From 3D Models to 3D Prints: an Overview of the Processing Pipeline <s> Non-planar approaches <s> Abstract This paper proposes a multi-material virtual prototyping system for digital fabrication of heterogeneous prototypes. It consists mainly of a topological hierarchy-sorting algorithm for processing slice contours, and a virtual simulation system for visualisation and optimisation of multi-material layered manufacturing (MMLM) processes. The topological hierarchy-sorting algorithm processes the hierarchy relationship of complex slice contours. It builds a parent-and-child list that defines the containment relationship of the slice contours, and subsequently arranges the contours in an appropriate sequence, which facilitates toolpath planning for MMLM by avoiding redundant tool movements. The virtual simulation system simulates MMLM processes and provides stereoscopic visualisation of the resulting multi-material prototypes for quality analysis and optimisation of the processes. <s> BIB001 </s> From 3D Models to 3D Prints: an Overview of the Processing Pipeline <s> Non-planar approaches <s> This paper proposes a topological hierarchy-based approach to toolpath planning for multi-material layered manufacturing (MMLM) of heterogeneous prototypes. The approach facilitates control of MMLM and increases the fabrication efficiency of complex objects by generating multi-toolpaths that avoid redundant tool movements and potential collisions. It uses a topological hierarchy-sorting algorithm to group complex multi-material slice contours into families connected by a parent-and-child relationship. Subsequently, a sequential toolpath planning algorithm generates multi-toolpaths for sequential deposition of materials without redundant tool movements. To reduce build time further, a concurrent toolpath planning algorithm generates collision-free multi-toolpaths to control the tools that deposit materials concurrently. It uses parametric polygons to construct tool envelopes for contour families of the same material property to simplify detection of tool collisions. The tightness of polygons can be controlled to suit the processing speed and the optimality of the resulting concurrent toolpaths. The proposed approach has been implemented as an integral part of a multi-material virtual prototyping (MMVP) system that can process complex slice contours for planning, stereoscopic simulation, and validation of multi-toolpaths. It may be adapted for subsequent control of MMLM processes. <s> BIB002 </s> From 3D Models to 3D Prints: an Overview of the Processing Pipeline <s> Non-planar approaches <s> This paper presents an approach to concurrent toolpath planning for multi-material layered manufacturing (MMLM) to improve the fabrication efficiency of relatively complex prototypes. The approach is based on decoupled motion planning for multiple moving objects, in which the toolpaths of a set of tools are independently planned and then coordinated to deposit materials concurrently. Relative tool positions are monitored and potential tool collisions detected at a predefined rate. When a potential collision between a pair of tools is detected, a dynamic priority scheme is applied to assign motion priorities of tools. The traverse speeds of tools along the x-axis are compared, and a higher priority is assigned to the tool at a higher traverse speed. A tool with a higher priority continues to deposit material along its original path, while the one with a lower priority gives way by pausing at a suitable point until the potential collision is eliminated. Moreover, the deposition speeds of tools can be adjusted to suit different material properties and fabrication requirements. The proposed approach has been incorporated in a multi-material virtual prototyping (MMVP) system. Digital fabrication of prototypes shows that it can substantially shorten the fabrication time of relatively complex multi-material objects. The approach can be adapted for process control of MMLM when appropriate hardware becomes available. It is expected to benefit various applications, such as advanced product manufacturing and biomedical fabrication. <s> BIB003 </s> From 3D Models to 3D Prints: an Overview of the Processing Pipeline <s> Non-planar approaches <s> In layered manufacturing, geometrical gap occurs between the computer-aided design (CAD) model and the fabricated, due to the stair-stepping effect. It will deteriorate the surface quality of the microstructure. By reducing the stair steps, the surface quality of a layered manufactured microstructure can be improved. In this research, dithering method is introduced to improve the surface quality of a microstructure in projection microstereolithography. The dithering method can represent grayscale effect with monochrome image, so it is applicable to the digital micromirror device for ultraviolet, which allows monochrome image as an input format. By adjusting the beam intensity using the dithered image, it is possible to control the cure depth of the resin within a layer. In a cross-section, the region to be dithered is obtained by the difference of the cross-sections between the current layer and the previous layer. It is called a compensation area (CA) in this research. The gray level to be applied is determined considering the CAD model and the curing characteristics of the resin. The dithered CA is then combined with the current cross-section. The parabola-shaped microstructures, using dithered and non-dithered cross-sectional images, have been fabricated and compared. The dithering method has shown good performance for improving surface quality and reducing the fabrication time. <s> BIB004 </s> From 3D Models to 3D Prints: an Overview of the Processing Pipeline <s> Non-planar approaches <s> Abstract Fused deposition modeling (FDM), one of the rapid prototyping techniques is a promising technology for rapid manufacture of end use parts direct from CAD files and with the proliferation of cheaper machines, is likely to play a vital role in future polymer processing, challenging traditional processes such as injection moulding in some cases. Research evidence suggests that the road and layer structures would have significant influences on the mesostructure and consequent mechanical behaviour of the resulting polymer part. While adaptive slicing and other deposition schemes have been attempted for different reasons, it is believed that an appropriate deposition scheme is essential to ensure the best inter-road and interlayer connectivity, resulting in a continuous network of polymer chains, as in the case of the traditional processes. The current research proposes the curved layer deposition for FDM, in particular for thin shell-like parts, to ensure fibre continuity. Mathematical models are developed for curved slicing, implemented in a few case studies, parts are printed, and test results suggest marked improvement in the mechanical characteristics of curved layer parts. <s> BIB005 </s> From 3D Models to 3D Prints: an Overview of the Processing Pipeline <s> Non-planar approaches <s> 3D printing hardware is rapidly scaling up to output continuous mixtures of multiple materials at increasing resolution over ever larger print volumes. This poses an enormous computational challenge: large high-resolution prints comprise trillions of voxels and petabytes of data and simply modeling and describing the input with spatially varying material mixtures at this scale is challenging. Existing 3D printing software is insufficient; in particular, most software is designed to support only a few million primitives, with discrete material choices per object. We present OpenFab, a programmable pipeline for synthesis of multi-material 3D printed objects that is inspired by RenderMan and modern GPU pipelines. The pipeline supports procedural evaluation of geometric detail and material composition, using shader-like fablets, allowing models to be specified easily and efficiently. We describe a streaming architecture for OpenFab; only a small fraction of the final volume is stored in memory and output is fed to the printer with little startup delay. We demonstrate it on a variety of multi-material objects. <s> BIB006 </s> From 3D Models to 3D Prints: an Overview of the Processing Pipeline <s> Non-planar approaches <s> Fused Filament Fabrication (FFF) is an additive manufacturing (AM) method that relies on the thermal extrusion of a thermoplastic feedstock from a mobile deposition head. Conventional FFF constructs components from stacks of individual extruded layers using tool paths with fixed z-values in each individual layer. Consequently, the manufactured components often contain inherent weaknesses in the z-axis due to the relatively weak thermal fusion bonding that occurs between individual layers, as well as poor surface finish in shallow sloped contours. This study demonstrates the use of Curved Layer FFF (CLFFF) tool paths in tandem with a commercially available parallel, or delta, style FFF system to allow the deposition head to follow the topology of the component. By incorporating a delta robot and CLFFF tool paths in this way, improvements in the surface finish of the manufactured parts has been observed, and time costs associated with Cartesian robot based CLFFF manufacturing have been notably reduced. Furthermore, employing a delta robot provides additional flexibility to CLFFF manufacturing and increases the feasibility of its application for advanced manufacturing. The study has also demonstrated a viable approach to multi-material FFF by decoupling support structure and part manufacture into regions of CLFFF and static z tool pathing in an appropriate fashion. <s> BIB007 </s> From 3D Models to 3D Prints: an Overview of the Processing Pipeline <s> Non-planar approaches <s> We demonstrate a new approach for designing functional material definitions for multi-material fabrication using our system called Foundry. Foundry provides an interactive and visual process for hierarchically designing spatially-varying material properties (e.g., appearance, mechanical, optical). The resulting meta-materials exhibit structure at the micro and macro level and can surpass the qualities of traditional composites. The material definitions are created by composing a set of operators into an operator graph. Each operator performs a volume decomposition operation, remaps space, or constructs and assigns a material composition. The operators are implemented using a domain-specific language for multi-material fabrication; users can easily extend the library by writing their own operators. Foundry can be used to build operator graphs that describe complex, parameterized, resolution-independent, and reusable material definitions. We also describe how to stage the evaluation of the final material definition which in conjunction with progressive refinement, allows for interactive material evaluation even for complex designs. We show sophisticated and functional parts designed with our system. <s> BIB008
There has been a number of interesting attempts at moving beyond standard layers for additive fabrication. These attempts usually focus on a specific technology since they exploit properties such as the ability to perform Z-motion during deposition (fused filament fabrication) or the ability to partially cure material (SLA/DLP). In the context of stereo-lithography (SLA) Pan et al. [PZZC12, PC15] exploit the formation of a meniscus when an object moves out of the resin tank. The meniscus is cured to fill the creases between two layers. Repeating this process produces smooth, accurate surfaces. Park et al. BIB004 show how dithering can cure resin partially and produce slanted surfaces along a layer. In the context of filament deposition, Chakraborty et al. [CRC08] proposed study curved layer deposition with the objective of strengthening shell-like parts by aligning the toolpaths with the surface. The mechanical properties of the parts are discussed in BIB005 . An interesting question is then to combine flat and curved layers, as discussed in  BIB007 . A key challenge of curved layers deposition is that the curvature of the paths is large as they flow along the surface several millimetres up and down. This has been demonstrated for specific parts, however in a general setting this degree of freedom is challenging to exploit. First because a novel type of slicers have to be developed, and second because the current design of deposition nozzle complicates the task: collisions between already printed paths and currently deposited paths become possible [CRC08, AT15]. Beam and truss structures are not very well suited for layer by [WPGM16] propose a more general approach using a 5DOF printer (see Figure 14) Machines based on filament extrusion can mount multiple extrusion heads, in which case motion planning during deposition becomes more complex. Choi and Cheung BIB001 input a triangle mesh per material (packed in a single colored STL file) that are sliced independently. The contours in each layer are grouped by inclusion order, so as to reduce redundant motions during deposition. This is later extended to motion planning for multiple, independent deposition devices. The challenge is to coordinate the movements of the nozzle depositing different materials, so as to avoid collisions or even deadlocks BIB002 BIB003 . In the context of computer graphics, Hergel and Lefebvre  also consider the case of slicing and toolpath planning for multiple materials. Instead of inputing multiple meshes, materials are selected by a slice shader which is executed on every point of each layer. Contours are extracted using an image space approach (see Section 3.4.2.2). The motion planner is optimized for visual quality, hiding potential defects in less visible regions of the part. Reiner et al. [RCM * 14] achieve a visual grading of colors on multi-filament printers, interleaving the deposited filaments in sine wave patterns along the surface (see Figure 15 ). Some technologies apply colors on entire layers (inkjet on powder  and inkjet on laminated paper [Mco05]). At slicing time, a raster RGB color image is applied to the layer contour. Since the colors are often specified from the surface (e.g. vertex colors or 2D texture map), they are propagated inside from the contour within a thin shell. However, the technology would be able to color the entire volume, even though opacity of the material limits the potential. Multi-jet technologies, the deposit droplets of different resins -have a wider variety of materials and colors. Open-FAB BIB006 allows to models complex multi-material geometries with voxels, and streams slices to a high-resolution multi-jet printer. This forms the basis of a complete modeling system for multi-material modeling BIB008 