From 3D Models to 3D Prints: an Overview of the Processing Pipeline <s> Vector case <s> Abstract There are several manufacturing applications in which a tool needs to move along a prescribed path performing machining operations. The path is typically described by a sequence of curves. For the entire process, the tool must move along each curve exactly once. For typical paths, significant time may be wasted in the movement between the end point of one curve to the start vertex of the next one along which the tool must operate. Normally, this non-machining motion is a straight-line motion. A good process plan would minimize the time wasted on such motion. An excellent application of this problem is found in the increasingly popular Layered Manufacturing (LM) methods. We first introduce a Genetic Algorithm (GA)-based approach to solve this problem. Next, we present a new strategy using a combination of the Asymmetric Traveling Salesman Problem and Integer Programming (TSP-IP) to solve it. Based on the pros and cons of these approaches, two enhanced GA formulations are developed. We compare the performance of the different techniques, with a view to their application to real-time path planning in LM applications. <s> BIB001 </s> From 3D Models to 3D Prints: an Overview of the Processing Pipeline <s> Vector case <s> Compared to conventional manufacturing processes, fused deposition (FD)‐based layered manufacturing processes have dramatically reduced the part design and manufacturing time. However, it is necessary to further enhance their process productivity. To this end, improvement of FD process efficiency is studied in this paper. A build time analysis is conducted and the deposition parameters that can be used to speed up fabrication processes are identified. The tool path‐based deposition planning approach is extended for ensuring layer quality when the build process is expedited under adjusted deposition parameters. A test part was built to demonstrate the proposed approach. <s> BIB002 </s> From 3D Models to 3D Prints: an Overview of the Processing Pipeline <s> Vector case <s> This paper describes an algorithm for minimizing the non-productive time or ‘airtime’ for milling by optimally connecting different toolpath segments. This problem is formulated as a generalized traveling salesman problem with precedence constraints and is solved using a heuristic method. The performance of the heuristic algorithm and the amount of improvement obtained for different problem sizes is also presented. This algorithm has been implemented in an automated process planning system and can be applied easily to other areas of path planning optimization like fused deposition modeling and laser cutting. <s> BIB003 </s> From 3D Models to 3D Prints: an Overview of the Processing Pipeline <s> Vector case <s> We present a detailed description of a zigzag algorithm for pocket machining. The algorithm is capable of computing correct zigzag tool paths for multiply-connected planar areas (“pockets”) bounded by a wide class of curves. It features a number of optimizations with respect to geometrical and technological objectives. In particular, a near-optimum inclination of the tool path is automatically determined. The underlying geometric principles are simple enough to allow the algorithm to be included in a numerical control computer. <s> BIB004 </s> From 3D Models to 3D Prints: an Overview of the Processing Pipeline <s> Vector case <s> The book presents new optimization algorithms designed to improve the efficiency of tool paths for five-axis NC machining of sculptured surfaces. The book introduces the reader to fundamental issues involved in the tool path planning such as the kinematics of five-axis machines, types of 5 axis machines, part surface representation, machining strips, optimal tool orientation, gouging avoidance and forward step error. It also introduces new methods of optimization based on research conducted by the authors, including schemes performed in the spatial domain, angular domain as well as procedures to optimize the initial setup. The book can be used by undergraduate and graduate students and researchers in the field of NC machining and CAD/CAM as well as by corporate research groups for advanced optimization of cutting operations. <s> BIB005 </s> From 3D Models to 3D Prints: an Overview of the Processing Pipeline <s> Vector case <s> One of important researches in Rapid Prototyping (RP) is to optimize the path planning which affects the efficiency and building quality of RP system. But it is very difficult to solve its optimization by traditional methods. Genetic algorithms (GAs) are excellent approaches to solving these complex problems in optimization with difficult constraints. The classic path-planning optimization problem has been shown to be a NP-complete problem. To obtain optimized boundary path result, a genetic algorithm (GA) is developed and implemented. The GA features optimized chromosome code, constrained crossover operator, constrained mutation operator and fitness evaluation function. Finally, experiments of the optimized boundary path based on the GA are conducted with satisfactory results. <s> BIB006 </s> From 3D Models to 3D Prints: an Overview of the Processing Pipeline <s> Vector case <s> This paper presents an algorithm to automatically generate optimal tool-paths for the wire and arc additive manufacturing (WAAM) process for a large class of geometries. The algorithm firstly decomposes 2D geometries into a set of convex polygons based on a divide-and-conquer strategy. Then, for each convex polygon, an optimal scan direction is identified and a continuous tool-path is generated using a combination of zigzag and contour pattern strategies. Finally, all individual sub-paths are connected to form a closed curve. This tool-path generation strategy fulfils the design requirements of WAAM, including simple implementation, a minimized number of starting-stopping points, and high surface accuracy. Compared with the existing hybrid method, the proposed path planning strategy shows better surface accuracy through experiments on a general 3D component. <s> BIB007 </s> From 3D Models to 3D Prints: an Overview of the Processing Pipeline <s> Vector case <s> Rapid prototyping and product customization have become more convenient with the emergence of 3D printing technologies. In extrusion deposition based 3D printing, objects are built by connecting many lines of filament, layer by layer. The efficiency of the printing process can be improved by optimizing motion paths of the printing nozzle. In this paper, a 3D printing path optimizer based on Christofides algorithm is proposed. Experiment results show that the proposed optimizer can significantly reduce the length of motion paths compared to a nearest neighbor-based optimizer using in consumer 3D printers. <s> BIB008 </s> From 3D Models to 3D Prints: an Overview of the Processing Pipeline <s> Vector case <s> Three dimensional (3D) printing can be used to manufacture many different objects range from toys to hi-tech robot parts. This paper investigates 3D printer trajectory planning to improve the speed of the printing process. The printing speed mainly depends on the motion speed and path of the printing nozzle. We use triangular and trapezoidal velocity profiles to minimize the transition time between print segments. In this work, several algorithms that were originally proposed as solutions for conventional traveling salesman problem are modified to adapt to the new problem. The proposed modifications are designed to obtain time-efficient trajectories for the printing nozzle. <s> BIB009
In the vector case the machine instructions amount to a piecewise linear toolpath (in some cases arcs) along which the printer must deposit, melt or sinter the printing material. The machine toolpath must be prepared for each slice, both for its outer contour and the interior. If the part comes in the form of a boundary representation (e.g. a STL file), particular attention must be paid to distinguish between the interior of and the exterior of the shape [VFLS13]. The generation of a machine toolpath for AM has a clear analogy with CNC pocket milling, where the material inside an arbitrarily closed boundary on a flat surface is removed to a fixed path BIB005 BIB004 . In the general case a machine toolpath intended for milling is, however, not suitable for additive manufactur- ing. In FDM the deposition of material along the path poses some additional challenges. The path has to be designed in such a way that the deposition of material is as regular as possible, thus avoiding under and over deposition. For the same reason the overlap between adjacent paths needs a much finer control, in order to cover the slice area with a uniform layer of deposited material BIB002 ( Figure 16 ). In powder bed technologies such as laser sintering and melting the heat control is fundamental to guarantee quality results. Many printers generate multiple melting pools at a time, in order to better distribute heat and avoid huge thermal gradients, that would generate rough surfaces and warping BIB007 . We discuss here the requirements for a quality machine toolpath and the major differences between available algorithms and technologies. 3.5.1.1. Continuity In FDM and other material deposition processes it is important to keep the amount of material deposited along the path constant. To this end, many authors observed that controlling the amount of material being deposited when a path begins or ends is very difficult. Recent works aim to reduce as much as possible the number of disconnected paths which are necessary to cover the slice area, so as to minimize discontinuities in the deposition process [ZCG * 16]. 3.5.1.2. Geometry Not only the endpoints of a machine path but also the geometry of the path itself may affect the quantity of material locally deposited in FDM. Long and low curvature paths are to be preferred to short paths with sharp turns. In the latter case the speed of the nozzle would decrease, thus increasing the time necessary to complete the deposition and possibly triggering under or overfilling of the filament [JHF * 14]. 3.5.1.3. Patterns Internal volumes of parts are a major factor in time and material consumption, in particular for FDM where the motion of the print head -a relatively heavy mechanical device -is slow. To save time and material, several infill patterns have been proposed. These patterns are usually specific to FDM due to the aforementioned continuity and geometry requirement, but also due to the overhang constraints. See Section 3.3.2.3 for an in-depth discussion of sparse infill patterns. 3.5.1.4. Domain split In order to be able to process arbitrarily complex slices, most of the approaches proposed in the literature use a divide-and-conquer strategy, partitioning the slice into a set of pockets to operate on, rather than trying to cover the whole area with a single connected curve. 3.5.1.5. Performances Besides the method used to decompose the domain and the particular curves used to fill each pocket, another important factor in the definition of an efficient toolpath is the reduction of the so called machine airtime, that is, the time necessary to move the nozzle (or the laser) from the end of a curve to the beginning of the subsequent one. Given a toolpath composed of a set of disconnected curves, the machine airtime can be minimized by acting on to three separate variables: the order in which the curves are processed, the path that takes the nozzle from the end of a curve to the beginning of the next one, and the orientation of each curve (from begin to end, or from end to begin). For the latter, notice that for closed loops, the machine may chose any point within the loop as a starting point. The path planning optimization problem has been shown to be related to the Travelling Salesman Problem (TSP), which is NP-complete. Many methods have been proposed for its approximate solution, using genetic algorithms BIB006 BIB001 , the Christofides algorithm BIB008 BIB009 or other heuristics BIB003 .