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.

What Garden Furniture Should You Consider?

Have you ever been sitting in you living room, there is a sudden gust of wind and your inexpensive plastic patio table chairs have flown past the window, ending up in next door’s garden? That’s not good for outdoor furniture hey? If only you had spent a bit more money and purchased heavier furniture. Well, for a number of reasons we recommend cast aluminium garden furniture.

One of the best websites we have found for metal patio furniture is Outside Edge as they are the only aluminium backyard furniture factory left in the UK who make their products from scratch, from raw metal to ‘finished in the box’.

We can honestly say that these table and chairs or loungers are not going to be found buried in the side of your garden shed! Aluminium has a good weight to it but it is not painfully heavy like cast iron and of course it cannot rust.

metal garden chairs

So you can pick the furniture up yourself and move it anywhere on the patio with relative ease, but it will not degrade outside.

So what is cast aluminium? This metal, when molten, is nice to work with as it runs smoothly which is perfect for casting. A mold is made using either sand, iron or steel, of the desired object in the negative. The liquid aluminium runs into the mold and when cooled forms the required part. It is removed from the mold and cleaned.

The parts are then put into a jig for repeated accuracy and welded together. For example a standard garden chair would have the following cast parts: back, seat, legs x 4 and arms x 2. The cleaned chair should then go off to be powder coated and baked in an oven, before being packed.

Most people actually think that the metal furniture that they see in garden centres or in other peoples homes is made from iron or steel but of course most of the modern garden furniture nowadays is made from aluminium. There are a number of reasons as to why this versatile element is used in many industries; such as the aeronautical industry, ship building, cars, boats, engine parts, cookware, filtration, architecture and more. As mentioned it is light but strong and can be readily worked. Aluminium also conducts heat quickly and efficiently, for instance it can be used for radiators.

One down side is that although aluminium never rusts it does oxidize. This only happens to the very beginning of the surface and does not affect the structural qualities of the metal. This is counteracted by lacquering and/or painting, particularly powder coating. Once the product is coated, for backyard furniture, aluminium is perfectly suited because there is virtually no up keep and washing is easy with pressurized water.

Unlike wood, aluminium patio furniture can be left outside all year round as it will not deteriorate or blow away!  Outside Edge offers garden sets in a variety of shapes and sizes from 2 seat bistro sets to huge 14 seater extending garden suites. There are loungers and benches but our particular favourite is the modular corner dining set. They also provide accessories like parasols, cushions and all weather covers.

Outside Edge’s cast aluminum is undeniably practical but it’s also design led. There are classic Victorian designs, contemporary shapes and even some standard municipal patterns. So go to their website, as we feel sure there is something for everyone!

aluminium sofa set