The likeliest failure origin for advanced ceramics parts, prepared by fused
deposition, is a void from improper fill. Adequate filling of each cross-section is dependent upon the deposition toolpath. Cross-sectional spaces are conventionally
filled with pre-defined parameters. We propose that adaptive build parameters will
control variations in geometry and property of a part. Voids, overfilling,
incomplete bonding and excess traversing can be suppressed by adjusting the fill
parameters for cross-sectional areas. Improved build parameters and toolpath
allows for faster build time and components ofj full density. Some implementations
are discussed and presented.Mechanical Engineerin

Chard, Jeffrey A.

Clancy, Richard B.

Gasdaska, Charles J.

Jamalabad, Vikram R.

English

Texas ScholarWorks

TOPOLOGy.DRIVEN IMPROVEMENTOFFDCBuILDPARAMETERSVikram R. Jamalabad, Jeffrey A. Chard<\ Charles 1. Gasdaska, Richard B. ClancyAlliedSignal Inc. Research & Technology101 Columbia Road, .Morrisfown, NJABSTRACTThe likeliest failureiorigin for advanced ceramics parts, prepared byjuseddeposition, is a voi~frQ111 improper fill. Adequate filling of ear:h cross-section isdependent upo~thede12.Qsition t9Qlpath. ..Cross-sectional spacesareco~ventionallyfilled withpre-definedpqrameters... We propose that adaptive buildparameterswillcontrol variations in geometry .and property .of apart. Voids,overfilling,incomplete bonding and excess traversing caflbe. suppressed bY~dj~stin~t~efillparameters for cross-sectional areas. Improved· build parametersalJdtoolpathallows for faster build time andcomponents ofjulldensity. Some implementationsare discussedandpresentedINTRODUCTIONFused Deposition of Ceramics (FDC) as a process has been shown to be a.feasible andcompetitive technique for ceramic componentmanufacture... Descriptions ofthe FDeprocess,its advantages, limitations and achievementsaredocurnented innumerous articles [1, 2 & 3].To briefly describe the process, the relevant zone (Figure •• 1) involves the •• deposition of athermoplastic material, loaded .with ceramic powder, in the form ofa bead or "road".• Roadsare deposited exclusively in the areas that the part "exists", effectively building up the part.The part thus. deposited is called a "green" part .and this completes the FDe stage of.theprocess.The theffiloplasticis then removed from the green artifact by a binder burnout (BB())process. The resi<iualpowder is packed closely ~110ugh.toretain the shape of theinitiaLpartbut has no strength~nd has a volume density of --55%. The part, in the ."brown" stage,is thenconsolidated by. sintering to fullclensity (--99.9%). The last stage. of sintering. results inshrinkage of about. 17%.in X and Y·and abollt 19% inZ·[4].The entire process is driven automatically· by computer-generated code and firmware.enabled by.computational algorith.rn.s. In this article, ..weyvill discuss some work done atAlliedSignal over the past few months relating to software for toolpath generation.a Graduate Student at Department of Me.chanical Engineering, University of Wisconsin, Madison319Commercial software. for toolpathgenerationrefiects. the absence of a large family ofSFF users interested in fully dense\Parts. We are currently using QuickSlice™, the BOftwareprovided by Stratasyslnc., for the generation oftoolpatbcodes to drive the FD.M1650™ at our10c~tion'iWhileithis is a.nexcellent· package for most .purposesitl~?ks•the ability to .generatetoolpathcodestofullyd~nsifyparts.Painstaking developll.1entof toolpathscomes to naughtwhella.l)1inorpanuneterj~\?hanged. We are in the process of developing technology that willremove the intensive/and iterative nature of toolpath design that is currently required of FDC.Work at the University .• of Illinois [51.• and to some extent Case Western Reserve [6] isbeginning to look ataspects of toolpath layout that is relevant to SFF.tz(a) (b)Figure 1: Schematic of the Fused Deposition (FD) process. To the left (a) is a close up ofa straightenedliquefier, showing the materialbeingdepositedona flat platen to form the part. At right (b), the tool isshown traversing a path that describes the cross section of the part.True three-dimensional parts are reduced to a single-dimensional road representationthat cumulatively draws, or describes,the original part. The 3-d object (contemporarily in"StL" form) is partitioned into two-dimensional "slice" representations. Each slice is thenused to generate the single dimensional road that will describe the slice. The conversion from3-d to I-d results in a loss of information at each step of the process. (Forinstance, the fin of ablade in 3-d is nota "fin" feature in I-d, but a series of locations for the tool to visit. Whilethere may be no way to retain the required information at the road build level, determining thetoolpath in accordance with the features that. are to be built would lead. to a better buildprocess. These procedural losses are compounded with systemic lossesthat occur during thebuild itself. Systemic losses include voids and improper filling, incomplete bonding, etc [7]that need to b~accounted for prior to generating the toolpath.In the current work, we attempt to treat these problems at the· slice level. Sincemostcurrent SFF techniques, not just FDC,· rely on 2-d slices (albeit of varying thickness or shape),this is a fair assumption. Considering each slice to be the elemental structure, four issues arecrucial to the effort:)P> Adequate filling of the particular slice with no unintended voids (systemic).)P> Adequate bonding of all the deposited roads (systemic)320» Retention of2-d features to the best ability ofmachine control (procedural).» Incorporation ofmulti-material deposition to leverage 2-head machine (procedural).Given a particular material and shape to be built, the only parameters.at our disposal arethe road deposition path and the flow-control of the material. . While thetwo parameters arelinked, there is an inherent i~dependencejn t~e deposition path from the process (it isapplicable to any SFF technique using I-d fabrication procedures). The flow control aspect istied in with the material being deposited or the SFF process that is being used. In this paperwe will concentrate on our efforts of the road path.FEATURE DEFINITION AND VOlD REMOVALOne of the limiting features of Fused Deposition of Ceramics is the formation of voidsduring slice filling. These voids can be caused. by both·•.• procedural and systemic causes.Systemic loss of fill information was dealt with toa larger degree in earlier.work·[7]. In thecurrent work, procedural .causes are addressed. The solutions to these problems includesystemic errors as well. It should be noted that surface finish of parts (primarily the stair-casing effect) is not considered here. We believe that·the limits of surface improvement havealmost been reached in our process and physical machining .is the only alternative •for furtherimprovement.To keep the surface finish ofthe generated path as smooth aspossible, the build procedureutilizes a perimeter depositionfollowed by an optional contourdeposition and subsequentrastering of the internal areas of theslice. Features that are smallerthan the road· width cannot bedefined. The focus here is todefine these features better withimproved·deposition procedures.Figure 2: Two features, the fin "A" and the hole "b", can bebuilt independently but not necessarily together. We can. look at features ontwo scales - the macro-and, forthe lack of a better term, the meso- levels. Artifacts such as "thin·fins" and "small holes" aredefined as macro level .. features. We will address problems associated with interactions ofmacro-level features. Meso level features would include sharp edges andfillets..\Vhilethisdefinition is rather arbitrary, it can be considered to be a. separation between preservation oftopology and the exact recreation of features."Macro" Features:. Consider the .. construction of two features in a particular cross-section (Figure 2). The. features that the· designer wants· to create. are the fin "A"and the hole"b". While these features can be built in the first configuration, the process may not be able tobuild the second configuration due to the road width being too wide. Moreover, even if thefeature were possible with a fine road deposition, the time to build the entire part with thosesettings would increase significantly.321figure 3 . describes the .phenomenon in atoolpath generated by QuickSlice™ . An expectedhole/is notcomplet~djllithefin·structure. Unlessthickness. of the road is .• la.rger than designed, thegaP11lay norfil!. Moreover, a· seam will> bepresent at the juncture of the two sharply turningroads.Figure 3: Toolpath generated byQuickSlice™ for the interacting featuresof Figure 2 and the resulting part.Figure 4 shows an alternate approach thataUows the designer to choose the level of featuredefinition. Feature loss occurred here when twooffset contours intersected, as shown in Figure 3. During toolpath generation, the designer isqueried at such locations and given the option of continuing with the conservative procedureof Figure 3 or the overfills ofFigure 4.The advantage ofthis pro.cedureisthat qesign intentc;an be maintained w}rile the Part isbeing.. manufacture<l. AssUIl1in~ that the hole is. a Jocating. pin with the outer surfaceunimportant, the toolpath of Figure ~~a)wolJId by appropriate. For the case where thesmoothness of the outer surface is mqre important, the hole can be deformed as long as it isretained (in this case the feature is a hole of some shape).o oFigure 4: Two optionsoftheine:w toolpathgiving the designer control over feature retention.On the left the circularity of the hole is preserved, while on the right the fin surface is preserved.Mtls()~I~velFeatu..es: Another drawback of the FD procedure for making parts is therounding ofcOrners that occurs during material deposition. This may not besl.1ch.a bad thingfor engineering applications, esp~cially ceramics, as sharp corners are sites for stressconcentration.•..•Ho}Vy~er, the rolll1ding~ftheinternal roads le~ds to.sl.1b-pyrimeter voids*andothyrincomplete fiUing.. "Dog-earing" and offsetting of the rasters has been used successfuUyin suppressing these problems [7]. APart from offsetting, a preliminary form ofthe dog-earingis· available in QuickSlice™ tOl.1sers through an escape sequence. However, the effects ofboth dog-ea.ring and offsetting depend upon the angle of incidence of the raster & perimeterand the width of the road. The. algorithms being currently developed and implemented takethesy.empirically observed parameters into account.• Sub-perimeter voicis are persistent errors occurring below theperimetric road due to incomplete filling - the causeof these voids is uncertain, but some of the parameters that treat them have been identified and implemented [7].322(b)(a)We use dog"'earing basedon earlier work[lbid], offsettingand rounding similar to [8 & 9]and some proprietarytechniques to develop theperimeter and internal roads.The intent of dog-earing here isslightly different from when itwas introduced in [7]. >Inthepresent case we use dog-eatingto make sure that all points onFigure 5: Impro"ement in perimeter feature definition. the perimeter. are at most thespecified distance from the original shape. Definition of features such as sharp comers orrounded fillets is thereby achieved. We assume that material and deposition properties areconstraints, established for properbuildt . These properties are used to establish flow criteriarequired in the FD machine. Specifically, we will look at sharp comers (outer and innerperimeter).The effect of these alterations has ramificationsthat extend to internal road patterns. Assuming that thesame part is .created strictlybyrastering, Figure 6describes the toolpath asgenerate~lbyour al~orithms.The rastering •.r?add~posits .l11~t~~tlLinthe •areas thatregular rastering (as done by Qui9kSliceTM)wpl4lq nQt,Overfilling does occur,especially •.•i~th~areassuch~the lower right comer andinthenotcharea (as\Yiththecontouring build pattern ofFigure5(b)).• I-J0'Wever,overfilling can be contained. while under-filling isintolerable.Using standard procedures, the perimeter for a double wedge shape generated byQuickSlice™ is shown in Figure 5(a). The part obtained is made constructed as two separatepieces welded together at the bottom. The top C0rtlers are defined as rounds. Apart from theinaccurate part,.the weld area has notches on both surfaces. Utilizing the dog-earing procedureand rounding, the perimeter defined by the altered algorithm is shown in Figure 5(b). The partis now built as a single piece, which ensures that. knit..lines or notches will not be created.Figure 6: Raster toolpath for a double-wedge with no external perimeter.INTERNAL PATH DETERMINATION - OPTIl\1IZATIONOF TOOLPATHSDeposition of the material in the internal sections of the slice requires adjustments thatextend beyond the geometry of the cross-section. For the shape defined in Figure 7, the filldefined by QuickSlice™ is optimized for ensuring minimal fill. With the techniques used int These properties include the material composition, temperature of deposition, environmental temperature, etc.323the last section, the fill for the slice improves to the extent that voids from incomplete paths,inter-lamellar and sub-perimeter voidst are filled in.We arecurre~tlyimplementing a procedure by which the contour of the slice perimeterwill4ir~ctthe internalraster.pattern.As the raster intersects an offset of the contour, adecision\ofbreakingthe raster orincluding the contour path is made. Comparing Figure 7(a)with 7(b), th~ .effects ofthis method Call be seen. Around the hole, the standard rasteringt~c1lnjqlleterminates. This causes voids in the part, especially in the region just above the hole(Figwe7(a)). In Figure 7(b),thearea above the hole is filled in with the contour followingraster.iMoreover, the hole is also adequately defined. A negative aspect of this method is theoverfilling that occurs at the lower end of the curve where the offset road will interfere withthe straight raster. Figlll'e 7(c) shows the internal raster laid down taking into account thegeometric features· of the slice. Note the dog-earing of individual comers of the internalrastering. This ensures that the part is completely filled in./7o(a) (b) (c)Figure 7: Internal raster filling with progressively increasing quality offill.Another advantage of the procedure· is the reduction in the number of separate roads.From the analogy of a wel4ing procedure, it is important to keep the material flowingcontinuously. Stopping and •starting lea4s to higher chances of errors and under-fills. InFigures 7(b) and (c), there is a single toolpath that rasters the entire area. Tests have shownthat this comparison is valid for more complex shapes.CONCLUSIONSWe have shown that utilizing the artifacts of geometry from the original shape of theslice help in ensuring that c~rtain. levels of feature are maintained in parts built by FusedDeposition. Dog-earing and rounding perimeter curves enables adequate filling of slice-sections. We have also demonstrated that using curves conformal to the perimeter as part ofthe internal raster improves the surface definition and the internal fill. A reduction in thenumber of stops and starts in the interior is also observed, thereby reducing the problemsassociated with startup.t Sub-perimeter voids are removed in response to the angle of incidence and road width.324Further work is being conducted in determining the flow·parameters of the roads basedupon the geometric aspects of the slice. The order ofroad deposition and multiple materialsare areas that are to be developed.ACKNOWLEDGEMENTSThis work has been performed as part of the "Solid Freeform Fabrication of AdvancedCeramics Phase II" project, funded by the Office of Naval Research and The .DefenseAdvanced Research Projects Agency under Contract #N00014-97-C-0154. We appreciate thesupport and advice of Dr. Stephen Fishman, program l110nitor·and Dr. William Coblenz andMr.· Kevin Lyons, program managers for the project. We would.also like to.thank the input ofMr. Milt Ortiz, AlliedSignal Engines, Dr. Philip 1. Whalen, AlliedSignal Inc., and ProfessorVadim Shapiro, University of Wisconsin.REFERENCES[1] Comb, J. W., Priedeman, W. R. and Turley, P. W., "FDM technology process improvements",Proceedings ofthe Solid Freeform Fabrication Symposium, Austin, TX, August 1994,pp. 42-9.[2] Agarwala, M. K., et al., "Structural quality of parts processed by Fused Deposition", Rapid PrototypingJournal, Vol. 2, No.4, 1996, pp. 4-19.[3] Agarwala, M. K., et al., "FDC, Rapid fabrication of structural components", The American CeramicSociety Bulletin, Vol. 75, No. 11, November 1996, pp. 60-5.[4] Clancy, R. B., et al., "Fused Deposition of Ceramics: Progress towards a robust and controlled processfor commercialization", Proceedings of the Solid Freeform Fabrication Symposium, Austin, TX, August1997, pp. 185-94.[5] Yardimci, M. A., Personal communication, Morristown, NJ, July 1997.[6] Choi S. et al., "CAD· and control technologies for computer-aided manufacturing of laminatedengineering materials", Proceedings of the Solid Freeform Symposium,Austin, TX, August 1997, pp. 643-52.[7] Jamalabad, V. R. et al., "Process improvements in Fused Deposition of Ceramics (FDS):Progresstowards structurally sound components", Proceedings of the 1996 ASME Design for· .. ManufactureConference,Irvine, CA, August1996,pp. 18-22.[8] Tiller, W.and Hanson, E. G., "Offsets of two-dimensional profiles", IEEE Computer Graphics andApplications, Vol. 4, No. 9,September 1984, pp. 36-47.[9]Held, M., "A geometry-based investigation of the tool path generation for zigzag pocket machining", TheVisual Computer, VoL 17, No. 5/6, 1991,pp. 296-308.325326

Topology Driven Improvement of FDC Build Parameters

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Topology Driven Improvement of FDC Build Parameters

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