injection molding.

Moisture Does Not Simply "Evaporate" into Bubbles—It Is "Dragged" to the Surface

The essence of silver streaks is a series of elongated microscopic bubbles arranged along the melt flow direction on the product surface. The gas inside these bubbles comes mainly from water vapor.

The moisture absorbed by PC pellets from the air is not simply attached to the surface—it diffuses into the interior of the pellets and dissolves in the free volume between molecular chains. The purpose of drying is to let this internal moisture slowly diffuse to the pellet surface and then be carried away by dry air. If drying time is insufficient or the temperature is wrong, moisture remains inside the pellets.

When these moisture-containing pellets enter the barrel, the situation changes. Barrel temperatures are typically between 280°C and 320°C, and water is in a gaseous state at this temperature. In molten PC, water exists as dissolved gas or microscopic bubbles. As the screw advances and the melt is injected into the cavity, these water vapor bubbles flow along with the melt. During filling, the bubbles are stretched into ellipses by the melt front and eventually contact the cold mold wall. After cooling and solidifying, they leave a series of shiny bubble points on the product surface—these are silver streaks.

A key observation is that silver streaks usually appear near the gate or on the product surface, rather than being uniformly distributed. This is because bubbles migrate to the surface with the melt flow, and the shear near the gate is most intense, making gas release most likely there.

PC's Hydrolysis Sensitivity: It Is Not Just "Water Turning into Gas"

If silver streaks were only a physical defect caused by water vapor bubbles, the problem would be relatively simple. PC's trouble is that moisture also triggers chemical degradation at high temperatures—hydrolysis.

PC's molecular backbone contains ester bonds. Under high temperature (230-320°C) and in the presence of water, ester bonds undergo hydrolysis, molecular chains are cut, and molecular weight decreases. Research explicitly points out that PC is sensitive to moisture at high temperatures and prone to hydrolysis, forming silver streaks and flow marks. A classic study on PC hydrolysis also showed that high-temperature, high-humidity environments reduce PC's molecular weight and impact strength.

The consequences of hydrolysis are twofold. On one hand, molecular chain scission produces low-molecular-weight fragments and gases, which further increase the severity of silver streaks. On the other hand, the decrease in molecular weight means the material's mechanical properties—impact strength and tensile strength—have already been irreversibly damaged. So silver streaks are not just an "appearance problem"—they are simultaneously a "performance problem."

PC's requirement for moisture content is extremely stringent. In industry practice, PC's moisture content typically needs to be controlled below 0.02% (i.e., below 200 ppm). Patent data shows that even after hot-air drying at 120°C for 4 hours, the pellets may still retain about 130 ppm of moisture. This level may not produce silver streaks under ordinary molding conditions, but under more demanding molding conditions or higher temperatures, it will cause yellowing and performance decline. Therefore, for materials like PC, an ordinary hot-air dryer is often not enough—a desiccant dryer is needed to lower the dew point sufficiently and control moisture content within a safe range.

The Formation Process of Silver Streaks: From Dissolution to "Tearing"

Putting the entire chain together, the process by which inadequate PC drying leads to silver streaks is as follows:

Step one: moisture dissolves inside the pellets. PC's ester bonds are polar and have an affinity for water molecules. Moisture diffuses into the pellet core—it is not simply surface attachment.

Step two: inadequate drying leaves residual moisture. If drying temperature is too low, time is too short, or the drying air dew point is not low enough, the moisture inside the pellets is not completely driven out. If dried PC is exposed to humid air, it will reabsorb moisture.

Step three: water vaporizes in the barrel and triggers hydrolysis. Residual moisture vaporizes into steam at barrel temperature, while some moisture undergoes hydrolysis with PC's ester bonds, producing low-molecular-weight fragments and additional gas.

Step four: bubbles in the melt are elongated. Water vapor and gases from hydrolysis form microscopic bubbles in the melt. When the melt fills the cavity, these bubbles are stretched into ellipses along the flow front.

Step five: bubbles contact the cold wall and solidify. The elongated bubbles reach the mold surface, cool and solidify at the cold wall, forming silver streaks arranged along the flow direction.

Why Are Other Materials Not as "Delicate" as PC?

Non-polar materials like PP and PE have no polar groups on their molecular chains, have no affinity for water molecules, and moisture can hardly enter the interior of the pellets. Even if a small amount of surface moisture enters the barrel, the amount of water vapor produced is very small and insufficient to form obvious silver streaks.

PA (nylon) has stronger hygroscopicity than PC, but PA's hydrolysis sensitivity is somewhat lower than PC's. Although PA's amide groups also absorb water, the hydrolysis rate of amide bonds at high temperatures is not as fast as that of PC's ester bonds. However, PA also requires drying; otherwise, silver streaks and bubbles will also appear.

PC's particularity lies in this: it possesses both high hygroscopicity and high hydrolysis sensitivity. Ester bonds readily form hydrogen bonds with water (hygroscopicity) and are readily cut by water at high temperatures (hydrolysis). These two characteristics combined together make PC's drying requirements stricter than most materials.

Inadequate Drying: Silver Streaks Are Only the First Problem to Surface

Silver streaks are the most visible manifestation of inadequate drying, but they are not the only problem. The same moisture also causes:

• Yellowing: acidic substances produced by hydrolysis combined with oxidation reactions cause the product to yellow
• Declining mechanical properties: molecular weight decreases, impact strength and tensile strength are damaged. This damage may not immediately appear in factory inspection, but will be exposed during use
• Unstable dimensions: changes in molecular weight cause shrinkage rate changes, making dimensional tolerances difficult to guarantee for precision parts