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.
Polymers fall into two broad categories regarding their moisture behaviour: hygroscopic and hydrophobic.
Understanding this distinction is essential for anyone working with plastic resins.
When polymers, especially hygroscopic ones, are exposed to high processing temperatures, retained moisture can trigger hydrolytic degradation. This leads to:
In severe cases, even a moisture content as low as 0.02% to 0.1% by weight can degrade product quality.
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.
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.
Effective polymer drying depends on:
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:
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.
Neglecting proper drying can result in:
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.
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.
Learn more about the importance of drying polymers, types of dryers and various drying methods used throughout the industry.
This short course on the Academy covers key concepts such as hydrolysis, hygroscopic and non-hygroscopic polymers, desiccant, dew point and more. Also get access to a downloadable guide to drying, with guidance on drying practice for different polymer types.
