At first, pattern is placed on moulding board, which fits the flask being used. The lower moulding box is placed on the board with the pin down as shown in Figure 20.15 (a). Moulding sand, which has previously been tempered, is filled over the pattern. The sand should be pressed around the pattern with the fingers and then the box is filled completely. The sand is then firmly packed in drag part of the box by means of a hand rammer. For ramming the sand near the wall of the flask, the peen end rammer should be used first, additional sand being placed into the drag as the sand is settled down. The inside area of the drag is then rammed with the butt end of the rammer. The ramming should be optimum. If the mould is not sufficiently rammed, there will be chance of breaking of the mould during handling. On the other hand, it will not permit the vapour and gas to escape from the mould if it is rammed too hard.
Figure 20.15 Steps in Mould Making
When ramming has been completed, the surplus sand is levelled off with the help of strike off bar. In order to ensure the escape of the gases, few small vent holes are made through the mould with the help of vent wire. The completed lower half of mould, i.e., the drag is then rolled over and pattern is exposed. The upper surface of the mould is first smoothed over with a trowel and is then covered with a fine coating of dry parting sand to prevent the sticking of the sands in the cope.
The cope is now placed on the drag as shown in Figure 20.15 (b), the pins on either side holding it in proper position. A sprue pin and riser is placed at right position. The rest of the operations such as filling, ramming, and venting of the cope are repeated in the same manner as before.
Riser and sprue pin is removed. The cope half of the flask is lifted off and set to one side. Before the pattern is withdrawn, the sand around the edge of the pattern should be moistened with a swab so that the edges of the mould will hold firmly together when the pattern is withdrawn.
To loosen the pattern, a draw spike is driven into it and rapped lightly in all directions. The pattern can then be withdrawn by lifting up the draw spike. Before mould is closed again, a small passage, i.e., gate must be cut from the mould at the bottom of the sprue opening. Now mould is closed for pouring the molten metal.
Leave a Reply