Key takeaways
- The order of diagnosis matters more than the list of causes. Mechanics first, slicer last.
- Wet filament causes at least five different symptoms, and all five are routinely misdiagnosed.
- Change one setting at a time and reprint exactly one test piece. Change three at once and you learn nothing.
- If a print that used to work no longer does, look for what changed — not at the settings table.
Most of the time spent fixing a 3D print is not spent on the fix itself, but on fixing the wrong thing. People start in the slicer because that is where the most buttons are, while the cause is a spool that has sat open for two weeks.
This article puts the order back. Work from the top down, and in most cases you will not need to reach the bottom.
The order of diagnosis
- 01
Mechanics first
Is the bed level, with the right Z offset? Are the belts tight? Is there a gap anywhere in the filament path? Has the nozzle bore worn wide? No setting can compensate for a machine that is wrong.
- 02
Material second
How long has the spool been open, and at what humidity? Measure the diameter at five points 10 cm apart. In a humid climate, this step fixes more faults than any other.
- 03
Temperature and flow third
Print a temperature tower and a flow calibration. These are the two settings with the largest effect, and also the easiest to check.
- 04
Speed and acceleration fourth
Drop speed by 30 % and reprint. If the fault disappears, the machine cannot keep up with the flow or the frame is ringing — it is not the material.
- 05
Fine slicer settings last
Retraction, coasting, wipe, overlap. These refine a print that already works. They do not rescue one that is wrong at the root.
Quick-reference table
| Symptom | Check first | Then | Last |
|---|---|---|---|
| First layer won't stick | Z offset and bed level | Clean the bed with IPA | Raise the bed 5–10 °C |
| Part lifts mid-print | Chamber heat, draughts | Add an 8 mm brim | Switch to a lower-shrink material |
| Layers split apart by hand | Nozzle too cool | Wet filament | Turn the part fan down |
| Stringing, fine hairs | Moisture — dry it first | Drop the nozzle 5–10 °C | Increase retraction |
| Rough surface, popping sounds | Wet filament, almost certainly | Uneven filament diameter | Clean the heater block |
| Under-extrusion, thin walls | Nozzle worn wide or partly blocked | Extruder idler tension | Calibrate flow and e-steps |
| Nozzle clogs mid-print | Heat creep | Oversized or dusty filament | Hot end heatsink and fan |
| Layer shift on one axis | Loose pulley on the motor shaft | Acceleration too high | Stepper driver current |
| Curled corners, soft edges | Not enough cooling | Temperature too high | Slow the outer walls |
| Holes in the top surface | Too few top layers | Under-extrusion | Raise infill overlap |
| Small features off-size | Material shrinkage compensation | Flow calibration | Drop the temperature 5 °C |
| Overhangs droop | Part cooling fan | Overhang speed | Add support or reorient the part |
The most misdiagnosed fault: wet filament
Moisture appears in five rows of the table above. That is not a coincidence. Water absorbed in the filament flashes to steam inside the nozzle, and the steam does three things at once: it tears the extrusion apart, it breaks pressure control, and it weakens the bond between layers.
If you are about to raise retraction to cure stringing, dry the spool first. You will save an afternoon.
A quick test: extrude 100 mm at printing temperature with the fan off and watch the strand from the side. Dry filament comes out straight, smooth and glossy. Wet filament shows bubbles, or curls and snaps. Temperatures and times for each material are in how to dry filament.
First layer won't stick
This is the most common fault and the one least related to the slicer. In order:
- Z offset. The first layer should be squashed slightly flat, not laid gently on the bed. A correct line has a slightly flattened top and fuses into the line beside it.
- Skin oil. A PEI bed loses grip from fingerprints, not from age. Wipe it with IPA before each print and don't touch the surface.
- Bed temperature. If the part holds in the middle but lifts at the corners, the problem is shrinkage, not adhesion: raise the bed temperature and add a brim.
- First-layer speed. Print the first layer at 20–25 mm/s. The plastic needs time to bond.
- Surface type. PETG grips smooth PEI so hard it can tear pieces out of the sheet; PLA grips poorly on a dirty textured plate. The plate is a variable, not a constant.
Layer separation: the part peels along the layer lines
This is the most expensive fault, because the part looks fine until it carries load. Three causes, in order of frequency:
- Nozzle too cool. Bonding between layers is a diffusion process: polymer chains have to cross the boundary while it is still hot. Print at the upper end of the published window.
- Too much part cooling. With PETG, ABS, ASA, PC and nylon, a strong fan cools the interface before it can weld. Turn the fan down to the minimum your overhangs will tolerate.
- No chamber heat for materials that need it. With ABS, ASA, PC and nylon, an enclosure is not optional — the print will crack along a layer line as the lower section cools and shrinks.
Nozzle clogs and heat creep
A clog right at the start is usually dust or carbon in the nozzle bore. A clog partway through a long print is almost always heat creep: heat travels up out of the melt zone, softens the filament where it should still be rigid, and the filament swells and jams in the tube.
Fixing heat creep, in order
- Hot end fan
- Check the heatsink fan is still running and not clogged with dust
- Temperature
- Drop 5–10 °C if the surface allows it
- Retraction speed
- Lower it — fast retraction pulls softened filament up into the cold zone
- Dwell time
- Avoid leaving a hot nozzle stationary, for example during a colour change
- Filament path
- Check the PTFE tube is seated hard against the heat break, with no gap
When a print that used to work no longer does
This case needs a completely different approach. Don't open the settings. Look for what changed:
- A new spool, or an old one left open longer.
- The bed was cleaned with something different, or not cleaned at all.
- The weather. A rainy week in a humid climate is enough to change how an open spool prints.
- A firmware or slicer update reset a profile.
- The nozzle has run a few hundred hours of fibre-filled material.
In our own workshop, that five-line list explains most cases of "the printer suddenly prints badly". Settings rarely change by themselves; everything around them does.
Frequently asked
Why won't my first layer stick to the bed?
Check, in order: a Z offset too high so the plastic isn't squashed, skin oil on the bed, a bed temperature too low, and a first layer printed too fast. Wipe the bed with IPA before every print and run the first layer at 20–25 mm/s.
How do I fix layer separation in a 3D print?
Raise the nozzle temperature to the upper end of the published window, turn the part cooling down to the minimum your overhangs tolerate, and dry the filament. With ABS, ASA, PC and nylon, use an enclosure — without chamber heat the part will crack along the layers as it cools.
How do I stop stringing?
Dry the filament first, because moisture is the most common cause. Then lower the nozzle temperature by 5–10 °C. Only increase retraction after those two steps, because too much retraction causes under-extrusion, especially with flexible filament.
Why does my nozzle clog during long prints?
Usually heat creep: heat climbs out of the melt zone, softens the filament where it should be rigid, and it swells and jams. Check the hot end's heatsink fan first, then lower the temperature and slow retraction.
What causes layer shift?
Nearly always mechanics: a pulley loose on the motor shaft, a slack belt, or acceleration set high enough that the motor skips steps. Check the pulley grub screws by hand before changing anything in the slicer.
In what order should I troubleshoot a print?
Mechanics first (bed level, belts, filament path, nozzle), then material (moisture, diameter), then temperature and flow, then speed and acceleration, and only last the fine slicer settings. One variable at a time, with one small test print each.