## 5 Design Recommendations
### General Requirements and General Influences  
Even though process settings have a huge influence on compatibility of material combinations some minimum requirements have to be fulfilled in order to be used in the same MEX process. All thermal requirements for both materials have to be able to be met in theory. Especially refractory deformation temperature of both materials may not be exceeded inside the build volume during the manufacturing process in order to maintain geometric accuracy of the part. If any material has a minimum requirement for build volume temperature that is exceeding the softening temperature of the other used material, this combination of materials is impossible to process. An example for this is the combination of PLA and PEEK. While the former has a softening temperature of about 60 \({}^{\circ}\)C, \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Influencing Factor** & \begin{tabular}{c} **Mechanical** \\ **Adhesion** \\ \end{tabular} & \begin{tabular}{c} **Diffusion** \\ **Theory** \\ \end{tabular} & \begin{tabular}{c} **Adsorption** \\ **Theory** \\ \end{tabular} & \
\begin{tabular}{c} **Polarization** \\ **Theory** \\ \end{tabular} &
\begin{tabular}{c} **Thermal** \\ **Stresses** \\ \end{tabular} \\ \hline
**Process-Specific** & & & & & & \\ Build Chamber & & & & & \\ Temperature & & & & & \\ Build Plate Temperature & & & & & \\ Extrusion Temperature & & & & & \\ Infill & & & & & \\ Perimeter Shells & & & & & \\ Air Gap & & & & & \\ Flow Rate & & & & & \\ Raster Angle & & & & & \\ Length of Line Section & & & & & \\ Extrusion Width & & & & & \\ Layer Thickness & & & & & \\ \hline
**Pretreatment** & & & & & \\ Filament Drying & & & & & \\ Plasma Treatment & & & & & \\ Tempering & & & & & \\ Mechanical Coupling & & & & & \\ Surface Smoothing & & & & & \\ Ambient Temperature & & & & & \\ Humidity & & & & & \\ Machine Calibration & & & & & \\ \hline
**Material** & & & & & \\ Polarity & & & & & \\ Thermal Diffusivity & & & & & \\ Chain Length & & & & & \\ Thermal Expansion & & & & & \\ Glass Transition & & & & & \\ Temperature & & & & & \\ Wettability & & & & & \\ Melting Temperature & & & & & \\ Crystallinity & & & & & \\ Filler & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: System of relationships of proposed influencing factors on the most relevant adhesion phenomena and on thermal stresses (\(\blacktriangleright\) Influence determined experimentally; \(\bigcirc\) Proposed influence). the latter requires the build volume to be heated to at least 60 \({}^{\circ}\)C in order to minimize warping and delamination, making the material combination not worthwhile. In the same vein, material combinations may only be unsuitable in certain orientations or in certain parts of the build volume. Two materials may for example pose different requirements for the temperature or nature of the build plate that may be incompatible with each other. This means those materials can never be combined successfully in the first layer of the manufacturing process, while processability may be possible above a certain critical height, where the thermal influence of the build plate is below a critical value. Furthermore, those combinations can therefore never have a vertically oriented interface throughout the whole part, since this would require combination of both materials in every layer. However, horizontal or tilted interfaces may still be possible to produce. Moreover, wettability is a crucial factor in compatibility of materials. As a minimal requirement for utilizing a certain material combination in the same process, wettability of one of the materials on the other needs to exceed a certain value. If the angle of contact is too big, the interface zone will inevitably fail, since only minor forces can be transmitted. This means that design measures like interlocking features have a chance to improve interface strength only if wettability is above a certain critical value. On top of that, a variety of environmental factors has high influence on bonding strength.