Current Rapid Prototyping systems are primarily aimed at small-sized objects containing many
shape details. In this paper a Rapid Prototyping technology is presented that is aimed at largesized objects having a complex, freeform outer shape. This new technology builds the model out
ofthick layers, each having freeform outside faces. The paper will present: an overview of current
methods to produce large prototypes, the basics of the new method, the technology used to
produce the layers, the toolpath planning and finally the overall system design.Mechanical Engineerin

Broek, J.J.

Horvath, I.

Lennings, A.F.

Smit, A.

Vergeest, J.S.M.

English

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Prototyping large-sized objects usingfreeform thick layers of plastic foam.A.F. Lennings, J.J. Broek, 1. Horvath, A. de Smit, J.S.M. VergeestDelft University of TechnologySubfaculty of Industrial Design EngineeringJaffalaan 9, 2628 BX Delft, The NetherlandsEmail aJ.lennings@io.tudelft.nlABSTRACT.Current Rapid Prototyping systems are primarily aimed at small-sized objects containing manyshape details. In this paper a Rapid Prototyping technology is presented that is aimed at large-sized objects having a complex, freeform outer shape. This new technology builds the model outof thick layers, each having freeform outside faces. The paper will present: an overview of currentmethods to produce large prototypes, the basics of the new method, the technology used toproduce the layers, the toolpath planning and finally the overall system design.1. INTRODUCTION.Most Rapid Prototyping systems build a model by stacking a large number of thin layers. Each ofthese layers has vertical front faces, which results in a so-called staircase effect on the surface ofthe object (zero order approximation). The thickness of each consecutive layer has to be verysmall in order to achieve a model of sufficient preciseness. This thin layer technology is wellsuited for prototypes of small-sized objects. For larger products however, the time and costinvolved are far too high: a different approach is needed. A number of solutions to this problemthat have been proposed are discussed in section 2. One of these being the combination of layerswith ruled front faces (first order approximation) and a higher layer thickness, to enhance speedwithout loosing preciseness.In section 3 of this paper we will present our new approach: a prototyping system based on higherorder approximation (e.g. second order, i.e. circular, or better). Using this technology consecutivelayers can be produced with outer surfaces that smoothly fit together. This way very high qualitymodels can be produced at moderate cost. The actual technology to be used is based on cuttingpolystyrene slabs using a flexible (hot) blade as a cutting device. The shape of the cutter can becontrolled during the cutting process, for which the theoretical base is given in section 4. Section5 describes the method to calculate the toolpath and the shape of the cutter for every tool position.The overall system design is presented in section 6, and finally in section 7 the results will bediscussed and conclusions will be given.972. RELATED RESEARCH.Rapid Prototyping is the process of automatically creating a physical prototype, based on a 3DCAD model, and within a short time. For this automatic creation of physical models threefundamentally different methods are possible [Kruth, 1995]:- Incremental (adding material), commercially available in LMT systems.- Decremental (removing material), commercially available in CNC systems- Deforming (changing the shape of flexible material), no commercial RP systems available.Next to these three base methods a number of hybrid methods have been developed too, forinstance stacking layers (incremental) that have been cut out (decremental).Incremental processes are mainly used for relatively small but complex (many shape details)models. A large advantage of these processes is that the price of the prototype is (almost)independent of its complexity. However, when the desired model has to be large, like one or morecubic meters, incremental techniques tend to be slow, complicated because of the small workingenvelope of these systems, and very expensive. So for the production oflarge models decrementalmethods are better suited, using large CNC milling machines. Currently easy-to-use CAMsoftware is available, especially developed for Rapid Prototyping purposes [Lennings, 1997]. Thisapproach is best suited for styling block models: detailed inside geometry is more suited forincremental processes. Thus, for the creation of models that both are large and contain manydetails no solution is available yet.Figure 1: Available slicing techniques.HigherFirstOrder of Approximation:ZeroFixedLayerThicknessDAdaptiveLayerThicknessIn recent years some steps forward were made inthe field of large shape prototyping. Recently asystem called Topographic Shell Fabrication(TSF) became commercially available, based onthe incremental principle [Formus, 1997]. Ituses sand as the building material, which is thenbound using wax. This system is able to buildmodels up to about 3.6 x 2 x 1.3 meters. Themodels produced by this technique are quiteheavy and can melt by sunlight because of thewax used. The main application of the techniqueis to create molds for lay up techniques.Furthermore, some new hybrid techniques havebeen developed or described. Research projectson the Universities of Utah [Thomas ea., 1996],Queensland [Hope ea., 1996, 1997a] and Delft[de Jager ea., 1996, 1997a, 1998] have deliveredsystems, algorithms and techniques that can beused to create large shapes out of thick layers.The main problems to be solved here were: howto build the shapes with as few layers as98possible, and how to avoid the staircase effect which also exists in thin layer systems but is ofmuch more. importance in thick layer systems.The solution for the first problem is adaptive slicing. Several algorithms have been developed[Hope ea., 1997b], [de Jager ea., 1997b] and [Marsan ea., 1996] that are able to generate layerswith variable thickness from a CAD model. The thickness of each layer depends on the localshape that has to be approximated and the tolerance that is given. An extruded shape can beproduced using one layer or a small number of the thickest possible layers, while a free formedshape of the same height. has to be built using many more layers to approximate the shapeaccurately.The second problem, on the staircase effect, can be reduced by using layers with sloped frontfaces (first order approximation) instead of the classical method which uses layers with verticalfront faces (zero order approximation) [Hope ea.] and [de Jager ea.]. Production of layers withsloped front faces can be done using existing cutting equipment, like 4 axis hot wire cutters, lasercutters or waterjet cutters. Using these two solutions combined results in fewer layers needed toproduce a model at a given accuracy: see the two leftmost columns of figure 1.3. A NEW APPROACH.Building a model by stacking a number of layers is in fact a perfect example of a discreteprocedure: the layer either is or is not present, causing the staircase effect mentioned before. Assaid before: sufficient for small models, but very inefficient in case of large models. The use ofsloped front faces does offer some improvement here, however is still insufficient for high qualityprototypes. In our proposed method we use a higher order approximation for the front faces ofeach layer, to be created using an analog procedure.In contrast to discrete methods, analog methods create shape by reproduction of a master, like amold for injection molding (3D example). The craftsman l110delmaker regularly uses 2D analogtools while creating a freeform model: all sorts of 2D templates that are being used as scrapingtools. [Trudeau, .1995]. A foam model of a sphere can· for instance be created by scraping. with. acircle-formed tool, using ·.. one or .more layers.•In case of a more complex shaped geometry thefront faces of the layers can.also be created by scraping with circular tools, which i$ second. orderapproximation. In many cases a better approximation is possible using freeform tools (higherorder approximation). The use of analog tools is very efficient: complex shapes can be producedin a short time. Especially when combined with adaptive layer thickness: see figure 1.However, the disadvantage of using an analog tool is that the tool first has to be created, becausean infinite amount of possible toolshapes exists. In our proposal this problem is. covered by usinga computer controlled flexible tool, that can inde.ed be. set to an infinite number of shapes. Thistoolis a heated flexible blade:. a thin, flexible metal blade that is supported at both ends by activesupports. By changing the length of the blade, the position and the orientation of the supports,theshape of the flexible blade can be influenced (figure 2). As the blade is heated it can be used tocut slabs ofpolystyrene foam, thus producing thick layers with freeform front faces.99xe.H=OH=Od.H = Ha--c.H=OM=OH=O~~-,--H = I-\,--b.M = M"mH=OM =M"M="1.H=O--yFigure 2: Possible shapes of the flexible blade.These layers have to bestacked (either auto-matically or by hand) toproduce the actual largeprototype. At theconnection between twolayers now a continuoussurface is created, with ahigh order continuity.Where standard LMTmethods produce adiscontinuous outersurface, and the ruledfront face method a zeroorder continuity(positional), the newmethod creates a firstorder continuity (tangential), or even a second order continuity (same curvature). Using thismethod will result in a large prototype of high quality, produced within a short time and atmoderate cost.4. OVERALL DESIGN OF THE NEW SYSTEM.Based on the idea of using a heated flexible blade to produce layers with freeform front faces, acomplete Rapid Prototyping system has been designed, which is presented in figure 3. It consistsof three phases. First the preparation phase, in which the geometry is imported, analyzed and ifneeded segmented. Segmentation is needed in case the total prototype cannot be manufactured inone piece because of size and/or geometry problems. We will not discuss the process ofsegmentation in this paper. Next the computing and simulation phase, in which the layers to bemanufactured are calculated and the result is simulated. Especially important for a successfulsystem is the layer thickness calculation (slicing), which has to produce layers of variablethickness in such a way that each layer can indeed be produced. For each layer the freeform frontface (nominal shape) has to be approximated by curves that can be produced by the flexible blade.Finally the third phase: the physical fabrication phase, in which the prototype is actually built on areal (designed, built and tested) machine.This overall system design consists of many processes, for all of which methods, algorithmsandphysical tools have to be developed. It is not possible to treat each of these processes in detailhere. In this paper we will discuss the most crucial items of the systems design, which are themodeling of the flexible blade and the curve matching of the blade-curve with the nominalgeometry.100calculationtool positionsimulatiomflayer fubricationcalculationtool pathPreparation phaseComputing andSimulation......-.;;.;.----;;;;;;; - - -( insertS!Physical fabricationand assemblypreliminmyshaping slabssupportstructurefabriCationinsertS?Figure 3: The overall design of the flexible cutting system.1015. THE FLEXIBLE CUTTING BLADE.As mentioned in section 3, the flexibleblade is •attached to active supports, ofwhich.both orientation·and position can bechanged. A schematic view of cuttingblade.andisuppQrts is given in figure 3.Each support is capable of transferring awinding force (to set the length of theblade), a bending moment· (to· set therotation) .. and ·a lateral force (to set theposition). It must also isolate the heatedblade in order not to loose too muchenergy.From a mechanical point of view, theflexible blade behaves like a 'physicalspline' ,which takes up its shape following Figure 4: The cutting unit, consisting of athe law ofminimum strain energy.· For the flexible blade attached to two active supports.commonly used blades the ratio betweenthe largest cross sectional dimension andthe actual length is very small « 0.01), which means that the blade is in fact a very tender bar. Onthis basis a physically based model of the flexible blade has been created, resulting in a non-lineardifferential equation which has no exact analytical solution. Details about these calculations canbe found in [Horvath, 1998-a].The problem could be solved using age0111etrically based modeling. Again the issue was to find acurve of a given length that interpolates two points ata certain angle and takes up its minimalstrain energy. For this calculation the assumptions are made that, irrespective of its cross section,the curved blade can be substituted by its profile curve, and that the curve is planar. The resultingB-Spline curve r(u) is called the profile curve of the flexible blade [Horvath, 1998-b].6. CURVE MATCHING.As said in section 3, the flexible cutting blade will be used to create layers with freeform frontfaces, which are used to build the prototype. A geometric description of each layer is derivedfrom the CAD model in a slicing process (see .section 6). A cross-section of the freeform frontface of the layer at a certain location is given by a curve q(v), called the shape curve. As thegeometry of the front face will vary following the layers circumference, an infinite number ofshape curves exists for each layer. Obviously only a limited number of curves will be processed,interpolating the front faces geometry in-between.102}figureS: The given (solid) shape curveq(v) that describes the CAD geometryneeds to be approximated by someportion of the (dashed) profile curve r(u)that describes the flexible blade.In order to calculate the toolpath for a certainlayer, a number of shape curves q has to beselected for processing. Next for each shape curvean appropriate profile curve r has to be found thatcan. be used to manufacture the layer. Curve rshould approximate curve q within a certaintolerance O. The evaluation of what can beachieved with shape curve r consists of shiftingcurve q along curve r, in such a way that the start-and endpoints b and c ofq remain on r (see figure4). For each shifted r the deviation 0 between thetwo curves can be measured by comparing anumber of points on each curve. This process iscalled curve matching. To find a suitable curve ra number of profile· curves that potentially matchwill have to be evaluated. This can be done byjust evaluating profile· curves from a library oneby one (current project stat~s) or by developing aconvergent process to find the desired profilecurve in a few iterations. The curve matchingprocess is described in more detail in [Horvath,1998-b].7. DISCUSSION AND CONCLUSIONS.tc\\\\r(u) ...\IIII~The objective of our research and development has been to come up with an effective technologyfor the fabrication of large-sized, freeformphysical models, based on higher order shapeapproximation. This technology proved to be feasible.The geometry of the flexible blade can be properly generated/calculated with the assumption ofminimum strain etlergy, specifying as boundary conditions the position and the tangency of eachsupport.Further work will focus on the slicing algorithms, the optimization of the process parameters andthe calculations and the development of the machine to be actually used.LITERATURE.* ForulUs,"TSF from Formus."PrototypingTechnology International, 1997 No 2, pp. 95.* Hope, R.L., Riek, A.T., Roth, R.N."Layer Building with Sloped Edges for Rapid Prototyping of Large Objects."103Proceedings 5th European Conference on Rapid Prototyping and Manufacturing. ,pp. 47-58Ed Dickens, P., Helsinki, 1996.* .De J~ger, P.J., Broek, J.1. andVergeest, lS.M.."Using adaptive ruled layers for Rapid Prototypil1g: principles and first results."Proceedings 8th Solid Freeform Fabrication Symposium, pp. 585-592Ed. David L. Bourell ea. Univ. of Texas, Austin, 1997, ISSN 1053-2153* De Jager, P.l"Development of a New Slicing Method to Improve Layered Manufacturing."Ph.D. thesis, Delft University Press, 1998, ISBN 90-407-1625-0* Horvath, I., Vergeest, J. S. M.,Juhasz, I. 1998-a"Finding the Shape of a Flexible Blade for Free-form Layered Manufacturing of Plastic FoamObjects." Proceeding of the ASME 1998 Design for Manufacturing Conference,Atlanta,Georgia, USA, September 13-16, pp. 25, (Status: Submitted for review).* Horvath, I., Vergeest, l S. M., Broek, l J., de Smit, A., 1998-b"Tool Profile and ToolPath Calculation for Free-form Thick-layered Fabrication."Computer Aided Design, Vol. X, No. Y, 1998, pp. 26, (Status: Submitted for review).* Kruth, J."Rapid Prototyping, aNew Application of Physical and Chemical Processes for MaterialAccretion Processes." Proceedings International Symposium for ElectroMachining, pp. 3-28EPFL, Lausanne, Switzerland, 1995.* Lennings, A.F."Advantages of using CNC milling for Rapid Prototyping"Proceedings 30th International Symposium on Automotive Technology & Automation (ISATA)Florence, Italy, June 1997, pp. 113-120Ed D. Roller, Automotive Automation Ltd, Croydon, 1997, ISBN 0 947719 96 2.* Marsan A., Dutta, D.,"Survey of Process Planning Techniques for Layered Manufacturing."Proceedings of 1997 ASMEDesign Engineering Technical Conference.ASME, New York, 1997, CD-ROM #DETC97/DAC3988.* Thomas, C.L., Gaffney, T.M., Kaza, S., Lee, C.H."Rapid Prototyping of Large Scale Aerospace Structures."Proceedings 1996 IEEE Aerospace Applications Conference, Vol. 4.IEEE, 1996, ISBN 0-7803-1396-6* Trudeau, N."Professional Model making."Watson-Guptill Publications, New York, 1995.104

Prototyping Large-Sized Objects Using Freeform Thick Layers of Plastic Foam

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Prototyping Large-Sized Objects Using Freeform Thick Layers of Plastic Foam

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