Sagging overhangs: why the underside droops

What is happening

An FDM printer lays molten plastic in layers. When there is empty space under a new layer, the plastic has nothing to rest on, and unless it has time to cool it droops under its own weight. Rather like a shelf hanging out slightly over thin air.

It is easy to see on larger overhangs and on long bridges. The underside is rough, rippled or has hanging strands, even when the rest of the part is clean.

What causes it

• A large overhang angle. The flatter the surface, the less support it has.
• Slow cooling. Soft plastic flows down before it sets. Good cooling helps.
• A hot nozzle. Overheated plastic is runnier, see nozzle temperature.
• Material. PLA cools faster and handles overhangs better than PETG, which is runnier and pulls strings.

How to avoid it in the design

Stick to the rule that an overhang up to about 45 degrees prints without support. Steeper surfaces can be turned into a bevel, see chamfering edges, or the model can be rotated so that the overhang disappears. Support can also come from a rib or wall that you leave in the model.

For round holes with a horizontal axis, a teardrop or diamond shape is used, which turns the upper part of the hole into a roof at 45 degrees. You only get an exact circle approximately that way, so for precise fits allow for tolerances.

How we deal with it

For every uploaded model we create supports automatically, and their cost is in the price per part. A support holds the plastic, but leaves a rougher surface where it touched. If the visible face matters to you, rotate the model or adjust it so that it does not need supports.

You do not need to set up supports. If you have a face that is meant to be smooth, tell us about it before ordering.