Moulding Process involves shaping foundry sand around the suitable pattern in such a way that pattern can be withdrawn to leave a cavity of the require shape in the sand. To facilitate this procedure the sand mould split into two or more parts. For casting of simple shapes, a two-part mould can be used, each half being contained in a box like frame. The upper frame is called “cope” and the lower frame as the “drag” while the two halves fitted together is called “box “or “flask”.
For production of hollow sections, entry of the liquid metal is prevented by having a core in the corresponding portion of the mould cavity. Projections on the pattern for locating core in the mould are called “core prints”.
For casting s like a wheel with a groove at the rim a 3 part mould is used. The middle part is called “cheek.”
Preparation of Mould
Moulds are made by hand if number of moulds to be prepared is small. If a large number of simple moulds required, then moulding machines are used.
To remove pattern easily from mould a parting compound, e.g. ninwetting talc, is dusted on the pattern. Fine grain facing sand is used to obtain good surface on casting. Normally, a dead weight placed on the cope flask to prevent the cope flask from floating due to hydrodynamic liquid metal. For a large mould, care should be taken to prevent the sand from falling of the cope flask when it is lifted to remove pattern. This can be done by providing extra supports, called gaggers, within the cope flask. For a casting with re-entrant surface, e.g. a wheel with a groove at the rim, the mould can be made in three parts as shown in figure below. The part between cope and the drag is termed as the cheek. For an easy escape of the gases, vent holes are provided in the cope flask.
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Three Part Mould |
The moulding machines operate on one or a combination of the principles are shown in the figure. In jolt ramming, the mould is lifted through a height of about 5 cm and dropped 50-100 times at a rate of 200 times per minute. This causes somewhat uneven ramming, but is quite suitable for horizontal surfaces. On the other hand, squeezing is found satisfactory for shallow flask. The sand slinging operation is very fast and result in uniform ramming.
Classification of Moulds
1. On the basis of material.
(i) Green sand mould
(ii) Plastic mould
(iii) Metal mould
2. On the basis of Method of making.
(i) Shell mould
(ii) Investment mould.
Metal moulds are permanent in the sense that a large number of castings can be made from a single mould; on the other hand, moulds of refractory materials can be used only once. Generally, the green sand moulds are used.
Moulding Sand
Moulds are made using special sand which is normally a mixture of sand, clay, water and some organic additives. Usual composition of green sand is 70-85% sand, 10-20% clay, 3-8% water and 1-6% additives. Clay together with water acts as a binding agent and imparts strength to the moulding sand. The organic additives burn out to high temperature of molten and make room for the moulding sand to expand.
The casting process is greatly dependent on the mould and therefore on the properties of moulding sand, for producing a good casting.
Properties of Moulding sand
(i) Strength: i.e. compressive strength of sand.
(ii) Permeability or porosity: It permits the dissolved gases, steam and gases generated in mould at high temperature to escape.
(iii) Flowability or plasticity: It is required for the sand to take proper shape of the pattern for faithful reproduction of its shape during moulding.
(iv) Refractoriness: it measures the ability of the sand to withstand the molten temperatures.
Effect of water- content on moulding sand properties.
There is an optimum water percentage from strength and permeability considerations. At low water content, clay powder, being finer than the sand, fills up the voids and reduces permeability. A high water content will cause the moist clay to forma coating on the sand grains, and enhances permeability by keeping grains farther apart. Beyond optimum range, the excess water fins up the voids and reduces the permeability. 