Casting

Casting

To produce castings, liquid (molten) metal is poured into moulds. After the metal has solidified, a replica of the workpiece based on the mould pattern is obtained. The work is carried out in the foundry. The mould is produced in the moulding shop. The mould-maker uses a pattern made from the workpiece lor this purpose. The casting is fettled (cleaned up and projecting flash or moulding sand removed) in the

casting cleaning shop.

Patterns

Patterns are made from wood, metal or plastics. For the larger patterns, air dried, dense and knot-free softwood is used; for smaller, more complex patterns hardwood is preferred. Metal patterns, made from cast iron, steel, brass or light metal, are of course more expensive, but have a longer life.

ln pattern making, allowance has to be made for the fact that casting metals contract or shrink on cooling. The pattern is therefore larger than the casting by the amount lost in shrinkage. At points to be machined later, the pattern-maker must add 0.5 to 10 mm. To avoid damaging the mould when taking out the pattern, this is tapered to make it 2% to 5% smaller internally and permit easy withdrawal. The finished wooden pattern is painted with pattern-maker’s lacquer.This gives it a smooth surface and protects it against moisture. The colours for painting are standardised. Patterns for cast iron are painted red, those for cast steel blue and for light metals green; core marks are shown ln black and faces to be machined in yellow.

Mould

We distinguish between sand casting and chill casting. The sand mould can be used only once. and is destroyed when removing the casting. For chill casting, the permanent steel or cast iron mould can be used repeatedly. Castings produced in this way have a clean surface and an accurate form. Fast cooling in the steel mould makes the surface particularly hard and wear-resistant. In order to obtain a hollow casting, acore must be placed in the mould. This is produced in a two-pan or more complex die (core box).

Flask moulding.

Flask moulds are normally rectangular cast iron frames with handles. They have pins and lugs so that the relative positions of the top and bottom flasks are accurately located on assembly. The sand mould is built up by the moulder, usually with the aid of a multi section pattern. He places one half of the pattern on the pattern board (Figure 9.30), positions the empty, frame shaped bottom flask moulds on it and fills it with sand. The sand is tamped firmly down, and the mould reinforced by inserting wires and pins. The bottom flask is then rotated, the second half of the pattern placed on the first half and the top flask aligned correctly. After the sprue and risers have been inserted, sand is lamped in and the top flask is lifted. The pattern is removed with great care and a core is inserted if needed. When the top and bottom flasks are re assembled, a cavity corresponding to the shape of the workpiece is produced.

Moulding machines are used only for mass production, to simplify the physically onerous work of the moulder and to save time.

Melting and casting

The metals are melted in suitable furnaces, and their quality modified or improved as necessary. The cupola furnace, also referred to as the shaft furnace, is mainly used in the iron foundry. It consists of a shaft 6 to 9 m high and about 1.5m in diameter. The molten metal is collected in the forehearth, run off into ladies, brought to the mould and poured into it. Casting stops when the riser is full. After having cooled sufficiently, the mould is destroyed and the casting taken out and cleaned.

In the casting cleaning shop the runner, the risers and the casting seams are removed by knocking off with hammers and chisels, sawn off or ground away. On steel castings, oxy-acetylene flame cutting

may even be used. The core is then driven out, and the surface of the casting cleaned with steel wire brushes, or by sand blasting, waterjet, emery wheel etc.

Casting processes

Pressure die casting. Non-ferrous heavy and light metal alloys are often mass-produced by the pressure die casting process. ln this casting process, the molten material is forced at high pressure into a

steel mould. Even the smallest cavities of the precision-machined steel moulds are filled densely and effectively. The accuracy andsurface quality are so high that the term "finish casting" is often applied to these items. Only the sprue and casting seams need to be removed. Casting pressure is between 20 and 3000 bar. A distinction is made between the cold-chamber and hot chamber processes.