We've been making things for a long time. For most of that time, objects were crafted using our bare hands or with the help of tools. Today, there are new computer-driven manufacturing tools like 3D printing. There are also many types of 3D printing technologies, and each one requires different design considerations. For example, liquid-based printers (like SLA) require holes that are oriented for the liquid to drain from cavities.

However, the most popular Additive Manufacturing technology for desktop printers is called Fused Deposition Modeling, or FDM. You can think of FDM like a robotically-controlled hot glue gun that extrudes threads of plastic to build a 3D object layer-by-layer. 

Because FDM builds 3D objects layer-by-layer, each printed layer needs to be supported by the layer below it. If there's nothing to attach to, the filament will just fall. 

Even if there is a layer to print on, if there’s not enough surface area for the extruded thread to attach to, something called Overhang occurs. Overhang is a common reason for print quality issues. 

This is where the 45-degree rule comes into play. Very little quality issues occur for angles less than 45 degrees (or more than 135 degrees) relative to the horizontal (depending on the printer).

Angles of 45 degrees allow for the extruded thread of plastic to be supported by about 50% of the previous thread of plastic.

Here we see a design printed vertically, with no 45-degree angles:

Notice the slight overhang that occurred:

But when designed for 45-degree angles, there's no overhang:

However, if designing with 45 degrees angles is not possible:

Reorient the design
Sometimes adjusting the model orientation can eliminate overhang. Sometimes printing the object upside-down, or on its side can remove the need for supports. It’s worth trying different orientations in your slicer of choice, and looking if the layers require support. Just keep in mind the effect of orientation on strength (see orientation, below). 

Cut the model into different pieces that individually don’t need supports
If you are making a box, for example, it’s better to take the top off and print it upside down, next to the rest of the box rather than wasting material for support. 

Add supports Manually:
Most slicer programs automatically add supports, but you can add them manually into the design if you want more granular control over where supports are located. Manual supports should typically have a gap of ~0.125mm-0.2mm to detach cleanly. When building manual supports, remember to consider gap distance.

In summary: Wherever possible, incorporate 45-degree angles into your design.

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