Top Z Distance vs Bottom Z Distance Explained: Slicer Mechanics and Interface Physics
By Sophia Al-Mansoor
In modern slicers like Bambu Studio, OrcaSlicer, PrusaSlicer, and Ultimaker Cura, the support interface acts as a sacrificial buffer. Yet print geometry dictates that this buffer works in two opposing directions. When an overhang extends outward, your machine builds a support pillar topped with a dense support roof. The machine then introduces a tiny vertical void before extruding the actual model. This gap is your top Z distance.
[MODEL OVERHANG] <-- Extruded on air; drops downward
======================== <-- Top Z Distance (Air Gap)
[SUPPORT ROOF] <-- Cool, hardened solid foundation
[Support Structure]
Gravity works against you here, pulling the molten plastic of the overhang downward into the clearance void. If the gap is too small, the perimeter bonds permanently to the support roof. If the gap is too wide, bridging sag turns the underside of the model into loose, stringy spaghetti.
Bottom Z distance functions in the reverse scenario. When a feature overhangs another section of the same print, such as the chin of a bust protruding over its chest, supports must anchor directly on the model's upward-facing sloped or horizontal walls. This transition zone is the support floor.
[Support Structure]
[SUPPORT FLOOR] <-- Molten plastic drops downward
======================== <-- Bottom Z Distance (Air Gap)
[MODEL SURFACE] <-- Pre-existing solid model ceiling
When the nozzle lays down the bottom layer of a support structure on top of your model, gravity pulls that molten support material downward into the cold surface below. The hot nozzle tip radiates heat directly into the printed wall beneath it. Because the nozzle squishes the first layer of support down, standard top Z clearance settings will weld the support floor firmly to your model's visible top skin.
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