Effects of Build Orientation and Raster Angle on the Tensile Behavior of 3D Printed Parts
In our previous posts we identified the importance of two slicing parameters, namely layer height and layer width, and the build orientation when using fused filament fabrication (FFF) manufacturing. The last post discussed the effect of layer width and layer height on the stiffness of 3D printed parts. In this post, we discuss the importance of the build orientation and raster angles for end use parts. These parameters are crucial to how the part will perform under different loadings, but they are often overlooked when developing parts using FFF manufacturing.
Advancements in the FFF manufacturing process have given users the ability to change almost every aspect of the printing process. Each part is built layer-by-layer, producing anisotropic mechanical properties for stiffness and strength of 3D printed parts.
The build orientation is defined by how the part is oriented on the build plate. Changing the build orientation modifies how the layers exist within the part. Within these individual layers, the user can modify the raster orientation. Raster orientation defines the direction of the individual bead paths within a layer.
We used the unique build/raster orientations in specimens for tensile strength testing outlined in the ASTM D638 testing standard. As shown in Figure 1, each specimen is given a unique name based on its build plate and raster orientation. It is important to note that each specimen is printed at 100% infill but contains voids between print beads as shown in Figure 2.
In order to quantify the effects of build orientation and raster angle on tensile behavior we focused on 4 test configurations, XY0, XY90, XY45, and ZX0.
1. XY0 specimens test for the part strength when the load is parallel to the bead path. In reference to Figure 2, this configuration would be loading in the XX direction.
2. XY90 specimens test for the part strength when the load is perpendicular to the bead path. In reference to Figure 2, this configuration would be loading in the YY direction.
3. XY45 specimens test for the part strength when the load is at a ±45 degree angle to the bead path.
4. Standing ZX90 specimens test for the part strength when the load is perpendicular to the layers. In reference to Figure 2, this configuration would be loading in the ZZ direction.
These 4 configurations provide us with insight on how specific sections of a 3D printed part will act under a load. In general, the XY45 specimen is a good representation of the shell, while the XY0 specimen tells us how the continuous outer wall will respond under tensile load. The XY90 measures the bead-to-bead bond strength within a layer and the ZX90 measures the layer-to-layer strength.
The stress-strain plots in Figure 3 express just how important build orientation and raster angle can be to the strength of an end use part. For tensile loading, the XY0 specimen tests produced an average ultimate strength of 35 MPa. That strength is almost three times higher than the ZX90 tensile specimens that had an average ultimate strength of 12 MPa. This large difference in strength comes from how the beads are loaded as shown in Figure 2. In the XY0 specimen, each bead experiences a homogeneous stress/strain field. The bond between beads has little or no effect on the mechanical performance. In contrast, for the ZX90 and XY90 specimens, the ultimate strength is dependent on how well the beads fuse together. This bead-to-bead bonding produces an inter-layer or debonding failure mode within the printed specimen. Lastly, the XY45 specimen produced a unique stress-strain plot when compared to the XY0 and XY90 stress-strain curves. The XY45 specimen has a mixed failure mode of both bulk material failure and debonding. This print configuration is more representative of a printed part because most parts do not contain a uniform print direction.
In summary, many of the interesting aspects of build direction and raster orientation presented here share a striking resemblance to the mechanical behavior of continuous fiber composite materials. Understanding material behavior extends far beyond characterizing the bulk material and is an essential aspect of providing the end user with a superior 3D manufactured part.