Views: 1 Author: Site Editor Publish Time: 2026-01-15 Origin: Site
Hollow pellets are a common defect in plastic pelletizing processes. If not addressed in time, they can negatively affect product appearance, bulk density, and downstream processing performance. This article explains the main causes of hollow plastic pellets and provides practical, production-proven solutions to help improve pellet quality and process stability.
Why Do Hollow Pellets Occur?
Understanding the root causes is the first step toward effective prevention. The most common reasons include:
1. Excessive moisture in raw materials
Moisture trapped in plastic resin vaporizes during heating, forming gas bubbles inside the pellets.
2. Improper plastification temperature
Temperatures that are too high or too low can lead to incomplete melting or thermal degradation, resulting in internal voids.
3. Poor degassing performance
Gases generated during melting or degradation cannot escape efficiently from the melt.
4. Overly rapid cooling
When the pellet surface solidifies too quickly, internal shrinkage occurs, creating hollow structures.
Three Practical Ways to Eliminate Hollow Pellets
1. Control Raw Material Pretreatment Strictly
Drying is critical.
Although common plastics such as PP and PE have relatively low moisture absorption, drying is still necessary when materials are stored in humid environments or when recycled content is used.
ABS, PS, and similar materials should be pre-dried using a hot-air dryer
Recommended drying temperature: 70–85°C (adjust based on material type)
Drying time: 2–4 hours
After drying, materials should be sealed to prevent re-absorption of moisture
2. Optimize Processing Parameters
(1) Precise Temperature Control
Typical processing temperature ranges:
PP: 180–200°C
ABS: 180–210°C
LDPE: 180–200°C
HDPE: 220–240°C
PS: 180–220°C
Proper temperature control ensures stable melt flow and adequate plastification.
(2) Screw Speed and Pressure Adjustment
Moderately increase screw speed to enhance shear heating and melting efficiency
Adjust die head pressure to ensure proper melt compaction
For PE materials, increasing back pressure can significantly improve plastification quality
(3) Ensure Effective Vacuum Degassing
Regularly inspect the vacuum venting system
Make sure vents are clean and unobstructed
For recycled materials, sufficient degassing is especially important to remove trapped volatiles
3. Equipment Maintenance and Process Optimization
Screw inspection: Replace severely worn screws to maintain stable conveying and mixing
Die head optimization: Increasing die pressure can improve pellet density, especially for high-density materials such as HDPE
Cooling system adjustment:
Avoid excessive or sudden cooling
Recommended cooling water temperature for PP: 30–50°C
PE pellets can use water cooling, but water temperature must be carefully controlled
Practical Tips from Production Experience
Start with small trial runs when adjusting parameters before full-scale production
Adjust settings based on material characteristics:
PP: high shrinkage, requires optimized cooling
ABS: pay attention to volatile content and degassing
PE: processing parameters should match density grade
PS: avoid excessive shear due to brittleness
Conclusion
Hollow pellets are rarely caused by a single factor. In most cases, they result from a combination of moisture, temperature, degassing, and cooling conditions. By improving raw material preparation, fine-tuning extrusion parameters, and maintaining equipment properly, most hollow pellet issues can be effectively resolved.
Because every production line and material formulation is different, process optimization should always be based on real operating conditions and systematic testing.
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