Plastic products develop silver streaks, what is the reason and how can plastic manufacturers eliminate them?

(1) Defect Features

The main manifestation of silver streaks on plastic parts is long, needle-like white frost-like fine lines on the surface of the plastic product.

(2) Causes and elimination methods of defects in plastic manufacturing

The common form of silver streaks is needle-like silver-white stripes formed by elongated air bubbles, which are further divided into degraded silver streaks and moisture silver streaks, all of which are formed by gases precipitating from the front end of the material flow. Degraded silver streaks are formed by the overheating degradation of the melt, while moisture silver streaks are formed by the vaporization of water contained in the raw materials. The specific analysis is as follows.

1) Injection mold

(1) If the cooling water channel of the mold leaks and seeps into the mold cavity, the moisture will vaporize into water vapor at high temperatures, thereby generating moisture silver streaks. To prevent this, mold cavity leakage should be strictly prevented.

(2) If the exhaust system is designed improperly and cannot quickly exhaust the gas in the melt, causing it to accumulate in the mold cavity, these gases may enter the melt and produce silver streaks. In this case, the exhaust system should be checked and, if necessary, corrected.

(3) If the gate is too small, and the cross-sectional area of the main runner and the sub-runner is too small, the melt will flow at a high speed, causing the temperature to rise, accelerating degradation, increasing degraded gases, and increasing the possibility of silver streak formation. In this case, the gate should be enlarged, and the cross-sectional area of the main runner and the sub-runner should be increased.

2) Injection molding process

(1) If the melt temperature is too high, it will promote the degradation of plastic, producing gas and thereby generating silver streaks. In this case, the melt temperature should be lowered.

(2) If the holding pressure time of the melt in the cavity is too long, the generated gas will accumulate and produce silver streaks. In this case, the holding pressure time should be appropriately shortened.

(3) If the injection speed of the melt is too fast, the local temperature of the melt will rise sharply, accelerating degradation and increasing gas production, leading to silver streak formation. In this case, the injection speed can be appropriately reduced.

(4) If the melt lingers in the barrel for too long, the gas production will increase, increasing the possibility of silver streak formation. In this case, the dwell time of the melt in the barrel should be appropriately shortened.

(5) Since silver streaks are formed by excessive gas entering the melt, adopting measures such as using a vented screw can reduce or even eliminate silver streaks.

3) Raw materials

(1) If the moisture content of the raw materials is too high, excessive water vapor will easily form silver streaks after heating. To prevent this, the raw materials must be thoroughly pre-dried according to the moisture content requirements.

(2) Mold release agents can also produce a small amount of volatile gas, providing conditions for the formation of silver streaks. To address this, the amount of mold release agent should be reduced.

(3) Reducing degradation can effectively prevent the formation of degraded silver streaks. Therefore, it is important to use resins with uniform particle size, reduce the amount of recycled materials, remove debris from the raw materials, and clean the residual material in the barrel.