Dryer Dew Point Too High: Causes and Fixes

A high dew point means the dryer isn't delivering air dry enough to pull moisture out of a hygroscopic resin (PET, PA, PC and similar materials), which directly causes hydrolysis, bubbles and streaking in the finished part. The most common causes are a saturated or damaged molecular sieve, an air leak in the system, insufficient airflow through the hopper, or a faulty sieve regeneration cycle.

TL;DR — check these first:

• Measure the actual dew point at the dryer outlet, not just the controller readout
• Check the age and condition of the molecular sieve
• Check hose, hopper lid and fitting seals for moist-air leaks
• Check whether airflow through the hopper is adequate for the resin's volume and pellet size
• Confirm the sieve regeneration cycle actually completes fully

Is a saturated or damaged molecular sieve the cause?

The molecular sieve absorbs moisture from the air; if it's saturated, old, or damaged (e.g. clogged by resin dust), it can no longer hold dew point at the setpoint.

How to confirm: Dew point drifts upward slowly over time (weeks/months) rather than being stable; the regeneration cycle runs the same as before, but the result is worse.

Fix: Replace the molecular sieve per the dryer manufacturer's recommended interval; check that the sieve is protected from resin dust by an adequate pre-filter.

Is an air leak causing the high dew point?

Any point where moist ambient air enters the closed dry-air loop (a damaged seal, a poorly sealed hopper lid, a cracked hose) directly raises dew point.

How to confirm: Dew point jumps suddenly rather than drifting gradually; the problem often worsens in more humid ambient conditions (e.g. after rain, in wetter seasons).

Fix: Check and replace seals on the hopper, hoses and fittings; confirm the hopper lid seals properly; check hoses and connectors for physical damage.

Is insufficient airflow through the hopper the cause?

If airflow through the hopper is too low for the resin's volume or pellet size (e.g. finer pellets need a different flow than coarser ones), air doesn't pass evenly through all the material, leaving part of it under-dried even though dew point at the dryer outlet reads fine.

How to confirm: Dew point at the dryer itself is fine, but material at the bottom or top of the hopper stays moist; the problem depends on sample position within the hopper.

Fix: Check and adjust airflow per the resin manufacturer's recommendation for the specific material type and pellet size; confirm the hopper isn't overfilled beyond the recommended level.

Is a faulty regeneration cycle the cause?

The sieve regeneration cycle (heating and cooling alternating sieve vessels) must fully complete each phase for the sieve to actually regenerate; an interrupted or shortened cycle leaves the sieve partially saturated.

How to confirm: Check whether heating and cooling times in the regeneration cycle match factory settings; check regeneration heaters and valves for proper operation.

Fix: Service or replace regeneration heaters/valves; check and correct the regeneration cycle timer.

Cause overview

Perotec Systems supplies Plastic Systems dryers and material conveying systems for the regional market. Browse the full dryers and dosing range or read How Proper Drying and Dosing Reduces Scrap in Production. Contact us with your material and dryer data — we help diagnose the cause.