SPECIAL CASTING PROCESS
SOME IMPORTANT CASTING PROCESS ARE:-
- Slush casting
- Investment casting
- Centrifugal casting
- Shell casting
- Die casting
SLUSH CASTING:-
This is an open casting method, where molten metal is poured in the casting cavity in open condition. It is allowed to cool for sometime and then mould is inverted. The unsolidified molten metal moves out on inverting the mould and solidified metal retains in mould. By breaking the mould the solid casting can be obtained. This type of casting does not necessarily require cope and drag arrangement. Only drag part is enough to carry out casting.
INVESTMENT CASTING
Being the most ancient casting technique, this process is used to produce those shapes which is difficult to obtain through die casting. The pattern used in investment casting is made up of either wax, ceramics or sometimes may be of steel. Generally wax is used as pattern as it melts easily. In designing mould, care is ought to be taken to overcome shrinkage of wax, ceramics etc.
The casting is closed and hence geometry of cast is complex in nature. Initially pattern of wax is dipped in abrasive slurry which constitutes binders, water and silica. It is dipped till it acquire required thickness over the pattern. Then it is allowed to cool down. After that it is heated in so that wax melts down and we get solid refractory casing.
Now molten metal is poured in the refractory and allowed to solidify. After solidification of metal, the outer refractory material is broken and finally we obtain solid casting.
CHARACTERISTIC
Such casting is generally done for obtaining intricate shape with complexity. Good shape, surface finish can also be obtained with such casting. Close dimensional accuracy is achieved, hence ornaments etc can be casted. Surgical instruments are also casted. Here mould is made out of cement concrete. As we know the thermal conductivity of concrete is higher then that of sand, surface hardening occurs. Highly complex shape with good finishing can be obtained by this process. Good dimensional tolerance with high accuracy is also characteristic of this process. Sections with thin diameter is also possible with this process. But on the other side it is expansive process.
CENTRIFUGAL CASTING
AFTER LEARNING INVESTMENT CASTING NOW WE WILL FOCUS ON CENTRIFUGAL CASTING.
As the name suggest centrifugal casting deals with rotation of mould which causes flow of molten metal in outward direction (away from axis of rotation). The mould is rotated at high speed of about 2800 rpm and at the same time molten metal is poured in the mould. The molten metal under centrifugal action is thrown to the inner wall of cylindrical mould. At high rotational speed the thin grained layer of solidified metal is obtained.
When the molten metal is poured in the mould box and the mould box starts rotating about its axis, the molten metal flows in outward direction (away from center). This type of casting provides many benefits over other casting. These castings are not in direct contact with atmosphere hence no environmental effect is there. Second thing is that, any size dimension (length, diameter and thickness) can be obtained. And at the same time one mould can be used for many casting for varying thickness need. Also no cope and drag arrangement is needed in this type of casting. Due to high centrifugal rotation of molten metal, the composition becomes highly homogeneous and uniform, hence good machinability is obtained. In casting it is very difficult to cast thin cylinders, for such purposes centrifugal casting is opted first.
The rotation continues till the molten metal is solidified. The impurities under centrifugal action is thrown to outer diameter which is machined after solidification of cast. The figure below depicts centrifugal casting arrangement.
This method is used to produce large hollow section
The thickness of casting depends on the amount of metal poured in for casting. Such casting is free from casting defects. The density of decreases as we move from outer wall to the center of casting. The figure below depicts the density distribution of product.
Disadvantage:-- Poor machinability.
- Only cylindrical component can be produced.Limited usage for other shapes.
- Maximum diameter of casting is limited three meter and length of fourteen meter.
DIE CASTING
AFTER STUDYING CENTRIFUGAL CASTING NOW WE WILL STUDY DIE CASTING.
BASIC CHARACTERISTICS- Such process is used for non ferrous alloys (zinc, copper etc).
- It is characterized by forcing the molten metal under gravity or under high pressure into the mould cavity.
- Used for high volume of production.
- Cost of casting is high.
- Material used as mould box is metal.
PROCESS OF DIE CASTING:-
THERE ARE TWO METHODS OF DIE CASTING
- GRAVITY DIE CASTING
- PRESSURE DIE CASTING
1. GRAVITY DIE CASTING:-
The molten metal is poured via runner and it enters mould cavity. Since the mould box is made up of metal, the molten metal may adhere to the mould box. To avoid this we sprinkle silica sand (parting sand) on the wall of mould box before pouring molten metal. After the molten metal solidifies, the mould box is separated and casting is obtained. The pouring of molten metal can be controlled by tilting of arrangement to distribute molten metal well.
2. PRESSURE DIE CASTING
As shown in figure, there is a gooseneck through which molten metal is pressurized by hydraulic plunger into the mould cavity against the die. High compression locking of about 550 tonnes is applied on the molten metal in the mould box. The latest technology includes use of computerized system like microprocessor chips which automatically reads cooling time extraction time. The molten metal is allowed to cool down under high pressure and casting is obtained. Fast cycle time is obtained with such die casting.
ADVANTAGES:-- Good surface finish
- Good dimensional accuracy
- Fast production rate.
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