Study On Microstructure Of 3D Printed Sand Mold Based On Computed Tomography And 3d Reconstruction

3D printing(3DP),an additive manufacturing technology for sand molds,has been widely used and rapidly developed in the trial production of new casting products.The method shortens the traditional casting process and provides greater flexibility for sand mold design.However,there is a major contradiction between strength and gas generation in the sand mold preparation process,which is the key factor restricting the development of 3D printing sand mold technology.Computed Tomography and its three-dimensional reconstruction are new methods to study the internal structure of materials.The CT scan technology and 3D reconstruction technology are used to analyze and study the microstructure and characteristics of 3D printed sand mold,and the 3D reconstruction technology of sand mold CT scan image is developed.The following aspects are mainly studied in this paper.The 3D reconstruction technology for sand CT images is developed using the VTK library and Open CV library on Python platform.In view of the characteristics of sand CT image,such as high resolution,many noise points and not obvious gray distinction,image preprocessing functions were developed,including a series of filtering processing,such as open operation,closed operation,expansion,corrosion,median blur,etc.The gray value distribution analysis method of the whole image is used to distinguish the difficult components in sand mold.A series of 3D reconstruction modules of slice images,including stack reconstruction,contour reconstruction and volume rendering reconstruction,were developed for the study of the internal microstructure of sand molds.The internal structure can be observed by any section cutting.By changing the threshold value,different components of sand mold can be extracted,reconstructed and STL format file output of surface discrete model can be realized.The industrial CT analysis was carried out for the sand core made by hot core box.1200 images of 1200×1200 pixels were obtained.The 3D reconstruction technology was used to carry out the 3D reconstruction of the sand core,and the 3D model of the sand core was obtained.It is found that each sand grain is uniformly wrapped by binder with high roundness.The binder produces the bonding point at the sand grain contact point.The industrial CT analysis was used to analyze the 3D printed sand mold.Sand grains,binder and void were extracted and their 3D models were constructed.Different from the sand core made by hot core box,the binder film in the 3D printing sand mold covers sand grain,but it is rough following the contour of sand with uneven thickness.There are also many gaps on the binder film.Binder can’t seep into the contact point of closely compact sand grains.Thus,each sand grain cannot be completely covered by binder.The sand grains isn’t closely compacted,and the compactness in three directions are different,with lower compactness in the direction perpendicular to the printing direction.These phenomena are determined by the layerwise printing and binder jetting method.They caused the low and directionally uneven strength of 3D printed sand mold.Through the comparison of the 3D printing mold and sand core made by hot core box,measures to improve the quality of 3D printing mold are proposed,in order to make the binder cover each sand as far as possible,sand grain bed can be heated during3 D printing or the printed sand mold can be heated so as to liquidize the binder and let it to fill the sip among sand grains under the effect of surface tension.These measures can improve the strength of sand mold.