How to Fix Over-Extrusion

Over-extrusion is when the printer pushes out more plastic than the slicer planned. Prusa describes the result as scarring of the bottom layers and unnecessary buildup of material on the top layers, with an uneven surface that stops parts from joining seamlessly. OrcaSlicer's wiki lists excess material, rough surfaces and dimensional inaccuracies. This 3D printing guide covers the fixes in order. It is based on maker documentation, not our own print tests, so treat values as starting points.

1. Check the first layer height

A nozzle set too close to the bed squashes plastic out sideways and can look exactly like over-extrusion. Bambu Lab's first layer guide says a line squished until it creates raised edges, or a dragging nozzle, means the nozzle is too low. Follow how to fix a bad first layer and re-level or recalibrate if needed (bed leveling guide).

2. Check for loose hardware

Bambu Lab describes a mechanical cause on the A1: thick, inconsistent lines and scattered blobs in the first layer test can come from four loose screws on the hotend heating assembly, which also interfere with bed probing and give incorrect Z heights. Bambu's wiki gives the fix for that printer (re-tighten the screws with an H1.5 hex key). On other printers, check that the nozzle and heater block are fitted as the maker describes. Also confirm the nozzle diameter set in the slicer matches the fitted nozzle (nozzle sizes explained).

3. Calibrate flow

Flow, called the extrusion multiplier in PrusaSlicer or flow ratio in OrcaSlicer and Bambu Studio, scales how much filament is pushed for each move. Prusa notes over-extrusion is a common problem with many filament brands and that the default for PLA is 1, with adjusted values usually between 0.9 and 1.1.

Prusa's precise method uses calipers and a 40 x 40 x 40 mm cube printed in vase mode at a normal layer height of 0.1 to 0.2 mm. Measure the wall thickness at three points in the middle of each wall and average them. Then extrusion multiplier = 0.45 divided by the average measured thickness. Prusa states this assumes a 0.4 mm nozzle, 0.45 mm extrusion width and a multiplier of 1 for basic materials such as PLA, ABS and PETG. Enter the new value in the filament settings and reprint to check.

Prusa's visual method needs no tools: print the test cube at 0.20 mm layer height, inspect the top layer by sight and touch, and change the multiplier by 1 to 2% at a time. Decrease it if there is too much material near the perimeters.

OrcaSlicer's flow ratio calibration prints a set of blocks with different modifiers and you choose the smoothest top surface. The Archimedean chords method makes eleven blocks from -0.05 to +0.05 in 0.01 steps. OrcaSlicer's wiki also warns Bambu Lab printer users not to select the Flow calibration option, to avoid conflicts with the printer's own system. Flow comes after temperature, volumetric speed and pressure advance in OrcaSlicer's recommended calibration order, so a rough surface can also come from those. See the OrcaSlicer beginner guide and how to calibrate a 3D printer.

4. Check temperature and filament

Overheated plastic runs more freely, and wet filament extrudes unevenly. Bambu Lab says moisture can cause uneven extrusion because absorbed water turns to steam in the hotend. Dry the filament. Keep the nozzle temperature within the range printed on the spool, and change it in small steps. Flow values calibrated for one brand and color often do not carry over to another, so repeat calibration for each filament.