How to Fix Over-Extrusion on Bambu Lab Printers

Over-extrusion is too much plastic per line: blobs and zits on the walls, bulging corners, rough shiny ridges on top surfaces, and dimensions that measure slightly large. It is the mirror image of under-extrusion, and the main lever is the flow ratio.

Why it happens

The slicer calculates exactly how much filament each line needs; the flow ratio scales that amount. If the ratio is too high for your actual filament — or the filament is thicker than the profile assumes — every line carries excess material that has to go somewhere. It squeezes out sideways and upward, and the nozzle ploughs through it on the next pass.

For reference, Bambu’s stock profiles use flow ratios between roughly 0.92 and 1.00 depending on material: PLA Basic at 0.98, PETG and ABS at 0.95, PC at 0.94, TPU at 1.0. Values far outside that band are a red flag.

Quick fixes

Advanced tuning: calibrate instead of guessing

Bambu Studio has a built-in flow rate calibration that prints test patches and lets you pick the smoothest one — use it instead of nudging numbers by hand, especially with non-Bambu filament:

• Run Flow rate from the Calibration tab, keep the result (it should land near 0.92–1.02 for rigid filaments).
• Follow with Flow Dynamics / pressure advance calibration — wrong pressure advance mimics over-extrusion at line starts and corners. Rigid filaments on Bambu printers typically calibrate to a K value around 0.02–0.06.
• X1-series printers can do automatic flow calibration with the built-in Micro Lidar; on P1 and A1 machines run the manual version.

How to confirm it really is over-extrusion

Slice a simple 20–30 mm calibration cube and look at the top surface and the vertical walls:

• Top skin with ridges the nozzle visibly dragged through, often shiny and rough at once — over-extrusion.
• Corners that bulge outward and outer dimensions consistently 0.1–0.3 mm large — over-extrusion (or pressure advance set too low).
• Walls with tiny regular gaps or a matte, under-filled look — the opposite problem; head to the under-extrusion guide.

Calipers settle arguments: measure the cube. Consistently oversized in X and Y with mushy detail means flow; oversized only at the bottom is elephant foot.

Also check when it started. Over-extrusion that appeared with a new spool points at the filament (different brand, different real diameter, or a generic profile whose 1.0-ish flow guess doesn’t fit). Over-extrusion that appeared after you edited a profile points at the edit — compare against Bambu’s stock values and revert anything you can’t explain. Broken imports are common enough that our profile health check flags flow ratios outside 0.7–1.3 as almost certainly corrupted.

When it isn’t really over-extrusion

• A single fat first layer squeezed outward at the bottom is elephant foot, not a flow problem.
• Blobs only where layers start and stop are seam artifacts — try repositioning the seam before touching flow.
• Stringy wisps plus blobs together often mean wet filament: moisture makes melt sputter and ooze. If the spool is old, dry it before recalibrating — calibrating with wet filament bakes the error in.