The choice of parting surface is also one of the key factors for the casting process. If the casting is not selected properly, the quality of the casting can not be guaranteed, and the process of mold making, molding, core making, box closing and even cutting is complicated. Therefore, on the premise of ensuring the quality of casting, the selection of parting surface should simplify the process as much as possible and save manpower and material resources. It has been proved that the following principles should be considered in the selection of parting surface:
(1) The mold should have the least parting surface and be shaped or divided into one parting surface as much as possible. Because of one more parting surface, more errors will be added to the mold, reducing the accuracy of the casting. If the casting has only one parting surface, a simple two box molding method can be used. Figure 3-5 shows the valve cover casting. In the mass production, in order to facilitate the operation in the process of molding and box closing, the parting method in figure I is adopted, with only one parting surface. It must be pointed out that when selecting the parting surface in practice, it is necessary to proceed from the reality and sometimes adopt more than two parting surfaces for some large and complex castings or castings with special requirements, which is beneficial to the quality of castings and the simplification of process.
(2) The selection of parting surface should make the number of core and movable block as small as possible, so as to simplify the process of mold making, molding and box closing.
(3) All or large castings shall be placed in the same sand box as far as possible, so as to ensure the accuracy of castings. If there are many machining surfaces of castings, the machining datum plane should also be in the same sand box with most machining surfaces. Two parting schemes of valve disc casting. The scheme in the figure makes the whole casting in a sand box, which is conducive to ensuring the mutual position and the upper and lower concentricity.
(4) In order to facilitate the molding, lower core, box closing and inspection of casting wall thickness, the cavity and main core should be located in the lower box as far as possible. However, the lower box cavity should not be too deep, and efforts should be made to avoid the use of hanging core and large sand.
(5) The parting surface shall be flat and straight as far as possible to simplify the mold manufacturing and molding process.
(6) Make full use of the height of sand box.
The above principles are often contradictory to each other in terms of specific castings, and are difficult to fully meet. Therefore, when determining the pouring position and parting surface, we should consider comprehensively, pay attention to grasp the main contradiction, and try to solve the secondary contradiction from the technological measures.