# Process-Dependent Influences on Adhesion in Multi-Material Extrusion
## 4 Results
### Tensile Testing
#### 4.2.1 Influence of Relative Nozzle Placement  
Figure 7 shows the influence of slight over-extrusion or under-extrusion of the first layer after a material change on the composite strength achieved by bringing the nozzle closer (positive values) or further away (negative values) from the last extruded layer. When the distance between the nozzle and the already extruded structure differs from the proper layer height, the necessary volume calculated by the slicer and the volume confined by the plane of the nozzle are not equal, which results in the corresponding over- or under-extrusion. This means that slight deviations due to mechanical switching from one nozzle to another or the changing of extruder tools can affect relative extrusion amount. These influences have been eliminated in this test as described above. As seen in Figure 7, this difference affects composite strength to a large degree. As shown, slight over-extrusion increases composite strength, while under-extrusion decreases composite strength. This mechanism is expected to occur due to the differing  
Figure 6: Tensile curves of reference specimens with relative nozzle placement of \(-0.00\) mm. Each color represents a different specimen. Figure 7: Influence of relative nozzle placement on composite strength of ABS–PLA specimens. forces pressing the material into the micro- and macro-mechanical pores and gaps thus affecting mechanical adhesion. As a liquid polymer is pressed more strongly onto the layer underneath due to over-extrusion, the pores are filled more thoroughly and thus composite strength increases. However, if the distance between the nozzle and the surface further decreases, an opposing mechanism is expected to take place at reductions close to the layer height. Since all the force is ultimately applied by the feeder, and the required force for extrusion increases with decreasing nozzle distance, slippage of the feeding mechanism and grinding of the filament are expected to occur once the force is high enough, leading to a reduction in strength. Hence, a maximum is expected to exist, but it could not be found within the scope of this investigation. Increasing the distance between the nozzle and the surface results in an opposing effect, and the pores are filled less thoroughly. For higher distances, this effect is expected to be further magnified by the formation of larger gaps due to under-extrusion. Force is then distributed less evenly on the interface zone due to these holes, causing true stress at the interface zone to increase, and resulting in lower effective composite strength. Fractures occurred almost exclusively at the material interface for all nozzle distances, akin to the fractured surfaces shown in Figure 8. Minor discolorations of ABS and white ABS residue at the PLA side indicate areas of high stress. These indicators are ambiguous, however, as all the samples show about the same amount of discolorations, independent of their respective interface strength. The comparatively low composite strength at \(-0.02\) mm, however, cannot be explained by the mechanisms of adhesion. Due to the relatively low standard deviation, it is suspected that the seal of the Ziplock bag might have been faulty, so the specimens were subject to humid air while being stored, which might have negatively affected the interface. Overall, nozzle distance has been shown to greatly influence composite strength as changes of only \(\pm 0.04\) mm resulted in a difference in composite strength of about 20%. Hence, nozzle distance should be considered at all times during manufacturing of multi-material or multi-color parts. Many printer designs include multi-nozzle print heads or multiple tools that require some form of mechanical switching and therefore show some variance in nozzle placement each time a material change occurs, as described above.