MACHINE MOLDING AND ITS MAKING

Molding processes may be classified as hand molding or machine molding according to whether the mold is prepared by hand tools or with the aid of molding machines. Hand molding is found to be economical when only a few castings are required. Hand molding is slow and it requires considerable skill to produce good castings .

On the other hand, the use of the molding machine is advisable when the large number of repetitive castings are to be produced since hand molding is more time consuming, laborious, and becomes expensive. The dimensions of machine cast castings are more accurate, in other words , it is possible to produce castings to close tolerances. As a consequence, the weight of the castings is reduced and material saved. The working time per mold is similar than that required for hand molding, this means that, related to same shop area , the output of castings is increased per unit of time, In fine, machine molding offers higher production rates and better quality casting in addition to less heavy and lower costs, and no specialized knowledge or skill is required on the part of the operator.

A molding machine performs two important functions - it packs the sand and draws the pattern. Molding machines are, therefore classified according to (1) the method of compacting the molding sand , and (2) the method of removing the patterns. In any case, one or two pattern are fastened to pattern plate which is installed in the molding machine. The patterns are made of metal, plastics or any other suitable material.

Classification Of molding machines according to methods of compacting the sand,

a) Squeezer machine.

b) Jolt machine.

c) Jolt - Squeeze Machine.

d) Sand Slinger.

e.) Straight -draw molding machine.

f.) Stripping-plate molding machine.

g.) Turn-over molding machine.

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THROTTLE GOVERNING OF STEAM TURBINES

Steam Pressure at inlet to a steam turbine is reduced by throttling process to maintain the speed of the turbine constant at part load and hence this method of governing is called "Throttle Governing". Construction : Throttle governing system consists of a centrifugal governor, a lever , an oil pump , a pilot piston , control valve , a relay piston and a throttle valve. The throttle valve is moved by a relay piston. The relay piston is actuated by pilot piston control valve. There are two piston valves covering ports in the pilot piston control valve without any overlap. These piston valves are operated by lubricating oil supplied by a gear pump at 2 to 4 bar. The oil returns to the drain from this chamber. Working : When the turbine works on full rated load, the throttle valve will remain open. When the load is decreased, the energy output of the turbine becomes in excess and the turbine shaft speed increases. Hence, governor sleeve will lift. The upward movement of the

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Mechinopedia For Mechanical Engineers

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