Machinery's Handbook, 31st Edition
1500 SAND CASTING 7) After the ramming is finished, excess sand is leveled and removed with a straight bar ( strike rod ). 8) Vent holes are made in the drag, using a vent rod , to the full depth of the flask and the pattern to facilitate removal of gases during pouring and solidification. 9) The finished drag flask is turned upside down, exposing the pattern. 10) The cope half of the pattern is placed on the drag pattern, using locating pins. The cope flask also is located with the help of pins. Dry parting sand is sprinkled all over the drag surface and on the pattern. 11) A sprue pin, used in making the sprue passage, is located some distance from the pattern edge. A riser pin is placed at an appropriate place. 12) Filling, ramming, and venting of the cope are done in the same manner. 13) The sprue and riser are removed. A pouring cup or basin is made at the top to pour the liquid metal. 14) The pattern is removed from the cope and drag. 15) Runners and gates are made by cutting the parting surface with a gate cutter (a piece of sheet metal bent to the desired radius). 16) The core for making a central hole is placed into the mold cavity in the drag, where it is anchored and supported by core prints. 17) The mold is now assembled and ready for pouring. Sand Casting Operation.—After the mold has been shaped and the cores have been placed in position, the two halves (cope and drag) are closed, clamped, and weighed down to prevent separation of the mold sections. The flasks are then subjected to pressure to counteract the force of buoyancy. (Buoyancy results from the weight of liquid metal being displaced by the core, according to Archimedes’ law.) The force tending to lift the cope is equal to the weight of the displaced liquid less the weight of the core. This can be expressed as: (8) where F b represents the force of buoyancy; W m is the weight of molten metal displaced; and W c is the weight of the core. The order of sand casting operation is as follows: 1) A suitable flask or molding box is selected, and space is allowed for ramming in sand. 2) The drag is placed on the molding board upside down. Then the lower part of the pattern is placed on the board inside the flask. Space is left for cutting. 3) Sand is added to fill in the flask and cover the pattern; it is firmly packed using hammers. After properly ramming in sand, the excess is leveled off with straight strike- off bar. 4) Using a vent rod, the vent holes are made in the drag to the full depth of flask to allow gases or air to escape during pouring and solidification. 5) The cope half of pattern is placed over the drag and positioned with locating pins. The cope flask is placed on the drag. The sprue pin is provided for sprue passage, located at a distance from the pattern. And the riser pin is positioned on the pattern. F W W b m c = −
6) The ramming, filling, and venting processes are repeated. 7) The cope is filled with sand and proper ramming force applied.
8) Excess sand is cut off from mold surface. The riser pin and sprue pin are removed. 9) The molding box is opened, and the pattern is removed from the cope and drag. One or more dry sand cores are mounted in position.
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