The Crucial Quartet: A Deep(er) Dive into the Four Key Drying Parameters

The purpose of this article is to expand on the drying parameters discussed in the Mo’s Corner article here: https://www.motan-group.com/en/mos-corner/mos-corner-news/detail/15-what-are-the-most-important-parameters-for-drying-plastics/

Introduction: Properly managing the drying parameters for resin materials is crucial to ensure their quality and performance in various industrial applications. These four parameters—time, airflow, temperature, and dewpoint—play a pivotal role in achieving the desired moisture content in the resin pellets. In this thought experiment, we will explore the consequences when three are set optimally, while the fourth is either too high or too low.

1. Time: Good airflow, good temperature, good dewpoint, Short dry time

When the drying time is too short, even with optimal airflow, temperature, and dewpoint, the result is inadequate drying. Inadequate time means there hasn't been sufficient opportunity for moisture to migrate from the interior of the pellets to the exterior, where it can be drawn away by the airflow. This lack of thorough drying can lead to processing defects and product quality issues.

Good airflow, good temperature, good dewpoint, Long dry time

When the drying time is too long, the pellets will become overdried. Much of the water has diffused out of the resins and carried away by airflow. Moisture content in the pellets will accordingly be lower than recommended and can lead to difficulties in molding and even processing defects.

2. Airflow: Good dry time, good temperature, good dewpoint, Too little airflow

Insufficient airflow, even when the drying time, temperature, and dewpoint are optimal, results in inconsistent drying. It's akin to placing a container of pellets in an oven, where the pellets near the top dry quickly due to direct contact with hot air, while those inside the container remain moist due to the lack of airflow. Inconsistent drying can compromise the resin's quality and processing performance.

Good dry time, good temperature, good dewpoint, Too much airflow

Conversely, excessive airflow at optimal drying conditions can lead to higher costs without significant benefits. Excessive airflow won't necessarily dry the resins faster because moisture still primarily diffuses from the pellet interior to the exterior. Moreover, high airflow increases operational costs and can dissipate heat from the drying equipment, potentially affecting the drying temperature. Taken to the extreme, too high of airflow can cause a fluidized bed condition which leads to unpredictable and inconsistent drying due to the departure from the mass flow condition that drying bins are designed around.

3. Temperature: Good dry time, good airflow, good dewpoint, Too low temperature

Setting the drying temperature too low results in inadequately dried resin. Drying relies on thermal energy to enhance moisture diffusion from the pellet interior to the exterior. When the temperature is too low, the diffusion rate is reduced, leading to a slow drying process. This situation can be compared to running a clothes dryer on a 'fluff' mode, where the clothes remain damp after the cycle completes.

Good dry time, good airflow, good dewpoint, Too high temperature

Conversely, excessively high temperatures, even when the drying time, airflow, and dewpoint are optimal, can yield poor material quality. The high temperature may overdry the resin and potentially cause degradation of the resin or additives, leading to processing defects. At the extreme high end, the resin can surpass its glass transition point leading to the entire material column turning solid inside the bin, which can be very difficult if not impossible to clean out.

4. Dewpoint: Good dry time, good airflow, good temperature, Too low dewpoint

A very low dewpoint leads to well-dried resin but at an increased operating cost. To maintain an excessively low dewpoint, the dry air generator must regenerate the desiccant more frequently than necessary. Dry air's role is to draw away moisture from the resin after it diffuses from the pellet interior. A lower dewpoint can theoretically absorb more moisture, but this won't occur if the moisture hasn't diffused from the pellets yet.

Good dry time, good airflow, good temperature, Too high dewpoint

Conversely, an overly high dewpoint can hinder adequate drying. A high dewpoint can slow or even halt the drying process by reducing the ability of airflow to remove moisture that has diffused from the pellets. The resulting under-dried resin will result in processing defects such as splay and reduced mechanical strength.

Conclusion: In conclusion, understanding and optimizing these four drying parameters are essential for achieving consistent and high-quality resin drying processes. Finding the right balance among time, airflow, temperature, and dewpoint is key to ensuring that resin materials meet the required standards and perform effectively in various applications.

For assistance in finding the perfect processing conditions for your parts, and for the equipment to make it happen, consider talking to the experts at Motan! Check out our website at https://www.motan-group.com/us/ for more information, and you can reach out to me and I will be happy to help you directly or get you into contact with a Motan representative in your area.