## 2 Related Work
### Variability in AM  
Commodity polymers such as PLA, acrylonitrile butadiene styrene (ABS), and polyethylene terephthalate glycol (PETG) are common materials for MEX. The anisotropic properties of high-performance materials are more complex and still require research into thermal management [19]. However, commodity polymers such as PLA are available at a low cost and are thoroughly explored [26; 27; 28; 29]. Numerous studies have identified uncertainty in the performance of components produced by polymer MEX [30; 31; 32]. Further work has identified the three main areas of the MEX process that contribute to the performance variability of components: the feedstock material, the print orientation and raster angle, and the process parameters [27; 28; 33; 34]. These areas are indicative of the three different categories of variability in MEX; (1) process and material variability, which cannot be controlled by the operator but needs to be considered when comparing printed parts; (2) configuration variability, which can be controlled by selecting process parameters based on existing literature, and (3) orientation variability, where the designer needs to consider the mechanical properties based on layer orientation. This is particularly relevant for GD. 1. Uncontrollable process and material variability. [PERSON] et al. analyzed the effect of PLA color on the UTS of MEX parts [30]. A natural (un-dyed) filament was found to have the best UTS of 57.16 MPa, whilst a gray one performed worst with a UTS of 50.84 MPa. Black, blue, and white filaments all sat in the middle of the range. [PERSON] et al. give a more general overview in their work 'Not all PLA filaments are created equal: an experimental investigation' [31]: across the range of 11 filaments tested, the authors found a \(\pm\)17% range in density, \(\pm\)5% variation in Young's modulus, and \(\pm\)10% variability in yield strength. As noted by [PERSON] et al., most AM research is conducted on a single printer, and if AM is to become a widespread manufacturing technique, it is necessary to determine the variation between components produced on different printers [32]. To this end, [PERSON] et al. produced 90 lattices across three Carbon M2 AM machines. Hardware was also switched out on each machine between prints to further assess variability. Following geometric measurements of all the lattices produced, it was found that the choice of the machine not only'significantly impacts' the thickness, length, and height of components but also the variance of component dimensions [32]. 2. Controllable configuration variability. In 2020, [PERSON] et al. investigated the effect of raster angle on MEX PLA components' UTS and fracture behavior using the J-integral [29]. The team tested four different raster angles of \(0/90^{\circ}\), \(15/-75^{\circ}\), \(30/-60^{\circ}\), and \(45/-45^{\circ}\). Of these, the \(45/-45^{\circ}\) raster components performed best, with greater elongation before fracture and a J-integral more than \(4\times\) that of the \(0/90^{\circ}\) raster components. [PERSON] et al. compared the relative effects of build orientation, raster angle, infill type, infill density, and layer height in polyethylene terephthalate-glycol (PETG) [33]. They found that all the variables except the layer height had a strong correlation with component UTS. Similarly, [PERSON] et al. found that increasing infill density and layer height together gave a \(5\times\) rise in energy absorption for PLA specimens undergoing ballistic impact tests [35]. The final area where variation emerges in the AM process lies in the high sensitivity of components to machine parameters. [PERSON] et al. attributed the 17% variation in UTS observed between identical components in their experiment to fluctuations in extrusion temperature [36]. Nozzle temperature studies suggest that the highest possible nozzle temperature that does not cause material degradation provides the best mechanical properties [37]. 3. Orientation variability for component design.