PVC Pellet Black Spots: Causes And Solutions

1. What Are Black Spots in PVC Pellets?

Black spots refer to small black particles or discolorations found on the surface or inside PVC pellets, often caused by contamination or degradation during processing.

2. Main Causes of Black Spots

• Incomplete cleaning: Residual material left inside the screw or barrel during product changeover.

• Contaminated raw materials: Impurities or carbonized particles in recycled or filled PVC.

• Screw carbonization: Overheating leads to material degradation and carbon buildup.

• Environmental contamination: Dust, dirt, or human-induced impurities entering the production line.

• Overheating: Excessive processing temperature or prolonged residence time causes melt burning.

3. Solutions to Prevent and Eliminate Black Spots

• Thorough screw cleaning during product changeover — pay special attention to cleaning the “three zones”: filter screen, screen plate, and die head.

• Strengthen raw material inspection, especially for filled compounds, recycled PVC, or additives prone to contamination. Avoid using materials with visible impurities.

• Optimize barrel temperature and residence time based on each formulation to prevent overheating and material carbonization.

• Maintain clean working conditions — keep floors moist to reduce dust and isolate machines producing different colors.

• Regularly clean auxiliary equipment such as water tanks and pelletizers to prevent buildup.

• Clean screen changers and die plates every time the filter is changed to avoid residue carbonization.

• Avoid feeding floor scrap into the pelletizer and control static electricity to prevent dust adhesion.

• Inspect and maintain the heating system regularly; fix temperature control issues promptly.

• Perform regular screw extraction and cleaning to remove residual material and prevent long-term carbon buildup.

Summary:

Black spots in PVC pellets mainly result from carbonization, contamination, or improper temperature control. Regular cleaning, raw material inspection, precise temperature management, and strict hygiene standards are key to ensuring high-quality, defect-free PVC granules.

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