Thermoplastic polyurethane (TPU) is a hygroscopic material, which means TPU pellets can absorb moisture from the surrounding air during storage and handling.
If this moisture is not removed before injection molding or TPU extrusion, it can cause bubbles, silver streaks, rough surfaces, unstable processing and even a loss of mechanical properties.
For this reason, proper TPU drying is an important step before processing.
Why Does TPU Need to Be Dried?
TPU resin gradually absorbs moisture when exposed to ambient air.
During melt processing, excessive moisture can affect the polymer and lead to defects in the finished product. In some cases, the surface defect is obvious; in others, the molded or extruded product may look acceptable while its mechanical properties have already been affected.
Common problems caused by insufficient TPU drying include:
- Bubbles or voids
- Silver streaks or splay marks
- Rough or foamy surfaces
- Unstable extrusion
- Poor surface appearance
- Reduced tensile and tear properties
- Inconsistent processing performance
This is why drying TPU pellets before processing is especially important for products requiring smooth surfaces, transparency or stable mechanical performance.
TPU Drying Temperature and Time
There is no single drying condition suitable for every TPU grade.
Drying temperature and time depend on the TPU chemistry, hardness, formulation, initial moisture level and processing requirements.
A practical starting reference for many commercial TPU grades is:
| Drying Parameter | Typical Reference Range |
|---|---|
| TPU drying temperature | 80-110°C |
| Drying time | 2-4 hours |
| Target moisture | ≤0.05% for many grades |
| More demanding grades | ≤0.02% may be required |
| Dryer type | Desiccant / dehumidifying dryer preferred |
| Drying air dew point | Around −40°C for demanding applications |
These values should be treated as general processing guidelines rather than fixed settings. Always check the Technical Data Sheet (TDS) of the specific TPU grade before processing.
Harder grades, specialty formulations and materials that have been exposed to humid air for a long period may require different drying conditions.
Why Is Dryer Dew Point Important?
Temperature alone does not guarantee effective TPU drying.
A conventional hot-air dryer may continuously circulate air that still contains significant moisture. In a humid production environment, this can make drying slow or incomplete.
A desiccant dehumidifying dryer provides low-dew-point air, helping moisture move from the inside of the TPU pellet into the surrounding dry air.
For applications with strict moisture control, a drying-air dew point around −40°C is commonly used.
Effective TPU drying depends on four important factors:
Temperature + Dry Air + Time + Airflow
Increasing the temperature alone is not always the solution. Excessive drying temperature can cause discoloration, yellowing or thermal degradation.
How to Dry TPU Before Injection Molding
For TPU injection molding, moisture often appears as surface defects such as silver streaks, splay marks or bubbles.
A typical preparation procedure is:
- Keep TPU pellets sealed until they are ready to be used.
- Transfer the material to a desiccant dryer.
- Dry according to the recommended temperature and time in the TDS.
- Keep the dried material protected from ambient humidity.
- Feed the material directly into the injection molding machine whenever possible.
If surface defects continue after drying, moisture should be checked before immediately changing melt temperature, injection speed or mold conditions.
Correct material preparation can eliminate many apparent “processing problems” before machine parameters are adjusted.
How to Dry TPU Before Extrusion
Moisture control can be even more critical during TPU extrusion, especially for:
- TPU tubing and hoses
- Wire and cable jackets
- TPU film and sheet
- Profiles
- 3D printing filament
Moist TPU pellets may result in bubbles, rough surfaces and unstable extrusion performance.
Because extrusion is a continuous process, it is also important to prevent dried material from absorbing moisture again while waiting in the hopper.
A closed material conveying system or dryer mounted close to the extruder can help maintain stable moisture levels throughout production.
Does Polyester TPU and Polyether TPU Require Different Drying?
Both polyester TPU and polyether TPU should normally be dried before melt processing.
However, it is not accurate to determine the drying temperature only from whether the TPU is polyester-based or polyether-based.
The correct condition depends on the individual TPU grade, including:
- Hardness
- Molecular structure
- Additive system
- Pellet size
- Initial moisture content
- Processing method
Therefore, the grade-specific TDS should always take priority over a general drying table.
How to Store TPU After Drying
Drying TPU correctly is only half of the process.
After drying, hot TPU pellets should not be left exposed to ambient air for long periods because the material can begin absorbing moisture again.
For stable production:
- Keep unused TPU in sealed packaging.
- Close opened bags immediately after use.
- Store dried pellets in sealed containers or a closed dryer hopper.
- Avoid leaving dried TPU exposed in open material trays.
- Re-dry material that has been exposed to humid air for an extended period.
Moisture reabsorption can be particularly fast in warm and humid production environments.
How Can You Tell If TPU Is Not Dry Enough?
The most common warning signs include:
Bubbles during extrusion
Moisture can vaporize during melt processing and create bubbles or voids.
Silver streaks during injection molding
Splay or silver marks are frequently associated with excessive material moisture.
Rough extrusion surface
Moisture can contribute to unstable melt flow and poor surface quality.
Unexpected loss of mechanical properties
TPU may sometimes show limited visible defects while molecular degradation has already affected performance.
For critical applications, appearance alone should not be used to determine whether TPU is sufficiently dry.
Frequently Asked Questions
Can TPU be processed without drying?
It is generally not recommended. Even TPU pellets taken from opened packaging may contain enough moisture to affect processing quality.
What is the recommended TPU drying temperature?
Many TPU grades are dried within approximately 80-110°C, but the correct temperature depends on the specific grade. Always follow the supplier's TDS.
How long should TPU pellets be dried?
A common starting point is approximately 2-4 hours, although longer drying may be necessary if the material has absorbed significant moisture.
Can TPU be dried at a higher temperature to save time?
Higher temperature does not always mean better drying. Excessive heat can result in yellowing, oxidation or material degradation.
What moisture content is suitable for TPU processing?
Many commercial TPU grades specify moisture levels around 0.05% or lower, while some grades and demanding processing conditions may require moisture below 0.02%.
Conclusion
Proper drying is one of the simplest ways to improve TPU processing stability and final product quality.
Before injection molding or extrusion, processors should control not only TPU drying temperature and drying time, but also dryer dew point, airflow and material storage after drying.
Most importantly, drying conditions should always be matched to the specific TPU resin grade and processing method rather than relying on one universal setting.
Correct moisture control can help reduce bubbles, silver streaks, rough surfaces and unnecessary production troubleshooting. For grade selection or processing support, contact Longyixing Chemicals.
