Aluminium Casting Workshop – 4.6.2019
In sand casting, the positive mould of the casting object is imprinted on both sides of a two-part sand mould. A thin layer of powder separates the two parts so they can be taken apart easily. The object to be cast must not have any inclusions and should also be tapered. This is important for later when removing the positive from the sand so that the mold isn’t damaged. As the two parts come together again, molten metal is poured into the cavity the object has left behind. Alternatively, it is possible to create a model from an easily melting rigid foam such as Styrofoam or Styrodur. They may be covered completely by sand and are simply melted away by the hot metal. Very complex shapes can be achieved this way. However, the surface structure of said positive mould is rather rough due to the pores of the foam.
With both techniques, it is very important to make sure that the sand is well compacted while building moulds. We used a very fine oil sand, which is particularly suitable for casting moulds. The sand sticks together very well and therefore stays in shape when condensed into a wooden or metal frame for the mould. When the mould is filled with molten metal, the sand burns. Most of the sand however can be reused for the next mould. Only the burnt parts have to be carefully scraped from the mould.
The most important parts to be included into the mould are an appropriately large pouring cup as well as air ducts and a riser. The pouring cup ensures that the metal is well distributed in the mould. The metal’s own weight pushes it evenly into thin parts of the mould. It is best to install the cup in a way so that the metal can flow in from the side. Otherwise air bubbles might form while pouring similar to the way that foam is produced when pouring beer. The air ducts also prevent air bubbles by letting the enclosed air escape. The riser ensures that cavities (so called “Lunker”) are avoided in the metal when it cools down. This is particularly important for larger castings.
When casting aluminium, it is important not to pour the metal at a temperature that is too hot. Aluminium contracts strongly when it cools down. When poured too hot, dents may appear in the casting due to the strong shrinkage. Aluminium is a light metal and has a relatively low melting point. It is between 580 and 680 degrees Celsius, depending on the aluminium alloy. The kiln is lined with rock wool and refractory mortar so that as little heat as possible is emitted. A gas burner brings the kiln to the desired temperature.
When this temperature is reached and the aluminium is liquid in the crucible, it is removed from the furnace with gloves and tongs. The metal is poured into the moulds.
Sand casting is particularly suitable as a technique for lower quantities and thus for model making or custom-made products. Sand casting moulds are easy to produce in comparison to permanent steel casting moulds (so-called chill moulds), but they have to be produced anew for each casting.
It is also possible to use moulds made of gypsum chamotte. However, it is important that the plaster mould is fired before the aluminium casting. The plaster still contains chemically bound water, which can cause the mould to explode when hot metal is poured into it. We were eager to try out this new learned technique for our own model making process.
workshop by with Carsten Theumer (BURG)
text: Tatjana Simbürger & Emma Brix