Why Are the Corners on My 3D Print Rounded or Bulging?

Rounded or outward-bulging vertical corners on a 3D print usually happen because the nozzle slows for the direction change while melt pressure remains too high. That extra plastic is deposited near the corner instead of being reduced in step with motion. Pressure advance, external-wall acceleration, flow, temperature, cooling, and the actual model path can all contribute. The fastest diagnosis is to check whether the swelling runs up the full vertical corner, appears only at the first few layers, sits only at the seam, or is followed by ripples along the wall.

Do not start by changing every speed setting. Print one plain square coupon with the seam forced away from the corner, mark its printer orientation, and compare the corner with the middle of each wall. A full-height bulge on most corners points toward pressure and corner motion. A wide bottom edge is elephant foot. Repeating echoes after a corner are ringing. A radius visible in the slicer preview is model or toolpath geometry, not a printer fault.

Short answer: what should you check first?

1. Confirm where the defect exists. Is the corner swollen for the full height, only near the build plate, only at one seam, or only after the printer has been running for a while?
2. Inspect the sliced outer-wall path. Zoom into the corner and verify that the model and generated toolpath are actually sharp enough for the result you expect.
3. Separate bulging from ringing. A bulge is concentrated at the direction change; ringing creates diminishing waves after it.
4. Check pressure advance or linear advance. Use the printer or slicer maker's supported calibration, and record the current value before changing it.
5. Check actual external-wall speed and acceleration. Requested speed alone does not show how hard the machine enters, decelerates for, and exits the corner.
6. Validate flow and temperature. General over-extrusion or a too-hot, slow outer wall can make the corner problem larger.
7. Print a controlled square coupon. Change one variable at a time and verify dimensions as well as appearance.

Read the corner pattern before changing settings

1. Residual nozzle pressure is the main cause of full-height corner bulges

Filament behaves like a compressed spring inside the melt path. The extruder builds pressure while a wall is moving at speed. As the toolhead decelerates for a 90-degree turn, extrusion demand falls, but pressure does not disappear instantly. Plastic continues leaving the nozzle and collects near the corner. The toolhead then accelerates onto the next wall, where pressure must build again.

Pressure advance, sometimes called linear advance, anticipates these speed changes. It reduces commanded extrusion before deceleration and adds it during acceleration. A value that is too low leaves rounded, swollen corners and may make the wall just after the corner slightly heavy. A value that is too high can create thin lines, corner gaps, weak starts, or an underfilled appearance. That is why blindly increasing the value is not a sound fix.

Calibrate at the conditions that expose the problem

Use the machine, firmware, or slicer maker's supported pressure-advance procedure. Keep the same nozzle, filament type, temperature, realistic flow range, and outer-wall behavior that produced the defect. A value derived from a slow PLA line test may not transfer cleanly to a hotter PETG profile, a flexible filament, a larger nozzle, or a high-flow job.

Record whether the value belongs to the printer, toolhead, material profile, or filament preset. Some systems store it in firmware; others inject it from the sliced file. Two active sources can overwrite each other. Confirm the command that reaches the printer instead of assuming the number visible in one screen is authoritative.

2. Corner speed and acceleration control how visible the pressure error becomes

A printer cannot instantaneously change direction at a square corner. It approaches the vertex, slows, redirects motion, and accelerates away. Higher external-wall acceleration shortens that event but increases motion demand. Lower acceleration gives the pressure system more time near the corner. Either can expose a poorly matched pressure-advance value, so the answer is not simply to make acceleration as low or as high as possible.

Inspect the slicer's speed and acceleration previews if available. Also check small-perimeter rules, overhang rules, and firmware motion limits. A short wall may never reach the requested speed, while a long wall may arrive at the same corner with much more stored melt pressure. This explains why a small calibration cube can look acceptable while a large enclosure corner bulges.

Use outer-wall settings as a controlled diagnostic

For one test, lower only the external-wall speed and use a known, moderate acceleration. If the bulge changes substantially, the defect is tied to the motion-and-pressure window. Do not treat the slower print as the final answer until pressure advance is checked. A production profile should be stable across the wall lengths and corner conditions the part actually contains.

3. General over-extrusion makes corners heavier

Pressure advance can be correct while the part still carries too much material overall. If wall centers measure thick, top surfaces look crowded, seams are heavy, holes shrink, and every corner is oversized, validate flow before chasing a corner-only parameter. Filament diameter assumptions, extruder calibration, line width, and material-specific flow can all matter.

Use the over-extrusion troubleshooting guide when the excess is not limited to direction changes. Correct the broad flow problem first, then repeat the corner coupon. Lowering global flow solely to sharpen a corner can leave the straight walls thin and weak.

4. Excess heat can soften or smear a nominally correct corner

A sharp corner remains close to the hot nozzle as the path changes direction. If temperature is unnecessarily high, the outer wall is moving slowly, or the layer has little time to cool, the corner may stay soft enough to round or swell. The symptom is more likely on small towers, thin shells, upper tips, and corners repeatedly reheated by adjacent lines.

Compare a one-object print with a two-object print that increases cooling time without changing the part. Inspect fan ramps and minimum-layer-time rules. Reduce temperature only within a range that preserves layer bonding and stable extrusion. PLA often tolerates more part cooling than ABS, ASA, nylon, or other materials that depend on controlled chamber heat, so use the material maker's range and real strength checks.

Do not confuse soft corners with wet filament

Moisture can cause popping, foamy extrusion, roughness, stringing, and unstable flow, but it rarely creates four clean, repeatable vertical corner bulges by itself. Drying is justified when those corroborating symptoms exist or the spool history supports it. A geometry-locked corner defect with smooth wall centers is stronger evidence for pressure and motion.

5. The model or sliced path may already contain the radius

FDM cannot reproduce an infinitely sharp mathematical edge, and the nozzle has a finite line width. Even so, a well-sliced external square should not show a large unexplained swelling. Zoom into the CAD model, exported mesh, and outer-wall preview separately. A fillet may have been added intentionally. A low-resolution mesh can facet a curve or alter a small corner. Path simplification, arc handling, variable line width, and minimum-feature rules can also change the generated route.

Print the same plain square from a trusted primitive. If the primitive is sharp but the original model is rounded, the machine is not the first suspect. Re-export the model with appropriate mesh resolution or use a supported geometry format that preserves curves. Do not add negative compensation until the source geometry and path are understood.

6. A seam bump at one corner is a different problem

If one vertical corner carries a stack of small blobs while the other corners are acceptable, the seam is likely dominating the appearance. The nozzle ends and begins each perimeter near that point, so restart pressure, wiping, seam gap, flow dynamics, and seam placement matter. Random seams can spread the evidence across the model and make the defect look less structured.

Force the seam to the center of a straight wall for one test. If the corner cleans up and the bump moves with the seam, use the site's seam-bump troubleshooting guide. Do not compensate every corner for a restart defect that exists at only one location.

7. Ripples after the corner are ringing, not just corner bulging

Ringing appears as repeated vertical echoes that fade away from the direction change. It comes from vibration in the printer, part, or motion system. A pressure bulge is concentrated at or immediately around the corner. Both can appear together because the same fast direction change excites motion and changes melt pressure.

Use the ringing and ghosting guide when multiple waves follow the edge. Check belt condition, pulley security, frame rigidity, toolhead play, acceleration, and the machine's supported input-shaping procedure. Input shaping can reduce motion echoes; it does not replace correct pressure advance.

8. A wide lower corner is usually elephant foot

Elephant foot spreads the first few layers outward because the initial layer is over-squished, the bed is too hot for too long, or the lower plastic remains compressed under the growing part. The upper vertical corner may be accurate. Pressure-related bulging usually continues much farther up the wall.

Measure the corner at the bottom, middle, and top. If only the lower edge is wide, use the elephant-foot troubleshooting guide. Check first-layer height, Z offset, bed temperature, leveling, and slicer compensation. Raising pressure advance will not repair an over-squished base.

9. Bowed walls and rounded corners need separate measurements

A part can look corner-heavy because the center of each wall curves inward. Infill pull, cooling shrinkage, heat-softened shells, and changing internal toolpaths can bow a wall even when corner deposition is reasonable. Put a straightedge across the cooled wall center. If the middle is recessed or the entire side waves, measure that shape before tuning the vertex.

The wavy or bowed walls guide separates heat, shell, infill, airflow, and machine-direction causes. Wall thickness also changes how internal pressure reaches the visible surface; the wall-thickness and perimeter guide is the stronger route when a thin functional shell cannot hold its intended shape.

A controlled test sequence for rounded or bulging corners

1. Keep the failed part and mark printer front, X, and Y on its underside.
2. Let it cool fully, then photograph each corner under the same raking light.
3. Measure width across wall centers and across the widest corner region at the bottom, middle, and top.
4. Inspect the source model and sliced outer-wall path at high zoom.
5. Note whether the seam sits on the worst corner and whether ripples follow it.
6. Print a plain square coupon with the seam forced to one wall center.
7. Use the same nozzle, filament, temperature, line width, wall order, flow, speed, and cooling as the failed job.
8. Run the supported pressure-advance calibration and record the previous and candidate values.
9. Reprint the square while changing only pressure advance.
10. If needed, test one moderate external-wall speed or acceleration change without altering flow and temperature.
11. Rotate the coupon 45 or 90 degrees if the defect appears axis-specific.
12. Repeat the winning setting twice, then verify hole fit, wall thickness, layer bonding, and corner dimensions on the real part.

Fixes that match the evidence

• All vertical corners bulge while walls are otherwise accurate: calibrate pressure advance for the actual material and wall conditions.
• Bulging changes strongly with wall length or speed: keep external-wall motion inside a stable range and confirm firmware acceleration limits.
• Wall centers are also thick and top surfaces are crowded: correct broad over-extrusion before fine corner tuning.
• Small upper corners soften: improve layer cooling or layer time and use an appropriate nozzle temperature for the material.
• Only one seam corner is heavy: diagnose restart pressure and seam behavior; test seam placement away from the corner.
• Ripples follow the corner: diagnose ringing and motion rather than relying on pressure advance alone.
• Only the first layers spread: correct elephant foot, Z offset, and lower-layer heat.
• The radius exists in preview: correct CAD, export, or slicing geometry before changing the printer.
• The wall center bows inward: test shell, infill, heat, and airflow causes separately from corner deposition.

What not to change first

• Do not keep increasing pressure advance until the corner looks sharp. Excess correction can create gaps, thin starts, and weak walls.
• Do not lower global flow from one corner symptom. Confirm excess material on straight walls and other features.
• Do not replace belts because the corner is swollen. Belts matter more when echoes, backlash, slip, or axis-specific evidence is present.
• Do not use input shaping as a pressure-advance substitute. The two controls address different parts of the direction-change event.
• Do not apply XY compensation before measuring the pattern. Global compensation can damage accurate walls, holes, and fits.
• Do not cool every material like PLA. A cosmetically sharper corner is not useful if layer bonding or warping gets worse.
• Do not judge a hot part on the plate. Let it cool before measuring dimensions and wall shape.

Frequently asked questions

Should I increase pressure advance to fix bulging corners?

Possibly, but only after a supported calibration shows the current value is low. Increase it in controlled steps and watch for corner gaps or thin line starts, which indicate too much correction.

Why are my calibration cube corners rounded but the dimensions are close?

Wall-center dimensions can be close while residual nozzle pressure swells only the vertices. Measure across the wall centers and inspect the corner profile separately. The slicer preview and a seam-relocated coupon help separate path geometry from extrusion dynamics.

Can printing slower make corners sharper?

It can reduce the visible defect in some profiles, but slower motion also changes how long the nozzle spends near the vertex. Use it as a diagnostic, then match pressure advance and acceleration to the wall speeds you intend to run.

Does input shaping fix rounded corners?

Input shaping reduces vibration and ringing. It does not directly remove excess melt pressure at a slowdown. A print can need both input-shaping validation and pressure-advance calibration.

Why is only one corner bulging?

The seam may be placed there, a fan may cool that machine side differently, or one axis may behave differently. Move the seam and rotate the coupon while keeping its bed location marked.

Are perfectly sharp FDM corners possible?

FDM corners always reflect nozzle diameter, line width, layer height, material, and motion limits. The goal is a controlled, repeatable corner within the part's dimensional and visual tolerance, not an infinitely sharp mathematical vertex.

Next steps

Save the failed part, the exact project, and the winning coupon. Record printer, firmware, nozzle, material, temperature, flow, line width, external-wall speed, acceleration, pressure-advance value, cooling, seam position, bed orientation, and measurements at the bottom, middle, and top. The dimensional-accuracy guide is the next route when the corner issue is part of a broader fit problem. The print-quality problems guide is better when several defects appear together.

For repeated functional parts, define where the corner is measured, the acceptable radius or maximum bulge, part temperature at inspection, and which faces are cosmetic before releasing a batch. If building and validating that process in-house costs more than the job supports, the printer-versus-service guide provides the decision checkpoint. JC Print Farm is the relevant expert handoff when the file revision, material, quantity, critical fits, visible faces, and acceptance method are already defined.