Knowledge

Sand casting is a casting method that produces castings in a sand mold. Steel, iron and most non-ferrous alloy castings are available sand casting method. As sand casting molding materials used in cheap and easy to get, casting manufacturing simple, single piece of casting production, batch production and mass production can be adapted to, for a long time, has been the basic process of casting production.

 

Manufacture of sand materials known as modeling materials for the manufacture of sand materials are customarily referred to as sand, for the manufacture of sand core modeling materials known as core sand. Usually sand is made of raw sand (sand or river sand), clay and water mixed in a certain proportion, of which about 9% clay, water is about 6%, the rest of the raw sand. Sometimes a small amount of additives such as coal powder, vegetable oil, wood chips, etc. are added to improve the performance of the sand and core sand.

The quality of molding sand and core sand directly affects the quality of castings, molding sand quality is not good will make the castings produce porosity, trachoma, sticky sand, sand and other defects. Good sand should have the following properties:

1) Air permeability

After the high temperature metal liquid is poured into the casting mold, the mold is filled with a large amount of gas, which must be smoothly discharged from the casting mold, and the performance of the sand mold that allows the gas to pass through the mold is called air permeability. Otherwise, the casting will produce porosity, underpouring and other defects. The air permeability of the casting model is affected by the sand particle size, clay content, moisture content and sand compactness and other factors. The finer the grain size of the sand, the higher the clay and moisture content, the higher the compactness of the sand, the worse the permeability.

2) Strength

The ability of sand to resist external damage is called strength. Sand must have a high enough strength in order to molding, handling, box process does not cause collapse, pouring will not destroy the casting surface. The strength of the sand should not be too high, otherwise the casting will be defective due to the decrease of air permeability and yielding.

3) Fire resistance

High-temperature metal liquid is poured into the casting to produce strong thermal effects, so the sand should have the ability to resist high-temperature thermal effects, i.e., refractory. If the fire resistance of the molding material is poor, the casting is easy to produce sticky sand. The more SiO2 content in the sand, the larger the sand particles, the better the refractoriness.

4) Plasticity

It refers to the ability of sand to deform under external force and to maintain the existing shape after removing the external force. The plasticity of the molding material is good, the molding operation is convenient, and the shape of the sand mold is accurate and clear.

5) Concession

When the casting condenses, the volume shrinks, and the sand should have a certain ability to be compressed, which is called concessionality. The concession of the sand is not good, the casting is easy to produce internal stress or cracking. The tighter the sand, the worse the concessions. In the type of sand to add wood chips and other things can improve the concessions.

Chemical hardening sand casting process is characterized by:

① chemical hardening sand strength than clay sand type much higher, and made of sand in the hardening to have a fairly high strength after the film, do not need to repair the type. As a result, the casting can more accurately reflect the size and contour shape of the mold, and is not easy to deform in the subsequent process. The resulting castings have high dimensional accuracy.

② Because of the viscosity of the binder and hardener are not high, it is easy to mix with the sand, sand mixing equipment, lightweight structure, small power and high productivity, sand processing work can be partially simplified.

③ The mixed sand has good fluidity before hardening, and the sand is easy to be pounded when modeling, thus it does not need a huge and complicated modeling machine.

④When molding with chemical hardening sand, molding materials such as wood, plastic and metal can be selected according to production requirements.

⑤ The content of binder in chemical hardening sand is much lower than that of clay sand, in which there is no powdered auxiliary materials, such as the use of the same grain size of the original sand, the gap between the sand particles is much larger than that of clay sand. In order to avoid casting metal penetration between the sand particles, sand or core surface should be coated with good quality paint.

(6) Chemical hardening sand with water glass as binder is low cost and odorless in the working environment. But this casting after pouring metal sand is not easy to collapse; used old sand can not be directly recycled, subject to regeneration, and water-glass sand regeneration is more difficult.

(7) with resin as a binder for the chemical hardening sand cost is higher, but after pouring the casting is easy to separate from the sand, the casting cleanup workload is reduced, and most of the sand used can be recycled recycling use.

Sand casting is one of the casting process, sand casting casting is generally used by the outer sand and core combination. As the sand casting molding materials used in cheap and easy to get, casting manufacturing simple, single piece of casting production, batch production and mass production can be adapted to, for a long time, has been the basic process of casting production. At present, internationally, in all castings production, 60 to 70% of the castings are produced with sand, and about 70% of them are produced with clay sand.

Sand casting compared to other casting methods, low cost, simple production process, short production cycle. So like the car's engine cylinder block, cylinder head, crankshaft and other castings are produced with clay wet sand process. When the wet type can not meet the requirements and then consider the use of clay sand table dry sand, dry sand or other sand. Clay wet sand casting casting weight can be from a few kilograms until dozens of kilograms, while the clay dry casting production can weigh tens of tons. Because of the above advantages of sand casting, its application in the foundry industry is becoming more and more widespread. In the future, it will play an increasingly important role in the foundry industry.

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