3D print under-extrusion — gaps, missing lines, weak walls

If your prints come out with visible gaps between adjacent extrusion lines, holes in the top layers, or walls you can snap with your fingers, you're under-extruding. The printer thinks it's laying down 100% of the material it's been told to, but something in the path from spool to nozzle is reducing the actual flow.

1. Partial nozzle clog

The most common cause. A tiny piece of burnt plastic or grit narrows the nozzle bore, so less plastic comes out than expected. Often the printer still prints fine at slow speeds and only fails at faster ones.

• Heat the nozzle to 15°C above normal print temperature.
• Do a cold pull: push a length of cleaning filament or nylon through by hand, drop the nozzle to 90°C, then pull the filament straight back out. The tip shows whatever was stuck inside.
• Repeat until the pulled tip comes out clean and shaped like the nozzle bore.
• If cold pulls don't fix it, just swap the nozzle. They're £2-5 and designed to be consumable.

2. Worn drive gear or slipping

If the drive gear (the toothed wheel that grips the filament) has been chewing filament, the teeth clog with dust and it can't push reliably. Symptoms: extruder clicks, filament has a flattened bite mark but no clean teeth pattern.

• Unload filament, release the idler, brush debris out of the gear with a stiff brush or pin.
• Check idler tension — spongy or uneven pressure means the idler spring is tired.

3. Too-cold nozzle for the speed

Material flows faster through hot plastic than cold. If you've turned print speed up without bumping temperature, the hotend can't melt plastic fast enough.

• Rule of thumb: every 20 mm/s increase in speed wants roughly 5°C more heat.
• Temperature tower printed at your actual speed will tell you the sweet spot for each filament.

4. E-steps miscalibrated

E-steps is the firmware setting that converts "rotate the extruder motor N steps" into "extrude X mm of plastic." If this is wrong, everything is slightly off.

• Mark 120 mm of filament from the top of the extruder.
• Command the printer to extrude 100 mm.
• Measure what's left above the mark. If it's 20 mm, e-steps are right. If it's more than 25 mm, e-steps are low.
• Adjust with M92 E<new_value> and save with M500.

5. Flow / extrusion multiplier too low in slicer

Different filament brands extrude slightly differently. A profile that worked with Hatchbox PLA may under-extrude slightly with eSun or Sunlu.

• Print a single-perimeter cube (no top, no infill) with the problem filament.
• Measure the wall thickness with calipers. It should match nozzle diameter (0.4 mm line = 0.4 mm wall).
• If it's 0.36 mm, bump flow to 100% × (0.4 / 0.36) = 111%.

6. PTFE tube friction or kink

On printers with a PTFE tube between extruder and hotend (Bowden) or buffer and extruder (CFS-style), a kink in the tube adds drag. The motor can't fight it, so less plastic pushes through.

• Pull the tube out of the couplers at both ends.
• Check for bends, grit on the inside, or a ragged cut at the end that's catching the filament.
• A tube with a sharp bend in it is done — replace it. They're pennies to buy.

Things that look like under-extrusion but aren't

• Gaps only in infill, solid walls and top layer fine. That's an infill-pattern issue, not extrusion. Try gyroid or cubic in the slicer.
• Top layer pin-holes but walls strong. Not enough top layers — set to 5 minimum on a 0.2 mm layer height.
• Walls separating between perimeters. That's a wall-overlap setting, not extrusion.

When to send it in

If cold pulls, nozzle swaps, e-step calibration and flow tests all look right but the printer still can't hit full flow, you may have a failing hotend heater (can't keep up with demand) or a thermistor reading high (firmware thinks it's at temp when it isn't). Both are bench-level diagnoses. Post the printer in, we'll scope the thermistor and verify the heater wattage, and return it printing properly.