Gaps, Thin Lines and Weak Walls (Under-Extrusion)

Likely causes, most common first

1. Nozzle too close on the first layerCommon
   Prusa lists a first layer calibrated too close first: the filament is blocked from pushing through, so the first layer looks starved. See Z Offset.
2. Partial clog in the nozzleMost common
   Debris or particles from filled filaments narrow the nozzle. Bambu Lab notes a partial clog shows as filament curling when you extrude by hand and as inconsistent extrusion. Prusa says a cold pull often clears it.
3. Printing faster than the hotend can meltVery common
   Every hotend and filament has a maximum volumetric speed. The OrcaSlicer wiki says it depends on the material, machine, nozzle diameter and extruder, and that even brand and color of the same material change it. Above that limit quality falls apart and the nozzle can clog.
4. Nozzle temperature too lowCommon
   Bambu Lab says printing too cold can cause under-melting and clogs. Prusa notes that its specialty steel nozzles may need 5 to 10 °C more Starting point than brass.
5. Extruder gears dirty or idler tension wrongCommon
   Prusa says to check the drive gears for dirt or misalignment and that an idler that is too loose or too tight can both cause under-extrusion.
6. Wrong slicer profile for the filament or nozzleCommon
   Prusa advises starting from the stock profile for your machine, filament type and nozzle size. A profile meant for a different nozzle or material asks for the wrong amount of plastic.
7. Filament problems: diameter, moisture, particlesOccasional
   Prusa gives the industry diameter tolerance as plus or minus 0.05 mm Rule of thumb and says to keep filament dry, especially PETG, ABS and soluble filaments. It also notes wood or metal filled filaments usually need at least a 0.4 mm nozzle, and some need 0.6 or 0.8 mm Starting point.
8. Hotend cooling problem (heat creep)Occasional
   Prusa lists insufficient hotend cooling as a cause, because filament then softens too far above the nozzle. This usually shows as extrusion that fades or stops partway through a print.
9. Flow setting slightly lowLess common
   Prusa describes minor under-extrusion as small gaps between lines or perimeters, fixed by extrusion multiplier calibration. It is the last thing to change, after the hardware checks out.

Fix it, one change at a time

1. Check the profile. Make sure the slicer has the right printer, nozzle size and filament type selected, and start from the stock profile for that filament.
2. Check the temperature. Use a nozzle temperature inside the range printed on the filament label. If you are near the bottom of it, raise it a few degrees and retest, or run a temperature tower to find the best point in the range.
3. Slow down, or find your flow limit. Print the same part at a lower speed. If that fixes it, run the slicer's max volumetric speed test: OrcaSlicer's defaults run from 5 to 20 mm³/s in 0.5 steps Starting point, and it recommends setting the limit 10 to 20% below the test result Rule of thumb because the test is a best case.
4. Clear any partial clog. If the strand in the air curled or sputtered, follow your printer maker's cleaning procedure, usually a needle from below and a cold pull. See Nothing Coming Out of the Nozzle for the safe order. The nozzle is hot enough to burn badly; heat it only through the printer's menu.
5. Check the extruder. With the printer off and cold, clean plastic dust from the drive gear teeth and set the idler tension to the maker's default position. See Extruder Clicking or Skipping if the extruder clicks.
6. Check the filament. Measure the diameter with calipers at a few points along the spool, dry it if it may be damp (see Popping, Steam and Bubbles From the Nozzle), and use a nozzle large enough for filled filaments.
7. Only then calibrate flow. If gaps remain with good hardware, a good temperature and a safe speed, adjust the flow ratio or extrusion multiplier in small steps using a flow calibration test, and save it to that filament's profile.

Confirm it is fixed

Print a small box with a solid top, at your normal speed. The top surface should be closed with no gaps between lines, the walls should be solid when held up to a light, and a thin wall should not break easily by hand. Then print one of the parts that failed. If it is good at first but fades later in long prints, the problem is heat creep rather than flow.

Why it happens

The slicer works out how much filament each line needs and the printer simply turns the extruder motor that far. Prusa points out that the printer has no sensor for how much plastic actually leaves the nozzle, so anything that gets in the way (a narrowed nozzle, plastic too cold and thick to flow, gears slipping on the filament) quietly turns into thinner lines. Nothing warns you until the print looks wrong.

Speed is the cause people miss. Doubling print speed doubles the volume of plastic the hotend must melt every second. Each hotend can only heat so much filament per second, and the OrcaSlicer wiki calls that its maximum volumetric speed. Beyond it, the plastic in the middle of the filament is not fully melted, back pressure rises, and output drops. That is why a print can look fine on slow outer walls and starved on fast infill, and why raising temperature helps a little and slowing down helps a lot.

This is also why the flow setting comes last. Turning up flow to cover a partial clog or a speed that is too high only hides the cause, and the print over-extrudes the day the cause goes away. Once the hardware is right, see Blobby Walls and Rough Tops for the opposite problem and how the same calibration catches both.