From 3D Models to 3D Prints: an Overview of the Processing Pipeline <s> Different types of external supports serve different purposes: <s> Abstract This paper presents a robust algorithm to generate support for fused deposition modeling (FDM). Since many flaws appear in most stereo lithography (STL) models, this algorithm utilizes slice data as input. A top-down approach was used to calculate the support slice layer by layer. The generation algorithm was described in detail including the slice grouping, oriental bounding box (OBB) calculation, offsetting, and Boolean operations. Several cases are given to validate the efficiency and robustness of the procedure. The algorithm provides necessary support not only for hanging surface but also for hanging vertexes and edges with O ( n ) time complexity, where n is the number of layers. The algorithm fully utilizes the parts' self-support ability and reduces support volume to the maximum extent. This slice data based algorithm has the same efficiency as the STL based algorithm but is more stable, which significantly enhances the robustness of the support generation process. <s> BIB001 </s> From 3D Models to 3D Prints: an Overview of the Processing Pipeline <s> Different types of external supports serve different purposes: <s> Fused Filament Fabrication (FFF) is the process of 3D printing objects from melted plastic filament. The hot plastic exits a nozzle and fuses with the part just below, adding a layer of material to the object being formed. However, filament can only be deposited on top of an existing surface. Therefore, overhangs require a disposable support structure to be printed, temporarily supporting the threads of plastic that would otherwise hang in empty space. Existing techniques for support generation fall into two categories: The first allow for very reliable prints by enclosing the bottom of the object in a dense structure, at the expense of increased material usage and build times. The second generate thin hierarchical structures connecting to the surface in a sparse number of points. This uses less material, at the expense of reliability: the part might become unstable, the structure itself may become difficult to print, the bottom surface quality degrades. The user therefore has to correct the structure and its parameters for each new object. We propose to exploit the ability of FFF printers to print bridges across gaps. Since bridges are always supported by pillars at their extremities, they are both stronger and more stable than hierarchical tree structures. Our technique first selects the points to support based on overhang and part stability during the entire print process. It then optimizes for a printable scaffolding composed of bridges and vertical pillars, supporting all points. The result is an automated support generation technique using little material while ensuring fine surface quality and stability during the printing process. <s> BIB002
• Local deposition technologies can only deposit material on existing surfaces below. Thus, surfaces appearing mid-air and surfaces at an excessive overhang require support just below them. • Shapes may move or deform during the fabrication process. This typically happens when fabricated objects are imbalanced and when the raw material (powder, resin) cannot sustain the weight of the print. Another source of distortion are stresses from thermal gradients. To reduce these issues supports acting as fixtures are necessary. • Some processes can generate a large quantity of heat, in particular metal printing. This excessive heat accumulation results in shape distortions and residual stresses. In such a case, additional supports may act as heat diffusers. We next review approaches from the literature and discuss their use for each type of support. First, however, let us take a closer look at when and why supports are required. Islands, overhangs, and self-supporting surfaces. A first situation that requires support is illustrated in Figure 6 , left. After dividing the shape into layers, one of the slices contains an island -a solid region that appears while not being supported from below. During additive fabrication from bottom to top, the material forming the island will not attach to already solidified material. For technologies using material deposition, the material forming the island will fall. As a consequence, material deposited onto the next layer above will also fall, and this will cascade into a catastrophic failure. For technologies using layer solidification, non solidified material below (e.g. powder) will usually be able to support the island -however weight might accumulate over several layers and a heavy disconnected component may start to sink down. For technologies using resins (SLA), the island is problematic: it will typically end up floating in the viscous resin, cascading into a complete print failure. Overhangs also can produce problematic cases with material deposition. The material is typically added progressively along deposition paths. The right illustration of Figure 6 shows a cross section of the deposited paths for an overhang region. Due to the layering, at some excess overhangs the deposited material will simply fall (red rectangles in the Figure) . It is interesting to note, however, that until some threshold angle the deposited material will have a sufficient bonding surface with the layer below. This self-supporting property stems from the bonding between successive layers and allows for overhangs to exist up to some maximum angle without requiring support. Overhangs are less of an issue for technologies employing layer solidification -even though excessive overhangs may distort due to change in material properties during solidification, or to auxiliary motion during preparation of the next layers. The width of the difference determines which regions are selfsupporting  . This can also be detected conveniently and efficiently with two-dimensional morphological operations in image space BIB001   . As discussed in  care must be taken however with some specific configuration, where some protruding regions might be mis-classified as supported (see Figure 1 .8 in this publication). BIB002 performs the detection directly at the toolpath level, verifying whether each deposited segment is supported by at least half its width from below. This first analysis generally leads to a compact set of points to be supported. Several approaches then select a subset by downsampling [ER07, CLQ13, DHL14, HWC14]. Generating a support structure. Once the surfaces requiring support are determined, the support geometry is computed. The main trade-off is between print time, material use, and reliability. For instance, for filament printers the traditional approach consists in extruding the mesh facets requiring support downward, thus defining a large support volume. The support volume is usually printed with a weak infill pattern (see also Section 3.3.2.3). This still uses a significant amount of material and time, but it is very reliable: the support typically has a large area of contact with both the part and the print bed, ensuring the print stability in most cases. In this context several approaches modify the support volume to reduce its size. Huang Support removal. External support structures are meant to be detached from the part after the print is completed. Depending on the materials involved (e.g. printing on metal) this operation can be extremely challenging. Furthermore, residual material can remain attached to the surface, badly affecting the surface finish. In polymer AM processes, to alleviate the supports detaching problem, thermoplastic materials that dissolve in alkaline baths are used  . Hildreth et al.  have recently shown that similar approaches are possible even for stainless steel printing, were the differences in the electrochemical stability between different metals can be exploited to dissolve carbon steel supports. Jhabvala et al. [JBAG12] exploited the pulsed laser radiation to print metal supports that are both faster to print and easier to remove; the system supports only SLM printers. A valid alternative consists in trying to orient the part in such a way that the supports necessary to sustain it during the print will stick only to the least salient portions of the shape, as proposed in [ZLP * 15] (Section 3.2.2).