When the temperature of the casting surface drops below the solid phase line temperature, the casting surface solidifies a layer of hard shell enclosing the inner liquid metal, and the inner runner of the casting system is frozen.
When further cooled, the liquid metal in the hard shell shrinks due to the decrease of temperature, which supplements the solidification shrinkage of the hard shell. At the same time, the solidified hard shell shrinks due to the decrease of temperature. If the volume reduction caused by liquid metal shrinkage and solidification shrinkage in the hard shell is equal to the volume reduction of the surface of investment casting, the solidified shell is still in close contact with the liquid metal in the hard shell, and no shrinkage hole will occur. However, the liquid shrinkage and solidification shrinkage of the alloy far exceed the solid shrinkage of the hard shell, so the liquid will fall off from the top of the hard shell. In succession, the hard shell will be thickened, the liquid level will continue to decline, all the metal solidification, in the upper part of the casting formed an inverted cone-shaped shrinkage hole. When the temperature drops to room temperature continuously, the volume of the whole casting decreases continuously, and the absolute volume of shrinkage hole decreases relatively, but the relative volume of the casting remains unchanged. If the riser is provided at the top of the casting, the shrinkage hole will be moved to the riser.
In summary, the basic reason for the centralized shrinkage is that the liquid shrinkage and solidification shrinkage of the alloy is much larger than the solid shrinkage; the conditions for the centralized shrinkage are that the casting solidifies layer by layer from the surface to the inside, and the shrinkage holes are concentrated in the final solidification part.
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