Design for Additive Manufacturing Using LSWM: A CADTool for the Modelling of Lightweight

and Lattice Structures

Alessandro Ceruti1(✉), Riccardo Ferrari2, and Alfredo Liverani1

1 DIN Department of Industrial Engineering, University of Bologna, Bologna, Italyalessandro.ceruti@unibo.it

2 School of Engineering and Architecture, University of Bologna, Bologna, Italy

Abstract. This paper presents the development of a CAD conceived to supportthe modelling of lightweight and lattice structures just from the initial stages ofthe design process. A new environment, called LWSM (acronym of LightWeightStructures Modelling), has been implemented in Python programming languagein an open-source CAD software to allow the fast modelling of several sandwichstructures or the filling of solid parts with cubic and tetrahedral lattice structureswhich can be produced by Additive Manufacturing (AM) techniques. Severaltests have been carried out to validate the tool, one of which is included in thepaper. The design of a bracket component inside LWSM using a traditional densegeometry and a lattice structure is described. The use of Design for AdditiveManufacturing (DfAM) functions helps the user in the design of innovative struc‐tures which can produced only with AM technologies. A significant change in theshape of the part respect to traditional solutions is noticed after the use of DfAMfunctions by experimenters: FEM analysis confirms a strong weight reduction.

Keywords: CAD · Lattice structure · Additive manufacturing · Lightweightstructures · FEM analysis

1 Introduction

One of the challenges of modern engineering is to obtain lightweight structures and tocompress as much as possible the “time to market” of products. Making optimized, lightand easy to manufacture parts is needed for environmental and commercial purposes inall industrial fields, but in particular within manufacturers of transportation (air, sea,rail) systems, and biomedical application where it is important to replicate structuresbelonging to nature. For instance, there is a positive lever action in aircraft design whenstructural weights are reduced [1]: to spare the structural mass of 1 kg can lead to obtaina reduction of 3 or even more Kg of the aircraft total mass. In this way, a smallerpropulsion system is required to fly and the fuel burned per passenger is reduced, witha beneficial effect on the environment pollutant emission and economy. Also other kindsof industry are interested in reducing structural weights: the cabin floor of the ItalianVivalto train, produced by Ansaldo Breda, is made of aluminum [2] to reduce thecoaches weight and consequently the electric energy required for locomotion. Nature

© Springer International Publishing AG 2017G. Campana et al. (eds.), Sustainable Design and Manufacturing 2017, Smart Innovation,Systems and Technologies 68, DOI 10.1007/978-3-319-57078-5_71

presents several examples in which a lightweight structure is obtained by using sparsetruss based structures [3]: bones are typical examples of such a kind of structures. Theengineering practice can benefit from the imitation of natural structures whose shapehave been optimized by the evolution through a thousand of years long design. On thecontrary, the traditional manufacturing processes are mainly based upon the removal ofmaterial, operation leading to weighty parts. Raw shapes like that obtained through thelamination process or the foundry are usually worked with chip removal operations(lathe, drill, milling machine) to obtain the shape of a part. Nowadays a new revolutionis spreading in manufacturing: Additive Manufacturing processes for powders, like theElectron Beam Melting (EBM) or Selective Laser Sintering (SLS) allow the generationof complex and sparse structures obtained by melting material layer by layer. Structuresbased on the repetition of thin closed beam based elements (called lattice) [4], squaredhole shapes, undercuts, sparse elements are now possible with very few limitations tothe designer. Unlike traditional machines, there is no need to keep into account theworking trajectory of a tool in the space, thus enabling the manufacture of more complexshapes. From a design point of view, AM is effective [5] when the concept “the designdrives the shape” is adopted, since there is no limit to shapes which the designer canselect. Complex geometries are usually obtained after an optimization process which isusually carried out to guess the lightest shape respecting the requirements on loads andconstraints of the problem. On the other hand, traditional design requires to follow therule “the shape drives the design” since it is not possible to obtain some shapes: thedesigner can model only parts which can be further properly machined and he mustremember this while shaping a part. The challenge of the next years will be to developa “new” class of tools and to train designers able to design parts exploiting the capabilityof AM to produce complex but structurally efficient shapes. CAD software packageswhich have been conceived to support traditional manufacturing processes must beimproved to support the AM designer with new tools to allow the fast design and sketchof the structures typical of the AM process. New geometries and shapes can be producedwith AM, so that among “Design for X” techniques, the concept of Design for AdditiveManufacturing (DfAM) has been introduced to stress the attention on how AM haschanged (and will further change in next years) the current product development cycle.From a research point of view, a bulk of literature deals with the design of lattice struc‐tures [6, 7] and topological optimization [8, 9]. However, several general purpose CADsystems do not still handle lightweight or lattice structures in an effective and efficientway. The aim of this paper is to describe the implementation of a CAD environment,called LightWeight Structures Modelling (LWSM), developed to support the designerwhile sketching lightweight and lattice structures. It is a matter of fact that current partsdesign workflow is based upon the CAD modelling of components conceived as to beobtained by chip removal machines; in the further, this parts are imported in externaloptimization environments [10] in which a topological optimization or the change fromdense to lattice or graded structures is carried out. The LWSM tool we present has beenwritten in Pyton programming language and implemented in the open source FreeCADsoftware in order to contribute to the development of tools able to support the everydayuse of functions AM oriented, and to allow designers to think in an “Additive way” [11].

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2 New Structures and CAD Systems

AM can be used to produce smart structures [12] based on three strategies: densitygraded materials, dense shapes obtained through topological optimization, lattice struc‐tures based parts. First two kinds of structures do not require particular attentions froma CAD perspective since they can be modelled by solid materials, eventually with achange in properties (like mass or elastic modulus) depending on the location in thebody reference axis: the material can be considered locally isotropic, even if propertiesare position-dependent. Lattice structures require much more attention: when the sizeof the cells which are repeated to obtain the part is large, they can be sketched one byone, like usually done for nerves or thin appendices in solid bodies. Also in case of smallcells there are no problems because the material can be considered both in CAD and inFEM as isotropic with a dense visualization. When the size of lattice structures cells areintermediate new functions are required. On the one hand the designer can’t waste timeto model a large number of small complex structures. On the other, a dense uniformvisualization doesn’t give an idea of the structure and can’t be used to represent thecomponent shape in a CAD system. There are problems also in FEM packages since themeshing of 3D structure based on small cellular elements requires a lot of nodes andsolid elements to be discretized. Studies [13, 14] are focused on how to implement asort of equivalent material to be applied to dense bodies to speed up analyses whiledescribing in a proper way cellular structures. However, CAD systems must present thelattice structure in a realistic way and the modelling of these structures should be carriedout in a very short time to reduce the user workload. A similar concept applies whensandwich structures based parts are modelled because also in this case tools and model‐ling functions to help the designer are required. From our point of view, lattice/light‐weight modelling functions are required in CAD tools, so that parts can be shaped usingthe typical AM structures just from the beginning of the prototyping workflow. To opti‐mize or to convert dense parts to lattice cells after the modelling in external tools canlead to losses in efficiency and can drive the final shaping towards sub-optimal solutions.A new class of structures, impossible to produce just a few years ago are now availableto the designer: these geometries are usually based upon the repetition of small cells orelements and present a quite high geometrical complexity, so that the generation of apart conceived implementing DfAM concepts should be supported by specific functionsdeveloped to reduce the CAD user workload.

3 FreeCAD LWSM Environment

An environment to support the design of lightweight structures has been developed inthe FreeCAD software. FreeCAD is an open source CAD in which it is simple to imple‐ment new additional environments respect to what already available in the downloadablerelease (part design, sketching, assembly, FEM analysis just to cite some of them). Theprogramming of new environments is simple due to the capability of recording actionscarried out by the user during a working sessions: commands are translated into a code

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called “macro” which can replicate in an automatic way what the human user didmanually. In this way, it is possible to automate complex procedures or actions.

A palette function has been designed for the environment, called LWSM. In this firstimplementation of LWSM there are 4 functions to implement the lattice based discre‐tization of a solid body, and 4 functions to sketch parametric sandwich panels. TheFig. 1 presents an image of the LWSM menu integrated in FreeCAD commands panel.There are two separate palettes in which 4 commands are grouped for each one. Thefirst palette includes commands useful to modify the internal structure of alreadymodelled 3D bodies, filling them with lattice/regular structures. They are: “3D Partfilling with extruded hexagons”, “3D Part filling with extruded cylinders”, “3D Partfilling with cubical lattice structures”, “3D part filling with tetrahedral lattice structures”.The following Figs. 2 and 3 present some examples based upon the filling of a solidshape given by the Boolean union of a cube with a 100 mm long edge and a cylinderwith an height of 50 mm to show how these functions can work with whatever solidshapes.

Fig. 1. LWSM menu palette with implemented functions

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Fig. 2. “3D part filling with extruded hexagons” function

Fig. 3. “3D part filling with cubical lattice structures” function

This set of functions in which lattice/regular structures are applied to alreadymodelled 3D dense parts works in the following way: the user selects the body whosesolid structure must be changed; a box inscribing the object is computed by using func‐tions available in FreeCAD; a “negative” lattice structure is sketched in all the box;

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finally a Boolean intersection between the body and the “negative” lattice structure iscarried out to obtain the final shape.

The command “3D Part filling with cubical lattice structures” for instance is basedupon the discretization of the body with elements based on a cube in which spheres liesat the 8 corners, connected along the edges by cylinders. Both the element size and radiiof spheres/cylinders can be set by the user. The second palette contains another set offunctions for the automatic design of structures where the user selects through a menudimensions and properties of the structure he/she wants to model. In this case “Createa sandwich with hexagonal cells”, “Create a sandwich with triangular cells”, “Create aNavtruss like sandwich”, “Create a lattice box” are names of implemented functionswhose symbols can be seen in Fig. 1 top-right corner.

Just to provide some examples, Figs. 4 and 5 show the control menu to set the datarequired for modelling and an example of the final structure which can be obtained (intransparency to show the internal structure features).

Fig. 4. “Create a sandwich with triangular cells” function

Fig. 5. “Create a Navtruss like sandwich” function

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To give an idea of how these 4 functions have been modelled, they are based uponthe implementation of the parametric modelling of structures imitating what carried outby the user during the sketch of sandwich/lattice structures. It is worth noting that allstructures obtained through the LWSM tool are solid and can be used for FEM analysis,eventually using the FEA tool of FreeCAD or exporting the solid model in “step” fileexchange format and importing it in external FEM codes in the further. Moreover,structures can be saved in STL extension, in order to be manufactured using AMmachines.

4 Case Study

The tools we developed have been tested in several case studies to evaluate the effect ofthe implementation, just from the CAD modelling, of functions useful for the “DfAM”.In this paper, we present an example related to the design of a bracket with three bosses,loaded with a lateral force of 1000 N. The position, diameter, and thickness of bossesis shown in Fig. 6, as well as the constraint and load positions. The material selected forthe application are stainless steel powders which are commonly used in EBM machines.

Fig. 6. Load, constraints, and dimensions for the case study

Aim of the design is to obtain a structure able to withstand a lateral load of 1000 Nin the upper boss, provided the other two bosses are completely constrained. The parthas been obtained by sketching bosses, adding a connecting shell with a thickness setequal to 20 mm and finally converting it into the lattice structure. At the end, bosses andthe lattice shell have been glued together with a Boolean operation, and holes have beenobtained using the “create pocket” command in the PART DESIGN environment ofFreeCAD. As reported in Fig. 7a, the lattice cubic structure selected for the modellingpresents cells with edge of 20 mm, equal to the plate thickness, and beams with a 4 mmdiameter. A following FEM analysis, carried out in the FreeCAD Calculix solver, andin other FEM to be confident in the solution, showed a maximum stress of around150 MPa (Fig. 7b). The FEM analysis has been carried out meshing the lattice structure

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(which is made of thin elements and requires a lot of nodes) and bosses with 3D tetra‐hedral elements: a quite large number of nodes (around 350,000) has been obtained, butwidely within commercial FEM computational capabilities.

(a) (b)

Fig. 7. Lattice based bracket: modelling and FEM analysis

If the model had been modelled as a dense metallic part it would have resulted asincluded in Fig. 8a where bosses have been welded to a plate with a constant thickness.

(a) (b)

Fig. 8. Dense shell based bracket: modelling and FEM analysis

As an alternative, the milling of the component from a raw rectangular plate wouldbe also possible. In this “traditional” scenario, the thickness of a plate assuring the samemaximum stress of 150 MPa would have been defined after a set of FEM analysis, asdid in this research. The Fig. 8b presents an image of one of the FEM analyses made toproperly set the thickness of the plate, which resulted equal to around 7.4 mm to obtaina stress level similar to that obtained in the lattice based shape. The dense “traditional”

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model mass is around 5.2 kg (7.4 mm thickness), compared to the 4.4 kg of the latticesolution (20 mm lattice elements thickness). From a technological point of view, it isworth noting that the AM choice presents advantages over the traditional manufacturing.When the bracket is obtained by milling a thick block of steel a long time for machiningshould be assumed (together with high costs for machining). When it is obtained bywelding bosses to a laser cut plate, additional calculations should be carried out to verifyrequirements on welding strength according to the ISO rules for welded components.In addition to this, the welding should be controlled with Non Destructive Inspectionsin case of high value components and a precise welding operation would require a toolto refer the components one another. Moreover, the welding could deform the compo‐nent due to the high temperatures reached during the working. So, even if the AM can’tbe considered a cheap technology in case of small lots of components, it can be compet‐itive due to the sparing in weights and the short “time to market” of the component.LWSD can be used also for the design of hybrid components in which a part in AM isadded to dense parts obtained with traditional machines, as Fig. 9 shows.

Fig. 9. Hybrid AM with LWSM

5 Conclusions

The modern engineering is more and more concerned in reducing the environmentalfootprint of human habits and needs: both buildings and industrial products mustcontribute to reduce pollutant emissions and save energy during the whole lifecycle. Atindustrial level, the Additive Manufacturing can be an answer to the need for lightweightstructures in engineering. Potentialities of AM are magnified when a proper and “AMoriented” design is carried out. Instead of using dense structures typical of subtractivetechnologies, graded or lattice structures should be adopted by the designer, just fromthe phase of component modelling within CAD software packages. A large part ofcurrent CAD systems can’t effectively support the user in the design and sketching ofthis kind of new structures; parts, modelled as dense bodies are usually imported inexternal tools where the optimization or the conversion into lattice structure is carriedout. An environment to support the design of lightweight structures has been imple‐mented in the FreeCAD software and tested. Several functions have been implemented.After an evaluation phase, it is straightforward that this tool can be useful for studentsand professional designers to implement new structures which can benefit of AM capa‐bilities. The user can “think additive” just from the modelling phase to completelyexploit potentialities which AM can offer with proper shapes. A case study of a boltedplate that was modelled in traditional and lattice structure demonstrated the usefulness

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of the LWSM tool since it is possible to model complex structures in an effective way.As a matter of fact, using a traditional structure (mass = 5.2 kg) instead of a lattice basedone (mass = 4.4 kg) an increase of the 18% in mass is noticed. Future works will addnew functions to the tool and include the experimentation of the LWSM by structuraldesigners when the weight reduction is of paramount importance as for transportationor biomedical applications.

References

1. Raymer, D.P.: Aircraft Design: A Conceptual Approach (Aiaa Education Series) 5th edn.,ISBN-13: 978-1600869112 (2012)

2. Metra Consulting, Aluminum extrusions for the railway sector. http://www.metra.it/aluminium/EN/case-history-industry-unimaginable-sectors-vivalto-a-high-capacity-double-decker-train-railway-sector-259.aspx. Accessed Nov 2016

3. Boruciński, M., Królikowski, M.: Design of non-uniform truss structures for improved partproperties. Adv. Manufact. Sci. Technol. 37(4), 77–85 (2013)

4. Hadi, A., Vignat, F., Villeneuve, F.: Design configurations and creation of lattice structuresfor metallic additive manufacturing. In: 14eme Colloque National AIP PRIMECA, Mar 2015,La Plagne, France (2015)

5. Staiano G., Gloria A., Ausanio G., Lanzotti A., Pensa C., Martorelli M.: Experimental studyon hydrodynamic performances of naval propellers to adopt new additive manufacturingprocesses. Int. J. Interact. Des. Manuf. 1–14 (2016). ISSN: 1955-2513, doi:10.1007/s12008-016-0344-1

6. Aremu, A.O., Brennan-Craddock, J.P.J., Panesar, A., Ashcroft, I.A., Hague, R.J.M., Wildman,R.D., Tuck, C.: A voxel-based method of constructing and skinning conformal andfunctionally graded lattice structures suitable for additive manufacturing. Add. Manuf. 13, 1–13 (2017)

7. Rosen, D.: Computer-aided design for additive manufacturing of cellular structures. Comput.Aided. Des. Appl. 4(5), 585–594 (2007)

8. Bendsoe, M.P., Sigmund, O.: Topological Optimization, Theory. Methods and Application.Springer-Verlag, Berlin (2004)

9. Taggart, D.G., Dewhurst, P.: Development and validation of a numerical topologyoptimization scheme for two and three dimensional structures. Adv. Eng. Softw. 41, 910–915(2010)

10. Altair Optistruct software. http://www.altairhyperworks.com/product/OptiStruct. AccessedNov 2016

11. Ceruti, A. Liverani, A., and Bombardi, T.: Augmented vision and interactive monitoring in3D printing process. Int. J. Interact. Des. Manuf. (IJIDeM) 1–11 (2016). doi:10.1007/s12008-016-0347-y

12. Martorelli, M., Gerbino, S., Lanzotti, A., Patalano, S., Vitolo, F.: Flatness, circularity andcylindricity errors in 3D printed models associated to size and position on the working plane.In: Eynard, B., Nigrelli, V., Oliveri, S.M., Peris-Fajarnes, G., Rizzuti, S. (eds.) JCM 2016.LNME, pp. 201–212. Springer, Cham (2007). doi:10.1007/978-3-319-45781-9_21

13. Ruegg, A.W.: Implementation of generalized finite element methods for homogenizationproblems. J. Sci. Comput. 17, 671–681 (2002)

14. Gonella, S., Ruzzene, M.: Homogenization of vibrating periodic lattice structures. Appl. Math.Modell. 32(4), 459–482 (2008)

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