PETG Extrusion Problems Explained

Why PETG is harder than PLA: The physics

PETG fails in ways PLA doesn't. Understanding why helps you fix it:

• Higher viscosity: PETG is thicker/stickier at high temps. It clogs nozzles more easily if temp is slightly wrong.
• Sensitive to temperature: PLA works at 200–220°C. PETG needs 240–260°C. A 5°C difference causes extrusion inconsistency.
• Thermal inertia: PETG cools slower, so plastic in the nozzle stays hot longer and can ooze between prints.
• Shrinkage: PETG shrinks 0.5–1% as it cools. This affects layer bonding and can cause warping.
• Abrasive: PETG wears brass nozzles faster than PLA. Worn nozzles jam more easily on PETG.

Practical result: PLA is forgiving. PETG requires dialing in temps, speeds, and pressure advance carefully. Get one setting wrong and it jams or extrudes poorly.

Problem 1: Nozzle jam (extrusion stops mid-print)

Symptoms: Print starts fine, then 10–30 minutes in, extrusion stops. Nothing comes out of the nozzle, but the stepper motor keeps trying (you hear clicking/grinding).

Root causes (in priority order)

1. Nozzle temperature too low (40% of jams): Cold PETG is thick and clogs the nozzle opening.
2. Filament sensor triggered jam (Prusa): Printer pauses due to detected blockage, thinking there's a jam when there isn't.
3. Flow rate exceeds safe limit (25% of jams): Trying to push too much plastic too fast. Hot-end can't extrude fast enough.
4. Worn or damaged nozzle (15% of jams): Rough nozzle edge or partially clogged opening traps filament.
5. Wet filament (10% of jams): Moisture boils in the hot-end, creating steam pockets that jam extrusion.
6. Retraction too aggressive: Pulling filament back and forth too hard can grind it, sending plastic dust into the hot-end.

Fix sequence (do in order)

Step 1: Raise nozzle temp by 5–10°C (try first, fastest fix)

• Example: 245°C ? 250°C
• Hotter plastic flows easier and is less prone to jamming
• Resume print from pause (if available) or restart

Step 2: Check and reduce flow rate

• Use volumetric flow calculator to verify settings
• Keep flow under 12–15 mm³/s for safety
• If you're above that, reduce print speed or layer height
• Restart print from beginning

Step 3: Swap the nozzle (if temperature didn't fix it)

• Nozzle cost: $3–7
• Heat nozzle to 280°C, unscrew old nozzle (takes 1 minute)
• Install new nozzle, cool, and resume printing

Step 4: Dry filament (if jam persists)

• PETG absorbs moisture over time
• Bake filament at 60–70°C for 4 hours
• Or use a filament dryer if available
• Moisture in PETG causes steam in the hot-end, which blocks extrusion

Step 5: Reduce retraction distance

• If using Bowden printer: reduce from 5mm to 4mm
• If using direct drive: reduce from 1.5mm to 1mm
• Aggressive retractions can grind filament

Step 6: Run a hot-end clean (nuclear option)

• Remove filament, heat nozzle to 280°C
• Push a cleaning filament or brass wire through the opening
• (Advanced: disassemble hot-end and soak in acetone if clogged with burnt plastic)

Problem 2: Blobs and zits (surface defects)

Symptoms: Small blobs of plastic appear randomly on the print surface. They don't affect structure but look ugly.

Root causes

1. Nozzle temp too high: PETG oozes between moves, creating small blobs when extrusion resumes.
2. Cooling fan too low: Plastic doesn't cool fast enough between layers, oozes out.
3. Retraction not working: Filament doesn't retract when moving, plastic leaks out of nozzle.
4. Print speed too slow: Nozzle dwell time too long, plastic oozes.

Fix sequence

Step 1: Lower nozzle temp by 5°C

• Example: 250°C ? 245°C
• Cooler plastic is less runny and oozes less
• Print a test cube

Step 2: Increase cooling fan to 20–25%

• PETG needs low cooling (10–15%) for layer bonding, but 20% is acceptable if blobs are the issue
• Test on a cube with overhangs (shows cooling effect)

Step 3: Verify retraction is enabled in slicer

• Open slicer (Cura, PrusaSlicer, etc.)
• Search for "retraction" in settings
• Ensure retraction is enabled (usually on by default)
• Check distance is appropriate: 5mm for Bowden, 1–1.5mm for direct drive

Step 4: Increase print speed by 10 mm/s

• Faster extrusion = less dwell time = less oozing
• Example: 90 mm/s ? 100 mm/s

Problem 3: Weak extrusion (thin lines, gaps, sparse output)

Symptoms: Extrusion comes out thin and inconsistent. You see gaps between lines, sparse infill, weak perimeters. Part is structurally weak.

Root causes

1. Nozzle temp too low: Cold PETG doesn't flow well, comes out thin.
2. Flow rate set too low in slicer: "Flow" or "extrusion multiplier" < 1.0.
3. Clogged nozzle (partial): Partially blocked nozzle restricts plastic flow.
4. Filament diameter not set correctly: Slicer thinks you have 1.75mm filament but you have 2.85mm (or vice versa).
5. Extruder slipping: Stepper can't push filament through, so it skips.

Fix sequence

Step 1: Raise nozzle temp by 10°C

• Example: 240°C ? 250°C
• Hotter plastic flows easier
• Print a test cube

Step 2: Check flow multiplier in slicer

• Open your slicer profile (Cura, PrusaSlicer, etc.)
• Search for "flow" or "extrusion multiplier"
• Should be set to 1.0 (100%)
• If it's 0.9, increase to 1.0

Step 3: Verify filament diameter

• Slicer setting: Usually 1.75mm or 2.85mm
• Check your filament package for actual diameter
• Wrong diameter = wrong extrusion amount

Step 4: Swap nozzle

• If temperatures are correct but extrusion still weak, nozzle is clogged
• New nozzle ($5) usually fixes it

Step 5: Check extruder gear tension (Bowden printers)

• If stepper is slipping, the tension screw on the extruder is too loose
• Tighten slightly (1/8 turn) and test
• Don't over-tighten or you'll grind filament

Problem 4: Stringing (thin webs between parts)

Symptoms: Thin plastic threads appear between separate objects on the print bed. Looks like a spider web.

Root causes

1. Nozzle temp too high: Hot plastic oozes during travel moves.
2. Retraction not working: Filament isn't pulled back during travel, plastic leaks.
3. Travel speed too slow: Nozzle lingers between parts, oozing plastic.

Fix sequence

Step 1: Lower nozzle temp by 5–10°C

• Example: 250°C ? 240–245°C
• Cooler plastic oozes less
• Print a multi-part test (castle or tower)

Step 2: Verify retraction is enabled and tuned

• Check slicer: retraction should be on
• Bowden: 5–6mm distance, 45 mm/s speed
• Direct drive: 1–1.5mm distance, 25 mm/s speed
• Increase retraction distance by 0.5–1mm if still stringing

Step 3: Increase travel speed

• Slicer: "Travel speed" or "max speed"
• Increase to 150+ mm/s (moves between parts fast, less dwell)

PETG vs PLA: Why settings are so different