Metal casting

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Metal casting (or metalcasting) is the process in which a metal is melted from preexisting forms or obtained from a reaction (e.g. thermite) in order to be brought to a liquid form, then delivered (poured) into a mold which contains a hollow cavity of the intended shape. The metal and mold are then cooled, and the metal part (the casting) is extracted. Casting is most often used for making complex shapes that would be difficult or uneconomical to make by other methods, though for the amateur is often used to convert scrap metal into bars and ingots, which are easier to store and handle.

General

Metal casting involves heating a metal or an alloy in a crucible until it melts, removing the floating slag then pouring the molten metal in a mold and letting it harden then cool. Metal casting depends on the type of metal you want to cast, and its fairly easy to do for metals with a very low melting point like lead, tin, zinc, only needing the flame of a gas stove, while other metals like aluminium, copper and alloys like brass and bronze need a hotter flame to melt. Traditional techniques include lost-wax casting, plaster mold casting and sand casting. There are other inconveniences, like fuming, flammability, corrosion of the crucible when casting metals.

Metal casting conditions for each metal and alloy

Alkaline metals

Alkali metals are very reactive and can catch fire in molten state if exposed to air. They are best melted under inert atmosphere or in an inert solvent, like mineral oil or high-boiling point ethers, like dioxane. To pour them in a mold, a preheated metal or glass syringe is used to suck them and then inject the molten metal in the mold. This process limits air contact.

Alkaline earth metals

Alkaline earth metals are also very reactive, but due to their high melting point, they cannot be melted and cast like the alkali metals. Magnesium is usually cast in an inert atmosphere, and the mold is covered with sand or magnesia to prevent it from catching fire as it's cooled. A similar technique is used for the other metals.

Rare earth metals

Similar to magnesium, rare earth metals and lanthanides are difficult to cast in air, as they're very reactive and require inert atmosphere.

Refractory metals

Due to their very high melting point, these metals are impossible to melt using a gas burner and few crucibles can hold them in liquid form. To melt these metals, an arc furnace is used, and the molten metal is added in a special mold made of refractory ceramic.

Ferrous metals

Ferrous metals (iron, cobalt, nickel) require high temperatures to melt, which can be achieved using coke or acetylene gas. MAPP gas can also be used. For casting, lost wax casting is widely employed.

While not ferrous metals, chromium and manganese have similar melting points, and can be cast in similar ways, though chromium metal is somewhat refractory, making it difficult to handle.

Precious metals

Copper, silver and gold are not very reactive and do not require protection against oxidation, nor do they fume in air. Their melting points are all around 1000 °C which can be achieved using a homemade furnace, however, due to their high thermal conductivity, they will cool rapidly, meaning you will have to heat them to higher temperature and pour them in the mold fast, before they solidify. Lost wax casting is commonly used for casting.

Platinum group metals, due to their very high melting point cannot be melt economically using a gas flame, and an arc furnace is usually employed.

Post-transition metals

Zinc and cadmium can be easily melted on a gas stove due to their low melting point, but they tend to fume during melting, and if inhaled, the fumes may cause metal fume fever. Cadmium is also very toxic. Tin and lead are also easy to cast and will not fume, unless heated to very high temperatures. Aluminium requires a bit more heat to melt, while indium, bismuth and thallium do not, though thallium, like cadmium is very toxic and working with this metal should be avoided. Gallium, due to it's very low melting point for a metal, will melt if simply hold in a hand. Like bismuth, gallium expands upon solidification, which means you will have to use a mold whose dimensions are a bit smaller than the object you intend to create to compensate for the dilation.

Mold making

There are several important mold making techniques:

  • Sand casting: Green or resin bonded sand mold.
  • Lost-foam casting: Polystyrene pattern with a mixture of ceramic and sand mold
  • Lost-wax casting: Mineral wax pattern with a sacrificial hardened and cured ceramic/sand mold
  • Ceramic mold casting: Plaster mold
  • V-process casting: Vacuum with thermoformed plastic to form sand molds, no moisture, clay or resin required
  • Die casting: Metal mold
  • Billet (ingot) casting: Simple mold for producing ingots of metal, normally for use in other foundries

How to cast metals

Materials needed

In order to cast metals, you will need the following:

  • Scrap metal (or a thermite mixture)
  • A heat source
  • A crucible
  • Tongs
  • Flux
  • A mold (either fixed or sand)
  • A water (or oil/sand) bucket
  • Protection equipment

Procedure

  • Take the crucible and fill it with the desired metal
  • Place it in the furnace
  • Turn on the heating (be it either gas, electric, or radiation)
  • Wait for the metal to completely melt then add the flux
  • Prepare the mold
  • Remove the slag from the molten metal surface
  • Preheat the mold
  • Grab the crucible holding the liquid metal (use pliers)
  • Carefully pour the liquid metal in the mold
  • Wait for it to solidify
  • Cool the hot solid metal by dumping it in a water or oil bucket
  • Break apart the mold (this can be done before quenching if you use the lost-wax casting method)
  • Take the cooled metal and remove any defects as desired

Safety

Metal casting is a hazardous activity, as it involves molten and hot metals, which pose a severe fire and burning hazard.

Some metals, like zinc and its alloys will fume when molten, which, if inhaled will cause metal fume fever.

Toxic metals like lead will not fume significantly during the smelting operation, but may release oxide particles in air, which are very toxic if inhaled.

ALWAYS preheat the mold before adding the molten metal! Moisture is known to violently react with most molten metals, like aluminium. Insufficient heating of the mold may cause it to explode when molten aluminium is poured in it.[1][2] However, if poured in a large volume of water, molten aluminium does not usually explode.[3]

ALWAYS WEAR PROPER PROTECTION WHEN MELTING AND CASTING METALS! This includes fire resistant gloves, apron, face shield, respirator, protection goggles.

See also

References

  1. http://www.youtube.com/watch?v=PHFGBtsfLSM&t=5m20s
  2. http://www.youtube.com/watch?v=A796N_YZTm8&t=0m12s
  3. https://www.youtube.com/watch?v=JEEOkMW1CYI

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