Blow Holes
(i) Excessive moisture in the moulding sand.
(ii) Low permeability and excessive fine grain sands.
(iii) Cores, neither properly baked not adequately vented.
(iv) Extra hard rammed sand.
(v) Rusted and damp chills, chaplets and inserts.
(vi) Excessive use of organic binders.
Remedies
(i) Control moisture content.
(ii) Use clean and rust free chills, chaplets and metal insert.
(iii) Bake cores properly.
(iv) Proper use of organic binders.
(v) Cores and moulds should be properly vented.
(vi) Moulds should not be rammed excessively hard.
Porosity or Pin Holes : Surface reactions sometimes cause subsurface porosity or pinholes. In aluminium alloys containing more than about 1% magnesium, magnesium reacts with water vapor of the mould to form H2 gas resulting in hydrogen filled pinholes at the surface, as shown in figure.
In steel castings subsurface pinholes may result from incomplete de oxidation of the molten metal.
Causes
(i) High pouring temperature.
(ii) Less flux used.
(iii) Gas dissolved in metal charge.
(iv) High moisture and low permeability in mould.
Remedies
(i) Ensure effective degassing.
(ii) Reduce moisture and increase permeability.
(iii) Increase flux proportions.