Study On The Preparation And Molding Process Of Coated Sand For Laser 3D Printing

Based on the special forming method and process characteristics of laser printing technology,this paper uses two kinds of thermoplastic phenolic resins,PF-1904 and PFR-1350 mixed in a 1:1 ratio as a binder for solving the low initial strength of laser sintered parts,and the problems such as easy to break or defect during the process of emoving powders and transferring.More systematic researches have been made on the manufacturing process of resin coated sand for laser printing technology and the selective laser sintering process.With the washed silica sand of 70/140 mesh and through experiments,the main process factors affecting the performance of the resin coated sand are determined as raw sand temperature,phenolic resin content,mixture temperature when adding curing agent,and mixing time after adding curing agent.The whole process of coating silica sand is analyzed and optimized by orthogonal test.The result shows that the influence of each process parameter on the properties of coated sand is: resin content>raw sand temperature>mixture temperature when adding curing agent>mixing time after adding curing agent,and obtain the optimum combination of coating process parameters: raw sand temperature of 180°C,resin content of 3wt.%,mixture temperature of 110°C when adding curing agent,mixing time of35 s after adding curing agent,with a tensile strength of 3.6MPa at room temperature and gas evolution 18ml/g,which fully able to meet the requirements of casting production.In addition,under the premise of reaching the technical index and strength in line with the requirements of application,the resin content of ceramsite coated sand is only 2wt.%,and gas evolution significantly reduced.Compared with silica coated sand,the comprehensive performance of eramsite coated sand sintered parts is obviously improved,especially suitable for molding sand molds(cores)for complex thin parts.In this paper,SLS-500 point scanning laser rapid prototyping machine self-developed by North University of China is used to systematically study the selective laser sintering process parameters on the resin coated sand by Orthogonal Test.Through the Method of Range Analysis in Orthogonal Experimental Design,we found that the influence of sintering process parameters on the sintering characteristics of the resin coated sand is:laser power > hatching distance > scanning speed> layer thickness,and the optimum sintering process parameters are:laser power 24 w,scanning speed 1400mm/s,hatching distance 0.20 mm,layer thickness0.30 mm.Experiments have verified that the initial room temperture tensile strength of the sintered specimen reaches 0.62 MPa under the optimum sintering process parameters.Studies on the post-curing treatment process of the sintered parts show that when the holding temperature of 220°C and the holding time of 15 mins,the tensile strength of the sintered specimen can reach 3.5MPa or more,which can meet the requirements of the casting process.