VISIBLE SEAM SCARS

A perimeter loop must start somewhere and end somewhere. Where it starts, there's a brief moment where pressure is building up and extrusion is slightly thin. Where it ends, there's a brief moment where residual nozzle pressure leaves a slightly thicker deposit. These two transitions happen at the same XYZ point on every layer because the slicer aligns them - either at the sharpest corner of the geometry, at a fixed angle, or at the closest point to the previous layer's seam. The scar is more visible when the start/stop transition is more abrupt. With no firmware compensation, the start has visible under-extrusion and the end has visible over-extrusion, both at the same point. Pressure Advance / Linear Advance smooths these transitions - the firmware predicts the pressure delay and adjusts extrusion ahead of speed changes, so the actual deposited width stays uniform across the start and end. Scarf seams (also called staggered seams or zigzag seams) take a different approach: instead of starting and ending at the same point on every layer, the slicer ramps the extrusion up over a short angled distance and ramps it down at the end. The seam becomes a zigzag pattern instead of a vertical line. With the right ramp length, the seam is essentially invisible. This is now standard in OrcaSlicer and Bambu Studio for most filaments.