How to Tune Pressure Advance or Linear Advance for Sharper Corners

Corner bulging, blobs at every seam, and walls that swell just before each turn all trace back to one thing: pressure lag inside your hotend. The fix is pressure advance (the Klipper term) or linear advance (the Marlin term), a setting that predicts how molten plastic behaves when the print head speeds up and slows down. Tune it with a single calibration print and your corners go from rounded mush to crisp 90 degree edges.

Quick Answer

Slice a tall hollow square test tower, sweep pressure advance from 0.0 to 0.1 in 0.005 steps, then measure the height where the corners are sharpest and the seam blobs disappear. For PLA that value usually lands around 0.02 to 0.06, and for PETG around 0.04 to 0.08. Set it once per filament type and the artefacts are gone.

Why corners bulge in the first place

Your extruder builds up pressure inside the melt zone while printing a straight wall. When the head reaches a corner and decelerates, that trapped pressure keeps pushing plastic out for a fraction of a second, so the corner gets a fat lip. When the head accelerates away again, the melt zone is briefly starved, so the wall just after the corner runs thin. Pressure advance models this lag and adjusts the extruder ahead of the speed change, easing off before a slowdown and pushing earlier into a speed up.

The same maths drives linear advance in Marlin, set with the K factor. The naming differs but the goal is identical: keep extruded line width constant no matter how the print head changes speed. If you are choosing or upgrading a machine, the current crop of FDM units in the 3D printer range at Evetech ship with either firmware, so the method below applies to almost anything you buy.

Running the calibration print

1. Generate the test pattern. In Klipper, use the built in tower method: slice the square_tower model with zero infill, a coarse layer height, and a high print speed near 100 mm/s so pressure effects are exaggerated and easy to read. In Marlin, print a multi K factor calibration pattern (the kind that lays down a fan of lines at stepped K values) so you read several settings from one print.
2. Disable interfering features. Turn off any dynamic acceleration control, input shaping smoothing on extrusion, and scarf or coasting seam tricks for the test. Those settle the same artefacts pressure advance fixes and will muddy your reading.
3. Print at one temperature and one filament. Pressure advance changes with melt viscosity, so a value tuned for PLA at 200C will be wrong for PETG at 240C. Calibrate per material.
4. Read the result. On the tower, scan up the corners. Below the ideal value you will see blobbing and bulging at every turn. Above it, corners go rounded and the wall leading into each corner looks starved or gappy. The correct height is where line width stays even all the way around.
5. Apply and verify. Enter the value (SET_PRESSURE_ADVANCE in Klipper, or M900 K in Marlin) and reprint a quick test cube. Confirm the seam blob is gone and corners are square before you commit it to your slicer or printer config.

PLA vs PETG vs flexibles

Lower viscosity filaments need less compensation. PLA tends to settle low, often 0.02 to 0.05, because it flows readily and the melt zone pressure bleeds off quickly. PETG runs stickier and holds pressure longer, so it usually wants a higher value, frequently 0.04 to 0.08. Flexible filaments like TPU behave differently again and often need their own careful sweep because the filament itself compresses in the path. Treat every new spool family as its own calibration job, save the values, and load them per profile.

When pressure advance will not fix it

If corners still bulge after a clean calibration, the cause is usually elsewhere: an over extrusion problem from a wrong flow rate, a partially clogged nozzle adding inconsistent back pressure, or a loose extruder gear slipping under load. Confirm your extrusion multiplier and nozzle condition first, because pressure advance only corrects the timing of extrusion, not the amount. A worn nozzle or PTFE liner will sabotage even a perfect value, and replacement nozzles and maintenance bits sit in the 3D printing accessories best sellers if you need to rule the hardware out.

Frequently Asked Questions

Is pressure advance the same as linear advance?

Functionally yes. Pressure advance is the Klipper name and linear advance is the Marlin name for the same compensation that adjusts extruder pressure ahead of speed changes. The calibration approach differs slightly between firmwares but the goal, even line width through corners, is identical.

How often do I need to recalibrate?

Recalibrate whenever you change filament type or brand, switch nozzle diameter, or make a large temperature change. The value is tied to melt viscosity and flow path, so a fresh material family deserves a fresh sweep. The same spool across reprints does not need retuning.

Why are my corners rounded after tuning?

Rounded corners usually mean the value is set too high, so the extruder eases off too aggressively before each turn and under extrudes into the corner. Drop the value in small steps and reprint until the corner is square without a blob returning.

Does a higher print speed need a higher value?

Not exactly. The ideal value is mostly about melt viscosity, not speed, but faster printing makes pressure lag more visible, which is why calibration prints use high speeds. A correctly tuned value holds across your normal speed range.

Can I just copy a value from someone online?

Use it as a starting point only. Hotend design, extruder type, filament brand, and temperature all shift the ideal figure, so a copied number gets you close but rarely nails it. The calibration print takes a few minutes and removes the guesswork.