Tracing under-extrusion from the spool to the nozzle tip

Global, local or only at corners

Bambu Lab splits under-extrusion into two patterns: global, where lines are sparse across the whole surface, and localized, where gaps appear in one area ^[3]. A third pattern points straight at one setting: if plastic is missing only at corners, the pressure advance value is likely wrong and a flow dynamics calibration is needed ^[3]. Prusa adds that the printer has no feedback on how much filament actually leaves the nozzle, so the firmware cannot detect the shortfall on its own ^[1].

Symptom, likely cause, what to check

Diagnostic order, from spool to nozzle

1. Spool and path. Confirm the spool turns freely and is not tangled, and inspect the PTFE tube for debris, kinks or wear ^[3].
2. Filament diameter. Prusa cites ±0.05 mm as the industry-standard diameter variation and notes that filled filaments such as wood can vary more ^[1]. Bambu Lab adds that filament can thicken where it was bent or crushed and thin where worn gears chewed it; trim those sections ^[4]. The slicer’s Diameter value feeds every extrusion calculation, so it has to match the filament ^[8].
3. Extruder gears and tension. On MK-series printers Prusa checks both Bondtech gears for dirt or misalignment and notes that an idler screw set too loose or too tight causes under-extrusion ^[1]. On Nextruder printers, a gearbox that was opened may need its alignment procedure ^[2], and debris in the drive gear grooves can be cleared with compressed air, a brass brush or a toothpick ^[5].
4. Free-extrusion test. Bambu Lab heats the nozzle to 220 °C and extrudes PLA with the extrude button: a long, smooth strand dropping straight down means a clear channel and working gears; a short, rough strand means a narrowed nozzle or a faulty gear ^[4].
5. Nozzle and hotend. If the nozzle is seated correctly, a cold pull often clears a partial clog ^[1]. Check that the hotend fan is oriented correctly and not blocked; poor cooling lets filament melt too high ^[1]. Clearing methods are in the clogged nozzle guide.
6. Temperature and speed. For filaments without a PrusaSlicer profile, Prusa suggests trying ±5–15 °C, and 5–10 °C more for hardened or stainless steel nozzles ^[1]. Bambu Lab recommends about 10 °C more for its fastest speed mode, or slowing down ^[3].
7. Calibration values last. Flow ratio, then e-steps or rotation distance, as described below.

Volumetric speed: the hidden ceiling

Every hotend melts a limited volume of plastic per second. OrcaSlicer’s Max volumetric speed (filament_max_volumetric_speed) caps print speed for each filament to prevent clogs and under-extrusion ^[7]. Its calibration prints a test from 5 to 20 mm³/s in 0.5 steps by default; the limit equals the start value plus the measured height of the good section times the step. Because the test is a best case, the guide suggests setting 10–20% lower ^[6].

Convert that limit into a print speed for your line width and layer height with the volumetric speed calculator, and see max volumetric speed calibration for the full method.

Filament-side causes

Three filament issues show up as under-extrusion even when the printer is fine. Damp filament prints poorly, and Prusa singles out soluble filaments, PETG and ABS; it recommends returning spools to their bag with silica gel or keeping them in a dry box ^[1]. Symptoms and drying are covered in the wet filament guide. Leftover plastic from a hotter material also blocks flow, so Prusa loads the new filament at the previous material’s temperature, for example PLA at the PETG or ABS temperature ^[1]. Filled filaments need the right nozzle: Prusa specifies at least a 0.6 mm nozzle and 0.2 mm layers for Woodfill, and a hardened nozzle for carbon fiber or kevlar blends ^[1].

Flow ratio comes after the hardware

Prusa positions extrusion multiplier calibration as a fix for minor under-extrusion, the small gaps between lines and perimeters. It also notes that total flow equals the firmware flow percentage (M221) multiplied by the PrusaSlicer Extrusion multiplier ^[9]. Raising flow to hide a partial clog or dirty gears gives a value that drifts as the fault changes: Bambu Lab’s calibration notes say to clean the hotend and extruder first, run cold pulls if quality has declined, and dry the filament before calibrating ^[10]. The procedure itself is in flow rate calibration.

E-steps and rotation distance

If every filament under-extrudes by a similar amount, the extruder’s steps per millimeter may be off. Klipper’s measure-and-trim method marks the filament about 70 mm above the extruder, extrudes 50 mm with G1 E50 F60, and scales rotation_distance by the actual over the requested distance. The slow rate matters, because fast extrusion builds pressure that skews the result. If the two distances differ by more than about 2 mm, repeat the test ^[11]. On Marlin, the equivalent value is set with M92 E<steps> and stored with M500 when EEPROM settings are enabled ^[12]. The rotation distance calculator does the arithmetic, and the procedure is in e-steps and rotation distance.