In the previous two posts in this additive engineering series, we talked about the purpose of support structures and how they work. Here we’ll start to talk about where we can get away without it on overhangs, bridges, and cantilevers.
As can be seen in the figure below, the weld trace-width is quite a bit wider than the layer thickness although a couple layers beneath the latest layer get re-melted. Therefore, the laser can focus on the solid part and have something to draw heat away, and have a part of the weld bead hang off the edge a bit with each layer. By doing this over and over again you can get an overhang. The rough rule of thumb is that you can go 45° before you need supports. This angle can vary depending on the material, power, laser speed, geometry, and approach direction of the recoater blade.
The rough rule of thumb is that you can go 45° before you need supports. This angle can vary depending on the material, power, laser speed, geometry, and attack of the recoater blade. For example, if the overhang is for a cone that is closing up and getting smaller as the layers progress, you can overhang more as the geometry will tend to hold itself together well. Also, if the overhang faces the oncoming recoater blade, the pressure from the new pile of powder will cause it to lift a little and get progressively worse as the layers continue.
If you overhang too far, there is not much material to draw the heat away and the overhang will tend to want to lift. The powder on the underside can also be more partially welded or the laser may even penetrate through and start drawing in larger chunks of metal powder. You can get away with this for a few layers but as this lifting stacks up, it can get to a runaway state where it is lifting quite a bit above the focus plane. The featured image of this post attempts to demonstrate this.
The image below shows some features being printed at different angles. Vertical walls will be very smooth, but the farther the feature tries to hang over, the more roughness you will see on the bottom surface until it tips far enough that supports are needed.
If you have a sudden expansion in cross section, the process can recuperate after a few layers but it can only withstand a cantilever of about 500 microns (.020″). The image below demonstrates what this might look like after a few layers. The faint dashed line shows the intended geometry. On the left, the part had a short cantilever where there was no support beneath the feature. In that area, the weld bead is very deep for that first layer so the overprint will be on the order of 250 microns beyond the intended surface (somewhat like we see in between supports like you can see on the bottom of this example). Sometimes I will shift my surfaces a little in anticipation of this overprint.
I was going to try to cover bridges in this post but I think that will need to be covered in the next post. Thanks for reading. If you want to go to the beginning of this series just click here.