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Sand Casting Material

Modeling Material Performance

The material used to make the sand mold is called molding material, the material used to make the sand mold is conventionally called molding sand, and the molding material used to make the sand core is called core sand. Generally, molding sand is made by mixing raw sand (mountain sand or river sand), clay and water in a certain proportion. Among them, clay is about 9%, water is about 6%, and the rest is raw sand. Sometimes a small amount of additives such as coal powder, vegetable oil, wood chips, etc. are added to improve the performance of molding sand and core sand.

The quality of molding sand and core sand directly affects the quality of castings. Poor quality of molding sand will cause castings to produce defects such as pores, sand holes, sand sticking, and sand inclusion. Good molding sand should have the following properties:

After the high-temperature molten metal is poured into the mold, the mold is filled with a large amount of gas. These gases must be smoothly discharged from the mold. The property of the sand that allows gas to pass through is called gas permeability. Otherwise, the casting will have defects such as porosity and insufficient pouring. The air permeability of the mold is affected by factors such as the particle size of the sand, the clay content, the moisture content and the compactness of the sand mold. The finer the sand size, the higher the clay and moisture content, the higher the compactness of the sand, and the worse the air permeability

The ability of molding sand to resist external damage is called strength. The molding sand must have high enough strength to not cause collapse in the process of modeling, handling, and closing, and it will not damage the surface of the mold during pouring. The strength of the molding sand should not be too high, otherwise it will cause defects in the casting due to the decrease of air permeability and concession.

The high-temperature molten metal has a strong thermal effect on the mold after being poured, so the molding sand must have the ability to resist the high-temperature thermal effect, that is, fire resistance. If the molding material has poor fire resistance, the casting is prone to sand sticking. The more SiO2 content in the molding sand, the larger the molding sand particles and the better the fire resistance.

It refers to the ability of molding sand to deform under the action of external force, and to maintain the existing shape completely after the external force is removed. The plasticity of the modeling material is good, the modeling operation is convenient, and the formed sand mold has an accurate shape and a clear outline.

When the casting is condensed, the volume shrinks, and the molding sand should have a certain ability to be compressed, which is called concession. The retreat of molding sand is not good, and the casting is prone to internal stress or cracking. The tighter the molding sand, the worse the concession. Adding wood chips and other things to the molding sand can improve the concession.

The Most Complete Sand Foundry Materials And Alternative Materials

Since humans began to master the casting technology more than 5,000 years ago, the modeling materials used have been based on silica sand. The application of castings in all aspects has promoted the progress and development of civilization and promoted the emergence of the industrial revolution. Since then, with the rapid development of science and technology, a variety of new materials have continuously emerged, and the casting technology has also changed with each passing day. However, today, silica sand is still the most important molding material in the foundry industry of all countries in the world. At present, the world’s foundry industry consumes 45-50 million tons of raw sand every year, of which silica sand accounts for more than 95%. The most desirable feature of silica sand is its abundant reserves, low price and easy availability, which is unmatched by any other ore sand. In addition, silica sand also has some characteristics that can adapt to casting conditions, such as:

  1. It has a sufficiently high refractoriness to withstand the pouring temperature of most casting alloys.
  2. The particles are relatively hard and can withstand the impact and friction of sculpting and pressure during modeling and the impact and friction of old sand regeneration.
  3. At a high temperature close to its melting point, it still has enough strength to maintain its shape.

However, silica sand also has many shortcomings, mainly in the following aspects:

  1. Poor thermal stability, with phase change at around 570°C, accompanied by large volume expansion, which is the source of various “expansion defects” in castings It is also the main factor affecting the dimensional accuracy and surface roughness of castings.
  2. The chemical stability at high temperature is not good, and it is easy to react with FeO to produce fusible iron olivine, resulting in sand sticking on the surface of the casting. 
  3. When using it to prepare clay green sand, after repeated use, the surface of the sand grains accumulates a metamorphic sintered layer, which affects the quality of the castings and has to be discarded and renewed. Moreover, the regeneration of this waste sand is difficult.
  4. Under the condition of using various high-strength and small-dosage binders, when the old sand is recycled, the silica sand is easier to fragment, and the dust generated by the fragmentation is very harmful to human health. The fatal weakness of silica sand. 

Today, with increasing requirements for casting quality and stricter regulations on environmental protection and cleaner production, “silica sand is not the ideal raw sand” has gradually become everyone’s consensus, and the search for alternative materials for silica sand has become an important issue in the current foundry industry. For one, every industrial country attaches great importance to this.

One of the ways to find substitute materials for silica sand is to use suitable natural non-siliceous sand. Another way is to develop artificially manufactured granular materials. Here, I would like to briefly review the development of these two aspects, and put forward some preliminary ideas on the promotion and application of artificial sand in the foundry industry in my country.

In the process of industrial modernization, the foundry industry has been exploring the application of non-siliceous sand for nearly a hundred years. The application of various non-siliceous sands plays a very important role in the advancement of casting technology and the improvement of casting quality. However, due to the impact of resource conditions and certain constraints, so far, on the one hand, there is no natural non-siliceous sand that has the possibility of replacing silica sand on a large scale; on the other hand, from the current trend of technological development, The replacement of non-siliceous sand with various man-made materials has become a topic on the agenda.

The development and application of various artificial sands has also experienced more than 50 years of development. In the past ten years or so, the application of artificial sand has increasingly shown its advantages in improving the quality of castings, reducing siliceous dust pollution, and saving resources. All types of work related to “sand” in the foundry industry are paying more and more attention to artificial Application of sand. 

In terms of special casting technology, artificial sand not only has good effects in V method modeling and lost foam casting applications, but also has become a hot research topic in investment casting to replace zirconium sand in surface refractories. .  

In the aspect of sand casting, artificial sand is not only suitable for resin-bonded sand, but also replaces silica sand in clay green sand, and there are also many successful examples.

In terms of different casting alloys, artificial sand is not only suitable for the production of steel castings, but also for iron castings and non-ferrous alloy castings.

The natural non-siliceous sands used in the foundry industry mainly include forsterite sand, aluminum silicate sand, zircon sand and chromite sand.

1. Forsterite Sand

The application of forsterite sand was earlier. In 1927, Norway, which is rich in forsterite mines, first used it to prepare molding sand for the production of steel castings. In the case of relatively poor early protection conditions, the use of this raw sand can prevent workers from suffering from silicosis. Moreover, it can be used to produce high-manganese steel and high-chromium-nickel steel castings, which can significantly improve the surface quality of the castings. 

However, this kind of sand is made of crushed ore rock, and its grain shape is not good, and its cleavage is well developed, it is easy to fragment, the molding sand is difficult to recycle, and it is not suitable for regeneration treatment, and the price is several times that of silica sand. This is the two main reasons restricting its promotion and application.

2. Aluminum Silicate Sand

Aluminosilicate sand, including sand with kyanite, sillimanite and andalusite. In natural minerals, the three are often present at the same time, with different crystalline structures, but the same chemical composition (Al2OSiO4), which is a homogeneous and multi-like variant. The aluminum silicate sand is round-grained, with a Mohs hardness of 6-7, which is convenient for regeneration and has good adaptability to various resin binders. Judging from the above characteristics, aluminum silicate sand is an excellent raw foundry sand. It has been reported in the United States for production. However, so far, apart from the production in Florida and Western Australia, no other reports have been reported. Resources available for exploitation.

3. Zirconium Sand

The basic component of zirconium sand is zirconium silicate (ZrSiO4), which has a variety of characteristics suitable for casting raw sand, such as:

  1. The surface is smooth, there are few cracks and pits, so the amount of binder consumed is small.
  2. Among all kinds of natural foundry sands, its thermal expansion rate is the smallest, and its thermal stability at high temperatures is excellent.
  3. Not wetted by molten metal.  
  4. It has good thermochemical stability and is basically not corroded by metal oxides at the pouring temperature of general casting alloys.
  5. The acid requirement is very low, and it is suitable to use acid-hardened resin as a binder.
  6. High thermal conductivity.

The use of zircon sand in casting production was first seen in the 1940s. At that time, its high thermal conductivity was used as the modeling material for aluminum alloy castings for airplanes to produce dense castings. In the early days, there were also foundries in my country that used zirconium sand as the base sand of clay green sand to produce small high-quality steel castings on the molding production line.  

However, there are not many zircon sand reserves in the world, mainly produced in Australia and South Africa. In recent years, the demand for ceramics, refractories, and sunscreens has increased significantly. The price soars so fast that it is shocking and restricts The application in foundry production.

Although zircon sand is an ideal modeling material in terms of its performance, in fact, it is extremely rare to use it to prepare sand. At present, it is mainly used as the aggregate for casting coatings and the refractory material for the surface layer of high-alloy steel castings in investment casting. Even in investment casting, although the amount is small, it is by no means a long-term solution. In 2006, Japan took the “research and development of zirconium sand substitute materials for investment casting” as one of the strategic basic technology research and development projects in the foundry industry, and the state allocated funds to support the implementation of the research team formed by related enterprises. The preliminary result is that man-made mullite ceramsite can be used instead of zircon sand.

The research and development of artificial sand has been more than 50 years so far, and it has gradually entered the practical application stage in the past 10 years, and has shown a good momentum of development in the foundry industry of various countries.

1.Carbon Sand   

The earliest carbon-grained sand used to replace silica sand in the United States was angular particles obtained by crushing graphite electrode blocks. Although this kind of carbon sand has many advantages, it has many disadvantages due to its poor grain shape. For example, the specific surface area of ​​the sand particles is large, and the amount of binder required increases; the prepared sand has poor fluidity and is difficult to pound; It is easy to break when used. Therefore, failed to promote the application.

In 1960, J. Gentry in the United States produced spherical carbon sand from calcined liquid petroleum coke, and tried it in casting production, and achieved good results. In 1984, it began to put into commercial production in California, USA, and promoted to the foundry industry. Since 1989, it has been operated by American Colloid Company. Since then, its application has been gradually promoted.  

Characteristics of carbon-grained sand The main characteristics of spherical carbon-grained sand

  • The thermal expansion is small, and the casting sand will not produce expansion defects when it is used to prepare the molding sand.
  • The particles are spherical, the molding sand has good fluidity, is easy to pound, and has good air permeability.
  • The molding sand and core sand used for preparation have good demoulding performance, even if there are deep recesses on the pattern, it is easy to come out.
  • No dust hazard from siliceous powder.
  • It is not wetted by molten metal, and does not interact with metal oxides, and the castings will not produce sticky sand defects.
  • The particles are not easily broken, and the recycling rate is higher than that of silica sand.

Application of carbon-grained sand

Various binders used in the foundry industry can be used for carbon-grained sand.

Molding sand and core sand prepared with carbon particles are suitable for any molding process and core-making process; it can be used to manufacture iron castings and various non-ferrous alloy castings. If the mold or sand core is coated with appropriate coatings, it can also be used to manufacture castings. Steel parts.  

Although the sulfur content in carbon sand is relatively high (about 5%), all sulfur is tightly combined with carbon and has no effect on the quality of castings and the environment.

The price of carbon sand is lower than zirconium sand, but much higher than silica sand.

Generally, it is used in conjunction with silica sand. The core is made with 50:50 mixed sand. Under the same sand shooting conditions, the compactness of the core is improved, so that the dimensional accuracy of the core can be improved. Moreover, the sand core can also be painted without coating.

Use clay green sand with carbon sand for modeling. The amount of coal powder in the sand can be reduced according to the proportion of carbon sand in the mixed sand, which can reduce the harmful gas emitted during casting.

When the mixed sand is used to manufacture ductile iron castings, the high thermal conductivity of carbon grain sand and the high compactness of the mold can reduce or even eliminate shrinkage porosity defects.

A foundry in the United States once evaluated the durability of carbon sand under production conditions. Clay green sand is prepared with carbon sand, and it is modeled with a shock pressure molding machine and a double-sided template. The sand is turned 2 to 3 times a day, and only bentonite and water are added each time the sand is mixed. After 8 months of verification, the amount of sand has not been reduced, and the particle size composition of the carbon sand in the system sand has basically not changed.

In the early 1960s, the United States also used graphite particles instead of silica sand to make molds and adopted differential pressure casting to produce high-quality magnesium alloy castings for aerospace use. The pressure difference of the protective gas is used to make the magnesium alloy liquid enter the mold.

The main points of the molding process are: use artificial graphite particles with an AFS fineness of about 47 (roughly equivalent to 40/70 mesh) as the base sand, add bentonite and water to mix, and the wet compressive strength of the mixture is about 40kPa. The mold is dried at 120°C for 24h. 

Then, the mold is assembled and placed above the sealed smelting crucible. The sprue below the mold is aligned with the riser tube and fastened to the crucible lid. The pressure difference between the argon gas in the crucible that protects the magnesium alloy liquid and the protective argon gas in the mold is used to make the magnesium alloy liquid rise steadily into the mold.

2.Enstatite sand   

Enstatite sand is made by smelting iron-nickel alloy slag. The development of enstatite sand can be said to be the reuse of waste. The price of finished sand is only slightly higher than that of silica sand.  

The raw ore for smelting iron-nickel alloys is weathered from nickel-containing peridotite. The chemical composition of the slag produced during smelting is also similar to natural peridotite. The main components are SiO2 and MgO. It also contains a small amount of iron oxide, aluminum oxide, calcium oxide and other ingredients. From the perspective of mineral composition, enstatite is the main component, and some forsterite and diopside are also included. There is no free MgO, CaO and SiO2.  

At present, all the manufacturers of enstatite sand are those who smelt iron-nickel alloys. The main production plant in the United States is GreenDiamond Products. In Japan, Pacific Metal Hachinohe, which produces iron-nickel alloys, cooperates with Yamakawa Sangyo Co., Ltd., which produces casting materials.

The varieties of enstatite sands. Currently, there are two types of enstatite sands with different grain shapes.

Early enstatite sand was made by quenching molten slag into water, breaking it, and then crushing it. The particles made in this way have sharp-angled shape and difficult to control particle size. They are not suitable for casting sand and are mainly used for sandblasting and construction.  

Around 1985, the Japanese Yamakawa Industry Co., Ltd. improved the production process of this enstatite sand so that it can be used as raw sand for foundry. The main point of the process is: the molten slag is quenched into water, cooled, and crushed to obtain sharp-angle enstatite particles, which are then placed in a proprietary trough grinder to add water to grind to improve the particle shape and surface properties.

In view of the poor grain shape of the angular enstatite sand, the prepared molding sand is not easy to pound. In 1998, Japan's Yamakawa Co., Ltd. and Pacific Metals Co., Ltd. worked together to successfully produce spherical enstatite sand. The main point of the preparation process is: do not quench the molten iron-nickel alloy slag into water for cooling, and use air instead. After being blown and cooled, it will become fine spherical particles with a particle size of 0.5 mm or less. Then add water to grind in a trough-type ore mill to obtain round-grained sand that is very suitable for casting raw sand. The trade name is "sun beads".

The characteristics of enstatite sand The main characteristics of enstatite sand

  1. The surface is clean.
  2. Less thermal expansion.
  3. No harmful silicon dust is produced.
  4. Because it is made of molten material, it has a solid texture and good durability.
  5. Spherical enstatite sand also has other advantages of spherical sand

Application of enstatite sand The refractoriness of enstatite sand is low

Mainly used in the manufacture of iron castings. It can also be used to manufacture high manganese steel castings with lower melting points.  

The angular enstatite sand has poor grain shape and is not easy to pound, so the thermal conductivity is low. The coated sand used for preparation is not easy to harden. In actual production, angular enstatite sand is rarely used alone to make coated sand, and it is often used in conjunction with silica sand.  

Angular enstatite sand can replace forsterite sand to make high manganese steel castings. Eagle Creek Foundry in Oregon, USA, used forsterite sand molded with water glass to manufacture high manganese steel castings. After switching to enstatite sand, since the consistency of the quality of this raw sand is better than that of forsterite sand, the stability of the mold quality is better. In addition, compared with forsterite sand, it is easier to regenerate and reuse, the consumption of raw sand is reduced, and the production cost is reduced.  

Spherical enstatite sand is a better modeling material. The compound coated sand used can significantly reduce the amount of resin added, thereby reducing production costs and reducing gas emissions. However, due to its high acid requirement, when using acid-hardened resins, a stronger hardener is required.

It is especially worth mentioning that the surface of spherical enstatite sand is very smooth. When the old sand is regenerated, only slight friction can remove the adhesive film on the surface of the sand. When used for molding sand and core sand of various organic binders, the sand regeneration process consumes less energy, and the old sand regeneration rate is high, which can bring many benefits. Even the water glass bonded sand, which has always been a problem in sand regeneration, has better results after using spherical enstatite sand.

At present, there are three types of aluminum silicate artificial sand: sintered ceramsite, low-density ceramsite and fused mullite sand.

(1) Sintered Ceramsite

First developed by the American CarboCeramics company in the 1980s, the trade name is "Ceramacore", which is artificially fired ceramic spherical particles, which were originally used as proppants in the oil and gas industries. In the early 1990s, the United States and Japan successively used it in the foundry industry as a substitute for zircon sand. The Japanese manufacturer is ITOCHU Ceramics Co., Ltd., and the name of the product is "Cerabeads", referred to as CB sand.

There are two kinds of granulation methods used by ITOCHU Ceramics: coarser granules, use a rotary granulator; 0.2~1mm granules, mix the powder according to the required ingredients, add the binder to form a slurry, and then spray Shape and dry at the same time. For powders that require fluidity and good filling properties, such as refractory materials used in investment casting surface coatings, surface sintering treatment is required. Put the prepared powder into a high-temperature flame to melt the sharp corners.  

As raw sand used in the foundry industry, sintered ceramsite has the following advantages:

  1. The particles are spherical, with good fluidity and easy to pound.
  2. Good air permeability, when used in oil and gas industry, both oil and gas can pass through. As foundry sand, it is especially suitable for lost foam casting and V method molding process.
  3. Low thermal expansion rate, comparable to zirconium sand. Foreign countries have tried ceramsite mixed with sand to cast molten metal at 1650°C, and no expansion defects were seen in the castings.
  4. Good regeneration and reuse performance. 

In the United States, sintered ceramsite was first used in the lost foam casting process. For example, Citation Casting Company is a major manufacturer that adopts the lost foam process. After switching to ceramsite as the filling sand, the effect is very good. According to reports, the factory believes that the use of ceramsite will greatly reduce the energy consumed for modeling, and the dimensional accuracy of castings will be significantly improved, especially the durability of the ceramsite.

American Ashland company attaches great importance to the various advantages of sintered ceramsite, and signed an exclusive sales agreement with CarboCeramics in 1997.  

In principle, sintered ceramsite can be applied to various existing binders. However, due to the rough and unsmooth surface, when low-viscosity binders (such as various resins) are used, more binders will be adsorbed on the surface. Therefore, the bonding bridges between the sand grains are finer, and the strength of the molding sand is lower when the amount of binder is the same.  

Both the United States and Japan have foundries that use ceramsite to prepare bentonite-bonded green sand to produce ductile iron castings, and satisfactory results have been obtained.  

The problem of using sintered ceramsite is the high price, and the current supply of product size specifications are not yet complete.  

(2) Due to the low thermal conductivity of low-density ceramsite

When manufacturing thin-walled iron castings, adding ceramsite to the clay green sand or core sand not only facilitates the filling of molten iron, but also avoids the occurrence of excessive Cold structure is an important measure that helps to produce ultra-thin wall (2.5~3mm) cast iron parts, which is very beneficial to meet the lightweight requirements of automobiles and other electromechanical equipment.  

According to reports in the United States, the composition of the low-density ceramsite used is roughly: 25%-40% alumina and 55%-75% silica. Its thermal conductivity is about 0.15~0.25W/m•K, which is only about 1/3 of that of silica sand. The bulk density is 0.35~0.45g/cm3, and the softening temperature is 1200~1600℃. It can be mixed into molding sand or core sand, and can also be used to prepare thermal insulation coatings or heating riser sleeves.

Regarding the manufacturing method of low-density ceramsite, it has not been reported abroad. From the reported data on its composition, thermal conductivity, bulk density, softening temperature, and the morphology under the scanning electron microscope, the author believes that it is likely to be power plant processing. A by-product of smoke and dust is commonly known as "floating beads" in my country.  

(3) Melted mullite sand

(the "jewel sand" produced in my country) In the development of artificial sand, my country has its own innovation, that is, the development of molten mullite sand. The area of ​​Luoyang, Henan, my country is rich in high alumina bauxite resources. More than ten years ago, the production company cooperated with colleges and universities to develop mullite artificial sand. Kailin Casting Material Company, Jinjue Casting Material Company, Baozhu Sand Casting Material Company and other enterprises Production, this kind of product was originally called "Baozhu Sand", and later Jinjue Company called it "Yuzhu Sand", and they are actually the same product. The manufacturing method of Baozhu sand is: select high-quality bauxite raw materials and melt them in an electric arc furnace. When the molten liquid flows out of the furnace, it is blown away with a compressed air stream. After cooling, spherical or nearly spherical particles are obtained. smooth. As far as the manufacturing method is concerned, it is basically the same as the method used by Japan's Yamakawa Corporation to manufacture "sun beads", but the chemical composition is different.

"Baozhu sand" has a variety of excellent properties, can be applied to various casting alloys and various casting processes, and the price is lower than zirconium sand and chromite sand. At the beginning of its appearance, "Baozhu Sand" quickly attracted the attention of foreign foundry industries. Early products were mainly exported to Japan and sold from Japan to other countries. In recent years, my country's foundry industry has also begun to use, and achieved good results.

The grain shape of orb sand is spherical or close to spherical, and the color is black-brown. The particle size is between 0.053~2.0mm (270~6 mesh), and customers can make specific regulations on the particle size distribution according to their own process requirements. In addition, the production plant can also supply fine powder below 0.053mm (270 mesh).  

The main properties of precious pearl sand:

  1. The refractoriness is above 1800℃, which is much higher than enstatite sand. The high temperature chemical stability is far better than silica sand. It can be used for cast steel, cast iron and various non-ferrous alloys.
  2. It is an fused material with dense texture and better durability than sintered ceramsite.
  3. The expansion rate after heating is about 1/5 of that of silica sand, which is roughly the same as zirconium sand and slightly lower than chromite sand.

As a molding material, it can effectively prevent expansion defects in castings. The price of gemstone sand is relatively cheap, about 1/3 of zircon sand and 1/2 of chromite sand.

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