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Robot assisted additive manufacturing: A review
Robotics and Computer Integrated Manufacturing 59 (2019) 335–345
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Robotics and Computer Integrated Manufacturing journal homepage: www.elsevier.com/locate/rcim
Full length Article
Robot assisted additive manufacturing: A review Pinar Urhal, Andrew Weightman, Carl Diver, Paulo Bartolo
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T
School of Mechanical Aerospace and Civil Engineering, The University of Manchester, UK
ARTICLE INFO
ABSTRACT
Keywords: 3D printing Additive manufacturing Multi-axis printing Robots
The additive manufacturing and the robotic applications are tremendously increasing in the manufacturing field. This review paper discusses the concept of robotic-assisted additive manufacturing. The leading additive manufacturing methods that can be used with a robotic system are presented and discussed in detail. The information flow required to produce an object from a CAD model through a robotic-assisted system, different from the traditional information flow in a conventional additive manufacturing approach is also detailed. Examples of the use of robotic-assisted additive manufacturing systems are presented.
1. Introduction Manufacturing is a process through which different raw materials are transformed into final products through the use of different fabrication methods [1]. In recent years, new manufacturing methods, such as additive manufacturing and advanced robotics, were developed and increasingly used by different industrial sectors, radically changing the way products are made. Additive manufacturing (AM) describes a group of processes that produce objects by depositing materials in a layer-by-layer way [2]. This technology, which is in its infancy, emerged in the late 1980s under the name of rapid prototyping. It was initially used to produce conceptual models to discuss design ideas, for form and fit applications or the production of architectural or anatomical models [3]. Materials were limited to few polymers, ceramics and metals [4]. Gradually the technology developed from rapid prototyping to rapid tooling allowing the direct or indirect fabrication of tools for injection moulding, thermoforming or blow moulding applications or for the fabrication of electrodes for electrical discharge machining applications [5]. Finally, additive manufacturing moved from rapid prototyping or rapid tooling to rapid manufacturing enabling the fabrication of final and fully functional products [6].Currently, additive manufacturing comprises seven different techniques (Table 1) enabling to process all types of materials including biological (cells and biomolecules), smart and functionally graded materials [7–8]. Through additive manufacturing, it is possible to produce an object of virtually any shape without the need of tooling [10–11]. Complex objects can be produced in one single process step, eliminating production steps and accelerating time to market with marginally increasing production costs. Moreover, additive manufacturing disrupts ⁎
the traditional supply chain, allowing for products to be produced closer to the point of use at the time of need, which limits material waste and improves both economies of scale and lead time [12]. It also dramatically reduces the time between design creation and prototyping by reducing the effort and the scheduled impact caused by iterative design and by increasing organisational alignment to accelerate decision-making [13]. Additive manufacturing has been successfully applied to a wide range of sectors including fashion, aerospace, aeronautics and defence, healthcare and construction [14]. Although recent developments show the increase use of AM systems for the fabrication of large-scale structures, most of the commercially available AM machines are three-axis Cartesian coordinate robots or gantry systems with limited construction platform dimensions [15–16]. Depending on the object shape and dimensions, support structures can be required which is increasing the fabrication time, material consumption and fabrication costs [17]. Moreover, the three-axis additive manufacturing machines are associated with a strict layer-by-layer fabrication approach creating objects with a typical stair-step effect [18]. Apart from all the benefits and drawbacks of conventional additive manufacturing applications, multi-axis robot-manipulated manufacturing methods which are widely used for welding and pick-andplace tasks, offer better quality and consistency, maximum productivity, greater safety for repetitive tasks and reduced labour costs [19–21]. The flexible functionality of robots serves the dynamic demands of manufacturing [22]. The combined use of multi-axis robot systems and additive manufacturing technologies offers the possibility for multi-axis additive manufacturing and the fabrication of complex geometries in different manufacturing environments. This review paper discusses the concept of multi-axis robot assisted additive manufacturing. This is an emergent
Corresponding author. E-mail address: [email protected] (P. Bartolo).
https://doi.org/10.1016/j.rcim.2019.05.005 Received 12 July 2018; Received in revised form 5 May 2019; Accepted 6 May 2019 Available online 14 May 2019 0736-5845/ © 2019 Published by Elsevier Ltd.
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area significantly growing. The leading printing principles that can be used with a robotic system are presented and discussed in detail. The information flow required to produce an object from a CAD model through a robotic-assisted system, different from the traditional information flow in a conventional additive manufacturing approach is also detailed. Examples of the use of robotic-assisted additive manufacturing systems are presented. Finally, research challenges and future perspectives are presented and discussed.
The majority of robotic-assisted extrusion-based systems are being used to produce freeform organic shapes, building construction elements for design purpose and usually large objects. Table 2 summarises the main materials and printing principles used by robot-assisted extrusion additive manufacturing. Zhang et al. [27] used an ABB robot to control a filament-based extrusion printing head (Fig. 1a). A computational platform called Robot Studio (developed by ABB) was used to allow process fabrication simulation, optimisation and visualisation (Fig. 1b). ABS parts were produced (Fig. 1c), but the system can be extended to other materials such as the use of carbon fibres to produce composite parts. Similarly, Wu et al. [28] used the 6-DOF UR3 robotic arm to produce polymeric parts from polylactic acid (PLA) in a filament form, with minimal or no support structures. A computational tool was developed in C++ to decompose the models into multiple support-free sub-models, printed in a collision-free sequence. Printing directions are determined by the software based on the decomposition strategy. Researchers from The University of Michigan (USA) used a robotic extrusion system to create a monolithic elastic net using a thermoplastic elastomer (TPE). The screw assisted extruder head was mounted on a seven-axis KUKA robot and the material, contrary to a FDM-like system, was supplied in the form of pallets providing high versatility in terms of the range of materials to be used, features definitions and material
2. Extrusion-based process Material extrusion is an additive manufacturing technique in which material is selectively deposited through a nozzle or orifice [23]. This technology developed by Scott Crump under the name of Fused Deposition Modelling (FDM) creates parts by extruding material (normally a thermoplastic polymer in a filament form) through a nozzle controlled by a computer into an XY platform [24]. After Crump's initial work the technique was further developed being able to process materials not only in a filament form but also in a pallet form [2]. Currently, the extrusion heads are classified as pressure-assisted and screw-assisted techniques enabling to print a wide range of polymeric materials, polymer-based composites, concrete, food and biological materials [25–26]. Table 1 Additive manufacturing methods [9].
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Table 1 (continued)
properties (Fig. 2). The system was developed as part of the Infundibuliforms project aiming to develop lightweight kinetic surfaces to create reconfigurable spatial enclosures with minimal amounts of material [29]. The high degree of freedom of the robotic systems allows the fabrication of complex objects without support structures, which improves accuracy, reduces post processing steps and minimises material waste. Other relevant examples include parts produced by the ABB-Robotstudio project (Fig. 3a) [30] and by the Institute for Advanced Architecture of Catalonia (Spain) [31] using an ABB 2400 L robotic arm (Fig. 3b). In the latter case, the system operates with thermosetting polymers and the printing head comprises two reservoirs containing two different chemical materials that are pumped into a mixing nozzle prior extrusion. During the mixing process, the two materials start a chemical reaction increasing its viscosity. Similarly, researchers from the Politecnico Di Milano (Italy) inspired by the pultrusion technique to create composite structures, used a six-axis robotic arm to print continuous fibres pre-impregnated with a thermosetting
resin. The curing process of the resin is performed by applying light using an optical fibre attached to the robotic arm. Due to the size and shape of building elements, robotic-assisted extrusion has been extensively explored in the fields of architectural design and construction using a wide range of materials. Inspired by the snails’ shell building process, Felbrich et al. [32] used an extrusion printing head mounted on a 6DOF robotic arm to produce freeform thermoplastics shells at an architectural scale. Craveiro et al. [33] used an ABB robot and two pumps working together to create functionally graded concrete elements using different combinations of concrete and cork (Fig. 4). The aim was to develop building elements with improved thermal efficiency, minimising the amount of concrete and reducing the environmental impact of the construction elements. The code was generated using HAL, a visual programming language grasshopper plug-in, combining the tool path information with the required material composition. Similarly, Gosselin et al. [34] developed a system based on an extrusion print head mounted on an ABB 6620 6-axis robotic arm. 337
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Table 2 Robotic-assisted extrusion based systems. Project name
Primary materials
Printing principles
Robotic additive manufacturing process simulation [27] RoboFDM: A robotic system for support free fabrication [28] Infundibuliforms: Kinetic systems [29]
Filament form polymer Filament form polylactic acid (PLA)
6 DOF ABB robot arm coupled extrusion printing head 6 DOF UR3 robot arm coupled with extrusion head
Pallet form of plastic and rubber mixture (Thermoplastic elastomer-TPE) Clay and ceramic mixture Thermosetting polymer
7-axis KUKA robot arm coupled with screw assisted extruder ABB robot arm coupled with extruder head ABB 2400 robot arm coupled with an extruder head with two reservoirs and a mixer nozzle 6 DOF robot arm coupled with extrusion head
Materially informed design to robotic manufacturing [30] Anti-Gravity Additive Manufacturing [31] A novel rapid AM concept for architectural composite shell construction [32] A design tool for resource efficient fabrication [33]
Thermoplastic polymer
Large scale 3D printing of ultra-high performance concrete [34] Robotic multi-dimensional printing [35]
Concrete ABS plastic
Large scale 3D printing [36] Mobile robotic fabrication [37] Large scale printing with cable-suspended robot [38]
Concrete Filament form fibre composite Polyurethane foam
Concrete and cork mixture
The system includes two peristaltic pumps, one for the pre-mix and another for an accelerating agent and a premix mixer (Fig. 5). The fabrication process includes two main steps. First, a mortar pre-mix with appropriate rheological behaviour is prepared and kept in a syringe mixer. Then the pre-mix is pumped to a screw using a peristaltic pump.
ABB robot arm coupled with an extruder head with two pumps ABB 6620 6-axis robot arm coupled with an extruder head with two pumps 6-axis KUKA robot arm coupled with an extruder head with multiple nozzles Small mobile robots and a lightweight extrusion system Small mobile robots and a lightweight extrusion system 6 DOF Cable suspended robot
Robotic-assisted fabrication systems are also opening new routes for in situ fabrication expanding the robotic fabrication process behind the constraints of the production platform [39–41]. Inspired by the
Fig. 3. (a) Ceramic printing for ABB Robotstudio project [30] (b) Thermosetting polymer printing [31].
Fig. 1. (a) Filament-based printing head controlled by an ABB robotic arm; (b) RobotStudio platform; (c) Printed polymeric part [27].
Fig. 4. Concrete-cork AM System [33]: (a) Robot controller, (b) ABB teach pendant, (c) Arduino developed module, (d) Functionally graded extruded component, (e) Concrete pumps, (f) ABB robot.
Fig. 2. FDM-like system with a seven-axis KUKA robot arm, (b) FDM printing head, (c) TPE net deposition [29]. 338
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Fig. 8. (a) Building process of multi-robotic system, (b) Diagram of the process [37].
Fig. 5. Robotic Extrusion Printing System [34]: (a) 6-axis robotic arm, (b) peristaltic pump for pre-mix, (c) peristatic pump for accelerating agent, (d) premix mixer, (e) screw assisted print head.
Fig. 6. (a) Robotic system with 6-axis KUKA arm (b) Printing head for main and auxiliary curves [35]. Fig. 9. (a) Cable robot system, (b) Final foam product [38].
followed by grip robots that are clamped onto the footprint and extend the structure (Fig. 7b). Finally, a vacuum robot moves over the printed structure and reinforces it by applying additional layers of material (Fig. 7c). At the University of Stuttgart (Germany) [37] researchers proposed a multi-robot system of cooperative mobile machines operating within the context of the surfaces of existing architectural elements such as façets, walls and ceilings, anchoring new tensile filament structures to these surfaces (Fig. 8). This project uses low payload agile machines, which can be easily used to print different types of materials by incorporating a lightweight extrusion printing head. Cable-suspended robotic systems are also being used to produce large-scale parts. An example is the research conducted at the University Laval (Canada) [38] that used a six DOF cable suspended robot to produce a large statue from polyurethane foam (Fig. 9).
Fig. 7. (a) Foundation robot and printing phase, (b) Grip robot and printing phase, (c) Vacuum robot and printing phase, (d) Printing phases of the mini builders system [36].
3. Photo polymerisation processes
geometric morphology of the spider silk and aiming to increase the structural performance of architectural structures, researchers from the Tongji University (China) [35] used an FDM-like printing head coupled with a 6-axis KUKA robotic arm (Fig. 6). They developed a strategy based on the combination of primary curve structure and auxiliary curves contact with the main curve, instead of using a conventional layer-by-layer deposition method. In order to simultaneously print the primary curve and auxiliary curves, one principle and three secondary nozzles were designed to print ABS material. Additionally, small mobile robots and cable driven robots are also being tested. The Institute for Advanced Architecture of Catalonia (Spain) [36] developed the concept of mini builders, using a family of three small mobile robots able to print concrete (Fig. 7). One or more foundation robots create the footprint of the structure (Fig. 7a),
The use of robotic systems to print photo-curable polymers is not significant and limited to hydrogels and medical applications. Li et al. [42] developed a robotic arm based hydrogel additive manufacturing system for in-situ printing in the medical field. The system, designed to directly operated during surgical operations, was used to print photocurable poly (ethylene glycol) diacrylate. The system includes a 3DOF desktop robot arm called Dobot version 1.0 and a light-curing system that consists of a micro-valve, two symmetrically positioned ultraviolet (UV) light sources and a nozzle holder (Fig. 10). The process starts with the ejection of material droplets to form a continuous line controlled by the movement of the robot arm. During the printing process the material is simultaneously exposed to UV light for curing (solidification), being the light source positioned. The material flow, size of the droplets 339
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Fig. 10. (a) Robotic arm printing platform, (b) Double-light-source ink jet light curing nozzle system [42].
intervals [49]. The adjustment of these parameters depends on the materials to be processed being the most commonly use material Inconel 625, Ti6Al4V, nitinol, stainless steel, aluminium and tool steel [50]. DED presents some limitations in building complex shapes, efficient powder delivery (in the case of powder form materials) and control of material properties [2]. Several research centres are developing DED systems. The Research Centre for Advanced Manufacturing (RCAM) at Southern Methodist University (USA) developed a robot controlled laser-based direct metal deposition (LBDMD) system that couples a 6-axis robot arm with an additional 2-axis tilt and rotatory positioning system (Fig. 11a) [51]. Focusing on the cost reduction and investigating the deposition rates, researchers from the University of Cranfield (UK) explored wire and plasma arc additive manufacturing for large (1.2 m) Ti6Al4V structures using a seven-axis KUKA robotic system (Fig. 11b) [15]. Wire arc additive manufacturing is also being investigated by researchers from the University of Wollongong (Australia) (Fig. 11c) [51],Technical University of Ilmenau (Germany) [52] and Indian Institute of Technology Bombay (India) [53]. At the Oak Ridge National Laboratori, Bandari et al. [54] are exploring the concept of laser wire DED for aerospace applications. Extensive experimental work was performed in order to optimise processing conditions (e.g. laser power, wire feed rate, robotic travel speed and inter-layer cooling time) minimising residual stresses and part distortion. Particular attention was payed to the cooling time between deposited layers of clamped and unclamped (high distortions being observed in unclamped parts) and inter-layer cooling time, which can be used to control the cooling conditions and consequently part distortion (long inter-cooling periods result in significant distortions). There are also several companies commercialising DED systems. The Laser Engineered Net Shaping (LENS) from OPTOMEC (USA) was the first commercially available DED system [55]. The LENS system consists of multiple powder feeders delivering metal powders through nozzles using argon as a carrier gas and a high-power ND-YAG laser [56–57]. The printing process is performed in a chamber purged with argon to avoid oxidation. Companies such as DMG Mori (Japan) and Mazak (Japan) are offering hybrid machine solutions combining laserbased DED and milling systems [58–59]. Alternative to lasers, electron beams are also being explored to fuse metallic material in a wire form which is fed into the path of an electron beam in a vacuum environment to additively build parts and features. The use of a vacuum environment eliminates impurities and improves mechanical properties. Additionally, the high temperatures associated to the electron beam processes reduce residual stresses. An example is the system developed by Sciaky Company (Fig. 11d), based on a dual wire feed system that allows to build graded parts by combining two different metal alloys in a single melt pool [60].Table 3 summarises the main characteristics of the machines being commercialised.
Fig. 11. (a) Robot controlled laser based direct metal deposition system, (b) Wire and plasma arc additive manufacturing, (c) Wire arc additive manufacturing system, (d) EBAM system from Sciaky.
and the position of the light source are key parameters [43–44]. Authors used two light sources symmetrically positioned to ensure the rapid solidification for both sides of the ejected material. 4. Directed energy deposition According to the ASTM F2792-12a, DED represents a group of additive manufacturing processes that uses thermal energy “to fuse materials by melting as they are being deposited” using a laser, electron beam or plasma arc as the energy sources [45]. This is the only multi DOF additive manufacturing technique able to process metallic materials either in a powder form (coaxial powder feeding) or in the wire form. The process has been used to create full dense and high-performance new parts or new geometries in existing parts [46]. It has been also used to repair large tools such as injection moulds. By changing the materials supplied by the nozzles, it is possible to create multi-material and functionally graded structures, which represents a key advantage of this technique [47]. DED has also been used to create thermal and chemical resistant coatings [48]. The process requires the correct adjustment of several process parameters such as laser power, scanning speed, powder feed rate, working distance, preheating temperature, carrier gas flow, shielding gas flow and cooling 340
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Table 3 Commercialised directed energy deposition machines.
Fig. 12. Hybrid manufacturing platform (a) Part printing process, (b) Part surface milling process [64].
Fig. 13. (a) Hybrid manufacturing platform, (b) Additive manufacturing head, (c) Subtractive manufacturing head [63].
5. Hybrid robot assisted systems
multi-axis CNC machining techniques [63]. Keating and Oxman [64] developed a system using a KUKA robotic arm to sequentially move a building platform between an extrusion-based printer and a milling system. First, large parts were printed from polyurethane foam; later subtractive milling and sanding processes were used to increase the surface resolution of the printed part (Fig. 12). Similarly, researchers from the University of Illinois (USA) developed a platform combining an extrusion-based system and a milling system with a 6-DOF robotic arm aiming to eliminate the need for support structures and to reduce both manufacturing time and material waste (Fig. 13). In this case instead of changing the place of the building platform, the manufacturing tool is changing [63].
In a hybrid manufacturing system, two different manufacturing processes such as additive and subtractive methods are combined and work as a single process on a platform, or a group of related processes can be placed in a separated environment [61]. Combination of different manufacturing processes can provide better surface integrity, reduced tool wear and production time. In addition, hybrid processes allow the fabrication of parts which are not able to be economically produced by separated processes [62]. In terms of the multi-axis robotic applications, this is an emergent area with very few examples available, most of them based on the combined use of additive manufacturing and 341
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Fig. 14. Information flow of conventional Additive manufacturing process.
Fig. 16. 6DOF multiplane printing system information flow [79].
6. Information flow
Uniform slicing is a process of intersecting the STL file with a set of horizontal planes with the same layer thickness. Each horizontal plane yields a planar contour that is piecewise linear. Adaptive slicing is a scheme that uses variable layer thickness based on the geometry change of the model along the build direction. This approach results in a reduction of build time and improves surface finish [73]. Slicing of an STL file, used due to the simplicity of the associated algorithms presents several disadvantages such as high computational cost (due to the large number of triangles usually considered) or low dimensional accuracy (due to the use of triangles to approximate the part surface instead of using analytical models) [74]. Direct slicing of cad models using different data formats (B-Rep, STEP, NURBS) have been also explored [75]. However, more sophisticated and complex algorithms are required to produce slices. Direct slicing is also software dependent [76]. Additionally, both uniform and adaptive slicing methods were developed for “conceptual” additive manufacturing systems based on the fabrication of planar “slices”. The use of robotic systems to assist the printing process allows multi-directional printing. Therefore, different research groups are currently developing multi-directional slicing algorithms to decrease the usage of support structures during the fabrication of overhangs or complex shapes [77]. However, these novel slicing algorithms are not effective in parts with holes or depression features. Typical additive manufacturing process flow chart from product design to actual part is presented in the Fig. 14. The information flow of robotic additive manufacturing systems is slightly different from the conventional three axis additive manufacturing methods. A unified strategy is difficult as in this case the information flow depends on the robot and its specific language. An example provided in the Fig. 15 showing the information flow for a six DOF extrusion system. The code was developed by researchers at Virginia Tech [78] and an ABB IRB 1200-7 robotic arm was used. Unlike the traditional AM methods, the system utilises a parser to pull out the information of movement and extrusion (feed rates and heating control) and finally, the information is sent to the robot. Similarly, researchers from the Florida Institute of Technology (USA) [79] developed an information flow system (Fig. 16) for a new 3D printer with a multi-plane layering capability. The system utilises a 6 DOF Motoman SV3X robot arm and an existing fused deposition filament machine allowing extrusion in multiple planes. An interfacing system was developed to facilitate the coordination between FDM system and robot arm. The printing process starts with loading a text file which includes the extruder parameters and instructions to control the robot arm to the interfacing software.
Traditional additive manufacturing methods start with the generation of a 3D solid CAD model [65]. The model is then tessellated into the standard STL (STereoLithoraphy file) format, which is finally sliced into multiple cross sections that are sent to the additive manufacturing system [66]. The STL file is conceptually simple and easy to generate but presents problems related to its size and numerical accuracy [67]. It is also not possible to specify material properties, so the fabrication of multi-material structures requires the use of several STL files [68]. This format consists of a list of connected triangular planar facets representing the outer surface of an object. Each facet is defined in terms of its vertices and a unit surface normal vector directed away from the interior of the part [69]. The vertices of the triangular faces are ordered according to the right-hand rule [70]. There are two STL file formats: ASCII (American Standard Code for Information Interchange) and binary. The difference between these two files is the format of the data definition. The quality of these files depends on the following parameters [2]:
• Chordal tolerance, which numerically describes the maximum dis•
tance between the actual part surface and the tessellated surface of the STL file. Angle control, which influences the tessellation of curves with relatively small radius in comparison to the overall size of the CAD model.
The quantity and size of the triangles determine how accurately the surface mesh represents the product. As the number of triangle increase and the relative triangle size decreases, the shape begins to be more accurate [71]. Finally, the STL model is sliced into a set of cross-section creating the SLI file format. The slicing process can be divided into two main types [72]:
• Uniform slicing • Adaptive slicing
7. Challenges and conclusions Flexibility, productivity and agility are the key elements of today's competitive manufacturing environment [80]. With the development of automation technologies, multiple degree of freedom robots are
Fig. 15. Information flow of 6-DOF extrusion platform [78]. 342
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promising systems for the implementation of flexible, productive and reconfigurable manufacturing methods, performing several tasks from basic handling operations to grinding, cutting, drilling, welding and polishing [81]. Moreover, current additive manufacturing systems have great potential to reduce time to market, increasing customisation, widening the design option compared to traditional methods. However, limitations of existing AM processes regarding product size, slow built rates, the need of support structure for regions with overhang, drive researchers to develop new fabrication strategies. Effectively, the threeaxis layer-by-layer manufacturing nature of conventional additive manufacturing systems and the limited working envelope are major drivers of change [63–83]. As described in this paper, adding an extra degree of freedom to current additive manufacturing systems by using a robotic system allows changing the direction of material accumulation during the fabrication process, building curve and overhang features without printing support structures. Unlike the conventional gantry system that characterises most of the commercially available AM machines, limiting part size, robotic arms can be placed anywhere and can perform printing within large workspace area. An example is the use of robotic-assisted additive manufacturing system for the fabrication of large building construction components. Both three-axis layer-by-layer and robotic systems presents adavantages and disadvantages. Robotic systems, despite allowing reduced levels of accuracy compared to threeaxis layer-by-layer systems (e.g. vat-photopolymerisation processes) are ideal to create large parts, to produce parts in a global inert environment or to allow in the same working space the combination of multiple techniques (e.g. robotic systems printing different materials and robotic systems performing inspection tasks) [84]. However, some problems are still limiting the use of robots in the field of additive manufacturing. Controlling both the robotic and additive manufacturing systems at the same time is one of the main problems. Current slicing algorithms are not able to generate G-code data compatible with the robot language and there are no standards regarding the information flow linking a CAD system and the roboticassisted additive manufacturing processes. Moreover, the quality of final AM products is mainly related to the accuracy of the motion systems of the AM machines. Therefore, the use of a robot manipulator to control an AM system can be resulted in poor part quality because of the low accuracy of robotic systems. For the fabrication of very large objects multiple robots can also be used increasing productivity but putting also some issues related to security, interface areas for the activities of these robots and to avoid obstacles. This will the case of construction, where multiple roboticassisted additive manufacturing systems could be used both off-site and on-site. In the last case there it is also possible to develop more sustainable manufacturing approaches by using materials available in the vicinity of the construction area. The rapid development of vision systems offers new opportunities to develop integrated systems allowing robots to “see” and adapt their functions in real-time due to changes in the surrounding environment. This will be also facilitated by the introduction of artificial intelligence (AI) into the robotics system, allowing to develop smart systems, optimising production processes and selecting the most suitable fabrication strategy to produce a part with improved properties. In the medical field, it is expected that robotic-assisted AM systems will be used for in situ printing of skin or cartilage replacements or bone grafts.
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Acknowledgements The first author acknowledges the support received from the Ministry of Education of Turkey to conduct her PhD. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.rcim.2019.05.005. 343
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Full length Article
Robot assisted additive manufacturing: A review Pinar Urhal, Andrew Weightman, Carl Diver, Paulo Bartolo
⁎
T
School of Mechanical Aerospace and Civil Engineering, The University of Manchester, UK
ARTICLE INFO
ABSTRACT
Keywords: 3D printing Additive manufacturing Multi-axis printing Robots
The additive manufacturing and the robotic applications are tremendously increasing in the manufacturing field. This review paper discusses the concept of robotic-assisted additive manufacturing. The leading additive manufacturing methods that can be used with a robotic system are presented and discussed in detail. The information flow required to produce an object from a CAD model through a robotic-assisted system, different from the traditional information flow in a conventional additive manufacturing approach is also detailed. Examples of the use of robotic-assisted additive manufacturing systems are presented.
1. Introduction Manufacturing is a process through which different raw materials are transformed into final products through the use of different fabrication methods [1]. In recent years, new manufacturing methods, such as additive manufacturing and advanced robotics, were developed and increasingly used by different industrial sectors, radically changing the way products are made. Additive manufacturing (AM) describes a group of processes that produce objects by depositing materials in a layer-by-layer way [2]. This technology, which is in its infancy, emerged in the late 1980s under the name of rapid prototyping. It was initially used to produce conceptual models to discuss design ideas, for form and fit applications or the production of architectural or anatomical models [3]. Materials were limited to few polymers, ceramics and metals [4]. Gradually the technology developed from rapid prototyping to rapid tooling allowing the direct or indirect fabrication of tools for injection moulding, thermoforming or blow moulding applications or for the fabrication of electrodes for electrical discharge machining applications [5]. Finally, additive manufacturing moved from rapid prototyping or rapid tooling to rapid manufacturing enabling the fabrication of final and fully functional products [6].Currently, additive manufacturing comprises seven different techniques (Table 1) enabling to process all types of materials including biological (cells and biomolecules), smart and functionally graded materials [7–8]. Through additive manufacturing, it is possible to produce an object of virtually any shape without the need of tooling [10–11]. Complex objects can be produced in one single process step, eliminating production steps and accelerating time to market with marginally increasing production costs. Moreover, additive manufacturing disrupts ⁎
the traditional supply chain, allowing for products to be produced closer to the point of use at the time of need, which limits material waste and improves both economies of scale and lead time [12]. It also dramatically reduces the time between design creation and prototyping by reducing the effort and the scheduled impact caused by iterative design and by increasing organisational alignment to accelerate decision-making [13]. Additive manufacturing has been successfully applied to a wide range of sectors including fashion, aerospace, aeronautics and defence, healthcare and construction [14]. Although recent developments show the increase use of AM systems for the fabrication of large-scale structures, most of the commercially available AM machines are three-axis Cartesian coordinate robots or gantry systems with limited construction platform dimensions [15–16]. Depending on the object shape and dimensions, support structures can be required which is increasing the fabrication time, material consumption and fabrication costs [17]. Moreover, the three-axis additive manufacturing machines are associated with a strict layer-by-layer fabrication approach creating objects with a typical stair-step effect [18]. Apart from all the benefits and drawbacks of conventional additive manufacturing applications, multi-axis robot-manipulated manufacturing methods which are widely used for welding and pick-andplace tasks, offer better quality and consistency, maximum productivity, greater safety for repetitive tasks and reduced labour costs [19–21]. The flexible functionality of robots serves the dynamic demands of manufacturing [22]. The combined use of multi-axis robot systems and additive manufacturing technologies offers the possibility for multi-axis additive manufacturing and the fabrication of complex geometries in different manufacturing environments. This review paper discusses the concept of multi-axis robot assisted additive manufacturing. This is an emergent
Corresponding author. E-mail address: [email protected] (P. Bartolo).
https://doi.org/10.1016/j.rcim.2019.05.005 Received 12 July 2018; Received in revised form 5 May 2019; Accepted 6 May 2019 Available online 14 May 2019 0736-5845/ © 2019 Published by Elsevier Ltd.
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area significantly growing. The leading printing principles that can be used with a robotic system are presented and discussed in detail. The information flow required to produce an object from a CAD model through a robotic-assisted system, different from the traditional information flow in a conventional additive manufacturing approach is also detailed. Examples of the use of robotic-assisted additive manufacturing systems are presented. Finally, research challenges and future perspectives are presented and discussed.
The majority of robotic-assisted extrusion-based systems are being used to produce freeform organic shapes, building construction elements for design purpose and usually large objects. Table 2 summarises the main materials and printing principles used by robot-assisted extrusion additive manufacturing. Zhang et al. [27] used an ABB robot to control a filament-based extrusion printing head (Fig. 1a). A computational platform called Robot Studio (developed by ABB) was used to allow process fabrication simulation, optimisation and visualisation (Fig. 1b). ABS parts were produced (Fig. 1c), but the system can be extended to other materials such as the use of carbon fibres to produce composite parts. Similarly, Wu et al. [28] used the 6-DOF UR3 robotic arm to produce polymeric parts from polylactic acid (PLA) in a filament form, with minimal or no support structures. A computational tool was developed in C++ to decompose the models into multiple support-free sub-models, printed in a collision-free sequence. Printing directions are determined by the software based on the decomposition strategy. Researchers from The University of Michigan (USA) used a robotic extrusion system to create a monolithic elastic net using a thermoplastic elastomer (TPE). The screw assisted extruder head was mounted on a seven-axis KUKA robot and the material, contrary to a FDM-like system, was supplied in the form of pallets providing high versatility in terms of the range of materials to be used, features definitions and material
2. Extrusion-based process Material extrusion is an additive manufacturing technique in which material is selectively deposited through a nozzle or orifice [23]. This technology developed by Scott Crump under the name of Fused Deposition Modelling (FDM) creates parts by extruding material (normally a thermoplastic polymer in a filament form) through a nozzle controlled by a computer into an XY platform [24]. After Crump's initial work the technique was further developed being able to process materials not only in a filament form but also in a pallet form [2]. Currently, the extrusion heads are classified as pressure-assisted and screw-assisted techniques enabling to print a wide range of polymeric materials, polymer-based composites, concrete, food and biological materials [25–26]. Table 1 Additive manufacturing methods [9].
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Table 1 (continued)
properties (Fig. 2). The system was developed as part of the Infundibuliforms project aiming to develop lightweight kinetic surfaces to create reconfigurable spatial enclosures with minimal amounts of material [29]. The high degree of freedom of the robotic systems allows the fabrication of complex objects without support structures, which improves accuracy, reduces post processing steps and minimises material waste. Other relevant examples include parts produced by the ABB-Robotstudio project (Fig. 3a) [30] and by the Institute for Advanced Architecture of Catalonia (Spain) [31] using an ABB 2400 L robotic arm (Fig. 3b). In the latter case, the system operates with thermosetting polymers and the printing head comprises two reservoirs containing two different chemical materials that are pumped into a mixing nozzle prior extrusion. During the mixing process, the two materials start a chemical reaction increasing its viscosity. Similarly, researchers from the Politecnico Di Milano (Italy) inspired by the pultrusion technique to create composite structures, used a six-axis robotic arm to print continuous fibres pre-impregnated with a thermosetting
resin. The curing process of the resin is performed by applying light using an optical fibre attached to the robotic arm. Due to the size and shape of building elements, robotic-assisted extrusion has been extensively explored in the fields of architectural design and construction using a wide range of materials. Inspired by the snails’ shell building process, Felbrich et al. [32] used an extrusion printing head mounted on a 6DOF robotic arm to produce freeform thermoplastics shells at an architectural scale. Craveiro et al. [33] used an ABB robot and two pumps working together to create functionally graded concrete elements using different combinations of concrete and cork (Fig. 4). The aim was to develop building elements with improved thermal efficiency, minimising the amount of concrete and reducing the environmental impact of the construction elements. The code was generated using HAL, a visual programming language grasshopper plug-in, combining the tool path information with the required material composition. Similarly, Gosselin et al. [34] developed a system based on an extrusion print head mounted on an ABB 6620 6-axis robotic arm. 337
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Table 2 Robotic-assisted extrusion based systems. Project name
Primary materials
Printing principles
Robotic additive manufacturing process simulation [27] RoboFDM: A robotic system for support free fabrication [28] Infundibuliforms: Kinetic systems [29]
Filament form polymer Filament form polylactic acid (PLA)
6 DOF ABB robot arm coupled extrusion printing head 6 DOF UR3 robot arm coupled with extrusion head
Pallet form of plastic and rubber mixture (Thermoplastic elastomer-TPE) Clay and ceramic mixture Thermosetting polymer
7-axis KUKA robot arm coupled with screw assisted extruder ABB robot arm coupled with extruder head ABB 2400 robot arm coupled with an extruder head with two reservoirs and a mixer nozzle 6 DOF robot arm coupled with extrusion head
Materially informed design to robotic manufacturing [30] Anti-Gravity Additive Manufacturing [31] A novel rapid AM concept for architectural composite shell construction [32] A design tool for resource efficient fabrication [33]
Thermoplastic polymer
Large scale 3D printing of ultra-high performance concrete [34] Robotic multi-dimensional printing [35]
Concrete ABS plastic
Large scale 3D printing [36] Mobile robotic fabrication [37] Large scale printing with cable-suspended robot [38]
Concrete Filament form fibre composite Polyurethane foam
Concrete and cork mixture
The system includes two peristaltic pumps, one for the pre-mix and another for an accelerating agent and a premix mixer (Fig. 5). The fabrication process includes two main steps. First, a mortar pre-mix with appropriate rheological behaviour is prepared and kept in a syringe mixer. Then the pre-mix is pumped to a screw using a peristaltic pump.
ABB robot arm coupled with an extruder head with two pumps ABB 6620 6-axis robot arm coupled with an extruder head with two pumps 6-axis KUKA robot arm coupled with an extruder head with multiple nozzles Small mobile robots and a lightweight extrusion system Small mobile robots and a lightweight extrusion system 6 DOF Cable suspended robot
Robotic-assisted fabrication systems are also opening new routes for in situ fabrication expanding the robotic fabrication process behind the constraints of the production platform [39–41]. Inspired by the
Fig. 3. (a) Ceramic printing for ABB Robotstudio project [30] (b) Thermosetting polymer printing [31].
Fig. 1. (a) Filament-based printing head controlled by an ABB robotic arm; (b) RobotStudio platform; (c) Printed polymeric part [27].
Fig. 4. Concrete-cork AM System [33]: (a) Robot controller, (b) ABB teach pendant, (c) Arduino developed module, (d) Functionally graded extruded component, (e) Concrete pumps, (f) ABB robot.
Fig. 2. FDM-like system with a seven-axis KUKA robot arm, (b) FDM printing head, (c) TPE net deposition [29]. 338
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Fig. 8. (a) Building process of multi-robotic system, (b) Diagram of the process [37].
Fig. 5. Robotic Extrusion Printing System [34]: (a) 6-axis robotic arm, (b) peristaltic pump for pre-mix, (c) peristatic pump for accelerating agent, (d) premix mixer, (e) screw assisted print head.
Fig. 6. (a) Robotic system with 6-axis KUKA arm (b) Printing head for main and auxiliary curves [35]. Fig. 9. (a) Cable robot system, (b) Final foam product [38].
followed by grip robots that are clamped onto the footprint and extend the structure (Fig. 7b). Finally, a vacuum robot moves over the printed structure and reinforces it by applying additional layers of material (Fig. 7c). At the University of Stuttgart (Germany) [37] researchers proposed a multi-robot system of cooperative mobile machines operating within the context of the surfaces of existing architectural elements such as façets, walls and ceilings, anchoring new tensile filament structures to these surfaces (Fig. 8). This project uses low payload agile machines, which can be easily used to print different types of materials by incorporating a lightweight extrusion printing head. Cable-suspended robotic systems are also being used to produce large-scale parts. An example is the research conducted at the University Laval (Canada) [38] that used a six DOF cable suspended robot to produce a large statue from polyurethane foam (Fig. 9).
Fig. 7. (a) Foundation robot and printing phase, (b) Grip robot and printing phase, (c) Vacuum robot and printing phase, (d) Printing phases of the mini builders system [36].
3. Photo polymerisation processes
geometric morphology of the spider silk and aiming to increase the structural performance of architectural structures, researchers from the Tongji University (China) [35] used an FDM-like printing head coupled with a 6-axis KUKA robotic arm (Fig. 6). They developed a strategy based on the combination of primary curve structure and auxiliary curves contact with the main curve, instead of using a conventional layer-by-layer deposition method. In order to simultaneously print the primary curve and auxiliary curves, one principle and three secondary nozzles were designed to print ABS material. Additionally, small mobile robots and cable driven robots are also being tested. The Institute for Advanced Architecture of Catalonia (Spain) [36] developed the concept of mini builders, using a family of three small mobile robots able to print concrete (Fig. 7). One or more foundation robots create the footprint of the structure (Fig. 7a),
The use of robotic systems to print photo-curable polymers is not significant and limited to hydrogels and medical applications. Li et al. [42] developed a robotic arm based hydrogel additive manufacturing system for in-situ printing in the medical field. The system, designed to directly operated during surgical operations, was used to print photocurable poly (ethylene glycol) diacrylate. The system includes a 3DOF desktop robot arm called Dobot version 1.0 and a light-curing system that consists of a micro-valve, two symmetrically positioned ultraviolet (UV) light sources and a nozzle holder (Fig. 10). The process starts with the ejection of material droplets to form a continuous line controlled by the movement of the robot arm. During the printing process the material is simultaneously exposed to UV light for curing (solidification), being the light source positioned. The material flow, size of the droplets 339
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Fig. 10. (a) Robotic arm printing platform, (b) Double-light-source ink jet light curing nozzle system [42].
intervals [49]. The adjustment of these parameters depends on the materials to be processed being the most commonly use material Inconel 625, Ti6Al4V, nitinol, stainless steel, aluminium and tool steel [50]. DED presents some limitations in building complex shapes, efficient powder delivery (in the case of powder form materials) and control of material properties [2]. Several research centres are developing DED systems. The Research Centre for Advanced Manufacturing (RCAM) at Southern Methodist University (USA) developed a robot controlled laser-based direct metal deposition (LBDMD) system that couples a 6-axis robot arm with an additional 2-axis tilt and rotatory positioning system (Fig. 11a) [51]. Focusing on the cost reduction and investigating the deposition rates, researchers from the University of Cranfield (UK) explored wire and plasma arc additive manufacturing for large (1.2 m) Ti6Al4V structures using a seven-axis KUKA robotic system (Fig. 11b) [15]. Wire arc additive manufacturing is also being investigated by researchers from the University of Wollongong (Australia) (Fig. 11c) [51],Technical University of Ilmenau (Germany) [52] and Indian Institute of Technology Bombay (India) [53]. At the Oak Ridge National Laboratori, Bandari et al. [54] are exploring the concept of laser wire DED for aerospace applications. Extensive experimental work was performed in order to optimise processing conditions (e.g. laser power, wire feed rate, robotic travel speed and inter-layer cooling time) minimising residual stresses and part distortion. Particular attention was payed to the cooling time between deposited layers of clamped and unclamped (high distortions being observed in unclamped parts) and inter-layer cooling time, which can be used to control the cooling conditions and consequently part distortion (long inter-cooling periods result in significant distortions). There are also several companies commercialising DED systems. The Laser Engineered Net Shaping (LENS) from OPTOMEC (USA) was the first commercially available DED system [55]. The LENS system consists of multiple powder feeders delivering metal powders through nozzles using argon as a carrier gas and a high-power ND-YAG laser [56–57]. The printing process is performed in a chamber purged with argon to avoid oxidation. Companies such as DMG Mori (Japan) and Mazak (Japan) are offering hybrid machine solutions combining laserbased DED and milling systems [58–59]. Alternative to lasers, electron beams are also being explored to fuse metallic material in a wire form which is fed into the path of an electron beam in a vacuum environment to additively build parts and features. The use of a vacuum environment eliminates impurities and improves mechanical properties. Additionally, the high temperatures associated to the electron beam processes reduce residual stresses. An example is the system developed by Sciaky Company (Fig. 11d), based on a dual wire feed system that allows to build graded parts by combining two different metal alloys in a single melt pool [60].Table 3 summarises the main characteristics of the machines being commercialised.
Fig. 11. (a) Robot controlled laser based direct metal deposition system, (b) Wire and plasma arc additive manufacturing, (c) Wire arc additive manufacturing system, (d) EBAM system from Sciaky.
and the position of the light source are key parameters [43–44]. Authors used two light sources symmetrically positioned to ensure the rapid solidification for both sides of the ejected material. 4. Directed energy deposition According to the ASTM F2792-12a, DED represents a group of additive manufacturing processes that uses thermal energy “to fuse materials by melting as they are being deposited” using a laser, electron beam or plasma arc as the energy sources [45]. This is the only multi DOF additive manufacturing technique able to process metallic materials either in a powder form (coaxial powder feeding) or in the wire form. The process has been used to create full dense and high-performance new parts or new geometries in existing parts [46]. It has been also used to repair large tools such as injection moulds. By changing the materials supplied by the nozzles, it is possible to create multi-material and functionally graded structures, which represents a key advantage of this technique [47]. DED has also been used to create thermal and chemical resistant coatings [48]. The process requires the correct adjustment of several process parameters such as laser power, scanning speed, powder feed rate, working distance, preheating temperature, carrier gas flow, shielding gas flow and cooling 340
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Table 3 Commercialised directed energy deposition machines.
Fig. 12. Hybrid manufacturing platform (a) Part printing process, (b) Part surface milling process [64].
Fig. 13. (a) Hybrid manufacturing platform, (b) Additive manufacturing head, (c) Subtractive manufacturing head [63].
5. Hybrid robot assisted systems
multi-axis CNC machining techniques [63]. Keating and Oxman [64] developed a system using a KUKA robotic arm to sequentially move a building platform between an extrusion-based printer and a milling system. First, large parts were printed from polyurethane foam; later subtractive milling and sanding processes were used to increase the surface resolution of the printed part (Fig. 12). Similarly, researchers from the University of Illinois (USA) developed a platform combining an extrusion-based system and a milling system with a 6-DOF robotic arm aiming to eliminate the need for support structures and to reduce both manufacturing time and material waste (Fig. 13). In this case instead of changing the place of the building platform, the manufacturing tool is changing [63].
In a hybrid manufacturing system, two different manufacturing processes such as additive and subtractive methods are combined and work as a single process on a platform, or a group of related processes can be placed in a separated environment [61]. Combination of different manufacturing processes can provide better surface integrity, reduced tool wear and production time. In addition, hybrid processes allow the fabrication of parts which are not able to be economically produced by separated processes [62]. In terms of the multi-axis robotic applications, this is an emergent area with very few examples available, most of them based on the combined use of additive manufacturing and 341
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Fig. 14. Information flow of conventional Additive manufacturing process.
Fig. 16. 6DOF multiplane printing system information flow [79].
6. Information flow
Uniform slicing is a process of intersecting the STL file with a set of horizontal planes with the same layer thickness. Each horizontal plane yields a planar contour that is piecewise linear. Adaptive slicing is a scheme that uses variable layer thickness based on the geometry change of the model along the build direction. This approach results in a reduction of build time and improves surface finish [73]. Slicing of an STL file, used due to the simplicity of the associated algorithms presents several disadvantages such as high computational cost (due to the large number of triangles usually considered) or low dimensional accuracy (due to the use of triangles to approximate the part surface instead of using analytical models) [74]. Direct slicing of cad models using different data formats (B-Rep, STEP, NURBS) have been also explored [75]. However, more sophisticated and complex algorithms are required to produce slices. Direct slicing is also software dependent [76]. Additionally, both uniform and adaptive slicing methods were developed for “conceptual” additive manufacturing systems based on the fabrication of planar “slices”. The use of robotic systems to assist the printing process allows multi-directional printing. Therefore, different research groups are currently developing multi-directional slicing algorithms to decrease the usage of support structures during the fabrication of overhangs or complex shapes [77]. However, these novel slicing algorithms are not effective in parts with holes or depression features. Typical additive manufacturing process flow chart from product design to actual part is presented in the Fig. 14. The information flow of robotic additive manufacturing systems is slightly different from the conventional three axis additive manufacturing methods. A unified strategy is difficult as in this case the information flow depends on the robot and its specific language. An example provided in the Fig. 15 showing the information flow for a six DOF extrusion system. The code was developed by researchers at Virginia Tech [78] and an ABB IRB 1200-7 robotic arm was used. Unlike the traditional AM methods, the system utilises a parser to pull out the information of movement and extrusion (feed rates and heating control) and finally, the information is sent to the robot. Similarly, researchers from the Florida Institute of Technology (USA) [79] developed an information flow system (Fig. 16) for a new 3D printer with a multi-plane layering capability. The system utilises a 6 DOF Motoman SV3X robot arm and an existing fused deposition filament machine allowing extrusion in multiple planes. An interfacing system was developed to facilitate the coordination between FDM system and robot arm. The printing process starts with loading a text file which includes the extruder parameters and instructions to control the robot arm to the interfacing software.
Traditional additive manufacturing methods start with the generation of a 3D solid CAD model [65]. The model is then tessellated into the standard STL (STereoLithoraphy file) format, which is finally sliced into multiple cross sections that are sent to the additive manufacturing system [66]. The STL file is conceptually simple and easy to generate but presents problems related to its size and numerical accuracy [67]. It is also not possible to specify material properties, so the fabrication of multi-material structures requires the use of several STL files [68]. This format consists of a list of connected triangular planar facets representing the outer surface of an object. Each facet is defined in terms of its vertices and a unit surface normal vector directed away from the interior of the part [69]. The vertices of the triangular faces are ordered according to the right-hand rule [70]. There are two STL file formats: ASCII (American Standard Code for Information Interchange) and binary. The difference between these two files is the format of the data definition. The quality of these files depends on the following parameters [2]:
• Chordal tolerance, which numerically describes the maximum dis•
tance between the actual part surface and the tessellated surface of the STL file. Angle control, which influences the tessellation of curves with relatively small radius in comparison to the overall size of the CAD model.
The quantity and size of the triangles determine how accurately the surface mesh represents the product. As the number of triangle increase and the relative triangle size decreases, the shape begins to be more accurate [71]. Finally, the STL model is sliced into a set of cross-section creating the SLI file format. The slicing process can be divided into two main types [72]:
• Uniform slicing • Adaptive slicing
7. Challenges and conclusions Flexibility, productivity and agility are the key elements of today's competitive manufacturing environment [80]. With the development of automation technologies, multiple degree of freedom robots are
Fig. 15. Information flow of 6-DOF extrusion platform [78]. 342
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promising systems for the implementation of flexible, productive and reconfigurable manufacturing methods, performing several tasks from basic handling operations to grinding, cutting, drilling, welding and polishing [81]. Moreover, current additive manufacturing systems have great potential to reduce time to market, increasing customisation, widening the design option compared to traditional methods. However, limitations of existing AM processes regarding product size, slow built rates, the need of support structure for regions with overhang, drive researchers to develop new fabrication strategies. Effectively, the threeaxis layer-by-layer manufacturing nature of conventional additive manufacturing systems and the limited working envelope are major drivers of change [63–83]. As described in this paper, adding an extra degree of freedom to current additive manufacturing systems by using a robotic system allows changing the direction of material accumulation during the fabrication process, building curve and overhang features without printing support structures. Unlike the conventional gantry system that characterises most of the commercially available AM machines, limiting part size, robotic arms can be placed anywhere and can perform printing within large workspace area. An example is the use of robotic-assisted additive manufacturing system for the fabrication of large building construction components. Both three-axis layer-by-layer and robotic systems presents adavantages and disadvantages. Robotic systems, despite allowing reduced levels of accuracy compared to threeaxis layer-by-layer systems (e.g. vat-photopolymerisation processes) are ideal to create large parts, to produce parts in a global inert environment or to allow in the same working space the combination of multiple techniques (e.g. robotic systems printing different materials and robotic systems performing inspection tasks) [84]. However, some problems are still limiting the use of robots in the field of additive manufacturing. Controlling both the robotic and additive manufacturing systems at the same time is one of the main problems. Current slicing algorithms are not able to generate G-code data compatible with the robot language and there are no standards regarding the information flow linking a CAD system and the roboticassisted additive manufacturing processes. Moreover, the quality of final AM products is mainly related to the accuracy of the motion systems of the AM machines. Therefore, the use of a robot manipulator to control an AM system can be resulted in poor part quality because of the low accuracy of robotic systems. For the fabrication of very large objects multiple robots can also be used increasing productivity but putting also some issues related to security, interface areas for the activities of these robots and to avoid obstacles. This will the case of construction, where multiple roboticassisted additive manufacturing systems could be used both off-site and on-site. In the last case there it is also possible to develop more sustainable manufacturing approaches by using materials available in the vicinity of the construction area. The rapid development of vision systems offers new opportunities to develop integrated systems allowing robots to “see” and adapt their functions in real-time due to changes in the surrounding environment. This will be also facilitated by the introduction of artificial intelligence (AI) into the robotics system, allowing to develop smart systems, optimising production processes and selecting the most suitable fabrication strategy to produce a part with improved properties. In the medical field, it is expected that robotic-assisted AM systems will be used for in situ printing of skin or cartilage replacements or bone grafts.
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Acknowledgements The first author acknowledges the support received from the Ministry of Education of Turkey to conduct her PhD. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.rcim.2019.05.005. 343
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