Troubleshooting and Safety

A method for fixing a failed print

• 1. Look at where it failed: the first layer, a certain height, a particular feature, or at random. A fault at one height points to the model or a cooling change; the first layer points to the bed; random faults point to the filament path or electronics.
• 2. Check the cheap causes first: a clean bed, dry filament, a clear nozzle, a tight nozzle, loose belts, a tangled spool, the right profile.
• 3. Change one thing at a time and print a small test; keep a log.
• 4. Use the right calibration test (a first-layer pattern, a flow cube, a temperature tower, a retraction test, a calibration cube): see Calibration and Maintenance.
• 5. If it will not go away, suspect a hardware fault: a worn nozzle or gear, a partial clog, a slipping belt, or an overheating driver.

Symptoms, causes and fixes

Symptom	Likely causes	Try
First layer will not stick	Bed not level, nozzle too high, dirty bed, bed too cool, first layer too fast	Re-level, lower the Z-offset, clean with isopropyl alcohol, raise the bed temperature, slow the first layer to 20 mm/s (calibration)
Part comes loose during the print	Small contact area, warping, a dirty bed, a nozzle that knocks the part	Add a brim, clean the bed, use glue stick, reduce the speed and travel over the part, raise the bed temperature
Warping and corners lifting	Cooling and shrinkage, draft, low bed temperature	Enclose the printer, add a brim or mouse ears, raise the bed temperature, reduce the fan for ABS, avoid drafts
Stringing and oozing	Temperature too high, wet filament, retraction too low, travel too slow	Dry the filament, lower the temperature by 5 to 10 °C, tune retraction and travel speed
Under-extrusion (gaps, thin walls, weak parts)	Flow limit exceeded, partial clog, worn extruder gear, wrong flow rate, temperature too low	Lower the speed, check Q against the hotend limit, clear the nozzle (cold pull), check extruder tension, raise the temperature
Over-extrusion and blobs	Flow too high, e-steps wrong, temperature too high	Calibrate the extruder and flow; reduce the temperature
Clicking extruder	Clog, too much speed or retraction, nozzle too close to the bed, a tangled spool	Clear the nozzle, slow down, raise the Z-offset, check the spool
Layer shifting	Loose belt or pulley, head collision, driver overheating, too much acceleration	Tighten belts and pulleys, check for curling parts the nozzle hits, lower the speed or acceleration, cool the drivers
Rough or ringing surface (ghosting)	Speed and acceleration too high, loose parts, resonance	Reduce outer wall speed and acceleration; tighten the frame; use input shaping
Z-banding (regular horizontal lines)	Bent or wobbling Z lead screw, loose coupler, inconsistent layer heights	Straighten or replace the screw, tighten the coupler, use a layer height that is a multiple of the full step
Elephant foot	First layers squashed outward, bed too hot, Z-offset too low	Lower the bed temperature, raise the Z-offset slightly, chamfer the base
Holes too small / dimensions wrong	Shrinkage, over-extrusion, steps per millimeter	Calibrate flow and steps, compensate holes in the model
Sagging overhangs and drooping bridges	Too hot, too little cooling, overhang too steep	Raise the fan, lower the temperature, slow down, add supports or redesign
Gaps between the infill and walls	Too little overlap, under-extrusion, wall width mismatch	Raise the infill overlap, check the flow, use a wall thickness that is a whole number of lines
Pillowing (holes in the top surface)	Too few top layers, too little cooling	Add top layers (5 or more), more fan, lower the infill gap
Blobs and zits at the seam	Seam position, pressure advance not set, over-extrusion	Move the seam to a corner, set pressure advance, adjust the retraction
Nozzle clog	Dust, debris, heat creep from poor cooling, wet filament	Cold pull, check the heatsink fan, use clean, dry filament, replace the nozzle
Print stops or shows “thermal runaway” / “heating failed”	Loose heater or thermistor, a draft, broken wire, a failing heater	Check and re-seat the wires and connectors, shield the printer from drafts, replace the part: never disable the protection
Prints fail at the same height	Model feature (a bridge or thin section), a filament tangle, a temperature or fan schedule, a Z-wobble	Look at the model and slicer layer view at that height

Extrusion problems in depth

Most extrusion problems come down to the filament path (dry, clean and free to feed), the hotend (hot enough and free of debris) and the flow demanded (within what the hotend can melt). The flow demand is Q = line width × layer height × speed (see Slicer Settings and Print Quality): if the hotend cannot keep up, the extruder clicks or skips and the part shows gaps. Try a slower speed or higher temperature before assuming a mechanical fault.

• Wet filament pops, hisses, strings and gives a rough, fuzzy surface; dry it (see Filament Materials).
• Heat creep softens the filament above the heat break and jams it, especially in a warm enclosure or with a failed heatsink fan or slow prints with long retractions.
• A worn nozzle (abrasive filaments or many kilograms) gives under-extrusion and rough walls; replace it.
• Filament diameter: measure it; a 1.70 mm filament extrudes 6 % less volume than 1.75 mm (5.63 %), which a flow calibration corrects.
• Extruder gear and tension: a gear full of filament dust, or an idler that is too loose or too tight, causes slip or flattened filament.

Adhesion, warping and layer problems

• Adhesion: start with the first layer (Z-offset, bed temperature, clean bed). A brim or glue stick solves most remaining cases, and a different build surface for the material (see FDM Printer Hardware) solves the rest.
• Warping: plastics shrink as they cool, and the top of a part cools and shrinks while the bottom is held to the bed. Keep the part warm (enclosure, bed temperature), cool it slowly (low fan), and give it more adhesion area (a brim) and fewer sharp corners.
• Layer splitting and cracking in ABS, ASA and nylon: the part cooled too fast; raise the nozzle temperature and the chamber temperature and reduce the fan.
• Weak layer bonds: too cool a nozzle, too much cooling, too thick a layer for the nozzle, or wet filament. Print hotter and slower, or orient the part differently (see Strength, Accuracy and Fits).

Safety

The main hazards are heat and fire, electrical faults, fumes and particles, chemicals (resin, solvents, isopropyl alcohol), moving parts and sharp tools. Printers are safe in practice when they are well built, properly set up and not left unattended for long, and when their protection features work.

• Hot parts. The nozzle runs above 200 °C and the bed up to 110 °C. Let them cool before reaching in; a molten plastic burn sticks to the skin.
• Moving parts. Keep fingers, hair and clothing away from moving heads, belts and gears; power down before working inside.
• Sharp tools. Use a sharp scraper and knife carefully, cut away from your body, and wear eye protection when removing supports and sanding.
• Noise, trip hazards and weight. Place the printer on a stable, level, non-flammable surface; keep cables clear.

Fire and electrical safety

Most printer fires start with heater circuits: a thermistor that falls out (so the firmware thinks the nozzle is cold), a loose connector that heats up, or a failed power-switching device that leaves a heater on.

• Thermal runaway protection in the firmware on every heater (nozzle, bed, chamber).
• Inspect connectors and wiring regularly for discoloration, melting or loose pins, especially the bed, heater cartridge and power connector; use ferrules or crimped lugs and tighten the screw terminals; replace damaged connectors.
• A smoke detector in the room (and ideally a fire extinguisher suited to electrical fires, such as CO₂ or ABC powder); fire-resistant surface beneath; no flammable materials near the printer.
• Do not leave a printer running unattended for long prints unless it has a fire-protection set-up you trust: some owners use a smart plug with a thermal or smoke trigger, a thermal fuse on the bed or a camera.
• Use a quality, correctly rated power supply with a fuse and an earth connection; do not modify mains wiring unless you are competent.
• Replace cheap unbranded heaters, power supplies and thermistors that show heat damage.
• Water and metal dust: keep liquids away from the electronics.

Fumes, particles and ventilation

Heated plastics emit ultrafine particles and volatile organic compounds. The amounts differ by material and temperature: ABS and ASA give off styrene and other irritants; PLA emits fewer, with lactide; nylon emits caprolactam; resin printers emit resin vapors. Emissions are highest during heating and at the highest temperatures, and the concentration in a room depends on how fast the air is changed: the steady concentration is inversely proportional to the ventilation rate.

• Print ABS, ASA, nylon and polycarbonate only in a ventilated space or an enclosure vented outside or through a good filter; keep the printer out of bedrooms and living areas.
• Enclosures contain emissions: add a filter (activated carbon and HEPA) or an exhaust duct.
• Print with lower-emission materials where possible; avoid very high nozzle temperatures; use a clean nozzle (burnt plastic emits more).
• Do not stay in the room during long ABS prints, and open a window afterwards.
• Sanding, drilling and cutting printed parts makes plastic dust: wear a P2 (FFP2) mask and work wet or with extraction, especially for filled filaments (carbon fiber, glass fiber).

Resin, powder, tools and chemicals

• Resin: a skin irritant and sensitizer, harmful to aquatic life: gloves, eye protection, ventilation, and proper disposal (cure leftovers; do not pour liquids down a drain). See Resin and Powder Processes.
• Isopropyl alcohol and acetone are flammable: small quantities, sealed containers, no ignition sources, good ventilation.
• Powder (nylon, metal) is a dust and combustible-dust hazard: closed handling, extraction and respiratory protection; metal powders can be pyrophoric.
• Glues and solvents: read the safety data sheet; cyanoacrylate bonds skin and eyes in seconds; epoxy is a skin sensitizer.
• Food contact: printed parts are generally not food safe: layer lines trap bacteria, and additives and nozzle materials vary.
• Children and pets: keep hot machines, resin, solvents, sharp tools and small parts out of reach.
• See Workshop Safety for protective equipment and shop practice.

Related

• 3D Printing and Additive Manufacturing: the overview of this guide.
• Calibration and Maintenance: the tests that find the cause.
• Slicer Settings and Print Quality: flow, cooling and retraction settings.
• Filament Materials: drying, temperatures and fumes by material.
• Workshop Safety: protective equipment, ventilation and fire.

FAQ

Is it safe to leave a printer running overnight?

It is a risk. Printers can be left running when they are well-built with working thermal runaway protection, inspected wiring and a smoke detector in the room, but the safest practice is to be present or nearby. Never leave a modified or untested printer unattended.

Are the fumes dangerous?

Printing emits ultrafine particles and, for ABS and ASA, styrene and other vapors, which irritate and are harmful at high or long exposure. Ventilate or filter, keep the printer out of bedrooms and do not breathe fumes directly.

Why does my print fail halfway through?

Look at the height of the failure: a model feature, a cooling or fan change, a filament tangle or run-out, a power glitch or an overheated driver. A first-layer problem would have failed earlier.

Should I use a fire extinguisher on a printer fire?

First cut the power if it is safe to do so, then use a CO₂ or dry-powder extinguisher suitable for electrical fires; do not use water on an energized printer. If the fire is not instantly under control, leave and call the fire service.

References

• Stephens, B. et al., “Ultrafine particle emissions from desktop 3D printers”, Atmospheric Environment, 2013 (emissions).
• Printer manufacturers’ safety instructions; Marlin and Klipper documentation on thermal protection.
• NFPA and local fire-safety guidance for smoke detectors and extinguishers; safety data sheets for resins, solvents and adhesives.

Last reviewed: October 2026. This is general guidance, not a safety certification: follow the printer maker’s safety instructions and your local rules.