Sand casting is a process used to make metal parts, but it can also be used to make other materials such as glass. It involves pouring a vat of molten metal material into a prepared sand mould. A sand mould is a cavity that is similar in shape to the desired product. Because the sand casting process is compatible with complex designs, sand cast products vary in shape, size and weight. In addition, the process is compatible with a wide range of metals and the use of sand makes it a cost effective option. The main costs for many sand foundries are the production of the mould and the purchase of the metal material.
Green sand contains sand (75% to 85%), clay (5% to 11%), water (2% to 4%) and other additives. The clay and water act as binders. Greensand for casting is suitable for high volume production and can be reused after casting. However, greensand casting is not suitable for close tolerance designs and sand castings require secondary finishing.
The Dry sand contains clay or other binders that are formed into a mould and then fired. Dry sand is suitable for ferrous and non-ferrous parts with complex designs and tight tolerances. As a result, dry sand is widely used in many sand foundries to produce parts such as engine blocks and transmissions. Although popular, dry sand cannot be reused.
Resin sand, or furan sand, is a mixture of silica sand or other aggregate material with a synthetic resin binder that creates stable and strong moulds for sand casting metal parts. The Resin sand has greater strength and dimensional stability, making it more suitable for parts that require precision and consistency. In addition, resin sand has a shorter curing time, requires less secondary finishing due to fewer surface defects, and is less likely to deform the mould when the molten material is poured.
This type of foundry sand uses a sodium silicate binder to consolidate the sand into the mould. It is therefore preferred for parts with complex and delicate patterns. Water glass moulds are less flexible in use and operation, but are dimensionally stable, low cost and have a short curing time.
This is a mixture of silica sand, core oil (such as mineral oil or linseed oil) and binders (such as dextrin and sodium silicate). It is suitable for making core material because of its high compressive strength.
A pattern is a reusable component made of materials such as wood, metal, plastic polymers, etc. It is divided into two halves, each with a design, which when combined form the finished product. To allow for the shrinkage of the final sand casting, slight adjustments are made to the design and the metal paths are integrated with the pattern, usually slightly larger. A draft allowance is also added to make the pattern easier to remove, with all surfaces parallel to the direction of pattern removal being slightly chamfered inwards.
Sand casting moulds are made by packing patterns with four ingredients: base sand, binders, additives and release agents. Base sand, such as silica sand, is used to produce the purest moulds and cores. Binders, such as clay and water, help to hold the sand together, while additives help to improve surface finish, strength, cushioning or refractory properties. Finally, release agents can be liquid or fine powders to facilitate pattern removal.
Like patterns, sand moulds for metal casting have two halves. You call the bottom half the drag mold and the top half the cover mold. After the sand solidifies, remove the pattern, then reassemble and clamp the two halves to form a cavity for the product when you pour the molten material. To achieve a better surface finish, apply a refractory coating to prevent the molten metal from affecting the mold. Moulds may also contain cores that form internal channels in the finished product.
The material is melted by any suitable method. The molten material is then poured smoothly and quickly into the mould to eliminate turbulence, which can cause casting defects and hinder solidification. Some materials that are extremely sensitive to oxygen require the use of a shielding process, such as an argon shield, to prevent air from entering the molten metal.
On proper solidification of the molten material, shakeout occurs. A shakeout operation involves splitting the sand and removing the metal casting in sand. Depending on the type of casting sand, you can reuse some.
Final processing for sand casting products includes machining operations such as honing and grinding for removing the gates, runners, and risers. Furthermore, you can subject the part to heat treatment or other surface finishing options like sandblasting.
Are you looking for a reputable partner for your parts fabrication and machining projects? Look no further. At PROTO MFG, we specialize in CNC machining and related technologies, including sheet metal fabrication, rapid prototyping, etc. Whether it is a project with a simple design or parts with complex geometries, do not hesitate to contact us today!
