Cooling curves-

Cooling curves are very useful in depicting some of the important features of the extend of superheat, and this superheat indicates the total time available for the liquid metal to fill the cavity before solidification begins. The higher the superheat, the more is the time available for pouring.

Solidification occurs in two stages-

Nucleation and grain growth. Once a stable nucleus is formed in the liquid metal, the second phase of solidification, that is, grain growth begins by acquiring atoms from the liquid metal. Nucleation generally starts on the mould walls since the extraction of heat is maximum at the walls. In the next stage of solidification grain growth starts in a direction opposite to the direction of heat transfer, that is, the liquid metal begins to freeze on to the thin skin and the thickness of solidified metal begins to freeze on to the skin and the thickness of solidified metal begins to grow towards the centre. The grain growth rate depends on the cooling rate and only those grains which have a favourable orientation in the cooling direction grow.

Design Considerations in Grinding-

In general, the design consideration for grinding operations are

similar to those for machining but certain special considerations are

required. Since these are finishing operations, the amount of material

to be removed for archiving the required tolerance and surface finish

should be kept to a minimum. Further, the design should allow the entire

grinding operation to be completed without repositionning of the

workpiece. Also, operations requiring frequent wheel dressing should be

avoided. This is particulerly important for form grinding.

Solidification and cooling-

After pouring the molten metal into the mould cavity, it is allowed to freeze and cool before the casting in taken out. The solidification process is extremely important since it dictates the structure of the cast metal and hence controls the properties of the casting.
The cooling curve of a metal depicted that the liquid metal cools quickly to the freezing point and then remains at a constant temperature until the metal loses its heat of fusion and solidifies completely. Further cooling occurs only after the solidification process is complete.
In casting, the temperature of the molten metal reaches freezing point first at the mould walls and then the solidification front moves towards the center through the liquid metal.

Pouring time-

The pouring time or the pouring rate of a casting depends upon the casting material and the configuration of casting since the cooling rate varies form material to material and is significantly affected by the volume and surface area of casting. If the pouring rate is low, the molten metal may start solidifying before the mould cavity is filled. On the other hand, higher pouring rate may cause erosion of mould cavity and turbulence. The pouring rate is, however, chosen to ensure complete filling of mould cavity before solidification starts.

Pouring Temperature-

In general, the higher the pouring temperature, the higher is the fluidity of molten metal because of decreased viscosity. But at higher pouring temperature, the amount of dissolved gases increases significantly. Therefore, the pouring temperature should be kept low to avoid defects due to trapped gases. It is clear that gas solubility and fluidity are two conflicting requirement. The pouring temperature is, however, decided on fluidity consideration alone and the temperature of the molten metal at the time of pouring is so chosen that every corner of the mould cavity is filled before solidification begins.

Design of Riser-

Risers are designed and located in such away that the volume of riser is sufficient to compensate for shrinkage and that it solidifies after the casting. A riser volume of approximately three times the shrinkage volume is usually
Considered adequate. The location of riser is selected to promote directional solidification. Invariably, more than one riser is required and the number and their locations depend upon the configuration of the desired casting.

Risers-

Risers are reservoirs of molten metal to feed the casting during solidification to compensate for solidification shrinkage. Shrinkage in casting occurs when the temperature of the liquid metal drops from pouring to freezing temperature, metal changes from liquid to solid state and the solid-phase temperature drops from freezing to room temperature. The function of the riser is to compensate for compensates for stage.

Gating System-

Gating system serves to deliver molten metal to all corners of mould cavity and is designed to minimize turbulence and erosion. The molten metal is poured into the pouring basin and it moves through the sprue to enter the runner. The sprue is usually tapered down to prevent aspiration of gasses through the sprue. The sprue well serves to dissipate the kinetic energy of falling molten metal before it enters the runner through the choke. The choke is provided to avaid the creation of vacuum, since the liquid metal tends to contract when there is a sudden change in the flow direction.

Gating System-

Gating system serves to deliver molten metal to all corners of mould cavity and is designed to minimize turbulence and erosion. The molten metal is poured into the pouring basin and it moves through the sprue to enter the runner. The sprue is usually tapered down to prevent aspiration of gasses through the sprue. The sprue well serves to dissipate the kinetic energy of falling molten metal before it enters the runner through the choke. The choke is provided to avaid the creation of vacuum, since the liquid metal tends to contract when there is a sudden change in the flow direction.

Melting and Pouring-

Several types of furnaces are used for obtaining molten metal. Cupola is extensively used for melting cast iron primarily because of lower initial cost as well as melting cost. Induction furnaces and side-blow converter are used for melting other foundry alloys.
The most common way of pouring the molten metal into the mould cavity is through the use of ladles. Molten metals react with oxygen to form metal oxides and these must be separated out from the molten metal entering the mould cavity. Since slag can be easily separated from molten cast iron, top pouring ladles perform satisfactorily. For steels, bottom pouring ladles are generally used for effective slag separation.

Single casting mould-

Single casting moulds are destroyed for taking out the casting and are generally classified as expendable moulds. Moulds used for repeated castings are made of metals or graphite and are obviously much more expensive. Such moulds are classified as permanent moulds and are suitable for large scale production.

About mould-

The performance of a mould is generally evaluated in terms of green strength dry strength and permeability. Green and dry strengths refer to the ability of the green mould and dry mould to retain its shape, and are evaluated in terms of the stress required to rupture a standard specimen under compression. Permeability is a measure of porosity and is evaluated as air flow rate through a standard specimen under a prescribes pressure.

The basic requirement that moulding materials must meet are-

• Refractoriness to withstand the pouring temperature
• Cohesiveness to retain the moulded shape.
• Permeability to permit gases to escape
• Collapsibility for easy removal of casting