The pouring position of a casting refers to the state and orientation of the casting within the mold during pouring. It directly affects the structure of tooling (such as patterns and core boxes) and the procedures for operations like core setting, mold closing, and cleaning, and may also influence subsequent machining. The pouring position can differ from the molding (closing) position and the cooling position of the casting. Horizontal pouring, vertical pouring, and inclined pouring do not represent the pouring position itself, but only indicate the spatial orientation of the parting line during pouring.
The pouring position is generally determined after selecting the molding method. It should be comprehensively considered in conjunction with the casting's size, structural characteristics, alloy properties, production batch, and the enterprise's production conditions. The proposed process plans should be analyzed and compared to select the appropriate pouring position. A correct pouring position should ensure the production of sound castings while facilitating molding, core making, and cleaning operations. The general principles for determining the pouring position are as follows:
(1) Critical sections should be placed at the bottom.
During pouring, impurities such as gases, slag, and sand particles tend to float upward. Consequently, the upper part of a casting is prone to defects like gas porosity, slag inclusions, and sand inclusions, meaning the likelihood of casting defects is significantly higher in the upper part than in the lower part. Furthermore, during solidification, the lower part of the casting benefits from the static pressure of the molten metal above and receives feeding, resulting in a denser structure. Therefore, critical sections of a casting should be placed at the bottom whenever possible. For instance, the guide rail surface of a machine tool bed is a critical surface that must be free from defects like sand holes, gas porosity, slag inclusions, cracks, and shrinkage porosity, and requires a dense, uniform structure with guaranteed hardness. Although the guide rail section is relatively thick and large, for a gray cast iron machine tool bed, placing the guide rail surface facing down at the bottom is the optimal pouring position.
(2) Important machining surfaces should face downward or be placed on the side.
As the upper part of a casting is prone to defects like gas porosity, slag inclusions, and sand inclusions, the bottom surface or vertical side surfaces have a lower probability of such defects. Therefore, important surfaces of a casting should face downward or be placed on the side. When a machining surface must be placed facing upward, the machining allowance should be appropriately increased to ensure no defects remain after machining.
For cylindrical castings like crane drums and rolls, the important surface is the outer cylindrical surface, which requires uniform structure and freedom from defects after machining. The optimal pouring position for these is to have the outer cylindrical surface in a vertical orientation. For gears, the tooth surface is the primary working surface and should face downward to ensure a dense structure and prevent casting defects.
(3) Large flat surfaces should face downward or be inclined.
When pouring a casting with a large flat surface, the molten metal rises slowly within the mold cavity. This prolonged exposure to radiant heat can cause sand to fall from the mold, leading to defects like sand inclusions, sand holes, and gas porosity on the flat surface. Hence, large flat surfaces of castings should ideally face downward. For large plate-like castings, inclined pouring can be adopted. This helps concentrate the molten metal flow during filling, increases the rising speed of the molten metal level, and helps prevent defects like sand scabs and inclusions. During inclined pouring, the value H (the height difference) is generally controlled within the range of 200-400mm, depending on the flask size.
(4) Ensure adequate mold filling capacity for the casting.
Thin-walled sections of a casting offer greater resistance to flow, cool faster, and have poorer filling capacity. These thin sections should be placed in the lower part of the mold, on the side, or below the ingate to prevent defects like misruns and cold shuts, such as in motor end covers and crankcases.
(5) Favor the intended solidification sequence.
The pouring position should be conducive to the desired solidification sequence. For castings made of alloys with high solidification shrinkage or with uneven wall thicknesses prone to shrinkage cavities and porosity, the choice of pouring position should prioritize achieving directional solidification. The thicker sections of the casting should preferably be located at the top or on the side to facilitate the placement of risers for feeding. For example, the thicker sections of a cylinder head are placed at the top to allow for riser placement, enabling directional solidification from the bottom upwards.
(6) Facilitate core positioning, stable support, and ease of core setting, mold closing, and inspection.
The pouring position should facilitate the positioning and stable support of sand cores, ensuring smooth venting. Hanging cores (cores supported from the cope), hanging core assemblies, or cantilevered cores should be avoided as much as possible. Hanging cores may cause the cope sand to collapse or fail during mold closing and pouring. Placing hanging core assemblies in the cope is operationally inconvenient. Cantilevered cores are unstable and prone to displacement due to the buoyant forces of the molten metal during filling, leading to shape inaccuracies. Therefore, they should be avoided. Additionally, the choice of pouring position should consider the convenience of core setting, mold closing, and inspection.
(7) Strive to make the mold closing position, pouring position, and casting cooling position consistent.
Inverting the mold is not only labor-intensive but can also easily cause core shift, sand drop, run-out, and other defects. Therefore, during process design, the mold closing position, pouring position, and casting cooling position should ideally be consistent to avoid inverting the mold after closing or pouring. Sometimes, for the convenience of molding, a method like "mold horizontally, pour vertically" is adopted. For instance, when producing nodular iron crankshafts, the mold may be placed vertically after pouring so that the riser is at the top to aid feeding. Or "mold horizontally, pour inclinedly," such as inclined pouring for plate castings. When the pouring position is inconsistent with the closing position (or cooling position), this should be clearly indicated on the casting process drawing.
The above are general principles for determining the pouring position. Often, all these principles cannot be simultaneously satisfied or embodied in a single casting. Sometimes, these principles may even conflict with each other in the implementation for the same casting. This requires the process designer to flexibly determine the pouring position based on the casting's structural characteristics, quality requirements, and production conditions, in order to facilitate production operations, avoid or minimize casting defects, and ensure casting quality.
