Permeability is a property of
foundry sand with respect to how well the sand can vent, ''i.e.'' how well gases pass through the sand. And in other words, permeability is the property by which we can know the ability of material to transmit fluid/gases. The permeability is commonly tested to see if it is correct for the
casting
Casting is a manufacturing process in which a liquid material is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified part is also known as a ''casting'', which is ejected ...
conditions.
Affecting factors
The
grain size
Grain size (or particle size) is the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may also be applied to other granular materials. This is different from the crystallite size, which refe ...
, shape and distribution of the foundry sand, the type and quantity of bonding materials, the density to which the sand is rammed, and the percentage of moisture used for tempering the sand are important factors in regulating the degree of permeability.
Significance
An increase in permeability usually indicates a more open structure in the rammed sand, and if the increase continues, it will lead to penetration-type defects and rough castings. A decrease in permeability indicates tighter packing and could lead to blows and pinholes.
Testing procedure
* On a prepared mould surface as a sample, permeability can be checked with use of a mould permeability attachment to permeability meter, readings such obtained are of relative permeability, and not absolute permeability. The relative permeability reading on a mould surface is only used to gauge sample-to-sample variation.
* On standard specimen as a sample
** For sands that can be compressed, e.g.: bentonite-bonded sand, also known as green sand, a compressed or rammed sample is used to check permeability.
** For sand that cannot be compressed, e.g.: Resin-coated sands, a freely filled sample is used. To check such a sample, user may have to use an attachment to the permeability meter called a core permeability tube.
The absolute permeability number, which has no units, is determined by the
rate of flow of air, under
standard pressure
Standard temperature and pressure (STP) are standard sets of conditions for experimental measurements to be established to allow comparisons to be made between different sets of data. The most used standards are those of the International Union ...
, through a rammed cylindrical specimen.
DIN
DIN or Din or din may refer to:
People and language
* Din (name), people with the name
* Dīn, an Arabic word with three general senses: judgment, custom, and religion from which the name originates
* Dinka language (ISO 639 code: din), spoken by ...
standards define the specimen dimensions to be 50 mm in diameter and 50 mm tall, while the
American Foundry Society defines it to be two inches in diameter and two inches tall.
[AFS Mould and Core test handbook] rammed cylindrical specimen.
formula is
PN = (VxH)/PxAxT
where
* V = volume of air in ml passing through the specimen
* H = Height of the specimen in cm
* A = Cross sectional area of specimen in cm2
* P = Pressure of air in cm of water
* T = Time in minutes
American Foundry Society has also released a chart where back pressure (P) from a rammed specimen placed on a permeability meter is correlated with a Permeability number. The Permeability number so measured is used in foundries for recording permeability value.
References
{{DEFAULTSORT:Permeability (Foundry Sand)
Casting (manufacturing)
Metallurgy