PETG foaming and moisture

PETG is highly hygroscopic — it absorbs moisture from the air. When wet PETG is extruded, the trapped water flashes to steam and the print comes out foamy and bubbly. This is the most common PETG-specific issue.

Symptoms

• PETG comes out of the nozzle looking foamy, full of bubbles
• Irregular extrusion with small “puffs” of steam
• Poor layer-to-layer adhesion
• Visible bubbles on the surface of the finished part
• Crackling / popping sound during extrusion

Possible causes

1. Wet pellets — PETG absorbed moisture, often during storage or after the bag was left open
2. Insufficient drying — temperature too low, time too short, or dryer left tightly closed
3. Low-quality / aged PETG — old batches or batches exposed to humidity
4. Flame-retardant grades — the retardant package makes the polymer more sensitive to moisture than plain PETG

Diagnosis

Step 1 — The batch

• How long has this batch been open?
• Was it dried before printing, at what temperature, for how long?
• Is it plain PETG or a flame-retardant grade?
• What is the room humidity? Below 60% RH is the target

Step 2 — Visual inspection of the pellets

• Transparent, glassy pellets → likely dry
• Yellowish, or puffed instead of a clean cut granule → already compromised
• A visible droplet inside a granule → trapped water, that batch is ruined

Fix

Drying procedure

Never run the dryer open

Do not leave the lid, the tank or any part of the dryer open while it runs — not “a crack at the top to let the moisture out”.

A dryer works by holding heated air around the pellets in a closed volume. Open it and it draws in room air continuously, and room air is what put the water in the material in the first place. The machine then runs for hours, gets hot, and dries nothing.

If you have been doing this, the material was never dry, however long the cycle ran.

1. Load pellets into the Dryer
2. Set the temperature to 60–70°C, depending on the grade
3. Run it for at least 4 hours — 6 is the figure to aim for
4. Check on it. The real limit is not a number on the dial, it is the pellets packing together. If they start clumping, drop the temperature. If nothing is dropping out of the dryer hopper while it still looks full, the material has compacted — stir the lower part by hand or with a stick
5. Feed the printer from the dryer. Dried pellets tipped back into a bag or a bucket pick moisture up again within hours

If pellets are too far gone

If the print is still foamy after a full 6-hour cycle:

• The batch is likely ruined (old, contaminated, or stored badly)
• Replace with a fresh sealed bag
• Contact the supplier if it’s a recent batch with quality issues

Flame-retardant grades

• The retardant package takes up more moisture than plain PETG. Give it a longer cycle than the figures above
• Always dry before use, even straight from a freshly opened bag
• Yellowish pellets, or a visible bubble inside a granule, mean the batch is already wet
• If the part does not actually need flame retardancy, plain PETG is far more forgiving. See PETG