During the drying process, if particles cannot be completely dried, we need to look for factors that affect the drying effect of the drying oven. There are generally four types:

1. Drying temperature

Heat is the key to unlocking the combined force between water molecules and hygroscopic polymers. When the temperature is higher than a certain level, the gravitational force between water molecules and polymer chains will be greatly reduced, and water vapor will be carried away by dry air.

2. Dew point

In a dryer, wet air is first removed to contain very low residual moisture (dew point). Then, reduce the relative humidity of the air by heating it. At this point, the vapor pressure of the air is relatively low. By heating, the water molecules inside the particles break free from the binding force and diffuse towards the air around the particles.

Dew point readings can help identify some issues, so the dew point value of the dry air should be monitored throughout the entire drying process. The dew point reading of the dryer during normal operation should be a straight line within the range of 20 ° F to 5O ° F. Of course, small fluctuations caused by changing the desiccant are normal. If the dryer operates normally, the dew point at the inlet of the air should be at least 30 ° F lower than the dew point at the outlet of the return air.

On the other hand, after the replacement of the desiccant, the dew point immediately reaches a peak, indicating that the desiccant is not sufficiently cooled before being placed, making it unable to adsorb moisture well. After cooling, the dew point of the desiccant will drop to normal standards. If the desiccant is not properly cooled, it will lead to temperature peaks, and sudden temperature changes will reduce the drying ability of the desiccant for thermosensitive materials such as ionomers, amorphous polyester, and certain nylon grades.

If the dew point reading is normal after the replacement of the desiccant bed, but the dew point rapidly rises before the end of the drying cycle of the desiccant, it indicates that the ambient air may have entered the closed air path, causing the desiccant to absorb moisture prematurely. Another possibility is that the regeneration of the desiccant is incomplete or contaminated. If the dew point reading is close to the dew point reading of the return air, it indicates that the regeneration gas circuit has completely failed or the desiccant has been severely contaminated.

3. Time

In the air surrounding the particles, the absorption of heat and the diffusion of water molecules to the surface of the particles require a certain amount of time. Therefore, the resin supplier should provide a detailed description of the time it takes for a material to effectively dry at an appropriate temperature and dew point.

4. Airflow

The dry hot air transfers heat to the particles in the drying silo, removes moisture from the surface of the particles, and then sends the moisture back to the dryer. Therefore, there must be sufficient airflow to heat the resin to the drying temperature and maintain this temperature for a certain period of time.

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