Design For 3D Printing – Overhangs
It would be nice if every model were easy to 3D print, but the fact is that some models are easier to print than others. Today we’ll look at one of the challenges when printing a 3D model – overhangs.
3D printers deposit filament (plastic) in thin, horizontal layers. The ‘next’ layer is printed on top of the layer below it, like stacking pieces of paper one on top of another. But the layers are not always exactly lined up with each other. Sometimes the layer being printed is “shifted” a bit from the layer below it, creating a situation where the printer is being asked to print that layer on “thin air”. This is an overhang.
Printers can actually compensate for overhangs. The software used to turn the model into a printable file is called a “Slicer” (because it slices the model into thin, horizontal layers which the printer can print). Modern Slicers are able to detect overhangs. When detected, the Slicer will print the overhanging layer so that it just slightly overlaps the layer below it. That overlap gives the overhanging layer support. However its not a cure all.
Imagine a tall building that goes straight up in the air. If we printed that building, because its exactly straight up and down, each higher layer would line up exactly with the layer below and there would be no overhang. Now imagine that same building is tilted, like the Leaning Tower of Pisa. Each higher layer would overhang the layer below it. How much it would overhang depends on how much the building is leaning. Lean a little, then there’s a little bit of overhang. No problem! Lean a lot, and there’s a lot of overhang. Uh oh! There comes a point where the “overhang angle” is just too steep and no matter how hard the Slicer tries, the printer itself can no longer compensate. Some printers are better at compensating for overhang than others, but at some “overhang angle” all printers will fail.
So what happens then? Do we just give up on the part? Of course not! There are 2 solutions, and as you may expect, one is better than the other.
A common solution is to tell the Slicer to generate “support material”. Let’s go back to our building. If our building leans over a lot we could build a scaffolding underneath. As we build up layers, we could actually rest the layers on the scaffolding. Once we’re done we can remove the scaffolding and our building, albeit leaning, would be complete. Slicers can do the same thing. We can tell them to generate support material to “hold up” the overhangs so they can be printed. But there are (many!) problems with that approach. Printing support material not only takes extra plastic (think extra cost to print) but also increases the time required to print the part, which in turn reduces productivity (again, think extra cost to print). Once the part is printed, the support material must be removed, which is a labor intensive operation and can sometimes damage the part, meaning we have to start over (and yet again, think extra cost to print).
So why do people create models that have overhangs? Sometimes it’s unavoidable, sometimes it’s just that they don’t realize the problems that are inherent in 3D printing, and sometimes they just don’t understand what the solution is. These are not unreasonable since the person modeling the part may have no experience actually performing a 3D print and so would not be aware. But what can be done? The answer is to “build in” supports.
Let’s look at another example. Instead of an entire building let’s just look at a wall on the outside of a building. As we go up the side of the wall every 10 feet or so is a flower box. The flower box is rectangular, about 3 feet long and sticking out from the wall by about 1 foot, meaning it has a 1 foot overhang since there is nothing underneath the flower box but air. To complicate things the angle of the overhang is extremely steep, in this case 90 degrees. That’s as steep as you can get, and we know our printer will not do a good job. So we can either resort to supports (yuk!) or we can “design for 3D printing” and “build in” supports. But how?
Imagine that underneath the flower box we put a wedge. The wedge is the same length as the flower box (3 feet) and the same depth (1 foot). Further, we design our wedge so that it sticks out from the building at a 45 degree angle, like this:
This effectively changes the angle of our overhang from 90 degrees to 45 degrees, which our printer can easily do. Granted it adds an extra (triangular) block to the bottom of our flower box, but it eliminates the need for supports which in turn results in a print that is better looking and will print more reliably. Simply put, the designer needs to 1) identify overhangs and 2) reduce or eliminate them.