Permeability (foundry sand)

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.^[1]

• 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, through a rammed cylindrical specimen. DIN standards define the specimen dimensions to be 50 mm in diameter and 50 mm tall,^[2] while the American Foundry Society defines it to be two inches in diameter and two inches tall.^[3] 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.