Casting

Casting

SAND CASTING

The process of sand casting is very old going back to the Bronze Age; the technique has changed very little since. It involves making a suitable void in compacted sand which is then filled with molten metal.

Several ingenious methods have been used to increase the range of shapes possible. For small castings several may be grouped in one moulding box and linked with runners fed from one common pouring gate.

Using this method of a special moulding board with patterns fastened to it provides another clever technique to further extend the range of shapes possible. The face of the board is made shaped instead of flat. Another shaped board the mirror image of the first must be used to shape the other half of the mould. Using this technique the joint line between the top and bottom mouldings does not have to be in straight line. This greatly extends the range of possible shapes but the manufacturing costs are again increased.

Sand Casting Process

This process is best suited to large casting where surface finish is not important or which will be machined later. Thin sections are not really suitable as the molten material starts to cool before the mould is completely filled, forming “cold shuts”.

The first stage in sand casting is to make a pattern in wood or metal of the shape to be cast. The items could be cast one at a time or several at once depending on the shape, number required and size.

To show the stages in sand casting imagine we needed to cast a tube as shown below.

First we make a pattern of a slightly larger size allowing for shrinkage. Pattern makers have special rulers one for each common metal that has a shrinkage allowance built in.

A core box is required to mould the core for the inside shape. The box is made in two halves with dowel pins to locate them together. The core is moulded from green sand mixed with linseed oil which is baked in an oven after moulding to harden it.

Half the pattern is placed on a flat moulding table and a suitable sized moulding box placed around it. The half pattern and moulding plate is dusted with parting sand, a fine dry sand which stops the green sand adhering to the surface and allows the pattern to be removed easily.

Green sand is tipped over the pattern and rammed down taking care to evenly compress the sand. The sand is not green in colour but is called green because it is new sand that has not been used for casting before.

Once the pattern is covered with a thick layer of green sand, the remaining space is filled with backing or used sand and the excess removed by scraping a bar across the box top.

Our shape would be filled at one end and vented at the other, but larger or more complex shapes could require several vents to ensure the mould is completely filled with molten ...