Study On The Key Technology Of Rapid Forming Casting Molds And Cores By Micro-Jetting And Bonding

Rapid forming casting mold (core) is the important field in the study and application of rapid prototyping technology. Compare with selective laser sintering (SLS), using bonding particles layer upon layer (3DP) to manufacture casting molds having some excellent advantages, such as small equipment investment, low operation cost, long operating life, simple maintenance and good acclimatization, is more suitable for rapidly casting metal parts. At present, the main problems of the technology are as follows:the precision and strength of formed sand molds is a little low, so there are needs to be improved by post-processing and pouring technology; the processing ceramic core has a high sintering shrinkage rate, so there are needs to be improved by pre-sintering and infiltration technology, which extend the manufacturing cycle and increase the difficulty of operation. In order to search for new ways to solve the problems so that the key point of improving the rate of rapid forming molds (cores) in the research of micro jetting and bonding rapid prototyping technology can be found. The topic takes Washing Silica Sand and Ceramacore as moulding sand materials, Water glass, Phenolic resin and nano-zirconia anhydrous ethanol dispersion solution as binder, calcia powder as ceramic core materials. This topic studies the problems of key technology of rapid forming high performance casting molds (cores) based on micro jetting and bonding, and establishes the theoretical foundation for low cost rapid manufacturing casting of metal parts, having a great significance.(1) The topic have developed the micro-jetting rapid prototyping equipment and control system with micro piezoelectric double printhead and single forming cylinder for solving the problems of low precision of ejecting drops and the formed powder layer’s shifting and cracking, which have a bad influence on the casting precision and strength. The topic takes the position compensation method, optimized the control strategy and increased the jetting precision based on the model of relation among the droplet placement position error, the printhead scanning speed and the installation height. According to the model of mechanism of the formed powder layer in powder-bed progress and the pressure distribution characteristics of powder in the stock bin, the topic has developed the device in dynamic dropping style. The pressure of the powder in the bottom tends to zero, which lower the crushing stress and shearing stress of the powder layer in the powder-bed progress significantly.(2) To guarantee the process of micro jetting and bonding forming, the fluidity of ceramic powder material has to meet the need of servo falling powder pavement process, as well as the surface tension and viscosity of the binder has to be satisfied with the jetting conditions of the micropiezoelectric printhead. Physical modified method was used to improve the fluidity of calcia and adjust the surface tension and viscosity of the bonding material. Mixed by Ball mill-drying-crushing process and controlled the particle size among150-300mesh, the repose angle of the calcia ceramic powder could be reduced to36.8°, which was under the40°limit of process requirement. The precondition to improve the strength of fabrication part is to maximize the density of powder’s layer by optimizing the equipment’s parameters of powder pavement. Orthogonal test was adopted to research the discipline and mechanism on how the servo falling powder pavement equipment’s pave velocity and hopper seam width influenced the density of powder with various particle size. The results showed that the max density could be acquired under the pave velocity of100m/s and hopper seam width of2mm with the precondition that this state wouldn’t affect the forming efficiency.(3) Further research was adopted about how the casting sand’s angle coefficient and size distribution, bonding material’s surface tension and viscosity affected the axial accuracy and property of micro jetting and bonding formed sand molds. The research results show that ceramacore sand molds with1.1angle coefficient was the best, the dimension error on X, Y, Z axes were0.18mm,0.11mm,-0.04mm, tensile strength was0.88MPa, air permeability was176.8. While the saturation level of phenolic resin binder was the same, density of ceramacore layer with70-140mesh was the best, quality of formed sand mold was the best. As with the moulding sand’s particle size decreased, the pore size of sand layer decreased, adhesive penetrating power decreased, which made binder’s permeation distance decrease, as well as the sand mold’s axial dimension. The one composed by water glass had lower viscosity, curing rate and cohesion strength, higher penetrating power in sand layer, longer crosswise penetrating time, lower bonding strength of bonding bridge, which make the axial accuracy and strength are both lower than the one composed by phenolic resin. Reducing the surface tension from50mN/m to42mN/m and raising the content of phenolic resin from55%to60%improved the forming accuracy by50%,50%,40%, which was0.11mm,0.05mm,-0.03mm, as well as the tensile strength by10.39%, which was0.85MPa. Binder solution’s viscosity and surface tension affect its penetrating power in sand layer, which affects the crosswise penetrating distance; binder’s cohesion strength has great impact on the strength of bonding bridge among adjacent sand particles, which means that increasing the binder solution’s mass fraction can increase the binder’s cohesion strength, strengthen the single bonding bridge, so as the sand mold’s strength.(4) Used nano-zirconia ethanol dispersion and PVP ethanol solution as a binder, respectively, comparative studied the influence and mechanism of dispersion jetting amount on the precision and performance of calcia ceramic core green and sintered body. The results show that the dimension error and relative density of calcia ceramic core green body increased with the jetting amount of binder, the accuracy and relative density of the core formed by using dispersion were greater than formed by using PVP solution. When the dispersion jetting amount was20%, calcia ceramic core green body sintered at1300℃got the best precision and performance, the sintering shrinkage rate was2.11%, bending strength was14MPa. After jetting into the layer of calcia, nano-zirconia particles aggregat in the pores of layer and cover the surface of calcia particles, which makes the core density increases, as well as decreases the sintering shrinkage.