Seam Placement

Why there is always a seam

Each wall is a closed loop, and the nozzle has to start and stop it somewhere. Bambu Lab describes the seam as the gap between the start and end of each perimeter, which leaves a vertical line of marks on the surface and is unavoidable in FDM printing (spiral vase mode is the exception). The marks come from the extrusion stopping and starting: a slight bulge, a small gap or a blob. You cannot remove the seam, only decide where it goes and how small it is.

Bambu Lab notes that seams hide well on surfaces with concave and convex corners, and that circular surfaces such as cylinders cannot hide them.

Where the settings are called what

What it controls	PrusaSlicer	OrcaSlicer	Bambu Studio	Cura
Where the seam goes	Seam position: Nearest, Aligned, Rear, Random	Seam position: Aligned, Aligned Back, Nearest, Back, Random	Seam position: Nearest, Aligned, Back, Random	Z Seam Alignment: User Specified, Shortest, Random, Sharpest Corner
Corner preference	Not confirmed	Not confirmed	Not confirmed	Seam Corner Preference: Hide Seam, Expose Seam, Hide or Expose Seam, Smart Hiding
Overlapping, ramped seam	Scarf joint placement	Scarf joint seam	Scarf seam type	Not confirmed
Gap left at the seam	Seam gap distance	Seam gap	Seam Gap	Not confirmed

Not confirmed means we did not find the name in that slicer's documentation; the feature may still exist. In PrusaSlicer the seam position is under Print settings, Layers and perimeters; some seam options appear only in Expert mode.

The placement options

• Nearest: Prusa and Bambu Lab both describe it as choosing a corner where possible (concave first), otherwise the point closest to where the nozzle already is. It minimizes travel. On a part with sharp corners the seam can all but disappear; on a round part it wanders.
• Aligned: the same corner-seeking choice, but each layer picks the point nearest the previous layer's seam, so the seam stays in one vertical line. Good for parts with a corner that faces away from view.
• Rear or Back: puts the seam at the back of the part (toward the back of the bed). Prusa says its Rear option still avoids overhangs and favors corners; Bambu Lab suggests it for a part with a show face, such as a mask. OrcaSlicer's Aligned Back is like Aligned but avoids the front-facing side.
• Random: a different point on every layer. Prusa says this makes the seam less noticeable and adds overall strength, but gives a rougher surface with small scattered zits, and suits smooth cylinders rather than parts with sharp edges.
• Cura names its options User Specified, Shortest, Random and Sharpest Corner, with a separate Seam Corner Preference for how corners are used.

Because Rear and Back place the seam toward the back of the bed, rotating the model on the plate moves the seam. Turning a part so its least visible corner faces the back is often the quickest fix.

Seam painting

When no automatic option puts the seam where you want, paint it. PrusaSlicer's seam painting tool (since version 2.3) gives manual control over where seams go, and Bambu Studio offers manual seam painting as well. Paint along an inside corner, a sharp edge or the side that will face a wall.

Making the seam smaller

• Seam gap. The slicer stops the loop slightly short so the excess at the join has somewhere to go. Bambu Lab expresses it as a percentage of nozzle diameter, with a default of 15 percent Starting point. OrcaSlicer says a larger gap reduces bulging and a smaller one makes the seam less visible, and that 0 to 15 percent is typically right on a well-tuned printer Starting point.
• Scarf seams. Instead of butting the start and end together, the slicer ramps the line up and overlaps it like a scarf joint in wood. Prusa, OrcaSlicer and Bambu Lab (since Bambu Studio 1.9) all offer it for outer contours, optionally holes. Prusa notes the trade-offs: more print time and poorer results at sharp corners and overhangs. That is why the slicers can apply scarf seams only on smooth perimeters, leaving corners to hide a normal seam.
• Wipe on loops. OrcaSlicer's Wipe on loop moves the nozzle slightly inward at the end of the outer loop to tuck the seam in; it needs more than one wall.
• Wall order. OrcaSlicer notes that printing the outer wall first gives less consistent Z seams than inner-first orders (see Walls and Perimeters).

If the seam still bulges with a sensible gap, see Retraction and pressure advance.

Seams and strength

Prusa and OrcaSlicer both describe the seam as a weak point. Staggered inner seams offset each inner wall's seam from the one outside it so the weak points do not line up, which Prusa says makes the part stronger and OrcaSlicer says can help water-tightness. For a functional part, a random seam spreads the weakness further, at the cost of looks.

What to change first

1. Part with corners: Aligned or Nearest, and rotate the part so the seam corner faces away from view.
2. Part with a show face: Rear or Back, or paint the seam.
3. Round part: paint the seam on the least visible side, or try a scarf seam. Use Random only if scattered zits look better than a line.
4. Seam still bulges: adjust seam gap, then check retraction and pressure advance.