There are many factors that affect the casting shrinkage, such as the type and composition of the alloy, the resistance to cooling and shrinkage of the casting, and the difference of cooling conditions. Each foundry should determine the appropriate casting shrinkage according to its own production conditions. Generally, cast iron is 1%.
The thickness of the metal layer left on the machining surface of the casting to be cut is called machining allowance.
The machining allowance shall be determined according to the casting size, the casting precision achieved by the molding method, the pouring position of the machining surface and the casting structure.
There are also many factors that affect draft angle. Such as: casting size, pattern height, pattern surface roughness, molding method, etc. Generally 30 ‘- 3 °.
4.Minimum cast out hole and groove
The minimum casting hole and groove shall be determined according to the surface roughness, draft angle, hole depth, etc. The minimum hole and groove of 12-15mm can be cast in the assembly line of iron castings.
5.Process correction amount
For the round boss and chessmen that need to be processed, the size of the design drawing is often small, and the size needs to be increased artificially when casting, which is the process correction amount.
6.Negative number of typing
For the large castings produced by dry mold and resin sand, stone wool rope or other things are often used to seal the parting surface when the box is closed to prevent fire, which is easy to pad up the sand mold and affect the casting size. Therefore, when designing patterns, the pattern size is often artificially reduced at the parting surface, which is the negative number of parting.
In the process of casting large flat and bed castings, due to the uneven cooling speed, the castings often deform after cooling. In the manufacturing of pattern, the reverse deformation pattern is made according to the opposite direction of the possible deformation of the casting, so that the result of the cooling deformation of the casting just offsets the reverse deformation. The pre deformation is the inverse deformation.
8.Negative number of cores
When making core with large core box, because the contact area of the box dividing surface is too large, the sand core is always more or less covered, and the size of the sand core is larger than that of the mold. Therefore, when the core box is designed, a certain size is artificially reduced, which is the negative number of sand core. Generally 0.2-0.3mm for core shooting mechanism core.