In sand casting, the casting process performance of the mold is directly related to the quality of the casting. The analysis of the characteristics of the Moldless sand casting mold is the basis for obtaining good performance castings, which is helpful to the subsequent Moldless mold design and mold module division based on composite forming. The traditional pattern modeling generally adopts dense mold structure, which is divided into upper and lower parts according to the pattern structure design, placement position and parting rules. The sand box is used to accurately fit and locate the upper and lower molds. In Moldless sand casting, 3D printing of sand mold can get rid of the limitation of molding in the sand box. It is necessary to divide the mold module according to the structural characteristics of the casting, and then assemble the mold module after printing. The modeling principle of sand mold 3D printing and stacking layer by layer subverts the design rules of traditional mold, which not only makes the mold tend to modular design, but also has great potential in saving molding sand. Relevant scholars have proposed design cases of hollow mold, including tetrahedral truss structure, surface stiffener structure, cooling channel type, etc., as shown in Figure 1. The mold size of sand mold NC milling usually depends on the size of the selected sand box, and is also related to the strength of the mold. When the outer wall thickness of the mold is relatively small, the sand box can ensure the stability of the prefabricated sand block in the handling, box closing and other processes, while too small mold wall thickness is easy to cause mold cracking or sand collapse.
The design of mold cavity of Moldless sand casting is also different from that of traditional mold, including casting (mold lifting) inclination, main structure of mold cavity and runner position arrangement. Among them, the setting of casting slope is necessary in the traditional mold. A reasonable casting slope is convenient for mold lifting or sand core removal. A unified casting slope can simplify the machining process of castings and improve production efficiency. Moldless sand casting can effectively avoid the increase of wall thickness due to casting inclination, resulting in the increase of metal consumption. At the same time, it can achieve high surface accuracy. For castings with high surface accuracy requirements, it can reduce machining allowance and further improve the machining feasibility of thin-walled castings. Therefore, when facing the forming of thin-walled castings, the volume of the mold cavity can be appropriately reduced. In terms of gating system design, complex sprues such as parabolic, hyperbolic and spiral can be applied to the mold, as shown in Figure 2, so as to improve the casting defects caused by the flow rate and turbulence of liquid metal during mold filling.
In addition, thermodynamic factors should be considered in the mold design of Moldless sand casting, that is, the casting process should be optimized according to the thermodynamic parameters of the mold module, and the free shrinkage, semi hindered shrinkage and hindered shrinkage of different parts of the casting should be calculated, so as to design the shrinkage rate of different mold modules to ensure the uniform shrinkage of the casting as a whole. For the division of Moldless sand casting and composite mold module based on composite forming, the following rules shall be followed:
(1) The divided mold module should be more conducive to the cleaning of the cavity than the traditional dense mold;
(2) The mold module shall have regular contact surface, the contact surface of the load-bearing part shall be set as a horizontal plane, and the division quantity shall be reduced as far as possible to improve the assembly accuracy between each mold module;
(3) For the parts requiring high surface accuracy in castings, the mold forming method with higher accuracy grade shall be adopted as an independent mold module, and the molding sand material with smaller particle size shall be selected to meet the requirements of high-precision mold surface. In addition, the subdivision of this part shall be avoided as far as possible;
(4) The divided mold module shall avoid slender structure module and fine module, so as to avoid damage to the module during processing and assembly and affect the casting forming.
