Study On Preparation And Forming Pcocess Of The Coated Sand For Selective Laser Sintering

The selective laser sintering technology enables overall fast and accuratepreparation of complex sand mold (core), provides an excellent way for all kinds oflarge complex thin-walled precision castings, has an important role of improving therapid response, manufacturing capability and manufacturing level of the key parts suchas aviation, aerospace and automobile industries areas. Laser sintering formability ofthe prepared coated sand material is less than ideal, mainly phenolic resin content ofcoated sand is too much, the initial intensity of the laser sintering parts is low,especially the fine structure of laser sintering is still very difficult to form and couldnot achieve practical industrial requirements of mechanical properties and gasevolution.The pearl sand of angular coefficient≤1.1is used as the laser sintering raw sand,designed and prepared the sand material of different size composition using thermalcoating process, make the experiments of laser sintering coated sand, study the affectof particle properties and sintering process parameters on the forming of laser sintering,and further focused on the law of laser sintering coated sand in conditions under thesame and different laser energy density. The main contents and conclusions are asfollows:The pearl sand is the laser sintering raw sand, the mobility of spherical sand isgood, could reduce the amount of resin, powder spreading of laser sintering moresmooth and uniform; adjustment gradation of coated sand can improve the shoppowder density of laser sintering, with the decrease of coated sand particle size, shoppowder density of laser sintering decreases, that of the adjacent five sieve sand withinthe average particle size at the range of0.053~0.150mm increased by21.3%comparedto the adjacent three sieve sand.Design and prepare a series of coated pearl sand material, obtained the best ofthermal coating process parameters for the coated sand of laser sintering throughorthogonal test: sand temperature of the resin accession to120℃and mixed grind time80s, the mixture temperature of reinforcing agent95℃and mixed with grind time60s.Surface morphology analysis of the prepared coated sand showed that phenolic resinevenly coated in the surface of pearl sand, the differential thermal analysis (DTA)showed that the curing temperature of pearl sand is150~160℃, decompositiontemperature of organic matter is250℃.The law of laser sintering process parameters and powder spreading parameters onthe forming accuracy and formability are studied, the key process conditions of laserrapid prototyping for high-performance coated sand mold (core) is got. The matchesvalue of laser rate and scanning speed directly impact on the laser sintering quality ofcoated sand, scan spacing value of0.15~0.20mm, the shop powder thick with thereasonable range of0.15~0.25mm. Study of particle characteristics on laser sintering accuracy and mechanicalproperties shows particle size and gradation of the coated sand have a significant effecton the laser sintering of coated sand mold (core). As particle size decreases, theintensity of laser sintering parts gradually reduced; the adjacent five sieve sand ofaverage particle size less than0.105mm can effectively guarantee the forming accuracyof the sintered parts.The overall regulation of laser rapid prototyping process through the energy densityis proposed, provided the key technical measures and scientific guidance for improvingthe surface quality, controling forming mechanism and increasing the sintering strength.As the laser energy density increases, the mechanical properties of samples graduallyincreases, the dimensional accuracy obviously decrease after the laser energy densitygreater than0.032J/mm~2, the most appropriate laser sintering domain is of0.024~0.032J/mm~2. The study also found: sintering effect of the same laser energydensity has obvious range difference, when laser energy density is less than0.024J/mm~2the sinter performance equivalence; when the laser energy density isgreater than0.024J/mm~2, as the increase of P and v values under the conditions of thelaser energy density, the strength of the sample is slowly increasing and then has aslight downward trend.The effects of adding the inorganic binder on mechanical properties and gasevolution of laser sintering rapid prototyping of coated sand show that at a low resincontent adding the inorganic binder to improve the early strength of laser sintered parts,and can substantially reduce the gas evolution of coated sand mold (core); in the coatedsand resin content of1.5wt%by adding3.0wt%inorganic binder, the early strengthof laser sintered parts increased by50%; gas evolution is10.90g/ml only increasing10%,that have a significant effect in improving the quality performance of the coatedsand, achieve the laser sintering and precision forming of high strength and low gasevolution of the complex shape coated sand mold (core).