Seam Placement and Gap

Theoretical Background

The Physics of the Z-Seam

In Fused Filament Fabrication (FFF), every layer is printed as a continuous extrusion path. The point where a perimeter loop starts and ends is called the Z-seam. The formation of a visible seam is dictated by two physical phenomena:

1. Pressure Relaxation Lag: When the nozzle completes a perimeter loop, the extruder stops feeding filament. However, because of the residual pressure inside the melt zone, plastic continues to ooze out for a fraction of a millisecond.
2. Surface Tension and Wetting: The molten polymer has high surface tension. When the nozzle stops to lift (the Z-transit phase), the molten plastic naturally clings to the nozzle tip, pulling upward and forming a tiny pimple or blob.

Mathematically, if we denote the volumetric flow rate as $Q(t)$, ideal flow stops at $t_{end}$. In reality, the flow decays exponentially: