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

Wet pellets — PETG absorbed moisture, often during storage or after the bag was left open
Insufficient drying — temperature too low, time too short, or dryer left tightly closed
Low-quality / aged PETG — old batches or batches exposed to humidity
PETG flame-retardant V0 — the flame retardant additive makes the polymer more sensitive to moisture than standard PETG

Diagnosis

Step 1 — Ask about the pellets

How long has this batch been in use?
Was it dried before printing? At what temperature, for how long?
Is it standard PETG or PETG flame-retardant V0?
What’s the room humidity? (target < 60% RH)

Step 2 — Visual inspection of the pellets

Transparent, glassy pellets → likely dry
Yellowish, with bubble-like shape instead of clean cut filament → already compromised or low quality
Visible water droplet inside the pellet → trapped water, pellet is ruined

Fix

Drying procedure

Load pellets into the Dryer
Set temperature to 65°C — never higher (above 65°C pellets aggregate)
Run for 3 to 5 hours minimum
Keep the dryer slightly open at the top to let moisture escape
Start printing immediately after drying — don’t leave dried pellets exposed to air

If pellets are too far gone

If after 5 hours of drying the print is still foamy:

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

Special case — PETG flame-retardant V0

The flame-retardant additive holds onto moisture more aggressively than standard PETG
Always dry before use, even straight from a freshly opened bag
If the application doesn’t strictly need flame retardancy, prefer standard PETG — it’s far more forgiving

How to prevent

Reseal pellets after each use (clip, ziplock, or original bag with desiccant)
Never leave the hopper open between prints
Print in an environment with < 60% RH when possible
After a print pause longer than 1 day, dry again before resuming
Keep an eye on the dryer — running too hot causes aggregation, running too cool fails to remove water

Notes

Pattern observed across 24+ customer cases — the most common PETG-specific complaint
Often combined with Extruder motor skipping when the extruder also struggles with the wet, irregular flow
Rule of thumb: if PETG looks foamy, it’s wet until proven otherwise