Additive manufacturing enables CAD-models to be directly manufactured without the need of forming tools. Without toolmaking, additive manufacturing processes are highly flexible and have quasi-unrestricted constructive freedom [11]. Fused-deposition-modeling (FDM) and selective-laser-sintering (SLS), as seen in Fig. 1, are two commonly used additive manufacturing processes.
The FDM process uses plastic filaments which are melted and deposited on a building platform by a heated nozzle. The building platform is then lowered down for the deposition of the next layer and repeated [12, 13]. In the SLS process, material in powder form is processed. The powder covers the entire build platform and is formed into a layer by a laser in a sintering process. To create the next layer, the build platform is lowered and a pusher is used to distribute fresh powder from a reservoir onto the lowered build platform. The next layer is produced by sintering the powder over again. [14]
Direct screwing refers to screwing processes in which the screw forms a nut thread in the joining partner when it is screwed in. Plastic direct screwing requires special screws whose geometric characteristics differ from those of metric screws. The main difference lies in the design of the thread flank, which has a small flank angle of 30° or less compared to the usual 60° for metric screws. Alternatively, asymmetrical shaped thread flanks are available. Asymmetrical thread flanks consist of a flat angle of 15° at the top to improve load-bearing capacity. Furthermore, they consist of a radius profile at the bottom which promote the displacement and flow of plastic [15, 16].
The joining process is based on torques and time for the screw-in process respectively. Characteristic features of the joining process are the screw-in torque, screw’s headrest, tightening torque, and over-torque as seen in Fig. 2 [17, 18].
While forming the first thread, the tapping torque increases. Further screwing results in increased friction between screw and material with every imbedded thread. The friction is displayed by the thread friction torque which increases linearly. The screw-in torque (\({T}_{S}\)) is corelates with the headrest of the screw. After the headrest, further turning results in a drastic increase of the torque. If the screw is turned too far, the joint fails due to bursting of the dome from shearing of the thread or screw breakage. At this point, the torque is at its maximum: over-torque (\({T}_{O}\)). The tightening torque (\({T}_{T}\)) can be determined from the over-torque and screw-in torque by the following functional equation [8, 17].
$${T}_{T}={T}_{S}+\frac{1}{3}\dots \frac{1}{2}({T}_{O}-{T}_{S})$$
Based on the DVS-2241 standard, a range for the tightening torque between the two factors is recommended for sufficient safety against failure during the joining in a real process.
There are limited publications on the combination of the two technologies. In a publication by the German automotive manufacturer, Daimler, and the German University of Duisburg-Essen [19], screw tests were carried out on domes composed of PA12 using the SLS process. For this purpose, Fieger et al. manufactured samples following the DVS 2241standard and used a screw with two different diameters. Furthermore, the geometry was varied in size. The screw-in and over-torque are within a suitable range for the automotive industry and a reproducible tightening process is given. Additionally, the study showed additively manufactured screw domes can be screwed five time repeatedly without failure. It is not clear whether the influence of orientation of the domes on the building platform was considered. In a publication by the KTP of the University of Paderborn [6], domes made of ABS using the FDM process were investigated. The domes were manufactured on edge (0°), diagonally (45°) and horizontally (90°) on the building platform as seen in Fig. 3.
For the geometry, the DVS 2241 standard was followed. The study showed that the alignment on the building platform has a significant influence on the strength of the connection. According to Moritzer et al, the lowest strength is at 0° and the highest at 90.
New findings from a recent publication [7] of the KTP, which can be regarded as the basis of this publication, show that the degree of anisotropy of the additive manufacturing processes plays a significant role in the achievable strengths. Thus, significant differences between the individual additive processes could be identified (Fig. 4) dependent on the process.
As the FDM process deposits individual layers from a filament, strength differences between the build space orientations are present. This is not present for the SLS process. All build space orientations have nearly identical strength values. The difference lies in the anisotropy of the components which is strongly influenced by the process. Unlike the FDM process, the layer structure in the SLS process is created by heating powder with a laser. As a result of the sintering process, the components are more porous than FDM components, but they are significantly more isotropic. Hence, it is nearly impossible to distinguish between individual layers. As a result, less delamination occurs in SLS parts than in FDM parts and a more uniform strength is measured between the three build orientations.