sand casting

Probably the best known and most commonly used of the casting processes, it has a combination of flexibility and low tooling cost which make it ideal for low and medium volume work. Manually assembled “sand boxes” also mean that sand casting can produce some quite complex shapes, although it should be noted that there is a necessity for draft angles to enable the tooling to separate properly when removing the cores.

The two most commonly used processes for sand casting are green sand and resin or air set.

The green sand process can be used to produce castings in a wide variety of materials. These clay-bonded sands have provided the principal medium from which moulds for sand casting have been produced for centuries. In essence, the mould material consists of sand, usually silica in the quartz form, clay and water. The water develops the bonding characteristics of the clay, which binds the sand grains together. When the mould is used in its moist condition it is referred to as “green”, hence the name. The subsequent sand casting surface finish can appear quite rough as the consistency of the sand can vary. If the mould is dried at a temperature just above 100°C (212°F), the majority of the free moisture will be removed. This is the principal of the dry sand moulding process. Removal of the free moisture is accompanied by a significant increase in the strength and rigidity of the mould. This enables the mould to withstand much greater pressures and so, traditionally, the dry sand process has been used in the manufacture of large, heavy sand castings.

The relative abundance of sand and clay minerals means that the material costs for the process are low. The mould material is reclaimable, with between 90 and 95% of the sand being recycled, although new sand and additions are required to make up for the deliberately discarded loss. From an environmental view the process is also acceptable, although problems may arise from the use of organic additions to the sand and from the generation of fine silica dust.

Air set, or resin sand casting uses organic and inorganic binders that strengthen the mould by chemically adhering to the sand. This type of mould gets its name from not being baked in an oven like other sand mold types. This type of mould is more dimensionally accurate with a slightly better surface texture but is slightly more expensive.

With both methods, the sand mixture is packed around a master "pattern" forming a mould cavity. Air-set moulds often form a two-part mold having a top and bottom, termed cope and drag. The sand mixture is tamped down as it is added, and the final mould assembly is sometimes vibrated to compact the sand and fill any unwanted voids in the mould. The pattern is then removed leaving the mould cavity ready to accept the molten metal. Once sufficiently cool, the sand box is split and the sand casting removed for final finishing.

There are also automated sand casting systems such as the Disa process, which allow high volume production of parts with very low component costs.

Cast material

• Grey iron
• Ductile iron
• SG Iron
• High Chrome Iron
• White irons
• Low Carbon Alloy Steels
• Aluminium Alloys
• Al-Si Alloy
• Al-Mg Alloy
• Copper based alloys

Tolerances and sizes

Please note: The above tolerances can vary dependant on shape and material.