Bridges, overhangs, and where support becomes necessary

The angle at which unsupported plastic stops holding itself up.

Unsupported plastic can hold its own shape only up to a certain angle away from vertical, because each new layer is partly resting on the layer below it and partly hanging out past its edge, and past roughly forty-five degrees from upright there is no longer enough of the layer below to hold the new one up before it sags.

The mechanism behind bridging and overhangs

Every new layer a fused filament printer lays down is molten plastic set directly onto whatever is beneath it, and if the layer below offers a full, flat surface to land on, the new layer is supported along its entire width the instant it is deposited. An overhang asks for something different: part of the new layer's width has nothing beneath it at all, only open air, and that unsupported part has only its own stiffness, and whatever it is still attached to sideways, to keep it from drooping down into the gap before it cools enough to hold its own shape. As the angle of the overhang leans further from vertical, more of each new layer's width goes unsupported, until the amount of layer hanging in open air overwhelms the small amount still gripping solid material, and past that point gravity pulls each new layer down before it has time to stiffen, and the surface finish collapses into sagging streaks and gaps rather than a clean overhang. A bridge, a horizontal span printed across a gap with support at both ends rather than leaning out from one side, survives at a much steeper effective angle than a one-sided overhang, because each new short length of plastic is only ever asked to span the small remaining gap between two anchored ends rather than to hold itself up entirely unsupported in open air.

The icing comparison

Piping icing onto a cake in a spiral, building a small dome ring by ring, only works because each new ring sits mostly on top of the ring laid down before it, still soft but held up by that support while it firms. Tilt the piping bag too far outward and try to build each ring further out than the one below it, hanging further and further into open air, and the icing begins to droop and sag before it can set, curling downward under its own weight rather than holding the shape it was piped into. A printed overhang droops for exactly the same reason, since each new bead of plastic is being asked to hold a shape in open air using only its own brief stiffness, and past a certain angle that stiffness runs out before the plastic has cooled enough to help itself. Piping the icing more slowly, giving each ring a moment longer to firm before the next one is added, buys exactly the same margin a slower, more heavily cooled print does, trading time for a steeper angle the material can still be trusted to hold.

The one number worth remembering

Most common printing plastics can manage an unsupported overhang of about forty-five degrees from vertical without noticeable sagging, since at that angle roughly half of each new layer still sits on solid material below it, and steeper angles need either a slower print, cooling the plastic faster right where it lands, or a temporary support structure printed underneath purely to be broken away afterwards.

What follows from this

Knowing that angle changes how a shape gets designed for printing in the first place, since a part redesigned to lean no further than that limit, or split so no single overhang exceeds it, can often be printed cleanly with no support at all, while the same shape printed at a steeper angle either needs support material that has to be removed afterwards or comes out with a visibly sagging underside exactly where the angle exceeded what the plastic could hold on its own. Turning a part on its side before printing can sometimes convert a steep overhang into a manageable one without changing the design at all, simply by changing which direction is treated as up while the layers are being built.