To produce cavities within a metal casting; as required for liquid cooling in engine blocks and cylinder heads. Special negative forms (core boxes) are used to produce sand cores. Cores can be made up from oil bonded sand, often a basic oil such as linseed oil can be used to make effective sand cores. Once made in the negative shaped core box the sand core is baked in an oven until it is cured. Cores are inserted into the mould box after removal of the pattern from the sand. Whenever possible, foundry pattern are designed and made to avoid the use of cores, reducing the use of cores will reduce the mould set-up time and thus help to reduce labour costs in the foundry. With a completed sand mould at the appropriate moisture content, the box containing the sand mould is then positioned ready to fill with molten metal; i.e. ferrous iron (cast iron) steel, or none ferrous bronze, brass, or aluminium alloy. After filling with molten metal the box is set aside until the metal is sufficiently cool to enable the removal of the hot casting from the sand mould. A rough casting is produced that, in the case of iron or steel, may still be glowing red. When casting with metals like cast iron or lead, which are significantly heavier than the moulding sand, the flasks, called the cope & drag is covered with a heavy plate to prevent a problem known as mould floatation. Floating the mould occurs when the pressure of the metal pushes the sand above the mould cavity out of shape, causing the casting to fail. Sometimes mould box clips or flip locks are used to lock the boxes together to prevent loss of metal due to the drag mould loating. After casting, the inner cores are broken up by rods or shot and removed from the casting. The metal from the sprue and risers is removed from the rough casting.Various heat treatments may be applied to relieve stresses from the initial cooling and to add hardness; in the case of steel or cast iron, by quenching in water or oil. The casting may be further strengthened by surface compression treatment such as shot peening, which adds resistance to tensile cracking and smoothes the rough surface, cast iron engine conrods are treated in this manner. The above methods and techniques can be applied to the hobby foundry, as well as the commercial set up, you have to think about what you are doing and the application of internal sand cores to your project. Need content? You may use this article at your website, or in your newsletter. The only requirement is inclusion of the following sentence - Article by Col Croucher of www.myhomefoundry.com - the definitive source for hobby metal casters of all ages. While at the web site you can download a free ebook & subscribe to the hot metal ezine to begin you're understanding and intrigue of metal craft. |
