## 4 Results
### Mechanical Adhesion  
As can be deduced by comparing peeling resistance of regular PLA/NinjaFlex specimens to ones with interlocking structures, mechanical interlocking has very high influence on adhesion in multi-material MEX. A depiction of a test specimen with these interlocking features can be seen in Figure 11.  
Figure 10: Comparison of different additional measures in order to increase the interface strength of selected material combinations. Figure 9: Overview of peel resistance of different material combinations. While the unchanged interface only shows a peeling resistance of about 2.4 kN/m integrating interlocking features results in a peeling resistance that is more than 3 times as high. While the influence of all other adhesion mechanism stays the same for both cases, macroscopic interlocking features enhance mechanical adhesion drastically, as bigger depressions can be filled more easily by molten material. Since machine calibration is not changed, it can be assumed that in this case about 6 kN/m can be transferred via mechanic adhesion alone. As seen in Figure 11, this heightened bonding strength even results in the tensile strength of both materials being exceeded in some parts of the specimen, which results in noticeable material residues in the very left part of the specimen. This highlights the advantage of AM over conventional manufacturing techniques. By utilizing the geometric freedom of AM through creating structures not manufacturable by conventional means, adhesion can be highly increased. However, in order to utilize this, a bare minimum requirement for the material combination is that material B needs to be able to wet material A. Otherwise the combination would already fail during the manufacturing process. On the other hand, it can be seen that surface roughness observably only plays a minor role in mechanical adhesion for multi material MEX. While maximum profile height Rc of the specimen XT with embedded carbon fiber is about 200 \(\upmu\)m and thereby contrary to expectation more than 70 \(\upmu\)m smaller than XT without embedded fibers, peel resistance only differs negligibly, as seen in Figure 9.  
As can be seen in Figure 12 specimens made out of XT without embedded fibers show deep crevices that even allow view onto the next lower layer. Ideally, those voids could be filled with different material at the layer of interface, creating interlocking features comparable to the ones fabricated out of PLA and NinjaFlex. One would expect those features to improve peel resistance drastically. Since XT with embedded fibers has no such holes, as shown in Figure 12 and therefore no way of forming deep interlocking features, yet peeling resistance is almost identical, one can only conclude that no such structures were formed during fabrication. Possible reasons for this behavior might be that molten NinjaFlex is not mobile enough to fill crevices above a certain depth or below a certain width, meaning higher roughness has no more positive effect. Figure 11: PLA/NinjaFlex specimen with interlocking features after roll peeling tests. On the other hand, peeling resistance of the combination of smoothened PLA and NinjaFlex is lower than that of the regular PLA/NinjaFlex combination. While specimens out of smoothened PLA show a surface roughness of about 26 um, the regularly printed specimens have a roughness of about 35 \(\upmu\)m.  
Figure 13 shows the process of determining surface roughness exemplarily for regular and smoothed PLA, while the same method of measuring was used for XT. It can be seen that the hot nozzle ripped parts of the structure out of the specimen during the smoothing process, leaving behind smooth plateaus where ridges used to be. Those smooth plateaus on the one hand offer less surface area for other adhesion mechanisms to be effective and on the other hand result in less interlocking features between the materials.