CASTING PROCESS AND PATTERN ALLOWANCE

                       THE CASTING PROCESS

What is casting?

It is the most ancient process of manufacturing. In this process the molten metal (liquid form) is poured in the mould cavity.

The shape of casting to be made resembles the mould cavity. The size of the mould cavity is kept slightly greater than that casting to be obtained. This is done for tolerance purpose and for machining to be done after casting solidifies.

As the molten metal is poured in the mould cavity, it takes the shape of mould cavity and solidifies
After solidification we obtain the casting.

 What are various terms related to casting?

 The various terms that relates to casting process are:-

Mould
Pattern
Core
Flask
Baking sand
Facing sand
Gating system 

What is mould?

It is nothing but the cavity in which the molten metal is poured. The size of mould is kept slightly larger than the casting to be obtained. This is done for tolerance purpose and machining allowance purpose. It may be made up of metal or sand.

What is pattern?

What impression is to be obtained in casting is provided by pattern. This pattern consists of various designs that we want on the casting after it solidifies. It may be of wood, metal or plastic (depending upon the need).

 What is core?

When we want to obtain a hollow cavities in a casting .
Metal flow around the core to form cavity.

What is baking sand?

This is generally used or burned sand, it covers most mart of refractory material which is found in mould.

What is gating system?

fig:-gating system

The gating system ensures the total flow of molten metal from starting to the end till it solidifies in mould cavity. The figure shows the gating system.

 The various part of gating system
Pouring cup
Sprue
Runner
Gates
Riser

POURING CUP 

It is nothing but reservoir in which molten metal is poured. Here impurities can be filtered out.

SPRUE

It is the tapered section through which the molten metal flows to the runner. The tapered section provides velocity head to the flow of molten metal.

RUNNER

It is trapezoid section in horizontal direction and connects to in-gates through which molten metal flows to cavity. This provides continuous flow of molten metal to cavity.

GATES

 
It signifies the entry point through which the molten metal enters the cavity.

RISER

After the mould cavity is filled, the excess of molten metal rises in the riser.Hence it signifies that mould cavity is completely filled.

PATTERN ALLOWANCES

As described the casting obtained has same shape as that of pattern. But the size of pattern is kept slightly greater than that of casting.

• Size of pattern = Size of casting + allowance
• Allowance is the tolerance provided.

There are various types of allowance :-

Machining allowance
Distortion allowance
Shrinkage allowance
Draft allowance
Shake allowance

SHRINKAGE ALLOWANCE

In this case the pouring temperature of molten metal is nearly 200 degree above melting point of metal.
The molten metal is solidified in the mould. This happens in three stages Super heated liquid to saturated liquid- evolves sensible heat.
Saturated liquid to solid-freezes and gives latent heat.
Free zed solid to solid at room temp-Gives sensible heat.
The casting as it passes through each and every stage shrinks while cooling.

    2.  MACHINING ALLOWANCE

• In order to provide smooth and fine surface of the casting, machining is done. For this reason some allowance is given so that surface finish can be provided to casting. This is called machining allowance.

• It is usually specified as 2mm/side - 4 mm/side.

     3.  DISTORTION ALLOWANCE

It is process of distorting the pattern in opposite direction. As we know that during cooling of mould stresses are developed in it. This will effect the cast as stresses will be induced in it also.

• When a V-shape or U-shape is casted, there may be distortion in the shape.
• Pattern allowance is done to take care of it.

     4.  DRAFT ALLOWANCE

• This is done in order to avoid the braking of upper surface of the mould.
• After removing the pattern the above surface is still in contact of mould. And due to shake of pattern, mould may break.
• To avoid such problem vertical faces are kept inclined or say tapered section is provided on the upper surface for easy removal of material known as draft angle. More is the complex structure greater is the draft angle.