For good permeability of moulding sand, the following is not a desirable characteristics:

Explanation:

Properties of Moulding Sand

• Proper moulding sand must possess six properties.
• It must have porosity, flowability, collapsibility, adhesiveness, cohesiveness or strength, and refractoriness.
• The properties are determined, not only by the chemical composition, but by the amount of clay matter in the sand, by its moisture content, and lastly by the shape and size of the silica sand grains.
• The permeability of the moulding sand is influenced by: the quality and quantity of clays and quartz in the mixture, moisture content, degree of compaction etc.
• The higher the percentage of silica content of a moulding sand mixture (that is lower clay content), the better its permeability and vice versa.
• The larger the grain size, the better its permeability and vice versa.
• The distribution of grain size in a particular sand mix greatly influences permeability.
• If all the sand particles in a mix are of the same size, then the bulk density is low and the permeability is higher because of the voids between the sand particles.
• Angular grains that are randomly oriented will have a lower bulk density than spherical grains and thus higher permeability.
• A sand mixture containing grains of all sizes, from the largest that can be used to the smallest ones, will have a bulk density approaching the true density of the sand.
• In this case, the permeability of the mixture will be very low, because the smaller particles will fill up the interspaces of bigger grains.