Sunday, May 18, 2008
Sand molded casting
A sand casting is a cast part, which is produced by forming a mold out of a sand mixture and pouring a casting liquid (often molten metal) into the mold. The mold is then air-cooled until the metal solidifies, and the mold is removed. Sand Casting is basically done in these steps:
1. Place a pattern in sand to create a mold2. Incorporate a gating system3. Remove the pattern4. Fill the mold cavity with molten metal5. Allow the metal to cool6. Break away the sand mold and remove the casting.
There are two main types of sand molding. "Green sand" is a mixture of silica sand, clay, moisture and some other additives. The "Air set" method uses dry sand that is bonded to other materials other than moist clay, using a fast curing adhesive. When these chemicals are used, they are collectively called "air set" sand castings to distinguish these from "green sand" castings. Many chemicals and mixtures have been designed for this use. Two general sand types are Natural bonded (bank sand) and synthetic (lake sand). And because synthetic sand's composition is more controllable it is preferred.
With both methods, the sand mixture is packed around a master "pattern" in order to form a mold cavity. If necessary, a temporary plug is placed to form a channel for pouring the fluid to be molded. Air-set molds often form a two-part mold having a top and bottom. The sand mixture is tamped-down as it is added, and the final mold assembly is sometimes vibrated in order to compact the sand and fill any unwanted voids in the mold. Then the pattern is withdrawn along with the channel plug.
Then the casting liquid (typically hot molten metal) is poured into the mold cavity left by the pattern. After the metal has solidified and cooled, the casting is separated from the sand mold. The mold is often designed to be single-use. There is typically no mold release agent, and the mold is generally destroyed in the removal process.
The accuracy of the casting is limited by the type of sand and the molding process. Sand castings made from coarse green sand impart a rough texture on the surface of the casting, and this makes them easy to recognize. Air-set molds can produce castings with much smoother surfaces. Surfaces can also be ground and polished, for example when making a large bell.
After molding, the casting is covered in a residue of oxides, silicates and other compounds. This residue can be removed by various means, such as grinding, or shot blasting.
During casting, some of the components of the sand mixture are lost in the thermal casting process. Green sand can be reused after adjusting its composition to replenish the lost moisture and additives. The pattern itself can be reused indefinitely to produce new sand molds. The sand molding process has been used for many centuries to produce castings manually. Since 1950, partially-automated casting processes have been developed for production lines.
1 Simple manual sand casting process
1.2 Molding box and materials
1.5 Design requirements
2 Types of Sand Castings
2.1 Green Sand
2.2 Cold Box
2.3 No Bake Molds
3 Essential improvements of the foundry technology
4 Fast molding & sand casting processes
4.1 Mechanized sand molding
4.2 Automatic high pressure sand molding lines
4.2.1 Horizontal sand flask molding
4.2.2 Vertical sand flaskless molding
4.2.3 Matchplate sand molding
5 Decorative use of wood patterns
6 Alternative casting methods
7 See also
9 External links
ORIGIN Foundry sand consists primarily of clean, uniformly sized, high-quality silica sand or lake sand that is bonded to form molds for ferrous (iron and steel) and nonferrous (copper, aluminum, brass) metal castings. Although these sands are clean prior to use, after casting they may contain Ferrous (iron and steel) industries account for approximately 95 percent of foundry sand used for castings. The automotive industry and its parts suppliers are the major generators of foundry sand.
The most common casting process used in the foundry industry is the sand cast system. Virtually all sand cast molds for ferrous castings are of the green sand type. Green sand consists of high-quality silica sand, about 10 percent bentonite clay (as the binder), 2 to 5 percent water and about 5 percent sea coal (a carbonaceous mold additive to improve casting finish). The type of metal being cast determines which additives and what gradation of sand is used.
The green sand used in the process constitutes upwards of 90 percent of the molding materials used.(1)
In addition to green sand molds, chemically bonded sand cast systems are also used. These systems involve the use of one or more organic binders (usually proprietary) in conjunction with catalysts and different hardening/setting procedures. Foundry sand makes up about 97 percent of this mixture. Chemically bonded systems are most often used for “cores” (used to produce cavities that are not practical to produce by normal molding operations) and for molds for nonferrous castings.
The annual generation of foundry waste (including dust and spent foundry sand) in the United States is believed to range from 9 to 13.6 million metric tons (10 to 15 million tons).(2) Typically, about 1 ton of foundry sand is required for each ton of iron or steel casting produced.