CASTING PROCESS

CASTING PROCESS

1.INTRODUCTION

The basic of casting processes Is feeding of molten metal Into a cavity of the required shape In a mould, followed by cooling to produce a solid object. The various processes differ principally In the way In which tho mould Is formed. In some cases e.g., sand moulding, a mould Is made up for each cast and subsequently broken up to remove the casting. In other cases, e.g., die casting, a permanent mould Is used repeatedly for a succession of casts and the casting Is removed after each cast without damage to the mould. Provision Is made for the feeding of the casting by forming a gating system in the mould. At the same time, an enlarged opening (a pouring cup) Is made to enable easy pouring, and channels are cut to allow the metal to flow out of the mould cavity after filling it, thus maintaining a metallostaUc head during solidification.


After solidification is complete, tho roughcasting Is removed from the mould, cores are
knocked out and the feeders & risers are cut - loose sand etc. Is removed and the casting Is now ready for machining operations. The preparation of the roughcasting for machining Is known as fettling.

For any permanent mould processes, metal moulds can be used. Those are formed complete with requisite running and rising system. Tho dlfflcuHios Involved In the production of metal moulds is largely responsible for the high costs of the permanent mould processes.

During the solidification process, control of the direction of the movement of the solid-liquid growth front can be achieved by the control of heat flow In the mould. Solidification can be initiated by the use of chills;  these are metal Inserts which conduct heat away more rapidly than the mould material. by using Insulating materials or exothermic compounds, heat flow conditions can be controlled to delay freezing In a particular part of the mould. The moulds used for sand casting may be of the 'green sand'  or  'dry sand' types. Silica sand Is generally used as mould material because of its refractory properties,. low cost and availability. In the green sand mould, the sand Is given plasticity by means of a clay binder than may be present In the sand or be added to it. Adequate quantity of water Is added to hold the mixture bonded. In a dry sand mould, the sand Is given plasticity In a similar fashion, but the. mould Is dried before the metal Is poured Into it. In making a mould, a pattern, usually of wood, Is placed In a flask and moulding sand Is rammed around it. Ordinarily, the mould is made In two halves, the upper portion being called the 'cope' and bottom portion the 'drag'.

The pattern Is designed so that it can be removed without disturbing the sand, leaving pattern-shaped cavity In the sand. Cores of baked oil-bonded sand are placed In the mould cavity to form Internalcavities of the casting as Illustrated In the figure above.

Pressure dye casting differs from permanent mould casting In that a positive pressure Is maintained on the metal In the mould during solidification. Metal moulds are used and the
liquid metal is usually forced into the mould under pressure.   For higher melting point alloys, e.g. aluminium based alloys injected at temperatures above 500°C, cold chamber dies casting is employed. Here a transfer chamber adjacent to the mould is charged with moHen metal, which is then forced into the die. Both vertical and horizontal machines are available.
V\lhereas hot chamber machines use pressures of 300-400 lb/in-2. considerably higher pressures in the range of 4000- 15000 Jb/in-2 are employed with cold chamber machines.

2.FULL MOULD CASTING

The full mould, or cavity less, the casting process is a recent development, the basic concept of which is very simple. The pattern Is made from a suitable combustible or vaporise material, usually expanded polystyrene, and the mould Is formed around H. In this case pattern is not removed and the molten metal Is poured, progressively decomposing it as filling proceeds until the whole of the mould Is filled with the metal.   It Is similar to the  Investment casting process In that the pattern Is destroyed as part of the process. However, master• moulds can be used to mass-produce pattern (Including cores,  runners and risers) by conventional polystyrene - forming procedures.

3. GENERAL ORIGIN OF DEFECTS

1. The casting design
2. The technique of manufacture - the method
3. The application of the technique - the wor1onanshlp

3.1GeneralCateaories of Defects
1. Shaping faults arising in the pouring
2. Inclusions and sand defects
3. Gas defects
4. Shrinkage defects come due to volume contraction in the liquid state and some times during solidification.
5. Contraction defects occur always after solidification.
6. Dimensional errors
7. Compositional errors and segregation.
Apart from those, there can be specific defects for certain types of alloys or types of castings, e.g. chilled surface in grey cast iron, moulting in malleable iron etc.

3.2 Shaping Faults arising during the pouring
Mlsrun or short-run
Cold shuts
Cold Laps
Most severe
Severe. Less severe

Reasons:
1.  Metal too cold for the type of casting involved
2.  An excessive chill from the mould surface, either due to heavy chilling or too high moisture in green sand.
3. Inadequate rate of mould filling relative to the freezing rate of the casting. Some times this may be due to low pouring speed and not sufficient gating, the backpressure of gases in a badly vented mould cavity.
Extended castings of high surface area to volume and thin castings are susceptible to this type of defect.
Alloys showing poor fluidity, high freezing range alloys and those carrying strong oxide films are susceptible.

3.3 Inclusions  and Sand Defects
Non-metallic inclusions can be of two types:-
1.  Indigenous 2.   Exogenous

Indigenous Inclusions:  They are products of reactions within the melt.  Normally dispersed throughout the casting. They are small in size.

Exogenous  Inclusions:  Result from the entertainment of non-metallic during pouring.  They may vary In size.  They may include dross, slag and flux residues, refractory fragments, and the mould materials.   They tend to be Concentrated In upper surfaces of the castings and areas adjacent to the ingots.   Indigenous inclusions are characteristics of melting practice. The size, shape, and distribution of these inclusions affect the properties.

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