What is the 45 degree rule for 3D printing?

The 45 degree rule is both a blessing and a curse for 3D printers everywhere. If you’ve spent much time with fused deposition modeling (FDM) printers, you’ve probably encountered this cruel intersection of geometry and physics, with drooping or ruined prints the unfortunate fruit of your negligence. Of course, if you are a novice, you may have never encountered the 45 degree rule and its consequences, even if you have already reviewed what to know before purchasing a 3D printer.

The 45 degree rule comes about because your 3D printer is fighting against the primordial force of gravity. What is the rule? The 45 degree rule refers to the angle at which a construction can safely support its layers while building an overhang. With angles up to 45 degrees, about half of an extra layer sits on top of the layer below, giving it enough support to hold its shape while it cools.

Best case scenario, you’ll end up with a warped or warped print (some of which can be cleaned up with a hobby knife and a little elbow grease). Worst case scenario, that great office project will be completely ruined and you’ll have to start printing from square one after making adjustments.

The overhang problem lies in the inherent nature of FDM printers. These 3D printers work by heating and placing thin layers of thermoplastic material on top of each other until a complete creation emerges. Sharp corners sabotage this process because each layer needs a foundation underneath; otherwise, it will succumb to gravity before it can cool enough to maintain its shape.

Although a good rule of thumb, the 45 degree rule is not set in stone. There are ways to build overhangs at steeper angles; you just need to give your printer a little help. Consider improved cooling and air circulation to help freeze layers in place before the next one is laid. Along the same lines, you can increase the cooling time of each layer before printing the next one. You can also lower your layer height to speed up cooling or rotate a model so sharper angles are more gradual.

Alternatively, you can print at full speed in your current setup and produce overhangs at almost any angle you can imagine using a simple hack: supports. Supports are exactly what they sound like: thin pieces of plastic that hold a layer while it cools. Once the print is complete, you can break or cut them and clean the area where they were attached to your build with a craft knife.

Supports are a great solution for printing internal cavities or arcs, such as in a hollow box with a recessed ceiling for example. Without supports, the printer would attempt to fill long areas of empty space and end up producing drooping filament strands. Instead, a dense grid of supports directly beneath the hollow roof can help circumvent the challenges posed by the difficult geometry.

A more advanced example is so-called shaft supports, which are common in FDM and resin workflows. If you printed something like a dragon with wings spread, the tree supports would branch upwards from a few base points and split into thinner arms that would only touch the necessary overhangs. This reduces contact damage compared to block supports and makes cleaning much easier. To test the effectiveness of the supports, pick up a few models with dramatic overhangs from one of the free project sites scattered around the Internet.