Overhangs and Supports

The 45 degree rule

A 3D printer cannot print in mid-air. Each layer has to sit on the layer below it, so when a surface leans outward, every new layer hangs a little further past the last. Up to about 45 degrees from vertical, each layer still rests largely on the one beneath and the surface prints cleanly. Flatter than that, edges droop and curl, and the part needs supports.

45 degrees is a conservative rule, not a hard limit. Prusa says a printer can cleanly print overhangs of 45 to 60 degrees depending on the nozzle diameter and settings, and less than 45 degrees with a small 0.25 mm nozzle. Cooling, print speed, and material all move the limit, so treat 45 degrees as the safe design value.

Resin printing follows a similar rule of thumb. Formlabs' orientation advice says parts in stiff, rigid resin can support themselves when a surface is steeper than 45 degrees from horizontal. Printer design guides list shallower limits for small test features, so 45 degrees is the safe value for real parts.

Bridges

A bridge is a horizontal span printed across open air between two supported points, such as the roof of a slot or a window. Because the plastic is anchored at both ends, short bridges print without supports. Long ones sag in the middle.

If a span is too long, a single support placed in the middle turns one long bridge into two short ones, which Prusa notes print close to perfectly while the support stays easy to remove. Changing the orientation so the span disappears is often better still.

Support styles

When an overhang cannot be designed out, the slicer adds support material: a scaffold printed under the overhang and removed after printing. PrusaSlicer offers three styles, and other slicers offer similar options.

Two settings shape how supports behave. Interface layers are a denser top surface on the support, so the overhang rests on something close to flat. The contact gap is a small vertical space between the support and the part that lets the support break away; Prusa finds 50 to 75% of the layer height works well. Supports can also be limited to grow from the build plate only, which keeps them off the part's own surfaces.

How supports mark a surface

Supports always leave a trace. Prusa notes that a face printed directly on the build plate is flat and smooth, while a face printed on top of supports looks more inconsistent and rough. Supports that start on the part itself can also leave small imperfections on the face they grow from.

So decide which faces matter before the part is printed. Put cosmetic and mating faces where supports cannot touch them: facing up, standing vertical, or flat on the build plate.

Designing supports out

The cheapest support is the one never printed. Prusa's first suggestion is to change the orientation or split the model into parts that each need fewer supports. After that, reshape the features that cause them.

• Chamfer the underside of ledges and shelves at 45 degrees, so they grow out gradually instead of starting in mid-air.
• Use chamfers, not fillets, on edges that face the build plate. A fillet starts almost flat, which Prusa notes creates a very steep overhang and a poor surface.
• Give horizontal holes a pointed, teardrop-shaped top, so the roof of the hole never becomes a flat overhang.
• Keep bridges short, or add a rib or pillar to the design to break up a long span.
• Split a part with many overhangs into simpler pieces that each print flat.

SLA supports and orientation

Resin parts are supported for a second reason. On a typical resin printer, the part hangs from the build platform and is pulled away from the film at the bottom of the resin tank after every layer. Supports hold the part against that force as well as under its overhangs, and each support touchpoint leaves a small mark. Formlabs' orientation advice comes down to five points.

• Print directly on the build platform when the part has a flat face that can sit there. Formlabs says these parts warp least and use fewer supports, though the bottom layers can come out slightly oversized.
• Keep the area of each layer small and avoid sudden jumps in area between layers. Tilting a part, often to about 45 degrees, spreads a large flat face over many small layers and reduces warping and curling.
• Put the larger, heavier end of a long part closest to the platform, so more supports hold it early in the print.
• Face cosmetic, detailed, and mating surfaces away from the supports.
• Avoid suction cups. A hollow or cup-shaped part can seal against the tank and tear free. Fill the hollow, add drain holes, or reorient the part.