Rapid prototyping manufacturing of silica sand patterns based on selective laser sintering.pdf

Journal of Materials Processing Technology 187188 (2007) 614618 Rapid prototyping manufacturing of silica sand patterns based on selective laser sintering J.L. Songa,b, Y.T. Lia, Q.L. Dengb, D.J. Hub aSchool of Materials Science and Engineering, Taiyuan University of Science SLS; Silica sand patterns; Sintered quality 1. Introduction Selective laser sintering (SLS) is one of the important branches of rapid prototyping manufacturing (RPM) that inte- grates computer engineering, numerical control technology, lasertechnologyandmaterialprocessingtechnology.Itwasalso one of the most signifi cant breakthroughs in recent 20 years. In the processing of SLS, laminated additive manufacturing method can be utilized to fabricate three-dimensional arbitrary shaped components from CAD models without the involving of special tools and dies. Therefore, production fl exibility and processing speed can be greatly improved, lead time and total cost can thus be reduced. Compared with other RPM technolo- gies,awiderangeofmaterialssuchasorganicpolymers,waxes, metals and ceramics can be used in SLS, the post processing is simple, and it is less time consuming. As a result, it has been highly focused since the invention in the end of 1980s and has been rapidly developed 15. Presently, the hot researching spot in the fi eld of selective laser sintering is mainly focused on the processing of metals and ceramic materials, and notable achievements have been Corresponding 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 sand patterns based on selective laser sintering was studied. Effects of process parameter on the forming qualities were investigated combiningexperimentalstudiesandmicroanalysismethods,and qualifi ed silica sand patterns have been obtained. 2. Experimental of selective laser sintering 2.1. Experimental system and materials Theexperimentalsystemconsistedofa3kWcross-fl owCO2lasermachine, a self-designed and built powder layering device, a set of computational mod- eling software and SIEMENS numerical control system. The accuracy of the numerical control system was 0.1mm. The principle of selective laser sintering system is shown in Fig. 1. In the SLS process, powders were scanned along the preplanned 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 of one layer thickness. Then the powders were preplaced on the previous sintered layers with the powder layering roller, and fi nally the whole silica sand pattern can be sintered by scanning layer by layer. The experimental materials used were self-prepared silica sand-phenol formaldehyde resin (PF resin) compounds. The main components of the silica sands were SiO2:99%, Al2O3:0.22%, and micro-content of TiO2, melting point is 1750C, the bonding agent was PF resin with grain size of 200meshes and softening point of 105115 C, the solidifi ed agent was 812% methenamine. 0924-0136/$ see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2006.11.108 J.L. Song et al. / Journal of Materials Processing Technology 187188 (2007) 614618615 Fig. 1. Principle of the SLS system. 2.2. Experimental methods In order to investigate the infl uences of process parameters on the quality and dimension accuracy of the sintered samples, the length, width and height of the multi-track laser sintered samples were respectively calibrated as L, W and H. The effects of laser power P, scanning speed F, overlapping , laser beam diameter D and powder mixture ratio on the sintered qualities and accuracy were studied respectively under conditions that other parameters were fi xed, the experimental results were average values of multiple experiments. After the sin- teringofthesamples,micromorphologiesofthesurfaceunderdifferentworking conditions were observed under KEYENCE three-dimensional microscopes. 3. Results and discussion 3.1. Effects of laser power on the sintering quality The forming mechanism of selective laser sintering silica sands lies in the absorption of laser beam energy. Under the heating action of laser beam, bonding agents are softened and molten, after the solidifi cation, silica sand grains are inserted in 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. At the same time, because the power of the laser machine jumps in a range of several watts, which leads to an obvious error while analyzing the infl uences of laser powers. On the other hand, because the electric current is relatively steady, it can be used as thereferencevalueoflaserpower.Ithasbeenobservedfromthe experiments that at an electric current of 0.8A, bonding agents PFresinhasnotbeencompletelymelted;atanelectriccurrentof 1.0A, the sintered effect was the best; while the electric power increases 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) 614618 bonized and presented a black color. Therefore, the optimized electric current under experimental conditions was 1.0A. Fig.2showstheeffectsofdifferentlaserpowersonthemicro morphologies of the sintered samples. Fig. 2(a) shows the mor- phology of the original silica sands. It showed that the grains of the original silica sands were basically uniform regular semi- transparent crystals presenting on a color of milk white and pale yellow. After the mixing and pressing, the outside of the silica sands were packaged by the bonding agents, the gaps of the sil- ica sand grains were fi lled with bonders. Fig. 2(b) is the micro morphologyunderlowerpower,itcanbeseenthatthesurfaceof thesamplepresentsacolorofpaleyellowunderlowpower,parts of the binders are not fully melted, and the bonding strength is not enough. Under medium power, the surface of the sample is inadeepbrowncolor,thesilicasandgrainsaremosaickedinthe solid binders and forms a semitransparent bonded body with an ideal sintered effect, as shown in Fig. 2(c). Fig. 2(d) shows the sintered surface under higher laser power, because the energy is too large, local areas on the surface present black brown colors and the binders has been carbonized. 3.2. Effects of scanning speed F on the dimension of the sintered samples Fig.3indicatestheinfl uenceofscanningspeedonthedimen- sion of the sintered samples at a laser beam diameter 3mm, overlapping width 0.5mm, laser power 12W, and powder ratio of silica sands and PF resin 14:1. With the increasing of scan- ningspeed,thelength,widthandheightofthesampledecreases gradually. The reason of this result is that with the increasing of the scanning speed, the dwelling time of the laser beam on the scanning spot shortened correspondingly, while the laser power is constant, the actual input energy in a unit time reduces, and the heat affected zone has also been reduced, which result in a dimension lessening of the sample. 3.3. Effect of overlapping , laser beam diameter D and powder mixture ratio on the height of the samples Intheexperiments,laserbeamdiameterDwassetto3mmby adjusting of the defocusing amount, overlapping of adjacent scanning tracks varied in the range of 0.52.0mm, laser power P was 12W, powder ratio was 11:1, and scanning speed F was 650mm/min. With the increasing of overlapping, sinter- ing depth increased. While the overlapping was increased, the absorption of laser energy increased, more quantities of binders were melted, and the depth of the softened binders increased. Underconditionsthatotherexperimentalparameterswerefi xed, laserbeamenergydensitydecreasedwiththeincreasingoflaser beamdiameter,andthesintereddepthreducedrapidly,asshown in Fig. 4. Fig. 4 also shows the relationships between powder mixture ratio and the sintered dimensions under condition of laser spot 3mm, laser power 12W, overlapping 1.1mm and scanning speed 650mm/min. It can be seen that with the increasing of , the length, width and thickness of the sample varies slightly, but not very signifi cantly. That is with the increasing of the bonding Fig. 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 softened and melted areas and the depth of the mixed powder decreased correspondingly.Therefore,thecompressivestrengthofthesin- tered samples can be enhanced with higher binder content, but too much bonding agents will result in a large amount of defor- mation and contraction, which results in the fl uctuation of the dimensions. The solidifi cation contraction of PF resin results in a difference between actual dimensions and the designed ones of the sintered samples, but there still have some rules to be fol- lowed. The powder mixture ratio of the subsequent experiments is selected as 11:1. 3.4. Post processing of the sintered samples and quality analysis Beforethepostprocessing,thereareresidualunmeltedbond- ing powders in the selective laser sintered samples, and the distribution of bonding agents is not homogeneous, the strength of the samples is low, and thermal holding process is required. Thesandpatternscanonlybeusedincastingafterthehardening J.L. Song et al. / Journal of Materials Processing Technology 187188 (2007) 614618617 Fig. 4. The relation of overlapping, spot size and powder ratio to the dimension of the sintered samples. process, in which the congregating phenomena of the bonding agent can be eliminated, water and gasifi able matters can be volatilized, and binders can be uniformly distributed when the sintered sample is holding at about 250C for 30min. In order to improve the strength of the sintered samples without car- bonization, the post processing temperature should not surpass 300C. Because selective laser sintering is a laminated material additive manufacturing process, the slicing thickness and pro- cess parameters have a signifi cant effect on the quality of the not controlled properly and the “stepping effect” will appear. The dimension accuracy and surface fi nish of the samples will be greatly affected. Simultaneously, in the processing of SLS, due to the non-uniform heating and contraction, deformation is liable to occur. To avoid these defects and improve the surface fi nish, the slicing thickness should be thin enough, and reason- able process parameters should be selected. The single layer thickness also has an effect on the property of the sintered com- ponents. Thin slicing layer will result in a higher dimensional accuracy and mechanical strength, but the manufacturing time will 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 diffi cult. Moreover, laser power density is decided by the laser power and spot size, andheatingtemperatureandperiodofthepowdersaredepended on laser power density and scanning speed. Under conditions of low power density and rapid scanning speed, parts of the pow- dershavenotenoughtimetomeltcompletely,andthestrengthof the sample is lower. Whereas, the powder temperature is exces- sively higher, the binding agents will be scorched and gasifi ed, and the sintered surface will become rough, bonding properties between layers and the sintering quality will be reduced. According to the above analysis, it is concluded that better sinteringeffectscanbeobtainedwithamediumlaserpowerand lower scanning speed. The optimized process parameters are as follows: electric current 1.0A, scanning speed 650mm/min, laserbeamdiameter3mm,overlapping0.5mmandpowdermix- ture ratio 11:1. Sand patterns sintered with previous parameters are shown in Fig. 5. 4. Conclusions Selectivelasersinteringofsilicasandshasthecharacteristics of high fl exibility, shorter lead time, lower cost, procedure cen- tralization and forming without dies. It is especially suitable for 618J.L. Song et al. / Journal of Materials Processing Technology 187188 (2007) 614618 thedevelopmentofcomplexshapedcastingsandtheproduction of single and small lot pieces. Process parameters have important effects on the property and accuracy of the sintered samples. With appropriate input laser power, powder ratio and overlapping, the surface accuracy anddimensionprecisioncanbeimproved,theinterlayerbonding strength and the mechanical strength of the whole components can also be improved. Bondingagentscanbeuniformlydistributedandthestrength of the samples can be enhanced with the post processing and holding, but the holding temperature c