Skills books › 3D Printing and Making at Home

Why 3D prints fail, and how to fix them

Most failed prints have one of a few causes: a dirty or badly levelled plate, wet filament, the wrong temperature, a partial clog or a loose belt. The quickest way to a fix is to find the cause first, and then change one thing at a time.

A method that works

1. Look carefully at the failed print. Where did it go wrong: the first layer, a certain height, one side, or every layer?
2. Ask what changed since the last good print: a new spool, a new file, a cold or humid day, a new setting?
3. Check the simple things first. Is the plate clean and level? Is the filament dry and feeding freely? Is the nozzle clean?
4. Change one thing, and print a small test piece, not the whole part.
5. Write down what you changed and what happened.

Changing five settings at once may fix a print by luck, but it teaches you nothing and the fault comes back.

The print will not stick, or the corners lift

A print that does not stick at all almost always has a first-layer problem. Clean the plate with isopropyl alcohol, level it while hot, and lower the nozzle height slightly.

Warping happens as plastic cools and shrinks, pulling the corners up. It is worst with ABS and ASA, with large flat parts and in cold, draughty rooms.

• Block draughts: close nearby windows, or use an enclosure.
• Check that the bed temperature is right for the filament.
• Add a brim in the slicer.
• Round the corners of the design slightly.

Stringing

Fine hairs between parts usually mean wet filament, so dry it first. Then lower the nozzle temperature 5 °C at a time, and increase retraction a little.

Thin lines, gaps and weak walls

This is under-extrusion. Likely causes are:

• a partial clog;
• a tangled or stuck spool;
• wet filament;
• a nozzle that is too cold for the print speed;
• a worn extruder gear that slips on the filament;
• the wrong filament diameter set in the slicer.

Blobs and rough walls are the opposite problem: the flow is set too high, or the nozzle is too low on the first layer.

Clearing a clog with a cold pull

A clicking extruder and plastic that barely comes out point to a clog.

1. Heat the nozzle to printing temperature and push some filament through by hand.
2. Let it cool to about 90–120 °C for PLA, so that the filament is soft but not runny.
3. Pull the filament out firmly and steadily. The tip often brings the dirt out with it.
4. Repeat until the tip comes out clean.

Never poke a hot nozzle with your fingers.

Layers that shift sideways

A layer shift usually comes from a loose belt or pulley screw, the nozzle hitting a curled-up part, or printing too fast. Tighten the belts and screws, check that nothing rubs, and slow down.

Heating errors

If the screen shows a thermal runaway or temperature error, switch the printer off at the wall and let it cool completely. Check that the temperature sensors and heater wires are firmly in place and undamaged, and replace damaged parts before you print again. Never disable the safety feature to get rid of the error.

Monthly care prevents most faults

With the printer off and cold, wipe away dust and plastic bits, check belt tension and screws, lubricate only as the maker's guide says, and look over the wiring for cracks or scorch marks.

This guide is based on Chapter 9 of 3D Printing and Making at Home, which includes a full table of print faults and their causes.