Casting Aluminium – what are the processes used by foundries?

casting aluminium

Casting Aluminium

There are thousands of foundries across the world that are casting aluminium. All manner of products that we all use are metal cast, we use them everyday and often without realizing it. Our cars have cast engine parts, our cookers use cast metal components, our houses, planes, bridges, ships, railways, pots and pans, drainage, fireplaces all use cast aluminium solid parts.

In fact without the use of foundries the industrial revolution would never have happened! Early on, the ancient Chinese cast many items in iron and bronze. Of course the iron age saw the first castings by man, but eventually when mass production at low cost produced pig iron the casting industry exploded!

In the 1920s, aluminium casting came to the fore, even though it had been discovered in the nineteenth century. The iron magnates, like Carnegie,  had in fact concealed the casting aluminium discovery so that that they could protect there own iron monopolies.

Aluminium, when cast, is strong in all dimensions, it is light and of course it will not rust. A third lighter than iron, stronger than iron and it does not corrode, unlike iron! There are basically 3 types of aluminium casting methods; sand casting, gravity die casting and pressure casting.

Casting Aluminium – Sand Method

The oldest form of casting dating back to the iron and bronze age.  A pattern of the object that desires to be cast is surrounded by sand (green wet sand is often used), with the sand kept in position with a metal or wooden frame. The sand is packed in tightly and half of the box is temporarily lifted out of position. The pattern or object is carefully removed and the cast box half is replaced with the pattern cavity intact.

Pouring holes are channeled into the sand so that the molten aluminium, when poured, will enter and completely fill the cavity. The molten metal takes approximately 30 seconds to solidify and the box mold is removed. Casting aluminium tongs must be used as the metal will still be hot!

The resulting cast is cleaned off, fettled and then ready for use. As with all casting the pattern result will shrink approximately 3%. So if a cast of an original piece is used, the product will always be three percent smaller. If a pattern is used, the pattern maker can allow for shrinkage and so makes the pattern 3% larger.

The sand cast method is slow but very easy to execute. So it is very good for short run production as the molds do not cost much. If large production numbers are needed this method is not really suitable as the labour costs are high. For instance the Jardine Garden Furniture Foundry in Lancashire, England makes a 61cm diameter round outdoor table. If they sand cast this table, 2 men can make about 12 tables a day. If the aluminium table is made through gravity die casting, which is semi automated, 1 man can make 90 tables a day!

Please see our next aluminium casting article which will describe the gravity die casting method.