< SODIUM SILICATE BONDED SAND
GREEN SANDS & DRY SANDS
Green sand moulding is the oldest method of sand casting, in it’s various guises this process has been in widespread use for something like 4-5000 years. The description GREEN in this instance refers to the material condition of the sand rather than it’s colour. The sands selected for green sand moulding are of a high SILICA type, washed and graded according to end use. The binder used for bonding the sand is a clay like substance of a CALCIUM or SODIUM base, known as ‘bentonite’. Coal dust and charcoal may also be added to the sand/bentonite mixture, as both these carbon based materials significantly improve the surface finish of the cast.
By combining appropriate proportions of binder with the sand, and carefully controlling moisture levels, a plastic, easily moulded medium is produced. Unlike sodium silicate and many of the other bonded sands described in this section, green sand does not need the addition of a catalyst to harden the mould before casting. In some cases sections of the mould (usually the core), or even the entire mould, may be fired in a KILN to harden the sand. If the mould is kiln fired, the production process is more usually referred to as DRY SAND MOULDING.
Green sand moulding has the advantage of requiring little in the way of specialist milling machinery, the medium can be adequately prepared for use by hand. The additional savings derived from having no CATALYST requirement, as well as the ability to recycle the sand without expensive equipment, makes this process attractive to studio and college based workshops working to a tight budget.
One disadvantage of the green sand process, is that the correct storage and preparation of sand for use, is both a fairly skilled and time consuming job. A number of modern commercial founders, especially those casting short run work in ferrous metals still retain green sand moulding facilities (usually this is carried on in parallel to other sand processes). In general though, the use of green sand has been in steady decline, having been more or less replaced by CO2 and oil based (petrobond) sand systems. Both these modern systems are easier and safer to use than green sands – safe because oil and gas versions hold little or no water content to potentially generate violent steam reactions in the mould.
AIR SET/RESIN & OIL BONDED SANDS >
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