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Metal Casting

Sand Casting:

The processes for the casting of metals can be divided into two main groups : casting with expendable molds and casting with permanent molds which can be reused a large number of times i.e Chill casting, pressure die casting, etc. It is necessary to make a model of the casting to be produced. Such a model is called a pattern in founding. The mold is then produced from the pattern. Wood, plaster, metal and plastics are materials used for pattern making. Except for very simple castings, the pattern will generally comprise two or more parts : the actual pattern and the cores which will form the cavities and recesses in the casting.

In casting with expendable molds, the individual pattern parts are first made by hand or by mechanical means and then assembled. The molding materials i.e., those used for constructing the actual molds in which the metal will be cast are usually mineral substances such as sand, cement, fireclay, plaster, etc., in conjunction with bonding agents such as Sulphite solution, oil, water glass, synthetic resins, etc. which give the molds the necessary strength and dimensional accuracy.

The bonding action may be achieved by drying or by chemical curing. In dry sand molding the mold is baked, in green-sand molding the mold is used with sand in the damp (green) condition. The metal is poured from above into an open mold. Closed molds, which are the more usual kind, are filled through a special system of channels (called runners or gates) which are generally so contrived that the metal enters at a low point and rises in the mold. When the metal has solidified and cooled, the casting is removed from the mold, and the runners and risers which ensure that the mold is properly filled and compensated for shrinkage are detached from the casting. The latter is then cleaned up by abrasive blasting, tumbling, grinding and cutting.

A sand casting or a sand molded casting is a cast part produced by forming a mold from a sand mixture and pouring molten liquid metal into the cavity in the mold. The mold is then cooled until the metal has solidified.

Sand casting is used to make large parts typically Iron, but also Bronze, Brass, Aluminum. Molten metal is poured into a mold cavity formed out of sand, natural or synthetic.

Sand casting can be used for most metals. There are many sand casting processes and special processes derived from this method. These are known by various names such as box molding, open sand molding, pit molding, template molding, etc. The most widely used method for making comparatively small castings is box molding (Fig.1).

In this method the pattern is embedded in the sand or other mold material within a molding box which usually comprises an upper and a lower part, the sand being compacted by ramming, pressure or vibration. Then the box is opened, the pattern removed, the cores inserted, the box closed again and casting carried out. For the casting of very large, heavy and intricate components the pit molding process is employed. Here the mold is built up in a casting pit. To give the sand greater strength when used as a mold material for large castings, cement may be added to it (cement-sand method). For symmetrically shaped castings the mold is sometimes formed by means of a template, a metal plate cut to the desired profile for producing a certain shape when it is moved along. e.g., slid along a guide track or rotated on a pivot (Fig.2).

Shell Molding

A substantial saving of mold material may be effected by using a molding box whose shape roughly corresponds to that of the pattern it encloses, so that only a relatively thin layer of material is needed. The molding sand (or other mold material) is introduced into the box by a blowing device and is compacted to form a shell-like mold around the pattern.

In plaster molding, the pattern made of metal or plastic is surrounded by a paste of gypsum plaster which is removed when it has set and is then assembled to form the mold which will receive the metal. With plaster it is possible to make molds of high precision, but this material has the disadvantage of having only low permeability to gas, which may give rise to difficulties in casting. This drawback can be overcome by the addition of foaming agents which increase the porosity of the plaster.

An important advance was achieved with the shell molding process, invented by Croning and patented in 1944 (Figs.3 and 4). Its principle consists in making a thin shell -only a few millimeters thick – around a pattern and then assembling the parts of the shell to form the mold for the metal. A suitable mold material is a mixture of 95% fine quartz sand and 5% synthetic resin and a hardening agent. Alternatively, a sand whose grains have been precoated with a resin may be used, the advantage of this technique being that segregation of the material is thereby prevented. The metal parts that make up the pattern are mounted on a metal base plate, and together they are heated to about 250 oC. Mold material is heaped on the pattern or deposited on it by a blowing device.

The temperature causes the mold material to form a thin adherent coating (“shell”) on the pattern by melting the resin constituent in immediate contact with the pattern. The rest of the mold material (in which the resin has not melted, so that the material is still of a loose granular constitution) is them removed, and the shell is cured by heating for a short time to 450 oC. With the help of the hardening agent the shell thus attains the necessary strength to serve as a mold to receive the metal and is detached from the pattern (Fig.3).

The hollow core is produced by a similar method (Fig.4). The halves of the shell and the cores are assembled to form the mold and are gripped in special holding devices or are glued together with a special adhesive. For casting, a number of molds may be arranged in stacks or installed side by side in a box, the voids between the molds being packed with steel balls to hold them firmly in position. When the casting metal is poured, the resin in the shell is burned away by the heat released from the metal so that only the sand remains, which can afterwards be easily shaken or knocked off the solidified casting.

Shell moulding is a process for producing simple or complex near net shape castings, maintaining tight tolerances and a high degree of dimensional stability. Shell moulding is a method for making high quality castings. These qualities of precision can be obtained in a wider range of alloys and with greater flexibility in design than die-casting and at a lower cost than investment casting. The process was developed and patented by Croning in Germany during World War II and is sometimes referred to as the Croning shell process.

Shell moulding process is an efficient, economical method of producing steel castings. The shell process is ideally suited for medium to high volume production of castings ranging in weight from a few ounces up to 80 pounds.