# Review and New Aspects in Combining Multipoint Moulding and Additive Manufacturing
## 3 Possible Additive Manufacturing Methods
### Additive Manufacturing on Curved Surfaces
#### 4.2.1 Flat Slicing Methods  
Currently, most slicing algorithms divide the component in plane layers of the same height. These layers are oriented parallel to the construction platform [28]. Therefore, the positioning of the component in the building space is an important process parameter regarding to the surface quality of the finished part, as all edges of the part that are not oriented parallel or perpendicular to the building platform, will show a more or less pronounced staircase effect [30]. [PERSON] et al. [75], therefore, develop an improved calculation model to predict the surface quality more precise, depending on the position of the component in the building space. This model can be used to reduce the staircase effect and improve the surface quality. [PERSON] and [PERSON] [66] develop an improved slicing algorithm using an adaptive layer thickness. The ideal layer thickness is selected based on the current outer contour of the component. On the one hand, this makes a significant improve of the surface quality possible and at the same time allows a significant increase of the construction speed, compared to a uniform reduced layer thickness required to achieve the same surface quality. A similar approach is chosen by [PERSON] et al. [76] but allows the user to set limits so that he can determine the required layer thickness himself. [PERSON] et al. [77] also use an adaptive slicing process, but at the same time extend it by the additional possibility of varying the width of the extrusion strand, as shown in Figure 14. Whereas in the standard process, all extrusion strands have the same width and height (see Figure 14 top). The concept proposed in [PERSON] et al. [77] has the opportunity to use different layer heights and extrusion strand widths in one part. As shown in the bottom part of Figure 14, it is possible to produce finer walls of the part in combination with relatively rough extrusion strands inside the part. This increases the surface accuracy according to [PERSON] et al. [77] by 38-55% and simultaneously reduces production times by 53%. [PERSON] et al. [51], [PERSON] et al. [52], [PERSON] et al. [78], and [PERSON] et al. [79], on the other hand, develop a system called \"TruSurf\". \"TruSurf\" in the first step cuts layer edges at an angle according to the object contour instead of straight cuts (see Figure 15). In comparison of the standard slicing (see Figure 15, left) and the \"TruSurf\" slicing (see Figure 15, right), a significant increase in accuracy can be achieved. Moreover, this allows significantly greater layer thicknesses and thus shorter construction times. In addition, they extend the process that not only straight, but also curved cuts are possible and thus the contour accuracy is further increased. The variable curvature of the knife proves to be problematic. The method shows significant improvements in surface accuracy but has been developed for the laminated layer manufacturing and not yet been applied to extrusion processes. [PERSON] et al. [80] allow the theoretical proof of these advantages. [PERSON] et al. [81; 82] show another calculation method of the reduced form error for sloping boundaries compared to the standard way. On the other hand, [PERSON] and [PERSON] [83] show that despite these extensions, more layers are required to achieve the surface quality than the previous methods indicate. Figure 14: Layer configuration in standard fused filament fabrication and the process variation approach (based on the work in [77]). [PERSON] et al. [84] show another problem: the support structures required for overhangs. Support structures lead to a higher material requirement and have to be removed after production. They develop a method for creating these support structures directly in the CAD tool. This generates a continuous process that reduces the necessary support structures. [PERSON] et al. [85,86] choose another way and mount the building platform on a 6-axis robot underneath a fixed extruder.