MOLDING

A mold is a void or cavity created in a compact sand mass with the help of a pattern, which is the near replica of the casting to be made. This cavity is filled with molten metal, which, on solidification, results into a casting. The pattern nearly resembles with the shape and size of the casting to be made. The process of creating the cavity or making of mold (in the sand) is termed molding. It consists of compacting molding sand around a pattern (made of wood, plastic or metal) enclosed in a mold box or flask. Molding-related activities include: pattern making, preparing molding sand mixture, core making, baking of mold and core and preserving molds in case of dry sand molds and metallic molds. Essential features of a sand mold and few terms related to the mold.

Pouring basin or pouring cup is an enlarged cup-shaped cavity made at the top end of a vertical sprue to help easy feeding of molten metal into the mold from the laddle during the process of casting.

Sprue is a vertical entry for the molten metal to flow downward to runners and finally to the mold cavity.

Runners are the channels to carry molten metal from the sprue to mold cavity.

Risers are specifically designed to store some additional metal during pouring so that when the casting shrinks during its solidification, the metal from the risers may flow to the casting to compensate for its shrinkage. Risers are designed to keep the metal in them in molten state until the casting is fully solidified. This way, the risers are the last to solidify. Staic Risers also help in escaping out (from the mold) of the entrapped gases and slag which may be dissolved (or in suspension) with the molten metal. Risers may be open risers or blind risers. Open risers indicate the complete filling of the mold by overflowing the molten metal from the top of the risers during casting.

Core is the insert (made of sand) placed in the mold cavity to form hollow regions misert within the casting or to define the interior surfaces of casting. Cores are also used on the outside of casting to form features such as deep external pockets or contours. Cores are made of sand and some binding material and are usually baked for strength before use. Metal cores are used with permanent metallic molds.

Vents are passages to carry off air and gases produced in the mold cavity when the molten metal comes in contact with the mold walls and the cores, since both are usually made from sands having additives which, on burning, generate lots of gases.

Gates or ingates are the entry ends of the runners connecting the mold cavity or risers with the main runners.

Various casting processes (discussed later) need different types of molds which may be broadly categorized as follows.

(a) Expendable molds such as sand mold, plaster mold, shell mold, investment casting mold, etc., which are used only once (for casting) as these have to be broken up for removing the casting from them.

(b) Permanent molds are made from steel or graphite and are so designed that they can be used repeatedly for a number of times to produce a large number of castings (as the casting can be removed from the mold without breaking it). Examples of such molds include: molds for die-casting, continuous casting and centrifugal casting.

(c) Composite molds are made of two or more different materials, such as sand and graphite molds are used for casting aluminium alloy torque convertors.

SALIENT FEATURES OF A GOOD SAND MOLD

A good sand mold should possess the following specific properties to ensure production of sound castings.

(i) Properly designed gating system to provide adequate and turbulence free flow of metal in the mold.

(ii) Generate minimum amount of gases on interaction of molten metal with mold.

(iii) Good venting capability to vent out gases generated during casting.

(iv) Good capability of resisting erosive action of hot metal flowing in the mold to ensure correct shape and size of casting.

(v) Higher refractoriness to withstand high temperatures of molten metal without the fusion of sand. (vi) Good strength to resist weight of molten metal.

(vii) Collapsibility for giving way to free contraction of solidifying casting for avoiding hot tears and cracks in casting.

(viii) Capability to provide specific rate of cooling for the casting so that a casting of desired structure may be produced.

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