In PVC extrusion production, black lines and black spots on the finished product are common quality issues that can significantly impact aesthetics and performance. These defects are often caused by PVC degradation, contamination, or improper machine operation. A key factor in preventing them lies in the proper adjustment of the barrel and screw.

Why Do Black Lines and Black Spots Appear?

PVC is a thermally sensitive material that can degrade if subjected to excessive heat or shear stress. Common causes include:

1. Thermal Degradation – High barrel temperatures or hotspots can burn PVC, producing black marks.
2. Contamination – Residues from previous production, metal particles, or degraded PVC.
3. Screw and Barrel Issues – Incorrect screw speed, compression ratio, or flight depth can generate uneven melt.
4. Material Stagnation – PVC sitting too long in the barrel may carbonize and cause black lines.

For a deeper understanding of PVC screw and barrel design, see our article on PVC barrel screw design features.

How to Adjust the Barrel and Screw

1. Control Barrel Temperature

• Lower barrel zone temperatures slightly to avoid thermal degradation.
• Ensure even heat distribution; uneven heating creates local hotspots.
• Use reliable thermocouples and monitor temperature profiles regularly.

2. Optimize Screw Speed and Feed Rate

• Reduce screw speed to minimize shear stress.
• Maintain consistent feed to prevent material stagnation.
• Adjust the compression ratio if your screw allows, to reduce local overheating.

3. Check Screw Geometry

• Use a low-shear screw specifically designed for PVC.
• Ensure flight depth and barrier/mixing sections are optimized to achieve uniform melt without degrading the material.

4. Maintain and Clean the Barrel

• Remove carbonized residues regularly to prevent black spots.
• Inspect barrel wear and nozzle buildup, which can trap material and cause defects.
• Consider bimetallic barrels or surface coatings to resist corrosion and wear.

5. Use Stabilizers and Additives

• Heat stabilizers in PVC help reduce decomposition during extrusion.
• Ensure proper mixing of stabilizers to achieve a consistent melt.

6. Inspect Material Quality

• Ensure raw PVC is clean, dry, and free from foreign particles.
• Proper drying reduces moisture-induced degradation and black marks.

Preventive Measures

• Regularly inspect and maintain screws and barrels.
• Use PVC-specific screws and barrels designed for low-shear, gentle mixing.
• Monitor barrel temperature zones with precise instrumentation.
• Use high-quality, contamination-free PVC resins.

For additional guidance on preventing wear and defects, refer to injection molding screw and barrel wear tolerances.

Final Thoughts

To solve black lines and black spots in PVC extrusion, proper barrel and screw adjustments combined with good maintenance practices and high-quality materials are essential. Optimizing temperature, screw speed, feed rate, and using PVC-compatible screws ensures smooth extrusion, consistent product quality, and longer machine lifespan.

Expert PVC Screw and Barrel Solutions

At Zhoushan Juyuan Machinery Co., Ltd., we specialize in PVC screws and barrels designed to minimize degradation and defects in extrusion processes. Our expert team can recommend the best design and adjustment settings for your machine to produce smooth, defect-free PVC products.

📧 Email: cxj.job@gmail.com
📞 Phone/WhatsApp/WeChat: +86 13905808474
🌐 Website: https://juyuan-machinery.com/

👉 Contact us today to optimize your PVC extrusion process and eliminate black lines and black spots!