Troubleshooting common 3D print defects: symptom, cause, fix

By now you have probably seen most of these: hairs between parts, walls with gaps, a print that stepped sideways, a base that flared out so wide the part would not fit. This lesson puts the common defects in one table with likely causes and fixes, then covers the five you will meet most often.

The numbers assume an FDM printer with a 0.4 mm nozzle and 0.2 mm layers. If your slicer names a setting differently, look at what the setting does rather than what it is called.

How to narrow it down fast

Before you touch the slicer, rule out the cheap causes.

1. Is the filament wet? Popping at the nozzle, bubbles in the line: dry the spool first (see wet filament). Moisture sits behind several rows in the table.
2. Is the first layer good? Squashed or floating, it affects the whole part.
3. Change one thing at a time and reprint a small test piece, not the real six-hour job.

Similar-looking problems have different causes. Layers that crack apart (separation) and layers that step sideways (shift) are fixed in different places. Look carefully before you act.

Symptom, likely cause, fix

What you see	Likely causes	Try in this order
Fine hairs between parts of the print (stringing)	Temperature too high, too little retraction, wet filament, old filament residue on the nozzle	Lower temperature 5–10 °C at a time, tune retraction, dry the filament, clean the nozzle
Thin walls with gaps, lines not touching (under-extrusion)	Partial clog, wet filament, dirty extruder gears or wrong idler tension, temperature too low, first layer squashed	Cold pull, dry filament, clean the gears, adjust temperature by ±5–15 °C, redo Z-offset
Part looks swollen, edges not crisp, top surface rippled (over-extrusion)	Flow / extrusion multiplier too high, filament diameter not what the slicer thinks	Lower flow 2–5 % at a time, measure the filament with calipers
Layers step sideways partway up (layer shift)	Belt too loose or too tight, loose pulley on a motor shaft, nozzle hitting a curled part, too fast	Adjust belts, tighten pulley set screws, slow down, fix the curling overhang
Nothing comes out, or it stops mid-print (clog)	Fully blocked nozzle, heat creep, filament ground through by the gear	Cut and reload filament, cold pull, check the hotend fan, lower room or enclosure temperature
Small bumps on the walls (blobs / zits)	Start and end point of each loop, not enough retraction or wipe, wet filament, flow too high	Move the seam to the back or a corner, enable wipe, dry filament, lower flow
Base flares out wider than the part (elephant foot)	First layer squashed, bed too hot, nozzle too hot	Enable elephant foot compensation at about 0.1–0.2 mm, raise Z-offset slightly, lower bed 5 °C, add a chamfer to the model
Horizontal cracks, layers coming apart	Temperature too low, too much fan, draughts or a cold room, layer too tall for the nozzle	Raise temperature 5 °C at a time, reduce fan, enclose ABS/ASA
Rough or wavy side walls, repeating pattern near edges (ringing)	Too fast, high acceleration, printer vibrating, loose belts	Lower speed and acceleration, put the printer on a solid surface, adjust belts, see speed vs quality
Holes or pinholes in the top surface, infill showing through	Too few top layers, infill too sparse, under-extrusion	Use 4–6 top layers, raise infill, fix under-extrusion first
Part comes off the bed or corners lift	Dirty bed, Z-offset too high, bed too cold, draughts	See bed adhesion and warping
Rough, drooping underside on overhangs	Past 45°, too little fan, too fast	More fan, slower overhang speed, supports or a different orientation

The five most common problems follow, in more detail than the table allows.

1. Stringing

Strings appear when the nozzle crosses a gap and plastic keeps oozing. The causes are plastic that is too runny (too hot, or wet) or retraction that does not pull back far enough. The Prusa Knowledge Base suggests, in order:

• Lower the nozzle temperature 5–10 °C and see if the strings fade. Do not go so low that layers stop bonding.
• Retraction length. Direct-drive extruders usually need no more than about 2 mm; Bowden setups need more, for example the Prusa MINI preset uses 3.2 mm. Flexible filament (TPU) usually needs longer retraction than rigid filament.
• Retraction speed. Faster reduces strings, but too fast and the extruder motor skips.
• Keep Lift Z low and leave wipe while retracting enabled.
• Try another spool. If the strings vanish at once, the first spool was wet.

Worked example: PLA at 215 °C leaves hairs between two posts. 205 °C thins them; raising retraction from 0.8 to 1.2 mm (direct drive) clears them. If layers start separating at 205 °C, go back to 210 °C and let retraction do the work.

2. Under-extrusion

Walls with gaps, lines not touching their neighbours, a porous top. Prusa lists these causes:

• Partial clog. Old filament residue or burnt material; fix with a cold pull (steps in the next lesson).
• Wet filament, especially PETG, ABS and soluble filament.
• Extruder gears. Plastic shavings packed into the teeth, or an idler screw that is too loose or too tight; both reduce output.
• Temperature, too low or too high. If the filament has no slicer profile, try ±5–15 °C.
• Hardened or stainless steel nozzles conduct heat worse than brass and may need 5–10 °C more.
• First layer squashed. A nozzle pressed into the bed cannot extrude.
• Filament diameter. The industry tolerance is ±0.05 mm, so extruded volume varies between spools.

Only when all of these pass should you raise flow / extrusion multiplier, 2–5 % at a time. Flow that masks a partial clog works for one print and then fails again.

3. Layer shift

The part steps sideways partway up. Prusa says this is “most often caused by the wrong tension of the belts or pulleys not being secure.”

• Belts. Too loose and too tight are both a problem. A plucked belt should sound like a low bass string and give a little when pinched; some printers show a tension number in a menu.
• Loose pulley is the main cause of repeating staircase shifts. Tighten both set screws, one of them against the flat of the motor shaft.
• Nozzle hitting the part. A curled overhang in the nozzle’s path, or very sparse infill letting the top sag. More fan, or a different orientation.
• Obstructions. Debris wrapped around a belt, a cable catching the frame, dry or scored smooth rods.
• Too fast for the machine. Lower speed and acceleration for large prints.

A shift at the same height every time, always in X, is usually a loose X pulley. Random shifts on large fast prints are usually speed.

4. Blobs and zits

Small bumps usually sit at the start or end of each loop (the seam), where the extruder stops and restarts and nozzle pressure is released. From the Simplify3D guide:

• If the bump is at the start of a loop, the extruder is priming too much after a retraction. Reduce the prime distance (names vary by slicer: extra restart distance, retract restart extra).
• If the bump is at the end of a loop, enable coasting at about 0.2–0.5 mm, or enable wipe so the nozzle keeps moving while it retracts.
• Cut unnecessary retractions with options such as only retract when crossing perimeters and avoid crossing perimeters.
• Seam position. Put it at the back or on a corner. The bumps are still there, but out of sight.

If blobs get steadily worse through a spool, PETG in particular, suspect moisture before anything else.

5. Elephant foot

The first layers come out wider because the plastic is squashed against a hot bed and spreads sideways. Mating parts get stuck at the base.

• Enable elephant foot compensation. In PrusaSlicer it is under Print Settings → Advanced (Advanced or Expert mode). Prusa’s own profiles have it on, and about 0.2 mm works well with a 0.4 mm nozzle. OrcaSlicer has a setting of the same name; Cura uses a negative initial layer horizontal expansion.
• Z-offset. If the first layer is a translucent sheet with the bed showing through, it is squashed too hard. Raise it 0.02–0.05 mm at a time (see bed leveling).
• Lower the bed 5 °C or the nozzle 5–10 °C so the first layer sets before it can keep spreading.
• Add a chamfer of 0.3–0.5 mm to the base edge in the model. This fixes it at design time and works with any slicer.

Do not overdo compensation: thin walls in the first layer disappear, and the brim detaches from the part. In the preview, the first layer should be only slightly smaller than the second.

Common mistakes

• Changing several settings at once. If it works you do not know why; if it comes back you do not know what broke.
• Raising flow to fix under-extrusion that is really a partial clog or wet filament. The next spool then over-extrudes.
• Chasing stringing with lower temperature until layers separate. One problem traded for a worse one.
• Tightening belts as hard as they go. Too tight makes the motor skip and wears the bearings. A good belt is a low note, not a high one.
• Not writing down what you changed, so every new spool starts the hunt again.

Some rows come back as the printer ages: belts loosen, nozzles wear, fans fill with dust. The last lesson covers routine maintenance and safety, so you open this table less often.

Sources

• Prusa Knowledge Base: Stringing and oozing
• Prusa Knowledge Base: Layer shifting
• Prusa Knowledge Base: Under-extrusion
• Prusa Knowledge Base: Elephant foot compensation
• Simplify3D: Blobs and zits