Why does my print need supports? Overhangs, the 45° rule and auto supports

Every layer needs something underneath

A printer builds a part by laying beads of molten plastic on the layer below. As long as each bead lands mostly on the previous one, it bonds and cools in place. Where there is nothing under it, it droops, curls up or falls as a string. Supports are the scaffolding a slicer adds under those areas, printed with the part and removed afterwards.

So the short answer to "why does my print need supports?" is that somewhere, the part has plastic that would be laid over air.

Where the 45° rule comes from

Overhangs are measured from vertical. A vertical wall has every layer directly on the one below. Tilt it outward to 45° and each 0.2 mm layer steps out by 0.2 mm, about half the width of the 0.4 to 0.45 mm line it sits on. Half a line of support is enough: the new bead bonds to the edge of the one below and holds. Past 45°, each layer hangs further out, less of it rests on anything, and the underside gets rough, then droopy, then fails.

45° is a rule of thumb, not a physical limit. Good part cooling, PLA and a slow outer wall print 50° or 60° acceptably; PETG and ABS, which print with less fan, droop sooner. Thinner layers help too, because at the same angle each layer steps out less.

Shapes that fool people

• Bridges are not overhangs. A flat span held at both ends, like the top of a slot or a doorway, prints across the gap in straight lines pulled tight between the two sides. Slicers print bridges with their own speed and fan settings, and spans of a few centimeters usually work without supports.
• Horizontal holes have an overhang at the top. The top of a round hole on its side gets steeper until it is flat. Small holes print fine anyway; larger ones sag at the top. A teardrop-shaped hole, with a 45° point at the top, avoids it.
• A fillet on a bottom edge is an overhang. A rounded edge where the part meets the bed, or under a ledge, starts almost flat. A 45° chamfer does the same job and prints cleanly, which is why designers put fillets on top edges and chamfers on bottom ones.
• Islands always need support. A feature that starts in mid-air, such as the tip of a hook pointing down, has nothing to start on.

How automatic supports decide

In SpeakCAD's online slicer, Supports has three settings:

• Auto (only where needed) hands the decision to OrcaSlicer's automatic supports, which are generated only where an overhang is steeper than the threshold in the print profile. After slicing, the result tells you whether any supports were printed and how much they weigh.
• Always applies the 45° rule strictly: the same automatic supports, under every overhang past 45°, small ones included, whatever the profile allows. Short bridges still print without.
• Never switches them off. Anything past the limit prints over air.

The threshold comes from the print profile, and slicers express it their own way: in OrcaSlicer's profiles it is the slope of a face measured from horizontal, typically 30°, and faces flatter than that get support. That is why a part that breaks the 45° rule by a few degrees often slices with no supports at all on Auto, and why Auto sometimes surprises you the other way, with a small support inside a horizontal hole or under a chin you had not noticed.

Leave Auto on by default. Choose Always when your printer or material droops on overhangs that Auto leaves in the air, such as PETG or ABS with little fan, or when an overhang must keep its exact shape. Choose Never when the part was designed to print without supports and the only overhangs are small holes or short bridges that print fine on their own: you avoid scars inside the holes.

Tree or normal, and where supports may stand

• Support style: Tree grows branches from a few points and reaches each overhang with thin tips. It uses less material, touches the part in fewer places and comes off more easily. It suits curved and scattered overhangs.
• Support style: Normal (grid) builds straight columns under the whole overhanging area. It uses more material but is steadier under a large flat overhang, such as the underside of a shelf.
• Only from the build plate lets supports stand on the bed only, never on the part. There are no marks on the part's upper faces and no supports inside horizontal holes, but an overhang above another part of the model gets nothing.

Avoiding supports altogether

The best support is the one you do not print. In order of effort:

1. Turn the part. Put the largest flat face on the bed and let overhangs become walls. Optimise the orientation does it for you, turning the part to its flattest base and fewest overhangs. For parts where the direction of the load matters more, see which settings matter for a functional part.
2. Accept a little droop where nobody looks. The underside of a hidden ledge can print rough. Choose Never and check the result.
3. Change the design. Chamfers instead of ledges, teardrop holes, a part split into two pieces that each print flat. The design rules guide lists them.
4. Choose the material. PLA, with full cooling, handles steeper overhangs than PETG or ABS; print settings by material covers the differences.

When the fix belongs in a part you did not design, SpeakCAD's AI editor can rebuild the STL as an editable model and make the change you describe, such as a chamfer under a ledge. A new part described from scratch is designed with its overhangs kept under 45° and a flat base on the bed.

Read the result before you print

Every slice in the online slicer tells you whether supports were used and what they weigh. If the number surprises you, change the orientation or the support settings and click Slice again: re-slicing the same file with other settings within 10 minutes spends nothing. Try it with your part.