Proper resin drying is one of the most important steps in producing high-quality plastic parts. Yet even with a drying system in place, manufacturers still encounter moisture-related defects including splay, bubbles, brittleness, poor surface finish and inconsistent processing.
The problem often isn’t the dryer itself. In many cases, improper drying results from incorrect material handling, drying parameters, equipment sizing or environmental conditions.
If your resin isn’t drying as expected, here are eight common causes to investigate.
Proper resin drying depends on several factors working together, including dryer type, airflow, dew point, drying temperature and material handling. If even one of these variables is incorrect, moisture-related defects can quickly appear in your finished parts. Use the checklist below to identify the most common causes of improper resin drying and learn how to correct them before they impact production quality.
1. You’re Using the Wrong Type of Dryer
Not every resin requires the same drying method.
Many processors assume heated air alone is enough to remove moisture. While that works for non-hygroscopic materials, it is ineffective for hygroscopic resins because moisture is absorbed inside the plastic pellet rather than remaining on the surface.
Common hygroscopic materials include:
- Nylon
- ABS
- PET
- Polycarbonate
- PBT
- Acrylic
- Polyurethane
These materials require heated, low-dew-point dehumidified air to properly remove internal moisture.
Materials such as polypropylene, polyethylene, PVC and polystyrene generally only require removal of surface moisture.
Solution: Match your drying technology to the resin you’re processing.
2. Your Dryer Isn’t Maintaining a Low Enough Dew Point
One of the most overlooked causes of poor drying is inadequate dew point.
The dew point measures how dry the process air actually is. Lower dew point air has a greater ability to absorb moisture from resin.
Many engineering-grade resins require process air with a dew point around -40°F (-40°C) or lower.
If the dryer cannot consistently maintain this level, moisture removal slows dramatically.
Signs include:
- Splay
- Silver streaks
- Gas bubbles
- Surface defects
- Brittleness
Solution: Verify dryer performance and routinely monitor dew point during production.
3. The Drying Temperature Is Incorrect
Drying temperature directly affects how quickly moisture can migrate from inside the resin pellet.
If temperatures are too low:
- Drying takes much longer
- Moisture remains trapped inside pellets
If temperatures are too high:
- Resin may degrade
- Additives may break down
- Sensitive materials like nylon can become overdried
For example, ABS dries much faster at 208°F than it does at 160°F.
Solution: Always follow the material supplier’s recommended drying temperature rather than relying on generic settings.
4. The Resin Isn’t Spending Enough Time in the Hopper
Even with the correct dryer, insufficient residence time prevents complete drying.
Moisture doesn’t disappear immediately. Even when the correct drying temperature and dew point have been achieved, water molecules still need time to migrate from the center of each pellet to the surface. Removing material from the hopper too early often results in parts that exhibit the same moisture-related defects as resin that was never fully dried. It must migrate from the center of each pellet to the surface before it can be removed by dry air.
Depending on the resin, recommended drying times typically range from:
- 1 to 4 hours
- Longer for specialty engineering materials
If throughput exceeds hopper capacity, material may leave the dryer before reaching its target moisture content.
Solution: Size drying hoppers based on throughput and required residence time.
5. Your Resin Started with More Moisture Than Expected
Recommended drying times assume a typical starting moisture level.
Unfortunately, resin doesn’t always arrive in the same condition.
Initial moisture content varies because of:
- Storage conditions
- Shipping
- Regional humidity
- Seasonal weather
- Packaging
Two containers of the same material may require different drying times depending on how much moisture they absorbed before arriving at your facility.
The amount of moisture a resin can absorb is much higher than the moisture level required for processing. For example, nylon can absorb up to 1% moisture, while PET can absorb up to 0.4% under saturated conditions. Before processing, however, these materials must be dried to much lower moisture levels to prevent defects.
For example, PET should typically be dried to 0.005% (50 PPM) or less before processing, while polycarbonate should be around 0.02% (200 PPM). Even though resin pellets may appear dry, internal moisture can still cause splay, bubbles, brittleness and reduced mechanical properties if proper drying targets are not achieved.
Solution: Use a moisture analyzer whenever possible rather than assuming incoming resin meets expected moisture levels. Verifying the initial moisture content allows processors to adjust drying time as needed and produce more consistent part quality.
6. Airflow Isn’t Correct
Temperature alone does not dry resin.
Airflow performs two important functions:
- Delivers heat
- Carries moisture away
Too little airflow limits moisture removal.
Too much airflow can create other problems including:
- High return air temperatures
- Reduced desiccant efficiency
- Material flow problems
- Higher energy consumption
Solution: Ensure airflow matches both the dryer and hopper size. Modern dryers with automatic airflow control help optimize drying while reducing energy use.
7. Your Resin Has Reabsorbed Moisture
Many hygroscopic materials begin absorbing moisture again almost immediately after leaving the dryer.
Even briefly exposing dried resin to humid plant air can increase moisture content before processing.
This often occurs when:
- Material sits in open containers
- Dryers remain disconnected from the machine
- Production stops for extended periods
- Resin is transferred between containers
Solution: Keep dried resin in sealed conveying systems or enclosed drying hoppers until it reaches the process machine.
8. You’re Drying Mixed Materials Together
Blending materials can complicate drying.
Common examples include:
- Virgin and regrind PET
- Materials with different drying temperatures
- Materials with different drying times
- Additive packages that change moisture behavior
A blended material should never be dried using “average” drying conditions.
Solution: Dry materials separately whenever possible before blending.
Warning Signs Your Resin Isn’t Dry
If you’re seeing any of these defects, excessive moisture could be the cause.
Injection Molding
- Splay
- Silver streaks
- Voids
- Brittleness
- Foamy melt
- Poor shot consistency
- Lower viscosity
Extrusion
- Surface roughness
- Gas bubbles
- Wave patterns
- Surging
- Poor dimensional control
- Polymer degradation
Don’t Forget: Overdrying Can Also Cause Problems
Many processors focus only on removing moisture, but drying resin for too long can also reduce part quality.
Overdrying may lead to:
- Brittle nylon parts
- Loss of surface gloss
- Additive degradation
- Higher viscosity
- Increased material waste
Modern drying systems with airflow control and temperature setback features help prevent overdrying during production interruptions.
Need Help Optimizing Your Resin Drying Process?
Whether you’re processing hygroscopic engineering resins or commodity materials, selecting the right drying system is essential for producing consistent, high-quality parts.
AEC offers a complete range of resin drying solutions, including desiccant dryers, compressed air dryers, drying hoppers and centralized drying systems for injection molding, extrusion and blow molding applications. If you’re looking to deepen your understanding of resin drying, moisture management and drying best practices, explore our educational resources in the AEC Knowledge Center.
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