In most additive layer processes, the build direction is usually the Z axis. However, orienting a part to this axis may affect build time, part strength, and even total build cost.
Let’s look at a typical powder-bed additive process. A heat source melts or sinters a layer of powder according to a specific pattern. The layer that has been melted plays an important role for the layer that will be on top of it. The previously melted layer must be strong enough to support the upcoming layers that will rest on top of it. In addition, it must help conduct heat away from the new layer.
With powder bed metal additive manufacturing, heat from the laser creates brief “weld pools” when the laser moves over the metal powder. The result is that the heat often affects the layers that have been sintered earlier. The heat slightly re-melts the earlier layers, helping to connect the layers together and resulting in a strong weld.
With overhangs, part of the area below the weld pool will be unmelted metal powder and thus, less thermally conductive, so heat from the melt pool will cool more slowly, increasing the level of sintering of the surrounding powder. This means that more material could attach to the bottom surface of the overhanging region, giving a rougher surface finish as a result of the slower cooling of powder below the overhang.
Orientation options
Experts in additive will advise that overhangs that are less than 45 degrees to the build plate should have support. They will also advise designing and orienting a part so that you minimize the need for supports as this can lower build and post processing costs.
Some suggestions to large overhangs may include:
–A design with tapered material to reduce supports, possibly increasing the part mass. The downside of this option is that it may require post-process machining or wire erosion.
–Angle the overhang to be at least 45 degrees and therefore self-supporting
–Invert the orientation so that you can use short supports under the bottom face. This option may also reduce build time, but may increase post-process finishing.
–Orient the part so that it lays flat on the build plate. This will reduce the build height. If you are doing batches of parts, though, it will limit the room available for nesting parts. Some parts may be more prone to resider stress too.
Many vendors offer build preparation software that can help designers evaluate various build orientations.
Orientation of features
How you orient a part will affect the surface finish, which may an important consideration for parts with various features. Accurately produced features on parts happen most often when the features are oriented so that they are on the top surface of a part. Some refer to this as the “up-skin.” Accordingly, aspects of a part that are on the bottom tend to be known as “down-skins” are usually have less definition.
Regarding orientation, designers might also want to keep in mind the potential effects of the dosing wiper that applies new layers of powder during a build. The wiper helps create a pressure wave in the powder bed as material is pressed down. Some features of a part may catch on the wiper, resulting in a failed build. Rotating the part can reduce these effects.
In summary:
— create self-supporting designs
–consider residual stress and surface finish with part orientation
–note that orientation also affects build time and costs
— complex geometries can complicate part orientate as the designer considers the trade-offs between surface quality, details, build time/cost and support structures.
