Why Does Plastic Degrade During Extrusion and How to Prevent It?
Why Does Plastic Degrade During Extrusion and How to Prevent It?
Introduction
Plastic degradation is one of the most common challenges in extrusion processing. During production, excessive heat, high shear stress, oxygen exposure, and long residence time can cause polymer chains to break down.
This degradation negatively affects:
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Product appearance
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Mechanical strength
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Surface quality
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Melt stability
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Production efficiency
Understanding why degradation occurs and how to prevent it is essential for maintaining stable production quality.
What Is Plastic Degradation?
Plastic degradation refers to chemical or physical deterioration of polymer materials caused by excessive thermal, mechanical, or oxidative stress.
Common signs include:
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Yellowing
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Black specks
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Burn marks
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Odor generation
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Reduced strength
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Brittle products
Main Causes of Plastic Degradation
1. Excessive Shear Heat
High compression ratios and aggressive screw geometries create excessive friction.
This generates shear heat that may exceed the material's thermal stability limit.
2. Long Residence Time
If material remains inside the barrel for too long, degradation risk increases significantly.
High L/D ratios combined with low throughput may worsen this issue.
3. Overheating
Improper temperature control can quickly damage heat-sensitive materials such as PVC.
4. Oxygen Exposure
Air trapped inside the barrel can accelerate oxidation.
This is especially problematic in recycled material processing.
How Screw Design Influences Degradation
Compression Ratio
| Compression Ratio | Effect |
| Low | Reduced shear heat |
| Moderate | Balanced melting |
| High | Increased degradation risk |
L/D Ratio
Longer screws improve melting but increase residence time.
Most extrusion systems use:
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24:1 – 30:1 for standard applications
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Lower ratios for heat-sensitive materials
Mixing Sections
Improper mixing designs may create localized overheating.
Careful mixing structure optimization is essential.
Material-Specific Considerations
PVC
PVC is highly sensitive to heat.
Even small temperature increases can cause severe degradation.
Recycled Materials
Recycled plastics are often already partially degraded.
Additional thermal stress further reduces stability.
High-Filler Materials
Abrasive fillers increase friction and heat generation.
Practical Methods to Prevent Degradation
Optimize Temperature Profile
Use gradual heating zones.
Avoid sudden temperature spikes.
Reduce Screw Speed
Lower RPM reduces shear heat.
Improve Cooling Systems
Stable barrel cooling helps maintain consistent processing conditions.
Use Proper Screw Design
Lower compression and optimized geometry reduce excessive stress.
Common Signs of Degradation
| Symptom | Possible Cause |
| Yellowing | Overheating |
| Black Spots | Burned material |
| Odor | Thermal decomposition |
| Brittle Products | Polymer chain breakdown |
| Surface Roughness | Melt instability |
Future Trends in Low-Degradation Extrusion
Manufacturers increasingly adopt:
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Smart temperature monitoring
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Energy-efficient screws
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Advanced cooling systems
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Low-shear screw designs
These technologies improve stability while reducing scrap rates.
Conclusion
Plastic degradation during extrusion is caused by a combination of thermal, mechanical, and oxidative factors.
By optimizing screw design, controlling temperature, reducing shear stress, and selecting suitable processing conditions, manufacturers can significantly improve product quality and production stability.
Preventing degradation not only improves product appearance and performance but also reduces waste and operating costs.