Mould making is the crucial step in the production of castings. Making a mould properly means a casting half done. A sand mould is formed by packing sand into each half of the mould. The sand is packed around the pattern, which is a replica of the external shape of the casting. When the pattern is removed, the cavity that will form the casting remains. Any internal features of the casting that cannot be formed by the pattern are formed by separate cores which are made of sand prior to the formation of the mould.
Sand is the most commonly used mould material for casting. Other materials used include metals, plaster and ceramics. Sand moulds are made in moulding boxes or flasks which are metal frames with sufficient adhesive area to support sand. In large flasks additional adhesive area is made available by providing cross bars in the flask.
Most moulds are made in two part flasks, the top part called “Cope” and the bottom part called “Drag”. Copes and Drags are held in definite relation to each other by aligning pins and lugs. For some moulds additional intermediate boxes called “Cheeks” may be required. If the production rates are high, moulds may be arranged in the form of stack moulds with a number of such intermediated boxes.
Also lubrication is often applied to the surfaces of the mould cavity in order to facilitate removal of the casting. The use of a lubricant also improves the flow the metal and can improve the surface finish of the casting. The lubricant that is used is chosen based upon the sand and molten metal temperature.
Characteristics of Mold Making:
1. The mould should be strong enough to resist erosion by the flow of the metal and to take the weight of the metal.
2. The mould material should not produce too much of gases as the gases may enter the mould cavity and get entrapped with metal or violently boil out the metal.
3. The mould should be made in such a way that gases generated are vented out of the mould.
4. The mould should be refractory enough to withstand the high temperature of liquid metal.
5. The cores should be collapsible enough to permit contraction of the metal after solidification.
6. The passages for the flow of liquid metal into the mould should be designed to provide smooth non turbulent flow of metal, effective trapping of slag and proper directional solidification.
7. There should be a proper riser system in the mould to provide sufficient extra metal to account for the liquid and solidification shrinkage.
8. The mould material and moulding process should be such that cleaning of castings is facilitated.