Engineering Insights: Thermoplastic Drying Part 2

Drying Methods and Key Process Considerations

Moisture behavior plays a critical role in thermoplastic performance, especially during high-heat processes like injection molding or extrusion. Part one of this series on thermoplastic drying explores the fundamental differences between hygroscopic and hydrophobic resins and why moisture sensitivity should be a key factor in material selection.

This second installment focuses on drying technologies and processing conditions that support optimal material performance. From understanding time and temperature control to selecting the right drying method, this guide highlights the practices and tools that processors can use to protect resin integrity and maintain consistent part quality.

Why Proper Thermoplastic Drying Matters

Drying thermoplastic resin prior to processing is not just a best practice. For hygroscopic materials especially, drying is a required step as embedded moisture can lead to hydrolysis, inconsistent viscosity, surface defects and weakened mechanical performance.

“Even if you’ve selected the right material for the job, that job can fail if the resin is not properly dried,” said Myles Wegner, Technical Development Engineer at M. Holland. “Drying is a key step to protect polymer integrity, reduce waste and maintain a stable, repeatable process on the manufacturing floor.”

Whether preparing a moisture-sensitive resin like nylon or a more forgiving material like polystyrene, proper drying plays a vital role in production success. It helps ensure consistent part quality and preserves the mechanical properties needed for performance. Stable flow behavior during processing minimizes the risk of defects, while fewer moisture-related issues translate to reduced scrap, less rework, and lower overall production costs.

Drying Temperature and Time: What Plastics Processors Should Know

While selecting the right resin is essential, it’s the drying conditions — specifically time and temperature — that ultimately determine whether any resin performs to its full potential.

“Time and temperature control are non-negotiable for hygroscopic materials,” said Myles. “It’s important to know the precise time and temperature required for drying the resin, as drying it improperly — whether too long or at the wrong temperature — can negatively affect material performance.”

Moisture removal processes must balance thoroughness with control. Exposing material to excessive heat or drying it for too long can degrade the polymer, alter its flow characteristics and impact color or surface finish. For example, nylon 66 typically requires four to six hours of drying at 80–100°C. Drying for too short a time will leave residual moisture that compromises mechanical properties and part consistency. On the other hand, overheating the material or extending the drying cycle unnecessarily can cause oxidative degradation, resulting in brittleness or discoloration.

Material manufacturers provide specific drying guidance tailored to the resin’s specific moisture absorption capacity and thermal stability. Following manufacturer guidelines helps plastics processors avoid introducing variability and material waste into an otherwise stable process.

“Every resin has a defined range that balances moisture removal with thermal stability. It’s important to stay within that window,” Myles confirmed.

Temperature Control
Thermoplastics are sensitive to overheating. Drying at temperatures above the manufacturer’s recommended range can lead to:

• Thermal Degradation: Overheating can cause the polymer chains to break down, leading to a loss of mechanical properties such as tensile strength and elongation. Certain resins, such as polycarbonates or polyesters, may begin to degrade at temperatures even slightly above their recommended drying range. Excess heat may cause impact modifiers in polycarbonate to crosslink, affecting impact resistance, viscosity and other material properties.
• Viscosity Changes: Excessive drying temperatures can alter the resin’s melt flow index (MFI), affecting its flow behavior during molding or extrusion. Significant increases in Melt Flow often correlate to a change in molecular weight and a decrease in mechanical properties.
• Color Variation: Inconsistent drying temperatures may cause uneven coloring or discoloration in the thermoplastic material, especially when attempting to color plastics.
• Surface Defects (Bubbles, Blisters, or Cracking): Uneven heating or excessive drying temperature can cause the formation of bubbles, blisters, or cracks on the surface of the thermoplastic.
  

Time Control
Drying time is equally important as temperature. Too short, and moisture will not be fully removed. Too long, and oxidation or degradation may occur, especially in hygroscopic materials like nylon. Inconsistent dwell times can also lead to uneven moisture levels within a hopper or between batches.

Time control becomes even more critical when working with regrind or filled materials, which may take longer to dry.

“Proper material drying is not just about hitting a target temperature,” Myles confirmed. “Processors need to allow enough time for moisture to escape. That’s something plastics manufacturers test for and validate for every resin grade.”

Thermoplastic Drying Methods

Processors and engineers must balance production goals with the resin’s requirements. Each resin comes with carefully validated drying specifications, and skipping or guessing at those numbers can lead to scrap, inefficiency or downstream failure.

“Drying is your first line of defense against material failure,” said Myles. “It’s not just about removing water — it’s about preserving the polymer’s strength, consistency and color.”

Drying method selection should be based on the resin’s moisture profile, processing volume, energy requirements and available equipment. Ensure dryer sizing, temperature control, and dwell time align with both the manufacturer’s recommendations and your facility’s throughput goals.

The breakdown below covers the most common drying methods and their uses.

Closing the Loop on Moisture Management

Resin drying is a foundational part of thermoplastic processing. Selecting the right method and running it according to the resin manufacturer’s specifications ensures your material will perform as intended, your processes will run smoothly, and your products will meet quality expectations.

In case you missed it, read part one of the thermoplastic drying series to learn more about moisture behavior in hygroscopic and hydrophobic materials.

Engineering Insights: Practical Plastics Solutions

Technical plastics pros like Myles are sharing their expertise in M. Holland’s Engineering Insights series. To ensure you don’t miss future technical insight, subscribe to receive the latest news and insights from M. Holland.

Disclaimer: The information provided in this post is intended solely for general guidance. Outcomes vary based on individual circumstances, and M. Holland Company, LLC does not ensure a specific result. Clients shall use their own independent skills and expertise when testing any application of technical support. M. Holland is not responsible and will not be liable for any discrepancies between expected and actual outcomes. M. HOLLAND DISCLAIMS ALL WARRANTIES, WHETHER EXPRESS OR IMPLIED, INCLUDING WITHOUT LIMITATION ANY WARRANTIES OF MERCHANTABILITY OR FITNESS OF A PRODUCT FOR A PARTICULAR PURPOSE.