Your plastic granulating equipment is running, but the finished pellets do not look right. Some contain visible bubbles. Others have tiny internal voids that only become obvious when the pellets are cut open. Operators adjust temperatures, increase vacuum, and change pelletizing conditions, yet the defects keep returning. For recycling plants, this can mean rejected material, unstable downstream processing, and higher production costs. At AMIGE, I believe the solution starts by identifying where the gas or voids originate—not simply changing the extruder settings.
Bubbles and internal voids in recycled plastic pellets can result from residual moisture, trapped air, volatile contamination, polymer degradation, ineffective melt degassing, unstable extrusion, or unsuitable pellet cooling and cutting conditions. Different plastics behave differently, so the correct solution depends on the material and processing system. At AMIGE, we evaluate feedstock preparation, drying, screw design, vacuum degassing, melt filtration, and pelletizing conditions together. Our goal is to help recycling plants improve pellet consistency and select practical equipment configurations that match their production requirements.
A pellet should be solid plastic.
Not a miniature air mattress.
And if the bubbles keep returning, the real problem may begin long before the melt reaches the die.
Why Do Bubbles Form Inside Recycled Plastic Pellets?
Bubbles appear when gas or vapor becomes trapped in the polymer melt or pellet during processing.
Possible sources include:
• Residual water
• Entrained air
• Volatile substances
• Contaminated feedstock
• Polymer degradation
• Inadequate degassing
• Unstable extrusion conditions
However, not every internal void comes from moisture.
Some defects may develop during cooling or solidification.
For this reason, I recommend examining the defect pattern before adjusting equipment.
For example, surface bubbles, internal cavities, and irregular pellet shapes may have different causes.
How Does Residual Moisture Affect Pellet Quality?
Moisture is one of the most common problems in recycled plastic processing.
Washed film, flakes, or rigid plastic pieces may carry water into the extruder.
When heated, this water can vaporize.
If the vapor is not removed effectively, it may create bubbles or unstable melt flow.
The risk depends on the polymer.
PE and PP are not highly hygroscopic, but surface moisture can still cause processing problems.
PET, PA, and certain engineering plastics can absorb moisture internally and may require more controlled drying.
At AMIGE, we recommend measuring moisture rather than assuming the material is dry because it feels dry.
Can Trapped Air Cause Internal Voids in Pellets?
Yes.
Low-bulk-density materials such as LDPE film can carry significant air into the feeding system.
Irregular feeding may also create unstable melting conditions.
Possible contributing factors include:
• Loose film feeding
• Inconsistent compaction
• Poor screw filling
• Unstable feed rate
• Inadequate venting
For flexible film recycling, a suitable feeding or compaction system can help improve material delivery into the extruder.
However, compaction alone does not guarantee bubble-free pellets.
The complete feeding, melting, and degassing process must be matched correctly.
Why Is Vacuum Degassing Important in Plastic Pelletizing?
A Vacuum Degassing System helps remove certain gases and volatile substances from molten plastic.
It can be useful when processing washed or contaminated recycled materials.
However, performance depends on:
• Screw design
• Melt temperature
• Material throughput
• Vacuum conditions
• Venting-zone design
• Feedstock characteristics
If the melt is not properly prepared before reaching the venting section, gas removal may be inefficient.
An unsuitable vacuum system may also experience material carryover or unstable operation.
At AMIGE, we prefer evaluating the extrusion process before recommending additional vacuum equipment.
A bigger vacuum pump is not automatically the answer.
Can Contaminated Feedstock Create Bubbles and Pellet Defects?
Yes.
Recycled plastic may contain substances that release gases during extrusion.
Examples include:
• Residual cleaning chemicals
• Oils
• Organic contamination
• Printing inks
• Adhesive residues
• Degraded polymer
• Incompatible materials
Some contaminants may volatilize during heating.
Others may remain as solid defects.
This is why washing, sorting, and material preparation are important before pelletizing.
A Plastic Melt Filtration System can remove certain solid contaminants.
However, filtration cannot remove every gas or dissolved substance.
We should not expect a screen changer to solve a degassing problem.
How Does Extrusion Temperature Affect Bubble Formation?
Temperature influences melting, viscosity, degradation, and gas behavior.
If the temperature is too low for the material and equipment, melting may become incomplete.
If thermal exposure is excessive, degradation or additional volatile formation may occur.
Important parameters include:
• Barrel temperature profile
• Actual melt temperature
• Screw speed
• Residence time
• Throughput
• Melt pressure
The correct settings depend on the polymer and formulation.
I do not recommend using one universal temperature profile for LDPE, HDPE, PP, PET, and PA.
A temperature adjustment should follow a diagnosis, not guesswork.
Why Do Some Pellets Develop Voids During Cooling?
Not every internal cavity originates inside the extruder.
Cooling and solidification conditions can also influence pellet structure.
Depending on the pelletizing method, relevant factors may include:
• Die temperature
• Melt uniformity
• Cooling-water conditions
• Strand stability
• Cutting conditions
• Pellet dimensions
For example, unstable strand formation or unsuitable cooling conditions may contribute to irregular pellet geometry.
However, cooling-related cavities should be distinguished from gas bubbles through appropriate inspection.
Changing cooling conditions will not remove moisture already trapped in the melt.
How Can You Identify the Real Cause of Bubble Defects?
Before replacing equipment, I recommend a systematic investigation.
First, inspect the incoming material.
Second, measure moisture using a suitable method.
Third, review feeding stability.
Fourth, check melt temperature, pressure, and degassing conditions.
Finally, examine the finished pellets.
A useful diagnostic sequence is:
Feedstock Inspection → Moisture Testing → Feeding Review → Extrusion Evaluation → Pellet Inspection
For persistent defects, laboratory examination of pellet cross-sections and material contamination may help identify the cause.
When checking extrusion equipment, follow the manufacturer’s shutdown, pressure-release, isolation, and hot-surface safety procedures.
Never open a pressurized melt system.
When Should You Upgrade Your Plastic Granulating Equipment?
An upgrade may be worth considering when:
• Bubble defects continue after proper drying.
• Feeding remains unstable.
• Degassing capacity is insufficient.
• Melt temperature cannot be controlled consistently.
• Pellet quality fails to meet customer requirements.
• Existing equipment cannot achieve the required capacity.
However, replacing the complete line may not be necessary.
The solution may involve better drying, improved feeding, revised screw design, suitable degassing, or changes to pellet cooling.
At AMIGE, we prefer identifying the actual production bottleneck before recommending additional equipment.
What Should Buyers Confirm Before Requesting a Pelletizing Solution?
Before recommending equipment, we normally ask for:
• Polymer type
• Material photos or videos
• Feedstock form
• Contamination level
• Moisture condition
• Required capacity
• Current extrusion configuration
• Pelletizing method
• Target pellet quality
• Local voltage
For example:
“We process washed LDPE film at 500 kg/h. The finished pellets contain visible bubbles, and we need more stable quality for downstream film production.”
This gives our engineers a useful starting point.
Conclusion
Bubbles and internal voids in recycled plastic pellets can result from moisture, trapped air, volatile contamination, ineffective degassing, unstable extrusion, or cooling conditions. The correct solution begins with identifying the defect source. At AMIGE, we help recyclers evaluate material preparation and equipment configurations for more consistent pellet quality.

Frequently Asked Questions
1. Why do recycled plastic pellets contain bubbles?
Possible causes include residual moisture, trapped air, volatile contamination, polymer degradation, inadequate degassing, and unstable extrusion conditions. The actual cause should be identified through process and material evaluation.
2. Can a vacuum degassing system eliminate bubbles in plastic pellets?
Vacuum degassing can remove certain gases and volatiles, but it cannot guarantee bubble-free pellets. Proper drying, feeding, melting, and cooling conditions are also important.
3. Does PE or PP need drying before pelletizing?
PE and PP are not highly hygroscopic, but washed material may carry surface moisture. Excessive water should be removed before extrusion according to the process requirements.
4. Can melt filtration remove the causes of bubbles?
Melt filtration can remove certain solid contaminants, but it does not remove every gas or volatile substance. Filtration and degassing perform different functions.
5. What information does AMIGE need to recommend a pelletizing solution?
Please provide material type, photos or videos, moisture condition, contamination, required capacity, existing equipment configuration, pelletizing method, target pellet quality, and local voltage.

Call to Action
Are bubbles and internal voids reducing the quality of your recycled plastic pellets?
Send AMIGE your material photos or videos, polymer type, moisture information, required capacity, current pelletizing process, and examples of defective pellets.
Our engineering team can help evaluate your material preparation, feeding, drying, and pelletizing requirements and recommend a practical equipment configuration for your recycling project.
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#VacuumDegassing #PlasticExtrusion #PlasticRecycling #MeltFiltration #RecyclingMachinery #AMIGE
