Why Do Plastic Raw Materials Need Drying? Dryer Types and Buying Guide
Introduction
Injection molding and extrusion operators all know that raw materials "need to be dried," but few truly understand why drying is necessary, what the specific consequences of skipping it are, or how to set drying temperature and time correctly. Many quality problems — silver streaks, bubbles, rough surfaces, strength loss — are actually caused by insufficient or incorrect drying, but operators typically look for the cause elsewhere and overlook this most fundamental step.
This article explains why plastic raw materials need drying, the specific consequences of skipping it, the operating principles and differences of each dryer type, the logic for setting drying parameters, and common drying mistakes.
Why Plastic Raw Materials Need Drying
Plastic hygroscopicity
Plastic raw materials absorb moisture from the surrounding environment during storage and transport — especially engineering plastics with strong hygroscopicity, which can absorb enough moisture to affect processing quality within just a few hours in a high-humidity environment.
Plastic moisture absorption occurs in two ways: surface adsorption and internal absorption. Surface-adsorbed moisture is relatively easy to remove — standard hot-air drying handles it. Internally absorbed moisture requires longer drying times and more precise temperature control — which is why drying requirements vary so dramatically between different materials.
Hygroscopicity differences between plastic types are large: PA (Nylon), PC, PET, and PBT are strongly hygroscopic and require the strictest drying; PP, PE, and PS have low hygroscopicity and are less affected by normal storage environments, but still require drying after extended storage or in high-humidity conditions.
What happens to moisture during processing
When plastic raw material enters the heated screw zone, temperature rapidly rises to the melt temperature (typically 200–300°C). Residual moisture in the material instantly vaporizes at this temperature, generating large amounts of steam.
Steam inside the molten plastic cannot escape quickly enough — it is encapsulated within the melt and passes through the die or mold together with it. After forming, the steam condenses, creating visible defects on or inside the finished part.
Consequences of Skipping or Insufficient Drying
Appearance defects
Silver streaks: silver-white streaks on the part surface — traces left by vaporized moisture escaping at the surface. The most common and most easily spotted defect from skipping drying, particularly visible on dark-colored parts.
Bubbles: visible inside or on the part surface — steam trapped inside the part during forming. Bubbles not only affect appearance but reduce part strength, most noticeably in thin-wall sections.
Rough or hazy surface: parts that should be smooth and glossy show surface roughness or foggy opacity — caused by moisture interfering with melt flow and surface forming behavior.
Property loss
At high temperatures, moisture does not only vaporize — it also causes hydrolytic degradation of some plastics, especially moisture-sensitive engineering plastics (PC, PET, PA). Hydrolysis breaks polymer chains — molecular weight decreases, and the part's mechanical strength, toughness, and heat resistance all drop noticeably. This degradation is irreversible: even if appearance is acceptable, properties are already compromised.
Processing difficulties
High-moisture raw material behaves differently in the screw than dry material — prone to screw slippage, unstable fill, and injection pressure fluctuations. Operators must continually adjust processing parameters, reducing production efficiency.
Dryer Types and Operating Principles
Hot-air dryer (hopper dryer)
The most basic drying equipment: electric heating elements heat ambient air, which is then blown into the hopper. The heated air passes through the gaps between plastic pellets, carrying moisture away.
Advantages: simple structure, low cost, easy maintenance. Suits low-hygroscopicity materials (PP, PE, PS).
Disadvantages: uses ambient plant air, which in Taiwan's rainy season or coastal locations may have relative humidity above 80%. Heating this air increases its temperature, but the air still carries its own moisture. When ambient humidity is high enough, the moisture delivered by the hot air can actually exceed what it removes — the material gets wetter, not drier. Unreliable for strongly hygroscopic engineering plastics.
Dehumidifying dryer
Built upon hot-air drying by adding a dehumidifying rotor (also called a molecular sieve or honeycomb rotor) that removes moisture from the air before heating. Air with a very low dew point (typically below −40°C) is then heated and delivered to the hopper. Regardless of ambient humidity, the air entering the hopper is very dry — stable, reliable drying that is not affected by environmental conditions.
Advantages: drying performance is not affected by ambient humidity; suitable for all hygroscopic materials — essential for PC, PA, PET, PBT, and other highly hygroscopic engineering plastics. Can control raw material moisture content to extremely low levels (typically below 0.02%), enabling stable processing quality.
Disadvantages: higher equipment cost than hot-air dryers; the dehumidifying rotor requires periodic regeneration (removing adsorbed contaminants through high-temperature cleaning); additional energy consumption.
Vacuum dryer
In a sealed chamber, vacuum is applied to the raw material — reducing ambient pressure so moisture can vaporize and escape at a lower temperature.
Advantages: lower drying temperature than hot-air drying, suitable for temperature-sensitive materials (some heat-sensitive engineering plastics, recycled regrind). The sealed environment means material has no contact with outside air — suitable for materials prone to oxidation or sensitive to environmental exposure.
Disadvantages: high equipment cost; typically small capacity; primarily batch processing — not suitable for applications requiring continuous high-volume supply.
Setting Drying Temperature and Time
Drying temperature and time are not arbitrary settings. Too low and drying is ineffective; too high causes material degradation or pellet clumping. Every material has corresponding recommended parameters.
Drying temperature
Temperature should be set within a range that effectively removes moisture without softening or degrading the material. Reference drying temperatures by material:
PP, PE: 70–80°C (low hygroscopicity — dry only when needed) / ABS: 80–85°C / PC: 110–120°C / PA6 (Nylon 6): 80–90°C / PA66 (Nylon 66): 80–90°C / PET: 160–170°C / PBT: 120–130°C
⚠️ These are reference ranges — always follow the drying specifications provided by your raw material supplier. Different grades and formulations of the same material may have slightly different recommended temperatures.
Drying time
Drying time depends on the material's moisture level and hopper capacity. Basic principles: strongly hygroscopic materials need longer drying time — generally 2–6 hours; weakly hygroscopic materials typically need only 1–2 hours. Larger hoppers with more material loaded require more time for hot air to reach every pellet.
The longer raw material has been exposed to high humidity after opening, the more moisture it has absorbed. If a bag has been open for more than 24 hours, increase the standard drying time by 30–50%.
Hopper loading quantity
Many operators habitually fill hoppers to the top — assuming more material means less risk of running out — but overfilling actually degrades drying performance. Hot air must penetrate the entire pellet bed to dry every pellet; too deep a pellet column means the air arriving at the bottom has already lost temperature. Recommendation: do not load more than 80% of hopper capacity, allowing hot air to circulate more effectively.
Common Drying Mistakes
Mistake 1: Using a hot-air dryer for PC or PA
PC and PA are strongly hygroscopic — only a dehumidifying dryer can control moisture content to the level needed for processing. Using a hot-air dryer for these materials in Taiwan's climate may achieve essentially no drying effect. Parts still show silver streaks and bubbles; operators cannot find the cause and continually adjust processing parameters to no avail.
Mistake 2: Setting drying temperature too high
Setting the temperature too high to shorten drying time causes pellets to begin softening in the hopper — pellets stick together and clump. In severe cases the hopper outlet blocks and the machine cannot feed. Degradation-sensitive materials (POM, PC) may also begin degrading at excessive temperature — actually worsening material quality.
Mistake 3: Leaving dried material in the hopper too long
Dried material in an open hopper will re-absorb moisture from ambient air over time — especially strongly hygroscopic materials, whose moisture content can recover to non-conforming levels within a few hours. Use dried material promptly after drying. If temporary storage is needed, verify that the hopper has effective sealing and insulation design to prevent moisture re-absorption.
Mistake 4: Using the same drying parameters for regrind and virgin material
Recycled regrind may have already undergone some degree of degradation during recovery and has lower temperature tolerance than virgin material. Applying virgin-material drying temperatures to regrind may accelerate degradation, further reducing regrind quality. Recommended: reduce regrind drying temperature by 5–10°C below virgin material settings and compensate with extended drying time.
Related articles: Dehumidifying Dryer vs. Hot-Air Dryer: Suitable Materials for Each Dryer Type; Plastic Regrind vs. Virgin Resin — Cost, Quality, and Carbon Emission Comparison.