# Process-Dependent Influences on Adhesion in Multi-Material Extrusion
## 4 Results
### Tensile Testing
#### 4.2.2 Influence of Line Alignment  
Figure 9 shows the composite strength of specimens fabricated with a different line alignment configuration from the one depicted in Figure 4. It is clear that the typically used perpendicular line orientation is ill suited for the material interface, as it shows the lowest composite strength. Comparing the perpendicular orientation to the aligned orientation, Student's \(t\)-test results reveal that the influence is significant (\(p\) = 0.014). Because of the fluctuations in nozzle distance due to the waviness of the extruded layer, the overall strength was reduced. Consequently, the aligned orientation showed the smallest distance between the nozzle and the layer underneath, as each line was extruded right atop the ridge of another line and, correspondingly, its composite strength showed the highest value by about 35% compared to the usual perpendicular printing orientation. While the shifted configuration had an overall higher distance between the nozzle and the part than the perpendicular orientation, as the lines were extruded right into the valleys of the layer underneath, the resulting composite strength was still higher. Comparing the shifted and perpendicular configurations in Student's \(t\)-test, however, shows no significant influence (\(p\) = 0.350). This is suspected to be due to the higher overlap area between each line having an overall positive effect. However, the composite strength of the \"shifted\" line placement is about 12% lower than the \"aligned\" orientation, presumably due to the higher nozzle distance.  
\begin{table}
\begin{tabular}{l l l l l l} \hline  & **SS** & **Df** & **MS** & **F** & \(p\)** \\ \hline Between Groups & 2.474 & 3 & 0.8247 & 5.72 & 0.005388 \\ \hline Within Groups & 2.8832 & 20 & 0.1442 & & \\ \hline Total & 5.3572 & 23 & & & \\ \hline \end{tabular}
\end{table}
Table 2: ANOVA analysis results of relative nozzle placement test results shown in Figure 7, excluding values for nozzle placement at \(-\)0.02.  
Figure 9: Influence of relative line placement on composite strength of ABS–PLA specimens.  
Lastly, deliberate over- and under-extrusion basically had no effect on the composite strength of the ABS-PLA specimens compared to the aligned configuration as shown by the \(t\)-test results (\(p=0.426\)). This is presumably true due to insufficient filling of the rifts, resulting in a similar strength as the \"shifted\" configuration. The effect of mechanical interlocking, however, seems small compared to the effect of adhesion in this case, which might not be true for every material combination, especially materials with lower melting viscosity which might be able to fill the gaps more easily and create strong mechanical interlocks. In addition, since less compatible materials showed less adhesion overall, even incomplete interlocking at the interface might still be beneficial in those cases.  
In addition, the fracture images of the over- and under-extruded specimens show major discolorations of the ABS and even delamination of the under-extruded layer, as shown in Figure 10. Even though the overlap between the under-extruded ABS layer and the over-extruded PLA layer might have increased, the cross-sectional area of the last ABS layer still decreased. This would mean that the same force was distributed in a smaller area and real stresses increased, resulting in an earlier failure. Since ABS is prone to delamination in the first place, different material combinations might show better results and more research is needed to unequivocally rule out the positive effect of deliberate under- and over-extrusion.