The casting process

The use of castings in the Street Furniture industry allows the designer a greater amount of freedom and flexibility than is possible with the use of fabricated materials.

Cast products can be made from virtually any metal type. In reality these would fall into the following categories;
1. Ferrous metals - iron & steel
2. Copper base - brass & bronzes (gunmetal is a type of bronze)
3. Light alloys - also known as non-ferrous, i.e. aluminium alloys, etc.

Flexibility of design comes from the ability of a cast product to take very complex geometrical shapes easily, whilst retaining known mechanical properties.

From a drawing, timber or metal patterns are produced. The type of pattern is dependant upon a number of factors such as the complexity of the shape, type of metal the product is to be made from and the quantity of castings to be produced.

A pattern is basically the finished item produced in a workable material i.e. timber. Holes and internal shapes needed in the casting are created by the use of cores. The cores can also be used to add internal strength when the casting is structural and the designer wishes to maintain a clean aesthetic shape on the outside.

The pattern is then rammed with sand at the foundry (the type of sand used also depends on the type of metal to be cast). The sand forms a refractory shape to hold the liquid metal until it has solidified. This is called a mould and consists of two halves of solid sand brought together. Weights are placed on the top to stop the pressure of the molten metal separating.

Scrap iron, alloys and chemicals are heated to a predetermined temperature then a sample is taken to ascertain that the liquid metal is chemically correct. The molten metal is now poured into the mould. Once poured the mould has to be left to cool before the casting can be knocked out. In most cases this is a minimum of 8 hours, sometimes even longer dependant upon the size of the casting.

When the casting has cooled it is knocked out from the sand mould. When the casting comes out it still has the running system, joint line and other surface spikes attached, these are then ground off (Fettled) in order to give the casting a neater appearance.

The product will still need more factory finishing including further fettling, filling or possibly machining, prior to finish painting.