unit 2 casting

8/13/2019 Unit 2 Casting

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DESIGN OF SPRUE

Casting

In casting , a sprue is the passage through which a molten material is introduced into a mold , and

the term also refers to the excess material which solidifies in the sprue passage. In sand casting , the sprue is formed by a dowel which is removed from the sand to make the hole

into which the metal is poured.

FunctionSprues can serve as filters, heat sinks , and as feeders. Bronze in particular has a high shrinkage rate asit is cooling; a sprue can continue to provide molten metal to the casting, provided it is large enough toretain its heat and stay liquid, as metal in the main casting cools and shrinks.

The design of the sprue and runner system can be also utilized to trap unwanted dross and sand fromcontinuing into the main cavity; this may include adding porous material to the runners, or designing the

sprue to eject the dross to the side of the sprue using cyclonic separation . ]Sprue design

The design of gating and runner is also essential for casting. The design can be done by using verticaland bottom gating.

For bottom gating

where:

= Time for filling

= Area of mold

= Area of gate

= Acceleration due to gravity

= Total height

= Height of mold cavity

This equation may change if the height of gating is equal to height ofcasting material.

Then the equation will be:

or, simplified,
http://en.wikipedia.org/wiki/Castinghttp://en.wikipedia.org/wiki/Castinghttp://en.wikipedia.org/wiki/Castinghttp://en.wikipedia.org/wiki/Molding_(process)http://en.wikipedia.org/wiki/Molding_(process)http://en.wikipedia.org/wiki/Molding_(process)http://en.wikipedia.org/wiki/Sand_castinghttp://en.wikipedia.org/wiki/Sand_castinghttp://en.wikipedia.org/wiki/Sand_castinghttp://en.wikipedia.org/wiki/Dowelhttp://en.wikipedia.org/wiki/Dowelhttp://en.wikipedia.org/wiki/Dowelhttp://en.wikipedia.org/wiki/Heat_sinkhttp://en.wikipedia.org/wiki/Heat_sinkhttp://en.wikipedia.org/wiki/Heat_sinkhttp://en.wikipedia.org/wiki/Bronzehttp://en.wikipedia.org/wiki/Bronzehttp://en.wikipedia.org/wiki/Bronzehttp://en.wikipedia.org/wiki/Drosshttp://en.wikipedia.org/wiki/Drosshttp://en.wikipedia.org/wiki/Drosshttp://en.wikipedia.org/wiki/Cyclonic_separationhttp://en.wikipedia.org/wiki/Cyclonic_separationhttp://en.wikipedia.org/wiki/Cyclonic_separationhttp://en.wikipedia.org/wiki/Cyclonic_separationhttp://en.wikipedia.org/wiki/Drosshttp://en.wikipedia.org/wiki/Bronzehttp://en.wikipedia.org/wiki/Heat_sinkhttp://en.wikipedia.org/wiki/Dowelhttp://en.wikipedia.org/wiki/Sand_castinghttp://en.wikipedia.org/wiki/Molding_(process)http://en.wikipedia.org/wiki/Casting


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where:

= Time for filling

= Area of mold

= Area of gate

= Acceleration due to gravity

= Total height (Height of gating + height of mold cavity)

= Height of mold cavity

DESIGN OF RISER

Riser is a source of extra metal which flows from riser to mold cavity to compensate for shrinkage which

takes place in the casting when it starts solidifying. Without a riser heavier parts of the casting will haveshrinkage defects, either on the surface or internally.

Risers are known by different names as metal reservoir, feeders, or headers.

Shrinkage in a mold, from the time of pouring to final casting, occurs in three stages.

1. during the liquid state2. during the transformation from liquid to solid3. during the solid state

First type of shrinkage is being compensated by the feeders or the gating system. For the second type ofshrinkage risers are required. Risers are normally placed at that portion of the casting which is last tofreeze. A riser must stay in liquid state at least as long as the casting and must be able to feed the castingduring this time.

Functions of Risers

Provide extra metal to compensate for the volumetric shrinkage Allow mold gases to escape Provide extra metal pressure on the solidifying mold to reproduce mold details more exact

Design Requirements of Risers

1. Riser size: For a sound casting riser must be last to freeze. The ratio of (volume / surfacearea) 2 of the riser must be greater than that of the casting. However, when this condition doesnot meet the metal in the riser can be kept in liquid state by heating it externally or usingexothermic materials in the risers.

2. Riser placement: the spacing of risers in the casting must be considered by effectively calculatingthe feeding distance of the risers.

3. Riser shape: cylindrical risers are recommended for most of the castings as spherical risers,although considers as best, are difficult to cast. To increase volume/surface area ratio the bottomof the riser can be shaped as hemisphere.


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DESIGN OF SPRUE


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DESIGN OF GATING SYSTEM


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unit 2 casting

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