# Process-Dependent Influences on Adhesion in Multi-Material Extrusion
## 2 Multi-Material Material Extrusion and Mechanisms of Adhesion
The extrusion amount can be controlled via two mechanisms--either by adjusting the extrusion multiplier in the slicing software in order to change the amount of extruder steps calculated by the software, or by fine-tuning the nozzle distance in the first layer after a tool change to change the volume constrained by the moving nozzle. Reducing the nozzle distance by 5% of the layer height, for example, has the same effect as increasing the extrusion multiplier in the slicing software to (100.0%/95.0%) = 105.3%. However, the nozzle distance is necessarily subject to fluctuations due to the mechanical tool switching mechanism of most printers. While the relevant literature includes several investigations regarding the influence of extrusion temperature or other process parameters on interface strength, the influence of nozzle distance, especially with regard to fluctuations due to tool switching, is commonly neglected. [PERSON] et al. examined the lap shear strength of multi-material combinations depending on extrusion width, line direction, line overlap, and print speed using an Ultimaker 3 printer with a nozzle-switching mechanism but did not take the variance of the switching mechanism into account [28]. Similarly, [PERSON] et al. conducted peel tests of PLA-Nylon specimens fabricated with an Ultimaker 3 without considering the nozzle height fluctuations [29; 30]. [PERSON] et al. investigated the lap shear strength of the PMMA-PC material combination fabricated with an E3D toolchanger without taking the precision of the tool-switching mechanism into account [31]. Similarly, [PERSON] et al. conducted tests on the tensile interface strength of TPU and conductive PLA fabricated with an E3D toolchanger, giving no thought to the switching mechanism [32]. [PERSON] et al. used an X400 by German RepRap GmbH to fabricate roll peel specimens of a variety of material combinations without taking the precision of the leveling of both nozzles into consideration [33]. [PERSON] et al. described the challenges of maintaining leveling of each nozzle in a self-developed multi-nozzle head, even without an active switching mechanism [34] and consequently improved their setup by including capacitive sensors in a subsequent publication [35]. However, the publication was unclear on the exact deviations in offset between each nozzle and how they might have affected adhesion. Since the fluctuations in the nozzle-switching mechanism should only pose a problem in horizontal interfaces with a multi-nozzle or multi-tool setup, publications investigating the strength of vertical multi-material interfaces [36; 37], as well as printer setups utilizing a single nozzle for multi-material AM, do not have the same problems, as no tool switching occurs and fluctuations are limited to the motion system. While single-nozzle machines might cause other problems like contamination due to insufficient replacement of the materials during material changes, they are not part of this investigation. In order to determine the effect of nozzle placement on adhesion, this publication tried to determine the extent of fluctuations for two common printers--an Ultimaker 3 and an E3D toolchanger. The latter was enhanced by a mechanism that could reliably determine nozzle distance and compensate fluctuations in order to investigate the influence of nozzle height fluctuations on interface strength. This mechanism was subsequently used to adjust the extrusion amount in the very first layer after tool changes and determine its effect on interface strength. Another aim was the investigation of the effect of extrusion temperature and build chamber temperature on multi-material adhesion. Between the mechanisms of diffusion shown in Figure 1, diffusion is most related to temperature as it is the driving factor.