Quick Answer
Dew point is the temperature at which moisture in the air begins to condense into liquid water. In plastic resin drying, dew point is used to measure how dry the process air is. The lower the dew point, the lower the moisture content of the air and the greater its ability to absorb moisture from the resin. For hygroscopic materials such as ABS, nylon, PET, and polycarbonate, maintaining a low dew point is critical for achieving proper drying and preventing defects such as splay, bubbles, brittleness, and poor surface finish.
Why Does Dew Point Matter in Plastic Resin Drying?
When processors talk about resin drying, temperature often gets most of the attention. Operators focus on setting the correct drying temperature and residence time, believing those factors alone determine whether a material is dry enough for processing.
However, there’s another variable that can have just as much impact on final moisture content:
Dew point.
Understanding dew point can help molders and extruders improve part quality, reduce scrap, and maximize the performance of their drying equipment.
What Is Dew Point?
Dew point is the temperature at which moisture in the air begins to condense into liquid water. Equal to 100% relative humidity, it represents the point where the air can no longer hold all of its moisture.
In plastics processing, dew point is used as a measurement of how dry the process air is. The lower the dew point, the lower the moisture content of the process air and the greater its ability to absorb moisture from the resin.
Think of dew point as a measure of the air’s drying power. Lower dew point air has a greater capacity to remove moisture and carry it away from the material.
Why Isn’t Heated Air Alone Enough?
One of the most common misconceptions in plastics processing is that heat alone will dry resin.
For non-hygroscopic materials, heated air is often sufficient because moisture remains on the surface of the pellet. However, hygroscopic materials absorb moisture internally within the pellet structure. Common hygroscopic resins include:
- ABS
- Nylon
- PET
- Polycarbonate
- PBT
- Acrylic
- Polyurethane
To properly dry these materials, heat must work together with low-dew-point air. The heat helps free moisture from the polymer structure, while the dry air absorbs and carries that moisture away. This is why hygroscopic materials typically require dehumidifying dryers rather than simple hot-air dryers.
Lower Dew Point = Better Moisture Removal
A useful way to understand dew point is to compare two drying systems operating at the same temperature.
Imagine ABS being dried at 180°F.
One dryer supplies air at a +51°F dew point.
Another dryer supplies air at a -10°F dew point.
Although both dryers operate at the same temperature, the lower dew point system removes moisture more effectively because the air contains less moisture and has a greater capacity to absorb additional moisture from the resin. As a result, the resin reaches a lower moisture content in less time.
What Dew Point Should a Plastic Resin Dryer Maintain?
Many engineering-grade resins require process air with a dew point around -40°F or lower to achieve proper drying.
If the dryer cannot consistently maintain that level, processors may experience:
- Splay
- Silver streaks
- Surface defects
- Gas bubbles
- Brittleness
- Inconsistent part quality
For this reason, monitoring dew point is just as important as monitoring drying temperature and residence time.
How Is Low Dew Point Air Created?
Modern dehumidifying dryers remove moisture from the process air before it enters the drying hopper. This is typically accomplished using:
- Desiccant bed dryers
- Desiccant wheel dryers
- Membrane compressed-air dryers
These systems remove moisture from the air stream and create the low-dew-point conditions necessary for drying hygroscopic materials. As the dry air passes through the hopper, it absorbs moisture from the resin and carries it away from the material.
The Four Factors That Determine Drying Performance
Proper resin drying requires four variables to work together:
- Drying Temperature – Provides the energy needed to release moisture from the polymer.
- Dew Point – Determines how much moisture the process air can absorb.
- Airflow – Delivers heat and carries moisture away from the material.
- Residence Time – Allows sufficient time for moisture to migrate from the inside of the pellet to the surface.
If any one of these variables is incorrect, moisture-related defects can appear in the finished product.
What Happens When Dew Point Is Too High?
Without a sufficiently low dew point, moisture removal slows significantly because the process air already contains more moisture and has less capacity to absorb additional moisture from the resin. This can occur even when the drying temperature is correct.
When excessive moisture remains in the resin, processors may encounter a variety of production and quality problems.
Injection Molding Defects
- Voids
- Splay
- Silver streaks
- Foamy melt
- Reduced viscosity
- Poor shot consistency
Extrusion Defects
- Surface roughness
- Gas bubbles
- Wave patterns
- Poor dimensional control
- Polymer degradation
These defects can increase scrap rates, reduce productivity, and negatively impact final part performance.
Optimize Your Drying Process
Achieving proper resin drying requires more than heat alone. Drying temperature, airflow, residence time and dew point must work together to ensure moisture is effectively removed from the material.
AEC drying solutions are engineered to provide precise temperature control, reliable low-dew-point performance and efficient material handling for a wide range of plastics processing applications. Whether you’re drying ABS, PET, nylon, polycarbonate or other engineering resins, the right drying system can help improve quality, reduce waste and maximize productivity.
Want to learn more? Explore AEC’s drying solutions and educational resources to better understand how moisture management impacts plastics processing.
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