Why do some materials require drying before use, while others do not?

Technical Service Team

Ultrapolymers Group

Drying polymers before processing is a critical step in ensuring the performance, consistency, and quality of plastic components.

The necessity stems from the way different polymers, interact with moisture in the environment. Whether you're working with injection moulding or extrusion, understanding why you should dry polymers before moulding can help prevent costly defects and maintain mechanical integrity.

Hygroscopic vs. Hydrophobic Polymers

Polymers fall into two broad categories regarding their moisture behaviour: hygroscopic and hydrophobic.

• Hygroscopic polymers, such as nylon 6 or polyesters, absorb moisture from the surrounding air.
• Hydrophobic polymers, such as polypropylene, naturally repel moisture. However, once additives like mineral fillers are introduced, even these polymers can exhibit moisture sensitivity.

Understanding this distinction is essential for anyone working with plastic resins.

Effects of Moisture in Plastic Polymers

When polymers, especially hygroscopic ones, are exposed to high processing temperatures, retained moisture can trigger hydrolytic degradation. This leads to:

• Decreased melt viscosity
• Inconsistent flow during moulding
• Visual defects like splash marks or voids
• Reduced tensile strength and impact resistance

In severe cases, even a moisture content as low as 0.02% to 0.1% by weight can degrade product quality.

Invisible Threats: Moisture Without Surface Defects

Not all moisture-related issues are visible. For example, materials like polyesters may look perfectly moulded but still suffer from reduced mechanical performance due to internal moisture. This reinforces the importance of proper drying of plastic resins, regardless of visible signs.

Condensation and Handling Risks

When polymers move from cold storage to a warmer processing area, condensation can form on the granules. This additional moisture poses a risk if not addressed through ideal drying conditions for plastics.

How to Dry Polymers Properly

Effective polymer drying depends on:

• Material type
• Drying temperature and time
• Drying equipment, such as desiccant dryers or vacuum dryers
• Processing rate

It’s vital to avoid under- or over-drying, as both can affect the strength and flow characteristics of the material. Using insulated hoppers or small-capacity feed systems can minimise moisture pickup after drying.

Tips:

• Follow recommended polymer drying temperatures
• Monitor drying time for specific materials
• Store materials in low-humidity environments

Do All Plastics Need Drying?

No. Not all plastics need drying, especially truly hydrophobic ones stored in optimal, controlled conditions. However, for materials like ABS, PET, or polycarbonate, drying plastics for injection moulding or extrusion is non-negotiable.

Consequences of Not Drying Polymers

Neglecting proper drying can result in:

• Mechanical failure
• Reduced product life span
• Cosmetic imperfections
• Increased scrap rate

Choosing the Right Dryer for your Polymer

Selecting the correct drying method depends on both the polymer’s moisture behaviour and your processing environment. The most common types of dryers for polymers include desiccant dryers, hot air dryers, and vacuum dryers. Desiccant dryers are ideal for hygroscopic materials, actively removing moisture from within the pellets using a dehumidified air loop. Hot air dryers are typically used for non-hygroscopic plastics, where surface moisture is the primary concern. Vacuum dryers, while more energy-efficient and faster, are often reserved for sensitive or high-throughput applications due to their higher upfront cost.

Conclusion

Drying polymers is not just a best practice—it’s often a requirement for maintaining product quality and performance. From understanding the humidity effects on polymer processing to selecting the best methods for drying polymers, careful moisture control ensures a smooth manufacturing process and durable end products.

For tailored advice on drying plastics for extrusion or moulding, or to get help choosing the right drying equipment for polymers, consult with a materials expert or your polymer supplier.

To find out more please check out our infographic on moisture absorption in polymers which helps illustrate how different materials behave and why proper dryer selection is essential to prevent defects and maintain part performance.