Study On The 3D Printed Hollow Sand Molds

Mold is necessary for the forming of castings.Traditional molds are dense,which leads to the difficulty in the cooling control of castings during the casting process and low cooling efficiency of heavy castings after solidification.A hollow mold proposed in this dissertation provided the possibility of cooling control of castings.It can improve cooling efficiency,reduce stress and deformation,improve the microstructures and the properties of castings,and evidently reduce the usage of molding material and the discharge of waste materials.This is in accordance with the trend of intelligent and environmental protection.A systematic research was conducted on the hollow sand mold in this dissertation.A hollow mold based on 3D printing was proposed,which consists of a shell and an external hollow structure.An intelligent closed-loop control system for this kind of hollow mold was established with real-time monitoring and control of the temperatures of mold and castings.Cooling control of this kind of mold can be achieved through temperature difference feedbacks,which further achieves the cooling control of castings.Design methodology for this kind of hollow sand mold was established.A program for its generation for castings was developed based on the STL file and finite differential meshes of a casting.The shell with varying thickness was formed in accordance with the thickness variation of a casting.The output STL file of the hollow sand mold can be directly applied for 3D printing.The thermo-mechanical coupled numerical simulation of castings and hollow sand molds during the casting process was realized using ProCast and Ansys.A numerical simulation study of a stress frame specimen with hollow molds was carried out.The temperature,stress and deformation of the casting and different hollow molds were obtained.Results showed that hollow molds had the advantages of high cooling efficiency and low stress and deformation level by controlled cooling.The optimization of the hollow mold structure was performed.Influences of a hollow mold on a stress frame and a tapered plate under natural cooling,overall controlled cooling and locally controlled cooling were studied by casting experiment with aluminum alloy A356.Results of the study showed that the cooling time of the stress frame specimen was improved by over 20% under natural cooling conditions and a further 32% under overall controlled cooling conditions by using the hollow sand mold.Meanwhile,under controlled cooling conditions,the deformation of the casting was reduced by 43%,the eutectic silicon was refined,tensile strength was improved by 17%,yield strength by 11%,and elongation by 67%,Meanwhile,the usage of molding sand was reduced by over 60%.The obtained castings had a smooth surface with no flash or rag.This kind of hollow mold was applied to an Aluminum hub casting.Controlled cooling for this casting was realized and sound castings were obtained.The usage of molding sand was significantly reduced.Influences of cooling control on the cooling efficiency,stress and deformation,microstructure,and mechanical properties of the hub casting were analyzed.Under controlled cooling,the cooling efficiency of the aluminum alloy hub was improved by 62%,the residual stress of the casting's surface was transformed to compressive stress,and its tensile,yield strengths and hardness were improved as well.