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Journal of Materials Processing Technology 187188 (2007) 614618Rapid prototyping manufacturing of silica sand patternsbased on selective laser sinteringJ.L. Songa,b, Y.T. Lia, Q.L. Dengb, D.J. HubaSchool of Materials Science and Engineering, Taiyuan University of Science & Technology, Taiyuan 030024, ChinabSchool of Mechanical and Power Engineering, Shanghai Jiaotong University, Shanghai 200030, ChinaAbstractInrecentyears,rapidprototypingmanufacturingtechnology(RPM)hasgraduallybecomingmatured,andhasbeenwidelyusedinthefabricationof functional and practical metal and ceramic components. However, researches on the selective laser sintering (SLS) of silica sands were verylimited.ExperimentsonrapidprototypingmanufacturingofsilicasandswerecarriedoutbasedonSLS.Micromorphologiesofthesinteredpatternsunder different working conditions were observed with three-dimensional optical microscopy (OM). Influences of process parameters such as laserpower, scanning speed, overlapping rate, laser beam diameter and powder mixture ratio on the dimension accuracy and sintered qualities wereinvestigated systematically. It indicated that “stepping effect” and deformation of the sintered samples reduced through the decreasing of slicingthickness and the optimization of process parameters. Finally, qualified selective laser sintering silica sand patterns were obtained. 2006 Elsevier B.V. All rights reserved.Keywords: Rapid prototyping manufacturing; SLS; Silica sand patterns; Sintered quality1. IntroductionSelective laser sintering (SLS) is one of the importantbranches of rapid prototyping manufacturing (RPM) that inte-grates computer engineering, numerical control technology,lasertechnologyandmaterialprocessingtechnology.Itwasalsoone of the most significant breakthroughs in recent 20 years.In the processing of SLS, laminated additive manufacturingmethod can be utilized to fabricate three-dimensional arbitraryshaped components from CAD models without the involvingof special tools and dies. Therefore, production flexibility andprocessing speed can be greatly improved, lead time and totalcost can thus be reduced. Compared with other RPM technolo-gies,awiderangeofmaterialssuchasorganicpolymers,waxes,metals and ceramics can be used in SLS, the post processing issimple, and it is less time consuming. As a result, it has beenhighly focused since the invention in the end of 1980s and hasbeen rapidly developed 15.Presently, the hot researching spot in the field of selectivelaser sintering is mainly focused on the processing of metalsand ceramic materials, and notable achievements have beenCorresponding author. Tel.: +86 351 6963339; fax: +86 351 6963369.E-mail address: jlisong (J.L. Song).achieved. However, as a kind of abundant and cheap material,researchesontheSLSofsilicasandwerestillverylimited68.In this paper, rapid prototyping manufacturing of silica sandpatterns based on selective laser sintering was studied. Effectsof process parameter on the forming qualities were investigatedcombiningexperimentalstudiesandmicroanalysismethods,andqualified silica sand patterns have been obtained.2. Experimental of selective laser sintering2.1. Experimental system and materialsTheexperimentalsystemconsistedofa3kWcross-flowCO2lasermachine,a self-designed and built powder layering device, a set of computational mod-eling software and SIEMENS numerical control system. The accuracy of thenumerical control system was 0.1mm. The principle of selective laser sinteringsystem is shown in Fig. 1. In the SLS process, powders were scanned along thepreplanned tracks by laser beam according to the CAD model of the compo-nents. After scanning of one layer, the piston was lowed down for a distance ofone layer thickness. Then the powders were preplaced on the previous sinteredlayers with the powder layering roller, and finally the whole silica sand patterncan be sintered by scanning layer by layer.The experimental materials used were self-prepared silica sand-phenolformaldehyde resin (PF resin) compounds. The main components of the silicasands were SiO2:99%, Al2O3:0.22%, and micro-content of TiO2, melting pointis 1750C, the bonding agent was PF resin with grain size of 200meshes andsoftening point of 105115C, the solidified agent was 812% methenamine.0924-0136/$ see front matter 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.jmatprotec.2006.11.108J.L. Song et al. / Journal of Materials Processing Technology 187188 (2007) 614618615Fig. 1. Principle of the SLS system.2.2. Experimental methodsIn order to investigate the influences of process parameters on the qualityand dimension accuracy of the sintered samples, the length, width and height ofthe multi-track laser sintered samples were respectively calibrated as L, W andH. The effects of laser power P, scanning speed F, overlapping , laser beamdiameter D and powder mixture ratio on the sintered qualities and accuracywere studied respectively under conditions that other parameters were fixed, theexperimental results were average values of multiple experiments. After the sin-teringofthesamples,micromorphologiesofthesurfaceunderdifferentworkingconditions were observed under KEYENCE three-dimensional microscopes.3. Results and discussion3.1. Effects of laser power on the sintering qualityThe forming mechanism of selective laser sintering silicasands lies in the absorption of laser beam energy. Under theheating action of laser beam, bonding agents are softened andmolten, after the solidification, silica sand grains are insertedin the bonders and constituted a solid state bonding skeleton.Because the softening point of the silica sand is rather lower,only about 110C, and the optimized heating hardening tem-perature is about 250C, the required power is quite lower. Atthe same time, because the power of the laser machine jumps ina range of several watts, which leads to an obvious error whileanalyzing the influences of laser powers. On the other hand,because the electric current is relatively steady, it can be used asthereferencevalueoflaserpower.Ithasbeenobservedfromtheexperiments that at an electric current of 0.8A, bonding agentsPFresinhasnotbeencompletelymelted;atanelectriccurrentof1.0A, the sintered effect was the best; while the electric powerincreases to a value of 1.2A, the surface of the sample was car-Fig. 2. Effect of laser power on the micro morphology of the sintered samples: (a) original silica sand particles (b) low power, (c) medium power, and (d) high power.616J.L. Song et al. / Journal of Materials Processing Technology 187188 (2007) 614618bonized and presented a black color. Therefore, the optimizedelectric current under experimental conditions was 1.0A.Fig.2showstheeffectsofdifferentlaserpowersonthemicromorphologies of the sintered samples. Fig. 2(a) shows the mor-phology of the original silica sands. It showed that the grainsof the original silica sands were basically uniform regular semi-transparent crystals presenting on a color of milk white and paleyellow. After the mixing and pressing, the outside of the silicasands were packaged by the bonding agents, the gaps of the sil-ica sand grains were filled with bonders. Fig. 2(b) is the micromorphologyunderlowerpower,itcanbeseenthatthesurfaceofthesamplepresentsacolorofpaleyellowunderlowpower,partsof the binders are not fully melted, and the bonding strength isnot enough. Under medium power, the surface of the sample isinadeepbrowncolor,thesilicasandgrainsaremosaickedinthesolid binders and forms a semitransparent bonded body with anideal sintered effect, as shown in Fig. 2(c). Fig. 2(d) shows thesintered surface under higher laser power, because the energy istoo large, local areas on the surface present black brown colorsand the binders has been carbonized.3.2. Effects of scanning speed F on the dimension of thesintered samplesFig.3indicatestheinfluenceofscanningspeedonthedimen-sion of the sintered samples at a laser beam diameter 3mm,overlapping width 0.5mm, laser power 12W, and powder ratioof silica sands and PF resin 14:1. With the increasing of scan-ningspeed,thelength,widthandheightofthesampledecreasesgradually. The reason of this result is that with the increasing ofthe scanning speed, the dwelling time of the laser beam on thescanning spot shortened correspondingly, while the laser poweris constant, the actual input energy in a unit time reduces, andthe heat affected zone has also been reduced, which result in adimension lessening of the sample.3.3. Effect of overlapping , laser beam diameter D andpowder mixture ratio on the height of the samplesIntheexperiments,laserbeamdiameterDwassetto3mmbyadjusting of the defocusing amount, overlapping of adjacentscanning tracks varied in the range of 0.52.0mm, laser powerP was 12W, powder ratio was 11:1, and scanning speed Fwas 650mm/min. With the increasing of overlapping, sinter-ing depth increased. While the overlapping was increased, theabsorption of laser energy increased, more quantities of binderswere melted, and the depth of the softened binders increased.Underconditionsthatotherexperimentalparameterswerefixed,laserbeamenergydensitydecreasedwiththeincreasingoflaserbeamdiameter,andthesintereddepthreducedrapidly,asshownin Fig. 4.Fig. 4 also shows the relationships between powder mixtureratio and the sintered dimensions under condition of laserspot 3mm, laser power 12W, overlapping 1.1mm and scanningspeed 650mm/min. It can be seen that with the increasing of ,the length, width and thickness of the sample varies slightly, butnot very significantly. That is with the increasing of the bondingFig. 3. Effect of scanning speed on the dimension of the sintered samples.agents, the dimensions of the sintered samples slightly increase.Due to the reduction of the bonding agent contents, the connec-tion bridges among silica sand powders reduced, the softenedand melted areas and the depth of the mixed powder decreasedcorrespondingly.Therefore,thecompressivestrengthofthesin-tered samples can be enhanced with higher binder content, buttoo much bonding agents will result in a large amount of defor-mation and contraction, which results in the fluctuation of thedimensions. The solidification contraction of PF resin results ina difference between actual dimensions and the designed onesof the sintered samples, but there still have some rules to be fol-lowed. The powder mixture ratio of the subsequent experimentsis selected as 11:1.3.4. Post processing of the sintered samples and qualityanalysisBeforethepostprocessing,thereareresidualunmeltedbond-ing powders in the selective laser sintered samples, and thedistribution of bonding agents is not homogeneous, the strengthof the samples is low, and thermal holding process is required.ThesandpatternscanonlybeusedincastingafterthehardeningJ.L. Song et al. / Journal of Materials Processing Technology 187188 (2007) 614618617Fig. 4. The relation of overlapping, spot size and powder ratio to the dimensionof the sintered cess, in which the congregating phenomena of the bondingagent can be eliminated, water and gasifiable matters can bevolatilized, and binders can be uniformly distributed when thesintered sample is holding at about 250C for 30min. In orderto improve the strength of the sintered samples without car-bonization, the post processing temperature should not surpass300C. Because selective laser sintering is a laminated materialadditive manufacturing process, the slicing thickness and pro-cess parameters have a significant effect on the quality of thenot controlled properly and the “stepping effect” will appear.The dimension accuracy and surface finish of the samples willbe greatly affected. Simultaneously, in the processing of SLS,due to the non-uniform heating and contraction, deformation isliable to occur. To avoid these defects and improve the surfacefinish, the slicing thickness should be thin enough, and reason-able process parameters should be selected. The single layerthickness also has an effect on the property of the sintered com-ponents. Thin slicing layer will result in a higher dimensionalaccuracy and mechanical strength, but the manufacturing timewill be prolonged. If the layer is too thin, uniformity and com-Fig. 5. Sand patterns sintered by selective laser sintering.pactibility of powder layering will become difficult. Moreover,laser power density is decided by the laser power and spot size,andheatingtemperatureandperiodofthepowdersaredependedon laser power density and scanning speed. Under conditions oflow power density and rapid scanning speed, parts of the pow-dershavenotenoughtimetomeltcompletely,andthestrengthofthe sample is lower. Whereas, the powder temperature is exces-sively higher, the binding agents will be scorched and gasified,and the sintered surface will become rough, bonding propertiesbetween layers and the sintering quality will be reduced.According to the above analysis, it is concluded that bettersinteringeffectscanbeobtainedwithamediumlaserpowerandlower scanning speed. The optimized process parameters areas follows: electric current 1.0A, scanning speed 650mm/min,laserbeamdiameter3mm,overlapping0.5mmandpowdermix-ture ratio 11:1. Sand patterns sintered with previous parametersare shown in Fig. 5.4. ConclusionsSelectivelasersinteringofsilicasandshasthecharacteristicsof high flexibility, shorter lead time, lower cost, procedure cen-tralization and forming without dies. It is especially suitable for618J.L. Song et al. / Journal of Materials Processing Technology 187188 (2007) 614618thedevelopmentofcomplexshapedcastingsandtheproductionof single and small lot pieces.Process parameters have important effects on the propertyand accuracy of the sintered samples. With appropriate inputlaser power, powder ratio and overlapping, the surface accuracyanddimensionprecisioncanbeimproved,theinterlayerbondingstrength and the mechanical strength of the whole componentscan also be improved.Bondingagentscanbeuniformlydistr