3D printer not printing properly: how to work out what is wrong

When a print fails or looks wrong, the temptation is to change ten settings at once. Don't. You will never know which one fixed it, and you will probably make something else worse.

We work through faults in the same order every time: bed, nozzle, filament, slicer. That order is not random. The bed and nozzle are physical and you can see them with your eyes. Filament is the next most common cause. Slicer settings come last because they are easy to change but easy to blame unfairly.

Start by describing the fault out loud

Before you touch anything, say what is actually wrong. "It is not printing properly" is not a fault. "The first layer is not sticking on the left side of the bed" is a fault. "The walls have gaps every few layers" is a fault. "The part looks fuzzy and there are little hairs between the towers" is a fault.

Each of those points to a different cause. If you cannot describe it, take a clear photo in good light from two angles. Half the messages we get start with a blurry phone picture and end with the customer realising the answer when they take a better one.

First layer problems

If the first layer is the issue, almost nothing else matters yet. A bad first layer guarantees a bad print.

Check the bed is clean. Skin oils, release agent from new build plates, and old glue stick all stop filament gripping. Wipe the plate with isopropyl alcohol, 90 percent or stronger, on a lint-free cloth. Let it dry. Do not touch the printing area with your fingers afterwards.

Now check the bed temperature against the material. PLA wants 55-65 deg C on most surfaces. PETG likes 70-80 deg C. ABS and ASA need an enclosure and 100-110 deg C. If you are running PETG at PLA temperatures the corners will lift every single time.

Then check the nozzle gap. Auto-levelling probes drift, and a machine that was perfect last month may need a fresh Z-offset today. Run a single-layer test square. The lines should touch each other with no visible gaps and no ridges where they squish into the next pass. Too high and you see strings of spaghetti. Too low and you see a translucent smear that the nozzle is dragging through.

Layer and wall problems

If the first layer is fine but the rest of the print is wrong, the fault is usually extrusion or motion.

Gaps between walls, missing top layers, or thin patchy areas usually mean under-extrusion. Causes, in order of how often we see them:

1. The nozzle is partially blocked. Heat it to printing temperature, push fresh filament through by hand, and watch the strand come out. It should drop straight down. If it curls, the nozzle is worn or coked up inside. 2. The extruder is slipping. Open the extruder, look at the filament where the gear bites it. You should see clean teeth marks. If you see chewed-up grooves or a flat polished patch, the gear is not gripping properly. Check tension and clean the gear with a brass brush. 3. The PTFE tube has pulled out of its coupling, leaving a gap where filament leaks pressure. Push it firmly back in and clip the coupling. 4. The filament is wet. PETG and nylon absorb water from the air. A spool that has sat open for a month will hiss and pop as it prints, and the surface will look rough. Dry it at 50-60 deg C for four to six hours in a filament dryer or low oven.

If the print is shifting layers sideways partway up, that is mechanical. A loose belt, a slipping pulley grub screw, or a stepper driver overheating and skipping. Tighten the belts so they twang like a low bass note, not a slack rubber band. Check that every pulley grub screw is on the flat of its shaft.

Stringing, blobs, and surface mess

Thin hairs between parts are stringing. Your retraction is too low, your print temperature is too high, or your filament is wet. Drop the nozzle temperature by 5 deg C and test again. If it improves, keep going down in 5 deg steps until layer adhesion gets weak, then come back up one step.

Blobs and zits on the surface usually come from pressure advance being wrong, or from the slicer doing a bad job of seam placement. Move the seam to a corner or hide it in a back face if the model allows it.

Ghosting, which is faint echoes of features repeated next to them on the wall, is vibration. Slow the outer wall speed down. On most machines, 30-40 mm/s for the outermost perimeter gives you a clean surface even if the rest of the print runs faster.

When the printer just stops

If the machine halts mid-print with no error, the most common causes are a thermal runaway protection trip, a microSD card that has corrupted, or power dropping out. Check the screen for any error code and write it down before clearing it. Re-slice and try again from a freshly formatted card. If it halts at the same height twice running, something in the gcode is triggering it.

If the hotend reads a wildly wrong temperature, suspect the thermistor. They fail by going open-circuit and the printer will refuse to heat. A replacement is a cheap part but you have to be careful about getting the right type for your board.

What not to do

Do not buy a new printer because the old one is misbehaving. A five year old machine that is fighting you will fight a brand new one too, because the problem is almost always something you can fix.

Do not flash unknown firmware to try to fix a mechanical fault. Firmware does not fix loose belts.

Do not stack five changes at once. Change one thing, print a calibration cube, look at it, then decide what to change next.

When to mail it in

If you have worked through the bed, nozzle, filament and slicer and the printer is still not behaving, post it to us and we will take a look. We see a lot of Crealitys, Bambus and older Prusas in the workshop and most faults come down to a worn hotend, a tired extruder, or a board issue that is not obvious from outside. Send a short note describing what you have already tried so we do not repeat your work and charge you for it. Get in touch first through our contact page and we will tell you what to send and how to pack it. Turnaround is usually within a few working days once it arrives.