Types of Sand to Make a Mold for Casting

Written by michaelyn erickson
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Types of Sand to Make a Mold for Casting
Sand can be shaped and moulded into a fairly hard structure. (sand patterns image by Greg Pickens from Fotolia.com)

Sand casting is an old foundry technique, and according to Custom Part.net it is the most widely used casting technique in the world. This process is used to create large metal parts, components and pieces, but hobbyists often create small household foundries of their own. Because the moulds must often be destroyed to remove the part, sand is a handy material. All sand used for casting is mixed with a binder to solidify the mould before the metal is poured in.

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Green Sand

According to Custom Part.net, green sand is the least expensive and most commonly used sand for casting. Green sand is composed of 90 per cent silica sand, three per cent water and seven per cent clay. Green sand is also referred to as water bonded sand, according to Casting Hobby, because it uses water to bond the sand and clay. Commonly used clay is bentonite, which is found in hobby and home-foundry green sands. Binders are sometimes used instead of clay to make green sand. Examples of industrial binders include resin and collagen-based materials.

Oil-Bonded Sand

Oil-bonded sand is foundry sand that uses oil instead of water as a bonding agent. Hobbyists like oil-bonded sand because it's easier to use, it doesn't steam like water-bonded sand and it produces a finer finish. Because of the sand, this type of casting doesn't produce clean surfaces. The sand is too porous to get a smooth finish. Oil has more of a gluey character than water. It can fill in the pores between the sand particles and that's why oil-bonded sand will give a slightly finer surface texture.

Dry Sand

Dry sand is just silica sand mixed with an organic binder. These moulds are first baked in an oven before being used to cast metal. The heat strengthens and hardens the sand mould, according to Industrial Metal Casting.com. This type of sand casting ensures more precise dimensions and fewer inaccuracies. Castings with more complicated designs are made easier in dry-sand foundries. This process is mostly used to make large items such as engine blocks and construction parts.

No-Bake Sand

No-bake sand is mixed with a liquid resin or plastic that hardens at room temperature. The mixture is created in a high-speed mixer, then poured into a box that contains a prototype of the item that you want to make. This "positive" is made of wood or metal. The sand is packed around the item either by hand or with a machine that applies compression and the mixture is allowed to harden. It is cut into two halves, the "drag," or bottom half, and the "cope," or top half. According to General Foundry, no-bake sand moulds will harden within a few minutes. Then the positive is removed and any cores are set in place. The mould is sealed shut and the metal is poured in.

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