Study On The Materials And Process Of Forming Casting Molds And Cores By Three Dimensional Printing

Three Dimensional Printing(3DP),as one of the most important addictive manufacturing technology,presents unique potential in manufacturing sand moulds(cores)with high speed,large size in terms of its abundant materials,small equipment investment,low operation cost and so on.However,the development of 3DP in fabricating sand moulds(cores)is limited due to the existing problems,such as the difficulty in maintaining the printhead,the poor stability of the process and the large gas evolution.This paper proposed a novel solution for forming sand moulds(cores)by 3DP based on solvent method and investigated the influence of material composition and process parameters on sand moulds(cores)performance.The main research contents are as following:Proposed a novel process based on solvent method to form sand moulds(cores)by 3DP.Traditional adhesive binder was replaced with liquid solvent and the forming process was finished with the dissolving and separating of the solvent.The sand powders were prepared by pre-coated technique and the properties of the powders were characterized by corresponding instruments.The ethanol based binder was used as printing liquid material.According to the properties of the phenolic resin,the post-processing scheme of the green parts at low temperature was scheduled.The prototype of 3DP was designed based on HP ink-jet printer.Studied the influence of material composition(resin/ethanol)and process parameters(spray volume/layer thickness)on the performance of sand moulds(cores).In the aspect of material composition,it was found that the tensile strength and gas evolution of 3DP specimens increased with the increasing of resin.The tensile strength increased with the increasing of ethanol,while the gas evolution was not affected by the ethanol content.On the other hand,in terms of the process parameters,the tensile strength increased with the increasing of spray volume and decreasing layer thickness while neither had impact on the gas evolution.For the sands with a resin content of 2.5%,an ethanol content of 90%,a layer thickness of 0.25 mm and twice of the spray volume per layer,the tensile strength of the green and post-processing parts was 0.48 MPa and 3.787 MPa,respectively.The gas evolution was 18.54ml/g.In addition,the dimension in X,Y,Z direction showed positive deviation and the deviation was less than 0.35 mm or 0.3%.Finally,the 12 CrMo turbine part was manufactured by this process and no obvious surface defects,such as pores or shrinkage,were found after shot pinning.Because the strength of 3DP green parts was relatively low,the raw coated sand was improved by adding ?-plaster and the effect of ?-plaster on the sand moulds(cores)property was studied.With the increasing of ?-plaster the tensile strength first increased then decreased,while the gas evolution increased obviously and the air permeability decreased at the same time.For the sand moulds(cores)with a resin content of 1.5%,the tension strength of the green parts reached up to 0.931 MPa with the addition of a 3% ?-plaster,which was three times of the raw coated sand and the tensile strength of the green parts was well enhanced.The amount of gas evolution was13.3 ml/g at 850? and air permeability was 50.07.This paper studied a novel process based on solvent method to form sand moulds(cores)by 3DP and the feasibility of forming sand moulds(cores)by 3DP with the coated sand and ethanol-based binder was validated.This process could solve the jet problem and had good stability,which provided a new method for manufacturing sand moulds(cores)by 3DP and was meaningful for the promotion and engineering application of 3DP technology.