Design of Metal Mold for Sand Casting

According to the casting structure and sand casting process, the metal mold for the box and cover is designed. The mold of the box mainly includes left mold block, right mold block, push plate, ejector pin, upper mold core pulling block and other mold parts, which are vertically divided, and the metal core is fixed on the left mold block (fixed mold); The box cover mold only has moving and fixed molds for vertical parting, push plate and ejector rod.

According to the principle of “temperature equalization method”, the wall thickness of the metal mold of the box is 15~30mm. However, because the sand casting is longer, its height and width are smaller, and the cavity structure is deeper, the mold base plate is thicker. The thickness of the metal mold (movable mold+fixed mold) should be 140mm, which is the height of the sand casting (90mm) plus the height of the bottom plates of the movable mold and fixed mold (25mm each). Then, according to the shape of sand castings and the installation method on the hard mold machine, the moving and fixed mold structures are designed, equipped with push plates and ejector pins. The metal mould of the box cover was designed by the same method.

The following four points should be noted during the mold making process.

(1) Dimension of mold cavity:

Where, Ax is the dimension of the cavity, mm; Ap is the average size of sand casting (after considering the minimum allowable size and tolerance), mm; K is the comprehensive linear shrinkage,%, generally 1%; δ Is the coating thickness, mm, generally 0.1~0.3; Δ Ax is the manufacturing tolerance of the cavity, mm.

Since the box and the box cover are assembled parts, it is required that the contour lines of the fitting surface edges of the two sand castings can be well matched after machining. The selection of mold size shall be based on the size of the rough drawing of sand castings, and then the machining allowance shall be adjusted.

(2) Minimize the resistance in the mold demoulding process. The dimension of the mold in the length direction is significantly longer than that in the width and height directions. Therefore, minimize unnecessary hole structures in the length direction.

(3) Die material The die material is heat-resistant die steel. The die block that contacts with aluminum liquid to form sand castings uses 5CrMnMo steel forgings, and the gate and riser use No. 45 steel.

(4) The surface requires nitriding process treatment or QPQ process treatment for the surface of moving and fixed mold blocks and pellets of the mold, and the surface of the mold cavity shall be polished to the required degree, so that defects such as backward draft mold and pits cannot be formed during manufacturing. The main body of the die is finally treated by QPQ process.

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