The structure of a part should not only help ensure the quality of the casting but also take into account the convenience of operations such as pattern making, molding, core making, and cleaning. This simplifies the casting process, improves production efficiency, and reduces the cost of the casting. The basic requirements are as follows:
1. Improve the surface structure of the casting to facilitate pattern drawing and molding
Design bosses, flanges, and ribs on the casting wall reasonably to avoid undercuts. Otherwise, additional sand cores may have to be added during machine molding, or during manual molding, bosses and ribs that hinder pattern drawing may need to be made into loose pieces. Both scenarios increase the workload of molding (core making) and pattern manufacturing to varying degrees. The arrangement of ribs should facilitate pattern drawing; ribs that can be perpendicular to the parting line should be designed as such. On non-machined surfaces, the inner and outer walls of the casting perpendicular to the parting line should have draft angles to facilitate pattern drawing. Appropriate draft angles should be designed along the direction of pattern drawing (i.e., perpendicular to the parting line). For bosses that are perpendicular to the parting line and located close to a flange, they should be connected to the flange to facilitate the removal of loose pieces and ease pattern drawing during molding.
2. Improve the internal cavity structure of the casting to reduce sand cores
The internal cavity of a casting generally needs to be formed by sand cores, which increases the labor hours for core making, tooling requirements, production costs, and cleaning workload. Unreasonable structural design of ribs, bosses, and flanges in the casting cavity is often a significant reason for the increased number of sand cores and process complexity.
3. Simplify or reduce the number of parting lines
If a casting adopts the structure shown in Figure 2-18a, a non-planar parting line process (curved parting surface) must be used, increasing the cost of pattern and mold plate manufacturing. Figure 2-18b shows an improved casting structure that allows molding using a simple central flat parting line.
The sleeve casting structure shown in Figure 2-19a requires three-part molding with two parting lines, making the molding process more difficult, difficult to automate, and resulting in low production efficiency. Figure 2-19b shows an improved sleeve casting structure that requires only one parting line and two-part molding, simplifying the molding process, improving molding efficiency, and making it easier to achieve automated production.
4. Facilitate the fixing and venting of sand cores
Figure 2-20a shows the original design of a support frame casting. Sand core No. 2 is cantilevered, making positioning and fixing difficult; core supports may sometimes be needed for fixing, and venting is not smooth.
Figure 2-20b shows the improved structure. The cantilevered core No. 2 and the shaft hole core No. 1 are integrated into a single core. Issues such as core positioning, fixing, and venting are resolved, making it easier to ensure the quality of the casting.
For thin-walled castings and castings subjected to air pressure or hydraulic pressure, the use of core supports is generally undesirable. If the structure cannot be changed, process holes can be designed in the casting to increase support points for the core prints, as shown in Figure 2-21. By adding process holes to the piston part, both sand cores can be fixed better, ensuring they do not shift during pouring and maintaining the dimensional accuracy of the internal cavity.
5. Reduce the workload of casting cleaning
Casting cleaning includes operations such as removing risers and flash, cleaning residual sand from the casting cavity, and surface blasting. Compared to molding and core making processes, the mechanization level of cleaning is relatively low, involving significant manual labor and harsh working conditions. Therefore, attention should be paid to reducing the cleaning workload during casting structure design.
Structures that reduce the number of parting lines, design cleaning holes, reduce casting stress, and prevent deformation are all beneficial for casting cleaning. In addition, the design of the casting contour should facilitate cleaning, structures that make it easier to cut off risers and reduce the number of risers, and facilitate the removal of sand cores. For example, the cast steel housing shown in Figure 2-22 has a structural improvement that facilitates the cutting and removal of risers. For the casting shown in Figure 2-23, the improved structure makes cutting risers and removing cores easier, reducing the cleaning workload.
6. Separate Casting and Combined Casting
Large and complex castings can be divided into several simple castings, which are then joined together by welding or bolting after casting. This process method is called separate casting. This method can simplify the casting process, making it possible for enterprises to produce large castings when constrained by factors such as melting capacity and hoisting/transportation capabilities.
Contrary to separate casting, to improve production efficiency and reduce casting costs, many small, simple parts like small bushings can be connected into a single long casting blank. This facilitates casting production and machining, improving product quality. This process method is called combined casting.
