Iron is a metal present abundantly in the earth’s crust. However, it has to be extracted from its ore, processed and only then can it be converted to the required. The ore will be subjected to a process called smelting, whose immediate product obtained is called pig iron. Pig iron has high carbon content. Pig iron, on further treatments provides cast iron. Cast iron comprises of lesser carbon content compared to pig iron.Cast iron is first heated to a very high temperature, until it melts. This molten iron has to be solidified using various moulds to obtain the desired commodity. Cast iron finds wide applications in construction sites, cookware, and decorative features and in industries. Let’s take a tour on the moulding process.
The first step in the process of moulding is pattern making. A pattern of the required shape and dimensions is prepared with the help of iron or steel or even using plastics. Usually for fabrication of iron, steel patterns in sand moulds are preferred.
Moulds for casting iron can be classified as sand mould, blow mould, compression mould and extrusion mould. Technically moulds comprise two halves. Each half is fabricated separately. One half of the mould is called cope and the other half, drag. When the two halves are brought together (i.e.) placed one over the other, a hollow cavity is created between them, into which iron in the molten state in poured and allowed to solidify so as to obtain the required casting.
A typical sand mould comprises of 85% sand, 7.5% bentonite, 3.5% coal dust and 4% water. Bentonite is an impure form of clay mainly composed of montmorillonite. The patterns used in the mould are mostly made of wood, metal or plastic. Iron during the process of solidification, will contract. Because of uneven cooling, the pattern must be larger than the original size required. The basic processes in sand moulding are as follows
1. A pattern is placed in sand to form a mould.
2. The pattern along with the mould is placed on a gating system. The gating system usually comprises of a gate through which molten metal flows into the mould. The pattern incorporated is then removed.
3. The molten metal is now poured through the gates provided by the gating system and then allowed to cool.
4. The sand covering the mould is now removed to obtain the casting.
The casting flask or the moulding box consists of many segments latched together. Sand is packed around this flask by a process termed as ramming.
Sand moulding is of two types namely green sand moulding, where green sand, which is nothing but sand in its wet state, is used and dry sand moulding, where the whole set up is baked to remove moisture.
In the moulding process, the two halves of the mould are separated well before too much of cooling occurs. This is done in order to prevent the metal casting from sticking with the mould.
Thus, after the molten metal solidifies to the desirable extent, sand covering the mould is excavated and the required casting is hence obtained.
Post time: 03-11-2017