What Is Sand Casting - Its Introduction, Production Process, Applications, Mold, Patterns 

Introduction :

The traditional method of casting metals is in sand molds and has been used for millennia. Sand casting is the most widely used casting process, accounting for a significant majority of the total tonnage cast. Sand casting is still the most prevalent form of casting; in the United States alone, about 15 million tons of metal are cast by this method each year. Nearly all casting alloys can be sand cast; indeed, it is one of the few processes that can be used for metals with high melting temperatures, such as steels, nickels, and titaniums. Its versatility permits the casting of parts ranging in size from small to very large and in production quantities from one to millions. Typical applications of sand casting include machine bases, large turbine impellers, propellers, plumbing fixtures, and a wide variety of other products and components.

Basically, sand casting consists of

• placing a pattern (having the shape of the desired casting) in sand to make an imprint,
• incorporating a gating system,
• removing the pattern and filling the mold cavity with molten metal,
• allowing the metal to cool until it solidifies,
• breaking away the sand mold,
• and removing the casting.
The casting must then be cleaned and inspected, and heat treatment is sometimes required to improve metallurgical properties. The cavity in the sand mold is formed by packing sand around a pattern, and then removing the pattern by separating the mold into two halves. The mold also contains the gating and riser system. In addition, if the casting is to have internal surfaces, a core must be included in the mold. Since the mold is sacrificed to remove the casting, a new sand mold must be made for each part that is produced.

PATTERNS AND CORES :

Sand casting requires a pattern a full sized model of the part, enlarged to account for shrinkage and machining allowances in the final casting. Materials used to make patterns include wood, plastics, and metals. Wood is a common pattern material because it is easily shaped. Its disadvantages are that it tends to warp, and it is abraded by the sand being compacted around it, thus limiting the number of times it can be reused. Metal patterns are more expensive to make, but they last much longer. Plastics represent a compromise between wood and metal. Selection of the appropriate pattern material depends to a large extent on the total quantity of castings to be made.
Patterns can be designed with a variety of features to fit specific applications and economic requirements. One-piece patterns, also called loose or solid patterns, generally are used for simpler shapes and low quantity production; they generally are made of wood and are inexpensive. Split patterns are two-piece patterns, made such that each part forms a portion of the cavity for the casting; in this way, castings with complicated shapes can be produced. Match-plate patterns are a common type of mounted pattern in which two-piece patterns are constructed by securing each half of one or more split patterns to the opposite sides of a single plate. In such constructions, the gating system can be mounted on the drag side of the pattern. This type of pattern is used most often in conjunction with molding machines and large production runs to produce smaller castings. Cope-and-drag patterns are similar to match-plate patterns except that split pattern halves are attached to separate plates, so that the cope and drag sections of the mold can be fabricated independently, instead of using the same tooling for both.

MOLDS AND MOLD MAKING :

Most sand-casting operations use silica sand (SiO2) as the mold material. Sand is inexpensive and is suitable as a mold material because of its high temperature characteristics and high melting point. In making the mold, the grains of sand are held together by a mixture of water and bonding clay. A typical mixture is 90% sand, 3% water, and 7% clay. Other bonding agents can be used in place of clay, including organic resins (e.g., phenolic resins) and inorganic binders (e.g., sodium silicate and phosphate). There are two general types of sand: naturally bonded (bank sand) and synthetic (lake sand). Because its composition can be controlled more accurately, synthetic sand is preferred by most foundries.
To form the mold cavity, the traditional method is to compact the molding sand around the pattern for both cope and drag in a container called a flask. The packing process is performed by various methods. The simplest is hand ramming, accomplished manually by a foundry worker. In addition, various machines have been developed to mechanize the packing procedure. These machines operate by any of several mechanisms, including
(1) squeezing the sand around the pattern by pneumatic pressure;
(2) a jolting action in which the sand, contained in the flask with the pattern, is dropped repeatedly in order to pack it into place; and
(3) a slinging action, in which the sand grains are impacted against the pattern at high speed.

The major features of molds in sand casting are as follows:

1. The flask, which supports the mold itself. Two-piece molds consist of a cope on top and a drag on the bottom; the seam between them is the parting line. When more than two pieces are used in a sand mold, the additional parts are called cheeks.

2. A pouring basin or pouring cup, into which the molten metal is poured.

3. A sprue, through which the molten metal flows downward.

4. The runner system, which has channels that carry the molten metal from the sprue to the mold cavity. Gates are the inlets into the mold cavity.

5. Risers, which supply additional molten metal to the casting as it shrinks during solidification. Two types of risers-a blind riser and an open riser.

6. Cores, which are inserts made from sand. They are placed in the mold to form hollow regions or otherwise define the interior surface of the casting. Cores also are used on the outside of the casting to form features such as lettering on the surface or deep external pockets.

7. Vents, which are placed in molds to carry off gases produced when the molten metal comes into contact with the sand in the mold and the core. Vents also exhaust air from the mold cavity as the molten metal flows into the mold.

For proper functioning, mold sand must be clean and preferably new. Several factors are important in the selection of sand for molds, and certain tradeoffs with respect to properties are involved. Sand having fine, round grains can be packed closely and, thus, forms a smooth mold surface. Although fine grained sand enhances mold strength, the fine grains also lower mold permeability. Good permeability of molds and cores allows gases and steam evolved during the casting to escape easily. The mold also should have good collapsibility to allow the casting to shrink while cooling and, thus, to avoid defects in the casting, such as hot tearing and cracking.

THE CASTING OPERATION :

After the mold has been shaped and the cores have been placed in position, the two halves (cope and drag) are closed, clamped, and weighted down to prevent the separation of the mold sections under the pressure exerted when the molten metal is poured into the mold cavity.

After solidification, the casting is shaken out of its mold, and the sand and oxide layers adhering to the casting are removed by vibration or by sand blasting. Castings also are cleaned by blasting with steel shot or grit. The risers and gates are cut off by oxyfuel-gas cutting, sawing, shearing, or abrasive wheels; or they are trimmed in dies. Gates and risers on steel castings also may be removed with air carbon-arc cutting or torches. Castings may be cleaned further by electrochemical means or by pickling with chemicals to remove surface oxides.