Drying Polymer Materials Before Extrusion: Why It Matters
Proper drying of hygroscopic polymers before extrusion is essential to prevent material degradation and maintain high-quality output. Moisture-sensitive polymers absorb water from the air, and if not dried correctly, they can lose their mechanical strength, show surface defects, and suffer from reduced molecular weight.
Why Some Polymers Must Be Dried
Polymers like nylon, polyesters (e.g., PET and PBT), and polycarbonate (PC) absorb atmospheric moisture rapidly. During extrusion, high temperatures cause this moisture to break down polymer chains, resulting in:
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Reduced molecular weight
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Weakened mechanical properties
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Lower thermal performance
To avoid these issues, manufacturers must dry the polymer properly before feeding it into the extruder.
Common Hygroscopic Materials That Require Drying
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Nylon (PA)
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Polyesters (PET, PBT)
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Polycarbonate (PC)
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Acrylics
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Ultem®
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Polysulfone
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Noryl®
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ABS (Acrylonitrile Butadiene Styrene)
Any moisture left in the material turns into steam during extrusion, leading to:
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Surface defects like splay or bubbles
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Void formation
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Foamy or porous appearance in the final product
Handling Pre-Dried Polymers
Some materials, such as nylon, are shipped in moisture-proof containers and arrive ready for use. However, once the container seal is broken or improperly resealed, the material will begin to absorb moisture again. In such cases, you must redry the resin before processing.
Special Considerations for Polyesters
Polyesters, especially PET, are extremely sensitive to moisture. To protect these resins:
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Use dehumidifying dryers with dew points below –40°F (–40°C)
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Transport materials using dry air
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Maintain a blanket of dry air or nitrogen in the feed hopper
Mixing and Additives: A Critical Step
If your formulation includes additives or fillers, you must ensure they are also moisture-free. Moisture-sensitive materials quickly reabsorb water once exposed to the atmosphere. If you can’t dry additives with the base resin, use:
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Individual feeders
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Isolated handling systems
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Sealed blending environments to avoid reabsorption
Can You Use a Vacuum-Vented Extruder?
In some cases, you can process slightly moist resins in a vacuum-vented extruder, which removes moisture in the venting zone. However, this method isn’t suitable for all polymers. Some materials begin degrading before the vent can fully extract the moisture.
Avoid Overdrying Polymers
Overdrying also causes problems. For example:
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Nylon 6,6 turns yellow when overdried
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It loses its mechanical strength
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The product develops undesirable discoloration
Always follow manufacturer-recommended drying times and temperatures to maintain the material’s optimal performance.
