Overdrying Resin

By understanding the behavior of different resins and implementing smart drying practices, manufacturers can protect material integrity and reduce waste.

How overdrying occurs – and why it’s bad

Overdrying occurs when resins are exposed to drying conditions—typically elevated temperatures and dry air—for longer than recommended time periods. This can lead to a range of issues, particularly when additives are involved. Some resins are particularly sensitive to overdrying.

Consider this example: A third-shift operator neglects to turn off the dryer, which runs all night. If a temperature setback isn’t in use, or airflow isn’t adjusted, the resulting high temperatures will degrade the material sitting in the drying hopper.

Different materials respond differently to overdrying – but the final results are equally problematic:

- Nylon: If nylon is overdried, usually beyond 12 hours, moisture content falls significantly. Viscosity therefore increases, because water in nylons acts like a plasticizer. Processing becomes harder, resulting in part quality inconsistencies.
- Polybutylene terephthalate: This material typically becomes brittle and loses surface gloss when dried over eight hours, with black and glossy parts turning dull gray. Furthermore, excess drying can degrade or remove additives like heat stabilizers, decreasing melt viscosity.
- Additives: Performance-enhancing materials used in resins – including plasticizers, lubricants, flame retardants and stabilizers – are more likely to degrade or evaporate when overdried because these materials often have low molecular weight.

Examples of overdrying

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Steps to take to avoid overdrying

Some options for protecting your resins from overdrying include the following best practices:

Reduce size dryer hopper(s): Ensure the residence time in the dryer matches the resin’s recommended drying time. If the hopper is too large, retrofit it with a level sensor to control the amount of material being loaded or replace the drying hopper with one that is sized properly for the material residence time.

Reduce process airflow: Today’s dryers allow manual or automatic airflow regulation. In manual mode, users can adjust airflow up or down to tune temperature and airflow for minimal energy usage. This reduces air flow to only the amount needed for proper drying of the resin while maintaining product quality. In automatic mode, the control will adjust to the material demand of the process. Air flow will be automatically reduced when the dryer senses a change in the consumption rate. This allows the drying process to continue at optimum conditions but reduces the airflow (and in some cases the process temperature) to prevent overdrying of the resin. Doing this also reduces energy consumption and maximizes the energy efficiency of the process. If the dryer senses demand has stopped, there will be a further reduction to maintain the material inside the drying hopper and minimize the dryer’s energy usage.

Use temperature setback: If molding is delayed after the resin is dried, reduce the dryer temperature while keeping the dryer running. This prevents moisture reabsorption without continuing to dry the resin aggressively. Every processor would establish the preferred temperature setting to use during an idle period.

Shut down when idle: If the machine is idle for more than four hours, turn off the hopper to avoid unnecessary drying.

Store resins properly: After drying, store resins in sealed containers or low-humidity environments to prevent moisture regain before the next round of processing.

Smart controls help avoid overdrying A control can be set to adjust the airflow of the process air to maintain the process setpoint temperature and lower the airflow to minimize the drying effect. By leaving the material at the process temperature setpoints during downtime, operators can resume drying knowing that the material is at the proper setpoint. Another smart control option is temperature setback mode, which will lower the temperature of the process setpoint and keep the airflow constant.

For critical industries like the military, aerospace and medical, inline moisture analyzers monitor moisture content of the resin constantly during processing.

In conclusion

Overdrying resin is as detrimental as underdrying, often leading to irreversible loss of mechanical properties and unusable material. By applying proper drying practices tailored to each resin type, manufacturers can prevent waste, maintain structural integrity and ensure consistent part quality.

RESIN DRYING