“Black streaks,” also known as “black lines,” refer to the black-colored streaks or lines that appear on the surface of plastic parts. The main cause of black streaks is the thermal degradation of the molding material, which is common in plastics with poor thermal stability, such as PVC and POM.
Effective measures to prevent the occurrence of black streaks include preventing the melt temperature inside the barrel from becoming too high and reducing the injection speed. If there are scars or gaps in the barrel or screw, the material adhering to these parts can overheat, leading to thermal degradation. Additionally, cracks in the check ring can also cause thermal degradation due to melt retention, so it is important to be cautious when working with high-viscosity or easily degradable plastics.
The reasons for the occurrence of black streaks are primarily related to factors such as the melt temperature being too high, the screw speed being too fast, excessive back pressure, eccentricity between the screw and barrel causing frictional heat, inadequate or excessive temperature at the nozzle orifice, instability or poor dispersion of colorant, residual melt sticking in the nozzle head, dead spots in the check ring/barrel causing overheating of the material, contamination in the recycled material in the feed throat, a too-small injection port, metal blockages in the nozzle, and excessive residual material leading to prolonged melt residence time.
To improve the issue of black streaks, the following measures can be taken: lowering the barrel/nozzle temperature, reducing screw speed or back pressure, conducting machine maintenance or replacing the machine if necessary, appropriately increasing the nozzle diameter or reducing its temperature, replacing or adding diffusers, cleaning residual material from the nozzle head, inspecting the screw, check ring, or barrel for wear, checking or modifying the feed throat material, adjusting the injection port, or clearing foreign objects from the nozzle, and reducing the amount of residual material to shorten the melt residence time.
Location: Ningbo Chenshen Plastic Industry, Yuyao, Zhejiang Province, China
Date: 27/09/2023
Post time: Oct-30-2023