Surface Finish and Sand Types used to achieve Fine Finishes

The type of molding sands used will determine the surface finish of sand casted products. Molding sands, also commonly known as foundry sands, are defined by eight characteristics:

1. Refractoriness
2. Chemical Inertness
3. Permeability
4. Surface Finish
5. Cohesiveness / Bond
6. Flowability
7. Collapsibility
8. Availability

Refractoriness

This refers to the sand being used ability to withstand the heat or temperature of the molten liquid being poured without breaking down. In cases where aluminum alloy is being casted, sand is required to withstand a temperature of 1 202°F (650°C), but when casting steel, sand needs to withstand a temperature of 2 730°F (1 500°C).

Choosing the right sand casting materials here is essential, as an insufficient sand will melt and fuse to the casting.

Chemical Inertness

It is important that the sand used does not react to the metal being poured, as it will fuse to the casting. This factor is important in the case of highly reactive metals such as titanium and magnesium.

Permeability

This characteristic refers to the sand's ability to exhaust gasses. When the molten metal is poured, gasses, such as nitrogen, steam, hydrogen and carbon dioxide, is produced. All these gasses must be properly expelled as it can cause defects, including gas holes and blow holes, in the final cast.

When it comes to permeability, it is also important to note that for each cubic centimeter (cc) of water added, 16 000 cubic centimeter (cc) of steam is produced.

Surface Finish

Both the size as well as the shape of the sand particles will determine the surface finish achieved for sand casted products. Finer particles will produce a better surface finish, but it is important to note that as the sand particles become finer and the surface finish improves, the permeability of the sand used, becomes worse.

Cohesiveness / Bond

The cohesiveness of the sand is the sand's ability to maintain its given shape after the pattern has been removed.

Flowability

The flowability of the sand has to do with the sand's ability to flow into tight corners and intricate details without making use of special processes.

Collapsibility

The sand needs to be easily stripped off the casting after it has solidified. A sand with a poor collapsibility will strongly adhere to the casting itself.

In cases where casting metals contract quite a bit during cooling or metals that have long freezing temperatures, a sand that has a poor collapsibility will cause defects such as hot tears and cracking. Additives are used to improve collapsibility.

Availability

When it comes to the cost of the sand used in the sand casting process, it is important to note that, for every ton of metal that is poured, between three to six tons of sand is needed.

Sand can be screened and reused, but this causes the particles to become fine, and prolonged screening and ruse will eventually cause the sand particles to become too fine.

For large castings, two different sands are often used, as it is a more economical choice and the second sand does not come into contact with the casting itself. The sand in contact with the casting is known as facing sand, the second sand being called backing sand.

Types of Sand Casting

Sand casting is divided into:

• Green Sand Casting:
  This is a low cost method of casting which does not deliver a very good surface finish.
• Resin Sand Casting:
  This casting method is widely used in the automotive industry as it delivers a good surface finish and dimensions. Also visit our Resin Cores for Sand Casting page.
• Sodium Silicate Bonded Sand Casting:
  This process is better for steel castings and is able to cast products of over 10 kilograms.

Sand Mold Production

The four main components of making a sand mold includes:

1. The Base Sand
2. A Binder
3. Additives
4. Parting Compound

The Base Sand

This is the type of sand used to produce the mold or core and is without a binder. With a bonding agent, sand particles will not bond together and is still unusable.

The following sands make up the sand casting materials used as base sands:

1. Chromite Sand:
   This is a solid solution of spinels and have a low percentage of silica, a very high thermal conductivity and a very high fusion point of 3 360°F (1 850°c). It is however an expensive base sand and is mostly used to produce cores or with a more expensive alloy steel casting.
2. Chamotte Sand:
   With a fusion point of 3 180°F (1 750°c), Chamotte sand has a low thermal expansion and is the second cheapest sand to use - note though that it is still higher than the cost of silica sand. Often used to cast larger steel work pieces, this sand limited to dry sand molding processes and has a very coarse grain, resulting in surface finish problems, though mold washes are performed to overcome most of these surface finish issues.
3. Olivine Sand:
   A mixture of orthosilicates of iron and magnesium, it has the advantage of a low thermal expansion, high fusion point as well as high thermal conductivity. Being from silica is one of its main advantages and can be used with a number of basic metals such as manganese steels, and is popularly used in Europe.
4. Silica Sand:
   The most commonly used, silica sand comes from natural occurring locations such as river beds and beaches, or is made by crushing sandstone. Pure silica has a fusion point of 3 200°F (1 760°C), but due to impurities, it has a lower melting point. In cases where a high melting point is necessary (such as for steels), a minimum of 98% pure silica sand is required, but in cases where low melting points are required (such as for those of non-ferrous metals and cast iron), lower purities of between 94% and 98% are used.
   Because of its abundance, it is the most inexpensive sand to se, but it has a high thermal expansion (causing defects when used in high melting point scenarios), and a low thermal stability (which in turn can cause unsound sand casted products).
5. Zicron Sand:
   Containing one third silica, it has the highest fusion point (4 710°F / 2 600°C), an extremely low thermal expansion and a high thermal conductivity. Commonly used in casting alloy steels and other expensive alloys, a mold wash is applied in order to improve the surface delivered. This sand is however expensive and not as readily available.

A Binder

Added to the base sands in order to bond the sand particles together, the following are popularly used in the sand casting process:

1. Clay and Water:
   Most commonly used and a binder, there are also two types of clay commonly used, namely bentonite (the most popular) and kaolinite.
2. Oil:
   Linseed, marine and vegetable oils are used as binders in order to produce sand casted products. Mostly phased out simply due to its increased costs, oils need to be carefully baked between 212°F and 392°F (100°C and 200°C) in order for the mold to cure. If overheated, the oil used as a binder will become brittle, wasting the entire mold.
3. Resin:
   Natural or synthetic gums with high melting points, the two most commonly used includes phenol formaldehyde (PF) and urea formaldehyde (UF). Phenol formaldehyde (PF) has a higher resistance to heat than the other does and is also less expensive. It also has a good collapsibility, low gassing and is capable of delivering a good surface finish.
   Cold-set resins are resins making use of a catalyst instead of heat in order to cure the binder. Popular because of the various different properties that can be achieved by mixing it with various additives.
4. Sodium Silicate:
   A high strength binder, it is used with silica molding sand and is cured by using carbon dioxide gas. Able to be used a room temperature, this binder is fast, but its high strength leads to shake-out difficulties and the possible occurrence of hot tears.

Additives

Additives are added to the sand casting materials used in order to:

• Improve the surface finish delivered.
• Improve the dry strength.
• Improve refractoriness.
• Improve the 'cushion properties'.

Up to 5% of 'reducing agents' can be added to the molding material in order to prevent wetting. These reducing agents include coal powered, fuel oil, pitch and creosote. It is added in order to:

• Prevent the liquid / molten metal from sticking to the sand particles, leaving these sand particles on the casting itself
• Improve the surface finish of the sand casted products.
• Decrease the occurrence of burn-on defects and metal penetration.

The above is achieved by creating gasses at the surface of the mold cavity, which in turn prevents the molten metal from sticking to the sand. It is important to note that reducing agents are not used with steel castings because these agents can carburize the metal during the sand casting process.

Up to 2% 'cereal binders' can be used in order to increase the dry strength as well as the surface finish achieved. Cereal binders include molasses, dextrin, sulphite lye and starch. The dry strength is the strength of the mold after it has cured and, cereal binders are able to improve collapsibility as well as reduce shake-out time. Cereal binders are however expensive additives.

Up to 2% iron oxide powder can be used in sand casting materials in order to prevent the occurrence of cracks and metal penetration. This leads to an improved refractoriness. Zicron flour and silica flour (or fine silica) is also capable of improving refractoriness, especially in ferrous metal castings. This additives do however greatly reduce permeability.

Up to 3% of cushioning materials can be added to help reduce hot cracks, scabbing, hot tear defects when casting high melting temperature metals. Cushioning materials include straw, saw dust, powdered husks and wood flour.

These additives are beneficial as they burn-off when the molten metal is poured, creating voids in the mold, allowing it to expand. These additives are also beneficial because they reduce shake-out time and increase collapsibility.

Parting Compound

Prior to casting, a parting compound is applied to the pattern so that the pattern can be removed from the mold easily. This parting compound can either be a liquid or an extremely fine powder with a particle diameter of between 75 micrometers and 150 micrometers (0.0030 inches and 0.0059 inches).

Commonly used liquids include:

1. Water based silicon solutions
2. Mineral oil

Commonly used powders includes:

1. Talc
2. Dry silica
3. Graphite

Back to Main Page: Sand Casting

Further Suggested Reading:

• Hollow Core Sand Casting
• Metals that can be Sand Cast
• Resin Cores for Sand Casting
• Sand Casting Tolerances
• Ultra Large Sand Casting