## 4 Discussion
The gap height analysis shows that the surface pattern of the side faces due to the stacked print layers results in the average gap height of the side-face contacts being almost twice as high as that of the bottom face contact, which explains the significantly lower contact ratio. The use of the side surface as a contact surface has another major disadvantage. Due to inhomogeneous component shrinkage in the binder-jetting process, the green parts have to be positioned in such a way that the z-axes from the printing process are aligned. Otherwise, the components do not shrink in the same direction, which causes a serious misfit and induces stresses that in turn have a negative effect on the joining result. Since the shrinkage in the printing plane is almost homogeneous, this disadvantage does not affect the top or bottom sides. The use of an initial contact force leads to a slight improvement in the contact ratio. The linear increase in the applied force does not show a linear response related to the contact ratio after sintering. Rather, the contact ratio approaches a maximum. The authors hypothesize that the general usage of a low contact force breaks local roughness peaks and thus increases contact, but beyond this, increasing the pressure does not yield a significant improvement. The simulation gives insights into the formation of the gap geometry that is formed by elastic and plastic deformation caused by an initial contact pressure. This contact pressure can occur deliberately or as a result of the production process of the green parts. The distribution of the gap height influences the sintering process, for not only do the contact cells form the sinter bridges between the two green parts, but the cells with a small gap height are relevant for the bond as well. During sintering, cells with a small gap height may form additional bonds between the two parts, strengthening the overall bond strength. It is also known from the MIM sector that further contacts can be created by so-called rolling and tilting processes of the powder particles during sintering [41]. It is found that for the upwards-facing surface, the amount of cells with a small gap height is much greater than the amount of small cells of the sidewards surface (compare Figure 17). This phenomenon might be responsible for the difference between the contact ratio of the simulation and the corresponding experiment (sinter joint after initial contact pressure). This leads to the conclusion that the contact ratio for the smoother surface is greater. It is concluded that the surface roughness is the dominant factor when it comes to comparing contact ratio. The deviation in planarity of the specimen surfaces was tracked down to the special curing process of the protoype printers, but can also occur in other devices due to a variety of reasons. Due to the differences in deformation between the edge area and the center of the build area, it is assumed that the generation of negative pressure for the extraction of the evaporated solvent through openings in the build platform is the cause of the deformations, since there are no extraction openings in the edge area on the build platform. The powder areas where binder has been introduced have a lower density than the surrounding powder bed (green part density \(\approx 4.5\,\mathrm{g/cm^{3}}\), powder bed density \(\approx 4.7\,\mathrm{g/cm^{3}}\)). Although the density of the green parts is lower than that of the surrounding powder bed, the authors assume that the introduced binder closes the pore network of the powder. Thus, the air can be drawn more easily through the non-wetted areas, which could lead to the free powder compacting more strongly than the bound powder in the outer edges of the parts when negative pressure is applied. In the edge areas of the part, this could result in a shear force that warps the parts, which are still uncured and easily deformable at this point.