UNDER
CONSTRUCTION
Sand Systems:
Green Sand
Oil Bonded Sand
Unbonded Sand
Sand Casting Tools:
Sand Storage
Flasks
Riddle
Molding Boards
Ramming Tools
Muller
Mixer
Fluffer
Miscellaneous Tools
What Is Sand?
Sand, gravel, clay, and silt are collectively known as sediment. What distinguishes sand from gravel and silt is grain size. Gravel is any material larger than 2 millimeters in its largest dimensions. Sand is any material between 2mm and 0.06mm. Silt is material which is finer than sand, but feels gritty when rubbed between your teeth. Clay is the finest material, and pure clay will feel smooth on your teeth.
What Makes Sand Good For Metal Casting:
Sand has several properties of sand that make it useful in foundry work. You have witnessed one property if you have ever seen anyone make sand castles out of damp sand. Dry sand pours like sugar, and will not hold a shape. Damp sand will clump or hold its shape when molded. Why does this happen? The following website gives you the basic explanation of why damp sand holds its shape:
http://whyfiles.org/shorties/sandcastle.html .The next property is refractoriness. This is the ability of sand to withstand the heat of the incoming molten metal and its ability to resist thermal shock. Good foundry sand should not fracture from sudden thermal expansion Generally, the higher the refractoriness, the lower the resistance to thermal shock. The metal you are casting will determine the tradeoff you will be willing to accept.
There are other properties that are important to the hobby caster. "The Navy Foundry Manual" of a good source of information about these properties.
Types of Sand
The chemical composition of sand is largely dependent on the source of the material. Two types of sand of interest to the hobby caster are silica sand and Olivine sand. Silica sand is essentially a high purity quartz (SiO2). Silica sand has other characteristics, such as shape and size. Another type of sand is Olivine or Magnesium iron silicate (MgFe)2SiO4. Olivine is useful to molders because of its high fusion point, low uniform thermal expansion, and resistance to thermal shock. Also, unlike silica, Olivine is not known to cause silicosis. Chromite and Zircon sands are also used in molds.
Foundry Sand
The type of sand that you use in your mold is dependent the surface finish you want on your casting, the alloy you are pouring, and the sand system that you use. The two types of sand systems that I use are green sand and oil tempered sand. Where the surface finish of the casting is not critical, a green sand system can be used. Silica sand is commonly used in green sand systems because of its low cost and availability. When using green sand you are limited to sand that is course enough to allow the steam from the sand to escape when the molten metal comes into contact with the mold. Finer grain sand can be used with oil tempered sand because oil is used to bind the sand rather than water. Oil is not nearly as explosive as water when it comes into contact with molten metal. I use a 125 mesh Olivine sand with my oil tempered sand. Sometimes you choice will depend on availability from local suppliers. Sometimes, you just have to "make do". In sand casting, the sand is compacted into a flask around a pattern. The pattern is then removed to produce a cavity to "cast" the metal. More complicated patterns may require binders other than water of oil. Binders other than water and oil are also used in sand molds. Sodium silicate is commonly used to make sand cores.