Investigation Into The Preparation And Intermolecular Cooperativity Effect In Composite Wax Powder For Selective Laser Sintering

Combined with investment casting,Selective Laser Sintering（SLS）can be used to produce the precision castings with the thin walls and complex shape parts,leading to a broad application prospect.For the polymer materials,such as polystyrene and polycarbonate,being used in SLS casting investment mold,some toxic and harmful gases and dusts are formed during demoulding.As a result,the environments are polluted and the human healths are endangered.Furthermore,unfortunately,the traditional investment casting wax powder has low melting point,and thus it is difficult to control sintering process.Also,it will be severely warpad.All the above deficiencies,however,are expected to be solved efficaciously by the the investment of the casting materials for SLS based on polyethylene wax.Focusing on the microstructure,the intermolecular interactions and their cooperativity effects among the components in the composite wax powders are discussed by means of quantum chemical density functional theory and ab initio calculation theory,and the nature of the compatibility among components of the SLS investment molding material with polyethylene wax is revealed,and the reasons for improving the properties of the composite wax powder molding parts are further explored.Based on above theoretical investigations,the composite wax powder material for SLS used in thin-walled complex castings was developed,with the related experimental and molding technological investigations.The main contents are as follows:（1）We choose Mⁿ⁺···coronene···CH₄(Mⁿ⁺=Li⁺,Na⁺,K⁺,Be²⁺,Mg²⁺or Ca²⁺)as the model systems of composite wax powder of PEW/C/SDBs.We employ B3LYP,M06-2X and MP2 methods with 6-311++G**basis set to make a research of interaction energies and cooperativity effects in the ternary complex.The cooperativity effects were found,which were in the orders of coronene···Li⁺>coronene···Na⁺>coronene···K⁺and coronene···Be²⁺>coronene···Mg²⁺>coronene···Ca²⁺.The essence of cooperativity effects were revealed by the analyses of RDG（reduced density gradient）and AIM（atoms in molecules）.The cooperativity effect can make the compound wax powder interact tightly with each other,which may cause significant changes in the mechanical properties of the molded parts from the micromechanism,such as the improved compatibility,reduced stress and strain during sintering process,etc.（2）In order to explore the effect of materials compatibility on graphene oxide（GO）replacing carbon black on the compatibility of materials,the ternary complex Na⁺···Graphene oxide（GO）···CH₄ was selected as a composite wax powder model system to evaluate the interaction and cooperativity effect between the Na⁺···σ/πand H-bonding interactions in the composite wax powder at the M06-2X/6-311++G（2d,p）and MP2/6-311++G（2d,p）levels.The interactions and cooperativity effects in Na⁺···GO···CH₄ were stronger than those in Na⁺···coronene···CH₄.Therefore,it could be inferred that,if graphite powder or carbon black was replaced by GO,the interactions and compatibilities could be strengthened among various components,and thus the performance of the sintering parts could be further improved.（3）Based on the theoretical calculation results,the PEW/C/SDBS and PEW/GO/SDBS composite wax powders were produced.The performance testing and structural characterization were carried out,verified the theoretical calculation results,and performed sintering experiments.After showing the formation of trimers,the stretching vibration peak of the C-H bond was obviously strengthened,and the O-H bond vibration peak was disappeared,and the peak of C-OH was moved toward the blue region,and the trimer had an extra strong peak at 1104.9 cm^–1,Shows that the interaction between molecules was strengthened and the synergistic effect was verified.Compared the peak at 1104.9 cm^–1 of PEW/GO/SDBS with that of 1109.5 cm^–1 of PEW/C/SDBS,both the strength and area of the peaks were significantly enhanced,indicating that the cooperativity effect of the interactions among components was more significant when carbon black was instead of GO.From DSC,the melting peaks of the two composites were gentle and wide,which will beneficial to control the sintering molding process.Compared with PEW/C/SDBS,the viscosities,bending and compressive strengths of test pieces of PEW/GO/SDBS composite wax powder were all improved.This indicates the cooperativity effect in PEW/GO/SDBS is more significant,and it was due to the interactions among the functional groups of GO,Na⁺in SDBS and C-H bond of polyethylene wax.（4）The optimum sintering parameters of two kinds of wax powders,PEW/C/SDBS and PEW/GO/SDBS,were obtained by orthogonal test.With the optimal sintering process parameters,the shrinkage and warpage of the sintered parts were effectively controlled.Compared with PEW/C/SDBS,for PEW/GO/SDBS the warpage rate was lower,and bending and compressive strengths were strengthened.These results indicate that a significant synergistic effect has occurred between the components of the composite wax powder,the material compatibility has been improved,and the stress and strain have been reduced.Furthermore,the compatibility was further improved by GO replacing carbon black.（5）The mechanical properties of the molded parts were greatly improved after the immersion wax treatment.The wax pattern made from the composite wax powder can be dewaxed by the steam method.The investment casting made of composite wax powder has good performance,such as complete frame,smooth surface and high dimensional accuracy.No obvious defects,such as cracks,air holes and inclusions,were found in the nondestructive testing.It is shown from above results that the composite wax powder can be used in the investment casting of the thin-walled complex castings without environmental pollution.In this work,the intermolecular interactions and their cooperativity effects between the components of the materials were investigated by the theoretical calculation,and from the microscopic level of view,the essential reason for the influence of the components on the properties was deeply discussed.A new environmentally friendly polyethylene wax composite was developed,which has the important theoretical and practical significance for the selection of the components and process optimization of SLS molding and casting.The research combines computational material science with additive manufacturing molding materials,and provides a new idea for the research and development of new materials for additive manufacturing.