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Freezing time

Chrvorinov's rule is based on the assumption that the freezing time is governed by its (V/A)2ratio or

known as modulus. It stated that the freezing time of the feeder must be at least as long as thefreezing time of the casting. In other words, the modulus of the feeder must be equal to or greater

than the modulus of the casting. Generally, Chvorinov's rule is the basis for calculating the size

of riser for short freezing range alloys such as steel or pure metals.

Cooling curves are important in controlling the quality of a casting. The most important part of the

cooling curve is the cooling rate which affects the microstructure and properties. Generally speaking, an

area of the casting which is cooled quickly will have a fine grain structure and an area which cools slowly

will have a coarse grain structure.

By consideration of Chvorinov's rule, a good shape for a riser would be one that has a long freezing time

or a small surface area per unit volume.A sphere would make the most efficient riser but presents

considerable difficulty to the pattern or mould maker who must remove the pattern from the mould. Asa result, the most popular shape for a riser is a cylinder, in which the height-to-diameter ratio is varied

depending on the nature of the alloy, the location of the riser, the size of the flask, and other variables.

Risers should be located so that directional solidification occurs. Since the thickest regions of a casting

are the last to freeze, the risers should be located so as to feed into these heavy sections.

Riser Design

The function of a riser is to supply addition molten metal to a casting to ensure a shrinkage porosity free

casting. Shrinkage porosity occurs because of the increase in density from the liquid to solid state of

metals. To be effective a riser must solidify after the casting and contain sufficient metal to feed the

casting or portion of a casting. Casting solidification time can be predicted using Chvorinovs Rule.

Chvorinovs Rule provides guidance on why risers are typically cylindrical. The longest solidification

time for a given volume is the one where the shape of the part has the minimum surface area. From a

practical standpoint, the cylinder has the least surface area for its volume and is easiest to make.

****copy from

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+freezing+time&source=bl&ots=z2p-V4CQW0&sig=gPFthHCOtYkEuVfvj0W_-

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Starting from " cylindrical-shaped riser are common as they loose low amount of heat...........

(i)........

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This expression is used for the simplest method for desiging a riser. It is called the modulus method.

While modern computer methods make it easier to optimize the riser, an initial guess of the correct

geometry is needed. The modulus method provides the best method for obtaining that initial guess.

With modern CAD packages it is easy to obtain the volume and surface area of the part. Many packages

will calculate these values for you. If you do not have access to a modern CAD package then the casting

geometry is broken up into a series of cubes, cylinders, and other prismatic entities and their volumes

and surface areas are added together.

To ensure a riser can feed a casting or casting section, its maximum feeding volume should bechecked

against the casting or section volume.

The location of a riser is very important for its performance. Riser should always be located at in-gates

or in hot spots within the casting. Hot spots are portions of a part that are thicker than the surrounding

geometry and therefore cool faster than the rest of the casting.