Cima, Michael J.

Sachs, Emanuel M.

English

Texas ScholarWorks

THREE DIMENSIONAL PRINTING:FORM, MATERIALS, ANI) PERFORMANCEMichael J. Cima and Emanuel M. SachsMassachusetts Institute ofTechnology, Departments of Materials Scienceand Mechanical.EngirteeringAbstractThree dimensionalprinting (3DP) is a process for rapid prototyping of functionalcomponents. Powdered materials are depositedin layers, and areas.are selectively boundby d~position ofa binder byink-jet printing. The unbound powderis removed followingsequential application of layers, resulting in a three dimensional part. The·process is wellsuited to materials such as ceramics.•and some metal alloysthatare normally processedvia a particulate state. Here we reviewrecentprogresstoward application of3DPtech-nology for the production ofmetal casting topling. Complex-shaped cores and shellswith high dimensional tolerance were prepared and used to produce castings of high tem-perature alloys.IntroductionInitial interest in rapid prototyping teehnology was motivated primarily by theneed to realize complex shapesin three dimensions. These renderings gave designers animportant tool to speed the developmentof product concepts.• Increased competitioninthe manufacturingindustri~shas caused pressure to reduce the timeto market for prod-ucts and created furtherint~rest infreeformfabricationofobjects. Thedevelopmentandcommercialization ofstereo··lithography•.• [1] •. wasa••milestonebecause•• ofthe direct·link itprovided betweenthe modern computer-aided-design(CAD)environment and realob-jects of complex shape. Its success, however, islimitedto materials that can be derivedfrom.photopolymerizable .monomers. Th.e••·added material constraints..••ofa.particularappli-cation pose significantdemands on the· chemical design of the monomer..• Thus, •• onlyap"plications thatcan use these plastics directly, or where they can be usedasmolds,willbeimpacted significantly.An alternative•approachis to make. true prototypes ofcomponents .ortoolingthatare. composed ofthe same materials as thoseingeneraluse.iThisisthe ba~is ofnewmethods like.selectivelaser.sintering(SLS) [2] •. atldthree dimensional printing (3DP) [3].Both SLS••and.3DP construct c0tnponents>from powders and therefore could potentiallyrep1icat~anYma.terial thatisnonnaHy.proc~ssedfrorrraiPartioulate st~te.Particulate Forming Technology and Rapid PrototypingParticulate forming processes are used primarily for parts composed of ceramicsor certain alloys. Solidification or deformation forming techniques for these materials187require conditions that are either unobtainable or may severely compromise the micro-structural integrity of the component. Most often, powder forming technology involvescompounding fme particles of the material with a binder and, in some cases, a liquid vehi-cle. The mixture is then molded into the desired shape by a number of methods, includ-ing injection molding, pressing, or slip casting. The methods vary greatly in their shap-ing strategies, but they all result in an intermediate state where powder of the desired ma-terial is held together by a binder which is either fugitive or may react with the matrixpowder during subsequent heat treatment.The materials objective in 3DP is to produce green components that are identicalto those prepared by conventional methods. 3DP is unique in that a solid object is cre-ated by ink-jet printing a binder into selected areas of sequentially deposited layers ofpowder. Each layer is created by spreading a thin layer of powder over the surface of apowder bed. The powder bed is supported by a piston, similar to that used in the SLSprocess, which descends upon spreading of each powder layer. Another feature which issimilar to other rapid prototyping technologies is that the instructions for each layer arederived directly from a CAD representation of the component. The area to be printed isobtained by computing the area of intersection between the desired plane and the CADrepresentation of the object. Thus, the individual sliced segments are joined to form thethree dimensional structure. The unbound powder supports temporarily unconnected por-tions of the component as the structure is built but is removed after completion of print-ing. The binder used can be exactly the same as used in conventional fabrication of theobject or may be designed to ultimately yield the same binder through chemical or physi-cal changes that take place in the powder bed after printing. In this way, true prototypescan be constructed since both the macro- and microstructural features of the productioncomponent are reproduced. A more complete description of 3DP equipment can befound in other publications [3].FounQry Materials and 3DPWe report here our recent progress toward the manufacture of investment castingtooling by 3DP. The materials used for this application are composed of refractory pow-ders such as fused silica, alumina, or zircon and a bonding material that is usually glass.The glass bond fuses when the mixture is heated and forms glass bridges between neigh-boring particles. Conventional foundry practice involves machining dies for wax posi-tives of the component and dies for cores to define hollows within the casting. The ce-ramic cores must be injection molded and fIred before being inserted into the wax die.The wax is then molded around the core. A ceramic shell is created on the outside of theresulting wax positive by sequential dipping of the wax positive in a ceramic slurry. Thewax. is then melted out of the shell followed by fuing to fuse the glass bond and give theshell some strength. Molten metal can then be poured into the shell and solidified. Theceramic shell is then removed by impact, vibration, or gritblasting. Ceramic cores are re-moved from the casting by a combination of mechanical action and chemical attack.Caustic solutions of sodium or potasium hydroxide are frequently used to dissolve theglass bond within the ceramic.188The glass bondin the shell is usually produced by colloidal silicain the ceramicslurry. Aqueous colloidal silica solutions contain as much as 20 vol% silica in the formof nanometer sized. particles. The particle suspensionisstabilized againstaggregation bythe developmentofcharge on the·surface .oftheparticles. The charg~iscreated by selec-tive adsorbtion of charge-determining ions and is commonly controlled by adjusting thepH of the solution.The materials used in investment casting must be highly optimized to meet allpro-cess requirements. The strength•must be.high enough to withstand. stress during casting,yet low enough to accommodateshrinkage during solidification and to .be removed fromthe castpart after processing. Italsomqst notreactwiththe1110lten metal and not changedimensions even with extended exposure to hightemperature. Lastly, cores must be sus-ceptible. to chemicalattackso·that they maybe removed·from the. interior of the castparts.Thesetnaterials requirements demandthat successful rapidprototyping methodsuse materials thatare as similar to those in currentpractice as possible. We currently ink-jet print colloidal silica and spread layers. of refractory. powder such as alumina. The re-sult is a material thatis remarkably similar to investment casting refractories. Shown inFigure 1 are<strengthresultsdetenninedbyJ()\.lf-point bending ofprinted bars [4]. Thebars were.printed at line spaciIlgsof 0.13mm,O.19 mm, a.ndO.2.S mm, and at relativebinder volumes (totalvolume·of silica and.water to total volume of the bar) of 40%,50%,and 60%. Thelatter parameter can be adjusted by changing the printing velocity at a con-Figure 1 Maximum bending strength (in MFa) as a funcnon of binder volumepercentage and print line spacing.1890.020 0.015 0.023stant binder flow rate. Finally, the bars were fired to 15000C prior to testing. Measure-ments were repeated four times at each combination of parameters. Also shown are con-tour lines derived by second order regression of the data. The strength is observed to varyfrom 12.3 to 18.7 MPa, values comparable to those in the foundry industry [5], andstrength depends largely on the amount of binder used. The binder content at maximumstrength is consistent with the literature on refractory shells [5]. Increasing amounts ofbinder serve to strengthen the component but eventually decrease its strength as the glassbecomes the major phase.Table I reports the dimensional changes of print~d plates of refractory material atvarious stages of processing [4]. The intended width of the plates was 38.100 mm. Aprint line spacing of 0.19 mm and 50% volume fraction of binder were used. Two fIringtemperatures, 10000e and 1500oe, were chosen as representative of the range requiredfor casting many metal alloys. The shrinkage was small and very reproducible. Theshrinkage occurring at high temperature may be related to the softening of the glass bondor reaction of the silica with alumina to form mullite.Figur~Z This photoshowsaQompletesheJ.landan as-printed cross sectionof the samepart The shell consists of apoul'ipgfunneLconnected by a gate to a casting cavity. Thematerial is alutnina powder and colloidal silica binder.190Figure 3 A cobalt-chromeshellinto which it was cast.section of theShown in Figure 2 is a photo of a complete shell and an as-printed cross sectionof the same part. The shell was printed directly from a computer file description whichwas compatible with commercial stereo lithography systems. The shell consists of apouring funnel connected by a gate to a casting cavity which has the shape of a hemi-sphere and contains several holes. Figure 3 shows a successfully cast cobalt-chromealloy made in the shell shown in Figure 3. Because this shell was made with rather largelayer height (0.178 mm) in order to speed its building rate, the surface finish would needto be improved for general applicability to the casting industry.Figures 4 and 5 show a nickel superalloy casting made from a printed shell withintegral core. The detail view in Figure 5 shows the hollow casting which requires that acore be printed during the 3DP process. The core was supported at the round cross sec-tion on top and at the rectangular cross section at the left when the part is viewed as inFigure 5. The core was printed in situ during normal 3D printing of the shell. A shellwith integral core like that shown in Figure 4 cannot be constructed by conventionalinvestment casting technology. This example illustrates how 3D printing may provide de-signers with prototypes that can not be produced any other way.A second example where 3DP can provide parts that cannot be constructed byconventional means is the production of hollow cores. Figure 6 shows I-core test sam-ples produced by 3DP. This simple shape is used to test whether a material can be suc-cessfully leached from the interior of a casting. The I-cores pictured are unusual, how-ever, because they were constructed to be hollow. Long and narrow cores are often diffi-191Figure .4•• ·Thepart ontherightisacolllPletesneUwithlntegralcore.•• The metal wouldbe poured into the. Jowe1" front •..•. The parton theJeftisahollQw casting (see detail below)which was poured into a shelllike the one shown. The casting is shown in the orienta-tion thatit would·have in the .shell.FigureS AdetaiLof thehollow(casting poured into·the shell with integral core •shownabove. Note that the core which created the intenalcavity was supported at the roundcross section on top and at the rectangular cross section at the left.192Figure 6removal by Inc:rea~slnlgwhile simultaneously reducing the amountcult to remove because they provide little cross sectional area by which toattackandre-move the material. Hollow coresincreasethesunaceareaofmaterial which can beattacked while simultaneously reducing the amount of material which mustbe removed.ConclusionsMethods to rapidly prototype functional components directly from a CAD modelare now a reality. Conventional powder-processed components such as ceramics can beproduced by techniques like three dimensional printing. We have demonstrated the pro-duction of shells and cores for investment casting by 3DP. Even at this early stage of de-velopment, 3DP has revealed itself as a method toroduce components that cannot be pro-duced by conventional processing methods. There seem to be no conceptual obstacles toapplying 3DP to other materials. We are now producing components from powderedmetals and have recently prepared complex-shaped silicon carbide preforms for metal ma-trix composites.Acknowle~~.tl1e~tsThe authors acknowledge the support ofthe Strategic ManufacturingInitiative ofthe ·National.ScienceF~undation andthe•. MIT/Industry ••Consortiumfor Three. Dimen-sional Printing.•.••.. The authors also acknowledge the work ofan extremely talented groupof researchers: D. Brancazio, J.Bredt, A. Curodeau, M.·.Esterman, T. Fan, C. Hartis,K.193Kremmin, A. Krinninger, A. Lauder, 1. Lee, S. Michaels, H. Tuerck, S. Zhang, and P.Williams.References1. T. Wohlers, "Creating Parts by Layers," Cadence, 5/8973-62. C. Deckard, J. Beaman, "Recent Advances in Selective Laser Sintering," 14th Conf.on Prod. Res. and Tech., U. of Mich. 1O/87,pp. 447-523. E. Sachs, M. Cima, P. Williams, D. Brancazio, "Rapid Tooling and Prototyping byThree Dimensional Printing" 1990 Trans. NAMRI/SME 41-454. M. Esterman, "Characterization of the Powder/Binder Interaction in the Three Di-mensional Printing Process," S.M. Thesis, Department of Mechanical Engineering,Massachusetts Institute ofTechnology, 19905. R.L. Rusher, "Strength Factors of Ceramic Shell Molds, Part I and II," Cast MetalsResearch Journal 10.(4)(1974) 149-53, and 11(1)(1975) 21-26194

Three Dimensional Printing: Form, Materials, and Performance

https://repositories.lib.utexas.edu/bitstream/2152/64335/2/1991-23-Cima.pdf

Three Dimensional Printing: Form, Materials, and Performance

Abstract

Similar works

Full text

Available Versions

Texas ScholarWorks