Minghe

Minghe

Welcome To Sand Casting Org

Free To Post, Free To Register By Minghe Sand Casting Org

Basic Material

The basic raw materials for making sand molds are foundry sand and molding sand binder. The most commonly used foundry sand is siliceous sand. When the high-temperature performance of silica sand cannot meet the requirements of use, special sand such as zircon sand, chromite sand, and corundum sand are used. In order to make the finished sand mold and core have a certain strength and not be deformed or damaged during handling, molding and pouring of liquid metal, it is generally necessary to add a molding sand binder in the casting to bond the loose sand particles together to form molding sand. The most widely used molding sand binder is clay, and various drying oils or semi-drying oils, water-soluble silicates or phosphates and various synthetic resins can also be used as molding sand binders. The outer sand molds used in sand casting are divided into three types: clay green sand, clay dry sand and chemical hardened sand according to the binder used in the sand and the way it builds its strength.

Clay green sand molds use clay and an appropriate amount of water as the main binder for molding sand. After the sand mold is made, it is directly combined and poured in a wet state. Wet casting has a long history and is widely used. The strength of green sand depends on the clay slurry formed by mixing clay and water in a certain proportion. Once the molding sand is mixed, it has a certain strength. After being pounded into a sand mold, it can meet the requirements of molding and pouring. Therefore, the amount of clay and moisture in the molding sand are very important process factors.

The Advantages Of Clay Green Sand Casting

  1. Clay is rich in resources and low in price.
  2. Most of the used clay wet sand can be recycled and reused after proper sand treatment.
  3. The cycle of manufacturing the mold is short and the work efficiency is high.
  4. The mixed molding sand can be used for a long time.
  5. After the sand mold is pounded, it can still tolerate a small amount of deformation without being damaged, which is very beneficial for drafting and core setting.

The Weakness Of Green Sand Casting

  1. To coat the viscous clay slurry on the surface of the sand grains during sand mixing, high-power sand mixing equipment with kneading action is required, otherwise it is impossible to obtain good quality sand.
  2. Since the molding sand has very high strength after being mixed, the molding sand is not easy to flow during modeling, and it is difficult to pound. It is laborious and requires certain skills when modeling by hand, and the equipment is complicated and huge when modeling by machine.
  3. The rigidity of the mold is not high, and the dimensional accuracy of the casting is poor.
  4. Castings are prone to defects such as sand washing, sand inclusion and pores.

At the beginning of the 20th century, the foundry industry began to use roller-type sand mixers to mix sand, which greatly improved the quality of clay green sand. The new high-power sand mixer can achieve high efficiency and high quality in sand mixing work. The emergence of the shock compaction molding machine, which mainly focuses on compaction, has significantly improved the compactness and uniformity of the mold. As the requirements for the dimensional accuracy and surface quality of castings increase, high-pressure molding machines that mainly compaction have appeared. The use of a high-pressure molding machine to make clay green sand molds can not only improve the dimensional accuracy of the castings and improve the surface quality, but also simplify the action of the compact mold and shorten the cycle, so that the entire process of molding and molding can be high-speed and automated. A new type of molding machine with gas impact pressure, using the thixotropy of clay slurry, can obtain a very compact mold by instantaneously applying a pressure of 0.5 MPa. These developments are important conditions for the clay green sand casting to adapt to the requirements of modern industry. Therefore, this traditional process has been used to produce a large number of high-quality castings.

Clay dry sand molds have a slightly higher wet moisture content than those used in the production of this sand mold. After the sand mold is made, the surface of the cavity should be coated with refractory paint, and then placed in an oven for drying, and after it has cooled down, it can be molded and poured. Drying clay sand molds takes a long time and consumes a lot of fuel, and the sand molds are easily deformed during the drying process, which affects the accuracy of castings. Clay dry sand molds are generally used to make steel castings and larger iron castings. Since chemically hardened sand has been widely adopted, dry sand types have tended to be eliminated.

Chemical hardening sand The molding sand used in this type of sand is called chemical hardening sand. The binder is generally a substance that can polymerize molecules and become a three-dimensional structure under the action of a hardener, and various synthetic resins and water glass are commonly used. There are basically 3 ways of chemical hardening.

  1. Self-hardening: The binder and hardener are both added during sand mixing. After the sand mold or core is made, the binder reacts under the action of the hardener to cause the sand mold or core to harden by itself. The self-hardening method is mainly used for modeling, but it is also used to manufacture larger cores or cores with small production batches.
  2. Aerosol hardening: Add binder and other auxiliary additives when mixing sand, do not add hardener first. After modeling or core making, blow in the gaseous hardener or the liquid hardener atomized in the gaseous carrier to disperse it in the sand mold or core to cause the sand mold to harden. The aerosol hardening method is mainly used to make cores, and sometimes it is also used to make small sand molds.
  3. Heat hardening: Add binder and latent hardening agent that does not work at room temperature when mixing sand. After the sand mold or core is made, it is heated. At this time, the latent hardener reacts with certain components in the binder to generate an effective hardener that can harden the binder, thereby hardening the sand mold or core. The heating hardening method is mainly used to make cores in addition to the manufacture of small thin-shell sand molds.

The characteristics of the chemical hardening sand casting process are:

  1. The strength of chemically hardened sand mold is much higher than that of clay sand mold, and after the sand mold is made, it will be released after hardening to have a relatively high strength, and it does not need to be repaired. Therefore, the mold can more accurately reflect the size and contour shape of the pattern, and it is not easy to deform in the subsequent process. The obtained castings have high dimensional accuracy.
  2. Because the viscosity of the binder and hardener used is not high, it is easy to mix with the sand grains. The sand mixing equipment has a light structure, low power and high productivity, and the sand processing part can be simplified.
  3. The mixed molding sand has good fluidity before hardening, and the molding sand is easy to pound during molding, so there is no need for a large and complicated molding machine.
  4. When modeling with chemically hardened sand, the pattern material can be selected according to the production requirements, such as wood, plastic and metal.
  5. The content of binder in chemically hardened sand is much lower than that in clay sand, and there are no powdered auxiliary materials. If raw sand with the same particle size is used, the gap between sand grains is much larger than that in clay sand. In order to prevent the metal from penetrating between the sand grains during casting, the surface of the sand mold or core should be coated with high-quality paint.
  6. The chemical hardening sand with water glass as the binder has low cost and no smell in the working environment. However, the molding sand is not easy to collapse after the metal is poured into the mold; the used sand cannot be directly recycled and must be recycled, and the regeneration of sodium silicate sand is more difficult.
  7. The cost of chemically hardened sand using resin as a binder is relatively high, but after pouring, the casting is easy to separate from the sand, the workload of cleaning the casting is reduced, and most of the used sand can be recycled and reused.

In order to ensure the quality of castings, the cores used in sand casting are generally dry cores. According to the different binders used in the core, the core is divided into clay sand core, oil sand core and resin sand core.

Clay Sand Core Is A Simple Core Made Of Clay Sand

Oil sand core cores made of core sand with dry oil or semi-dry oil as the binder are widely used. The oil has low viscosity, the mixed core sand has good fluidity, and it is easy to compact when making the core. But the strength of the newly made core is very low, and it is generally accepted by a profiling core board, and then baked in an oven at 200-300°C for several hours, and the oil is oxidized by air to harden it. The shortcomings of this core making method are: the core is easily deformed during demolding, handling and baking, resulting in reduced dimensional accuracy of the casting; long baking time and high energy consumption.

Resin sand cores are various cores made of resin sand. After the core is hardened in the core box, it can be taken out to ensure that the shape and size of the core are correct. According to different hardening methods, the manufacture of resin sand cores is generally divided into two methods: hot-box core-making and cold-box core-making. ①The hot core box method for core production: Appeared in the late 1950s. Usually furan resin is used as the core sand binder, and a latent hardener (such as ammonium chloride) is also added. When making the core, keep the core box at 200-300°C. After the core sand is injected into the core box, the ammonium chloride reacts with the free formaldehyde in the resin to generate acid at a higher temperature, so that the core will harden quickly. It takes about 10-100 seconds to establish the release strength. The core is made by the hot core box method, and the dimensional accuracy of the core is relatively high, but the process equipment is complicated and expensive, energy consumption is high, irritating gas is discharged, and the working conditions of the workers are also very poor. ②Cold core box method to make cores: Appeared in the late 1960s. Use urethane resin as the core sand binder. When the core is made by this method, the core box is not heated, and amine steam is blown into it for a few seconds to harden the core. This method is superior to the hot box method in terms of energy, environment, and production efficiency. In the mid-1970s, the furan resin cold box method, which was hardened by blowing sulfur dioxide, appeared again. The hardening mechanism is completely different from the urethane cold core box method, but the process characteristics, such as fast hardening and high core strength, are roughly the same as the urethane cold core box method.

Footnotes

  1.    Footnote 01

    Li Changhe Yang Jianjun. Metal Technology. No. 16 North Chenggen Street, Donghuang, Beijing: Science Press, first edition in May 2014. Fourth printing in July 2016: 34~36.

  2.    Footnote 02

    Chen Qiaosheng. A three-layer sand box for sand-turning foundry in a copper furnace and its casting process: CN, CN103008560A[P]. 2013.

  3.    Footnote 03

    Wu Daowei. Comprehensive utilization of raw materials for foundry foundry from coal slime[J]. Mineral Resources Conservation and Utilization, 2000(4).

  4.    Footnote 04

    Foundry Professional Society of Chinese Mechanical Engineering Society. Casting Handbook: Volume 5 Casting Process. Beijing: Mechanical Industry Press, 2003

  5.    Footnote 05

    Li Hongying, Zhao Chengzhi. Casting process design: Machinery Industry Press, 2005

  6.    Footnote 06

    Ai Yunlong and so on. Engineering materials and forming technology: Machinery Industry Press, 2016: 202

  7.    Footnote 07

    Foundry Professional Society of Chinese Mechanical Engineering Society. Casting Handbook: Volume 6 Special Casting. Beijing: Mechanical Industry Press, 2003

Footnote 01

Li Changhe Yang Jianjun. Metal Technology. No. 16 North Chenggen Street, Donghuang, Beijing: Science Press, first edition in May 2014. Fourth printing in July 2016: 34~36.

Footnote 02

Chen Qiaosheng. A three-layer sand box for sand-turning foundry in a copper furnace and its casting process: CN, CN103008560A[P]. 2013.

Footnote 03

Wu Daowei. Comprehensive utilization of raw materials for foundry foundry from coal slime[J]. Mineral Resources Conservation and Utilization, 2000(4).

Footnote 04

Foundry Professional Society of Chinese Mechanical Engineering Society. Casting Handbook: Volume 5 Casting Process. Beijing: Mechanical Industry Press, 2003

Footnote 05

Li Hongying, Zhao Chengzhi. Casting process design: Machinery Industry Press, 2005

Footnote 06

Ai Yunlong and so on. Engineering materials and forming technology: Machinery Industry Press, 2016: 202

Footnote 07

Foundry Professional Society of Chinese Mechanical Engineering Society. Casting Handbook: Volume 6 Special Casting. Beijing: Mechanical Industry Press, 2003

On this page