Cima, Michael J.

Michaels, Steven

Sachs, Emanuel M.

English

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METAL PARTS GENERATIONBYTHREE DIMENSIONAL PRINTINGSteven Michaels, Emanuel M. Sachs, Michael 1. CimaMassachusetts Institute of TechnologyAbstractDimensional Printing is arapid prototyping process in which powderedmaterials are deposited in layers and s~lectively joined with binder from an ink-jet styleprinthead. Unbound pOWder is removed upon process completion, leaving a threedimensional part. Stainless steel and tungsten parts have been created from metal powderwith the 3DP process. The parts have green properties similar to those produced by metalinjection molding. A tooling insert made from 316L stainless steel powder was used toinjection mold a polypropylene part. The 3DP process is easily adaptable to a variety of..........."....,... J, >.)O systems, allowing the production of metallic/ceramic parts with novel""'''' 1-', '''.. , This paper will discuss the use of the 3DP process to produce injection......v ..""'...,,,.,., tooling and end-use parts.IntroductionMetal parts to be used as end-use parts and injection molding tooling inserts haveusing the 3DP process. This paPer describes a process where 316L stainlesspOWder is selectively bound with a latex emulsion binder using the 3DP process,producing a green part with strength comparable to parts produced by metal injectionmolding (MIM). A series of post-processing steps similar to those found in powdermetallurgy processing were used to obtain an all-metal part which was then used as atooling insert to injection mold a polypropylene part.The 3DP ProcessDimensional Printing (3DP) is a process for the rapid fabrication of threedimensional parts directly from computer models [1]. A solid object is created by printinga sequence of two-dimensional layers. The creation of each layer involves the spreading ofa thin layer of powdered material followed by the selective joining of powder in the layerby ink-jet printing of a binder material. A continuous-jet printhead is raster scanned overeach layer of powder using a computer controlled stepPer motor driven x-y table.Individual lines are stitched; together to form 2D layers, and the layers are stitched togetherto form a 3D part. The printing nozzle has a circular OPening 46 Jlm in diameter. Thenozzle is stimulated by a piezoelectric transducer vibrating at 60 kHz to break the streaminto droplets 80 Jlm in diameter. Commands to modulate the binder stream are derived fromCAD data. The powder bed is lowered at the completion of each layer by lowering thebottom of the rectangular cylinder which contains the bed. Figure 1 is a drawing of the3DP system. Unbound powder temporarily supports unconnected portions of thecomponent, allowing overhangs, undercuts and internal volumes to be created. Theunbound powder is removed upon process completion, leaving the finished part.244Fast AxisPowder andPistonBinderSupplyFigure 1. The 3DP System.Figure 2. CAD Rendering of an Injection Molding Tooling Insert.245Printing a Stainless Steel PartThe overall process to create a metal part can be divided into several steps. First,the green part is printed using the 3DP system by using a temporary organic binder. Thepart is processed using techniques similar to those used in MIM. The organic binderthermally decomposes in an inert gas furnace. Subsequent fIring at high temPeraturesinters the part and increases its strength. Firing schedules can be devised to densify thecomponent or increase the particle-particle bonding without densification. Figure 2 is aCAD drawing of the part printed in these experiments. Only the top portion of this partwas printed in order to reduce printing time.3D PrintingThe powder used in these experiments was a 316L spherical stainless steel powderwith a size range of 15 - 30 flm [2]. This powder exhibited a typical packing density of57% when spread into layers during the 3DP process. The thickness of each layer was175 flm in all the experiments.An aqueous acrylic copolymer emulsion was used as the binder [3]. This emulsionis self-crosslinking and cures by drying in air to form a high durometer solid The binderwas diluted with water to 25% acrylic solids by weight. The binder was filtered through1.2 flm capsule filters under pressure before then passing through the printhead. Totalflowrate was 1.2 cc/minute. The individual printed lines which are stitched together toform each layer were spaced 175 flm apart. Preliminary experiments showed that properchoice of flowrate and binder dilution yielded parts with high strength and stiffness whilestill being easy to print.Initial attempts at printing with this combination of powder and binder wereunsuccessful because of significant particle rearrangement. The binder stream cut a largetrench into the powder surface upon impact and capillary action would then draw thepowder up into large agglomerates. Figure 3 is a strobe photograph of this impact effect.The printhead, not visible in the photograph, is traveling right to left. Ejected powder canbe observed on the powder surface over an area as wide as 5 mm.Several methods were tried to give the powder surface enough cohesive strength toresist deformation during printing. First, the entire powder layer surface was sprayed withwater from an ultrasonic sprayer prior to printing in order to bind the layer together viacapillary tension. The desired cohesive strength was obtained, but the presence of moisturein the top layer of powder greatly enhanced transport of the binder material in the powderbed. The resulting binder "bleeding" caused a complete loss of edge definition in eachprinted layer and produced an unacceptable part.The final solution was to pretreat the metal powder with a thin coating of metal salt.A slurry composed of metal powder and dilute aqueous salt solution was blended, driedand sieved to create the coated powder. During the printing process, a layer of treatedpowder was spread, misted with ultrasonic water mist and dried with forced hot air. Thepresence of the metal salt significantly increased the cohesive strength of the layer.Presumably, the metal salt on the powder surface recrystallized during wetting and drying,forming interparticle bonds which were strong enough to resist binder stream impact andthe subsequent interparticle capillary forces. Less than 0.05 weight % salt ( based on finalpart weight) was required to achieve this cohesive effect. Figure 4 shows the dramaticimprovement which the powder surface locking process has on the printing process.The entire powder bed was placed in an oven after completion of the printingprocess and fired at 100°C for one hour to completely cure the acrylic binder. The greenpart was then removed from the powder bed. Salt bonded material is extremely weakcompared to the polymer bonds in the green part. Thus, the remaining attached powderwas easily blown off with compressed air. Excess powder was sieved and reused.246Figure 3. Printing Into Untreated Stainless Steel Powder.Figure 4. Printing Into Stainless Steel Powder Treated with Metal Salt.247DebindingBinder was removed from the green part by thermal decomposition an inert gastube furnace [4]. The binder polymer chains are broken by heating during thermaldecomposition and.the binder is evolved as a gaseous product. Binder removal from 3DPparts is similar to debinding of MIMparts but is inherently faster because less binder isrequired in the 3DP process. Atypica13DP green part is 10% by volume binder, leavingapproximately 30% open porosity. MIM parts, being solid metallbinder composites withlittle openporosity, must be debound very slowly to prevent the formation of internal gaspockets which can rupture the part. No such limitation exists for Printed parts.Debinding was done in an argon atmosphere tube furnace. Parts were heated to400°C to burnouttheacrylic binder,followed by firing at loo0°C to skeletonwith sufficientstrength to be handled afterdebinding. A typical process requiressix hours.lJpon ..completion ofthedebinillng process, parts. exhibited dimensional changeof +/-0.2% along the xandyaxes and up to +2% in the z axis. (The x axis is along theprinted line,· the y axis is across the printed line, and the z axis is across the printed layers. )SinteringMetal skeletons to be handled after the debinding stepdescribed above. Tooling were,however, sintered to higher density. Variousfiring schedules were used to parts with final densities between 65% and 92% oftheoretical. Figure 5is a photograph of a green and sintered part. In this case, the part hadbeen sintered to 78% of theoretical density, resulting in obvious shrinkage.Injection MoldingA 3DP part was used to demonstrate plastic injection molding from a SFF generateddie. The 3DP part was machined to fit into an aluminum runner assembly block andinstalled into an Engel EC88, 25 ton injection molding machine located at MIT. The matinghalf of the mold was a simple cavity cut into an aluminum block which gave the injectionmolded part additional thickness and strength.Approximately 40 polypropylene parts were injection molded. Melt temperatureand pressure were 230°C and 1200 psi, respectively. Figure 6 is a photograph of thetooling insert and injection molded part. Part removal from the 3D printed mold was madedifficult due to the absence of knockout pins in the mold, the lack of draft angle in the moldfeatures, and the relatively rough finish of the infiltrated mold surface. These factorscontributed to the rough finish on the plastic parts.EconomicsThe green parts produced by the 3DP process can be compared directly to thoseproduced by metal injection molding. In both cases, green parts are approximately60 vol. % metal, held together with a polymeric binder. The most important costassociated with MIM is that of the hard tooling required for each part. The analogous costin 3D printing is the 3D printing machine itself. The 3DP machine is, however, a universaltool capable of producing any shape. A calculation of the tooling cost associated with 3Dprinting reveals the economic feasibility of producing end-use 3D printed metal parts. Aproduction capacity 3D printing machine has an estimated price of $200,000. Over a20,000 hour service life, an additional $50,000 would be spent on maintenance andoPerating costs. The powder printing surface on such a machine might typically measure30 cm x 30 cm. The vertical build rate for a 32 nozzle printhead would be 2.5 cm per hour.Many small parts distributed throughout the powder bed could be printed simultaneously.Over 140 green parts with overall dimensions of 2.5 cm x 2.5 cm x 2.5 cm could be248Figure 5. A 3D Printed Part Sintered to 78% Density (left) and a Green Part (right).Figure 6. A Tooling Insert and an Injection Molded Polypropylene Part.249printed together, requiring an hour to produce one "layer" of parts. The associated toolingcost per part would be $ 0.09. A MIM mold to produce the same part would costapproximately $50,000. In this case, 3D printing can economically comPete with metalinjection molding in batch sizes of up to 500,000 parts.The rough·estitnateabovedernOIlStratestbatthe3DP process is a viable method formetal part production. An important aSPect of this analysis, however, is that there is nominimum batch si~e associated withthe 3DP process. Batches of 10 or 10,000 all have thesame tooling cost . Most importantly, the "tool" can be redesigned as quickly as new CADinformation can be downloaded.Conclusions3D printinghasthe capabilityt9 produce awidevanety ofcomplex metal parts to beused as tooling and end-use parts. In addition, nearly any material system which can beprovided in powder form can be used in the 3DPprocess to produce parts with novelmaterial compositions.. To date,. green parts have been printed with iron, stainless steel,tungsten, tungsten/nickel alloy, tungsten carbide and tungsten carbide/cobalt alloy.Areas .of future work include an investigation of the parameters related to partdimensional accuracy, surface finish and material properties. Methods for increasing thegreen density of as-printed parts need lobe in investigated. A better understanding of theballistic interaction of the binder stream and the powder layer is needed, as well as newmethods for preventing powder rearrangement during the printing process.References1. Sachs, E., Cima, M., Williams, P., Brancazio, D., and Cornie, 1., "ThreeDimensional Printing: Rapid Tooling and Prototypes Directly From a CAD Model",Accepted for publication in the Journal of Engineering for Industry.2. Ultrafine Powder Technology, Woonsocket, RI 028953. HA-16, Rohm and Haas, Philadelphia, PA 191054. German, R.M., Powder Injection Moldini, Metal Powder Industries Federation,Princeton, NJ, c. 1990.250

Metal Parts Generation by Three Dimensional Printing

https://repositories.lib.utexas.edu/bitstream/2152/64405/2/1992-28-Michaels.pdf

Metal Parts Generation by Three Dimensional Printing

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