Common Print Errors
Diagnose FDM print problems by (1) identifying when the defect starts, (2) doing a few quick hardware/material checks, then (3) applying the smallest safe change and re-testing on a small calibration print. Most failures come from first-layer setup, inconsistent extrusion, cooling/overhang handling, or motion (belts/pulleys/collisions).
TL;DR
Match the defect to when it starts (first layer, early layers, mid-print, end), then apply one small fix and re-test on a quick calibration print. Most “mystery” failures are either wrong first-layer Z/bed cleanliness, inconsistent filament flow, bad cooling on overhangs, or loose motion parts causing shifts.
Step 1: Identify when the defect begins
The start time of a defect is your best clue. First layer problems point to bed cleanliness, bed temperature, Z offset, and first-layer speed. Defects that begin on early overhangs point to cooling, overhang speed, and supports. Problems that appear suddenly mid-print often indicate a motion event (collision, belt slip) or a feed issue (tangle, clog). Issues that get worse late in the print often come from the part getting taller and wobblier, heat building up in small features, or supports losing contact.
Step 2: Fast checks before changing slicer settings
- Confirm the slicer profile matches the filament (PLA/PETG/TPU) and nozzle size.
- Make sure the spool can unwind freely; check for tangles and sharp bends in the filament path.
- Check the extruder drive gear: clean teeth, correct tension, no clicking or grinding.
- Clean the nozzle tip (remove plastic boogers that can catch the print) and clean the build surface (remove oils/glue residue).
- Verify bed leveling/mesh is actually enabled for the print; if unsure, run a first-layer test square.
- Inspect motion hardware: belts feel firm, pulleys/grub screws tight, no wobble in the gantry or bed.
- Confirm part-cooling fan behavior: turns on when commanded, ducts not blocked, airflow aimed at the nozzle exit (not the heater block).
Symptom to first-fix map (smallest safe change first)
Print won’t stick, lines look round, or corners lift in the first few layers
Likely cause: Z offset too high (not enough squish), dirty/greasy bed, bed too cold, first layer too fast, first layer too little extrusion
Fix: Clean the bed, slow the first layer, raise bed temp slightly, then adjust Z offset until lines touch and look slightly flattened (no gaps between adjacent lines).
Warping on larger parts (edges lift, base curves upward)
Likely cause: Uneven cooling/drafts, bed temp too low, too much fan early, not enough bed contact area
Fix: Add a brim, block drafts/enclose, keep fan low/off for the first layers, and increase bed temp a small amount.
Stringing/wisps between features
Likely cause: Nozzle too hot, wet filament, retraction too low, long travels across open air
Fix: Dry the filament first, then lower nozzle temp in small steps and tune retraction; reduce long open-air travels with combing/travel avoidance where appropriate.
Gaps, under-extrusion, weak top layers, or infill not connecting
Likely cause: Partial clog, filament slipping, printing too cold/too fast, inconsistent filament drag from spool path
Fix: Check spool drag and extruder gear cleanliness/tension, increase temperature slightly or reduce speed, then clean the nozzle (cold pull/nozzle clean) if symptoms persist.
Blobs/zits, pimples, or random rough patches
Likely cause: Moisture popping, nozzle too hot, inconsistent feed resistance, seam placement/pressure changes
Fix: Dry filament, reduce temperature slightly, ensure the spool feeds smoothly, then move/align the seam to a less visible edge and avoid sharp speed changes if possible.
Layer shifts (sudden offset in X or Y)
Likely cause: Loose belt/pulley, nozzle collision with a curled edge/support, acceleration too high, stepper/driver overheating
Fix: Tighten belts and pulleys (especially grub screws), look for warping/supports causing collisions, then reduce acceleration/jerk and verify electronics cooling/airflow.
Supports fail or overhangs look droopy
Likely cause: Not enough cooling, overhang speed too fast, printing too hot, support interface too sparse or Z gap too large
Fix: Increase part cooling, slow overhangs/bridges, lower nozzle temp slightly, and adjust support interface density and Z gap; re-orient the part if it reduces overhang angle.
Poor layer bonding; part snaps along layer lines
Likely cause: Printing too cold, too much fan, too few walls/top-bottom layers, orientation puts load across weak layer adhesion
Fix: Increase nozzle temp slightly, reduce fan for stronger bonding, add perimeters and top/bottom thickness, and re-orient so real loads go across layers (not between them).